Twin spray dryer installation.
(By courtesy of Niro Atomizer.)
CHEMICAL AND PROCESS ENGINEERING SERIES
Spray Drying
An Introduction to Principles,
Operational Practice and Applications
K. MASTERS
LEONARD HILL BOOKS LONDON
AN INTERTEXT PUBLISHER
Published by
Leonard Hill Books
a division of
International Textbook Company Limited
158 Buckingham Palace Road, London SWI W9TR, and
24 Market Square, Aylesbury, Bucks
© K. Masters 1972
All rights reserved. No part of this publication may be reproduced,
stored in a retrieval system, or transmitted, in any form or by any
means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the copyright owner.
First published 1972
ISBN 0 249 44093 8
c
Printed in Great Britain by J. W. Arrowsmith Ltd
To My Loving Wife, Ben to
for her patience and understanding during the many long
months of writing, and for her crowning the completion of
the book with a supreme gift, Caroline, a sister for Stephen.
Preface
The importance of spray drying, as illustrated by the rapid expansion
of spray drying facilities in all regions of industry, has not been reflected in
literary coverage of developments in industry: I have long felt a book
dealing specifically with the spray dryer operation is long overdue, In all
standard text books and handbooks on chemical and mechanical engineering, spray drying has been restricted to limited coverage under the section
headed 'Drying', and in most of these cases coverage consists of a few paragraphs following detailed handling of the more traditional drying operations,
for example, tray, rotary and drum drying. This, tdo';Carii be said of books
dealing specifically with drying where the text on spray rying is general in
nature. *
The true status of spray drying has not been truly voice
of applications being varied—ranging from low prodUction - rates,m the most
delicate pharmaceutical operations to high tonnage- Procinction :;:t1
inorganic chemicals. The purpose of this book is toprovide' a:sre erence
.,
medium covering spray drying principles, equiprnent, operatiOnat practice
and applications. By discussing these aspects in various degreeS Ofdetai
I hope to focus more attention on spray drying as a highly specialized
operation, fully established in many fields of inthistry.
The script has been prepared for both personnel associated with spray
drying and students of industrial drying. 1 have tried to relate the current
knowledge of the subject in practical terms, with worked examples to show
that theoretical principles, when backed up by practical experience, can be
often applied to predict, assess, and optimize spray dryer performance, to
* (a) Publications devoted solely to spray drying: see Marshall (35), Lykov and Lenochik
(478;) (b) recent publications containing chapters on spray drying: see Dombrowski and.
Munday (30), Williams-Gardner (376), Nonhebel and Moss (479); (c) literature reviews"on
spray drying: see Masters (I), McCormick (494).
ix
X PREFACE
select the most suitable dryer designs for a particular application, and to
show how limitations in existing designs can be rectified through equipment
modification. Theoretical models used to describe the spray dryer operation
are not discussed in detail. The majority of important equations are just
stated, and not derived. To those readers who wish to study aspects of spray
drying in greater depth, adequate references are given.
The choice of units for this book presented a difficult decision. In my
experience of spray drying installations, personnel concerned with their
operation do not use or have not as yet become familiar with the SI units
system due to become standard in the mid-seventies, Based upon the practical emphasis intended for the script I decided to maintain existing units,
joining in line with the latest editions of standard chemical and mechanical
engineering textbooks and handbooks. Many of these established books are
either American or English in Origin, thus based upon the British unit
systems. However, there are numerous personnel with spray drying interests
operating in areas using metric systems. As a compromise I have tried to
utilize both systems, by duplicating data where convenient, and by presenting
worked examples in either one set of units or the other, Perhaps this is far
from ideal, and I expect comment on this decision but I feel this approach
will familiarize the text to a greater number of people than had I used one
set of units throughout. Most people are familiar with both sets of units,
but feel more competent in one than the other, although they possess the
ability to convert units quickly.
Worked examples have been included in the text to illustrate use of
theoretical principles, empirical equations and operation of equipment.
Calculations throughout have been conducted using a slide rule or logarithmic tables, items that are readily available to personnel from management
to plant operator levels. Only in chapter 7 (example 7.4) dealing with droplet
motion in air was a computer used to eliminate the need to rework constantly
a given equation.
The writing of this book would not have been possible without the cooperation of the management of Aktieselskabet Niro Atomizer, Copenhagen,
(N.A.), and permission to use much of the pictorial material owned by N.A.
I wish to put on record my indebtedness to the late Dr. K. Nielsen, who as
Head of the Research and Development Department of N.A. encouraged
the commencement of the project, and who made available to me an extensive literature survey on spray drying.
I also wish to acknowledge the help of so many of my colleagues, whose
critique of the text proved so useful in the final preparation of the text for
the publisher. All I duly thank. Certain of my colleagues assisted so generously that special thanks are due ; Dr. S. Hovmand, Mr. M. Petersen,
for their assessment and suggestions in the first eleven chapters, Mr. Aa,
PREFACE xi
Moller for his opinions on atomization, Mr. M. Troldborg, whose experience
in the field of auxiliary equipment and operational practice I drew heavily
upon, Mr. J. Due Jensen for his unselfish assistance in tabulating patent
coverage of spray drying, Messrs. E. Houghton-Larsen, H. Justesen, and
J. Storm for their invaluable suggestions for improving the section on spray
drying operations, last but not least Mr. J. Pilegaard for the services of the
Information Department.
I wish to express my thanks to my wife and sister who assisted with the
preparation of the text, and to all the secretaries, who typed the text so well,
no mean task dealing with a tongue far removed from their native Danish.
My thanks also go to the ladies who assisted in preparing drawings, especially
Mrs. Lundgaard on whom the bulk of the work fell.
While acknowledging such assistance, I must emphasize that the interpretation of spray drying presented here is solely mine, and does not
necessarily agree with the official view of my employer (N.A.).
It is hoped that my handling of the script encourages greater use of the
spray drying operation and that, by providing a reference book, I have
helped fill the literature gap that exists as far as spray drying is concerned.
If these goals are achieved, the energies expended will be fully rewarded,
since there can be no disputing that spray drying today is a subject in its
own right.
Spray drying is a relatively new technique, but is already broad in scope.
The selection of material had to be a matter of personal choice. However,
to those readers who feel the text could be enhanced by clarification of any
part, or by further addition, I would be grateful to hear their suggestions.
K. Masters
Skovlunde
Denmark 1972
Contents
Preface
List of figures
List of tables
Introduction
ix
xxv
xxxiii
1
SECTION 1—BASIC PRINCIPLES, DEFINITIONS
I SPRAY DRYING FUNDAMENTALS
2 REPRESENTATION OF SPRAYS
2.1 Terminology
2.1.1 Droplet
2.1.2 Particle
2.1.3 Agglomerate
2.1.4 Size
2.1.5 Particle shape
2.1.6 Size distribution
2.1.7 Mean size
2.2 Methods of Data Presentation
2.2.1 Tabular form
2.2.2 Graphical form
(a) histograms
(b) size frequency curves
(c) cumulative plots
2.3 Analysis of data
2.3.1 Mean diameters
(a) most frequent diameter
(b) arithmetic mean diameter
11
23
23
23
23
23
25
25
25
25
26
27
27
27
27
28
28
28
29
29
X1V
CONTENTS
(c) geometric mean diameter
(d) harmonic mean diameter
(e) median diameter
2,3.2 Distribution functions
(a) normal distribution (Gaussian)
(b) log-normal distribution
(c) square root normal distribution
(d) empirical distribution functions
(i) Nukiyama–Tanasawa distribution
(ii) Rosin–Rammler distribution
(iii) upper limit function
2.3.3 Mean droplet diameter transformations
3 DRYING TERMS AND PRINCIPLES
3.1 Common Terms and Principles
3.2 Applying the Psychrometric Chart to the Spray Drying
Process
4 PERFORMANCE OF SPRAY DRYERS
4.1 Introduction
4.2 Heat and Mass Balances over Spray Dryers
4.3 Dryer Efficiency
4.4 Residence Time in Drying Chamber
5 EQUIPMENT INCORPORATED IN SPRAY DRYERS
5.1 Introduction
5.2 Passage of Air and Product through a Spray Dryer
5.3 Equipment Items Making up a Spray Dryer Layout
5.3.1 Equipment comprising the feed system
5.3.2 Atomizer
5.3,3 Equipment comprising the supply air system
5.3.4 Air disperser and drying chamber
5,3.5 Equipment for separation and recovery of product
from exhaust air
29
30
30
33
33
34
34
35
35
35
37
38
42
42
47
54
54
55
59
62
65
65
65
66
66
68
70
72
78
SECTION II—THE PROCESS STAGES OF SPRAY DRYING
6
ATOMIZATION
6.1 Introduction
6.2 Basic Mechanism of Atomization
6.3 Classification of Atomizers
6.4 Application of Basic Mechanisms to Commercial Atomizer
Conditions
6.5 Rotary Atomization
6.5.1 Introduction
6.5.2 Flat smooth disc atomization (vaneless)
87
87
87
91
92
93
93
93
6.6
CONTENTS
XV
(a) flow over smooth flat vaneless discs
(b) droplet size from smooth flat vaneless discs
6.5.3 Wheel atomization (atomizer wheels)
(a) introduction
(b) flow over a vaned wheel
(i) radial velocity
(ii) tangential velocity
(iii) resultant release velocity
(iv) angle of liquid release
(c) effect of operating variables on droplet size
(i) effect of liquid feed rate on droplet size
(ii) effect of peripheral speed on droplet size
(iii) effect of liquid viscosity on droplet size
(d) prediction of mean droplet diameter and spray size
distribution
(i) mean droplet diameter
(ii) size distribution
(iii) weight distribution of sprays
(e) atomizer wheel performance during spray drying
(i) air pumping effects
(ii) drying air entry
(iii) particle shape change
(f) atomizer design requirements
(i) atomizer drive
(ii) liquid distributor
(iii) atomizer wheels
(g) applications of atomizer wheels
6.5.4 Disc atomization (vaneless bowls, cups, plates)
(a) introduction
(b) fluid flow over vaneless discs
—radial velocity
—tangential velocity
(c) droplet size prediction
(d) performance of vaneless discs
(e) applications of vaneless discs
Nozzle Atomization
6.6.1 Introduction
6.6,2 Centrifugal pressure nozzle
(a) theoretical principles
(b) spray patterns
(c) fluid flow from centrifugal pressure nozzles
(d) nozzle operating characteristics
97
100
100
100
102
103
104
104
104
105
108
109
112
113
114
118
126
127
127
129
130
135
135
141
142
148
148
148
150
151
151
152
154
157
158
158
159
159
162
165
170
XV1
CONTENTS
(i) relation between flow rates, pressure and
density
(ii) spray angle
(iii) effect of operating variables on droplet size
—effect of capacity on droplet size
—spray angle and droplet size
—effect of pressure on droplet size
—effect of feed viscosity on droplet size
—effect of surface tension on droplet size
—effect of orifice size on droplet size
(iv) droplet size distribution
(e) prediction of nozzle performance
(f) prediction of mean droplet size of sprays from
centrifugal pressure nozzles
(g) nozzle operating features and nozzle applications
(h) nozzle assemblies for commercial use
6.6.3 Pneumatic (two-fluid) nozzle atomization
(a) theory
(b) design features
(c) droplet release and spray angle
(d) spray characteristics
(e) effect of variables on droplet size
(i) effect of air–liquid ratio
(ii) effect of relative velocity on droplet size
(iii) effect of viscosity on droplet size
(iv) effect of air density on droplet size
(f) prediction of mean droplet size
(i) Nukiyama–Tanasawa equation
(ii) Gretzinger–Marshall equation
(iii) Kim–Marshall equation
(iv) Kim–Marshall equation (three-fluid nozzle
atomizer)
(g) comparison between the spray characteristics of
two- and three-fluid nozzles
(h) pneumatic nozzle assemblies and their operation
(i) advantages and disadvantages of pneumatic
nozzles
(j) applications
6.6.4 Rotating pneumatic cup atomizers
(a) introduction
(b) effect of variables on droplet size
(c) prediction of mean droplet size
170
170
171
171
172
172
173
174
174
174
175.
182
188
193
196
196
197
199
200
201
201
202
202
202
202
206
211
217
221
221
222
227
228
229
229
231
232
CONTENTS
XVii
(d) prediction of droplet size distribution
6.7 Sonic Atomization
6.8 Atomizer selection
7 SPRAY-AIR CONTACT (MIXING AND FLOW)
7.1 Introduction
7.2 General Principles
7.3 Droplet Trajectory Characteristics
7.3.1 From rotary atomizers (atomizer wheels)
7.3.2 From nozzle atomizers
7.4 Droplet Moverhent in Drying Chambers
7.4.1 Droplet release velocities
7.4.2 Droplet deceleration
(a) from rotary atomizers
(b) from nozzle atomizers
7.4.3 Terminal velocity (spherical droplets)
7.4.4 Droplet movement : distance–time relationships
(a) droplet movement in non-rotating air
(i) motion from rotary atomizers
(ii) motion from nozzle atomizers
(b) droplet movement in rotating air
(i) large diameter chambers
(ii) small diameter chambers
7.4.5 Product deposition on chamber wall
(a) in small chambers
(b) in large chambers
8 DRYING OF DROPLETS/SPRAYS
8.1 Introduction
8.2 Evaporation of Pure Liquid Droplets
8.2.1 Evaporation of single droplets
(a) droplet evaporation under negligible relative
velocity conditions
(b) droplet evaporation under relative velocity
conditions
(c) evaporation under high air temperature conditions
8.2.2 Evaporation of sprays of pure liquid droplets
8.3 Evaporation of Droplets Containing Dissolved Solids
8.3.1 Evaporation of single droplets
(a) vapour pressure lowering effect
(b) effect of dried solid formation in droplets
(c) droplet evaporation times
8.3.2 Evaporation of sprays containing dissolved solids
8.4 Evaporation of Droplets Containing Insoluble Solids
233
234
235
241
241
247
249
249
251
251
251
252
252
253
256
261
261
261
267
271
271
273
274
275
276
279
279
281
282
282
284
293
297
302
302
302
303
305
309
310
XViii
CONTENTS
8.5
9
Drying in Media Other than Air
8.5.1 Drying in gases
8.5.2 Drying in superheated vapours
8.5.3 Drying in superheated steam
8.6 Dried Product Properties on Completion of Drying
8.6.1 Effect of operating variables on product properties
(particle size and bulk density)
(a) effect of feed rate
(b) effect of feed solids
(c) effect of feed temperature
(d) effect of surface tension
(e) effect of inlet drying temperature
(f) effect of spray/air contact velocity
8.6.2 Formation of hollow particles
8.7 Size Differences between Product before and after Drying
8.7.1 Procedures to predict dried particle size distribution
(a) graphical method
(b) mathematical method
8.8 Size Distributions of Wet Sprays and Resulting Dried
Particles
SEPARATION AND RECOVERY OF DRIED PRODUCT FROM AIR
9.1 Introduction
9.2 Selection of Equipment for Separating Powder from
Exhaust Drying Air
CONTENTS
313
314
314
315
316
11
316
316
316
316
317
318
319
319
319
320
320
320
322
324
324
327
10
CONTROL SYSTEMS
1 0, 1 Control System (based upon feed rate regulation)
10.2 Control System (based upon heat input regulation)
10.3 Feed System Controls
10.3.1 Systems for rotary atomizers
10.3.2 Systems for nozzle atomizers
10.4 Interlocks
10.5 Fully Automatic Spray Dryers
10.5.1 Programmed start-up
10.5.2 Full automation of spray dryer and feed
pretreatment
10.6 Precaution Against Fire and Explosion
10.7 Control Systems for Preventing Fire/Explosion
Conditions in Deposit Formations
335
336
338
339
339
339
340
341
341
344
347
349
352
352
352
356
357
357
364
365
371
378
378
380
384
385
385
386
388
390
392
393
398
SECTION IV—SURVEY OF AUXILIARY EQUIPMENT
12
SECTION III—OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
11.1 Determination of Air Flow
11.1.., Supply air to dryer
11.1.2 Exhaust air leaving dryer
11.2 Measurement of Air Velocity in Ducts
11.2.1 Pitot tube measurement
11.2.2 Orifice/venturi measurements
11.2.3 Anemometer measurements
11.3 Determination of Powder Loading in Air Flows
11.3.1 Manual sampling
(a) cyclone probe (B.C.U.R,A.) method
(b) internal filter thimble method
(c) external filter thimble method
11.3.2 Continuous sampling
(a) light intensity methods
(b) electrostatic methods
(c) CEGRIT sampler
11.3.3 Determination of dried product losses from
mass balance techniques
11.3.4 Reducing powder losses from spray dryer
exhaust systems
11.4 Assessing Acoustic Environment (Noise Levels)
11.5 Measurement of Droplet and Particle Size
xix
SURVEY OF AUXILIARY EQUIPMENT
12.1 Air Heaters
12.1.1 Steam air heaters
12.1.2 Fuel oil air heaters
12.1.3 Gas air heaters
12.1.4 Electric air heaters
12.1.5 Liquid phase air heaters (thermal fluid heaters)
12,2 Fans
12.2.1 Fan requirements on a spray dryer layout
12.2.2 Fan parts and principles
12.2.3 Fan characteristics
12.2.4 Fan laws
12.2.5 Fan efficiency
12.2.6 Fan horsepower
12.2.7 Effect of altitude on fan performance
12.2.8 Fan mounting
12.2.9 Fan noise
401
401
401
404
406
406
406
407
408
408
410
411
414
415
418
418
418
XX
CONTENTS
CONTENTS
12.2.10 Fan selection
Mechanical Powder Separators
12.3.1 Dry collectors
(a) dry cyclones
(i) cyclone performance
(ii) overall cyclone efficiency
(iii) pressure drop over cyclones
(iv) cyclone layouts
(b) bag filters
(i) principles
(ii) bag filter designs
(iii) bag filter performance
(iv) pressure drop over bag filters
(v) bag filter fabrics
(vi) use of bag filters
(vii) bag filter layouts
(c) electrostatic precipitators
(i) principles
(ii) performance of electrostatic precipitators
(d) gravity settlers
12.3.2 Wet collectors
(a) wet scrubbers
(i) principles
(ii) wet scrubber designs
12.3.3 Powder removal from mechanical separators
(dry collectors)
(a) hoppers
(b) powder removal valves
12.4 Pneumatic Powder Conveying
12.4.1 Principles
12.4.2 Pneumatic conveying systems in practice
12.4.3 Operational difficulties with pneumatic
conveying systems
12.5 Pumps
12.5.1 Pump duties in spray dryers
12.5.2 Pump types and uses
12.6 Washing Equipment
12.3
419
419
420
420
422
424
427
431
432
432
432
435
436
436
436
437
437
437
438
439
440
440
440
443
445
445
445
451
451
455
456
458
458
460
460
SECTION V—APPLICATIONS OF SPRAY DRYING IN
INDUSTRY
13
14
APPLICATIONS OF SPRAY DRYING (A LITERATURE SURVEY)
APPLICATIONS IN THE CHEMICAL INDUSTRY
465
498
Plastics, Resins
14.1.1 Polyvinylchloride (P.V.C.)
14.1.2 Resins
14.1.3 SBR lattices
14.2 Ceramic Material
14.2.1 Wall tile and floor tile ceramics
14.2.2 Oxide ceramics
14.2.3 Plastic bodies
14.2.4 Glass
14.2.5 Carbides
14.3 Washing Powder
14.3.1 Heavy duty detergents
14.3.2 Light duty detergents
14.3.3 Soap powder
14.3.4 Soda products
14.3.5 Bleach powders
14.3.6 Optical brighteners
14.3.7 Perfumes for washing powder
14.4 Pesticides
14.4.1 Herbicides
14.4.2 Fungicides
14.4.3 Insecticides
14.5 Dyestuffs—Pigments
14.5.1 Titanium dioxide
14.5.2 Kaolin (china clay)
14.6 Fertilizers
14.6.1 Ammonium nitrate
14.6.2 Ammonium phosphate
14.6.3 Superphosphates
14.7 Mineral Ore Concentrates
14.7.1 Sulphide ores
14.7.2 Iron ores
14.7.3 Cryolite
14.8 Inorganic Chemicals
14.9 Organic Chemicals
14.10 New and/or Specialized Applications in the Chemical
Industry
14.10.1 Abrasives
14.10.2 Catalysts
14.10.3 Cement
14.10.4 Metal powders
14.10.5 Sodium carbonate
14.1
XXI
499
499
501
502
502
505
506
509
510
510
511
512
514
515
515.
517
517
518
518
520
521
521
521
522
523
526
526
528
530
533
535
535
537
537
537
538
539
539
540
542
543
xxii
15
CONTENTS
14.10.6 Sodium hydroxide
14.10.7 Titanium tetrachloride
APPLICATIONS IN THE FOOD INDUSTRY
15.1 Milk Products and Eggs
15.1.1 Milk products
(a) skim milk
(b) whole milk
(c) whey
(d) fat-enriched milk (milk replacer)
(e) sodium caseinate
(f) baby foods
(g) coffee/tea whiteners
(h) cheese
(i) ice-cream mix
(j) high fat powder for bakery use (also butter)
(k) buttermilk
15.1.2 Eggs
15.2 Beverages, Flavouring Compounds, Meats, Protein from
Vegetable Sources
15.2.1 Beverages
(a) instant coffee
(b) instant tea
15.2.2 Flavouring compounds
15.2.3 Meats
15.2.4 Edible protein from vegetable sources
(a) soya beans
(b) peanuts
(c) potato fruitwater
15.3 Fruits and Vegetables
15.3.1 Fruits
(a) tomatoes
(b) banana
(c) citrus fruits
(d) soft fruits
(e) other fruits
15.3.2 Vegetables
(a) beetroot
(b) asparagus
(c) peas
(d) mashed potato
15.4 Carbohydrates
15.4.1 Corn products
CONTENTS
543'
543
545
545
545
547
553
556
560
562
564
564
564
565
565
565
566
568
568
568
572
572
573
573
573
575
575
576
576
577
581
583
583
583
584
584
584
584
584
585
585
16
17
18
15.4.2 Wheat gluten
15.4.3 Sugar products
APPLICATIONS IN THE PHARMACEUTICAL-BIOCHEMICAL
INDUSTRY
16.1 Introduction
16.2 Enzymes
16.3 Antibiotics
16.4 Sera, Vaccines, Plasma and Substitutes
16,5 Micro-organisms
16.6 Yeast
16.6.1 Production of food yeast from sugar cane
molasses
16.6.2 Active yeast (brewers')
16.6.3 Barley powder
16.7 Vitamins
16.8 Pharmaceutical Gums
APPLICATIONS IN INDUSTRIES UTILIZING CHEMICALS FROM
TIMBER
17.1 Applications in the Tannin Industry
17.1.1 Manufacture of tannin from woods
17.1.2 Manufacture of tannin from barks
17.1.3 Manufacture of tannin from fruits
17.2 Applications in the Cellulose Industry
17,2.1 Concentrated sulphite waste liquor
17.2.2 Li gn osulphonates
APPLICATIONS IN THE OFFAL AND FISH INDUSTRIES
18.1 Spray Drying of Slaughterhouse By-products
18.1.1 Blood
18.1.2 Glands and tissues
18.1.3 Gelatin and glue
18.1.4 Excreta
18.2 Spray Drying of Fish Wastes
18.2.1 Stickwater (fish solubles)
18.2.2 Whole fish meal
18.2.3 Normal fish meal
18.2,4 Edible fish flour
18.2.5 Fish hydrolysates
SECTION VI—APPENDICES, NOMENCLATURE,
REFERENCES, INDEX
19 APPENDICES
A.1 Conversion Factors
XXiii
586
586
587
587
592
594
595
597
597
598
600
600
600
600
602
602
602
603
604
604
604
604
606
606
606
609
609
609
609
609
611
612
612
612
617
617
XXiV CONTENTS
A.2 Temperature Conversion Tables
A.3 Properties of Dry Air
A.4 Vapour Pressures of . Water
A.5 Psychometric Charts
1. Normal temperatures
2. Elevated temperatures
A.6 Humid Heat-Humidity Relation for Air-Water Vapour
System at Atmospheric Pressure
A.7 Temperature-Latent Heat of VaporizationTemperature Relation (Water)
A.8 Liquid Flow over Vaned Atomizer Wheel
A.9 Thermal Conductivity of some Insulation Materials
A.10 Sieve Sizes
A.11 Particle Size Distributions of Standard Powder Grades
for Use when Optimizing Cyclone Product Loading
20 NOMENCLATURE
21 REFERENCES
22 hsTDEX
620
622
623
624
624
625
626
List of Figures
627
628
630
631
633
634
640
654
Frontispiece Twin spray dryer installation
0.1 The process stages of spray drying
0.2 Layouts of spray dryers and associated suspended particle
processing equipment involving atomization
0.3 Dried powder forms produced by spray drying
1.1 The features of the process stages involved in spray drying
1.2 Equipment involved in a standard spray dryer layout
1.3 Atomizers in operation
1.4 Product-air flow in spray dryers
1.5 Air temperature distributions in co-current and counter-current flow spray dryers
1.6 Product discharge from co-current flow spray dryers with a
rotary atomizer
2.1 Range of droplet/particle sizes obtained in spray drying
operations
2.2 Size distribution of droplets and particles
2.3 Droplet size distribution represented in histogram form
2.4 Frequency and cumulative curves showing mean droplet
diameter values in example 2.1
2.5 Log normal distribution on probability paper
2.6 Droplet size distribution of a centrifugal pressure nozzle
(grooved core swirl) conforming to a square root normal
distribution
2.7 Rosin-Rammier size distribution
2.8 Comparisons of log normal, square root normal and upper
li mit distributions for a centrifugal pressure nozzle
Size
data for example 2,2
2.9
3.1 Graphical relation between common drying terms
XXV
3
5
7
12
13
15
17
18
20
24
26
27
30
35
36
37
38
41
43
XXVI
LIST OF FIGURES
3.2 Two applications of the psychrometric chart in spray drying
3.3 Drying characteristics of droplets during spray drying
4.1 Effect of feed solids content on dryer heat load
4.2 Dryer data for calculation of heat and mass balances
4.3 Drying process represented in example 4.2
5.1 Product and air passage through a spray dryer
5.2 Spray dryer layouts with rotary and nozzle atomization
5.3 Feed system involving a feed preheater
5.4 Basic types of atomizers
5.5 Atomizer positioning in drying chamber
5.6 Co-current flow dryers
5.7 Air disperser types creating rotary air flow around atomizer
5.8 Counter-current flow dryer
5.9 Mixed flow dryers
5.10 Primary product discharge from drying chambers
5.11 Total product discharge from drying chambers
5.12 Drying chamber wall cooling
5.13 Equipment for separation and recovery of airborne powders
5.14 Dry product conveying systems on spray dryers
6.1 Ohnesorge chart showing liquid jet disintegration as a
function of Reynolds number
6.2 Classification of atomizers
6.3 Mechanisms of atomization
6.4 Mechanisms of atomization (represented photographically)
6.5 Atomization mechanism for rotating discs
6.6 Liquid flow to and from edge of atomizer wheel
6.7 Effect of feed rate on mean droplet size for atomizer wheel
6.8 Effect of atomizer wheel peripheral speed on mean droplet
size
6.9 Effect of atomizer wheel speed of rotation on mean droplet
size (aqueous sprays)
6.10 Effect of feed viscosity on wheel atomization
6.11 Curves for prediction of spray size distributions from
atomizer wheels after the Herring-Marshall relation
6.12 Size relationship between droplet and particle in example 6.4
6.13 Spray weight distribution from rotary atomizers
6.14 Air flow through vaned atomizer wheel due to air pumping
effects
6.15 Pressure distribution at vaned atomizer wheel due to air
pumping effects
6.16 Arrangement for supplying cool air to atomizer wheel
6.17 Positioning of rotary atomizer in relation to air disperser
LIST OF FIGURES
48
49-50
55
56
61
66
67
69
70
71
73
74
75
76
77
79
80
81
83
89
91
95
96
98
102
108
109
111
113
119
125
126
128
129
130
131
XXVii
6.18 Atomizer drives
136
6.19 Rotary atomizer drives for atomizer wheels
139
6.20 Rotary atomizer with variable speed control through a
fluid coupling
140
6.21 Spindle pressurizing
141
6.22 Positioning of liquid distributor with atomizer wheel
142
6.23 Liquid distributor designs
143
144
6.24 Designs of vaned atomizer wheels
6.25 Abrasive wear on vaned atomizer wheel
146
6.26 Atomizer wheel with bushings for abrasive and corrosive
147
feeds
6.27 Disc atomizer (plate type, multiLtier)
149
6.28 Disc atomizer (cup type)
149
155
6.29 Effect of feed rate on mean droplet size from disc atomizer
6.30 Effect of atomizer disc speed on mean droplet size
156
6.31 Effect of number of plate tiers of a disc atomizer on particle
size distribution
157
158
6.32 Size distributions from atomizer wheels and discs
6.33 Centrifugal pressure nozzles
160
6.34 Inserts for generating liquid rotation within centrifugal
161
pressure nozzles
161
6.35 Centrifugal pressure nozzle construction
162
6.36 Spray angle characteristics
6.37 Velocity diagram for liquid flow from a centrifugal pressure
166
nozzle
168
6.38 Variation of feed rate with square root of nozzle pressure
6.39 Feed rate, feed pressure, feed density ratios for centrifugal
pressure nozzles
171
6.40 Effect of variables (feed rate, feed viscosity) on Sauter mean
172
diameter
173
6.41 Effect of feed viscosity on spray droplet size distribution
6.42 Typical droplet size distribution of a centrifugal pressure
174
nozzle used in spray drying
175
6.43 Nozzles with tangential feed entry
176-7
6.44 Nozzle dimensioning relations after Doumas and Laster
6.45 Nozzle dimensioning relations after Dombrowski and Hasson 180-1
6.46 Range of orifice sizes per given nozzle body for optimum
189
atomization conditions
6.47 Performance of centrifugal pressure nozzles (rated on
192
aqueous feed) on 42 % TS skim milk concentrate
194
6.48 Positioning of nozzles in relation to air disperser
198
6.49 Designs of pneumatic nozzle heads
XXViii LIST OF FIGURES
6.50 Effect of nozzle operating variables on spray mean droplet
size
6.51 Nozzle designs used in pneumatic nozzle investigations
6.52 Nukiyama-Tanasawa plot for converging two-fluid nozzles
operating on aqueous feed
6.53 Nukiyama-Tanasawa distribution (a typical plot)
6.54 Gretzinger-Marshall plot for converging two-fluid nozzles
6.55 The Gretzinger-Marshall droplet size relation expressed as a
nomograph
6.56 Plot of film thickness against feed rate
6.57 Dimensions of nozzle design in example 6.15
6.58 Size distribution for two-fluid nozzle (after Kim-Marshall)
6.59 Piping connections for pneumatic nozzles
6.60 Feed systems for pneumatic nozzles (two-fluid)
6.61 Feed and air distribution rings for pneumatic nozzles
6.62 Two-fluid paste nozzle with screw feeder
6.63 Atomization mechanism for a pneumatic cup atomizer
6.64 Pneumatic cup atomizer head
6.65 Pneumatic cup atomizer and air disperser positioning in
drying chamber
6.66 Particle size distributions of rotary and nozzle atomizers
operating on silica and alumina based catalysts
6.67 Modifications of the atomizer drive to handle liquid, powder
or liquid-powder feeds
7.1 Air disperser for rotary atomizers (in co-current flow dryers)
7.2 Ceiling air disperser with rotary (vaned wheel) atomizer in
co-current flow dryer
7.3 Air disperser for nozzle atomizers in co-current flow dryers
7.4 Air disperser for counter-current flow dryers
7.5 Air dispensers and atomizers in mixed flow dryers
7.6 Plot of R f I pv 2 against Reynolds number for spherical particles
7.7 Plot for CD Re 2 against Re for spherical particles
7.8 Plot of drag coefficient against Reynolds number
7.9 Areas of likely wall impingement of partially dry product
in small dryers
7.10 Impingement area control by air disperser adjustment in
large dryers with rotary atomization
8.1 Drying rate curve
8.2 Configuration of droplet evaporating in high temperature
surroundings
8.3 Estimation of temperatures of droplets containing dissolved
solids
LIST OF FIGURES XXiX
203
204
207
208
211
212
214
217
220
223
224
225
226
230
231
232
236
239
243
244
245
246
247
250
257
260
277
278
280
295
302
Effect of crust properties on evaporation times
Characteristics of droplet undergoing drying (drying air
above boiling point)
8.6 Characteristics of droplet undergoing drying (drying air
below boiling point)
8.7 Temperature profile in example 8.4
8.8 Temperature of droplet undergoing evaporation in air and
superheated vapour
8.9 Effect of feed concentration on bulk density of spray dried
products
8.10 Effect of inlet drying air temperature on bulk density of
spray dried products
8.11 Graphical determination of size distribution of dried product
from that of wet spray
9.1 Product discharge from conical and flat chamber bases
9.2 Exhaust ducts layouts with primary product discharge from
conical chamber base
10.1 Outlet air temperature chart for dryer operation under
manual and automatic control
10.2 Control system A: outlet temperature control by regulation
of feed rate, inlet temperature control by regulation of
fuel combustion rate
10.3 Control system B: outlet temperature control by regulation
of fuel combustion rate, manual regulation of feed rate
10.4 Control system for pressure nozzles (control system A)
10.5 A combined block and schematic diagram for a fully
automated spray dryer
10.6 Automatic feed solids content measuring equipment used in
fully automatic spray drying plant in the dairy industry
10.7 Block diagrams for automatic control of combined
evaporator and spray dryer systems
11.1 Pitot tube with inclined manometer
11.2 Positions for velocity measurement by pitot tube in circular
duct (equal duct areas)
11.3 Positions for velocity measurement by pitot tube in
rectangular ducts (equal duct areas)
11.4 Pitot tube pressure difference (P D )-air velocity nomograph
11.5 Permissible positions in ducts for pitot tube measurements
and air sampling
11.6 Location of venturi and orifice meters in inlet drying air duct
11.7 Positions selected in spray dryer layout for various air flow
measurements illustrated in example 11.2
8.4
8.5
304
306
307
312
315
317
318
321
325
326
336
337
339
340
342
344
346
357
359
360
361
363
364
366
XXX LIST OF FIGURES
11.8 Inlet duct zoning for anemometer measurements in
example 12.2
367
11.9 Flow conditions at sampling nozzle
372
11.10 Effect of incorrect sampling rate on observed powder loading
in air stream
373
11.11 Basic layout for sampling powder laden air
373.
11.12 Velocity distributions and profiles existing in ductwork
arrangement
375
11.13 Layout of B.C.U.R.A. cyclone probe for sampling powder
laden air
378
11.14 Layout for measuring powder loadings in air flows using
the filter thimble sampler with orifice plate for sampling
rate control
382
11.15 Filter thimble sampler for measuring powder loadings in
air flow
383
11.16 Layout for measuring powder loadings in air flows using the
filter thimble sampler with gas meter for sampling rate
control
384
11.17 Layout for external filter thimble equipment
385
11.18 Measuring powder-air loadings by light intensity methods
386
11.19 Layout for sampling equipment based upon contact
electrification
387
11.20 The CEGRIT sampler for measuring powder loading in air
flows
389
11.21 Sound pressure level against frequency plot for assessing
acoustic environment of equipment
395
12.1 Relation between saturated steam pressure and temperature
402
12.2 Indirect air heaters
404
12.3 One- and two-fan spray dryer layouts
407
12.4 Principle parts of a centrifugal fan
409
12.5 Performance curves characteristics for radial, forward
curving, and backward curving blades
410
12.6 Fan performance chart for example 12.1
411
12.7 Relation between fan horsepower and installed fan motor
horsepower
415
12.8 Basic designs of industrial cyclones
421
12.9 Theoretical and actual grade efficiency curves
423
12.10 Grade efficiency curve for cyclones specified in example 12.7
424
12.11 Measuring pressure drop over a cyclone
427
12.12 Cyclone layout in parallel
431
12.13 Grade efficiency curve for a standard bag filter
432
12.14 Designs of bag filters
434
LIST OF FIGURES xxxi
12.15 Electrical power system for an electrostatic precipitator
438
12.16 Grade efficiency curve for a dry electrostatic precipitator
439
12.17 Gravity settler
440
12.18 Designs of wet scrubber
441
12.19 Grade efficiency curves for wet scrubbers
442
12.20 Hoppers
446
12.21 Powder removal valves
448-9
12.22 Vortex air-lock
451
12.23 Powder removal from cyclones, equipment in practice
452
12.24 Pneumatic conveying layouts
453
12.25 Determination of air rate and duct size for pneumatic
conveying
455
12.26 Classification of pumps
458
12.27 Feed system layouts
459
12.28 Automatic washing equipment
461
14.1 Examples of unit operations associated with wet preparation
as compared with spray drying process
502
14.2 Spray dryers for ceramic pressbody
503
14.3 Micro-photographs of pressbody
504
14.4 Particle size distributions of wall tile material produced by
atomizer wheel and pressure nozzle atomization
505
14.5 Spray dried steatite
506
14.6 Flow diagram for production of ferrite by spray drying
508
14.7 Flow diagram for preparation of spray dried jiggering bodies
510
14.8 Flow diagram for spray drying carbides in closed cycle
511
14.9 Flow diagram for spray drying of detergent formulations
516
14.10 A semi-closed cycle spray dryer for herbicides
520
14.11 Flow diagram for spray drying titanium dioxide
523
14.12 Flow diagram for producing kaolin by spray drying
524
14.13 High tonnage spray dryers on kaolin in Georgia, USA, each
producing 15-25 tons per hour
525
14.14 Production of ammonium nitrate prills by spray cooling
528
14.15 Flow diagram for production of mono-ammonium phosphate
by spray drying
529
14.16 Flow diagrams for the production of double superphosphate
fertilizers
532
14.17 De-watering operations for flotation concentrates
533
14.18 Flow diagram for spray drying flotation concentrates
534
14.19 Twin spray dryer complex in Western Australia, producing
536
bone dry nickel-concentrate
14.20 Flow diagram for manufacture of cement utilizing spray
541
drying
XXXii LIST OF FIGURES
15.1 Products derived from milk
15.2 Spray dryer layouts for dairy products
15.3 Forms of spray dried skim milk powder
15.4 Rewet instantizer (shown with agglomerating tube mounted)
15.5 Spray dryer with vibro-fluidizer for instant skim milk, whole
milk, milk replacer, ice-cream mix, cheese, cream powders
15.6 Processing stages in the spray drying of whey
15.7 Spray dryer producing non-caking whey powder
15.8 Flow diagram for the production of spray dried sodium
caseinate
15.9 Processing diagram for production of egg powders
15.10 Flow diagram for production of instant coffee powder by
spray drying
15.11 Process stages in the production of protein from vegetable
sources using spray drying
15.12 Flow diagram for the production of potato protein by spray
drying
15.13 Flow diagram for the production of tomato powder by spray
drying
15.14 Dehumidified enclosed band conveyor for cooling spray
dried tomato powder
15.15 Processing stages in production of banana powder by spray
drying
16.1 Single stage aseptic spray drying system
16.2 Two stage aseptic spray drying system
16.3 Flow diagram for aseptic spray drying plant with two-fluid
atomization
16.4 Spray dryer with rotary atomizer for aseptic drying
16.5 Flow diagram for the production of fodder yeast from
molasses
18.1 Stages in the processing of animal blood by spray drying
18.2 Flow diagram for the production of dried fish solids
A.5 Psychrometric charts for air-water vapour system at
atmospheric pressure
A.6 Humid heat-humidity relation for air-water vapour system
at atmospheric pressure
A.7 Latent heat of vaporization-temperature relation (water)
A.8 Liquid flow over vaned atomizer wheel
546
548
550
554
555
557
559
List of Tables
563
566
569
574
576
578
580
582
589
591
593
595
599
607
610
624-5
626
627
628
2.1 Calculation of mean diameters in example 2.1
2.2 Mean diameters based upon number, surface and volume
2.3 Distribution functions
2.4 Mean diameter transformation
2.5 Data for example 2,2
2.6 Calculation of Sauter mean diameter in example 2.2
5.1 Advantages and disadvantages of dry and wet powder
recovery
6.1 Prediction of droplet size following liquid jet disintegration
6.2 Droplet size prediction for smooth flat vaneless discs
6.3 Values of A = 0*(1) 2 ) 112
6.4 Values of radial velocity and liquid film thickness at edge of
vaned atomizer wheel (example 6.1)
6.5 Operating conditions covered by investigators in drawing
up equations (6.26-6.31) to predict spray characteristics
from atomizer wheels/discs
6.6 Effect of operating variables on droplet size from atomizer
wheels
6.7 Relation between mean droplet size and wheel speed for
different van loadings for low viscous feeds
6.8 Sauter mean diameter predicted from viscosity data (13, 42)
(figure 6.10) (a comparison with experimentally
determined values)
6.9 Relations to predict spray characteristics from atomizer
wheels
6.10 Comparison between experimental and predicted Sauter
mean droplet sizes
6.11 Values of constant in equation (6.30) for given dryer size
32
33
34
39
39
40
82
90
101
103
105
106
107
111
113
114
115
116
XXXiV
LIST OF TABLES
LIST OF TABLES
6.12 Range of operating variables applicable to curves A-E in
figure 6.11
6.13 Relations to predict droplet size from vaneless discs
6.14 Correlations to predict mean droplet size from centrifugal
pressure nozzles
6.15 Nozzle recommendation: enquiry form
6.16 Key to figure 6.48
6.17 Typical performance of internal mixing type nozzles
6.18 Correlations for predicting droplet size from converging
pneumatic nozzles
6.19 Experimental conditions covered by pneumatic nozzle
droplet size correlations
6.20 Relationship between dispersion coefficient, Sauter mean
diameter and slope of the Nukiyama-Tanasawa plot (6)
6.21 Values for example 6.15
6.22 Values for example 6.16
6.23 Comparison of predicted mean droplet sizes
6.24 Tabulated data showing pneumatic nozzle flexibility
6.25 Correlations for predicting droplet size from pneumatic cup
atomizers
6.26 Relation between Sauter mean diameter and maximum spray
droplet size
7.1 Relation between Reynolds Number (Re) and drag coefficient
(CD ) (for spherical droplet)
7.2 Spray trajectory relations for rotary atomizers
7.3 Values of K factor
7.4 Theoretical droplet trajectories in example 7.3 (from rotary
atomizer)
7.5 Theoretical droplet trajectories in example 7.4 (from nozzle
atomizer)
8.1 Proposals for the equation Nu (or Sh) = 2.0 + K(Re)x(Pr
or Sc)
9.1
117
153
11.1
205
11.2
206
11.3
11.4
209
215
219
220
228
233
12.1
12.2
12.3
12.4
12.5
12.6
13.1
234
13.2
249
251
255
14.1
267
14.2
15.1
16.1
16.2
270
286
Values of the integral f
in equation (8.23)
J C D Re l * 5
288
Fractional evaporation of aqueous droplets undergoing
decelerated motion in dry air (379°F)
8.4 Evaporation times for pure liquid droplets
8.5 Values of fl for different temperature driving forces
8.6 Spray evaporation history
8.7 Evaporation of aqueous droplets in atmospheres of air and
superheated steam
289
291
291
300
8.3
on
9.3
9.4
184-5
190
195
200
d(Re)
8.2
9.2
316
XXXV
Collection efficiency of primary separators on low and high
328
powder/air loadings
Suitability of powder-air separation equipment in spray
330
dryer installations
331
Relative costs of separation equipment in spray drying
Dimensions of separation equipment to handle 70 000 m 3 /hr
332
air at 80°C
Effect on Pd reading of pitot tube misalignment in the air
360
stream
Equipment for determination of powder loading in ducted
377
air flows
379
Efficiency of B.C. U R.A. cyclone
Available methods of measuring droplet size distribution of
397
sprays and particle size distribution of dried product
405
Efficiencies of indirect fuel oil air heaters
414
Typical fan efficiencies
425
Data for example 12.7
430
Dimensions of industrial and model cyclones
435
Recommended face velocities for bag filter designs
437
Properties of bag filter fabrics
Products suitable for suspended particle processing systems,
471-5
involving atomization
Recent patents and patent applications dealing with products
476-97
to which spray drying or spray cooling is applied
Particle size distribution of oxide ceramics according to
507
atomizer used
530
Methods of double superphosphate manufacture
553
Properties of ordinary and instant milk powders
594
Uses of spray dried enzymes
598
Relative compositions of fodder yeast and muscular protein
Introduction
Spray drying is by definition the transformation of feed from a fluid
state into a dried form by spraying the feed into a hot drying medium. It
is a one step continuous suspended particle processing operation. The
feed can be either a solution, suspension or paste. The resulting dried
product conforms to powders, granules or agglomerates, the form of which
depends upon the physical and chemical properties of the feed and the
dryer design and operation. Spray drying is a procedure which in many
industries meets dried product specifications most desirable for subsequent
processing or direct consumer usage. Intensive research and development
during the last two decades has resulted in spray drying becoming a highly
competitive means of drying a wide variety of products (1). The range of
product applications continues to expand so that today spray drying has
connections with many things that touch our daily lives. The extent of
this will become apparent during the course of this book.
Spray drying has moved into all major industries ranging from production
in the most delicate of conditions laid down in food and pharmaceutical
manufacture right through to the high-tonnage outputs within such heavy
chemical fields as mineral ores and clays. There are many products and
articles in use around us each day to exemplify the extensive usage of spray
drying. This is apparent if we consider just one aspect of common interest
to us all, namely our home living.
From foodstuffs to home fittings, spray drying has many associations.
Each product requires different powder requirements to be met during
manufacture. For example, the housewife may be concerned only with
the taste and price of the foodstuffs she buys and the quality of the household
aids she uses, but it is most likely one would find a wide range of foodstuffs,
equipment and fittings within her home having direct and indirect connections with the spray drying operation. Foodstuffs may well be instant
2
INTRODUCTION
coffee, coffee whitener, dried eggs, milk, soups, babyfoods, perhaps even
powdered cheese and fruits. These are examples of products with direct
connections. Spray dried foodstuffs appeal to the eye, retain nutritive
contents and are easy to use, through being readily dry mixed and reconstituted. This is irrespective of their dried forms which are highly diverse.
Milk powders can be in agglomerated (instant) form, whereas eggs, soup,
coffee whitener have powdery, and fruits granular, forms. Apart from dried
foodstuffs that are consumed directly, there are a variety of spray dried
products used in cuisine. Examples include condiments (garlic, pimento),
flavouring compounds, rennet, and ingredients in biscuits and cakes. Meat,
vegetables and fresh fruit are foodstuffs with indirect connections with spray
drying. Meat may be from a slaughtered animal reared on feeds based
upon spray dried skimmilk, whey or fat enriched milk (replacer). Whereas
appearance might not be so crucial here, particle size and consistency
must be conducive to animal digestion. All vegetables and fruit can be
connected with spray dried fertilizers and pesticides used in cultivation.
Here good spreading characteristics are the powder requirements, placing
emphasis on particle size distribution and moisture content of the powder.
Passing from foodstuffs to general household commodities, many examples
can be cited. Perhaps the best known spray drying application is household
detergents; but also spray dried soaps and other surface active agents
and optical brighteners are available. In the bathroom cabinet spray
dried pharmaceutical products, and even cosmetics, are likely to be found.
Pharmaceuticals, e.g. antibiotics, are produced under the most aseptic of
conditions as finely divided powders, which are often tableted prior to
marketing. The spray dried powder form is ideal for rapid assimilation
into the body organs. Many cosmetics rely on spray drying to provide
constituents in such articles as face powders and lipsticks. Applications
to home fittings and furnishings are also extensive. Wall tiling is formed
by pressing coloured spray dried clays. Paints contain spray dried pigments.
Electrical insulation material is spray dried prior to pressing into parts for
electronics and electric power supplies. Also in the electronics field spray
dried ferrites enjoy wide use, being found in pressed form in telephones,
radio, television, etc. Many household aids are powered by an electric
motor with a ferrite rotor. All these pressing operations demand strict
particle size distributions that can be met by the spray drying operation.
No such survey of spray dried products in the home is complete without
mention of (a) plastics, as many household plastic utensils originate from a
manufacturing process that includes a spray drying stage; (b) fabrics, as
spray dried dyestuffs provide the vivid colours of furnishings and clothing;
(c) stationery, as spray drying provides many materials for printing while
spray dried kaolin is used in paper making itself ; (d) shoes, bags and leather
INTRODUCTION
3
wear as spray dried tannin is closely associated with the curing of leather ;
(e) starches, as the extensive processing of this, one of mankind's most
basic materials, often includes a spray drying stage. Spray dried starch
and its derivatives (sugar, syrup) are widely used in ice cream, confectionary,
desserts, jellies, preserves, frozen fruit, soft drinks. In non-food manufacture,
spray dried starch is used in textiles, papermaking, printing and adhesives.
Examples could go on, but the point has surely been made. The variety
of spray dried products illustrates the ability of spray drying equipment to
meet a wide range of powder product requirements.
Spray drying involves atomization of feed (hereafter aqueous unless
stated otherwise) into a spray, and contact between spray and the drying
medium (hereafter termed air unless stated otherwise) resulting in moisture
evaporation. The drying of the spray proceeds until the desired moisture
content in the dried particles is obtained, and the product is then recovered
from the air. These four stages are illustrated in figure 0.1.
I STAGE I
'ATOMIZATION
L
_J
IR
1
0
AIR
STAGE 2
SPRAY-AIR
LcONTACT
r
r
-
1
STAGE 4
DRIED
POWDER
RECOVERY
STAGE 3
!MOISTURE
EVAPORATION
FROM SPRAY
-
DRYING
CHAMBER
L
.1
U
L
Figure 0.1, The process stages of spray drying.
4
INTRODUCTION
INTRODUCTION
The word 'atomization' can be confusing initially. 'Atomization' has no
association with atoms and nuclear physics but covers the process of liquid
bulk break-up into millions of individual droplets forming a spray. The
energy necessary for this process is supplied by centrifugal, pressure, kinetic
or sonic effects. During spray—air contact, droplets meet hot air and a
rapid moisture evaporation takes place. If the spray drying plant is properly
designed the outcome will be dried particles suspended in the drying air,
from which an efficient particle removal is essential. Explaining the process
through viewing an actual installation can be unrewarding as the spray
drying operation cannot be separated into distinct processes for individual
viewing.
Any form of dryer provides means of moisture removal by the application
of heat to the feed product and control of the drying medium humidity. A
spray dryer is no exception. Heat is applied as a heated atmosphere and
evaporation is promoted by spraying the feed into this atmosphere. Humidity
control is by air flow and temperature regulation. Although the vast
majority of cases employ hot atmosphere to drive moisture from each
spray droplet, there are cases where the delicacy of the operation demands
that the drying medium is just warmed over atmospheric temperatures.
This is a variation of the basic spray drying concept, and is termed low
temperature spray drying. A further variation is to spray the product into
freezing air, whereupon the individual droplets are frozen for subsequent
moisture removal through sublimation under vacuum. This variation of
spray drying is termed spray freeze drying. Should the temperature of the
air permit only solidification of the spray droplets, the process is termed
spray cooling.
A still further variation is to utilize the chamber as a spray reactor. For
liquids that react chemically with a given gas to form a new compound, the
contacting of gas with liquid in an atomized state gives rise to fast reaction
rates because of the large surface area of the droplets. The process is termed
spray reaction.
The spray drying chamber need not only be used to produce dry products;
use as a spray concentrator has long been recognized. In fact the use of
drying and concentrating liquids by atomizing techniques can be traced
back to a one hundred year old patent (2). The historical background of
spray drying has been recently described by Hatfield (130).
While most of the spray drying operations utilize a drying medium of
air and exhaust the air to atmosphere (open cycle), there are cases where
an inert gas, e.g. nitrogen, is used as the drying medium instead of air. The
inert gas prevents risk of explosion where flammable or .explosive solvents
are involved. The inert gas is constantly re-used in the drying process
through re-cycling within a closed system. The evaporated liquid absorbed
5
into the drying gas in the drying chamber is recovered in a scrubber—
condenser system. Organic solvents and diluents can thus be evaporated
and recovered. For products that form potentially explosive powder—air
mixtures in the chamber of an open cycle drying system or where the exhaust
air contains pollutants or odours, the semi-closed drying system can be
used. In the former case, explosions are prevented by lowering the 0 2
content in the drying chamber by control of the combustion air to a direct
air heater and exhaust air bleed (see page 522). In the latter case the pollutants
and odours in the exhaust air bleed, which forms a small percentage of
dryer air requirement, are deactivated prior to atmospheric emission.
The spray drying layout and its possible modifications to closed cycle and
semi-closed spray drying, spray cooling, spray reaction and spray freeze
drying are illustrated in figure 0.2.
In introducing the subject, emphasis has been given to the ability of the
spray drying process to handle a wide range of products, and meet the
combustion
air
ine t gas
make-up
hot
heater
DRYING
CHAMB ER
DRYING
CHAMBER
p
A OPEN CYCLE
B CLOSED CYCLE
C
SEMI-CLOSED CYCLE
SPRAY DRYING
otIT.05ph0dC
air
COOLING
REACTION
FREEZING
CHAMBER
CHAMBER
CHAMBER
SPRAY COOLING
SPRAY REACTION
SPRAY FREEZING
KEY: f—feed, p--product, s—scrubber/condenser/cooler.
Figure 0.2. Layouts of spray dryers and associated suspended particle processing equipment
involving atomization.
6
INTRODUCTION
specifications laid down by diversified industries. Such range of application
has led to dryer designs becoming less standardized as each product has
to be treated individually and handled in specialized ways to meet the
dried product specifications. Whatever dryer design the product demands,
the advantageous features of spray drying are retained.
These advantages can be listed as :
(1) Continuous in operation.
(2) Adaptable to full automation.
(3) Dried product specifications met through dryer design and operational
flexibility.
(1) Required product form (particles as spheres, cenospheres, fines,
agglomerates) (figure 0.3).
(ii) Required product properties (dusty or dustless, degree of flowability,
wettability, etc., also see below).
(4) Applicable to both heat sensitive and heat resistant materials.
(5) Economic in operation.
Dried product specifications are related to the properties of :
(a) Particle size distribution. This governs product appearance, packing, and
subsequent processing requirements.
(b) Bulk density and particle density. This is closely related to packing
container requirements. Spray drying maintains constant product density
and eliminates the need for batch mixing to meet specified densities.
(c) Appearance. This is vital from the consumer angle.
(d) Moisture content. This governs powder quality, colour, flowability,
shelf life, packing and subsequent processing requirements.
(e) Friability. This is related to handling and packing.
(f) Dispersibility. This is related to rate of solubility on reconstitution.
(g) Colour, aroma, taste. This is related to overall powder quality.
(h) Activity. This is related to any heat degradation occurring with biochemical products.
Economic operation is closely associated with drying temperatures and
solids content in the feed. The higher the drying temperatures and solids
content, the greater the thermal efficiency' of the process. For example,
based on an ambient and outlet drying air temperature of 68°F (20°C) and
185°F (85°C), increase in inlet drying air temperature from 275°F (135°C)
to 1202°F (650°C) increases the overall dryer efficiency from about 44
to 90 Z. Increase in feed solids (for a given production rate) from 50 % to
60 % reduces the heat load by nearly 50 %. However, many spray dryers
operate at low feed solids and comparatively moderate drying temperatures
as these conditions are dictated by the feed properties. In these cases,
successful operation is based not so much on thermal efficiency but on the
ability of the process to dry delicate products without degradation.
INTRODUCTION
7
(a)
(b)
(c)
Figure 0.3. Dried-powder forms produced by spray drying. (a) Fine particles (dusty) (noninstant skim milk). (b) Spheres (Aluminium oxide). (c) Agglomerates (instant skim milk).
8
INTRODUCTION
• The book is divided into five sections. Section I contains five chapters
on basic principles and includes important definitions. Section II deals
with the process stages of spray drying, where individual chapters are
given to atomization, spray—air contact (mixing and flow), drying of sprays,
and dried product recovery. Section III, entitled 'Operational Practice',
describes control systems and common operational measurements. Section
IV briefly surveys some of the equipment used on spray dryers. Section V
concludes the book with descriptions of established, new or interesting
products that are spray dried nowadays. Products are classified according
to their induStry.
A literature survey, including recent patents covering spray drying
applications, is given at the beginning of section V (chapter 13).
Section I
BASIC PRINCIPLES, DEFINITIONS
1 Spray Drying Fundamentals
Spray drying consists of four process stages (figure 0.1):
(A) Atomization of feed into a spray.
(B) Spray—air contact (mixing and flow).
(C) Drying of spray (moisture evaporation).
(D) Separation of dried product from the air.
Each stage is carried out according to dryer design and operation, and,
together with the physical and chemical properties of the feed, determines
the characteristics of the dried product. The spray homogeneity following
atomization and the high rates of moisture evaporation (spray—air mixing
and flow) enable the temperature of the dry product to be considerably
lower than the drying air leaving the drying chamber. The product is thus
not subjected to high temperatures, and when separated from the drying
air is devoid of any heat degradation. The basic physical principle of
`evaporation causes cooling' is very pertinent to the operation.
Figure 1.1 shows diagrammatically what features each process stage has.
Figure 1.2 indicates the actual equipment involved to carry out each stage
in a standard layout, represented by a rotary atomizer, cyclone separators
and pneumatic product conveying. Feed is pumped from the product
feed tank (1) to the rotary atomizer (5). The atomizer is located within a
ceiling air disperser (12), through which drying air enters the drying chamber
(15). The air is drawn from atmosphere by a supply fan (8) and is steam
heated (10). Following the drying of the spray in the chamber, the majority
of dried product falls to the base of the chamber and enters a pneumatic
conveying system (20) via a powder offtake (18). Conveying air is filtered (19).
The product fines remain entrained in the air and pass (27) to cyclones (28)
for separation. The fines enter the pneumatic conveying system. Exhaust
air passes to atmosphere via the exhaust fan (31), Dried powder in the
SPRAY DRYING FUNDAMENTALS
BASIC PRINCIPLES, DEFINITIONS
pff
f7 d
ULLl
SPRAY EVAPORATION
;61,
SPRAY-AIR
L)
Figure 1.1. The features of the process stages involved in spray drying (shown diagrammatically).
PRODUCT SEPARATION
h,
oax
z
13
ao
Sti
FEED -ATOMIZATION
12
Equipment items
Feed system
1. Product feed tank.
2. Water feed tank for use during dryer
start-up and shut down.
3. Three way valve and filter.
4. Feed pump.
Atomizer system
5. Rotating vaned wheel atomizer.
6. Atomizer motor drive.
Supply air and spray-air contact system
7. Filter at drying air intake.
8. Air supply fan.
16. Insulated chamber wall.
17. Chamber pressure detector.
Product recovery system
18. Powder discharge unit at chamber base.
19. Filter at conveying air intake.
20. Pneumatic conveying system duct.
21. Transport cyclone.
22. Powder hopper.
23, Rotary valve.
24. Pneumatic conveying system exhaust
duct.
25. Conveying air fan (transport fan).
26. Outlet drying air temperature measuring
element.
27. Exhaust drying air duct.
28. Main powder recovery cyclone.
29. Rotary valve.
30. Exhaust air duct from cyclone.
31. Exhaust drying air fan.
32. Exhaust air flow damper.
33. Exhaust air stack.
34. Air hood.
35. Spray dryer control panel.
9. Air flow damper.
10. Air heater (here shown as a steam-air
heater).
11. Inlet drying air temperature measuring
element.
12. Air disperser.
13. Cooling air outlet (air disperser cooling),
14. Air disperser cooling fan.
15. Spray drying chamber.
Figure 1.2. Equipment involved in a standard spray dryer layout. (By courtesy of Niro
Atomizer.)
pneumatic transport system is collected at the base of the transport cyclone
(21). Instrumentation items include air temperature measurement (11) (26),
chamber pressure measurement (17) and dampers (9) (32) for adjustment
of air flows.
14
BASIC PRINCIPLES, DEFINITIONS
SPRAY DRYING FUNDAMENTALS
15
(A) Atomization of Feed into a Spray
The formation of a spray (atomization) and the contacting of the spray
with air are the characteristic features of spray drying. The selection and
operation of the atomizer is of supreme importance in achieving economic
production of top quality products. The atomization stage must create a
spray for optimum evaporation conditions leading to a dried product of
required characteristics.
Rotary atomizers and nozzles are used to form sprays. With rotary
atomizers centrifugal energy is utilized. There are two categories of rotary
atomizers : (a) atomizer wheels, (b) atomizer discs. Wheel designs are
available to handle feed rates up to tens of tons feed per hour. With nozzle
atomization, pressure, kinetic or (less common) sonic energy is utilized.
There is a wide range of nozzle sizes and designs to meet spray drying needs.
Feed capacities per nozzle are lower than per rotary atomizer, leading to
nozzle duplications to meet high feed rate requirements.
The selection of the atomizer depends upon the nature of the feed and
the desired characteristics of the dried product. In all atomizer types,
increased amounts of energy available for liquid atomization result in
sprays having smaller droplet sizes. If the available atomization energy is
held constant but the feed rate is increased sprays having larger droplet
sizes will, result. The degree of atomization depends also upon the fluid
properties of the feed material, where higher values of viscosity and surface
tension result in larger droplet sizes for the same amount of available
energy for atomization.
(i) Rotary Atomizers (Utilization of Centrifugal Energy)
Feed is introduced centrally on to a wheel or disc rotating at speed. The
feed flows outwards over the surface, accelerating to the periphery. Feed,
on leaving the periphery, readily disintegrates into a spray of droplets.
Rotary atomizers form a low pressure system. A wide variety of spray
characteristics can be obtained for a given product through combinations
of feed rate, atomizer speed and atomizer design. Designs of atomizer
wheels have vanes or bushings. Vanes are high, wide, straight or curved;
bushings circular or square. Vaned atomizer wheels are used in many and
varied industries, producing sprays of high homogeneity. Atomizer wheels
with bushings are used in more specialized fields, e.g., for handling abrasive
feeds. Wheels can be operated to produce sprays in the fine to medium
coarse size range (see figure 2.1). Peripheral velocities reach 550 ft/sec
(154 m/sec) in industry. Designs of disc include vaneless plates (discs),
cups, and inverted bowls. Discs are used to meet coarse spray requirements.
Rotary atomization is discussed in detail in chapter 6. The spray form
leaving a rotary atomizer (vaned wheel) is shown in figure 1.3(a). Rotary
atomizers are reliable, easy to operate and can handle varying loads.
(a)
(b)
Figure 1.3. Atomizers in operation. (a) Rotary atomizer (vaned wheel). (b) Nozzle atomizer
(pressure nozzle).
16
BASIC PRINCIPLES, DEFINITIONS
(ii) Pressure Nozzles (Utilization of Pressure Energy)
The feed concentrate is fed to the nozzle under pressure. Pressure energy
is converted to kinetic energy, and feed issues from the nozzle orifice as a
high-speed film that readily disintegrates into a spray as the film is unstable.
The feed is made to rotate within the nozzle resulting in cone shaped spray
patterns emerging from the nozzle orifice. Sprays from pressure nozzles
handling high feed rates are generally less homogeneous and coarser than
sprays from vaned wheels (see figure 2.1). At low feed rates spray characteristics from nozzles and wheels are comparable. Duplication of nozzles
allows fine sprays to be obtained in nozzle dryers, but nozzles are generally
used to form coarse particle powders having good free flowability. The
spray form leaving a pressure nozzle is shown in figure 1.3(b). Variation of
pressure gives control over feed rate and spray characteristics. Pressure
nozzles have operating pressures up to 10 000 p.s.i. (680 atm). Pressure
nozzles are described in detail in chapter 6.
(iii) Two-fluid Nozzles (Utilization of Kinetic Energy)
The feed concentrate and atomizing gas (usually air) are passed separately
to the nozzle head. High air velocities are generated within the nozzle
for effective feed contact which breaks up the feed into a spray of fine droplets.
Sonic velocities are often generated. The air stream is rotated within the
nozzle, and feed is contacted either within the nozzle (internal mixing) or
as the liquid emerges from the orifice (external mixing). The nozzles operate
successfully at low pressures (up to 100 p.s.i. (7 atm)). The feed system
includes pressure pumps, although at low feed rates the suction caused by
the air flow ejector effect will be sufficient to draw liquid into the nozzle.
Adjustment of the air flows controls the degree of atomization at constant
feed rate. Two-fluid nozzles have the advantage of handling high viscous
feeds and produce sprays of medium coarseness, but of poor homogeneity.
For low viscous feeds, fine particles can be produced, although the resulting
dried powder may be of agglomerates. There is much likelihood for high
occluded air content within the particles. Two-fluid nozzles are discussed
in chapter 6. Two-fluid nozzles are generally applied in small diameter
chambers where fine atomization is required. However, they are expensive
to operate and low production per dryer unit is obtained.
Atomizing air can be combined with a rotary atomizer to form a pneumaticcup atomizer.
(8) Spray-Air Contact (Mixing and Flow)
The manner in which spray contacts the drying air is an important factor
in spray dryer design, as this has great bearing on dried product properties
by influencing droplet behaviour during drying. Spray-air contact is
SPRAY DRYING FUNDAMENTALS
17
AIR OUT
PRODUCT IN
ATOMIZER
AIR PRODUCT
IN
IR IN
ATOMIZER
ATOMIZER
PRODUCT IN
AIR
OUT
AIR PRODUCT
OUT OUT
PRODUCT
ial
CO- CURRENT
FLOW DRYER
Al
IN
Al R
IN
PRODUCT
OUT
b
COUNTER-CURRENT
FLOW DRYER
AI R
AND —
PRODUCT
OUT
MIXED FLOW
FLOW DRYER
Figure 1.4. Product—air flow in spray dryers.
determined by the position of the atomizer in relation to the drying air
inlet. Many positions are available. The spray can be directed into hot
air entering the drying chamber as shown in figure 1.4(a). Product and air
pass through the dryer in 'co-current' flow,* so called after the inlet-outlet
layout for air, feed, and dried products (3) (35). This arrangement is widely
used, especially in cases of handling heat sensitive products. Spray evaporation is rapid, the drying air cools accordingly, and evaporation times are
short. The product is not subject to heat degradation. Product temperature
is low during the time the bulk of the evaporation takes place as droplet
temperatures approximate to wet bulb temperature levels. When the
desired moisture content is being approached, each particle of the product
does not rise substantially in temperature as the particle is then in contact
with much cooler air. In fact, low temperature conditions prevail virtually
throughout the entire chamber volume, in spite of very hot air entering
the chamber. This is shown in figure 1.5(a), which illustrates a typical air
temperature distribution within co-current flow dryers. Both rotary and
nozzle type atomizers can be used in co-current flow dryers. With the
former type, the radial trajectory of droplets from the atomizer periphery
can be controlled, so excessive product deposits on the drying chamber
wall are prevented.
* Quotation marks are used as spray—air movement in reality is often far from co-current,
e.g. at the point of actual spray—air initial contact and in any areas of back mixing in the drying
chamber.
18
BASIC PRINCIPLES, DEFINITIONS
SPRAY DRYING FUNDAMENTALS
Alternatively, the spray can be contacted with air in 'counter-current'
flow (figure 1.4(b)). Spray and air enter at the opposite ends of the dryer
(3) (35). This arrangement offers dryer performance with excellent heat
utilization, but it does subject the driest powder to the hottest air stream
(see air temperature distribution, figure 1.5(b)). It readily meets granular
powder requirements of non heat-sensitive products. Counter-current flow
is commonly used with nozzle atomization, since an upward streamline
flow of drying air reduces the fall velocity of the large droplets in the spray,
enabling sufficient residence time in the drying chamber for completion of
evaporation,
AIR
120
FEED
AIR
230
1
I I
220
160
40
I
220
60
130
FEED
th
60
150
180
FEED
180
130
100
120
120
40
60
240
100
90
270
90
100
BO
BD
300
100
300
80
80
100
\ 100 —14.-AIR
330
AIR
350
AIR
PRODUCT
a. CO-CURRENT DRYER
(C) Drying of Spray (Moisture Evaporation)
240
90
110
105
PRODUCT
the hot areas around the air disperser. Air flow in the drying chamber is
discussed in chapter 7.
The selection of how best to contact the spray cloud with drying air is
dependent upon the product involved. For example, in the 'countercurrent' arrangement, the hottest drying air contacts the driest particles as
they are about to leave the chamber. If the dried product can withstand
presence in a very high temperature, and a high bulk density product is
required, this layout is highly suitable. The product particles will be of
low porosity due to the reduced tendency of the droplet to expand rapidly
and fracture during evaporation. If the particle cannot withstand such
high temperature conditions alternative contacting methods must be
employed and the `co-current' system may be suitable. The hottest drying
air contacts droplets at their maximum moisture content. The rapid
evaporation prevents high droplet temperatures. However, the droplets
undergoing such a high evaporation rate may well expand or fracture to
give non-spherical porous particles, and the product will often have a
low bulk density.
100
40
50
110
110
210
230 AIR
100
19
350
AIR
PRODUCT
b. COUNTER - CURRENT DRYER
Figure 1.5. Air temperature distributions in co-current and counter-current flow spray dryers.
(AU temperatures in degrees centigrade.)
There are dryer designs that incorporate both `co-current' and 'countercurrent' layouts, i.e. mixed flow dryers, figure 1.4(e). In this type coarse
free flowing powder can be produced in dryer chambers of relatively small
size.
In all cases, the movement of air predetermines the rate and degree of
evaporation by influencing (a) the passage of spray through the drying
zone, (b) the concentration of product (particle population) in the region
of the dryer walls, and (c) the extent to which semi-dried droplets re-enter
As soon as droplets of the spray come into contact with the drying air
evaporation takes place from the saturated vapour film, which is quickly
established at the droplet surface. The temperature at the droplet surface
approximates to the wet bulb temperature of the drying air. Evaporation
takes place in two stages. At first there is sufficient moisture within the
droplet to replenish that lost at the surface. Diffusion of moisture from
within the droplet maintains saturated surface conditions and as long as
this lasts, evaporation takes place at a constant rate. This is termed the
first period of drying. When the moisture content is reduced to a level,
where it is insufficient to maintain saturated conditions, the so-called
critical point is reached and a dried shell forms at the droplet surface.
Evaporation is now dependent upon the rate of moisture diffusion through
the dried surface shell. The thickness of the dried shell increases with time
causing a decrease in the rate of evaporation. This is termed the falling
rate period or second period of drying.
Thus a substantial part of the droplet evaporation takes place when the
droplet surfaces are saturated and cool. Drying chamber design and air
flow rate provide a droplet residence time in the chamber, so that the desired
droplet moisture removal is completed and product removed from the
dryer before product temperatures can rise to the outlet drying air temperature of the chamber. Hence there is little likelihood of heat damage to the
product.
20
BASIC PRINCIPLES, DEFINITIONS
SPRAY DRYING FUNDAMENTALS
During evaporation, the atomized spray distribution will undergo change.
Different products exhibit different evaporation characteristics. Some tend
to expand, others collapse, fracture or disintegrate, leading to porous
irregular shaped particles. Others remain a constant spherical shape or
even contract, leading to denser particles. The extent of any particle shape
change and hence the dried powder characteristics is closely connected to
the drying rate, and it follows that meeting desired powder characteristics
requires close consideration to the drying chamber design.
Drying of sprays forms the subject matter for chapter 8.
21
System (2). Total recovery of dried product takes place in the separation
equipment (figure 1.6(b)). This system places great importance on the
separation efficiency of the equipment, but the system is often utilized, as it
does not need a product conveying system.
Separation of dried product from the air influences powder properties
by virtue of the mechanical handling involved during the separation
stage. Excessive mechanical handling can produce powders having a high
percentage of fines. Separation effects in drying chambers is discussed
fully in chapter 9. Separation and recovery equipment is covered in
chapter 12.
( D) Separation of Dried Product from the Air
Product separation from the drying air follows completion of the drying
stage, when the dried product remains suspended in the air. Two systems
are used to recover the product. These are illustrated in figure 1.6 for the
case of co-current flow dryers with a rotary atomizer and cyclones.
System (1). Primary separation of dried product takes place at the base
of the drying chamber (figure 1.6(a)). During operation, the majority of
product falls to the base of the chamber, while a small fraction passes out
entrained in the air and is recovered in the separation equipment. Such
equipment is usually cyclones, Bag filters are also installed as are electrostatic precipitators and wet scrubbers. The choice of equipment is dependent
upon the powder loading of the air leaving the drying chamber, and accept,able efficiencies of recovery. With this system, a classification of powder
is achieved, as the coarse particles are recovered at the chamber base and
the finer particles from the separation unit. This form of powder classification can be utilized. However, normally the two powder offtakes are combined and conveyed to a single discharge area.
PRODUCT Ill
AIR IH AIR IN
ATOMIZER
AIR
ovr
AJR AND
ENTFtA INED
POWDER
U.
b.
PRIMARY
PRODUCT
DISCHARGE
SECONDARY
PRODUCT
DISCHARGE
TOTAL
PRODUCT
DISCHARGE
Figure 1.6. Product discharge from co-current flow spray dryers with a rotary atomizer.
Meeting Dried Product Requirements in Spray Drying
The meeting of dried product requirements calls for close attention to all
four process stages. All stages effect dried product properties to a degree.
Atomization technique and feed properties will have a bearing on particle
size distribution, bulk density, appearance, and moisture content. Spray—
air contact and resulting evaporation, in fact the drying operation, will have
a bearing on bulk density, appearance, moisture content, friability and retention of activity, aroma and flavour. Techniques for product—air separation
will determine the degree of comminution the powder undergoes following
completion of drying. Many operational variables associated with atomization and the drying operation offer means of altering dried product characteristics. The important variables are summarized below.
(1) Energy Available for Atomization
Increase in energy available for atomization will create smaller droplet
sizes at constant feed conditions. Increase in rotary atomizer speed, nozzle
pressure, or air—liquid flow ratio in two-fluid nozzles will decrease mean
spray droplet size. The spread of droplet sizes in the spray distribution
may not be appreciably changed. Producing greater amounts of fine
particles can often form a product of higher bulk density. The greater
numbers of smaller particles fill the voids between the larger, and smaller
particles may well be more dense.
(2) Feed Properties
Increase in feed viscosity through increase in feed solids or reduction in
feed temperature will produce coarser sprays on atomization at fixed
atomizer operating conditions. Surface tension effects appear minor.
Increase in feed solids effects evaporation characteristics where generally
an increase in particle and bulk density results.
(3) Feed Rate
Increasing feed rate at constant atomizer operating conditions will produce
coarser sprays and dried products.
22
BASIC PRINCIPLES, DEFINITIONS
(4) Atomizer Equipment Selection
Rotary atomizers and nozzles exhibit different spray forming characteristics,
which can be utilized to meet necessary product requirements. Selection
can only be made with reference to the product concerned. However,
modern rotary atomizer and nozzle designs are flexible and can produce
similar spray characteristics over a wide range of industrial products and
dryer capacities, although there are applications where certain atomizer
techniques are so established that they are considered standard.
(5) Atomizer Equipment Design
This is closely connected with item (1) above. If the energy available is
made to act on reduced feed bulk, finer sprays will result. Many spray
properties can be achieved by alteration to the vane design of an atomizer
wheel, where number, height, width, length will determine the amount of
liquid at the point of atomization at the wheel periphery. Similarly, duplication of nozzles, at fixed pressure will create finer sprays through reduced
liquid bulk per nozzle.
(6) Air Flow
Air flow rate controls to a certain extent product residence time in the
drying chamber. Increased residence time leads to greater degree of moisture
removal. Reduction in air velocity assists product recovery from drying
chamber. Air flow has bearing on product handled and its dried properties.
(See earlier, (B) Spray—Air Contact.)
(7) Drying Temperature
Inlet. Increase in inlet temperature increases the dryer evaporative capacity
at constant air rate. Higher inlet temperatures mean a more economic
dryer operation. Increased temperature often causes a reduction in bulk
density, as evaporation rates are faster, and products dry to a more porous
structure.
Outlet, For a fixed moisture content and dryer design, outlet temperature
must be kept within a narrow range to maintain the powder packing and
flow requirements. Increase in outlet temperature decreases moisture
content at constant air flow and heat input conditions. Operation at low
outlet temperature to produce powder of high moisture content is used,
where agglomerated forms of powder are required. This is often utilized
in making special 'instant' powders. Auxiliary equipment (e.g. fluid-bed)
is required to handle the high moisture powder leaving the dryer.
2 Representation of Sprays
Spray droplets and dried particles are an inherent part of spray drying.
It is vitally important that droplets and particles can be represented in a
manner for easy reference. Accepted terminology is available to express
mean size and size distributions. Particle size distribution features as a
most important dried product specification. Particle size is closely related
to droplet size, but is rarely equal, due to droplet behaviour during drying.
Ranges of droplet and particle sizes met in spray drying are shown in
figure 2.1.
2.1. Terminology
2.1.1. Droplet
This refers to the state of subdivision of feed on being sprayed from the
atomizer. As long as the surface moisture remains in the spray, the spray
is said to be composed of droplets.
2.1.2. Particle
This refers to the state of subdivision of dried product. The shape of the
particle depends upon how the droplet was formed during atomization and
how the droplet/particle behaved during drying.
2.1.3. Agglomerate
An agglomerate is composed of two or more particles adhering to each
other. Agglomeration is often desired in the spray drying operation as
agglomerated particles show improved dispersibility characteristics.
Agglomerates can be formed through two or more droplets coalescing in the
proximity of the atomizer and drying in this state, or through partially
dried droplets adhering to each other in the lower regions of the chamber,
due to their sticky surfaces. Agglomerates when specifically desired are
created in the drying chamber by (a) contacting the evaporating spray
24
BASIC PRINCIPLES, DEFINITIONS
REPRESENTATION OF SPRAYS
25
with dry product fines, or (b) installing special agglomerating equipment
directly to the drying chamber (e.g. fluid-bed instantizer).
2.1.4. Size
The size of a particle/droplet/agglomerate is the representative dimension
that expresses the degree of comminution. For spherical particles the
diameter represents its size. For non-spherical particles, its size can be
represented by an 'apparent' diameter. This is the mean distance between
extremities of the particle measured through the particle centre of gravity.
Size can also be based on area, volume or weight (see below 'Size Distribution' and 'Mean Size').
2.1.5. Particle Shape
The complexity of the atomization mechanism and the distortions a droplet
undergoes during drying result in the majority of spray-dried products being
comprised of non-spherical particles. Wide variations in shape are often
evident The ratio of measured maximum to minimum particle diameters
often defiii -e hape. To express divergence from sphericity a shape factor is
used, defined as ifie-rattobetween the actual surface or volume of the particles
and the total surface or volum-d-obtaiffed from size measurement techniques,
e.g. microscopic analysis or sieving, where spherical particles are assumed.
2.1.6. Size Distribution
Droplets and particles comprising sprays and dried products are never of
one size. The atomizer cannot form totally homogeneous sprays. Spray
droplets are subjected to different shape distortions depending upon
their drying characteristics and travel within the dryer. Dried particles
and droplets from a spray have a range of sizes termed their size distribution.
A great number of methods have been devised over the years for measuring
size distributions. Microscopic analysis (with manual or automatic counting), sieving, sedimentation, elutriation, light absorption and automatic
sensing equipment (e.g. Coulter Counter analysis) are the established
methods. Counting procedures express the number of droplets or particles
within a suitable size group (increment). Size distributions can be represented
by a frequency or cumulative distribution curve. If occurrence is given by
number, a number distribution results, or if given by area-volume-weight
corresponding to a given diameter, area-volume-weight distributions
result. Number size distribution is the outcome of microscopic analysis—
weight distribution the outcome of sieving. Methods of size analysis are
reviewed in chapter 11. Droplets forming a size distribution of a spray
from a centrifugal pressure nozzle, and particles forming a size distribution
of a spray dried ceramic material are shown in figure 2.2.
2.1.7. Mean Size
The mean droplet or particle size represents a single value, most suited to
be representative of the whole distribution. No value is adequate to express
-
26
BASIC PRINCIPLES, DEFINITIONS
••
•• •
• • • 1. ;••
•
■ .
•
■
•
•
•
•
•
•
OS
•
•
■
••
. • • 11'0
'•
••
• •
• • .11 •
■
•• •
• • '
•
••
'••
•
•• • •
.0 • •
•
•
'.
:. :
••
■
• •
■
• •••
•
•
*
- •• •
500p
(a)
SDX centrifugal pressure nozzle
type SE-622 300 p.s.i.g., 26.5 U.S. gal/hr water
Sauter mean diameter = 65 micron
(b)
Figure 2.2. Size distribution of droplets and particles. (a) Droplet size distribution. (By courtesy
of Delavan Manufacturing Company.) (b) Dried particle size distribution (ceramic material).
(By courtesy of Niro Atomizer.)
a size distribution, and the mean size must be used with other parameters
to define more fully size characteristics. There are many types of mean
diameter parameters. The type selected depends upon the information
required about the spray or product, i.e. length, area, volume, shape, etc.
The pertinent characteristic varies according to how the droplet or particle
data is to be used. Details on mean size parameters and their calculation
follow in this chapter.
SIZE RANGE
•
• •
27
For a comprehensive account of spray data presentation, the reader is
referred elsewhere (4) (5) (6) (35).
2.2.1. Tabular Form
This is the most precise and general method of presenting droplet size data.
Tables can show a listing of size against one or more ways of expressing
their distributions, e.g. size frequency or size cumulation. However, large
amounts of data can make tabular form unwieldy and difficult to interpret.
2.2.2. Graphical Form
The use of graphical presentation offers many advantages in spite of the
accuracy of tabular presentation. Graphs present data in a form whereby
approximate values of deviation and skewness of data from a given mean
can be assessed quickly. Furthermore, graphs are more manageable than
long lists of data in tabular form. Graphs also provide constants that
describe the size distribution. The relationship of a size distribution to a
certain mathematical function can be seen at a glance from graphical
representation.
(a) Histograms
The histogram is the simplest way of representing size distribution of a
spray. It is a plot of the percentage number of droplets or particles in
a given size range (size increment). The histogram gives an immediate
indication of the droplet size, which constitutes the majority of the distribution. A typical histogram is shown in figure 2.3.
OCCURRENCE OF DROPLETS
•
REPRESENTATION OF SPRAYS
w
o_
D.
min
DROPLET SIZE
(D
Figure 2.3. Droplet size distribution represented in histogram form.
2.2. Methods of Data Presentation
There are two basic forms of representation available to express the characteristics of size and size distribution. These forms are summarized below.
(b) Size Frequency Curves
Curves are more practical when a large number of size increments are
used to express the size distribution. Size frequency curves can be considered
28
BASIC PRINCIPLES, DEFINITIONS
REPRESENTATION OF SPRAYS
a smoothed out form of histogram (figure 2.3) and give the relative frequency
of a variable within a certain group of data (measurements of diameter,
volume, surface area, etc.). The frequency curve shown in figure 2.4 is a plot
of the number of droplets with a diameter within certain size increments
(fN (D)) against diameter (D). The subscript N indicates a frequency of
occurrence according to number. Droplet frequency is generally stated as
a percentage diameter. The frequency curve is expressed as
f
fN (D) d(D) = 100 %
(2,1)
p,
If the ordinate of the plot is the volume or surface area corresponding to a
given diameter the resulting curve is skewed due to the weighting influence
in the large-diameter range.
(c) Cumulative Plots
A further method of representing size distribution is the cumulative distribution plot formed by plotting the cumulative frequency percentage of particles
or droplets greater or less than a given size, against the size. Cumulative
percentage can be represented on linear or probability paper. Cumulative
plots are shown in figure 2.4 and figure 2.5. Particle or droplet diameter,
surface, area, volume, weight can form the basis of the curve.
2.2.3. Mathematical Form
This is the representation of data in a manner most suitable for size prediction. Mathematical functions express the form of distribution (see table 2.3).
29
duly calculated have a number basis. These are described below. Calculation of mean diameters is illustrated in example 2.1.
(a) Most Frequent Diameter (DO
The most frequent diameter often characterizes a spray or dried powder
sample satisfactorily. This size can be seen from tabular results. When
represented graphically Df corresponds to the highest value on the frequency
curve. It also appears as the point of inflection for data represented on a
cumulative curve. Dr is shown in figure 2.4.
(b) Arithmetic Mean Diameter (D AM )
The arithmetic mean diameter (D AM ) is defined as the sum of the diameters
of separate particles/droplets divided by the number of particles/droplets.
This mean diameter is most significant when the particle or droplet size
distribution is not overbalanced by either very large or very small particles—
droplets. DAM can be defined in terms of the frequency curve, where
D rnax
-J—
D AM — 100 •
Dmi„
D • fN (D)d(D)
(2.3)
The arithmetic mean can be readily calculated from sampling data (see
example 2.1), since
DAM
=
ED . fN (D)AD
EfN (D)AD
(2.4)
where
2.3. Analysis of Data
A distribution function and two parameters can be used to represent spray
data. These parameters consist of a mean diameter of some form, and a
measure of the size range of particles involved.
2.3.1. Mean Diameters
A mean diameter is a mathematical value that represents the complete
spray. This value can be a measure of number, length, area, or volume.
A general equation that defines a mean diameter is given by
(Dpoa
_p
fN (D)A(D)
EDP . fN (D)A(D)
(2.2)
where D is the mean diameter and p q are integers (or zero). f (D) is the
function representing the spray size. For number (arithmetic) mean :
p = 0, q = 1: surface mean p. = 0, q = 2: volume surface mean p = 2,
q = 3. AD is the size class increment and is usually taken as uniform.
When analysing sprays from atomizers, microscopic counting of a given
spray sample is the technique most commonly used. The mean diameters
ED . fN (D)(AD) = summation of every given size (range) multiplied by its
percentage occurrence;
• EfN (D)(AD) = summation of given size occurrence, which is 100 for
sampled data.
(c) Geometric Mean Diameter (DGM)
The geometric mean diameter (D GM ) is defined as the nth root of the product
of the diameters of the /7 particles/droplets analysed. D GM is always smaller
than DAM . The geometric mean for droplet and particles according to their
sizes D 1 , D 2 ,. , D„, (where fN (D) is the percentage occurrence of a given
size in a sample) is represented by
DGm — 10..sly(Dfi N(DI)
DJ'
DiiN(D.))
(2.5)
DGM is of particular value since in the log-normal distribution the geometric
mean is the diameter having the highest frequency. Defined in terms of
logarithms
log DGM = ffiEfN(D) (log D)
(2.6)
30
REPRESENTATION OF SPRAYS • 31
BASIC PRINCIPLES, DEFINITIONS
(d) Harmonic Mean Diameter (D HM )
The harmonic diameter is defined as the number of particles/droplets
divided by the sum of the reciprocals of the diameters of individual particles/
droplets. It is related to specific surface, and is of value where surface area
is the important product characteristic. D HM is always smaller than DGm
For occurrence of a given size stated as a percentage, - the harmonic diameter
is represented by
100
Dflm =
(2.7)
IfN(D)/(D)
(e) Median Diameter (D M )
The median diameter is defined as the diameter above or below which 50 %
of the number of droplets/particles lie. The median diameter divides the
area under the frequency curve into two equal parts. It is represented by
the 50 % line on the cumulative curve (figure 2.4). D M is especially useful
when an excessive number of very large and very small particles are present.
The mean diameters defined above are divergent. The parameter selected
is appropriate to the circumstances of sampling. In general
D,,m
DM
(2.8)
DAM
The relation between the mean diameters follows no definite rule, as any
relation depends upon the skewness or bias of the distribution. The calculation of the mean diameters defined above is illustrated in example 2.1.
EXAMPLE 2.1
Spray from a nozzle atomizer is sampled and analysed by microscope. The
numbers of droplets occurring in size increments of 20 micron were determined. 500 droplets were counted. The resultant distributions are given
in table 2.1, column 4. Calculate :
1. Most frequent diameter
2. Arithmetic mean diameter
3. Geometric mean diameter
4. Harmonic mean diameter
5. Median diameter
The computations required are shown in table 2.1 and the distribution is
shown graphically as a frequency and cumulative curve in figure 2.4.
1. The most frequent diameter as seen on the frequency curve or in table 2.1,
column 4, is 70 micron.
2. The arithmetic mean diameter as defined in equation (2,4) becomes, for
uniform size increments
Column 7
9608
=
Column 5
100
96.08 micron
3. The geometric mean diameter as defined by equation (2.6) becomes
log DGM
Column 8 190.38
— 19038
100
100
D Gm = 80 micron
4. The harmonic mean diameter as defined by equation (2.7) results in
o 9n
7
E <
22
100
20 18
80
16 -
0
1412-
60
10 40
a
6
4
20
20
40
60
80
100
120
140
160
180
200
220
240
CUMULATIVE PERCENT LESS THAN DROPLET SIZE (b)
7
co.
DROPLET SIZE CD) MICRONS
Figure 2.4. Frequency and cumulative curves showing mean droplet diameter values in
example 2.1.
100
100
—
— 67 micron
DHM _— Column 9
1 486
5. The median diameter
(i) corresponds to the 50 % line on figure 2.4
= 83 micron
(ii) from table 2.1, column 6:
less than 90 micron, greater than 70 micron
Apart from mean diameters based on the frequency of size occurrence
other mean diameters based on surface, volume or mass are available. For
each chosen basis, the appropriate values of integers (p . q) as given in
equation (2.2) must be used. Table 2.2 shows four such mean diameters.
REPRESENTATION OF SPRAYS
BASIC PRINCIPLES, DEFINITIONS
33
Table 2.2. Mean Diameters Based upon Number, Surface
and Volume
AD
CD AD 0 AD Net CD oo eN eN
oo
m
CD AD
00 et DON oo
• c.,.4
(-14
,-. CD CD CD CD CD c, et
C7 CD 6
6 e5 6 6 c5 c)
cp
ati
r
(4p (471 rq
• •D AD ct AD- oo UD - •D et et ion N
00 UD 0' CD er et 00
c7 ,-. 00 on
(.0
• oo r4-- r- rl r- cN
00
.1
--,
( 1 -4 ( ) - i e5
m
rl
ob
tr)
cn on
Mean diameter
D
Surface mean
D
d
c? cu u? u? op c? u? up c?
oo ,t r- O. ep
r- -Li
• C7 ,t
r- oo oo CN
CN CD
9
r4 et vn
r 4 ,
O
0 N
:
N op 4 r-
tfl
1—c
r ,?m ,0t r- vl ,?N 0
9-1
CD CD CD \AD r4 oo r4 VD VD Do CD r4
• Nr r
et DO N rt CN
c) N
00 N
on AD
') et
N (-4 rcq r4m •D
m
VD oo qs,
LE
NcV
NN
NNN
c7 r,
cry
0
N
Cq
00
-
C7 r- en ,t ,r) c?
CD
•
-
0 CD CD 0 0 CD CD CD 0 CD CD 0
• rn vn r- CN w
(-1 Min
w. r-
N N
CD CDCD CD CD CD CD CD
AD 00 CD rl
CD CD CD CD CD (-4
cg Ng
c
- c; c; "c .cT rCg
Cr)
AD 00 CD N 1 UD 00
°
AM
sm
CN
0 0
PI, 0 r4 00
C7 0 - CD et UD UD < 4 (7
N - CD CD r- C. vn 00 CD DO N r- CD
uD
( V r- r4 et et rn C7 oa AD of
CN
•
Mathematical definition
Arithmetic mean
(linear)
Volume mean
Table 2.1. Calculation of Mean Diameters in Example 2.1
32
r
cg
N
O
Volume—surface
(Sauter) mean
D
vm
D
vs
ED . fN (D)AD
EfN (D)AD
(2.4)
LD 2 fN (D)AD1 112
EfN(0)AD
(2.9)
ED 3 fN(D)AD
EfN (D)AD
(2.10)
(D 3 fN(D)AD)
D2fN(D)AD
(2.11)
Analysis of spray dried powder using sedimentation or sieving techniques
results in mean diameters based on volume or weight. Where the latest
light absorption techniques are used, diameters are based on surface. The
volume mean diameter (D VM ) usually exceeds the Sauter mean diameter
by 15-20 %.
The two mean diameters commonly used in the spray drying operation
are the median diameter (D M ) and the Sauter mean diameter (D„) (equation
2.11).
The median diameter is that diameter above or below which lies 50 %
of the number or volume of droplets/particles. The Sauter mean diameter
is that droplet or particle having the same surface-to-volume ratio as the
entire spray or powder sample. The parameters pre-suppose spherical
droplets or particles.
2.3.2. Distribution Functions
Many authors have prepared mathematical relationships to represent
size distributions. The existence of varying forms illustrates that no one
mathematical distribution fits the size range of droplets or dried particles
produced in the spray dryer operation. The more common distributions
are given below. Their mathematical definition is given in table 2.3. d(N) is
the number of droplets or number fraction in size increment. d(D), B and C
are constants.
(a) Normal Distribution (Gaussian)
Several distributions are based on the normal (Gaussian) distribution. The
mathematical form is given in table 2.3, equation (2.12). Plotting the number
of droplets or frequency against their size results in a 'bell-shaped' curve.
The distribution is of limited use in spray drying. (S N is the number standard
deviation.)
34
BASIC PRINCIPLES, DEFINITIONS
REPRESENTATION OF SPRAYS
Table 2.3. Distribution Functions
d(N) =
. exp
i
(2.12)
Nog D - log D Gm ) 2 1
2,s.
(2.13)
[(,/D - V D Gm ) ]
(2.14)
r
d(D) SN , 02m)
Log-normal
1
d(N) _
d(D) - D . S G V(27E) . exP
Square root normal
I
d(N) =
d(D)
2 . V(27c . D . SG). exP
Upper limit
d(N) =
99.599 -
_
_
95 -
_
_
- Df
1
1
d(N)
= B . D 2 . e -(c •' )
Nukiyama-Tanasawa
d(D)
vi) = 100 - twii5R)q)
Rosin-Rammler
2S G
rlog ((D rii.„ - D)/D Gm l '
. ex
d(D) D . S G V(27r)
L
2,5?,
P
L
2. A
z
I-
90 as 70 60 50
40 -
_
_
_
(2.15)
20 (2.16)
(2.17)
(b) Log-normal Distribution
The log-normal distribution has a theoretical relation to droplet formation
and is applicable to the spray drying process. The distribution has two
parameters, the geometric mean size (D G ,) and the geometric standard
deviation (S G ). The mathematical form is given in table 2.3, equation (2,13).
The 'distribution gives a straight line on a plot of size (D) as abscissa on
log-scale against the number per cent oversize or undersize as ordinate on
probability scale (figure 2.5). The geometric mean diameter corresponds
to D at the 50 % line. The geometric standard deviation is the size ratio at
,
the corresponding oversize values of 50 % and 15 9 % or the size ratio at
the corresponding undersize values of 84.1 % and 50 %. Use of the undersize
plot to determine geometric mean size and geometric standard deviation is
shown in example 2.2.
The log-normal distribution enables determination of the geometric
mean size and deviation, and with this data other mean diameters (shown in
table 2.4) can be calculated. This is possible since the nature of the lognormal distribution law means the geometric standard deviations on number,
weight, surface area bases are equal. The log-normal distribution is applicable
to represent sprays from rotary (vaned wheel) atomizers.
(c) Square Root Normal Distribution
The square root normal distribution equation (2.14) is similar in form to
the log-normal distribution (equation 2.13). Both distributions have a
mean and standard deviation. (VD) merely replaces (log D). The square
root normal distribution is used with centrifugal pressure nozzles, as the
CUMULATIVE
Normal
Equation
No.
Form
Distribution
35
'v_.
10
5-
_
1 0.5 -
_
005
DROPLET SIZE (0) MICRONS
100
Figure 2.5. Log-normal distribution on probability paper.
distribution often represents spray size data with greater accuracy than the
log-normal distribution. A spray of a centrifugal pressure nozzle is shown
represented by the square root normal distribution in figure 2.6.
(d) Empirical Distribution Functions
(i) Nukiyama—Tanasawa Distribution. The distribution (equation (2.16)) is
proposed as suitable for representation of sprays from pneumatic nozzles.
For the distribution to apply, a plot of (log 1/D' . d(N)/dD) against D' will
be a straight line, the slope of which gives the value of the constant (C).
Values of the dispersion coefficient (q) can vary between e and 2, depending
upon nozzle design. For a given spray sample, values of q are selected by
trial and error to determine the value that gives the actual spray distribution
the closest agreement with a linear relationship. The higher the value of q,
the narrower the size distribution.
For small capacity nozzles, q can be taken as unity : the distribution is
described in more detail under the section on pneumatic nozzle atomization
(chapter 6) where it is plotted. Procedure on applying the distribution is
given by Lewis et al. (7).
(ii) Rosin—Rammler Distribution. The Rosin-aammler distribution is
widely quoted to express size distributions especially from nozzle sprays.
36
REPRESENTATION OF SPRAYS
BASIC PRINCIPLES, DEFINITIONS
10
37
-3
CUMULATIVE PERCENT
BY WEIGHT LESS THAN D
99.9
99.5
99
95
90
99
10
70
60
50
40
30
20
10
5
10
0.0
0.1
0
2
4
6
8
10
1 1 11
(DROPLET DIAMETER 1'1/2 1 ( : )
12
14
16
R
MICRONS
Figure 2.6. Droplet size distribution of a centrifugal pressure nozzle (grooved core swirl)
conforming to a square root normal distribution. Feed rate: 120 kg/hr (aluminium silicate).
Feed pressure: 50 atm. Orifice diameter : 1.1 mm. Spray angle: 45p. Grooved core swirl insert.
10°
10
100
DROPLET DIAMETER
It is empirical, and relates the volume per cent oversize (V D ) to droplet
diameter (D), It is a special case of the Nukiyama—Tanasawa Distribution.
The mathematical form is given in table 2,3, equation (2.17). Rearranging
equation (2.17) we obtain
100. (
=D
VD
(2.18)
( MICRONS )
Figure 2.7. Rosin—Rammler size distribution.
Thus it follows that the value on the ordinate of figure 2.7 corresponding
to Li equals
log
100
36.8
= log 2.75 = 0.43
L/ R
or
log In /100\
VD
q
ID
./T)
(2.19)
For a spray distribution to follow this function, the plot of log (100/V D )
against (D) will give a straight line on log-log paper. This is shown in
figure 2.7. The slope represents the dispersion coefficient (q). The higher
the value of (q), the more uniform the distribution. The Rosin—Rammler
mean (D R ) can be obtained directly from the curve as it is the droplet diameter
above which lies 36.8 % of the entire spray volume.
(iii) Upper limit function. This is a development of the log-normal distribution. The distribution is expressed in equation (2.15). Proposed by Mugele
and Evans (6), it contains a third parameter, the stable maximum droplet
size, which permits greater flexibility in fitting experimental data. The
upper limit distribution is considered justified as all sprays are made up of
droplet sizes that have a maximum size, i.e. the upper limit size. Nelson and
Stevens (8) discussed the merits of the distribution in relation to the normal
and square root normal distributions. The upper limit distribution places
practical limits on the minimum and maximum droplet sizes, unlike the
square root distribution, which gives meaningless values in the maximum
38
BASIC PRINCIPLES, DEFINITIONS
REPRESENTATION OF SPRAYS
size range. A comparison of the three distributions has been made (8) for
the case of a centrifugal pressure nozzle. This is shown in figure 2.8. The
upper limit and square root distributions approximate typical experimental
droplet size distributions for a centrifugal pressure nozzle.
99.95
99.9 -
I
1
i
T
- 99.5 _
d
0.51n 2 SG
(2.20)
_
D SM (Surface mean)
ln DGm
2
1.01n SG
(2.21)
:- --3 --- -
D vm (Volume mean)
In DGm + 1.5 lri 2 SG
(2.22)
95
I-
90
PERCENT LESS
I
80
70
60
50
40
30
_
1
-
Dvs (Volume—surface (Sauter) mean)
2
in DGm
In SG
(2.23)
The spray was sampled, the droplets counted and sized. Data is given in
table 2.5. Calculate the Sauter mean diameter.
Table 2.5
z
_
10
Measured size (micron)
3
,
il 2
-
▪
5
-
1 /
0•5 - 3 /2 1
/ /
•
-
1
-
0.1
0
20
40
Number counted
-
/
20
9
Transformation
(expressed in natural logarithms)
Mean diameter
111 DGm
2
99
Table 2.4. Mean Diameter Transformation
DAM (Linear or arithmetic mean)
i
39
60
80
100
DROPLET SIZE ID)
120
140
Less than 5
5-15
15-25
25-35
35-45
45-55
55-65
65-75
75-85
3
18
130
220
105
48
16
11
6
160
MICRONS )
Figure 2.8. Comparisons of log-normal, square root normal and upper limit distributions for a
centrifugal pressure nozzle (based upon Nelson and Stevens (8)): I. log-normal distribution;
2. upper limit distribution; 3. square root normal distribution (shaded area represents typical
experimental data for the centrifugal pressure nozzle).
2.3.3. Mean Droplet Diameter Transformations
If the size distribution of a sampled atomized spray obeys (or is in close
agreement with) the log-normal distribution, the transformation between
mean diameters can be used. This is shown in table 2.4. Example 2.2
illustrates the use of the transformation (table 2.4).
EXAMPLE 2.2
An atomizer in a pilot plant unit is spray drying a low viscous liquid at a
feed rate of 1 lb/rnin. A resultant spray is found satisfactory as the dried
product meets the specifications. Data is required on the spray distribution,
so that the atomization conditions 'can be selected for a scaled-up unit.
Method 1: (use of equation 2.11)
The computation steps are given in table 2.6.
For uniform size increments
Column 8
DVS = Column 7
129 983 x 10 3 )
= 43.4 micron
6889 5 x 102
.
Method 11: (use of equation 2.23)
This method prevents the tedious computations leading to columns 7
and 8 in Method I.
Draw the cumulative per cent undersize curve from data in columns 2
and 6 respectively. This is shown in figure 2.9. Spray distribution lies on a
straight line, hence following the log-normal distribution. From curve
Geometric mean diameter (D GM ) = D
= 24.2 micron
84.14
39
Geometric standard deviation (S G ) = D 50 =
1.62
24.2 =
REPRESENTATION OF SPRAYS
BASIC PRINCIPLES, DEFINITIONS
41
999
"e),
00
-do,—.c op
2?
x
8 CD CD efl
8 P. cri;
00
Lu
Ni
0
r—
CD 8 8
•
0r
9 47 4? 7' O a 9
r- on 0
N
N MD DO CN CP, CN CD
,r
CD
00
00
MD
N
CD ‘ID CT CD CD
6,■ Cn pi)
rq
r.•- )
995
99
95
•
0
90
•
80
70
60
84.14%
▪
50
w
-
40
30
CUMUL ATIVE PERCENT
0 0 on
co op r op 0
6 ob 6 6 ken .0 on cn 71on oo
• r4 00 ro.1
vn cn
N
00
Table 2.6. Calculation of Sauter Mean Diameter in Example 2.2
40
20
.
24.2 microns
10
5
y
..•••••
••••••
0.1
2
3
4
6 8 10
DROPLET SIZE 101
on
',ID
•
rn
c,
rvn vn vn vn
39 microns
50%
20 30 40 60 80 100
MICRONS
Figure 2.9. Size data for example 2.2, conforming to a log-normal distribution (cumulative
per cent undersize curve for calculating geometric mean diameter and geometric standard
deviation).
Using equation (123) and substituting
In D vs = in 24.2 + 2.5 In' 1.62
C H VD
erl 00 CD CD ken 00
-
rn. Cl CD .4
log io D vs = log„ 24.2 + 2.5 . 2-303 logi o 1.62
= 1.3838 + 0.255
vn
N CD CD CD CD CD CD CD CD
vn ,47
Q n
N
= 1.638
therefore
Dvs --- 43.5 micron
a)
• E
o2a
(:)
lfl
in
V-) 4-1.
rn
1
•
1
.4 ) kr)
vn
V-1
1
vn vn
N 00
1
vno t
N m Cr vn MD N
DRYING TERMS AND PRINCIPLES
3 Drying Terms and Principles
of the droplet depend upon whether bound or unbound moisture is
evaporated, as each has distinct features. As long as unbound moisture
exists, drying proceeds at a constant rate, and will continue while the rate
of moisture diffusion within the spray droplet is fast enough to maintain
saturated surface conditions. When diffusion and capillary flow can no
longer maintain these conditions the critical point is reached, and the
drying rate will decline until the equilibrium moisture content is attained.
The equilibrium moisture content will remain unchanged while product
is exposed to the same atmospheric humidity and temperature.
Figure 3.1 relates these terms to the drying of a spray droplet in a constant
humidity air medium. Other terms used frequently in describing mechanisms
are outlined below.
100
In the spray drying operation, the liquid to be removed is invariably
water, although the removal of organic solvents in closed cycle operations
is becoming more widespread. The drying principles involved for both
water and non-water systems are the same, and thus in this chapter, the
case of water evaporation into air will be taken to illustrate the drying
principles involved.
3.1. Common Terms and Principles
The removal of water from a feed to the extent of leaving its solids content
in a completely or nearly moisture-free state is termed drying. The moisture
in the feed can be present in two forms : bound and unbound moisture.
The nature of the solids and accompanying moisture determines the drying
characteristics. The bound moisture in a solid exerts an equilibrium vapour
pressure lower than that of pure water at the same temperature. Water,
retained in small capillaries in the solid, adsorbed at solid surfaces, as
solutions in cell or fibre walls, or chemically combined with the solids
(product water of crystallization) falls in the category of bound moisture.
The unbound moisture in an hygroscopic material is that moisture in
excess of the bound moisture. All water in a non-hygroscopic material is
unbound water, which exerts an equilibrium vapour pressure equal to
that of pure water at the same temperature. The equilibrium moisture is
the moisture content of a product when at equilibrium with the partial
pressure of water vapour of the surroundings. The free moisture is the
moisture in excess of the equilibrium moisture and consists of unbound
and some bound moisture, Only free moisture can be evaporated.
The mechanism of moisture flow through a droplet during spray drying
is diffusional, supplemented by capillary flow. The drying characteristics
43
0
BOUND
MOISTURE
0
UNBOUND
MOISTURE
›0
z
T
0
EOUILIBRIU
LI] 1.10ISTUR
or
0
MOISTURE
REMOVED
DURING
EVAPORATION
MOISTURE CONTENT
(free moisture)
wt moisture
unit wtdrysolid
SPRAY
DROPLET
FROM
ATOMIZER
Figure 3.1. Graphical relation between common drying terms.
Dry air is atmospheric air exclusive of accompanying water vapour, as
apart from wet air, which is inclusive of water vapour. The water vapour
content varies considerably from day to day and from place to place.
Properties of dry air are given in Appendix 3, Properties of wet air are given
by humidity (psychrometric) charts.
Psychrometric charts relate air temperature and humidity, or air enthalpy
and humidity. The former is the more commonly used. The abscissa is the
dry bulb temperature (DB), the ordinate absolute humidity (H a ) and the
parameter relative humidity (H„,). Air volumes, total heat content and
wet bulb (WB) temperature (adiabatic saturation (AS) temperature) are also
indicated. Parentheses are used here since the air—water system is unique,
as for all practical purposes the (WB) and (AS) temperatures can be considered the same, and the wet bulb and adiabatic cooling lines coincide.
The (WB) exceeds the (AS) temperature in all other systems. Psychrometric
44
DRYING TERMS AND PRINCIPLES
BASIC PRINCIPLES, DEFINITIONS
charts for air—water vapour systems covering normal to elevated temperatures are given in Appendix 5.
Humidity defines the moisture content of the air. The amount of water
vapour present in the air is independent of the air pressure (P T ) but it does
depend upon the temperature of the air with which it is mixed, i.e. according
to Dalton's Law. Thus for air—water systems, the absolute humidity (H a )
is related to the partial pressure (A y ) of the water vapour in the air in the
following way
45
force it provides resisting drying. Vapour pressure is governed solely by
temperature, and is directly related to drying rates. The vapour pressure
of water at any temperature provides the forward driving force for drying.
For- the air—water systems, the vapour pressure at a saturated surface
(PwB ) is related to that of water in the surrounding air (P,,) by equation (3.4).
The equation can be used to calculate wet bulb temperatures, see example 3.2
P,03
=P+
29 C,PT
A
AT
(3.4)
18
18( p w
29 PT — pw
(3.1)
Ha =
C, = specific heat of vapour at constant pressure,
1 = latent heat of vaporization at wet bulb temperature,
AT = temperature difference between surface and air (i.e. wet and dry bulb
temperatures)
where H a is expressed as
weight of water vapour i lb \ gr
unit weight of dry air \ lb
lb
or
(g
kg
Absolute humidities for air—water systems at atmospheric pressure are
read directly from psychrometric charts. Absolute humidities at pressures
other than atmospheric, i.e. H a at pressure B, are given approximately by :
1
Ha(B) = Ha{cA-) + 0.622P4
1
1 PT — P‘,) (760 —
(3.2)
Pressure A = 760 mmHg
where Pp' = vapour pressure of water at wet bulb temperature. Relative
humidity (H„,) is the water vapour content expressed relative to the water
content at saturation at the temperature of the mixture. The mixture
humidity at saturation is designated 100 %. Alternatively the relative
humidity can be expressed as the ratio.
Ti re' = 100
partial pressure of water in air at temp (T)
vapour pressure of water at temp (T)
where
(3.3)
The addition of heat to a wet droplet is insufficient by itself to promote
satisfactory drying. Removal of moisture depends upon the humidity of
the surrounding drying air. To maintain high drying rates cool humid air
must be moved from around a droplet and replaced with hot low humid air.
The partial pressure of water in air is the pressure it would exhibit if
existing alone in the same volume and at the same air temperature. The
sum of partial pressures of components in a gaseous mixture equals the
total mixture pressure. The partial pressure of vapour is the vapour pressure
at dew point temperature (see equation (3.3)). Alternatively from Dalton's
Law, the partial pressure is the multiple of total pressure and mol fraction
of vapour. The significance of partial pressure concerns the reverse driving
The dew point (saturation temperature) is the temperature to which wet
air must be cooled at constant pressure before liquid will form through
condensation. At the dew point, the saturation vapour pressure equals the
partial pressure of water vapour in the air mixture. The dew point is not
the temperature where all water vapour is condensed to leave dry air. If
the air is cooled below the dew point, condensation continues. The amount
of condensation depends upon the degree of cooling. The air is at a state
of saturation at lower temperatures and the absolute humidity decreases
by the quantity of water condensed. The dew point is related to vapour
pressure data where the vapour pressure at dew point equals
Pw . = P,.s x lire,
( 3.5)
where
Pw = vapour pressure at dry bulb temperature
humidity expressed as a decimal
Hrei relative
=
The dry bulb temperature of air is recorded by an ordinary thermometer
when in thermal equilibrium with the air surroundings. The wet bulb
temperature is recorded under steady state conditions by a thermometer,
whose surface is saturated with liquid water and simultaneously exposed
to a mixture of air and water vapour. The usual technique is to place a
clean cloth around the thermometer bulb and thoroughly moisten the cloth.
The thermometer must be placed in a strong air current (velocity > 900 ft/min
(4.5 misec)) (379).
The wet bulb temperature is lower than the dry bulb temperature. The
difference (or depression) is proportional to the moisture evaporation
from the wet cloth surface. The wet-bulb temperature in fact, corresponds
46
BASIC PRINCIPLES, DEFINITIONS
DRYING TERMS AND PRINCIPLES
to the temperature at which the air would normally be saturated without
any change in its heat content. The upper limit of wet bulb temperature is
the dry bulb temperature. The lower limit is the dew point. For saturated
mixtures of air, wet and dry bulb and dew point temperatures are the same.
The wet bulb temperature does not represent conditions of thermal equilibrium. It represents simultaneous heat and mass transfer the dynamic
equilibrium between the rate of heat transfer to the thermometer bulb and
the rate of mass transfer from the bulb.
The driving force for moisture evaporation from a saturated surface is
the difference between the water vapour pressure at the temperature of the
surface and the partial pressure of water vapour in the surrounding air
A y ). The driving force can equally well be expressed in terms of the
difference in humidity at the saturated surface (H„) and the humidity of
the air (H a ), i.e. H, — H a . Equation (3.1) relates the partial pressure of
water vapour in the surrounding air to H a . At the saturated surface, the
partial pressure of water equals its vapour pressure and thus substitution
of p, for P„, in equation (3.1) gives the expression for
H. The rate of mass
transfer from a saturated surface equals :
dW
dt
k A(H — H a ) = kgA(PwB Pw)
g
(3.6)
w
For dynamic equilibrium the rate of heat transfer is equal to the multiple
of the rate of mass transfer and latent heat of vaporization. The rate of
heat transfer from a saturated surface equals
dQ
dt
W
h A(T
)=
A
(3,7)
— T, B ) = k g (Hw H a )A
(3.8)
a
Tw B
dt
combining equations (3.6) and (3.7)
The coefficients h„ k g are incorporated with the humid heat (C s ) in the
Lewis Number. This is unique in being unity for air—water vapour systems
and results in the adiabatic cooling line and wet bulb line being the same
on the psychrometric chart. This greatly simplifies dryer calculations of
water containing solids dried in air.
The humid heat (C s ) is the heat required to raise the temperature of a
unit mass of air and its vapour one degree at constant pressure. It is expressed
by
C s = 0.24 + 0.45H a
(3.9)
The humid heat is used to calculate the heat for raising the temperatures of
air—water vapour mixtures where
Q a = ifiC s AT
47
(3.10)
Equation (3.10) is valid so long as condensation or vaporization does not
take place. The relationship between humid heat and humidity for air—water
vapour systems at atmospheric pressure is given in Appendix 6.
The enthalpy (heat) of a mixture of air and its water vapour is the sum of
the air enthalpy and vapour enthalpy. Enthalpies are relative to a given
reference level (Tr ) taken as air and saturated liquid water at 32°F (0°C).
For air—water vapour systems
(3.11)
Q a = (0.24 + 0.45H a )(T — 7) + ~.H a
where A is the value at the reference temperature. The relationship between
temperature and the latent heat of vaporization of water is given in Appendix
7.
Drying characteristics of droplets during spray drying are best illustrated
by plotting the variations in evaporation rate and accompanying changes
in droplet temperature and vapour pressure as droplet moisture decreases.
Figure 3.3(a) illustrates the droplet temperature change for a droplet containing 50 % moisture contacted with hot air 290°F (143°C) (humidity
50 gr/lb (7 g/kg), equivalent WB temperature = 105°F (40°C)). The plot
shows that although a spray dried product comes into contact with hot air,
at no stage during the process does the product temperature become excessive
to cause product degradation. The product is removed from the dryer
long before the product temperature has time to rise to approach the temperature of the exhaust drying air. Figure 3.3(b) shows the vapour pressure
change during the droplet drying. The vapour pressure during the initial
drying period is that at the WB temperature 105°F (40°C), which is 2.24 in
Hg (see Appendix 4). Figure 3.3(c) shows the rate of drying curve. The
ordinate expressed in weight moisture evaporated per unit time per unit
area is often termed the drying flux. The falling rate curve shown shows the
basic phases. Different shapes are obtained for different products and
different conditions.
For more detailed treatment of drying principles the reader is referred to
Perry (91), Coulsen and Richardson (98), Blackadder and Nedderman
(500), and Cremer (501).
3.2. Applying the Psychrometric Chart to the Spray Drying Process
The psychrometric chart provides all data pertaining to a wet product
in contact with air. Common applications include determining the evaporative capacity of a dryer, droplet/particle temperatures and necessary air
conditions for product conveying and handling packing.
•▪
-
48
ZE
cc
ci
BASIC PRINCIPLES, DEFINITIONS
DRYING TERMS AND PRINCIPLES
DRY BULB TEMPERATURE AT DRYER OUTLET
160-
300
—1
cc
Z• t
)_ 800
49
LL
LI_ 600
TEMPERATURE
150-
700
MOISTURE CONTENT AT
DRYER OUTLET
140-
0
cc
la- 500
cc 300
I
CRITICA
PO I N r L
130-
DROPLET
ENTERS
DRYER
EVAPORATION
AT DECREASING
RATE
120-
EVAPORATION AT CONSTANT
RATE
110 25
3 2
u_
i=}
100
1—
=
DRY
BULB
TEMP
15 20 25
5
TEMPERATURE °F
55 60
30 35 40 45 50
MOISTURE CONTENT PERCENT
IN DROPLET
EQUILIBRIUM
MOISTURE
CONTENT
IU
WET BULB
o
a
o.
HUMID
HEAT
378
c0180
Lu
cc
iri
D
ci
1418 ;< cc
z¢
oc)140 7
cc
)ci
(
ris
2.6
c
.._
1_
a.1
AIR AT DRYER OUTLET
160' F 0,8 15 R H 4CeF D.P
2.4 -
0
I-
ci
WO
Lu Z
.4,-.
Ee
105.5
rx
U °-
12
L_ =.0
a.
-135
cox
uct_
D LI
0
2-24 _
❑
2.2 -
EVAPORATION AT CONSTANT RATE
2.D -
❑
yr
to
a'
L.
a
-130
ix
EVAPORATION AT
DECREASING RATE
1-6
1
1
MOISTURE CONTENT
LEAVING DRYER
VAPOUR PRESSURE OF
MOISTURE IN AIR AT
DRYER OUTLET
lu
1422
14`
0
30
50
55 60 65 70
DRY BULB TEMPERATURE (°F)
75
80
85
95
90°F
DRY BULB
100
DROPLET
ENTERS
DRYER
CRITICAL
POINT
1-0 -
1• 2
5
10
15
20
25
30
35
40
45
50
55
Moisture content of droplet %
b.
Figure 3.2. Two applications of the psychrometric chart in spray drying. (a) Estimating droplet
surface temperature, (b) Conveying air conditions.
b.
Figure 3.3. Drying characteristics of droplets during spray drying. (a) Variation of droplet
temperature during drying. (b) Variation of droplet vapour pressure during drying.
50
DRYING TERMS AND PRINCIPLES 51
BASIC PRINCIPLES, DEFINITIONS
EXAMPLE 3.1
The dew point of air sampled at the base of spray dryer is 115°F (46°C),
If the chamber pressure is 10 mm of water below atmospheric, what is the
air humidity in this region?
Data : Barometric pressure 29.92 in Hg (760 mm Hg), vapour pressure of
water (Pw ) at 115°F = 2.995 in Hg (from Appendix 4). Now
er
0
a.
u.1
U cc
CC
FD2
1
2
CONSTANT RATE
INTERNAL
MOVEMENT
CRITICAL
OF MOISTURE
POINT
CONTROL'LIN G
UNSATURKHED
SURFACE
DRYING
crz
cob
u0_
UZ
DRYING RATE
FALLING RATE
0
0
25
1 mm water = 0.0029 in Hg
chamber pressure = 29.92 — (10 x 0.0029)
29.891 in Hg
DROPLET
ENTERS
DRYER
,
PT = 29.891 in Hg
H
MOISTURE CONTENT OF DRIED
PARTICLE LEAVING DRYER
I
15 20 25 30 35 40 45
b
The humidity is calculated from equation (3.1) by substitution of (at Dew
Point)
Pw p,, = 2.995 in Hg
T
I
= 0.066 lb water
lb dry air
EXAMPLE 3.2
50
MOISTURE CONTENT.( PERCENT)
C.
Figure 3,3 continued. Drying characteristics of droplets during spray drying.
drying curve.
2.995
18F
L29.891
— 2.995
a=
(c) Rate of
The determination of the theoretical moisture removal from a spray
dryer is illustrated in example 3.2. The droplet surface temperature is
shown in figure 3.2(a) by a droplet contacted with air 262°F (128°C) containing 35 gr/lb moisture, It is that temperature where the adiabatic
(wet bulb) cooling line (which passes through the drying air conditions)
intersects the dew point line. Data on air conveying conditions are often
essential where dried product specifications impose stringent control of
wet air in contact with product, i.e. where product is highly hygroscopic.
Figure 3.2(b) illustrates how the dew point (DP) 69°F (20.5°C) of conveying
air 90°F (32°C) DB, 75°F (24°C) WB can be obtained, so as to ascertain
whether this DP is likely to be reached during plant operation, and whether
an air heater will ever be necessary to prevent condensation conditions.
The figure also shows the total humidity to be 106.5 gr/lb (15.2 g/kg), If
experience has shown the dry product to rapidly take in moisture from such
wet air, some form of air conditioning must be used to lower the absolute
humidity.
Droplets of aqueous slurry are sprayed from a nozzle into air at 240°F
(116°C). An air sample is drawn off the atomization zone and the dew point
determined. Assuming the droplet surfaces are still saturated at the point
of air sampling, calculate the droplet temperature. Barometric pressure =
2992 in Hg.
With the above assumption, the droplet temperature is the same as the
wet bulb temperature. Equation (3.4) can be applied using a trial and error
method to calculate the temperature (T s ), where
P,„ = 3.446 in Hg
PT = 29.92 in Hg
C, = 0.24 BTU/lb °F
AT = (240 — Ts ) °F
Substituting values in equation (3.4)
A
3.446 = P„
11.6 7:
Trial and error method.
Let
TS = 140°F
2 = 1014 BTU/lb
AT = 100°F
PwB (from Appendix 4) = 5.881 in Hg
52
BASIC PRINCIPLES, DEFINITIONS
DRYING TERMS AND PRINCIPLES
Substituting in equation (3.4)
3.446 = 5.881 — 1.144
3.446 0 4.737
By further trial and error, the value of is = 131°F gives satisfactory equality
of the equation
Ts = 131°F
= 1019 BTU/lb
P„ = 4.647 in Hg
3.446 4.647 — 1.242 = 3.405
droplet temperature = 131°F (55°C).
EXAMPLE 3.3
A spray dryer is supplied from atmosphere with air containing 50 gr/lb
dry air (7 g/kg). Air enters the dryer at 300°F (149°C). For maintenance of
the specified moisture content in the product, the humidity of air leaving the
dryer is maintained at 15 % relative humidity. Calculate the theoretical
quantity of moisture that can be removed in the dryer, and the temperature
drop through the dryer.
Although the air is heated to 300°F, it still retains a moisture loading of
50 gr/lb. The maximum theoretical amount of moisture that can be taken
up by the dryer is the amount required to saturate the drying air. in practice,
saturation is never achieved, and for most spray dryer operations, where
outlet temperatures are well above atmospheric temperatures, the relative
humidities are kept low to enable rapid completion of product drying.
Hence, using Appendix 5.2. For inlet air, T = 300°F, H a = 50 gr/lb, the
corresponding wet bulb temperature is 106°F (41°C). Now : 106°F WB is
equivalent to 360 gr/lb. Moisture required to saturate drying air = 360 —
50 = 310 gr/lb (44 g/kg),
For an outlet relative humidity of 15 %, the final air conditions can be
obtained from the chart (Appendix 5.2). For the ideal case, the total heat
in the air is the same initially as it is finally, if the product is considered
not to receive heat or release heat to the air during the drying.
Therefore, reading along adiabatic cooling line : at 15 % RH, the corresponding dry bulb temperature is 166.5°F (75°C) having a dew point of 96°F
(36°C). For dew point of 96°F, equivalent moisture loading = 260 gr/lb
(37 g/kg).
Therefore
(a) Theoretical moisture removal in the dryer = 260 — 50 = 210 gr/lb
of dry air (30 g/kg).
(b) the temperature drop through the dryer = 300 — 166.5 = 133.5°F
(56°C).
53
In reality to achieve the above moisture removal, inlet drying temperatures
above 300°F are required to supply the added heat to raise the temperature
of the product within the dryer, and to allow for the heat losses from the
dryer structure.
The calculation accounts for heat supplied by the drying air for moisture
evaporation. The additional product temperature requirements can be
calculated from the specific heat data of the dried product. Heat losses
from the dryer can be calculated from standard procedures involving convection, and radiation losses from the dryer structure. Such procedures
are to be found in standard heat transfer textbooks (9, 10).
EXAMPLE 3.4
Air at 96•5°F (36°C) and 30 % RH enters a cooler which supplies air to the
base of a spray dryer. The presence of cooled air completes product crystallization. The product leaves the spray dryer with a sticky texture, although
at low moisture content. What is the temperature the cooler exit air must
exceed to prevent saturated air passing to the dryer base?
Method I (From Appendix 4)
Vapour pressure of water at 96.5°F (36°C) = 1.74 in Hg
Substituting in equation (3.5)
Vapour pressure at dew point = 1.74 . 0.3 = 0.522 in Hg (13.25 mm Hg)
Dew point corresponding to this vapour pressure = 60°F
Method II
The above result can be read directly off the psychrometric chart. At the
intersection of the dry bulb temperature line of 96.5°F (36°C) and relative
humidity curve of 30 %, draw the horizontal line to intersect the saturation
line. Point of intersection is the point 60°F.
The cooler exit air temperature must exceed 60°F (16°C).
55
4,1. Introduction
Spray dryer performance is an expression of the thermal efficiency of the
process and in practical terms defines the ability of the dryer to produce the
desired product specification economically. Dryer design is directed towards
achieving desired dried product properties at highest possible thermal
efficiencies. The thermal efficiency of a spray dryer depends upon the
operation temperatures. It is defined as the ratio
heat used in evaporation
heat input
As it is normal practice not to recover the heat content in the air exhausted
from the drying chamber, the thermal efficiency is increased by increasing
the temperature of the air entering the chamber and operating the dryer
at an outlet temperature as low as the process allows. The whole basis of
economic operation is the utilization of heat passed into the dryer, i.e. the
dryer heat load, or the heat input required for producing a unit weight of
dry product. The heat load is proportional to the evaporation rate, and for
a given rate is greatly affected by the solids content in the dryer feed. As
seen from figure 4.1 it is economically important to operate at as high a
solids content as possible. For a given production rate, increase in feed
solids content from 10 to 25 % will result in a 66.6 % reduction in heat load.
Even increasing the solids content from 25 to 50 % will result in a similar
heat load reduction. For feeds of high solids content (50 % and over) substantial reductions in heat load can still be obtained by continued solids
content increase. Increase from 50-60 % will result in a reduction of heat
load of nearly 50 %. To utilize these savings atomizer development seeks
designs to atomize feeds of higher solids content satisfactorily at high feed
rate.
MOISTURE FOR EVAPORATION
HEAT LOAD
4 Performance of Spray Dryers
UNIT WT OF BONE DRY SOLIDS IN•FEED
PERFORMANCE OF SPRAY DRYERS
10
6
6
4
2
0
0
20
40
60
80
100
PERCENTAGE SOLIDS IN FEED
Figure 4.1. Effect of feed solids content on dryer heat load.
4.2. Heat and Mass Balances over Spray Dryers
Air—product flow and temperature data for assessing dryer performance
are obtainable from heat and mass balance data. For continuous operation
with negligible product hold-up in the drying chamber, the mass input of
air and feed per given time equals mass outputs of air and product. Heat
input of air and feed equals heat output of air and product plus the heat
losses from the drying chamber. For semi-continuous operation, shu
down if often due to build-up of product in the dryer. The difference i
product input and output equals the accumulation.
Heat and mass balances are drawn up below with reference to figure 4.
For calculation of air feed and product enthalpies, a reference temperatur
of the water freezing point is used. It is common to base moisture balance
on a unit weight of bone dry product. Suppose M s wt units/hour of dry soli
enter the spray dryer in a feed (solution, slurry or paste) containing (W s ) i w
units moisture/unit dry solid. The feed is dried to give solids leaving th
dryer with a moisture content of (Ws ), wt units moisture/unit dry solid. The
feed temperature when atomized is (Ts ), and product is discharged at a
temperature of (Ts ), . Drying air is supplied to the dryer at a rate of G a wt
units dry air/hour at temperature (71. The inlet absolute air humidity is
56
57
PERFORMANCE OF SPRAY DRYERS
BASIC PRINCIPLES, DEFINITIONS
Heat input = heat outlet + heat loss
(%) (Td ( )(1)
(4.4)
thus
Ga(Qa)i + M5(Qs)1 = Ga(Qa)2 + WW2 + QL
(4.5)
where
(T)( 0° ( 1-12 )
)
Q L = heat losses from the dryer outer cladding and structural supports.
The heat loss is expressed by the standard heat transfer equation _
Q L = UAAT
( Ms ) ( 5)2 ( Gs ) ( Ws )
Figure 4,2. Dryer data for calculation of heat and mass balances.
H 1 , and increases during the dryer operation to H2 . The air leaving the
dryer is (Ta ) 2
(4.6)
For well-insulated drying chambers heat losses are low. For non-insulated
drying chambers, or chambers designed with cooling air jackets to maintain
cool dryer walls (when handling many hygroscopic and heat sensitive
products) heat losses from the dryer are high. Q L becomes a significant term
in equation (4.5). The enthalpy of the feed (42 5 ) 1 as it enters the atomizer is
the sum of the enthalpy of the dry solid and the moisture as liquid.
Thus
(4.7)
(Qs)1= CDs(AT) + (Ws)iCwAT
where
Moisture balance
Moisture entering in feed
Moisture entering in hot air
Moisture leaving the dryer in the dried product
Moisture leaving in the exhaust drying air
CDS = heat capacity of dry solid
(MS) (141s)1
C w — heat capacity of moisture (in liquid form)
AT = difference in temperature between feed
temperature and the reference temperature
level (water freezing point)
(G a)H,
Ms(W5)2
GaH 2
For no product accumulation in the chamber
input = output
(4.1)
m5( 14701 + G.(111) = Ms ( W)2 + Ga H2
(4.2)
thus
or
Ms((W)1 (Ws)2) = Ga(H2 H1)
By similar procedure.
Enthalpy or heat balance
Enthalpy of air entering dryer -- G a (Q a ) 1
Enthalpy of feed entering dryer = M 5 (Q S ) 1
Enthalpy of exhaust drying air = G a (Q a ) 2
Enthalpy of dried solid = M5(Q02
(4.3)
The enthalpy of the drying medium (Q a ) whether entering or leaving the
dryer is expressed in terms of the humid heat, absolute humidity, and the
latent heat of vaporization of water at freezing point, where
Q a = QA T) +
(4.8)
If additional air is added to the chamber either in the form of cooling or
conveying air, the enthalpy of this air flow must be added to the left-hand side
of equation (4.8). The equivalent weight of air must be added to the value of
(G a ) on the right-hand side of equation (4.5).
EXAMPLE 4.1
2060 lb/hr of dried product (4 % moisture) is produced from a co-current
flow spray dryer incorporating a rotary atomizer. Atmospheric air (75°F)
(50 % RH) is heated to 350°F before entering the dryer. Air is exhausted
from the chamber at 176°F. Feed (45 % solids by weight) is pumped to the
58
BASIC PRINCIPLES, DEFINITIONS
PERFORMANCE OF SPRAY DRYERS
atomizer at 80°F. Dried product temperature is 115°F. What is the air rate
if heat losses from the dryer are estimated at 95 000 BTU/hr?
2060 x 96
Feed rate to dryer M, =
Enthalpy balance
Enthalpy entering dryer
100
89.2G a + 1980(77.8) BTU/hr
x - = 4400 lb/hr
45
100
Enthalpy leaving dryer
Moisture in feed (1470, = 55
45 = 1.22 lb/lb dry solid
Moisture in driedproduct (WO ,
4
59
36.7(1980) + G a (34.6 + 1140H 2 )
From equation (4.5)
-- 0.042 lb/lb dry solid
89.2G a + 1980(77.8) = 36.7 x 1980 + G a (34.6 + 1140H 2 ) + 95 000
Heat capacity of dry solids C ps = 0.4 BTU/lb °F
Absolute humidity of inlet air H, = 0.0095 lb/lb dry air (75°F, 50% RH)
Enthalpy of inlet air (equation (4.8)) (reference temperature 32°F)
Combining equations (A) and (B)
54.6G a - 13 700
1140
(C)
G a H 2 = 2340 + 0.0095G a
(D)
G a H2 =
(Qa)i = (0.24 + 0.451-1 1)(aD1 - 32) + 1075.2(H 1 )
= 77.7 + 10.2 = 89.2 BTU/lb
Enthalpy of outlet air (exhaust)
(B)
54.6G a - 1140H,G a = 13 700
From equations (C) and (D)
(Qa)2 = (124 + 0.45H 2 )(176 - 32) + (1075.2H )
54.6G a - 13 700 = 1140(2340 + 0.0095G a )
2
= (34.6 + 1140H 2 ) BTU/lb
54.6G a - 13 700 = 2 670 000 + 10.8G a
Enthalpy of feed entering dryer (equation (4.7))
G a = 60 600 lb/hr
(Q s )i = 0.4(80 - 32) + 1.22. 1. (80 - 32)
Humidity of outlet air (H2)
= 19.2 + 58.6 = 77.8 BTU/lb
11 2
Enthalpy of dried product leaving dryer
2340 + 0.0095G a
-
Ga
2340
60 600 + 0.0095
(Q s ), ---- 0.4(115 - 32) + (0.042)1 . (115 - 32)
= 33.2 + 3.5 = 36.7 BTU/lb
= 0.0386 + 0.0095 = 0.0481 lb/lb
(337 gr/lb dry air)
Moisture balance
at 176°F-relative humidity = 15 %.
NB. If operation had been adiabatic, i.e. no heat loss from the dryer, the
required air rate for the given conditions becomes 58 600 lb/hr.
Basis Dry solids dried per hour = 2060 x 96 = 1980 lb/hr
00
Moisture entering in feed
Moisture leaving in product
.'. Moisture evaporated
Moisture gained by air
= 1980 x 1.22 1b/hr
= 1980 x 0.042 lb/hr
= 1980 x (1.178) = 2340 lb/hr
= moisture lost from feed
4.3. Dryer Efficiency
From equation (4.3)
G a (H 2 - 0.0095) = 2340
(A)
Hot air enters the dryer chamber, and due to moisture evaporation from the
spray, the air temperature falls during air passage through the drying
chamber. If the chamber is well insulated, the heat losses to the surroundings
can be assumed negligible, and the fall of air temperature follows the
60
BASIC PRINCIPLES, DEFINITIONS
PERFORMANCE OF SPRAY DRYERS
adiabatic cooling line, as depicted on the psychrometric chart (Appendix 5)
and figure 4.3. Maximum possible evaporation from a given air flow is
obtained, if the air is exhausted in its saturated state. This never occurs in
actual spray dryer practice, although there are cases when producing very
moist powders, when approach to saturation can be close. Relative humidity
is generally low at the dryer outlet. Example 3.3 (page 52) shows calculation
of the theoretical moisture that can be removed in a spray dryer for a given
set of air conditions.
If the air is considered to enter a spray dryer at a temperature T 1 after
being heated from atmospheric temperature To , and that during the drying
process the air temperature falls and air is exhausted at a temperature of T2,
the efficiency of process can be expressed as follows
EXAMPLE 4.2
Atmospheric air at 74°F (23°C) and 30 % RH is heated to 300°F (149°C) and
passed to a spray dryer. The air is exhausted at 180°F (82°C) and 9 % RH.
Determine :
(a) maximum thermal efficiency (ideal case),
(b) overall thermal efficiency,
(c) evaporative efficiency,
(d) degrees of heat and percentage heat not used for evaporation.
The drying process can be represented on a psychrometric chart, figure 4.3.
cr)
z
0)
LD
(a) Overall thermal efficiency (n o „ raii ) is defined as the fraction of total heat
supplied to the dryer used in the evaporation process. It can be approximated
to the relation
[T, —
I
/overall =
z
0
T21
T, — T o
61
100
(4.9)
•
350
where T2 = the outlet temperature if the operation was truly adiabatic.
Inspection of equation (4.9) shows a rapid increase in overall efficiency
on increase in inlet temperature for fixed outlet and ambient conditions.
For outlet and ambient temperatures of 185°F (85°C) and 68°F (20°C) the
overall efficiency is 43.5 % at an inlet temperature of 275°F (135°C); 74 %
at 518°F (270°C) ; 87.5 % at 1004°F (540°C); and 89.5 % at 1202°F (650°C).
(b) Evaporative efficiency (ri e „,) is defined as the ratio of the actual evaporative capacity to the capacity obtained in the ideal case of exhausting air at
saturation. It can be approximated to the relation
Lu
I—
I-0 210
0
38
55 74
T.
105
300
TT
160 189
TEMPERATURE ° F
l evap
T,
[ Ti
—
T2
Tat_
100
(4.10)
where Tsai = the adiabatic saturation temperature corresponding to the inlet
air temperature (T1 ).
Although high efficiencies are obtainable for higher inlet air temperatures,
there are limitations to how high temperatures can be raised and yet still
obtain better performance. Firstly there is a temperature level above which
the heat effects on the atomized spray become too severe and where product
will degrade. Secondly extra cost of air heating beyond a certain level
may not be justified and thus the inlet air temperature may reach a maximum
from an economic viewpoint.
Figure 4.3. Drying process represented in example 4.2.
(a) Maximum thermal efficiency (ideal case) is obtained by exhausting the
drying air in a saturated state. The air temperature at saturation is obtained
from the intersection of the adiabatic cooling line through (T 1 ) and the
100 % relative humidity line. From figure 4.3 wet bulb temperature of
drying air = 105°F (41°C), hence :
heat used in evaporation
heat input
300 — 105 100 =
300 — 74
195
100 86 .4%
226 1
This is given by equation (4.9). T2 is
(b) Overall thermal efficiency (n,ove
calculated as follows : The absolute humidity of exhaust air is 210 grulb
62
BASIC PRINCIPLES, DEFINITIONS
PERFORMANCE OF SPRAY DRYERS
(30 g/kg), corresponding to 180°F (82°C) at 9 % RH. For adiabatic conditions, absolute humidity level in the exhaust air remains the same, and the
required temperature is where the absolute humidity line intersects the
adiabatic cooling line used in (a) above. From figure 4.3 this temperature
is 189°F (87°C). Thus the equation (4.9)
[ 300 — 181
//overall
111
100 = [--
300 --74
1891
=
""
300
L — 105
100 = [
111
1100 = 57 %
195
300 — 180]
9
[
[
9
-= 7.5%.
120
4.4. Residence Time in Drying Chamber
Drying chambers are designed to handle an air volume containing sufficient
heat for drying the spray droplets, and to provide an air residence time
sufficient for droplets to be dried to particles of desired moisture content.
Initial drying rates are very high in the first period of drying when the
majority of moisture is evaporated in very short time intervals, but during
the second period of drying, rates fall off quickly and more time is required
to evaporate product down to low moisture content. Many extra seconds
are required to evaporate the last few percentages of moisture to bring the
final moisture down to the desired moisture content. Drying rate curves
for a droplet can give information on residence times necessary in drying,
but due to difference in evaporation characteristics of single droplets and
sprays, residence time requirements are usually established on pilot plant
dryer facilities.
The minimum residence time of product can be assumed to be the average
residence time of air. This is calculated by dividing the chamber volume
by the total air rate. Air volume at the outlet temperature is used. Chamber
designs incorporate cylindrical, conical or rectangular forms, and chamber
volumes (V) can be calculated from :
Cone
Cylinder
(4.13)
where D c h is chamber diameter, b = breadth, L = length, h height.
For conical based drying chambers of cylindrical height (h') and cone
angle (60°) the volume can be expressed as (dimensions in ft)
V = 0.7854DL(h' 0.2886k h )
100 = 49.2
(d) (i) degrees of heat lost to evaporation due to the heat losses from chamber
and product heating = 189 — 180 = 9°F (5°C)
(ii) percentage heat lost =
V = bLh
(4.14)
2261
(c) Evaporative efficiency (n evap,) • This is given by equation (4.10). The
saturated air temperature is 105°F. Thus substituting in equation (4.10)
11
Rectangular (box)
63
V = A-hD c2,
(4.11)
hD 2h
c
(4.12)
=
4
Most of the product during passage through the •dryer has a residence
time much higher than the average air residence time. This is due to (a) particles enter recirculating air flow regions in the chamber, (b) particles remain
suspended at the chamber wall or fall at lower velocities due to air velocities
being lower than the average air velocity through the chamber. For drying
chambers with primary product discharge from the chamber and secondary
separation from the powder recovery section, only the fines fraction passing
to the recovery section will experience the minimum residence time. This
fraction contains fine particle sizes, and their size will enable drying to be
completed to the moisture content of the product recovered from the
primary product discharge point, even though the primary product will
have experienced greater residence times. For drying chambers with total
product discharge at the recovery section, a greater proportion of the
product will experience residence times in the order of the average air
residence time. Drying chambers must be sized, therefore, to provide a
residence time optimum for a given product. The optimum residence time
is the time for completion of desired moisture removal at minimum temperature increase of dried product temperature. It follows that coarse sprays
require longer residence times in the dryer than fine sprays for the same
inlet air temperature. Where low drying air temperatures are required
for highly heat sensitive products, transfer driving forces are low and
residence times are very long. This can lead to drying towers of great
height.
Dryer designs are available that cover the entire range from short (5
seconds) to long (several minutes) residence times. The longer the residence
time the larger the structure for a given evaporative capacity. Products
that can withstand turbulent air handling and contact with hot air temperatures can be effectively spray dried in mixed flow dryers (cyclone type), see
figure 5.9(a), at short residence times. For products that can withstand
hot temperatures, but a minimum of air turbulence, tall drying towers of
high residence time are required as near free falling droplet velocities conditions are achieved in the dryer. For the many products that are dried in
co-current flow dryers with rotary air flow, intermediate residence times
(20-40 seconds) prevail.
64
BASIC PRINCIPLES, DEFINITIONS
EXAMPLE 4.3
126 000 lb/hr air pass through a co-current flow conical based spray dryer
(60° cone) of 24 ft diameter and 20 ft cylindrical height. If the outlet drying
temperature is 176°F what is the minimum residence time in the chamber?
Volume of chamber given by equation (4.14).
Substituting in values
V = 12 200 ft 3
5 Equipment Incorporated in Spray Dryers
Air rate = 126 000 lb/hr
000 359
Volume at 176°F = 126
x 636= 33 700 ft 3 /min
60
29 x 492
Minimum residence time =
12 200 x 60
= 217 sec
33 700
5.1. Introduction
Many spray dryer designs are available to cover the wide range of product
applications. Although designs are diversified, each design contains standard
equipment, which can be classified into four categories :
A. Heating of the drying air ; air heaters with accompanying fans, air filters,
dampers and ducts.
B. Atomization of feed into a spray; atomizer with feed supply system of
pumps, tanks and feed pretreatment equipment.
C. Contacting of air and spray, and drying of spray ; drying chamber
complete with air disperser, product and exhaust air outlets,
D. Recovery of dried product ; complete product recovery system with
product discharge, transport and packing. Air exhaust system with fans,
dampers and ducts.
Equipment items are discussed generally in this chapter. More detailed
discussion of the more important equipment items is given in section IV
(chapter 12).
5.2. Passage of Air and Product through a Spray Dryer
The passages of air and product through a spray dryer are shown diagrammatically in figure 5.1. The air intake is from atmosphere, unless the
spray dryer is operating under closed cycle with a special gaseous medium,
when recycling is employed. The air is first filtered, then heated. The
filter can be dispensed with, if entry of atmospheric dust into the heater
and drying chamber does not jeopardize dried product quality. Direct or
indirect air heating can be used depending upon the product spray dried.
66
EQUIPMENT INCORPORATED IN SPRAY DRYERS
BASIC PRINCIPLES, DEFINITIONS
67
AIR
EXHAUST
OLOINE
FEE D
FILTER
PUHP
ATOMIZER
0-
PRODUCT
AIR DISPERSER
ATOMIZER
(rotary )
L-
FEED SYSTEM
AIR
HEATER
LID .ED
AIR
AIR
FILTER
AIR
DRIARR
HEATER
CHAMDER
AIR
PRODU4
RECOVERY
FEN
i RTA'RE
—P-
EEPUIPMENT
EXHAUST
FAN
FAN
FILTER
FEED TANK
AIR SUPPLY SYSTEM
CYCLO'vE
AIR EXHAUST SY51EN
Figure 5.1. Product and air passage through a spray dryer.
D
PUMP
FILTER
Flow of air through the heater is by a supply air fan located at the heater
inlet. In some dryers especially those of smaller capacity, the exhaust fan
is sized to draw sufficient supply air through the heater. In a one fan system,
the drying chamber operates under a higher negative pressure than when a
two fan system is used. A supply fan can balance the pressure in the chamber.
The dryer feed, i.e. a solution, suspension or paste is pumped or gravity
fed to the atomizer located within the drying chamber. The feed must be
strained. The atomized spray readily undergoes drying within the chamber.
Dried product of required moisture content is discharged at the base of the
chamber or borne away with the exhaust air for recovery in the product
recovery system. Air is drawn from the chamber and through the product
recovery system by the exhaust fan. The product recovery system separates
out entrained powder prior to venting the air to atmosphere. The exhaust
air can contain much heat, and a heat recuperator can be installed before
the air passes to atmosphere. The temperature of the outlet air is controlled
through adjustment to the inlet air temperature at constant feed rate, or
through adjustment of feed rate at constant inlet air temperature (see
chapter 10 on dryer control).
- Equipment items mentioned in figure 5.1 are standard to basic spray
drying layouts. Two such layouts, featuring rotary atomization (figure 5.2(a))
and nozzle atomization (figure 5.2(b)) illustrate the position of equipment
items in relation to the drying chamber.
PRODIJE T RECOVERY
a,
AIR
ExFIALLST
AIR DISPERSER
ATOMIZER
(nozzle)
AIR
NEATER
FAN
F AN
FILTER
:yc LONE
FEED
TANK
1=1
PUMP FILTER
5.3. Equipment Items Making Up a Spray Dryer Layout
5.3.1. Equipment Comprising the Feed System
The feed system comprises of suitable holding and feed tanks, strainers or
fine filters, and feed pumps. A feed pump transfers the product to the
ERY
b.
Figure 5.2. Spray dryer layouts with rotary and nozzle atomization. (a) Layout with rotary
atomizer, (b) Layout with nozzle atomizer.
--0-.10--
atomizer either directly or via a constant head feed tank. The holding tanks
are of sufficient volume to enable continued plant operation, even though
supply of product to the tanks may be intermittent. It•is quite usual for
two holding tanks to be used alternatively thus assuring constant product
supply to the dryer. The feed strainer or filter is important as all matter
likely to jeopardize the performance of the dryer through partial or total
blockage of the atomizer must be removed. The feed system is designed
to enable easy cleaning and maintenance, Piping is normally laid out to
enable recycling of product or washing fluids through the complete feed
system via the holding tanks. Choice of metallic piping depends upon the
feed. With food products, for example, stainless steel is used throughout.
Applications in the chemical field often require specially lined holding
tanks and corrosion resistant pumps, pipes and pipe fittings. The pumps are
sized and selected for each application. Pumps most widely used are the
rotary positive displacement type. Diaphragm pumps are suitable when
feeds contain irregular shaped insoluble solids. Further details on pumps
and pump selection are given in chapter 12.
There are
cases where pretreatment or preheating of the feed
is required prior to atomization. Pretreatment can be the blending in of
additives, or feed dosification to maintain required feed properties, such as
a pH value. Preheating invariably is conducted to lower the feed viscosity
in order to ensure required atomizer performance. Preheaters can be of the
plate, shell and tube, scraped surface or spiral tube type. The type is selected
according to the feed, heat transfer rates, available space and cleaning
requirements. Pretreating and preheating equipment is connected into the
feed system just before transfer of product to the atomizer. A piping diagram
for a feed system involving a preheater is shown in figure 5.3. Feed from
holding tanks is transferred through the preheater into a smaller feed
tank prior to pumping to the atomizer. Feed from the preheater can be
pumped directly to the atomizer, but the inclusion of a small intermediate
(atomizer) feed tank aids flexibility in the feed system, by providing possibilities for dosification of products that cannot withstand passage through
a preheater, feed recycling and visual control of feed supplies to the atomizer.
A water tank for dryer start-up and shut-down is mounted by the atomizer
feed pump.
5.3.2. Atomizer
The atomizer is often regarded as the heart of the spray drying process.
The ability of the atomizer to produce sprays of desired droplet size distribution is a most important factor in determining the success of the process.
Two basic types of equipment are used rotary atomizers and nozzle atomizers
(figure 5.4). Main design and operation features were introduced to the reader
in chapter 1. Atomizer equipment is discussed in greater detail in chapter 6.
69
Figure 5.3. Feed system involving a feed pre eater.
EQUIPMENT INCORPORATED IN SPRAY DRYERS
BASIC PRINCIPLES, DEFINITIONS
CONDEN SATE
68
70
EQUIPMENT INCORPORATED IN SPRAY DRYERS
BASIC PRINCIPLES, DEFINITIONS
71
COUPLING TO MOTOR DRIVE
FEED PIPE
GEARING
TOP
BASE
LIQUID DISTRIBUTOR
SIDE BASE
W
-
VANED WHEEL
SIDE TOP
a.
FEED PIPE
CONNECTION
I FEPIALEI
NOZZLE ORIFICE
z
NOZZLE HEAD
END
MIDDLE
Figure 5.5. Atomizer positioning in drying chamber.
Figure 5.4. Basic types of atomizers. (a) Rotary atomizer (atomizer wheel). (b) Nozzle atomizer.
Whatever atomization technique is used, the atomizer is positioned
within the drying chamber and operated so that the spray has intimate
contact with the drying air. Contact can be made as the air flows upwards,
outwards or downwards. In each case moisture evaporation from the
spray in the vicinity of the atomizer is fast enough to dry the feed sufficiently
before the spray comes into the neighbourhood of the chamber wall. Insufficient drying leads to semi-wet deposits on the wall. Atomizer positioning in the dryer is illustrated in figure 5.5.
5.3.3. Equipment Comprising the Supply Air System
The supply air system delivers drying air to the chamber. Equipment
involved are filters, heaters, dampers, ducts and fans.
The supply air is filtered unless possible resulting contamination of the
spray dried product by dust in the air can be tolerated. Atmospheric air
is the usual supply source. In cases of closed cycle drying, the exhaust
drying medium is filtered, re-cycled, passed through conditioning equipment
and returned to the heater.
Filter media are capable of removing airborne particles above 5 micron.
The filter housing can consist of cages in which squares of filter medium are
clamped. The filter squares are removed periodically for manual washing.
More automatic arrangements are available where the pressure drop over
the filter is measured continuously. On reaching a set pressure drop value
the filter roll moves forward bringing a clean filter surface into operation
thereby reduding the resistance over the filter. The filter rolls are replaceable.
72
EQUIPMENT INCORPORATED IN SPRAY DRYERS
BASIC PRINCIPLES, DEFINITIONS
Air heaters are either the indirect or direct type. Heating sources can
be steam, fuel oil, gas, heat transfer fluids or electricity. Selection depends
upon the product being spray dried and fuel availability. Groups of products
can be classified according to their heater requirement.
Group A. ProduCts that can withstand high temperature and be contacted
with products of combustion. Oil or gas fired direct heaters are applicable. Examples of such products include clays, and mineral ore concentrates, and many inorganic materials. Successful use of direct oil
heaters in many cases requires clean products of combustion. Burning
of high quality oils is essential.
Group B. Products that can withstand high temperatures, but must remain
free of contact with products of combustion. Oil or gas fired indirect
heaters are applicable. Examples of such products include various
inorganic salts and fertilizers.
Group C. Products that can withstand products of combustion but are
subjected to limitations in temperature due to chemical changes or
likelihood of ignition. Oil or gas fired direct air heaters are applicable,
subject to a built-in automatic high temperature alarm system. Dilution
of combustion products is used to maintain suitable air temperatures.
Examples of such products include various organic and inorganic
salts.
Group D. Products which cannot be subjected to high temperature conditions or be contacted with products of combustion. Indirect air heaters
must be used. In small units, liquid phase or electrical heating is often
applied. Products in this grouping include foodstuffs and many fine
chemicals.
Heaters are discussed in chapter 12.
Supply air fans are of the centrifugal type. Backward curved impellers
are used as high volume air movement is achieved at low pressures. Fans
are discussed in chapter 12. Dampers are of the butterfly type.
5.3.4, Air Disperser and Drying Chamber
Air enters the drying chamber through the air disperser. The function of
the air disperser is to provide the required heat for drying, control droplet/
particle travel during drying and remove the vaporized moisture from the
chamber. Where the air disperser and atomizer are located together, air
flow can influence the droplet size distribution of the spray. The function
of the drying chamber is to provide air/particle residence times for obtaining
desired dried product moisture levels without heat degradation and unwanted wall deposits. Product discharge must be continuous and the
method of discharge conducive to the desired form of dried product. Drying
chambers are designed to discharge the majority of the product at the
base (primary product discharge) or to convey all the product with the
73
7:n
‘+
Cd
a
Cd
g
sir
45
=o a
❑,
t„.1
5
74
BASIC PRINCIPLES, DEFINITIONS
EQUIPMENT INCORPORATED IN SPRAY DRYERS
exhaust air to a product separation and recovery unit (total product
discharge).
Dryer designs fall into three categories, co-current, counter-current, and
mixed flow dryers . (as defined in chapter 1). Figure 5.6 illustrates four types
of co-current flow drying chambers. The air disperser and atomizer are
located either at the top, base or end of the chamber. Types (a) and (b) are
the most common. Type (a) has a downward rotary air flow. The air
disperser creates air rotation through tangential entry and/or flow over
angled vanes. Spray is quickly drawn into a similar rotary motion. Such
air disperser designs are shown in figure 5.7 (see also chapter 7, figures 7.1,
7.2, 7.3). Type (b) has a downward streamline flow. The air disperser creates
non-rotary flow by use of perforated plates and/or straightening vanes.
The spray acquires a streamline flow motion once the deceleration of droplets
from the nozzle is completed. In type (c) the air must prevent dried particles
settling out in the upward flow. In type (d) the reverse is required, namely
air flow permits dried particles to fall out of the air and fall to the chamber
base. In counter-current flow dryers (figure 5.8) the atomizer and air disperser are located at opposite ends of the drying chamber. Counter-current
75
AIR OUT
FEED IN
PRODUCT
OUT
Figure 5.8. Counter-current flow dryer.
IT
Cliornbrr rob!
A lomizer
....----.
■....
An,ked
win es
flow is mainly restricted to nozzle atomization in tall, narrow diameter
towers. The air disperser creates various degrees of air rotation on entry
into the chamber (see figure 7.4) but due to chamber height, the rotary
motion cannot be sustained in the upper region of the chamber. In mixed
flow dryers, air and product are subjected to both co-current and countercurrent flow conditions during passage through the drying chamber. This
is accomplished by :
(i) Air flows in two directions while the product flows in one. The air inlet
and outlet ducts plus the atomizer are located at the top of the chamber as
shown in figure 5.9(a).
a.
(ii) Air flows in one direction while the product flows in two. The atomizer
sprays up from the chamber base into the incoming drying air. Product
discharge is at the chamber base. This is shown in figure 5.9(b).
d.
Figure 5.7. Air disperser types creating rotary air flow around atomizer. (a) Tangential entry
(roof mounted). (b) Angled vanes creating swirl (centrally mounted). (c) Angled vanes; banjo
inlet (roof mounted). (d) Tangential entry; double inlet (roof mounted).
In figure 5.9(a) high inlet velocities develop a high degree of swirl to obtain
high evaporation rate conditions. The residence time within the dryer is
very short (5-10 sec). The vigorous cyclonic motion created within the
chamber sweeps the walls clean, while at the same time separates out the
product from the air. Product is discharged at the chamber base. Due to
the high powder—air ratio within the chamber a fair percentage of product
remains entrained in the air and is carried out at the top of the dryer. In
76
EQUIPMENT INCORPORATED IN SPRAY DRYERS
BASIC PRINCIPLES, DEFINITIONS
AIR OUT
FEED IN
77
figure 5.9(b) the feed is sprayed upwards into a rotary air flow created by a
ceiling air disperser. The spray travels upwards and decelerates to rest.
The spray then becomes fully under the influence of the air flow and passes
back down the chamber. This type of design is often designated the 'fountain
spray dryer'. With the 'fountain' design, product can be either conveyed
away with the exhaust drying air, or undergo separation in the conical base
of the chamber and be discharged separately. Specialized patented drying
chamber designs are covered in references (418) to (433)(514).
PRODUCT AND AIR FLOW
PRODUCT AND AIR FLOW
I
I
AIR
OUT
AIR
OUT
OUT
PRODUCT
OUT
PRODUCT OUT
CONVEYOR
PRODUCT OUT
b.
❑.
ASR IN
SWEEPER SWEEPER DRIVE
PRODUCT AND AIR FLOW
SWEEPER
AIR OUT
\
AIR OUT
Ati
111
PROD T OUT
SWEEPER DRIVE
C.
PRODUCT
OUT
b.
Figure 5.9. Mixed flow dryers. (a) Cyclonic. (b) Fountain layout.
Figure 5,10. Primary product discharge from drying chambers. (a) From conical base chamber.
(c) From double conical base chamber with
sweeper.
(b) From• flat bottom chamber with sweeper.
78
BASIC PRINCIPLES, DEFINITIONS
With the majority of chamber designs, dried product passes to the base,
which is equipped for either primary or total product discharge. In primary
product discharge (figure 5.10) the greater proportion of dried powder is
discharged directly from the chamber base. Only a fines fraction remains
entrained in the exhaust air for recovery in the powder separation unit.
Figure 5.10(a) shows primary product discharge from a conical chamber
base. Air is exhausted from the side of the chamber cone. Figure 5.10(b)
shows primary product discharge by a sweeper from a flat-bottom chamber.
The swept area is large and product is subjected to mechanical handling.
Primary recovery of product is from the conical base due to the separation
effect of the cyclonic air flow created within the cone. Primary recovery of
product from a flat-bottomed chamber is due to the product settling out
from the air flow and falling at a terminal velocity to the floor. Both designs
can handle lumpy products. Thermoplastic and high fat content powders
are best handled in conical based dryers. There are blockage tendencies
with a screw conveyor system in flat bottom designs. The main features of
primary product discharge from both conical and flat bottom chambers
can be combined to give a conical based design of reduced chamber height.
Figure 5.10(c) shows this development of the conical base design where the
lower part of the cone is inverted to give a double coned or W-shaped
chamber. The chamber has a central exhaust air duct. A sweeper guides
product into a rotary valve. Efficient primary product separation is still
maintained with the arrangement. Use of a sweeper does subject product
to some mechanical handling but this is reduced to a minimum as the actual
swept floor area is small in relation to the chamber cross-sectional area.
In total product discharge (figure 5.11) the air velocity in the exhaust duct
must be high enough to convey the product to the separation unit. Due to
the requirement for pneumatic conveying, this form of discharge is not
recommended for spray dried products that leave the chamber as sticky
lumps, although the conveying of product can be assisted by introducing
cooler atmospheric air into the exhaust duct.
Refinements to chamber design are used to handle special products ((435)
to (441), (502)). For sticky (thermoplastic) products chamber wall temperatures must be kept low. Wall air cooling, (figure 5.12(a)) or introduction of
secondary cooled air to sweep the lower chamber walls, (figure 5.12(b), (c))
can be used. For products having the tendency to remain on the chamber
wall, electrical hammers or vibrators are mounted on the wall to constantly
dislodge the product.
5.3.5. Equipment for Separation and Recovery of Product from Exhaust Air
Dried product that is entrained in the air leaving the drying chamber
must be effectively separated and recovered to maintain maximum output
from the dryer operation and prevent air pollution through excessive
EQUIPMENT INCORPORATED IN SPRAY DRYERS
a.
79
PRODUCT,
AIR OUT
SWEEPER
PRODUCT,
AIR OUT
PRODUCT,
AIR OUT
(i)
b,
Figure 5.11. Total product discharge from drying chambers. (a) From conical chamber base.
(b) From flat chamber base; (i) sweeper side discharge, (ii) sweeper central discharge.
powder release to atmosphere. Dry and wet equipment can be used. Product
recovered dry is transported to further processing or to the dryer baggingoff area. Product recovered wet may be recycled to the dryer feed pretreatment, or simply considered as waste. Dry equipment of cyclones,
bag filters or electrostatic precipitators are used as the main separation/
recovery stage. Generally product quality is not affected, although the
comminuting effect of flow throligh cyclones can reduce the particle size
of products exhibiting fragile structures. Where air with traces of product
BASIC PRINCIPLES, DEFINITIONS
EQUIPMENT INCORPORATED IN SPRAY DRYERS
81
CODLING AIR OUT
(a) Double wall cooling air jacket. (b)Rotating
PRODUCT OUT
AIR/PRODUCT OUT
leaves the main stage and must be cleaned prior to exhausting to atmosphere,
wet equipment, e.g. wet scrubbers, wet cyclones,-irrigated fans are installed
as a secondary stage. Often product concentrations in scrubber liquids
are too low for economic concentration and re-drying, and thus the liquids
are passed to effluent treatment. The range of dry and wet equipment is
shown in figure 5.13, and their features are listed in table 5.1.
0
CODLING AIR IN
80
AIRBORNE POWDER
WET SEPARATION
!AIR CLEANING)
DRY RECOVERY
E
k
DRY
CYCLONES
BAG
FILTERS
ELECTROSTATIC
PRECIPITATORS
WET
SCRUBBERS
WET
CYCLONES
IRRIGATED
FANS
Figure 5.13. Equipment for separation and recovery of airborne powders.
Cyclones are widely used due to their low cost and virtual total lack of
maintenance. Cyclone efficiency is normally high enough with most spray
dried products to dispense with the need for a secondary stage. It must
be borne in mind, however, that even the highest efficiency cyclone operating
under optimum operational conditions cannot reach 100 % efficiency, and
if complete separation is required alternative systems must be used. Such a
system is often a bag filter which is virtually 100 % efficient. However, the
advantage of being a single unit exhausting powder-free air to atmosphere
is offset by maintenance difficulties. Should bag rupture occur or product
leakage result through incorrectly mounted bag clamps, access into the
bag assembly is not too easy. Bag inspection in a dust laden environment
makes isolation of the faulty bag(s) a time consuming task, and down-time
often proves to be lengthy.
In choosing the most suitable separation system, particle size, structure,
loading in the air, air rate, required efficiency of separation, available power
and space are factors to be considered (see also chapter 9). When all the
product leaves the chamber airborne, powder loadings are high and a
battery of high efficiency cyclones are required to reach high degrees of
recovery. A 99.5 % efficiency, however, still gives a measurable product loss
if production rates are high. When complete dry recovery is essential a
bag filter might well be added after the cyclones or installed to replace the
cyclones entirely. When primary discharge of powder in the chamber is
adopted, only the fines fraction remains entrained in the air and passes to
the separation unit. If bag filters are used, near total recovery of fines is
•
Table 5.1. Advantages and Disadvantages of Dry and Wet Powder Recovery
Dry powder recovery
Wet powder recovery
Advantages
1. Powder recovery in useable form.
2. No equipment corrosion.
3. Available equipment capable of removing virtually all the airborne particle sizes from the spray dryer.
4. Simple layout (e.g. cyclones).
5. Hygienic (e.g. cyclones).
6. Capable of removing sub micron powder particles (e.g. bag
filter).
7. Operates in a dry floor area.
1. Handles high temperature exhaust air.
2. Handles high humidity exhaust air.
3. Corrosive gaseous medium recovered and neutralized.
4. No explosion dangers.
5. Available equipment small in size compared with dry
collectors.
6. Soluble material recovered and pumped back to the dryer.
7. High efficiency of powder recovery.
Disadvantages
1. Not suited to hygroscopic materials.
2. Some equipment types have humidity and temperature operating
limitations.
3. Powder atmospheres can cause an explosion hazard.
4. Cleaning sequence can be complicated (e.g. bag filters).
5. Equipment can block-up under adverse operating conditions
(e.g. cyclones).
6. Strict maintenance required (e.g. bag filter).
7. Equipment wears with abrasive powders (e.g. cyclones).
SNOIIINIJR0 `salaiDmuct DISVII
oc
1. Insoluble powders are discharged as effluent liquid, unless
settling and filtration equipment is available.
2. Scrubber liquid can cause an effluent problem.
3. Small micron sizes are not wetted and thus are lost from the
equipment.
4. Greater corrosion risk to the equipment.
5. Scrubber water can freeze in cold weather.
6. Recovery of product in dry form requires further treatment.
7. Flooding can occur, and a drained floor area is required.
8. Unhygienic-unsuitable for many food products.
4 '5,
'
P
ut
❑
4,
.
cr
•0
o
co
m
m
ci
0
co
og
0
0..
-I `6' '5 0
.......
54
,
O
C
0
CD
co
co
F
-
C a>
CD
V' 0 0
m
o
7P
n
CD
ID
C)
g
EQUIPMENT INCORPORATED IN SPRAY DRYERS
S.
crq 0
0
-4
co
•-e
C
q
CX)
84
BASIC PRINCIPLES, DEFINITIONS
(e.g. dyestuffs) or (c) are likely to cause an extreme air pollution problem
(e.g. herbicide dusts causing flora damage in the surroundings), cyclones
may well be followed by bag filters and/or scrubbers to ensure negligible
losses. This is done even though primary recovery from the chamber may
result in only a small percentage of the powder production entering the
recovery section.
When primary product recovery is from the chamber, there is more than
one place of product discharge within the dryer layout. To assist ease of
:product handling for packing it is usual to combine the numerous powder
discharge places within a common transport system. The transport system
can be pneumatic or mechanical.
A pneumatic system is shown in figure. 5.14(a). Powder discharged from
both the chamber and cyclone bases feed a common pneumatic system.
Product is conveyed to a transport cyclone, from which the final packaging
of the product takes place. The conveying air can be exhausted to atmosphere or blown back into the spray dryer system. Figure 5.14(a) shows the
conveying air returned into the main ductwork of the exhaust drying air.
Alternatively conveying air can be returned to the drying chamber. Pneumatic conveying is discussed further in section IV (chapter 12).
Mechanical conveyor belt systems are suitable for granular, non dusting
products. The system is shown in figure 5.14(b). It is used to transfer product
to silo storage, or additional treatment processes, or to areas for direct
bagging-off. Although such systems eliminate the problems of handling
conveying air, they tend to result in a dirty operation. They cannot be
considered where any degree of hygiene is demanded. Conveyor belt
systems are found mainly in the chemical industry. Conveyor belts and
other, equipment for mechanical conveying have been recently discussed
(1970) by Buffington (11).
Section II
THE PROCESS STAGES OF
SPRAY DRYING
6 Atomization
CI. Introduction
The atomization stage in spray drying produces from liquid bulk a spray
of droplets having a high surface to mass ratio. The dried product that
results from moisture evaporation of atomized spray can be made to possess
the desired particle size distribution through control of the atomization
variables.
The ideal spray is one of small individual droplets of equal size. Heat
and mass transfer rates and drying times are then the same for all droplets
in the spray, ensuring uniform dried product characteristics. Droplets of a
spray evaporate quickly and the short drying times involved maintain low
droplet temperatures due to the cooling effect that accompanies evaporation.
No product deterioration can take place due to heat, if correct atomization
is combined with a suitable drying chamber design to give a product residence
time just sufficient for completion of moisture removal. The ideal requirement of an atomizer is to produce homogeneous sprays. Such sprays have
not yet been obtained at industrial feed rates, although atomizer types are
available producing sprays that closely approach homogeneity, when operating with liquids of certain physical characteristics at low feed rates.
6.2. Basic Mechanism of Atomization
The mechanism of atomization has been studied by many workers, but
remains a controversial subject in spite of much published data. Various
theories predominate, each receiving experimental support. The mechanisms
now accepted as applicable to commercial atomization conditions are the
outcome of work by pioneers on the stability and collapse of simple liquid
jet (ligament) systems. A dripping tap or the slow release of liquid from a
pipette are examples of the simple jet system initially studied, and on which
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
atomization theories are based. The formation of each drip illustrates
that the basic atomization steps of liquid break-up depend upon forces
acting on and within the jet. These forces destroy the jet's initial stability.
It is gravity in the case of the dripping tap. Simple relationships can express
basic break-up. Extension to more complicated conditions enables atomization mechanisms pertaining to spray drying to be deduced. However,
conditions occurring in commercial atomization are extremely complex
and theoretical relationships cannot be considered adequate. Apart from
irregular liquid break-up, due to non-ideal liquid distribution at the atomizer
edge, there is the complex interaction between the spray leaving the atomizer
and the surrounding air, unknown stresses within each droplet the instance
after leaving the atomizer, and the influence of droplets on each other within
the spray cloud.
The development of a mechanism theory applicable to commercial
atomization can readily be followed and one man is credited with the
innovation. Lord Rayleigh (12) published in 1878 the mathematical break-up
of non-viscous liquid jets under laminar flow conditions. The conclusion
drawn at that time stated that a jet becomes unstable and ready to disrupt
if its length is greater than its circumference. Although these conditions
do not exist in rfiality, an actual liquid jet being turbulent, viscous and
subjected to surrounding air influence, the general conclusions were accepted
in later theories. Tyler (13) working with mercury jets came very close to
Rayleigh 's predictions. Weber (14) extended the predictions to include
viscosity, surface tension, and liquid density effects. In the 1930's jet disintegration had reached the stage where the effects of air resistance on jet
break-up were being considered. Haeniin (15) reported that orderly wave
formation at the jet surface caused by a high velocity air flow become
completely irregular on velocity increase. Castleman (16) also analysed
high velocity disturbances, and suggested the degree of atomization was
controllable by the relative velocity of air and liquid. In direct continuation,
jet stability was found to be a function of Reynolds number. Ohnesorge (17)
is credited with the Reynolds Number relationship. The tendency of the jet
to disintegrate is expressed in terms of liquid viscosity, density, surface
tension and jet size. Ohnesorge condensed his findings by suggesting that
the mechanism of liquid break-up could be expressed in three stages, each
stage characterized by the magnitude of a dimensionless number Z'. The
Z' number was the ratio of the Weber/Reynolds numbers :
V
i
p 1 d.) 1 i 2
a
x
( Vd n p,) -
(P)adr,) 1I2
Vi = jet velocity, p, = liquid density, d o = nozzle (jet) diameter, c = surface
tension, I/ = liquid viscosity. Z' is plotted against Reynolds number in
89
figure 6.1. The graph shows three zones :
Zone 1. At low Reynolds numbers the break-up of liquid jets shows the
Rayleigh mathematical prediction.
Zone 2. At intermediate Reynolds numbers, the break-up of liquid jets is
by jet oscillations with respect to the jet axis. The magnitude of these
oscillations increases with air resistance until complete disintegration of the
jet takes place.
Zone 3. At high Reynolds numbers, complete atomization occurs at the
orifice from which the jet emerges.
WEBER NUMBER
REYNOLDS NUMBER
88
2
1
10
10
4
3
10
10
6
5
10
10
REYNOLDS NUMBER ( Re)
Figure 6.1. Ohnesorge chart showing liquid jet disintegration as a function of Reynolds number.
From figure 6.1 atomization mechanisms can be deduced. Atomization
mechanism at low Reynolds number is through oscillations created at the
liquid jet surface. At intermediate Reynolds numbers, the degree of atomization depends upon the wave motion set-up in the jet. At high Reynolds
numbers, the disintegration is so rapid that direct atomization is obtained
at the atomizer edge. This is an oversimplification of the process of liquid
atomization, since in commercial atomization conditions all mechanisms
act simultaneously influencing the spray characteristics to varying degrees.
For further details on mechanism of liquid jet disintegration, reference
should be made to Giffen and Muraszew (18), Miesse (19) and Richardson
(481). The various atomization theories arising from work on liquid jet
instability are summarized by Friedman, Gluckert and Marshall (20). A
comprehensive survey and critique of atomizer literature, up to and including
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
91
December 1966 has been reported by Lapple, Henry and Blake (25). The
dependence of jet instability and disintegration on liquid properties, jet
velocity and orifice size is illustrated in table 6.1, which contains correlations
proposed by investigators to predict droplet size following jet disintegration.
‘46 ,43 1/40 +43
6.3. Classification of Atomizers
Droplet size equation
Atomization results from an energy source acting on liquid bulk.* Resultant
forces build up to a point where liquid break-up and disintegration occurs
and individual spray droplets are created. The different atomization
techniques available concern the different energy forms applied to the
liquid bulk. Common to all atomizers is the use of energy to break-up
liquid bulk. Centrifugal, pressure, kinetic energy are the common forms
and each classify the atomizer device. Sonic and vibratory atomizers are
less common, with work into possible spray drying applications within the
early stage of development.
ATOMIZERS
. INCORPORATING
CENTRIFUGAL
ENERGY
Droplet size parameter
ROTARY
ATOMIZERS
VANED
WHEELS
VANELESS
DISCS
CUPS BOWLS PLATES
PRESSURE
ENERGY
KINETIC
ENERGY
SONIC
ENERGY
PRESSURE
NOZZLES
PNEUMATIC
NOZZLES
SONIC
NOZZLES
I
CENTRIFUGAL
PRESSURE
NOZZLES
SWIRL
CHAMBER
GROOVED
CORE
TWO-FLUID
INTERNAL
MIXING
THREE-FLUIE
SIRENS
WHISTLES
1
EXTERNAL
MIXING
ROTARY PNEUMATIC ATOMIZERS
Figure 6.2. Classification of atomizers.
Merrington, Richardson (24)
90
The classification of liquid atomizers used in spray drying is shown in
figure 6.2. Rotary atomizers feature high velocity discharge of liquid from
the edge of a wheel or disc. Pressure nozzles feature the discharge of liquid
under pressure through an orifice. Pneumatic nozzles feature the break-up
* Minimum energy for atomization is the requirement to create new surface. The theoretical
power (PK ) can be expressed (18) (29):
PK = AO
where A = net average area created per unit time, e = surface tension.
Based upon thermodynamic considerations, power requirement to create new surface is
only a very small proportion of that expended in an atomizer during spray dryer operation.
92
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
6.5. Rotary Atomization
of liquid on impact with high velocity air or other gaseous flow.' Sonic
nozzles feature break-up of liquid through sonic excitation. For each class
of atomizer, a wide range of designs has been developed to handle the
diversity of feed materials and to meet dried product characteristics.
6.4. Applications of Basic Mechanisms to Commercial Atomizer Conditions
The mechanisms as stipulated by Ohnesorge (17) apply to the break-up of
simple jets. Before these mechanisms at high Reynolds numbers can be
tonfidently applied to commercial atomizer conditions, a link between
simple jets and the liquid form leaving a rotary atomizer or issuing from a
nozzle orifice had to be established. Liquid at the edge of a rotary atomizer
or in a pneumatic nozzle is subjected to high velocity contact with air.
Castleman's (16) analysis on high velocity disturbances thus applies, and
by drawing on his conclusions can relate the mechanism of atomization in
these commercial atomizers to simple jet break-up mechanisms of Ohnesorge
(17). Liquid issues from a centrifugal pressure nozzle as a conical sheet due
to the swirl imparted to the liquid within the nozzle. Experimentation has
shown a similarity between the break-up of jets and the disintegration of
conical sheets of liquid. Observations on a simple centrifugal nozzle have
established that the conical sheet (the dispersion of which is measured as a
cone or spray angle) breaks up into droplets a short distance from the orifice,
when undulating disturbances in the cone eventually result in its collapse.
As pressure is increased, break-up occurs nearer the Orifice, the behaviour
of which is analogous to the findings of Ohnesorge for simple jets.
The mechanisms applicable to conditions under which commercial
atomizers operate can be summarized as the immediate disintegration of
liquid at the atomizer edge. Disintegration at the atomizer edge is the
result of turbulence in the issuing liquid and the action of air forces, whilst
being opposed by viscosity and surface tension forces in the liquid. The readjustment of sheer stresses within the liquid once the droplet is airborne is
another factor contributing to further disintegration of the droplet. The
atomization mechanisms are still not fully understood. The role played by
air friction at the atomizer edge is especially interesting, since successful
atomization of liquid in turbulent flow can be obtained in vacuum at low
feed rates. Much research (25, 35) has been undertaken on subjects ranging
from simple jet disintegration to the atomization characteristics of basic
atomizer devices, e.g. low speed rotating discs. Much of the uncertainty
results from the experimental difficulties involved. When it comes to the
investigation of commercial atomizers, the process of liquid disintegration
proceeds too fast to distinguish the individual phases of liquid break-up.
The role of each liquid property is thus still obscure.
93
IR
6.5.1. Introduction
In rotary atomization, the feed liquid is centrifugally accelerated to high
velocity before being discharged into an air—gas atmosphere. The liquid is
distributed centrally on the wheel/disc/cup. The liquid extends over the
rotating surface as a thin film. Rotary atomization is often termed centrifugal
atomization, but this can be a little misleading as centrifugal energy is also
utilized to a certain degree in centrifugal pressure nozzles, where liquid is
given rotational motion.
The degree of rotary atomization depends upon peripheral speed, feed
rate, liquid properties and atomizer design. Maximum centrifugal energy
is imparted to the liquid when the liquid acquires the wheel/disc peripheral
speed prior to discharge. If a flat smooth disc is rotating at high speed and
liquid is fed on to its top surface, severe slippage occurs between the feed
liquid and disc. The velocity of liquid from the disc edge is much lower
than the disc peripheral speed. Conditions of no slippage occur at very
low speeds, but the available centrifugal energy permits only the smallest
of feed rates to be satisfactorily atomized. To prevent slippage in commercial atomizers, radial vanes are used. The liquid is confined to the vane
surface, and at the periphery the maximum liquid release velocity possible is
attained. The maximum release velocity is the resultant of radial and
tangential components acquired by the liquid. Alternatively slippage can
be reduced by increasing friction between liquid and rotating surface. This
is often done by feeding liquid on to the lower surface of a disc shaped as an
inverted plate, bowl or cup. As the liquid is flung outwards due to centrifugal
force, the liquid film is pressed against the disc surface. Both techniques
are used to handle large feed rates, although the vaned designs (atomizer
wheel) are selected in cases where fine sprays are required. However, many
spray dryer operations call for large particle sizes, and thus the inverted
disc types enjoy wide usage.
Smooth flat vaneless discs (with feed on top surface) are rarely used in
spray drying operations. Only a brief description will be given (6.5.2).
Their operation, however, goes some way to explain the basic atomization
mechanism involved in rotary atomizers.
Atomizer wheel designs are dealt with in detail (6.5.3). These are the
most widely applied of rotary atomizers. Vaneless discs of the plate and cup
type are discussed afterwards (6.5.4).
6.5.2. Flat Smooth Disc Atomization (Vaneless)
For a flat smooth vaneless disc, the degree of atomization obtainable
will depend upon the magnitude of feed acceleration toward the edge over
the smooth surface. The final liquid release velocity depends upon the
94
THE PROCESS STAGES OF SPRAY DRYING
slippage between liquid and the rotating surface. The extent of slippage
depends upon feed rate and physical properties of the liquid, namely the
viscous drag and the ability of the liquid to wet the surface.
The disintegration of liquid into droplets from a rotating disc is governed
by:
(a) the viscosity and surface tension of the liquid,
(b) inertia of the liquid at the disc periphery,
(c) frictional effects between the liquid droplets and surrounding air
at the point of droplet release from the disc,
(d) readjustment of shear stresses within the liquid droplet once the
droplet is airborne.
At low disc peripheral speeds, the liquid viscosity and surface tension are
the predominating factors. The higher the disc speed, the more inertia
and air friction contribute to the mechanism of droplet formation. Due
to liquid slippage over the disc surface, liquid inertia never becomes high
and as release velocities are low, air frictional effects are minimized too.
Only in the commercial atomizer designs of vaneless discs do inertia and
air friction play a dominant role at high feed rates and high liquid release
velocities.
For smooth flat vaneless discs, when viscosity and surface tension predominate the mechanism of atomization, sprays are formed by individual
formation and release of droplets from the disc edge. This is shown diagrammatically in figure 6.3(a), Feed rates and disc speeds are very low.
Sprays consist of two or three prominent droplet sizes, i.e. parent droplet
and two satellites.
With continued disc speed increase and higher feed rates, the direct
droplet mechanism changes to one of ligament break-up. The point concentrations of liquid that gave rise to direct droplet formation contain
more liquid, and ligaments begin to extend out from the disc edge. The
ligaments again disintegrate into sprays of parent and satellite droplets
as illustrated in figure 6.3(b). Larger parent droplets are formed from higher
feed viscosities and surface tensions. Increases in feed viscosity increase
the proportion of satellite droplets in the spray. Spherical droplets are
formed irrespective of the surface tension value.
The mechanisms illustrated in figure 6.3(a and b) are controlled by the
physical properties of the feed liquid. The transition between the predominance of liquid properties and of inertial forces occurs when the liquid
ligaments join to form a liquid sheet extending from beyond the disc edge
(figure 6.3(c)). The mechanism of liquid atomization at the transition
between liquid ligament and sheet formation is shown photographically
in figure 6.4. Atomization by liquid sheet formation is often referred to as
the velocity spraying mechanism (26). The mechanism is accentuated by
ATOMIZATION
95
0
0
0
0
a
O
0
O
a.
0
❑
.
6„
0
.„
o
p
o
a
DISC EDGE
c.
Figure 6.3. Mechanisms of atomization. (a) Direct droplet formation. (b) Ligament formation.
(c) Sheet formation.
highly viscous liquids. The sheet disintegrates giving a spray of broad
droplet size distribution. With further increase of disc speed at constant
feed rate the liquid sheet retracts towards the disc edge. If feed rate is increased with disc speed, velocity spraying continues. If slippage over the
disc is minimized or prevented, liquid release velocity from disc edge becomes high enough to enable air frictional effects at the liquid—air interface
to become the controlling atomization mechanism. The mode of disintegration of the retracting sheet makes the formation of completely homogeneous
sprays appear a remote possibility. Use of high viscosity and surface tension
96
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
97
reduces the broad distribution as does increase of disc peripheral speed at
constant feed rate. To produce narrow distribution, high speeds are required
at low disc loadings.
Bauer and Kruger (27) in a recent article (1969) review current knowledge
of atomization mechanisms from vaneless discs. Theory pertaining to
flow over the disc and release of liquid from the disc edge is discussed.
Photographic evidence of the mechanisms involved have assisted visual
appreciation of the atomization process.
Once the droplets have left contact with the disc edge and are airborne
further disintegration can take place due to readjustment of shear stresses
set up within the liquid during liquid flow over the disc surface. Droplet
disintegration by this mechanism is difficult to distinguish from disintegration by air friction effects, which also causes droplet distortion when droplets
are airborne. In spray drying the air friction effects are considered the
major mechanism, but atomization in low vacuum has shown the mechanism
of shear stress readjustment to occur when liquid flow is turbulent.
(a)
(a) Flow over Smooth Flat Vaneless Discs
The mechanism of liquid atomization has been reported widely on flat
vaneless discs. With the degree of atomization dependent upon the velocity
of liquid release, which in turn depends upon the slippage between liquid
and surface, Frazer (28) defines the occurrence of severe slip by the relation
(6.7), where M = mass feed rate (lb/hr), p = feed viscosity (cp) and d = disc
diameter (ft)
( M
(6.7)
1440
mpd )
Quite high feed rates that can be handled on a disc. For example a 6 in
(150 mm) disc operating on water can take 38 lb/min (17 kg/min) before
extreme slippage conditions commence. Atomization by spray drying
standards would be poor, illustrating the additional factors like air—liquid
frictional effects that make for effective liquid disintegration.
The time (t) taken for liquid to reach the disc edge under non-slippage
conditions at very low feed rates has been expressed mathematically by
Seltzer and Marshall (29) (N = disc rotation)
t =
(b)
Figure 6.4. Mechanisms of atomization (represented photographically).
tion. (b) Transition between ligament and sheet formation.
(a) Ligament forma-
0
27EN
(6.8)
B is the number of radians a liquid element must pass before disengagement.
0 is defined as the theoretical hyperbolic path travelled by the liquid element
where B = cosh' (r d /ro ).
Equation (6.8) was evolved from a set of simultaneous non-linear ordinary differential equations proposed by Seltzer and Marshall (29), for disc
98
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
conditions where both slippage and frictional resistances occur under
laminar flow conditions. The flow characteristics were defined by
d er
dt 2
127r1.0\ dr \ 2
Qp, ndt)
r (10)
2
0
(6.9a)
d dO\ 27;u0 2 1
d& dr
0
(6.9b)
La)
dt
dt
Qp, r
= disc rotational speed (2IEN), Q = volumetric feed rate, r = radial
distance from centre of disc, t = time for element of liquid to move from
r o to r, dO/dt = rotational speed of feed.
dt r dt )
This can be reduced to equation (6.8) through integration, by substitution
of P = dr/dt.*
Equation 6.8 applies to the 'ideal' disc, Frazer (28) proposed functions
to express flow conditions over an ideal disc surface, where liquid break-up
at the disc edge is due to ligament or sheet disintegration. The transition
between the two disintegration mechanisms is stated as functions of two
dimensionless parameters Az and B' where
y \ 1/6
(pcni)
d od
r
1/2
)
13' is expressed in terms of the Z' number (see figure 6.1).
The parameters show the roles played by viscosity and surface tension
on resulting liquid break-up conditions. The transition is represented on
figure 6.5 by a line of separation.
as checked out by substituting these values in equation (6.9b). If frictional
viscous forces are also ignored = 0), equation (6.9a) becomes
N72
p
=
dr
d er
=
dt 2
pd) 1/2
o-
A r =Nd
-
° [(61i)
For conditions of non-slippage
dO
N =—
dt and d ( da ) —
— 0
dt dt
d e r dP dP dr
dP
dt 2 = dt = dr
= P dr
Thus
dP
P— =
(6.10)
dr
N2r
P dP = N 2 r dr
Integrating
P2 N 2 r 2
+
T=
2
C
when
ATOMIZATION
THROUGH LIQUID
SHEET FORMATION
t = '0,
rry
r ro
C
—
P2
Ar2( r 2
11
.'
Q
N dt
001
1
1
1
1
1
41
1-0
Integrating
"
ce•d )
dr
Nt = fro (r2 rD1/2 = cosh' ?1
( A421 /4)
r
rD dr
dt
N(r 2 rW/2 dr
dt
dr
(r 2 rg) 112
P
J
N2r2
2
or
z
0001
dr
=0
dt
Therefore
ATOMIZATION
THROUGH LIQUID
LIGAMENT FORMATION
.0
99
ro
05
Figure 6.5. Atomization mechanism for rotating discs (28). (Relationship between operating
variables and atomization mechanism.)
for N in rev/min
aNt = cosh' (1
ro
100 THE PROCESS STAGES OF SPRAY DRYING
Equation number
oc
N
1-4
tll
N
Ld
Sheet disintegration
Ligament break-up
cis
Oyama, Endou (34)
Walton, Prewett (33)
N
Direct droplet formation
where p.q.r.s are respective integers, the values of which depend upon
atomizer operating conditions.
6.5.3. Wheel Atomization (Atomizer Wheels)
(a) Introduction
The mechanism of atomization applicable for atomizer wheels is similar
to that discussed for flat smooth vaneless discs in section 6.5.2. Mechanisms
are shown photographically by Marshall (35) and Kurabayasi (36). At
low speeds and feed rates, viscous and surface tension forces predominate
to give direct droplet formation. For intermediate speeds, the disintegration
of liquid ligaments extending out from the vane edge is by centrifugal and
to a lesser extent by gravitational forces. In the commercial range of conditions (high liquid flows at high wheel peripheral speeds), liquid disintegration occurs right at the wheel edge by frictional effects between air and the
liquid surface as liquid emerges as a thin film from the vane. The mechanism
does not lend itself to the formation of homogeneous spray formation,
although sprays of small droplet sizes can be produced. Increasing the
viscosity and surface tension of the liquid will act to increase the spray
uniformity, but at the expense of a low mean droplet size. However, for
general operational circumstances in spray drying, adjustment to the feed
II II II
aa
by
Cl
Atomization mechanism
D cc N -13 : D cc o- g: D d': D cc QS
ATOMIZATION 101
Table 6.2. Droplet Size Prediction for Smooth Fiat Vaneless Discs
(b) Droplet Size from Smooth Flat Vaneless Discs
Six equations to predict droplet size from smooth flat vaneless discs are
given in table 6.2. Other equations in this field are given by Lapple (25)
and Dombrowski (30). All are applicable to the simplest of systems involving
low feed rates of low viscous liquids. Surface tension and liquid density
appear as the only liquid properties of influence, which is misleading as
viscosity is of prime importance. This is illustrated in equation (6.13), where
the author claimed close agreement between experimental and predicted
results for low viscous liquids but not for high viscous liquids. Droplet size
appears inversely proportional to the first power of disc speed for these
simple systems.
The conditions of experimentation produced uniform sprays, although
the presence of satellite droplets was evident. The D max value in the Bar (32)
equation refers to the parent droplet. Frazer (28), Walton and Prewett (33)
and Hege (31) give the average diameter of the spray. Oyama and Endou (34)
use the Sauter mean diameter. In table 6.2; D is microns, N is rev/min,
3
3
d is cm, o- is dynes/cm, p is g/cm , Q is cm /sec.
The equations of table 6.2 are of limited interest to spray drying conditions.
However, the equations indicate the relationships between droplet size
and the operation variables; relationships that do hold into commercial
atomization conditions.
102
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
liquid properties is not possible or desirable. To obtain optimum droplet
size uniformity for a given feed rate the maintenance of the following six
conditions is essential :
i. Vibrationless wheel rotation.
ii. Large centrifugal forces compared with gravitational forces (i.e. high
peripheral speeds).
iii. Smooth vane surfaces.
iv. Uniform liquid feed rates.
v. Complete wetting of the vane surface.
vi. Uniform distribution of feed to the vaned wheel.
(b) Flow over a Vaned Wheel
Liquid fed on to a wheel moves across the surface until contained by the
rotating vane. Liquid flows outwards under the influence of centrifugal
force and spreads over the vane wetting the vane surface as a thin film. At
very low liquid vane loadings, the thin film can split into streams.
Unlike the flat smooth vaneless disc, no liquid slippage occurs on a wheel
once liquid has contacted the vane. The vanes, whether radial or curved,
prevent transverse flow of liquid over the surface. Of the basic equations
developed in the previous section, equation (6.9a) is not valid. For vaned
wheels, only viscous resistance is acting.
103
Droplet travel from a wheel is shown in figure 6.6. The liquid film on
leaving the vane edge has radial (XY) and tangential (YZ) velocities giving
a resultant component (WY). The angle of release is less than 45° to the
wheel edge. For commercial atomizer wheels, the radial velocity component
is much smaller than the tangential, and release velocity approximates to
the peripheral wheel velocity, and at an angle of release approaching the
tangent to the wheel edge.
(i) Radial Velocity (14 Radial velocity of the liquid film at the wheel edge is
calculated from differential equations derived from consideration of forces
acting upon the liquid film (35). Differential equations are derived in
Appendix 8.
Marshall (35) suggests that for atomizer wheel sizes and peripheral
speeds used in industrial spray drying liquid acceleration along the vane
has ceased upon reaching the wheel edge. The radial velocity is given by
PJav 0)211/3
Vr =
(6.18)
3/2h2
The error in this simplification is minimal, but Marshall (35) has stated the
error can be reduced further if equation (6.19) is used where X has values
greater than 1.
DROPLET TRAVEL RELATIVE TO WHEEL EDGE
Z = X1/3
1
1
1
9X 27X" 218X 213
(6.19)
Where
X = r(R*o.) 2 )
LIOWO DISTRIBUTOR
and
R* =
3/.1h 2
R* 112
DROPLET TRAVEL.,
RELATIVE TO CHAMBER WALL.
v?.1-
1/2
A 5 in (125 mm) 24-vaned wheel atomizing water gives values of X as shown
in table 6.3. Equation (6.19) is valid for calculating radial velocities in this
case.
x
FASTENING
BOLTS
Table 6.3. Values of X = r(R*(o 2 )'I2
Wheel speed (rev min)
RESULTANT
v
LIQUID SLIPPAGE OVER
SURFACE UNTIL
CONTACT WITH VANE
Figure 6.6. Liquid flow to and from edge of atomizer wheel.
Feed rate (113/min)
15 750
20 600
24 000
4.0
7.0
10.0
28
16
11
32
18
13
35
20
14
104
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
Frazer (28) has forwarded an estimation of radial velocity (ft/sec) from a
vaned atomizer wheel.
P i n 2N 2 dQ 2 1/3
(6.20)
V, = 0.043 [ m/22112
1
Where d is ft, p. is cP, h is ft, N is rev/min, Q is ft /min, p, is lb/ft .
The graph of flow rate plotted against radial velocity is a straight line on
log-log plot.
(ii) Tangential Velocity (VT ). The vanes prevent slippage and the liquid
acquires the peripheral velocity of the wheel on release.
(6.21)
V, = rdN
(iii) Resultant Release Velocity (Tires). The resultant release velocity is the
square root of the sum of the squares of the radial and tangential components.
Tires
(6.22)
[ Vi + Vi?.]"2
(iv) Angle of Liquid Release (a). The angle of liquid release follows from
equation (6.22) by basic geometry
V
(6.23)
tan VT
In practice a is very small, and Tires approximates to the peripheral speed of
the atomizer wheel.
EXAMPLE 6.1.
Radial velocities and film thickness at the edge of a 10 in diameter 20 vaned
wheel are required. Vane height is lk in. If the feed specific gravity is 1.5
but the feed liquid viscosity can be varied from 50-100 cP by preheating,
calculate values at feed rates of 132, 264 lb/min at viscosities of 50, 80,
100 cP respectively. The wheel speed is 10 000 rev/min.
Assumptions : Equal liquid distribution to all vanes. No acceleration of
liquid on vane surface at wheel periphery.
For operating conditions of 132 lb/min, and feed viscosity 80 cP, substituting values in equation (6.18)
Q,
1.18 x 10 -3 ft 3 /sec per vane,
w = 1044 rad/sec
p i = 93.5 lb/ft'
33.5 ft/sec
Film thickness (b) is calculated from the continuity equation (no hold-up
on vane) (b = +2,/h Vr)
b = 3.5 x 10 -4 ft (0.107 mm)
Values of V, and b for other operating conditions are given in table 6,4.
Equation (6.20) can also be used. (Note how much smaller the radial velocity
is compared with the tangential component.)
Table 6.4. Values of Radial Velocity and Liquid Film Thickness at Edge of
Vaned Atomizer Wheel (Example 6.1)
3
3
105
Feed rate
(lb/min)
Liquid
viscosity
(cP)
Radial velocity Film thickness at wheel edge
at wheel edge
(mm)
(ft)
(ft/sec)
132
132
132
50
80
100
39
33.5
31
3.0 x 10 -4
3.5 x 10 -4
3.8 x 10 -4
9-15 x 10 -2
1.07 x 10 -1
1-16 x 10 -1
264
264
264
50
80
100
63
53
49.5
3.8 x 10 -4
4.5 x 10 -4
4.8 x 10 -4
1.16 x 10 -1
1.37 x 10 -1
1.47 x 10 -1
430 ft/sec (131 m/sec).
Peripheral velocity
(c) Effect of Operating Variables on Droplet Size
Operational variables that influence droplet size produced from atomizer
wheels are speed of rotation, wheel diameter, wheel design (number and
geometry of vanes or bushings), feed rate, viscosity of feed and air, density
of feed and air, surface tension of feed.
The effect of these variables on mean droplet size of a spray is best studied
through dimensionless analysis, where variables are grouped to represent
basic atomization mechanisms. Forms of Reynolds number (defining the
inertial and viscous force ratio on the liquid) and Weber number (defining
the inertial to surface energy ratio) are commonly used. Friedman, Gluckert
and Marshall (20) used the following groupings where the atomizer wheel
speed conveniently appears in one group only (M, = mass feed rate for
wetted periphery)
D vs
r
i Ma
\ p
Ni p b I M p lc I LV
\r
pNr 2 )
Groups having exponents a . b are related to Reynolds Number, the remainder to Weber number.
Effect of liquid properties and liquid release velocity on droplet size has
already been introduced under the section 6.5.2. Several investigators have
studied the effect of these variables and others, and report that the variables
of most influence on droplet size in commercial atomization conditions are
peripheral wheel speed, and liquid loading on the vanes (volumetric basis).
Atomizer wheel/disc
Viscosity (cP)
Density (lb/ft 3 )
Surface tension (dynes/cm)
Diameter (in)
Speed (rev/min)
Feed rate (lb/hr)
Vane number
Vane height (in)
,
Friedman,
Gluckert,
Marshall (20)
Herring,
Marshall (37)
Frazer (28)
Vaned
non-vaned
1-9000
62.4-88
74-100
2-8
860-18 000
8.5-800
2-24
0-015-1.31
Vaned
non-vaned
1
62.4
74
2-8
5000-32 500
0.2-50
8-24
0-31-1.28
Vaned
non-vaned
1-9000
62.4-88
74-100
2-8
860-18 000
33-4050
8-24
0.31-1-32
Masters,
Mohtadi (38)
Scot, Robinson,
Pauls, Lantz (39)
Vaned
Vaned
1
62-4
74
5
15 750-24 000
240-600
24
0.25
9.8-199-5
5327-8
2 11 900-41 300 ' ,4.2-37
- 24 .
0.23
THE PROCESS STAGES OF SPRAY DRYING
Table 6.5_ Operating Conditions Covered by Investigators in Drawing up Equations (626-621) to Predict Spray Characteristics
from Atomizer Wheels/Discs
Table 6.6. Effect of Operating Variables on Droplet Size from Atomizer Wheels
Wheel speed
Wheel diameter
Peripheral speed
Feed rate
Vane height
Vane number
Feed viscosity
Feed density
Feed surface tension
Herring,
Marshall (37)
Masters,
Mothadi (38)
N —°-82
N - ° -6
Q ❑-2
(Nd) —°.83
Q0.24
(Nd) °
Q0.18
- ❑ -1
b-o-1
n -(1-1
b- 0.12
n
0-1
µ— 0.2
p 0.5
)0 — ❑ -5
Frazer (28)
N - ❑ -6
d — 0-2
Q -2
❑
b --o-1
n
o•i
❑
N - -6
d - °• 2
d- 0.85
Scott, Robinson,
PauIs, Lantz (39)
OW -13-54
Q0-17
—0.02
0_ 0-1
NOLLVZCWOIY
Operating variable
Friedman,
Gluckert,
Marshall (20)
••
•
108
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 109
Table 6.5 lists investigators, and the operating conditions under which their
conclusions were drawn. The effects of operating variables on droplet
size, concluded from such work are summarized in table 6.6.
(i) Effect of Liquid Feed Rate on Droplet Size. Droplet size varies directly
with feed rate at constant wheel speed. The variation of Sauter mean droplet
size with feed rate is shown in figure 6.7. Droplet size varies between 0.17-0.2
power of feed rate. Friedman, Gluckert and Marshall (20) established a 0.2
power variation for feed rates up to 70 lb/min at constant wheel speeds
within the range 3500-14 000 rev/min where maximum peripheral speeds
reached 208 ft/sec (63 m/sec). Feed properties ranged from (a) specific
gravity (1-1.425), (b) viscosity (1-9000 cP) and (c) surface tension (74-100
dynes/cm).
2 00
1
I
I
I
overage slope = 0.2
150
200
CC
Li
1—
77 FT /SEC
<
1 00
•
90
—
J Ln
CL z
o 70
Or ct
O
60
Z X
7
(270 Kg/hr). Scott (39) studied the particular case of small diameter wheels
rotating at very high speeds and with low feed rates and found a 0.17 power
variation to apply.
(ii) Effect of Peripheral Speed on Droplet Size (including effect of speed of
rotation on droplet size). Droplet size is inversely proportional to wheel
peripheral speed at constant feed rate. This is shown in figure 6.8. Droplet
size varies inversely within a power range of 0.54--483 power of peripheral
speed. Herring and Marshall (37) established the 0.83 power relation within
a low speed range (50-305 ft/sec, 15-93 m/sec) at constant feed rates (20-50
lb/min, 9-23 kg/min). Masters and Mohtadi (38) showed within a higher
speed range (325-496 ft/sec, 100-150 m/sec) using a similar wheel design
that an inverse power relation of 0.7 fitted experimental data. Such findings
indicate that the rate of decrease of mean size with peripheral speed decreases
with increase of speed within the higher speed regions. It follows that, for
set feed conditions, there is a minimum droplet size for which further increase
of speed has no further effect.
50
ct
REST LT S EY FRI EDMAN,
734
FT/SEC
( 20 )
GLU CNER AND MARSHALL
150
RESULTS BY
HERRING AHD
MARSHALL
ISLE!
LLI
N
(7) -.100
U)
Z
Li]
•
LI)
( 37)
091)
325 FT/SEC
426 rrissc
49 6 FT/SEC
LLJ
—
40
30
4.0
AQUEOUS FEED RATE 11: ,
7.0
10.0
90
0_ L'
0
— 80
RESULTS BY MASTERS
AND MOHTA131 ( 3B )
•
70
LLI
60
15.0
MASTERS AND
/min
Figure 6.7. Effect of feed rate on mean droplet size for atomizer wheel.
Frazer (28) substantiated the 0.2 power variation working over a similar
range of variables. Masters and Mohtadi (38) atomized water at constant
wheel peripheral speeds in the higher regions up to 496 ft/sec (150 m/sec),
and established a 0.18 power variation to be more representative of the feed
rate—mean droplet size relation. Feed rates ranged up to 600 lb/hr
RESULTS NV
MONT AD!
i 35)
( SLOPE-041
50
40
200
300
400
500
600
700
PERI PH ERAL SPEED FT/ SEC
Figure 6.8. Effect of atomizer wheel peripheral speed on mean droplet size.
ATOMIZATION 111
110 THE PROCESS STAGES OF SPRAY DRYING
Scott (39) showed the power relation to be even lower than that stipulated
by the above investigators. When operating small diameter wheels at very
high speeds and low feed rates, the Sauter mean diameter was shown to vary
inversely with the 0.54 power of peripheral speed.
Peripheral speed is widely accepted as the main variable for adjustment
and maintenance of a specified droplet size. However, the precise role
played by the variable is still not clear. It has been shown by Wallmann and
glyth (40) that droplet size does not necessarily remain constant if equal
peripheral speeds are produced in wheel designs of various diameter and
speed combinations, operating at equal feed rates and fixed feed solids.
The wheel that subjects liquid to the greater centrifugal force produces the
s mallest droplets. In fact, plotting droplet size against a centrifugal factor
(c) defined as the number of times gravity, results in an inverse function,
from which follows
D cc (dN 2 ) -1
14 1/min
7.5 l/rnin
4.51/rnin
3.01/min
1.8 I /min
(6.25)
as
rg
42r 2 r 2 N 2
r3600
7r2r/V2
900g
7r 2 riN 2
5000
20000 30000
SPEED P .P. M
1800g
where N is rev/min.
Separate investigation of the relation between droplet size (D) and wheel
speed (N) at constant wheel diameter (d), and the relation between droplet
size and wheel diameter at constant wheel speed have not supported equation (6.25). Droplet size is inversely proportional to diameter, but both
Friedman (20) and Frazer (28) found D to vary as d - ", where Herring and
Marshall (37) found variation to support d' 85 . Similar discrepancies
were obtained from investigations into the droplet size-wheel speed relationship. Both Friedman and Frazer propose D cc N - ", whereas Meyer (41)
proposes .D cc N - ' 69 and Herring and Marshall propose D oc
The effect of speed on Sauter mean droplet size is shown from aqueous
spray data to follow an average exponent of —0.6 in figure 6.9.
A rule-of-thumb law of D oc N -2 is often applied in industry for assessing
droplet size change in atomizers that impart large centrifugal forces to the
feed liquid. Experimental data has not proven this otherwise as the low
exponents (-0.6 to —0.83) have been established only with relatively small
atomizers handling intermediate feed rates at low vane loadings. However,
size data predicted by using D cc N " or D cc (N d) 4 °' 83 and compared with
dried particle analysis of product atomized at industrially high feed rates
have suggested the low power relations ( — 0.6 to —0.83) to apply, but in
these cases vane loadings were low, as wheels contained numerous vanes.
It is interesting to note the low loading conditions upon which experimental
10000
Figure 6.9. Effect of atomizer wheel speed of rotation on mean droplet size (aqueous sprayi).
data have been obtained. Generally only low vane loading conditions and
low viscous feeds were used, but even in this range a decrease in the power
of 'N' accompanied a decrease in vane loading. This is shown in table 6.7
for low viscous feeds.
Meyer (41) produced atomization conditions of velocity spraying at low
D cc N -2 , but there is no
feed rates by using high viscous feeds.and found -2
real evidence to point to the relation of D cc N being more applicable
-c
to high industrial capacities than D cc N ".
Table 6.7. The Relation between Mean Droplet Size and Wheel Speed for
Different Vane Loadings for Low Viscous Feeds
Author
Vane loading
lb/hr ft
wetted periphery*
Power of
wheel speed
Scott et al. (39)
Masters, Mothadi (38)
Friedman, Gluckert, Marshall (20)
Herring, Marshall (37)
9-80
380-950
800-5500
500-14 500
D cc N -° * 54
D cc N-°. 6
D N -a
-° 8 3
D cc N *
' 6
*1000 lb/hr ft = 23.5 kg/hr cm. Wheel peripheral speed range: 100-500 ft/sec
(30-150 m/sec).
112
ATOMIZATION 113
THE PROCESS STAGES OF SPRAY DRYING
However, the trend in table 6.7 does indicate that if velocity spraying
conditions are produced by industrial capacities creating high vane loadings
with low viscous feeds or by use of higher viscous feeds at intermediate feed
rates, an exponent of N between - 1 and -2 can well be considered acceptable. A working relation of D cc N -1.5 would be justified where vane
loadings with low viscous feeds exceed 3 x 10 $ 1b/hr wetted ft (7050
kg/hr cm) for wheels rotating with peripheral speeds in the range 100-500
ft/sec (30-150 m/sec). Such vane loadings can occur in (a) wheels where flow
area is minimized to prevent air pumping effects of the wheel, and (b) wheels
with bushings for corrosive and abrasive feeds, where flow area is minimized
to reduce areas of wear (see under atomizer wheel designs). Further work
in this area of high vane loadings is required to substantiate exponent
values of N applicable to industrial conditions.
However, many industrial atomizers operate at low vane loading conditions due to the atomizer wheel design, which contain numerous high sided
vanes. The lower power values established experimentally are more often
than not adequate to represent industrial atomization conditions.
For maintenance of a specified droplet size at proposed higher feed rates
or increased feed solids, wheel peripheral speeds must be increased. From
a practical viewpoint, decrease in droplet size is more readily achieved by
increase in wheel speed than by increase in wheel diameter. It is advisable
to first consider if the atomizer drive has spare power capacity for handling
the feed rate at higher speeds, before considering fitting a wheel of larger
diameter and operating at the same wheel speed. Often the fitting of an
oversize wheel will cause the wheel perimeter to extend outside the atomizer
drive lower casing. This creates the possibility of product build-up on the
casing and wheel due to inevitable eddies caused by air movement around
the wheel edge protrusion.
(iii) Effect of Liquid 14scosity on Droplet Size. The variation of Sauter mean
diameter with liquid viscosity at constant feed and wheel speed is shown in
figure 6.10. Droplet size varies directly to the 0.2 power of viscosity (20).
Figure 6.10 is based upon results (20) conducted on a 5 in (125 mm) diameter
vaned wheel rotating at 14 000 rev/min. Viscosity of the feed liquid ranged
over 1. 0-15 000 cP. Slight variation to the values of liquid density and surface
tension accompanied the viscosity changes. Due to the inability to correct
for changes in density and surface tension, the slight variations were assumed
to have a negligible effect on the droplet size results.
At higher peripheral speeds the exponent 0.2 also appears valid. This
has been shown (42) by results with aqueous sprays formed from a 5 in
diameter wheel rotating at speeds between 15 750 and 24 000 rev/min. For
feed rate of 4-10 lb/min, sprays were sampled and the Sauter mean diameter
calculated. Such data, when substituted in the group (D vs N'/M") gave
SLOPE 02
0.2
0'l
c'?
Lo •
DOB
006
- 9 004
a
002
10.01 V
14 88 0 } c P" n
10
(14681
0-1
( 1488 )
0.01
194.88 )
ABSOLUTE VISCOSITY
Figure 6.10. Effect of feed viscosity on wheel atomization.
values for the group in close agreement with values predicted from figure 6.10.
Figure 6.10 thus provides a method to estimate Sauter mean diameter if
viscosity data is known. Comparison of experimentally determined Sauter
mean diameters with diameters predicted from figure 6.10 are shown in
table 6.8. For aqueous sprays, agreement is of a high order.
(d) Prediction of Mean Droplet Diameter and Spray Size Distribution
In spite of intensive investigation into the mechanism of atomization from
rotating atomizer wheels, the prediction of spray characteristics still remains
uncertain. The effect of individual variables has been established over a
li mited range and are presented in (c) above. There are few general correlations to express atomization features of commercial atomizer wheels.
Table 6.8. Sauter Mean Diameter Predicted from Viscosity Data (13,42) (figure 6.10)
(A Comparison with Experimentally Determined Values)
Wheel diameter
5 in (125 mm)
By experiment
Group
D„ 1\1'6
M 0.2
Sauter
mean size
(micron)
0.0484
0.0470
0.0485
0.0460
50.2
62.9
52.6
76.6
0.05
0.05
0.05
0.05
Aqueous
feed rate
(lb/min)
Wheel
speed
(rev/min)
Sauter
mean size
(micron)
Group
D. .1\1' 6
4.0
7.0
7.0
10.0
24 000
20 600
24 000
15 750
47.3
57.2
49.5
68.5
Where M is lbinnin, D„ is ft, N is R.P. M.
By prediction
M0'2
H4
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 115
Table 6.9. Relations to Predict Spray Characteristics from Atomizer Wheels
Reference
1
(28)
Equation
D vs = 4.2 x 10 4(N) - ' 6 (p i )
Number
1
°-2 6 °-1
° 5 ( )
• 1 11
( 71
-T
rz
(6.26)
Dvs = micron, N = rev/min, h = ft, M L = lb/hr,
d = ft, P i = lb/ft 3 , a = dynes/cm,
2
(20)
( Al)p 0.6(
Dvs =
p i Nr 2
= cP
) 0.2 ( cypin h) 0•1
Mp
M i;
(6.27)
K' = constant, value according to atomization conditions
(38)
(20)
for conditions shown in table 6.5 column (4) K' 0.37
for conditions shown in table 6.5 column (1) K' = 0.40
D vs ft, r = ft, Mi„ = lb/min.ft, N = rev/min,
Table 6.10. Comparison between Experimental and Predicted Sauter Mean Droplet Sizes
(droplet size in micron)
u l = lb/ft.min, a = lb/min e , p i = lb/ft 3 , h = ft
3
(20)
D ma „ = 3.0D vs
(6.28)
4
(38)
D95 = 1.4D vs
(6.29)
5
(37)
D v1,4 =
K mt
24
83
(Nd) * (nl ❑ 12
°
(6.30)
D vivi = micron, d = in, M = lb/min, h = in,
(39)
Dvs = 5240(M p ) °.171 (7tdN) - 0.537 011r °.°17
Wheel speed
(rev/min)
Equation number
24 000
20 600
15 750
Liquid feed rate (lb/min)
Peripheral
speed
(ft/sec)
486
426
325
(a)
(b)
10.0
7.0
4.0
(c)
(a)
(b)
(c)
(a)
(b)
(c)
70.6 48.5 47.3 78.0 54.5 49.5 83.8 57.0 52.0
76.3 53.4 53.7 85.4 60.3 57.5 91.7 63.4 60.0
894 63.5 58.2 100.0 96.6 64.9 107.3 76.0 68.5
(a) Equation (6.26). (b) Equation (6.27)• (c) Experimental results (38).
K = see table 6.11
6
at al. (20) for determination of the Sauter mean diameter of a spray are
applicable to pure liquids of low viscosity. These two equations are limited
by not being deduced from peripheral speed levels over 300 ft/sec. These
speeds are nowadays common in industrial applications. Furthermore, all
experimentation has been carried out on simple liquid systems, thus drawing
doubts on the applicability of the equations to likely atomization characteristics of complex solutions, slurries and pastes that form many spray dryer
feeds. However, comparison of experimentally derived values with those
predicted theoretically by these equations when applied to high peripheral
speed conditions (38) shows close agreement between measured aqueous
spray and the spray predictions by the Friedman equation (6.27) but not
with the Frazer equation (6.26). Such a comparison is shown in table 6.10.
(6.31)
Dvs = micron, Mp = g/sec,cm, d = cm, N = rev/sec,
ul = poise
Available correlations are- given in table 6.9 and conditions under which
they were determined are given in table 6.5.
Wheel performance at low peripheral speeds (50 ft/sec, 15 m/sec) is reported
and illustrated photographically by Golitzine (43), but although the account
is comprehensive, it cannot be applied to peripheral speeds of commercial
importance (200-500 ft/sec, 60-150 m/sec). Within this higher speed range,
droplet formation cannot be studied visually, and spray characteristics are
predicted from empirical equations, the validity of which has been established
experimentally from analysis of sprays.
(1) Mean Droplet Diameter. For peripheral speeds up to 300 ft/sec (90 m/sec),
the equations (6.26) and (6.27) proposed by Frazer (28) and Freidman
In fact the experimental results for mean droplet size follow the dimensionless groups of equation (6.27), but with a constant reduced from 0.4 to 0.37.
The Friedman relation between mean and maximum size equation (6.28)
cannot be considered totally reliable. Measured maximum droplet size,
when defined as the size below which 95 % of the spray is included appeared
only 1.4 times greater than the measured mean droplet size.
Equation (6.30) represents the volume mean diameter taken from the
relation proposed by Herring and Marshall for prediction of size distribution
4
data. The constant (92.5 x 10 ) represents an average value. Comparisons
between actual and predicted spray sizes have indicated that predicted
values more closely resemble actual values if different values of this constant
are used for various atomization situations. These values are given in
table 6.11 related to dryer sizes. The atomizer operating conditions that
relate to dryer sizes are given in table 6.12.
It follows from table 6.11 that in small diameter test dryers (case I) where
very high speed small diameter wheels are used at low feed vane loadings,
fine spray sizes are obtained. The value of the constant is low. In pilot plant
116
ATOMIZATION 1 17
THE PROCESS STAGES OF SPRAY DRYING
dryers (case II), a semi-industrial atomizer size is often employed, and at the
feed rates employed is really over-sized. Fine sprays are obtained when the
atomizer is run within its designed range, usually at peripheral speeds
300-550 ft/sec (90-170 m/sec). The value of the constant is also low. In
industrial dryers of medium capacity (case III), the atomizer operates at
rated capacity and speed. Sprays of high homogeneity are formed having
larger mean diameters. The value of the constant increases accordingly
and approximates to the average value quoted in equation (6.30). If the
atomizer is run specifically at low peripheral speeds large droplet sizes are
obtained and this is represented by increase in the value of the constant
(case IV). For industrial dryers of large capacity (case V), the atomizer
wheel has an extensive wetted periphery, and is sized to rotate with a high
peripheral speed. Low vane loadings are maintained and for low viscous
feeds, fine sprays of high homogeneity are produced, as depicted by a reduced
value of the constant. For all dryer sizes, values of the constant will vary
according to the product atomized and the wheel design. This is especially so
if the wheel design differs considerably from the more common vaned types.
When operating the dryer on a set product, the value of the constant for the
dryer size and product can be established by wet analysis of the atomized
spray. The values of the constant given in table 6.11 are offered as a guide to
atomization assessment and to obtain data for first design calculations.
Equation (6.31) has been proposed by Scott et al. (39) and represents the
Sauter mean diameter of sprays formed by a small diameter atomizer wheel
rotating at very high speeds. It is applicable to predicting droplet size in
laboratory sized dryers of small diameter. The influence of each variable on
the mean size is correlated by a simple exponential equation. Predicted
values of mean diameter show close agreement to measured values when
droplet sizes do not exceed 60 micron.
Ce
CN
-e
I
II
III
IV
V
Drying situation and dryer size
Dryers in general
Small diameter test dryers
Pilot-plant and, small scale production dryers
Industrial dryer, medium capacity, peripheral speed
greater than 200 ft/sec (60 m/sec)
Industrial dryer, medium capacity, atomizer peripheral
speed less than 200 ft/sec
Industrial dryer, large capacity
r
-
* This is an average value as used in equation (6.30).
Modified value of
constant for use in
equation (6.30)
99.0 x 10
4
86.0 x 10 4
ett
Ce
1
ems
5 0)
92.5 x 10 4 *
85.0 x 10 4
83.0 x 10 4
94.0 x 10 4
ix
71 •
Table 6.11. Values of Constant in Equation (6.30) for Given Dryer Size
Case
00
Ce
0
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 119
(it) Size Distribution. Size distribution data of sprays are best represented
by the relation of Herring and Marshall (37). The relation is represented
on square root probability paper by a plot of the group
0
0
0•8301/00•12
-4
C).
6
6
z
FOR DATA ON CURVES SEE TABLE
CO
WHERE X 1
1
1
1
I
I
I
I
Np
0,
0 0
o
I
d
ill
o o C7 0I
1
1 ,
C
❑ NVHI SS31 39tl1N3Dd3d 3wrrion 3Alivinmn3
0
- co
••■
1 1
LO O ca p
oa C•••
❑
0,
Q
p 0 C. 0
C ,
1
0 In
0 NVH.I. SS31 30VIN33d3c1 31•JMOA 3A 11V 1111.4'13
-
FOR DATA ON CURVES SEE TABLE 6.12
1
E
a,
WHERE X -
against the cum ulative volume percentage less than D, the selected droplet
h
size. Plots are s o wn in figure 6.11(a) for size distribution data obtainable
from laboratory and pilot plant dryer sizes, and in figure 6.11(b) for size
distribution data obtainable from production sized dryers. The range of
operational variables applicable to curves A-E are listed in table 6.12.
Data are largely lacking for atomization of liquids other than water at
high peripheral s peeds. Predictions of spray characteristics at high speed or
sizing of wheels t o meet given spray characteristics are best carried out by
applying the correlation on which figures 6.11 are based. Use of equation
(6,27) with a m odified constant, particular to the product atomized is often
adopted. The m odified constant can only be obtained from product drying
experience.
The Friedman-Gluckert-Marshall correlation does offer a quick method
to scale-up wheels to handle a higher feed rate yet maintain a constant size
distribution. Inspection of equation (6.27) shows that if vane height or
number of vanes is increased to maintain the same conditions of feed rate
per wetted periphery at increased feed rates, the predicted mean and maximum droplet size remains virtually the same. Thus if feed rate per wetted
periphery i s ma intained the same on wheel scale-up based upon a constant
wheel diameter, mean droplet size will be reproducible. For example, if a
50% increase in feed rate is required, without changing droplet size, the
vane height can be increased 1.5 times to maintain the same feed rate per
wetted periphery. All groups on the right-hand side of the equation (6.27)
remain the same for the new feed rate conditions, except for that last, where
there is an increase in value by the factor (1-5) 0 ' 1 or 1.04, Predicted mean
droplet size from the scaled-up wheel is only 4 % higher than size obtained
l
by the quick sca e -up method based upon equal feed rate/wetted periphery
conditions. This is subject to the authenticity of the Friedman-GluckertMarshall correlation for the atomized conditions considered.
EXAMPLE 6.2
An industrial atomizer, fitted with a 9 in wheel operates at 18 000 rev/min,
A fine atomization is required and the wheel contains 240 vanes (g in height).
The feed rate is 2200 lb/hr. Predict the mean and maximum droplet size
in the spray. The feed properties are p i = 70 lb/ft 3 , v= 74 dynes/cm,
pt i = 1 cP.
_C
L -"-
)
(6.30a)
X
x
10 4
0
(D(Nd)
mi..24.
21.63 B
118
6
120
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
Method I : Friedman equation (6.27),
Data
surface tension (o- ) --- 5 x 10 2 lb/min 2
viscosity (A) = 4 x 10 -2 lb/ft min
Function becomes
D x 21 100 x 1.71)
= 1.52D
4
2.37 x 10
Referring to figure 6.11(b), curve C
Total wetted periphery = 240 x 0.375 7.5 ft
12
50% cumulative = 94
95 % cumulative = 176
2200
Mass flow per total wetted periphery - (
= 4.9 lb/min ft
99.95 % cumulative = 289
60 x 7.5
Wheel radius = 0.375 ft
Substituting in values:
Mn
0'6
Lp 1 Nr2
r
1 0.2
r
1.84x 10'
4 x 10 - 210.2
= 0.382
4-9
[Ni p]
G. '
FL apinhl
M _1
therefore
94 = 1.52D v m
D vm = 62 micron
1•52D 95%
D 95 = 116 micron
D„, a „:D 99 . 95 % cum = 289 = 1.52D max
Dmax = 190 micron
Dvm:Dsoxciim =
10,6
4.9
2
L70 x 18 000(0.375) ]
D 95 : D 95 x cum = 176
Size Prediction (micron)
[5 x 10 2 x 70 x 7.5i"
= 2.54
4.9'
D
Method I
Method H
Average
Dvm
89*
62
75.5
(a) Sauter mean droplet size
Use equation (6.27) with K' = 0.37 as vane liquid loading is low.
Dmean
DVS
75.5
53*
64.25
Dvs = 0.37 x0.375 x 1.84x 10 -3 x 0.382 x 2.54 = 2.40 x 10' ft
D95
106
116
111
Dmax
226.5
190
208.25
= 75.5 micron
(b) Size below which includes 95 % of spray (equation (6.29))
D 95 •=•
1.4D v5 = 106 micron
(c) Largest droplet size (equation (6.28))
D max = 3.0D v5 = 226.5 micron
Method II: Herring-Marshall correlation (Equation (6.30), required units
shown in table 6.9)
Data as in Method I
(Nd) °. " = (18 000 x 9) 0 ' = 21 100
(nh)0•12
t24.
(240 x 0.375) 0 ' 2 = 1.71
/2200\ 0.24
= 137
121
* Denotes an estimated value as volume mean diameter
usually exceeds the Sauter mean diameter by 15-20%.
Values of D mean and D95 establish the spray homogeneity, and define the
particle size range expected in the dried product. It can be seen from the
results that a fair degree of agreement exists between the two methods for
predicting droplet size. Reviewing the results that established the two
methods (20, 37), values predicted by Method II showed differences from
values predicted by Method I by varying degrees. These differences were
explained by such factors as different sampling and droplet sizing procedures.
Practical experience has shown Method II to be more representative of
low vane loading atomization.
EXAMPLE 6.3
In developing the spray drying of an inorganic salt on a large-scale, pilot
plant drying tests were conducted to obtain data on dried product characteristics and suitable drying conditions. A 24 vaned (4.75 in diameter)
122
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
atomizer wheel (vane height 1 in) was used . Operating at a feed rate of
220 lb/hr, and wheel speed 12 000 rev/min, dried product was obtained
having a sieve analysis of
> 120 micron
0.5
>100 micron
2.5
> 75 micron
19.0 %
> 60 micron
56.0
> 44 micron
86.0
Estimate the change in size the spray droplet undergoes during drying.
The feed liquid is of low viscosity and the dried product contains a fair
percentage of spherical particles.
Solution. Assume feed density and viscosity approximates to that of water,
which justifies use of the Herring-Marshall relations.
Units given in table 6.9. Hence :
,
(N ❑ i" = 8860
ol h ❑ .12 = 1.46
(m0 24 = 1 . 37
❑
Substituting in Herring-Marshall relation (equation (6.30))
Yx„ um = D x 0.942
For these operating conditions :
Wetted periphery = 24 in
Feed rate per unit wetted periphery = 0.153 lb/min in
peripheral speed = 249 ft/sec
Based upon data, the predicted wet spray size distribution is obtained from
figure 6.11(a) (curve B), where
Ys = 44
Y50%=
83
Y95% = 137
Hence :
size analysis of wet droplets
D 5% = 46.7 micron
/3 50 .4 = 88.1 micron
D„cx = 145.5 micron
123
From plot of sieve analysis data of dry product
% =- 36 micron
D 5 ❑ % = 62 micron
D 93% = 96 micron
Estimated reduction in droplet size during evaporation = 24-34 %.
6.4
The product used in example 6.3 was next test dried in a larger capacity
dryer, an industrial spray dryer of medium size. The purpose of the tests
was to obtain additional size distribution data to enable the prediction of
atomization characteristics for spray drying the product at a feed rate of
21.5 ton/hr using a high capacity vaned atomizer wheel (7.5 in diameter,
24 vanes, 0.62 in wetted height) at 13 000 rev/min. The additional tests
were conducted with an 8.25 in diameter vaned wheel, having 4 vanes of
wetted height 0.62 in. Operating conditions and the sieve analysis of the
dried product are given below. The same feed solids concentration was
used throughout :
EXAMPLE
Feed rate (lb/hr)
Wheel speed (rev/min)
Sieve analysis
> 120 micron
> 100 micron
> 75 micron
> 60 micron
> 40 micron
Test I
1140
11 250
Test II
1230
8650
3%
10 %
28 %
44 %
73 %
12.5
27.5
53.0
64.0
84.0
Using the above data, estimate the droplet shape change that occurs during
evaporation, and predict the particle size distribution of the product obtained
from the high tonnage operation.
TEST
Applying the Herring-Marshall relation (6.30a) (units given in table 6.9)
Y`xcum = D x 0.7302
Now feed rate (M,) = 19 lb/min
Peripheral speed (VT ) = 408 ft/sec
Feed rate/unit wetted periphery (M r ) --- 7.66 lb/min in
Conditions suggest use of figure 6.11(b) (curve E)
Size analysis of wet spray
124
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
125
200
yx c u m
X
Wet spray
prediction
(micron)
5%
50%
95%
40
86
151
55
118
207
200
1 60
H 120
TEST II
Applying Herring—Marshall relation (6.30a)
Y%cum = D x 0•576
40
Now
M = 20.6 lb/min
VT = 311 ft/sec
00
80
120
1 60
200
240
260
Mp = 8.3 lb/min in
320
350
404
rwcRoNs
PREMTED WET DROPLET 512E
Figure 6.12. Size relationship between droplet and particle in example 6.4.
Conditions suggest use of figure 6.11(b) (curve E).
Yzcum
X
Wet spray
prediction
(micron)
5%
50%
95%
40
86
151
69
150
264
Applying the dried particle size data to the large tonnage operation,
and employing the Herring—Marshall relation.
Yx,cum = D x 0.384
Now
M = 805 lb/min
The relationship between wet droplet size and dry particle size is obtained
by comparing the above wet prediction with the corresponding dry product
sieve analysis. This is given below :
Test T
Wet
Dry
Wet
Dry
D5
55
118
207
20
56
113
69
150
264
28
74
145
D95
Mp = 54 lb/min in
Conditions suggest use of figure 6.11(b) curve E for wet spray size. Corresponding dried particle sizes are obtained from figure 6.12.
Test II
Micron
D 50
VT = 425 ft/sec
Wet
°4..
D5%
The relationship between wet droplet size and dry particle size is given in
graphical form in figure 6.12.
D 59%
D 95%
X
40
86
151
Dry
(micron)
108
224
394
51
121
205
126
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 127
Predicted dry product analysis becomes :
200 micron
X150
X120
> 100
> 75
> 60
> 40
7
27%
50%
66
85
93%
98%
(iii) Weight Distribution of Sprays. The weight distribution of sprays from
rotary atomizers has been reported by Adler and Marshall (26). Herring
and Marshall (37), Frazer, Eisenklam and Dombrowski (28) and Friedman,
Gluckert and Marshall (20). Attempts were made to predict the weight
distribution of a spray that falls within a given radial distance, The spray
from a rotary atomizer is ejected horizontally. Each droplet size in the spray
will decelerate by air friction at different rates. For still air medium conditions, droplets of different size will fall under gravity at different radial
distances from the atomizer. The distribution in the vicinity of the atomizer
is complicated by the air movement created by the atomizer rotation, but
at distances of 3 ft (1 m) or more below the wheel, air movement due to
the rotation is less significant, and a weight distribution can be obtained by
collecting droplets that fall into radial zones around the atomizer. A typical
weight distribution plot is shown in figure 6.13 for various atomizer designs
operating at equal peripheral speeds and feed rates. The sampling plan was
1 metre below the atomizer.
00
ATOMIZER DIAMIETER: 125mm
ATOMIZER 5PEE3 : 15,750 RPM
F-
A
z
cc
75
C
LU
0
0
1—
= 50
> 25
0
3
1
2
Radial distance from rotary atomizer
4
Figure 6.13. Spray weight distribution from rotary atomizers (sampling plane 1 metre below
atomizer): A, wheel: 24 high vanes; B, wheel: 24 low vanes; C, wheel; 8 low vanes; D, disc:
vaneless.
Four different designs are shown. Each has a diameter of 125 mm, and
speed of 15 750 rev/min. The feed rate is 1000 kg/hr. Figure 6,13 illustrates
the atomizer producing the coarser spray to traject the spray over longer
radial distances. The radial distance within which a given weight percentage
of spray falls can be taken as inversely proportional to speed to the power 0.2,
i.e. (N - 0 ' 2 ), and directly proportional to feed rate and diameter to the power
0 , 2, i.e. (ma, 2 d ° ' 2 ). This is an imprecise relationship, as the effect of the chamber walls, air flow, and wheel air pumping will influence the actual spray
trajectory. During spray dryer operation, the drying air flow completely
disrupts the theoretical spray trajectory and droplet fall. Data predicted
from collection of droplets following trajectory in still air must be used
with great caution if applied to actual dryer conditions. Droplet and spray
trajectories are discussed in more detail in chapter 7.
(a) Atomizer Wheel Performance During Spray Drying
The majority of experimental conditions under which spray analysis studies
have been conducted neglect influence of the drying air entry around the
atomizer and air flow caused by wheel rotation. Theoretical spray and
particle characteristics will differ from those in reality, as the liquid released
from the wheel periphery is subjected to (i) air flow of the wheel pumping
effects, (ii) drying air entry and (iii) any particle shape change that occurs
during drying.
(i) Air Pumping Effects. Little specific information is available on the effect
of air pumping on spray characteristics. The mechanism of spray formation
would suggest air flow by air pumping to assist atomization, but in a fashion
not conducive to homogeneous spray formation. Air pumping is seen more
as a primary source of energy loss than as a variable governing particle
size of the dried product. Energy losses in small wheels can be quite substantial when compared to atomization energy requirements. Air pumping
can effect product quality. It tends to aerate the liquid within the wheel,
resulting in low bulk density products. Wheel design must minimize this
effect if high bulk density products are required from low viscosity feed
liquids.
Air pumping effects contribute to spray-air mixing in the wheel proximity.
In pilot plant spray dryers, air pumping has a considerable influence on the
resulting air/droplet flow within the drying chamber. In industrial dryers
air pumping has a negligible effect on overall air/spray flow within the
chamber. Flow is governed solely by the air disperser design. Air pumping
can, however, contribute to the formation of deposits on the surfaces on the
wheel and atomizer drive casing. These deposits become scorched eventually,
as they lie in hot air regions.
The passage of air through a rotating vaned wheel is shown in figure 6.14.
The vanes move the air outwards creating regions of low pressure over the
128
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZER DRIVE
ATOMIZATION
129
CASING
AIR DISPERSER
LIQUID DISTRIBUTOR
AIR FLOW
PATTERNS
AI R FLOW
VANED WHEEL
A
Figure 6.14. Air flow through vaned atomizer wheel due to air pumping effects.
PRESSURE
DISTRIBUTION
wheel top surface. Air ejected from the vanes draws in air from beneath
creating a recirculatory air flow below the wheel. The likely pressure configuration around the atomizer in conjunction with a ceiling air disperser is
illustrated in figure 6.15. The pressure distribution is plotted over the wheel
centre line.
Drying air entering the dryer through the air disperser passes along the
outer surface of the atomizer casing and travels towards regions of lower
pressure. The hot air will flow into the clearance between the wheel top
surface and the base of the atomizer casing. Hot air enters the wheel causing
partial drying of product in the interior. Spray droplets leaving from the
upper surface of each vane will be entrained by the air flow into the clearance,
especially when the air flow is excessive. Semi-wet product will be deposited
on the metallic surfaces, dry out, and eventually be scorched by the hot air
environment, Spray droplets and air leaving the lower surfaces of each vane
possess sufficient momentum to further entrain air from beneath the wheel.
Local eddies are set up with the likelihood of product being deposited on the
lower surfaces of the wheel.
Air flow through the wheel must be restricted as much as possible for the
cleanest atomizer operation. This can be achieved by : (a) A minimum
clearance between the wheel and atomizer casing. (b) Vanes are sized to
provide a sufficient atomization surface, and yet to possess a cross-section
area that reduces the air pumping capacity of the wheel to a minimum.
(c) Restrictions to air flow are designed into the wheel top surface and liquid
distributor. (d) Replacement of vanes by annular orifices (bushings or jets)
CHAMBER PRESSURE
REGION OF LOW
PRESSURE AT WHEEL CENTRE,
Figure 6.15. Pressure distribution at vaned atomizer wheel due to air pumping effects,
so sized that the feed liquid substantially fills the orifices when air passages
become virtually negligible.
In case (d), atomization characteristics often deteriorate by use of jets
instead of vanes for feed rates rated on vaned wheel loadings. Orifices
restrict air flow, but atomization surfaces become insufficient to handle
the feed rate effectively.
The usual arrangement for vaned wheels to prevent excessive air pumping
effects in industrial sized dryers is shown in figure 6.16. There is a minimum
clearance between the wheel and the atomizer drive casing. The air pumping
capacity of the wheel is met by the wheel drawing cool air from outside
the atomizer. The cool air flows into the low pressure regions above the
wheel. Hot air and entrained spray are no longer drawn into the top
clearance. The cool air drawn into the wheel interior also prevents partial
drying of product within the wheel.
(ii) Drying Air Entry. The manner of drying air entry into the drying chamber
is determined by the air disperser design, and controls the evaporation
rate of the atomized spray. The actual position of the atomizer with respect
130
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
AIR
FEED
FA N
FEED
AIR
FILTER
131
6
. W
,
1
usur
AIR
LIQUID PEED
PIPE
B.
FEED
FEED
AIR
C
COOL AIR
FLOWS TO
WHEEL
FEED
VANED WHEEL
Figure 6.16. Arrangement for supplying cool air to atomizer wheel. Cool air supply to meet
air pumping capacity is obtained by: A. Use of a small fan. B. Suction of air through a filter
using wheel's own suction ability. A minimum clearance between wheel and atomizer casing,is
maintained (see C) to limit air flow over wheel top surface.
to the air • disperser is an important factor in the efficiency of the dryer.
Dried product properties of a given material will vary according to the
positioning of the atomizer. Figure 6.17 illustrates various rotary atomizer
positions adopted in drying chambers and shown in relation to the air
disperser. Further details on drying air entry and air flow within the drying
chamber are given in chapter 7.
(iii) Particle Shape Change. In all cases, it is the final size of the dried product
that is of interest to the spray dryer operator. Apart from the importance
of atomizer performance, it is essential to know the relationship between
the size of droplets before and after drying. Unfortunately, there is no sound
basis to draw conclusions as feeds vary so much in their physical properties
and sprays dry in different manners.
The relationship between the size of wet spray droplet and its size when
in dried form can be expressed by a shape change factor (13). The shape
change factor covers any droplet shrinkage or puffing during evaporation.
AIR
--b-
— AIR
AIR
Figure 6.17. Positioning of rotary atomizer in relation to air disperser.
The relationship between wet and dry forms is represented by
D w = /3DD
(6.32)
where D w is the droplet size on atomization, and D, is the particle size in
dried form.
The shape change factor (/3) will vary for each product and each droplet
size making up the spray distribution. Drying temperatures also effect the
magnitude of /3. Values of fi are determined experimentally for a given
product by observing the evaporation of a specific wet droplet size in constant
air temperature conditions. When atomizers produce wet spray distributions
132
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
that can be equated to a distribution function, values of /3 can be obtained
from sieve or other analysis of the dried product. Comparison between the
dry particle distribution and the predicted wet droplet distribution gives an
immediate value of )3 for a given wet droplet size or for a given wet droplet
size increment.
The shape change factor (/3) proves useful data in cases of required dryer
scale-up. It is not uncommon to ascertain what alterations are required to
atomizer control for maintenance of a given product quality (i.e. particle
size), when dryer capacity is enlarged, feed concentration varied, or atomizer
wheel design changed. The wet droplet size on atomization follows the
relation
-t
D w oc
(6.33)
. (d) . (N)Fr (h)' (n)
Equation (6.36) can further be modified to include the dryer evaporation
capacity, as the feed rate can be expressed in terms of the evaporation
capacity and feed moisture content.
Thus
100
100 — 100/(1 + 0.01H)1
M = E[
or
(6.37)
M=E
ofy
and dried particle size is related to the operating variables through equations (6.32) and (6.33).
Values of the exponent for each variable can be taken from table 6.6.
Exponent values generally used for low vane loading conditions are :
,.
=
13
-
m()•2
1V1
L4
2
“2
r...2
'f
2
1
(6.35)
A relation covering atomization variables and feed properties, and their
interaction for maintenance of a given particle size can be derived from
equation (6.35), as
A,f0.2
'" 1
1
-ill 2
-10 . 2 L0.1 0.1 ro - 0.01 [,40.210.1„0.1m-0.6] [R
n
1 iv
'4 2 it2 "2 2 P2
[ "1 "1
(6.36)
(6.39)
Change in feed solids of the feed concentrate while maintaining a constant
dryer evaporative capacity and atomizer wheel design. The required wheel
speed to produce the same dried particle size is obtained from equation (6.39)
where E 1 = E2, d 1 = d 2 , n 1 = n 2 , and h 1 = h2 .
(6.34)
. N o.6
m0.2
„.1.
2
[Ni' 6 {E 2 (1 + 0.01H 2 )/0.01H 2 } 0.2
{.E 1 (1 + 0.011/0/0.01H i l' 2 $ 2 4 2 4' 1 4 1
N2
(
2
4 '4 i n?' N
CASE 1
or
2D0 2.
1
dVh?' i ni' 1 NV
Equation (6.38) provides a useful correlation to select wheel speed for
maintaining particle sizes on alteration of feed properties and interchange
of wheel design. This is evident by rearranging equation (6.38)
m0•2
r
1- {E 1 (1 + 0.01H )/0-01H } ° ' 21 rE 2 (1 + 0.01.1/ )/0.01H I 'l /3/2(6.38)
1_
The above values cannot be considered applicable to all products. For
certain products (for example, those exhibiting thixotropic flow properties)
different values have been shown through experience to more closely represent the effects of any variable change on droplet size. The most suitable
power values must be applied. In the vast majority of applications the
above values will suffice.
By combining equations (6.32) and (6.33) and applying the above power
values
fiD D oc d ru, •
0.01H
where E = evaporation capacity of dryer, H = percentage moisture in feed
(dry solids basis).
By substituting equation (6.37) in equation (6.36)
1
Exponent (p) of feed rate (M):
+ 0.2
Exponent (q) wheel diameter (d):
— 0.2
Exponent (r) wheel speed (N):
—0.6
Exponent (s) vane height (h):
— 0.1
Exponent (t) number of vanes (n): —0.1
133
.6
[Nr{(1 +
+ 0.01.11- 1 )/0.01H 1 r'fl 2
(6.40)
CASE 2
Change in dryer evaporative capacity while maintaining the same atomizer
wheel and feed properties. The required wheel speed to produce the same
dried particle size can be predicted from equation (6.39) where d 1 = d2 ,
h i = h2 , n i = n2, Pi = 13 2
N2'6
2 —
[m- 0.6 (
1
2) °.21
El
(6.41)
134
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
135
Therefore
CASE 3
Change in evaporative capacity accompanied by change in feed solids, but
maintenance of the same wheel design. d 1 = d2 , h, = h2 , n 1 = n2 .
Thus from equation (6.39)
6
N2'
= 15 000'6
[0_600(1 + 0.01($2)) 0. 01(82)} ° ' 2 100/3 1
{800(1 + 0.01(122))/0.01(122)} 0 ' 2 95fi 2
= 3204 x 1.191 x 1.05
NO.6
NINE2(1 0-01H2)/0•01H2r.2/3I
{E 1 (1 + 0.011/0/0.01H11
.2
132
(6.42)
CASE 4
Change in evaporative capacity accompanied by change in feed properties
and use of different wheel design. The prediction follows equation (6.39).
EXAMPLE 6.5
A rotary atomizer is operating successfully at 15 000 rev/min in a dryer of
800 kg/hr evaporative capacity. If the same wheel and atomizer drive is
transferred to a dryer of 1600 kg/hr evaporative capacity and operated on
the same feed concentrate, what wheel speed must be applied to reproduce
the same spray characteristics?
In equation (639) d 1 = d2 , n, = n2, h 1 = h2, 16 1 = P2'
Hence equation (6.41) is applicable
N' 6 = 115 000
0.6
(1600/800) 02 1
N ° ' 6 = 368.1
N = 18 900 rev/min
EXAMPLE 6.6
Calculate the required wheel speed of the atomizer as in example (6.5) to
maintain product particle size if, for operation in the larger dryer, the feed
concentration is raised from 45-55 % solids by weight. It is estimated that
the factor governing the relation between wet droplet size and its corresponding dry particle form will decrease 5 % as solids concentration is raised
to the new level.
Equation (6.39) reduces to equation (6,42) as d i = d2 , n, = n 2 , h, = h2 .
Data :
N, = 15 000 rev/min
E, = 800 kg/hr
E2 = 1600 kg/hr
0.95/3,
H2 = 55/45 = 1.22 lb/lb (122 % moisture dry solids basis)
H 1 = 45/55 = 0.82 (82 %)
N2 = 22 630 rev/min
(f ) Atomizer Design Requirements
Atomizer design is focussed upon the need to produce homogeneous sprays
over a wide range of feed rates from a machine that is thoroughly reliable,
and requires a minimum of preventative maintenance. The rotary atomizer
must be dynamically balanced. Feed liquid distribution on to the wheel
must be uniform, with no splashing on any wheel surface. Atomizer rotation
must be vibration-free. Vibration causes likelihood of frequent mechanical
failure. Smooth running of atomizers is of particular importance in large
industrial dryers. Out-of-balance wheel rotation becomes more pronounced
in atomizers designed for high peripheral speeds and large wheel liquid
loadings. Flexibility is of great importance in atomizer design. The atomizer
must handle a wide range of feed materials and yet have an operation
controllable to produce any desired product specification. In meeting these
requirements, many designs are offered by manufacturers.
Atomizer design can be divided into. three parts : (i) The atomizer drive.
(ii) The liquid distributor. (iii) The atomizer wheel.
An industrial atomizer features a spindle rotating in heavy duty, high
speed bearings that are fully lubricated. The wheel is mounted on the spindle
and securely fastened. As the spindle and wheel rotate, feed material is
fed to the wheel centre via the feed pipe and liquid distributor. The basic
features of the atomizer design has been illustrated in an earlier chapter
(figure 5.4). The wheel speed, feed pipe diameter and size of liquid distributor
are selected according to the feed material atomized. The atomizer can
handle feeds in solution, suspension, paste or powder form. The atomizer
drive can be at fixed or variable speed. The feed pipe can be heated for
handling liquids that tend to solidify at normal temperature levels.
(i) Atomizer Drive. The atomizer drive rotates the wheel within a specified
speed range and loading conditions. The drive can feature (a) the turbine
principle, (b) gearing to gear up an electric motor, (c) a direct driven electric
motor (fixed speed) or electric motor with frequency converter for speed
control, (d) a belt drive. All have their advantages and disadvantages,
whether they be reliability or cost. All drives must ensure that the operating
speed of the atomizer does not correspond with the shaft speed, where
natural vibration occurs. The various drives are shown in figure 6.18.
136
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
ELECTRIC MDTCR
The power requirement for driving a vaned atomizer wheel is directly
proportional to the square of speed at fixed feed rate, and not the cube of
speed, the usual relationship for fan and pump performance. The wheel
acts as an air pump or fan, but the common law does not apply. Increase
in speed does not increase the liquid feed rate, as this is controlled externally
of the atomizer.
The theoretical net power consumption for vaned wheels is given by
equation (6.43) (35)
C04.1PRESSED AIR PIPE
(a)
137
PK -=` 6.65 x 10 -12 M LN 2 (2d 2
(b)
d,2,)
(6.43)
where PK is kw, N is rev/min, d, d d is in, M L is lb/inin.
In practice, power consumptions of atomizer wheels will exceed values
predicted in equation (6.43) as further power is required to overcome
friction within the atomizer drive, and to power air through the wheel as a
result of the air pumping effect. There is no general correlation available
applicable to all atomizers as the power requirements of an atomizer wheel
and atomizer drive system depends upon the designs employed. It is usual
procedure for atomizer manufacturers to express the power requirement
in the form of power per unit increase in feed rate (water basis) quoted at a
reference wheel diameter and speed.
The power variation with changes in feed rate and wheel diameter at a
constant reference speed is expressed by
(c)
1 2
PR = K n
u1. .1[1
s III i
DELT DRIVE
a
HEAR PDX
)
FEED PIPE
1i
—%115 mff
1111 u
IM-
I
,
,.
(d)
VANED WHEEL
IN
(6.44)
R uR
where
MAR DOE Muil.
—
1 "
ELECTRIC HOVER
FEED PIPE
IMItail ir RIFE71
-
Fi ,1
SPIIJILE
I 'MED WHEEL
(e)
PR = required power at reference wheel speed
K = constant for given atomizer wheel and drive system
(power requirement/unit increase in feed rate)
•
M, = operating feed rate
MR = reference feed rate on which K is based
d = operating wheel diameter
d R -= reference wheel diameter
The total power requirement of the atomizer (P K ) is given by
K
P
Figure 6.18. Atomizer drives. (a) Turbine drive. (b) Direct belt drive. (c) Direct drive (electric
motor with frequency converter). (d) Gear and belt drive. (e) Gear drive.
P
R + P1
where P, --- idling losses of the atomizer.
(6.45)
138
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 139
Power requirements at speeds other than the reference speeds are obtained
from equation (6.46) using P i, from equation (6.44)
Ni 2
P2 = PR [-
(6.46)
NR
where
N = operating wheel speed
NR = reference wheel speed
P2 = power for required operation
Atomizer Drives for Fractional Feed Rates. In spray dryers for laboratory use,
chamber diameters are small. Very fine sprays are required to enable completion of evaporation in the limited residence times available. To obtain
fine sprays small wheels are rotated at very high speeds. Where air turbine
drives are used, speeds of the order 50 000 rev/min are reached. Electric
motor drives with gearing enable 40 000 rev/min to be obtained with
reliability. A vaned atomizer wheel with turbine drive is shown in figure
6.19(a).
Atomizer Drives for Pilot Plant, Semi Industrial Industrial Feed Rates. For
plants up to and including industrial applications involving moderate feed
rates, belt drives often power the atomizer. If wheel liquid loadings are not
high (e.g. drying low solid concentration feeds) a belt can drive the spindle
directly. A V-belt drive to the atomizer via a gearing system is a more
robust unit, however, and can handle higher wheel liquid loadings. Figure
6.19(b) shows an atomizer with a worm gearing system belt driven from a
35 HP motor. The 82 in (210 mm) diameter wheel rotates up to 15 000
rev/min (peripheral velocity = 335 ft/sec (110 m/sec)), where feed rates up
to 2.0 ton/hr can be atomized. With reduction in wheel speed, atomizer
capacity can be increased to 4.5 ton/hr at 10 000 rev/min. V-belts can handle
up to 60 HP drive requirements.
At higher capacities, direct drives to the atomizer are required. High
frequency motors can be connected directly to the atomizer spindle and with
suitable control gear, wheel speeds can be easily varied (376). High frequency
motors can handle up to 125 HP drive requirements. Where synchronous
electric motors are used, the spindle is powered via a gear box incorporating
helical gears and vertical shafts. The gear box power transmission is a
robust and reliable form of atomizer drive and is most suited to handle
large feed rates. Atomizer speeds can be altered by change of the gear
train in the gear box, but it is more common and convenient to control
speed by a fluid coupling mounted between the atomizer gear box and the
electric motor. An atomizer with a fluid drive is shown in figure 6.20. The
positioning of the fluid coupling beneath the electric motor is illustrated.
-
—
(b)
(a)
WA
(c)
'
(d)
Figure 6.19. Rotary atomizer drives for atomizer wheels. (a) Turbine drive (wheel diameter
shown = 50 mm). (b) Belt and gear drive (wheel diameter shown = 210 mm). (c) Helical gear
drive (wheel diameter shown = 160 mm). (d) Epicyclic gear drive (wheel diameter shown =
210 mm). (By courtesy of Niro Atomizer.)
140
THE PROCESS STAGES OF SPRAY DRYING
The fluid drive system is best incorporated with electric motor horse powers
of 60 HP and upwards.
Figure 6.19(c) shows an atomizer powered through a gear box by a motor
mounted over the spindle. Wheel diameters reach 10 in (250 mm) for
100 HP drives. Feed rates of 6 ton/hr can be atomized at 18 000 rev/min.
With reduction in atomizer speed capacity can be increased to 16 ton/hr
at 10 000 rev/min.
In high production dryers, for example, atomizer feed rates up to 50 tons/hr,
the drive design shown in figure 6.19(c) is more than adequate. However,
a smaller diameter wheel is used (61 in, 160 mm) plus strengthened bearings
and gearing for transmitting a power drive of 200 HP. Feed pipes are also
enlarged. At such high liquid loadings a maximum speed of 13 000 rev/min
is imposed. During operation lower speeds are usually adopted (e.g. 11 000
rev/min). Atomization characteristics at these lower speeds are maintained
ATOMIZATION
141
by increasing the vane area of the atomizer wheel. Many industries demand
even higher feed capacities, and atomizers with 600 HP drives are now
appearing. Such drives incorporate epicyclic gearing systems (figure 6.19(d)).
High rotation speeds place great demands on the lubrication of the
atomizer. Safety alarm circuits must protect the lubrication system. Special
precautions are taken in the atomizer design to prevent oil penetration
down the spindle into the atomizer wheel. Many designs incorporate air
pressure protection via the spindle (figure 6.21). Miscellaneous auxiliary
equipment is available to assist control of the atomizer performance.
Tachometers can record spindle speed and detectors can meter spindle
vibration. Atomizer wheel drives feature in patents (381)(387). Other
patents cover a sterilizable (383) and pressurized drive (384).
VENTING
OF DRIVE
CASING
VENTING
OF SPINDLE
COMPRESSED
AIR OR NITROGEN
FILTERED AIR
UNDER PRESSURE
ELECTRIC
MOTOR
VARIABLE SPEED
DRIVE
( FLUID COUPLING1
LABYRINTH
ATOMIZER
DEAR BOX
AIR FLOW
DOWN SPINDLE
ATOMIZER
DRIVE
VANED WHEEL
Figure 6,20. Rotary atomizer with variable speed control through a fluid coupling.
Figure
6.21. Spindle pressurizing. Prevention of drying gases penetrating atomizer drive.
(ii) Liquid Distributor. The liquid distributor is placed around the base of
the spindle in close proximity to the wheel. This is shown in figure 6.22.
Liquid flows on to the wheel as near to the centre as possible. The liquid
distributor is often supplied from one or more feed pipes, and designs smooth
out the flow around the periphery of the distributor and ensure equal flow
142
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
143
FEED IN
ATOMIZER
DRIVE CASING
LIQUID
DISTRIBUTOR
O
VANES
HOLES
A-A
B-B
WHEEL
SPINDLE
Figure 6,22. Positioning of liquid distributor with atomizer wheel.
into each vane of the wheel. Methods used widely involve (a) imparting
rotary motion within the distributor and (b) the formation of a reservoir
within the distributor. The reservoir feeds several constant head outlets
in the form of holes drilled in the distributor plate. Liquid distributor
designs are shown in figure 6.23. Patented designs include (402) (403).
The distributor must be leak-proof. Liquid leakage on to the top surface
of the wheel results in problems of semi-wet or scorched deposits forming
on the atomizer wheel and drive surfaces.
The liquid distribution velocity to the wheel must not be so high that
splashing occurs. Splashing causes liquid to find its way onto the top of
the wheel resulting in problems associated with a leaking distributor. For a
selected wheel speed, the distributor must never permit any vane to approach
flooding conditions. Imperfect atomization and heavy deposit formation
on the atomizer result. Liquid velocities commonly used in distributors
range between 9-15 ft/s (3-5 m/s). The liquid distributor is designed together
with the atomizer wheel it accompanies. Use of an incorrect design can
cause flooding. Flooding prevails when liquid is distributed onto the wheel
surface in such a way that the bulk flow arriving on the wheel surface cannot
be accelerated away from the central area fast enough. Accumulation occurs
with flooding.
(iii) Atomizer Wheels. Numerous atomizer wheel designs are available.
This is illustrated from the available patents. Designs are claimed to give a
spray that dries to a product of desirable characteristics, General wheel
Figure 6,23. Liquid distributor designs. (a) Liquid distributor with tangential entry. (b) Liquid
distributor with angled slots forming a swirl chamber. (c) Liquid distributor with constant
head outlets.
design appear in patents (385) (386) (387) (388), a high bulk density milk
wheel (389), wear resistant wheels (44) (391) (393), multi-stage wheels (394)
(395) (396) and powder—liquid wheels (397) (398) (399).
Atomizer wheels produce sprays of high homogeneity over a wide range
of mean droplet sizes. Control of size distribution is carried out by change
in wheel speed. With excess power available for wheel speed increase, size
distribution can be maintained for higher feed rate conditions. Such flexibility of operation ensures dried product qualities to be as desired. Atomizer
wheels have many important operational features. The chance of clogging
is virtually non-existent given correct wheel selection. The projection of
spray into the drying air gives effective spray—air contact for achieving fast
drying rates. Maintenance is minimal. These features often tip the scales
in favour of rotary atomization in circumstances where alternative atomization techniques could just as well be applied to a product. The atomizer
wheel and the drive need precision manufacture, but the operation features
offset the high cost of the atomizer.
Atomizer wheels are designed to withstand the centrifugal forces they
are subjected to during rotation, and designs in fact carry suitable safety
144
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
factors. Due to the elasticity of the wheel material, distortion can take place
during rotation. Allowance for distortion enables wheel dismantling for
cleaning or parts replacement.
The wheel is placed on the end of the spindle. The spindle end is conical
so as to facilitate correct wheel centring, easy mounting and removal. The
wheel must then be securely fastened to the spindle with suitable locking
devices. The wheel is positioned with a suitable clearance between distributor and wheel. Under no circumstances must the wheel rub the distributor
during rotation. This can cause (a) sparks and initiate explosion within the
drying chamber, (b) atomizer drive spindle fracture, (c) hot spots on the wheel
surface, leading to formation of burnt particles in the product.
(a)
(b)
(c)
(d)
Figure 6.24. Designs of vaned atomizer wheels. (a) Straight vanes. (b) Curved vanes (wheel
top cover removed). (c) Wear resistant vanes (inserts), (d) High capacity vanes. (By courtesy
of Niro Atomizer.)
145
Selected Atomizer Wheel Designs: Standard Atomizer Wheel (Vaned). This is
the multi-purpose wheel having straight vanes (figure 6.24(a)). There are
usually 18-36 vanes. The design is cheap to fabricate and requires no special
liquid distribution techniques. The wheel is commonly supplied with pilot
plant and experimental dryers, being capable of operating over a wide range
of feed rates and periphery speeds. The straight channels are virtually
clog-free, but the wheel does pump large quantities of air. The vanes can
be shaped to form rectangular or oval flow channels.
Nozzle Atomizer Wheel. A nozzle wheel features small diameter orifices
(jets) instead of channels (476). The air pumping effects are substantially
reduced. The wheel finds application where product aeration must be
minimized. The limited orifice cross-sectional area for liquid discharge
allows only relatively small feed rates to be effectively atomized.
Atomizer Wheel for High Bulk Density Products. Present day operations involving high tonnage production often require high bulk density dried
products, e.g. dairy products. Designs with curved vanes produce high
bulk density powders at high feed rates, figure (6.24(b)). In the case of skim
milk powder, 7-10 % increase in bulk density can be achieved over a straight
radial vaned design. Apart from increase in bulk density, curved vanes
reduce the amount of occluded air in the product, a feature of special importance to products containing fat, e.g. whole milk powder.
Atomizer Wheel for Corrosive Feeds. In cases of liquids that are corrosive
but not abrasive, it is usually worthwhile to fabricate the whole wheel in a
metal resistant to corrosion. Hastelloy, tantalum and titanium are commonly used.
Atomizer Wheel for Abrasive Feeds. Problems of excessive wear on wheel
surfaces when abrasive feeds are atomized are overcome by incorporating
resistance surface (inserts) within the inner wheel body. Figure 6.24(c)
shows a wheel with resistant surfaces on each straight vane. The inserts
are replaceable. The layout permits particle size distributions, characteristic
of straight vaned wheels, to be obtained with abrasive feeds. The importance
of these surfaces is illustrated in figure 6.25 where the condition of a standard
straight vaned stainless steel wheel is shown after abrasive foreign matter
entered the feed accidentally and was passed to the atomizer for a few hours.
Use of inserts would have prevented such wear.
Atomizer Wheel for Abrasive and Corrosive Feeds. Abrasive and corrosive
feeds pose the gravest operating conditions for an atomizer wheel. The
high feed velocity over the wheel surfaces causes rapid wear unless the surfaces
are protected. All surfaces likely to come into contact with the feed must be
adequately protected from abrasion and corrosion. A wheel as shown in
figure 6.24(c) can be used, but if the feed is highly abrasive and corrosive,
regular maintenance and insert replacement is required. A novel and
146
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
147
wear over the entire surface. This is accomplished by rotating the bushings
90 degrees at a time. For further details on the maintenance of the wheel,
the reader is referred elsewhere (45).
The wheel design creates several thotand hours of operation per bushing
before replacement becomes essential. Such extended operation periods
apply to the most abrasive of mineral ore concentrates that are currently
spray dried in large tonnages (45)(46)(47). Examples include flotation
concentrates from the mineral ore industry. Molybdenum disulphide,
clays, fertilizers, cement and cryolite can also be handled.
For the wheel design in figure 6.26, any wear is restricted to the bushing.
To reduce the cost of bushing replacement, the wheel is fitted with the
minimum number of radial bushings for obtaining the specified dried product
particle size. Reduction in the number of bushings used in a wheel will
increase the coarseness of the resulting spray. Where coarse sprays are
acceptable, the minimum bushing requirement offers attractive savings in
operational costs. If fine dried particles are desired, the number of bushings
are simply increased. Wheels with up to 24 bushings are not uncommon.
Wear resistant bushings or surfaces used in wheel operation are
usually carbides, when feeds are abrasive and corrosive. In less demanding
Figure 6.25. Abrasive wear on vaned atomizer wheel.
recently patented antiwear arrangement (44) completely overcomes these
problems.
A wheel with such an arrangement is shown in figure 6.26. The atomizer
wheel features wear-resistant sintered bushings, so placed to protrude
within the inner chamber of the wheel. Such arrangement produces a
protective layer of solids to be formed right from the start of the spray
drying process. Solids separate out from the feed as a layer along the outer
wall of the inner chamber. The thickness of the layer corresponds to the
projection distance of the bushings. The protecting layer of solids, together
with the use of wear-resistant bushings and a wear-resistant base plate
enables the wheel body to be fabricated in stainless steel and yet possess a
long operating life. The atomizer wheel possesses the most effective and
cheapest built-in abrasion control, since the abrasive solids in fact provide
their own protective layer.
Use of this wear-resistant bushing and base-plate arrangement in a
conventional wheel body prolongs the life of the body by several hundred
ti mes when operating with abrasive and corrosive feeds. Wear occurs
slowly at the surface of the bushings in direct contact with feed concentrate,
but bushing life is quadrupled by repositioning the bushings to equalise
00.00000 0
Affair
MEN
Wheel hub
1. Wheel
2. Inner chamber
3. Wheel periphery
4. Wheel base
5. Wheel top plate
6. Annulus for feed distributor
7. Wear resistant bushing
8. Steel bushing
9. Bushing recess
10. 0-ring seal
11. Wear resistant base plate
12. Protective layer of concentrate
13. Outer wall of inner chamber
Figure 6.26. Atomizer wheel with bushings for abrasive and corrosive feeds (45).
148
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
conditions alumina is suitable. Ceramic bushings can withstand higher
centrifugal forces than carbides. For the severest of conditions silicon carbide
is adopted as a first-rate all-round performer. Tungsten carbide has good
wear resistance with neutral solutions, but it is not resistant to strong acids
or alkalies (i.e. PH values, 0-5 and 9-14). Acids attack the cobalt binder.
Tungsten carbide is susceptible to oxidizing acids. Chromium carbide has
less wear resistance but good corrosion resistance. Use of a nickel binder
renders the carbide inert to nitric acid.
Large Capacity Atomizer Wheel. A double wheel design is used for feeds that
provide no atomization difficulties, but where high feed rates must be
handled. The wheel consists of two levels (tiers) of vanes, see figure 6.24(d).
A double liquid distributor distributes the feed equally into each tier of vanes.
By using two or more tiers, liquid loadings on each vane can be maintained
low, achieving complete atomization while total feed rates are high.
(g) Application of Atomizer Wheels. Applications of atomizer wheels are
found in many major industries, including, for example, ceramics, dairy
products, dyestuffs, foodstuffs, inorganic chemicals, mining and pharmaceuticals. Such varied application is due to atomizer wheels being able to
handle all feeds that have good flow characteristics and a molecular structure
that permits break-up of liquid into individual droplets. Through the correct
choice of wheel design, feed liquids, slurries and pastes having either heat
sensitive, thermoplastic, abrasive, corrosive or high viscous properties
can be successfully atomized.
Although one atomizer unit is used per drying chamber, there is no
reason why multi-atomizer units cannot be applied in very large drying
chambers. Where multi-atomizer units are used, drying air is supplied
around each atomizer wheel.
Atomizer wheels are incorporated with nozzle atomizers (83) in a drying
chamber to produce products of special size distributions.
6.5.4. Disc Atomization (Vaneless Bowls, Cups, Plates)
(a) Introduction
Inverted bowls, cups and plates are classified as vaneless disc atomizers.
Unlike atomizer wheels, friction between the liquid and the disc surface
is increased to prevent liquid slippage over the surface. Liquid is fed on the
underside of the plate or the internal surface of the cup or bowl and is
pressed against the surface as liquid flows outwards to the edge. The disc ,
can consist of single or multi-tiered plates. A multi-tier plate disc is shown
in figure 6.27. A cup atomizer is shown in figure 6.28.
The difference between bowl, cup and plate-type atomizers lies in their
cone-angle. (The cone-angle is defined as the angle the release surface of
the atomizer makes with the axis of rotation.) The plate disc cone angle
approaches 180° giving the atomizer a flat design of large diameter. The
149
Figure 6.27. Disc atomizer (plate type, multi-tier). (By courtesy of Niro Atomizer.)
cup atomizer has a much smaller cone angle giving a longer design of smaller
diameter. The bowl atomizer lies in between.
At the operating speeds of rotation, feed liquid is distributed on to the
disc and attains rotation due to the friction between the liquid and disc
FEE D
LIQUID DISTRIBUTOR
Figure 6.28. Disc atomizer (cup type).
150
THE PROCESS STAGES OF SPRAY DRYING
surface. The liquid flows outwards, forming a thin liquid film over the entire
contact area. With no vanes present to restrain liquid flow, liquid issues
from the entire disc periphery.
The mechanism of atomization for plate, bowl and cup type atomizers
is similar to that already described in section 6.5.2. Disc design, rotation
rate, feed rate and liquid properties (density, surface tension and viscosity)
intercombine to influence the final disintegration of liquid into a spray. At
low feed rates and low rotation rates atomization is by direct droplet formation. This mechanism changes with increasing feed rate to one of
disintegration of liquid ligaments. When the ligaments can no longer accommodate the liquid supply arriving at the disc edge, a film is formed around
the complete disc periphery. Sprays are formed by the break-up of this
fil m. This is known as the velocity spraying mechanism. At low disc peripheral speeds, sprays of wide size distribution are formed. Increase in
peripheral speed improves spray homogeneity. The atomization mechanism
stages are illustrated in figure 6.3. Feed rate is expected to have a greater
influence if droplets are formed from the velocity spraying mechanism
rather than from disintegration of liquid ligaments. This influence is through
control of sheet thickness by the feed rate.
For optimum performance of vaneless disc atomizers, the liquid layer
should be uniform over the entire disc surface. These conditions are obtained
through uniformity of liquid supply, vibrationless rotation and smooth
disc surfaces. A further prime requirement concerns the manner of liquid
distribution on to the disc. The moment liquid reaches the disc surface,
it must undergo immediate centrifugal acceleration that renders gravitational
effects negligible. Centrifugal forces must exceed ten times gravity for effective
atomization. Atomization mechanism of cup atomizers is described by
Dombrowski and Munday (30).
(b) Fluid Flow over Vaneless Discs
Liquid flow over plate, bowl and cup type atomizers is similar. Liquid
extends over the atomizer surface as a thin film. Final release of liquid in
spray form is prominently in the horizontal direction. The lips of both types
can be shaped to provide a small liquid axial velocity component on discharge.
For discs operating under spray drying conditions, a liquid film is present
at the disc edge, but the film thickness decreases rapidly as the liquid moves
away from the edge. The film thickness, once extended outwards becomes
dependent upon flow rate, disc rotation and disc diameter rather than
liquid viscosity and density. The liquid properties govern film thickness
on the disc surface. Droplets formed by the disintegration of this film move
away from the disc with a velocity resultant of the radial and tangential
components acquired on point of release.
ATOMIZATION
151
Radial Velocity (V r). The radial velocity varies according to the Hinze and
Milborn (48) equation
[Pico 2 Q 2 sin 0 ' 13
(6.47)
Vr =
127r2rth
]
which rewritten in terms of disc speed, becomes
Vr
[2/3 1 Q 2 N 2 sin 01 1 /
341
=
3
3
(6.48)
3
where Tir is ft/min, p i is lbs/ft , Q is ft /rnin, d is ft, 1.4 1 is lb/ft min. For a
plate-type disc, the semi-cone angle approaches 90°, thus sin 0 = 1.
Frazer (28) regrouped equation (6.48) in terms of liquid viscosity expressed
in centipoise, and lir in ft/sec. The conversion factor becomes
11/3
=0.043
60 L3 4 x 10-2
The equation according to Frazer (28) for a plate-type disc thus reads
N2n211/3
(6.49)
yid
Tangential Velocity (V 1 ). The tangential velocity component depends upon
the velocity acquired by the liquid through frictional effects with the rotating
disc surface. The velocity can vary from that of the disc peripheral velocity
(nNd) at low feed rates to fractions of the peripheral velocity at high feed
rates. Conditions of severe slippage have been given in equation (6.7).
These are conditions where the tangential component becomes less than
one half of the disc peripheral velocity. As a guide for intermediate slippage
conditions, for values of (M/ tord) in equation (6.7) of 500, 1000, the corresponding liquid tangential velocity can be taken as 0.8, 0.6 of disc peripheral
velocity (14).
The release velocity (V resultant) and angle of release follow equations
(6.22) and (6.23).
EXAMPLE 6.7
A five inch plate type disc is atomizing 10 lb/min of liquid at 15 000 rev/min.
What are the liquid release characteristics at the disc edge?
Radial Velocity (K)
Use of Frazer equation (6.49) (sin 0 = 1) where
p i = 62.4 lb/ft'
N = 15 000 rev/min
Q = 10/62.4 ft 3 /rnin
d = 5/12 ft
/./ 1 = 1.0 cP
V= 0.043
152
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
Substituting in values
41 ft/sec
Tangential Velocity (V T )
Degree of slippage of liquid over disc. Referring to equation (6.7)
[10 x 60 x 121
---- 460
1 x x5
(
153
to break-up. The ligament diameter can be deduced theoretically by applying
continuity conditions to flow in each ligament. Flow per ligament per unit
time multiplied by the number of ligaments issuing from the disc edge
equals the total feed rate. Hinze and Milborn (38) propose a semi-empirical
relation for the number of ligaments (Z) present at the disc edge. Based on
experimental data from cups, Z can be equated to :
[p1 N2d31 5/ 12 [1 1/6
(6.54)
Z = 0.678
ti?
Partial slippage conditions as 460 < 1440. Hence use
where N is rev/min, d is ft, kt, is lb/ft min, a is dynes/cm.
VT = 0.8 Vildi se )
Disc peripheral velocity
Table 6.13. Relations to Predict Droplet Size from Vaneless Discs
VT(disc) = it dN = 326 ft/sec
Therefore
VT = 326 x 0.8 = 260 ft/sec
Release velocity ( V.iesuitant) (equation (6.22))
2 + 412)1/2
Vi-esuitant = (260
Atomization
mechanism Reference
[
Direct drop
formation
(49)
Ligament
formation
(48)
Sheet
formation
(49)
263 ft/sec
Number
Equation
3011
DAy =
“3
2
27Ep i @rdN) sin 0
D is ft, Q is ft 3 /min, p i is lb/ft 3 , N is rev/min, d is ft,
p i is lb/ft min
512 mad) — 111 11/2
(
( Q (piN2d3)
0.77
DAy =
2
Nd
a
Units as in equation (6.50)
-
Release angle (a) (equation (6.23))
tan -1 (41/260) = 9°
Note: The resultant discharge velocity can be in practice equated to the
tangential component, as 1/T »
(c) Droplet Size Prediction
Droplet size produced by vaneless disc atomizers varies according to the
atomization mechanism applicable to the conditions of disc operation.
Relations to predict droplet size are given in table 6.13. For other relations
the reader is referred to Dombrowski and Munday (30).
For the case of atomization by direct droplet formation (at low feed rates
and low disc rotation) droplet size can be equated approximately to the
thickness of the liquid film at the disc peripheral surface. Equation (6.50),
table 6.13 represents the order of droplet size formed. At the point of droplet
detachment, however, the retaining thread of liquid also disintegrates into
smaller satellite droplets, but these form only a small percentage of the total
liquid flow.
At higher feed rates, where droplet formation is by ligament disintegration,
the resulting droplet sizes are the resultant of mechanisms described in
section 6.5.2. Photographic evidence (48)(49) has shown ligament disintegration from vaneless discs to conform to the Rayleigh instability
theory, where droplet size can be obtained from the ligament diameter prior
1.06M
5
2
-6
1.91 x 10 (Nd) 1•
(CIA4)°'66
PI [(il/P1)
3
D is microns, M is lb/hr, p i is gicm , a is dynes/cm,
d is in, µ l is cP
D Ay =
6
6.657
DAy = [ Tr
(6.51)
(6.52)
°- 2 5
r
(31)
(6.50)
1-5
0.33
(6.53)
22
r(o
D is cm, r is cm, co is sec', p i is g/cm
3
The use of equations relating break-up of ligaments automatically involves
the limitations of the ideal system it described. Therefore for a vaneless
disc, droplets produced can be approximated to the diameter of the ligament.
A relation for obtaining droplet size in this manner is given in equation
(6.51). The elongation of the ligament as it moves away from the disc edge
will produce smaller droplet sizes than obtained from direct droplet formation.
On continued increase of feed rate, a point is reached when the ligament
cannot handle the volume flow and a sheet of lig aid forms around the disc
edge. Atomization is governed by the mechanism of sheet disintegration
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
(velocity spraying). The transition between ligament and sheet conditions,
as expressed by Hinze and Milborn (48) follows the relation
[ pig ] [ p1co 2d31 0.6 [ 1,1 2 0.167
--- constant
(6.55)
crd 3
p l ed
a
For values of the constant exceeding 1.77 (fpm units) sheet formation around
the disc edge prevails. The droplet size resulting from the disintegration of
the sheet can again be related to the sheet thickness at the point of break-up
although the nature of the liquid break-up by air forces results in greater
range of sizes produced. Sprays produced from sheet disintegration are
generally less homogeneous than from ligament or direct break-up
mechanisms. For sheet disintegration the average droplet size can be
calculated from equations (6.52) and (6.53).
EXAMPLE 6.8
Calculate the average droplet size obtained from the atomizer operating
as in example 6.7.
At 15 000 rev/min the atomization mechanism will be by disintegration
of liquid sheets at the disc edge.
Method I: Use of equation (6.52),
Data
d = 5 in
M = 600 lb/hr
= 1 g/cm 3
a = 74 dynes/cm
= 1 cP
N = 15 000 rev/min
substituting values in equation (6.52)
DAv
1.0[(1.0)
0.25
1.06 x 600
(74 x 600) 0.66 + 1.91 x 10 -6 (75 000)9"
1.06 x 600
(13 350) ° '
= 55 micron
(d) Performance of Vaneless Discs
The performance of vaneless discs follows similar trends described previously
for atomizer wheels.
Effect of Feed Rate on Droplet Size. Mean droplet size is proportional to
feed rate. The exponent has been shown to vary according to the atomization
mechanism involved, but no clear conclusions have been drawn. Results by
Waltman and Blyth (40), Friedman and co-workers (20), and Herring and
155
Marshall (37) have reported exponent values to vary between 0.1 and 0.4
for low viscous feeds (1-15 cP) feed rates 5-60 lb/min and disc peripheral
speeds between 150-210 ft/sec (46-64 m/sec). Meyer (41) operated inverted
bowls at higher peripheral speeds (460 ft/sec) and reported that increase in
viscosity from 330-6800 cP increased the exponent from 0.2 to 0.4 at feed
rates under 16 lb/min of a thixiotropic slurry.
Mean droplet size is generally accepted as being proportional to the
0.2 power of feed rate at constant disc speed, for commercial conditions
where low viscous Newtonian feeds are involved. This is shown in figure
6.29 as the variation of Sauter mean droplet size with feed rate of an aqueous
solution.
E
tij
10000 RPM
200
N
12000 RPM
U)
15000 RPM
23000 RPM
SAUTER MEAN
154
10
1.0
0.1
5.0
FEED RATE (got /min )
12 cm DISC. SINGLE TIER.
Figure 6.29. Effect of feed rate on mean droplet size from disc atomizer (plate type).
Effect of Disc Speed on Droplet Size. Mean droplet size is generally accepted
as being inversely proportional to the 0.6 power of disc speed under commercial operating conditions. This is illustrated in figure 6.30 by the Sauter
mean droplet size variation with disc speed of spray produced by a 10 cm
two-tier plate disc.
Effect of Peripheral Speed on Droplet Size. Mean droplet size is inversely
proportional to disc peripheral speed. The exponent has been shown to
vary according to the atomization mechanism. At low feed rates of low
•
156
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
2 TIER . 10 cm die DISC
S. 0 of FEED = 164
157
5.0 = 1.5
15,000 RPM
FEED RATE . 2 litres/ rni n
DISC DIAMETER .120 mrn.
500
959 5
99
r
A
i Y
SAL1TER MEAN
1.0 Kg /m in
SLOPE — 066
z 90
;T- 80
10 Kg/min
4 , 5 Kg/min
'I TIER
6 95
•
rt 70
I- 50
50
(..D 40
1— 30
1g 20
CL 10
20
2,000
S,000
10,000
w
5
•
L.)
1
20.000
DISC SPEED .(RPM)
Figure 6.30. Effect of atomizer disc speed on mean droplet size.
viscous feeds, direct droplet or ligament disintegration mechanisms apply
and exponent values between — 0.6 and — 0.8 have been reported by Waltman
and Blyth (40), Friedman and co-workers (20), and Herring and Marshall
(37). With creation of a sheet disintegration mechanism (velocity spraying)
by operating with high viscosity feeds (6800 cP) Meyer (41) reported an
increase in exponent to — 2.1, for a feed rate 10 lb/min (4.5 kg/min) and
peripheral speed 420 ft/sec (130 m/sec). Reduction of peripheral speed below
330 ft/sec saw a substantial reduction in exponent down to — 0.7. An
exponent of — 0.8 is generally accepted as the working value in commercial
operating conditions.
Droplet Size Distributions. Droplet size distributions of plate and inverted
bowl-type discs have been reported by Herring and Marshall (37) to follow
equation (6.30). The wetted periphery for plate discs equals nd per tier,
and replaces the vane wetted periphery in equation (6.30). The increase in
the number of tiers per disc will decrease the mean droplet size of the spray
provided liquid distribution to each tier is equal. Data from the spray
drying of sodium orthophosphate solution show the effect of the number
of tiers, as represented in figure 6.31.
For a given operating condition sprays formed from a plate-type disc
are coarser than sprays formed from vaned wheels. This difference in
coarseness is not so striking at plate speeds over 12 000 rev/min and feed
I
20
I
50
DROPLET SIZE ID)
100
200
micron )
Figure 6.31. Effect of number of plate tiers of a disc atomizer on particle size distribution.
rates giving low disc loadings. Under these circumstances, spray homogeneity compares favourably with that of sprays from vaned wheels. It is
at low speeds of rotation that atomization by plate-type discs produce
coarse sprays having greater homogeneity than that obtained from low
speed vaned wheels. Atomization appears more controllable at low speeds
when using vaneless discs. It is this characteristic that gives plate discs
their main area of application, the production of coarse powders without
presence of fines. This is illustrated in figure 6.32, where droplet size distributions for the atomization of sodium orthophosphate solution at low
speeds are plotted for a plate disc and vaned wheel respectively.
(e) Application of Vaneless Discs. Vaneless discs of the bowl- and plate-type
are often applied when coarse particle size powders are required at high
production rates. The multiple tiers of plates enable their use in high capacity
dryers. Atomizers of the cup type are at present used under conditions of
much lower liquid feed rates.
Applications include detergents, ceramics and clays, although the platetype design does not lend itself too well to abrasive materials.
158
ATOMIZATION 159
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZER SPEED 6000 RPM
ATOMIZER SPEED10,000 RPM
DIAMETERS 120MM
-- DIAMETERS 120 MM
A
_
\
.
6
SU
-
3 TIER PLATE DISC
_
3 TER PLATE DISC
6
6
8
.
.
24VANED WHEEL
-
a
24 VANED WHEEL
3
.
i
.
DROPLET
. , I f
t
50
100
200
SIZE ID) {MICRON
.
1
400
I
I
50
100
200 300
DROPLET SIZE I D1 I MICRON I
Figure 6.32. Size distributions from atomizer wheels and discs.
Vaneless discs fill the role of an atomizer for specialized applications.
Although overshadowed by vaned wheels and nozzle atomizers they are
used to advantage when coarse but fairly homogeneous sprays are required
at low feed rates, and/or when a low speed rotary atomizer drive is available.
Patents covering vaneless discs include (392), a heated disc (400) and a
rotary cup (401).
6.6. Nozzle Atomization
6.6.1. Introduction
The function of the nozzle atomizer is the acceleration and disintegration
of bulk liquid flow, terminating with the dispersion of the resulting droplets
to form a spray. A single orifice or ejector cannot be considered an atomizer.
The liquid is disintegrated if the liquid jets are turbulent enough, but the
droplets are not dispersed by the action of the nozzle.
Energy transfer during nozzle atomization is very inefficient (18) (30).
Arising out of attempts to improve (a) the degree of atomization, (b) nozzle
flexibility and (c) spray trajectory, various nozzle types have been developed.
Three types will be covered, namely, those utilizing either pressure, kinetic
or sonic energy for liquid bulk break-up.
The centrifugal pressure nozzle, which is based upon pressure energy
utilization is the most common of the three types. Spray size distribution
characteristics, energy transfer, the spray ' pattern and design simplicity
have established such nozzles in many and varied spray dryer applications.
The centrifugal nozzle is discussed in section 6.6.2. Nozzles based on kinetic
and sonic energy are discussed in sections 6.6.3 and 6.7.3. Atomizers that
incorporate both the principles of kinetic nozzles and rotating cups are
mentioned in section 6.6.4.
6.6.2. Centrifugal Pressure Nozzle
(a) Theoretical Principles
The principle of the pressure nozzle is the conversion of pressure energy
within the liquid bulk into kinetic energy of thin moving liquid sheets.
The sheets break-up under the influence of the physical liquid properties
and by the frictional effects with the medium into which the liquid sheet is
discharged. Invariably the medium is air. Liquid sheets range in thickness
from 0.5 to 4 micron. Sheet instability is rapid as any influence that causes
fluctuations in the order of this thickness promotes sheet disintegration.
Pressure energy conversion to kinetic energy* in a centrifugal pressure
nozzle is carried out in such a way that the resulting liquid motion is rotary.
This arrangement enables greater energy transfer leading to improved
atomization. The principle parts of a centrifugal pressure nozzle are shown
in figure 6.33(a). External views of typical nozzles are shown in figure 6.33(b).
The mechanism of sheet formation follows the principles described earlier
in section 6.2. Pressure applied to the liquid within the nozzle forces the
liquid out of the orifice. Rotation imparted to the liquid upstream of the
orifice develops a conical spray as liquid passes out from the orifice. The
sheet velocity is constant, and the thickness of the sheet diminishes as the
cone develops. There is a maximum sheet length for a given pressure.
Increase in pressure reduces the sheet length. Increase in viscosity acts to
lengthen the sheet, whereas the increase in surface tension has the opposite
effect. Increase in sheet velocity and increased turbulence caused by frictional effects with the discharge medium act to reduce sheet length. Reduced
frictional effects from spraying into a medium at sub-atmospheric pressure
result in elongation of sheet length.
Sheet lengths can hardly be seen visually in commercially sized nozzles
operating in spray dryers. High nozzle velocities and the resulting turbulence
on atomizing liquids of low viscosity provide very short lengths. High viscous liquids, subjected to high pressures which maintain high nozzle velocities
again prevent development of the sheet length to any degree.
* Less than 0.5% of applied energy is utilized in liquid break-up (18). Virtually the whole
amount is imparted to the liquid at kinetic energy. The horsepower requirement for pressure
nozzles can be expressed (30).
f ti = 7QAP x 10' HP
where
AP is p.s.i.
Q is imp gal/min,
160
THE PROCESS STAGES OF SPRAY DRYING
NOZZLE
BODY
ATOMIZATION
NOZZLE
HEAD
161
LIQUID FLOW
LIQUID FLOW
LIQUID
FLOW
ORIFICE
SLOTTED
SWIRL
INSERT
(a)
CI NCLI NED
SLOTTED
INSERT
A.
B.
r SPIRAL
GROOVE
INSERT
swIRLt61
INSERT
Figure 6.34. Inserts for generating liquid rotation within centrifugal pressure nozzles.
Three types of inserts are shown in figure 6.34. Centrifugal pressure
nozzles that incorporate tangential feed entry or use inserts are shown in
figure 6.35. The theory of centrifugal pressure nozzles is discussed in detail
by Marshall (35) and Green (482). A complete mathematical treatment is
presented.
ORIFICE
(b) (i)
(b)(ii)
Figure 6.33. Centrifugal pressure nozzles. (a) Principle parts. (b) Typical nozzles ; (i) Delavan
type (sdl) spray drying nozzle, (ii) Delavan type (sdx) spray drying nozzle.
The internal design of the centrifugal pressure nozzle is critical in forming
the liquid into thin conical sheets, capable of breaking up with a minimum
of external disturbance. Methods of imparting rotary motion within the
nozzle include use of spiral grooved inserts, inclined slotted inserts, swirl
inserts, or simple use of tangential flow entry. The slotted inserts feature
multiple feed inlets into the orifice, but each slot is of small cross-sectional
area, and prone to clogging. An effective feed strainer must be incorporated
in the feed line or nozzle body, and be regularly cleaned. The swirl insert
has a single feed inlet. Like the nozzle with tangential feed entry, a natural
free vortex flow is produced with a minimum of friction. The swirl channels
have comparatively large flow passages, and enable such nozzles to handle
high solids feed without undue wear or clogging. Highly abrasive or corrosive
feed, however, wear the nozzle whatever swirl mechanism is adopted,
although wear rates are minimized by use of special materials (e.g. carbides).
(Patented nozzle designs are given in references (442) to (450).)
SWIRL
CHAMBER
FLOW CHANNEL
TANGENTIAL
TO SWIRL CHAMBER
C.
b.
E
©
1
2
111E1
c.
1
©
3
4
EU ( 1
2 3
0
L,
a
6
5
1
7
8
0
6
7
a
Figure 6.35. Centrifugal pressure nozzle construction. (a) Centrifugal pressure nozzle with tangential liquid entry (62). (b) Centrifugal pressure nozzle with swirl inserts (61). (c) Centrifugal
pressure nozzle with inclined slotted inserts ; 1. nozzle, 2. washer, 3. orifice, 4. swirl insert, 4a. inclined slotted insert, 5. end plate of distributor, 6. liquid distributor, 7. washer, 8. nozzle body (61).
162
ATOMIZATION 163
THE PROCESS STAGES OF SPRAY DRYING
(b) Spray Patterns
Sprays formed by the designs shown in figure 6.35 have hollow cone patterns
symmetrical with respect to the nozzle axis. The pattern has a clearly defined
spray angle on discharge from the orifice but the angle decreases and becomes
less well defined away from the orifice due to effects of the ambient air
conditions in relation to the momentum of each droplet. The spray angle
is shown diagrammatically in figure 6.36. The hollow cone spray has an
air core at the centre of the orifice. The air core is formed as a result of
liquid rotation within the nozzle. Greater liquid rotation creates a wider
spray angle, and larger air core. Increase in spray angle signifies decrease in
nozzle capacity as shown in figure 6.36(d), by a plot of spray angle versus
nozzle discharge coefficient.
II
A
SPRAY
ANGLE
I
DROPLET POPULATION
PROFILES
TH EORETICAL SPRAY COVERAGE AT
RATED SPRAY ANGLE.
160
SOLID CONE
SPRAY
160
HOLLOW CONE
SPRAY
a.
P2
it
120
2
100
SO
2
A2 <
P
I
SO
Al
20
02
0.6
0.4
00
DISCHARGE COEFFICIENT C D }
c.
d
Figure 6.36. Spray angle characteristics. (a) Droplet population in solid cone and hollow
cone sprays. (b) Nozzle spray angle. (c) Effect of pressure on spray angle. (d) The relation
between discharge coefficient and spray angle for a typical hollow-cone centrifugal pressure
nozzle used in spray drying.
Hollow cone spray patterns can be transformed into solid cone spray
patterns by balancing the rotational motion with an axial liquid velocity
component. In solid cone sprays, droplets are distributed fairly uniformly
throughout the conical pattern although spray droplet population is highest
in the centre. Hollow and solid cone patterns are shown in figure 6.36(a)
with their spray droplet population. For equal feed rates and feed pressure,
droplet size distribution is more homogeneous in the hollow cone than solid
cone. The solid cone contains a higher proportion of larger droplets, and
these droplets are concentrated at the centre of the pattern.
Successive wear in the nozzle orifice can produce undesirable spray characteristics by creating non-symmetrical sprays. Spray pattern symmetry,
its importance in spray drying and its measurement is discussed by Tate (50).
Flat patterns from fan spray nozzles have not as yet been seriously applied
in spray drying, although much data have been acquired on fan nozzle
performance (30) (480). Flat patterns could be used to advantage in small
drying towers. The nature of the pattern with droplet travel in one plane
prevents interference between sprays in closely bunched nozzle assemblies
and prevents product deposits at the dryer walls due to a minimum of radial
droplet travel.
Nozzles having either hollow or solid cone spray patterns are used in
co-current, counter-current and mixed flow drying chambers. The hollow
cone nozzle; however, is especially suited for co-current systems as the
majority of droplets making up the spray lie on the extremity of the cone
and are readily contacted with drying air to initiate rapid evaporation.
Where solid cone sprays are used in co-current systems the spray droplets
present in the centre of the cone are surrounded by high humidity atmosphere
for longer time periods. This is due to the extended time for the spray dispersion to expose the central droplets to the hot drying air.
Solid cone sprays are readily applicable to counter-current flow systems.
The drying air meets the full face of the cone pattern. Contact between spray
droplets at the centre of the pattern is as good as at the extremity. Furthermore the larger droplets of the spray distribution present in the centre of
the cone pattern are moving in a prominently axial direction and thus show
less tendency to disperse outwards for early impact in a semi-wet state on the
dryer walls. The droplets making up the extremity of the spray cone are of
smaller size, and although they possess a radial velocity component on
leaving the nozzle orifice, their size and contact with drying air enables
completion of droplet surface drying before arrival at the dryer wall. Where
hollow cone sprays are used in counter-current flow dryers, greater degree of
wall deposits may occur, especially in small chamber volumes. Control of wall
deposits makes adjustment to the air disperser critical. The spray from a
hollow cone spray nozzle may possess a more uniform size distribution,
164
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
but with the droplets directed from the nozzle with a radial velocity component, and the high droplet population at the extremity of the pattern,
longer drying times can result for complete drying of the spray and this leads
to wall deposit conditions.
In mixed flow dryers, hollow cone sprays are best applied when the first
phase of the droplet-air flow is co-current, i.e. as in a layout shown in
figure 5.9(a). Where droplet-air flow is first counter-current (see figure
5.9(b)) use of a solid cone spray can lead to chamber operation with reduced
deposit formation. Solid cones enable droplets to remain in the centre of
the drying chamber until their highest trajectory is reached, and thus cause
the droplets to remain airborne for longer time periods during the second
phase of co-current flow. Increasing the airborne time of droplets just
fractions of a second can have significant effect on the degree of completion
of moisture removal from the droplets and the minimizing of deposit formation at the dryer walls.
The desirability of either hollow or solid cone sprays in particular types of
dryer exemplifies the role of spray aerodynamic properties in the formation
of the spray pattern as observed during dryer operation. Aerodynamic
spray properties, namely velocity of entrained air, droplet velocity and spray
force provide information for optimising spray dryers featuring nozzle
atomization. These spray properties govern (a) droplet population within
the drying chamber, (b) the extent of hot air-droplet mixing near the nozzle,
and (e) the rate of droplet penetration through the drying air towards the
chamber walls or base.
Knowledge is far from complete as to how momentum is distributed
within a spray pattern, between spray droplets and entrained air, or as to
whether the variation in droplet population within the spray pattern is
predictable at various distances from the orifice. The established aerodynamic laws of droplet motion cannot be directly applied to the spray
system as there is considerable flow interference between droplets, and the
shape of droplets can be distorted during flight. Air entrainment at a
given distance from the orifice tends to be independent of the spray pattern,
being governed principally by the laws of turbulent transfer in air jets.
For sprays of fine droplets, the majority of movement in the spray at the
nozzle orifice is converted into the momentum of the entrained air stream
within relatively short distances from the nozzle (i.e. within approximately
3 ft (1 m)). The total flow of air associated with the spray increases as the
spray broadens on moving away from the orifice in the pattern defined
by the spray angle. This increasing entrainment of air into the spray assists
rapid spray evaporation by creating intimate spray-air contact. The degree
of entrainment, however, does not depend upon the spray angle entirely,
as the droplet population throughout the spray pattern appears a factor of
165
great influence. Hence the air entrainment occurrence differs for solid and
hollow cone spray patterns of the same spray angle. For a solid cone spray,
air might not penetrate the centre of the spray pattern until droplets have
travelled quite some distance from the nozzle. For hollow cone sprays air
penetration is much more rapid and spray-air mixing more effective.
It is desirable to know the influence of entrained air on droplet motion
from nozzles as relative velocity between droplets and the air is a prime
factor in moisture evaporation rates from the spray. There appears some
relation between feed rate, operating feed pressure, cone angle and spray
pattern, but no generalized form is available at present. The spray ejection
velocity from the nozzle followed by air entrainment maintains droplet
velocities considerably higher than droplet terminal velocities over time
periods where the majority of moisture evaporation takes place. There are
inevitable errors caused by basing heat and mass transfer rates in nozzle
dryers on mean drying air velocity or droplet terminal velocities. However,
any error acts to give conservative evaporation times, but air entrainment
effects promote faster droplet trajectories to the wall, and trajectory time
may prove too rapid for completion of evaporation, with resulting wall
deposits.
Air entrainment into sprays (480) has been recently reported (1969) by
Benatt and Eisenklam (51). A model for entrainment was developed and
confirmed using centrifugal pressure nozzles on aqueous feed. A dimensionless entrainment parameter was introduced. The model enabled entrainment
rates (mass flows of entrained air) to be predicted.
(c) Fluid Flow from Centrifugal Pressure Nozzles
The flow of liquid from the orifice of a centrifugal pressure nozzle can be
illustrated by velocity vectors as shown in figure 6.37. At the outlet (c),
liquid at a given point on the orifice wall will have a component of forward
velocity (u„) and one of tangential velocity (u h ) imparted by the liquid rotation
produced by the tangential liquid entry. From basic geometry, the liquid
leaving this point will travel in a straight line at an angle (tan -1 u h /u„,) to
the nozzle axis. The orifice does not run full, the air core within the orifice
ranging from 0.4 to 0.8 of the orifice diameter.
Applying the continuity equation to liquid flow through the nozzle
configuration shown in figure 6.37 results in equation (6.56).
For two inlets :
2(gri nie1 } = 2gr 2 bu y
(6.56)
where :
b = liquid film thickness at orifice,
r = inlet feed channel radius,
r 2 = orifice radius
166
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
167
where
n = 0.5 for turbulent flow,
C v = velocity coefficient,
h = pressure head,
P = total pressure drop.
Feed flow through the nozzle, stated in terms of a discharge coefficient (C D )
can be deduced from equation (6.60).
The discharge coefficient represents the ratio of the actual liquid discharge
rate to that theoretically possible under ideal conditions of energy transfer.
The discharge coefficient is related to the velocity coefficient by an area
coefficient CA where CD = Cl/CA . For centrifugal pressure nozzles, the
presence of the air core reduces the area of liquid flow through the orifice
and CA tends to CD . The velocity coefficient approaches unity values
accordingly. Thus :
LIQUID FILM THICKNESS
I N ORIFICE
Volumetric flow rate (Q) = feed velocity x flow cross-sectional area.
(6.61)
Q = Vres C A A
where A = orifice area. Combining equations (6.60) and (6.61) for turbulent
flow:
1/2
Q = cv (2gh) 112 C A A = C v (2g—
Figure 6.37. Velocity diagram for liquid flow from a centrifugal pressure nozzle; r 1 = inlet
channel radius, r 2 = orifice radius, R2 = swirl radius, b = liquid film thickness at orifice.
The feed channel length is assumed long enough to create tangential and not
diffuse flow into the nozzle.
The velocity of the liquid entering the nozzle (V irile ,) is given by
MI
-
(6.57)
KM et 27tr2ipi
=
where M, = mass liquid flow rate. Thus follows from equation (6.56)
14,,
7'
(6.58)
rzu
From the geometry of the spray, the resultant velocity of the liquid sheet
leaving the nozzle (Vres ) is given by :
Vinlet
Vres = [4 + Ufl 112
n
(6.62)
P) 1 / 2
Q = CD A(2gh) 112 = CD A(2g—
(6.63)
Alternatively, the mass flow rate (M L } = Qp and equation (6.63) can be
rewritten
(6.64)
ML = CDA-( 2gPa12
The discharge coefficient depends upon the size of the orifice, and varies
with the dimensions of each nozzle design. It follows from equation (6.64)
that a plot of flow rate against the square root of the total pressure drop
results in a straight line, the gradient being a measure of
C D A(2gp) 112
(6.59)
For cases of very low friction losses in the nozzle, the tangential velocity
component for liquid leaving the nozzle can be calculated by applying the
law of conservation of angular momentum to the nozzle flow, Referring to
figure 6.37
uh r 2
(6.65)
VinIetR =
(6.60)
where R = mean swirl radius of feed inlet to swirl chamber.
Expressed in terms of pressure drop across the nozzle :
P
Vres
ies = Cv (2ghr = C,(2g— )
now CD = CVCA
CAA
168
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
Example 6.9 illustrates the basic calculations for determining flow characteristics from centrifugal pressure nozzles.
EXAMPLE 6.9
The variation of liquid feed rate with pressure for a centrifugal pressure
nozzle of design shown in figure 6.37 is given in the table below. Calculate
the flow properties of the nozzle when operating at 60 psig with a liquid of
density 70 lb/ft 3 .
tan 24° = C 0 A(2gp) 112
Feed pressure
P (psig)
P 112
115
140
160
185
20.25
30.25
42.25
56.25
4.5
5.5
6.5
7.5
Feed rate Feed pressure
P (psig)
M (lb/hr)
p112
8.5
9.5 .
10.5
72.25
90.25
110.25
205
230
250
Data :
25
0.5 x 12 x 3600
tan 24 f.p.s. wilts
1. Calculation of Discharge Coefficient (C D )
Substituting data in equation (A)
C D ---
25 tan 24
1
0.5 x 12 x 3600 X A(2gp) 1 / 2
-
25 x 0.4452
4
0.5 x 12 x 3600 x n(0.070) 2
144
(64.4 x 70) 1 / 2
= 0.34
Measured cone angle at 60 psig = 80°
2. Calculation of Inlet Velocity to Nozzle
At operating pressure of 60 psig : P' 12 --- 7.746. Mass feed rate (from graph)
M = 188 lb/hr. Inlet velocity to nozzle (equation (6.57))
Nozzle dimensions :
feed channel diameter to swirl chamber = 2 off: 0.052 in
orifice diameter = 0.070 in
internal diameter of swirl chamber = 0.302 in
The feed rate vs pressure data is plotted in figure 6.38. The gradient approxi-
1
188
44
1
= 25.3 ft /sec
= 3600 H 2(n/4)(0.052)2 x 70
=
3. Calculation of Resultant Velocity of Liquid Sheet Leaving Nozzle
Using equation (6.59) where C v is taken as 1.
300
(17. es )
[64.4 x 60 x 1441 1 / 2
= 89.2 ft/sec
70
250
4. Calculation of Forward Velocity (U,) from Nozzle
For a cone angle of 80° = 20 where from figure 6.37
_c° 200
k 150
LI,
Li
w
U-
(A)
The conversion factor to express the gradient in f.p.s units (for the abscissa
and ordinate scales of figure 6.38)
(
Feed rate
M (113/hr)
169
mates to
tan 0 = -
U,
100
U]
(B)
tan 40 = 0.8391
50
0-8391U,
Uh =
0
2
3
4
5
6
7
.
ps ig 2
1
2
(FEED PRESSURE)
I
9
10
1
. '
Figure 6.38. Variation of feed rate with square root of nozzle pressure (example 6.9),
Now resultant velocity (Fres ) = ( Uv2 + UD I / 2 and by combining equations
(6.59) and (B)
Vr „
therefore U, = 65.8 ft/sec
89.2 = (LT, + 0-83911/,2)" 2
170
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
L
5. Calculation of Film Thickness
From equation (6.58) where
1
h =
.2
171
v
'Tile,
r, U,
r 1 = 0.026 in
r 2 = 0.035 in
b=
(0.026 2 )
25.3
x
= 0.0074 in
0.035
65.8
5
20
30
P2
PRESSURE RATIO (7 , )
6. Calculation of the Percentage Conservation of Angular Momentum
From equation (6.65), where R = (0.302 — 0.052) 0.125
Uhr2
//inlet)?
0.8391 x 65.8
25.3
0.035
12
a.
12
X 100 = 61 %
0.125
(d) Nozzle Operating Characteristics
(i) Relation between Flow Rate, Pressure and Density. Performance of the
nozzle is effected by pressure, liquid density and viscosity. The flow rate
is directly proportional to the square root of pressure, and varies inversely
with the square root of liquid density. This relation is only approximate.
Q
1/2
Q2
(P2
(Pi) 112
Q1
\PI I
1P 2i
(6.66)
The relation between the ratio of two operating flow rates and the two
corresponding operating pressures is shown in figure 6.39(a). The relation
between the ratio of two operating flow rates and the two corresponding
liquid densities is shown in figure 6.39(b).
Any change in liquid density is usually associated with changes in other
liquid properties, and thus the above relation does not apply in all cases.
The effect of viscosity on flow rate cannot be clearly defined being dependent
upon the specific nozzle design and operating conditions involved. Increase
in viscosity can increase flow rates especially in conical spray nozzles, although the reverse trend can be observed in other designs. The exact effect
must be determined by experiment for a given nozzle design and operating
conditions.
(ii) Spray Angle. A minimum pressure is required to develop any form of
spray angle. The minimum is approximately 20 psig (1.4 atm). Increase of
pressure decreases the spray angle, but influence is slight, especially if
measured near the nozzle orifice. At higher working pressures above
200 psig (14 atm) the spray angle becomes reduced due to air entrainment
04
1.0
100
FEED DENSITY RATIO
b.
Figure 6.39, Feed rate, feed pressure, feed density ratios for centrifugal pressure nozzles,
effects. Viscosity has a significant effect where an increase produces a
narrower spray angle. At very high viscosities the spray angle is so reduced
that a plug stream of liquid results, and atomization ceases. The effect of
pressure on spray angle is illustrated in figure 6.36(c).
(iii) Effect of Operating Variables on Droplet Size
Effect of capacity on droplet size. Larger droplets accompany increase in
feed rate for increases occurring within the rated feed rate range of the
nozzle. This is illustrated in figure 6.40(a). If initial feed rate is well below
the rated flow, atomization is incomplete, as the liquid velocities within the
nozzle are inadequate. In this case increase in feed rate will decrease droplet
size until the rated capacity of the nozzle is reached.
172 THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 173
Spray angle and droplet size. Smaller droplets are formed from sprays of
larger spray angle. This is shown in figure 6.36(d). Increase in spray angle
decreases nozzle discharge coefficient. Decrease of discharge coefficient
decreases feed rate and this for a constant pressure reduces droplet size.
Effect of pressure on droplet size. By virtue of greater energy at higher
pressures, droplet size will decrease with increase of pressure, although
there are varying opinions as to the exact relation involved. For trend
NOZZLE INSERT : GROOVED CORE
SAUTER MEAN DIAMETER 1M ICRONS)
400
1.5 ato
3 ato
6.5ato
10 do
17.5 ato
200
900
50
10
estimation droplet size can be assumed to vary as the — 0.3 power of pressure
within the medium pressure range. At very high pressures, further increase
of pressure has virtually no effect on atomization. Inspection of figure
6.40(a) illustrates increase of pressure to reduce mean droplet size at fixed
feed rate. Pressure is plotted as a parameter.
Effect of feed viscosity on droplet size. Coarse atomization results from
increase in feed viscosity. Droplet size can be taken to vary with viscosity
to the power 0.17-0.2. The exact relation between spray mean droplet
diameter, droplet size distribution and feed viscosity is particular to each
nozzle design and dimension. General trends are illustrated in figure 6.40(b)
and 6.41 for a 0.79 mm orifice grooved core nozzle. Some increase in capacity
can also accompany viscosity increase, further assisting coarser atomization.
With viscosity increase the air core of the nozzle becomes increasingly
filled with liquid. At a given pressure, capacity will increase until the nozzle
operates with no air core. Capacity increases are small, however. Very
high viscous materials cannot be atomized by pressure nozzles.
100
AQUEOUS FEED RATE
NOZZLE INSERT:GROOVED CORE
NOZZLE PRESSURE: 29.5 ato
Kg/HOUR
NOZZLE INSERT - GROOVED CORE
FEED RATE - 0.715 Htros/mi n
NOZZLE PRESSURE • 29.5 ato
ORIFICE DIAMETER: 0.79mm
99.9
90
99.5
99
CUMULAT)VE VOLUME PERCENTAGE
z
FEED RATE , 0.715 iitersImin
ORIFICE DIAMETER: 0.79 mm
80
w
w 70
x
8
95
90
80
70
60
50
40
30
20
10
5
60
0.5
01
s:1
0
(b)
2
3
6
5
6
7
FEED VISCOSITY c.5
Figure 6.40. Effect of variables ((a) feed rate, (b) feed viscosity) on Sauter mean diameter.
W5
0
2
4
12
10
6
811
1,
'2
(DROPLET SIZE) 2 (MICRONS )1
14
Figure 6.41. Effect of feed viscosity on spray droplet size distribution.
16
174
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 175
Effect of surface tension on droplet size. Energy supplied to the nozzle has
to overcome both viscous and surface tension forces. Hence liquids of high
surface tension become more difficult to atomize. Surface tension values
of feed liquids that are nozzle atomized vary over a relatively small range,
and thus effects of surface tension changes on droplet size are much less
significant than for changes in viscosity.
Effect of orifice size on droplet size. Droplet size increases with the square of
orifice diameter, other nozzle parameters being held constant.
(iv) Droplet Size Distribution. The available range of droplet sizes obtained
with centrifugal pressure nozzles is extensive. Whether wide or narrow
distributions are produced depends upon nozzle dimensions, operating
pressure and feed conditions. A typical size distribution is given in figure
6.42 to illustrate the range of sizes obtained from a nozzle operating in
conditions well representative of a general spray drying application.
Droplet size distributions of pressure nozzles are fully described by
Marshall (35). Many sprays are represented in linear form when presented
as a square root normal distribution (380).
(e) Prediction of Nozzle Performance
Correlations to predict nozzle performance are available for given nozzle
designs. An overall working formula has as yet eluded investigators. Correlations utilize either known operational data or nozzle dimensions.
Doumas and Laster (52) and Dombrowski and Hasson (53) have proposed
methods to predict performance where only the nozzle dimensions are
known. The methods involve use of nozzle parameters established experimentally on nozzles having tangential swirl channel inlets as shown in
figure 6.43. Nozzles used by Doumas and Laster featured circular inlet
FEED PRESSURE . 300 p.s. 1.g
FEED RATE. 350 Lb /hr (VISCOSITY < 5 c.p. )
TANGENTIAL INLET SWIRL CHAMBER.
IN
CUMULATIVE PERCENT BY VOLUME LESS THAN
999 5
99.9
99.5
99
LIQUID IN
95
90
LIQUID OUT
80
a.
70
60
50
40
30
LIQUID IN
20
10
5
0.5
0-05
0
100
200
LIQUID
OUT
300
DROPLET DIAMETER D. (MICRONS )
Figure 6.42. Typical droplet size distribution of a centrifugal pressure nozzle used in spray
drying.
b.
Figure 6.43. Nozzles with tangential feed entry. (a) Nozzle used by Doumas and Laster (52).
(b) Nozzle used by Dombrowski and Hasson (53).
176
ATOMIZATION 177
THE PROCESS STAGES OF SPRAY DRYING
channels. Nozzles used by Dombrowski and Hasson featured inlet channels of rectangular cross section, so chosen due to the practical difficulties of drilling circular holes that are truly tangential to the swirl chamber.
The inlet channel length to diameter—width ratio in both types of nozzle
designs permitted full tangential and not diffuse flow into the swirl chamber.
1.00
co
• 0-80 c
2
060
0.40L
L.Lr
•
020-
n.
0
C.
2
4
{r
10
8
6
or
12
14
16
16
A'
Figure 6.44. Nozzle dimensioning relations after Doumas and Laster (52). (a) Relation between
discharge coefficient and nozzle dimensions. (6) Relation between mean spray angle and
nozzle dimensions. (c) Relation between the parameter (cc) (equation (6.68)) and nozzle dimensions.
Method of Doumas and Laster (52)
0
4
2
6
8
10
12
The feed rate through a swirl nozzle of design shown in figure 6.43(a) is
derived from conservation of momentum, Bernoulli and continuity equations. From the feed rate relation (equation (6.63)), the discharge coefficient
and spray angle can be expressed in terms of the nozzle dimensions expressed
1 112
as a parameter (Rr 2 lri)(r2 /R ) , where R = swirl chamber radius, r, =
radius of inlet feed channel, r 2 = orifice radius, R' = (R 1. 1 ).
The discharge coefficient expressed in terms of the nozzle dimensions
equals
a(1 — C0 112
(6.67)
C
D = (1
a + a2(x)2)1/2
for values of
2
14
R
r r2 ' 1 f
t
i t
0,
rr
re
r,
(6.68)
= 1 — —2
A' = r2
R
(6.69)
rt
From equation (6.67), CD and r c are independent of h, the head of liquid at
the nozzle. A direct relation between CD and the nozzle dimensions is shown
in figure 6.44(a). This enables the direct determination of the discharge
1 11
l
coefficient, where CD is plotted against the parameter [A (r 2 /R ) 1. The
mean spray angle (0) is calculated from the parameter (figure 6.44(b)).
The mean spray angle correlates to
2
4
6
-
b.
(2 2'
1R
8
10
(
1
/2
12
14
15
11
r R (r2 \ 1
16
9 = 43.5 log [14(
2
rl
R1
(6.70)
178
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 179
The air core diameter (r e ) is calculated from equation (6.68), which rearranges to
12
re = (1 — a)' r2
(6.71)
The calculation of re is simplified by obtaining values of a from figure
6.44(c), where A' in equation (6.69) is readily determined from the nozzle
dimensions.
EXAMPLE 6.10
Nozzle atomization with a centrifugal pressure nozzle (Swirl type) is used
to produce 500 lb of powder (moisture content 3.5 %). The feed concentrate
is 32 % TS, and specific gravity 1.14. The nozzle orifice is 0.112 in diameter.
Spray angle is 60° at 500 psig operating pressure.
After 500 hours of operation, atomization deteriorates, and the product
becomes off-specification. At the time when the control laboratory can no
longer grade the product as acceptable, the nozzle spray angle had increased
to 75° under normal feed conditions. Estimate from the above data the
possible wear that had taken place within the nozzle orifice.
The final diameter of the orifice can be calculated using figure 6.44.
Production rate = 500 lbihr
= 1505 lb/hr
The nozzle discharge coefficient (C D ) can be calculated from the spray
angle (0), using figure 6.44(a), (b). For 0 = 75°, figure 6.44(b)
1/2
= 3.8
rl
which from figure 6.44(a) Cr, = 0.18, now
Q = CD A(2gh) 112
Q
A=
(
1505
— 5.88 x 10' cu ft/sec
3600 x 1.14 x 62.3
r
2
ft2
144
h=
(500 + 14.7)33.9
= 1040 ft
14.7 x 1.14
r
2
144Q
= C (2gh) 112
D
r=
( 144 x 5 , 88 x 10'
n0.18(64.4 x 1040) 112 ]
r = 0.076 in
Original diameter of orifice = 0.112 in, area = 0.00985 in 2
Final diameter of orifice = 0.152 in, area =- 0.01814 in'
2
Therefore area increase of orifice due to wear = 0.0083 in .
Method of Dombrowski and Hasson (53)
The method was developed from investigations into the flow characteristics
of swirl nozzles of the design illustrated in figure 6.43(b). The inlet swirl
channels are rectangular. The nozzle pressures studied ranged over 50-150
psig (3.5-10 atm) on aqueous feeds. Operating feed conditions cover a
flow number range of 1-10 imperial gallons/hr (psig) l " 2 . The flow number
relationship is given by
Flow number (FN) = ( ( 2
)0.5)
x 9.
65
100
Feed rate of concentrate to the nozzle = 500
x
1 0
32
r22R\ I
where 1 atm = 33.9 ft of H 2 O
(6.63)
(6.72)
The nozzle parameter is given as (A/(d 2 x d M )) where A = area of inlet
swirl channel(s), d M = mean diameter of inlet vortex in swirl chamber,
d 2 = orifice diameter.
The mean diameter (d M ) is obtained from the swirl chamber diameter (d 3 )
and the swirl channel width (1), where d M = (d 3 /).
The nozzle parameter was developed from the Taylor (54) solution of the
mechanics of low viscous liquid flow through swirl nozzles. Spray angle
and discharge coefficient are directly related to the nozzle parameter, as
shown in figure 6.45(a), (b). The deviation from ideal flow conditions can
be corrected for in terms of the ratio of orifice length to diameter (L/d 2 ).
This correction is shown in figure 6.45(c).
The prediction of spray angle and discharge coefficient is straightforward
using figure 6.45. The ratio of orifice length to orifice diameter (L/d 2 ) is
obtained from the nozzle dimensions, and the correction factor (F 1 ) for the
effect of (L/d 2 ) on spray angle is obtained directly from figure 6.45(c). The
correction factor F, is combined with the nozzle parameter and the ratio
(L/61 2 ) to form a modified parameter
Fl
A _ W m \ 11212/3
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
The modified nozzle parameter forms the abscissa in figure 6.45(a), (b).
The spray angle can be read directly from figure 6.45(a) using a known value
of the modified nozzle parameter.
For prediction of the discharge coefficient, a correction factor F2 is used
to modify the apparent discharge coefficient, applicable to the case of
theoretical frictional flow. Firstly the modified nozzle parameter, as stated
above, is used to read off the apparent discharge coefficient from figure
50 - 150 pAr.g
0.5
18
p 04
15
z
1•4
uf-
181
NO1133k:1800
180
r.)
rr
cr 02
1.2
0.1
0-1
AQUEOUS FEED
10
10
0.5
100
7-)
ORIFICE LENGTH
ORIFICE DIAMETER
80
G,
Lu
ELI
Figure 6.45. Nozzle dimensioning relations after Dombrowski and Hasson (53). (a) Variation
of spray angle with modified nozzle parameter. (b) Variation of apparent discharge coefficient
with nozzle parameter. (c) Correction factors F 1 F2 for the effect of orifice length and diameter
on spray angle and discharge coefficient.
60
w 40
2
?<- 20
a.
Lt)
0
0.4
0•8
12
16
2-0
2.4
26
32
6.45(b). The actual discharge coefficient is finally calculated using the
correction factor (F2 ) where
2
(-zia2
j
MODIFIED NOZZLE PARAMETER
,
)'
F2(CD(apparent) — 0-05
CD(actual)
I
(6.73)
The value of (F2 ) is obtained from figure 6.45(c).
EXAMPLE 6,11
A swirl nozzle with two tangential, rectangular inlet channels is operating
at 7 atm at rated flow. The dimensions of the nozzle are as follows :
as
0q
APPARENT DISCHARGE COEFNCIENT
Orifice diameter (d 2 ) = 3 mm
Orifice length (1) = 1.5 mm
Mean swirl diameter (d m ) = 18 mrn
Swirl chamber diameter = 22 mm
Inlet channels — 4 x 4 mm
Predict the spray angle and discharge coefficient.
The nozzle type and dimensions suggest the Dombrowski and Hasson
method applicable :
1. Nozzle Parameter
1 A\
d 2 c/ 1„/ )
0
0.4
06
1.2
1.6
A
2.0
2.4
1
2.8
! 12
d
MODIFIED NOZZLE PARAMETER 7
F
_bl_) '2
'3
i i d2 .d m k d 2 i
r
i
3.2
=
32
56
4
7
0-57
2. Ratio
i dm 112
b.
2x4x4
3 x 18
ke12)
=
( 18y/2
-3-
= 6
112
= 2.4495
182
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION 183
2
3, Ratio : orifice length to diameter
The flow number is the ratio of (Q/P" ) but can be obtained directly from
the relation
1
(—1 ) = P
= 0.5
d2
FN = 293C D A
3
4. Modified nozzle parameter :
\ 'd m \ 1/212/3
Ft
= 0.39(0.57 x 2.4495) 213 = 0.487
A
[(d d m )
From figure 6.45(a)
spray angle = 75'
5, Now for a modified nozzle parameter of 0.487, the apparent discharge
coefficient (from figure 6.45(b)) CD(a
= 0.35. From figure 6.45(c):
pparent)
Correction factor (F 2 ) = 1.6 as
— = 0.5
d2
Hence : Actual discharge coefficient C D(ac ,„„, ) is calculated from equation
(6.73)
C
D(actual) = (1.6 x 0.35 — 0.05)
= 0.51
(f) Prediction of Mean Droplet Size of Sprays from Centrifugal Pressure
Nozzles
The factors governing the mean droplet size of sprays for a given set of
operating variables are very complex. An empirical approach is the only
way to establish relationships. Relationships between mean droplet size
and operating variables have been established through spray analysis at
various values of the nozzle design variables; orifice- diameter, ratio of
orifice length—diameter, liquid pressure, and discharge medium pressure.
Many of the correlations proposed apply specifically to one design of nozzle
and atomizing liquid, but empirical correlations are available for use in
first design and performance calculations. These are given in table 6.14
in four sections. The first three sections quote correlations found applicable
to nozzles having specific swirl chamber designs. The fourth section includes
empirical correlations of centrifugal pressure nozzles in general.
Nozzles with grooved inserts where liquid rotation is created by flow
through angled grooves or slots are covered by relations quoted by three
different investigators. Equation (6.74) proposed by Frazer, Eisenklam and
Dombrowski (55) expressed the Sauter mean diameter in terms of a flow
number (FN) governing volume flow rate (Q) and nozzle pressure (P).
(6.83)
when the orifice area (A) is in cm 2 .
Equation (6.75) by Moulton and Turner (56) resulted from fitting extensive
data into the correlation between Sauter mean droplet size and orifice
diameter, flow rate, surface tension and viscosity. Droplet mean size is
shown to be proportional to the 1.5 power of orifice diameter and inversely
proportional to the 0 , 44 power of flow rate. This correlation is unique
in attaching such importance to surface tension with a power relation up
to 0-6. It is interesting to note that viscosity shows little prominence. This
is rather misleading, although over the viscosity range investigated (less
than 3 cP) influence would be slight. Equation (6.76) by Nelson and Stevens
(8) expressed the volume mean diameter in terms of the fluid flow characteristics from the nozzle as defined by the Reynolds and Weber numbers.
The volume mean diameter exceeds the Sauter mean diameter by approximately 15-20 %. The factor (Y) in equation (6.76) is dimensionless and
consistency of units must be adhered to when calculating the Z-value.
Nozzles with swirl plates to create liquid rotation were also studied by
Frazer, Eisenklam and Dombrowski (55). Two equations (6.77) and (6.78)
are quoted. A wide range of liquid variables are covered.
Nozzles with simple tangential entry were studied by Moulton and
Turner (56). Equation (6.79) results from treatment similar to that given by
equation (6.75). The power values in both equations were found to be of
the same order. The average geometric standard deviation (S G ) for the
Moulton and Turner nozzles was correlated in terms of orifice diameter
(mm) where
SG = 0.173 + 0.126 log ic+ do
(6.84)
The similarity between the equations (6.75) and (6.79) gives greater
confidence when discussing the suitability of general correlations. Such
correlations (equations (6.80-6.83)) show the Sauter mean diameter in terms
of operating variables. Equation (6.81) by Tate and Marshall (57) interpreted
nozzle performance in terms of the axial velocity (U,) and the tangential
velocity ( U T ) components on liquid discharge.
The average axial velocity is given by the ratio of the volumetric flow
rate to area of liquid flow through the nozzle orifice, i.e.
u=
v
n(rgr2)
rc
(6.85)
where Q = volumetric feed rate, ro = orifice radius, rc = air core radius.
(i) Grooved
Equation
log io D vs = 1.808 +
6-94
Range
+ 0.138(FN)
AP
D vs = 16.56(4 -52 )(6 °-713 )(yr 55 )(Q
logio D vm = Y
do
and
(ii) Swirl plate
0-444)
where
Y = -0-144Z 2 + 0.702Z - 1.26
we ) 0.2( v) 12]
Z = log io (Re)
--G
Re
Dvs =
335(FN)• °9 v ° " 25
A p0.348
RFTSD 1
L AP
3-
D vs = 119.2
1-589)(61)-594)(4-22o)(2-0.537)
(iii) Tangential entry
Dvs 41-4(c1Q
=
(iv) General
D vs
- 0-5
157 --o
( r;)
0-45
597(0)
D vs = 286(d o + 0.17) exp
76704 .33
D MMD
(u,
-11(u .00-4.0
(d o is in, U v , UT is ft/sec)
13
Q
100 > AP > 25 psi
2.0 > FN > 0-05
0 = 60°
(6.74)
(55)
o- = 26-37 dynes/cm
p = 1.024 - 1.073 g/cm 3
- 0.9-2-03 cP
Q 1-30 US gal/sec
do = 0-7-1.5 nun
(6.75)
(56)
P 1000 psi
Q = 4.0-70 gal/hr
0 = 52-91°
(6.76)
(8)
p = 0.746 Wan'
AP = 12-300 psi
FN 0.5-2-0
v = 2.0-18-5 cs
= 22 dynes/cm
(6.77)
(55)
AP < 100 psi
FN = 10-500
B- 85°
(6.78)
(55)
(6.79)
(56)
(6.80)
(7 )
(6.81)
(57)
(6.82)
(58)
(59)
do = 1-4-2.03 mm
o- = 26-37 dynes/cm
Q = 1-30 US gal/sec
= 0.9-2-03 cP
p i = 1.024-1.073 g/cm 3
1-5
(ici do [P 1 pr .
0.0094U-1-1
Equation
number Reference
UT = 7-50 ft/sec
U v = 40-150 ft/sec
NOI1VZIWOIN
Nozzle swirl
chamber design
DNIANCE AVIldS .40 SIDVIS SSff3O2id UHI
00
Table 6.14. Correlations to Predict Mean Droplet Size from Centrifugal Pressure Nozzles
186 THE PROCESS STAGES OF SPRAY DRYING
For swirl chambers U T = 42
.4
—5w
where A s , = area of flow into swirl chamber.
For grooves
UT =
187
ATOMIZATION
For values of U, in ft/sec, equation (6.85) can be rewritten as (0.407Q/4)
where orifice diameter (d o ) is in and feed rate (Q) is US gal/min.
The air core can be calculated from equations (6.68) and (6.69) and figure
6.44(a). As a guide, for orifice sizes above 0.05 in diameter, air core diameters
are 0.7-0.8 orifice diameter. Below 0.05 in the air core decreases to 0.4
orifice diameter. Exact values depend upon nozzle designs and operating
conditions.
The average tangential velocity (U T ) is calculated from the flow rate,
flow area, and the swirl angle imparted to the flow.
Q cos a
(A g )n
Assume the dried product particle size distribution is similar to wet spray
distribution.
Method I (Lewis—Nukiyama—Tanasawa equation)
Substituting pilot plant operating values in equation (6.80)
D vs = 86 micron
86 = 157(
0.5
70
440
(6.86)
86 = 63 + 87
0.45
1
+ 597(
70 x 1.02
r
15
0.35
° 5
L.K N O.05(440/1.02) ' ]
F0.3361 "
KN
K N = 0.8.
(6.87)
For scaled-up dryer :
120
Feed rate per nozzle = 60
where (A g ) = groove cross-sectional area, n = number of grooves, a = angle
of grooves to plane 90° to the nozzle axis. For values of UT in ft/sec equation (6.87) can be rewritten as
0.32Q cos a
UT =
= 0.5 gal/min.
x 4
Increase in capacity obtained solely by pressure change
(A g )n
where (A g ) is in' and (Q) is US gal/min.
The much quoted Lewis—Nukiyama—Tanasawa relation (7) is given in
equation (6.80). It differs greatly in form from the Tate—Marshall equation .
as droplet size appears independent of the tangential velocity for constant
feed rates, orifice sizes and liquid pressure. Use of equation (6.80) requires
a value of the constant K N , which invariably necessitates experimental
determination or (as a last resort) estimation. Predictions of mean droplet
size using the two general equations are not consistent, but are usable in
first design calculations to establish the order of magnitude of the spray
droplet size. The Tate—Marshall correlation is the easier to apply. Example
6.12 illustrates predictiOn of mean droplet size using the two equations.
The Kim—Saunders equation (6.82) is proposed for large scale nozzles.
EXAMPLE 6.12
A centrifugal pressure nozzle operating successfully in a pilot spray dryer
is to be used in a larger unit. Operating with a feed rate of 21 gal/hr at
440 psi, analysis of the dried product gives a Sauter mean diameter of
86 micron. If the feed rate in the scaled-up dryer incorporates four similar
nozzles, and the total feed rate is 120 gal/hr, what mean droplet size is
likely to be obtained?
Feed data: p = 1.02 g/cm 3 , o- = 70 dynes/cm, p = 10 cP.
Nozzle data : d e = 0.05 in, U, = 57 ft/sec at 21 gal/hr, UT = 13.4 ft/sec.
Q = fl = 0.35 gal/min
= 440 psi
P2
N al 2
(6.66)
LQii
.5 2
— ) = 900 psi
Pressure required for new condition = 440 x( 0
0.35
Substituting in equation (6.80)
P = 900 psi
KN = 0.8
Q = 0.5 gal/min
D os = 71.5 micron
G.
Method II (Tate—Marshall equation)
Substituting values into equation (6.81) for nozzle operation in pilot plant
unit
10, s = 286(0.05 + 0.17) exp [57 — 0.0094(13.4)] = 286(0.22) exp 0.102
-= 70 micron.
For scaled-up dryer :
Tangential velocity component at nozzle = UT = 13.4 x
Axial velocity component at nozzle = U v = 57 x
0.50
= 19.1 ft/sec
0.35
0.50
= 81.5 ft/sec
0.35
188
ATOMIZATION 189
THE PROCESS STAGES OF SPRAY DRYING
Substituting values in equation (6.81) the mean size predicted at higher
pressure
13
0 0094 19.1 ]
D„ = 286(0.05 + 0-17) exp [81•5
= 286(0.22) exp — 0.02
= 62 micron
Size predicted by equation (6.80) D vs = 71.5 micron. Size predicted by
equation (6.81) D„ = 62.0 micron.
(g) Nozzle Operating Features and Nozzle Applications
Centrifugal pressure nozzles are employed in a variety of applications
where feeds are of low viscosity and contain no large solid particles in suspension. With correct nozzle selection both fine and coarse sprays can be
obtained. A comprehensive range of nozzles are offered by all nozzle manufacturers to meet any requirement of capacity and spray angle. •
The advantages of centrifugal pressure nozzles lie in their low cost, ease
of replacement and simple maintenance. Low costs result from nozzles
being machined from small metal bulk, usually of readily available metals,
i.e. standard stainless steels. All parts of nozzles are readily dismantled.
Manufacturers have achieved the fine art of being able to duplicate performance of replacement metering parts of a nozzle, i.e. orifices and swirl
inserts. This enables replacement parts in reconditioned nozzles to perform
consistently without constant adjustment to the feed system. It must be
pointed out, however, that for reasons of economic manufacture, slight
deviations in nozzle performance are inevitable from part to part, but
variations from rated flow or spray angle are only of the order 3-6 %,
which for industrial spray drying applications can be tolerated. Maintenance of nozzles is straightforward as their construction is basically simple.
All parts fit with small tolerances and there is little chance for assembling
nozzles incorrectly. Attention must be given to any packing seals as these
must always be in perfect condition. A leaking nozzle creates wet deposits
in the drying chamber.
The main disadvantages concern the need for high pressure pumps,
effective feed filtering (straining) to prevent orifice clogging, and severe
restrictions on the type of feed material the nozzle can handle without undue
wear. Nozzle durability has increased considerably over the last decade
by the use of modern materials, that are extremely abrasion and corrosion
resistant. Nozzle erosion must be prevented, as spray dissymetry results
from worn metering parts. Wide fluctuations in feed rate can then occur.
A poor quality product is produced, and there is extensive dryer down time
for nozzle maintenance.
Centrifugal pressure nozzles are prone to wear as high liquid velocities
are generated in the swirl chamber and orifice. The higher the feed pressure
and feed solids content, the greater the chance of nozzle wear. Wherever
high velocity flow occurs, the flow surface is fabricated in a hard material.
Tungsten and silicon carbides are highly anti-abrasive. Chromium carbide
resists oxidation or chemical attack, and is used to counter corrosive feeds.
Alumina is also used for anti-abrasive or anti-corrosive surfaces.
Centrifugal pressure nozzles are not too flexible and optimum operation
is within a narrow range of working conditions for a given orifice size and
swirl chamber. The flexibility of nozzle assemblies is improved by the design
of nozzle bodies to handle a wide range of feed rates through interchangeable
orifices and swirl inserts. This is illustrated in figure 6.46 where aqueous
feed rates are plotted at fixed pressure against orifice diameter at constant
swirl chamber size. Three swirl chamber sizes are given denoted by A, B, C.
FEED PRESSURE: 14 ata /2060 p.5.1
1000
0
0.1
I
l
i
50
0.5
NOZZLE ORIFICE DIAMETER
(mm)
Figure 6.46. Range of orifice sizes per given nozzle body for optimum atomization conditions.
190
THE PROCESS STAGES OF SPRAY DRYING
It can be seen that nozzle body flexibility increases with increased nozzle
size, as a greater range of orifice sizes can be fitted on the nozzle body and
the feed rate range duly extended. For the smallest nozzle (swirl body A)
the range of orifice size that can be fitted covers 0.7-10 mm, and only variation of feed rate between 25-65 litres/hr (5.5-14 g/hr) can be handled for
complete atomization. For the larger swirl body B, orifice sizes of 0.9-4.0 mm
can be incorporated and a feed rate range of 450 litres/hr (100 g/hr) is
achieved with the lower limit around 70 litres/hr (15 g/hr). The swirl body C
gives the largest nozzle the greatest flexibility, since any feed rate between
100-1000 litres/hr (22-220 g/hr) can be atomized if interchangeable orifice
sizes between 1 , 0-4.5 mm are available.
When operating nozzles, care must be taken in their handling. The metering parts are machined with great precision, and thus the slightest damage
must be avoided if nozzle performance is to be maintained. A streaky or
off-line spray results from damaged orifices or swirl inserts, and the situation
may well be aggravated by creating abnormal feed rates. To maintain nozzle
performance, periodic cleaning is essential. Reduction in feed rate or streaky
sprays may be due to a dirty nozzle, although with correctly filtered feeds,
the best of modern nozzle designs are not prone to clogging during operation.
Clogging is likely if product is allowed to dry out in the nozzle during shut
down. It is standard practice to dismantle the nozzles during shut down,
remove any hard deposits on the outer surface with a stiff brush and wash all
metering parts thoroughly. If the nozzle is not reassembled immediately,
the parts should be left in clean water or weak caustic soda solution.
Sizing nozzles for a required dryer performance is often complicated by
nozzle selection being made from brochures listing nozzles rated on aqueous
feeds. Product flow will invariably differ from that of water. It is advisable
to consult nozzle or spray dryer manufacturers for advice or for approval
of nozzle selections. Nozzle manufacturers offer comprehensive service as
regards performance of their nozzles on particular feeds. For enquiry into a
nozzle recommendation, a form similar to that shown in table 6.15 is filled
in and supplied to the manufacturer. For products like milk or coffee,
Table 6,15. Nozzle Recommendation : Enquiry form
A. Customer
Date of enquiry
B. Product details
Product to be atomized
% solid content
Feed temperature
Feed density
Feed viscosity at feed temperature
ATOMIZATION 19 I
Mass flow rate
Volumetric flow rate
Ph value
C. Other feed properties (delete where applicable)
Newtonian/non Newtonian flow characteristics
Corrosive/abrasive
Any special chemical structure? yes/no
Chemically active with stainless steels? yes/no
D. Spray dryer details
Dryer type and size and nozzle location
Feed pipe size to nozzle(s)
Inlet air temperature
Outlet air temperature
Number of nozzles in dryer
Available operating feed pressure
Dryer chamber dimensions
Co-current/counter-current/mixed flow
E. Dry product details
Production rate
Moisture content
Bulk density
Solubility
Mean particle size
F. Nozzle performance
(a) Any limitations on bulk density, if so, state :
(b) Desired particle size distribution
(c) Any limitation on number of nozzles mounted in dryer.
If so, state maximum and minimum number :
(d) Any safety hazards in handling product
(e) Any particular spray angle
G. Previous Nozzle experience in dryer (if any)
Manufacturer
Nozzle type
Number of nozzles mounted
Rated flow rate per nozzle on water
Wear history of nozzle
Any particular operational problems associated with
above no zzle(s)
For manufacturer use only
Recommended nozzle
Nozzle type
Number mounted in dryer
Pipe size
Metering parts—type
Operating pressure
Remarks
signed
192
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
that are spray dried in large tonnages using nozzle atomization, charts are
available to convert the nozzle water rating, stated in a brochure to the desired product rating. The product solids content must be known. A typical
conversion chart is shown in figure 6.47 for the case of a 42 % skim-milk
concentrate, atomized with a swirl insert nozzle. At the nozzle rated pressure
the aqueous feed rate is seen to exceed the milk feed rate as feed rates increase (dotted line abc). Pressure is represented as a parameter, and thus
performance of the nozzle at pressures greater or less than rated pressure
can be read off immediately. For selection of nozzles, when the feed rate,
feed solids and available operating pressure is known, the equivalent water
rating can be read off the chart (dotted line egf). This rating enables correct
nozzle selection from the brochures.
42% T S. SK1r'4 WU< CONCENTRATE,
25
. 200 CENTPOISE,
24
16
12
8
4
0
4
8
12
16
20
24
28
32
36
40
CONCENTRATE FEED RATE ( litres / mi
Figure 6.47. Performance of centrifugal pressure nozzles (rated on aqueous feed) on 42 % TS
skim milk concentrate. (Nozzles with swirl insert.)
Nozzle atomization is fully established as a mode of atomization in spray
dryers. Centrifugal pressure nozzles are used extensively in many industries,
where spray dried products are manufactured in large tonnages. Examples
of such products include milk, coffee, detergents, soaps, ceramics, catalysts
and many inorganic salts. The use of nozzle atomization for given products
is given under each product section. These are found in the chapters on
applications, chapters 13-18 (section V).
193
(h) Nozzle Assemblies for Commercial Use
A single nozzle assembly is used wherever capacity and particle size requirements of the dryer can be fulfilled without nozzle duplication. A single
nozzle layout simplifies the operation by its ease of control and ease of nozzle
observation.
In industrial sized dryers utilizing nozzle atomization, multi-nozzle
assemblies are invariably installed to handle high feed rates. Assemblies
are designed to provide equal operational conditions at each nozzle for
uniformity of feed atomization. Each nozzle is located so that constant
evaporation conditions are obtained around each nozzle. There are two
locations of nozzle assemblies commonly used in co-current flow spray
dryers. Nozzles are mounted around the periphery of the chamber, or nozzles
form a cluster and are positioned at the top of the dryer surrounded by a
ceiling (roof) air disperser. Both types have their advantages and disadvantages. The wall assembly permits ease of access to the nozzles from the dryer
side enabling nozzle replacement and maintenance to be carried out quickly
and simply. Observation of the performance of each nozzle is straightforward. Greater care has to be given in providing uniformity of flow to each
nozzle. This is especially so if slurries are being atomized as there is a tendency for the feed solids to settle out in the horizontal feed distributor ring.
However, this nozzle assembly is widely used and with much success. The
cluster assembly claims improved evaporation conditions by locating the
nozzles in the centre of the air disperser for optimum spray—air contact.
Access to the nozzle cluster, and observation of nozzles is more complicated
but such complications are offset by the advantages of uniform feed distribution to each nozzle.
The arrangement of nozzles with access to each nozzle through the dryer
wall is applied to dryers of counter-current and mixed flow type. Nozzles
arranged in clusters from a single feed pipe system are not so common.
For counter-current flow dryers, non-rotary air flow conditions often prevail
and the individual positioning of nozzles via the dryer wall enables convenient nozzle adjustment for creating effective spray coverage over the
chamber cross-sectional area. For mixed flow dryers, a nozzle cluster on a
single feed pipe gives poor nozzle accessibility. Nozzles are normally
mounted singly via the dryer cone wall. With this mounting only short feed
pipe lengths are required to position each nozzle in the central area of the
chamber. The advantage of easy accessibility to each nozzle is obtained
without the constructional difficulties of maintaining rigid nozzle positioning
over extended pipe projection lengths into the chamber.
The number of access points around the chamber wall or the number of
feed pipes making up a nozzle cluster can be minimized by the use of multihead nozzles. Each multi-head nozzle can operate 2-3 orifices effectively
FEED IN
AIR IN
AIR
IN
FEED
IN
ET?'
PRIXECT
OUT
D.
AIR OUT
ONIANCE AVIIdS AO SRO NUS
SSRDOIld al-II
AIR IN
FEED IN
FEED IN
AIR
IN
AIR IN -.'
G.
PRODUCT
OUT
H.
PRODUCT
OUT
Figure 6.48. Positioning of nozzles in relation to air disperser.
Table 6.16. Key to Figure 6.48
Air flow pattern in
chamber
Chamber design
Air disperser type
Conical base
Conical base
Conical base
Flat base
Conical or flat base
Roof
Tangential entry (top)
Roof
End
Base
F
Vertical co-current
Rotary
Vertical co-current
Rotary
Vertical co-current
Parallel
Horizontal co-current Rotary
Vertical counterRotary or parallel
current
Mixed
Rotary
G
H
Mixed
Mixed
Conical base
Conical or flat base
A
B
C
D
E
Rotary
Rotary
Conical base
Nozzle assembly
Single or multi heads, in cluster
Single or multi heads, in cluster
Single or multi heads, side mounted
Single, end mounted
Single or multi heads, in cluster or
side mounted
Roof
Single or multi heads, central location
Tangential entry (top) Single or multi head, in cluster
Tangential entry (side) 'Single or multi head, in cluster
NOLLVZINIOIV
Dryer type
196
THE PROCESS STAGES OF SPRAY DRYING
in spray dryers. Should one of the orifices function incorrectly, however,
the whole nozzle head has to be shut off, removed from the dryer and dismantled. This deprives the chamber of the throughput of 2 or 3 orifices
and can contribute to a marked reduction in dryer capacity if, for instance,
only four nozzle heads are mounted in the chamber. Reduction in capacity
need only be short lived, as with good access to the chamber, nozzle exchange
can be quickly accomplished. Where any reduction of capacity is undesirable
(even over short time periods) a stand-by nozzle arrangement is incorporated
in the feed system layout.
The use of multi-head nozzles is most effective when nozzles are mounted
in close proximity of the air disperser. If multi-head nozzles are mounted
at the chamber wall, spray is distributed with less uniformity over the chamber
cross section. The high droplet population created by each nozzle is less well
contacted by drying air. Air can channel past the droplet population limiting
full utilization of the dryer capacity.
Whatever the type of nozzle assembly selected for commercial use, all
contrive to meet the following features.
1. Ease of access to nozzles.
2. Ease of nozzle removal.
3. Uniformity of feed distribution.
4. Possibility to isolate each nozzle.
5. Possibility to visually observe nozzles in operation.
The positioning of nozzle assemblies in relation to the dryer air disperser
is illustrated in figure 6.48. The key is given in table 6.16.
6.6.3. Pneumatic (Two-Fluid) Nozzle Atomization
(a) Theory
Pneumatic nozzle atomization involves impacting liquid bulk with high
velocity gas. The mechanism of atomization is one of high velocity gas
creating high frictional forces over liquid surfaces causing liquid disintegration into spray droplets.
The theory of liquid break-up follows the principles given in section 6.2
as proposed by Castleman (16) amongst others. Liquid disintegration in the
presence of gaseous flow involves complex situations of liquid instability,
but the overall process can be considered to occur in two phases. The first
phase involves the tearing of the liquid into filaments and large droplets.
The second phase completes the atomization by further breaking these
liquid forms into smaller and smaller droplets. The entire process is influenced by the magnitude of the liquid properties ; surface tension, density and
viscosity, and the gaseous flow properties of velocity and density. Gaseous
media used primarily in pneumatic nozzle atomization include air and steam.
Inert gas, e.g. nitrogen, is chosen for specialized closed cycle spray drying
systems. (Hereafter, in section 6.6.3 the gaseous atomizing medium will be
ATOMIZATION
197
referred to as air, unless stated otherwise.) A high relative velocity between
liquid and air must be generated so that liquid is subjected to optimum
frictional conditions. These conditions are generated by (a) expanding the
air to sonic or super-sonic velocities prior to contacting the liquid or (b)
directing the air flow on unstable thin liquid sheets formed by rotating
liquid within the nozzle. Liquid break up is so rapid and effective that
sprays of low mean droplet size are formed. Both low and high viscous
liquids can be handled.
For small liquid feed rates, rotation of liquid within the nozzle is not
essential for complete atomization. High velocity air can readily penetrate
a low velocity solid liquid jet, causing the necessary turbulence and energy
transfer to form a spray of narrow spray angle. Sprays are characterized
by high homogeneity. At larger liquid feed rates, however, even high
velocity air cannot penetrate the thick solid liquid jets involved. Atomization is incomplete, and there is a wide droplet size distribution throughout
the resulting spray. A high percentage of liquid remains in the centre of the
spray as a solid jet, and on leaving the nozzle is ejected over large distances.
At higher commercial feed rates the liquid bulk must be formed first into
thin sheets to assist liquid instability, for effective air—liquid contact and
break down of liquid into ligament form or individual droplets. Unless the
sheet formation or 'feed prefilming' takes place ineffective atomization
results, even at high air velocities. A solid jet of liquid is simply accelerated
by the air so rapidly that the liquid acquires the air velocity, minimizing
the shear forces between liquid and air. The whole atomization mechanism
cm.pends upon these shear forces. Hence pneumatic nozzles often feature
centrifugal pressure nozzle designs in the liquid nozzle head to enable the
formation of conical liquid sheets through liquid rotation within the
nozzle.
(b) Design Features
Various design techniques are available to produce optimum conditions of
liquid—air contact for atomization. All feature prefilming of the liquid in the
feed nozzle orifice, and air annulus sizing for acquiring optimum relative
velocity between air and liquid at the air nozzle orifice. Four designs are
featured :
1. Contacting of air and liquid within the nozzle head (internal mixing).
2. Contacting of air and liquid outside the nozzle head (external mixing).
3. Combined internal and external mixing by using two air flows within the
nozzle head (three-fluid nozzle).
4. Contacting air and liquid at the rim of a rotating nozzle head (pneumatic
cup atomizer).
Internal, external and combined mixing designs of convergent nozzles
are illustrated in figure 6.49(a), (b) and (c). Convergent nozzle heads are the
198
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
AIR
SLOTTED
SWIRL
IN SERT
ORIFICE
FEED-A1R
MIXING
CHAMBER
OINTERNAL MIXING (TWO-FLUID)
FEED
SLOTTED
SWIRL
INSERT
AIR ORIFICE
FEED ORIFICE
b) EXTERNAL MIXING (TWO-FLUID)
AIR
PRIMARY
AIR
!Ar.4.12LIN
FEED
SECONDARY
AIR
•Nit
am=4
•
Psil■..-2z&iz
7Aka
1P 'iwk
i .2, 417.10,..w...womakires
illi
1111!"- --15
-.....
w.,...h...1,,
Im
-_
..,11 11i 1. ., , r:. 7 111111MIIIINk.
■
Nil IIIMIIIIIIIIMMIIII
.•....em..i.lik
4AImi v. m
199
most widely applied in spray drying. The pneumatic cup atomizer is illustrated and described in section 6.6.4 (page 229).
Internal mixing designs offer the advantage of achieving higher energy
transfer* than with external mixing. However, external mixing enables
greater control of atomization by independent control of both liquid and
air streams.
The combined mixing nozzle is a combination of basic internal and external
mixing designs. It incorporates the high energy transfer advantages associated with the former type, with the greater control of the spray characteristics
obtainable with the latter type. Combined mixing designs provide greater
amounts of available kinetic energy for atomization. It is therefore used
with liquids of high viscosity, where the advantages of being able to atomize
such difficult liquids offset the very low efficiency of the nozzle and the large
amount of air required for nozzle operation. Patented nozzle designs are
given in references (451) to (463).
Counter-contact between air and feed enables the highest nozzle efficiency
to be attained. Referring to figure 6.49(c) the feed and primary air flows are
subjected to rotation in different direction. Relative velocity on contact is
at its highest, optimum energy transfer is created, and feed break-up is
completed to form a spray of small droplets. The central air jet of secondary
air has little rotation but acts to initiate feed—air mixing and feed break-up.
The pneumatic cup atomizer is used for high viscous liquids or to obtain
very fine sprays from low viscous liquids. The prefilming of liquid at the
rotating nozzle edge provides an excellent uniform liquid sheet for contact
with the air flow.
(c) Droplet Release and Spray Angle
Pneumatic nozzles have great flexibility in producing small droplet sizes
over a wide range of feed rates, but the small droplet size does not prevent
the occurrence of extensive spray penetration from the nozzle head. On
release from the nozzle orifice droplets are carried forward by the momentum
of the spray and expanding atomizing air. The droplet size characteristics
can be varied over a wide range by adjustment of the feed—air flow ratio at
the nozzle head.
Spray angles are narrow and cannot be varied appreciably by adjustment
of the feed—air flow ratios to the nozzle. The maximum spray angle is of the
4
* Even so, less than 0.5 % of applied energy is utilized in liquid break-up (18). Virtually the
whole amount is imparted to the liquid and air as kinetic energy. The horsepower requirement
assuming isentropic expansion can be expressed (30):
c) COMBINED INTERNAL—
EXTERNAL MIXING (THREE-FLUID)
Figure 6.49. Designs of pneumatic nozzle heads.
Px = 0 • 136M A T {0.5Ma
2
+ 2.5[1 --- (
P
0.286
HP
P2
where MA is lb/sec, T is °R. Ma is Mach No., P1 /1)2 is initial to final pressure ratio,
200
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
order 70-80°, and is obtainable at the rated maximum feed rate and air
pressure for the larger throughput nozzles. The spray angle is decreased
by decreasing the feed rate at constant air pressure. Decrease in air pressure
will increase the angle as long as the maximum angle has not been obtained.
Spray angles can be held constant by modulating air pressure with the feed
rate.
With internal mixing nozzles, the spray angle changes with the feed—air
ratio. The higher the ratio, the larger the spray angle. A mass ratio of feed—air
25 :1, for example, may feature a spray angle of 70-75°, a reduction in this
ratio to 8 :1 might well produce an angle of, say 50-55°. A typical relationship
between feed rate, air rate, and spray angle for an internal mixing nozzle is
shown in table 6.17. The effect on nozzle performance of holding either air
pressure, feed rate and spray angle constant is shown.
Table 6.17. Typical Performance of Internal Mixing
Type Nozzles
Aqueous
Air
feed rate Air rate pressure
(lb/hr)
(cfm)
(atm)
Spray
angle
Constant
variable
50
100
150
1150
1300
1400
2.0
0.85
0.25
12.5
11.5
10.0
1.35
1.35
1.35
3.4
3.4
3.4
45°
75°
80°
40°
50°
55°
air
200
200
200
1.45
2.5
3.8
1.35
2.05
2.7
70°
60°
50°
feed
rate
constant
1150
1250
1350
10.0
12.0
14.4
2.6
3.4
4.1
45°
45°
45°
spray
angle
constant
pressure
constant
With more viscous feeds, larger mean droplet sizes are produced, but homogeneity is not so high. Pneumatic nozzles have operational flexibility,
producing either small or large droplets over a wide range of feed rates
according to the air rates (pressures) applied. This is quite unlike the centrifugal pressure nozzle, where capacity has a limited range for complete atomization.
Nozzle designs producing small mean aqueous droplet sizes (10-20
micron) are reported by Gretzinger and Marshall (63). Nozzles producing
coarse aqueous sprays of mean droplets of the order 300 micron are reported
by Chen and Kevorkian (64) for internal mixing nozzles. Gretzinger and
Marshall used feed rates of the order 1 lb/min (0.45 kg/min) and small orifice
nozzles, whereas Chen and Kevorkian used feed rates of 90-250 lb/min
(40-115 kg/min) and larger diameter orifices (7/16-1 in, 11-25 mm). Large
mass flow ratios of feed to air (10:1) were used.
The production of fine sprays is dependent, however, upon the liquid
orifice being circular, and the air annulus (or annuli) being concentric.
Eccentric positioning of the liquid orifice in relation to the air nozzle produces sprays of deflected spray angle. The spray contains streaks of partially
atomized feed. When orifices are completely true, droplets are distributed
uniformly throughout the spray cone. Solid cone sprays are produced.
(e) Effect of Variables on Droplet Size
Pneumatic atomization has received attention from many workers, but
available data do not appear to correlate to any close degree. Available
correlations apply to particular nozzle designs operating over restricted
working ranges. It does appear, however, that compared with alternative
atomization techniques, pneumatic nozzles have been found truly functionable only at flow rates considered moderate by other techniques. Although
this may be so, pneumatic nozzles can provide the all important spray
distribution when handling feeds of more exacting liquid properties.
Studies (65) have shown that the mean spray size produced by pneumatic
atomization follows the relation
D=
In many low throughput nozzles, where extremely fine sprays are produced (mean diameter 15-20 micron), the spray angle may be as low as 10-20°.
However, this angle exists at the nozzle head, and within a short distance, the
spray disperses.
(d) Spray Characteristics
Sprays of low viscous feeds are characterized by low mean droplet sizes
and high degrees of homogeneity. Formation of sprays having a mass
median diameter of 15-20 micron is well established for pneumatic nozzles.
201
A
( V i?ei
par
B rirr
M iq
(6.88)
where Vrei = relative velocity between air and liquid at nozzle head,
Mair/Mlig = mass ratio of air to liquid.
The exponents c4 and fi are functions of nozzle design, and A, B are constants involving both nozzle design and liquid properties.
(i) Effect of Air/Liquid ratio Of ah./M iiq) on Droplet Size. The mass ratio
is one of the most important variables to effect mean droplet size. Increase
in the ratio decreases droplet size. Where the nozzle is used to produce
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
203
a
FEED - WATER
0
40
160
120
80
200
240
280
SPRAY MEAN DROPLET DIAMETER ( MICRONS )
0.
AIR VELOCITY- SONIC
100
AIR--LIQUID MASS RATIO
coarse sprays, the mass ratio becomes the most significant variable as depicted
in equation (6.88).
Values of M ai ,./Miiq range from 0.1 to 10.0. Below 0.1 atomization
deteriorates rapidly even with liquids that are readily atomized. The upper
value of 10.0 is the limit for effective ratio increase to create smaller sizes.
Above ratio values of 10 atomization proceeds with excess energy expenditure without marked decrease in mean droplet size. Investigators have
indicated that droplets of 5 micron do not disintegrate into smaller sizes
in the presence of high velocity air but experimental sampling of sprays from
nozzles show that sizes down to 1 micron are apparent. The mean droplet
size approaches a limiting value asymptotically for increased values of the
mass ratio. This is shown in figure 6.50(a), where the effect of air—liquid
mass ratio on spray mean droplet diameter at various values of atomizing
air velocity is depicted. The effect of air—liquid mass ratio on spray mean
droplet diameter at various values of feed viscosity is shown in figure 6.50(b).
The limiting value decreases with the (approximately) 0.36 power of the total
nozzle air rate.
(ii) Effect of Relative Velocity on Droplet Size. Mean droplet size decreases
with increase in relative velocity. This can be seen from a cross plot of figure
6.50(a). Increase of the relative velocity between air and liquid at the point
of contact increases the air dynamic force (V 2 p a ) in equation (6.88) and more
energy is available for atomization. The effect of relative velocity becomes
most pronounced when fine sprays are being formed at low feed rates.
Hence the limiting mean droplet size is directly related to the air velocity.
In fact, mean droplet size varies inversely with the air velocity to the power
of 1.14, and inversely to the power 0.57 of the air dynamic force.
(iii) Effect of Viscosity on Droplet Size. Increase in viscosity increases
mean droplet size. This follows basic atomization theory. The complexity
of the atomization mechanism prevents a precise variation to be established.
The exponent of viscosity lies in the range 0.30-0.37. Kim and Marshall (66)
propose an exponent of 0.322.
(iv) Effect of Air Density on Droplet Size. Increase in air density at constant
air velocity increases the air dynamic force, resulting in a decrease in mean
droplet size. This follows from equation (6.88).
(f) Prediction of Mean Droplet Size
The ability to predict spray size characteristics is a necessary requirement
whenever pneumatic nozzles are to be applied in spray dryers. Correlations
expressing nozzle performance are given in table 6.18. The three correlations
for two-fluid nozzles represent the most quoted generalized equations
available. Figure 6.51 shows nozzle designs on which data for the correlations were obtained. The mean droplet size of sprays is related to the operating nozzle conditions. One correlation is given for three-fluid nozzles.
AIR-LIQUID MASS
202
80 -
6-0 -
4.0
2.0-
250cp
5 cp
40
80
120
160
200
240
SPRAY MEAN DROPLET DIAMETER (MICRONS)
Figure 6.50. Effect of nozzle operating variables on spray mean droplet size. (a) Effect of air-
liquid mass ratio and air velocity on spray mean droplet diameter (typical performance curves
for two-fluid nozzles at intermediate feed rates). (b) effect of air-liquid mass ratio, liquid viscosity
on spray mean droplet diameter (typical performance curves for two-fluid nozzles at intermediate feed rates).
•
•
,:".
17z frq .
• > 0 . 72 CIL ,9
•
P_.
c.
P
• CD
0
THE PROCESS STAGES OF SPRAY DRYING
FL
"
rn
ro „
• Iv
P
0
P
P
C)
CD
a t !-1 - 5 g”
r, g
a
!..-1
. <
1,3 t'- co
r
0
Cy
en
p
6 F.4-
'11
°
O. !=
7 n ••
ch G.
')
P Q.
o
C9
CD
vi
0-
N
ET' S0
I-,
N
8' ,:
z
0 i
t
O
31ZZOtq 0rrT1J-a3aHi
•
0
N
0
Fp' -,-
w co
0.
cp
4:
•-•1
0
k4;:p
.
-
Table 6.18. Correlations for Predicting Droplet Size (micron) from Converging Pneumatic Nozzles
Nozzle type
Two-fluid converging
Equation
number Reference
Equation
°.s
= 1410[o-
+ 191
ill
91
"5
x 10 3 1•5
( 0)9 1) °-5
Q.
where IIa is ft/sec, p, is lb/ft 3 , o is dynes/cm, i.i, is cP, Q, is ft 3 /min.
Dvs
(6.89)
(65)
(6.90)
(63)
(6.91)
(66)
(6.92)
(66)
v. pi
M i-
H
DM,, = 2600 H
M GL
04
a
where M, is lb/min, 11,, is P, L is cm, G. is lb/min ft 2 .
r ( ) 0-17( 1 ) ( m a i ml
249o- ' 1 4 . 3 2
=
OY
-/ MtvID
( v r2eip JO- 5 7A0. 3 6 4-16 + 12601_
[
1
;C I"
M1)
where
m= —1 at (-- 2-) <3
ML
in = —0.5 at (
Three-fluid converging
D
8140
1 0-"i4 -32 )
°.11
1
+ 1240
54
0
P?'"
Pia
saw)
mmD
where
m = —1 at ( M
--1 < 3
M,
.
Ma
M
m = —0.5 at Via) > 3
,
f)( VreOsecondary air
where M, is lb/min, V is ft/sec, p is lb/ft 3 , o- is dynes/cm, is cP, f,„ is wt fraction.
Va(av) = fw( Vrel)primary air + •( 1
NOIIVZIIAlaLY
>3
ML
M, is lb/min, A is in 2 , p i is cP, o- is dynes/cm, p is lb/ft 3 .
206
Nozzle dimensions
Liquid nozzle (in)
Air nozzle (in)
Air anulus area (in')
Equation (6.90)
GretzingerMarshall (63)
Equations
(6.91), (6.92)
K imMarshall (66)
Internal mixing
Sauter mean
7-97
< sonic
43-75
19-73
0.3-30
1-10
External mixing
Mass mean
5-30
sonic
65
50
1-30
1-25
3.8-22
0.01-0.666
10-100
External mixing
Mass mean
6-350
250-sonic
50-65
30-50
8-50
0.06-40
0.7-10
0.015-2.0
0.10-80
0054-0.217
0.145-0.279
0.0035-0.0124
0054-0-222
0.120-0.272
0.00354.054
0.02-1.0
-
wiipii
p
...."
.
_I . yo-2
QA
°
SAUTER MEAN DIAMETER (MICRONS 1
Equation (6.89)NukiyamaTanasawa (65)
III.
1000
Table 6.19. Experimental Conditions Covered by Pneumatic Nozzle Droplet
Size Correlations
Design
Mean size
Mean size range (micron)
Air velocity (ft/sec)
Liquid density (lb/ft')
Surface tension (dynes/cm)
Liquid viscosity (cP)
Mass ratio iM
- a iMI
— liqu i d,1
Air rate (ft 3 /min)
Liquid rate (lb/min)
Atomizing pressure (prig)
207
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
°
L . 500
3
-
A
100
-
-
-
10
0,
--'- . 2 7.1 6 3
.,
_
QA
0
E_
3
—
° -10
A
I
1
-
3.- =104
°A
1
.
i 1 i . k
10 -4
1
L
1
I
l
f
i
t
.
i
I
I
-2
-3
0
10
RELATIVE) SEC
0.008-0.08
0.040-0200
00012-0.030
Operating conditions applicable to the equations in table 6.18 are given in
table 6.19. Other droplet size relations are given by Lapple (25) and Dombrowski (30).
(i) Nukiyama Tanasawa Equation (65). Equation (6.89) expresses the
Sauter mean diameter in terms of nozzle operational variables. The equation
is not dimensionally consistent. It is plotted for aqueous sprays in figure
6.52. The equation is frequently cited for general application, but in fact
was established for feed rates only up to one pound per minute with small
diameter nozzles. Applicable nozzle designs are shown in figure 6.51(a).
There is doubt on the validity of the equation, if air velocities within the
nozzle approach sonic values. At low feed rates, values of the volumetric
ratio (Q a/Q i ) exceed 5 x 10 3 in most designs at sonic velocities, and thus the
second term in equation (6.89) has little effect on the mean droplet size.
The correlation reduces to
1(
—
(6.93)
D cc
-
Vair PI
The exact relationship between relative velocity and the Sauter mean
diameter is not clear over the complete range of operating conditions.
Experience has shown that air velocities of the order of sonic with mass airliquid ratio of ten are required to produce mean sizes under 25 micron.
Figure 6.52. Nukiyama—Tanasawa plot for converging two-fluid nozzles operating on aqueous
feed.
For equation (6.89) in metric units, values of VA are expressed in m/sec
and viscosity in poise. The constant in the first term becomes 585 and the
constant in the second term becomes 597.
The droplet size distribution for two-fluid nozzles studied by NukiyamaTanasawa followed the function :
dN
=
d(D)
(6.94)
exp ( - cDg)
where N = number of droplets smaller than D, b.c. = constants, m = 2,
q = dispersion coefficient.
Equation (6.94) can be rewritten as
dN
d(D) = HO' exp ( - CD's)
(6.95)
Mugele and Evans (6) show that equation (6.95) can be rewritten as
In ( D2
1
dN
D(D))
In qc (31 q )
( 3)
r-
1 DN
a
log10
—( D2 . d(D)
) = (log10
CD 0
(6.96)
(6.96a)
208
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
where a is the term containing the gamma function. A plot of log in (1/D 2 .
diVid(D)) against D 4 will be a straight line if the spray follows the distribution.
For Nukiyama—Tanasawa nozzle designs, q, the dispersion coefficient can
be taken as unity. The corresponding plot is shown in figure 6.53. The
Sauter mean diameter can be calculated from the value of slope (c = b/2.3).
Mugele and Evans (6) show the Sauter mean diameter to be represented by
1/
r--6 ) (r-5 )
(6.97)
q
q
The slope of the plot, figure 6.53, is the measure of c in equation (6.97).
DSMD =
9 (
1
10
■
0
10
\
\
upon nozzle dimensions and cannot be equated automatically to unity.
Experimentation has shown, however, that 'q' does remain constant per
given nozzle design. Values of 'q' can vary between 0.15 and 2.0. For a
given spray sample, values of 'q' are selected by trial and error to determine
the value that gives the actual spray distribution the closest agreement to the
linear relationship of figure 6.53.
The size distribution is best conducted by counting droplets that fall into
equal size increments (AD), i.e. droplets within the range 7.5-12.5 :12.5-17.5
micron, etc, where AD -= 5. The number of droplets (AN) per given size
increment (AD) is obtained. For every size increment, the value of log 10
[(1/D 2 )(AN/AD)] is calculated, and plotted against Dq, where q is the selected
value. Values of q are adjusted to obtain the best linear relationship.
The relationship between Sauter mean diameter and the slope of the
Nukiyama—Tanasawa distribution plot for values of dispersion coefficient
is given in table 6.20.
10 -
Table 6.20. Relationship between Dispersion
Coefficient, Sauter Mean Diameter and Slope
of the Nukiyama-Tanasawa Plot (6)
q
3
10
1,
5
50
100
150
200
DVS
2
1
0.5
0.33
0.25
10-4
10
209
1.5 b ° ' s
5 b'
110b -2
4080b -3
2.13 x 10 5 b -4
b
2.25D vi
5
Dv-si
10.514s5
a.33
16.0D s
21-5Dv-sO.25
250
SAUTER MEAN DROPLET SIZE (MICRONS )
Figure 6.53. Nukiyana-Tanasawa distribution (a typical plot).
The gamma function (r) is described in many textbooks on advanced
mathematics. For example, see (67). Function values are obtained directly
from tables (68). When q = 1, the gamma function of any integer is the factorial of the integer minus one. Thus for any integer (n), I",, = (n — 1) !
For determination of the Sauter mean diameter, where half integer gamma
functions are involved in equation (6.97), F() = m 112 . If other fractions are
involved in the gamma function, tables are used. The higher the value of `q'
that correlates spray data to the Nukiyama—Tanasawa distribution, the
narrower the distribution.
General application of the distribution to other two-fluid nozzle designs is
doubtful, if the dispersion coefficient is kept al nity. `q' appears dependent
EXAMPLE 6.13
A convergent two-fluid nozzle (internal mixing design) is atomizing a
Newtonian fluid of 5 cP viscosity at a rate of 48 lb/hr. What mean droplet
size is expected if the atomizing air is regulated to a mass air—liquid ratio of
five?
Liquid Data
o- = 70 dynes/cm
p i = 80 lb/11 3
Nozzle Data
liquid orifice diameter = 0.18 in
air annulus area = 0.03 in'
Air Data
Atomizing air pressure 50 psig at 80°F.
210
THE PROCESS STAGES OF SPRAY DRYING
211
ATOMIZATION
Low feed rate and nozzle design suggests Nukiyama-Tanasawa equation
applicable.
48
0-8
Liquid feed rate (Q L) = — = 0.8 lb/min — = 0-01 ft 3 /min
80
60
I
5
I
I
I
I
I
))
=2500 LL ,i ) (Po L
D
PLOT OF ,
104
4
I
I
4
I
I
I
1
"
-
I
I
5
1
I
4
l
•
•
...
••■
—
...
Mass ratio = 5.
••
—
MEDIAN DROPLET DIAMETER
Air rate (QA) = 4.0 lb/min
Air density = 0.323 lb/ft 3
QA
4.0
12.4 ft 3 /min
0-323 '=
Q r _ 0-01
12.4 = 8-1 x 10'
n0.18 2
Area of liquid orifice =
= 0.0254 in 2
4
Area of air annulus = 0.03 in 2
—
.-
—
—
..•
—
••
—
10
15 -
QA
1
.
Liquid velocity negligible : Air velocity taken as the relative velocity.
Substituting values in equation (6.89)
. 5
r
5
19
+ 1(70 x 80)• 5 ]
= 1-42[0-875] °.5 + 191
5
[(5600)' 5
x 101
1.5
104
1 045 [8-1 x 10-1]1.5
= 1.02(0-935) + 191(0-067)" 5 (0-81) 1 ' s
= 1.33 + 191(0-296)(0-729)
= 1.33 + 41 = 42.33 micron
Note: As the liquid properties are similar to water, figure 6.52 can be used
to obtain an estimate of the mean droplet size.
QL
QA
= 8-1 x 10'
J.
= 0-00111 (sec/ft)
v
From figure 6.52 D vs = approx. 40 micron.
I
I
7
1
1
1.
1
i
e
i
l
l
i
—_
I
I
1
1
1
1.
,
.
Figure 6.54. Gretzinger—Marshall plot for converging two-fluid nozzles,
.
po ,
i
AIR-LIQUID MASS RATFO
12 4
144
Air velocity = — x
990 ft/sec
60
0 03 -
1410 I
i
Dvs =
( 990 80]
cc
1.0
0 01
144
0 . 94 ft/sec
60 x 0-0254 -
Liquid velocity
4,', L.104 ,
i cm,gmm
-
(ii) Gretzinger-Marshall equation (63). Equation (6.90) expresses the mass
mean diameter in terms of the nozzle operating variables. The equation is
shown plotted in figure 6.54. Metric units are applied to the parameter (p a L).
The mass mean diameter correlation was drawn up from data using a
convergent external nozzle design at low feed rates. An applicable nozzle
design is shoWn in figure 6.51(b). Mean droplet sizes within the range 5-30
micron were obtained at feed rates up to 40 lb/hr. Presentation of the droplet
size data in the form of figure 6.54 permits the determination of air and liquid
rates for mean diameters within this size range. The correlation is of
particular interest, as it offers a nozzle design procedure. The design method
by Gretzinger-Marshall (63) is illustrated in example 6.15. The method is
restricted to co-current contact between air and liquid, and is not applicable
if air contacts the liquid at right angles. For rapid and approximate determination of droplet size, a nomograph shown in figure 6.55 can be used.
The nomograph was drawn up by Chattergee (69) for air-water systems.
Mean droplet size for a Gretzinger-Marshall type nozzle can be obtained
directly from the diameter of the nozzle wetted periphery (inches) and the
nozzle feed rate, stated in imperial gallons per minute. The nomograph
proves a handy method to determine the range in which mean droplet sizes
( micron) of low viscous feeds, i.e. feeds having similar flow properties to
water, are likely to be produced. The droplet size distribution data obtained
212
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
NOZZLE AQUEOUS
FEED RATE
From figure 6.54
DMMD = 7.6 micron
{IMP GALLSPER
NOZZLE WETTED
PERIPHERY
(INCHES)
30
ZD
From equation (6.99)
1.0
Dvs = 5.3 micron
D•6
20
(b) Using figure 6.55 (assume feed properties as those of water). Now wetted
periphery = 0.18 in
0.3
1.0
D
30
0
Feed rate =
006
04
004
0.03
02
DMMD = 6.5 micron
0-01
D. MASS MEDIAN DIAMETER
1 MICRONS?
Figure 6.55. The Gretzinger-Marshall droplet size relation expressed as a nomograph (69).
for the external mixing nozzle are not found to fit standard distribution
functions. Gretzinger and Marshall expressed the size distribution in terms
of the mass median diameter and the geometric standard deviation (S e ).
The geometric standard deviation could be expressed by
= 1.77 D
(6.98)
The relation between the mass mean diameter (.13„,,,m ) and the Sauter mean
diameter (D vs ) for use where comparison with other correlations expressing
Dvs is required, is given by
log D M ,, m = log D vs + 1.1513 log' (S G )
(6.99)
General use of the equations (6.90) and (6.98) is not recommended unless
nozzles conform to the test nozzle design of external mixing and low feed
rates are used.
EXAMPLE 6.14
Determine the mean droplet diameter if data in example 6.13 is applied to an
external mixing nozzle. Use (a) Gretzinger-Marshall curves, (b) nomograph.
Data as in example 6.13 ; mass ratio = 5.
(a) Diameter of wetted perimeter (equated to liquid orifice diameter)
0.18 x 2.54 = 0.467 cm
Parameter (p a L) =
0 323
48
62.3
= 0.0623 imp gal/min
x
80
60
From figure 6.55
0.02
G
213
'
)0.467 = 23.6 x 10 -4
62.4
The accuracy of the predictions depends upon values of air density at the
point of liquid contact, and the contact periphery. These values are often
subject to speculation.
Figure 6.54 is applicable in determining suitable nozzle designs and
operational nozzle data for a desired mean droplet size. The GretzingerMarshall method for calculating nozzle performance is illustrated in example
6.15.
The Gretzinger-Marshall method is as follows :
1. For the desired mean droplet size, use figure 6.54 to obtain corresponding
values of the parameter (p a L) and mass ratio (M a/M,).
2. Assume likely values of air density (p a ), temperature (Ta ) and liquid nozzle
wall thickness (t d) for the system under consideration.
3. Calculate values of L from parameter values in step 1.
4. Tabulate internal diameter values of the liquid nozzle.
5. Calculate the liquid flow Reynolds Number (Re) in each case. Tabulate
values (Re = 4/x x QpIp) where Q = volumetric flow rate per unit tube
width. Delete chosen conditions, where Re is greater than 1000.
6. Calculate liquid film thickness (t) using the volumetric flow rate Q,
where Q = pgt3 1311. This is plotted in figure 6.56 for (Re) numbers less
than 1000. Plotted on log-form it supplies a direct relationship between
Q and t.
7. For correct nozzle operation, film thicknesses must be greater than 0.03
cm, but less than 0.06 cm. This range represents the most favourable atomization conditions.
8. Calculate atomizing air rate.
9. Calculate atomizing air annulus (A = M alV a p a ).
10. Calculate atomizing air annulus cross-sectional area.
214
THE PROCESS STAGES OF SPRAY DRYING
1
1
1
1
-
1 f 1
1
1
i
1
1
ATOMIZATION
1 ,
I
I
f
I' 1
1 11
I
I
1
I 1 I i
215
Table 6.21
0-1
:
-
-
,
1
, 0
, .i 1-
-
'7.
I
II
Iv
V
Parameter
( x 10 4 )
(cm)
(cm)
30
25
20
15
10
5
1.000
0.833
0.677
0.500
0.333
0.167
0.880
0.713
0.557
0.480
0.213
0.047
III
Estimated
condition
M c./M I
1
2
3
4
5
6
0.32
0.38
0.48
0.70
1.00
2.00
L
VI
Re
td
VII
(cm)
FFLM TRIMNESS t I CM
-
0.01
^
.
-
1
I
0.046
0.050
0.054
0.056
0.074
0.120
-
Re < 1000
0-301
364
448
575
668
1500
6800
1
1
1
1
1 1
I
1
j
1
10 -4
1
1 1 1
1
1.
I
1D-2
[ GA N
i
i
l
l i
f
r
I
1
1
3. Calculate values of L (metric units)
1 1 1
to°
10 t
For condition 1 =
cufEt
Figure 6.56. Plot of film thickness against feed rate (for flow conditions in nozzle head).
30 x 10 -4
= 1.0 cm
0-003
Therefore liquid tube thickness - 0.06 cm
diameter of tube t d = L - (2 x 0.06) = L - 0.12 cm
11. Calculate atomizing air pressure (P) using the equation according to
Fliegner (70):
p
M a (T) 112
(0.533)C D A .
(6.100)
12, Calculate geometric standard deviation (S G ) using equation (6.98).
6.15
Design a small converging external mixing pneumatic nozzle to atomize
20 lb/hr water and produce a mass mean diameter of 20 micron. What is the
corresponding Sauter mean diameter? The nozzle is to be operated at sonic
velocity.
1. For 20 micron, values of (p a L) and (M a/M,) are given in table 6.21 using
figure 6.54.
2. Assume p a 0.003 g/cm 3 , Ta = 80°, liquid tube thickness (t d) = 0.0235
in = 0.06 cm.
EXAMPLE
Sonic velocity at 80°F = (540°R) = (gKRT/MW) 112 = Va
(32.2 x 1.4 x 1.545 x 10 3 x 540) 1 / 2
29
Va = 1159 ft/sec
4
Re = x Qp,
12
it
3
3
where Q is volumetric flow rate per tube width (cm /sec cm) p i = 1 g/cm ,
= 0.01 g/cm sec (1 cP). Q is calculated from the given mass feed rate,
3
where 20 lb/hr -= 0.32 ft /hr
4
1 ft 3 = 2.83 x 10 cm
Q
3
= 0.32 x 2.83 x 104
3
= 2.86 cm /sec
3600 x 0.88
1-0
4
Re = - x 2-86 x -= 364.
0.01
it
(6.10)
where K = specific heat ratio, R =. gas constant = 1.545 x 10 3 ft lb/°R,
MW = molecular weight = 29.
Va
where L = 1.0 cm, t d = 0.880 cm.
4. Values of ❑ for conditions (2-6) given in table 6.21, column (V).
5. The Reynolds Number can be calculated
Values of Re are calculated for all conditions. These are given in table 6.21,
column VI. Conditions 5 and 6 are eliminated as Re is greater than 1000.
6. Calculate liquid film thickness (t). The figure 6.56 where t is plotted
against (Qpip). Thus for condition 1
Qp
2.86 x 0.01
I)
1.0
= 2.86 x 10 - z
cm 3 2
cm sec)
216
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
Corresponding value of t = 0.046 cm. Calculate values of t in conditions
2-4. Values given in table 6.21, column VII.
7. For favourable atomization conditions suitable values of t lie between
0.03-0.06 cm.
Any of the conditions (1-4) are applicable for nozzle design to produce
the required mass mean diameter. Condition 4 will be selected for its highest
air-liquid flow . ratio. Experience shows that a high air-liquid ratio is the
surest requirement for effective atomization to sprays requiring such a small
mean droplet diameter.
8. Calculate atomizing air rate
- 0 70
Now M 1 = 0.333 lb/min. Hence M a = 0.233 lb/min.
9. Calculate air annulus (a). Now A = M alV a p a and by substituting values
in consistent units, A = 0.00406 in 2 .
10. Calculate diameter of outer air annulus (d o ). From nozzle geometry
A = (7r14)(4 - di) where outer diameter of liquid tube = (0.480 + 0.12) =
0.60 cm = d 1 . Hence
d i = 0.236 in
A = 0.00406 in'
0.00406
+ 0.236 2 = d1).
0.785
0.247 in = d o
11. Calculate atomizing air pressure (P). By applying the Fliegner equation
(6.100) where M a = 3.88 x 10' lb/sec, T = 540°K, CD -= 0.95, A = 0.00406
in 2 . P = 43.7 psia.
12. Calculate the geometric standard deviation (S G ) of the resulting spray.
Using equation (6.98)
S, = 1.77(20' 14 ) = 2.72
The Sauter mean diameter for the spray is given by equation (6.99)
log 20 = log D v , + 1.1513 log' 2.72
13010 = log, 0 D v , + 1.1513 log1 0 2.72
Dvs 12.1 micron.
The nozzle design for the specifications of the problem is depicted in figure
6.57.
AIR TEMPERATURE Ta = 80
217
0
AIR FLOW = 0.233 lb /rni n AIR
LIQUID FLOW =0333 Ib AnLIQUID '
AIR
AIR VELOCITY Va .1159 ft/sec
AIR PRESSURE P 43.7 psia
a
Figure 6.57. Dimensions of nozzle design in example 6.15.
(iii) Kim-Marshall equation (66) (Two-Fluid Nozzle Atomizer). Equation
(6.91) expressed the mass median diameter in terms of the nozzle dimensions
and operating conditions. The applicable nozzle design is shown in figure
6.51(c). It is applicable to convergent external mixing designs operating
with Newtonian liquids. The equation is considered acceptable in representing various designs of pneumatic nozzles since difference in designs are
allowed for in the value of the constants. The value of the powers appear
fixed if convergent designs are considered. The equation has been developed
from results obtained over a wide range of operating variables, and appears
applicable to commercial nozzle conditions. The dynamic force of the
atomizing medium and the mass air-liquid ratio are clearly shown to be the
two most important variables of nozzle performance. There is a similarity
with the Nukiyama-Tanasawa equation, but the Kim-Marshall equation
predicts larger droplet sizes especially in the smaller size ranges under similar
conditions. Closer agreement with the Gretzinger-Marshall equation has
been suggested if the second constant in equation (6.91) is increased from
1260 to 3100 (66).
Droplet size distributions from the Kim-Marshall test nozzles do not
comply with the logarithmic, square root probability or Rosin Rammler
distribution functions.
Analysis of spray data having the same order of D i, /,„ D , but obtained from
varying operating conditions, has shown percentage diameters of spray
droplets from convergent type nozzles to be linearly proportional to the
median diameters. Droplet diameters corresponding to 10, 25, 75 and 90 %
of the cumulative volume distribution can be obtained from the mass mean
diameter as :
D10 = 0.33 DMMD
(6.102a)
.
D25 = 0 57Dmmn
(6.102b)
D75 = 1•51DmmD
(6.102c)
D90 = 1 •98DMMD
(6.102d)
218
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
For comparison of data with alternative correlations involving the Sauter
mean diameter D vs , the relation
Dvs = 0.83D m „
Data
cr = 70 dynes/cm
(6.103)
= 5 cP
can be applied.
A distribution function for pneumatic nozzle designs in general has been
shown by Kim-Marshall to correlate with remarkable accuracy to the
modified logistic equation. Based on the logistic equation by Pearl-Reed
(71), the cumulative distribution (0,) less than a diameter D (volume basis) is
related to droplet size as
(1),
b(1 b)
b - exp (arD)
b
(D v
b=
1 - b
1 + c exp (rx*)
(6.105)
A plot of log [(1 - b)/(0,, - b)] against x* represents the volume distribution
curve as a straight line, where values of r and c are obtained from the graph
as the slope and intercept, where r = - 2.18, c = 6.67. Substitution of these
values in equation (6.105), the relative cumulative distribution less than D
becomes
c1) „, = (
1.15
)
Tiro = 990 ft/sec
p a = 0.323 lb/ft'
p i = 80 lb/ft 3
Ma = 5
(6.104)
where r is a parameter, and a and b are constants. Kim-Marshall used a
size parameter (x*) defined by the ratio (D/D MMD ). Equation (6.105) rewritten
in terms of (x*), where a = (D MMD ) - ' becomes
0.150
(6.106)
1 + 6.67 exp ( 2.18x*)
Equation (6.106) has an inflection point at
= 0.42 and x* = 0.87.
The constants (r and c) appear to hold for pneumatic nozzles of different
dimensions and design features. Equation (6.106) can be used to predict
size distributions of pneumatic nozzles.
Differentiation of equation (6.106) with respect to x*, yields the volume
frequency c/),, function.
16.7 exp ( 2.18x*)
[1 + 6.67 exp (- 2.18x*)] 2
(6.107)
Similar distributions on number and surface basis are derivable by use of
appropriate multiplying factors.
EXAMPLE 6.16
Compare the nozzle performance predicted in examples (6.13) and (6.14)
with a prediction using the Kim-Marshall equation.
219
A = 0.03 in 2
A. Sauter Mean Diameter (Dva ) using Kim-Marshall equation. Substituting
in equation (6.91)
)
249(70 0 ' 4 1 )(5 0.3 2 0.36
0-$7
2
DMMD
(80) ° '"
(0.03)
[(990) x (0.323)]
(5 )2 \ 0.17
1
c) - 0 5
+ 1260
x ((990)o.54)k-i
80 x 70 )
D M MD = 8.4 micron
From equation (6.103)
Dvs = 0.83 x 8.4 = 7.0 micron
B. Size Distribution of Spray
For cumulative distribution and frequency data equations (6.106) and (6.107).
Data for values of x* are given in table 6.22. The cumulative distribution
and frequency curves are plotted in figure 6.58 from data in table 6,22.
Table 6.22
X*
Dmicron
Cumulative
distribution
(relative)
0.25
0.5
0.87
1.0
1.5
2.0
2.5
2.1
4.2
7.3
84
12.5
16.8
21.0
0.086
0.208
0425
0.505
0.770
0.910
0.970
Frequency
(relative)
(volume basis)
0.410
0.525
0.650
0.400
0.160
0.068
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
10
0 ,8
)
06
- 0.4
inflection
point
(7.3 )(0.4 2 5)
> 0.2
- 0.2
w
CC
0
4
8
12
15
24
20
MASS MEAN DIAMETER Limp
RELATIVE CUMULATIVE DISTRIBUTION LESS
1 .0
(VOLUME BASIS I
220
H
z
Figure 6.58. Size distribution for two-fluid nozzle (after Kim—Marshall (66)), (a) Cumulative
plot. (b) Frequency plot.
Droplet sizes corresponding to 10, 25, 75 and 90 % of the cumulative
volume distribution can be calculated directly from equation (6.102).
221
The close agreement between the Gretzinger—Marshall and the Kim—
Marshall predictions but substantial difference with the NukiyamaTanasawa prediction is due to the design of nozzles used in the investigation.
The Japanese workers used internal mixing designs, while the American
workers used external mixing designs. Use of the correlations must be
restricted to the design of nozzle under investigation. The topic of nozzle
dimensions and the effect on spray size remains highly controversial. Nukiyama—Tanasawa report no effect on mean droplet size for increase in liquid
nozzle diameter in their designs. Gretzinger reports decrease in spray size,
whereas diverging nozzle designs show increased spray size.
D. Kim—Marshall equation (Three-Fluid Nozzle Atomizer)
Equation (6.92) correlates the performance of a three-fluid nozzle atomizer.
The nozzle design is shown in figure 6.51(e). The liquid flow is contacted on two
sides by two separate air flows. Such nozzles perform similar to two-fluid converging designs. Spray uniformity is improved by virtue of greater energy
being available for atomization. The overall efficiency of the nozzle is
lower. The form of equation (6.92) resembles equation (6.91). The exponent
of the air dynamic force increases to 0.72 from 0.57. This suggests the
mechanism of atomization for the three-fluid designs may differ from
conventional designs. Air rate remains the greatest influence on droplet size.
Inspection of the equation shows the velocity of contact between air and
liquid to be determined from the mass fractions (f,,,) of air flowing in the
primary and secondary nozzle annuli. The velocity is given by
D 14 = 2.8 micron
V = (MT/re ,) primary nozzle) + (1 — f,,(K e ,) secondary nozzle) (6.108)
4.8 micron
where f,, = fraction of total atomizing mass air rate passing to primary
nozzle.
(g) Comparison between the Spray Characteristics of Two- and Three-Fluid
Nozzles
The spray characteristics obtained by two- and three-fluid nozzles are
similar when atomizing low viscous Newtonian feeds at low to intermediate
feed rates. A single air stream is sufficient to complete liquid break-up
and achieve a fine atomization. The second air stream of the three-fluid
nozzle often raises the air—liquid flow ratio above the limit where further
increase of this ratio produces a reduction in mean spray size. Use of the
second air stream merely constitutes a waste of energy, except perhaps for
high feed rates of low viscous liquids, when a marginal improvement in
spray characteristics is obtained for this extra energy input.
Two-fluid nozzles are available from manufacturers' stocks in all sizes.
Small nozzles can produce sprays of mean size down to 30 micron on relatively low viscous feeds. Larger nozzles for commercial use produce coarser
sprays as higher feed rates must be handled per nozzle.
D25 =
D75 = 12.7 micron
D90 = 16.6 micron
C. Comparison of Mean Droplet Size Data
Results using the three proposed correlations, equations (6.89), (6.90), (6.91)
on identical nozzle operational data are given in table 6.23.
Table 6.23. Comparison of Predicted Mean Droplet Sizes
Correlation
Sauter mean diameter
(micron)
Nozzle design
Nukiyama—Tanasawa
Gretzinger—Marshall
Kim—Marshall
43.3
5.3
7-0
internal mixing
external mixing
external mixing
222
THE PROCESS STAGES OF SPRAY DRYING
For high viscous feeds, there is a great need for the second air stream of the
three-fluid nozzle, if complete atomization is to be achieved, and a fine
spray produced. For high feed rates, substantial air flows are required in
each nozzle stream. For non-Newtonian high viscous feeds, a two-fluid
nozzle will prove satisfactory only if feed rates are low. As feed viscosity is
increased, a two-fluid nozzle will commence to produce coarse irregular
atomization at a much lower viscosity level than with a three-fluid nozzle.
Hereby lie the main advantages of three-fluid nozzles in spray drying (326) ;
(a) the ability to atomize higher viscous feeds and yet form sprays of low
mean droplet size, and (b) to produce a narrower droplet size distribution
with low viscous feeds.
Comparing a two- and three-fluid nozzle of similar size and operating
under similar conditions, the three-fluid nozzle will generally produce mean
spray sizes 10-15 lower than two-fluid nozzle for mean sizes under 100
micron.
(h) Pneumatic Nozzle Assemblies and their Operation
Pneumatic nozzles are mounted individually around the chamber wall or
roof, or in cluster form located in close proximity to the air disperser.
Mounting positions are similar to those described for centrifugal pressure
nozzles. The desirable features of mounting as listed on page 193 also apply.
Nozzle access and nozzle removal is complicated by having both air and
feed lines to each nozzle. The piping arrangement becomes even more
complex with three-fluid nozzles, where for each feed line, two independently controlled air supply lines are connected to each nozzle.
Wherever nozzles are mounted in the drying chamber, the piping at the
nozzle body is covered by a sheath. The sheath forms a compact arrangement.
The sheath exposes a minimum of surface area to the drying air while providing a surface that is easy to clean. Nozzle piping connections for twofluid (both internal and external types) and three-fluid nozzles are shown in
figure 6.59. All supply connections are made outside the drying chamber,
so that the complete nozzle arrangement can be lifted out of the chamber
for cleaning or maintenance.
The feed system for pneumatic nozzles involves separate control of both
feed and gaseous medium (air/steam) supply. The basic feed system for twofluid nozzles is shown in figure 6.60(a). It is established practice to have air
flowing to the nozzle before feed is passed to the nozzle. This prevents the
nozzle spitting on start-up. A uniform spray and clean nozzle head conditions must be obtained right from the start of operation. Feed deposits
during start-up invariably increase during operation, causing deterioration
in atomization.
Control can be either manual or automatic. Compressed air from a
general supply line or local compressor is filtered and the pressure is adjusted
ATOMIZATION
SHEATH
SHEATH
a.
223
b.
C.
Figure 6.59. Piping connections for pneumatic nozzles. (a) Piping connection for a two-fluid
nozzle (external mixing). (b) Piping connection for a two-fluid nozzle (internal mixing).
(c) Piping connection for a three-fluid nozzle.
at a pressure reducing valve to correspond with the operating nozzle pressure. Air flow rate is adjusted by a control valve, and the actual flow rate is
observed on a metering device, e.g. a rotameter. When steam is used as the
atomizing medium, supplies to the nozzle must also pass through a strainer.
This is to prevent any scale or rust particles lodging in the nozzle annuli
creating mal-distribution of steam. Feed is pumped to the nozzle via a strainer.
All oversize particles, whether foreign matter or product lumps must be
removed as the slightest of blockages will give poor nozzle performance.
For internal mixing two-fluid nozzle designs, pressures in the feed and air
lines are virtually equal. For external mixing two-fluid nozzles, air and feed
flow are independent of each other and the pressure depends upon the annuli
areas and required flow rates. For three-fluid nozzles, the central air flow
contacts feed within the nozzle, and the pressure of these flows is closely
related. The velocity of feed entering the nozzle is much lower than the air
flow, which then acts with an ejector effect on the feed flow. Feed pressure
at the nozzle is slightly lower 1-i_an the air pressure.
This ejector effect often enables low feed rates to be handled by the nozzle
without the need for a feed pump. Figure 6.60(b), (c) shows two feed systems
224
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
PRESSURE
REDUCING
VALVE
ROTAMETER
CONTROL
VALVE
AIR
FILTER
FEED
WELL__
FLOW
METERING
DEVICE
COMPRESSED
AIR
FEED
FOSITNE
DISPLACEMENT
PUMP
a
FEED
ATOMIZING
AIR
NOZZLE
CONSTANT HEAD
FEED TANK
FEED
NOZZLE
STRAINER
OVERFLOW
CONSTANT HEAD
FEED TANK
FEED
DISTRIBUTOR
RING
225
Air and feed pressures for pneumatic nozzles are observed on standard
Bourdon-type pressure gauges. A diaphragm inlet to the feed pressure
gauge is required to prevent feed entering the gauge mechanism. The feed
rate is conveniently measured by a metering device of the magnetic flow type.
An automatic feed—air flow system is shown in figure 6.60(d). The air flow
is set manually at the control valve. The air pressure required at the nozzle
is prefixed by manual adjustment of the pressure reducing valve. The air
flow rate is metered continuously at an orifice plate or venturi. The pressure
difference is monitored and transmitted to the ratio recording controller.
The controller adjusts the feed rate so that the air—feed flow ratio is maintained. Any change in the air rate causes the air—feed ratio to move away
from the set value. The correct ratio is restored by adjustment to the feed
pump speed.
Distribution of air and feed is from circular rings mounted around the
exterior of the drying chamber or on the roof, figure 6.61. This arrangement enables feed and air flow to each nozzle to be shut off when removing
nozzles for cleaning or maintenance. If air is used as the gaseous atomizing medium, prefiltering can take place at the point of take-off from the
DRAIN
DRYING AI R
C.
b,
TWO -FLUID
NOZZLE
FEED
DRYING AIR
DRYING A IR
ATOMIZING AIR
FEED
ATOMIZING
AIR
ORIFICE PLATE
OR VENTURI
CONTROL
VALVE (MANUAL)
AIR OR STEAM
RATIO
RECORDING
CONTROLLER
MAGNETIC
FLOW METER
PREFILTERED
PEED
MONO PUMP
WITH VARIABLE
SFEED DRIVE
AND PNEUMATIC
CONTROL
a.
DRYING AIR
FEED
Figure 6,60. Feed systems for pneumatic nozzles (two-fluid). (a) Basic system. (b) Siphon feed
system. (c) Gravity feed system. (d) Automatic feed system (automatic air—feed ratio control).
C.
ATOMIZING
AIR
that do not require pumps. Liquid passes to the nozzle under either siphon
or gravity feed. These systems are straightforward to operate, and provide
uniform feed flow. Successful operation depends upon the maintenance of
the required siphon and liquid head.
b.
Figure 6.61. Feed and air distribution rings for pneumatic nozzles. (a) Two-fluid nozzle, roof
ring distributor. (b) Two-fluid nozzle, wall ring distributor. (c) Three-fluid nozzle, ring distributor in air disperser.
226
THE PROCESS STAGES OF SPRAY DRYING
supply main. If steam is used, there is always scale formation throughout
the pipework, and filters must be placed as close to the nozzle body as is
conveniently possible.
Modification to the standard feed system is required for paste-like
materials. Many such materials have poor flow properties yet are able to be
effectively atomized in pneumatic nozzles of the external mixing type.
Pumping pastes into a distribution ring layout is often not possible, or if a
degree of pumpability is attained, distribution to each nozzle is by no means
satisfactory. In these cases, thick pastes or filter cakes are screw-fed to the
nozzle. This involves a complex feed arrangement for each nozzle and therefore is most conveniently adopted in small to medium sized dryers, where a
single nozzle mounting is sufficient to meet the drying chamber evaporative
capacity.
A screw feed pneumatic nozzle is shown in figure 6.62. The thick paste
is fed continuously or dumped batchwise into the hopper. The paste is fed
to the nozzle orifice by two screw feeders, one in the horizontal and the other
in the vertical plane. Paste is pressed through an adjustable feed annulus
and immediately contacted with high velocity atomizing air. In the nozzle
head the air flows from an annulus within the feed annulus. Turba and
Nemath (72) describe this type of nozzle design.
AIR
FEED HOPPER
SCREW FEEDERS
NOZZLE HEAD
Figure 6.62. Two-fluid paste nozzle with screw feeder.
Nozzle cleanliness, whatever the nozzle design, must be maintained
at all times. Flushing the nozzle with water is often not sufficient to thoroughly clean the nozzle passages, and any remaining product will dry out during
dryer shut down. Dismantling the nozzles during shut down is the recom-
ATOMIZATION
227
mended procedure, and wherever labour conditions exist to permit such
practice, this should be carried out. It is useful to recall that dried out
product remaining in the nozzle head on dryer start-up can cause as many
performance problems as a dirty gaseous medium supply. Dryer shut down
or reduction in capacity due to mal-operating nozzles is more costly than
the extra time spent to ensure complete nozzle cleanliness.
(1) Advantages and Disadvantages of Pneumatic Nozzles.
The advantages of pneumatic nozzles lie in their capability to produce
sprays of high homogeneity and of small mean droplet size. These characteristics can be obtained over a wide range of operating conditions, while
handling either high or low viscous feeds. The nozzle design has its operational advantages over other nozzle types. The presence of liquid orifices
larger than those in centrifugal pressure nozzles reduces the likelihood of
nozzle clogging under normal working conditions. However, the nozzle
annulus or orifice for the gaseous medium flow is much smaller by comparison, and all foreign matter that can lodge in the nozzle must be prevented
from reaching the nozzle head. Filtered compressed air or clean steam is
essential. Pneumatic nozzles do not require high pressure pumping equipment, a notable advantage over centrifugal pressure nozzles, where high
pressure working can give operational and maintenance problems.
The disadvantages of pneumatic nozzles concern (i) the high cost of compressed air, (ii) the reduction in spray dryer capacity due to cold atomizing
air entering the chamber, and (iii) very low nozzle efficiency.
Re. 1. Where air compressors are required on site to supply the nozzle
system, a sizeable investment is usually involved. Once installment is completed, the cost of producing compressed air becomes expensive as consumptions are high. In small dryers, for example, where similar spray characteristics can be achieved by both pneumatic nozzles and a rotating atomizer
wheel, to achieve the same evaporative capacity from identical feeds successful operation of the nozzles may require 20 HP compressors compared with a
5 HP rotary atomizer drive. Performance of nozzles in straight competition
with rotary atomizers suffers badly on power consumption.
Re. 2. The reduction in available dryer evaporative capacity, due to cold
atomizing medium entering the chamber can also be significant. There
are many small spray dryer designs that can be operated successfully on
either pneumatic nozzles or atomizer wheels, but under similar heat input,
spray and feed conditions, the dryer evaporative capacity with pneumatic
nozzles can be as much as 20 % lower than the evaporation capacity achieved
by using an atomizer wheel.
Re, 3. The low efficiency is borne out from the remarks above concerning
air compressor operational costs. Defining efficiency as the ratio of energy
required to create liquid bulk break-up into droplets divided by the input
228
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
of energy supplied to the nozzle, the energy input is high and the efficiency
subsequently low. Efficiencies are of the order of a fraction of a percentage
to produce large droplets of 100 micron size. To produce sprays of droplets
of the order 10-20 micron, efficiencies are only a few hundreds of a percentage.
In spite of some notable disadvantages, pneumatic nozzles are often used
in spray dryers. Their ability to atomize high and low viscous feeds, and
produce fine or coarse sprays of special size distribution are particularly
attractive to industry.
(j) Applications
Pneumatic nozzles are used mainly to form very fine sprays of low viscous
Newtonian liquids. All nozzle manufacturers offer nozzles that cover a
wide range of capacities. Nozzles also find applications in atomizing
highly viscous liquids, whether they exhibit Newtonian or non-Newtonian
flow characteristics. Such feeds include slurries or pastes, gelatine, plastics,
glues and pre-gelatinized corn starches. Pneumatic nozzles are suitable
for products where flow properties cause sharp increases in viscosity with
shear. Such products cannot be atomized in pressure nozzles, but are successfully handled in pneumatic nozzles, where atomization is achieved by
application of very low shear stresses, e.g. certain p.v.c. emulsions.
The ability of the nozzles to form fine or coarse sprays makes them ideal
for use in small laboratory or pilot-plant dryers. Such flexibility is illustrated
in table 6.24 by a 3 mm orifice internal mixing nozzle operating on a 60
Table 6.24. Tabulated Data Showing Pneumatic
Nozzle Flexibility
Feed material: 60 % solid clay slip
Orifice diameter : 3 mm
1.2 metre diameter chamber
Inlet dryer temperature (°C)
Outlet dryer temperature (°C)
Air rate (NM 3 /hr)
Air pressure (atm)
Feed rate (kg/min)
Feed pressure (atm)
Moisture content ( % H 2 O}
Particle size analysis
% > 500 micron
> 250 micron
% > 120 micron
% > 60 micron
(2)
(1)
Coarse
Fine
atomization atomization
375
105
3.4
1.75
0.9
1.6
1.4
375
105
1.9
1.5
1.2
08
5.3
0
4
38
86
4
26
68
94
229
solids clay slip (feed viscosity = 40 centipoise) in a 4 ft (1.2 m) diameter
pilot plant dryer. Operational data to form a fine spray is given in column
(1), together with the particle size distribution. A 33 % increase in feed rate
accompanied by a 46 % reduction in air rate still maintained complete
atomization conditions and produced a coarse spray. The coarse spray
distribution is given in column (2).
Atomization performance of many non-Newtonian liquids in small
diameter dryers has been effectively reproduced with large capacities through
duplication of scale-up nozzles. The scale-up ability has established usage
of pneumatic nozzles in industrial-sized dryers.
Use of pneumatic nozzles for heavy pastes has great potential as it offers
marked advantages over alternative atomization techniques. Much development is being conducted into aspects of nozzle design for handling high paste
throughputs. A typical case is the Baran (73) careless nozzle, reported to
handle up to 8000 lb/hr (3600 kg/hr) of 1000 poise paste feeds. Areas to
which this type of nozzle design has been successfully applied include titanium
dioxide, calcium carbonate, clays, stearates, corn starch, organic and inorganic pigments.
6.6.4. Rotating Pneumatic Cup Atomizers
(a) Introduction
The combination of a rotating cup and an air flow directed at the cup rim
forms a pneumatic cup atomizer. It is an atomizer development for specific
cases of(a) obtaining very fine sprays from low viscous feeds and (b) atomizing
high viscous liquids (that exhibit flow characteristics not ideal for conventional atomization techniques) to sprays of coarse mean size.
The stages of atomization are shown diagrammatically in figure 6.63.
The atomization mechanism follows the principles of liquid disintegration
by high speed air jets. Liquid flow over the rotating cup produces a liquid
film at the cup rim. The high speed air issuing from the air annulus in close
proximity of the cup rim causes first film instability followed by film disintegration into droplets. The interaction between air and liquid causes excessive
mixing around the cup. Hot or cold air, or steam can be used as the atomizing
medium. The success of this atomizer is due to the rotating cup acting as a
prefilming device, forming at the cup rim thin liquid sheets of uniform
thickness, ideal for break-up into droplets by the impinging air flow. The
theory of operation has been extensively reported by Frazer, Dombrowski
and Routley (74).
A rotating pneumatic cup atomizer is shown in figure 6.64. Feed flows
to the rotating cup from a feed pipe mounted within the hollow atomizer
spindle, A liquid distributor mounted at the spindle end, distributes the
feed via circular holes on to the inner surface of the cup. A weir is sometimes
incorporated in the inner cup surface design to assist even distribution of
230
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
231
FEED PIPE
CUP
FEED
LIQUID
FLOW
AIR
ANNULUS
AIR
FLOW
FEED FILM
FILM
INSTABILITY
11 1 L ■
, r
7 , •:.-
LI QUID FILM
DISINTEGRATION
•
/
• / ?..
-
,.K \
AIR
SWIRL
VANES
Figure 6.63. Atomization mechanism for a pneumatic cup atomizer,
feed over the entire inner surface area. The liquid film streaming off the cup
is impacted by air flowing from the air nozzle annulus. The air velocity is
in the sonic range. Within the air nozzle, swirl motion is generated by directing air flow over inclined vanes. In the diagram sealed spindle bearings are
shown. These bearings can be air cooled when steam or hot air is used as
the atomizing medium.
Spray angles from pneumatic cup atomizers are small. Spray droplets
are carried rapidly forward with the momentum of the spray. Drying
chambers require an extended vertical height to compensate the trajectory
distance of droplets from the cup before effective spray-drying air contact
is achieved.
The effectiveness of the atomization mechanism enables difficult, high
viscous liquids to be atomized at feed rates high enough to be of interest to
industry where large air rates can be tolerated, i.e. Air—liquid mass ratios
greater than two.
In large dryers duplication of cups is possible, but is not applied if each
cup operation requires excessive volumes of air. Where duplication is used,
each cup has its own drying air supply. To prevent interference between
sprays, a prominently parallel air flow through the chamber is created by a
perforated plate air disperser. In dryers fitted with a single cup unit, an air
disperser creating a rotary flow pattern in the chamber can be successfully
used in tall narrow chambers. Air dispersers for pneumatic cup atomizers
are shown in figure 6.65.
LIJUID
DISTRIBUTOR
AIR ANNULUS
CUP LiP
. Figure 6.64. Pneumatic cup atomizer head.
(b) Effect of Variables on Droplet Size
The effects of variables on droplet size are similar to those discussed for
pneumatic nozzles. Increasing the air flow to the cup rim at constant feed
rate and cup rotation decreases the mean droplet size, and the overall spray
angle is narrower. Increasing the feed rate increases the mean droplet size,
232
THE PROCESS STAGES OF SPRAY DRYING
FEED
ATOMIZATION
FEED
FEED
233
Table 6.25. Correlations for Predicting Droplet Size from Pneumatic Cup Atomizers
4
COOLING
AIR
'0' 21
R
Dvs= 6 x 10_ + [ 1.40 0.5
[1 +
Pa
0.59e •5
D„ = 6 x 10 -4 + [ 0 .5
p a (ad +
0
21
0.0651F
1 .5_
.44/1
1 LVI-(05Vi
[
0.0651
Mr]
1+
10.5
5
(6,109)
VR + 1)
10.5
(6.110)
— V, + 1)]
in cgs units.
DRYING
AIR
the sheet thickness (s) at the cup lip, as obtained from equation (6.111).
AIR
AIR
s=
(a)
PRODUCT
PRODUC T
PRODUCT
ATOMIZING AIR
CUP MO TOR
DRIVE
PEED
BULK DRYING
AIR THROUGH
PERFORATED
PLATE AIR
DISPERSER
SWIRLING DRYING
AIR FLOW AROUND
EACH CUP ATOMIZER
3.PNEUMATIC CUP ATOMIZERS
PLACED EGUIDISTANT(601
FROM EACH OTHER
(b)
DRYING
AIR
Figure 6,65. Pneumatic cup atomizer and air disperser positioning in drying chamber. (a) Single
atomizer arrangements. (b) Multi atomizer arrangements.
but the effect is reduced in the presence of high air velocities. The air velocity
has more effect on droplet size than the cup rotation speed. Increased
cup speeds do not impart much more energy to the liquid due to liquid
slippage over the cup surface. Droplet size of low viscous feeds appears
little influenced by the air—liquid mass ratio for values of the ratio greater
than 3 : 1.
(c) Prediction of Mean Droplet Size
The Sauter mean droplet size obtained from the rotating pneumatic cup
atomizer can be calculated by equations (6.109) and (6.110). These equations
are given in table 6.25. Equation (6.109) expresses the mean size in terms of
[ 3Qv
1 113
2n V T sin 0
(6.111)
where 0 = half angle of divergence of the cup lip. Equation (6.110) expresses
the Sauter mean size in terms of the liquid flow rate and other operating
variables.
The correlations were developed over the following range of conditions :
Feed, kinematic viscosity (v)
surface tension (a)
density (p)
Feed flow rate
Air velocity
Air flow rate
Cup speed
Cup diameter
Measured Sauter mean diameter
Maximum diameter
5-165 cS
49-35 dynes/cm
50.5-52 lb/ft 3 (0.81-0.835 g/cm 3 )
10-1000 lb/hr (4.55-455 kg/hr)
95-650 ft/sec (29-197 m/sec)
50-1000 lb/hr (23-455 kg/hr)
1500-6000 rev/min
4 in (10 cm)
24-320 micron
210-630 micron
(d) Prediction of Droplet Size Distribution
Frazer, Dombrowski and Routley (74) report that sprays formed from
rotating pneumatic cups approximate to the Rosin—Rammler distribution.
The Sauter mean diameter and the maximum droplet size in the spray
depends upon the value of the mean size as given in table 6.26.
EXAMPLE 6.17
A 6 cm pneumatic cup atomizer is fed with 100 kg/h of a 40 % solids plastic
emulsion, and uses 180 kg/h of compressed air. The atomizer speed is 3500
rev/min. Air is directed 4 mm from the cup lip. The air annulus is 1 mm.
Using the data given below, predict the spray characteristics.
Data for equation (6.110):
3
a = 63.7 dyne/cm, p a = 1.21 x 10' g/cm , v R = 94.5, d = 6 cm, a = 0.4
17.3, Q = 26.2 cm 3 /s, M R = 1.8.
C111, VT
1100 cm/s, Va = 190 m/s, VR =
234
THE PROCESS STAGES OF SPRAY DRYING
ATOMIZATION
Table 6.26. Relation between Sauter Mean Diameter and
Maximum Spray Droplet Size
Sauter mean diameter (D„ Rato
Ratio
(micron)
maximum diameter (D „,„„)
mean diameter
D vs
10.0
80
6.0
4.5
4.0
3.4
2.8
24
2.1
2.0
20
30
40
60
80
100
150
200
250
300
Now D, = 6 x 10' + (A)(B)(C) ° ' S .
A=
r
B
[
C0'5=
' x 94.5' 1
) ' (0.4 x 6 + 0.42)0'25 = 226
0.59 x 63-7
L(1.21 x 10
1+
0 5
-3 ° 5
0.065
= 1.027
(1.8)1'5
10.5
26.2
[(1100) (0.5 x 17.3' — 17.3 + 1)
3
1.21 x 10 -5
D„ = 6 x 10 -4 + (226)(1.027)(1.21 x 10 -5 ) cm
39 micron
From table 6.26
Dm ax
= 6.2
D,,, a), = 242 micron
6.7. Sonic Atomization
Interest in the possible use of sonic atomization in spray drying has been
growing over the last decade. This has come about by the need to develop
a technique which can deal with the increasing number of liquids that cannot
be atomized successfully by wheels, nozzles or cups. Such liquids include
the non-Newtonian, high viscous and long molecular chain structured
235
products that form only 'strings' or filaments from rotary atomizers (not
individual droplets) and liquids that require excessive pressures for effective
atomization in centrifugal pressure nozzles. Attention was drawn to the
use of sonic energy, as the likely mechanism of atomization suggested the
formation of uniform droplet sizes. Sonic nozzle development has not
yet reached the stage where such nozzles are a real alternative to more
conventional atomizers already established in spray drying. However the
development is still at an early stage. The advantages of sonic nozzles
operating at low pressures and having wide liquid channels suggest suitable
use for abrasive and corrosive materials.
Perhaps the most immediate development of sonic energy use in spray
drying will be the employment of ultrasonics to promote increase drying
rates (82). Already small-sized spray dryers have been fitted with a multiwhistle liquid atomizer and ultrasonic sources placed in the dryer walls
at the level of the drying zone. A marked improvement in dryer capacity is
claimed.
The theory of sonic atomization is described by Antonevich (75). Patented
nozzle designs are given in (81) (464) to (468). The four main types are the
Hartmann mono whistle nozzle (80), stem jet nozzle (76), vortex whistle
nozzle, and mechanical vibratory nozzle (77)(78)(470)(471). Prediction of
spray characteristics is discussed by Peskin and Raco (79), Wilcox and
Tate (80) and Popov (77).
6.8. Atomizer Selection
The function of any atomizer is to produce as homogeneous a spray as
possible. When selecting from the various types of atomizer available,
consideration must be given to the design which produces economically
the most favourable spray characteristics for a given set of operational
conditions.
6.8.1. Selection based upon Desirability
The most desirable characteristics of any atomizer can be listed as :
a. simple construction
b. designs conducive to easy maintenance
c. available in both small and large sizes
d. spray size distributions controllable through adjustment to atomizer
operating conditions
e. operated by standard pumping equipment, gravity feed or siphon feed
systems
.1. handle feed without internal wear.
Certain atomizers, in spite of exhibiting some or all of these features, are still
not suited to certain applications due to accompanying undesirable features.
236
ATOMIZATION
THE PROCESS STAGES OF SPRAY DRYING
These undesirable features could be (a) the necessary feed system not compatible with the method of atomizer operation, (b) droplet release characteristics from atomizer not compatible with dryer chamber design, (c)
insufficient mounting space for atomizer.
6.8.2. Selection based upon Spray Characteristics
With proper design and operation, nozzles and rotary atomizers can produce
sprays having similar droplet size distribution. There is the general opinion
that atomizer wheels produce sprays of greater homogeneity, but this is not
always borne out in practice, especially at low feed rates, when fine sprays of
similar characteristics can be produced by wheels and nozzles. A coarse
droplet—particle requirement at industrial feed rates can generally best be
achieved with a centrifugal pressure nozzle and a fine droplet—particle
requirement with a rotary atomizer. Examples of the particle size distribution for spray dried silica and alumina based catalysts produced by rotary
and nozzle atomization, as shown in figure 6.66, illustrate this. The unbroken
lines indicate the two distributions to have similar form. The shaded areas
_
1
I
1
1 1 1 1 1
CUMULATIVE PERCENT LESS THAN D
99.5
95
I
1
1- 1 1 1 1 1
_
r
_
.er
4 I...
ip 00
–
–
_
70 _
90
gatIAI
A III r
lk
0
114
■0"-".
41 otiAi
M
ArusT
W
.
11 i I I
AS ;0111 1 r ll Iv
SA1 tra I I I illif
i:
ROTARY
ATOMIZER
{ wheel )
411100
_
ANtrilillifir
50
govil
,fflot
be
30 _
10 _
_
I
i
I
1
1
I
I
I
_
Ail
or
,
NOZZLE
ATOMIZER
4-ozig.---__...„,,ssow
1111
101.1—rf
'Aar
er„m7A0
a
lli
tgi
rigir
t•
opf 01 I
5 _
10
–
_
_
I
{pressure)
I
1
I
100
i
i
i
.._
_
i
1000
PARTICLE SIZE . D. {micron)
Figure 6.66. Particle size distributions of rotary and nozzle atomizers operating on silica and
alumina based catalysts.
237
indicate the variation of particle size that can be obtained by adjustment
to the atomizer operation. The shaded areas overlap indicating the two
atomizers to produce similar sprays under specially selected conditions.
6.8.3. Selection Basis
For a given spray drying application, the selection between rotary and nozzle
atomizers involves the following considerations :
1. The feed capacity range of the atomizer where complete atomization is
attained.
2. The power requirement of the atomizer to attain complete atomization
of the feed (atomizer efficiency).
3. The droplet size distribution at identical feed rates.
4. The maximum and minimum droplet size levels (spray homogeneity).
5, The operational flexibility.
6. The drying chamber design, most suitable for atomizer operation.
7. The feed properties most suitable for atomizer operation.
8. The atomizer experience available on product in question.
Re. 1. Designs of rotary and nozzle atomizers are available to cover all
capacity requirements in the low—intermediate—high feed rate ranges. It is
at very high capacities that rotating atomizer wheels emerge as the most
favourable method, in spite of the ability to reach such capacities through
duplication of nozzles in multi-nozzle atomizer systems.
Re. 2. The power requirements of atomizers are of the same order for most
spray drying conditions and rarely do such requirements have the ultimate
say in atomizer selection. Power input to atomizers greatly exceeds the
theoretical energy requirement for liquid bulk break-up into spray droplets
(18)(30). Efficiencies are very low and are normally ignored or considered
of secondary importance as long as the desired spray characteristics are
being obtained at rated capacities. Thus the extremely low efficiencies of a
three-fluid nozzle become of no consequence, if a high viscous liquid can be
successfully atomized only by such a nozzle.
Re. 3. Droplet size distribution from rotary and nozzle atomizers can have
similar characteristics at low to intermediate feed rates even though the
mathematical laws they follow are not always the same. At high feed rates
spray homogeneity is generally greater with rotary atomizers.
Re. 4. It is often the range in which the maximum, minimum or mean sizes
fall that is the important performance criteria. Vaned atomizer wheels,
two-fluid nozzles and pneumatic-cup atomizers are favoured for the finest
spray requirements, vaned atomizer wheels and pressure nozzles for general
use in the intermediate size range, and vaneless discs, or pressure nozzles
for coarse sprays.
Re. 5. Atomization by rotary atomizers is generally more flexible than
nozzle atomization from the operational standpoint. For a given rotary
238
THE PROCESS STAGES OF SPRAY DRYING
atomizer application, a wheel might handle wide variations of feed rate without much variation in the particle size of the product, and without the necessity of changing dryer operating conditions. Only alteration of wheel speed
is required. The extra power requirement for any speed increase is normally
available. For a given nozzle application, increase in dryer capacity requires
an increase in feed pressure, but there is an accompanying change in the
spray size distribution. When the spray characteristics are strictly specified
duplication of nozzles is required. If the available pressure is limited, but
spray characteristics are not critical, exchange of nozzle orifice size will
suffice.
Re. 6. When selecting atomizers, dryer design plays an important part.
From this viewpoint, nozzles are the more adaptable. The confined nature
of nozzle sprays enables any nozzle positioning in either co-current, countercurrent or mixed flow drying chambers with air dispersers creating rotating
or parallel air patterns. Rotary atomizers generally require a rotating air
pattern.
Re. 7. Atomizer wheels and nozzles are available to atomize low viscous,
non-corrosive and non-abrasive liquids with equal success. Wheels are
suited for handling corrosive and abrasive slurries and powder feeds (figure
6.67). For products that create pumping problems, under high pressure,
wheels are normally first choice, although pneumatic nozzles can also handle
such feeds. Often the pressure requirement of centrifugal pressure nozzles
is too high. A standard pneumatic nozzle might produce a 60 micron
mean droplet size operating at 100 psig (7 atm) at a feed rate of 10 lb/min
(4.5 kg/min). A centrifugal pressure nozzle may well require 1000-5000 psig
(7-35 atm) to create the same spray conditions. Pneumatic nozzles also
provide the best atomizing technique for liquids of long molecular chain
structure. These are usually high viscous and non-Newtonian materials.
Many high viscous Newtonian feeds can be atomized by wheels and nozzles,
following preheat for maximum reduction of viscosity.
There are circumstances where each type of atomizer is unsuitable. Should
the feed contain fibres, pressure nozzles are unsuitable. Should the feed
concentrate be unable to withstand impact, or the feed capacity be met
only if vast quantities of atomizing air are available, two-fluid nozzles become
unsuitable. Should the feed concentrate resemble a long-chained polymer,
which forms threads and not droplets from vaned atomizer wheels, this
technique becomes unsuitable.
Re. 8. Selection of an atomizer for a new spray dryer installation is based
normally on past experience of spray drying the product. In cases where
rotary and nozzle atomizers can be used with equal success, e.g. many milk
products, dryer manufacturers favour either one system or the other system.
Manufacturers will state their preference as the best method. The favoured
ATOMIZATION
239
method is often shown by the range of drying chamber designs offered as
standard by a manufacturer. Preferences have led to milk dryers in the
United States oi America being invariably fitted with nozzle atomization,
whereas in Europe rotary atomizers show much greater prominence.
However, there can be differences in the dried properties (e.g. bulk density,
particle shape) of product produced from nozzle or rotary atomizers. The
extent of these differences depends entirely on the product properties and
product treatment within the atomizer.
With new products there is no previous plant experience on which to base
atomizer selection. However, any new application is developed through
laboratory and pilot-plant testing, the results of which decide virtually the
most suitable atomization technique. Most spray dryer manufacturers have
extensive laboratory facilities equipped with units with both rotary and
nozzle atomization. The outcome of tests with various atomizing methods
gives an unbiased selection. The most suitable atomizer at testing levels
must have the same performance when scaled up to handle industrial
capacities if the choice is to be of any value.
In conclusion it is fair to point out that a clear cut choice between nozzle
and rotary atomization is rare. Circumstances arising from the projecting
of spray dryers usually dictate the atomizer choice (520). If the feed is
corrosive or abrasive, an atomizer wheel virtually decides itself, being a low
LIQUID FLOW
P - POWDER FLOW
L-
Figure 6.67. Modifications of the atomizer drive to handle liquid, powder or liquid—powder
feeds.
240
THE PROCESS STAGES OF SPRAY DRYING
pressure device with easy incorporation of wear resistant parts for prolonged
trouble-free operation. If dried product quality is well defined, the specifications may automatically call for a certain nozzle or special wheel design.
It may even call for use of dual atomizer systems, i.e. systems comprising
(a) both a rotary atomizer and nozzle mounted in the drying chamber (83),
(b) mounting of nozzles having different orifice and swirl chamber sizes, or
(c) atomizer wheels having tiers elliptically shaped or of different diameters.
If a range of products are to be handled in the same dryer, atomizer flexibility
is essential. Here the rotary atomizer is the usual preference since by mounting different wheel designs on the atomizer drive, various feeds can be
handled (figure 6.67) and different product specifications met (see p. 145). If
the projected operation is one of very high capacity, again the rotary
atomizer is the favoured choice, although multi-nozzle arrangements
operate successfully on filtered feeds. However, once faced with tens of tons
feed per hour rotary atomizers offer substantial operational advantages.
Of course, it need not be the feed and dried product properties that decide
the issue. Where dryer rebuilding is planned either to increase capacity or
handle different products, choice of atomizer is limited to that which fits the
existing dryer chamber design. The chamber dimensions, position of exhaust
air duct in relation to air disperser often decide the atomizer choice.
The arguments for and against the choice of rotary and nozzle atomization
are many and varied, but fortunately designs of atomizer have reached such
sophistication to enable hundreds of products to be successfully spray dried
(see chapter 13).
7 Spray-Air Contact (Mixing and Flow)
7.1. Introduction
The prediction and control of spray—air movement within the spray drying
chamber are important requirements for dryer design and performance.
The manner in which the spray, on leaving the atomizer combines with the
drying air determines the rate and extent of drying. The resulting spray—air
movement determines the time each droplet remains in the chamber.
Drying chamber and air disperser design must create a flow pattern, which
prevents the deposition of partially dried product at the wall and on the
atomizer. Wall deposits are caused by droplets travelling too rapidly to the
wall, thereby not allowing sufficient drying time to elapse. Atomizer deposits
result from local eddies. Eddies also cause re-entry of dried particles back
into the hottest air regions of the dryer, and even into the air disperser
where particles become scorched leading to contamination of the finished
dried product.
Much has still to be understood about how operating and design variables
can best be combined to produce the spray—air movement for optimum
drying conditions. The range of dryer designs and the unknown interdependence of the variables prevents a general relation being formed to
express their combined effect on spray—air movement. General reviews
of spray—air movement in spray dryers appear in the literature (1)(2)(35)
(84)(85)(86)(87). Arising from such studies, fine sprays can be considered
to move under the complete influence of the air flow throughout most of the
dryer volume. Once small droplets have left the atomizer, they attain the
velocity of the surrounding air in the proximity of the atomizer. How far
droplets travel until fully influenced by the air flow depends upon the droplet
size, shape and density. However, little data is available relating operating
variables (air rate, feed rate, atomizer operation and location) to the distance
242
THE PROCESS STAGES OF SPRAY DRYING
small droplets travel prior to attaining local air velocities. Coarse sprays
are more independent of the air flow. Droplet/particle size, form and density
determine product fall through the air.
The amount of published data on spray—air movement is limited and is
applicable mainly to small dryers. Air flow determination depends upon a
suitable experimental technique, and few are considered successful. Any
probing device automatically interferes with normal flow conditions,
especially in small diameter dryers. Data for small diameter dryers have
mostly been obtained from visual observations of light powder suspended
in air streams. Powder movement was then considered representative of
conditions existing during dryer operation.
The common approach to droplet movement is to calculate droplet
trajectory from the atomizer to the chamber wall using stepwise methods.
Air flow data is required and this is either predicted or obtained experimentally in equipment having analogous flow characteristics. Air velocity
measurements in pilot-plant size dryers have been conducted using anemometer methods. In industrial size dryers gas tracer techniques have been
shown possible (88)(483) and holography appears a new method of the
future.
Spray—air movement is classified according to the dryer chamber layout,
i.e. a 'co-current', `counter-current' or 'mixed flow' dryer with associated
atomizer (see chapter 5). The designation of 'co-current flow', 'countercurrent flow' or 'mixed flow' to spray—air movement in a dryer is in fact not a
true representation of actual conditions. For the case of a 'co-current flow'
dryer with rotary atomizer (figure 5.2), the spray leaves the atomizer to be
contacted obliquely by the entering drying air. Furthermore eddies within
the drying chamber, around the air disperser, and at the walls create local
areas of counter-current flow between spray and air.
Spray—air movement is governed by air disperser location and design,
atomizer location and operation, spray droplet behaviour when drying,
chamber dimensions, and method of powder—air discharge. However, it is
the air disperser that determines spray—air movement during the critical
first period of droplet drying. Correct air dispersing stands out as an essential for obtaining a successful spray dryer operation. Air dispersers were
introduced to the reader in chapter 5.
Air dispersers are shown in figures 7.1-7.4. Figure 7.1 shows three arrangements in co-current flow dryers with rotary atomizers. Figure 7.3 shows
arrangements for co-current flow nozzle dryers.
In figure 7.1(a) the air enters directly above the atomizer wheel. The
air flow is divided within the air disperser to give (a) rotational flow around
the atomizer and (b) a local flow down around the wheel edge to depress the
spray into an 'umbrella' cloud formation. This air disperser design gives a
SPRAY—AIR CONTACT (MIXING AND FLOW) 243
HOT AIR
I-tOT
COOL
AIR
VANED
APEX
Figure 7.1. Air disperser for rotary atomizers. In co-current flow dryers. (a) Ceiling air disperser, volute inlet for hot air. Air rotation controlled by angled vanes. Spray pattern controlled
by straightening cone. Rotary air flow in chamber. (b) Ceiling air disperser, volute inlet for
hot air, Tangential inlet for hot/cool air around walls. Rotary air flow in chamber. (c) Central
air disperser, air rotation controlled by angled vanes. Rotary air flow in chamber.
good control over radial droplet trajectory, although precise adjustment of
the air disperser vanes is required to prevent pronounced recirculation of air
into the top corners of the chamber. An actual air disperser and atomizer are
shown in figure 7.2. Alternative air disperser designs shown are (1) tangential
air entry at the top corners of the chamber, giving high wall velocities
(figure 7.1(b)), and (2) the entry of air vertically upwards underneath the
wheel (figure 7.1(e)). Introduction of air under the atomizer wheel is particularly advantageous in drying operations involving very high inlet air
temperatures, i.e. of the order 1400-1600°F (760-870°C). Hot air can be
readily introduced into the drying chamber without resorting to refractory
lined ductwork. The atomizer drive and chamber roof are also protected, as
neither is directly exposed to the high air temperature. Atomizer cooling
and roof construction of the drying chamber are simplified. The air disperser
in figure 7.1(c) is a vaned apex placed on top of an air-cooled duct. The vanes
create strong circulating air flow around the atomizer.
The initial contact between air and spray can be co-contact (i.e. atomizer
wheel and air rotation in the same direction) or counter-contact (rotations
244
SPRAY—AIR CONTACT (MIXING AND FLOW)
THE PROCESS STAGES OF SPRAY DRYING
245
b.
a.
ROT AIR
PERFORATED
SHEET
NOZZLE
H11 1 11 H M I
STRAIGHTENING
VANES
HOT AIR
PERFORATED SHEET
HOT AIR
HOT AIR
Figure 7.2. Ceiling air disperser (type (a) figure 7.1) with rotary (vaned wheel) atomizer in
co-current flow dryer.
in opposite direction). A more controlled air flow results from co-contacting,
but the greater mixing created by counter-contact enables coarser sprays
to be dried per given chamber size. Wall impingement of product often
decreases, as counter-contact acts to further reduce the radial trajectory of
spray. This increases fractionally the time the spray droplets remain suspended in the air flow, Such advantage is offset to a degree by the greater
tendency for deposit formation on the surfaces of the atomizer.
Air dispersers for co-current flow dryers with nozzle atomization create
rotary or non-rotary air flow in drying chambers depending upon dryer
design. Figure 7.3(a) shows four air dispersers creating rotary air flow.
Type (i) is flexible for both nozzle and rotary atomizer use. Type (ii) is in two
parts, where a central streamline flow promotes immediate contact between
spray and hot air, prior to the spray coming under the full influence of the
rotary air flow created by the other part. Type (iii) illustrates the use of wall
entry of hot air. Type (iv) shows a plenum air disperser for horizontal (hox
type) dryers. Each nozzle has its own hot air supply. Figure 7.3(b) represents
three air dispersers creating parallel (streamline) flow. Perforated plates or
straightening vanes are used. Air is dispersed over the entire cross-sectional
area of the drying chamber, types (i) and (ii), or concentrated just around the
PERFORATED
SHEETS
Ill
RIAU/
t h0
Figure 7.3. Air disperser for nozzle atomizers in co-current flow dryers. (a) Creating rotary air
flow in chamber; (i) ceiling air disperser, volute inlet for hot air. Air rotation controlled by
angled vanes (vertical dryer); (ii) ceiling air disperser, (two-part) tangential entry to outer part.
Streamline (Perforated plate) in inner part (vertical dryer); (iii) wall air disperser, tangential or
volute inlet for hot air (vertical dryer); (iv) end wall air disperser, plenum inlet (swirl plate,
horizontal dryer). (b) Creating parallel (streamline) air flow in chamber (vertical drye•); (i)perforated plate (over dryer cross-sectional area, vertical dryer); (ii) straightening vanes (over dryer
cross-sectional area, vertical dryer); (iii) perforated roof (vertical dryer).
246
THE PROCESS STAGES OF SPRAY DRYING
SPRAY—AIR CONTACT (MIXING AND FLOW) 247
HOT
AIR
(o
)
NOT
AIR (a)
a,
b.
b.
a,
C.
d.
Figure 7,4. Air disperser for counter-current flow dryers. (a) Wall air disperser, tangential or
volute inlet for hot air. (b) Central air disperser, angled vaned inlet for hot air, (c) Wall bustle
(plenum) air disperser, angled vaned inlet for hot air (vertical vanes), (d) wall bustle (plenum)
air disperser, angled vaned inlet for hot air (horizontal vanes).
C.
nozzle, as shown in the perforated roof disperser, type (iii). Dispersion of air
around the nozzle(s) leads to faster drying rates and better utilization of the
chamber volume. In type (iii) use of 4-5 perforated sheets can create plugflow conditions. The air pattern is controlled by the spacing of the sheets.
Air dispersers for counter-current flow are shown in figure 7.4. Although
tangential entry at wall (type (a)), and central dispersers (type (b)) are known,
air dispersers of the bustle (plenum) type are generally used (types (c) and (d)).
The bustle is located at the base of the cylindrical chamber section. Air
enters with a rotary motion through angled inlet tuyeres, but this motion
becomes depressed further up the chamber. Correct adjustment of the air
disperser is necessary to prevent hot air by-passing droplet fall. With optimum spray—air contact, counter-current flow dryers give a most efficient
utilization of heat available in the drying air.
Air dispersers for mixed flow dryers are shown in figure 7.5. Ceiling or
base dispersers are used depending upon the nozzle positioning. Type (a)
shows the 'cyclone-type' dryer with tangential air inlet around a top-
d,
Figure 7.5. Air dispersers and atomizers in mixed flow dryers. (a) Wall air disperser, tangential
or volute inlet for hot air (nozzle or cup atomizer). (b) Ceiling air disperser, volute inlet for
hot air (nozzle atomizer). (c) Base central air disperser, tangential inlet for hot air (nozzle,
wheel atomizer). (d) Base central air disperser, swirl vanes for inlet hot air (nozzle atomizer).
mounted nozzle. Type (b) shows the 'fountain-type' dryer with a ceiling air
disperser. Types (c) and (d) show base-mounted nozzle(s) surrounded by the
air disperser. Specialized patented air disperser designs are covered in
references (404) to (417).
7.2. General Principles
The manner of spray—air flow characterizes the droplet population throughout the chamber and bears important relation to the evaporation rate of the
spray, the optimum residence time of droplets in a hot atmosphere and the
extent of wall deposit formation.
248
SPRAY—AIR CONTACT (MIXING AND FLOW)
THE PROCESS STAGES OF SPRAY DRYING
Whatever the mode of atomization, each droplet in the resulting spray is
ejected at a velocity greatly in excess of the air velocities within the chamber.
However, droplet kinetic energy is soon dissipated by air friction, and direct
penetration is limited to short distances from the atomizer. In industrial
sized dryers, wall deposits are rarely caused by direct throw of product to the
wall, unless atomization has been incomplete. The droplets become influenced by the surrounding air flow, and movement is governed by the design
of air disperser. Any attempt to calculate chamber dimension requirements
is dependent on droplet path data and rate of drying.
Droplet travel in cylindrical (vertical) spray dryers from the time of release
from the atomizer to the point of contact with the chamber wall can be
regarded as one- or two-dimensional motion from a nozzle atomizer operating in a non-rotary air flow, and as three-dimensional motion from a rotary
atomizer operating in a rotary air flow.
It is possible to derive theoretical correlations to represent droplet motion.
The simplest correlations consider droplet mass and sphericity constant,
but as droplet mass and shape change during passage through the dryer,
actual travel is far from that predicted. Droplet motion under evaporating
conditions must be considered by taking into account factors which affect
droplet trajectory and heat and mass transfer. This leads to complex
correlations. Certain assumptions to the spray drying process are made to
render these correlations more workable. These include (a) heat transfer
between droplet and air is by forced convection, (b) droplets constituting the
spray are spherical, (c) spray is homogeneous, (d) the chance of coalescence
and break-up of droplets during trajectory is disregarded, (e) for dryers
with rotary air flow, the air proceeds through the dryer as a perfect cyclone
(velocity is constant in the axial direction but varies in the tangeitial direction), (f) for dryers with non-rotary air flow the air proceeds through the
dryer in parallel streamline flow.
For droplets moving relative to air there is a resisting force due to friction
between air and surface of the droplets (friction drag) and a drag force related
to droplet shape. The resulting relative movement between droplet and air
is dependent upon the variation in the resisting force and is controlled by the
extent of change of the droplet's physical properties during evaporation.
The basic principles of spray movement in spray dryers can be illustrated
with reference to a single droplet. At any given instant of droplet travel in a
vertical plane the forces acting on the droplet can be expressed as
6
2
= D3 (p — pPa)g
)g — C D a V r e
dt
6
2
droplet and air, A = area (= 70 2 /4 for spherical droplets), Vre , = droplet
velocity relative to air.
When gravitational forces and drag forces are equal droplet acceleration
becomes zero and the droplet velocity is constant. The constant velocity is
termed the terminal settling velocity. Under these conditions the total
resisting force (F) can be expressed in terms of particle size, air density and
drag coefficient
2
(7.2)
F = 1CDPR(Vrei) A
The value of the drag coefficient is dependent upon the droplet Reynolds
Number (Re) = (DV p a I kt a) as shown in table 7.1.
Table 7.1. Relation between Reynolds Number (Re) and Drag Coefficient (C D )
(For Spherical Droplets)
Laminar (streamline)
(St okian flow)
Semi-turbulent (transitional
flow)
Turbulent
Cp
Re
Flow region
less than 0.2
24R e -
from 0.2 to 500/1000
Approx : 0.4 + 40Re -1
greater than 1000
constant 0.44
The drag forces on droplets can be expressed over a wide range of velocities
by using the fluid flow representation of plotting the droplet Reynolds
Number (where droplet velocity is relative to air) and the dimensionless
group (R' p a V 2 ). R' is the force per unit projected area of droplet in a plane
90 degrees to the droplet motion. The plot is shown in figure 7.6. From
equation (7.2)
(7.3)
F = Ri A
and
( R!
1
p y 2 ) = 2- CD
(7.4)
For complete treatment of droplet/particle movement in air, the reader is
referred to Coulson and Richardson, Vol. 2, chapter 4 (89). Droplet/particle
movement in turbulent flows is reported by Clift and Gativin (486).
73. Droplet Trajectory Characteristics
dV
D3
249
(7.1)
where D = droplet diameter, C D =- drag coefficient, p w p a = densities of
7.3.1. From Rotary Atomizers (Atomizer Wheels)
Droplet trajectory, following horizontal release from the atomizer is
effected initially by the air swirl around the wheel (caused by wheel rotation)
250
SPRAY—AIR CONTACT (MIXING AND FLOW)
THE PROCESS STAGES OF SPRAY DRYING
drawn up by Herring and Marshall (37). Comparison between predicted
trajectory and actual dryer performance suggested equation (7.7) to be a
conservative estimate of droplet penetration. This was explained as the
relation does not consider reduction in droplet density during evaporation
or the effect of drying air flow. In equation (7.7) d is in, M is lb/min, N is
rev/min. Of the three equations quoted equation (7.6) is generally preferred
as it refers to maximum radial trajectory in still air conditions.
10 4
10
3
In
0
IN
>
Ec k.
"k"..
251
10 2
10
10 0
Table 7.2. Spray Trajectory Relations for Rotary Atomizers
10
-2
I
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
REYNOLDS NUMBER f Re
i
t
6
10
7
10
Author
8
10
Friedman, Gluckert, Marshall
Reference
(20)
R 10 = 04 (
log scale
2
and finally by the drying air flow. In small test dryers, wheel rotation contributes greatly to the overall flow pattern and droplet trajectory in the chamber,
but this is not so in industrial dryers as the influence of wheel rotation declines rapidly with radial distance. Air flow and droplet trajectory are
governed primarily by the air disperser.
The radial travel of droplets depends upon wheel design and speed, feed
rate, air disperser design and location. The effect of the operating variables
(atomizer dimensions, and speed, feed rate) on radial trajectory of sprays
has been studied by various workers (20) (26) (28) (37).
Results were expressed as the radial distance a given percentage of the
spray travels while falling a given distance. Relations expressing radial
trajectories are given in table 7.2. Maximum or near-maximum trajectory
decreases with atomizer speed and increases with feed rate (other variables
held constant). The relation by Friedman, Gluckert and Marshall (20)
(where r is ft, M is lb/min and N is rev/min) was established from 26 experimental runs within the range of operating variables given in chapter 6,
table 6.5. The effect of containing dryer walls was not considered. However,
the relation was proposed as being useful in assessing the variations in
trajectories when altering atomization conditions. The relation by Frazer,
Eisenklam and Dombrowski (28) (where d is ft, M is lb/hr and N is rev/min)
was established within a range of variables given in chapter 6, table 6.5.
Arising from their work on droplet penetration, a cumulative weight distribution curve for water droplets at any radial distance 3 ft beneath the atomizer wheel was drawn up. Cumulative weight distribution curves were also
dN.11.4 .O.25
2
Frazer, Eisenklam, Dombrowski
(28)
R. =
7.2d °.21 /14
N o•16
Herring, Marshall
(37)
R9 9
12e 2 J14 ° •
N O. 16
Figure 7.6. Plot of R'IpV against Reynolds number for spherical particles.
Equation
number
Equation
(7,5)
•
(7.6)
(7.7)
Notation:
d = wheel/disc diameter; M = feed rate, N = atomizer speed.
Rio = radial distance in feet 50% of spray is thrown before falling 10 in below atomizer.
R 99 = radial distance in feet, which includes 99 of the mass of the spray.
Rmax = radial distance in feet at which 99 % of the spray falls 3 ft below the atomizer.
For units and range of variables see table 6.5.
7.3.2. From Nozzle Atomizers
Spray droplets on emerging from the nozzle swirl chamber and orifice
are ejected forward with a high velocity component in the direction of the
nozzle axis. Droplets from pressure nozzles decelerate rapidly. Droplets
from pneumatic nozzles do not decelerate so rapidly as the droplets are
carried forward in the dispersing atomizing air flow. With small droplets,
radial velocity component on release is low, and spray trajectory to the wall is
limited. This enables drying towers of a narrow diameter to be used with
nozzle atomization.
7.4. Droplet Movement in Drying Chambers
Droplet movement in the drying chamber consists of (1) droplet release
from the atomizer, (2) droplet deceleration and (3) free falling motion or
motion under the influence of the drying air flow in the chamber.
7.4.1. Droplet Release Velocity
Droplet release from rotary and nozzle atomizers is described in chapter 6.
252
THE PROCESS STAGES OF SPRAY DRYING
SPRAY-AIR CONTACT (MIXING AND
7.4.2. Droplet Deceleration
Droplets leaving an atomizer at velocities in excess of the surrounding air
decelerate rapidly due to frictional forces acting on the droplet surfaces.
The rate of deceleration determines the penetration of droplets through the
air surrounding the atomizer. Giffen and Muraszew (18) drew up deceleration equations for single droplets, discharged into still air conditions where
gravity is neglected. Once the droplet has been discharged from the atomizer,
the droplet velocity decreases, and the droplet passes first through a turbulent,
then a semi-turbulent to a laminar flow region. If the droplet diameter is
large, a laminar flow might not be experienced due to a high terminal velocity.
Deceleration in each flow region is given by
dV =
dt
18y a
Laminar flow
(7.8a)
dV = 0.3pa 2 30p a v
dt
Dp, V D 2 pw
Semi-turbulent flow
(7.8b)
dV
dt
Turbulent flow
(7.8c)
D2 pw
_
Dp w
Calculation of droplet motion during deceleration requires use of the relation
2
between the droplet Reynolds number and the dimensionless group R' /p a V ,
see figure 7.6. Time and distance travelled by droplet during deceleration
is obtained by integration of equations (7.9) and (7.10).
(b) Droplet Deceleration from Nozzle Atomizers
For a droplet discharged from nozzle atomizers, whether in the upward or
downward direction, equation (7.10) can be used to develop the droplet
motion in the vertical plane. The cone angle will give a horizontal velocity
component, and motion in this direction is given by equation (7.9). If spray
discharge is upwards, both gravity and air frictional forces act together
in retarding the motion of the droplet in the vertical plane until the droplet is
brought to rest. Once the droplet begins to fall, gravity and air frictional
forces are in opposition.
For nozzles of small cone angle, movement of all particles is prominently
in the vertical plane, and droplet motion can be considered one-dimensional
in a gravitational field.
For a droplet moving downward in one-dimensional motion, equation
(7.10) can be rewritten
d
in
Integration of equations (7.8a, b, c) yield the deceleration time and droplet
penetration during each flow region. The above equations are often quoted
but they have limited practical use, owing to the simple system they describe.
More precise methods (though still having important practical limitations)
are available to describe droplet deceleration from rotary to nozzle atomizers.
(a) Droplet Deceleration from Rotary Atomizers
For a droplet leaving the edge of a rotary atomizer, the deceleration can be
expressed as equations of motion in the horizontal and vertical planes.
For the case of horizontal droplet release at a velocity Uh from the wheel/
disc edge, gravity acts immediately on the droplet giving a vertical velocity
component U.
The forces acting on the droplet are the apparent weight of the droplet
mg(1 p a/p w ) and the drag force R'A.
Equations of motion in the horizontal direction become
dUh =
dt
Pa V2
p aAU h (U )2; + UD1 I 2
(7.9)
Equation of motion in the vertical direction
dU,
—
dt
R'
p a T7
p a U,(U t2, + UD112 + mg(1
Pa
Pw
)
(7.10)
FLOW) 253
Ri
Uv
C
..- —
A)
2 Ap a U + mg (1 — Pw
dt
pa V
v
(7.11)
for spherical droplets, equation (7.11) becomes
[1.5pa
R. '
U„
Pa
1
+ g ( 1 — —)
2 U.
Dp w
dt = pa V
Pw
(7.12)
Integration of equation (7.12) yields the deceleration time of droplet travel.
By substituting U., = Rega/Dp a in equation (7.12)
R' 11.5p a \ I Re 2 14\
14,
d(Re) =
DPa dt
2
2
p a V k Dp„ \ D P!
1
+g
Pwi
(7.13)
Re-arranging and integrating
R
t=
" FDpa(N Pa)g
fRei
L
Pat).
1tt a l R'
g
Re
D 2 p w kp a V 2
dRe
(7.14)
Lapple and Shepherd (90) expressed equation (7.11) in terms of the drag
2
coefficient (C D ) using equation (7.4) [C D = 2(re /p a V )]. Equation (7.11)
can be written in terms of CD and droplet mass.
dU,
dt
p a C D Aq + g p, — Pa
k
2m
P.
(7.15)
254
THE PROCESS STAGES OF SPRAY DRYING
By expressing U, in terms of Reynolds number, equation (7.15) becomes
P. A dt = ( dRe
- C D Re 2 )
(7.16)
2Dm
(/) is the maximum value of C D Re 2 which a droplet of size D can attain when
falling freely.
For spherical particles at terminal velocity
4 g
=3
/4
Integration of equation (7.17) gives
/t a il
t
2Dm
255
Values of K for values of Z between 0-1.0 vary from unity at laminar flow
conditions, to 0.5 at extremely turbulent conditions. For conditions likely
to occur in the spray drying operation, K values will range from 0.8-1.0.
Some typical K values are given in table 7.3 for values of Z less than unity.
A complete list of values is given elsewhere (90).
SPRAY-AIR CONTACT (MIXING AND FLOW)
Pa(Pw Pa)D3
Table 7.3. Values of K Factor
Values of K for following values of 0
(7.17)
R 2
e
f (4) - C Re 2 ) -1 dRe
D
where Re l. , Re x is droplet Reynolds number on release from atomizer and
after time t. Equation (7.18) is an easier form of equation (7.14) to apply.
For laminar flow region (C D = 24Re -1 ) equation (7.18) becomes upon
integration (for spherical droplets)
t=
p s D 2 17,
V„\
(7.19)
18p a 17f. -
it aA t = 3 p a t =
[(4) / + Re 2 C D1- / 2 )(41 I / 2
In (01/2
R e2 c1/2)(01/2
2Dm
4D2pw
2(CDO) 1/2
Re C
p a A.
2Dm
t
3p t
K
4D 2p w - CDRe
1-
1 04
0
1.00
0.98
0.90
1.00
0.96
0.83
1.00
0.95
0.76
Calculate the time to decelerate a spherical 200 micron (6.5 x 10 -4 ft)
droplet sprayed down a dryer tower from a nozzle at 300 ft/sec. A nonrotating air flow exists down the tower. Assume droplet density and diameter
undergoes no appreciable change during deceleration.
Data:
p w = 62.3 lb/ft'
p a = 0.068 lb/ft'
= 1.3 x 10 -5 lb/ft sec
112
R ei c1:12)
(7.20)
Equation (7.20) can be applied to droplet discharge from a nozzle at high
velocity. To enable use of equation (7.18) over the entire droplet velocity
range, Lapple and Shepherd (90) introduced a factor K that enables direct
calculation of the velocity-time relation in the semi-turbulent flow region.
Equation (7.18) written in terms of the factor K becomes :
2.
In (1 - Z 1 \
10 3
EXAMPLE 7.1
where V is the droplet velocity after time t, and Vt is the terminal velocity.
For the turbulent flow region, CD = constant (0.44).
Equation (7.18) becomes
1 2
10 2
0.5
1.0
(7.18)
R e1
Z2
(7.21)
Subscripts 1, 2 indicate initial and final conditions. CD, Re, are values at final
condition, and
C
Z = [Re(1
) 11 1
Z2 represents a measure of the fractional approach to terminal velocity.
Reynolds number of droplet on release from nozzle = 1020. Consider
droplet motion to be in turbulent region until Re = 500 or V = 147 ft/sec.
Motion in Turbulent Region
CD = 0.44
Use the equation (7.20) to calculate time to reach 147 ft/sec
= 290 (calculated from equation (7.17) using above data)
Deceleration time in turbulent flow region is given by equation (7.20)
t
4(6.5 x 10 -4 ) 2 62.3
In A
2(0.44 x 290) 1 / 2 x 3(1.3 x 10-5)
where
2
A=
[290) 1 / 2 + 500(0.44) 1 / 2
(290) 1 / 2 - 1020(0.44) 1 /
1 2
1 2 x
(290) / - 500(0.44) /
(290) 1 / 2 + 1020(0.44) 1 / 2 ]
t = 7.5 x 10 -3 sec (negligible)
256
THE PROCESS STAGES OF SPRAY DRYING
SPRAY—AIR CONTACT (MIXING AND FLOW)
o
Terminal velocity conditions can be shown to be in the semi-turbulent flow
2
region (0.2 < Re < 500). Now 0 = maximum value of C D Re at terminal
velocity conditions.
From graph of C D Re 2 against Re (figure 7.7). Re = 7.2 (semi-turbulent
region).
I
U
Motion in the Semi-Turbulent Region
Time of travel is given by equation (7.21). Value of Z, (at Re = 500) CD =
0.48 (from table 7.1). Now
257
4
1 01
3X
Z = Re(C,10) 112 = 500(0.48/290) 1 / 2 = 20.4
DRAG COEFFMIENT - ( REYNOLDS
Zi = 414
Value of Z2
Re = 7.2 = (290/C D ) 112
Therefore
CD = 545
5.45) 1/2
Z2 =
= 0.986
290
7.2
10
0.972
Value of K is obtained from table 7.3. At Zz = 0.972 (0 = 290) K
t =
0.9
10 1
4(6.5 x 10 -.4 ) 2 62.3i
0.9 ) ln ( 1 — 414
3(1.3 x 10 -5 ) 1(5.45)(7.2)
1 — 0.972)
= 5 x 10 -1 sec
Deceleration time to terminal velocity
I
sec
le
7.4.3. Terminal Velocity (Spherical Droplets)
Terminal velocity conditions occur when the force of gravity acting on the
droplet is counter-balanced by the air frictional forces. The left-hand side of
equation (7.1) becomes zero, and equating the two forces
Tc
6
1
7 D2
2
D 3 (P,,, P a)g = — CDPa( 4 )V
(7.22)
REYNOLDS NUMBER
4
Vf = [ (Pw Pa)gpi
3
C Dp a
( Re )
2
Figure 7.7. Piot of C 0 Re against Re for spherical particles,
air friction (drag) forces (F) can be expressed :
F = 3ny a DT/f
2
The terminal velocity Tif is expressed by
10 1
(7.24)
The terminal velocity in terms of measurable variables
ll2
(7.23)
When values of yr correspond to a droplet Reynolds number less than 0.2,
V
2
9
= D 6 , — Pa)
181.ta
(7.25)
When droplet motion at terminal velocity corresponds to a Reynolds number
258
THE PROCESS STAGES OF SPRAY DRYING
SPRAY-AIR CONTACT (MIXING AND FLOW)
or
within the range 0.2-500, the air frictional (drag) forces (F) equal (357)
F = 37Ipa DV(1 + 0.15Re °
87
) (7.26)
Droplets formed during spray drying rarely feature sizes and densities to
give terminal velocity at high Reynolds numbers. Terminal velocities can
well be within the semi-turbulent flow region, in which case equation (7.27)
(in f.p.s. units) may be applied (91).
V=
0.153D 1 ' 14 g" i (p,
0900.290100.4
(7. 27)
Terminal velocities of droplets in spray drying chambers deviate from those
in the ideal conditions assumed above. Deviation from ideal conditions
occurs through :
(a) Interference Between Droplets and Between Droplet and Chamber Wall
Effects Droplet Motion. Air movement associated with the presence of a
large number of droplets discharged from an atomizer over extended time
intervals differs from air movement surrounding a single droplet. The
leading droplet of a spray gives energy to the surrounding air. The resulting
forward air movement reduces the air resistance acting on following droplet.
The overall effect is to create greater droplet penetration from the atomizer.
(b) Droplets are Liquid therefore not Rigid. Liquid droplets are subjected to
deformation due to various pressures acting over the surface, and internal
circulation of liquid within the droplet. Deformations cause increase in the
drag force, while internal circulation can act to reduce the drag by reducing
the velocity gradient at the droplet surface.
(c) Droplets are often not Spherical. Deviation's from sphericity can be
allowed for by incorporating a shape factor. The shape factor is defined
as the ratio of diameter - of sphere having the same volume to the diameter
of sphere having the same surface area For spray dried powders having a
high proportion of spheres (i.e. ceramics) the shape factor approaches 0.95.
For majority of powders where shapes tend to be spherical and not elongated,
the shape factor ranges between 0.5 and 0.85. The shape factor must be
estimated from microscopic droplet analysis.
(d) Droplets change Shape during Drying and can change from Solid -io Porous
and Hollow Forms. If droplet drying is rapid, hollow spheres can be formed
prior to the establishment of terminal velocity conditions. In applying the
terminal velocity equation, correction to the particle density must be carried
out. The density of a hollow particle is given by
surface area x wall thickness (b,,) x wall density
particle volume
259
PD
7I D2Pwanbw
—
_k n.)) 3
(7.28)
D
(p 0 replaces p w in equation (7.25) for particle travel at terminal velocity.)
Prediction of Terminal Velocity when Drag Coefficients are Unknown
In cases where droplet sizes are large enough to produce a terminal velocity
outside laminar flow conditions, terminal velocities are best determined
graphically.
Solving for the drag coefficient in equation (7.23) and expressing in logarithmic form
log C D = log ( 4g(P 3w—aPa) D) — 2 log Vf
Pa
(7.29)
Expressing the Reynolds number at terminal velocity in logarithmic form
Dpa
log Re = log
/La
+ log Vf
(7.30)
Eliminate Vf from (7.29) and (7.30)
log CD
2log Re + log
[4gD 3 p a (p, — p a )]
3
/4
(7.31)
Equation (7.31) gives a straight-line equation of slope 2. Passing through
Re = 1, therefore
CD —
(4gD 3 p a(p, — p a))
(7.32)
A similar expression can be obtained wlire droplet size , D does not appear in
the second term of the right-hand side of ±quation (7.31). By substituting in
iRettar
D3=
Pe Vf
and separating equation (7.31) becomes
469‘,
log CD = log Re + log (
POtta)
3p17,31.
(7.33)
Hence to calculate Vf where CD is unknown, equation (7.33) is plotted on a
CD—Re plot (figure 7.8). The terminal velocity is calculated from the point of
intersection of the two curves.
THE PROCESS STAGES OF SPRAY DRYING
SPRAY-AIR CONTACT (MIXING AND FLOW)
261
EXAMPLE 7.2
In a counter-current flow spray dryer with a nozzle spraying down the
chamber, a coarse particle size is being produced. All particles above 200
micron are to fall to the chamber base for direct product discharge. What
is the maximum velocity of drying air up the dryer to obtain this size separation?
Data:
Particle size 200 micron = 6.5 x 10 -4 ft
Average air temperature over the majority of dryer height = 176°F (80°C).
o
Figure 7.8. Plot of drag coefficient against Reynolds number.
260
Air viscosity =
= 1.4 x 10 -5 lb/ft sec.
Air density = (p a ) = 6.26 x 10 -2 lb/ft 3 .
Particle density = (PD) = 63.8 lb/ft 3 .
Air velocity must not exceed the particle terminal falling velocity. Using
equation (7.32) (Pr)
CD
4 x 32.2(6.5 x 10 -4 ) 3 (6.26 x 10 -2 )(63.8 - 0.0626)
3(1.4 x 10 -5 ) 2
= 246.4
Referring to figure 7.8.
At C D = 246.4 and Re = 1 draw line of slope - 2. At intersection with the
' a— Re curve, the Reynolds number = 6.6 (semi-turbulent motion). Theret ore
Re = 6.6 =
Dp
aV
f
ha
-5
V
f
IN3f1333(n DYHO
=
6.6(1.4 x 10 )
= 2.36 ft/sec
(6.56 x 10 -4 )(6.26 x 10 -2 )
The air velocity must be less than Vf .
7.4.4. Droplet Movement: Distance-Time Relationships
Distance-time relationships can be drawn up for droplet travel. in one- or
two-dimensional motion using equations (7.9) and (7.10) where droplet
velocities are relative to the air.
(a) Droplet Movement in Non-rotating Air
(i) Motion from Rotary Atomizers. Two-dimensional motion results from
droplets released from the edge of a rotating wheel. The release velocity is
sufficient to produce initial droplet motion in the turbulent flow region.
Under these conditions the distance-time relation in the horizontal plane
is expressed by Coulson and Richardson (89).
262
SPRAY—AIR CONTACT (MIXING AND FLOW) 263
THE PROCESS STAGES OF SPRAY DRYING
Sh = 1 1n ( Uh ct + 1)
(7.34)
where Sh = distance travelled in horizontal direction, and c= 0.33p a/Dp w .
When droplet enters the semi-turbulent region, no simple function
2
between Reynolds number and the dimensionless group (IV/p a V ) exists in
this range and the distance travelled in the horizontal plane is calculated by
graphical integration techniques.
Equation (7.9) can be written, neglecting the effect of gravity and resultant
velocity component in the vertical plane
m
d Uh
R'
=
Pa y
R'
x
—
dt = pay'
P
1 8pat \
D 2 p,
(7.40)
(
1 —* 0
13 2 p w
therefore
Sh(max)
D 2 p„Uh
18pa
(7.41)
d Uh = T18/1, dt
Uh
D2p,
(7.42)
integrating :
(7.37)
Dp w
By definition : Uh = Re(p./DP a ) = dsldt and by substitution into equation
(7.36) and subsequently integrating, a relation is obtained between t and Re.
3
18
exp
(7.36)
)
dU h ^ 3 x C D p a U il
2D 2 p, 1' 2
exp (
The maximum radial distance is attained when
h
In terms of the drag coefficient
dt
18 m,,
D 2 pw(1
The droplet velocity during streamline flow is represented by substituting
CD = 24/Re into equation (7.37).
1.5p a
—
Dp,
Uh
Sh
(7.35)
, Ap a U?
which, for the case of a spherical droplet
dU h
Once droplet motion lies within the laminar flow region droplet travel in
the horizontal direction is given directly by (42)
1
dRe
(R ! PaV2)(Re)2
(7.39)
Values of Uh can be determined, and by further graphical integration the
droplet travel in the horizontal direction obtained in terms of t.- Graphical
integration is a time-consuming operation, and often it is avoided through
neglecting time of droplet travel in the semi-turbulent region. This is considered valid for small droplet sizes, which undergo virtually instantaneous
deceleration, and time intervals in this region are extremely short. Where
time and distance of travel in the semi-turbulent region can be significant,
e.g. for large droplet systems, the time increment method by Lapple and
Shepherd (90) based on equation (7.37) is the most manageable.
18 at
± constant (C)
D 2 p,
(7.43)
at t = 0, C = log Ue where U e is the droplet velocity entering the laminar
region. Hence
Uh = U. exp (
(7.38)
Equation (7.38) is integrated graphically between the limits to obtain t as a
12
function of Re, where values of (K/p a I ) can be calculated from (357)
( R'
° 687
= 12Re -1 (1 + 0.15Re) '
2
PaY
log Uh =
18p t
(7.44)
2
D
pw
Droplet travel in the vertical direction is negligible in the turbulent and
semi-turbulent flow regions. With no vertical velocity component imparted
upon the droplet on release, vertical travel will be within the laminar flow
region. Distance travelled (S y) in the vertical direction can be expressed (89)
Sv =
bt
a
U,,
a
b
b
+ 2
a
a
UV \
a)
—at
(7.45)
where U,, is velocity entering the laminar flow region. In this case
U„ = 0; a = 1812 a/D 2 p.„,; b = g(1 — p al l%)
The velocity relation at any given time is obtained by integrating equation
(7.15) following substitution of CD = 24/Re. For the case of a spray drying
operation p, » p a . Hence
dU,=g
dt
(18paUvl
pD2
(7.46)
264
THE PROCESS STAGES OF SPRAY DRYING
SPRAY-AIR CONTACT (MIXING AND FLOW)
On integration using the integration factor
J
1811
p‘vD
22
and introducing the values a, b, as defined above :
Elv
= b + j,
a k
17\
a
(7.47)
Distance-Time Relationships by the Method of Lapple and Shepherd (90)
The difficulty in dealing with the semi-turbulent region in the above analysis
can be overcome by the method of Lapple and Shepherd. This involves
calculating travel in small selected time intervals using corresponding
values of the drag coefficient to express the flow region characteristics.
The calculation is based upon equations (7.9) and (7.10) expressed in terms
of the drag coefficient.
[3pa C D U U h ]
dt
4p, D
(7.48)
(p„, - P a) 3p„CDUU,1
dt
dU, =[g
4p,„D
Pw
(7.49)
dU h =
2
where U is the resultant velocity (U ii + UD 1 / . The method is as follows :
(i) A time increment is selected.
(ii) The droplet release velocity from the atomizer is calculated. Using this
velocity, the droplet Reynolds number is determined, and by referring to
figure 7.8 the corresponding value of the drag coefficient (C D ) is obtained,
(iii) The value of C D is substituted in equations (7.48) and (7.49) (dU h )(dU,)
are then computed, where for the initial time increment U = Uh , U, = a
(iv) Using the values of (dU h )(dU,) calculated in step 3, a velocity is estimated
at the end of the time increment.
(v) Values of (dUh ) and (dU,) are re-calculated using velocity conditions at
the end of the time interval.
(vi) Values of (dU h ) and (dU,) calculated for conditions at the commencement and termination of the time increment are then averaged. The average
value is used to obtain the second estimation of the droplet velocity at the
end of the time increment. The procedure is repeated until the calculated
average velocity increments (dU h )(dU,)correspond to those used in determining the velocity at the end of the time increment.
(vii) Movement in the next time increment is considered and the above
procedure is repeated where the velocities at the end of the first time increment is used as the initial velocities in the next time increment.
265
(viii) Calculations continue until velocities are reduced to terminal velocity
values. The velocity-time relationship is thus calculated.
(ix) The distance-time relationship is finally calculated by graphical integration where s
u dt.
An excessive number of trial and error approximations are required to
compute maximum radial distances using the above method. The procedure
can be simplified by using direct relationships (equations (7.40)-(7.47))
between droplet velocity and time in the later stages of droplet motion
within the laminar flow region, Deceleration of small droplets is very rapid
and velocities in the laminar region are obtained for many droplets after
fractions of a second. The number of trial and error approximations is
restricted to dealing only with the first 0-010-0.02 sec of motion. This is
illustrated in example 7.3.
Villadsen (92) has illustrated the Lapple and Shepherd method for a
droplet of 109 micron diameter. Rapid deceleration is again illustrated.
For a release velocity of 440 ft/sec (132 m/sec), the velocity in the horizontal
direction reduces to 25.5 ft/sec (7.8 m/sec) in 0.04 seconds, during which time
the droplet gains a vertical velocity of 0.23 ft/sec (0.07 m/sec).
EXAMPLE 7.3
Calculate the droplet trajectory over the first 0.04 seconds of a 60 micron
(1.97 x 10 -4 ft) droplet leaving a rotary atomizer. The wheel peripheral
velocity is 350 ft/sec. Consider the droplet aqueous, and the air viscosity
1.3 x 10 -5 lb/ft sec.
To apply the Lapple and Shepherd method, still air conditions are assumed.
Data:
Droplet release velocity = 350 ft/sec
Therefore
10 4 )(0-067)
Reynolds number = (1.97 x
U = 1.02U.
(1.3 x 10-)
Substituting data in equations (7.48) and (7.49)
Uh
r3x
0.067
CDUUh
L4 x 62.3(1.97 x 10-4)
At
(a)
= 4.1CD UUh At
AU =
[ 32.2 62.3 --0.067 \
62.3
= [32.2 - 4.1C D UUJAt
At U h = 350 ft/sec, U„ = 0.
3 x 0.067C D U tiv
4 x 62.3(1.97 x 10-4) At
(b)
266
THE PROCESS STAGES OF SPRAY DRYING
SPRAY AIR CONTACT (MIXING AND FLOW)
Motion in the horizontal direction
267
Re = 1.02 x 350 = 356
Now 0.016 0 0.0143, i.e. estimation not correct. Let U. = 0.013, UU, = 3.0,
CD = 0.74
CD = 0.62 (from figure 7.8)
AU, = (32.2 - 41 x 0.74 x 3)0.0005 = 0.0115 ft/sec
U = Uh
as
Average increment
U, = 0
Selected time increment (At) = 0.0005 sec. Substitute data in equation (a)
to calculate velocity increment AU h .
0.0115 + 0.0160
- 00135
2
AU, = -4.1 x 0.62 x 350 2 x 0.0005 = -156 ft/sec
0.0130 0.0135, therefore estimation of correct order. Proceed to next
increment.
Variation of U h with t, and U,, with t are plotted. Areas under the curves
give the distance travelled. Results are tabulated in table 7.4. Accuracy of
results depends on the closeness of the velocity estimations.
Estimate velocity at end of time increment. Suppose Uh after 0.0005 sec
is 200 ft/sec, Re = 205C, = 0.77 substitute in equation (a),
AU, = 4.1 x 0.77 x 200 2 x 0.0005
63 ft/sec
Average velocity increment
(156 -4- 63)
AU h
=
-
109
Table 7.4. Theoretical Droplet Trajectories in Example 7.3
(from rotatory atomizer)
2
now 350 - 109 = 241. However 241 0 200, i.e. estimation not correct.
Suppose U, = 230 ft/sec after 00005 sec.
Re = 235,
A Uh =
U = Uh = 230
CD = 0-74,
2
-
4-1 x 0.73 x 230 x 0.0005 = - 80 ft/sec
Average increment
-
(156 + 80)
=
2
-
118
Now 350 - 118 = 232, 232 230, therefore estimation of correct order.
For the next time increment, Uh = 230 ft/sec, At = 0.0005 sec. Repeat
above procedure.
Motion in the vertical direction
At t = 0, U, = 0. Therefore to calculate U, after 0.0005 sec, substitute in
equation (b)
U, = 32.2 x 0.0005 = 0.016 ft/sec
Let U, = 0.01, UU, = 230 x 0.01 = 2.3, CD = 0-74. Therefore
U„ = (32.2 - 4.1 x 0.74 x 2.3)0.0005 = 0.0126 ft/sec
Average increment
0-016 + 0-0126
- 0-0143 ft/sec
2
Droplet velocity (ft/sec)
Distance travelled (ft)
Time
(sec)
Horizontal
Vertical
Horizontal
0
0.0005
350
230
0
0.013
0
0.15
0
0.001
165
0.045
0.27
-
0002
0.004
0.006
90
50
30
0.085
0.120
0.146
043
057
0.65
00001
0.0003
00006
0-008
0.010
0.012
0.014 ,
20
15 11.5
8.8
0.168
0.185
0.200
0.214
0.71
0.76
0.79
081
0.0009
0-0012
0.0016
0.0020
Vertical
.
Example 7.3 exemplifies the rapid deceleration of a droplet after release
from the atomizer. In 0.014 seconds the horizontal droplet velocity decreases
to 8.8 ft/sec (2.7 m/sec). The radial distance of travel is less than one foot.
The velocity gained in the vertical direction and the distance travelled is
seen in table 7.4 to be negligible during this period.
(ii) Motion from Nozzle Atomizers
Nozzle point upward. For spray ejection upwards into a spray drying
chamber, the droplet velocities from the nozzle are sufficient for the larger
droplet sizes in the nozzle spray to be in turbulent motion. For the basic
case of one-dimensional motion in a gravitation field (air flow has no
rotation) droplet motion from the nozzle is expressed by Coulson and
268
SPRAY-AIR CONTACT (MIXING AND FLOW)
THE PROCESS STAGES OF SPRAY DRYING
Richardson (89) as
1
S, = -- In (cos f ct
c
U,
— sin f et)
f
(7.50)
(in consistent units). Where U,, = velocity of droplet leaving nozzle, and
=
( D(p, p a)4 112
0.33p a
(7.51)
Deceleration of the droplet takes place and when the droplet is in semiturbulent motion, time can be expressed in terms of the Reynolds number as
R eg [ ( Dpa(p
p jg)
Re y
+ ( 3-112 ' x
(7.52)
t
2D p,
paUZ
v
The Reynolds number is a function of t from which the vertical velocity
can be obtained. By further graphical integration the distance moved
for a required time interval is calculated.
Deceleration continues until the droplet experiences laminar flow. Form
drag is negligible and the droplet is brought to rest due to skin friction
forces only. While the droplet comes to rest and then accelerates within
laminar conditions the distance-time relation (irrespective of sense) is given
by equation (7.45).
As the droplet accelerates within semi-turbulent conditions the distance
ti me relationship form is similar to equation (7.52) except for a change of
sign. If the droplet is still large and heavy enough (even though evaporation
reduces weight during motion), to reach turbulent conditions, motion is
given by
1
S, rt + - 1n — f + u, + (f ' u„) e -21- 1
c 2f '
{
(7.53)
One-dimensional motion in the vertical plane occurs in nozzle drying
towers incorporating non-rotary air flow. Equations (7.50)-(7.53) are used
for the case of nozzle placed to face up the drying tower.
Nozzle pointing downward. For nozzles placed at the top of the dryer,
equations (7.52)-(7.53) express the downward motion. The equation is
selected according to the droplet Reynolds number. All velocity values
used in the equations are relative to the surrounding drying air velocity.
To calculate both vertical and radial movement (due to droplets leaving
nozzle with a radial velocity component fixed by the nozzle spray angle),
the Lapple and Shepherd (90) method can be used (equations (7.48) and
(7.49)).
Example 7.4 illustrates use of the Lapple and Shepherd method in calculating droplet trajectories from a nozzle atomizer. Movement in the vertical
269
direction is more substantial as the droplet leaves the nozzle with a high
axial release velocity. Results are presented in table 7.5 which gives the time
the chosen droplet size takes to complete its radial travel.
EXAMPLE 7.4
A co-current nozzle tower 8 ft 6 in diameter and 16 ft 5 in cylindrical height
is to be adopted for.the spray drying of a slurry. A coarse particle size is
required and a centrifugal nozzle has been' selected. At the required feed
rate of 2.35 US gal/min, the cone angle is 90°. The largest droplet size
formed in the nozzle spray is estimated not greater than 500 micron. Assuming that the drying air flow is non-rotary and of very low velocity and any
particle shape change that might occur is negligible, predict whether direct
i mpingement of clay slurry on the wall of the dryer will result with the
given nozzle.
Data: The initial vertical velocity of the spray droplets follows the relation (57)
=
4.07 x 10 -1 Q
da
ft/sec
Q = feed rate (US gal min), d, = orifice diameter (in), density of air (p a ) =
3
0.0523 lb/ft', density of slurry (p,„) = 93.5 lb/ft , air viscosity CUa) = 1.546 x
-5
10 lb/ft se; nozzle orifice diameter = 0.138 in.
The chance of impingement at the wall is best considered by calculating
trajectories at conditions of greatest droplet travel, i.e. no reduction in
droplet density.
Procedure : Diameter of largest droplet 500 micron = 0.00164 ft, initial
droplet velocity in the vertical direction (V,), (from given relation) V, 50 ft/sec, nozzle cone angle = 90°, initial droplet velocity in the horizontal
direction = 50 tan 90/2 = 50 ft/sec.
For motion in the horizontal direction
Substitution of data in equation (7.48)
❑ U h = -0.256C D UU hAt
For motion in the vertical direction (equation (7.49))
AU, = [32.2 - 0.256C,UU h ]At
The Reynolds number 5.55 U
where LP = LIZ + U,
2
Values of CD obtained for values of Re from figure 7.8.
Applying the Lapple and Shepherd method:
For time intervals of 0.01 sec, adopting trial and error procedures as
illustrated in example 7.3, U h decreases to 44.3 ft/sec (13.5 m/sec) and U,
270
SPRAY—AIR CONTACT (MIXING AND now) 271
THE PROCESS STAGES OF SPRAY DRYING
try
rn
r- rr- r4, C4 Ch t■ 4D 47
et 00 C.A Ch CD DO 00 C7 qp ma et N Q CR t- 4O VI
et
1-1
VI r■ DO 47 CD er r- CN aet
et 'Noo0" Cp
et ,t rn vD 00 C
C vp N 00 00 Do on 00 00 0'
CD CD CD 6 .4 6 CD ,4 CD CD C5 C7 6 c5 c5 c5 6a
'' 1" 0 "..0
un Ch Ch Ch CD m- N N CD N rn 4
,
r DO N o0 c!N N vn eh (:!r N u? VD N N DO 00 00 DO
6 6 T... *4 N 0.1 4 41 vn v5 VD vb v5 v5 v5 v5 v5 4D
j 4 .5? A s g '
trf
c5 o
4
,2;
E5 E5
CD -4 c5 c5 c) 6 ep 6 6 6 6 c) cp c) c)
N 01 CD CD CD et VD 00 CD Cl CD DO CD N CD 41 YO
cr, N oo
nr1 a0 C1, er vp rn
un ei rn N un ob Cr, on N on 42-r CD C7 C7 6 e5 c5 c5
1
1
1
1
1
1
1
1
N N in op
'.0' un N
CD CD Cp C0 CD 4D r4 cp CD C7 r- vp on 4
Lel tr.,
cD
,r kr)
r- r- r- r- r,
rrn cr, 47
on r4 CA
c,
Ch 00 VD et N 4D N
N N N CN 00 N 41
4D et
4? n..) C)1 4D
0' Ch r■ VD N
CD CD
u;) er cry cry iry ocs .4 4 4 ,L4 6 6 6 6 6 6 6 6
4-1 41
CD 0 CD 0
NCr, rq
vi 4
41 41 et 1.12 CD cr1 00 00 Cq
DO 00 IA
4 vp
CD 00
C, VI 00 C4 CN et VI C4
N (-A
N co un CD Li') rq
-4 -4
CD
N VI
N
r- CP( un rq
try
C7 01 T.,
"9 "9
"9 +D —
, ,
rcD c? on N N N v?
47 42 V?
6 6 CD 6 6 CD CD ,, ,Lr T4 ,4
„Li
• cp o cp c) cp cp cp 41 VD (-1 et ■tp co v, et CN et
• 00
0, 4 00
44 N
vn ob N
ce
,r
00 ,..rD r 4
00,C)'rn et et e7'
4 .1et on on rq r.4 (.1
4t,
0 oo oo N on r- N M oo —4 N 00 N 00 4D N DO Ch CD
V!)
6
-4 Le) , -1 v5
06 ob ob ob ob ob 6
r- 4O VI VI et on (-1 CA
•
,z) In - 0
a., 0 ‘7J-
%.0 N 00 ON 0
o- 4 e5 N 4 ob 6 v5 N 6 v5 N ob ob ob ob ob ob 6
4 1 ep ..71- on on rq
rn cp
c) ,t cr■
VI 7T
ra
et tr"
cn n.)
4 0 CD --I et cc 4? on r.:3
c?
CA
6 ei 6
ci1 C.4
c5
c)
c5
CD
6
rn
CA
—
CA et et VD CD CD CD CD CD CD CD 0 CD CD I
QQQQQ QQ C
CD CD CD 6 6 CD CD CD c5 c5 c5 c5 c5 c5 6 6 6
cD
cr) et 4D CD et CD CD C7 C7 CD CD CD CD CD 0 CD
Co o QQCD
N m et 4? 47) N 00 Crs C? C4
,
00 0 Ô-000 c5 o CD CD 6 CD 6 C7 T,
4
decreases to 44.6 ft/sec (13.6 m/sec). The droplet travel is given in table 7.5
covering the entire trajectory until radial travel ceases.
After 1.2 sec the maximum outward travel is reached, namely 6.84 ft
(2.1 m). Downward travel during this period is 14.5 ft (4.3 m). Considering
that evaporation of moisture from the droplet during the actual drying
process will act to reduce trajectory distances, the dryer dimensions are
concluded large enough to prevent direct impingement of the spray cloud
on the dryer wall.
(b) Droplet Movement in Rotating Air (Spiral Air Flow) (Droplet in a Centrifugal Field)
The above analysis and worked examples show droplet movement to be
very small during deceleration from any atomizer. Droplet path of travel
can be considered to be that of the air flow pattern in the chamber. If a
spiral air flow is created by the air disperser, droplets will travel as if they
were in a centrifugal field.
The general equations for a droplet moving in a rotating air flow is
given by (90)
dU i.
dt
dU,
dt
t11\ 2
r
cr a lq \ -1
ip a C D AU f U r \
2m
U ,U,\ • [P.CDAUPt — Ut)]
2m
r
(7.54)
(7.55)
where U rdroplet velocity in the radial direction, U, = droplet velocity in
velocity relative to air, li t = air
=
the tangential direction, Uf droplet
droplet
mass.
m
--velocity in the tangential direction,
It
has
been
shown
that if a rotating vaned
(i) Large Chamber Diameters.
atomizer wheel is used in conjunction with a rotating (spiral) air flow
pattern, spray droplets become fully influenced by the air flow in less than
six wheel diameters from the atomizer edge (38). For large chambers,
local flow effects around the wheel (caused by pumping effect of the wheel)
on droplet travel can be disregarded. Droplet motion in dryer designs
having diameters greater than six wheel diameters (i.e. all models above
laboratory size) can be simplified to a consideration of droplet retention
in a fully formed rotating (spiral) flow. The time for a droplet of given size
to travel radially to the chamber wall under the influence of centrifugal
forces can be calculated.
Equations (7.54) and (7.55) can be simplified for the case of spherical
droplets and laminar flow conditions.
As the spiral air flow is the governing influence on droplet travel during
the greater part of the droplet residence time in the drying chamber, motion
272
THE PROCESS STAGES OP SPRAY DRYING
SPRAY—AIR CONTACT (MIXING AND FLOW)
for a spherical droplet can be represented as (90)
dU r
dr —
WIF
r
dU,
dt
,U
Pa
w
IL
;X
fq.)1
2
U,
[181.1a (V,
U r)
D 2 p v,
r
[18,12.a U r]
2
(3,1)
(7.56)
(7.57)
for movement in the radial and tangential directions. Since droplet travel
becomes rapidly influenced by the air pattern, droplet tangential velocity
( U t ) approximates to air tangential velocity component (14). For droplet
travel in chambers larger than pilot-size models, the droplet radial velocity
is small due to the more extensive chamber diameters used and the mode
of air disperser selected. A further simplification can be made by neglecting
the tangential and radial acceleration.
Droplet radial velocity reduces to
ur =
V 2 69 , — POD 2
'
18 P a r
(7.58)
Equation (7.58) is the form of Stokes' Law for constant settling rate conditions in a centrifugal field. The tangential velocity (14) is dependent upon
chamber size and air disperser design. V is normally estimated. Data
from an experimental determination of the air pattern is rarely available,
but Edeling (93) has proposed the approximate relation for tangential
velocity in a chamber with rotating air flow.
)0.5
(7.59)
Y= Vt(wall)( —
R
c
The time (At) for radial travel between two points r„ r 2 can be deduced
from the radial and tangential. components. Now U, = dr/dt, V =
Substituting in equation (7.58) and integrating, the resulting correlation is
t=
18kt a in (r,/r,)
D 20) 2 (Pw — Pa)
(7.60)
where o) = average angular velocity.
The subscripts 1 and 2 refer to initial and final conditions so that r 1 and r2
are the distances of the droplets from the centre of rotation at given times.
When applying equation (7.60) radial increments (r 2
r 1 ) must be small
to avoid errors in co.
Application of equations (7.58) and (7.60) represents the theoretical case.
Although calculated values provide valuable information for estimating
trajectory paths and times, it must be stressed that irregularities within the
273
air flow pattern, changes in droplet properties as droplets undergo evaporation, and interference between droplets in the spray all act to produce an
actual trajectory differing from that theoretically predicted.
(ii) For Small Diameter Chambers. For droplet travel in small drying
chambers, the above analysis does not strictly apply. The droplet trajectory
to the chamber wall is more likely to be completed without the drying air
flow exerting so much influence.
Evaluation of trajectories from rotary atomizers and nozzles has been
reported by Gluckert (94) for the case of chamber diameters under 5 ft
(1.5 m). Droplet travel was assumed to follow the induced air flow from
the atomizer. Air movement caused by the atomizer becomes the controlling
factor in the trajectory path, not the drying air flow through the air disperser.
Gluckert measured the induced air flow. Trajectories are expressed by
equations (7.61-7.65).
Observed trajectories for droplets issuing from rotary atomizers were
represented in terms of the radial velocity of the induced air flow. The radial
7
velocity was expressed as the fraction of the initial velocity (1 /V,), where
V is the induced air stream velocity (ft/hr) and V 0 is the droplet release
velocity from the atomizer (ft/hr), R is the radial distance from centre of
chamber (ft), r d is the rotary atomizer wheel/disc radius (ft). The trajectory
is expressed as
[ b'
V = 1.2
vo
(R
. rd1"2
r id )
(7.61)
x —
R
where
b' = width of annular-slot air orifice (ft) —
( ML )
p a nDV
The trajectory was represented as a straight line on plotting
V
Vo
against
R (R r d }\
x
rd
b'
—
(
The trajectory time (t) to the chamber wall was determined on integration
of equation (7.61) (R, = chamber radius (ft))
t
r(it c r d /2) 2
2.4 V0 (hir d ) 112 ]
L
(7.62)
Observed trajectories for droplets issuing from pressure nozzles were
evaluated (94) from measurements of air flow produced by momentum
transfer from the spray to the surrounding air. The resulting trajectory
274
THE PROCESS STAGES OF SPRAY DRYING
SPRAY-AIR CONTACT (MIXING AND FLOW)
equation was given by (Y = distance from atomizer (ft), cr o = do( 9 .11),) 112 ft)
V ,do
Vito
and the trajectory time given by:
64 \ 2/3 V 2/3
6.4K o cro
(7.63)
(7.64)
(v = drying chamber volume (ft 3 )), Vv ❑ = axial velocity of liquid at nozzle
orifice (ft/hr).
Observed trajectories for droplets issuing from two-fluid (pneumatic)
nozzles were evaluated from the atomizing air-flow. The reduction in
droplet/air velocity in the axial direction was represented by
4
= 6.2
(7.65)
where V, = velocity of atomizing air at nozzle orifice (ft/hr). Equation (7.65)
is for nozzles operating with air velocities at the nozzle of 1000 ft/sec
(305 m/sec). Residence time (t) of droplets in the air stream per given distance
from the effective nozzle orifice (4) is given by :
t
17 ,2
12.4 VA
(7.66)
where Y,. = distance from nozzle to position four chamber diameters
away (ft).
7.4.5. Product Deposition on Chamber Walls
Product formation at the chamber wall falls into three main categories.
(1) Semi-wet deposits caused by the coarse droplets of the spray travelling
to the wall in a time insufficient for such droplets to acquire dry surfaces.
(2) Sticky deposits caused by the nature of the product at the temperature
of the dryer. (3) Surface dusting of wall by dry powder.
Semi-wet deposits occur on wall areas in direct path of droplets released
from the atomizer. These deposits can build up to break off as wet lumps
or stream down the wall. Deposits are caused by the very coarse droplets
in a spray or by incomplete atomization. Elimination of the deposits
requires correction to atomizer performance. The drying air flow has limited
influence. Sticky products occur over the whole chamber wall surface as
swirling powder within the chamber makes contact of product with the
confining walls inevitable. With many of these products, deposits build-up
to a certain thickness and then fall off to leave the chamber wall clean.
Surface dusting also results from the inevitable contact between dried
particles and wall. The degree of dusting depends upon the geometry of
275
the wall, wall cleanliness, local air velocity and any electrostatic forces
between particle and the wall. Dusting does not form hard layers and
powder can be easily removed by an air hose or light brushing.
Retention of product at the chamber wall over lengthy time intervals is
undesirable. It affects product quality and contributes to more frequent
dryer shut down for cleaning. Deposits on the wall can become scorched
and when dislodged, mix in and contaminate the entire product. Hard
layers often prove difficult to remove. Deposits contribute directly to
economic loss in the form of non-saleable product and loss of production
due to extended dryer shut down periods for cleaning.
Whilst the vast majority of spray dryers operate without chamber wall
deposits, there are a range of products that require a limited retention
time on the wall to complete moisture evaporation. Such products offer
great resistance to moisture removal at low moisture levels, and thus to
acquire a dry powder, wall residence times up to 5 minutes may be required
(e.g. fruit and vegetable powders). Products that are spray dried in this
way form porous agglomerates at the wall. They build up to a certain
thickness and then constantly break away from the wall in the form of
nodules. The control of such operations is delicate as degradation of the
product can easily occur if drying conditions do not produce product
properties conducive to continual and frequent break-away of product
from the wall. Wall cooling is necessary.
Wall deposits are more of a problem in small sized drying chambers
where radial distances from the atomizer and droplet residence times are
short. However, little data has been published on wall impingement characteristics.
(a) In Small Chambers
Deposits are always liable in the region of the atomizer wheel level due to
the proximity of the wall to the ejected droplets. Region of semi-wet deposition in small co-current flow dryers with rotary atomizers has been reported
by the author (95). Impingement at the wall is represented as an impingement profile using spray rake equipment. The profile represents the quantity
of droplets contacting the wall. Impingement is pronounced at the roof
and atomizer wheel levels. Variations in wheel speed, feed rate, air rate
and atomizer location alter the degree and areas of maximum impingement.
For a fixed atomizer position, increase in wheel speed acts to decrease
impingement at the top of dryer wall and increase impingement below
atomizer 'level for low feed rates. At higher feed rates, maximum spray
intensity occurs at the dryer roof level. In some cases a second area of
heavy deposits forms at the wheel level. The position of this minimum
i mpingement region changes with wheel speed. At low speeds, minimum
position is at the dryer roof. At high speeds it coincides with the wheel
276
SPRAY—AIR CONTACT (MIXING AND FLOW)
THE PROCESS STAGES OF SPRAY DRYING
level, For increase in feed rate, impingement increases at and above wheel
level. Impingement below this level decreases. For increase in air flow,
i mpingement above wheel level decreases. There is an increase in the
percentage of the feed impinging at the dryer wall due to the increased air
flow creating increased rotation in the chamber.
Regions of semi-wet deposition in nozzle dryers depend upon the nozzle
mounting and drying air flow. The location and forward direction of
nozzles has important influence. For a pressure nozzle at the top of a
co-current flow dryer the issuing spray expands on leaving the nozzle.
The downward drying air velocity, the nozzle feed rate and nozzle cone
angle determine the extent of spray expansion, and the level beneath the
nozzle where presence of spray at the dryer wall first occurs. For a two-fluid
nozzle likely impingement areas are lower down the dryer. Two-fluid
nozzles have a narrower cone angle, and the high speed atomizing air
projects the spray forward at higher velocities. The spray expands as it
entrains the surrounding air. Spray droplets move outwards to the dryer
walls. Even though areas of impingement are further down the dryer,
semi-wet deposits are more likely to occur than with pressure nozzles. The
atomizer air that surrounds the droplets on spray expansion prevents
intimate contact between hot drying air and spray, and lower initial evaporation rates result. Sprays from a pressure nozzle become more quickly under
the influence of the air flow, obtain better mixing and evaporate more
quickly than sprays from a two-fluid nozzle.
For nozzles in fountain-type dryers areas of likely impingement occur
at the top of the dryer cone. The larger particles in the spray will fall too
quickly out of the air flow on to the chamber cone. These larger droplets
may be moist as insufficient evaporation time has elapsed during fall to the
wall. A moist layer forms, which is accentuated by dry powder falling and
adhering to the moist particles. This can lead to chamber conditions prone
to heavy cone deposits.
Areas of likely impingement of partially dry product in small sized dryers
are shown in figure 7.9.
(b) In Large Chambers
Impingement areas of partially dry product in large drying chambers are
much easier to control than in the smaller designs. The increased chamber
size enables longer airborne residence times for droplet drying. The influence
of the atomizer on droplet travel becomes minimal and thus wall impingement of product can be counteracted by control of the drying air velocity
and direction. Such control is carried out by using the means of adjustment
designed into air dispensers for large chambers.
For co-current dryers with rotary atomizers and ceiling air disperser air
rotation or spray depression (umbrella shaped spray cloud) is controlled
277
C
>0044.0( AREAS
OF LIKELY IMPINGEMENT
Figure 7.9. Areas of likely wall impingement of partially dry product in small dryers. A. Pressure
nozzle (hollow cone), co-current flow. B. Two-fluid nozzle, co-current flow. C. Rotating disc/
wheel, co-current flow. D. Pressure nozzle, mixed flow.
to eliminate areas of semi-wet deposits (see figure 7.10). For co-current
nozzle dryers with non-rotary air flow, the air disperser is a perforated
sheet or straightening-vanes. Limited adjustment to the air disperser is
available for control of the dryer air flow pattern. The location of the
perforated holes in the sheet and distance between sheets can be re-arranged
to limit any local eddy flow around the atomizer. However, the nozzle
positioning is flexible. Slight re-positioning of the nozzles or change of
direction of projection is readily accomplished, and thus spray can be
directed away for areas of wall impingement. For nozzles in clusters and
278
THE PROCESS STAGES OF SPRAY DRYING
located centrally in the top of the chamber within a ceiling air disperser
creating air rotation, a wide range of impingement control is available.
The ability to control the rotation at the air disperser and the ability to
reposition the nozzles offers great scope for operating dryers without wall
impingement of partially dried product.
8 Drying of Droplets/Sprays
AIR
8.1. Introduction
Figure 7.10. Impingement area control by air disperser adjustment in large dryers with rotary
atomization. A. Impingement on roof and upper wall due to excessive air rotation, insufficient
spray cloud depression. B. Impingement on lower wall and cone due to excessive spray cloud
depression, insufficient air rotation.
For the vast majority of products, there is no tendency for powder to
adhere to the chamber wall if sufficiently dry. For these products, once
drying over extended time periods is completed the chamber can be cleaned
by an air sweep (permitting air to flow over wall surfaces) or by a light
brush-down. For products having tendencies to adhere to chamber wall,
forms of vibratory devices are mounted on the dryer to periodically dislodge
powder. For products that have stickiness characteristics, whether it be
due to moisture or heat, wall deposits will occur irrespective of air flow
pattern control. If vibratory devices on the chamber walls are inadequate,
introduction of secondary air around the walls can assist, or an air broom
system can be used (figure 5.12).
The evaporation of volatiles (usually water) from a spray involves simultaneous heat and mass transfer. With the contact between atomized droplets
and drying air, heat is transferred by convection from the air to the droplets,
and converted to latent heat during moisture evaporation. The vaporized
moisture is transported into the air by convection through the boundary
layer that surrounds each droplet. The velocity of droplets leaving the
atomizer differs greatly from the velocity of the surrounding air and,
simultaneously with heat and mass transfer, there will be an exchange of
momentum between the droplets and the surroundings.
The rate of heat and mass transfer is a function of temperature, humidity
and transport properties of the air surrounding each droplet. It is also a
function of droplet diameter and relative velocity between droplet and air.
Models to describe the droplet drying are to be found in many publications
on drying. Drying principles, factors controlling drying rates and drying
characteristics of droplets have been introduced to the reader in chapter 3.
The evaporation history for spray droplets commences with moisture
removal at near constant rate and constant droplet surface temperature
(first period of drying) followed by a decline in removal rate until drying
is complete (second or falling rate drying period). The rate begins to fall
off once the droplet moisture content is reduced to a level known as the
critical moisture content.
The majority of droplet moisture is removed during the first period of
drying. Moisture migrates from the droplet interior at a rate great enough
to maintain surface saturation. The wet bulb temperature represents the
droplet temperature. The evaporation rate can be considered constant,
although this is not strictly true. In the spray drying operation droplet
280
DRYING OF DROPLETS/SPRAYS
THE PROCESS STAGES OF SPRAY DRYING
evaporation commences with the immediate spray—air contact, and the
rapid transfer of moisture into the air is accompanied by lowering of the air
temperature. Any air temperature lowering reduces the driving force for
heat transfer, and the evaporation rate can begin to fall of even though
surface saturation is being maintained. However, it is common to refer
to the initial phase of droplet drying as the constant rate drying period.
Moisture migration lowers the moisture level within the droplet and a
point is eventually reached when the rate of migration to the surface becomes
the limiting factor in the drying rate. Surface wetness can no longer be
maintained, and a falling-off in drying rate results. The rate of moisture
migration is effected by the temperature of the surrounding air.
If the air temperature is so high that the temperature driving forces
permit evaporation to commence at a rate where migration of moisture
cannot maintain surface wetness from the start, the droplet will experience
little constant rate drying. A dried layer will form instantaneously at the
droplet surface.
This dried layer presents a formidable barrier to moisture transfer, and
acts to retain moisture within the droplet. Thus inlet drying temperatures
can readily influence the dried product characteristics. Increase in inlet
air temperature often results in a rapid formation of the dried outer layer.
This submits the droplet to higher surface temperatures than when lower
inlet air temperatures are used. A lower air temperature would mean a
lower initial drying rate, with the maintenance of a surface temperature
(equivalent to the wet bulb temperature) over longer time periods.
cc
LU
>—
o w
x IL/_
z
Cr
a
8.2. Evaporation of Pure Liquid Droplets
<
<ow
CC
c
The actual evaporation time for droplets contacted at a fixed air temperature depends upon droplet shape, chemical composition, physical structure
and solids concentration. The actual time is the sum of the constant rate
and the falling rate periods until the desired moisture level is reached.
The general drying characteristics are illustrated by a drying rate curve,
as shown in figure 8.1.
In phase A.B, the drying rate is established immediately the droplet
contacts the drying air. There follows a slight increase in droplet surface
temperature, and the drying rate increases in the milliseconds required for
heat transfer across the droplet—air interface to establish equilibrium.
In phase B.C, conditions of dynamic equilibrium are represented. Drying
proceeds at constant rate, which is the highest rate achieved during the entire
droplet evaporation history. Droplet surface is maintained saturated by
adequate migration of moisture from within the droplet to the surface.
In phase C.D, at point C, the critical point is reached and moisture within
the droplet can no longer maintain surface saturation. Drying rate begins
to fall, initiating the falling rate drying period. This period can form more
than one phase, if local areas of wetness remain on the droplet surface.
Phase C.D continues until no areas of wetness remain.
In phase D.E, resistance to mass transfer is wholly in the solid layer.
Evaporation continues at a decreasing rate until the droplet acquires a
moisture content in equilibrium with the surrounding air. Approach to
the equilibrium moisture content E is slow. In the spray drying operation,
product is usually removed from the dryer before the equilibrium moisture
content is reached. Droplet temperature rises throughout the two phases
of the falling rate period.
Figure 8.1 is diagrammatic. Drying curves in reality have no sharply
defined points. Some of the drying zones as shown may not even occur.
For example, in the spray drying of products that are heat sensitive, the
applied air temperatures are low and the phase A.B can well extend until
the critical point is reached. The drying in this case can be said not to
feature the customary constant rate period.
W
1.L1
o
281
o-
E
MOISTURE CONTENT (1-11
Figure 8.1. Drying rate curve.
( WEIGHT OF MOISTURE
PER UNIT WEIGHT OF
BONE DRY PRODUCT )
Conclusions drawn from studies on the evaporation of pure liquid droplets
form the basis for understanding the spray drying evaporation mechanisms.
The ideal case of evaporation of single pure liquid droplets can be modified
to deal with the deviations in the basic theory necessary to include the
presence of dissolved or insoluble solids.
The extent of moisture removal from a droplet present in a spray dryer
depends upon the mechanism governing the rate of evaporation and the
282
DRYING OF DROPLETS/SPRAYS
THE PROCESS STAGES OF SPRAY DRYING
residence time during which evaporation takes place. The residence time
results from the spray—air movement set up in the drying chamber. For the
greater part of droplet travel in the chamber, the droplets are completely
influenced by the air flow, and the relative velocities between droplet and
air is very low. The boundary layer theory states evaporation rates for a
droplet moving with zero relative velocity is identical to evaporation in
still-air conditions. Thus the mechanism of evaporation that is still-air
based upon boundary layer theory can be justifiably applied to many spray
drying conditions.
In the case where droplets move relative to the surrounding air, the
resulting flow conditions around the moving droplet influence the evaporation rate. In calculating transfer rates, these flow conditions plus the properties of the droplet are represented in combinations of the dimensionless
groups
283
droplet in still air can be expressed as
Sh = 2.0
(8.1b)
following the heat and mass transfer analogy (98).
For pure liquid droplets, equation (8.1) predicts that the rate of change
of the droplet surface will remain constant during evaporation.
The evaporation rate (c1147 Idt) in terms of mass transfer can be expressed
from equations (3.6) and (8.1), by substituting
K = ,12)
D
and
A=
dW
— = 277D,D(P,B — p,,)
(8.2)
dt
Group
Significance
Symbol
Reynolds (Re)
inertia force
viscous force
D„ p a
where ./), B = water vapour pressure at temperature of saturated droplet
surface, p w = partial pressure of water vapour in surrounding air.
For air—water system at atmospheric pressure
ila
D, = 0.258 cm 2 /sec at 26°C
Prandlt (Pr)
kinematic diffusivity
thermal diffusivity
C p i.t a \
= 0.305 cm 2 /sec at 60°C
Schmidt (Sc)
kinetic viscosity
molecular diffusivity
Nusselt (Nu)
Sherwood (Sh)
total heat transfer
conductive heat transfer
mass diffusivity
molecular diffusivity
Kd
The evaporation rate in terms of heat transfer can be expressed from equations
(3.6) and (8.1) by substituting h, = 2K d /D
dt
( h,D
K)
K gD
( D)
where D = droplet diameter, p a = density of drying medium, pa = viscosity
of drying medium, C, = heat capacity (constant pressure of drying medium),
Kd = average thermal conductivity of gaseous film surrounding an evaporating droplet, h, = convection heat transfer coefficient, K g = mass transfer
coefficient, D v = diffusion coefficient.
8.2.1. Evaporation of Single Droplets
(a) Droplet Evaporation under Negligible Relative Velocity Conditions
Experimental data (108) have shown that heat transfer to a spherical droplet
in still air can be expressed as
Nu = 2.0
27tDK,
dW
(8.1a)
Likewise has it been established that the mass transfer from a spherical
=
(Ta T)s
(8.3)
where Ta = air temperature, TS = droplet surface temperature.
Conclusions can be drawn from equations (8.2) and (8.3) as to the characteristics of pure liquid droplet evaporation. (a) The evaporation rate is proportional to diameter and not surface. (b) Absolute evaporation rates from
large droplets are greater than from small droplets. (c) Evaporation is
proportional to the square of initial diameter.
The evaporation time can be deduced from a heat balance over a spray
droplet. From equation (3.6)
d14.7
2 — = h AAT
dt
=heAAT
dt
— (1147
Now
itD 3 p i
284
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
285
and for heat transfer
Therefore
(Nu) = 2 + K 2 (Rer'(Pr)Y'
_hcnD2AT
41
dt
16
2
zp 1 3D 2 d(D) — h anD AT dt
6
A
AP'
dt =
(8.4)
d(D)
210 T
The term —(2p 1 /2AT) remains constant during the majority of droplet
residence time in the dryer, and integration of equation (8.4) yields the
evaporation time (t) (D, .= initial droplet diameter).
I
D
° d(D)
t = API
2AT JD 1 h c
(8.5)
AT is the mean temperature difference between the droplet surface and
surrounding air. It is best to apply the logarithmic mean difference (LMTD)
as defined by
LMTD =
AT. — AT.
2.303 log 10 (AM/AT' )
(8.6)
AT, and AT, are the temperature differences between droplet and air at
the beginning and end of the evaporation period.
The arithmetic mean (AMTD) can be used with little error if AM/AT 1 is
less than 2.
AMTD
AT, + AT,
2
(8.7)
Equation (8.5) can further be simplified where for negligible relative velocity
conditions h e = 2K d /D (equation (8.1))
t
19°
API
4KAT
T
Dd(D) = API (D,1
8K d A T
—
D 2,)
(8.8) .
(b) Droplet Evaporation under Relative Velocity Conditions
Evaporation rates increase with increase in relative velocity between droplet
and air due to the additional evaporation caused by the convection in the
boundary layer around the droplet. The total transfer coefficients for the
transfer from a spherical droplet can be expressed in terms of the dimensionless groups where for mass transfer
(Sh) = 2 + K i (ReNSc)Y
(8.9)
(8,10)
Equations (8.9) and (8.10) reduce to equation (8.1) when the relative velocity
is zero. There is much discussion over the power values of (x), (y), (ye),
and constants K„ K2.
Rowe, Claxton and Lewis (99) determined values of the above powers
and constants for spherical droplets/particles, and by comparison of data
from other investigations concluded
x = x' = 0.5
(8.11)
y = y' = 0.33
(8.12)
Equation (8.11) represents an average value since the power of Reynolds
number increases with Reynolds number from about 0.4 at Re = 1 to 0.6
at Re = 104.
The value of x accepted generally for evaporation conditions in spray
drying is 0.5. This is applicable to a Reynolds number range between
100 and 1000. Motion of small droplets in this range occurs only in the
first fractions of a second of travel, and thus much of the evaporation occurs
at droplet Reynolds number far below 100. According to Rowe (99) little
importance should be attached to having an exact value for the power of
Reynolds number. Various proposals of equations (8.9) and (8.10) are given
in table 8.1. The form most widely applied is the Ranz and Marshall (100)
equation.
"3 Dvpa °•5
(17 D .=
2.0 + 06 CP'12 a
(8.13a)
Kd
Kd
lia
„ 0.33 Dv Q.5
0.5
=
gD
2-0 + 0-6 '
(8.13b)
D,
PaDv
11 a
—
When applying the above equations, certain limitations must be taken into
consideration.
Steady state drag coefficients apply. It is convenient to apply the drag
equations at steady state to the case of accelerating or decelerating droplets.
In reality the drag coefficients (C D ), for accelerated motion can be 20-60
higher than values at constant velocity.
Heat transfer to evaporated moisture is neglected. For drying conditions at
high temperatures, much heat is taken up in heating the vapour as it is
transported outwards from the droplet surface. This effect is discussed
later (see page 293).
Droplet internal structure is stable. Any internal circulation, oscillation or
surface distortion of the droplet will increase heat and mass transfer rates
due to variations in the thickness of the boundary layer.
DRYING OF DROPLETS/SPRAYS
THE PROCESS STAGES OF SPRAY DRYING
&La
ss,,§8s.8
1;:4'
cn
on on on on on
o? v? on o? o? on o?
h o c Ch a o CD
Table 8.1. Proposals for the E(iiation Nu (or Sh) = 2.0 + K(Re)x(Pr or Sc)''
286
287
Droplets are stable in air flow. Droplets in a spray dryer are often subjected
to a swirling air flow. This causes droplet rotation, and such rotation reduces
the boundary layer, thus increasing evaporation rates.
(0 Evaporation Rate. Droplets released from an atomizer decelerate rapidly
to become completely influenced by the surrounding air flow. During
droplet deceleration considerable evaporation occurs. Equations by
Frossling (102) express evaporation during this period. The increase in
evaporation rate due to droplet motion is represented by Frbssling as the
second term on the right-hand side of equations (8.20).
Mass transfer
Ap
(I. + 0.276Re"Sc' 33 )
(8.20a)
2701C dA T(1 + 0.276Re"Sc" 3 )
(8.20b)
N = 2nDD
u? ‘4 vn un ken 47
0 o 6 6 6 6 6
on
rq rq
c) 41 un c) rq cN
t"? V7 yi vl o7
CD CD c) cb c5 c) o
d
die
tu
ms
tes
Sy
RT
Heat transfer
dW
dt
where N = transfer rate, dp = driving force in terms of partial pressures.
From equations (8.20) the Nusselt number for air system can be calculated
Nu =
coI a) &I ,f2r a) a) a)
tA "t73 '7)
g
cti
ri ariti#6
c)c 0 CD 000
a
rrJ
+al)
a.)
Kd
= 2(1 + 0 , 25Re")
=
,
'
27rD13, R 0.276Sc
t Ln
r- 00 a\
-1
c6 06 06 06 D6 06 00
Re" dt
(8.22)
where NEx = transfer rate during deceleration.
The fractional evaporation is a weight fraction, and is obtained by dividing
NEX by the droplet weight (expressed in moles). The Sjenitzer equation
to express the fractional evaporation (X w ) yields
X, = 4.42AHSc'"
m
(8.21)
Sjenitzer (107) developed equation (8,20a) to express the weight frar,tir,In (X,)
of a pure liquid droplet that evaporates due to- droplet relative motion.
The evaporation during droplet deceleration (from equation (8,20a)) equals
NEx
tn
kD
(Re )
CdRe1.5
(8.23)
Where AH = humidity gradient (approx. ApM,IRTp„),
= molecular
weight of liquid droplet. Values of the integral are given in table 8.2.
Inspection of equation (8.23) shows that subject to constant properties
of the system, the fractional •aporation during deceleration depends only
upon the droplet Reynolds number (at atomizer release) and the final
velocity obtained at the completion of evaporation. The final velocity is
288
THE PROCESS STAGES OF SPRAY DRYING
Table 8.2. Values of the Integral
d(Re)
C.Re 1.5
in Equation (8.23)
Re
5 x 10 4
1 x 10 4
5 x 10 3
1 x 10 3
5 x 10 2
1 x 10 2
5 x 10 1
1 x 10 1
5
1
d(Re)
CDR e "
0.030
0060
0.090
0.175
0.300
0.390
0.600
0.615
0.670
0.745
the terminal velocity of the droplet. This velocity, and hence the Reynolds
number, is dependent upon the droplet diameter at the onset of terminal
velocity conditions. For pure liquid droplets, the diameter changes during
motion.
Change of diameter produces change in deceleration rates and the extra
evaporation produced by the droplet motion can only be calculated by a
stepwise method. Values Of X will for the case of decreasing diameters
become smaller than values calculated from equation (8.23), which assumes
constant diameters during evaporation.
For low values of the humidity gradient (OH) droplet diameter change
during the short time interval of droplet deceleration can be considered
negligible. Sjenitzer (107) illustrated the application of equation (8.23) for
the aqueous droplet system using the assumption of negligible droplet
diameter change during the deceleration period and employing the deduction that the fractional evaporation during the deceleration period is
independent of the droplet release velocity when this is high. To comply
with the assumption, a low humidity gradient was selected (OH = 0.06).
Fractional evaporation due to droplet motion of water droplets ranging
in diameter from 100-1000 micron is shown in table 8.3. Values were
calculated for a drying air temperature of 379°F (193°C), droplet wet bulb
temperature of 109°F (43°C), using equation (8.23) and table 8.2.
Once droplet deceleration is completed and terminal velocity conditions
prevail, the Frossling equation (8.20) can be rearranged to obtain the weight
fraction of the droplet evaporated per unit length of travel (d W Id!).
DRYING OF DROPLETS/SPRAYS
289
Table 8.3. Fractional Evaporation of Aqueous Droplets
Undergoing Decelerated Motion in Dry Air 379°F (193°C)
(Fractional increase due to motion of constant diameter
droplet)
Fractional Evaporation x,„,%
(micron)
100
200
300
400
500
29.0
25.0
21.5
19.0
17.0
(micron)
600
700
800
900
1000
15.5
145
13.6
12.8
12.1
A
pp
dW
= LLD
(1 + 0.276Re"Sc')
di
ViD2 ART
(8.24)
where the terminal velocity Vi is calculated by equations in chapter 7 (7.25),
(7.27).
For aqueous droplet—air systems, equation (8.24) reduces to
H
W
4.6 x 10
(8.25)
2 (1 + 0.23Re')
di
fD
where dW 'di = fractional evaporation per metre length of fall, V, = terminal
velocity, OH = humidity difference.
A plot of equation (8.25) shows dW I& decreases rapidly with increasing
droplet diameter. In a spray distribution, the smaller droplet sizes will
dry more rapidly than the larger. This results in the possibility of over-dried,
small particles being present with larger particles of desired moisture
content.
Certain significant conclusions can be drawn from the Sjenitzer equations
1. A slight reduction in droplet size causes a marked increase in the fractional
evaporation. Thus drying chamber dimensions based upon constant
droplet size during evaporation will be conservative. The effect of droplet
size reduction further acts to decrease the chance of wall impingement of
semi-dried droplets.
2. If droplets remain a constant diameter the resulting evaporation will
act to reduce droplet density and hollow dried particles will form. Hollow
droplets fall at lower velocities. As the fractional evaporation is inversely
proportional to the droplet velocity, and evaporation on a weight basis
is equal, the fractional evaporation increases over that of a solid droplet
at the same rate of fall by a factor equal to the ratio of droplet volume to
its hollow air space volume.
290
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
3. For small sized droplets under 100 micron, evaporation during deceleration Can be considered insignificant compared with the free-falling evaporation during the remaining residence time in the air.
(ii) Evaporation Time. The evaporation time for a pure liquid droplet under
relative velocity conditions is obtained from equation (8.5) by substituting
values of h, from either the Ranz-Marshall equation (8.13a) or Ingebo
equation (8.14).
Using the Ranz-Marshall equation, the time of evaporation can be
evaluated as follows
t =
DAD)
2K d AT
JD
(2 + 0.6Re"Pr'
(8.26)
33
)
Equation (8.26) was integrated by Duffie and Marshall (22) for droplets
falling at terminal velocity, within the semi-turbulent flow region (Reynolds
number range 0.2-500). The terminal velocity was calculated for this
region using equation (7.27) (chapter 7)
D0
2#
[(DI, - D?)
t=
8KdAT
D 2.° 7
On
D i. "
±
fDioo I
where
=
[0153 ( gPa(P)122-
.33
P ) ("
1 0.5 [ CP Pay
a )
Kd
(8.28)
With D in equation (8.27) expressed in micron, equation (8.28) can be
expressed for. aqueous droplet-air systems in corresponding metric units as
0.36
13 = 0.071 H
11 2
ram - i.o 7
Table 8.4. Evaporation Times for Pure Liquid Droplets
(after Duffie and Marshall (22))
Droplet
diameter
D
(micron)
Evaporation times (sec)
= 2.0
600
500
400
300
200
100
50
3.2
2.5
1.8
1.0
0.6
0.2
<0.1
p iA N [
8K d A T
(8.29)
3
= 3.0
13 = 3.5
6.0
4.8
3.1
2.0
1.1
0.3
0.1
11.0
8.2
5.9
3.8
2.0
0.6
0.2
19.0
14.0
10.0
7.0
3.3
1.1
0.3
1
2)3
0 D2.o7d(D)
2
D, r 1 + /3D 1 '"
(8.30)
Evaporation times for values of /3 are given in table 8.4 for droplet diameters
between 50 and 600 micron. Values of /3 are obtained from the temperature
driving force (AT) between air and droplet surface. Values of # are shown
in table 8.5. The reference droplet temperature (Tr) for both tables is 130°F
(54.5°C).
For droplet temperatures that greatly differ from the reference temperature a correction factor can be applied to the predicted evaporation time.
The correction factor is given -by
L, -
Ta
where Prandlt number is taken as 0.73 and p i » Pa in g/cm . Ta is the absolute
air temperature. Equations (8.28) and (8.29) result in a straight line graph
on plotting 13 against Ta on log-log paper.
The second term of the right-hand side of equation (8.27) is a velocity
correction for droplets larger than 100 micron at terminal velocity in the
semi-turbulent flow region. The necessary correction for droplets under
100 micron is considered negligible. To express the entire evaporation
history of a pure liquid droplet, i.e. droplet evaporates away completely,
equation (8.27) can be integrated for droplet evaporation time during the
decrease in droplet size to 100 micron. The remainder of the evaporation
time, i.e. droplet size decreases below 100 micron until evaporation completion is represented by equation (8.8). Total drying time on summation of
= 2.5
equations (8.8) and (8.27) becomes
t =
(8.27)
291
T.
droplet
For evaporation of liquid droplets other than water, a correction factor
Table 8.5. Values of fl for Different
Temperature Driving Forces O T)
AT
/3(mm -1 ' 07)
(°F)
(°C)
2.0
2.5
3.0
3.5
800
480
290
200
426
249
143.5
93.5
292
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
can be applied following the use of table 8.4 and table 8.5. Values of /3,
obtained from table 8.5 or equations (8.28) and (8.29), are multiplied by
/3" 6 . The evaporation time corresponding to the modified /3 value, is
finally corrected by the factor ro .Awaieri•
Using the Ingebo equation (8.14) the time of evaporation can be evaluated
as follows (for Nusselt numbers greater than 10 and constant relative
velocity (101))
t
=
4KdAT
D1) . 4
X
1
0.6
Vreip agR T,
X
0.213
K doir)
Kd {vapour)
P1MMDV
(8.31)
0.5
]
where MM mean molecular weight of vapour and air, D v = vapour
diffusivity.
For droplets falling at terminal velocities in the Stokian region (Reynolds
number < 0.2), evaporation times can be expressed by equation (8.32).
The terminal velocity equation (7.25) (chapter 7) is applied.
P
[(D02
8K :iii:AT
2)6"
D° D'd(D)
Lop
(8.32)
PD1.5]
where
[ g ( pi
= 0.276
p
18/4
o pa l 0.5 rc pita l 0.33
(8.33)
Kd
°
°
(3' varies from 80-200 over the temperature range 32-540 F (0-282 C).
The second term of the right-hand side of equation (8.32) represents the
decrease in evaporation time of a droplet falling at terminal velocity compared with the time required for evaporation in still air conditions.
All the equations derived above to express droplet evaporation time are
based upon two important assumptions (369). These are (i) areas of heat
and mass transfer are equal for each droplet and (ii) all heat transfer to a
droplet is consumed as latent heat of vaporization. Such assumptions are
justified for evaporating conditions where drying temperatures are not
excessively high.
EXAMPLE 8.1
A spray dryer installation is applied to a titanium tetrachloride process
whereby a TiC1 4 content in combustion gases is increased by spraying
impure liquid TiC1 4 into the combustion gases. A purification takes place
as the clean gases pass to condensers, while solid impurities are collected
at the chamber base. If the atomizer produces a coarse spray, where the
maximum droplet diameter is 500 micron, calculate the maximum evaporation time of the spray.
293
Data
Dew point of TiCI 4 = 266°F
p i = 1.65 g/cm 3
A„ = 72 BTU/lb
Inlet temperature of combustion gases = 2000° F
Outlet temperature of combustion gases = 302 ° F
Feed temperature = 70°F
Now droplets evaporate completely to vapour. Temperature driving force
(equation (8.6))
A To = 2000 — 70 ° F = 1930°F
A T, = 302 — 266 ° F = 36°F
1894
LMTD =
2. 303 log10 1894
36
1894
— 478°F
2.303 x 1.7211
Mean air temperature surrounding droplet = 478 + 266 = 744 ° F. From
table 8.5 the corresponding value is taken as 2.5. Product correction
factor /3 0 ' 36 = 1.39 therefore overall value = 1.39 x 2.5 =- 3.46.
From table 8.4 take /3 as 3.5. Therefore at D = 500 micron, equivalent
water evaporation time is 14.0 sec.
Time correction for TiC1 4 droplet = , Pi Ai .
' wat r
e
Droplet temperature correction = [
1.65 x 72
970
Ta — 130 1
Tdroplet
744 — 130
744 — 266
614
478 — 1.28
Maximum evaporation time
14.0
( 1.65 x 72 )
1.28 = 2.2 sec
970
The evaporation time of 2.2 seconds assumes the effects of the high gaseous
temperature surroundings to be negligible. As discussed below, these
effects can be significant.
(c) Evaporation under High Air Temperature Conditions
For droplet evaporation at high air temperatures heat transfer to the
droplets no longer solely provides the latent heat of vaporization. Heat is
also transferred to the vapours moving away from the droplet interface
294
into the air flow. This can lead to decisive errors when calculating evaporation times by the equations mentioned above. The evaporation time
calculated in example 8.1 could well be invalid because the evaporation is
conducted at high temperature, and here the assumptions for the computation are no longer relevant.
The effect of high temperature on evaporation mechanism has been
analysed mathematically by Marshall (108) and Godsave (367). For an
evaporating droplet, the differential equation for conduction of heat through
the surrounding vapour layer can be drawn up.
Consider figure 8.2 where W' is the rate of evaporation (mass/unit time)
and x the radial distance from the centre of the droplet. The differential
equation for heat passing inwards through the gas film surrounding an
evaporating droplet with simultaneous transfer of vapour outwards through
the film can be shown to be
x 2d2T2 + 2x
dx
295
DRYING OF DROPLETS/SPRAYS
THE PROCESS STAGES OF SPRAY DRYING
W/C p \dT
0
4n1( d ) dx
DROPLET
SURFACE
MASS FLOW OUT (W) I
NEAT FLOW IN
(8.34)
where Kd = thermal conductivity of gas film surrounding droplet.
Integration (108) (367) of equation (8.34) between the radial limits of
droplet surface and bulk air flow enables the temperature to be expressed
as a function of distance through the layer.
E
T — Ts
Ta— Ts
— exp (
exp (
exp (
E
r
E
R
Figure 8.2. Configuration of droplet evaporating in high temperature surroundings.
(8.35)
— exp ( — —
r /)
2
where Ts = droplet surface temperature, Ta — bulk air temperature, t = any
1
temperature in the vapour layer, E = 14/ C p147rK„, r1 = droplet radius,
R2 = bulk air flow radial limit:
Values of E assume the thermal conductivity (K d ) to be constant over
the entire layer. K d actually varies according to the vapour composition
of the layer. An average value should be used. By defining the heat transfer
coefficient (h e') as (108) QI AAT or
Kd dTIdx r 1 )
(
hcD
2E1r,
Nu —
Kd
1
21 )
(7
1
(Nu)E= 0 =
1
1
(8.37)
_ r1 - R
The effect of vapour surrounding the droplet upon the Nusselt . number
may be estimated from the ratio,
1
AT
The Nusselt nuiriber Wu) can be expressed
s
Equation (8:36) forms the basis for expressing the actual and apparent
evaporation conditions. Actual conditions are given by equation (8.36).
Apparent conditions are obtained by expanding the exponential in equation
(8.36) and substituting E = 0 into the equation to give the Nusselt number
at negligible evaporation rate from the droplet.
1
[exp E(-1--1
r1
R2
(8.36)
Nu
Nu E _,
E(—
1
'l
exp E(-r/
(8.38)
296
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
where the Nusselt number ratio also represents the ratio of the actual and
apparent heat coefficients (11,(actual) and h c (apparent)). Hence
Correcting for mass transfer effects using the correction factor
a'
—1
ai
h c (actual) = h c (apparent)( ,
ea — 1
(8.39)
where a' is calculated from equation (8.43). Droplet relative velocity is
assumed low, whereby Nu = 2.
where (368)
a
,
WiC p 1
47c1C d rt
1
R2 I
(8.40)
If the radial limit (R 2 ) of the boundary layer is of the order of the droplet
diameter (D), (a') reduces to
W'C P
1
X—
a=
• 47rKd
D
Nu(ATC11
2
A.
LMTD = 478 ° F
C p = 0.133 BTU/lb°F
Calculation of correction factor :
T = 478 ° F,
ATC p
ATCP
= 0.88,
a' = 0.61,
(e' a
1+
=
1.88,
l 1) = 0.72
Now
t actua l = tapparent (
e a'
1
2.21
tactual = — = 5•1 sec
0 .72
Percentage increase in evaporation time due to mass transfer effects = 28 %.
(8.42)
The increase in droplet evaporation time due to heating of the surrounding
vapours at high temperatures can be obtained by dividing the apparent
evaporation time by the correction factor a' /(e" — 1). An expression for
a' can be derived (108)
a = In [1 +
Data
(8.41)
Equation (8.39) shows the influence of mass transfer on observed heat
transfer coefficients. Equation (8.39) indicates that at high evaporation
rates (i.e. E is large) a' is large and the value of actual Nusselt number may
be appreciably lower than apparent Nusselt number. At high temperatures,
the value of a' can reach 4.0, when the corresponding value of a'/(e" — 1)
becomes approximately 0.075. This indicates that values of h, (actual) can
be as low as one-tenth the value of h, (apparent).
The apparent Nusselt number (Nti) ED can be calculated directly from
the McAdams (10) relation for heat transfer to spheres
(Nu) apparent = 0.37 rvreiPay.6
297
(8.43)
The relative velocity between droplets and air in the spray dryer is very
low for a substantial part of the droplet evaporating time and the Nusselt
value correspondingly approaches 2.0. The term (A TC p/A) determines the
magnitude of a' and can be interpreted as the ratio (sensible heat transferred
to vapour/latent heat).
EXAMPLE 8.2
If the correction for high temperature operation is applied to the droplet
evaporation in example 8.1, what difference does this make to the predicted
evaporation time? Predicted evaporation time (example 8.1 = 2.2 sec).
8.2.2. Evaporation of Sprays of Pure Liquid Droplets
The evaporation characteristics of droplets within a spray differ from
evaporation characteristics of single droplets. Although basic theory
applies in both cases, it is difficult to apply this theory to the case of a large
number of droplets evaporating close to the atomizer. Any analysis of spray
evaporation depends upon defining the spray in terms of a representative
mean diameter and size distribution, the relative velocity between droplet
and its surrounding air, droplet trajectory and the number of droplets
present at any given time per given volume of drying air. Furthermore
there are grave difficulties in determining these factors in the vicinity of the
atomizer and spray evaporation data is subsequently limited. For sprays
moving at low velocities in low velocity air (counter-current flow dryers) or
at low relative velocities with high velocity air (co-current flow dryers), the
following points can be made.
1. Spray evaporation causes reduction in air temperature and evaporation
rate decrease.
2. Sprays of wide distribution evaporate initially more quickly than more
homogeneous sprays of the same chosen mean diameter. The increased
298
THE PROCESS STAGES OF SPRAY DRYING
evaporation is due to the smaller droplet sizes in the distribution. The
larger droplets evaporate much slower, and thus the overall spray evaporation time is longer.
3. No mean diameter parameter can represent adequately the droplets
during evaporation of a spray.
4. The size distribution gives the best representation of droplets during
the evaporation of a spray.
5. Size distribution of droplets in a spray changes during evaporation.
6. For homogeneous sprays, the droplet diameter parameter decreases
during evaporation.
7. For non-homogeneous sprays the droplet mean diameter will generally
show an initial increase prior to decrease until completion of evaporation.
Evaporation of sprays, whose droplets move at pronounced relative
velocities (for example, coarse nozzle atomization in co-current flow or
`fountain-type' dryers) shows additional features.
1. The droplets travel greater distances before a given fraction is evaporated.
2. Influence of relative velocity between droplet and air is more significant
on evaporation rates at higher release velocities from the atomizer and at
higher drying temperatures.
3. For droplet release from atomizers at high velocities, the relative error
of neglecting droplet velocity is greatest for the smallest droplet sizes in
the spray distribution. Small droplets evaporate virtually instantaneously,
and a large proportion of the evaporation is accomplished in the period of
droplet deceleration.
Notable contributors to spray evaporation analysis have been Probert
(110), Marshall (108), Shapiro and Erickson (111), Fledderman and Hanson
(112), Manning and Gauvin (113), Bose and Pei (114), and Dickinson and
Marshall (109).
Probert (110) presented a - theoretical analysis based upon a spray size
distribution following the Rosin—Rammler distribution. Spray droplets
were considered to have no relative velocity and temperature driving force
changes were assumed negligible during evaporation. The more homogeneous the spray the faster evaporation proceeds to completion, although
Probert showed coarse spray distributions may well have higher initial
evaporation rates. Work by Probert was extended by Fledderman and
Hanson to cover spray evaporation under conditions of relative velocity,
where the spray distribution was assumed to follow the NukiyamaTanasawa equation. The equations proposed by Fledderman and Hanson
proved extremely complex, rendering the equations of little practical value.
A practical method of spray evaporation evaluation has been proposed
by Marshall (108). An increment method is involved. The size distribution
is divided into small size groups. Each group is considered individually
DRYING OF DROPLETS/SPRAYS
299
while evaporation proceeds and the average diameter of the spray decreases.
The method was simplified to a spray following a logarithmic distribution,
evaporating under zero relative velocity conditions. The method was set
out for pure liquid droplets. The calculations, however, can prove useful
illustrating the nature of changes that do take place when the spray drying
operation does not involve pure droplets. The Marshall method involves
the calculation of the change of the average droplet diameter in each selected
size group over short time intervals. Short time increments are selected
to enable the assumption of droplet evaporation under the same temperature driving force during the selected time interval. Evaporation for each
size group in the distribution is considered, and a new distribution is drawn
up on which the evaporation during the second time interval is calculated.
Procedure continues until evaporation is complete.
The procedure as stated by Marshall is as follows
1. Plot the cumulative distribution curve for the spray under investigation.
This involves plotting the droplet size in the spray against the cumulative
percentage of the spray less than the droplet size.
2. Size-increments are selected and the average diameter specified. Using
the droplet size data, the frequency of droplet size occurrence is known and
mass fractions (F) corresponding to the size increment are calculated.
3. The initial temperature driving force between droplets and air is determined.
4. The time for complete evaporation is determined and designated t o ,
The time is obtained from equation (8.5), substituting k for Kd.
to
p I 2.(D3
Df)
8KdAT
(8.44)
Value of AT, obtairidd from step 3, is assumed to hold during the entire
droplet travel during the spray dryer. D, is zero for a pure liquid droplet.
5. On the basis of step 4, a suitable time increment (At,) is chosen for estimating the first increment of evaporation. Marshall proposes a suitable time
increment which enables complete spray evaporation of the first two size
groups.
6. The time increment At, is substituted from the calculated value t 0 . The
resulting time t,, i.e. (t 0 — At,) is substituted back into equation- (8.44) and
a value of D, the 'reduced' diameter of the droplet in each size group is
calculated. The reduced diameter is assumed to exist after t 1 seconds of
evaporation. The time increment must be selected so that constant temperature conditions (AT) over the time increment can justifiably be assumed.
7. The ratio (.13,1D 0 ) 3 is calculated. This ratio will give the numbers proportional to the fraction of the spray remaining in each size group. The
mass fractions calculated in step 2 are multiplied by this ratio.
300
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
8. The multiples of each size group are summed, i.e. EF❑ (D i lD ❑ ) 3 = Fl .
(F,/F❑ ) equals the fraction of remaining spray and (1 — F,/F❑ ) is the fraction
evaporated. If F❑ is normalized, F❑ = 1, and F 1 is the remaining fraction.
9. The incremental temperature drop over the first time interval is calculated
from a heat balance. Heat transfer effects to the vapour surrounding the
droplet are considered negligible. Hence
M air C p AT
(8.45)
where M ai ,. = mass air flow rate through the dryer, M ug = liquid evaporation rate, AT = temperature drop through the dryer. The incremental
temperature drop over the first time interval becomes (Ti — T2 )/(1 — F 1 ),
where F1 is the fraction of spray remaining after the first time increment
(At /).
10. The procedure (steps 1-9) are repeated for the new size distribution
formed after At 1 seconds.
11. The calculation proceeds with each new size distribution being considered until evaporation is completed.
The above procedure is time-consuming, but significant information
becomes available. The variation of spray mass fractions and drying air
temperature with time is computed.
Marshall (108) illustrated the procedure for a spray of geometric mean
diameter (120 micron). Maximum droplet size was 450 micron. The spray
was contacted with 530°F (277°C). The outlet air temperature was 230°F
(110°C). Table 8.6 illustrates the evaporation history of the given spray.
Table 8.6. Spray Evaporation History
Droplet size
(micron)
Droplet size
below which
stated %
of spray
occurs
geoTime
metric
t
max, mean
(sec) Dina, DGM 10 %
90%
°F
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
202
205
212
220
230
250
270
285
285
530
325
275
260
250
245
240
239
238
450
438
430
425
421
416
412
408
405
120
125
140
155
165
172
177
180
180
62
62
62
63
65
67
69
72
93
Drying air
temperature
277
163
135
127
121
118
116
115
114.5
Temperature
driving
force
T
°F
°C
400
180
150
130
120
115
111
108
106
204
82
66
54
49
46
44
42
41
Spray mass
fraction
remaining
1.0
0.30
0.15
0.10
0•07
0.06
0.05
0.04
0035
301
Conclusions applicable to spray dryer operation can be drawn from
table 8.6.
1. The majority of spray evaporation is completed in a short time interval.
For example, 90 % of the evaporation is completed during the first 1.5 seconds.
2. Rapid decrease in air temperature accompanies the evaporation. For
example, air temperature drops to within 30°F (17°C) of the outlet air
temperature during 1.5 seconds.
3. The mean size of the pure liquid spray increases with time due to the
rapid completion of evaporation of the smaller droplet sizes in the spray.
In actual spray drying operations, the variation of mean size with time will
depend upon the solid content in the spray droplets and whether drying
characteristics lead to particle expansion or retraction during evaporation.
Shapiro and Erickson (111) studied the mathematics of spray evaporation
for one-dimensional spray motion. General differential equations were
proposed to express spray behaviour, but lack of a comparison between
predicted and experimental results limits the usefulness of the work.
Manning and Gauvin (113) determined heat and mass transfer rates for
water sprays in co-current and counter-current air flows. Experimental
data justified the use of the Ranz and Marshall (equation (8.13)) and Frossling
equations (8.20) (8.21) for spray evaporation. Bose and Pei (114) studied
evaporation of water sprays in a co-current flow nozzle dryer. The relative
velocity between spray and air was concluded to be of great significance
to the resulting heat and mass transfer rates. A substantial part of the
evaporation takes place during spray deceleration. Large errors can occur
if relative velocities are equated to the terminal velocity and used as the
velocity term in the evaporation analysis.
Dickinson and Marshall (109) drew up computer programmes to calculate
the evaporation histories of water sprays. The conditions of both negligible
and significant relative velocity between spray and air were studied. Principle
parameters incorporated in the studies were a mean diameter, size distribudim of spray on release from atomizer, droplet population, drying air and
droplet temperatures and air velocity. Mathematical equations for spray
evaporation of non-uniform distributions were developed. Ideal conditions
of constant droplet temperature, ideal co-current and counter-current air
flow and droplets of pure liquid were assumed to prevail.
It would appear that future procedures for evaluating evaporation histories
will rely heavily on computer facilities, and the development of improved
experimental techniques to study droplet and spray evaporation. Techniques
for studying evaporation of sprays in three-dimensional movement would
be of particular value to meet demands of accurate input data for full utilization of the computer resources.
302
THE PROCESS STAGES OF SPRAY DRYING
Current manual step-wise methods will continue to be applied and
provide useful data on the evaporation characteristics of any given system,
once_ initial size distribution and vapour pressure data are known. It is
well to recall that for spray dryer design, considering the evaporation of
the largest droplet in the spray will generally suffice in sizing chambers.
8.3. Evaporation of Droplets Containing Dissolved Solids
Droplets containing dissolved solids evaporate at lower rates than pure
liquid droplets of equal size. The presence of dissolved solids decreases
the vapour pressure of the liquid and the vapour pressure driving forces
for mass transfer are reduced. The drying characteristics feature the formation of solid material at the droplet surface, and thus differ completely
from pure droplet evaporation where the droplet evaporates away completely.
8.3.1. Evaporation of Single Droplets
(a) Vapour Pressure Lowering Effect
With the depression of the vapour pressure of liquid containing dissolved
solids droplet temperatures will increase over the wet bulb temperatures
that prevail when droplets consist of pure liquid. The vapour pressure
lowering effect can be illustrated by plotting vapour pressure curves on the
humidity-temperature (psychrometric) chart (curves A and B on figure 8.3).
Curve A represents the saturation curve for pure liquid and B represents
the saturation curve for the liquid containing dissolved solids.
7
4_
Hs
Iw
,A
303
Figure 8.3 can be used to obtain the droplet surface temperature. (The
humidity is represented by partial pressure of water which for air-water
systems equals vapour pressure when air is saturated. Vapour pressure
curve for pure liquid is the saturation curve for pure liquid on the chart.)
The procedure is similar to that described in chapter 3. Drawing the
adiabatic saturation curve through the drying air conditions of Ta , p a ,
(Ha ), the intersection of the vapour pressure curve of the saturated solution
estimates the surface temperature of the droplet containing dissolved
solids (Ts ). Continuation of the adiabatic line will intersect the vapour
pressure curve for the pure liquid, and gives the droplet temperature (T w ).
( Ts Tw ) represents the increase in droplet temperature due to the presence
of the dissolved solids.
If the presence of dissolved solids has negligible effect on the vapour
pressure relationship (this is the case for many spray dried salts), there is
little difference in the vapour pressure curves for pure liquid and the solution.
The surface temperature of the droplet can be taken as the temperature
of adiabatic saturation in the case of water being the solvent, or the normal
wet-bulb temperature if other liquid solvents are being evaporated.
(b) Effect of Dried Solid Formation in Droplets
The formation of dried solid at some stage in the evaporation of a droplet
greatly alters the following evaporation history. Experimentation has
shown that droplets of solutions when initially contacted with drying air
in a spray dryer commence evaporation at rates that are more or less constant (first drying period). Ranz and Marshall (100) showed that surface
temperature of the droplet during this period can be equated to that of the
saturated solution, although the droplet surface concentration can be less
-1
than saturated. The average drying rate (dW/dt) (dimensions MT )
during the first period of drying can be expressed from equation (8.3) as
dW 27c1cD av AT
dt A
of
cc
H, j
DRYING OF DROPLETS/SPRAYS
(8.46)
where Da, = average droplet diameter.
The first period of drying ceases when the droplet moisture content
falls to a critical value, characterized by the dnitial presence of a solid phase
forming at the droplet surface. Then follows the falling rate period of
-1
drying where the average rate of evaporation (dW'/dt) (dimensions 1 ) is
expressed by equation (8.47).
H) fra
<°
of
•:E
0_
T
DROPLET TEMPERATURE
Figure 8.3. Estimation of temperatures of droplets containing dissolved solids. A. Vapour
pressure curve for pure liquid. B. Vapour pressure curve for saturated solution.
dW' -12K,4T
dt
ADC P s
(i.e. dW/dt dirldt x wt of dry solid).
(8.47)
304
DRYING OF DROPLETS/SPRAYS
THE PROCESS STAGES OF SPRAY DRYING
Movement of moisture from the interior to droplet surface becomes less
and less, due to the increasing resistance to mass transfer caused by the
solid phase becoming more extensive. The rate of heat transfer exceeds
that of mass transfer. The droplet begins to heat up. Sub-surface evaporation
occurs if the heat transfer is sufficiently high to cause vaporization within
the droplet. In spray drying operations, where the air temperature is above
the boiling point of the droplet solution, the liquid inside the droplet reaches
its boiling point, and vapour is formed. When the solid phase formation
forms a crust around each droplet, pressures can be set up within the droplet.
The effect of pressure depends upon the nature of the crust. Vapours will
be released if the crust is porous, but for non-porous, the droplet may rupture
or even disintegrate. Droplet residence times in the hottest drying zones
are short and -if droplet temperatures do not reach boiling point levels,
moisture movement within the droplet is by diffusional and capillary
mechanisms. Each material shows different drying characteristics in the
falling rate period. If droplets form a film of dried product at its critical
moisture content and this film is highly impervious to vapour flow, the
drying rate will fall sharply, and the evaporation time for complete volatile
removal is greatly extended. On the other hand, if a highly porous surface is
first formed, vapour continues to easily migrate to the droplet—air interface,
and the drying rates only gradually fall from the value established during
the first period of drying. This is illustrated in figure 8.4. Droplets can form
into a variety of shapes ; ruptured, expanded, collapsed, fragmented. For
each shape, drying in the falling rate period undergoes different rates.
I MPERVIOUS CRUST FORMATION
POROUS CRUST FORMATION
EVAPORATION
H
DROPLET EVAPORATION
COMPLETED TO
REQUIRED LEVEL
EVAPORATION TIME
Figure 8.4. Effect of crust properties on evaporation times.
305
Charlesworth and Marshall (115) have illustrated shape and composition
changes that spray dried droplets can undergo. Shape and texture of the
dried particles fall into two groups. Each group depends upon whether
the drying air temperature is above or below the boiling point of the droplets.
The characteristics of the two groups are shown in figures 8.5 and 8.6.
(c) Droplet Evaporation Times
Droplet drying times are equal to the sum of evaporation times in the
constant and falling rate drying periods, and are calculated using equations
(8.46) and (8.47). This requires data on the initial droplet diameter (D 0 )
and the droplet diameter at the critical moisture content (D o ). Four methods
are available to determine the shape change factor, and hence (D o).
(i) Experimentally determine the decrease in droplet size by observing
evaporating droplet until the critical moisture point is reached.
(ii) Determine the wet spray size characteristics (for convenience use a
mean size parameter) and compare with the mean size parameter obtained
from a dried powder sample. Manual sizing of individual particles will
offset sizing errors due to non-spherical particles, but as this is time consuming sieve analysis of powder is often used. It is a convenient method
but cannot differentiate between shape configurations.
(iii) Estimate a value, as 60-80 % of initial droplet diameter.
(iv) Use moisture content determination to calculate ratio of droplet to
dried particle diameter (22). See also example 8.4.
The critical moisture content is obtained from the moisture originally
occupying the droplet volume lost during the first period of drying. Subtraction of this value from the initial moisture content realises the critical
moisture content. One value can be used to define the critical moisture
content of all droplets in the spray. This is justified as Charlesworth and
Marshall (115) showed that for any solute the average critical moisture
content of a droplet has approximately the same value for all droplets,
and is independent of the initial droplet diameter, initial solute concentration and drying conditions.
The droplet surface temperature (Ts ) is calculated from the humidity
chart for the solution. If crystals are formed at the droplet surface, the
temperature can be calculated according to Williams and Schmitt (116).
Calculation of evaporation times based upon equations (8.46) and (8.47) is
illustrated in example 8.3.
EXAMPLE 8.3.
A 45 % by weight aqueous solution of a dissolved salt is spray dried to a
product of 4 % moisture. The inlet and outlet drying air temperatures are
350°F and 176°F. Calculate the drying time for a droplet of 196 micron.
The droplet undergoes a 10 % shrinkage during evaporation and the critical
moisture content occurs at 30 % moisture content.
NO CHANGE
BUBBLING
MINOR FRACTURE
LESS POROUS
RIGID
-
z
MAJOR FRACTURE --
LIQUID
INITIAL RARTVAL COMPLETE
DROPLET SOLID CRUST
CRUST
a
- COLLAPSE
SKIN REFORMS
- f NFL A TII'JG
NON POROUS
PLASTIC ---
0
THE PROCESS STAGES OF SPRAY DRYING
...POROUS
RIGID
CON T/NUED
INFLATION
- BUBBLING -
- COLLAPSE
--
BLISTERING -
0
SPONGE-LIKE
Figure 8.5. Characteristics of droplet undergoing drying (drying air above boiling point).
Based on Charlesworth—Marshall (115).
STRUCTURE
OF
CRUST
R4G10
NO CHANGE
NO CHANGE
LESS POROUS
RIGID
INITIAL PARTIAL COMPLETE
LIQUID
CRUST
DROPLET SOLID CRUST
— NO CHANGE
— FRACTURE
_CRYSTAL
FUR . GROWTH
PLIABLE
Am.
—COLLAPSING ---
—SHRIVELLING
Figure 8.6. Characteristics of droplet undergoing drying (drying air below boiling point).
Based on Charlesworth—Marshall (115).
SAV/IcIS/SITIcIMIG 40 DNIAX ❑
r POROUS
308
THE. PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
Data
3
density of liquid droplet = 1.03 g/cm
density of dried particle = 0.55 g/cm 3
latent heat of vaporization = 970 BTU/lb
thermal conductivity of liquid droplet = 0.38 BTU/hr ft 2 F/ft
thermal conductivity of drying medium = 0.0172 BTU/hr ft 2 ° F/ft
droplet temperature from humidity chart = 103°F
(a) Constant Rate Period (first period of drying) t ° , r ,
Average rate of evaporation is given by equation (8.46)
(b) Falling Rate Period (tf.r. )
Average rate of evaporation is given by equation (8.47). Assume droplet
temperature rises to 140°F. Air temperature falls 250°F 176°F. Logarithmic mean temperature difference = 78° F. Substitute in equation
dW'
dW
dt
2n0.381186 x 101
192
970
30.5
Initial moisture content of droplet (grams)
4 196x10
3
2
-43
4)3
1-03 2.23 x 10'
Solid content in droplet (grams)
41196 x 1013
0-45-
1.03= 1.82 x 10 -6
2
Moisture content at critical point (30 % by weight) (grains)
6
1.82 x 10 - 30=)
7.8 x 10'
(
70
Moisture removed (grams)
(2.23 - 0.78)10 -6 = 1.45 x 10 -6
t„
1.45 x 10 -6
= 1.1 x 10 -5 hr (0.04 sec)
131 x 10-
= 106 x 103
dW dW'
x (wt of dry solid) = 1.06 x 10
dt = dt
= 4.25 x 10
lb moisture
lb dry solid/hr
-6
lb/hr(19.4 x 10
-4
3(1.82 x 10-6
454
)
g/hr)
Moisture content at commencement of falling rate period (grams)
7.8 x 10 -7
Final moisture content (grams)
(7.8 - 0.759) x 10' = 7.04 x 10
(where 1 ft = 30.5 cm)
= 28.8 x 10 -5 lb/hr (1.31 x 10 -1 g/hr)
12 x 0-0172 x 78
-4
970( 177 x 10 ) 2
0-75 x 62.5
30 . 5
\
3
The average density of droplet during this period is taken as 0.75 g/cm .
Now evaporation rate
Initial droplet diameter = 196 micron
Critical droplet diameter = 177 micron
Average droplet diameter = 186 micron
Assume air temperature falls 100°F during this period, i.e. 350 250°F.
Droplet temperature = 103°F, and the logarithmic mean temperature difference (by equation (8.6)) = 192 ° F. Substituting in equation
309
-7
Hence
-7
7.04 x 10
= 362 x 10 -4 hr (1.3 sec)
tfr =
194 x 10 -4
'
Total drying time = 0.04 + 1.3 = 1.34 sec.
8.3.2. Evaporation of Sprays Containing Dissolved Solids
Theoretical considerations of heat and mass transfer of single droplets
apply to sprays of droplets. The extent of vapour pressure lowering due
to the presence of the dissolved salts will vary per droplet size. Resistances
to moisture transfer will also vary, as the solid phase will not appear simultaneously throughout the size distribution. Methods of spray evaporation
analysis are highly complex, and investigations in this field are limited.
Dloughy and Gauvin (117) are among the few investigators to study
specifically evaporation of sprays containing dissolved solids. They showed
that the total spray evaporation time could be predicted with a fair degree
of accuracy by employing a step-wise method of calculation.
Baltas and Gauvin (118) have shown that application of computer
programming to stepwise techniques offers the most promising method of
evaluating evaporation histories. Baltas and Gauvin studied evaporating
sprays of sodium nitrate. Many of the difficulties of the spray dryer system
that require simplifying assumptions (often unrealistic) were overcome by
310
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
the selection of a simple system of spray movement at terminal velocity
in the free fall zone of a single nozzle dryer. The computations were nevertheless complex, and they indicated that the difficulties in accurate prediction
of spray evaporation was not in the ability to programme the complicated
features of spray dryer operation (e.g. swirling air flow, temperature gradients,
multiple atomizers, chamber shape) but in the inability to obtain representative data of these parameters for including in the programming,
8.4. Evaporation of Droplets Containing Insoluble Solids
Feeds of insoluble solids form slurries and pastes (hereafter referred to as
suspensions). There are negligible vapour pressure lowering effects in
droplets containing insoluble solids and the temperature can be put equal
to the wet bulb temperature of pure liquid droplet during the first period
of drying.
Total drying times of droplets containing insoluble solids can be evaluated
by dealing separately with the two drying periods. Drying times for the
first period of drying are short compared with the following second period,
but in calculations where the first period is not considered negligible, Ranz
and Marshall (100) have reported equation (8.8) applicable, where K d is the
thermal conductivity of the evaporating liquid.
The drying time for the falling rate drying period cannot be reliably
expressed in equation form. It depends upon the nature of the solid phase.
Ranz and Marshall (100), however, have proposed a relationship in terms
of the critical moisture content (140, where
tf,
ADePs(We
—
W2) 2,1) Ps(We
6/0 Tay
—
W2)
12.K,A Tav
(8.48)
where W2 = final moisture content of the dried particle, Kd = thermal
conductivity of drying medium.
For drying of droplets at low Reynolds number, and negligible vapour
pressure effects, the total time according to Marshall (108) can be expresSed
by adding equations (8.8) and (8.48).
ttotal
APIM
81Cd (Ta — Ts )
+
AD!Ps(Wc — 1412)
12.K,ATa ,
(8.49)
Equation (8.49) represents the evaporation history in its most simplified
case. However, calculated values give close agreement with actual evaporation times in many instances and thus the equation is useful for obtaining
data for spray dryer chamber design. For a known flow pattern within the
chamber, the minimum residence time for spray evaporation can be cal-
311
culated, i.e. the time to evaporate the droplets to a state of dryness for
prevention of semi-wet product build-up on the dryer walls.
In applying equation (8.49) the thermal conductivity is calculated at the
mean film temperature surrounding the evaporating droplet. The film
temperature can conveniently be taken as the average between the exhaust
drying air temperature and the droplet surface temperature. The droplet
surface temperature is the adiabatic saturation temperature of the suspension
spray. The surrounding air temperature at the end of the first period of
drying is usually unknown. The driving force (AT) over the entire period
is most conveniently taken as the logarithmic mean temperature difference
between the inlet air temperature and the slurry feed temperature, and the
exhaust air temperature and the droplet surface temperature at the critical
point. The driving force (AT) for the falling rate period can be taken as the
difference between the exhaust air temperature and the droplet surface
temperature at the critical point although the surface temperature will
rise during the falling rate period. Alternatively air temperature at the
droplet critical point and droplet temperature rise during the falling rate
period can be assumed. The logarithmic mean temperature difference is
applied to both periods of drying. The droplet diameter at the critical point
(D c ) is usually an unknown value. Ideally this value requires data on the
evaporation characteristics of the suspension droplet to enable determination of droplet size change before solids form at the surface. In the absence
of such data, the methods described under evaporation times for droplets
containing dissolved solids (page 305) can be applied. The factor is taken
to express the percentage decrease in droplet diameter during the first
period of drying. Droplet size change is then considered negligible during
the second period of drying.
Example 8.4 illustrates a method for calculating evaporating times of
suspension droplets. The shape change factor and critical-moisture contents
are calculated from initial and final droplet/particle moisture contents.
EXAMPLE 8.4
A slurry containing 40 % by weight of insoluble solids is spray dried to a
moisture content of 1.5 %. Inlet and outlet dryer temperatures are 860°F
and 200°F. The saturated droplet surface temperature estimated from the
psychrometric chart is 148°F. Calculate the evaporation time for a 300
micron droplet in the spray.
Data
Density of slurry (p w ) = 76 lb/ft'
Density of dried product p d = 50.2 lb/ft'
Feed temperature of slurry = 80°F
Mean thermal conductivity
2
of gaseous film around droplet = 1.375 x 10' BTU/hr ft /ft °F
312
DRYING OF DROPLETS/SPRAYS
THE PROCESS STAGES OF SPRAY DRYING
The temperature profile of air and droplet through the dryer is established.
Air temperature falls rapidly on contact with the spray, but the rate of
decrease becomes less and less as the outlet temperature is approached.
The droplet temperature initially rises from the slurry feed temperature
to the saturated surface temperature, fixed by the humidity conditions of
drying air on contact with the spray. The droplet temperature is considered
constant during the first period of drying (t c. ,..). The droplet temperature
increases slowly during the second period of drying (t f „..).
A likely profile for the given example is given below in figure 8.7.
313
AT = (air) 860°F 350°F
148° F
(droplet) 80°F
LMTD = 426°F
2 1
t
970 x 62.3[(9.8 2 - 8.35 ) '13600
8 x 0.01375 x 426
,, = 1.2 sec
(b) Second period of drying (t Er. )
Substituting values in equation (8.48). Moisture content at critical point (W c).
3
Decrease in value due to 15 % shrinkage = *7[D' (0.85D) ]
CRITICAL POINT(c)
1000
860-F- Ta
TEMPER ATU RE
i 800
600
350 • F = Tac
n 3)
= 0.386( —
6
70
Moisture removed = 0 386 ( - )th
.
6
70
400
3
Moisture remaining = —6 -(0 6p w 0.386th)
AIR
148.F Tel
86F= TI C
3
200
= 200 - F
DROPLET
= 175- F
d
Critical moisture content =
(70 3 /6)(0-6p, - 0.386th)
o
3
(P i = 62.3 lb/ft )
DRYER
INLETii)
DRYER
OUTLEToi
Figure 8.7. Temperature profile in example 8.4, T. = air temperature, Td = droplet temperature.
Droplet Size Change During Drying
Moisture content of slurry = 1.5 lb/lb dry solid
Moisture content of dried product = 0.0152 lb/lb dry solid
Substitute values in equation (8.51)
DD
76
(1 + 0.0152)1 1 / 3
= 0.85
x
D w - 50.2 (1 + 1.5)
Droplet size shrinkage = 15 %.
(a) First period of drying (t° , r• )
Substituting value in equation (8.8)
D i = 9.8 x 10 -4 ft
D c = 8.35 x 10 -4 ft
OrD 3 16)0.4p,
0.386pi
= 1.5 -
04,
p
= 0.7 lb/lb dry solid
AT = (air) 350°F -* 200°F
175°F
(droplet) 148°F
LMTD = 94°F
970(0.7 - 0.0152) x 50.2(8-35 x 10
t fr 12 x 0.01375 x 94
-4 2
) 3600
= 5.4 sec
Total evaporation time = 1.2 + 5.4 = 6.6 sec
8.5. Drying in Media Other Than Air
Air is the drying media most widely used in spray drying operations, but
operations are becoming more frequent where drying is carried out in an
inert atmosphere. Nitrogen or superheated steam are used in cases of
extreme explosion hazard promoted by presence of solvents in the feed.
DRYING OF DROPLETS/SPRAYS
THE PROCESS STAGES OF SPRAY DRYING
Use of inert atmospheres will demand performing the spray drying in a
closed-cycle system to limit gas usage and recover feed solvents,
The effect of gaseous media on evaporation rates and the comparison
between air and inert atmospheres on drying rates can be observed by
inspection of equations (8.20a, b) where the mass and heat transfer around
a sphere is given.
For any gas having a higher diffusivity and thermal conductivity than
air, there will be an increase in the rate of droplet evaporation. For low
viscosity gases, the Prandlt number increases accordingly.
8.5.1. Drying in Gases
When evaporating in light gases (for example, hydrogen), the rate of evaporation is much higher than in air due to the higher rates of diffusion and
thermal conductivity. This advantage of light atmospheres is counteracted
by droplet trajectory being more extensive than in air. Evaporation times
may be considerably faster, but terminal velocities are correspondingly
higher, resulting in little change in overall dryer dimensions for a given
evaporative capacity.
S.5.2. Drying in Superheated Vapours
The mechanism of droplet evaporation in drying media of superheated
vapours is one of negligible mass transfer resistance and the droplet temperature approximating to the saturated vapour temperature, equivalent to the
dryer operating pressure. The mechanism differs from evaporation into
gases, where droplet temperatures are not so high, only equivalent to wet
bulb temperature levels.
Proposals for use of superheated vapours instead of gases or air in spray
drying arise from (a) marked improvement in the thermal efficiency of the
drying operation, and (b) the provision of inert atmospheres, where product
oxidation and likely explosion hazards are prevented. Use of superheated
vapours enables the volume of drying media leaving the dryer to be smaller
than when using gases.
For materials of poor heat conductivity superheated vapours prove
effective drying media. However, heat degradation of product will occur
if the product is sensitive to temperatures above the boiling point of the
liquid phase at the pressure of the drying operation. This is shown in
figure 8.8 where droplet temperature during droplet evaporation in superheated vapour is compared with that in air. Problems of condensation at
the dryer outlet and within the product separation equipment arise when
superheated vapours are used as drying media.
Correlations (equation (8.13)) for predicting heat transfer coefficients of
evaporating droplets in gases also apply to droplet evaporation in the
presence of superheated vapours (374). Evaporation and drying of droplets
in superheated vapours has been studied by Trommelen and Crosby (374).
315
BOILING PONT OF
SOLVENT AT DRYER
PRESSURE
TEMPERATU RE
314
WET BULB TEMPERATURE
FEED TEMPERATURE
EVAPORATION TIME
Figure 8.8. Temperature of droplet undergoing evaporation in air and superheated vapour.
A. Evaporation in superheated vapours. B. Evaporation in air.
8.5.3. Drying in Superheated Steam
Evaporation of aqueous droplets into superheated steam is only determined
by heat transfer to the droplets. There is no diffusional mass transfer in the
gas phase (107). Heat transfer rate is the limiting factor to volatiles removal.
Droplet surfaces are much higher than when evaporation takes place in air.
Actual temperature values depend upon steam pressure. The heat transfer
rate into steam is somewhat lower than into air. The Prandlt numbers are
almost identical (Pr air = 0.74, Prsteam = 0.78). The heat transfer coefficients
are given by equation (8.50)
2
---5_,= _-(8.50)
h e (s [ eam) = hc(air) x
3 Re m ,
0
' 5_
It follows from equation (8.50) that the heat transfer coefficient in steam is
approximately 15 % less than the coefficient in air. Overall evaporation
rates are lower with superheated steam than with air at 212° F (100° C). At
°
temperatures of 752°F (400 C) rates in air and steam become virtually
equal.
Dried products of high bulk density often result from evaporation in
superheated steam. The main problem with applying superheated steam
concerns condensation, especially at the dryer outlet and product handling
equipment. Operational difficulties often outweigh any improvements in
the dried product properties obtained by employing drying atmospheres
of superheated steam. Moreau and Passey (472) report successful application. A comparison of evaporation rates for aqueous droplets in air and
superheated steam are given in table 8.7.
•
316
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
0.8
Table 8.7. Evaporation of Aqueous Droplets in Atmospheres of Air and
Superheated Steam (after Trommelen—Crosby (374))
Air
Superheated steam
1250
395.6
118.4
6.1
0.54
5.26 x 10 -2
1260
392.0
208.4
6.25
0.405
416 x 10 - 2
317
U
8.6. Dried Product Properties on Completion Of Drying
8.6.1. Effect of Operating Variables on Product Properties (Particle Size and
Bulk Density)
Each operational variable of the spray drying process influences dried
product properties. The extent of each influence depends upon the product.
However, general conclusions can be drawn.
(a) Effect of Feed Rate
Increase of feed rate at constant atomizing and drying conditions increases
particle size (see figure 6.7) and bulk density of the dried product. There
need not be any change in the deviation of the particle size distribution.
(b) Effect of Feed Solids
Increase in feed solids increases the particle size of the dried product. As
feed solids are increased at constant drying temperatures and feed rates,
the reduced evaporation load will result in products of lower moisture
content. There will be greater tendency for such rapid moisture evaporation
that hollow dried particles result and overall bulk densities are lower.
However, this does not always occur because this effect is counteracted to a
certain extent by individual spray droplets being larger and the weight of
moisture per given droplet remaining approximately the same. The variation
of bulk density of spray dried coffee extract with increased solids is reported
by Duffle and Marshall (22), and Crosby and Marshall (119). The effect of
feed solids on bulk density of various spray dried products is shown in
figure 8.9.
(c) Effect of Feed Temperature
Increase in feed temperature has an effect on dried product properties if
lower feed viscosities are required for adequate feed pumping and atomization. For low viscosity liquids at room temperature levels, increases in
feed temperature have negligible effect. Increase in temperature will tend
to decrease the total heat requirement for spray evaporation, but the subsequent increase in feed heat content remains small compared with the heat
requirement for vaporization.
,
0.6
BULKDENSITY OF SPRAY DRIED PRODUCT
Droplet diameter (micron)
Air temperature (°F)
Droplet temperature (°F)
Relative velocity (ft/sec)
Rate of heat transfer (BTU/hr)
Rate of evaporation (lb/hr)
0.4
0.2
0
10
20
33
40
50
60
70
SOLIDS CONTENT IN FEED %.
Figure 8.9. Effect of feed concentration on bulk density of spray dried products (results of
95°C, 116°C (Duffle,
various investigators). a, b, Water dispersible dye, t i = 493°C, t r
= 20°C (Wallman, Blyth, 40); d, Sodium
Marshall, 22); c, Sodium silicate, t ; = 260°C,
silicate (Lamont, 370); e, Sodium silicate (Reavell, 371); f, gelatin (Lamont, 370); g, Soap (Jones,
372); h, Coffee, t i --- 212-215°C (Crosby, Marshall, 120).
Duffle and Marshall (22) report a trend of decrease in bulk density with
feed temperature increase for products forming spherical hollow particles,
but they present literature data that show an inconclusive effect.
(d) Effect of Surface Tension
Surface tension effects vary widely for each product. However they are
rarely pronounced. Surface tension values of most feeds for spray drying
fall within a range too narrow to have much influence on dried product
properties. Surface tension effects dried product properties by influencing
the mechanisms of atomization and drying. Low surface tension feeds
produce smaller spray droplets. Sprays contain an increasing proportion
of very fine droplets and spray distributions tend to become wider. Larger
droplets will dry, with the likelihood of hollow particle formation and low
bulk density. The fines fractions are dried to lower moisture levels. Their
blending in with the remaining product maintains bulk densities as high
as possible. High values of surface tension produce larger droplet sizes,
318
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
and size distributions tend towards being narrower. There is little difference
between the bulk density of products, having their natural surface tension
and products obtained from feed material of lower surface tension through
addition of surface wetting agents (at identical feed solids composition and
drying conditions).
(e) Effect of Inlet Drying Temperature
The effects of drying temperatures depend upon the drying characteristics
of the products. For droplets that tend to expand on drying, larger particles
of decreased bulk density result from increased drying temperatures. This
has been substantiated by Duffle and Marshall (22) and is shown in figure 8.10.
If however, the temperature increases to the extent that evaporation becomes
so rapid that the droplets puff, fracture or disintegrate, the resulting fragments
CD
0
a.
0.8
0
w
TY OF SPRAY
cc
0.6
319
compact to form higher bulk density powders. The effect of inlet drying
temperature on the bulk density of coffee extract has been reported by
Crosby and Marshall (119).
(f) Effect of Spray Air Contact Velocity
Increase in contact velocity between spray and air will create increased
mixing that will increase heat and mass transfer rates. At very high velocities,
droplet shape distortion may occur, droplets may fracture and even disintegrate or coalesce through impact with other droplets in turbulent flow
motion. With increased velocity, evaporation times become shorter, dried
product particles exhibit irregular shapes. Any bulk density change will
vary according to the product. There is no general trend.
8.6.2. Formation of Hollow Particles
The tendency of expansion and formation of hollow particles plays an
important role in the final bulk density of dried product. Hollow particles
can be formed from four mechanisms.
(a) A surface layer, semi-impervious to vapour flow forms at the droplet
surface. The droplet puffs out as vapour is formed within the droplet and
expands with the increase in droplet temperature.
(b) Moisture evaporates at a rate, faster than the diffusion of solids back
into the droplet interior. On completion of evaporation, air voids are
present, especially in the case of crystalline products.
(c) Liquid flows with accompanying solids to the droplet surface by capillary
action. This leaves the centre of the droplet as a void. This mechanism is
applicable to clay slip feeds.
(d) Air entrained in the feed contributes to air spaces within the droplet.
—
0.4
8.7. Size Differences hilti'ieen Product before and after Drying
tfr
Lu
0.2
-J
CD
CO
0
100
200
30D
400
500
INLET DRYING AIR TEMPERATURE
600
PC
Figure 8.10. Effect of inlet drying air temperature on bulk density of spray dried products
(results of various investigators). a, Sodium silicate, 25 % 60°C (Duffle, Marshall, 22); b, Sodium
silicate, 25 % 20°C (Wallman, Blyth, 40); c, Sodium silicate (Lamont, 370); d, Water dispersible
dye, 19.5% 95°C (Duffle, Marshall, 22); e, Dispersing agent, 36% 80°C (Duffle, Marshal], 22);
f, Soap (Lamont, 370); g, Gelatin (Lamont, 370); h, j, Santomerse, 20 % (Chu, Stout, Busche,
373); k, Coffee extract, 22 % (Crosby, Marshall, 120); m, Sodium sulphate, 20 % 80°C (Crosby,
Marshal], 120),
Evaporation calculations use the estimation of dried particle diameters at
the end of the first period of drying to represent the dried product diameter.
This assumption is valid for products whose surfaces are completely porous
to moisture flow. Inorganic salts come into this category. However, there
are many spray dried products whose dried product shape characteristics
have not been finalized until well into the second period of drying. Final
particle size and density for these materials are often difficult to predict
and the exact relation between wet spray and the resulting dried powder
requires experimental determination over a range of drying temperatures
and feed solids concentration.
The droplet size on atomization depends upon the mode of atomization,
physical properties of the feed, feed solids concentration and applied drying
temperatures. The particle size on completion of evaporation depends
upon the dried product properties. Shape and size changes will differ
THE PROCESS STAGES OF SPRAY DRYING
DRYING OF DROPLETS/SPRAYS
according to whether the droplet has an amorphous, granular or colloidal
form. The size changes that occur during evaporation can be expressed
in a mass balance over the droplet on dry solids basis. Thus :
Initial solid content per spherical droplet (subscript w)
=
p
6
(1 +1- W,
Final solid content per dried droplet (subscript D)
1
= —
6 7ED D- p 11)( 1
1
w2
)
WEIGHT DISTRIBUTION
OF SPRAY
60
40
20
180
D,
1(1 + W2) 1
P o (1 + WI ) x 1]
113
-
x
160
DROPLE
DIAMETER
(8.51)
80
0
(microns)
160
PARTICLE
DIAMETER
240
For the majority of spray dried products, W2 is low, so that 1/(1 + W2 )
tends to unity.
Denoting 1/(1 + W) as C, the weight fraction of solids per unit weight of
product in dried particle equation (8.51) becomes
I3
D,= [PD x CD TD
i,
p, C,
WEIGHT DISTRIBUTION
OF PRODUCT
80
The ratio of diameters can thus be expressed
DD
321
100
CUMULATIVE PERCENT
LESS THAN
320
(8.52)
All droplets are taken to contain the same proportion of solids.
8.7.1. Procedures to Predict Dried Particle Size Distribution
(a) Graphical Method
The weight distribution of a spray dried product can be conveniently
predicted from the wet spray data using the graphical method of Crosby
and Marshall (120). This is shown in figure 8.11 for the case of sprays that
undergo size decrease during evaporation.
The relationship between wet spray droplets and dried particles is given
by equation (8.52). The wet spray size distribution is determined by micro
scope size analysis or represented directly as a distribution function if the
atomization characteristics of the given atomizer follows a known distribution law (chapter 6).
Conversely the initial spray can be predicted from a size distribution of
dry product by sieve analysis.
(b) Mathematical Prediction
It follows from equation (6.32) that for every product, the wet spray size
equals the dried particle size times a factor 13 that depends upon the product's
physical characteristics, drying temperature and the initial and final moisture
contents.
D W = AD D
(6.32)
C
_
45°
Z
X160
if
80
w
0
cc
0 0
80
160
PARTICLE DIAMETER
(microns)
RELATIONSHIP BETWEEN
DROPLETS AND DRIED PARTICLES
Figure 8.11. Graphical determination of size distribution of dried product from that of wet
spray.
Equation (6.32) has been discussed in relation to atomization in chapter 6.
f can be estimated for an individual product, and can be used when predicting plant performance.
Prediction of plant performance by experimental determination of such
a factor has been reported by Moore et al. (121) for non-soapy detergents.
From work in a nozzle tower (6 ft diameter, 30 ft high, 1.8 m dia x 9 m) the
size of the dried product was found larger than the droplet in wet form.
The shape factor was expressed as a volumetric expansion factor. Experimental data established a relation between the volumetric expansion factor,
the outlet dryer temperature and wet spray droplet size. The relation led
to an interpretation of the evaporation events leading to the droplet shape
322
THE PROCESS STAGES OF SPRAY DRYING
change during drying. These events were listed as (a) rate of heat transfer
to droplet, (b) rate of heat loss by evaporation, (c) effect of droplet size on
the amount of expansion caused by the generation of a given amount of
vapour within the droplet, (d) modification of film properties and rate of
formation of the surface film due to changes in drying temperature and
(e) reduction in volume during the early stages of drying due to loss of
moisture.
The work (121) serves as an example of how mathematical predictions of
product characteristics from large dryers can be carried out from data
obtained experimentally on a pilot plant unit.
EXAMPLE 8.5
A product is being spray dried as a slurry of 35 % solids. If the feed concentration is to be raised to 50 % solids, what changes to the atomization stage
must be made to maintain the same dried particle size? The relation (f)
between the wet and dried product size can be taken as established in terms
of initial moisture content, where # = 1.5 — 0.015 100/(1 + 0.01 W). (W is
percentage moisture, dry basis.)
At 35 % solids
W.H= 185%
At 50 % solids
W.H= mo%
Hence
#35 = 1.5 — 0.015
100
2 .85/
= 1.5 — 0.53 = 0.97
100
)51 50 = 1.5 — 0.015( —)
2.00
= 1.5 — 0.75 = 0.75
From equation (6.32) D w , = /3 2 /fl 1 x D w 1 (D, is constant). For dried
particle size to be the same, atomization conditions must be changed to
obtain a reduction in wet spray size. Wet spray size must be reduced by
0.75/0.97 to 0.774D w .
8.8. Size Distributions of Wet Sprays wad Resulting Dritd -Particles
Size distributions of dried particles are mostly narrower than the size
distribution of the atomized wet spray. This has been established over a
range of feed temperatures and feed concentrations by Crosby and Marshall
(119) (120) for materials whose drying characteristics exhibit crystalline,
surface film forming and capillary mechanisms. In cases where the distribu-
DRYING OF DROPLETS/SPRAYS
323
tion of the wet spray more. or less follows a well-established distribution
function, the distribution of the dried particles shows marked deviation
from the function. If the wet spray is represented by the square root normal
function, the dried particles more clearly become represented by the lognormal distribution. Wet spray and dried particles follow the same distribution function only for low drying temperature and low feed solid conditions.
Experimenting with (a) sodium sulphate (drying characteristics representative of a crystalline product), (b) coffee extract (drying characteristics
representative of a film forming mechanism) and (c) clay slips (drying
characteristics representative of a capillary mechanism), Crosby and
Marshall (120) found the dried particle size did not exceed the wet spray
size.
The difference between the mean sizes of wet spray and dried particles
show different trends when feed solids concentration is changed at constant
inlet temperature. For crystalline products, the difference between the
mean sizes decreases as feed concentration increases towards saturation.
The effect becomes less pronounced at low drying air temperatures and
high feed temperatures. For products characterized by their film forming
properties, there is little difference between the mean sizes at 'high feed
concentration. At low concentrations, however, the difference is significant.
Decrease in feed concentrate acts to increase the difference between the
mean sizes. When feed temperature is varied, any change to the difference
between the mean sizes appears to be of lesser significance. Only at high
drying air temperatures does feed temperature exhibit any degree of influence.
SEPARATION AND RECOVERY OF DRIED PRODUCT FROM AIR
9 Separation and Recovery of
Dried Product from Air
325
a.
9.1. Introduction
The fourth and last stage of the spray drying operation is the separation of
dried product from the air. Separation is followed by the removal of product
from the dryer. The stage shall meet two important requirements of spray
dryers ; (a) an economic dry solids recovery and (b) an exhaust free of airborne particles. Economic recovery requires recovery of product in its
most useable and/or most saleable form. A clean exhaust in open-cycle
dryers is the requirement for prevention of discharge of airborne powder
to atmosphere in quantities that exceed local or national pollution standards.
A clean exhaust in closed-cycle dryers is the requirement for drying air/gas
return through the air heater without chance of scorching airborne powder.
Such scorching can lead to a source of ignition or passage of dark particles
into the drying chamber.
Separation of dried product can be carried out firstly at the chamber
base (primary product separation (discharge)) followed by separation of
fines in collecting equipment (secondary product separation (discharge)).
Alternatively, all product can be conveyed to collecting equipment (total
product separation (discharge)) (figure 9.1). The amount of primary product
separation depends upon the chamber design, atomization of product and
drying air flow. ProcilIct is either seban; red out of the air by the cyclonic air
flow set up at a conical chamber base, or by the ability of particles to fall
out of the air flow to a flat chamber base. Whatever method of separation
is used, some form of collection equipment is required after the drying
chamber. Collection equipment can be dry or wet types, e.g. cyclones,
bag filters, scrubbers, electrostatic precipitators. Choice of equipment (see
later, 9.2) is based primarily on cost, collection efficiency and product
treatment undergoing separation (comminution). Equipment was introduced
in chapter 5. Equipment performance is discussed in section IV.
b.
Figure 9.1. Product discharge from conical and flat chamber bases. (a) Primary product dis-.barge from chamber, secondary product discharge from collecting unit. (b) Total product
charge from collecting unit, 1. Spray drying chamber. 2. Product—air separation in collecting
unit. 3. Product discharge. 4. Clean air.
Recovery of dried product involves powder removal valves to discharge
powder into silos, packing machines or conveying systems. Valve selection
depends upon whether intermittent or continuous discharge is required.
Manual, flap or rotary valves can be used. Further details are given in
section IV.
For dryers with primary product discharge, the degree of separation in
the drying chamber is closely connected with the location and design of
duct leaving the chamber. This is especially so for chambers with a conical
base. Eight such exhaust arrangements are shown in figure 9.2. The arrangement selected depends upon the product form and collection equipment
installed. The arrangement giving a very low powder loading need not
necessarily be the most suitable. With cyclones, for instance, a very low
loading of 'fines' results in low collection efficiencies and appreciable loss of
fines. Using higher loadings of powder including slightly large particle
sizes would recover some of these fines due to the ability of large particles
326
THE PROCESS STAGES OF SPRAY DRYING
to entrain or agglomerate fines that would otherwise be lost. Of the arrangements shown in figure 9.2, those in which air is made to turn 180° (types
A, F, 0, H) to enter the duct give higher powder carry-over (hence lower
chamber separation) than those requiring only a 90° or less turn. This is.
particularly the case with type A as this arrangement has the duct inlet
in an area of high powder loading. Types F, G and H are not so prone to
higher powder carry-over as they are incorporated with nozzle atomizers
producing coarser sprays.
Figure 9.2. Exhaust duct layouts with primary product discharge from conical chamber base.
Type A features a downward facing outlet projecting deep into the
conical base. With the close proximity of the wall, and the air velocity
increase on approaching the duct entrance, the arrangement is susceptible
to re-entraining powder that has already separated out at the walls or base.
Re-entrainment effects can be severe where air leakage in at the chamber
base (chamber operating under atmospheric pressure) fluidizes powder
reaching the base discharge point. This fluidizing effect becomes of less
significance to type B, where air turns through 90° and the entrance is
further removed from the walls and chamber base. However, types A and
B do feature favourable points. They utilize the chamber volume fully
for evaporation, as the air must flow throughout the entire volume. This
SEPARATION AND RECOVERY OF DRIED PRODUCT FROM AIR
327
gives maximum residence time, and enables the air flow pattern created
by the air disperser to be maintained substantially throughout the chamber.
The downward facing exhaust duct has a minimum interference on this
air pattern, and in fact, acts to maintain the pattern down into the chamber
cone. Type C, on the contrary, draws air to the side of the chamber, greatly
interfering with the flow by creating a lopsided air flow pattern. Again
suction is in the neighbourhood of powder present at the walls. Type C
is only used when handling products that are sticky or heat sensitive or in
cases where any deposits forming on the exterior surface of the projected
duct is likely to create product quality deterioration and frequent dryer
shut down.
Type D, the upward facing duct can achieve lower levels of powder
carry-over than type B, since air is drawn from an area of low airborne
powder concentration. However, the chamber volume is more poorly
utilized. Type E requires no air turn into the duct and the location of the
duct entrance ensures even lower powder carry-over, but again utilization
of the chamber volume is poor. This aspect can be improved by adding a
cap to the duct (type F). The cap maintains the rotary air flow into the
lower half of the chamber. The presence of the cap requires air to turn
90°. Powder carry-over increases, but the cap discourages product falling
into the duct entrance. The cap attachment is only feasible for non-sticky
powders. The duct arrangement causes accessibility problems for cleaning.
Types G and H differ from the above as separation is achieved by educting the air from the dryer at the wall area located at the base of the chamber
cylindrical section. The exhaust duct can be tangential to the wall or leave
from an annular ring (plenum). With these two arrangements, air is exhausted where separated powder -congregates at the wall. Powder is continuously scrubbed by the air, and re-entrainment is inevitable. However,
these arrangements are used with coarse powders, and if lower wall or
plenum air velocities are maintained, primary separation of product can be
very high.
9.2. Selection of Equipment for Separating Powder from Exhaust Drying Air
Many spray drying applications are so established nowadays that certain
types of separating equipment are connected with certain products. This
has arisen from operational practice which has shown equipment to meet
demands in separation performance and operational costs. While the
majority of equipment selection is based upon experience, there are certain
rules of selection, although these rules may at times be rather general.
The desired separation efficiency depends upon the value of the product,
cost of recovery, and environmental pollution codes. If the product is
328
THE PROCESS STAGES OF SPRAY DRYING
SEPARATION AND RECOVERY OF DRIED PRODUCT FROM AIR
expensive and/or the dryer is in an area of strict pollution control, separation (as close to 100 % as technically possible) is essential. If the product
is cheap and/or the degree of recovery less critical, the efficiency is fixed at
the point where cost of further recovery per unit of recovered product
exceeds the value of the extra product recovered. Even though many types
of separation equipment cannot approach efficiency levels of 100 %, they
are nevertheless often acceptable on the grounds of suitability in the other
aspects of operational convenience, costs, and degree of maintenance
required. These aspects are just as important. Any limitation in collection
efficiency can always be rectified by installing secondary separating equipment, whereas limitations in these other aspects are not so readily rectified.
Whatever the performance, selected equipment must be able to handle the
product with a minimum of attention.
To justify installation of any separation equipment, (a) the separation
efficiency must be acceptable, (b) the product characteristics be such to
enable successful handling, (c) all operational features must fit in with those
of the drying chamber operation, (d) the product itself must render the
separation equipment investment and operational cost realistic, (e) the
necessary space requirements must fit in with the available area designated
to the separation equipment in the spray dryer plant layout.
Selection based on Collection Efficiency
The vast majority of spray drying installations produce particle sizes in
the range 1-400 micron. If the total production is conveyed to the separation
equipment, this is the range of particle sizes selected equipment must handle.
If some product recovery is achieved from the drying chamber, this will
remove the larger particle sizes. The selected separation equipment handles
Table 9.1. Collection Efficiency of Primary Separators on Low and High Powder/Air
Loadings
Type
Cyclones
Bag filters
Electrostatic precipitators
Operating on
Operating on
low powder—air loadings high powder—air loadings
90-96%
95-99.5 %
90-99
95-99%*
97-99.9%
95-99%
*This value can range over 99% in many transport cyclones (see figure 1.3, item 19) where
very high powder—air loadings exist. For example, tests with a 1250 mm diameter (D) transport
cyclone (wrap around inlet, cone height 2 x D) operating with an air throughput of 5700 Nm 3 /hr
at 30°C has established separation efficiencies exceeding 99% when handling 1000 kg/hr
dusty powder
Inlet powder loading: 175 g/Nm 3
Outlet powder loading: 191 mg/Nm 3
Collection efficiency : 99.89 %
329
low powder—air loadings, where the particle sizes involved range from
1-80 micron. For this range of particle sizes, equipment available for use
as primary separators (mounted directly after the drying chamber) have
efficiency levels exceeding 90 % as illustrated in table 9.1.
The efficiency of equipment used as secondary separators falls within the
ranges : for bag filters : 98-99.99 %, for scrubber (wet) : 80-96 %. Inlet powder
loadings to secondary equipment are low (under 250 mg/Nm 3 ).
Inspection of the above data shows that where collection efficiency is
critical (as in the case of products that cause environmental pollution even
in very small quantities), bag filters must be selected and installed in the
exhaust drying air either as a primary or a secondary collector. When
efficiency levels are not so critical, cyclones will prove the best selection.
Selection based on Product Handling Suitability
The suitability of equipment to handle a given product can be assessed by
noting the conditions under which the unit must operate. The unit must
cope with the exhaust drying air temperature and powder properties and
operate over long periods continuously. Cyclones can handle most products
and operate at high temperatures. Wet scrubbers can operate with sticky
products, and at high temperatures too, but have not the advantage of dry
recovery of the collected powder. Bag filters are severely restricted in the
operating conditions they can successfully withstand, but the handling of
the finest of powders is a great advantage.
The suitability of equipment types to spray drying conditions is summarized in table 9.2. It can be seen that no one type is ideal for all conditions.
Selection must be made on the basis of what type best fulfils the intended
application.
Selection based upon Operational Features
A small yet important point to bear in mind is that the operational procedure
of the separation equipment must fit in with the operational procedure of
the drying chamber. The equipment must operate continuously as long as
the drying process proceeds. For cyclones there are no problems, as long
as blockaging of the cyclone base does not take place. For bag filters,
proprietary equipment must have a cleaning arrangement to enable continuous operation of the drying chamber without wide fluctuations in air
throughput. For scrubbers, there are no operational problems as long as
sufficient, water supplies are available, and plugging of any jets or nozzles
is prevented through liquid filtering or straining. Electrostatic precipitators
create few problems, as designs are developed for continuous operation
over long time periods.
Selection based upon Costs
Costs are an important factor in all industrial operations, and this is certainly
so with spray dryers where the collection of airborne powder can constitute
SEPARATION AND RECOVERY OF DRIED PRODUCT FROM MR
THE PROCESS STAGES OF SPRAY DRYING
-0 2.3
g. j
* 1
•
.
m
.... I .
• I
• I I .•
•
1
1
C
Design used in spray drying plant
330
•
•
• 1•
•
* 1 1 1
1.
*
4
*
OF *
•
II
1
**
*
• **
•
11
1
g
r"
v
o.
1
C
t,
a sizeable proportion of total operating costs. The main purpose of the
selected unit is to recover as much product as possible, but the level for the
most economic recovery is seldom the highest, especially where product
worth is relatively low per unit weight. It is obtaining the final! 04 of recovery
that causes costs to soar, and thus equipment selected must strike the
happy medium of operating at an optimum efficiency. To go above this
efficiency level, the extra costs of recovery involved exceeds the value of the
product collected. If pollution of the environment is the criteria in fixing
a recovery efficiency, the optimum efficiency is that which meets the standards
set up to safeguard health and local surroundings.
It is very difficult to generalize on equipment cost suitability. Product
handled, and plant location changes proprietary equipment suitability from
place to place. Many types can have relatively low cost price, but high
operational costs, or low operational costs, but low collection efficiency.
A general guide to costs is given in table 9.3. As prices quickly change,
costs are presented on a relative basis.
Table 9.3. Relative Costs of Separation Equipment in Spray Drying (122)
(Dry cyclone cost used as reference level)
2
U
2
g
bf
Type
Cyclones
Bag filters
Spray chamber scrubber
Venturi scrubber
Impingement scrubber
Electrostatic precipitator
-0 u
t
›. 2
*
*
3
• **
•
*
*
11
1
if *
•••* ► *
0
zt,'E
..
0OO
**
331
•
•
Capital costs
Operating costs
(including maintenance)
1
4-5
1.5-2
2
4-5
5-6
1
5-6
4-4.5
7-8
2.5-3
1
Selection based upon Space Area Requirement
All types of separation equipment have differing space requirements. However, it is seldom that space limitations are such to reject a unit type if it is
suitable on all other aspects of selection. With selection of equipment for
new spray drying plant, the overall plant layout can be adjusted to accept
any dimensions of separation equipment. There may be difficulties if a
new plant has to be erected in an existing building or an old dryer modified
for increased capacity or change of product, but such problems can be
overcome by using two smaller units in parallel if a single unit requires too
much height. It is not always limited building height that decides multiple
small units. One large cyclone, for example, may handle the total air flow
and achieve acceptable collection efficiency, but its sheer size can mean
an extensive duct length between the drying chamber and cyclone inlet.
332
THE PROCESS STAGES OF SPRAY DRYING
Two cyclones in parallel give a better layout and cyclone supporting structures can be simplified, an added advantage.
The relative dimensions of collection equipment are illustrated in table
9.4 based upon design sizes to handle 70 000 m 3 /hr air at 80 ° C.
Table 9.4. Dimensions of Separation Equipment to Handle 70 000 m 3 /hr
air at 80° C
Type
Cyclones
Bag filter
Spray chamber scrubber
Venturi scrubber
Impingement scrubber
Number of
Dimensions
(1 x b x h) or (dia x h)
2
1
1
1
1
2.8 dia x 10.5 m
4 x 1.5 x 2.5 m
3 dia x 6 m
1 dia x 12 m
1.5 dia x 6 m
For further guidance on the selection of powder—air separation equipment, reference should be made to Stairmand (122) (507). Review of recent
equipment is available (123) (124) and modern trends in separation is
described elsewhere (125) (508) (516).
10 Control Systems
Operational practice involves (a) dryer operational procedures, (b) dryer
control, and (c) measurements to assess dryer performance.
Operational procedures include start-up, operation, shut down, cleaning
and maintenance. It is common to start up spray dryers on water (where
moisture is to be evaporated from the feed solution), but this is not essential.
The required inlet and outlet temperatures are established on water, after
which the system is switched over to feed under either manual or automatic
control. Operational procedure requires maintenance of the desired
product moisture content, usually by maintenance of a set outlet air temperature. Ensuring availability of feed, and handling the discharge of powder
leads to continuity of operation. Shut-down procedure often involves the
change-over from feed to water for the running down of the plant without
affecting the quality of product remaining in the dryer system. Cleaning
procedure involves a washing cycle that achieves plant cleanliness within
a minimum of time. Maintenance procedure is based upon scheduling to
prevent overlooking any equipment that can cause plant shut-down through
failure. Instructions for spray drying plants differ from product to product
and from design to design. Instruction manuals for a given dryer application are available from dryer manufacturers.
• Methods of dryer control depend upon dryer design and product. Control
systems are discussed in chapter 10. Measurements to assess dryer performance are described in chapter 11.
The aim of a spray dryer control system is the maintenance of desired
dried product quality, irrespective of what disturbances occur within the
drying operation and variations in feed supply. The most effective product
parameter to control is the moisture content. Equipment for continuous
measurement of moisture content is in an advanced state of development
but as yet is not widely used. Outlet air temperature (exhaust air from
drying chamber) is the parameter controlled. This temperature represents
product quality, i.e. bulk density, colour, flavour, activity as well as moisture
content.
Spray dryers can be controlled either manually or automatically. Manual
control is applied to small plant (laboratory, pilot-plant or small industrial
sizes) which are operated batchwise, often on a range of products. The
products must be easy to dry. Manual control can be applied to large
industrial units operating on such products, but the demands of continuous
operation on operating personnel, the maintenance of constant product
quality over lengthy durations of production makes automatic control
(semi or full) a virtual necessity. As shown in figure 10.1 outlet temperature
with smaller deviations from the control setting (+0.5 to 1.0°C) are maintained over longer periods by automatic control than by manual control.
If products are difficult to dry, automatic controls are utilized irrespective
of dryer size.
Control is accomplished by maintaining a set outlet temperature through
varying (a) the feed rate to the dryer, hereafter denoted control system A, or
(b) inlet drying air temperature, hereafter denoted control system B. Control
system A is the more widely used of the two systems. By applying automatic
control to the outlet air temperature, product moisture can be held within
very narrow limits.
336
OPERATIONAL PRACTICE
CONTROL SYSTEMS
4
P.M,
337
The control system is illustrated in figure 10.2. The system consists of
two control loops of quick response. Quick response loops are the desired
control characteristics to prevent adverse drying conditions.
AIR
FLUE
GASES
FEED
wu
rx,„.0.12&
umv..4"-
As
To
VARIABLE
SPEED DEAR
PUMP
4
INDIRECT OILFIRED
AIR HEATER
%A.
OIL
COMBUSTION MR
Figure 10.2. Control system A. Outlet temperature control by regulation of feed rate. Inlet
temperature control by regulation of fuel combustion rate.
Figure 10,1. Outlet air temperature chart for dryer operation under manual and automatic
control.
Theory of automatic control is considered outside the scope of this book.
For details, the reader is referred to available textbooks (126) (127). Automatic controlling and regulating systems are covered in British Standard
1523 part I (1967). Rasmussen (128) describes the design of instrumentation
for spray dryer control systems. Aikman (129) reports frequency-response
analysis of spray dryers. Hatfield (130) describes available control systems.
Patent coverage includes (473) (474) (475).
10.1. Control System A
(a) Outlet temperature control by feed rate regulation.
(b) Inlet temperature control by air heater regulation.
The air temperature in the exhaust duct is measured and transmitted
to the temperature indicating controller (TIC), which counteracts any
temperature deviations from the desired set point by varying the feed rate.
The temperature of air to the dryer air disperser is measured and transmitted to a temperature indicating controller. Any deviation from the
desired inlet air temperature is corrected by control of fuel and combustion
air to the burner (oil and gas fired air heaters), steam pressure in a steam—air
heater, or power input to an electric—air heater.
In the event of failure in the feed system (pipe or atomizer blockage, pump
damage, pump control failure) where feed supply to the atomizer is drastically
reduced or ceases, a safety system can be installed to prevent the outlet air
temperature rising above a known safety level, since the system can be
potentially dangerous for many products inasmuch that feed failure can
lead to rapid rise in outlet air temperature as the air heater continues to
function. A built-in safety system can shut down the air heater once a
certain outlet air temperature is reached, or water can be passed to nozzles
positioned as safety measure, for example in the dryer roof. These nozzles
338
OPERATIONAL PRACTICE
can be sized to act either as atomizer nozzles (and therefore restore the
normal outlet temperature on water feed while the feed system failure is
rectified), or as quenching nozzles to flood the chamber if the product
under high temperature conditions can cause a fire hazard through spontaneous combustion or explosion hazard through an extremely dry dusty
product swirling in the chamber.
Some safety systems switch feed over to water at the feed pump and
operate on a two temperature level safety system. For a partial blockage
of the feed system the outlet air temperature will rise more slowly, and on
reaching the first level (say 300°F (150°C)), feed supply is stopped and
switched over to water. If the blockage is minor, the water flow may dislodge
the blockage and after a short time when the normal outlet temperature
has been restored, so can the feed supply. If the blockage cannot be removed
by water, or water cannot pass to the atomizer (in cases of pump failure)
the outlet air temperature will continue to rise to the secondary safety level
after which the chamber is flooded by quench nozzles or the air heater is
shut down or a combination of both. To draw attention to rise in outlet
air temperatures, alarms can sound 20-40°F (10-20°C), before any unsafe
temperature level is reached.
The use of two level safety systems gives the opportunity of restoring
dryer conditions on water by using water to flush out a blocked feed or
enable feed system failures to be rectified. If feed system difficulties can
be overcome by a short operation on water feed, time will be saved by not
having to shut down and restart the dryer.
10.2. Control System B
(a) Outlet temperature control by air heater regulation.
(b) Feed rate held constant.
The control system is shown in figure 10.3 and is used particularly for
dryers with nozzle atomization where wide variations in feed rate cannot
be handled. The outlet air temperature is measured and transmitted to a
temperature indicating controller. To compensate for any deviation from
the desired outlet air temperature, the heat input to the dryer is adjusted
by the controller through regulation to the combustion rate in the gas or
oil air heater (if fitted), or steam pressure at the steam—air heater (if fitted).
From a theoretical viewpoint this is an inherently safe and acceptable
system as any increase in outlet temperature brings about a decrease in
heat input to the dryer. From a practical viewpoint, however, the system
has operational disadvantages of lengthy control lags due to time lags in
the heater control circuit. These time lags can increase outlet air temperature fluctuations. The system has the inability to handle effectively variations
CONTROL SYSTEMS
339
AIR
FLUE
GASES
FEED
0
PUMP
INDIRECT OILFIRED
HEATER
VARIABLE
SPEED
GEAR
OIL
COMBUSTION AIR
Figure 10.3. Control system B. Outlet temperature control by regulation of fuel combustion
rate. Manual regulation of feed rate.
in feed solids, but it can be improved by outlet temperature cascaded on the
inlet temperature controller which controls the heater.
Safety systems similar to those used in control systenLA are-used. When
using a two level safety system, the rise of outlet air temperature to the
first level will switch feed over to water flow to the atomizer. If water flow
cannot regain and maintain the desired outlet temperature, the heater is
automatically shut down.
10.3. Feed System Controls
1. Systems for Rotary Atomizers. Control system A is adopted with direct
feeding from a suitable positive displacement pump with variable speed
control, a centrifugal pump throttled by a control valve, or a gravity feed
constant head tank with a control valve in the outlet pipe. Control system B
is rarely considered.
2. Systems for Nozzle Atomizers. Both control systems are applicable, but
control system B is preferred. Control system B can be applied in two ways
with the feed pump on a fixed manual control setting. The outlet temperature controller can be cascaded on to the inlet temperature controller as
340
OPERATIONAL PRACTICE
CONTROL SYSTEMS
•
r-
AIR
FEED
FLUE
GASES
--------
OIL
J
INDIRECT
OIL- FIRED
AIR HEATER
341
alone for shut down and cleaning purposes. For example; for an 'air-sweep'
chamber cleaning operation. An alternative system for prevention involves
a vacuum switch mounted at the chamber ceiling in a powder-free area.
The switch is normally set to shut down the exhaust fan if chamber pressures
reach —4 in W.G. ( — 100 mm W.G.).
For prevention of heater damage. An interlock ensures that the burner
cannot be ignited unless the storage pump, burner fuel pump, combustion
fan and dryer fans are in operation. For indirect heaters a differential
pressure switch can be installed to check the correct air flow, and linked
to the combustion and supply fans.
Prevention of wet chamber wall. An interlock ensures that feed cannot be
passed to the rotary atomizer when not running. Furthermore the interlock
will cause the feed to cease on atomizer failure.
Precaution against burnt product. An interlock ensures that the main dryer
fans cannot be started unless the necessary cooling fans are in operation.
The fans cool potential hot areas in the chamber where build-up and scorching of product can take place.
10.5. Fully Automatic Spray Dryers
COMBUSTION
AIR
Figure 10.4. Control system for pressure nozzles (control system A).
shown in figure 10,4 or linked directly to the heater. Pressure safety systems
are incorporated to shut down the dryer if excessive pressures build up in
the feed system due to blocked nozzle(s). Control system A can be applied
in two ways. The outlet temperature controller sets the pressure control
loop in the feed system or is linked directly to the feed pump. It is usual
for each nozzle in a multi-nozzle assembly to be brought in manually during
start-up as the inlet temperature rises.
10.4. Interlocks
Interlocks are closely connected with the control and operation of the
dryer. Interlocks are installed to ensure safe start-up, operation and shut
down of the dryer. Some examples of typical interlocks are as follows :
For prevention of drying chamber damage. An interlock ensures that an
exhaust fan cannot be started before the supply fan. This is installed if there
is a chance of chamber collapse under the low negative pressure conditions
caused by an exhaust fan operating alone, The interlock is usually overridden when a chamber door is open. This allows the exhaust fan to operate
10.5.1. Programmed Start-up
Analysis of spray dryer control systems has reached the stage when it is
possible for the complete spray dryer installation to be started up from the
pressing of a single push button. The swing to full spray dryer automation
is motivated by the need to meet stricter product quality specifications, and
hold quality levels during extended periods of dryer operation, and achieve
lower running costs.
The combined block and schematic diagram for a fully automated spray
dryer operated on the control system A principle is shown in figure 10.5.
The dryer installation features a rotary atomizer, indirect oil fired air heater,
cyclone collectors and pneumatic conveying of the powder to automatic
weighing equipment at the powder discharge.
Control programming is through timing equipment. A sequence timer
starts up the dryer in the correct order. A typical start-up sequence for a
dryer with an oil-fired air heater is as follows : (a) atomizer start-up, (b) cooling
fans, (c) supply fan—exhaust fan, (d) transport fan, (e) rotary valves, (f) vibratory conveyor (if fitted), (g) oil burner pumps, (h) combustion air fan, (i) flue
gas exhaust fan, (j) oil burner—and at the start of product drying, electric
hammers or similar devices (if fitted) on the chamber wall.
With the initiation of the heat source to the dryer, a series of process
control follow automatically. Dryer feed is usually initially water, which
continues until (1) a temperature equilibrium has been reached in the dryer
342
OPERATIONAL PRACTICE
CONTROL SYSTEMS
_J
343
(inlet and outlet temperatures at desired level), and (2) there is sufficient
product in the feed tank to ensure continuity of operation. Once these two
conditions have been fulfilled, an air operated three-way valve changes the
feed automatically from water to feed product. The changeover is smooth
and little deviation occurs to the outlet temperature apart from the effect
of initial feed dilution in the feed line. The outlet temperature is controlled
by the feed rate and the inlet air temperature by the oil combustion rate.
Any fall in inlet temperature is counteracted by the servomotor introducing
greater quantities of oil and combustion air for an increased combustion
rate, and vice-versa for any rise in air temperature. Temperature deviations
are within 1-2°F (0.5-1.0°C). Such control contributes greatly to product
quality. The success of any fully automated system is reliability in operation,
and the ability of the control system to deal with failure on the dryer installation. Reliability is met by the use of established robust equipment whether
pneumatic, electrical or mechanical in operation. Where fault in equipment
does occur, interlocking and alarm systems limit operational dangers by
shutting down appropriate plant items and indicating the type of failure.
In the case of the layout depicted in figure 10.5, for example, a feed-back
system will prevent an operation from starting if the previous step is faulty.
If a motor fails during the start-up sequence, after the motor has received
the start-up impulse from the sequence timer, a control lamp will light up
to give visual indication of the fault. However, as the sequence timer proceeds
with the programme and submits the start-up impulse to the next step, a
feed back interlocking system is brought into operation. This means the
following step will not be started. Further protection against continued
start-up in a manner dangerous to the drying equipment is afforded by
interlocking the different start-up steps.
Although the start-up steps proceed automatically, the plant operator
can follow the switching-in sequence as a light comes on for each equipment
item in operation. This light is extinguished in cases of faulty equipment
operation, and a red lamp comes on to indicate the type of failure. Faults
in the system are also made known to the operator through siren alarms.
Faults covered by siren alarms can be faulty lubrication in a rotary atomizer,
excessive outlet temperature level, or low feed tank levels. It is common for
the alarm to sound after a short delay (5 sec) following a failure lamp signal,
since in many cases the failure lamp signal is due to a temporary condition,
e.g. a valve change-over at the feed tanks. In such cases there is no need for
the alarm signal to sound.
If a fault does occur in the control equipment itself, it is possible
to switch over to manual control and continue the production. Shut
down and cleaning programmes operate in a similar way via the sequence
timer.
344
CONTROL SYSTEMS
OPERATIONAL PRACTICE
The adoption of the above system to the spray drying of milk has been
reported (131).
10.5.2. Full Automation of Spray Dryer and Feed Pretreatment
The full automation of the spray dryer as described above will maintain a
uniform product quality as long as input feed compositions do not vary
widely during operation. To reduce variations in feed solid concentration,
attention must be paid to the pretreatment section that supplies the spray
dryer. The maintenance of constant feed solids to the spray dryer is best
achieved by coupling the pretreatment section to the spray dryer and
applying control system analysis to the two parts as one unit.
The coupling of an evaporator (pretreatment section) to the spray dryer
has been achieved within the dairy industry. Notable advantages achieved,
apart from control of feed conditions to the dryer include the elimination
of intermediate feed tanks and great precision in the dosing of additives.
Feed concentrate does not come into contact with ambient air.
C
d
b
Figure 10.6. Automatic feed solids content measuring equipment used in fully automatic
spray drying plant in the dairy industry. (By courtesy of Niro Atomizer.)
345
The control parameter is the total solids in the feed concentrate leaving
the evaporator and is based upon the continuous measurement of density,
or continuous weighing. Systems are based upon hydrometer measurement, refractometers or electronic sensors but all have disadvantages to
various degrees in accuracy, reliability or unsuitability to certain products.
Continuous weighing techniques of a given volume, however, have been
successfully applied. Such measuring equipment is shown in figure 10.6.
Concentrate from the evaporator flows via the intake pipe (a) through
a U-tube loop (b) pivoted on flexures (c) and connected by flexible connectors (d). Any change in density results in a proportional change of
force in the weigh beam (h), which is measured by a pneumatic force balance
system nozzle and flapper (e) and relay (f). (The diaphragm actuator (g) is
for the concentrate feed control valve.)
The milk or steam supply at the evaporator is controlled by signals from
the U-tube transmitter. Four possible control arrangements are shown in
figure 10.7. System A has the evaporator as the dominating unit. (The
evaporator is termed the master, spray dryer is termed the slave.) Systems
B — D are in reverse with the spray dryer playing the dominant role in the
control.
In system A, signals from the total solids detector (i.e. U-tube equipment
described above) control the input of raw feed to the evaporator. The
steam pressure is held at a fixed setting. The variations in the amount of
concentrate due to deposit formation in the evaporator are compensated
by moderate alterations to the spray dryer inlet temperature.
In system B the spray dryer inlet and outlet temperatures are fixed. The
total solids in the concentrate is kept at the required value through alteration of feed input. The evaporative capacity of the evaporator is changed
through alteration to the steam pressure supplied to the thermo-compression
stage, which itself is controlled by a level controller in the last effect of the
evaporator.
In system C steam pressure regulation is governed by the solid content
in the feed passing to the spray dryer. The raw feed input to the evaporator
is governed by level control on the last effect of the evaporator.
In system D steam is added to the finisher stage and is independent of
steam supply to the main evaporator stages. The total solids control the
steam pressure to the finishers, and the level controller regulates raw feed
input to the evaporator. This system has faster response characteristics.
Cleaning-in-place (C.I.P.) can be built into the control system to operate
automatically on evaporator shut down. There are considerable advantages
in establishing a fully programmed system for the spray dryer too.
Full benefit of evaporator and spray dryer automation is achieved only
when other sections of the factory are under automatic control, but the
346
CONTROL SYSTEMS
OPERATIONAL PRACTICE
0
control of complete plants that include a spray drying stage. Balls (132)
has compared electronic and pneumatic control systems of the future.
Level
Steam
Spray
dryer
Milk
Powder
10.6. Precaution against Fire and Explosion
To condenser
Evaporator stage
I
L ---------------
-8
Steam
Total solids
Level
Drying air
•
> 1 Stage
Milk .
II Stage
Spray
dryer
Finisher
Ut Stage
L_
To condenser
I
Outlet air
Total solids
Drying air
nSteam
C
Powder
-I—I/ Evaporator stage
L
) Milk
I Stage
1
1
.11 Stage
Finisher
Sigge
Powder
32z)
I
To condenser
I-72 Evaporator stage
L
8—
Outlet air
—Total solids
Level
■
Drying air
Steam
Mil k? I Stage
r21 Stage
Stage *:>
L
I— 7y Evaporator stage
Evaporator: .Masier"
A: Spray dryer: • Slave'
4
Finisher
(1 )
347
4
Spray
dryer
-8 —Total solids
Powder
Outlet air
Evaporator: „Slave'
13 — D: Spray dryer: „Mosier'
Figure 10.7. Block diagrams for automatic control of combined evaporator and spray dryer
systems.
time is fast approaching when instrumentation for a whole factory incorporating spray drying plant is centralized, and the control room likened to
those already established and publicized in the modern oil refining industry.
Electronic instrumentation is now being applied more widely to spray
dryers. More extensive usage appears inevitable with computors for process
Potential hazardous conditions can exist during spray drying operations
when (a) handling products that form flammable mixtures at certain powder—
air (or powder—drying medium) loadings (363), (b) handling solids associated
with flammable solvents, or (c) permitting ignition sources (spark generation)
to occur through faulty component performance. Rose (133) in a recent
paper (1970) has discussed dust explosion hazards, dealing with the subjects
of initiation and maintenance of explosion, minimizing hazards and eliminating explosion sources. In dryer design, therefore, even the remotest possibility
of fire and explosion must be considered and counteracted through allowance
in dryer construction design, proper explosion detection equipment, means
to discover danger signs that can lead to mishap, and of course, adequate
extinguishing equipment.
The outbreak of fire (364) is the resultant of the prior events of ignition,
and maybe explosion (in that order). Spontaneous flare-up can be the
`explosion'. If ignition sources are removed, the fire hazard no longer
exists. For any spray dryer operation, conditions that could cause ignition
must be fully appreciated by operating personnel. The operators' responsibility is always one of discovering potential ignition sources and nullifying
them.
Causes of Ignition
The causes of ignition are as follows :
1. Spontaneous combustion in certain product deposits.
2. Hot solid particles entering the dryer with the drying air.
3. Spark generation through friction between metallic surfaces or between
hard product deposits.
4. Electrical failure leading to electrical discharge.
5. Static electricity discharge.
Re. 1. Spontaneous combustion is associated with the burning of organic
matter by an oxidation process (exothermic) which proceeds at increasing
reaction rates due to its own heat generation. Spontaneous combustion
occurs in deposit masses. The process leading to flame break-out is often
slow as in the initial stages heat generated by the reaction is dissipated to
the surrounding product drying medium or metallic surfaces. Ignition will
only occur if the intensity of heat generation for a certain critical surroundings temperature is higher than the ability of the product to dissipate the
heat. The critical temperature can -vary with product layer thickness.
Ignition propagation time increases with decrease in critical temperature
348
OPERATIONAL PRACTICE
levels. The composition and critical temperature of the product together
with its location play a leading role in the mechanism toward flame generation through spontaneous combustion. For combustible products, there
is always the chance for spontaneous combustion to occur in any place
where product deposits out and remains in contact with the drying air.
As dry product contacts the hottest drying air in counter-current flow
dryer designs, these designs are more prone to spontaneous combustion
explosion than co-current designs. Potentially combustible products are
not dried in counter-current flow systems. The hotter the region in which
product deposits, the greater the risk of spontaneous combustion occurrence.
Critical deposit areas are at the chamber walls (although not near the humid
drying zone) and the exhaust duct (especially in horizontal bends where
product can readily deposit), Such deposits can form product layers of
several millimetres thickness and remain over considerable time periods.
The self-ignition process can be accelerated at any time by a complimentary
heat source. This could be a chamber illuminating light.
Under normal operating conditions, the extent of product build-up on
the drying . chamber wall is normally well below layer thickness levels for
self ignition. Nevertheless, thick isolated layers of product can be created
from non-operational conditions. These include : (a) Wet washing. Poor
chamber and duct cleaning can lead to wet patches of product remaining on
surfaces. These patches will dry out on heating up the dryer, providing a
good basis for further deposit formation. (b) Incorrect start-up/operation
procedure. If incorrect start-up or operation causes wet product to be
trajected against the chamber walls, these wet deposits on drying out can
create a basis for further deposits of dry product.
Re. 2. Not Particles. A great danger resulting from badly filtered drying
air is the possibility of red-hot particles entering the drying chamber. The
particles of foreign matter become red-hot on passage through the heater.
The danger is real for both direct and indirect heaters. If these particles are
large enough to penetrate the humid zone around the atomizer, they will
enter the less humid zones of the chamber while still relatively hot. Their
presence can spark off an explosion with resulting fire. The danger is
greatest in spray dryers operating at high inlet air temperatures, but products
liable to ignite or form mixtures with air within the limits of flammability
are rarely subjected to very high drying temperatures in spite of high drying
efficiencies that can be obtained. Red-hot particles can also be formed from
the return of powder fines to the drying chamber (e,g. for agglomerating
purposes) if there is a likelihood of powder entering or remaining in the
chamber air disperser. This is prevented by the introduction of fines into
the chamber via the atomizer, under the atomizer or through the dryer
roof (away from the air disperser).
CONTROL SYSTEMS
349
Re. 3. Friction between Surfaces. The contacting of metallic surfaces or
layers of hard deposits can generate high temperatures through friction.
Ignition temperatures can be reached. Fans or rotary valves are areas
where such conditions can occur. Where rotary atomizers are used, incorrect assembly or a badly cleaned atomizer wheel and liquid distributor
can constitute danger areas. Sticky deposits consisting of high solids
content form and eventually dry during the spray drying prOcess giving a
friction surface with the rotating wheel.
Re. 4. Electrical Failure and Electrical Discharge. Sparks readily create
explosions within powder—air mixtures having combustible properties. The
source of spark is commonly from damaged rotating equipment striking
casings (e.g. fans). Such conditions must always be avoided through preventive maintenance. Faulty electrical equipment is always a potential
source of sparks, but rarely causes an explosion as electrical equipment is
located away from the powder—air environment.
Re. 5. Static Electricity. Spark discharge through build-up of static electricity
could under certain circumstances create explosions (e.g. powder transport).
The spray dryer is normally a completely metallic structure, and when
earthed static electricity is rarely a problem. Earthing eliminates the build-up
of static electricity to levels that create spark discharge. Static electricity is
discussed in detail by Haase (134).
10.7. Control Systems for Preventing Fire/Explosion Conditions in Deposit
Formations
Prevention through Pressure Detection
The use of sensors that react to variations in pressure can be used in explosion
prevention control. The sensors mounted within the drying chamber are
extremely sensitive to pressure variations that are produced during the
first phases of explosion. These variations are sufficient to activate fire
extinguishing equipment. The success of the system is based upon explosions
not being an instantaneous occurrence but requiring a definite time period
to develop a destructive pressure. Only milliseconds are required but this
is sufficient to activate the explosion/fire suppressant. The sensor, on
detecting imminent explosion, activates through an electrical power unit
the dispersion of a suppressant. (Dispersion is often triggered by controlled
explosion.) From the instigation of ignition to explosion/fire suppression,
as little as 60 milliseconds may elapse. Further details are reported by
Isaacs (135).
Prevention through Air Analysis
Prior to ignition due to spontaneous combustion, many products smoulder
for lengths of time. The smouldering can release gas as a result of combustion.
350
CONTROL SYSTEMS
OPERATIONAL PRACTICE
Released gas can be detected through analysis of the exhaust drying air.
Continuous gas analysis can record the presence of any gas that characterizes
burning deposits. However, such analysis equipment is expensive. As
ignition follows some time after commencement of smouldering, steps can
be taken to shut down the plant for thorough inspection of the smouldering
deposit areas. If the product is characterized by rapid outbreak or flame
on smouldering, continuous gas analysis can activate automatic dryer
shut-down or chamber/duct dousing. Often with organic products the
evolved gases are pungent, and operating personnel may often be able to
detect the smell. This is certainly possible if dried milk smoulders where
pungent acrolein is evolved (375).
Prevention through Limiting Explosion Possibility
Although instruments are available to detect conditions constituting an
explosion hazard, these should never be required to operate since ways of
dryer operation are available to reduce to a minimum the possibility for
spontaneous combustion and explosion. There are precautions special to
each dryer application, but in general the following apply :
(a) The operating instructions supplied for correct dryer operation are
closely followed. Special attention is paid to maintenance of temperature
control during operation, and correct start-up and shut down procedure.
(b) Cleaning of equipment is thorough. Areas of likely deposit formation
are inspected for cleanliness before start-up. There is full control of parts
re-assembly.
(c) Regular cleaning of supply air filters. This includes washing of filters,
daily control of filter cleanliness and exchange of filter media when
demanded.
(d) Regular inspection of all electrical installations, mechanical equipment,
bearings, packings, etc., within the dryer vicinity.
(e) Regular inspection during operation of areas that are known to be prone
to deposit formation.
(f) Close control of the operation : (i) Control of temperatures and atomizer
loadings. (ii) Product quality—control of product cleanliness. Burnt or
discoloured product may originate from a smouldering deposit. (iii) Control
of product discharge rate to establish no accumulation of product in the
chamber.
(g) Odour in exhaust drying air. Personnel should have experience in
associating pungent smell with smouldering deposits.
Prevention through Temperature Detection
The use of thermostats placed at a wall or duct surface where deposit
formation is likely is a known method for minimizing fire damage. It will
not prevent fire however.
-
351
Precaution to limit Explosion Fire Damage
Extra precautions to limit damage are required for plant handling product
that tends to form deposits within the system and is prone to spontaneous
combustion. This is to limit damage in the event of explosion, where prevention was not possible. The following precautions can be made : (i) Installation of bursting discs in drying chamber wall. (ii) Installation of chamber
doors that burst open under sudden pressure rise. (iii) Modern fire fighting
equipment on hand and in operating order.
Extinguishing media can be water, steam, carbon dioxide or other special
chemicals. Chlorinated hydrocarbons have the disadvantage of producing
phosgene gas when contacting a heated metallic surface. Carbon dioxide
does present some limitations in personnel breathing in the working area,
especially in the air space under the spray dryer chamber.
Procedure in case of Explosion or Fire
On the break-out of fire, the following actions must be carried out as quickly
as possible : (i) Shut down fans and heater. (ii) Switch off the feed of concentrate, and pass water to the atomizer as soon as possible at maximum
rate. (iii) After plant has cooled down, continue feeding water. Product in
the chamber will inevitably be spoilt.
The causes of explosions in dried milk has been reviewed by Pisecky (375).
OPERATIONAL MEASUREMENTS
11 Operational Measurements
353
(a) Air heater characteristics (steam, fuel/gas, electricity).
(b) Evaporation rate.
(c) Supply fan characteristics.
(d) Pitot tube measurements in the inlet duct.
(e) Anemometer measurements in the inlet duct.
(f) Orifice/venturi (built into inlet duct) characteristics.
(g) Air dilution methods.
Re (a). Air Heater Characteristics
(1) Steam Heater. Reliable _air flow measurements can be obtained from
measuring the condensate from the steam—air heater :
From a heat balance over the heater tubes, the air flow rate (L a ) can be
expressed as
_ M e(H s— H )c
IA
(11.1)
CAut — Tin) x
where M, = condensate rate, H s = enthalpy of - steam entering heater,
H = enthalpy of condensate, I/ = heater efficiency (95 %), Cp = specific heat
of air (0.24), Tout = air temperature leaving heater, 1;„ = air temperature
entering heater.
N.B. The flow of condensate from the heater is always fluctuating. When
measuring condensate, a sufficient volume must be collected to compensate
this fluctuating flow. Calculations based upon insufficient condensate can
lead to marked inaccuracy in the calculated air rate. 3-4 condensate volumes
are collected to determine the mean flow rate. Prior to measuring condensate,
the heater must be operating under steady state conditions (at least z hour
after dryer start-up). The condensate temperature is measured as near to
the heater steam traps as possible. When measuring air temperatures
leaving the heater, readings are taken from all sides of the duct leading from
the heater. Temperatures often vary across the duct due to uneven heat
transfer throughout the heater. The thermometer bulb should be protected
against any radiation effects from the heater coils.
(ii) Oil or Gas—Air Heater. Air rates can be calculated from the heat generated
during fuel combustion.
The air rate (L a ) is expressed as :
a
The ability to asses dryer performance requires measurement of operational variables and product properties. In this chapter measurement of
operational variables, e.g. air flow, powder—air loading, conducted on the
dryer by operating personnel is discussed in greater detail than measurement
of product properties, e.g. moisture content, bulk density, particle form,
conducted under laboratory control procedures. This is due to the difficulty
in relating laboratory measurement techniques to given products. Moisture
content, for instance, is one of the most important dried product properties,
yet for a given product, determination can be carried out in various ways.
Every operator of a spray dryer standardizes his own procedures. Laboratory
techniques are merely touched upon and for further details, the reader is
referred to publications on laboratory practice.
11.1. Determination of Air Flow
Methods are available to measure air flow either continuously or manually.
For continuous measurement, the pressure difference over an item of
equipment, or built-in restriction (orifice, venturi) is calibrated in terms of
air flow. Manual measurements involve use of equipment performance
characteristics or special measuring equipment. The choice of technique
depends upon whether clean or powder laden air is involved, the type of
equipment installed, and air temperatures and humidities in the measurement area. There are three areas of measurement : (a) supply of air to the
air disperser, (b) air flow leaving the drying chamber, and (c) air flow in
pneumatic conveying systems (if fitted). Measurement of air flow has been
discussed in detail by Ower (136).
11.1.1. Supply Air to Dryer
The supply air rate can be determined from measurements based upon :
MQ Hf
L=
(11.2)
Cp(T.3.1 —
where M = combustion rate (weight of oil/unit time of Volume of gas/unit
time), Q H = calorific value of oil or gas used. Efficiendy values for direct
and indirect heaters are listed under heaters in chapter. 12.
The accuracy of the air flow- calculation depends upon accurate measurement of fuel and gas combustion rates, and reliable data on the fuel calorific
values.
a
354
OPERATIONAL PRACTICE
(iii) Electric Air Heater. Air rates can be calculated from the heat generated
by the elements of the heater, as given by the heater kilowatt rating, and the
increase in air temperature over the heater (1 KWH = 3413 BTU/hr
(860 Kcal/hr)). Heater efficiencies are of the order 95 %.
Re (b). Evaporation Rate
The air flow to the dryer can be determined from measurements of evaporation rate. The measurements can be based on water or feed product evaporation.
If a fixed feed rate of water is evaporated under steady state conditions,
the measured temperature drop in the drying chamber will be caused by the
water evaporation, plus heat loss through the chamber walls.
The air rate can be calculated from the measured inlet and outlet drying
temperatures by conducting a heat balance over the chamber (see chapter 4).
The heat input (drying air and water feed heat content) is equated to the
heat content in the exhaust air leaving the dryer, the heat loss through the
chamber walls and the heat used in moisture evaporation. If secondary
air enters the chamber, this also contributes to the temperature drop over
the chamber. The heat content of this air flow must be included in the heat
balance as a further heat input. Secondary air can be (a) the return of
transport air to the chamber via ducting or atomizer, (b) any cooling air
that enters the chamber, or (c) false air from leakage through door or
window.
The heat loss (Q L ) from the chamber walls is determined using the basic
heat transfer equation (Q = UAAT). The heat transfer or transmission
coefficient ( U) depends upon the insulation material used at the outer
2
chamber wall surface. This can vary from 0.2 BTU/ft /hr °F (1 Kcal/m' hr °C)
2
2
for well-insulated chambers to 2.0 BTU/ft /hr °F (10 Kcal/m hr °C) for
chamber walls having forced air cooling. Thermal conductivity of some
insulation materials is given in appendix 9.
Re (c). Supply Fan Characteristics
The air flow is obtainable from fan performance curves. Data on fan speed,
operating pressure and air temperature are required. Air rates are then
read directly from the curves. The reference temperature and atmospheric
pressure levels on which the curves are based rarely coincide with the
operating conditions. The operating data must be adjusted accordingly
when using the curves. This method can give inaccuracies, and should not
be used as the only measuring technique.
Re (d). Pitot Tube Measurements
If a suitable duct length exists within the supply air system, pitot tube
measurements can be conducted to determine the velocity profile over the
duct. The average velocity can be calculated and using the cross sectional
OPERATIONAL MEASUREMENTS
355
area of the duct the volume flow is readily deduced. The mass flow is calculated using the air density, which is dependent upon the duct air temperature. Further details on pitot tube measurements are given under air
velocity measurement (11.2.1).
Re (e). Anemometer Measurements
The anemometer is suitable_ as the supply air is clean. The inlet air duct
normally_has sufficient straight lengths for anemometer access and accurate
air measurement. Many types of instruments are available commercially
and are robust enough to cope with plant site conditions. A useful portable
instrument is the electronic anemometer, which measures velocity directly
(137). The anemometer is used in conjunction with a transistorized indicator
unit. Use of anemometers for measurement of air flow is illustrated later
in this chapter.
Re (f). Orifice/Venturi Characteristics
If a suitable duct length exists within the supply air system, an orifice or
venturi meter can be installed to give a continuous pressure drop reading
that is related to air flow. Figure 11.6 shows typical positioning of the
meters. Further details of measurement are given under air velocity measurements (11.2.2).
Re (g). Air Dilution Methods
Air flow rate can be measured by methods involving air dilution with gas
analysis. It is especially suitable for flow measurement in irregular shaped
ducts or in duct systems having a complexity of bends. The air flowing
through the duct is diluted with a known quantity of diluent gas, and the
diluent gas concentration is determined by gas analysis following thorough
mixing in the duct. This method is applicable to measuring air flow rate
from a direct heater. The method is illustrated in example 11.1.
Of the manual methods mentioned above, measurements based upon
heater characteristics, evaporation and pitot tube are usually the most
dependable. Pitot tubes (with manometer) can generally be made available,
which cannot be said for anemometers, Dilution methods are for special
duct layouts which prevent _ use of alternative techniques. Results from
fan characteristics should be treated with caution unless other techniques
can be carried out as a check.
EXAMPLE 11.1
In a pilot plant spray dryer, the supply air contains 0.24 % CO 2 by volume.
To measure the air rate, carbon dioxide is introduced into the duct from a
cylinder at constant rate before the heater. The dilution rate was measured
at 1.6 lb/min, After mixing, the air contains a 1.4 % CO 2 by volume. If
the air temperature after dilution is 76°F, what is the air flow into the drying
chamber at 300° F?
356
OPERATIONAL MEASUREMENTS
OPERATIONAL PRACTICE
Basis : 1 ft 3 of CO 2 free air. Supply air contains
0.24 \
100 0.241
--
—
0.24 parts CO2
99.76 parts CO, free
—
0 0024
ft 3 CO2
ft' CO, free
Air after dilution contains
1.4
100 — 1.41
3
1.4 parts CO, — 0.0142 ft CO,
3
ft CO, free
98.6 parts CO 2 free
Increase = 0.0142 — 0.0024 = 0.0118
ft 3
ft
normally available in the exhaust ducting. Further details on pitot tube
measurements are given under air velocity measurement (11.2).
Re (d). Anemometer Measurements
This method can only be used in duct work after a high efficiency dry powder
separator since clean air conditions are required. It is not recommended
if the exhaust air is saturated or corrosive. Details given for measurements
in the supply air duct apply here.
Re (e). Air Dilution Methods
Details given for measurements in the supply air duct apply here.
.6
CO, added =
11.2. Measurement of Air Velocity in Ducts
L
x 359 ft /min at N.T.P.
44
3
536
Volume at 76°F = 16 x 359 x
492
44
142.5 ft 3 /min
142.5 x 100
— 1210 ft 3 /min
Air rate before heater =
357
0.0118
99.76
760
3
Air rate into drying chamber (1210 x —) = 1715 ft fmin
536
11.1.2. Exhaust Air Leaving Dryer
The exhaust air flow rate can be determined from measurements based
upon:
(a) Pressure drop characteristics of the powder—air separator in the exhaust
system.
(b) Exhaust fan characteristics.
(c) Pitot tube measurements in the exhaust duct.
(d) Anemometer measurements in duct after the powder—air separator.
(e) Air dilution methods.
Re (a). Pressure Drop Characteristics of Powder Air Separator, e.g. Cyclones
Approximate air flow rates can be obtained from pressure drop characteristics of the powder—air separator, especially cyclones. Correlations relating
pressure drop and air flow depend upon equipment design, and are available
from equipment suppliers.
Re (b). Exhaust Fan Characteristics
The use of fan characteristics to determine air flow has been mentioned
under the supply fan characteristics (11.1.1). The same procedure applies
here.
Re (c). Pitot Tube Measurements
The pitot tube technique is commonly adopted as suitable duct lengths are
11.2.1. Pitot Tube Measurement
The velocity of air in dryer ducts is conveniently measured by a pitot tube
and inclined manometer where air velocities exceed 10 ft/sec (3 m/sec).
A pitot tube and accompanying inclined manometer (single limb) are
shown in figure 11.1. The pitot tube consists of two concentric tubes formed
into a 90° bend so that the nose points directly upstream when the tube
body is inserted through the duct wall. The small hole at the nose of the tube
faces directly into the air stream and measures impact (total) pressure . (head)
(Pt).
TOTAL PRESSURE
HOLES
,g// WA
',Kir wr■
STATIC PRESSURE
( HOLES
PITOT TUBE
ELLI PSOI PAL
NOSE
—
Figure 11.1. Pitot tube with inclined manometer.
358
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
The ring of small holes drilled into the side of the outer tube measures
the static pressure (P s ). The positioning of these holes in relation to the
tube nose is critical and is carefully chosen to eliminate unwanted pressure
effects set up between the impact and static pressure holes. It is thus important
that pitot tubes conforming to recognized standards are always used.
Tubes made from 18/8 stainless steel will cover virtually all temperature
conditions likely to be met on spray dryers (i.e. 18 % chromium, 8 % nickel,
0.6 % titanium steel welded points—can be exposed to 700°C).
For a pitot tube facing into the air stream the impact pressure will exceed
the static pressure. Thus the plastic hose fitted to the central tube is connected to the stud at the zero end of the manometer scale. The plastic hose
fitted to the outer tube is connected to the top end of the single-limb manometer. The pressure read on the manometer scale is thus the difference
between impact and static pressure, namely the dynamic pressure (P d ).
To conduct a velocity measurement the pitot tube is inserted in the duct
with the nose facing directly into the air stream. Care should be taken
when inserting the pitot tube to prevent blowing the fluid out of the manometer by ensuring the impact and static holes enter the duct simultaneously.
The most practical method to prevent loosing manometer fluid is the pinching of the plastic hoses while inserting the pitot tube. Alternatively, manometer balancing valves are obtainable commercially.
The pitot tube is clamped so that the nose is at a fixed position within the
duct. The differential pressure is read off the manometer. Slight fluctuations
always occur to the meniscus in the manometer and the mean value is taken.
This is the dynamic pressure for the position in the duct only where the
tube nose is placed. 20 seconds should always be allowed to elapse before
taking the mean value. This allows the meniscus fluctuations to settle down.
The air velocity (K) is given by the general equation
va =
2Pd g
where Va is m/sec, Patin°, is mm Hg, Tabs is '1K, Pc,Ps is mm WG,
K =
2
(a) For Circular Ducts
For duct diameters less than 2 ft (600 mm) 4 equal area zones are used.
Pitot tube readings are taken at distances from the duct access holes at
0.062D, 0.25D, 0.75D, 0.938D, where D is the duct diameter (see figure 11.2(a)).
ACCESS HOLE
FOR PITOT
TUBE
(11.3)
Pa
30
Patmas
x
Tra,
528
d
)
408
K1— 408 + Ps i
0.706 0
0.062
0 , 25
0.75
(11.4)
where va is ft/min, Pat ., is in Hg, Tab , is °R, P,P, is in WG. The correction
factor
0.044
0047
0.294 0
1/2
xP
10 350
10 350 + P,
but is equated to K2 1,
=
if Ps is less than 250 mm WG.
Values of I/a in equations (11.4) and (11.5) are local velocities, A series of
measurements is taken over duct area to form a velocity profile for determination of the average duct velocity. A profile is necessary as velocities
are rarely uniform over the duct. The positions at which pitot tube readings
are taken in the series of measurements depend upon duct size. Positions
are selected from dividing the duct into zones of equal area.
Equation (11.3) can be written correcting for variations in temperature and
pressure. In British units
= 4 x 103 ( K x
359
K, is a correction to the barometric pressure to allow for static pressure
in the duct. If. P, is less than 10 in WG K can be taken as 1.
In metric units
1/2
va = 4.05 ( 1(2 760
as
(11.5)
P
X 293
0.853 D
0.9360
0•956 D
D
A DUCTS UNDER
2 ft DIAMETER
600 mm)
B. DUCTS
EXCEEDING
2 ft DIAMETER
600 mm
Figure 11.2. Positions for velocity measurement by pitot tube in circular duct (equal duct
areas.
▪
360
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
For duct diameters greater than 2 ft (600 mm) 6 equal area zones are
used. Pitot tube readings are taken at distances from the duct access hole
at 0.044D, 0.147D, 0.294D, 0.706D, 0.853D, 0.956D, where D is the duct
diameter (see figure 11.2(b)).
Measurements should preferably be made along two traverses 90° apart.
_1.a
[ I2
ACCESS-•
HOLES
FOR
PITOT
TUBE --.
-1-
1-
-
-
Pitot Tube Misalignment
The ratio of pressure difference measured in a uniform air stream with the
pitot tube deviated at an angle 0 degrees from the air stream to the pressure
difference when the pitot tube is facing directly into the air stream is given
in table 11.1.
As the pitot tube is turned, there is an initial increase followed by decrease
in Pd recorded values until eventual negative response. At 90° to the air
stream there is a sharp negative response and this effect offers an approximate
method of detecting and measuring swirling air flows.
+
4
H
4
H
f
PI TOT TUBE
PRESSURE
DIFFERENCE
+
in WG
mmWG
e-100
4A AREA LESS THAW 2 ft ' or 0 , 2 r00
B.
4-
4-
+
•C
(b) For Rectangular Ducts
Pitot tube measurements should be taken at points shown in figure 11.3.
The minimum number of points for fairly uniform flow is
17
1 00
2
4 positions
duct areas less than 2 ft (0.2 m )
2
duct areas between 2-24 ft 2 (0.2-2.25 m ) 12 positions
20 positions
duct areas exceeding 24 ft 2 (2.25 m 2 )
-
5•0
1 40 - 60
- -7o
1 80
2.
--
qa
0
25.5
2 4.
30
37.
0
10
20
40
60
80
(PA
130
1 42
-
1 4
1r
1 4.
13.2
rz,r
- ISO
- 12.0
Angle of deviation
from air stream
0
53.331•1"
AIR
TEMPERATURE
AREAS 2-24 ft . or 02-2.25 rri
Table 11.1. Effect on P d Reading of Pitot
Tube Misalignment in the Air Stream
AIR VELOCITY
nysec.
Io
Figure 11.3. Positions for velocity measurement by pitot tube in rectangular ducts (equal duct
areas).
2
Iry
11.17
11-2.
10■6
- 110
VIC
400 200
•
( cl)0=0
210
470
1.0
1.05
0.9
0.6
0.0
- 0.6
361
Sao
G,
6•.
Figure 11.4. Pitot tube pressure difference (P LO-air velocity nomograph.
362
OPERATIONAL PRACTICE
Pitot Tube Calculations
Air velocities from measured P d values are calculated from equations (11_4)
and (11.5) or read directly from the nomograph (figure 11.4).
When calculating the average velocity, the square roots of the velocity
heads are often recommended to give a 'root mean square' value, but in
practice this is not necessary. No appreciable error is involved if an arithmetic average of the velocity heads is taken where the majority of readings
making up the velocity profile does not vary by more than +25 % from the
mean figure. The average velocity is multiplied by the cross-sectional area
of the duct to give the volume air flow. Mass air flows are obtained from
the air density at the point of measurement.
Although pitot tube instructions require the tube to point directly into
the air stream, an air velocity measurement of sufficient accuracy for spray
dryer applications can be obtained as long as the tube head alignment is
within 15° of the air.
Limitations in the Pitot Tube Methods
1. Presence of Airborne Powders. The pitot tube measurements are affected
by powder entrained in the air stream. A pitot tube should not be used in
powder conveying ducts, where powder—air loadings are high. Powder
loadings in air leaving powder separator units (e.g. cyclones, bag filters)
are normally low enough to enable pitot tube methods to be successfully
applied.
2. Low Air Velocities. The accuracy of the measurement depends upon a
measurable dynamic pressure difference recorded on the inclined manometer. Duct velocity must exceed 10 ft/sec (3 m/sec). Good accuracy
demands at least a Pd of 0.4 in WG (10 mm) equivalent to 40 ft/sec (12 m/sec).
Use of lighter manometer fluids will assist the recording of low Pd values.
3. Non Ideal Duct Layout. Ideal flow conditions for accurate pitot tube
measurements require six duct diameters of straight duct before the measurement point. This is to obtain uniform flow conditions at the point of measurement. However, it is uncommon in spray dryer layouts to have inlet and
exhaust duct systems with such unrestricted straight lengths of duct. This
is due to the presence of dampers and sharp bends. To conduct a velocity
measurement with a pitot tube, therefore, the duct layout must be studied
so as to select the most favourable measuring position. If the best position
available is still not ideal, determination of the velocity profiles along two
or four perpendicular planes across the duct is essential. Permissible
sampling positions, as shown in figure 11.5, are subject to every care taken
with the pitot tube measurements. Otherwise velocity profile data obtained
is of questionable value. There are limitations, however, to how much
careful velocity profile determinations can compensate for non-ideal duct
arrangements. The use of a pitot tube should not be attempted at (a) positions
OPERATIONAL MEASUREMENTS
363
S.P
NOT LESS
THAN ONE
DUC T
DIAMETER (D)
A.
NOT LESS
THAN 4 D
NOT LESS THAN 20
-
B.
C.
Figure 11.5. Permissible positions in ducts for pitot tube measurements of air sampling. (S.P,
sampling (measurement) plane.) A, Sharp bend (upwards). B, After fan. C, After damper.
closer than one duct diameter to any upstream bend or damper, or (b) at a
point where air flow is in swirling motion.
4. Liable to Damage. Any damage to the pitot tube holes will affect the
value of the dynamic pressure measured. Always before use, inspect the
holes for damage (e.g. dents or burrs).
5. Small Holes Tend to Block. The pitot tube will not function if the holes
are blocked. Before use, the tubes should be blown through via the connection nipples to ensure that the impact and static pressure holes are open.
364
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
11.2,2. OrificelVenturi Measurements
Orifice or venturi meters are for continuous measurement of air velocity.
Both types can be installed permanently in supply air ducting, figure 11.6.
Venturi meters are seldom considered in the exhaust duct system due to
the required run-in and run-out duct lengths for mounting. Orifice meters
should not be mounted in the exhaust system, as air is never 100 % powder
AIR DISPERSER
VEN TURI
365
free. This leads to deposit and cleaning problems at the orifice. Both
types of meters are based upon the acceleration of air through a constriction.
The kinetic energy increases with pressure energy decrease. The air rate
is measured from the resulting pressure drop, where the rate is approximately
proportional to the square root of the pressure drop. The pressure tappings
are located at the inlet of the meter and at the point of lowest pressure. For
the orifice meter, the upstream pressure tappings is approximately one duct
diameter and the downstream tapping approximately one half duct diameter
from the orifice. For the venturi the upstream tapping is approximately
one duct diameter from the entrance, with the downstream tapping at the
throat. Minimum run-in and run-out duct lengths are 4 x diameter run-in
and 9 x diameter run-out.
The venturi is generally preferred over the orifice. The venturi has a
higher discharge coefficient and is more efficient in operation due to a
greater proportion of the pressure drop being recoverable. It does constitute
a larger equipment item, but the duct to the dryer air disperser can normally
accommodate this.
Air flow through an orifice/venturi is represented by the basic equation
) 1/2 (20 p) 1/2
1
(11.6)
Vm = CD(
1 — (a/A?
Pa
,
MR HEATER
A . VENTURE ME TER
AIR DISPERSER
41110
where Vm = mean velocity at the orifice plate or venturi throat, a = area of
orifice or venturi throat, A = duct area.
Compressibility of the air must be allowed for if the pressure difference
is an appreciable fraction of the absolute pressure. Compressibility can be
taken into account by multiplying the discharge coefficient C, by an expansion factor (Ye ). For air flows in spray dryer ducts, values of Ye approach
unity. For pressure ratios (downstream divided by upstream) exceeding
0.8.
For an orifice :
•
> 0.934 where a/A = 0.3
ORIFICE PLATE
11111
AIR HEATER
DRYING CHAMBER
ROOF
B
> 0.91 where
a/A = 0.7
Ye = > 0.876 where
a/A --- 0 , 3
Ye = >0.836 where
a/A = 0.6
•
For a venturi :
ORIFICE METER
Figure 11.6. Location of venturi and orifice meters in inlet drying air duct.
11.2.3. Anemometer Measurements
An anemometer is used in duct flow conditions under 10 ft/sec (3 m/sec). It
is an instrument for low air velocity and applicable to cases where dynamic
pressures registered by pitot tubes are too small to allow accurate measurement. Anemometers are usually calibrated in feet of gas flow. To conduct
366
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
367
l000 mm
a measurement, the anemometer is placed in the air stream and is held in a
set position for a definite time. A stop watch is best used. Hence, if the
instrument is held for one minute the anemometer velocity reading will be
in feet/min (m/min).
The limitation of the instrument concerns the difficulty in keeping adjustment. It should be calibrated every time used. It is not suitable for hot,
powder-laden air flow or ducts handling corrosive air—gas mixtures.
1
+w
a
+-
E
1
12
+
13
±.
+
1—
19
±
+
-
H
+
-1—
H-
--
+
18
+
21.
4 + -F
4-
—I-
+
30
25
Figure 11.8. Inlet duct zoning for anemometer measurements in example 12.2.
Figure 11.7. Positions selected in spray dryer layout for various air flow measurements illustrated in example 11.2•
11.2.4. The Application of the Various Techniques of Air Flow Measurement
to Determine Air Rates in Spray Dryers, An Example
EXAMPLE 11.2
It is required during commissioning of a spray dryer to determine the air
flows. The dryer layout is shown in figure 11.7. A gas fired indirect heater
is used. The inlet temperature is 210°C and the outlet temperature 88°C.
The feed temperature is 48°C. Ambient air temperature = 5°C. It is
proposed to use the various techniques:
A. Air to dryer
(1) Anemometer
(2) Gas—air heater combustion rate
(3) Fan characteristics
(4) Evaporation
B. Air leaving dryer (5) Pitot tube measurement
(6) Pressure drop over cyclone
(7) Fan characteristics
The measurements are discussed below. The exhaust air composition after
the cyclone is not suitable for the anemometer.
A. Supply Air to Dryer
Method (1) Anemometer Measurement
A rectangular duct 1000 mm x 1300 mm before the supply fan was used
for the anemometer measurement. The measurement point in the duct was
considered not ideal and thus the duct was divided into 30 zones of equal
area (figure 11.8). Each zone measured 167 x 260 mm. Each zone area
was 0.0433 m 2 . The anemometer was held in each zone and the following
results obtained.
Zone
Anemometer
reading after
1 min
1
2
3
4
5
6
7
8
9
10
175
152
188
154
207
173
208
229
200
243
Zone
Anemometer
reading after
1 min
Zone
Anemometer
reading after
1 min
11
12
13
14
15
16
17
18
19
20
178
188
220
242
191
244
235
191
224
228
21
22
23
24
25
26
27
28
29
30
246
239
225
238
226
246
250
259
246
250
368
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
Summation of zone velocities EV = 6495 m/min
Total air flow L = EV .p air . A x 60 kg/hr
Pair at 5°C = 1.252 kg/m
3
L = 6495 x 1.252 x 0.0433 x 60
= 21 200 kg/hr
Method (2) Gas Fired Indirect Air Heater Characteristics
The air flow rate is calculated using equation (11.2).
Calorific value of gas = 8400 Kcal/Nm 3
Heater efficiency = 80 %
Gas consumption rate (average of five readings) = 165 Nm 3 /hr
L
165 x 8400 x 0.80
— 22 500 kg/hr
a — 0.24(210 — 5)
Method (3) Fan Characteristics
The fan speed is calculated from the electric motor drive and diameter
ratio of the fan and motor pulleys. A tachometer measures the motor
speed. In this example, the fan speed is 1980 rev/min.
To calculate the air rate from the fan characteristics curves the total
pressure (Ps ) developed by the fan is required where P, = P s + Pd. The
static pressure (Ps ) over the fan (use static pressure hole of pitot tube and a
U-tube manometer) = 120 mm WG. The dynamic pressure (P d ) can be
obtained using pitot tube, but flow in the inlet and outlet of the fan does not
give conditions for accurate measurement. Calculation of P d by equation
(11.7) was used.
p
APd =
(11.7)
2 g
For example
2
Fan inlet area at measurement point = 0.738 m
2
Fan outlet area at measurement point = 0.193 m
Air temperature = 5°C
Air density = 1.252 kg/m'
Let air rate be assumed = 23 000 kg/hr
From equation (11.7)
( 1
23 000 2
Pa = 1 1.252 1
2
2
2 9.81 1.252 x 3600 0.1932
AV = vou, —
To calculate velocity values, the air flow is first estimated from which
values of V° „, and V,„ are obtained from the outlet and inlet fan areas. The
calculated value of P c, is added to the measured value Ps to obtain the total
pressure (Pr ). With the value of P, and fan speed for ambient conditions,
the air flow rate is read off the fan characteristic curve. If the air flow rate
estimated initially to calculate P d does not agree with the value read of the
characteristic curve, further values of air rate are taken until a value is
obtained for complete agreement with the fan curve value.
1
0,7382 )
42 mm WG
= 42 + 120 mm WG -= 162 mm WG
For the fan in question, an air flow rate indicated on the fan curve
23 200 kg/hr. Estimated value 23 000 23 200 kg/hr. Values are close
enough to consider estimation correct.
Method (4) Evaporation Measurement
Air flow rate based upon evaporation of water feed is calculated from a
heat balance from which (with given metric units)
La
M L (595 + 0.46 Tu )
—
Cp(Ti
(11.8)
The accuracy of using equation (11.8) depends upon the correct data or
estimation of (TL ), i.e. how many degrees of the measured chamber temperature drop are due to heat transmission losses through the dryer structure
and introduction of secondary or false air.
For given measurement, water feed rate (M L ) = 940 kg/hr, and for an
insulated drying chamber, TL is estimated 7°C.
Substituting in above equation
L aa =
where
369
940(595 + 0.46 x 88)
— 21 700 kg/hr
0.24(210 — 88 — 7)
B. Exhaust Air from Dryer
Method (5) Pitot Tube Measurements
The duct diameter at point of measurement is 800 mm, 6 zones were used
(see figure 11.2). Measurements were conducted through traverses in the
horizontal and vertical planes. Data are tabulated below
T = 85 ° C
D = 800 mm
A = 0.504 m
2
370
OPERATIONAL PRACTICE
1
2
3
4
5
6
OPERATIONAL MEASUREMENTS
Pitot tube
measurement (Pd )
Horizontal
Vertical
(mm WG)
Zone
Distance from
duct access hole
(mm)
0.044D
0.147D
0.294D
0.706D
0.853D
0.956D
- 35
120
235
565
680
765
7.0
9.6
11.9
11.8
10.6
8.4
Average Pd
measurement
(mm WG)
Air velocity
(m/sec)
7.1
9.7
11.9
11.5
10.0
8.2
118
14.0
15.8
15.4
14.1
12.6
7.2
9.8
11.9
11.2
9.4
7.8
Using figure 11.4
Zone
1
2
3
4
5
6
Av = 13.9 m/sec.
Volume air rate = 13.9 x 0.504 x 3600 = 25 200 m 3 hr
p a (85°C) = 0.988 kg/m 3 . Therefore L a = 24 900 kg/hr
Method (6) Pressure Drop over Cyclones
This measurement requires the cyclone suppliers' correlation for air rate
in terms of cyclone diameter (d), static pressure drop, and air temperature.
The given single cyclone has a volute (wrap around inlet), and air rate (L a )
is given by (n = number of cyclone0
L a = 6700nd
(
AP,
p
X
T, + 273 760
kg/hr
where
= 87°C, AP, = 180 mm WG
d = 2.24 m
Pa s s = 760 mm Hg
760 1/2
the air rate = 6700 x 1 x 2.242 180
23 700 kg/hr
x
(
360
760
Method (7) Exhaust Fan Characteristics
Similar procedure adopted as for the air rate determination at the supply
fan (item 3).
= 1230 rev/min
Fan speed
Fan inlet duct area at measurement point = 0.436 m 2
2
Fan outlet duct area at measurement point = 0.490 m
= 85°C
Air temperature
= 250 mm WG
Measured static pressure (P,)
= - 3 mm WG (where
Calculated dynamic pressure (.1%)
estimated air rate corresponds
with value from the fan curve)
= 247 mm WG
Total pressure
-= 23 500 kg/hr
From fan curve
Conducting an Air Balance over the Dryer
Method (1) 21 200 kg/hr
Method (2) 22 500 kg/hr
Method (3) 23 000 kg/hr
Method (4) 21 700 kg/hr
22 100 kg/hr
Average
940 kg/hr
Evaporation
Air leaving dryer Method (5) 24 900 kg/hr
Method (6) 23 700 kg/hr
Method (7) 23 500 kg/hr
24 033 kg/hr
Average
Thus false air (by difference) = 24 033 - (22 100 - 940) = 993 kg/hr
Air to dryer
N.B. The fan characteristic method is often considered too inaccurate to
be of use. If this method is used, and the air flow value does not agree with
data from other methods, the value should be disregarded.
11.3. Determination of Powder Loading in Air Flows
1 2
/
371
Data on powder loading in air flowing through the dryer exhaust ducting
provide insight into the important operational aspects of 1. efficiency of
powder separating and recovery equipment, 2. powder losses from powder
separating and recovery equipment. Powder loadings in air can be determined by manual sampling or by continuous in-line measurement (510) (511).
Manual determination involves the sampling of air at a duct access
point where air flow is conducive to isokinetic sampling techniques. Air
is sampled for its powder content at positions selected across the duct
cross-sectional area for representative sampling. Sampling is conducted
372
OPERATIONAL MEASUREMENTS
OPERATIONAL PRACTICE
x
wz
cc
1-(r)
by drawing air plus entrained powder into a sampling nozzle of known size
and through a probe tube which is inserted into the duct. The sampling
rate is adjusted so that the air velocity entering the nozzle equals the air
stream velocity just outside the nozzle (figure 11.9). This is the requirement
for isokinetic sampling. The effect of incorrect sampling rate on observed
powder loading is shown in figure 11.10. Air passing out of the probe tube
is separated of its powder content. Knowledge of the air sampling rate
and the weight of entrained powder separated per unit time enables calculation of the powder loading. The basic layout for sampling is shown in
figure 11.11. In principle the method is simple enough, but in practice it
10 micron
1 micron
1.0
cc
LI]
0 cc
LLk
0
0
LU
cc
LU
tr)
POWDER
AIR
100 micron
z
DUCT
IN
lg /cc . 2 0 . C,.. 1 oto
BASIS : PARTICLE DENSITY
2.0
373
-J
0
1.5
1 0
05
0
SAMPLER
NOZZ LE
AIR VELOCITY I N SAMPLER NOZZLE
LOCAL AIR VELOCITY IN DUCT
STREAM
Figure 11.10. Effect of incorrect sampling rate on observed powder loading in air stream
A.
(after Stairmand (188)).
is complicated by the need of extreme sampling care to ensure that the
powder loading of the sampled air is representative of duct conditions.
Sampling must represent the average of powder particles passing through
the sampling plane. This presents no difficulty when the distribution of
particles is uniform throughout the duct cross-sectional area. However,
this is rarely so due to the duct-work layout. It is necessary to collect
B.
PROBE TUBE
SOLI DS
COLLECTING
DEVICE
SAMPLED
AIR
SAMPLING
PLANE
ACCESS HOLE
SAMPLING
NOZZLE
1
C.
Figure 11.9. Flow conditions at sampling nozzle. Flow profiles indicating isokinetic and non-
isokinetic sampling conditions. A. Conditions for isokinetic sampling. B. Non-isokinetic
conditions. Insufficient sampling rate (excess of large particles). C. Non-isokinetic conditions.
Excess sampling rate (insufficient large particles).
POWDER
LADEN AIR
SOL DS
RECOVERY
Figure 11.11. Basic layout for sampling powder laden air.
TO
SUCTION
FAN
374
OPERATIONAL MEASUREMENTS
OPERATIONAL PRACTICE
samples from a sufficient number of positions across the sampling plane to
account for variations in particle population (mass flow) over the sampling
plane. Flow of air and airborne powder are similar though not identical.
The extent of the difference depends upon the size distribution and particle
density of the powder, the air velocity and duct layout. In bends, the particles
tend to be centrifuged to the outside of the bend although the uniformity
is gradually restored by turbulent mixing. Uniformity is obtained 2-6 duct
diameters from a 90° bend. 3-5 diameters is sufficient to establish a
uniformity suitable for sampling.
If the air velocity is high and the bend sharp, the air flow may separate
from the inner wall of the bend and not begin to be corrected until 2 diameters downstream. The air in the dead space will tend to recirculate in
one large eddy. The eddy rotation is forwards near the main stream and
backwards near the wall. The rotation velocity is not high but may be
sufficient for reverse flow to be detected by a pitot tube. When selecting
a duct position to measure air velocities the selected position must be free
of eddy flow effects.
Typical velocity and likely powder loading over the cross-sectional area
of duct arrangements are illustrated in figure 11.12. Duct arrangements
considered are (1) straight duct, (2) following a sharp bend in a circular
duct, (3) following a fan, (4) following a damper.
The need to consider a number of sampling positions to obtain the
overall powder loading leads to a time-consuming and often tedious sampling
practice. However, failure to recognize variations in air velocity and powder
loadings over the duct area can lead to high sampling errors. If powder
sampling is merely conducted at the duct centre, under conditions of marked
non-uniform flow profile, sampling can be considered in error in excess of
± 75 %.
With correct attention paid to non-uniform flow profiles, sampling of
airborne powder can be in error less than + 20 %.
Continuous determination involves an in-place mounting of a powder
detector or collector unit. With detector units, the presence of solids in air
flowing across the path of unit sensors is registered on indicating equipment,
calibrated for the product in question. Presence of solids are commonly
detected by light intensity methods. With in-place collector units, the air
is continuously sampled for powder. The equipment design enables the
sampling rate to vary with fluctuations in air flow. A sampling nozzle is
often incorporated, fixed in a position selected as representative of the air
flow through the duct sampling plane. The quantity of powder collected
per unit time indicates the extent of powder loading in the air flow. A sharp
increase in collected powder signifies a marked increase in powder loading.
Such monitoring of powder levels can provide useful information in assessing
0
>
375
376
OPERATIONAL MEASUREMENTS
OPERATIONAL PRACTICE
day-to=day dryer performance. Once the equipment is installed it can
always be calibrated against a method giving an absolute powder loading
value. A useful in-place collector unit is the Vegrit' equipment (137), which
is applicable to many spray dryer exhaust systems.
Selection of Equipment for Determination of Powder Loadings in Air Flow
The selection of equipment for either manual or continuous sampling
depends upon the powder loading range likely to be met at the sampling location. Measurements to determine efficiencies of powder separator units
involve both high loading conditions on the inlet side and low loading
conditions on the outlet side. Measurements of powder loadings that constitute powder losses from spray dryer exhausts involve only very low
loading conditions.
For measurements in high powder loading conditions, equipment units
must be capable of handling large quantities of powder if any degree of
measurement accuracy is to be achieved. In manual sampling procedures,
there must be effective separation of powder from the sampled air. In
continuous detector systems, the sensors must be able to handle high powder
populations. For measurements in low powder loading conditions, equipment must handle very small amounts of entrained powder. In manual
sampling procedures this calls for near perfect separation efficiency of the
separator incorporated within the equipment. In continuous detector
systems, the sensors must be able to detect often minute particle sizes in
air containing low levels of powder entrainment.
Equipment Available
Equipment offered for manual sampling of airborne powder comprises a
sampling nozzle of known diameter, a probe on which the nozzle is attached,
a means of extracting powder from the sampled air, a means of regulating
and measuring the sampling rate and an air velocity determination set.
Equipment offered for continuous measurement is characterized by the
type of detecting systems. Most include sensors, amplifiers, converters and
recording equipment. Equipment offered for continuous monitoring not
based upon electronic units, are characterized by the sampling technique
system adopted.
Equipment suitable for determination of powder loadings in spray
drying exhaust air flows is listed in table 11.2. Low powder—air loadings
exist after main or secondary powder separator units, e.g. cyclones, bag
filters, electrostatic precipitators. High powder—air loadings exist before
main separator units or in powder conveying systems.
Choice of Equipment
The cyclone probe equipment is recommended as an all-purpose manual
sampler. It can be used with the vast majority of spray dried products.
It operates in both high temperature and high humidity air, is easy to operate,
377
Table 11.2. Equipment for Determination of Powder Loading in Ducted Air Flows
Powder—Air loading
Low
High
Manual sampling
Continuous
measurement
Cyclone probe* (B.C.U.R.A.
method)
Internal filter*
External filter*
Light intensity methods
Cyclone probe* (B.C.U.R.A.
method)
with (Large hopper attachment)
Light intensity methods
Electrostatic methods
CEGRIT sampler (137)
* B.S. 3405, 1961.
and high degrees of accuracy can be obtained by plant operator personnel.
The alternative filter methods demand. greater finesse of measurement.
Without great care, experimental errors can be high. Only where the bulk
of airborne particles have a size less than 3 micron does the filter method
become essential. The Cegrit equipment is recommended as an all-purpose
continuous (semi) sampler, for use with non-sticky particles in non-saturated
air conditions. Light intensity and electrostatic techniques are much more
expensive and their installation is only justified if powder loadings require
monitoring due to (a) slightest powder emissions to atmosphere cause
prohibited pollution levels, or (b) the product is of high value and 100%
recovery is economically essential.
Installation of equipment stated in table 11.2 involves careful and
important preparation. Firstly, an initial inspection of the spray dryer
duct-work is required to decide the most suitable sampling point. Arrangements must be made for making the necessary holes in the duct for equipment mounting, erect a working platform (if none is already available) and
provide a power source to operate the equipment.
When selecting the most suitable sampling position the ideal is a duct
location where a uniform air flow profile is likely to exist (i.e. at the end of
an extended straight duct section containing no flow interference such as
dampers). Where there are no such duct sections, a position must be chosen
where the powder population can be expected to be predictable and reasonably uniform. If this position is in a vertical duct position so much the
better. The air velocity at the sampling point must exceed 10 ft/sec (3 m/sec)
if a pitot tube is to be used for air velocity determination.
Access hole requirements will depend upon the chosen equipment. For
continuous methods, the equipment design will fix the type and size of
access holes required in the duct wall. For manual sampling, access holes
378
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
for circular ducts should be such that one sampling line lies on a diameter
in the plane of the duct and the other lies on the diameter at right angles
to it. Access holes for rectangular ducts should be at different levels in the
side of the duct. The number of required levels will depend upon the duct
size. Access hole locations are shown in figures 11.2 and 11.3.
11,3.1. Manual Sampling
(a) Measurement of Powder Loadings in Ducted Air Flows using the Cyclone
Probe (B.C.U.R.A,) Method (138)
Principle of Operation. A sample of air from the spray dryer exhaust system
is drawn into a sampling nozzle, attached directly to the inlet of a small
cyclone inserted completely within the duct (figure 11.13). The cyclone is
mounted at the end of the probe. Powder particles are centrifuged out of
the air sample and enter a detachable hopper. The air is drawn by a suction
fan through the cyclone, the probe tube, a flexible hose and a control valve.
The quantity of powder sampled is determined by weighing the hopper
(dry) before and after the measurement. The cyclone is of special design
having a high efficiency (see table 11.3), but particles smaller than one
micron are not collected but pass down the probe and through the fan.
A filter attachment can be used to collect such sizes, but for general sampling
on spray dryers, the attachment is not required. No appreciable error is
involved. If there is strong evidence of powder passing through the probe
tube to the fan, and the filter attachment is not available, a correction factor
can be applied to sampled powder, by stipulating that powder actually
collected represents 80-90 % of powder entering cyclone. (This would only
be considered when sampling very fine powders, where over 80 % of powder
collected is less than 10 micron.)
HOPPER
CYCLONE
■
SAM PL] N G
NOZZLE
E PROBE TUBE
PRESSURE TAPPI NG
-HOSE
LANE 0 F
DUCT WALL
PROBE
HANDLE
-
Af R PLOW
VALVE
MANOMETER `
Figure 11.13. Layout of B.C.U.R.A. cyclone probe for sampling powder laden air.
- SUCTION
FAN
379
Table 11.3. Efficiency of B.C.U.R.A. Cyclone
Particle size (micron)
Efficiency of collection
2
3
4
5
6
8
10
15
20
72
86
91
94
95
96
97
98
99
>99.2
Sampling is based upon the isokinetic principle. Air is drawn into cyclone
nozzle at such a rate that the nozzle air velocity is the same as the local duct
velocity. Maintenance of isokinetic sampling conditions is - very simple.
The cyclone itself is used as a flowmeter, and the pressure drop over the
cyclone (registered on a manometer) indicates the air flow through the
cyclone. Calibration of the cyclone enables the required manometer reading
for isokinetic sampling to be read directly from a chart, which is based upon
the pitot-tube dynamic pressure difference corresponding to the duct
velocity.
The location of the cyclone in the duct is selected and sampling procedure
conducted so that the powder content in the sampled volume is representative of the total powder content in the duct air flow.
The features of the method include :
1. The cyclone can be used at any temperature likely to be met in a spray
dryer exhaust system.
2. The cyclone can be used over long sampling times, as the collection of
powder does not increase the resistance to air flow through the cyclone.
3. Sampling control is straightforward as the cyclone acts as its own flowmeter.
4. A simple calibration chart relates duct velocity to required cyclone
pressure drop for correct sampling.
5. At least 5 grams of powder can be collected. Such measurable amounts
reduce experimental errors.
6. The cyclone can be used in humid air conditions so long as no entrained
wet droplets are present in the air flow.
7. Cyclone operation is not always reliable with sticky airborne powders.
For full details on the equipment, sampling procedure, and theory, the
reader is referred to Hawksley et al. (138), British Standard 3405, 1961,
Appendix 1, or instruction manuals (137).
380
OPERATIONAL MEASUREMENTS
OPERATIONAL PRACTICE
(b) Measurement of Powder Loadings in Ducted Air Flows using the Internal
Filter Thimble Method
Principle of Operation. A sample of air from the spray dryer exhaust
system is drawn into a nozzle attached to a filter thimble housing. The
filter housing is mounted within the duct. Powder particles entrained in
the sampled air are separated out in the filter thimble. Air is drawn through
the filter housing by a suction fan. Air sampling rates are measured by
(a) an orifice plate assembly, or (b) a gas meter assembly.
Air flow is adjusted to give isokinetic conditions at the nozzle. Sampling
procedure is such that the powder content in the sampled air volume is
representative of the average powder content in the ducted air flow. The
theory of measurement is described by Soo et al. (139).
The advantage of using a filter to remove entrained powder (as compared
with the cyclone method above) concerns the ability of the thimble to remove
all airborne particles down to 1 micron. There are, however, notable disadvantages, namely (a) sampling times are generally short as powder collection steadily increases the resistance to air flow through the filter to a point
where isokinetic sampling conditions can no longer be maintained, (b) constant readjustment to sampling rates is required due to increasing resistance
to air flow, (c) common paper thimbles are unsuitable for use in high humidity
conditions, (d) fractions of a gram are normally only collected, leading to
high possible experimental errors, (e) variations in paper thimble weight
due to atmospheric humidity can be significant.
Sampling with the internal filter method requires constant personal
attention paid to the equipment. However, due to the 100 % collection
efficiency the method is often selected for sampling air flows containing a
large proportion of particle sizes less than 10 micron, and/or where powder—
air loadings are very low, e.g. after a bag-filter.
Sampling by the internal filter method involves (a) selection of the most
suitable sampling position, (b) access into the duct to mount the probe
tube and conduct velocity measurements (usually by pitot-tube), (c) determination of the air velocity profile in the duct and (d) careful weighing of the
thimbles.
Paper extraction thimbles, used in the sampler, readily absorb moisture,
and since the weight of powder collected is small, any variation in thimble
weight due to moisture alone causes considerable error. It is therefore
essential to use a control thimble. The control thimble is subjected to the
same humidity variations as the test thimbles, and thus any weight change
in the control can be referred to the test thimbles. This procedure is based
upon the assumption that changes in humidity will affect the weight of all
the thimbles equally. While not absolutely true, the method is sufficiently
accurate if the thimbles are of the same material and approximately the
same weight.
381
An alternative method is the complete drying out of the thimbles in a
constant temperature oven. In this case, there is a chance of destroying
the paper thimbles should the oven be too hot. Experience has shown that
no improvement in accuracy is obtained with this method, except for
hygroscopic materials.
With hygroscopic materials, the test thimble is dried in the oven 230°F
(110°C) with a standard thimble. Both are weighed in weighing bottles,
or sealed test-tubes before and after the sampling. The control thimble is
placed in the oven for 10 minutes. If the standard thimble loses more weight
from the second heating, the oven is too hot and is destroying the paper.
The accuracy of the test thimble weight is thus doubtful. It is necessary to
resample and dry in a cooler oven or under vacuum.
EQUIPMENT DESCRIPTION
(i) The Internal Filter Thimble Equipment containing the Orifice Plate
Assembly for Air Sampling Rate Control
The equipment layout is shown in figure 11.14.
Principle of Operation. The sampling probe tube is mounted in the duct (C)
with the nozzle (A) facing directly into the air stream. Air is drawn through
the nozzle by a fan (H). The quantity of air is controlled by valve (E) so that
the air velocity into the nozzle is equal to that of local air velocity around
the nozzle.
The powder entrained in the air passing through the nozzle (A) is filtered
out in a paper filter thimble (B). From the difference in the thimble weight
before and after sampling, powder—air loading in the duct is calculated.
The air sampling rate is read by the pressure drop reading (K) over the orifice
plate (F). The sampling air temperature at the orifice is measured by the
thermometer (I).
A typical sampling probe tube, containing the filter housing and detachable
sampling nozzle, is shown in figure 11.15. Various designs of isokinetic
probe tubes are discussed by Denis (140).
The correct air sampling rate is determined from the local duct velocity
at the position of sampling, following a pitot-tube velocity measurement.
The pressure drop over the orifice that corresponds to the pressure difference
reading of the picot-tube is obtained from the orifice calibration chart
supplied with the orifice plate assembly. Values of orifice pressure drop
for isokinetic sampling conditions depend upon the operating variables of
orifice suction pressure and sampling air temperature. The suction pressure
is regulated by the bleed valve (0). The catchpot (L) is only included when
condensation occurs in the probe tube (D). The placing of the filter thimble
within the duct reduces the possibility of condensation fouling the filter.
Sampling times should not be less than 30 minutes per filter thimble.
A clean thimble is used at the same time as a control to determine changes
in thimble weight due to humidity variations.
382
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
0
FOCAL 111Fe
383
WALL
0
0
C
1:. SUCTION FAH
cd
O)
DO
E
1=4
k
rt
ti
t■—■
E
-c
C
ta
t.=
3,T2
.
❑
(.)
C
0
E
tu
tu
tr
(‘'
N Uy
,13
5
▪ E
6 2
E. D. C. B.
.
CA
AM FLOW
`)
Figure 11.15. Filter thimble sampler for measuring powder loadings in air flow,
0
"CI
H.Suction fan
0
1=1,
tx)
171
Probe tube
Suction control valve
Duct wall
CA
w
6.r
Filter thimble
rn
57_
E
03
+a)
(ii) The Internal Filter Thimble Equipment containing a Gas Meter for Air
Sampling Rate Control
The equipment layout is shown in figure 11.16.
Principle of Operation. The sampler is mounted in the duct (C) with the nozzle
(A) facing directly into the air stream. Air is drawn through the nozzle
by a fan (F). The quantity of air is controlled by diaphragm valve (G) so
that the air velocity into the nozzle is equal to that of air around the nozzle.
The powder entrained in the air passing through the nozzle (A) is filtered
out in a paper extraction thimble (B). From the difference in the thimble
weight before and after sampling, powder loss rate is calculated. The total
air volume sampled is calculated from the gas meter (H) reading, the air
temperature at the meter being measured at thermometer (T). The venturi
(D) and manometer (E) are used as a visual indication to any variation in
air sampling rate. 30 minutes sampling is sufficient for each measurement.
A clean thimble is used as the control to changes in thimble weight due to
humidity variations.
When condensation occurs in the probe tube and flexible tubing, the water
is collected and measured in a catchpot. This is to prevent water entering
384
OPERATIONAL MEASUREMENTS
OPERATIONAL PRACTICE
385
8
7
it
Figure 11.16. Layout for measuring powder loadings in air flows using the filter thimble sampler
with gas meter for sampling rate control.
the gas meter. Since the gas meter thus records a 'dry' air rate, a correction
to the sampling rate is required. The equivalent volume of water vapour is
determined and the sampling air rate (to be measured at the gas meter
to correspond with the required wet sampling air rate at the sampling
nozzle) is calculated.
(c) Measurement of Powder Loadings in Ducted Air Flows using the External
Filter Thimble Method
The external filter thimble method of sampling is identical to that described
for the internal filter thimble. Orifice plate or gas meter air sampling controls
can be used.
The difference in the two methods concerns the location of the filter
thimble, which for the external filter thimble method is outside the duct.
The arrangement allows easier access to the thimble for mounting or
removal as the probe need not be removed from the duct. It also allows the
nozzle to remain fixed in the duct, although sampling at a fixed nozzle
position in the duct requires a uniform velocity profile over the duct for
maintenance of good sampling accuracy. The equipment layout is shown
in figure 11.17.
Principle of Operation. Air is drawn through the nozzle (1) and through the
probe tube (2) into a filter housing (3), mounted outside the duct. Condensation is prevented in the probe tube and housing by a heating mantle (9)
wrapped around the sampler. Air flow is metered and controlled by an
orifice or gas meter (5). A pitot tube (7) measures duct velocity. Air sampling
rate is adjusted to give isokinetic conditions at the nozzle. The weight of
powder collected in the filter per unit time is determined and the total
powder loading in the duct is calculated using the ratio of nozzle-duct
areas.
11
1. sampling nozzle
2. probe tube
3. filter thimble housing
4. filter thimble
5. instrument case containing air sampling
rate controls
6. suction fan
7. Pitot-tube
8, inclined manometer
9. heating mantle
10. heating mantle control
11. duct wall
Figure 11.17. Layout for external filter thimble equipment.
The external filter thimble is not widely used unless the duct diameter
is small, and duct air humidity low. Duct diameter (d) should be less than
18 in (450 mm) and sampling conducted in a length of straight duct of at
least 10 x d, so that sampling in one position is justified due to uniform
velocity profile across the duct. Air humidity in the duct should be low to
minimize likely condensation problems in the filter housing, which can
occur even with the heating mantle in operation. The powder should be
non-sticky to prevent deposits forming in the probe-tube bend after the
nozzle mounting. Deposits lead to clogging and powder collected in the
filter will no longer be representative of actual duct conditions.
11.3.2. Continuous Sampling
(a) Continuous Monitoring of Airborne Powder by Light Intensity Methods
Continuous measurement of powder—air loadings using light intensity
methods have been successfully applied to the spray dryer field. Equipment
(141) (142) has been reported to measure loadings down to 0.005 grains/ft 3
(approx. 11 mg/NW). Equipment is usually mounted on the suction side
of the fan, or in ductwork having a negative pressure.
386
OPERATIONAL MEASUREMENTS
OPERATIONAL PRACTICE
OPTICS
LIGHT
SOURCE
DETECTOR
FRESH
AIR
BLEAD
RECORDER
Figure 11.18. Measuring powder—air loadings by light intensity methods.
Principle of Operation. The instruments are based upon the absorption of
light by airborne powder. A light source emits a strong beam across the
duct (see figure 11.18). A detector on the opposite side of the duct responds
to the light beam, and produces an electrical field signal that is proportional to
the energy it detects. The detector can consist of a filament whose electrical
resistance varies according to the energy it receives. The light source is
fixed in intensity and the electrical signal from the detector will depend upon
the amount of powder passing through the light beam. Powder particles
will reflect or absorb a proportion of the light energy. Fluctuations in
detected light energy are measured and transmitted to a recorder. The
recorder is calibrated for the powder type and powder-loading range likely
to be met in the given duct system. The accuracy of the instrument depends
upon maintenance of a constant light source, and detecting sensitivity.
All optical parts in the equipment must remain clean during operation.
By installing the equipment before the exhaust fan, clean atmospheric air
can be drawn into the layout to prevent dust and dirt settling on the lens
system and the light source. Light intensity equipment is available both
in Europe and U.S.A. (143) (144) (145).
(b) Continuous Monitoring of Airborne Powder by Electrostatic Methods
Continuous measurement of powder—air loadings using electrostatic
techniques is a new development in sampling equipment.
Two main techniques are used (i) separating powder from air by charging
by ion bombardment, (ii) detecting powder in air by charging by contact
electrification (146).
Technique (i) is more commonly known as electrostatic precipitation.
In equipment using this technique, air is sampled via a probe tube and
387
nozzle located at a fixed position in the duct. The sampling rate is adjusted
to maintain isokinetic conditions at the sampling nozzle. Sampled air is
drawn through a small electrostatic precipitator, where the air velocity is
reduced substantially to allow complete powder separation from the air.
The separated powder falls to a graduated container where rate of collection
can be monitored. Accuracy of measurement depends upon selection of a
sampling nozzle position fully representative of the powder—air loading of
the duct. For this reason, it is not recommended for sampling in large
ducts, or in duct systems having sharp bends and flow obstacles (i.e. dampers)
in the neighbourhood of the sampling point.
Technique (ii) features the sampling of air via a probe tube and nozzle
located within the duct. The sampling rate- is adjusted to meet isokinetic
conditions. However, instead of separating the powder from the air, as in
(i), the air is made to rapidly rotate so that the powder is centrifuged out
to touch an electrically insulated excitor tube. The excitor tube has carefully
selected electrostatic properties. An electrostatic charge is generated on the
tube by the particle contact. The charge is measured and transmitted as an
electrical signal to a recorder. The equipment layout is illustrated in figure
11.19. Once the sampled powder laden air has passed through the nozzle,
probe tube, and excitor tube, it is exhausted back into the duct. The magnitude of the charge depends upon the quantity of powder in contact with the
10
8
it
0
0
AIR FLOW
1. sampling nozzle
2. probe tube
3, detector housing
4. excitor tube
5. insulation material
6. suction fan
7. manometer
8. recorder
9. exhaust tube
10. duct wall
Figure 11.19. Layout for sampling equipment based upon contact electrification.
388
OPERATIONAL PRACTICE
excitor tube per unit time. For the monitoring of absolute powder—air
loadings, the system is calibrated for the product in question. This calibration is carried out by introducing known amounts of powder fines into
clean air prior to passage through the excitor tube. The detection of powder
by the excitor tube is very accurate. The probe tube and sampling nozzle ,
must be carefully located in the duct so that flow into the nozzle is representative of overall duct conditions. This technique is developed commercially (147). It is not suitable for sticky powders, or those that form deposits
in the probe or excitor tubes.
(c) Continuous Monitoring of Airborne Powder by the CEGRIT Sampler
The CEGRIT automatic sampling equipment is an easy-to-use monitoring
unit for airborne powders. It requires little personnel attention and provides
data on the fluctuations of powder—air loadings in exhaust ducts throughout
a plant operating day. Sampling through a probe tube and collection of
powder in a cyclone over long time periods is involved. Instantaneous
values of powder loadings cannot be obtained as in other automatic equipment (e.g. based upon light density measurements). The sampler operates
on a wide range of powders, but it is restricted to non-sticky powders,
where there is no likelihood of build-up in the probe tube and cyclone inlet.
The sampler is highly flexible, being operated at most temperatures likely
to be found in spray dryer exhaust systems. The sampler can be mounted
in either horizontal or vertical ducts. The sampler consists of a sampling
tube with nozzle, a highly efficient small-scale cyclone (2 in diameter) with
- collecting jars, and an ejector at the cyclone outlet. The cyclone can be
heated to prevent condensation problems within the powder collection
system. The sampler contains no moving parts, and can be mounted either
indoors or outdoors. It is suitable for sampling in ducts having an underpressure of 1 in (25 mm) WG or more, air velocities up to 70 ft/sec (21 m/sec)
and air temperatures up to 700°F (370°C). The sampler conforms to British
Patent 872 904 and is manufactured under licence (137). Sampling procedure
is based upon isokinetic. measuring techniques. Powder loadings in the
duct are determined by the rate at which the collected powder builds up
in the cyclone jar. The cyclone efficiency exceeds 99 % for most powder sizes
likely to be met. Both the ejector and sampling probe tube can be fixed
in any direction to meet the air flow.
Principle of Operation. Powder laden air is drawn through a sampling
nozzle and probe tube into a cyclone by an ejector device operated by the
suction of the duct itself. Air flow through the cyclone is continuous and
uniform, as resistance to flow remains constant in the absence of product
build-up. The air rate is controlled to give isokinetic sampling conditions.
Once set, isokinetic conditions are maintained irrespective of fluctuations
in air flow along the duct.
OPERATIONAL MEASUREMENTS
389
The sampler is shown in figure 11.20. Various nozzle sizes are available
to cover the range of duct air velocities likely. More than one collecting
jar is required to enable rapid changeover during operation. The pressure
drop over the cyclone is measured by a manometer (0-8 in WG).
The equipment samples at one position in the duct. The equipment is
best located in a straight section where a single central sampling point
represents flow conditions within the duct. For less ideal duct sections a
velocity profile over the duct is determined by a pitot-tube traverse to select
the position representative of the average powder air loading. The probe
tube length is sized so that when the sampler is mounted at the duct, the
sampling nozzle is located at the selected position. From the local air velocity
at the selected position the sampling nozzle is selected using the calibration
chart supplied with the equipment. The air velocity into the nozzle must
equal the air velocity past it, and yet the air flow rate through the cyclone
must be sufficient for efficient powder separation while not exceeding
8 in WG. The sampling rate is read off calibration curves that relate cyclone
pressure drop to pitot tube readings. A temperature correction is usually
involved.
1. sampling nozzle
2. probe tube
3. cyclone
4. powder hopper
5. heating mantle
6. heating control
7. manometer
8. pressure tappings
9. sampling flow control
10. ejector for air flow through cyclone
11. duct wall
Figure 11.20. The CEGRIT sampler for measuring powder loading in air flows.
390
OPERATIONAL. MEASUREMENTS
OPERATIONAL PRACTICE
11.3.3. Determination of Dried Product Losses from Mass Balance Techniques
The use of mass balance techniques to determine the product losses from
the recovery system of a spray dryer is often cited as a method for use where
sampling equipment is not available. Although the required weighing and
tank dipping operations seemingly offer a simple method, this technique
should not be used. Experimental errors involved in the measurement of
tank volumes, feed solid content, sack weighing and product moisture
content, when additive, can be very significant in a calculated loss value.
The source of errors can be in the measurement itself, e.g. the physical
li mitations of reading a scale exactly, or be one of precision, due to inaccuracies of the measuring equipment, e.g. inaccurate weighing balance
or thermometer.
The procedure for relating errors in measured quantities to the error
in the calculated quantity involves the general law for propagation of
errors (148). This law is expressed in the form of a partial differential equation.
For a variable (z) being a function of several directly measured quantities
x n ) is expressed as
(x 1 x 2 ...x n ) the error in z due to errors in (x i x,
Az =
az
ax ,
Ax, +
ez
ax,
Ax, +
+
az x
ax
(11.9)
where Ax ,Ax 2 ... A; may be considered as errors in the measured quantities x 1 x 2 x n (67). Equation (11.9) holds for any types of errors provided
they are small.
The possible errors in using mass balance techniques to determine losses,
where data are obtained by general methods of measurement under dryer
operation, are illustrated in the following example.
EXAMPLE 11.3
A spray 'dryer layout with cyclone separators has a reported excessive
powder loss following visual observation of the exhaust duct. In the absence
of sampling equipment, a mass balance was attempted over the spray dryer
to calculate the powder loss. A tank of stated volume (5000 litres) was set
aside and during the drying of this volume, every care was taken to account
for all the powder produced. The feed contained a solids content of 12.4 %,
45 sacks containing 15 kg powder were collected. 2 kg powder was swept
from the floor around the packing area. The powder moisture content was
7.7 %. What is the maximum percentage error possible in the powder loss
calculated by the mass balance technique?
Now powder accounted for following drying = (15 x 45) + 2 = 677 kg.
The loss is the difference between accounted powder and solids equivalent
fed to dryer.
391
Now equivalent solids is a function of measurable variables, feed volume,
solids contents and density where
ST =
F pFX F 5000 x 1.033 x 0.124
= 694 kg
Xp
0.923
(I )
The accuracy in solids feed to the dryer depends on the accuracy of each
variable. The error can be calculated by estimating the accuracy of the
measured variables and using equation (11,9).
Variable
Approximate value
Estimated
maximum error
5000 litres
1.033 g/cm 3
12.4
92.3 %
15 kg
2 kg
±20 litres
+0002 g/cm 3
±0.1 %
+0-1
±0.1 kg
±01 kg
Feed (F)
Feed specific gravity (p,)
Feed solid content (X F)
Product solid content (Xp)
Bagged powder (P)
Spilt powder (L a )
Error (AS T ) in feed solids is given by applying equation (11.9) to equation (I)
AS T =
OST
aST
DS T
AF + apF. Ap F +
= p,X FAF FX F
Xp
ax AXp
Fp F IKE FpFX F
Xp
1.033 x 0.124
0.923
DS
Xp
+
5000 x 1.033
AXF
0.923
X?)
5000 x 0.124
5000
AP s
0.923
'
5000 x 1.033 x 0-124
AX,
(0.923) 2
= 0.136AF + 673Ap F + 55AX F 7.4AXp
In order to obtain the maximum error, individual errors are summed.
S T(m „) = 0.136(20) + 673(0.002) + 55(0.1) + 7.4(0.1)
=10.3 kg
Now powder loss (L c ) is given by
Lc =
P — Lc
392
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
Applying equation (11.9) the maximum error is the sum of individual
errors
&L omax) = 1AS T + 45AP + 1AL,
= 10.3 + 4.5 + 0.1
14.9 kg
Powder loss calculated from mass balance
L =
5000 x 1.033 x 0.124
0.923
2 — (45 x 15)
= 17 kg
17
Percentage loss = 6
Maximum percentage error =
94 =
2.45 %.
14.9
= 87.5 %.
17
Powder loss calculated by the mass balance technique can be in error by
as much as + 87.5 %.
11.3.4. Reducing Powder Losses from Spray Dryer Exhaust Systems
The efficiency of a powder—air separator is virtually constant under ideal
operational conditions. However, efficiency levels can drop substantially
if dryer operation causes conditions to deviate from normal. Reduction
in efficiency is registered as high powder losses from the spray dryer exhaust
ducts.
Dryers are designed for a given recovery of solids. For products that can
cause damage to surroundings recovery must be 100 % and multiple separator
units in series are used. For non-toxic products that sell cheaply, 99.5 %
recovery often suffices as the cost of equipment operation to recover the
final 0.5 % is more than the value of the powder recovered. Cyclones are
generally used for these products.
For plant layouts where powder loss measurements establish a.loss value
higher than normal, it is unusual if the reason cannot be traced to plant
maloperation or poor equipment maintenance. Excessive losses from
cyclones are due to (a) overloading, (b) finer particle size of powder entering
the cyclone, (c) malfunction of the cyclone powder discharge equipment.
Re (a). This can be caused by : (1) Partial or total blockage of one or more
of a battery of cyclones, thereby overloading the cyclone(s) with clean
inlets. (2) Incorrect drying chamber operating pressure, leading to (for
dryers with primary powder discharge at the chamber base) a larger cycloneborne powder fraction. (3) Sharp reduction in air flow through the cyclone
393
handling a fixed powder quantity. (4) Sharp increase in air flow through the
cyclone leading to excessive turbulence and high re-entrainment losses.
Re (b). This can be caused by: (1) Too fine an atomization. (2) Too low a
feed viscosity. (3) Too low feed solids. (4) Reduced feed rate at constant
atomization conditions. (5) Excessive comminution of powder in pneumatic conveying systems (transport cyclones with air return to main cyclones
inlet).
Re (c). This can be caused by : (1) Blocked or partial blockage of cyclone
base. (2) Leakage of air into cyclone base via powder discharge equipment.
(3) Incorrectly adjusted Vortex air locks (see chapter 12).
For conditions of excessive powder loss, the feed, atomization, chamber
and cyclone conditions must be investigated. Order of investigation could
be as follows :
(a) Visually inspect cyclone for cleanliness. If deposits exist in inlet or outlet
ducts or cyclone sides, the cyclone must be coMpletely cleaned.
(b) Measure air flow through the cyclone by AP or other suitable methods.
If air flow is not as specified adjust air dampers to achieve correct flow.
(c) Measure ratio of powder leaving chamber and cyclone and determine
whether cyclone is overloaded. If overloaded, check (1) chamber pressure
(if low, re-establish correct pressure by adjustment of inlet/outlet dampers)
and (2) particle size distribution, bulk density, and moisture content of
powder at the point of bagging-off. If powder is finer, control feed solids,
feed rate, atomization conditions. If bulk density or moisture content is
lower, check for any increase in inlet and outlet drying temperatures.
If no real deviations are found during the above investigation, the fault
must lie at the cyclone base with excessive air leakage into the cyclone.
Check the clearance of rotors, rotor rotation in rotary valves or the pressure
drop over Vortex air locks, or correct seating of flap valves (i.e. according
to applicable equipment).
Excessive losses from bag filters are invariably due to broken or ill-fitting
bags, and for scrubbers insufficient water supply.
11.4. Assessing Acoustic Environment (Noise Levels)
A spray dryer being industrial processing equipment invariably produces
noise which, depending upon the sensitivity of the recipient can be distracting and annoying through interference with speech or wanted sounds.
Individual equipment items (fans, atomizers, pumps, pretreater, product
handling equipment) can produce noise above acceptable levels, but due
to flexibility in equipment layout, overall levels in operator working areas
need not be above safe working levels. Equipment items rarely emit noise
levels to cause temporary or permanent hearing damage to personnel in
OPERATIONAL PRACTICE
OPERATIONAL MEASUREMENTS
close vicinity for short periods. Noise levels around spray drying plants
can be contained to comfortable limits by good building and equipment
layout, correct equipment operation and regular equipment maintenance.
In the past few years, the subject of noise, and noise control has been
receiving increasing publicity (504). The resulting public interest has turned
to concern, with pressure now being applied to authorities for legislation
to limit noise. From the operators viewpoint, it is becoming necessary for
owners of spray drying equipment to measure and limit noise levels. Two
categories of noise must be dealt with : (a) External noise, radiated from a
dryer installation to nearby residential areas giving rise to complaints from
local property owners, and possibly infringing on local regulations, particularly at night. Particular care must be taken in locating open or semiopen plant. (b) Internal noise within plant working areas. Such noise levels
can affect plant personnel, and exceed permitted limits. With noise consciousness becoming more recognised, acoustic engineering (149) (150) is
now being applied in the design of spray drying installations..
11.4.1. Basic Nomenclature in Acoustics
Decibel (dB). The decibel (dB) is a logarithmic unit, and is defined in terms
of a pressure ratio. Strictly speaking the decibel is ten times the logarithm
of the ratio of two sound powers. Sound power is proportional to the square
of sound pressure (P) and hence
2
PI = 20 log , ( -1 )
dB = 10 log in ( —)
ic
Po
Po
(11.10)
where P0 is a reference sound level, P 1 is the actual pressure. The reference
level (P0 ) corresponding to 0 dB is taken as the pressure for the threshold
of hearing (0.0002 dynes/cm 2 or 0.0002 //bar (microbar).
Sound Pressure Level (SPL). A noise meter responds to sound pressure
variations giving an SPL reading normally in decibels. An SPL reading
does not completely represent the noise characteristics that are causing
the noise nuisance, since the human ear has a different response to different
frequencies. Noises of a given SPL value will sound quite different to a
human observer if they are at different frequencies.
Noise Rating Number (N or NR). This is a number to denote acoustic
environment. Each N or NR number represents a curve of sound pressure
levels (dB) plotted against frequency in cps (Hz) (figure 11.21). From the
slope of the curve, it can be seen that for a given N number a greater SPL
is acceptable at lower frequencies than at the higher ranges of frequencies
for the permitted acoustic environment not to be exceeded.
11.4.2. Assessing the Acoustic Environment for Safe Working
For the given working area, a noise level meter and frequency analyser is
used to measure noise and frequency. The measured data are plotted on the
395
110
SOUND PRESSURE LEVEL ( SPL } ( d B I
394
100
90
80
EQUIPMENT CREATING
70
AN ACOUSTIC ENVIRONMENT FOR SAFE WORKING
60
250
500
FREQUENCY
1000
cps
2000
4000
8000
REF 0.0002 MICROBAR
Figure 11.21, Sound pressure level against frequency plot for assessing acoustic environment of
equipment,
N-curve for permissible environmental conditions (figure 11.21). If the
curve plotted from measured data breaks the N-curve at any point, the
permissible conditions are exceeded.
The permissible N-curve depends upon the location area and plant
items. For internal noise in working areas, an N 85 level is generally agreed
by medical specialists and health authorities to be acceptable for personnel
working in the environment for eight hours per day. The N 85 level also
means that working near machinery exceeding this level has to be regulated,
and personnel be advised to wear ear protection. Alternatively, acoustic
attenuation can be applied to the machinery to bring the noise level below
N 85 when measured in working areas. The distance between machine and
point of measurement depends upon local codes of practice.
During measurement of noise from a single item of machinery, the effect
of background noise must be considered. During noise testing it is often
difficult or impossible to take tests with just the one noisy machine in operation. It is usual to take a reading with the machine first on, and then off
to obtain a background noise measurement. If the background noise is
10 dB lower in all frequencies than the figures in the first reading, then the
effect of the background can be .ignored. Should the background levels be
within less than 10 dB, then to obtain the effect of the noise source alone,
the two readings must be combined. Special tables and graphs must be
used for combining levels, since due to the logarithmic nature of dB figures,
direct addition and subtraction is not valid. (Consult Handbook on Acoustic
Engineering for tables and graphs) (151).
396
OPERATIONAL PRACTICE
11.4.3. Likely Areas having Noise Problems
(a) Small Rooms (Especially small rooms with hard floors and tiled walls)
It is common for hygienic reasons to place specialized equipment in individual small rooms with hard floors and tiled walls. Such arrangements
lead to rooms being acoustically live. Multiple reflections occur leading
to high reverberant sound fields. In this situation, use of acoustic tile on
wall and floors is of little value where only marginal noise reduction is
gained (2-3 dB at some frequencies). In such circumstances the necessity
of equipment isolation in a small room for hygienic reasons must be judged
against the working environment caused by the equipment, if noisy.
(b) Vicinity of Fans
Fan noise is a function of pressure development, and with the tendency to
increase working pressure there is consequently higher noise output. For
a given pressure requirement, there is not a significant difference between
different types or makes. When the effect of fan noise on the environment is
crucial, three approaches can be taken. (a) Use two fans working in series
instead of one, thus halving the pressure development load, and operating
at lower speed (multi-stage fans are not suitable where high powder loadings
are expected). (b) Arranging duct layout so that the fan can be mounted in a
separate room isolated from general working area. (c) Providing an acoustic
enclosure around the fan and motor, and ensuring fan support is stiff and
anti-vibration pads are used. Provision for heat disposal must be provided
in any enclosure.
(c) Buildings
For a spray drying plant housed in a well-made building having closefitting doors and windows, the external noise levels created by plant operation will be very low. The building will act as a large acoustic enclosure,
and stops effectively high frequency noise. Internal noise levels will depend
upon the noise generated from each equipment item, and the degree of
noise reflection at the building walls. A lightweight building is subject to
greater noise levels through transmittance of equipment vibration through
the structure. If structural panels are subjected to transferred machinery
vibration, the panels in turn will vibrate at their own natural frequency.
Large vibrating surfaces produce low frequency noise, which adds to the
general noise level in the plant. To prevent equipment vibrations being
transferred to the building structure, anti-vibration dampers and flexible
pipe and duct connections become standard fittings.
(d) Exhaust Ducts
Acoustic energy as exhaust fan noise leaks through the building shell and
passes to atmosphere via the exhaust duct. This leakage can cause a nuisance
as fan noise is beamed over areas adjacent to the plant. Such sound emission
covers a wide band of frequencies, and contains discreet low frequency
OPERATIONAL MEASUREMENTS
397
tones that are specially annoying to noise sensitive people. For exhaust
fans (centrifugal type) there is usually a strong low frequency component
of noise caused by the passage of each impeller blade passing the fan cut-off
or throat plate. This low frequency disturbing note is often at about 250Hz.
The actual value depends upon fan speed and the number of blades on the
impeller.
In theory, noise problems associated with exhaust duct operations can be
overcome by attenuation. However, in practice low frequency components
of noise are difficult to attenuate as the majority of barrier and absorbent
materials have a better noise reduction performance at higher frequencies
than they do at frequencies in the range 250 Hz. The choice of materials is
also limited due to the air conditions in the exhaust duct. Materials must
withstand high temperatures (up to 250°F (120°C)), high humidity (even dew
point conditions), often powder-laden air and corrosion (particularly if
mounted outdoors). The presence of powder-laden air leads to eventual
build-up on attenuation materials. Many acoustic materials are cellular
and prone to powder penetration. Cleaning becomes difficult. A most
effective noise barrier that is readily' washable is merely a concrete mass,
but excessive weight is non-compatible with lightweight building structures
used to house spray dryers nowadays.
Attenuators for exhaust fans are generally mounted on the building roof
although they can be mounted within the building subject to good access.
The construction materials are comprised of those having good noise
Table 11.4.
Measurement of
Available methods
Droplet size distribution of sprays
General (35) (152)
I mpaction coated slides (153) (154) (155) (156)
Sampling cells (26) (57) (63) (157) (158) (159)
(160) (161) (162)
Spray solidification (56) (163)
Spray freezing (8) (164) (165)
Photographic (166)
Light scattering (4)
Particle size distribution of dried
product
General (167)
Preparing samples (168)
Sieving (4) (169) (170) BS 1796 (1952)
Microscopy (4) (171)
Air elutriation (172) BS 3406 (1962), part 3
Sedimentation (4) BS 3406 (1963), part 2
Coulter counter (173)
398
OPERATIONAL PRACTICE
reduction performance and those with reduced performance but can withstand the operating conditions. Cellular materials are used with the attenuator generally lined with a washable surface (e.g. asbestos, plastics). Often
an attenuator is designed to act as a gravity settler also (chapter 12).
11.5. Measurement of Droplet and Particle Size
Data on particle size of dried products are invariably essential for control
of product quality. However, particle size is not always representative of
the wet droplet size on atomization. When selecting and testing, atomizers,
methods for droplet size determination are required. Full details of measurements are given in the literature. Notable references are given in table 11.4.
Section IV
SURVEY OF AUXILIARY EQUIPMENT
12 Survey of Auxiliary Equipment
12.1. Air Heaters
Five types of air heaters are available to heat drying air in spray dryers.
(1) Indirect steam air heater.
(2) Indirect oil or gas air heater.
(3) Direct oil or gas air heater.
(4) Electric air heater.
(5) Liquid phase air heater.
Direct air heaters are used where spray dried material can be contacted
with products of combustion. Indirect heaters are used where contact
with combustion products cannot be tolerated.
12.1.1. Steam Air Heaters
Steam air heaters are used in spray drying where inlet air temperatures,
up to and around 400°F (200°C) are required and steam is available at
pressures reaching 350 psig (25 atm). Hence steam air heating is widely
used in the food and dairy industries where steam supplies are essential for
evaporators and associated equipment. The drying air temperature depends
upon the steam pressure (figure 12.1) and is approximately 9-13°F (5-7°C)
lower than the steam temperature. As superheated steam is not ideal from
a heating viewpoint, dry saturated steam is used with condensate leaving
heater at 170°F (80°C).
Air heaters are constructed from rows of extended fin coils housed in an
insulated metal case. The heater is usually mounted on the discharge side
of the supply air fan. The first few rows at the air entry end are arranged to
act as a condensate section.
Heat load is calculated from the weight and specific heat of the air and
temperature rise. Steam consumption is calculated knowing the total heat
content of steam on entry and exit. Heater size depends upon the heat
402
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
130
9
120
110
100
7
90
6
80
5
70
I
50
v
a: 50
3
30
2
20
10
100
120
212
250
1 60
140
300
180DC
350'F
Figure 12.1. Relation between saturated steam pressure and temperature (steam pressure as
ordinate),
transfer properties of the tubes and fins, i.e. materials of construction.
For an air velocity of 1000 ft/min (5 m/sec) over the tubes, heat transfer
coefficient values of the order 10.5 BTU/°F hr ft' (50 Kcal/°C hr m 2 ) are
attained. A common form of tube construction is to use steel tubes with
wound-on steel fins galvanized externally, the tube bore being 0.55 in (14 mm),
and the overall diameter of the crimped fin 1.4 in (35 mm). Between 20 and
30 rows of tubes would be used to give an air exit temperature of 400°F
(200°C) and these would be grouped in blocks of 2 or 3 rows with the tubes
403
welded into top and bottom steel channel headers. An air velocity of
1000 ft/min (5 m/sec) over a heater of this type would give an air pressure
drop of 3 in WG (75 mm).
Steam air heaters are relatively cheap, but there are general maintenance
problems concerning corrosion, tube distortion and steam leakage. Correct
steam trapping and feed water treatment are required for a long heater
working life. It is essential to remove condensate as fast as it is formed.
Steam traps of the float or thermostatic type are used in conjunction with
strainers and provision for air release. Feed water treatment recommended
by boiler manufacturers should always be followed, but in addition the
condensate from the heater should be checked to ensure that it is not chemically aggressive. Corrosion resistant materials are available from tube
manufacture and stainless steel or cupro nickel are often used particularly
in the condensate section. The cost of using these materials must be considered against the cost of a heater made from steel and supplied with a
spare section, which can be used as replacements should corrosion take
place. Heaters can be manufactured so that replacements can be made with
the minimum of down time.
Positioning the heater is important to ensure that air flow on and off
is such that dead spots do not occur. No bends should be made close to
the heater and change in shape of the section ducting should be gradual.
The heater should be raised from ground level so that condensate piping
can be sloped to ensure rapid draining of water.
Various arrangements of control and steam trapping (174) are used on
steam heaters. One method of temperature control is to locate a thermostat
in the air supply duct which acts on a modulating valve in the steam line.
To obtain smooth temperature control, only a small part of the heater is
regulated, the main heater sections then operate on a constant load.
The ideal system for steam trapping is for each block of two or three
rows to have its own trap and for condensate to be collected in a flash
vessel and then taken to the condensate section.
However, it is possible to use group trapping of sections where the load
is steady and then allow the condensate to pass to a section, which acts
partly as a flash steam and partly as a condensate section. This gives a less
complicated piping layout which saves space and cost.
Apart from temperature indicating instruments, it is usual to fit a pressure
gauge on the steam line after the control valve so that a check can be made
on the steam condition at point of entry to the heater. A sight glass after
a steam trap will enable an operator to see if condensate is flowing. The use
of strainers placed before a steam trap or built into the trap will prevent
dirt from blocking up the trap. Strainers must be changed or cleaned at
regular intervals.
404
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
Operational data of steam air heaters can be applied to calculate the
air flow passing to the drying chamber. From heat balance over the heater
the air flow (L a ) is given by
Al(H
s
=
Id
a
—
H)
(12.1)
rl
—
where M c = condensate (steam) rate, H s = enthalpy of steam at heater
inlet,
enthalpy of condensate, n = heater efficiency (98-99 %).
A steam air heater mounted at the dryer roof of a co-current flow dryer
with rotary atomization is shown in figure 12.2(b).
405
removable for tube access for cleaning. Heaters are built in a range of stan6
dard sizes ranging from heat loads of 4 x 10 5 -10 7 BTU/hr (10 5 -2.5 x 10
Kcal/hr). Heater size limits higher loadings. A maximum air temperature
leaving this type of heater is approximately 750°F (400°C). The efficiency
of the heater depends upon heat load and air temperature. A guide to
efficiency is given in table 12.1. An indirect heater is shown in figure 12.2(a).
Table 12.1. Efficiencies of Indirect Fuel
Oil Air Heaters
Air temperature
To
Efficiency*
350-500°F
(175-250°C)
525-610°F
(275-325°C)
660-750°F
(350-400°C)
80-85
75-80
70-75
* Dependent on heat load.
(a)
(b)
Figure 12.2. Indirect air heaters. - (a) Indirect oil fired air heater. (The combustion fan is to the
left of the picture and the servomotor which controls the quantity of oil and air for combustion
is placed above it.) (b) Indirect steam air heater. (Mounted at air disperser level. Steam traps
and condensate piping are shown at the base.) (By courtesy of Niro Atomizer.)
12.1.2. Fuel Oil Air Heaters
In indirect fuel oil heaters, drying air and combustion gases have separate
flow passages. The combustion gases pass over or through tubes that act
as the heat transfer surface for the drying air. The heater consists of radiant
and convection sections. The combustion chamber is fabricated in heat
resistant steel. Galvanized tube is used to handle the combustion gas flow.
The body of the heater is fabricated in standard steels. The heater end is
Direct fuel oil air heaters consist of a combustion chamber that is lined
with refractory, and a mixing chamber. The combustion nozzle assembly
is mounted in the combustion chamber. The heater can be either horizontally
or vertically mounted. Heaters are built in standard sizes with heat loadings
4
6
from 2 x 10 5 -2 x 10 7 BTU/hr (5 x 10 -5 x 10 Kcal/hr). Heaters for
higher heat loadings are available, but outside standard sizes they are
expensive. Drying air temperatures between 400-1100°F (200-600°C) can be
obtained with fuel oil combustion. The cleanliness of the combustion gases
depend much upon the quality of the fuel oil. For low sulphur oils, the gases
can be so clean to enable use of direct air heater designs with such white
products as titanium dioxide (without discolouration) or coffee (with aroma
defect). The efficiency of direct fuel oil air heaters is of the order 95-98 %.
Economic operation of fuel oil air heaters requires the attaining of high
combustion rates with low percentage of excess air, avoidance of black
smoke and long refractory life. The conditions are fulfilled if (a) atomization
of fuel oil is complete, (b) mixing of oil spray and air is intimate and (c) oil
flame does not contact refractory within the heater.
Operational data can be applied to calculate the air flow passing to the
drying chamber. From the calorific value (Q a ,) of the fuel and fuel combustion
rate (W), the air flow (L a ) is given by
La
WQ c ,
Cp(To
T)'1
(12.2)
406
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
12.1.3. Gas Air Heater
In gas air heaters, the gas burner normally operates on the aerating principle.
Gas is mixed with part or all of the air required for combustion. The gas
flows from a jet at high velocity causing a reduction of pressure around the
jet into which combustion air readily flows to give intimate mixing between
gas and air. Often this mixing section can be in the form of a venturi, but
many types of efficient mixing forms are offered by manufacturers of gas
burners. Gas mixtures with air have limits of inflammability, above and
below which combustion cannot be sustained. For example, natural gas
mixtures must contain between 5 and 14 % oxygen by volume. The whole
field of gas combustion has been well investigated and further details are to
be found in textbooks on fuel technology (175).
The construction and performance of indirect gas air heaters is similar
to indirect fuel oil air heaters. Direct gas air heaters are less complicated
in construction, consisting of a cylindrical combustion chamber of insulated
heat resistant steel. Heaters are built in standard sizes for heat loads covering
the range 4 x 10 5 -2.5 x 10' BTU/hr (10 5 -6 x 10 6 Kcal/hr). Drying air
temperatures of 1500°F (800°C) can be readily attained, and this temperature
range covers most high temperature spray dryer operations. Subject to
satisfactory refractories, heaters can be designed to reach air temperatures
in the neighbourhood of the gas flame temperature 3600°F (2000°C).
Compared with other types of direct air heaters, gas fired are cheaper to
fabricate. They are being used more and more in spray dryer layouts due to
the increasing availability of natural gas.
Apart from complete gas air heater units, simply open ring-jet burners
can be mounted after an indirect air heater (steam) to boost drying air
temperature, or installed as the sole direct heating source in spray dryers of
medium air flow rate. With correct gas jet selection for the inlet duct air
velocity, combustion can be so clean to enable use in drying systems involving
coloured or aromatic products.
Efficiencies of gas air heaters are similar to those of fuel oil air heaters. To
calculate the air flow rate through the heater, equation (12.2) can be used
where Q c , is the calorific value of the gas.
12.1.4. Electric Air Heaters
Electric air heaters are common on laboratory and pilot-plant spray dryers.
The heater is cheap to build but expensive in operation. Air temperatures
up to 750°F (400°C) are attainable in practice.
12.1.5. Liquid Phase Air Heaters (Thermal Fluid Heaters)
Liquid phase air heaters are a relatively new method of air heating on spray
dryers. It is used in closed cycle drying systems where temperatures up to
750°F (400°C) are required in inert gas—organic vapour mixtures that constitute the drying gas. The system is also used in medium size spray dryers
407
where inlet drying air temperatures of 400°F (200°C) are required, but where
steam for heating is not available. The heater system consists of a special
oil or heat transfer fluid circulating at high speed through a boiler and air,
heater. The heater consists of rows of finned tube through which the fluid
flows. The boiler can be . gas or fuel oil fired. The main advantage of liquid
phase heating is the open pressureless system, capable of attaining relatively
high air temperatures. Heaters are available with heat loads up to 1.6 x 10 7
BTU/hr (4 x 10 6 Kcal/hr).
12.2. Fans
Spray drying is an operation that requires movement of air or other gaseous
media. This movement through the dryer is carried out by centrifugal fans.
One or two main fans are used in a spray dryer layout. For dryers that
operate with a single exhaust fan, the drying chamber is under a high negative
pressure. The majority of spray dryer designs, however, operate with the
two main fan system. Apart from the exhaust fan, a supply fan is mounted
on the inlet side of the drying chamber. This fan forces the inlet drying air
through the air heater and air disperser. By using the two fan system,
greater flexibility of chamber pressure operation is obtained. The supply fan
is sized to balance the exhaust fan, and the chamber can operate at near
atmospheric pressure if required. The operating chamber pressure often
controls the powder—air loading in the exhaust air leaving the chamber.
ONE -FAN SYSTEM
usually small dryers)
TWO FAN SYSTEM
Figure 12.3. One and two fan spray dryer layouts.
408
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
With a two fan system, the chamber pressure can be adjusted to give the
optimum powder—air exhaust loading for highest collection efficiency in
the powder recovery unit. One and two fan systems are shown in figure 12.3.
Only the principles of fan engineering are referred to here. For a detailed
account of the subject, the reader is referred elsewhere (176) (177).
12.2.1. Fan Requirements on a Spray Dryer Layout
(a) Exhaust Air Fan, always essential and mounted in the exhaust system
of the spray dryer. It is good practice to locate the fan after the powder
recovery unit, so that the fan operates on clean, powder-free air. The fan is
the largest on the unit, as it has to overcome the largest pressure losses of
the layout, namely over the powder recovery system.
(b) Supply Air Fan, mounted in the inlet duct system to the drying chamber.
It is mounted before the heater and thus runs with cool air. The fan supplies
drying air to the chamber. It is smaller in size than the exhaust fan and as the
pressure requirement is lower, it will use less power. The supply fan is sized
in relation to the exhaust fan, so that air flow through the dryer and the
drying chamber pressure is controllable.
(c) Transport Air Fan. A transport air fan is required for dryer installations
where product is conveyed pneumatically away from the chamber base and
powder recovery system to the product bagging-off or storage areas (silos).
The fan is sized to produce an air flow whose air velocity through the duct
will maintain powder conveying. A transport cyclone is invariably incorporated in the system to separate out the powder, and the fan is mounted
after the cyclone so as to operate on air of very low powder loading.
(d) Cooling Fans. These can be many and varied in application. Fans are
used to cool potential hot spots in the chamber structure, or prevent the
rise of temperature in certain dryer components. Cooling fans normally
require small horsepower drives.
Typical cooling fans used are (i) atomizer cooling, (ii) air disperser cooling,
(iii) duct cooling, (iv) chamber wall and roof cooling.
(e) Air Heater Fans. These include the fans to meet requirements for combustion air and flue gas exhaust on oil and gas fired installations.
12.2.2. Fan Parts and Principles
The essential parts of a centrifugal fan are (a) a rotating impeller, (b) its
housing (usually spiral), (c) impeller shaft and bearings, (d) inlet and outlet
flanges or spigots, (e) a sturdy base plate.
These parts are shown in figure 12.4. There are variations in impeller
design, each depending upOn how the blades are mounted to form the
impeller. Radial, forward curving or backward curving types are available.
The forward curving blade is concave in the direction of rotation. The
backward curving blade is convex in the direction of rotation. The blade
design defines the fan characteristics. Each type is never 100 % efficient
409
in operation due to air movement through the impeller, air, skin and bearing
frictions. The number of blades forming the impeller has a greater effect
on the fan volume—speed characteristics than on the fan efficiency.
IMPELLER
SHAFT
AND
BEARINGS
OUT LET
AIR FLANGE
INLET AIR FLANGE
or
SPIGOT
ROTATING
IMPELLER
(2 blades
IMPELLER --HOUSING
STURDY BASE PLATE
Figure 12,4. Principle parts of a centrifugal fan,
The pressure developed by a fan results from the centrifugal forces of the
air enclosed between successive blades, and the energy of the air leaving the
blade tip. The pressure developed by the fan depends upon the blade design.
At constant fan speed the backward curving blade produces the least
pressure, the forward curving blade the most. This can be seen from the
vector diagrams in figure 12.5.
The backward curving blade is used extensively in spray dryer applications.
It meets the requirement of high volume air movement at medium/low
pressures. Backward curving blades can be formed to achieve 80 % total
efficiencies (75 % static efficiencies), but although the blade has self-cleaning
properties, handling sticky entrained powder can cause deposit formation
on the back face of the blade. The optimum blade profile can be used for
supply fans as filtered air is handled and also for the exhaust fan if installed
after a high efficiency product recovery separator, e.g. bag filter. For transport fan requirements, where higher powder—air loadings are likely, use
of the optimum blade curve can result in deposit problems especially for
conveyed powder with stickiness tendencies. A compromise is often made
in the blade design. A blade profile that lies between the backward curving
and radial types is adopted to give better self-cleaning characteristics. A
reduction in fan efficiency results, the total efficiency of the modified blade
decreasing to 70 % (static efficiency = 65 ).
In many spray dryer installations, the modified blade design is used on all
fans throughout the plant thereby obtaining savings in costs through
standardization. However, in cases where total power requirement of the
plant is strictly specified, the optimum backward curving blade profile
can be used on all fans handling clean air. Only where high powder loadings
are expected, will the modified self-cleaning profile be adopted.
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
12.2.3. Fan Characteristics
Once a fan is installed in a given ducting system, the operating characteristics can be deduced by plotting the system characteristics on the performance curves. .These characteristics depend upon flow resistances or the
pressure required to deliver air through the system at a desired rate. The
system characteristic is therefore plotted with air flow rate as abscissa
and pressure as ordinate. Fan operation conditions are represented where
the system characteristic curve intersects the fan pressure curve.
EXAMPLE 12.1
A fan with backward curving blades operates at 1300 rev/min and 300 mm
total pressure drop and delivers 42 000 m 3 /hr of air. The fan horsepower
requirement is 65 HP. The system characteristics change and the pressure
falls to 200 mm at 42 000 m 3 /hr. Use the performance curves in figure 12.6
to determine the new fan operating conditions.
Fan characteristics are dependent upon the design of blade arrangement.
For a given fan design, the primary characteristics are represented on fan
performance curves, where pressure and horsepower are plotted against
air flow rate at a fixed impeller speed and air density. The difference in the
form of the performance curves for radial, forward and backward, curving
blades are shown in figure 12.5.
R
a . RADIAL BLADES
b. FORWARD CURVING
BLADES
0
LEI
Lc
m
w
m
0
a-
AIR FLOW
c BACKWARD CURVING
BLADES
t
PRESSURE —.
V
1
411
c.r)
z
z
— 300 P
1
▪ 250 P
▪
200 2
69
65
cc
cr)
Li)
1./1
cc
Z:13MOd 3S8OH
410
0
42000 47000
3
AIR FLOW —*
AIR FLOW —"-
Figure 12.5. Performance curves characteristics for radial, forward curving and backward
curving blades (for constant fan speed and air density).
For forward curving blades, inspection of figure 12.5(b) shows a characteristic not favourable to spray drying plant. Any change in the system
pressure that increases air volume flow can cause a marked increase in
horsepower requirement Thus power overload cover is required due to the
rapid rise of power for relatively small increases in air flow. The backward
curving blade, on the contrary, features power limiting characteristics
(figure 12.5(c)), and therefore is a good choice for spray drying plant.
Performance curves of industrial fans often include more data than is
shown in the simplified curves of figure 12.5. Fan efficiency and fan size are
also represented.
AIR FLOW RATE M /hr La
Figure 12.6. Fan performance chart for example 12.1.
Initial system characteristic is represented by line A. Line A intersection
with operation curve (N = 1300 rev/min) gives initial fan operating conditions, i.e. P 1 = 300 mm WG, (L a ), = 42 000 m 3 /hr, (FHP) 1 = 65 HP. The
system characteristic curve alters to form line B denoting a pressure decrease
to 200 mm. Reading from the curves, the new operating conditions are
(La)2 = 47 000 m 3 /hr, (FHP) 2 = 69 HP and a new total pressure of 250 mm
WO at 47 000 m 3 /hr with a constant fan speed of 1300 rev/min.
12.2.4. Fan Laws
All fan performance changes are governed by laws, which are independent
of the design of fan. They apply to a homologous series and a fixed point of
rating.
412
SURVEY OF AUXILIARY EQUIPMENT
(a) Law I. For increase or decrease in fan speed for a constant air density
and given fan design and size.
L a cc N: Volume flow and mass flow (L a ) vary as the fan speed (N).
P oc 1\1 2 : Pressure (11 varies as the square of fan speed.
HP oc 1\1 3 : Power (HP) varies as the cube of fan speed.
(b) Law II. For changes in fan size where geometrically similar fans are
involved, but fan blade tip velocity remains constant.
L a cc d 2 : Volume flow and mass flow vary as the square of the impeller
diameter (d).
P = K: Pressure remains constant.
HP cc d 2 : Power varies as the square of the impeller diameter.
(c) Law III. For changes in air density (p a).
L a = K: Volume flow remains constant (mass flow varies as the air
density).
P cc p a : Pressure varies as the air density.
HP cc p a : Power varies as the air density.
From the above laws, useful working relations can be developed.
(d) If the fan size is varied, but the fan speed is kept constant.
La cc d 3 : Flow varies as the cube of impeller diameter.
Pressure varies as the square of impeller diameter.
P cc d 2 :
HP oc d l i 5 : Power varies as the 1/5th power of impeller diameter.
(e) If the fan size is varied together with the fan speed.
La cc d 3 (N 1 /N 2 ): Flow varies as the cube of diameter and fan speed
ratio (N 1 /N 2 ).
P oc d 2 ( N1/N2) 2 : Pressure varies as the square of diameter and the square
of the fan speed ratio.
HP oc d 5 (N1IN2) 3 : Power varies as the diameter to the power 5 and the
cube of the fan speed ratio.
Use of the relations are illustrated in examples 12.2, 12.3 and 12.4.
EXAMPLE 12.2
An exhaust fan draws 40 000 kg/hr of air (80°C) through a cyclone system.
The static pressure drop over the system is 225 mm WG and the fan horse
power is 60 HP. It is planned to increase the dryer capacity by increasing
the air flow by 5 %. If the fan currently runs at 1200 rev/min, what new fan
speed is required? Predict the increase in pressure drop and power requirement.
N 1 = 1200 rev/min
SURVEY OF AUXILIARY EQUIPMENT
413
Using law I
N2
42 000
1260 rev/min
= 1200 40 000 =
1260) 2
P2 = 225(-12w3 = 248 mm WG
126
13
(FHP) 60 ( 1"06 = 69.5 HP
Increase in static pressure = 23 mm WG
Increase in FHP = 9.5 HP
Required speed = 1260 rev/min
12.3
If a fan produces 40 000 m 3 /hr in a given system, but is replaced by a geometrically similar fan whose impeller is 10% larger, what is the horsepower
requirement and pressure drop if the replacement fan is rotated at a speed
to give the same blade peripheral velocity? The original fan required 60 HP
and the static pressure drop was 225 mm WG.
EXAMPLE
d 2 = 1.1d 1
(L a) 1 = 40 000 m 3 /hr
Using law II
1.1 2
(L a ) 2 = 40 000(
1.0
) = 48 400 m 3 /hr
P2 = P„i.e. pressure remains constant
2
(FHP) 2 = 60( 1.1 ) = 72.6 HP
1.0
EXAMPLE 12.4
A fan rotating at 1320 rev/min draws 44 000 m 3 /hr of air at 80°C. Condition
changes- occur so that air flows through the fan at 50°C. What is the air
flow through the fan at the reduced temperature. Calculate static pressure
increase and fan power increase, if for the original condition P 1 = 270 mm
WG and 75 HP.
(L a) i = 40 000 m 3 /hr
Density of air at 80°C = 1 kg/m
(L a ) 2 = 42 000 m 3 /hr
Density of air at 50°C = 1.09 kg/m3
414
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
Using law III
I
I
.
1
1
1
.
415
1
WORKING TEMPERATURE 100°F 3 13° C 1
For new conditions, the volume flow remains the same. The total mass
flow increases as 44 000 m 3 /hr at 50°C --- 48 000 kg/hr.
160
1.09
P2 = 270
1.0
= 294 mm WG
1.09
1 0 = 81.8 HP
(FHP) 2 = 75 12.2.5. Fan Efficiency
Fan efficiencies of 100 % are never obtainable and the horsepower supplied
to the shaft of a fan is greater than the theoretical horsepower requirements
to move the air through a given system.
Fan efficiency (ri) is given by the ratio
Ls
Ls
a
10
z
fan output or theoretical horsepower
x 100
shaft horsepower (FHP)
The efficiency of a given fan depends on its point of rating and this varies
with air flow and pressure which in turn depends upon temperature and
altitude.
Efficiencies can be stated either as a total efficiency (based upon total
pressure) or as a static efficiency (based upon static pressure) and horsepower input. Static efficiencies are generally lower than total efficiencies
as illustrated in table 12.2.
Table 12.2. Typical Fan Efficiencies
Fan efficiency (%)
Fan blade design
Backward curving blade
Modified backward curving
blade with self cleaning
properties
Radial blade
Total pressure
basis
Static pressure
basis
80
75
70
60
65
55
Loss of efficiency is caused by friction and turbulence within the fan.
Further losses occur due to friction in bearings and glands, and drive losses
must be covered. In considering overall efficiency the electric motor efficiency must be known. The relation between motor horsepower and fan
horsepower is shown in figure 12,7.
10
100
INSTALLED FAN MOTOR HORSEPOWER
Figure 12.7. Relation between fan horsepower and installed fan motor horsepower.
In order to cover these losses and give a safety factor it is usual to select
a motor of a standard frame size approximately 15 % up on fan horsepower.
The horsepower values in figure 12.7 are based upon an average working
temperature of 100°F (38°C). Where temperatures are lower, cold start
conditions will demand higher horsepower and sufficient installed horsepower must be available. However, for warmer working conditions, less
power will be required. Fan horsepower values are reduced by 5 % for
temperatures at 115°F (46°C), while at 140°F (60°C) 24 % decrease is evident.
12.2.6. Fan Horsepower
The fan consumption in horsepower (HP) can be expressed in terms of the
air flow (L a ) and fan pressure (P). Horsepower can be based on either static
(Ps ) or total pressure (P T ). When efficiency is taken into account then the
figure will be the same in both cases.
L a PT
(HP) T 6356
(I P)s
= LaPs
6356
or 0.0001571,,PT
(12.3a)
or 0.000157L a Ps
(12.3b)
416
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
at 68°F (20°C) and barometer 30.00 in Hg. Weight of air at this condition
is 0.075 lb/ft 3 for a relative humidity of 62 % [i.e. B.S.848, part 1(1963)].
Where L. = air flow (cfm), P = fan pressure (in WG).
Incorporating the fan efficiency, it follows from equations (12.3a) and •
(12.3b) that the fan horsepower (FHP) is given by
The following results were obtained
Psord . 0 = 270 mm WG
PT(inlet) = — 250 mm WG
+ 10 mm WG
PS(oullet) =
FHPtotal pressure = (
L.Pr
L a PS
FHPstaiie pressure = (
6356gs
Horsepower is inversely proportional to absolute temperature.
EXAMPLE 12.5
Calculate the necessary installed motor horsepower for an exhaust fan
moving air at a rate of 113 500 lbs/hr at 176°F (80°C). The static pressure
requirements for the fan is 81 in WG. The barometer stands at 3000 in Hg.
The fan static efficiency can be taken as 70 %.
Solution based upon equation (12.3).
Wt of air =
30— 10 = 20 mm WG
1/2
Velocity = [2 x 9.81 x 20 Pwater
1000 Pair
= PD = PT — Ps
= 1891 lb/min
Now
36
1891
x
= 31 000 cfm
Air volume handled by fan —
0.075
528
at all temperatures
Static pressure at 68°F = (8.5) x
Fan horsepower =
If the fan horsepower is 67 HP, and the inlet and the outlet ducts are 900 mm
calculate
1. static and total pressure heads produced by the fan,
2. air flow rate,
3. efficiency based upon static pressure.
1. Static pressure produced by the fan = 10 — ( — (250)) = 260 mm WG,
Total pressure produced by the fan = 30 — ( 250) = 280 mm WG.
2. Air velocity given by r2gH1 112 , where H' is the velocity head (metres of
air)
113 500
60
+ 30 mm WG
PT(outlet) =
6356riT
Pwater
0.7
(at 80°C) = 10 3
Pair
636
= 10.6 in WG
5 28
31 000 x 10-6 x 0.000157
u = 19.8 m/sec
1r
2
3
Air flow rate = 19.8 x —
4 (0.9) x 3600 = 45 350 m /hr
= 73.5 at 68°F
at a temperature of80°C — 45 350 kg/hr
28
Fan HP at 176°F = 73.5 x636
L- = 61.0
To cover start up conditions size motor 73.5 + (15 %) = 85
Nearest size = 100 HP
EXAMPLE 12.6
During the commissioning of a spray dryer, the performance of the exhaust
fan under operating conditions is to be determined. This was carried out
with a pitot-tube assembly. The duct temperature was 80°C.
417
3. Work done by fan = mass air flow x static head
= 45 350 x
0 pair
45 350 260
kg m
3
x 10 = 3270
3600
sec
1000
= 42.7 HP
Fan efficiency =
42.7
x 100 = 64
418
SURVEY OF AUXILIARY EQUIPMENT
12.2.7. Effect of Altitude on Fan Performance
With the decrease in air density with increase in altitude, higher air volumes
must be handled by the fan for a given mass flow requirement. Care must
be taken in sizing fans for operation at altitude. Data specifications are
based upon sea level conditions and thus compensating factors must be
applied to prevent fans and motors being undersized.
A mass air flow at sea level is equivalent to a volume that will be 43 %
greater if the fan was operating at 7500 ft. (This is computed from taking
the atmospheric pressure at sea level at 760 mm Hg and the atmospheric
pressure at 7500 ft at 530 mm Hg where increase in volume is in the ratio
of 760/530 = 1.43.) Pressure drop will increase substantially if a fan
sized at sea level operates at altitude. Pressure drop varies as the square
of velocity and for the above example a 43 % increase in velocity will give
a 206 % increase in pressure drop. Larger fans and motor sizes are a characteristic of high altitude working due to the additional fan work load. Actual
system losses are lower with lower air density, and correct sizing can hold
velocities down to reasonable figures.
12.2.8. Fan Mounting
Fan operation can be marred by excessive noise and vibration if fans are
insecurely mounted. The larger and faster the fan the greater the possible
noise and vibration levels. Wherever feasible, fans should be mounted at
floor level on solid concrete foundations. However, in spray dryer layouts
it is not often convenient to have all fans at floor level, as an extensive duct
system would result. Fans mounted at elevated heights cannot have concrete bases due to load limitations of the building structure, but as an
alternative, as solid a base as possible must always be provided. Spring
loaded or rubber anti-vibration pads should be placed between the fan/motor
frame and the building structure to dampen vibration transmitted to the
structure. Failure to do this in steel building structures can give rise to
vibrations being amplified within the steelwork, leading to vibration and
noise problems away from the area of fan mounting. Vibration transmittance can be a problem if automatic control equipment is also mounted
on the building steelwork.
12.2.9. Fan Noise
Fan noise is becoming a factor of increasing importance to acceptable fan
operation, due to the more stringent noise level standards being set as
permissible in working areas. Fan noise will increase over normal levels, if
(a) the fan mounting is insecure, (b) the fan rotor is out of balance, (c) fan
bearings are on the point of failure, or (d) the fan is operating on increased
powder—air loadings, (e) incorrect original selection gives too high a speed
or poor efficiency.
SURVEY OF AUXILIARY EQUIPMENT
419
General maintenance should prevent frame mounting looseness and
frequent bearing failure. It is always good practice to check pulley belt
tension between motor and fan. The fulfilling of lubrication schedules will
do much to prolong bearing life. Out-of-balance operation often occurs
through build-up of product on the fan blades. The blades must be regularly
washed, and drainage cocks are invariably provided in the fan casing to
facilitate easy fan washing.
12.2.10. Fan Selection
Selection of fan design and fan size requires data on the operating conditions :
(a) Air flow rate as mass flow.
(b) Pressure (static and total).
(c) Air density (based upon temperature, altitude and moisture content).
(d) Available power.
(e) Available speed range and application, powder loading, etc.
Selection is made from fan curves. Manufacturers and suppliers have available performance curves for each design and size of fan offered. Selection
requires finding the fan, whose performance meets the operational data
requirements. There are many ways of presenting fan data. Performance
curves can apply to a single fan size, or be extended to cover performance
of a range of fan sizes of fixed design.
The fan design is first selected. This is comparatively straightforward
for a spray dryer fan, as the range of suitable designs is not extensive. Fans
are normally of the backward curving blade type. For the selected design,
the performance curves that cover a range of fan sizes and air rates are
inspected. With data on working conditions, the correct fan size is selected
automatically. If two or more fan sizes appear applicable, the fan is chosen
having the highest efficiency at the required working conditions. Note that
when inspecting performance curves, they are related to a standard temperature and barometer. The working pressure must be corrected to the
standard basis. Having fixed the fan size, the fan blade speed and efficiency
are read off the curves. The required horsepower is calculated using formulae that accompany the performance curves.
12.3. Mechanical Powder Separators
Mechanical powder separators for product separation and recovery (collection) are mounted directly after the drying chamber. Dry cyclones, bag
filters and electrostatic precipitators are used for dry powder collection.
Wet scrubbers, wet cyclones and irrigated fans are used as final air cleaning
units. Equipment is discussed from a practical viewpoint. For comprehensive description of equipment, theoretical principles of operation,
420
SURVEY OF AUXILIARY EQUIPMENT
mathematical treatment of power—air separation, the reader is referred
to other available publications (124) (172) (178) (179) (504).
12.3.1. -Dry Collectors
(a) Dry Cyclones
A dry cyclone is a centrifugal particle separator. The theory of operation
is based upon vortex motion where the centrifugal force acting on each
particle causes particle movement away from the cyclone axis. Particle
movement in the radial direction is the resultant of two opposing forces.
The centrifugal force acts to move the particles to the cyclone wall, while
the drag force of the air acts to carry the particles into the central air core.
The theory of particle movement in a spinning gas is given in detail elsewhere (178) (180). The centrifugal force is predominant, and particles pass
through the air stream, concentrating in the outer layer of air moving around
the inner surface of the cyclone body. The centrifugal force acting on each
particle is represented by the term mV,2 1r where m = mass of particle,
V, — tangential air velocity and r = any given radial distance from cyclone
axis. Inspection of this term indicates that particles just outside the central
air core of the cyclone are subjected to the greatest centrifugal force as
here the velocity is highest while the radial distance is small.
Time is required for product to move outwards and reach the cyclone
wall, and thus efficient cyclonic separation does depend upon an air residence
time in the cyclone being sufficient to permit the completion of product
travel to the cyclone wall before air flow turns upwards to the outlet duct.
Even when separation has been effected, the upward turn of the air flow
must be achieved without re-entrainment of product into the air stream.
Aerodynamic parameters of cyclones have been recently (1970) discussed
by Muschelknautz and Krambrock (181).
The most common form of dry cyclone, used in conjunction with spray
drying chambers is the reverse-flow type. The cyclone can operate at
either an over-pressure (fan on inlet side) or under pressure (fan on outlet
side). In spray drying layouts the fan is mounted on the outlet side, thus
enabling the fan to handle powder-free air. Powder and air pass tangentially
into the cyclone at velocities that are virtually equal. Powder and air swirl
in spiral form down to the base of the cyclone, separating the powder out
at the cyclone walls. Powder leaves the base of the cyclone. The clean air
spirals upwards along the axis of the cyclone and passes out of the top. Flow
through the cyclone is essentially a double spiral. The presence of the
upward spiral forms the central core. The central core diameter has been
related to the exit cyclone duct diameter (ID ex „) and ranges between D exi ,
to iD exit . Powder that has not been separated out by the time the air flow
direction turns upwards into the central core, will pass out of the cyclone
with the air. The double spiral flow concept is only a fair representation
SURVEY OF AUXILIARY EQUIPMENT
421
of actual flow patterns in the cyclone. Actual flow patterns are complex,
as there are secondary air flows between the two spirals. Although much
analysis of flow profiles within cyclones has been carried out using tracer, or
pitot-tube techniques (180) (182) the nature of the secondary air flow is
still unknown. The secondary air flow..is due to the interaction of axial and
radial velocity components, but there is much discussion as to whether the
flow is a single vortex covering the whole length of the cyclone, or a double
vortex. The complexity of flow within a cyclone results in particle sizes
separated out in cyclones to be smaller than predicted theoretically, even
when using realistic assumptions. Particle sizes above 20 micron can be
removed in practice. The possibility of sizes less than 20 micron being
collected falls off rapidly with decreasing size. High efficiency units for
removal of sizes 10-20 micron require precise proportioning of the cyclone
dimensions.
AIR
OUTLET
AIR
INLET
AIR
INLET
PRODUCT
OUTLET
a,
b.
Figure 12.8. Basic designs of industrial cyclones. (a) Tangential inlet, (b) Wrap-around inlet.
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
In the basic industrial cyclone design powder-laden air enters tangentially
setting up spiral motion within the cyclone. A modification of the basic
design is the 'wrap-around' inlet or volute inlet, where higher collection
efficiencies are claimed due to more controllable and uniform flow spirals
set up in the upper part of the cyclone. The tangential inlet and volute inlet
designs (figure 12.8) are widely adopted in spray drying installations. The
majority of airborne powder from drying chambers have a size distribution
within a range that these designs can handle. The volute inlet is often used
to handle very high air throughputs and achieve high collection efficiencies.
There are many variations of the above two designs. Special cyclones
feature helical shaped roofs, multi-cylindrical and cone sections, powder
by-pass techniques, vortex shields, special outlet duct designs and hoppers.
The helical shaped roof is aimed at reducing turbulence in the top of the
cyclone leading to higher efficiencies at lower pressure drops. Multicylindrical and cone sections of the cyclone body produce controlled and
uniform increase in velocity down the cyclone. Powder by-pass techniques
permit powder to by-pass the air flow in the top section of the cyclone, an
area which for conventional designs gives the greatest likelihood of powder
re-entrainment around the entrance of the outlet duct. Vortex shields act
to stop the downward spiralling air flow. Through precise adjustment reentry of separated powder into the upward outgoing air flow is minimized.
A further attachment is the hopper at the base of the cyclone. The hopper
again acts to prevent re-entry of powder back into the outgoing air flow.
Scroll outlet ducts are featured enabling air to leave the cyclone with a
minimum of resistance, thus contributing to low overall pressure drop.
(i) Cyclone Performance. The performance of cyclones when handling spray
dried products has great importance in the design, layout and operation of
spray drying plant. Performance depends on such factors as product particle
size, shape and distribution, bulk density and powder loading in the air.
Characteristics that establish whether cyclones are suitable for a spray
dryer layout include (a) a size parameter to express particle size that can be
collected in a cyclone, (b) the overall cyclone efficiency when handling a
product of known size distribution and particle form, (c) pressure drop
over the cyclone. These characteristics decide whether a single cyclone or
a cyclone battery can suffice and operate as the only collection unit, or
whether secondary equipment is required after the cyclone(s).
The Size Parameter. Two size parameters are used to define particle sizes
a cyclone can remove. These are the critical particle diameter and cut size.
The critical particle diameter is defined as the particle size that will be completely removed from the air flow (100 % collection efficiency). This diameter
has been adopted as a means of stating the separation expected from a given
cyclone design. Equations can be drawn up expressing the critical diameter
423
in terms of the air-flow properties and cyclone dimensions. There are
limitations to these equations as assumptions, often unrealistic are involved
in their development. Flow patterns within cyclones are complex, whereas
the equations express simple systems. No theoretical model is available
that adequately covers particle movement within cyclones. It is virtually
impossible with `todays state of the •are, to deal mathematically with the
flow effects due to particle interference, particle re-entrainment tendencies
and the varying velocity profiles within the cyclone. Strauss (178) lists
assumptions and quotes the available equations. However, predicted values
of critical diameter are not usually borne out in practice. There is no sharply
defined point where a particle size is either 100 % separated or 100 % lost
in the exhaust air. For the critical particle size to have real practical meaning,
the grade efficiency curve (figure 12.9) would have to follow the line ABC.
It does not, the line (curve AED) being representative of actual conditions.
Referring to figure 12.9 predicted critical particle diameter lies in the range
10-20 micron, but practical conditions indicate 100 % separation to be
achieved only for the much coarser particle size of 105 micron. The more
favourable parameter to express separation is the cut size.
15
105
C
0.
COLLECTION EFFICIENCY %
422
PARTICLE SIZE ( MICRON
Figure 12.9. Theoretical and actual grade efficiency curves.
The cut size or cut of the cyclone best represents the obtainable separation.
It is defined as the size for which 50 % collection efficiency is achieved.
(The 50 % level on the grade-efficiency curve gives the cut size.) Grade
efficiency curves, obtained experimentally are the most reliable source of
information to obtain size separation data. Grade efficiency curves are
424
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
drawn up by systematically operating a cyclone with a uniform particle
size dust. The deviation of such a curve from the theoretical case is shown in
figure 12.9. Area ABE represents particles that are expected to escape
separation, but are separated in practice. This extra degree of separation
is due to agglomeration effects and influence of larger particle movement.
Area CDE represents particles that should have been collected, but are not
due to particle interference, particle re-entrainment and eddy air flow
effects within the cyclone.
(ii) Overall Cyclone Efficiency. The overall efficiency of a cyclone is that
obtained when handling a product of definite size distribution. The grade
efficiency curve cannot be applied directly to spray dried product entering
cyclones. However, knowing the grade efficiency curve and the product
size distribution, the overall efficiency can be calculated.
It is common to know with a fair degree of accuracy the particle sizes
entering a cyclone from a spray drying chamber. For a layout shown in
figure 9.1(b), where all product is exhausted to a cyclone collection unit,
size data are obtainable from atomization and spray evaporation characteristics. For a layout shown in figure 9.1(a) size data are not so readily
available. However, the chamber design and dryer operation enables the
proportion of the total product throughput that is separated out in the
chamber to be often estimated or measured. The size range of this separated
fraction (chamber fraction) can be analysed. From the atomization and
chamber fraction particle sizes, the likely size distribution and quantity
of product passing to the cyclone(s) can be drawn up. A calculation of the
overall cyclone efficiency is illustrated in example 12.7.
100
EXAMPLE 12.7
A spray dryer produces 1000 kg/hr of dry product, 85 % of the production
is recovered from the chamber base, while the remainder passes to two
identical reverse flow cyclones. The grade efficiency curve for the cyclones
are shown in figure 12.10. Powder samples taken between the chamber
and cyclone inlet were analysed to give a size distribution of the airborne
product (table 12.3, column 2). Predict the quantity of product not separated
out in the cyclones.
The solution involves the grade efficiency curve to determine extent of
separation for each mean of the size range used during the analysis. The
overall efficiency is then calculated from summing the product of percentage
efficiency in each size range and percentage occurrence, and dividing by 100.
Table 12.3
(1)
(2)
Particle size
(micron)
Size occurrence
passing to cyclones
0-5
5-10
10-15
15-20
20-25
25-30
30-40
40-50
50-60
60-70
2
(3)
(4)
Grade efficiency
( %)
Overall efficiency
( %)
430
76.0
89.0
94.0
95.5
96.0
96.5
970
97.5
98.0
0.86
3.8
12.46
35.72
18.15
11.59
4.85
2.91
0.97
0.98
5
14
38
19
12
5
3
92.29
GRADE EFFICIENCY
100
Column (4) = [column (2) x column (3)1 x 10
60
425
-2
Overall efficiency = 92.3
1000 x 15
= 150 kg/hr
40 -
Product entering cyclones =
20
Product leaving cyclones = 150 x — = 1.155 kg/hr
100
100
7.7
0
10
20
30
40
PARTICLE SIZE
50
60
70
80
90
( MICRON I
Figure 12.10. Grade efficiency curve for cyclones specified in example 12,7.
100
Example 12.7 shows that a low overall efficiency value (i.e. in the neighbourhood of 90 %) does not automatically mean high quantities of product
lost with the exhaust air. The grade—efficiency curve indicates the cyclone
design to be of the high efficient type. The low overall efficiency is due to
426
SURVEY OF AUXILIARY EQUIPMENT
the fineness of the product the cyclone handles. Due to a low powder loading, only low weight loss levels from the cyclone actually result. In fact if
all production from the spray dryer was passed to cyclones for collection,
overall efficiencies of over 98.85 `)/, are required to improve upon the separation achieved in the example. This may be above the level of collection
achievable for handling the total production in cyclones even though increase
in powder load to a cyclone often acts to increase the overall efficiency.
In many cases, the increase in overall efficiency obtained by increasing the
cyclone powder loading does not result in a decrease in powder weight loss
from the cyclone.
Data pertaining to overall efficiencies of cyclones prove useful when
comparing possible cyclone performance with actual performance under
operating plant conditions. If it becomes evident during operation that
product losses from cyclone(s) are well above values predicted from overall
efficiency (while inlet product loading conditions remain approximately
the same) faulty cyclone operation can immediately be suspected. The most
common cause is leakage of air into the cyclone base. Leakage assists the
re-entrainment of product back into the outgoing air stream. Other causes
can be (a) change in the particle size distribution of the ingoing airborne
product, (b) cyclone overloading, i.e. where a battery of cyclones is involved,
partial blockage in one of the inlet ducts may be causing maldistribution of
product into each cyclone, (c) the cyclone base is partially or totally blocked.
This condition is readily evident by extremely high loss levels.
The highest obtainable efficiency of a cyclone depends upon the continuous
withdrawal of product from the cyclone base. If withdrawal is impeded
and product builds up within the cyclone base, very high loss levels will
result from the take-up of powder that settles at the blocked outlet.
Efficiencies are reported by Ter Linden (180) and Stairmand (182) to be
influenced by cyclone dimensions. If performance is not up to requirement,
modification to the dimensions can increase efficiency. Alteration to the
cylindrical height, exit duct protrusion within the cyclone, and cyclone base
design are the usual features that are modified. Efficiency has been reported
to increase as the ratio of cyclone diameter to exit duct diameter is increased.
A ratio value of three appears the upper limit for effective improvement.
Increasing the cyclone height in relation to the exit duct diameter appears
to improve efficiency, where a ratio value of 10-11 seems the optimum
order. Other dimensions influencing efficiency are (a) the precise positioning
of the exit duct entrance in relation to the inlet duct, and (b) the inlet and
outlet duct sizing. How much these dimensions contribute to collection
efficiency depend upon the design of cyclone inlet.
Increase of air throughput with the accompanying increase in air velocity
acts to improve efficiency (181) but any improvement can be offset by pro-
SURVEY OF AUXILIARY EQUIPMENT
427
moting greater turbulence within the cyclone and increasing product reentrainment. Increasing the product loading of the cyclone is often suggested
as a sure way to improve cyclone performance. This has been found by
experiment to be true to certain limits, as increase in the number of large
particles will entrain or agglomerate larger numbers of smaller particles
that would otherwise be lost in the exhaust air. However, this effect can
be more than compensated for by the presence of larger product concentration in the lower parts of the cyclone leading to greater degrees of product
re-entrainment. Optimum loadings are determined using standard powders.
Particle size distribution of standard powders are shown in appendix 11.
However spray dried products deviate from standard dusts in size, shape
and density, and reproducing optimum conditions in actual plant depends
upon extrapolation of test data to actual conditions.
(iii) Pressure Drop over Cyclones. The pressure drop characteristics of
cyclones have direct bearing on the economy of the spray drying operation.
Pressure drop losses influence power requirements, and decide fan sizing
for the exhaust air system. Cyclones used in spray drying applications
operate between 3-8 inches of water pressure drop (75-200 mm WG). The
effect of powder loadings on pressure drop in cyclones has been discussed
by Kriegel (183).
The total pressure drop is a combination of many individual losses.
Losses are due to friction and air expansion or contraction at the cyclone
Figure 12.11. Measuring pressure drop over a cyclone. Item A. Pressure tapping measuring
total pressure drop (APT). Item B. Pressure tapping measuring static pressure drop (APO.
428
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
inlet. The kinetic energy losses and static head losses within the cyclone can
be measured as dynamic and static pressure differences using a manometer.
One limb of the manometer is connected to the inlet cyclone duct, and the
other limb is connected to the outlet cyclone duct (figure 12.11).
If the pressure tappings in the inlet and outlet ducts record impact pressure,
and the inlet and outlet duct areas differ, the total pressure drop will be
recorded at the manometer. If the pressure tappings record the static
pressure drop (AP,), and the inlet and outlet areas differ, the dynamic pressure
difference (AP d ) can be calculated from the equation
APd =
1 p a (172
—
k r owlet — Vinle t)
2 g
(12.4)
where V = air velocity, p a = mean air density over the cyclone.
Adding (APd ) to the measured (APJ gives the total pressure drop (AP T )
across the cyclone. Use of equation (12.4) is shown in example 12.8.
EXAMPLE 12.8
A cyclone handles 22 000 kg/hr of air. The inlet duct area is 1 m 2 and the
outlet duct 0.5 m 2 . The measured total pressure drop is 140 mm WG.
There is a negligible temperature drop over the cyclone, the air temperature
remaining 80°C. What correction to the total pressure drop must be made
to obtain the static pressure drop?
Now equation (12.4) expressed in mm WG pressure drop and air flow
rate (L a ) in kg/hr becomes
11 L a
2
APd = 2 k 3600)
1
1
Pa
9.81 [19!(/0.uunet,
nF(A?
r.k—inlet)
where
V=
La
L a = 22 000 kg/hr
2
Aoutiet = 0.5 m
2
Pa = 1 kg/m' at 80°C
11220001 2 1 r
1
2
d = 2 3600 ) 9.811_1 x 0.5 2
1 37.3
= 2 9.81(4 — 1) = 5.7 mm WG
AP, = APT AP, = 140 — 5.7 — 134.3 mm WG
Methods of predicting pressure drops have been developed out of flow
theories by Ter Linden (180) Stairmand (182) and Shepherd and Lapple (184).
The limitations of being unable to adequately define the complex cyclone
air flow patterns are apparent, as predicted values from generalized equation
are not confirmed in practice. Cyclone manufacturers supply correlations
that have been • established from operational data. Various correlations
appear in the technical and trade literature for application with certain
designs.
For wrap-around inlet designs of dimensions conforming to cyclone
type 2, table 12.4, the pressure drop (AP) is given by the relation:
AP =
1.,
P c/4T
2
(12.5)
where K2 is a constant established in practice, T = air temperature, d =
cyclone diameter, / 3,3 = barometric pressure.
Expressed in metric units where AP is mm WG, L a = kg/hr, T = D C,
PB = mm Hg, d = metres, equation (12.5) becomes
760
L )2
(12.6a)
AP =
s ( T + 273) PO
6700 d'
Expressed in British units, where AP is in WG, T is °F, P„, is in Hg, d is ft,
and L a is lb/hr, equation (12.5) becomes
AP =
29.92
L )2
—(T
+ 460)
4
PB
9270 d
AP = 0.013v c2 p a
APT = 140 mm WO
AP
Hence
(12.6b)
The simplicity of the equations, whereby the pressure drop is related
directly to the cyclone air rate, air temperature, and barometric pressure
and only one cyclone dimension, the diameter, is obtained at the expense of
some variation between actual and predicted values. A comparison of
values has shown differences between values to be small enough to enable
practical use of equation (12.5).
For cyclones with the helical inlet design, trade literature reports the
pressure drop (in WG) to be predicted by (179)
paA
Hence
&l et = 1.0 m
429
12
(12.7)
where p a = air density (lbs/ft 3 ), ye = average inlet velocity (ft/sec).
For cyclones with the standard tangential inlet design (i.e. conforming
to the dimensions in table 12.4, type 5), the pressure drop (in WG) is predicted by (179)
(12.8)
AP — 0-024v c2 p a
C
S
aS
a,
a8
High capacity
SURVEY OF AUXILIARY EQUIPMENT
411
n.P.d
":5
6
c,
(f) °'
o
C:
A
S
0
N
6
N
SURVEY OF AUXILIARY EQUIPMENT
Cr
C
:1=2.
a
t:i
3
• 3
O
a
38
O
A
G a
`
N
0
L a = 35 000 kg/hr
aDk.
T = 80°C
AP =
a
0
3m
Pa = 758 mm Hg
45
N
6
431
EXAMPLE 12.9
A 3 metre diameter cyclone (design type 2, table 12.4) handles an air flow
rate of 35 000 kg/hr. The average air temperature over the cyclone is 80°C.
The barometer stands at 758 mm Hg. Predict the pressure drop. Using
equation (12.6a) where
d
99
5 E
a
(35 000) 2 1
(80 +
6700 34
760
= 120 mm WG
41
(iv) Cyclone Layouts. The mounting of a single or multi-cyclone arrangement after a spray drying chamber is well established. For a multi-cyclone
layout, the mounting is usually in parallel (figure 12.12). Where used in
series a powder classification is obtained. The choice between single or
multi-cyclone units is dependent upon the product application. However,
if the exhaust drying air can be handled in a single cyclone unit, and the
obtainable recovery efficiency of the order of 98 % is acceptable a single
cyclone system is adopted. The ease of product removal and handling, a
minimum of interconnecting ducts, and the ease in which the whole layout
can be cleaned are important advantages. However, exhaust air can be
divided and distributed effectively into two or more cyclones, A higher
overall recovery efficiency is obtainable from a battery of smaller cyclones
in theory, but any increase in efficiency over the single cyclone layout is
dependent upon even distribution of powder and air into each cyclone.
(95
-
0
.5 •
O
a
a
99
Table 12.4_ Dimensions of Indus
430
0
.94
Ma
a
91
1
9
N
N
N
-5,
N
N
6
A
(-9
9
C
C
4
;7
Tr,'
•cl
Uj
s .5
ro 74
1
3
cn Li .5
-iv 3
-a 0
be
a
En .5,
C
C
.5
.5
O
,t1
aCa
-•-••
5 co
N
ro
C
-'
„
;I 13
N
00
Figure 12.12. Cyclone layout in parallel.
432
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
In practice identical operational conditions within each cyclone of the
battery is very difficult to obtain and maintain. Cleaning also is more
difficult due to inaccessability, although the mounting of in-place cleaning
(CIP) obviates this difficulty.
(b) Bag Filters
(1) Principles. The bag filter is often the favoured equipment for collection
and recovery of powder entrained in the exhaust air of a spray dryer. A
bag filter consists of numerous bags, installed so that each bag receives
approximately equal quantities of air. The collection of bags is termed the
baghouse. The simple principle of passing powder-laden air through a
close woven fabric is one of the oldest methods of air cleaning, but, it remains
today an efficient means of powder separation, being able to remove particles
below 10 micron. With the correct fabric weave, 1 micron particle size
can be collected. The performance of bag filters depend upon (a) the type
of product handled, (b) the inlet powder loading in air, (c) bag fabric, and
(d) bag cleaning procedure. Modern bag filter designs prove reliable in
operation, but are subject to strict maintenance and inspection to ensure no
bag leakage. The slightest of leaks will quickly diminish the very high
collection efficiency levels the units are capable of achieving. A typical
grade efficiency curve of a bag filter is shown in figure 12.13. Greater than
99 % efficiency is obtained with 5 micron particles, 99 % efficiency is'obtained
with 1-2 micron sizes.
(ii) Bag Filter Designs. Bag filters can be classified into three types. Each
type is classified according to the bag cleaning operation.
•
100
EFFICIENCY
98 -
96
COLLECTION
94 -
92
90
0
2
4
PARTICLE SIZE
fi
8
( MICRON )
Figure 12.13. Grade. efficiency curve for a standard bag filter,
10
433
Type A : Manual cleaning on shut down.
Type B: Semi-continuous cleaning (periodic).
Type C: Continuous cleaning.
Type A. The bag filter is taken out of operation for cleaning. The time
between each shut down will depend upon bag area and how quickly the
product cake builds up on the bag. As spray drying systems require operation
to be continuous, frequent shut-down practice is not acceptable. This type
is seldom used, unless the spray dryer is operated batchwise. Such baghouses are a simple construction and form a low cost system.
Type B. Cleaning takes place periodically without shutting down the
equipment. Cleaning can be accomplished in two ways :
(1) The bag filter consists of a series of batch units mounted in parallel.
Each unit has many bags. Periodically one of the units is taken out of service
for cleaning, while the remaining units handle the total air flow. Each unit
is taken out of service in turn, when cleaning is carried out by vibrating the
bags. This can be done by swinging the bags to and fro, violent shaking in
the vertical plane, or by applying any form of eccentric movement. The
cleaning sequence is controlled by a timing device, which can be set to suit
the product handled and the powder loading conditions of the baghouse.
A complete cleaning cycle of 30-45 minutes is normal.
The bag filter requires bags to periodically handle increase in air rate to
compensate the loss of air flow to the unit of bags being cleaned. However,
the cleaning time per unit of bags can be small in relation to the overall
cycle time. For a filter of for example 3-units, the bags may be only out of
service 1-2 minutes for cleaning, every 30-45 minutes. The bag filter performance is greatly dependent upon the reliability of the timing device.
Failure in the cleaning cycle will result in excess cake layers building up
on the bags in filtering service. The increase in pressure drop over the filter
bag leads to reduced air flow and difficulties arising in the spray drying
chamber. Should minor maintenance be required on any unit of the baghouse, the timing device can be adjusted to extend the cleaning down time
cycle, so that work can be carried out on the bags or vibrating mechanism
without interrupting plant operation.
A series of units mounted in parallel is illustrated in figure 12.14(a).
(2) The bag filter can consist of a single unit, where each bag is cleaned
periodically during operation. It is not necessary to shut down any part of
the baghouse for cleaning. A common form of cleaning device for this type
of bag filter is the pulsed air jet (figure 12.14(b)). An air jet can be directed
into each bag, and each bag is frequently pulsed with a high pressure air jet.
The pressurizing of the bag prevents air flow through the bag, causing the
bag to vibrate or puff out and dislodge the product away from the bag fabric.
434
SURVEY OF AUXILIARY, EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
The pulses can be of very short duration; for example 0.1-0-2 seconds.
Every bag is pulsed once every 10-15 seconds. 90-100 psig (6-7 atm) air
pressure usually suffices. Solenoid valves actuate the air nozzles.
COMPRESSED AIR
SOLENOID VALVES
• GLEAN AIR
435
high volumes of powder-laden air handled per unit area of bag fabric.
The continuous cleaning also maintains low pressure drop conditions
over the bags.
(iii) Bag Filter Performance. The performance of a bag filter, judged by
the overall collection efficiency of airborne powder is the best of the collection
equipment used in spray drying plant. Achieving and maintaining such
high performance does depend, however, upon (a) maintaining a leak-proof
bag system, (b) high bag durability to resist rapid wear on each bag due to
the mechanical handling each bag receives during cleaning, (c) effective
bag cleaning procedure. Failure on these points lead to product losses
greater than those from dry cyclones.
The air velocity through the bag fabric is kept low (1-15 ft/min, 0.5-7.5
m/sec). For fine powders, air velocities are of the order 1.5-3.0 ft/min,
but the coarser the product, the higher the velocity that can be used. Furthermore the shorter the cleaning time per bag, the higher is the permissible
air velocity (filter face velocity). Table 12.5 shows typical face velocities
in the three types of bag filter designs considered above. The pulsed reverse
jet type can operate effectively up to 15 ft/min on particle sizes present in
exhaust air from spray dryers, due to the effective and rapid cleaning sequence.
-• NOZZLES PRODUCING AIR JETS 000 FILTER
DI-DW RING [MOVING UP AND DOWN
IN VERTICAL PLANE
OVER RAG SURFACE}
Table 12.5. Recommended Face Velocities for Bag Filter Designs
.--DAD FILTER
Bag filter design
Cleaning practice
Mode of cleaning
CLEAN AIR
OUT
POWDER LADEN
AIR
PRODUCT
Type A
Manually cleaned
Type B
Semi continuous
automatically
cleaned
c.
POWDER DUD
Face velocity
(ft/min)
1-3
Intermittent pulsed
air jet
3-6
Frequent pulsed
air jet
10-15
Shaking
3-6
Traverse air jet
6-12
Figure 12.14. Designs of bag filters. (a) Bag house consisting of separate bag filter units in
parallel, Each unit removed alternatively from system for reverse air flow cleaning, Units
1, 2, 3 filtering powder-laden air. Unit 4 on reverse air flow cleaning. (b) Bag filter using air
jet cleaning principle. (c) Bag filter using the air blow ring cleaning principle.
Type C
Type C. The air jet cleaning system with its short duration pulses can be
considered a continuous cleaning operation. However, there are designs
that are completely continuous. One established design is the use of blow
rings (figure 12.14(c)). Instead of a pulsed air jet, air flows continuously
from an air ring that traverses up and down the bags. The air ring blows
product away from the bag fabric. Air pressures are of the order 25-35 in
WG. The advantage of the blow ring system, apart from the continuous
product discharge achieved, and low air pressure system, concerns the
Table 12.5 gives the recommended face velocities. Use of too high a
velocity causes compaction of the product on the fabric. Compaction
causes increase in pressure drop over the bag and possible penetration of
product through the fabric. The increase of force on the bag also leads to
earlier fabric weave failure, and puts greater demands on the cleaning procedure. The product becomes impregnated within the weave to a greater
extent than is normally allowed for when sizing the unit and specifying
operating conditions.
Continuous
automatically
cleaned
436
Even with correct face velocities and effective cleaning procedures some
product particles remain in the fabric weave. In fact, their presence, to a
li mited extent, is a requirement for optimum filtering efficiency. After a
new bag has been in operation for a period, the powder content that is not
removed during the cleaning cycle remains at a constant level. The presence
of this retained product is shown by the pressure drop through a new unused
bag at a constant air flow rate to be less than the pressure drop over the
same bag when mounted in the baghouse following cleaning, and operating
on the same air flow. The constant powder content level in the fabric weave
is termed the equilibrium powder content of the bag fabric. The requirement of maintaining correct face velocities, that are invariably low, results
in large fabric areas for filtration. Bag filters associated with spray dryer
chambers of high evaporative capacity are large units.
(iv) Pressure Drop over Bag Filters. The pressure drop over bag filters used
in spray drying plant generally ranges between 1.5-8 in WG (40-200 mm
WG). The pressure drop value is difficult to predict. Data are obtained
experimentally and are made available by manufacturers of bag filter equipment supplied for spray dryers.
Semi-empirical methods are available to predict the order of pressure
drop for various bag fabrics. The methods are based upon the so-called
`channel theory and fibre drag theory'. Full details are given by Strauss (178).
(v) Bag Filter Fabrics. The fabric used in a bag filter is selected from considerations of cost, powder characteristics, and operating air temperature.
The more exacting the operating conditions for the bag, the more expensive
is the fabric. With spray drying plant, air temperatures the bag fabric must
withstand are not excessive, 265°F (130°C) being of maximum order, as
high temperature prevention is built into the dryer control system. However,
if temperatures in excess of this level are to be handled, bag fabrics are
available to meet these conditions (silicone treated glass fibre). The properties of various bag filter fabrics have been reviewed recently by Walling
(362). Table 12.6 lists properties of common natural/synthetic filter fabrics.
(vi) Use of Bag Filters. Bag filters are used where 100 % collection efficiency
is required from a one-stage separation unit. The complete range of available
bag filter designs are reported by Strauss (178). With the use of top quality
bag fabrics and maintenance of leakproof bags, the bag filter proves an
efficient unit. Maintenance is constantly being reduced through improved
baghouse design, as reported recently (1970) by Pring (186), and (1971) by
Akenhead (485). Capital costs can be as much as five times that of a high
efficient cyclone and running costs just as high, but these factors are often
compensated fully by the increased recovery of product. If the product
has a high value per unit weight, the substantial savings in reduced losses
quickly accounts for increased capital and running costs. Cases where
437
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
Table 12.6. Properties of Bag Filter Fabrics
Natural/
synthetic
fibres
1
Cotton
Wool
Polypropilene
Polyamides (nylon)
Acrylics
Polyesters
Fluorocarbons
(teflon)
Glass-fibre
Maximum
operating dry
air temperature
(°C)
Resistance to :
Flammability
Alkalis
Biologicals
90
90
1.00
110
120
130
H
M
H
H
M
H
L
L
H
H
H
1-1
H
L
H
L
L
M
260
310
H
H
L
Acids
H
L
L = low, M = medium, H = high.
a bag filter is not recommended are (1) handling air of high moisture loading,
(air near dew point), (2) hygroscopic or deliquescent airborne particles,
(3) products requiring high standards of hygiene in handling, (4) handling
very high air temperatures, (5) particles/agglomerates in stringy (fibrous)
form. A suitable exception in case (3) is egg white powder (p. 568).
(vii) Bag Filter Layouts. Bag filters are often mounted directly after the
drying chamber as the main separation unit. The efficiency of the unit is
very high, and secondary separation equipment is seldom necessary. High
efficiency of separation is dependent upon all bags in prime physical condition and correctly mounted. A baghouse, in view of maintenance requirements should be positioned to enable good accessibility in an area of good
lighting and ventilation.
(c) Electrostatic Precipitators
(1) Principles. Electrostatic precipitators are not used widely in spray drying
plant in spite of their forming a low pressure drop device capable of high
collection efficiency performance. Initial cost in purchase and erection is
high. Where installed electrostatic precipitators form main powder separation units. High air volume rates are handled.
There are two types of precipitators (a) plate type and (b) pipe type. The
plate type is ideal for dry powder collection. The pipe type is generally not
applicable to spray dryers being more suitable in handling volatilized
fumes, liquid or slurry droplets (504).
The principle of operation is the creation of a strong electrical field into
which flows powder-laden air. The high voltages required are produced
through rectifier and transformer units. The system is shown in figure
12.15.
438
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
439
100
80
)- 60
z
Ui
u_ GO
Lu
POWER
SUPPLY
RECTIFIER
CONTROL
TRANSFORMERRECTIFIER
UN IT
20
ELECTROSTATIC
PRECIPITATOR
Figure 12.15. Electrical power system for an electrostatic precipitator.
A large voltage difference between a discharge electrode and the collecting
earthed plate forms the desired electrical field strength. The earthed
plates are mounted in series with the discharge electrode suspended
between the plates. The whole plate system is enclosed. The powder-laden
air from the spray drying chamber enters the precipitator and flows between
the plates at low velocity. Discharge of air ions from the discharge electrode
combines with the powder particles. Powder particles obtain a negative
polarity, and move towards the positively charged collecting plates. On
contact with the plates the particle charge is dissipated. Particles are removed from the plate area by a tapping mechanism, and the powder falls
to the hopper beneath.
Wet washing of the plates is also possible. There is a very low pressure
drop over the system, as force is only applied to the particles.
Powder particles that adhere to the plates, and particles of high resistivity
(which prevent the electrical mechanism) are not suitable for such equipment. Spray dried products that can be effectively handled include cement,
bauxite, gypsum, mineral concentrates and some foodstuffs.
(ii) Performance of Electrostatic Precipitators. Electrostatic precipitators
can collect airborne powder with up to 99 % efficiency for particle distribution
sizes greater than 1 micron. The particle density appears to have little
effect on efficiency. High efficiency levels are obtained within the pressure
drop range of 0.25-1.0 in WG (6-25 mm WG). The typical grade efficiency
curve of an electrostatic precipitator is shown in figure 12.16. Plate face
velocities are of the order 2 ft/sec (0.6 m/sec).
Available methods for predicting performance are of doubtful validity
for many powders. For further details on performance, reference should
be made to Strauss (178). Performance data are given by Andersen and
Heinrich (187).
5
10
15
20
25
30
POWDER PARTICLE SIZE (MICRON I
Figure 12.16. Grade efficiency curve for a dry electrostatic precipitator.
(d) Gravity Settlers
Gravity settlers can rarely be used as main or secondary separation units of
a spray dryer as they are only suitable for collecting particle sizes exceeding
200 micron.
However, there is one collection need a gravity settler will readily meet,
especially in the food industry. This is the collection of product flakes
forming from deposits that gradually build up on the duct surfaces after
cyclones or bagfilters and within the air hood. As most powder separation
units installed in spray drying plant never achieve 100 % efficiency, there is
always the chance that the small quantities of products that do escape to the
dryer air hood will deposit out on the duct and air hood surfaces. Deposits
are caused by escaped particles becoming wet and sticky in the exhaust air
that may well be saturated on being cooled during passage to atmosphere.
Over a period of time a deposit layer will build up in the duct system. These
deposit layers are not particularly elastic, and after reaching a certain thickness will crack and break away. Break-away is severe during start-up or
shut down of the dryer - when deposit layers crack due to duct work expansion,
contraction or vibration. The start-up of a dryer may well create a high
concentration of product flake discharge from the air exhaust system. This
discharge causes fouling of the dryer roof and a pollution nuisance to surrounding property. To catch such flake discharge, gravity settlers are
installed on the dryer building roof over the exhaust air outlet duct. They
often replace the air hood.
The gravity settler is designed for very low pressure losses, and operation
does not call for a larger exhaust fan. The principle of operation is the
440
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
441
car
sudden reduction of air velocity and change in air flow direction. Wall
baffles assist flake fallout.
A gravity-settler is shown in figure 12.17. It consists of a lightweight box,
having a number of compartments formed by the wall baffles. It is often
a wood structure with a washable lining of plastic or rubber. Air flow
through the box is of the order of 15-20 ft/sec (4.5-6.0 m/sec).
CL EA N AIR
WATER
KU LES
WATER NET
DE-ENTFLumm
SCRUDDER
WATER
J el LE 7
INLET
impIND ELI EN E
BAFFLES
AIR OU T
❑
LIGHTS
1
SAFETY NET
POWDER LADEN
y- AIR
EXHAUST
❑RYI NG AIR
T
BAFFLE
PowDER LADEN
R
INEEwn
KVIENDLE
ACCESS DOOR
FOR CLEANING
A
BAFFLES
SCRUBBER WATER
SCESIDECR
ELATE.
=RIMIER 1.1 ER
CLEAN MI
TO
DRAIN
TRAP DOOR
IN BAFFLE
TO
DRAIN
Figure 12.17. Gravity settler.
After dryer operation, the flake deposits can be swept from the floor
using a brush or vacuum cleaner. Often the quality of the product is too good
for waste disposal. The product is sold just as an inferior grade. The box
has drainage, and wash water is on hand for periodic wet cleaning. This is
necessary especially where hygienic standards must be maintained (e.g.
when handling food products).
12.3.2. Wet Collectors
(a) Wet Scrubbers
(0 Principles. Wet scrubbers remove 'solid particles from a powder-laden
airstream with liquid. The contact between particles and liquid is carried
out by : (A) Flow of powder-laden air through a spray (figure 12.18(a),
spray chamber type). (B) Flow of powder-laden air through a plate, or
packing over which a head of liquid (usually water) is maintained (figure
12.18(b), (c), i mpingement type). (C) Flow of powder-laden air through
a venturi, into which liquid is injected traversely at the throat section
(figure 12.18(d), (e), venturi type).
Types (A) and (B) form low energy systems, while (C) is a high energy
system. All three types are used in spray drying plant, although type (B) is
not considered if the product powder does not readily dissolve in the liquid
or may cause possible clogging of the plates or packing. For operations
requiring hygienic operations, type (C) is generally preferred due to the
E.
DRAIN I Ras
POWDER
n.
LADEN
AIR
Figure 12.18. Designs of wet scrubber. A, Spray chamber type. B, Impingement type (plates).
C, Impingement type (packed). D, Venturi type (vertical). E, Venturi type (horizontal).
relative ease of cleaning and maintaining the unit free from deposits over
long periods of operation.
Wet scrubbers are invariably used as a secondary unit for removal of
particles that escape collection in a primary collection unit, e.g. cyclones.
It is an effective method of cleaning the exhaust air of last traces of powder.
Powder sizes down to 1 micron can be collected, and 99.5 % collection
efficiency is generally achievable on 5 micron particles. Typical grade efficiency curves are shown in figure 12.19.
Wet scrubbers are used more for air cleaning than product recovery.
This is due to the product recovery being in wet form. The solids content of
442
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
the scrubbing liquid requires further drying to recover product in dry form.
Although in many cases it is feasible to recirculate the scrubbing liquid
thereby increasing its solid content, rarely is it possible to reach solid content
levels that can be fed directly back to the spray dryer and dried economically.
Invariably concentration is required.
la)
80
EFFICIENCY %
60
40
20
5
10
15
20
25
30
POWDER PARTICLE SIZE ( MICRON )
G.
100
80
60
;•,,■
)2 40
uJ
(.7)
lL
11 1
- - 20
2
4
10
12
POWDER PARTICLE SIZE
14
15
18
20
(MICRON
b.
Figure 12.19. Grade efficiency curves for wet scrubbers.
Venturi scrubber.
(a) Spray chamber scrubber. (b)
443
(ii) Wet Scrubber Designs
Spray Chamber Type. A spray chamber wet scrubber is shown in figure
12.18(a). Powder-laden air enters the base of the chamber and passes
upwards through sprays. Sprays are formed from nozzles placed around
the circumference and/or in the centre of the chamber. The upward air
velocity ranges between 1.5-4 ft/sec (0.4-1.2 m/sec). The actual velocity is
selected according to the droplet sizes formed at the nozzles. The air velocity
permits the spray droplets to fall to the base of the chamber and not be
entrained in the cleaned air leaving the scrubber. The scrubber water can
be recirculated to the nozzles or directly discharged to the drain.
As a droplet falls through the powder-laden air, it contacts particles and
collects them by a mechanism of inertial impaction and interception.
Stairmand (188) reports on the optimum particle size for collection by a
falling droplet. For water droplets between 100-1000 micron, collection
efficiency of a 10 micron particle ranges between 70-80 Z. For a 3 micron
particle, collection efficiencies are low, up to 20 Vo .
A spray of prominently 1000 micron droplets is considered the most
suitable, as relative velocity conditions between droplet and particle are
more easily maintained in the scrubber in spite of droplet evaporation.
As efficiency of. collection is low for particles under 5 micron the removal
of fine particles from airstreams requires repeated impacts between particles
and droplets.
This is achieved by having multi-level nozzle mountings, and a nozzle
layout to ensure spray coverage over the entire scrubber cross-sectional
area. A typical grade efficiency curve for a spray chamber scrubber is shown
in figure 12.19(a).
The collection efficiency of the scrubber operating on low powder-air
loadings as a secondary separator of a spray dryer is relatively low at 55-70 Y o .
However, pressure drop losses are not high and power requirements low.
Impingement Type. Collection efficiency of small particles is increased if
the airborne particle contacts an extended liquid surface, rather than the
surface of droplets. In the impingement type scrubber, extended liquid
surfaces are created by liquid flowing over baffles or plates. The powderladen air impinges on these wetted surfaces. Designs of impingement
scrubbers are shown in figure 12.18(b), (c). Powder-laden air enters the
base of the unit and passes up in a flow counter-current with the scrubbing
liquid. Liquid passes from the base to drain or to a recirculation tank.
Clean air passes out at the top of the scrubber via a de-entrainer.
In figure 12.18(b) liquid flows from the top of the unit over a succession
of sieve plates. To achieve maximum collection efficiency on the plates,
the powder-laden air is best saturated on arrival at the lower plate. Water
mist jets or steam can be added to the inlet airstream to obtain saturation
444
SURVEY OF AUXILIARY EQUIPMENT
conditions, or as shown in figure 12.18(b) sprays of liquid are directed upwards under the lower impingement baffle plate. Figure 12.18(c) shows
liquid flow over a packed bed.
The collection efficiency of the impingement scrubber is higher than for
the spray type. When operating on most spray dried products at low
powder—air loadings, efficiencies up to 98 % are obtained. Pressure drop
losses over the unit tend to be higher than the spray type under identical
air and powder loading conditions.
Venturi Type. Designs of venturi scrubbers are shown in figure 12.18(d), (e).
Powder-laden air is accelerated through a venturi, and at the throat scrubber
water is ejected into the air as liquid jets. The air velocity in the throat is
high enough to break up the liquid jets and form sprays with droplets
dispersed throughout the air flow. The atomization of these liquid jets
follows the pneumatic nozzle principle. The scrubbing liquid is separated
out of the air in the body of the scrubber. A de-entrainer is fitted in the
outlet to prevent droplets leaving the scrubber with the cleaned air. The
separated water drains from the scrubber base for discharge to sewer or for
recirculation to the liquid jets.
The pressure drop over the scrubber is virtually representative of the
energy requirements of the scrubber. The energy used in the water jet
formation is a minor contribution. The overall pressure drop is high. The
high pressure drop levels can be reduced by preatomizing liquid in nozzles
instead of using simple liquid jets. However, the total energy consumption remains about the same due to the increase in energy required at the
atomizer nozzles to form fine sprays. However, efficiency levels do not
automatically increase. Fine droplets are less effective in particle collection
than coarse droplets formed from jets. This complies with spray chamber
scrubber theory.
There is effective contact between powder particles and liquid droplets
in the throat section of the scrubber. Collection efficiencies are of the order
of 90-95 % for particles above 1 micron. A grade efficiency curve for a typical
venturi scrubber is shown in figure 12.19(b). The obtainable efficiency
depends upon the liquid rate at the throat. Increase liquid flow increases
efficiency. The efficiency is also increased if the air entering the throat
section is of high enough humidity to enable saturation to occur within the
throat, which is at a lower pressure. Condensation will take place on the
particles as they move on out of the throat into a higher pressure region.
To assist achieving saturation within the throat steam can be blown into the
duct upstream. Efficiencies of venturi scrubbers have been recently (1970)
discussed by Calvert (189). The prediction of venturi scrubber performance
is reported in detail by Strauss (178). Operational data is given. Pressure
drops greatly exceed that of alternative scrubber techniques described above.
SURVEY OF AUXILIARY EQUIPMENT
445
To achieve a 95 % collection efficiency of particles above 1 micron, pressure
drop losses are of the order of 12-25 in WG (22.5-50 mm Hg).
The scrubber can be fabricated in carbon steel for non-corrosive conditions.
For exacting conditions, stainless steel or any suitable lining of plastic or
rubber is used.
The venturi scrubber is the most common type used as a wet secondary collector. It represents a lower capital expenditure than any of the other scrubber
types, while producing a performance of high collection efficiency for particle
sizes above 1 micron. The unit handles high air temperatures and humidities.
Most designs have no moving parts except where a pump is fitted to supply
a mounting of atomizing nozzles. Water rate determines the collection
efficiency.
There are operational disadvantages. The scrubber does require a
sufficient water supply, although water may be conserved through recirculation (not with spray nozzle mountings, as the eventual sludge can block
the nozzle orifices). Scrubber water discharge can cause an effluent disposal
problem, and many products may on this account have to be collected dry
in alternative equipment. The hygiene of scrubbers are often insufficient
for many foodstuff operations, as the warm, high humidity air enables
rapid bacterial growth.
12.3.3. Powder Removal from Mechanical Separators (Dry Collectors)
The technique of powder removal plays an important role in separation
efficiency especially if the separator operates at a high underpressure. Poor
efficiency results from powder being removed too slowly or too quickly,
and where air leakage occurs into the separator. Removal techniques are
described below with reference to cyclones.
(a) Hoppers
Hoppers are fitted to cyclones to reduce powder re-entrainment caused by
the intensive vortex area at the cyclone base. Two types are shown in
figure 12.20(a). For products that do not flow easily (dead products) the
sloped sides have to be steeper, or vibrators and electric hammers mounted
on the hopper to assist continuous emptying. For sticky products, some
form of scraper has to be used. The hopper is then cylindrical to provide
an easier scraping surface. A hopper with scraper is shown in figure 12.20(b).
Most hopper designs are offered as cyclone auxiliary equipment and used
where powder recovery is critical to the dryer operation.
(b) Powder Removal Valves
Many types of valves are used for powder removal. The type adopted
depends upon whether intermittent or continuous discharge is required.
For intermittent discharge : (i) Manual butterfly valves. (ii) Manual push—
pull valves. (iii) Self-actuating counter-weighted and mechanical operated
single or double flap valves. For continuous discharge : (iv) Rotary valves.
446
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
CLEANING
--------- I
PORTS
a.
main advantage of the exchangeable bucket system is the ease of ensuring that
the cyclone base is free of air leaks during operation. However, this advantage is greatly offset by excessive leakage that occurs during bucket
exchange, as a closed valve does not provide a seal between the low pressure
at the cyclone base and atmosphere. Often the instance of bucket exchange
can be seen from the high powder emission from the cyclone exhaust duct
and fluctuations to the inlet drying air temperature of the spray dryer.
These fluctuations occur in one fan dryer systems (figure 12.3). When a
cyclone bucket is exchanged, the accompanying air leakage past the valve
reduces the air flow drawn through the air heater. Heat input remains constant, and thus at each moment of bucket exchange, a sharp increase in inlet
drying temperature is witnessed. The suitability of the bucket removal
system is closely connected with the inlet drying temperature the product
can withstand in the chamber.
(ii) Manual Push Pull Valves. The manual push—pull valve is incorporated
with the cyclone bucket removal system (figure 12.21(ii)). The mounting of
a push—pull valve instead of a butterfly valve greatly reduces the air leakage
occurring during bucket exchange. However, the valve performance is
suspect on many products which tend to deposit on the sliding surfaces,
and render the valve inoperable by eventual clogging.
The disadvantage with the bucket system, whatever type of valve is used,
concerns the labour and constant attention the system requires during
dryer operation. The task often requires lifting substantial weights, and
should the bucket be allowed to accidentally fill, excessive powder losses
from the cyclone results.
Powder discharge from cyclones on industrial spray dryers of larger
outputs, employ either a large capacity hopper mounted under the cyclones,
or powder discharge directly to a conveying system (band or pneumatic).
This obviates the need for constant attention being paid to the cyclone base
operation. It does not matter if the discharge device mounted at the base
of the cyclone operates intermittently or continuously as long as operation
is self-actuating or mechanical.
(iii) Self actuating and Mechanical Flap Valves. Self actuating and mechanical double flap valves are shown in figure 12.21(iii). The self-actuating flap
valve is counter-weighted and opens automatically when there is a sufficient
weight of powder on the valve to overcome the counterweight. It is a simple
arrangement but has disadvantages. Its use is restricted to coarse freeflowing powders. The discharging operation is slow during which time
excessive amounts of air can leak into the cyclone. The slightest build-up
on the edges of the flap will prevent the valve from closing. This condition
will cause continuous and undetectable air leakage. Many of these disadvantages are overcome by the use of the double flap valve. The valve is
—
SCRAPER
MANHOLE
ROBUST
GEAR MOTOR
b.
Figure 1220. Hoppers. (a) Designs of cyclone hoppers. (b) Hopper with scraper, scraper speed
25-30 rev/min.
(v) Pneumatic air locks (vortex air locks). The choice is governed closely
by the product applications and type of mechanical separator. Valves
(i—iv) are preferred for separators other than cyclones.
(1) The Butterfly Valve. The butterfly valve is normally incorporated with
cyclones equipped with detachable buckets. In pilot plant or small industrial
size spray dryers the amount of powder passing to the cyclone(s) may be
small. This is especially so if primary product discharge from the chamber
is adopted. A butterfly valve and bucket are then mounted directly to the
cyclone base (figure 12.21(1)). The bucket remains fastened to the cyclone
until the bucket is approximately half full. During this period the butterfly
valve is open. Powder is removed by exchanging buckets. The butterfly
valve is closed during bucket exchange. The period between bucket exchange is readily determined from the dryer evaporative capacity. The
447
-
-
448
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
449
mechanically operated (often by an air piston or driven cam arrangement).
Each flap is timed to open and close in set sequence. Both valves are never
open at the same time. The sequence cycle can be set-up so that small
quantities of powder are frequently removed from the separator base.
A suitable sequence cycle could be as follows :
o sec
+ 5 sec
+ 20 sec
+ 25 sec
+60 sec
Top flap opens :
Top flap closes :
Bottom flap opens :
Bottom flap closes :
Recycle repeats.
Bottom flap closed
Bottom flap closed
Top flap closed
Top flap closed
PUSH- PULL
SLIDE DAMPER
BUTTERFLY
DAMPER
- HANDLE
v)
b.
(U
Figure 12.21. Powder removal valves. (a) For intermittent discharge; (i) butterfly, (ii) manual
push—pull, (iii) self-actuating flap valve, double flap valve (mechanically operated), (b) For
continuous discharge; (iv) rotary valve. (By courtesy of Niro Atomizer.)
AIR CYLINDERS
( DOUBLE-ACTING )
PISTON
COUNTERWEIGHT
mvAA OLVN EESTIC
I ME R
FLA
MANHOLE
FLAP
ARM
SELF-ACTING
MECHANICALLY
OPERATED
COMPRESSED
AIR
a.
Successful valve operation depends upon the flaps and gaskets remaining
clean. As with the self-actuating valve any build-up on either flap will cause
air leakage and reduce removal efficiency. However the mechanical opening
and closing of each flap in the double flap design causes jolting and vibration
to act on each flap thus dislodging potential product build-up on the flaps
and sealing gaskets. Double flap valves are not ideal for heavy sticky products, but hygroscopic products are successfully handled when the product
is in low humidity air.
(iv) Rotary Valve. By far the most common discharge arrangement is the
continuous product removal obtained with the rotary valve (figure 12.21(iv)).
The valve consists of a 6-8 bladed rotor in dust-tight bearings and sealed
shaft. The drive system is isolated from the valve interior eliminating
any chance of oil or grease contaminating the powder. The rotary 'valve
can be operated with many spray dried products, although there are severe
limitations regarding those which are sticky and hygroscopic. Any product
that tends to hang in the rotor sections and not fall quickly away will render
450
SURVEY OF AUXILIARY EQUIPMENT
the valve inoperable, as a rotating lump of product will form. For products
having a tendency to stick, however, an electric hammer mounted just
above the valve will generally assist the operation. The clearance between
the rotor and the valve body is often adjustable to suit the product in question.
A larger clearance is required for coarse granular products than for fine
grain products. Products having some sticking tendencies (e.g. those
containing fat) require a sizeable clearance. The necessary clearance constitutes the main disadvantage of the rotary valve. Any clearance means a
continuous air leakage through the valve if fitted to cyclones or other low
pressure separators. As the degree of leakage depends upon the clearance,
the wear of the rotor should regularly be inspected to check clearances are
not too excessive. Apart from leakage of air via the clearance, a fixed volume
of air (i.e. volume of each rotor section) enters every time the rotor section
passes up under the separator base. For high humidity levels in a separator,
entry of cold air can cause condensation on the metallic surfaces above the
valve. Any moisture formation in this region leads to product build-up.
An improved design of rotary valve that reduces air leakage to a minimum
has been reported recently (358). A tapered rotor facilitates easy and
precise adjustment of the clearance between rotor and housing to allow for
wear, variations in temperature and variations in product. Easy accessibility
for cleaning without need for special tools is a further important feature.
(v) Pneumatic (Vortex) Air Lock. A continuous discharge technique for
cyclones involving no mechanical operation is the pneumatic vortex air
lock (figure 12.22(a)). It can only be associated with a pneumatic powder
conveying. Air enters the lock tangentially and forms a vortex rotating
in the same direction (although this is not essential) as the cyclone vortex.
The vacuum (underpressure) in the middle of the vortex is the same as that
in the bottom of the cyclone, consequently there is a continuous flow of
powder from the cyclone into the air lock. Product passing to the cyclone
base is taken up by the air lock vortex and conveyed away. The system
operates on many spray dried products, and is only limited to the ability
of the product to be pneumatically conveyed. Efficient operation of the
air lock depends upon balancing the air rate through the system to give the
required underpressure within the air lock.
The air lock has a damper on the inlet side for adjustment of air rate
when balancing the air lock. Balancing is straightforward in a one air
lock—one cyclone (figure 12.22(bi)) and two air locks—two cyclones in parallel
systems (figure 12.22(bii)). Balancing air locks in series is more difficult.
The series layout shown in figure 12.22(biii) is difficult due to the differing
pressure conditions at the base of the chamber and cyclone.
The delicate balancing of the air lock for optimum efficiency constitutes
the main disadvantage of the system. Should the air lock falter, excessive
SURVEY OF AUXILIARY EQUIPMENT
451
CONVEYING AIR
AND PRODUCT ia4p1 OUT
FLANGE TO
CYCLONE BASE
CONVEYING AIR (a)
IN.
a.
AIRBORNE PRODUCT
FROM CHAMBER
a+p
a #p
a+p
if
SINGLE
(ii) IN PARALLEL
( Hi) IN SERIES
Figure 12.22. Vortex air-lock. (a) Layout. (b) Arrangements.
powder losses from the cyclones results. When an air lock falters, the reason
is invariably concerned with the accompanying conveying system. The air
lock is best controlled by maintaining the pressure drop level established
at the time of balancing. A low pressure drop reading indicates a reduction
in conveying air rate through the lock. Reduction is normally caused by a
partially blocked conveying duct, or lumps of product falling from the
cyclone and building-up in the centre of the air lock.
Actual equipment in use for powder removal from cyclones is shown in
figure 12.23.
12.4. Pneumatic Powder Conveying
12.4.1. Principles
Pneumatic conveying is a common technique for transporting dried product
within a spray dryer installation and is applied to : (a) Conveying powder
from the base of the drying chamber and/or mechanical powder separators
to a packing or storage area (figure 12.24(a)). (b) Conveying powder, usually
fines, back into the drying chamber usually for agglomeration purposes
(figure 12.24(b)).
452
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
453
Figure 12.24(a) represents a low pressure system. The positioning of the
conveying fan makes it a 'draw-through system'. Centrifugal fans are used.
Air velocities in the ducts are fairly low (65-80 ft/sec, 20-25 m/sec). The
air—product weight ratios are high. Large duct diameters are used. The
draw-through system enjoys widespread use. The fan is mounted after the
transport cyclone and runs on powder-free air minimizing fan cleaning and
maintenance.
Figure 12.24(b) represents a high pressure system. A blower is incorporated
instead of a fan. The blower is usually of the rotary type (vane or cycloidal
■
A
.
a
Figure 12.23. Powder removal from cyclones, equipment in practice. (a) Hopper with scraper.
(b) Automatic double flap valves (air operated). (c) Rotary valve. (d) Vortex air lock.
Figure 12.24, Pneumatic conveying layouts. A, low pressure (vacuum or draw-through)
system. B, Positive pressure system.
454
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
(Rootes)). The conveying system is operated at an overpressure. Leakproof connections between the conveying duct and the drying chamber or
recovery unit require special rotary valve designs. Ducts are small with
standard pipe (2-3 in diameter) often forming the conveying system. To
maintain adequate conveying conditions, high air velocities are used. The
system is capable of operating on low air—product weight ratios, but this
feature is not always utilized, unless further powder addition to the system
supplements.the small quantities of fines normally produced during drying.
The system is often called the `fines-return system'. The blower is placed
before the powder inlet.
Pneumatic conveying systems are highly flexible being able to transfer
product anywhere in the dryer installation. Setting up a conveying system
only requires having nearby structures for duct support and the possibility
of making holes in walls, floors or ceilings for duct traverse. A conveying
system involves little maintenance expense. Trouble-free operation is
achieved, where required conveying velocities are maintained and where
powder being conveyed has no tendency to stick to duct bends.
Although the minimum conveying velocity is often thought to be the
terminal velocity, this is never true in practice. This is because (a) Stokes
law does not often apply due to many conveyed particles having sizes over
100 micron, (b) particles must be accelerated and not just suspended in the
duct system, (c) the conveying velocity must overcome friction between
particles and between particles and duct/pipe surfaces and (d) minimum
conveying velocities differ for transfer in horizontal or vertical runs.
Friction increases substantially with increase in product—air weight ratios.
If powder is conveyed in quantities to give a product—air weight ratio of 6,
friction losses can reach three times the losses created for air passing alone
through the system. On the other hand, if the quantity of product conveyed
is small, as in a low pressure system, and an air—product weight ratio of 4:1
prevails, friction losses barely exceed the losses incurred by clean air conveying. Friction losses can be calculated using the standard D'Arcy or
Fanning formulae for duct/pipework given in all chemical and mechanical
engineering textbooks. These formulae really apply to incompressible
fluids, but they may be used for air when the total pressure drop is less than
1/10th of the initial absolute static pressure. Friction of product along the
conveying duct/pipe wall surfaces is less for vertically than horizontally
mounted duct/pipe runs, but the slip between individual particles is greater
and is reflected in a greater static pressure requirement to create the necessary
lift. In all cases the critical velocity for pneumatic conveying will exceed the
terminal velocity by at least a factor of four. No general factor magnitude
can be stated as this will depend upon each product conveyed. Minimum
velocities for conveying are dependent upon product bulk density, particle
455
size and shape. For details on the theory of pneumatic conveying the reader
is referred to Molyneux (190), Zenz and Othmer (191), Kraus (192), Coulson
and Richardson (193) and Murty and Rey (194) or any chemical engineering
textbook covering the unit operations. Pressure drop calculations for
powder transport in straight ducts and bends have been recently described
(1970) by Schuchart (195). Equations and an extensive reference list is
given.
12.4.2. Pneumatic Conveying Systems in Practice
Conveying velocities exceeding 65 ft/sec (20 m/sec) are recommended for
most spray dried products. The conveying ducts are sized to give the recommended air—product weight ratio found in practice to prevent product
WT RATIO _
POWDER;AIR
POW
RATE
CONVEYING
AIR VELOCITY
100 -
1 '
rrvsec)
DUCT
DIAMETER
(cm
}
50 -
2CONVEYI NG
AIR RATE
( rrr/sec)
4-
.1.0
10
.
-01
5-
15 20 -
-10
—
3050701-
100 -
Figure 12.25. Determination of air rate and duct size for pneumatic conveying. Guide: Given
powder rate (1), weight ratio (2), join to obtain conveying air rate (3). Given recommended
velocity (4) and calculated air rate (3) join to obtain duct diameter (5).
456
SURVEY OF AUXILIARY EQUIPMENT
deposition in the system. Estimation of duct size and air rate is given in
figure 12.25, based upon data by Murty and Rey (194).
A conveying duct system should not have a duct/pipe run larger than
absolutely necessary. There must also be a minimum of 90° bends. Extended
duct lengths and bends increase pressure drop losses, leading to greater
power requirement to operate the system and extensive duct surfaces to
maintain clean. Where bends are inevitable, these should be widely curved
to reduce friction losses and the possibility of product build-up and duct
blockage. A vertical bend with a horizontal approach is most prone to blockage if a product has the tendency to adhere to duct surfaces. A horizontal
bend is also prone but not to the same degree. A vertical bend with a vertical
approach in any system will be the region of least likelihood of blockaging.
Velocity losses in conveying systems have been reviewed by Haag (196).
It is always poor operational practice to hammer ductwork heavily in
order to free a blockage. Invariably once a blockage has been discovered,
it is in too advanced a state to be cleared by hammering the duct. Little
effect is achieved and any resulting dents in the duct increase frictional effects
and aggravate the deposit formation tendencies. A heavily dented duct
(especially an upward vertical bend with a horizontal approach) can, for
instance, give constant trouble. If a product has the tendency to deposit
out at the bend or in a straight duct section reappraisal of the conditions
is required, i.e. air rate, air temperature and humidity, powder temperature.
If deposit formation persists in local areas even under the best possible
conditions, special duct surface treatment should be applied to reduce
the friction between powder and duct surface.
12.4.3. Operational Difficulties with Pneumatic Conveying Systems
Problems arising in pneumatic conveying can usually be attributed to
(a) insufficient conveying air, (b) powder too moist, (c) powder too warm,
(d) air too humid, (e) duct cleanliness insufficient.
Any one, or combination of these conditions can lead to a blocked pneumatic system.
Re (a). Insufficient conveying air can arise from a dirty filter through which
air is drawn into the conveying system. These filters are usually washable
or can be cleaned by a vacuum cleaner. A dirty filter will register as a high
pressure drop over the filter material. The pressure drop over the filter should
always be checked as part of the daily operational routine.
Re (b). Too moist a powder is heavy and tends to cake into lumps. Under
these conditions the conveying velocity, stipulated for a free-flowing powder,
is insufficient to convey away lumps. The lumps fall to the duct floor or
lodge in duct bends. Once a lump causes an obstacle in the duct, build-up
rapidly continues until duct/pipe blockage results. Moist powder can arise
from deviations in drying air temperatures, atomization, drying air rate and
SURVEY OF AUXILIARY EQUIPMENT
457
poor air dispersion in the chamber. These deviations are readily apparent
on the dryer control instruments.
Re (c). Too warm a powder has increased tendency to cake together and
lump. This leads to conditions as mentioned in Re (b). Powder temperature control must be carried out frequently on products that have caking
tendencies due to heat. Powder temperature is best measured during powder
packing. The product temperature in the bag or other container remains
constant during normal operation and any marked deviation can be readily
detected. Powder leaving a spray drying chamber or recovery unit is warm,
and often the pneumatic conveying system is used to cool the product while
it is conveyed to the packing or storage area. This cooling is often necessary
to prevent powder caking.
To obtain optimum cooling conditions, the inlet conveying air temperature
must be as low as possible. Normally conveying air is drawn from atmosphere within the dryer base vicinity. For indoor plants, the ambient temperature at the conveying air inlet often rises during the day, but these
increases are normally small and do not effect the degree of product cooling
to any marked extent. If the conveying air temperature does rise significantly over ambient conditions, the cause is often traced to excessive
amounts of warm drying air passing from the drying chamber into the conveying system. This results from either (a) too high an overpressure in the
drying chamber blowing warm air into the conveying system, (b) too low
an underpressure in the conveying system drawing air from the drying
chamber or (c) a dirty filter at the air intake. Conditions (a) and (b) are
remedied through adjustment to the air flow dampers of the dryer system.
Condition (c) is discussed under Re (a).
Re (d). Conveying systems handling hygroscopic products require conveying
air of low humidity. Any increase in humidity above that specified as
permissible will result in the product taking up moisture from the air with
eventual lumping and increased likelihood of deposit formation within
the duct horizontal runs and bends. When operating a low humidity air
conveying system, air conditions entering the system can be continuously
monitored at the dryer unit control panel. Alternatively for periodic control, a wet and dry bulb thermometer can be used in the air duct leading
to the powder entry point. Deviations in humidity must be rectified quickly
as moist deposits formed in the duct and left for some time cannot be removed just by'return of acceptable air conditions. Failure in air conditions
often requires plant shut-down followed by inspection and washing of the
bends in the conveying duct system.
Re (e). Incomplete washing of a duct system can lead to poor conveying
performance. Should product remain as a deposit lump in a duct after
washing, the lump will dry out and act as a nucleus for further rapid build-up
458
SURVEY OF AUXILIARY EQUIPMENT
SURVEY OF AUXILIARY EQUIPMENT
once the system is in operation again. Extra time spent on thorough cleaning
is always repaid in longer periods of trouble-free operation.
Pneumatic transport and its hazards have been recently reviewed by
Smith (325).
459
The transfer pump can also supply product directly to the atomizer
feed pump (figure 12.27(b)), The use of the transfer pump and an atomizer
feed pump in series is common, where heavy pastes are involved. Preheating
is necessary to assist feed flowability and enable continuous pumping
performance when the height of the atomizer above the pump is large.
12.5. Pumps
Pumps are required in the feed system to either transfer product within the
feed system or supply product to the atomizer. These duties involve variable
speed pump drives, throttling of pump discharge or recirculation of a proportion of the pump discharge. Various types of pumps are used due to the
variety of feeds that are encountered. There is an applicable pump design
for feeds that are in solution, slurry, suspension, melt or paste form. Pump
selection is based not only upon the feed properties, but on the atomizer
incorporated in the feed system. Where rotary atomizers or two fluid nozzles
are used, low pressure pumps suffice. Where pressure nozzles are employed,
high pressure pumps are required.
A
TO
ATOMIZER
PUMPS FOR SPRAY DRYERS
CENTRIFUGAL
POSITIVE DISPLACEMENT
B
ROTARY
SCREW
RECIPROCATING
GEAR
LOBE
El
PISTON
TO
ATOMIZER
PLUNGER
DIAPHRAGM
Figure 12.26. Classification of pumps.
12.5.1. Pump Duties in Spray Dryers
Pump duties are illustrated in the feed system layouts shown in figure 12.27.
(a) Transfer Pump Duty
This involves the transfer of feed from main feed tanks through preheating
or pretreating equipment into a smaller atomizer feed tank. The atomizer
feed tank supplies the atomizer feed pump. Transfer pump duties call for
pump characteristics of constant head and constant discharge, or variable discharge with nearly constant head. High pumping rates are required if
recirculation, is to maintain mixing in the main feed tanks. Much lower
pumping rates are involved if level control in the atomizer feed tank brings
the transfer pumping rate to equal the atomizer pumping rate (figure 12.27(a)).
C
TO
ATOMIZER
D
Figure 12.27. Feed system layouts. A, Preheater and level controlled feed tank. B. Preheater
and direct piping to atomizer. C. Direct feed system (variable speed pump or throttling valve).
D. Constant head feed system to atomizer.
460
SURVEY OF AUXILIARY EQUIPMENT
(b) Atomized Feed Pump Duty
This involves one pump to supply the atomizer from the main feed tanks
(figure 12.27(e)). Under steady state drying conditions, the required pump
characteristics are of constant capacity at constant head. In practice,
dryer control calls for slight variations in pump capacity at constant head.
Pumping requirements to the atomizer are generally low when compared
with pumping requirements in many chemical process industries.
(c) Gravity Feed Tank Pump Duty
This involves the pumping of feed to a constant head tank that supplies
the atomizer (figure 12.27(d)). This calls for a constant head and constant
capacity. Often high capacity pumping is required to assist feed mixing by
creating a high overflow rate.
12.5.2. Pump Types and Uses
The main classes of pumps for spray dryers are shown in figure 12.26.
Rotary pumps of the screw type meet the requirements for the majority
of installations. They are successfully used on such diversified products as
milk, tomatoes, pharmaceuticals and clay slips. Gear pumps are used with
heavy pastes. Diaphragm pumps are used with slurries containing large
irregular insoluble solids. Centrifugal pumps are used with both solutions
and slurries. High pressure dual or triple piston or plunger pumps are
successful for pressure nozzle atomization arrangements.
Details of these pumps are to be found in textbooks on the unit operations.
Recent literature includes developments in screw pump performance and
design by Polley (197) and analysis of likely pump breakdowns by Clements
(198). Pumps for special conditions of low suction pressure are discussed
by Pearsall and Scobie (199). Hattiangadi (200) reports on specifying centrifugal and reciprocating pumps.
SURVEY OF AUXILIARY EQUIPMENT
K
461
AR
MAINS
WATER
A.
MAINS
WATER
NOZZLES
OR
JETS
12.6. Washing Equipment
Washing equipment for spray dryer installations include (a) equipment
handled manually, (b) equipment continuous in operation (non-manual),
(c) equipment for cleaning-in-place (CIP).
The most common manual equipment in use is the spray water jet gun.
Available designs operate on cold water, or utilise steam to form hot water.
Hand operated spray guns are available from numerous nozzle manufacturers. With manual washing, thorough cleanliness can be obtained, but
it does depend upon good accessibility to all plant items, and sufficient
time within a production programme to enable operators to complete a
thorough washing.
Continuous operating equipment is often based upon tank-cleaning
requirements. A common equipment is the rotating nozzle head. The
ROOF LAY OUT
B.
CHAMBER LAYOUT
Figure 12.28. Automatic washing equipment. (a) Rotating nozzles/jets. (b) Roof mounted
nozzles/jets.
462
SURVEY OF AUXILIARY EQUIPMENT
nozzle arrangement is lowered into the chamber or cyclone (figure 12.28(a)).
Water is pumped to the rotating head (consists of 2 or 4 nozzles) at a high
rate. The head acts as a water turbine. The nozzles rotate in the vertical
plane while moving around the vertical axis. The water flow determines
the nozzle rotation rate. Over a fixed time interval, every point on the
chamber surface is impinged directly by a water jet. The wash water can
be recirculated or passed directly to drain. For washing a co-current drying
chamber (having rotary atomizer), the atomizer is first removed and the
cleaning nozzle arrangement lowered into the chamber. This is convenient
since the atomizer is withdrawn anyway to dismantle the wheel/disc and
liquid distributor. The nozzle arrangement is normally located one-third
the way down the chamber. A plate is fitted on to the supporting hose to
prevent water being sprayed into the air disperser. The arrangement is
ideal for cleaning large surface areas. It is unsuitable for cleaning items of
li mited surface area (e.g. ductwork), or chambers with central air dispersers.
CH' equipment involves the mounting of water nozzles at strategic positions
in the dryer installation. The nozzles douse surfaces that have become coated
with product during operation. The nozzle heads can be placed in position
prior to the cleaning operation and removed for the drying operation, or
can be a permanent fitting. CIP of a drying chamber is shown in figure
12.28(b). The use of CIP methods are becoming more and more popular
in drying plant, especially those handling food products (201).
Section V
APPLICATIONS OF SPRAY DRYING
IN INDUSTRY
13 Applications of Spray Drying
(A Literature Survey)
This chapter forms an updated version of a literature survey compiled by
the author in 1968 (1). References up to and including April 1972 are now
given. Products that have been spray dried on industrial or pilot plant scale
are given in table 13.1. A comprehensive list of patents concerning spray
dried products is given in table 13.2. Patents date from 1965 inclusive,
although a few older patents are given where of special significance.
Two decades of research and development have made spray drying a highly
competitive means of drying, whether the situation is a delicate operation
under sterile conditions or a high tonnage production of chemical products.
The range of products suitable for spray drying continues to expand. Unit
sizes and capacities have increased substantially through advancements in
technical know-how and an appreciation of spray dryer mechanisms,
especially in the atomization field. The chemical industry continues to
broaden its range of application. The dairy industry shows increased plant
capacity with the domestic demand for instant products and the wide-spread
utilization of milk products as constituents of animal feedstuffs. The food
industry remains an active field following the trend to food dehydration.
The chemical industry already applies spray drying to a great extent. The
spray drying of p.v.c. plastics is a typical example. Emulsion and suspension
(e-p.v.c. and s-p.v.c.) are the two main types requiring drying. Spray drying
is the only practical way for drying e-p.v.c. Particles are colloidal and the
emulsion is fed to the dryer at 30-60 % solids. Either rotary or nozzle
atomization is employed. Excessive inlet temperatures are not used due
to the heat sensitivity of the product. The dried product consists of hollow
globules from tiny cohesive particles. The degree of cohesion is regulated
by the drying temperatures.
s-p.v.c. consists of larger particles. Mechanical dewatering is possible
in a centrifuge from which a 75-80 % solids having a very loose consistency
466
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
(p.v.c. snow) is obtained. Spray drying can then be conducted with a rotary
atomizer of special design for non-liquid feeds. Such a method is of great
interest to firms having large capacity e-p.v.c. spray drying, but require
limited s-p.v.c. production. p.v.c. is discussed further in chapter 14 (14.1).
The ceramic industry widely utilizes spray drying especially in the production of ceramic pressbody following the wet process, The advantages of
direct transformation of the slip into an immediate pressable material are
in this instance particularly attractive. The physical properties of spray
dried material are ideal for pressing and firing operations, thus decreasing
rejection percentage. The properties of materials spray dried for dry pressing
have been reviewed by Storm (202). With the development of wear resistant
atomizing devices either rotary or nozzles can be applied. Choice is dependent upon the most suitable particle size for the pressing operation. Wheel
atomization has been used in large capacity units producing clays, ferrites,
aluminium oxide, and wall-tile material. Nozzle atomization is widely
applied to steatites, carbides and wall-tile material. Use of spray drying
for ferrites has been reviewed (203) (204) (205) (206). Different types of
spray dryers for ceramic products have been reviewed recently (1969) by
Ries (207). Other recent publications on spray drying in the ceramic industry
include Helsing (208) on oxide-ceramics, Lengersdorff (209) on ceramic
jiggering bodies, and Storm (210) on electronic press bodies, and an alumina
patent (211). Ceramics are discussed further in chapter 14 (14.2).
For detergent manufacture, dryer design is dependent upon product
type. Heavy-duty detergent units feature pressure nozzle atomization in
tall and slender counter-current drying towers. Light-duty detergent units
can be co-current designs. Special product handling prevents particle
fracture. Solids content of the feed can be up to 65 % and the poWder is
dried to 10-12 % moisture content. Inlet drying temperature range to
660°F (350°C). Silvis (212) and more recently (1970) Duckworth (213)
report all aspects of detergent drying. Fundamentals have been given by
Milwidsky (214) and Chaloud et al. (215). The adjustment of slurry
formulations is reviewed in further articles (216). Improved preparation
techniques of sodium tripolyphosphate for detergent slurries is based on a
novel plant design featuring a combination of spray dryer and calciner (217).
Further references to slurry preparation include improved dryer operation
(218) and detergent processing (219). Moore (121) reports on the effect of
drying variables on detergent powder characteristics. Detergents are
discussed further in chapter 14 (14.3).
In the field of agricultural pesticides, spray drying is applied to fungicides,
insecticides and herbicides. Rotary atomization (atomizer wheels) is
preferred as the resulting fine powder grain-size enables even dosage distribution. Ethylenebisdithiocarbamates of zinc and manganese, colloidal
APPLICATIONS OF SPRAY DRYING
467
sulphur, copper oxychloride are examples of spray dried fungicides. In the
production of herbicides, the sodium salts are usually spray dried. The
sodium salt of di-chloropropionic acid requires strict outlet drying temperature control as product decomposition is likely if variations of only
10°F (6°C) occur. Automatic control and built-in water spray safety devices
feature the plant design. Further examples of spray dried herbicides include
the sodium salt of methyl chlorophenoxy-acetic acid and the series of products having similar chemical structures. Pesticides are discussed further
in chapter 14 (14.4).
Both dyestuffs and pigments are spray dried. There has been much recent
interest in these products. Dryer designs use rotary atomization (atomizer
wheels) as a fine grain size is normally required. Maximum inlet drying
temperatures of 500°F (260°C) for organic dyestuffs are generally imposed.
Inorganic pigments are dried at much higher temperatures. For coarser,
non-dusty specifications, nozzle atomization is applied. Dyestuffs are
often produced in small quantities, and the cleaning aspects of dryer design
receive careful consideration. It is advantageous, however, to use one
drying unit for one product. Dye-slurry drying with rotary atomizer has
been reviewed (220). Kirady (221) reports experimental spray drying of
pigment slurries with pressure nozzle atomization. The use of two-fluid
nozzle atomization in the spray drying of aqueous acrylic polymer dispersion
paints is claimed to advantage (222). Feed solids up to 45 % and low outlet
drying air temperatures result in a 4 % moist Product bearing the exact
proportions of solid resin, pigment and pigment extender. The use of high
velocity air in two-fluid atomization applications is claimed to improve
the final quality of aqueous dispersion paint composition (223). Kaolin clay
was one of the first inorganic materials to be spray dried in vast tonnage
for use as a white pigment in paints. Other important uses include paper,
plastics, rubbers, pesticides and ceramics. Clarke (224), Fanselow (225),
Agnello (226) and Freeport kaolin (227) report spray drying operations on
kaolin. Titanium dioxide (228) is another white pigment extensively spray
dried. Either rotary or nozzle atomization can be used. Product collection
is by bag filters. Dyestuffs and pigments are discussed further in chapter 14
(14.5).
The past few years have seen spray drying become established in the
mining industry. Recent articles on spray drying of minerals (45) and spray
drying in the mineral processing industry by Damgaard-Iversen (229) have
described the developments that enabled spray drying to be accepted as a
heavy industry operation. Kleeman (46) describes the spray drying of nickel
concentrate ores in Australia. Spray drying of copper, molybdenum and
nickel ores in U.S.A. has also been recently reported (47). Mining applications are discussed further in chapter 14 (14.7).
468
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
New applications in the chemical industry are reported for silicon carbide
abrasive (230), carbon black (231), super-phosphate fertilizers (232) (233),
phosphorous-phosphoric acids (234), toners (235) (236) and cement slurry
(237) (238). The successful application (239) to spent sulphite liquors, a
by-product of the pulp and paper industry, has enabled marketing of powder
for uses including dispersing agents in insecticides, gypsum board manufacture, leather tanning, furnace clays, and animal feed and mineral ore
pelletizing. The conversion of crills from the pulp into a useful product can
be accomplished by spray drying the crills in aqueous suspension (240).
Application in the cellulose industry is discussed further in chapter 17.
Application of spray drying offers almost limitless possibilities for the
food industry. Entwistle (241) and Patsavas (242) have reviewed spray
drying from the food industry's viewpoint. Dairy products continue their
rapid growth through the diversity of specialized purposes to which such
powder can now be utilized. This has been accompanied by increased
demand for more stringent milk powder specifications. Spray drying of
milk was the subject of a symposium organised by the Australian Society of
Dairy Technology (243) (244). The definition of stable powder forms most
suitable for specific production applications is reported in further literature
(245) (246) (247) together with the development of processing techniques for
high tonnage production of premium grade powder of special physical
characteristics. The structure of milk powders and new forms of marketed
powder are reviewed elsewhere (248). Instant powder forms the basis of a
further review (246). For recent developments in spray dryer design for
milk the reader is referred to (249)(250) (506) (518).
Recent developments (1970) in the agglomeration of milk powder to instant
products have been reported by Bergmann (251). Some recent patent claims
for instant powder production include agglomeration during nozzle atomization (252) (253), lecithin dosage (254) (255) (256) (257) (258) and use of
binding agents (259). The other main by-prodncts of the dairy industry,
apart from the skim milk is whey formed from cheese making and from the
manufacture of casein. Spray dried whey powder is now utilized as a fodder
additive and as a constituent in the confectionery industry. Three spray
drying processes for producing ordinary and non-caking whey are discussed
by Galsmar and Bergmann (260). Two methods are reported for fresh
whey, which includes virtually all types of cheese-whey where further bacterial
acidification has been stopped by cooling. One method is reported for
acid whey (pH less than 4.6). Weber (261) reviews methods of whey utilization. Protein from whey using spray drying has been reported elsewhere
(262) (498).
Special techniques for the spray drying of other dairy products include
drying of fat-containing milk products (263) (264), ice-cream mix (265),
APPLICATIONS OF SPRAY DRYING
469
sour cream (266), cheese (267), dietetic products (268), casein (269) and
buttermilk (270). Dairy products are discussed further in chapter 15 (15.1).
The role of spray drying in the coffee industry is given by Sivetz (271).
Recent literature is limited, as at present more attention is being paid to
freeze drying techniques. However, improved continuous extraction techniques are yielding improved quality extracts for either spray or freeze
drying. Improved flavour and aroma retention is claimed with a spray drying
process which includes coffee fines stabilization through encapsulation of
the fines with extract (272). Beverages are discussed further in chapter 15
(15.2). Flavour retention in spray drying preconcentrated liquid foods,
e.g. coffee, tea, milk, fruit juices, is discussed by Thijssen (477).
Spray drying is applied in the food processing industry. The production
of tomato powder from concentrated pastes is growing with drying facilities
in Europe, U.S.A. and North Africa. The techniques of tomato spray drying
have been reported (273) (274). Descriptions of industrial installations are
available (275) (276) (277) (278). Adoption of spray drying to citrus fruit
(279) has enabled improvements in the quality of fruit powders reports
Urbanek (280). The mechanism of spray drying is suited for such heat
sensitive products, and the use of a carrier gum and inert gas packing aids
flavour retention and product handling of the hygroscopic powder. The
production of soluble citrus fruit powder free from preservatives using spray
drying techniques results in retention of natural fruit flavours (281). A
successful application to raspberries is also evident (282). The spray drying
of tomatoes, apple, banana and vegetables, where skim milk is used as a
carrier is reported by Breene and Coulter (283). For flavouring materials,
an imitation brown sugar processed by spray drying has been marketed
(284). There is news of a production of a low caloried sweetener based on
dextrin and at least one noncaloric sweetening agent (285). Pressure nozzle
atomization is employed with up to 65 % feed solids. The dried powder
contains 6-10 % moisture. An experimental investigation into the spray
drying of flavouring materials is reported by Brooks (286). Measurements
were made on the loss of ethyl-caprylate dispersed in aqueous gum arabic.
Details on spray drying in the flavouring industry is reported elsewhere (287).
A process for the preparation of dry flavouring materials from crustaceans
for human consumption is reported by Gray (288) to utilize a spray drying
stage. Dry free-flowing glucose production from starch conversion also
includes a spray dryer (289) (290). The dryer features a special fines return
system, whereby powder is returned to the atomization zone. Spray drying
of molasses for animal fodder is described (291) (517). Feed contains
3-8 % kaolin, and is atomized by a rotating disc into a chamber having air
swept walls. The development of spray drying to potato processing has
resulted in three patents (292) (293) (294). A pneumatic paste nozzle is
470
APPLICATIONS OF SPRAY DRYING
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
featured for atomizing the mashed potato, and the process is claimed to
prevent the breakage of potato cells, which results in the formation of a
glutinous mass on reconstitution. A 9-inch bowl atomizer in a co-current
flow drying tower features the patent coverage (292) (293) where powder at
5-6 % moisture (bulk density 0.5-0.6 g/cm 3 ) yields prime mashed potato
flavour on reconstitution with water/milk. The bowl design and operating
conditions prevent excessive shearing effects that cause cell rupture. The
spray drying of starch (both pregelatinized and coldsetting starch) is reported
(295). High pressure nozzle atomization is used in the production of pastry
mixes (296). The production of high protein food additives from soyabean,
plant extract (297) (298) is growing in importance. Drying of liquorice
extract has been recently reported (519).
The modification of basic spray designs to meet sterile processing conditions is reported (299). The use of one and two stage units are described to
deal with the low feed solid contents usually dried. Blood serum application
is referred as an example. A laboratory dryer design for biological materials
is described by Freeman (300). A thesis on spray drying enzyme rennet has
been presented by Shah (301). Amylase drying appears elsewhere (302).
Processing of cellulose to produce protein using micro-organisms, involving
spray drying is reported recently (303). Application in the biological industries are discussed further in chapter 16.
By utilizing spray drying for processing abattoir products, a hitherto
considered waste can be converted into highly profitable commodities.
Abattoir by-products include blood, hair, glands, tissue and organs. Bergsoe
and Fakstorp (304) describe the production of dehydrated glands and tissues,
and layout the flow sheets for blood processing to ordinary and soluble
blood powder, blood cell powder and blood albumin. The production of
high protein foodstuffs by spray drying of meat extracts and hydrolysate
is noted. Gelatine drying in spray dryers is also practised (305).
In the following chapters 14-18, many of the above mentioned products
are considered in more detail. Chapter 14 deals with the important applications in the chemical industry, i.e. plastics, ceramics, detergents, pesticides,
dyestuffs, fertilizers, mineral ore and sundry organic and inorganic chemicals.
Chapter 15 is devoted to applications in the food industry with special emphasis given to milk products, eggs, extracts, fruit and vegetables and some
carbohydrates. Chapter .16 describes the spray drying of pharmaceuticals
and yeast in the biochemical industry. The tannin and cellulose industries
are briefly mentioned in chapter 17. The chapters on various applications
are completed with descriptions of spray drying in the offal and fish industries
(chapter 18).
471
Table 13.1, Products Suitable for Suspended Particle Processing Systems, involving
Atomization. (Spray Drying, Spray Cooling, Spray Reaction, Spray Purification.)
14. Chemical industry
14.1. Plastics, resins
AB and ABS latex
Melamine formaldehyde resin
Phenol formaldehyde tesin
Polyacrylate-emulsion-type
Polyacrylonitrile
Polyethylene
Polyvinyl acetate
Polyvinyl butyrate
Polyvinyl chloride emulsion-type
Polyvinyl chloride suspension-type
Polyvinyl pyrrolidone
Polyvinyl toluene Rubber latex
SBR latex
Urea formaldehyde resin
14.2. Ceramic materials
Aluminium oxide
Beryllium oxide
Carbides
Carborundum
Electro-porcelain
Enamels
Floor tile material
Ferrite
Glass powder
Insulator material
Iron oxide
Kaolin
Silicium oxide
Spark plug material
Steatite
Titanates
Tungsten carbide
Uranium oxide
Wall tile material
Zirconium silicate
14.3. Detergents and Surface active agents
Alkyl-aryl sulphonates
Detergent enzymes
Dispersing agents
Emulsifying agents
Fatty alcohol sulphate
Heavy-duty detergents
Light-duty detergents.
Mono- and dipotassium orthophosphate
Mono- and disodium orthophosphate
Nitrilo tn-acetic acid salts
Optical brightener
Phosphates
Saponine
Soap
Tetra potassium polyphosphate
14.4. Pesticides
Calcium arsenate
Copper oxychloride
Cuprous oxide
2,4 DBA sodium salt
2,4,6 TBA sodium salt
DDT with filler
2,4 dichloro-phenoxyacetic acid
2,4 dichloro-phenoxypropionic acid
mono-methylamine salt
Dichloro-phenoxypropionic acid
Dichloro-propionic acid sodium salt
Diemthyl-dipyridyl dichloride
Lead arsenate
Mangano-ethylene-bis-dithiocarbamate
Methyl-chloro-phenoxyacetic acid
Methyl-chloro-phenoxypropionie acid
Methyl-chloro-phenoxybutyric acid
sodium salt
Sodium aluminium fluoride
Sodium fluoride
Sodium methyl arsenate
Sodium penta-chlorophenolate
Sulphur colloidal
Zinc-ethylene-bis-dithiocarbamate
Zinc diethyl dithiocarbamate
Zinc dimethyl dithiocarbamate
14.5. Dyestuffs, pigments
Basic dyes
Ceramic colours
Chrome-yellow
Copper oxide
Cosmetic colours
Dyestuff intermediates
Formulated dyestuffs
Food colours
Indigo dye
Ink pigments
472
APPLICATIONS OF SPRAY DRYING
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Table 13.1—continued
Inorganic pigments
Iron oxide
Kaolin
Lithopone
Milori blue
Organic pigments
Paint pigments
Paper colours
Phthalocycanines
Plastic pigments
Soluble and microdisperse textile dyes
Titanium dioxide
Water colours
Zinc chromate
Zinc potassium chromate
Zinc tetroxychromate
14.6. Fertilizers
Nitrogen fertilizers
Ammonium salts
Urea
Phosphoric acid fertilizers
Superphosphates
Potash fertilizers
Two component fertilizers
(N–P,
P–K)
Three component fertilizers
(N–P–K)
14.7. Mineral flotation concentrates
Copper ore
Cryolite
Iron ore
Lead ore
Manganese ore
Molybdenum ore
Nickel ore
Silver ore
Tungsten ore
14.8. Inorganic chemicals
Abrasive grits
Aluminium (metallic)
Aluminium chloride
Aluminium hydroxide
Aluminium oxide
Aluminium phosphate
Aluminium silicate
Aluminium sulphate
Ammonium chloride
Ammonium molybdate
Ammonium nitrate
Ammonium phosphate
Ammonium sulphate
Antimony sulphide
Arsenic oxide
Barium carbonate
Barium chloride
Barium hydroxide
Barium sulphate
Barium titanate
Bauxite waste liquor
Bentonite
Beryllium dioxide
Bismuth aluminate
Bismuth carbonate
Borax
Boric acid
Boron phosphate
Calcium carbonate
Calcium chloride
Calcium hydroxide
Calcium chromate
Calcium hypochlorite
Calcium nitrate
Calcium phosphates
Calcium silicates
Calcium sulphate
Catalysts
Cement
Chrome-iron oxide
Chromium sulphate
Copper chloride
Copper oxide
Copper oxychloride
Copper sulphate
Copper sulphide
Cupric oxide
Cryolite
Feldspar
Ferric sulphate
Ferrous sulphate
Graphite
Gypsum
Kaolin
Lead zirconate
Lithopone
473
Table 13.1—continued
Lithium chloride
Magnesium aluminium silicate
Magnesium carbonate
Magnesium chloride
Magnesium hydroxide
Magnesium phosphates
Magnesium sulphate
Magnesium trisilicate
Magnesium uranate
Manganese carbonate
Manganese chloride
Manganese oxide
Manganese sulphate
Molybdenum disulphide
Nickel carbonate
Nickel chloride
Nickel sulphide
Potassium acetate
Potassium carbonate
Potassium bicarbonate
Potassium bichromate
Potassium carbonate
Potassium chlorite
Potassium chromate
Potassium metaphosphate
Potassium nitrate
Potassium permanganate
Potassium phosphate
Potassium silicate
Powdered metals
Silicon dioxide
Silicon carbide
Sodium aluminate
Sodium aluminium sulphate
Sodium arsenate
Sodium bicarbonate
Sodium bichromate
Sodium bisulphate and sodium sulphate
Sodium bisulphide
Sodium borate
Sodium carbonate
Sodium chloride
Sodium hypochlorite
Sodium fluoride
Sodium formate
Sodium hydroxide
Sodium perborate
Sodium phosphate
Sodium silicate
Sodium silicon fluoride
Sodium sulphate
Sodium thiosulphate
Thorium carbonate
Thorium nitrate
Titanium dioxide
Titanium tetrachloride
Tungsten carbide
Uranium dioxide
Zinc arsenate
Zinc carbonate
Zinc chloride
Zinc sulphate
Zirconates
14.9. Organic chemicals
Aluminium stearate
Aluminium triformate
Amino-naphthol-sulphonic acid
para-Aminosalicylic acid
Amino acid
Ascorbic acid
Ammonium-di-nitro-ortho cresol
Barbituric acid derivate
Barium ricinoleate
Calcium acetate
Calcium butyrate
Calcium gluconate
Calcium caseinate
Calcium lactate
Calcium proprionate
Calcium saccharates
Calcium stearate
Carboxymethyl cellulose
Chloramine
Chlorohexidine gluconate
Chloromycetine succinate—calcium salt
Chlorophyll
Choline salts
Citric acid
Dicyandiamide
Dicyclohexylphthalate
Diethyldiphenylurea
2,4 dichlorophenoxy-acetic acid sodium
salt
Dodecylbenzenesulphonate sodium salt
Glutamic acid
474
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Table 13.1—continued
Glyoxal
Lactose
Lysine
Maleic acid
Methyl acrylic acid
Methyl arsenic acid–disodium sulphate
Mono-carbonic acid
Mono-ethanol aminosulphate
Monochloroacetic acid sodium salt
Oxalic acid
para-nitro phenol
Pantholtenates
Perfumes
Potassium acetaldehyde sulphoxylate
Potassium acetate
Potassium isopropylxanthogenate
Potassium phthalate
Potassium sorbate
Quinoline sulphonate
Sodium acetate
Sodium benzene sulphonate
Sodium benzoate
Sodium di-oxalate
Sodium dimethyl dithiocarbamate
Sodium monochloroacetate
Sodium salicylate
Stearic acid
Tartaric acid
Thiocarbamates
Thionine
Urea
Waxes
Xanthates
Zinc stearate
15. Food industry
15.1. Milk products and eggs
Baby food
Butter (high fat products)
Buttermilk
Casein
Caseinates
Cheese
Cocoamilk with sugar
Coffee whitener
Cream
Egg—Egg white
Egg yolk
Whole egg
Ice cream mix
Malted milk
Milk replacer
Mixed milk products
Skim milk
Whey
Whey mother liquor
Whole milk
15.2. Food products and plant extracts
Artichoke extracts
Beef broth
Bouillon
Cake mixes
Camomile tea
Cereals, pre-cooked
Chicken broth
Chlorophyll
Cocoa mixtures
Coffee-extracts
Coffee-substitute extract
Decaffeinated coffee extract
Fat flour mixtures
Fish proteins
Flavourings
Food colouring
Garlic
Hip-extract
Lactose
Liquorice extract
Malt extract
Meat puree
Milk coffee mixture
Pimento
Plant protein
Proteinhydrolysate
Rennet
Soup mixes
Tea extract
Whiteners (coffee/tea)
15.3. Fruit, vegetables
Apricots
Asparagus
Banana
APPLICATIONS OF SPRAY DRYING
Table 13.1—continued
Beans
Beetroot
Carrot
Citrus fruits
Mangoes
Onion
Peaches
Soft fruits
Tomato
15.4. Carbohydrates and similar products
Baking compounds
Corn steep liquor
Corn syrup
Glucose
Gum arabic
Maize gluten
Pectin
Sorbitol sorbose
Starches
Sugar/gelatin mixtures
Total sugar
Wheatflour
Wheat gluten
16. Biochemical industry
16.1. Pharmaceutical products
Antibiotics and moulds
Bacitracin
Penicillin
Streptomycin
Sulphatiazole
Terramycin
Tetracyclin
Dextran
Enzymes
Hormones
Lysine (Amino acids)
Pharmaceutical gums
Sera
Spores
Tabletting constituents
Vaccines
16.2. Yeast products
Brewers yeast
Fodder yeast
Yeast extracts
Yeast hydrolysates
17. Industries utilizing chemicals from
timber
17.1. Tannins
Tannins from bark
Mangrove extract
Mimosa extract
Oak extract
Pine and fir extract
Tannins from wood
Quebracho extract
Chestnut extract
Tannins from fruits
Dividivi extract
Myrobalan extract
Tannins (synthetic)
17.2. Cellulose
Sulphite waste liquor
Lignosulphonates
18. Offal and fish industries
18.1. Slaughterhouse products
Animal protein
Blood
Bone glue
Excreta
Gelatin
Glands
Tissues
18.2. Fish products
Fish flour
Fish hydrolysates
Fish meal
Fish pulp
Fish stickwater (solubles)
Whale stickwater (solubles)
475
(Dates given for both patent specifications and applications are the dates of publication. If equivalents in various countries have
been published preference has been given to US. or British, if possible.)
Product
group M.
(See table 13.1)
Product
14 Chemical industry
14.1 p.v.c.
Plastics,
Resins
Rubber
lactices
Acrylic
paint
Chlorinated
polyisoprene
Epoxy
polyamide
a.o.
Organopolysiloxanes.
p.v.ac.
paint
p.v.ac.
Polyethylene
Polyethylene
Various
Patent or
application no.
Brit. pat.
952 628
U.S. pat.
3 194 781
U.S. pat.
3 325 425
German pat. appl.
2 003 528
Brit. pat.
1 138 180
U.S. pat.
3 383 773
U.S. pat.
3 287 290
Danish pat
106 467
Brit. pat.
1 033 657
Japanese pat. appl.
70.05146
Danish pat.
107 901
Patentee and/or inventor(s)
Date
Farbwerke Hoechst Aktiengesellschaft
March 18, 1964
Du Pont de Nemours and Company, Hedberg, J. G.,
O'Donnell, H. B., Peet, G. W.
Monsanto Company, Bray, W. J.
July 13, 1965
June 13, 1967
Imperial Chemical Industries Ltd., Felstead, E.
Sept 17, 1970
Yarsley Research Laboratories-Ltd., Lanham, B. J.,
'Tyke', V. C.
Dec. 27, 1968
Owens Illinois, Inc., Nugent, D. C.
May 21, 1968
Monsanto Company, Bray, W. J.
Nov. 22, 1966
Edison Societa per Azioni
May 8, 1967
Eastman Kodak Company, Wright, J. F., Shahan, N. D.
June 22, 1966
Sakamoto, N.
Feb. 20, 1970
Rohm & Haas Company
July 17, 1967
-11.•
a.
Alusn,amNI ONIAlltaAvuds30 SNOLINDrIcicni
Table 13.2. Recent Patents and Patent Applications dealing with Products to which Spray Drying or Spray Cooling is Applied
Table 13.2—continued
14.2 Ceramic
materials
14.3
Detergents
and surface
active agents
Product
Patent or
application no.
Patentee and/or inventor (s)
Date
Oxide
ceramics
Plastic
bodies
Calcium
zirconate
a.o.
Cellular
clay
material
Brit. pat.
1 081 730
German pat. appl.
1 268 551
French pat.
1 452 220
General Motors Corporation, Fenerty, M. J., Somers,
A. V.
Rheinisch-Westfdlische Isolatoren-Werke GmbH.,
Draeger, K.
Societe de Fabrication d'elements Catalytiques
Aug. 31, 1967
French pat.
1 537 564
Synfibrit GmbH
Aug. 23, 1968
Detergents
U.S. pat.
3 357 476
U.S. pat.
3 369 304
U.S. pat.
3 380 922
Brit. pat.
1 159 643
Brit. pat.
1 189 543
Brit. pat.
1 208 372
German pat. appl.
1 961 450
Colgate-Palmolive Company, Toffiemire, R. H.
Dec. 12, 1967
Monsanto Company, Clark, S. G., Feierstein, H. E.
Feb. 20, 1968
Purex Corporation, Ltd., Shields, G. G., Patterson,
C. B.
Mo och Domsjo AB
Apr. 30, 1968
July 30, 1969
Colgate-Palmolive Company
Apr. 29, 1970
Colgate-Palmolive Company
Oct. 14, 1970
Unilever, N. V., Bauer, H. E.
July 2, 1970
Detergents
Detergents
Detergents
Detergents
Detergents
Detergents
May 16, 1968
Sept. 9, 1966
ONIAIICI AV MS JO SNOLLVDMIcIY
Product
group no.
(See table 13.1)
Table 13.2—continued
–4
cc
Product
Detergents
Detergents
Detergents
Detergents
Detergents
Detergents
Optical
brighteners.
Emulsifier
14.4
Pesticides
14.5
Dyestuffs and
pigments
General
Dyestuffs
Dyestuffs
Patent or
application no.
Patentee and/or inventor(s)
Date
Henkel & Cie GmbH
Nov. 5, 1970
Witco Chemical Company, Inc., Mausner, M. L.,
Rainier, E. T.
Colgate–Palmolive Company
Nov. 10, 1967
Feb. 27, 1970
Colgate–Palmolive Company
May 22, 1970
Unilever, N. V, Kerkhoven, F. J., Troost, S.
Sept 22, 1968
Colgate-Palmolive Company, Reinish, M. D.
Oct. 26, 1968
Farbenfabriken Bayer Aktiengesellschaft, Kleinheidt,
E. A., Meier, E., Scholermann, W., Schonol, K.
Aktieselskabet Grindstedvxrket, Jensen, K. S.,
Andreasen, J.
Dec. 10, 1969
Dutch pat. appl.
70 01446
German pat. appl.
1 812 574
Farbwerke Hoechst A.G.
Aug. 7, 1970
Riedel-de Haen A.G.
June 11, 1970
British pat appl.
1 233 479
Brit pat
1 064.924
Brit. pat
1 069 380
Farbenfabriken Bayer Aktiengesellschaft, Mols, H.,
Hornle, R., Raab, H.
Farbenfabriken Bayer Aktiengesellschaft, Breig, K.,
Blum, W., Muller, G., Steinmetz, G.
Farbenfabriken Bayer Aktiengesellschaft, Breig, K.,
Blum, W., Miiller, G., Raab, H.
May 26, 1971
German pat. appl.
1 922 451
French pat.
1 501 615
French pat. appl.
2 011 190
French pat.
1 590 487
Dutch pat. appl.
67 04145
Danish pat appl.
1807/68
Brit. pat.
1 173 806
Danish pat. appl.
112 247
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Product
group no.
(See table 13.1)
Nov. 25, 1968
Apr. 12, 1967
May 17, 1967
Table 13.2—continued
Product
group no.
(See table 13.1)
Product
Patent • or
application no.
Dyestuffs
Belgian pat.
677 910
Carbon
U.S. pat
black
3 433 660
Electrostato- U.S. pat.
graphic toner 3 326 848
Electrostato- U.S. pat.
graphic toner 3 338 991
Kaolin
U.S. pat.
clay
3 372 043
Opacifiers
German pat. appl.
2 019 765
Zn yellow
Polish pat.
a.o.
58 625
14.6
Fertilizers
General
(prilling)
General
(prilling)
General
(prilling)
General
(prilling)
General
(prilling)
U.S. pat.
3 334 160
Brit. pat.
1 209 454
Brit. pat.
1 209 886
German pat.
1 227 429
German pat.
1 228 231
Patentee and/or inventor(s)
Date
Farbenfabriken Bayer Aktiengesellschaft, Breig, K.,
Blum, W., Muller, G., Raab, H., Steinmetz, G.
Cabot Corporation, Jordan, M. E., Hardy, J. F.
May 31, 1966
Xerox Corporation, Clemens, C. F., Lenhard, M. J.
June 20, 1967
Xerox Corporation, Insalaco, M. A., Clemens, C. F.,
Lenhard, M. J.
Engelhard Minerals & Chemicals Corporation,
Fanselow, J. R.
The Champion Paper Company, Ltd., Vassiliades, A. E.,
Nauman, E. F., Shroff, S.
Instytut Cheminieorganicznej, Bednarczyk, K.,
Chejduga, A., Bandrowska, C.
Aug. 29, 1967
March 18, 1969
March 5, 1968
Nov. 12, 1970
Nov. 5, 1969
2
Gulf Oil Corporation, Wisneski, P. M., Mosier, C. F.
Aug. 1, 1967
Fisons Fertilizers Limited, Bradley, J. K., Sherwin, K.A.
Oct. 21, 1970
(
Fosfatbolaget AB
Oct. 21, 1970
Pzt
Badische Anilin- & Soda-Fabrik Aktiengesellschaft,
Zapp, K. H.
Badische Anilin- & Soda-Fabrik Aktiengesellschaft,
Jockers, K., Gettert, H.
Oct. 27, 1966
Nov. 10, 1966
0,
,.
Product
group no.
(See table 13.1)
Product
Patent or
application no.
General
German pat. appl.
(prilling)
1 263 707
General
French pat.
(prilling)
1 589 598
Ammonium U.S. pat.
nitrate
3 533 776
Ammonium German pat. appl.
nitrate
1 964 614
Ammonium U.S. pat.
phosphate 3 214 260
Ammonium Danish pat. appl.
phosphate
113 926
Mixed
U.S. pat.
fertilizers
3 539 326
Mixed
Brit. pat.
fertilizers
1 104 253
Mixed
French pat.
fertilizers
1 519 801
Mixed
Dutch pat. appl.
fertilizers
69.06224
Calcium
German pat. appl.
ammonium
1 907 361
nitrate
Urea
German pat. appl.
1 246 681
Patentee and/or inventor(s)
Date
Potasse & Engrais Chimiques, Quanquin, B., Le Clerc,
G. M., Pierre, J.-L.
Norsk Hydro-Elektrisk Kvelstofaktieselskab,
Friestad, I. A.
Fisons Fertilizers Limited, Coates, R. V., Curtis, M. F.,
Harris, G. J., Smith, P. S.
Dzerzhinsky Filial Nauchno-Issledovatelskogo
I Konstruktorskogo Instituta Khimicheskogo
Mashinostroenia, Vagin, A. A.
Nissan Chemical Industries, 0i, Y., Kusanagi, S.,
Arata, M.
Norsk Hydro-Elektrisk Kvxlstofaktieselskab, Friestad,
I. A., Skauli, 0.
Mitsui Toatsu Chemicals Incorporated, Otsuka, E.,
Takada, M., Matsuo, K., Murozono, H., Nagayama, T.
Fisons Fertilizers Limited, Chapman, J. D., Dee, G. T.,
Janikowski, S. M., Peters, R. H.
Ugine Kuhlmann, Gittenait, M.
March 21, 1968
March 31, 1970
Oct. 13, 1970
Oct. 8, 1970
Oct. 26, 1965
May 12, 1969
Nov. 10, 1970
Feb. 21, 1968
AWISCIUNI NI 9NIIAII0 A.V1MS AO SNOIIVOlicicIV
Table 13.2—continued
April 5, 1968
Stamicarbon N. V.
Oct. 27, 1970
Ruhrchemie AG, Weitendorf, K. F., Traenckner, K. C.
Aug. 27, 1970
The Lummus Company, Summerville, R. N.
Aug. 10, 1967
Table 13.2—continued
Product
Urea
14.7
Minerals—
ore flotation
concentrates
14.8
Inorganic
chemicals
Catalysts
Catalysts
Catalysts
Catalysts
Catalysts
Cement
Cement
Cement
Lead
powder
Sodium
carbonate
Patent or
application no.
Patentee and/or inventor(s)
Date
French pat.
1 475 268
The Chemical and Industrial Corp.
March 31, 1967
Swedish pat. appl.
325 242
A/S Niro Atomizer, Damgaard-Iversen, J.
June 29, 1970
U.S. pat.
3 347 798
U.S. pat.
3 433 587
Belgian pat.
741 001
Danish pat.
107 416
Danish pat.
113 217
German pat. appl.
2 048 178
German pat. appl.
2 044 457
Danish pat.
108 487
Danish pat. appl.
115 422
U.S. pat.
3 202 477
Badische Anilin- & Soda-Fabrik A.G., Baer, K.,
Sperber, H., Goehre, 0., Leibner, G.
Engelhard Minerals & Chemicals Corporation, Haden,
W. L., Dzierzanowski, F. J.
Nitto Chemical Industry Limited, Nakamura, Y.,
Saito, S., Sasaki, Y.
The Standard Oil Company
Oct. 17, 1967
March 18, 1969
Apr_ 1, 1970
Sept. 25, 1967
National Lead Company, Granquist, W. T.
March 3, 1969
A/S Niro Atomizer, Damgaard-Iversen, J., Kruse, F.
Apr. 15, 1971
A/S Niro Atomizer, Gude, K. E., Lund, B.
March 11, 1971
Dyckerhoff Zernentwerke A.G.
March 28, 1967
Associated Lead Manufacturers Limited, Kinsell, D.
Oct. 6, 1969
Diamond Alkali Company, Loeffler, J. E. jr., Springer,
R. A., Bolick, E. L.
Aug. 24, 1965
ONIAIICI AVIIcIS JO SINIOLINDI1c1c1V
Product
group no.
(See table 13.1)
Table 13.2—continued
Product
Iron
sulphate
Alkali
metal
perborate
Alkali metal
perborate
Aluminium
oxide
Aluminium
oxide
Aluminosilicate
Aluminosilicate
Calcium
chloride
Calcium
chloride
Metal
oxides
Metallurgical
powders
Nickel and
Cobalt
sulphate
Patent or
application no.
U.S. pat.
3 195 981
U.S. pat.
3 510 269
Dutch pat. appl.
69.10222
U.S. pat.
3 411 878
Swedish pat. appl.
313 553
U.S. pat.
3 472 617
Japanese pat. appl.
70.19167
U.S. pat.
3 339 618
U.S. pat.
3 196 930
U.S. pat.
3 273 962
Brit. pat.
1 110 386
Belgian pat.
734 953
Patentee and/or inventor(s)
Date
British Titan Products Company Limited,
Hansford, K. R., Roberts, A. L., Evans, A. W.
Deutsche Gold- und Silber-Scheideanstalt, Bittner, F.,
Simmerback, E., Dahm, F. L., Schaller, A.
Dec. 10, 1962
May 5, 1970
Deutsche Gold- und Silber-Scheideanstalt
Feb. 17, 1970
Produits Chimiques Pechiney-Saint-Gobain, Graulier,
M., Michel, M.
Universal Oil Products Company, Moehl, R. W.
Nov. 19, 1968
W. R. Grace & Co., McDaniel, C. V., Maher, P. K.,
Pilato, J. M.
Universal Oil Products Co.
July 1, 1968
Oct. 14, 1969
July 1, 1970
Wyandotte Chemicals Corporation, Bowden, J. H.,
Terry, C. T.
Knapsack-Griesheim Aktiengesellschaft, Ebert, H.,
Giesswein, K., Harmsen, E., Vomberg, H.
Monsanto Company, Walsh, R. J.
Sept. 5, 1967
Cabot Corporation
Apr. 18, 1968
A/S Niro Atomizer, Nielsen, K., Nielsen, H. B.
Aug. 29, 1969
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Product
group no.
(See table 13.1)
July 27, 1965
Sept. 20, 1966
Table 13.2—continued
Product
group no.
(See table 13.1)
Product
Brit. pat.
1 189 656
U.S. pat.
3 383 172
French pat.
1 431 784
U.S. pat.
3 538 200
U.S. pat.
3 338 671
U.S. pat.
3 385 661
U.S. pat.
3 387 924
U.S. pat.
3 499 476
Brit. pat.
1 026 675
Brit. pat.
1 158 178
German pat.
2 011 336
Patentee and/or inventor(s)
Date
Fisons Fertilizers Limited, Smith, P. S.
Apr. 29, 1970
Deutsche Gold- und Silber-Scheideanstalt, Biegler, H.,
Kallrath, G.
Produits Chimiques Pechiney-Saint-Gobain,
Golovtchenko, M. V.
Shell Oil Co., Hite, J. R.
May 14, 1968
Feb. 7, 1966
FMC Corporation, Marschall, H. L., Coykendall, J. W.
Aug. 29, 1967
Central Glass Co., Hayakawa, M., Yasutake, Y.
May 28, 1968
Knapsack Aktiengesellschaft, Hartlapp, G., Heymer, G.,
Traulsen, K.
Knapsack Aktiengesellschaft, Hartlapp, G., Klee, H.,
Koch, J.
Buttner-Werke Aktiengesellschaft
June 11, 1968
Apr. 20, 1966
Albright & Wilson (Australia) Limited
July 16, 1969
FMC Corporation, Herink, J. F.
Oct. 1, 1970
Nov. 3, 1970
March 10, 1970
APPLICATIONS OF SPRAY DRYING
Potassium
chloride
Silicon
dioxide
Sodium
bisulphate
Sulphur
molten
(prilling)
Tripolyphosphate
Tripolyphosphate
Tripolyphosphate
Tripolyphosphate
Tripolyphosphate
Tripolyphosphate
Tripolyphosphate
Patent or
application no.
Table 13.2—continued
14.9
Organic
chemicals
Product
Borate
sequestering
agent
Diphenylolpropane
Fumaric
acid a.o.
Urea,
sodium
glutamate
a.o.
15 Food Industry
15.1 Milk
products and
eggs
15.1.1
General
Milk
products
General
General
General
General
Patent or
application no.
00
Patentee and/or inventor(s) .
Date
U.S. pat. appl.
3 539 463
W. R. Grace & Co.
Nov. 10, 1970
Dutch pat. appl.
66.16683
Japanese pat. appl.
70.32217
Japanese pat. appl.
70.22926
Stamicarbon N.V.
May 27, 1968
Ueno Seiyaku KK.
Oct. 17, 1970
Nippon Shiryo Kogyo KK.
Aug. 3, 1970
U.S.A. (Secretary of Agriculture), Hanrahan, F. P.,
Bell, R. W., Webb, B. H.
Foremost Dairies, Inc., Hutton, J. T., Nava, L. J.,
Shields, J. B., Kempf, C. A.
Koopmans Meelfabrieken N.V., Giddey, C.
May 25, 1965
Jan. 25, 1966
Industrie-Werke Karlsruhe AG
Sept. 28, 1966
Dr. Max E. Schulz
Nov. 10, 1966
U.S. pat.
3 185 580
U.S. pat.
3 231 386
Brit. pat.
992 758
Brit. pat.
1 044 268
German pat.
1 228 567..
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Product
group no.
(See table 13.1)
May 19, 1965
Table 13.2—continued
Product
group no.
(See table 13.1)
Product
General
General
Skim
milk
Whole milk
Whole milk
Whole milk
Whole milk
Whole milk
Whey
Whey
Whey
Whey
German pat. appl.
1 262 114
Japanese pat. appl.
70.32223
U.S. pat.
3 410 701
U.S. pat.
3 164 473
U.S. pat.
3 278 310
U.S. pat.
3 300 315
U.S. pat.
3 301 682
Brit. pat.
1 005 825
Brit. pat.
1 196 501
U.S. pat.
3 222 193
Brit. pat.
1 137 228
Brit. pat.
1 212 258
German pat. appl.
1 492 787
Patentee and/or inventor(s)
Date
Foremost Dairies, Inc., Hutton, J. T., Nava, L. I.,
Shields, J. B., Kempf, C. A.
Meiji Milk Prods. Co. Ltd.
Feb. 29, 1968
Etablissements Laguilharre, Ciboit, J. J.
Nov. 12, 1968
Dairy Foods Incorporated, Shields, J. B., Nava, L. J.,
Kempf, C. A.
Borden Company, Williams, A. W., Busch, A. A.
Jan. 5, 1965
Foremost Dairies, Inc., Nava, L. J., Hutton, J. T.,
Shields, J. B., Kempf, C. A.
Carnation Company, Loo, C. C.
Jan. 24, 1967
Nestle's Products Limited
Sept. 29, 1965
Battelle Memorial Institute
June 24, 1970
U.S.A. (Secretary. of Agriculture), Hanrahan, F. P.
Dec. 7, 1965
Foremost Dairies, Inc.
Dec. 18, 1968
Emery Carlton Swanson, Swanson, E. C., Henderson,
R. J., Kyle, R. C.
Flachroste Berching GmbH, Schanze, R
Nov. 11, 1970
Oct. 17, 1970
Oct. 11, 1966
Jan. 31, 1967
APPLICATIONS OF SPRAY DRYING
Whole milk
Patent or
application no.
Sept. 11, 1969
-1=.•
00
Jl
Product
group no.
(See table 13.1)
Product
Whey
Whey
Whey
Whey
Whey
Whey
Whey
Whey
Baby food
Coffee/tea
whitener
Coffee/tea
whitener
Cheese
Ice cream
mix
Patent or
application no.
German pat. appl.
1 921 793
German pat. appl.
1 943 422
German pat. appl.
2 007 960
Dutch pat. appl.
67.05296
Dutch pat. appl.
70.04843
U.S. pat.
3 522 054
German pat.
2 027 679
French pat.
2 012 982
East German pat.
29 870
U.S. pat.
3 241 975
U.S. pat.
3 321 318
Brit. pat
1 147 105
U.S. pat. •
3 215 532
,
continued
DC
41
Patentee and/or inventor(s)
Date
Ralston Purina Company, Arndt, R. H.
Nov. 27, 1969
Kraftco Corp., Engel, M. E.
March 5, 1970
Kraftco Corp., Singleton, A. D., Eggen, I. B.
Sept. 24, 1970
Foremost Dairies, Inc.
Oct. 16, 1967
Home Products Corp.
Oct. 6, 1970
Cavroy, P. G. P., Rambaud, M. E M., Cousin,
C. M. J. E., Savignac, G. G. E.
A. Bratland
March 7, 1967
Stauffer Chemical Company, Effinger, R. H., Schwartz,
M. G.
Hampel, Dip1.-Milchwirt Heinz-Georg, Hempel,
Dr. Hans-Christoph
Henry S. Brochner
July 11, 1969
Koopmans Meelfabrieken N.V., Giddey, C.
May 23, 1967
Societe Laitiere du Pays d'Auge `Solaipa'
Apr. 2, 1969
Top-Scor Products, Inc., Bassett, H. J.
Nov. 2, 1965
Dec. 17, 1970
July 5, 1966
AILISfICENI NI °MAIM AVILIS do SHOLLVOIlddV
Table 13.2
March 22, 1966
Product
group no.
(See table 13.1)
Product
Ice cream
mix
High fat
powders
(butter)
High fat
powders
(butter)
High fat
powders
(butter)
Casein
Cultured
buttermilk
Honeyrnilk
Milk
protein
concentrate
Sour cream
Patent or
application no.
Patentee and/or inventor(s)
Date
U.S. pat
3 357 840
Brit. pat.
1 041 465
Roberts Dairy Company, Fisher, C. D.
Dec. 12, 1967
Commonwealth Scientific and Industrial Research
Organization
Sept. 7, 1966
Canadian pat.
704 131
Commonwealth Scientific and Industrial Research
Organization, Hansen, P. M. T., Linton-Smith, L.
Feb. 16, 1965
German pat.
1 201 669
Dr. Max E. Schulz
Apr. 28, 1966
Brit. pat.
1 113 871
U.S. pat.
3 468 670
U.S. pat.
3 357 839
U.S. pat.
3 218 173
Vitamin Inc.
May 15, 1968
H. C. Christians Co., Nilsson, C. A.
Sept. 23, 1969
David Torr
Dec. 12, 1967
Crest Foods Co. Inc., Loewenstein, M.
Nov. 16, 1965
Beatrice Foods Co., Noznick, P. P.
Dec. 12, 1967
Armour and Company, Hull, M. E., Kline, R. W.
Nov. 11, 1969
Kuriyama
May 20, 1970
U.S. pat.
3 357 838
Sweet cream U.S. pat.
buttermilk
3 477 853
Whipped
Japanese pat. appl.
cream
70.14108
powder
DNIAUCt AMIS 10 SNOLLVD1IddY
Table 13.2—continued
Table 13.2—continued
15.1.2
Egg
products
Product
General
Whole eggs
Whole eggs
Egg white
Egg white
Egg white
Egg white
Egg white
15.2 Beverages,
flavours,
meats, edible
proteins
15.2.1
Beverages
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Patent or
application no.
Patentee and/or inventor(s)
Date
U.S. pat.
3 222 194
U.S. pat.
3 475 180
Japanese pat. appl.
70.13426
U.S. pat.
3 170 804
U.S. pat.
3 207 609
U.S. pat.
3 287 139
U.S. pat.
3 362 836
Brit. pat.
1 194 941
Norris Grain Company, Gorman, J. M., Hannah, V. H.
Dec. 7, 1965
Tillie Lewis Foods, Inc., Jones, R. E.
Oct. 28, 1969
Taiyo Foods Company Limited
May 14, 1970
U.S.A. (Secretary of Agriculture), Kline, L.,
Sugihara, T. F., Meehan, J. J.
National Dairy Products Corporation, Gorman, W. A.,
Stearns, C. K., Weisberg, S. M.
Hercules Incorporated, Ganz, A. J.
Feb. 23, 1965
Searle & Company, Scott, D.
Jan. 9, 1968
Armour and Company, Sebring, M.
June 17, 1970
U.S. pat.
3 361 571
U.S. pat.
3 361 572
U.S. pat.
3 406 074
Hills Bros. Coffee Inc., Nutting, L., Chong, G. S.
Jan. 2, 1968
Hills Bros. Coffee Inc., Nutting, L., Chong, G. S.
Jan. 2, 1968
The Kroger Co., Klein, P., Raben, I., Herrera, W. R.
Oct. 15, 1968
Sept. 21, 1965
Nov. 22, 1966
A.IIISclUNI NI oNIAli ❑AVMS JO SNOLLVDIlddY
Product
group no.
(See table 13.1)
41.
00
00
Table 13.2—continued
Product
Patent or
application no.
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
Coffee
(instant)
U.S. pat.
3 421 901
U.S. pat.
3 436 227
U.S. pat.
3 458 319
U.S. pat.
3 458 320
U.S. pat.
3 514 300
U.S. pat.
3 529 968
U.S. pat.
3 535 119
Brit. pat.
1 099 810
Brit. pat.
1 214 283
German pat. appl.
1 807 308
German pat. appl.
1 933 591
German pat. appl.
2 000 315
French pat.
1 566 489
Patentee and/or inventor(s)
Date
General Foods Corporation, Mahlmann, J. P., Migdol,
Jan. 14, 1969
N. R., Kaleda, W. W.
The Folger Coffee Company, Bergeron, R. J., Hair, E. R., Apr. 1, 1969
Joffe, F. M.
General Foods Corporation, Block, H. W., Henshall, N.
July 29, 1969
The Folger Coffee Company, Niven, W. W.
July 29, 1969
Mc°, S. k, Mishkin, A. R., Marsh, W. C.
May 26, 1970
The Procter & Gamble Company, Hair, E. R., Dasher,
D. F.
Klein, P., Raben, I., Herrera, W. R.
Dec. 18, 1967
The Procter & Gamble Company
Jan. 17, 1968
General Foods Corporation
Dec. 2, 1970
Societe des Produits Nestle S.A., van Sise, J. W.
July 24, 1969
Oct. 20, 1970
Jan. 22, 1970
The Procter & Gamble Company, Lombana, C. A.,
Strang, D. E.
A/S Niro Atomizer, Mengel, P. AA., Houghton-Larsen, E. July 23, 1970
Ottonia Etablissement, Bach, H., Bottger, M., Pabst, H.
AV1MS JO SNOLINDIlddV
Product
group no.
(See table 13.1)
May 9, 1969
00
Table 13.2—continued
Product
Coffee
(instant)
Coffee
(instant)
Tea
(instant)
Tea
(instant)
Tea
(instant)
Tea
(instant)
Cocoa/coffee
Fruit
beverage
15.2.2
Flavouring
compounds
Patent or
application no.
Danish pat. appl.
3418/70
Danish pat. appl.
3419/70
U.S. pat.
3 451 823
Brit. pat.
1 117 102
Brit. pat.
1 117 103
Brit. pat
1 209 055
Brit. pat.
1 106 638 and 39
U.S. pat
3 395 021
Cola
U.S. pat.
beverage
3 510 311
Encapsulation German pat. appl.
1 255 081
Jelly
U.S. pat.
dessert
3 264 114
Meat
Brit. pat.
flavour
1 084 619
Patentee and/or inventor(s)
Date
A/S Niro Atomizer, Houghton-Larsen, E., Hansen, 0.
Jan. 2, 1972
A/S Niro Atomizer, Hansen, 0.
Jan. 2, 1972
Afico S.A., Mishkin, A. R., Marsh, W. C., Fobes, A. W.,
Ohler, J. L.
Nestle's Products Limited, Giddey, A., Vuataz, L.
June 24, 1969
June 12, 1968
Nestle's Products Limited, Giddey, A., Vuataz, L.
June 12, 1968
Tenco Brooke Bond Limited, Plaskett, L. G., Miller,
J. M.
Rau, Willy
Oct. 14, 1970
March 20, 1968
General Foods Corporation, Glicksmann, M., Farkas,
E. H.
July 30, 1968
Clay, W. C., Swaine, R. L., Beusch, D. W.
May 5, 1970
Roha-Werk Walter Biihner & Co., Pilarczyk, W.
Nov. 30, 1967
General Foods Corporation, Glicksmann, M.,
Schachat, R. E.
International Flavors & Fragrances, Inc.
Aug. 2, 1966
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Product
group no.
(See table 13.1)
Sept. 27, 1967
Product
group no.
(See table 13.1)
Product
15.2.3
Meat
15.2.4
Edible
proteins
Green
plants
Soya beans
Soya beans
Soya beans
Soya beans
a.o.
Tempa
Patent or
application no.
Patentee and/or inventor(s)
Date
French pat.
1 568 051
Charier-Vadrot, Pierre
May 23, 1969
Danish pat. appl.
116 340
Brit. pat.
1 202 678
Brit. pat
1 210 926
German pat.
2 043 490
Japanese pat. appl.
70.31344
U.S. pat.
3 489 570
Budapest Miiszaki Egyetem, Iloilo, J., Zagyvai, I.,
Koch, L.
Ralston Purina Company
Dec. 29, 1969
Aug. 19, 1970
Fuji Oil Co. Ltd.
Nov. 4, 1970
Rohm and Haas Company
July 1, 1970
Fujisawa Pharm Co. Ltd.
Oct. 9, 1970
Beatrice Food Co., Noznick, P. P., Ludsas, A. J.
Jan. 13, 1970
U.S. pat.
3 222 193
French pat.
1 568 929
French pat.
1 467 207
Brit. pat.
1 108 547
U.S. pat.
3 298 838
United States of America (Sec. of Agriculture),
Hanrahan, F. P.
Charier-Vadrot, Pierre
Dec. 7, 1965
May 30, 1969
Baudot, Georges
Jan. 27, 1957
Baudot, Georges
Apr. 3, 1968
Villarreal, Jorge Rivera
Jan. 17, 1970
15.3 Fruits
and vegetables
15.3.1 Fruits
General
General
Almonds
Almonds,
nuts a.o.
Citrus fruits
ONIAWG AV)IdS JO SNOLLVDrIcIdY
Table 13.2—continued
Product
group no.
(See table 13.1)
Product
Coconut
Raspberries
Tomato
15.3.2
Vegetables
15.4
Carbohydrates
Corn products
Patent or
application no.
Brit. pat.
1 144 256
German pat. appl.
1 225 952
U.S. pat.
3 353 969
Patentee and/or inventor(s)
Date
Beatrice Foods Co., Noznick, P. P., Bundus, R. H.
March 5, 1969
Meyer-Frohlich, Dr. Hans
Sept. 29, 1966
Beatrice Foods Co., Noznick, P. P., Bundus, R. H.
Nov. 21, 1967
General Foods Corporation, Hollis, F. Jr., Borders, B.
Nov. 30, 1965
General Foods Corporation, Sienkiewicz, B., Hollis,
F. Jr.
American Potato Company, Rainwater, J. H., Beck, R. G.
July 19, 1966
July 20, 1967
Rohm & Haas GmbH, Krebs, J., Hoppe, G.
Jan. 4, 1968
Beatrice Foods Corporation, Bundus, R. H.,
Luksas, A. J.
May 11, 1965
Mashed
potato
Mashed
potato
Mashed
potato
Mashed
potato
Onions
U.S. pat.
3 220 857
U.S. pat.
3 261 695
German pat. appl.
1 245 272
German pat. appl.
1 258 251
U.S. pat.
3 183 103
Dextrin
and
sweetener
Pregelatinized
starch
U.S. pat.
3 320 074
Afico S.A., Gebhardt, H.T.
May 16, 1967
U.S. Pat.
3 332 785
Boehringer Ingelheim GmbH, Kuckinke, E., Buchta, K.
July 25, 1967
AILISIICINI NI ONIANCE Ad2IdS 40 SNOI1V311cIdV
Table 13.2—continued
Table 13.2—continued
Sugar
Patent or
application no.
Patentee and/or inventor(s)
Date
Dextrins
(encapsulating agents)
U.S. pat.
3 455 838
National Starch and Chemical Corporation, Marotta,
N. G., Boettger, R. M., Nappen, B. H., Szymanski, C. D.
July 15, 1969
General
U.S. pat.
3 271 194
Brit. pat.
1 112 553
Japanese pat. appl.
70.07945
Dutch pat. appl.
70.01026
U.S. pat.
3 477 874
Belgian pat.
772 311
Brit. pat.
1 072 816
U.S. pat.
3 533 805
German pat.
1 203 585
French pat.
1 437 582
U.S. pat.
3 519 482
Yokohama Seito Kabushiki Kaisha and Takara
Kabushiki Kaisha, Oikawa, H.
Nippon Shiryo Kogyo Co. Ltd., Niimi M., Furukawa,
T., Masada, H.
Yokohama Sugar Refining Co. Ltd.
Sept. 6, 1966
March 20, 1970
Hayashibare Company
July 28, 1970
Karl Kroyer, A/S Niro Atomizer, Repsdorph, I.,
Kroyer, K. K. K., Damgaard-Iversen, J.
A/S Niro Atomizer, Hansen, 0.
Nov. 11, 1969
Foremost Dairies Inc.
June 21, 1967
Foremost-McKesson, Inc., Nava, L. J., Palmer, G. M.,
Hutton, J. T.
Brennerei and Chemische Werke Tornesch, Jessen,
Dr. V., Hinck, K. H., Menzel, Dr. M.
M. Jacques Lespagnol
Oct. 13, 1970
CPC International Inc., Walson, G. P.
July 7, 1970
Product
General
General
Amylose
Glucose
product
Glucose
product
Lactose
Lactose
Molasses
Molasses
Starch
hydrolysate
,
May 8, 1968
Dec. 13, 1968
Oct. 21, 1965
March 28, 1966
ONIAIICE AV 4S 40 SNOI11011c1c1V
Product
group no.
(See table 13.1)
Table I3.2—continued
Product
Starch
hydrolysate
Starch
hydrolysate
Starch
hydrolysate
Sucrose
Topping mix
Sugar
product
colouring
Other carbohydrates
Cake mix
Cake mix
Pastry mix
Polysaccharides
16
PharmaPharmaceutical– ceuticals
biochemical
Pharmaindustry
ceuticals
Patent or
application no.
Dutch pat. appl.
70.00560
Brit. pat.
1 169 538
Danish pat. appl.
2531/70
Brit. pat.
1 191 908
U.S. pat.
3 414 980
German pat. appl.
1 932 583
U.S. pat.
3 383 217
Brit pat.
1 113 806
U.S. pat.
3 257 213
Danish pat.
116 501
Brit. pat.
944 771
German pat.
1 932 583
Patentee and/or inventor(s)
Date
CPC International Inc.
Sept. 29, 1970
Lyckeby Starkelsegiradling AB, Conrad, E., Frostell, G.
Nov. 5, 1969
Lyckeby Starkelseforadling AB, Conrad, E., Theander, 0.
Jan. 18, 1971
Nippon Shiryo Kogyo Co. Ltd., Niimi, M.,
Furukawa, T., Masada, H.
National Dairy Products Corporation, Nezbed, R. L.
May 13, 1970
Dec. 10, 1968
Boehringer Mannheim. GmbH, Rieckmann, P.,
Groppenbacher, G., Schellhorn, J., Rothe, W.
Jan. 7, 1971
The Pillsbury Company, Meade, R. E., Greenberg, S. I.
May 14, 1968
The Pillsbury Company
May 15, 1968
The Procter & Gamble Company, Colby, E. E.
June 21, 1966
The Pillsbury Company, Halleck, F. E.
Jan. 19, 1970
S. A. Orsymonde, Lafon, V.
Dec. 18, 1963
Boehringer Mannheim GmbH, Rieckmann, P.,
Groppenbacher, G., Schellhorn, J., Rothe, W.
Jan. 7, 1971
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Product
group no.
(See table 13.1)
Table 13.2—continued
Product
group no.
(See table 111)
Product
Enzymes
Enzymes
Enzymes
Enzymes
Active yeast
Active yeast
Food yeast
Food yeast
Vitamins
Vitamins
German pat. appl.
1 442 107
Danish pat. appl.
5370/68
Danish pat. appl.
804/69
Danish pat. appl.
1770/70
Danish pat.
108 847
Japanese pat. appl.
70.33753
U.S. pat.
3 407 072
German pat. appl.
1 227 413
French pat appl.
1 594 829
Danish pat.
114 040
Danish pat. appl.
117 265
German pat.
1 767 499
Danish pat. appl.
2755/68
Patentee and/or inventor(s)
Date
Instituto de Biologis y Sueroterapia, Sornovilla, N. M. U .
Oct. 24, 1968
Pfizer & Co., Ayers, R. C., Kolb, C. A.
May 7, 1969
Aktiebolaget Astra, Delin, P. S., EkstrOm, B. A.,
Sjoberg, B. O. H., Thelin, K. H., Nathorst-Westfelt, L. S.
Aktiebolaget Astra, Delin, P. S., Kiessling, K. H. F.,
Thelin, K. H., Nathorst-Westfelt, L. S.
Arbejdernes Fmllesbageri A/S
Sept. 15, 1969
March 7, 1967
K. Takai
Oct. 29, 1970
Toyo Jozo Co. Ltd., Aizawa, M., Matsuda, T., Omura,
Amano, I., Nakamura, T.
Standard Brands Incorporated, Johnston, W. R.
Oct. 22, 1968
Oct. 27, 1966
Distillers Co. (Yeast) Ltd.
July 17, 1970
Unilever N.V., Burt, A. W. A.
May 19, 1969
Commercial Solvents Corporation, Miescher, G. M.
Apr. 6, 1970
Oct. 11, 1970
E. Merck Aktiengesellschaft, Niirnberg, Dr. E., Pich, Dr. C. not yet
published
Dec. 16, 1968
F. Hoffmann-La Roche & Co. Aktiengesellshaft,
Klan', Dr. H.
APPLICATIONS OF SPRAY DRYING
Lactic acid
bacteria
Medicinal
yeast
Active yeast
Patent or
application no.
Product
group no.
(See table 13.1)
17.2
Cellulose
industry
18
Offal and
fish industries
Gelatin
Fish products
Product
Patent or
application no.
Resins and
U.S. pat.
crills in
3 364 101
paper pulp
Resins and
Danish pat. appl.
crills in paper 117 738
pulp
Patentee and/or inventors}
Date
A/S Niro Atomizer, Eek, T.
Jan. 16, 1968
Tee-Pak, Inc., Bridgeford, D. J.
May 25, 1970
Gelatin/
vitamins
Gelatin/
vitamins
Brit. pat.
1 142 147•
Swiss pat.
389 505
Olaxo Laboratories Limited, Clegg, J. B., Elliot, L. G.
Feb. 5, 1969
F. Hoffmann-La Roche & Co., Klaui, Dr. H.
July 15, 1965
Crustaceans
U.S. pat.
3 264 116
French pat. appl.
2 014 817
Dutch pat appl.
69.14370
Danish pat. appl.
5568/69
Gray, Robert D.
Aug. 2, 1966
Louis Sanders S.A.
April 24, 1970
Koninklijke Zout-Organon N.V.
Oct. 26, 1970
Litton Industries Inc., Anderson, E. E.
Apr. 22, 1970
Fish protein
Fish protein
Whole fish
AILLSCIGNI NI ONIAIKI AVIRIS 10 SNOIIV3I1c1cIV
Table 13.2—continued
Product
group no.
(See table 13.1)
Product
Patent or
application no.
U.S. pat.
Miscellaneous Fatcarbohydrate 3 514 298
products
U.S. pat.
Fat
3 295 986
emulsion
U.S. pat.
Fat
3 383 219
emulsion
Danish pat. appl.
Fat
emulsion
1805/68
U.S. pat.
Shortening
3 393 075
Patentee and/or inventor(s)
Date
Beatrice Foods Co., Noznick, P., Tatter, C. W.
May 26, 1970
General Foods Corporation, Saslaw, I. M., Brady, J. J.
Jan. 3, 1967
Patterson, Bernard A.
May 14, 1968
Abbott Laboratories, Chapman, J. D.
Oct. 27, 1968
Nippon Oils & Fats Company Limited, Hayashi, Y.,
Takama, N.
July 16, 1968
ONIAIIGAV MSJO SNOIIVDrIddV
Table 112—continued
APPLICATIONS IN THE CHEMICAL INDUSTRY
14 Applications in the Chemical Industry
499
new area of application, establishing spray drying in the hydrometallurgical
field with the handling of very high flotation concentrate feed rates to produce
powders for smelting or chemical refining. Categories 14.8, 14.9 on inorganic
and organic chemicals lists product groupings. New and/or specialized
applications in the chemical industry (category 14.10) conclude the chapter,
and include examples of spray cooling, spray reaction, and spray purification.
14.1. Plastics, Resins
It is in the chemical industry that applications of suspended particle processing systems involving atomization (spray drying, spray cooling, spray reaction,
spray purification) have grown most in the last decade. The time has long gone
since detergent spray drying was the lone application of real capacity in the
chemical industry to counteract the spray drying of milk and coffee in the
then dominant food industry. To-day, applications in the chemical industry
are extremely varied as shown in table 13.1 (chapter 13) where nine main
categories are featured. It is impossible in the scope of this book to deal with
all the applications mentioned. There is only space to briefly mention
established and new applications, considered of wide appeal. Whenever
possible, additional references are given to supplement the text.
Category 14.1 on plastics, resins has evolved through the increasing use
of synthetics. The spray drying of p.v.c. stands prominent in this category.
Category 14.2 deals mainly with ceramics, where the spray dried product
has shown itself ideal for pressing and sintering operations. In fact, spray
drying has rationalized the production of pressbodies. Category 14.3 is
devoted to detergents and washing agents, an area of application where
spray drying first became established in the chemical industry. Category 14.4
deals with pesticides and here spray drying applications are widening as the
product characteristics are proving ideal for effective and economic utilization, especially crop spraying. Category 14.5 (dyestuffs, pigments) is another
of expanding interest. The dried product has all the desired properties,
especially redispersibility, Category 14.6 on fertilizers is a relatively recent
application, where spray drying has been incorporated with established
production techniques to improve the economy of these techniques. The
industry is dominated by prilling, which, being a spray cooling mechanism,
is carried out in equipment similar to tall spray drying towers. Mineral ore
concentrates, discussed in category 14.7 is the latest and a most important
14.1.1. Polyvinylchloride (p.v.c.)
Vinyl chloride, produced from ethylene is polymerized catalytically in
autoclaves to form polyvinylchloride (p.v.c.). There are three main polymerization techniques (359): emulsion polymerization (e-p.v.c.), suspension
polymerization (s-p.v.c.) and bulk polymerization of liquid vinyl chloride
(producing a dry polymer) (359). e-p.v.c. and s-p.v.c. are the two main types
requiring drying. The nature of polymerization determines the type produced. Emulsion polymerization techniques give a particle size range sufficiently small for the particles to remain in colloidal suspension. Emulsions
in water of uncoagulated p.v.c. are being produced under such names as
paste polymer, paste resin or latex. s-p.v.c. particles are the result of large
particle growth during polymerization, giving a granular polymer that
lends itself to centrifuging, filtering and rinsing.
Spray drying is the only practical way for drying e-p.v.c. Spray dryers
can be operated to give the desired p.v.c. particle size distribution and degree
of case hardening. Moisture contents down to 0.1 % can easily be achieved
from the dryer chamber. s-ps.c. is sometimes dried in available dryers
with powder rotary atomizers. Alternative drying processes are used,
however, for large capacity s-p.v.c. drying. Drum drying is the popular
method, but flash/fluid bed dryers offer a number of advantages that are
now becoming recognized by many producers.
(a) Emulsion p.v.c.
The polymerized p.v.c. particles move within the colloidal range. Predewatering is not possible. Solids content can be increased by multi-stage
vacuum evaporation. e-p.v.c. is spray dried applying either rotary or nozzle
atomization to obtain a white powder of very low moisture (0.01-0.1 %).
Feed solids range from straight emulsion (30 %) to pre-evaporated emulsion
55-60 %. Drying temperatures vary greatly. Maximum hot inlet air temperatures are around 575°F (300°C), but there are kinds of p.v.c. that are
so heat sensitive that inlet temperatures may not exceed 330°F (165°C).
However, all p.v.c. require drying chamber conditions that prevent retention
of particles in an environment whereby scorching takes place. Presence
of scorched particles is readily visible in the white powder. Areas around
500
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
the air disperser are the most likely source of scorched particles. HoweVer,
it must always be borne in mind that it is possible for such particles to be
present in the feed and it is clear that these particles cannot be diminished
during the drying process.
The size of dried p.v.c. particles is usually critical to the p.v.c. use. Product
demands can be 90 % greater than 80 micron, or 90 % less than 60 micron
(with mean size less than 40 micron). The mode of atomization is chosen
according to the size requirement. Coarse powders are produced by pressure
and two-fluid nozzles, or rotating inverted plates or cups. Fine powders
are produced by rotating atomizer wheels of special design, or two-fluid
nozzles. Where very fine particle sizes are required (e.g. 100 % less than
30 micron) pneumatic sieving of spray dried powder is applied. The atomizer
is selected to produce a dried powder form, giving a minimum of oversized
particles.
Powder consists of hollow globules formed of cohesive colloid particles.
The degree of cohesion is adjusted by means of temperature. Thus in the
manufacture of dry product of paste quality, only slight cohesion is required
and thus low temperatures are used. For strong linking demands higher
temperatures are applied, but there is a maximum above which the individual
particles melt to form fully sintered particles. When sintering or 'case
hardening' of coarse particles is used, spray dried p.v.c. is marketed as such
(without being pelletized) so as to enable the user to make his own formulations through dry mixing of p.v.c. with fillers, pigment stabilizers, etc. Case
hardening effects can occur during atomization, but use of atomizer wheels
and nozzles of special design can reduce this effect to be negligible if so
desired. For many applications, paste-p.v.c. powder is mixed with a plasticizer. The viscosity of this mixture generally depends on influence of heat
during spray drying, particle size, and mixing time. Although smaller particles lead to a lower mixture viscosity, grinding of powder to form smaller
particles can result in higher viscosities.
Cyclones and bag filters are used to recover airborne powder. The powder
is rather brittle and the comminution effect associated with flow through
cyclones can be utilized if even finer particles are required. However,
product recovery from cyclones usually is insufficient to prevent powder
emission levels that cause air pollution, and thus in these cases bag filters
are installed.
(b) Suspension P.V.C.
s-p.v.c. contains larger particles than e-p.v.c. Mechanical de-watering is
possible. By centrifuging, p.v.c. of 75-80 % solids can be obtained. This has
a consistency of a filter cake and is termed p.v.c. (suspension) snow. It can
be dried in a spray dryer using a special powder rotary atomizer. The
atomizer wheel is of open design, and a powder chute feeds p.v.c. into the
APPLICATIONS IN THE CHEMICAL INDUSTRY
501
wheel. The use of a spray dryer is usually restricted to manufacturers who
have large e-p.v.c. capacity, but who also demand smaller amounts of
s-p.v.c. For plants processing only s-p.v.c., flash /fluid bed dryers are used.
14.1.2. Resins
Resins (polycondensates) are widely used today in powder form (496).
For certain synthetic resins, a conversion to powder is required because the
removal of water, organic solvents or diluents is necessary for further processing of the resin. Other resins are converted into powders because the
liquid resin tends to undergo a slow change in characteristics due to continued polymerization. Powders can be stored practically indefinitely
without changes in characteristics.
Resins in powdered form are often preferred to liquid resins because
of the much easier packing and handling. Powders require less freight
expenses for transportation over long distance. However, the resin is
eventually used in liquid form, but spray dried powder is readily reconstituted
to liquid thereby meeting this requirement.
Melamine urea, and phenol formaldehyde resins are dried in co-current
flow dryers with air-broom attachment and dried product handling in cool air.
S.B.R. and A.B.S. (Acrylonitrilebutadienestyrene) resins use similar equipment. Polycarbonate can be dried in counter-current flow nozzle towers.
Melamine/Urea Formaldehyde Resins
Melamine and urea formaldehyde resins are spray dried in large quantities
(306). The resin powders are used in laminating, wet and dry adhesives in
paper textile and leather treatment. The largest outlet is the board industry ;
paper, composite, straw board and plywood etc. In the manufacture,
formaldehyde is mixed with melamine (or urea) in a stirred glass lined kettle.
Catalysts are added. Condensation is recorded by temperature and pH
measurements. Reacting time depends upon the specific resin and product
characteristics required. The finished liquid is pumped out of the kettle
through a filter into holding tanks that are agitated and steam jacketed.
The liquid resin (50-70 % solids) (120°F, 50°C) is pumped to the spray dryer
with air broom (figure 5.12). A rotary atomizer (vaned atomizer wheel) is
used (V1 = 140-160 m/sec). Inlet drying temperatures are 390-530°F
(200-275°C), and outlet 150-165°F (65-75°C). At this outlet air temperature
the resin particles are dry, but soft and tacky. Cold air is introduced at the
dryer outlet to cool the particles well below the softening point and form
a free-floWing powder. Cyclones are normally used to remove product from
the exhaust air. The powder bulk density is 0.6-0.8 g/ml. Higher density
can be obtained if the product is pulverised, and blended prior to packing.
Spray dried resins are free-flowing powders of negligible moisture. Spray
drying is well suited to handle the heat sensitive nature of the product. One
of the first large resin spray dryers has been described by O'Connor (307).
502
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
14.1.3. SBR Lattices
Products with a styrene/butadiene ratio of at least 67 : 33 can be spray dried
without the addition of fillers in e-p.v.c. spray dryer designs.
14.2. Ceramic Material
The use of spray drying in ceramic manufacturing commenced in the midfifties, and today over 50 % of the total European volume of wall and floor
tiles and electronic ceramics are made from spray dried pressbodies. With
spray drying, the processing of slip preparations to finished pressbody is
continuous without product hold-up. The unit operations involved in a
typical example of processing are illustrated in figure 14.1. Raw material is
blunged, milled, screened and mixed prior to drying, pressing and firing.
In traditional operations of wet preparation the production of pressbody
requires filter pressing, drying, crushing, rewetting and classifying, but
modern operations with spray drying reduce these five stages to only one.
RAW MATERIALS
PRE-TREATMEN T
WET-MILLING
BLUNGING
SCREENING
APPLICATIONS IN THE CHEMICAL INDUSTRY
503
Spray dryers utilize either rotary or nozzle atomization. For dryers with
rotary atomizers, slip is fed to the wheel by gravity or low pressure diaphragm
pumps. Wheel periphery speeds are low to produce a coarse atomization.
For large capacity wall-tile dryers, speeds are adjusted to around 300 ft/sec
(90 m/sec). Wear resistant atomizer wheels incorporating either carbide
or alumina bushings (figure 6.26) are used as the slip is abrasive. The wheel
atomization is insensitive to fluctuations in feed characteristics and the
wheel itself is not liable to clog. The particle size is controlled through
regulation of wheel speed. The dryer installation is shown in figure 14.2(a).
Direct oil, gas, or solid fuel heaters raise the air temperature upwards of
1100°F (600°C) prior to entering the drying chamber. Co-current flow
conditions are used. The majority of the product leaves the chamber base,
and the air is exhausted via a cyclone. The cyclone powder is normally
mixed with the main chamber product in a common transport system.
Alternatively, powder can be recycled to the drying chamber or to the slip
preparation section. A wet scrubber of the liquid recirculation type is
fitted after the cyclone to clean air of any powder escaping separation in the
cyclone prior to passing to atmosphere. Scrubber liquid can be concentrated
through recirculation, and passed back to the dryer feed tanks.
For rather coarse powders, nozzle atomization can be used in low capacity
dryers. The flow diagram is shown in figure 14.2(b). The slip is pumped by
a diaphragm pump to nozzles operating at 150-300 psig (10-20 atm). Wear
resistant nozzle orifices are essential.
MAGNETIC SEPARATION
spray drying
processing
traditional wet
processing
a.
ROTARY
ATOMIZATION
FILTER PRESSING
DR Y, ING
CRUSHING
SPRAY DRYING
RE-WETTING
AIR
CLASSIFYING
b.
SILL
PRESSES
NOZZLE
ATOMIZATION
SORTING
DRYING
FIRING
Figure 14.1. Examples of unit operations associated with wet preparation as compared with
spray drying process.
Figure 14,2. Spray dryers for ceramic pressbody; (a) with rotary atomization, (b) with nozzle
atomization.
504
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE CHEMICAL INDUSTRY
Dried pressbody must be free-flowing in order to rapidly fill the pressing
dies. Spray dried pressbody has a controllable particle size distribution,
consists of spherical particles, and with the absence of fines it is ideal for
pressing operations. Moisture content and bulk densities are maintained
constant, and the particle structure can be controlled by the spray dryer
operation. The importance of binders in spray dried pressbody is discussed
by. Hoffman (484).
Particle shape and size distribution vary widely from pressbody produced
by traditional techniques. Figure 14.3(a) shows a microphoto of a pressbody
prepared according to the traditional wet process of filter pressing, drying,
milling, etc. The pressbody consists of irregular shaped particles, has
numerous very large particles and a noticeable amount of fines. The spray
dried pressbody (figure 14.3(b)) has spherical particles and a narrower
particle size distribution. These characteristics meet the correct degree of
free flowability required for conveying, handling and pressing operations.
505
14.2.1. Wall Tile and Floor Tile Ceramics
Recent advances in spray drying have enabled successful application of
spray drying in the field of wall and floor tile production. Developments in
both the chemical and mechanical preparation of highly concentrated
slips and in the atomization of these slips to form a spray dried product of
desired characteristics have rendered spray drying economically competitive
to the traditional wet processing methods.
The important product characteristics of average particle size, particle
size distribution and moisture content of the finished pressbody can be
adjusted in the spray dryer operation, within limits, through regulation of the
slip properties, atomizer and drying conditions. Both wheel and nozzle
atomizers are used. Typical screen analyses from industrial spray dryers
operating with tile material are given in figure 14.4. The curves show the
practical upper size limits for wheel and nozzle atomizers. There is an
absence of fines. The spherical shape of the dried particles yields a freeflowing product which fills the pressing dies both uniformly and rapidly,
thus securing the same density in all parts of the tile.
In the tile manufacturing industry the nozzle atomizer generally has its
optimum performance at capacities lower than 1.5-2 t/hr water evaporation.
The rotary atomizer is more suitable in handling higher capacities.
FEED 56-60% T.S.t 3% dispers/ng agent 1
(a)
1
1
1
1
1
1
r
'
,
i
t
..
-
99
.
.
95
..
..
-.
...
75
50
I
I I
I
25
I
I
5
I
1
I
I
1
10
I
I
The flexibility of the spray dryer in producing pressbodies of various
characteristics is important as pressing operations often require a specific
pressbody quality to overcome product sticking to the dies. Sticking leads
to non-uniformity in ceramic surfaces (especially tile surfaces). The preparation of the slip requires care to prevent impurities that can effect pressbody quality. The inorganic salts in water used to prepare the slip, and the
salts added as deflocculants (to enable increase in solids content by reducing
slip viscosity) remain in the pressbody. Combinations of salts can cause die
sticking and choice of deflocculant is vital.
1-
Ili
Figure 14.3. Micro-photographs of pressbody. (a) Pressbody produced by traditional wet
process. (b) Pressbody produced by spray drying. (By courtesy of Niro Atomizer.)
i
I
(b)
CUMULATIVE PERCENT LESS THAN SIZE D.
1
50
PARTICLE SIZE
100
ea)
. micron)
Figure 14.4. Particle size distributions of wall tile material produced by atomizer wheel and
pressure nozzle atomization. A. Atomizer wheel 210 mm diameter 8300 rev/min, B, Pressure
nozzle (swirl insert) 8 kpfern2.
506
APPLICATIONS IN THE CHEMICAL INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
14.2.2. Oxide Ceramics (Ferrite, Aluminium Oxide, Steatite, Glass)
Spray drying is used extensively in manufacturing oxide ceramics. Slips
are sprayed into drying air at temperatures normally between 575-1375°F
(300-750°C). The drying process is accomplished in 2-5 seconds, and after
..discharge from the dryer, the powder particles have a shape related to the
original shape of the droplets formed during atomization.
Spray dried powder is suitable for both isostatic pressing and the more
conventional die pressing. A sample of steatite press powder is shown in
figure 14.5. Spray dryers for oxide ceramics feature rotary or nozzle atomizers. Plant layout is similar to that described earlier in relation to figure
14.2. Feeds can be heat sensitive, e.g. glass and ferrite with organic binders
and lubricants—softeners.
1mm
Figure 14.5. Spray dried steatite. (By courtesy of Niro Atomizer.)
Dryers are normally of limited size. Rotary wheel atomization is used
where particle size distributions having a mean size within the range 50-150
micron are acceptable. Nozzle atomization is selected for sizes within the
range 250-300 micron. The rotary atomizer holds many practical operational advantages as described by Helsing (208). The choice of atomizer is
generally governed by the oxide ceramic powder size requirement. Where
size distribution is critical both types can be incorporated in the drying
chamber. Table 14.1 illustrates the particle size distribution of oxide
507
ceramics produced by nozzle atomizers. Where both atomizers are used
simultaneously, feed rates to each atomizer are approximately equal.
Table 14.1. Particle Size Distribution of Oxide Ceramics according to Atomizer Used
Atomization type
% by wt greater
than (micron)
420
250
177
120
60
40
Wheel
Nozzle
%)
( %)
Combined
wheel-nozzle
( %)
0
6
27
55
84
94
6
60
76
86
96
98
6
32
52
73
94
96
Powders produced by rotary atomizers are widely used for isostatic
pressing. The press powders are highly free-flowing and fines-free (no particles smaller than 15 micron). In die pressing flowability is important to
enable the powder to rapidly fill all edges and cavities in the die during the
pressing. Press powder prepared by conventional methods cannot produce
such flowability. There are greater amounts of rejects. Due to low degree
of abrasiveness, dies for pressing spray dried ferrite, for example, can be
used for up to five times as many pressings as when conventional powder is
available. Figure 14.3 illustrates pressing characteristics of conventional
and spray dried powder.
For steatite, pressing often requires a constant bulk density (1.0-1.2 &m')
to meet existing tooling requirements. Strict control of the dryer outlet
temperature and atomization of feed can meet specifications of constant
bulk density. The characteristics of electronic ceramic pressbodies have
been discussed by Storm (210).
(a) Ferrites
Ferrites are nowadays used in many areas. Electronics, communications
and entertainment industries use soft ferrites in television, radio, lighting,
telephones, and high frequency welding. The automotive industry uses
hard ferrites in small motors that power windows, seats, etc. Power tools,
cordless electrical goods, and a range of household appliances use hard
ferrite. The ferrite industry is very extensive, based upon the use of permanent
magnets (hard ferrite) and electromagnets (soft ferrite). Hard ferrites (barium
and strontium iron oxides) and soft ferrites (manganese-zinc and nickel-zinc
iron oxides) are manufactured from spray dried material by dry pressing.
508
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE CHEMICAL INDUSTRY
The ferrite press powder produces a pressing devoid of imperfection such
as cracks and voids, which could cause poor electrical and magnetic properties. The green strength of the pressing has been found to increase when
using spray dried powder. This is an important feature since it is possible
to subject the pressed (but not yet fired) components to high mechanical
loads without fracture. Binders and lubricants, added to improve component
strength can thus be reduced. Polyvinyl alcohol is commonly used as a
binder, with softening agents of glycerine or ethylene glycol.
The flow diagram for a ferrite production line is shown in figure 14.6.
Raw material is analysed for foreign matter, then wet milled, spray dried
and calcined. Calcined powder is reground by wet milling with lubricants,
binders and deflocculents added. The resulting slurry is spray dried. Two
RAW MATERIALS
Fei 0 4 MnCO Zn O
spray dryer
( rota ry{wh ee t) atomizer
otory calciner
LUBRICANT DEFLOCCULEN T BINDER
pneumatic
conveying
spray dryer
( nozzle atomizer}
bolt mill
slurry teed tank
PRESSI NG POWDER
Figure 14.6. Flow diagram for production of ferrite by spray drying.
509
spray drying stages are utilized. A spray dryer with rotary (wheel) atomization is selected for the initial drying of green materials prior to calcining.
A spray dryer of similar capacity with nozzle atomization produces the
final pressbody of rather coarse nature from the calcined material.
(b) Aluminium Oxide
Aluminium oxide is used in vast quantities as spark plug insulating material.
Completely homogeneous pressing is required for optimum electrical
insulation performance. During production, there is direct transfer of
powder from the dryer to the presses. Feed slurry to the dryer is approximately 70 % solids and contains additives to give powder improved pressing
performance. Organic binders (e.g. polyvinyl alcohol) are preferred nowadays to wax binders, as the press powder is soft enough to maintain clean
die presses but not soft enough to create turning difficulties when machining
insulators on the lathe. Rotary (wheel) atomizers are used, where coarse
atomization is achieved with low peripheral speeds (V T = 90-100 m/sec).
Drying outlet temperatures are in the range 230-240°F (110-115°C) to give
powder moisture contents between 0.1 and 0.25 %. Powder size distribution
is narrow, an accepted distribution being of the order of 75-80 % greater
than 60 micron, 12-20 % greater than 125 micron.
(c) Steatite
Steatite is produced generally on spray dryers with nozzle atomization.
High bulk density requirements for pressing are achieved through control
of slip composition and drying conditions. Steatite is used as insulator
material. Homogeneity of the pressing powder is critical in achieving constant electrical insulating properties. Pressing produces a pressbody of
smooth surface, which has the advantage of less adherence of surface dust
and improved electrical insulation chracteristics. Increase in bulk density
is achieved by increase in slip concentration. Concentrations as high as 85
have been successfully dried. Deflocculants are added to the slip. Where
organic or volatile deflocculants (that volatilize during the drying or firing)
cannot meet pressing requirements, electrolytic deflocculants are used.
These deflocculants are carefully chosen so that they cause no adverse
effect on the mechanical or electrical properties of the ceramic product.
14.2.3. Plastic Bodies (Ceramic Jiggering Bodies for Fine Stoneware and Art
Ceramics)
This is a relatively new application of spray drying in the ceramic industry.
Jiggering bodies require 15-25 % moisture contents. Products with such
high moisture contents cannot be produced directly in spray dryers, and
thus part of the slip is dried to 5 % moisture followed by a mixing of powder
with slip to obtain the desired moisture content. The layout is shown in
figure 14.7. When using a planetary mixer to mix the powder and slip, an
extremely homogeneous body with very exact water contents is achieved
510
APPLICATIONS IN THE CHEMICAL INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
PREPARATION OF CERAMIC SLIP
in
AIR LIFT
gaol
,
11[111 1[1 1 1J1 1 11
SPRAY
SILO
WEIGH INC
VESSEL
MIXER
Figure 14.7. Flow diagram for preparation of spray dried jiggering bodies.
in only 5 minutes of effective mixing time. Mixing proportions, to obtain
100 kg of 20 % moisture jiggering body, would be 123 kg of slip at 65 % solids,
of which 73 kg is spray dried to 50 kg of 5 % moisture powder, which is then
mixed with the remaining 50 kg of slip to form the jiggering body.
Use of spray drying in the pottery industry has been described by Lengersdorff (209) and Helsing (509).
14.2.4. Glass
Spray dried glass powder is used for insulating material. The powder must
have a form whereby bubbles formed during sintering escape readily. Each
particle must have a high physical strength and be extremely free-flowing.
Acrylic resins are used as binders. Traditional binders like polyvinyl alcohol
make the particles too brittle. Carboxymethylcellulose and a long range
of low molecular binders destroy the free-flowability of the product and
thus are not suitable.
14.2.5. Carbides (Tungsten, Titanium, Tantalum, Niobium)
Carbide suspensions (carbide in organic milling liquid) are being spray
dried nowadays in increasing quantities. Compared with older processing
techniques, handling and labour costs are reduced and there is less risk of
contamination. Carbides of the chemically pure analysis grade are readily
obtainable by spray drying, The greater control of particle size achievable
during the spray drying process is a further notable advantage in obtaining
powders for such applications as pressings and flame spraying.
511
With the presence of organic liquids in the feed, a closed cycle spray drying
system is adopted. The drying gas is a mixture of nitrogen and milling liquid
vapour. The carbide is ball milled with an organic milling liquid, and a
binder (2 %), e.g. hydrocarbon wax, added as lubricant. The suspension of
carbide (75 % solids) is transferred from feed tanks to the spray dryer under
nitrogen pressure. Alternatively, a feed pump system can be used. The
closed cycle dryer is shown in figure 14.8. The drying chamber is a 'fountaintype' (see figure 5.9) with nozzle atomization. The suspension is atomized
at the nozzle into a relatively coarse spray. The outlet temperature depends
upon the organic milling liquid. A bone dry carbide is obtained from the
chamber base. Particles that are exhausted with the gas leaving the chamber
are recovered in a cyclone. (In the case of tungsten carbide the amount
passing to the cyclone is very small.) The cleaned gas enters a scrubbercondenser, where the gas is cooled by cool milling liquid. The milling liquid
in the feed evaporated during drying is condensed. The remaining gas is
heated prior to returning to the drying chamber. The gas heater is of the
indirect type, i.e. electric or liquid phase oil—heat exchanger. As with all
closed cycle systems involving explosive mixtures, there is pressure and
temperature instrumentation throughout the dryer layout. An oxygen
analyser monitors the oxygen level in the drying gas. Sealed cleaning systems
are also designed into the dryer.
GAS DISPERSER
GAS H EATER
( I NDIRECT }
COOLER
DRYING
CHAM BER
PRESSURE
NOZZLE
N
FEED
SYSTEM
SCRUBBER/
CONDENSER
2
MILLI NG
LIQUID
MAKE- UP
MAKE- UP
DRY
CARBIDE
Figure 14.8, Flow diagram for spray drying carbides in closed cycle.
14.3. Washing Powder (Detergents, Soaps, Surface Active Agents)
Washing powders are one of the best known of spray dried applications.
The free-flowing, non-dusty, non-caking products in bead form are produced
in either co-current or counter-current flow dryers with rotary or nozzle
512
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
atomization. The use of counter-current flow with nozzle atomization is
the preferred layout for high bulk density household products. Feeds are
homogenized and fine-filtered prior to passing to nozzles operating at high
pressure. Air vessels are used to dampen pump pulsations. Hot air from
direct fired air heaters enters from a circular channel at the base of the
cylindrical part of the tower with slight rotary motion. The exhaust air is
drawn from the top of the tower. Inlet temperatures vary according to
product. Up to 750°F (400°C) is used for some detergents, but lower temperatures are used for fine washing agents, the exact value being determined
by the organic content in the feed. Product recovery from the drying tower
is very high (99 %). The fines fraction is usually returned to feed preparation.
Washing powders fall into four groups:
Heavy Duty Detergents (high and low foaming).
Light Duty Detergents.
Soap Powders.
Soda Products.
Detergent formulations consist basically of (a) an active ingredient, (b)
sodium tripolyphosphate, (c) silicates, (d) sodium sulphate. The active
ingredients can be cationic, anionic or nonionic. The most common are
anionic, namely fatty alcohol sulphates, primary and secondary alkyl
sulphonates and alkyl aryl sulphonates. Synthetic detergents are currently
based upon linear alkyl benzene sulphonates, obtained from n-paraffins
via alkylation and sulphonation. Formulations have been recently (1970)
discussed by Duckworth (213). The composition of detergents vary according
to their use. Light duty products have a low (or even no) phosphate and
high sulphonate content whereas the proportions are opposite for heavy
duty detergents. Heavy duty products are for tough cleaning jobs in the
home and in industry. Light duty products are for dish washing, light
household cleaning and laundering of delicate fabrics.
Low foam products have a soap constituent and low sulphonate content,
and are especially used in automatic washing machines. Soap powders
rarely contain high proportions of soap, but are sold containing 45% soap
with soda and additives. Soda products consist of sodium carbonate and
waterglass.
14.3.1. Heavy Duty Detergents
Synthetic detergents are widely used in cleaning of all types and operate
successfully with hard water. The coarse powder granules in bead form are
free-flowing, non-dusty and readily dissolve in water. Shelf-life is excellent,
with no tendency to instability or lumping. The feed can be formulated
batchwise or continuously. The preparation technique has definite influence
on the final dried properties. Precise weighing/metering, mixing, homo-
APPLICATIONS IN THE CHEMICAL INDUSTRY
513
genizing and de-aeration, takes place before spray drying. As high a solid
content as possible is used (50-65%) to give the highest powder bulk density
and best economic use of the spray dryer. The feed must be free of air and
free of material that can clog the nozzle atomizer. The product is dried in
a counter-current flow drying tower at inlet temperature 660-750°F (350400°C), outlet 195-230°F (90-110°C). Nozzle atomizing pressure is in the
range 30-60 atm. The dried powder leaves the base of the drying tower and
is transported via a conveyor belt to an air-lift. During passage on the belt
product dosing is carried out. Dosing can be products that would be
damaged during spray drying, e.g. lauryl alcohol, enzymes (505). Bleach in
the form of sodium perborate is also added at this stage, and sometimes a
specially protected enzyme product (enzyme encapsulated in wax). Perfume
is added after the following air lift. Physical properties of heavy duty detergents can be generalized as
Active ingredient, e.g. alkyl benzene sulphonate
14-20 %
Foam booster, e.g. lauryl alcohol
0-5 %
Sodium tripolyphosphates
30-45 %
Anti-soil redeposition agents, e.g. sodium carboxymethyl0.1-1.5%
cellulose
Anti-corrosion agent, water glass
5-8
Filler, e.g. sodium sulphate
10-15
Optical brightener
01-05 %
Perfume
0.1-0-2%
Bleach (sodium perborate)
6-15%
Enzymes
0.3-0.75 %
Perborate decomposition preventives (ethylenediamines of
%
fatty acids)
Moisture content 8-13
0-30-0.35 g/cm 3 (after spray drying)
Bulk density
0.35-0.40 g/cm 3 (after perborate dosing)
2-3%
Size distribution > 1500 micron
10-20
> 500
50-75
> 250
85-95%
> 120
95-100
> 60
The addition of sodium silicate reduces corrosion in washing equipment
and pitting of enamel glosses. Carboxymethylcellulose (CMC) prevents
the redeposition of soil especially in the case of cotton garments. Optical
whiteners are used in all formulations, as these absorb ultraviolet light and
reflect it as visible light making the fabric appear brighter.
The addition of enzymes to synthetic heavy duty detergent powder is a
new development that has created a substantial market. The development
514
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
arose out of enzyme pre-soak powders. Proteolytic enzymes are used. The
majority of protease preparations are fermented products of the microorganism, bacillus subtilis. The enzyme is added as a spray dried product
(503). The activity of the enzyme depends upon temperature with optimum
activity between 120-160°F (50-70°C).
The addition of ethylenediamines of fatty acids (e.g. ethylenediamine
tetra-acetic acid) acts as organic sequestering agents. These organic agents
tend to sequestrate the metallic salts naturally present as trace impurities
in other materials that catalyse the decomposition of hydrogen peroxide.
These agents are added to act as perborate stabilizers.
Low foam washing powders have been developed to meet the increasing
use of automatic washing machines. Synthetic detergents foam substantially
under the severe agitation produced in washing machines. Low foam powders are based upon soap. The powder is free-flowing, readily dissolvable,
and in fact the physical properties are similar to heavy duty detergents.
The particle size distribution is about the same, but bulk density is around
0.35 g/cm 3 . Spray drying procedure is similar to heavy duty detergent.
A low foam formulation can be generalized as
5-7 % soap
4-5 % alkyl aryl sulphonate
4 % ethylene oxide condensation product
8-9 %
silicate (water glass)
10-15 % filler, e.g. sodium sulphate
35-40 % phosphates
12-15 % perborate
Traces of CMC, enzymes, optical brightener and perfume
The moisture content is approx. 8 %.
14.3.2. Light Duty Detergents
Light duty detergents are used less than the heavy duty. They find wide
application only in dish washing and light household cleaning (janitoring).
Light duty detergents are produced in nozzle towers with either co-current
or counter-current flow. The particle size is much finer than heavy duty
products. Inlet drying temperatures are also lower 480-530°F (250275°C). Outlet temperature is in the range 210-230°F (100-110°C). Nozzle
pressures are around 20 atm. The physical properties are
30-35 %
alkyl aryl sulphonates
0-5 %
fatty alcohol sulphates
50-60 %
sodium sulphate
Moisture content 0-3 %
Bulk density
0.15-0.20 g/cm 3
Size distribution > 1500 micron
0
X500
10-15%
APPLICATIONS IN THE CHEMICAL INDUSTRY
> 250
> 120
> 60
515
40-60
70-80
95-100
14.3.3. Soap Powder
Soap powder for hand use requires low temperature operation and is usually
flash dried under vacuum. The spray drying of a 60 % soap feed with wheel
atomization (320-170°F, 160-75°C) or nozzle atomization (440-195°F,
225-90°C) in co-current flow dryers is rarely used these days.
Products based upon soap and soda are varied in composition. Products
consisting of mainly soap and soda are spray cooled, but such products
these days require added constituents which have increased the water
content so that spray drying must be used. A typical formulation may well
contain 45 % soap, 20 % soda, 5 % silicates, 5 % phosphates, 23 % moisture
and 2 % as CMC, optical brightener and perfumes. Such a product consists
of particles 20 % > 300 micron, 50 % > 200 micron and 80 % > 85 micron.
This is much finer than synthetic detergents. If the soda content is in the
range 20 40 %, the powder is dusty.
The spray drying of neat soap, and soap-soda formulations, are described
by Stockmann (308). A dryer with two-fluid atomization of neat soap is
cited.
14.3.4. Soda Products
Soda bleach contains sodium carbonate and water glass. Formulations
can be made more forceful by the addition of a little soap or active ingredients.
These products are relatively dusty, but of high bulk density, 0.6 g/cm 3 ,
and are produced by spray cooling or spray drying with either rotary or
nozzle atomization. Dryers are similar to those used for soap powders.
Spray Dryer Layout for Washing Powders
In current high tonnage production of spray dried detergents, continuous
mixing of ingredients are used to form formulations that are spray dried in
counter-current drying towers with nozzle atomization. Many spray
towers have built-in flexibility of both co-current and counter-current air
flows in order to handle special formulations and achieve specific bulk
density.
The plant layout for detergent formulations consists of a feed preparation
section (automatic feeding, proportioning and mixing) feed pumping to the
nozzle atomizer, dried powder conveying and dosing, followed by thorough
blending, screening and packaging. The layout is shown in figure 14.9.
Linear alkyl benzene sulphonate paste, the preparation of which has been
recently discussed by Duckworth (213), is metered into a slurry preparation
tank together with metered sodium silicate solution, and solid phosphates,
sulphates and additives. The blending of these ingredients has been described
516
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
SOLIDS
FROM
SILOS
APPLICATIONS IN THE CHEMICAL INDUSTRY
EXKAUST AIR
SLURPS'
ADEIN
CRUTCH.
FILTERS
RONDO NIZER
MGM
PRESSURE
FEED
PUMP
WITH FILTER
AND AIR VESSEL
DIRECT FIRED
AIR HEATER
DETERGENT
PAC KIRA
Figure 14.9. Flow diagram for spray drying of detergent formulations.
by Silvis (212) in detail. The slurry preparation tank acts as a coarse mixer.
Here lumps are broken down and air pockets are eliminated. Materials
after blending are conveyed to an ageing vessel.
Mixing is carefully controlled to prevent aeration of the slurry. Feed
slurry passes through a coarse filter, homogenizer (colloid mill) and fine
filter. Deaeration of product is carried out if necessary. The slurry of
constant solid content and viscosity is ready for spray drying. The handling
of product in the feed treatment section plays a large role in the quality of
the dried product (e.g. granulation, degree of fines, etc.).
The slurry (120-180°F, 50-80°C) is fed to the spray dryer by a high pressure
pump. Counter-current product—air flows are mainly used, although
parallel co-current and mixed flow can be used where a definite air flow
profile results in a special quality of detergent bead. Counter-current flow
systems give product bulk densities 0.2-0.4 g/cm 3 and moisture contents
6-15 %. Co-current flow systems give low bulk densities 04-0.15 g/cm 3
with moisture content 3-8 %. Mixed flows give high moisture contents
20-25 %.
The vast majority of powder leaves the base of the chamber. The entrained
fines fraction is recovered from the exhaust air in cyclones or bag filters.
517
These fines are returned to the wet zone in the drying chamber for agglomeration or reslurried.
The main product is belt conveyed to an air lift. Any after-drying dosing
is carried out on the belt. This can be organic foam boosters (lauryl alcohol),
enzymes (proteolytic) and sodium perborate. The air lift raises the powder
to a storage hopper from where gravity feed takes the product through
screens, a perfuming chamber to packing machines.
14.3.5. Bleach Powders
Active ingredients in bleach powders can be divided into four groups
(a) inorganic chlorine releasing compounds, (b) organic chlorine releasing
compounds, (c) oxygen releasing compounds, (d) optical brightening agents.
The groups are fully discussed by Milwidsky (309). Group (a) consists of
salts of hypochlorous acid, group (b) compounds are very varied and new
chemicals are proposed month by month (chloramines, chlorinated isocyanuric acids). Group (c) are reactant products of hydrogen peroxide and
inorganic salts. Sodium perborate is well known as a bleaching additive
in washing powders. Perborate dosing is carried out after spray drying as
incorporation of the product before drying is difficult. Bleaching activity
requires quite hot temperatures as perborate is stable in cold water. Stability
in storage is often assisted by addition of magnesium sulphate, or organic
sequestrating agents (ethylenediamine tetra-acetic acid) that prevent decomposition of hydrogen peroxide. Optical brighteners in group (d) are
discussed separately below.
Household bleach powders are produced by spray drying in co-current
flow dryers with rotary atomizers. Granular powders can be produced
by maintaining a high powder moisture content in the drying chamber to
form agglomerates that are after-dried in a fluid-bed mounted at the chamber
base. Low outlet air temperatures are used. The final product is granular
and non-dusty. Dryers with air-broom attachments (figure 5.12) and pneumatic product cooling are also used but products are not so granular due
to comminution effects during conveying.
14,3.6. Optical Brighteners
These products are chemicals for brightening fabrics during washing.
The mechanism is one of ultra-violet absorption, which is reflected as
visible light. Hence fabrics appear brighter than untreated fabric when
viewed in daylight. These products are fluorescent dyes which tend to
fluoresce in the blue and ultra-violet end of the spectrum. There is a difference
between optical brighteners and blues of the older technique. In bluing,
the blue pigment absorbs yellow and red light falling on fabric, thereby
removing the apparent yellowish tint of linens, etc. Blues (e.g. ultramarine,
phthalocyanimes) reflect less light than they receive, whereas an optical
brightener reflects the same amount of light in a different shade. Optical
518
APPLICATIONS IN THE CHEMICAL INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
brighteners are spray dried in co-current flow dryers with rotary atomizers
or low pressure nozzles. Choice of brightener depends upon the presence
of powder bleaches, as many are susceptible to oxidation.
14.3.7. Perfumes for Washing Powders
Spray drying of perfumes involves micro-encapsulation. The droplets of
perfume oil are dispersed within a protective colloid (emulsion) and when
dry, the colloid forms a protective skin to prevent fragrance losses due to
diffusion into atmosphere. Colloids generally used are gum arabic or
dextrin. They are water soluble. Each microcapsule contains numerous
minute droplets of perfume oil. A microphoto of an encapsulated fragrance
is shown in an article by Barreto (310).
The spray dryer for perfumes incorporates either nozzle or rotary atomization in a co-current flow chamber. The drying procedure is straightforward. It is the preparation of the perfume feed that determines the success
of the operation. The feed consists of the perfume oil, the emulsion and
water. The ratio of oil to emulsion is important as too little emulsion leads
to poor encapsulating during spray drying and fragrance losses. Too much
emulsion leads to a high viscosity feed that is difficult to dry due to poor
atomization. General feeds to the dryer contain 40 % solids.
The spray drying process is ideal for perfumes, as during the evaporation
process droplet temperatures are low and fragrance volatilization is negligible
during the time it takes for the protective skin of the droplet to harden.
Spray dried perfumes are used in the manufacture of soap powders,
synthetic detergents, cleansers and chemical specialities. These are of
special interest in chlorinated products since they do not react with other
chemicals of the formulation. The powders are free-flowing, easily blended
and require no special packaging, as long as water is absent.
14.4. Pesticides
Spray drying has been adopted worldwide for pesticide preparation using
either open, semi-closed or closed cycle drying systems. Spray dryers are
designed for a given pesticide, or with built-in flexibility to enable the drying
of a number of formulations. Pesticides can be divided in herbicides,
fungicides and insecticides.
Herbicides are (a) phenoxy derivatives, (b) halogenated acid derivatives,
(c) benzoic acid and phenyl acetic acid derivatives, (d) others (e.g. chlorates).
Fungicides are (a) dithiocarbamates, (b) colloidal sulphur, (c) copper compounds, (d) thiuram disulphides. Insecticides are (a) chloride containing
compounds (D.D.T.), (b) arsenates, (c) thiophosphates (parathion), (d)
others,
519
Examples of pesticides that are successfully spray dried
HERBICIDES
Methyl-chloro-phenoxyacetic acid sodium salt (MCPA-salt)
Methyl-chloro-phenoxybutyric acid sodium salt (MCPB-salt)
Dichloro-phenoxybutyric acid sodium salt
Methyl-chloro-phenoxyacetic acid (MCPA)
Dichloro-phenoxypropionic acid
Dichloro-phenoxypropionic acid mono-methylamine salt
Dichloro-propionic acid sodium salt
Dimethyl-dipyridyl-dichloride
Chlorates
FUNGICIDES
Mangano-ethylene-bis-dithiocarbamate
Zinc-ethylene-bis-dithiocarbamate
Zinc-dimethyl-dithiocarbamate
Zinc-diethyl-dithiocarbamate
Tetramethyl-thiuram-disulphide
Copper oxychloride
Cuprous oxide
Colloidal sulphur
INSECTICIDES
Pentachloro-phenolate
Sodium pentachloro-phenolate
Sodium methyl-arsenate
Sodium aluminium fluoride
Calcium arsenate
Lead arsenate
Sodium fluoride
DDT
Thiophosphates
Most pesticides are dried as liquids. Where feeds are filter cake, powder
atomizer wheels are used (figure 6.67).
Operating conditions are varied due to the complex range of pesticide
formulations. Due to the toxicity of pesticides, prevention of environmental
pollution is one of the most important problems of manufacture. The degree
of hazard caused by pollution depends upon the product. Insecticides
being poisonous to humans must not contaminate working areas. Herbicides
and fungicides are not poisonous to humans but emission to atmosphere
can readily cause damage to surrounding flora, especially with herbicides.
Such conditions are prevented by the use of semi-closed or closed cycle
spray drying layouts. A suitable semi-closed cycle dryer is shown in figure
14.10. The drying medium is air, which is exhausted via a scrubber system.
520
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE CHEMICAL INDUSTRY
INDIRECT AIR
HEATER
CONDENSER/COOLER
HERBICIDE
Figure 14.10. A semi-closed cycle spray dryer for herbicides (a part of the exhaust air is used
as combustion air in heater).
Part of the scrubber air is recycled through an indirect oil/gas fired air
heater to the drying chamber. The remainder is used as combustion air in
the heater. Any herbicide present in the air leaving the scrubber is decomposed (inactivated) during passage through the heater combustion zone.
The quantity of air passed through the heater combustion zone equals
the air entering the dryer layout as powder conveying air. Closed cycle
systems (figure 0.2) are used where an organic solvent in the feed requires
an inert gas drying medium. For recovery of solvent following evaporation in the drying chamber, a condenser-cooler replaces the scrubber.
If the toxicity of the product and plant location permit open cycle dryers,
very low emission levels can still be obtained by use of scrubbers in series
with cyclones or bagfilters.
14.4.1. Herbicides
The sodium salt of organic herbicide formulations is generally spray dried
for products used for spraying (crop dusting) operations. Examples of such
salts are methyl-chlorophenoxyacetic acid-sodium salt (MCPA-sodium
salt) and dichloropropionic acid-sodium salt. The free acid form of herbicide
formulations is spray dried using powder wheel atomizers (figure 6.67).
Free acids are manufactured as intermediate products.
All herbicides require efficient exhaust cleaning air equipment. Cyclones
are usually used as the primary collector, but these are followed by bag
filters or wet scrubbers in open cycle systems or scrubbers (cooler condensers)
and air heaters in semi-closed and closed cycle systems. MCPA-sodium
salt is often dried in semi-closed cycle systems. A 50 % liquid feed is atomized
with a vaned wheel (VT = 500 ft/sec, 150 m/sec) in a co-current flow dryer.
Inlet hot-air temperatures are of the order 500°F (260°C). The MCPA
521
(pure acid) can be dried as a filter cake using a powder atomizer wheel at
lower speeds (VT = 360 ft/sec, 110 m/sec). Drying air temperatures are
lower (280°F, 140°C). Dichloropropionic acid sodium salt is spray dried
in special designs that need very exact control of the exhaust air temperature.
Low moisture content levels in the powder are required. If air temperature
falls and the powder becomes moist, decomposition occurs with product
condensation in the bag filters.
14.4.2. Fungicides
Fungicides are generally spray dried with rotary atomizers (vaned wheel).
Two important fungicides, the ethylenebisdithiocarbamates of zinc and
manganese are spray dried in this way. Close control of exhaust air temperatures is required for the manganese form. If powder becomes too dry
(moisture less than 2 %) product heat degradation occurs through product
smouldering.
14.4.3. Insecticides
DDT (Dichloro-diphenyl-trichlorethane). DDT melt is spray cooled. Cocurrent flow chamber is used. Inlet air temperature of 140°F (60°C) and
feed temperatures of 195°F (90°C) are observed. A rotary atomizer forms
a spray with 100 % greater than 60 micron and approx. 30 % greater than
500 micron. Spray cooling eliminates the stickiness associated with conventional methods. The powder bulk density ranges between 0.35-0.42
g/cm 3 . The powder is quite brittle. It has excellent solubility.
14.5. Dyestuffs—Pigments
Organic dyestuffs can be classified as soluble dyes (direct, acid, basic), vat
dyes (indanthrene dyes, indigo, indigosol dyes), azoic dyes (naphthols, diazo
compounds), microdisperse (formulated) dyes and reactive dyes. Organic
dyestuffs are highly suited to spray drying.
For organic dyestuffs a maximum hot drying air temperature of 480°F
(250°C) applies for the vast majority of products. Rotary and nozzle atomization is applicable. Hollow particles or agglomerates are formed. These
are rather brittle and are easily dispersed into dissolvers or similar aggregates. Desired moisture or residual diluent content can generally be achieved
by the spray drying operation, but for extremely low moisture contents,
after-dryers (pneumatic or fluid-bed) are incorporated at the base of the
drying chamber. Cyclones are used as the product separation and recovery
equipment. These are easily cleaned enabling different dyestuffs to be dried
in the same unit. Wet scrubbers are mounted after cyclones to remove
traces of dyestuff leaving the cyclones. It is usual these days that, on account
of cleaning requirements, each product has its own spray dryer. In these
cases bag filters are often used instead of cyclones. Co-current flow spray
522
APPLICATIONS IN THE CHEMICAL INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
dryers with rotary atomization is the standard lay-Out for most dyestuffs.
For special requirements, e.g. coarse, non-dusty, or extremely heat sensitive
products, nozzle atomization is applied in tall co-current flow towers having
parallel (streamline) air flow conditions. To create and maintain this air
flow, the air disperser is a perforated plate. The exhaust air is drawn from
the tower via a ring having six equidistant apertures. The apertures converge
to two exhaust ducts. Product recovery from the drying tower is approx.
98 %. For coarse product requirements, the fines are returned to the wet
zone around the nozzles for agglomeration.
Organic and inorganic pigments can be spray dried. Rotary and nozzle
atomization is applicable. Particle characteristics are similar to dyestuffs.
Pigments are used in rubber, paint, ink and plastics manufacture. Wellknown inorganic pigments include titanium dioxide, kaolin, zinc chromate,
iron oxide and carbon black.
For dyestuffs or pigments that occur as suspensions in organic liquids or
as dispersions in resin, closed cycle dryer layouts are used (figure 0.2(b)).
If the dried product can cause powder explosion conditions in air, semiclosed cycle dryer layouts are used, designed on the self-inertizing principle.
The recycled drying air/gas is heated by direct gas firing at an open ring
burner. A controlled percentage of excess air is used at the gas burner. The
layout is similar to figure 0.2(c) but after the cooler/condenser, a volume of
exhaust gases equal to that used as combustion air is bled from the system.
The 0 2 content in the dryer is thus held low preventing explosion hazards.
The 0 2 content in the dryer is approximately 1/5 the excess combustion air
(i.e. 2 % 0 2 content for 10 % excess combustion air).
14.5.1. Titanium Dioxide
White thixotropic filter cake (up to 60 % solids) is dried at inlet/outlet
temperatures (1110/250°F, 600/120°C). The moisture content of the powder
is 0.5 %. Co-current flow dryers are used with rotary atomizers, or mixed
flow with nozzle atomization. Choice of atomizer depends upon requirements to particle size distribution of the dried product. The flow diagram
is shown in figure 14.11. The filter cake is fed into a disperser to worksoften the paste. Vigorous agitation reduces the titanium dioxide viscosity
to a level whereby a pumpable smooth paste is formed. Diaphragm pumps
are used to supply the rotary atomizer. The pump must have large suction
and discharge parts. A strainer/disintegrator can be placed between the
pump and atomizer to prevent any undispersed product passing to the
atomizer where plugging can occur. Use of a flooded disintegrator for spray
dried titanium dioxide has been reported by Young and Ireland (311).
Direct gas fired heaters (open jet ring) are used, with combustion controlled to prevent formation of soot that can taint the white powder. The
dried powder is collected in a bag filter. The dryer is normally controlled
523
by maintaining a constant feed rate and controlling gas combustion rate to
maintain a constant outlet temperature and prevent bag filter damage.
Safety devices to safeguard the bag filters against excess heat are standard.
The bags are of polyacrylonitrile or polyester fibres. The powder is often
micronized after drying. Hence there is little need for a coarse powder
unless sold in this state. Titanium dioxide is most widely used as a white
pigment in paints, paper, etc.
Ti TANIUM
DIOXIDE
FILTER CAKE
STRAINER —
SI N TE GRA TOR
DIRECT
AIR HEATER
DISPERSER
DIAPHRAGM
PUMP
SPRAY DRI ED
Ti 0
PIGMENT
2
HIGH SHEAR FEED PREPARATION
[ work-softening of thixotropic Ti 0 2 ]
Figure 14.11, Flow diagram for spray drying titanium dioxide.
14.5.2. Kaolin (China Clay)
Kaolin clay was one of the first inorganic materials to be dried in spray
dryers of large capacity. Figure 4.13 shows a complex of four spray dryers,
(30 ft (9 m) in dia), each producing 12-15 tons dried kaolin per hour. The
purity and uniformity of spray dried kaolin has established wide usage in
the paper, paint, plastics, rubber, and ceramic industries.
Kaolin is a hydrous aluminium silicate (Al 2 0 3 .2Si0 2 .21-1 2 0). It is a
leading white pigment consumed in vast tonnages. The bulk goes to the
paper industry as coating material in high gloss and quality printing grades
and as a low cost filler. The process diagram for kaolin is shown in figure
14.12. Crude kaolin from the mine is milled, crushed and slurried in a
blunger with water and chemical dispersants (sodium silicates or phosphates).
The resulting clay slip (25 % solids) is fed through sets of degritting wet
cyclones, screened and left to settle in large settling tanks for up to 14 hours.
The bottoms are passed through wet cyclones and pumped to the filter
clay plant. The fine clay slip drawn off the top of the tank consists of approx.
524
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE CHEMICAL INDUSTRY 525
Figure 14,13. High tonnage spray dryers on kaolin in Georgia, U.S.A., each producing 15-25
tons per hour. (By courtesy of Nichols/Niro Atomizer.)
85 `)/, less than 2 micron particle size. This is immediately treated with acid,
flocculating the slip, and bleached (zinc or sodium hydrosulphite). pH is
adjusted to 3.5 with sulphuric acid. The clay slip changes from a cream
to blue—white colour. After bleaching, the slip is thickened to 12% solids,
passed through 150 mesh screens and centrifuged to 25 % solids. The solids
are increased to a 60-65 % solids cake in a vacuum filter. For spray drying the
solids cake must be transformed into a free-flowing slurry, a requirement
for acceptable atomization. The solids content must, however, be held
high to achieve an economic spray dryer operation. A free-flowable slurry
is obtained by the addition of dispersants (tetrasodium pyrophosphate,
tripolyphosphate or sodium hexametaphosphate) on a 0.3 % dry solids
basis. The feed slurry is passed through 150 mesh screens and a magnetic
separator before entering the spray dryer.
The spray dryer is a co-current flow conical based design. Rotary atomization is employed using abrasion resistant wheels (figure 6.26). Drying air
(oil or gas heaters) at 1100°F (600°C) enters the dryer via a central air disperser located just beneath the wheel. This arrangement prevents local
hot spots on the roof, which could cause product discolouring. Drying air
is exhausted at 250°F (120°C) and product falls to the base of the chamber
for discharge into conveyors. The dried product temperature is around
526
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
160°F (70°C). Clay fines that are exhausted with the air are recovered in
cyclones or bag filters. The dried clay (1 % moisture) consists of microspheres and are non-adhesive. It is an essentially 100 % pure, free-flowing,
high bulk density product. There are several references to spray dried
kaolin in the literature (224) (226) (312).
14.6. Fertilizers
Application of spray drying in the fertilizer industry is restricted to a few,
but important products, e,g. ammonium phosphate and superphosphates.
The well-known process of manufacturing phosphate fertilizers by reacting
phosphate rock with phosphoric acid has been modified by the introduction
of spray drying. Although based upon the traditional wet process, it differs
in having part of the phosphate rock—phosphoric acid mixture spray dried
before granulation. Other important fertilizers, e.g. compound N—P—K
types, ammonium nitrate, and nitro chalk, are produced by the spray cooling
principle. The reader is referred elsewhere (233) for flow diagrams of industrial fertilizer production utilizing spray processes.
14.63, Ammonium Nitrate
Ammonium nitrate is mostly produced by direct neutralization of nitric
acid with ammonia according to the basic equation :
NH 3 + HNO 3 = NH 4 NO 3 + 20 600 cal
The neutral solution of NH 4 NO 3 is concentrated to more than 90 %, in
most cases by utilization of the heat of neutralization. The concentrated
solution is then treated according to either of the three methods in order
to obtain a dry product.
(1) Graining is normally effected by evaporation of the ammonium nitrate
to 98 %. The product obtained is discharged into a jacketed kettle equipped
with heavy plows that keep the material stirred as it cools and solidifies.
The mass breaks apart into grains which are subsequently cooled, screened
and mixed with anticaking agent before bagging.
The particles produced by this method are suitable for munition use, but,
due to the wide range of particle size distribution and the small average
size, not for fertilizer use.
(2) Crystallizing of ammonium nitrate is performed in continuous vacuum
crystallizers. The crystals are dried in rotary dryers. The crystallization
process is relatively safe, and yields products of a shape suitable for fertilizer
use. The investment cost, though, is rather high.
(3) Spray cooling of ammonium nitrate is the cheapest method for obtaining
a dry product from the solution, both with regard to operational cost and
investment. As early as 1918, a German application was filed for a patent
APPLICATIONS IN THE CHEMICAL INDUSTRY
527
covering the production of particulate dry ammonium nitrate through
spray cooling. For atomization, a two-fluid nozzle was applied using either
cold or hot compressed air. The atomized ammonium nitrate was cooled
in a chamber by means of cold air.
In 1927 (313), a method for spray cooling of ammonium nitrate was
patented in the United States. The atomization was effected by means of a
fountain pressure nozzle, and the final product was claimed to be characterized by low density and low hygroscopicity. The bulk density was controlled by adjustment of the concentration of the ammonium nitrate solution, its temperature, and the degree of atomization. Atomization could for
example be adjusted to give a diameter between 400 and 2000 micron
for 95 % of the particles. After solidification, the particles were dried in a
rotary dryer. A similar plant, however, provided with rotating disc atomizer
instead of nozzles, was patented in 1938 (314). The rotating disc atomizer
was claimed, among other advantages, to facilitate the adjustment of the
particle size and thereby of the bulk density.
The purpose of the above methods was to produce an ammonium nitrate
powder well suited for munitions. However, in 1946 (315) the first spray
cooling method for production of ammonium nitrate in granular form
suitable for use as fertilizer was published. This method was called `prilling'.
The concentrated ammonium nitrate solution is atomized at the top of a
high tower. Falling through the tower, the droplets cool and solidify to
pellets or prills when meeting cold air in co-current or counter-current flow.
Figure 14.14 shows the flow sheet for a prilling plant. The concentrated
ammonium nitrate solution is atomized at the top of a tall tower. The
atomization can be effected by pressure nozzles adjusted to spray either
upwards or downwards. In the original patent it is stated that the solution
is preferably sprayed upwardly into the tower at an angle of about 45°
to the horizontal. Later research work, however, indicated that this angle
is not decisive and that the same result is obtainable by downward nozzle
spraying f.inst. at an angle of 60° to the horizontal downward. The atomization can also be effected by rotary atomization, f.inst. by means of a rotating
spray bucket. The spray bucket consists of perforations. Clogging of the
holes is prevented by a blade shaped element which is rotated on the inside
of the perforated wall. The concentration of the ammonium nitrate solution
of 99.5-98.8 % results in high density prills (0.95 g/cm 3 ). A 95-97 % concentrate gives low density prills (0.75 g/cm 3 ).
During the prilling process, very little moisture is removed. It is, therefore,
necessary to after-dry the product, either in a rotary dryer, or in a fluid-bed,
if the solution contains more than 0.3-0.5 % moisture. Before coating with,
for example, clay to prevent caking during storage, prills are cooled in a
rotary cooler, or fluid-bed.
528
APPLICATIONS IN THE CHEMICAL INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
CONC. NH4 NO3 SOLUTION
ATOMIZING DEVICE
AIR OUTLET
PRILLING TOWER
COOLING AIR
COOLING AIR
MEM III
DRYER
COOLER
FINES
SCREENS
AMMONIUM
NITRATE
FRILLS TO
COATING AND
BAGGING
Figure 14.14. Production of ammonium nitrate prills by spray cooling.
Nitro-chalk, a mixture of ammonium nitrate and calcium carbonate,
is spray cooled similar to ammonium nitrate. A slurry consisting of about
one part ground chalk to two parts molten ammonium nitrate is atomized
either by means of nozzles or a rotary atomizer.
14.6.2. Ammonium Phosphate
Ammonia and phosphoric acid (free of aluminium and iron phosphates)
are reacted at 120°F (50°C) in a spray absorber. The diammonium phosphate
((NI1 4 ) 2 HPO 4 ) liquor is concentrated, centrifuged and spray dried. Further
addition of phosphoric acid will form the monoammonium phosphate
(NH 4 )H 2 PO 4 for spray drying. The reactions are given by
529
(180-190°C), but is stable at 210-250°F (100-120°C). Diammonium phosphate will decompose at about 180°F (80°C) to ammonia and free phosphoric
acid giving a sticky product. Triammonium phosphate is only stable under .
pressure. Normal commercial grades of monoammoniurn phosphate
contain some diammonium phosphate (= 5 %).
Spray drying of ammonium phosphate produces a free-flowing low
dusting product. Moisture content (3-5 %) and powder temperatures are
controlled to prevent tackiness and caking in storage. In the spray drying
of monoammonium phosphate, a 60 % solids feed is pre-heated to 190°F
(90°C) and pumped to a co-current flow dryer. A coarse product is required,
although particle size is not critical as the spray dried product is not the end
product, being further granulated with other chemicals. Pressure nozzle
(15 atm) or low speed rotary atomization is used. Inlet drying temperatures
up to 750°F (400°C) are obtained from a direct fired gas or oil burner. Outlet
temperature is in the range 230-380°F (110-195°C). High powder temperatures can cause an after-crystallization following drying leading to cakingtendency conditions. If free acid is present, the product will cake at moisture
contents above 5 %. If there is no free acid, products of moisture content
up to 9 % will remain free-flowing. A flow diagram with nozzle spray drying
is shown in figure 14.15.
Spray dried product particle size, 50 % over 300 micron, is the normal
requirement, but adjustment of atomization conditions can produce coarser
or finer products.
Ammonium phosphates are a prime fertilizer source.
NH
3
H3 P O 4
nozz
H PD
3 4
(NH4 ) H 2 PO4
4
NH4 2
air
2NH 3 + H 3 PO 4 (NH 4 ) 2 HPO 4
MONO AMMONIUM
.
PHOSPHATE ,
(NH 4 ) 2 HPO 4 + H 3 PO 4 2NH 4 H 2 PO 4
Control of pH values in the recirculating liquor determines the type of
ammonium phosphate formed. Monoammonium phosphate forms below
a pH 5, diammonium phosphate above this value.
Strict control of outlet temperature is required during spray drying.
Monoammonium phosphate will decompose at a temperature of 355-375°F
„
REACTORCONCENTRATOR
CRYSTALS
TO FLASH
DRYER
CENTRIFUGE
SECONDARY
REACTOR
SPRAY
DRYER
Figure 14.15. Flow diagram for production of mono-ammonium phosphate by spray drying,
530
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE CHEMICAL INDUSTRY
14.6.3. Superphosphates
The processing of double superphosphate features as an important application of spray drying within the phosphate fertilizer industry. Double
superphosphate is the end product of phosphate rock, i.e. phospherite or
apatite treatment with phosphoric acid.
The principal reaction between tricalciumphosphate and phosphoric acid
results in the desired monocalciumphosphate monohydrate
Ca3 (PO 4)2
4 H 3 P0 4, + 3H 2 0 3CaH 4 (PO 4 ) 2 . H 2 0
The reaction mixture contains several other compounds including dicalciumphosphate, CaHPO 4 , tricalciumphosphate, Ca 3 (PO 4 ) 2 , calciumfluophosphates, CaP0 3 F.
The reaction may be effected either with electric furnace phosphoric acid
having a high P 2 0 5 content (about 60 %), or with wet process phosphoric
acid having a lower P 2 0 5 content (30-47 %). In the latter case, the water
originating from the dilute phosphoric acid must be removed, either before
mixing with phosphoric rock, i.e. the dilute phosphoric acid must be concentrated, or after the initial phase of the reaction between phosphate rock
and acid has taken place, i.e. the product must be dried. The steps involved
in the three different methods of double superphosphate manufacture are
shown in table 14.2.
The reaction between phosphoric acid and raw phosphate is influenced
primarily by the concentration of acid, the type and particle size of phosphate
rock, the time and temperature of mixing, and the duration storage of
mixture. The most significant factors are concentration of acid, and storage
ti me of reaction mixture.
Table 14.2. Methods of Double Superphosphate Manufacture
Steps involved
1 Manufacture of
phosphoric acid
2 Concentration
of acid
3 Mixing of acid
and rock
4 Drying of product
5 Final curing
Electric furnace
phosphoric acid
x
II
III
Wet process
acid
Phosphoric
acid
Acid concentrated before
Product dried
after reaction
x
X
x
x
x
x
x
X
X
531
In order to obtain the maximum conversion of the phosphate raw material
into the required phosphate form (principally monocalciumphosphate),
it is necessary to store the reaction mixture for some days or weeks in a curing
pile. During this period the reaction goes to completion resulting in a
decreased moisture and free acid content in the final product.
Many attempts have been made to develop quick-curing processes both
for double superphosphates and normal superphosphates. These have been
based upon curing at elevated temperatures, and applying the fact that
reaction rates are increased if the dilute phosphoric acid is concentrated in
contact with the ground phosphate rock.
The principal features in the continuous production of quick-cured
granular double superphosphate are the use of relatively dilute (39 % P2 0 5 )
phosphoric acid (this produces a fluid reaction slurry), and recirculating a
very large proportion of the dried product for mixing with the reaction
slurry. Figure 14.16(a) shows a simplified process flow sheet. The phosphate
rock and the dilute phosphoric acid are mixed in the reaction tank at a
temperature of 175-212°F (80-100°C). The slurry from the reaction tank
goes to a blunger in which it is mixed with dry, recirculated fines. The
blunger is discharged into the rotary dryer where the reaction—started in
the reaction tank— is completed and the moisture content reduced to 2-3 Z.
The product is discharged from the dryer at a temperature of 200-212°F
(95-100°C). The dry product is classified as fines, finished product, and
oversize. For each ton of finished product leaving the system approximately
10 to 20 tons of fines and pulverized oversize are recycled to the blunger.
Recently, the procedure has been modified by the introduction of spray
drying. The processing includes a spray drying stage, and differs from the
process described above on two main points. Firstly, the major part of the
reaction mixture (55-60 %) is spray dried and thus converted into a freeflowing powder before being mixed with the remaining reaction mixture
(45-40 %). Secondly, the proportion of dry product that must be recirculated
is considerably smaller.
Figure 14.16(b) shows a simplified process flow sheet. The phosphate
rock and dilute phosphoric acid (30-32%) are mixed in two reaction tanks
at a temperature of about 195°F (90°C). From reaction tank I, the slurry is
pumped to the spray drying plant equipped with a rotary atomizer, and
dried in co-current flow of hot air (inlet temperature 900-1100°F (500600°C)). The spray dried powder is discharged from the dryer at a temperature of about 210°F (100°C).
In the blunger, the spray dried powder is mixed with slurry from tank II
and discharged into the rotary dryer. The granular product leaves the rotary
dryer with a moisture content of about 3 %. .It is then classified as fines,
finished product, and oversize. The oversize is pulverized and returned
532
APPLICATIONS IN THE CHEMICAL INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
PHOSPHATE ROCK
PHOSPHORIC ACID
spray dried powder, the granules formed in this way are more uniform than
those from the process described previously, and the amount of fines and
oversize product, therefore, much smaller.
(b) The spray dried powder leaves the drying chamber at a temperature of
210-230°F (100-110°C). Although the drying time is short, it appears that
the conversion of phosphate rock into available phosphates is accelerated
at this stage. As yet, insufficient information is available on the actual
operation conditions of existing plants to verify this point.
Spray drying of the reaction mixture of phosphate rock and phosphoric
acid presents some technological problems due to the stickiness of the
product, and the possibility that the monocalciumphosphate may revert
to an insoluble form through overheating. It is absolutely necessary for
strict control of the air pattern within the drying chamber. The slurry is
extremely abrasive and corrosive and necessitates use of special diaphragm
or centrifugal pumps. Direct or gravity feed to the wear resistant atomizer
wheel is used.
REACTION TANK
BLUNDER
ROTARY DRYER
HOT AIR
SCRE ENS
DOUBLE SUPERPHOSPHATE
(FINISHED PRODUCT )
RECYCL
FINES
RECYCLE RATE = 10 to 20 x FINISHED PRODUCT RATE .
REACTION TANK I
533
REACTION TANK II
PUMP
HOT AIR
14.7. Mineral Ore Concentrates
SPRAY DRYER
The application of spray drying to flotation concentrates is a completely
new technique in the hydrometallurgical industry. In the conventional
de-watering of dried concentrate preparation (figure 14.17), a vacuum filter
and rotary dryer follow the thickener to produce a 7 % moisture product.
BL UN DER
ROTARY DRY ER
b.
HOT AIR
—
1
THICKENER
SCREENS
1
RECYCLE
DOUBLE
SUPERPHOSPHATE
( FIN ISH ED PRODUCT
VACUUM Fl LTER
1
UNDER FLOW
70% SOLIDS
Fl NES
A
1
RECYCLE RATE — FINISHED PRODUCT RATE
2
Figure 14.16. Flow diagrams for the production of double superphosphate fertilizers. (a)
Conventional process. (b) Spray drying process.
to the blunger together with the fines. Only about 0.5 tons of fines and
pulverized oversize product is returned to the blunger for every ton of
finished product leaving the,system.
This substantial reduction in the amount of recycled product is mainly
due to the following.
(a) In the blunger, and also in the first section of the rotary dryer, the slurry
adheres to the surface of the spray dried product (and the returned fines)
building up the granules in layers. Due to the free-flowing properties of the
B
SPRAY
DRYER
ROTARY DRYER
DRIED FLOTATION
CONCENTRATE 7% H 2 O
DRIED FLOTATION
CONCENTRATE
SELECTIVE - BONE DRY
TO 5% H2O
Figure 14.17. De-watering operations for flotation concentrates, A. Conventional system with
thickener, vacuum filter and rotary dryer producing dried concentrate with 7 % moisture.
B. Spray drying system with only thickener and spray dryer producing dried concentrate with
moisture selective between bone dry.and 5%.
534
APPLICATIONS IN THE CHEMICAL INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
The advent of spray drying substitutes vacuum filters and rotary dryers
and produces a dried concentrate of moisture content controllable to any
value between bone dry and 5 %. The spray drying system is continuous,
and enables an efficient concentrate handling system from the thickener
to the smelter and eliminates dust losses. The advantages of the spray dryer
system have been reported recently (47) (229).
Heating of the drying air in spray dryers handling flotation concentrates
is by direct fired heavy fuel-oil heaters. Pulverized coal burners or coal
stokers are also used, and where available, exhaust gases from diesel powered
electric generators, and smelting furnaces as these then minimize fuel requirements. Co-current flow dryers utilize rotary atomizers. Flotation concentrates are abrasive and even corrosive, and the atomizer has a wear resistant
wheel (figure 6.26). Dried product is discharged from the base of the drying
chamber. Fines exhausted from the chamber with the air are collected in
cyclones, bag filters, electrostatic precipitators, scrubbers or selected combinations of such equipment. Electrostatic precipitators are preferred where
abrasiveness of the dried concentrate leads to excessive cyclone wear, or
where water shortage prevents use of scrubbers. Concentrates dried to
bone-dry levels are stored in silos and transferred by pneumatic handling.
Concentrate of higher moisture content levels are transported by belt
conveyors.
FROM
FLOTATI ON
OPERATION
THICKENER
THICKENER UN DERFLOW
IOR WATER I FOR REWET TI NG
1
EXHAUST
AIR
FEED TANK
SPRAY
DRYER
-
AMBIENT AIR
-
AVAILABLE
WASTE HEAT
CON BUSTI ON1-1MR
-
E
=AV
AIR
C OO LING
HEATER A IR
PRODLE
COLL Ec
TOR
❑ .CYCLONE.SCRUBDER
B BAOFILTER
c ELECTROSTATIC
PRECIPITATOR
DRIED PRODUCT
S ILO
TO
CHEMICAL
REFINING
TO
FLASH
SMELTER
COMPACTOR (FLAKER I OR
REW ET TING TO NON-DUSTING
PRODUCT
TO
REVERBERATORY
SMELTER
Figure 14.18. Flow diagram for spray drying flotation concentrates.
535
14.7.1. Sulphide Ores (Copper, Nickel, Molybdenum, Tin)
The flow diagram for the spray drying of sulphide flotation concentrates is
shown in figure 14.18. Concentrates up to 70 % solids by weight are dried
to a product of moisture content required for reverberatory smelting, flash
smelting or chemical refining.
Inlet drying air temperatures up to 1830°F (1000°C) are used in the spray
dryer. Direct air heaters are employed. Where available, waste heat can
act as a supplementary heat source, thus minimizing fuel requirements of
the heater. The drying air is passed to the spray drying chamber via a central
air disperser, the outlet of which is just beneath the atomizer. The central air
disperser is a most effective design to handle very high air temperatures,
and provide optimum spray—air contact for rapid moisture evaporation
from the spray. The concentrate is gravity fed to the atomizer. Direct
feed with automatic flow control at a diaphragm valve is also applicable.
The exhaust air temperature from the drying chamber is controlled to a
value set by the desired dried product moisture content. The spray drying
system causes no measureable loss of sulphur. Primary product discharge
takes place at the base of the drying chamber. Powder fines are recovered
from the exhaust air in mechanical separators. On large dryers, electrostatic
precipitators are installed. When cyclones are favoured, the exhaust air is
scrubbed after the cyclone. The scrubber water can be recirculated via a
settling tank to increase the solid content. The scrubber water is returned
to the thickener, from which solids pass back into the dryer system.
Operation of one of the first spray dryers on sulphide ores has been
described in the literature (46). Two spray dryers each 30 ft (9 m) in diameter
are reported producing 45 tons of dry concentrate per hour from a 65 %
solids feed (figure 14.19). The feed is atomized by a 9 in (225 mm) wear resistant atomizer wheel rotating at 8400 rev/min.
Potential applications of the above process include many minerals at
present being processed by froth flotation. These minerals include lead,
zinc, manganese, tungsten and silver ores.
14.7.2. Iron Ores
Spray drying is becoming applied to iron flotation concentrates, replacing
the conventional processing of vacuum filter and rotary dryer. Spray drying
systems enable more economic and effective use of binders, and produce a
superior product for pelletizing. A co-current flow spray dryer with rotary
atomizer is used. The layout is similar to figure 14.18. The dried product is
belt conveyed to pelletizing and sintering equipment.
The underflow from the thickener can be pumped directly to the spray
dryer at 72-75 % solids, or first mixed with vacuum filter cake to increase
solids to 80 %. _Bentonite and limestone are added (if required) to the slurry.
The binder, when added to a liquid base is normally more active than when
536
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Figure 14.19, Twin spray dryer complex in Western Australia, producing bone dry nickel-
concentrate. (Direct heater to right of drying chambers. Cyclones, scrubber to left of drying
chambers.) (By courtesy of Niro Atomizer.)
dry mixed (i.e. in the conventional process). This means savings in binder
requirements. Based upon a unit weight of dry concentrate for pelletizing,
binder requirements for the spray drying system can be up to 50 % lower
than for the conventional system. The conventional system requires 0.31.0 % bentonite, 2% limestone addition, and dry mixing in a 'mullet.' mixer.
The concentrate is dried to 5 % moisture in the spray dryer then pelletized
and sintered to bone dry in a travelling grate kiln. The final product is
homogeneous and completely free of 'mini-balls'. Recirculation of under/
oversize from the pelletizer is avoided. This compares favourably with the
conventional process where the concentrate leaves the rotary dryer at 7 %
moisture, then 2 % water is added as a fine spray prior to pelletizing. A
large recirculation of fines from the pelletizer often occurs.
The spray dried concentrate of 5 % moisture is optimum for pelletizing.
By achieving a product of good pelletizing characteristics at a moisture
APPLICATIONS IN THE CHEMICAL INDUSTRY
537
content lower than in the conventional process, the spray drying system
enables reduction of the evaporative load on the sintering equipment.
14.7.3. Cryolite
Cryolite is sodium aluminium fluoride (Na 3 A1F 6 ) and as a white mineral
is found mainly in Greenland with lesser deposits in the Ural mountains
and Colorado. It is used in the aluminium, enamel, glass and welding electrode industries. Crude cryolite has many impurities, the most important
being quartz, siderite, galena, chalcopyrite, fluorspar, zinc blende and
hagemannite. The removal of impurities is carried out in four stages. The
mineral is crushed and the known impurities are often sorted out by hand.
Ferrous materials are removed by magnetic separators. The next stages are
selective grinding and screening. The hard quartz resists grinding and
hence is removed during screening. The ground cryolite is further separated
from the impurities by flotation techniques. The ground cryolite and accompanying minerals pass to flotation cells, where through aeration and reagent
addition, one or more of the impure minerals float to the cell surface for
removal with the bubbles and foam. The flotation process is repeated several
times, until only the cryolite and fluorspar remain. A final reagent dosage
effects the separation of the flotation cell and is removed. The cryolite
suspension is spray dried and packed in bags as 94-98 % pure cryolite.
Co-current flow dryers with rotary atomizers are used. The flow diagram
is similar to figure 14.18.
Cryolite lowers the melting point of aluminium oxide from 4000-1750°F
(2200-950°C), thus enabling aluminium smelting to use standard furnaces.
Although synthetic cryolite is widely used nowadays in the aluminium
industry, there is still strong opinion within the industry that the natural
product is indispensable especially during start-up of smelting furnaces.
The natural product is the more stable and more reliable. Other uses of
powder cryolite fines are in fluxes, and as filling material for grinding wheels.
14.8/14.9. General Inorganic and Organic Chemicals
The vast majority of inorganic and organic chemicals that have a solid
form at atmospheric conditions can be obtained from fluid feeds in suspended
particle processing equipment involving an atomization chamber. Equipment design is so flexible that subject to the feed being atomized satisfactorily
diverse product properties can be handled, whether it be in open, semiclosed or closed cycle dryers, spray reactors or spray coolers.
The list of products given in table 13.1 cannot be considered complete.
New products from day to day are being test dried successfully to increase
the range of product applications.
538
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
14.8. Inorganic Chemicals
Inorganic chemicals fall into the following categories.
1. Aluminates
2. Arsenic compounds
3. Carbonates, Bicarbonates
4. Chlorides
5. Chlorites, Hypochlorites
6. Chromates
7. Cyanides
8. Fluorides
9. Fluorosilicates
10. Hydroxides
11. Manganates
12. Nitrates
13. Oxides
14. Phosphates
15. Silicates
16. Sulphates, Sulphides, Sulphites, Thiosulphates
17. Uranates
18. Zirconates
14.9. Organic Chemicals
Some applications have already been discussed in previous sections where
products in the form of dyestuffs, plastics, rubbers, resins, pesticides are
mentioned. Carbohydrates and organic pharmaceuticals fall into the food
and biochemical fields and are mentioned in chapters 15 and 16. Organic
products form three main categories.
1. Organic acids which include amine acids, salicyclic acid, citric and
maleic acids, ascorbic acid.
2. Organic salts which include phthalates, stearates, salicylates, benzoates,
butyrates, gluconates, lactates, sacharates, sorbates and many others.
3. Nitrogen containing compounds, which include hydrazine, chloramines,
ureas.
14.10. New and/or Specialized Applications in the Chemical Industry
The products described below are given individual attention due to their
special relation to suspended particle process systems that involve a chamber
with an atomizer.
APPLICATIONS IN THE CHEMICAL INDUSTRY
539
The products selected are
Abrasives, require special atomizing and handling equipment
Catalysts, require meeting strict specifications of particle size and shape
Cement, a new field of application
Metal powders, a new field of application
Sodium carbonate, an example of spray reaction
Sodium hydroxide, an example of spray cooling
Titanium tetrachloride, an example of spray purification.
The first four products involve the chamber as a dryer. The final three
products are examples of utilizing spray drying equipment in the other
suspended particle processing of spray reaction, cooling and purification.
14.10.1. Abrasives
Coarse and fine grits for abrasive materials can be manufactured by spray
drying grit suspensions. Abrasives require grits of narrow particle size to
conform with international standards. Grit is subjected to a series of washing
operations, where a suspension of finer and finer grit sizes is obtained from
successive washing. The suspension can be formed into slurries by sedimentation techniques and then spray dried to give a dried product of uniform
moisture and narrow size distribution.
The spray drying operation is one of high temperature with inlet temperatures of the order 1000°F (550°C) and outlet temperatures 350-390°F
(175-200°C). The feed slurries are particularly abrasive and require wear
resistant atomizer wheels (as in figure 6.26). Use of low pressure nozzles
with boron carbide inserts has also been reported (230) but monthly insert
replacements were necessary.
Silicon carbide is a typical abrasive that is spray dried.
14.10.2. Catalysts
Spray drying is established as an important stage in the preparation of
catalysts. The choice of atomizers, atomizer operation and drying chamber
design enables the specific powder qualities for catalyst effectiveness to be
readily obtained, whether coarse granules or fine powders are required.
Spray dried products consist of free flowing spherical particles of constant
bulk density, constant residual moisture, and fixed size distribution. These
physical properties render the product well suited for subsequent fluid-bed
processing or dry pressing into catalyst pellets. Solutions, slurries, gels
or pastes of catalysts and catalyst carriers can be dried to products of closely
controlled characteristics. Where the catalyst is dispersed in a toxic or
inflammable solvent closed-cycle spray drying is applied.
Spray dried catalysts form three important categories (a) catalysts for
inorganic reactions, e.g. ammonia, hydrogen, sulphuric acid synthesis,
carbon—oxygen reaction, (b) catalysts for oil and gas refining, e.g. cracking,
540
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
reforming, desulphurization, and (c) catalysts for organic reactions, e.g.
hydration, dehydration, oxidation, polymerization and condensation.
It is impossible to generalize catalyst drying as catalyst requirements are
so varied. Invariably co-current or mixed flow dryers are used. Atomizer
wheels, pressure nozzle or two-fluid nozzle atomization is selected after
particle size requirement. Inlet temperature can be high (1110°F, 600°C)
or relatively low. The development of wear resistant atomizer wheels has
established rotary atomizers as the preferred atomization technique.
The spray drying of silica-alumina petroleum cracking catalysts has been
described by Shearon (316). General catalyst manufacture involving spray
drying has been reported by Placek (317).
14.10.3. Cement
The spray drying of cement slurry is an example of one of the latest applications in the heavy inorganic chemical industry although its potential has
long been recognized (318). Spray drying has been incorporated in cement
manufacture in two ways. (a) Modifying existing facilities to increase production (semi-wet process). (b) Rationalizing new cement plant designs
(dry process).
In (a) the addition of a spray dryer to an existing kiln increases the kiln
capacity and improves overall heat economy. In (b) the kiln length can be
substantially reduced per given cement clinker output. With a spray dryer
attachment a kiln operates only as a calciner.
In the conventional wet process of cement manufacture, limestone and
clay are homogenized to form a 60% solids aqueous slurry. The slurry
is introduced into a rotary kiln and contacted with hot combustion gases
in a counter-current flow. Clinker is produced from the kiln, after the slurry
has undergone three stages ; (i) moisture evaporation, (ii) vaporization of
some solid components (e.g. alkalis), and (iii) calcining. The clinker passes
to rotary cooling, ball milling and gypsum addition. The kiln exhaust gases
pass to powder—air separation equipment for recovery of fines and alkali
fraction.
A spray dryer can be incorporated with the kiln, so that the kiln receives
powder as well as slurry, or just powder alone. Introduction of powder
increases the heat economy and output per given kiln size.
In the semi-wet process applied to existing plant, figure 14.20(a), exhaust
gases from the kiln are ducted to a spray drying chamber at 575-660°F
(300-350°C), into which cement slurry is sprayed. The slurry is pumped to
a clay/limestone/water homogenizing tank as a 60 % solids content feed
and mixed with powder fines to raise solids content to a maximum 70 %.
The slurry, now a paste, is atomized by a rotary atomizer with a wear
resistant wheel. Nozzles can also be used (50 % solid) with effective feed
filtering although anti-abrasive parts replacement is more frequent. Spray
APPLICATIONS IN THE CHEMICAL INDUSTRY
541
slurry
2
130)150 C
-
300350 . 0
alkali
powder
raw
materials
slurry
cement
gypsum
max 70% TS
a.
b.
raw
mot erlots
Figure 14.20. Flow diagram for manufacture of cement utilizing spray drying. (a) Semi-wet
process. (b) Dry process. 1. Kiln. 2. Spray dryer. 3. Cyclone. 4. Electrostatic precipitator.
5. Homogenizer (mixer). 6. Rotary cooler. 7. Ball mill.
dried powder falls as a bone dry product from the chamber base and is
introduced back into the kiln towards the firing end of the kiln. Exhaust
gases leaving the spray dryer pass to powder separator equipment. Cyclones,
bag filters, scrubbers and electrostatic precipitators can be used. Cyclones
are recommended to remove the coarse fines, with a multi-stage electrostatic
precipitator to cover the finer fines and alkali content. The outlet temperature of gases from the spray dryer is adjusted to give a bone dry product
542
APPLICATIONS OF SPRAY DRYING IN. INDUSTRY
and meet the needs of the fines recovery equipment. Where electrostatic
precipitators are utilized outlet temperature ranges between 260-300°F
(130-150°C). Powder from the cyclone and the first electrostatic precipitator
recovery stage is mixed with cement slurry to form a 70 % maximum pastelike feed. The high alkali content fines are discarded, being used for fertilizer
additives or land fill.
The capacity of the kiln is increased by reducing the evaporative load.
More 'efficient utilization of heat in the gases leaving the kiln leads to improved operating economy.
In the dry process (figure 14.20(b)), only powder is passed to the kiln. This
means that when designing a new plant the kiln associated with the spray
dryer will be only 70 % of the length required for the wet process if the kiln
were to operate under conventional wet process operation. Marked reduction in costs are possible by incorporating a spray dryer. The dry process
differs from the semi-wet process since the kiln acts only as a calciner.
Exhaust gases leaving the kiln are much higher leading to a more economic
spray drying operation. Slurry from the homogenizing tank (60 % solids
( max. 70 %)) is pumped to a rotary atomizer (wear resistant wheel). Bone
dry powder leaves the chamber. Powder recovered from the exhaust air
in the cyclones and electrostatic precipitator is mixed with the chamber
powder (except alkali fraction from electrostatic precipitator) and passed
back to the kiln. Clinker from the kiln is cooled, ground in a ball mill and
gypsum added prior to storage.
14.10.4. Metal Powders
The formation of metal powders by spraying melts which are spray cooled
is a new and interesting development in the field of metallurgy (319) (492).
Powders can be compacted to give metal pieces improved quality. With
steel powders, properties unattainable before are acquired. High speed
tools do not suffer from segregation. They have very fine and uniform
grain size, and high dimensional stability during heat treatment. By powdercompacting techniques, casting operations become obsolete.
The scope of metal powders is immense, especially as hydrometallurgical
methods are being more and more applied for extracting methods from
ores, and the final product is in powder form.
A steelmaking process to include the production of steel powder has been
recently reported (1970) (319). Based in Sweden, the steel is melted in a
high frequency furnace. The molten steel is tapped to a tundish located
at the top of a spray cooling tower (30 ft (9 m) in height). The molten steel
flows to two-fluid nozzles. The steel particles cool on falling through an
argon atmosphere to prevent oxidation. The powder (mean size 500 micron)
freely flows from the tower base to capsules ready for compaction. Compaction is first carried out at 4000 atm at ambient temperature followed by
APPLICATIONS IN THE CHEMICAL INDUSTRY
543
100 atm at 2000°F (1100°C) in argon. The compacted steel billet is ready for
forging or rolling in the conventional way.
14.10.5. Sodium Carbonate
Sodium carbonate can be produced in a spray drying chamber used as a
spray reactor (figure 0.2) (493): Interest has been growing in this process
during recent years since sodium carbonate is being used in increasing
amounts in detergents (replacing phosphates), and caustic soda (the starting
material) is becoming more and more available as a by-product in the
manufacture of chlorine by electrolysis.
Liquid caustic soda (35-50 % concentration) is sprayed into a hot gas
atmosphere (1110°F, 600°C) containing carbon dioxide. The caustic soda
spray reacts with the carbon dioxide to form sodium carbonate.
2NaOH + CO, = Na 2 CO 3 + H 2 O
The CO, rich gas can be low sulphur content combustion gases. To achieve
as complete a conversion as possible the CO, content in the hot gas should
not be less than about 14 % wt. Waste gases from steam boilers where
fuel combustion is stoichiometric are applicable. If waste CO, is available
it can be used following heating to 1110°F (600°C).
Solids recovered from the chamber contain 99 % Na 2 CO 3 or better,
0-1 % NaOH and traces of Na 2 SO 4 . The moisture content is approximately
0.1 %, and bulk density 0.6 &m'.
Standard spray dryer layouts are used with nozzle or rotary atomization.
Vapour and combustion gases are exhausted via cyclones. Atomization
is controlled to produce a granular dried product (100-300 micron).
14.10.6. Sodium Hydroxide
Sodium hydroxide in pellet form is manufactured by the spray cooling
process.
Sodium hydroxide melt at 660°F (350°C) is atomized by low pressure
nozzles to give a coarse atomization (500-1500 micron) in a mixed flow
(fountain layout) or counter-current flow cooling chamber. Ambient air is
introduced into the chamber. The temperature of the exhaust air is of the
order 330-350°F (165-175°C) well above the critical 230°F (110°C) level.
The latent heat of fusion is 126 BTU/lb (70 Kcal/kg). The product is deliquescent and requires special handling. Gravity flow feed systems are
generally adopted.
14.10.7. Titanium Tetrachloride
In the manufacture of titanium tetrachloride, a spray dryer is operated as a
spray purifier. During the f;' - anium ore heat treatment, a gaseous mixture
containing TiC1 4 , CO, CO 2 ' 2 , HCl and inerts is evolved. This mixture
passes to a spray chamber at 1830°F (1000°C). Impure liquid titanium
tetrachloride (impurities of iron, vanadium chlorides, etc. can reach 20 %)
544 APPLICATIONS OF SPRAY DRYING IN INDUSTRY
is sprayed into the hot ingoing gases. Vaneless (plate-type) discs, rotating
at low speeds (VT = 200-300 ft/sec, 60-90 m/sec) are used as atomizers.
Vaned wheels are not suitable due to erosion effects on the vanes due to
i mpure feeds. Nozzles are less suitable as the impurities tend to block the
nozzle internal passages. The atomized sprays are relatively coarse with
vaneless disc atomization, but the large droplets are readily evaporated
as the liquid feed has a low latent heat of vaporization (A = 72 BTU/lb,
40 Kcal/kg). A 500 micron droplet evaporates in less than 1 second. Small
diameter chambers can thus handle high feed rates.
The titanium tetrachloride enriched gaseous mixture leaves the chamber
and passes to condensers for tetrachloride recovery. An amount of solid
material originating from the impurities in the liquid feed is recovered from
the chamber base.
15 Applications in the Food Industry
15.1. Milk Products and Eggs
15.1.1. Milk Products
Raw materials derived from milk are shown in figure 15.1. They are utilized
in the milk powder industry to produce products that can be listed as
follows :
Skim milk
Coffee/tea whitener
Whole milk
Cheese
Whey
Ice cream mix
Fat enriched milk
Butter
Sodium caseinate
Buttermilk
Baby food
Skim milk, whole milk, whey and fat enriched milk are still the most
i mportant from the aspect of quantities spray dried. However, current
trends show increased drying of caseinate, for use in luncheon meat, sausages
and similar products, while also finding a market in special diets for those
having difficulties in acquiring the daily protein requirement through normal
diet. There is also increasing interest in spray dried baby foods and beverage
whiteners. Spray dried cheese is widely used in the powdered soup and
biscuit industries. Cream powder is an important raw material in ice cream
mix, and can be successfully spray dried in spite of powders having a fat
solids content between 48-70 %. Dairy products having fat contents up to
82 % have been spray dried, namely butter. Spray dried buttermilk is used
as an ingredient in animal feedstuffs.
The rapid growth of the milk powder industry during the last decade has
evolved through the diversity of specialized purposes for which such powder
can now be utilized. However, this widening range of applications has also
been accompanied by the increased demand for milk powder of more
546
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE FOOD INDUSTRY
stringent specifications. Recent research papers published from the milk
industry have, therefore, been concerned with the definition of stable powder
forms most suitable for specific production applications. These theoretical
studies have been accompanied by development of processing techniques
for high tonnage production of premium grade powder having special
physical properties.
C
BUTTER
BUTTER MILK
'N.
Churning
t
1
C
SKIM MILK )
Coagulation —+ CHEESE
Precipitation
CASEIN
WHOLE MILK
Processing
Solution
1C-
CASEINATE
R0CESSE ❑ CHEESE
)
Evaporation
Crystallization
Separation
MILK SUGAR
MOTHER LIQUOR
Figure 15.1. Products derived from milk.
Dairies with spray dryer installations handling over a million kilograms
of milk per day are nowadays by no means unique (321). Large capacity
spray dryers are used, and this has led to more economic spray drying of milk.
Use of inlet hot-air temperatures up to 750°F (400°C) is possible with
secondary cool air introduced lower in the chamber. Without cool air,
480°F (250°C) temperature appears maximum, but lower temperatures are
547
usually used to obtain optimum powder quality. Milk is never dried with its
original moisture content, but evaporated to concentrations of 45-52 %
prior to spray drying. Concentration is carried out due to economics, as
i about ten times cheaper in a modern evaporator than in
water evaporation is
a spray dryer. Furthermore a concentrate effects spray dried product quality
in a positive way.
A further area of high capacity drying is whey. Whey powder from fresh
whey is well used in the foodstuff industry, as ingredients in powdered soups,
cake mixes, baby foods. However, it is the processing of raw whey (quarg or
cottage cheese whey) that has increased overall whey drying capacities.
With the vast quantities produced, the pollution problem its discharge creates
is a problem that must be tackled and spray drying is one possible solution.
Apart from products dried in high capacities, there is a range of specialized
products. Diet powder preparations, for example, can contain the daily
human needs of protein, vitamins and calories. Other convenience food
products are ice-cream powder, milk-cocoa powder, cake mixes and even
custard powder.
Spray dried milk products are redissolved or dry mixed when utilized.
The important powder characteristics are flowability, wettability, sinkability,
dispersability and solubility, Skim milk is dusty when in ordinary spray
dried form, but recent developments in skim milk powder agglomeration
have overcome these problems. Whole milk powder, with its high fat content
demands quick solubility and powder flowability, and this has been accomplished by addition of additives, e.g. lecithin. Spray dried whey powder is
very hygroscopic in ordinary form, but modern processing techniques are
now able to produce non-caking, non-hygroscopic varieties.
Drying of milk products has been reviewed recently by Noyes (322),
Minifie (323), Shebler (244), Crossley (324), Hynd (489), Knipshildt (490),
Antonsen (518) and Pisecky (506).
(a) Skim Milk
There are several ways of spray drying skim milk (329). Each is adapted to
meet specified physical characteristics of powder, i.e. varying grades of
agglomerated (instant) powder. Three main methods involve (a) a spray
dryer with pneumatic conveying system (basic system), (b) a spray dryer with
vibrated cooling bed, (c) a spray dryer with vibrated after-dryer and cooler
(straight-through system). Vibrated after-dryers and coolers operating on
the fluid-bed principle are hereafter termed vibro-fluidizers. Common to all
methods is a skim milk concentrate of 45-52 % solids feed to the dryer, use of
indirect gas/oil heaters or steam heaters. Skim milk powder contains approx.
4 % moisture.
(i) Ordinary Skim Milk Powder. Ordinary skim milk powder is produced on
a plant layout shown in figure 15.2(a). This is in fact the basic layout, with a
548
APPLICATIONS IN THE FOOD INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
AIR OUT
ALR OUT
AIR
HEATER
after-cooler
( vibro-fluidizer}
FE D
SYSTEM
b.
ATOHEZER
Alk
FeED
offerdryer
C.
aftercooler
vibro-fluidiier
P R RCt
TC
auU
Figure 15.2. Spray dryer layouts for dairy products. (a) Layout with pneumatic cooling system
(basic). (b) Layout with cooling bed system (aftercooler, vibro-fluidizer). (c) Layout with straightthrough instantizer system (after dryer-cooler, vibro-fluidizer).
pneumatic conveying system. Drying air is introduced at the top of the
chamber around a rotary atomizer. The majority of powder leaves the
conical chamber base, while entrained powder passes to the main cyclones
(or bag filters). The chamber and cyclone powders are mixed and cooled in
a pneumatic conveying system prior to bagging-off Skim milk powder
549
produced in such a plant consists of small single particles and is very dusty.
Dissolving is difficult as the powder lumps.
A typical microphoto of the powder is shown in figure 15.3(a). The high
content of small particles gives the powder its dustiness, poor wettability and
dispersibility, but high bulk density. The properties of ordinary skim milk
powder are given in table 15.1.
Nozzle atomization can be used in the basic layout. The drying chamber
may differ from that shown in figure 15.2(a) as nozzles also fit in vertical
towers and horizontal co-current chambers (box dryers). Pressure nozzles,
similar to those shown in figure 6.33 atomize 40-50 % concentrate at pressures
of 170-200 atm. Usually nozzle powder has a higher bulk density than
powder from rotary atomizers. Ten nozzles are often incorporated to
produce 1500 kg/hr of dry powder. However, such nozzle duplication, likely
abrasion and maintenance of high pressure pumps often make the rotary
atomizer the preferred technique.
Skim milk powder produced from a rotary atomizer (atomizer wheel,
figure 6.24(b)) can have 15-35 % occluded air, while nozzle powder has
7-13 %. Powders have a mean particle size of about 60 micron. Atomizer
wheel powder can closely approach nozzle powder through increase in feed
concentrate, by controlling feed temperature prior to atomization, and
adjustment of atomizer speed to produce a mean particle size in the optimum
range. Surface tension can affect occluded air content by its influence on
particle rupture and ballooning during drying. The protein content and the
state of the protein has marked influence on surface tension.
The physical structure of dried milk is given by King (327) (328).
(iii) Instant Skim Milk Powders. The dissolving of milk powder in water
features a rather complicated mechanism. Each individual particle has
initially to be wetted, then sink into the liquid in order to be finally dissolved.
For a mass of particles there is the problem of dispersing them through the
liquid. For skim milk powder the wettability and rate of solubility of each
individual particle are very high, but when a large number of particles is
present lumping occurs. Each lump is surrounded by a semi-impervious
layer of milk solids, which delays the penetration of water into the dry
powder within the lump. This lump formation is the reason for the poor
dispersibility of an ordinary milk powder.
Powder properties that are of importance for the reconstitution process
can be summarized as follows :
1. Wettability. This is the ability of a milk powder particle to adsorb water
on its surface, i.e. to be wetted. It is through this solids-water contact that the
reconstitution process commences.
2. Sinkability. This is the ability of milk powder to sink down into the water
after it has been wetted.
550
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE FOOD INDUSTRY
(a)
3. Dispersibility. This is the ability of milk powder to be distributed throughout the water. This means the ability of the lumps or agglomerates to fall apart.
4. Solubility. This term is used for, two different properties : (a) The rate of
dissolving. (b) The total solubility (expressed, for example, as the solubility
index).
During the 'instantizing' process the original single particles of the raw
product (usually skim milk powder or as another food powder preparation)
are transformed into porous agglomerates. When these porous agglomerates
come into contact with the Water, water quickly enters the pores to wet the
particles. Subsequently, the particles sink below into the water, disperse, and
finally dissolve.
Agglomeration alters the physical state of the powder to such an extent
that the rate of wetting, sinking, and dispersing is so increased that reconstitution occurs very quickly. The powder is instant, and dissolves readily in
cold water. The alteration of the physical powder conditions from single
particles to agglomerates also means alteration of properties other than
instant properties. For instance, there is a considerable drop in bulk density,
also thermo-stability is effected. Instant milks for use in coffee and tea form
an extensive market for agglomerated products, and high thermo-stability is
often demanded. High thermo-stability prevents coagulation when instant
milk powder is added to hot, slightly acid liquids containing tannin. However,
there are markets that require a low-heat skim milk powder without 'cooked'
flavour for reconstitution purposes.
The basic layout (figure 15.2(a)) can be adapted to produce agglomerated
powder. Agglomerated powder can be produced by incorporating a cooling
bed system at the base of the chamber. Powder of 'superior' instant properties
can be produced by so-called 'straight-through instant process', a special
development of the vibro-fluidizer for after-drying and cooling. The vibrofluidizer is mounted at the base of the chamber.
Powders produced by the cooling bed and straight-through systems
exhibit instant properties. These systems produce powder in a single process
directly from fresh milk and thus depend upon regular milk supplies. For
processing that does not rely on milk supplies and can be operated year
round, stored skim milk powder is instantized by a separate rewetting
process. Rewetting gives a considerably higher degree of agglomeration . and
therefore has even better instant properties than 'straight-through' powder.
Four stages are involved in the rewetting process.
1. Powder is wetted with water, either in the liquid or vapour phase. The
surface of the particles is uniformly wetted.
2. Wetted powder is retained in the chamber for moisture stability. The
residence time is sufficient for the moist clusters to settle out at the base of the
chamber.
.
(c)
(d)
Figure 15.3. Forms of spray dried skim milk powder. (a) Ordinary skim milk powder, layout
figure 15.2(a). (b) Dustless skim milk powder, layout figure 15.2(b). (c) Instant skim milk powder,
layout figure 15.2(c). (d) Prime rewet instant skim milk powder, layout figure 15,4. (By courtesy
of Niro Atomizer.)
551
552
APPLICATIONS IN THE FOOD INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
3. The clusters are dried, reducing the moisture content to the desired level,
and cooled in a vibro-fluidizer.
4. The clusters are screened.
Equipment for rewetting is termed re-wet instantizer. The re-wet
instantizer forms a separate plant layout, and is not an attachment to a spray
dryer. It can operate in conjunction with the spray dryer, up-grading ordinary
spray dried powder. .
Dustless Skim Milk Powder from Spray Dryer with Cooling Bed System. The
layout is given in figure 15.2(b). The drying chamber is as in the basic layout
but here the fines formed during production are returned to the atomization
zone for agglomeration with the evaporating spray. Dried particles pass out
of the chamber into a cooling bed (vibro-fluidizer) for powder cooling,
transport and classification. The air velocity in the cooling bed is high
enough to separate off the fines for return to the atomization zone, but is low
enough to prevent formation of extra fines through agglomerate breakage.
The powder leaving the cooling bed possesses a low dustiness and improved
dispersibility. The cooling bed is used for both fat and non-fat products.
A microphoto of skim milk powder produced on this layout is shown in
figure 15.3(b). The extent of agglomeration can be seen typified by the
presence of particle-adherence of small to large, and very low content of
fines.
Instant Skim Milk Powder from Spray Dryer with Straight-through Instantizer
System. The plant layout is shown in figure 15.2(c). Instant powder is
obtained by agglomeration in two separate stages. The initial agglomeration
of fines is carried out in the atomizing zone, amidst the evaporating spray.
Further agglomeration occurs within the drying chamber. The spray dryer
is operated to maintain a slightly increased powder moisture content at the
dryer outlet. Under these conditions the powder possesses stickiness
properties which readily promote agglomeration through self-adhesion of
particles. The moist warm powder passes out of the chamber base into the
vibro-fluidizer for completion of drying and subsequent cooling. A microphoto of skim milk powder produced on this layout is shown in figure 15.3(c).
The granular characteristics of the powder are evident. It has a coarser
particle size (mean 300-400 micron) than powder from the cooling bed
system. It, therefore, possesses a higher degree of wettability and dispersibility. The properties of powders produced on a dryer with pneumatic
conveying system (ordinary) and straight-through system (instant) are
summarized in table 15.1.
Prime instant Skim Milk from Rewetting Skim Milk Powder. An improved
instant product results from a further processing of powder produced on a
spray dryer with pneumatic conveying system. The skim milk powder is
rewetted in a vertical agglomerating tube, then dryed and cooled.
553
Table 15.1. Properties of Ordinary and Instant Milk Powders
(Typical Figures)
Whole milk
Ordinary
Moisture content
2.3
Bulk density
0.62
Solubility index
0.1
Wettability
more than 10 hr
Dispersibility*
Skim milk
Instant
Ordinary
Instant
Rewet
2.6
0.59
0.1
40 min
46.3 g
3•8
0.64
0.1
More than I hr
4.3
0.55
0.1
10 sec
48.4g
4.0-4.5
0.28
0•I
3-6 sec
>49g
* Standards for Grades of Dry Milks (including Methods of Analysis, Bulletin 916), A.D.M.I.,
Chicago, 1965.
t Mohr, W., Milchwissenschaft, 15, 215 (1960).
The vertical agglomerating tube layout incorporates a surface agglomeration procedure. Surface agglomeration is accomplished by treating powder
with steam or warm moist air. The surface of the individual dry particles will
be wetted by condensation, thus creating the required stickiness. If contact
between powder and air takes place, for instance in a vortex, agglomerates
are formed. This procedure is the most suitable for skim milk, as pure milk
solids require considerable mechanical impact if large porous agglomerates
are demanded.
The equipment is shown in figure 15.4. The powder is fed from a silo
into a vertical agglomeration tube into which warm humid air is introduced
simultaneously. The surface of each particle becomes moist and sticky.
The particles form agglomerates of required size. The agglomerates pass
into a small chamber and are contacted with drying air. The surface of the
agglomerates are cooled in a vibro-fluidizer mounted at the chamber base.
The dried agglomerates pass to a sieving unit from which the product is
discharged with the desired particle size limits. Fines collected from the
sieve are returned to the agglomerating process together with fines from the
drying chamber and vibro-fluidizer.
A microphoto of skim milk powder produced on a rewet instantizer is
shown in figure 15.3(d). The product consists of big porous agglomerates.
It is this structure which gives the product excellent instant properties.
The product is also non-dusty, due to absence of fines, which gives the powder
the additional advantage of having a low bulk density ( 0.3 g/cm 3 ). (For
other properties, see table 15.1.)
(b) Whole Milk
Whole milk can be spray dried in dryers used for producing ordinary skim
milk powder. A layout as shown in figure 15.2(a) is applicable or preferably
the cooling bed system figure 15.2(b). The powder gains in flowability due to
554
A
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
PPLICATIONS IN THE FOOD INDUSTRY 5 55
atomizer{ for citernistive
arrangement}
Gif L0
EPod INC
.i .. E
AIR HEATER
PAOOLMT
out
COOLING MA
Figure 154 Rewet instantizer (shown with agglomerating tube mounted),
a low content of free fat resulting from the more lenient mechanical handling
in the vibro-fluidizer than in the pneumatic conveying system. The fat
content renders the whole milk powder particles sticky and therefore electric
hammers operate at the chamber wall to prevent deposit formation. Rotary
or nozzle atomization is used. Atomizer wheels with curved vanes (figure
6.24(b)) rotate at peripheral speeds between 400-500 ft/sec (120-150 m/sec).
Centrifugal pressure nozzles (figure 6.33) operate in the range 100-140 atm.
Whole milk concentrate of 40-50 % solids is usually dried with inlet hot air
temperatures, 340-390°F (170-200°C). The final powder contains 2.5 %
moisture.
Whole milk powder can be instantized by the straight-through process
(figure 15.2(c)) followed by lecithin addition (506). Agglomeration of whole
milk powder improves its dispersibility but not its wettability. This is caused
by the content of free fat in the powder which, even if in minute quantities is
sufficient on migration to the surface of each particle, to render the particle
water repellent (hydrophobic). The migration of fat can be prevented by
physical means, i.e. the addition of small quantities of surface active agents,
e.g. lecithin. Lecithin improves the wettability making the powder instant
in cold water. The use of lecithin to improve on instant properties has been
known for some years, but in most countries the use of additives to milk
powders is not permitted. However, the International Dairy Federation has
Figure 15.5. Spray dryer with vitro-fluidizer for instant skim milk, whole milk, milk replacer,
ice-cream mix, cheese, cream powders. (By courtesy of Niro Atomizer.)
556
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
recently dealt with this problem and recommends the addition of lecithin to
fat-containing milk powders (515).
(c) Whey
Whey has been known for decades as a product of high nutritive value, but
until recently it has been considered a troublesome by-product, the majority
of which has been discharged as effluent to river courses. The application of
spray drying to whey and the development of markets using whey powder
as ingredients in foodstuffs for human and animal consumption has transformed the dairy and cheese-making industries.
There are two main groups of whey :
Sweet Whey. This is also termed cheese whey and is produced during cheese
making, when rennet is used. Sweet whey forms a very large family of
products. Their compositions may vary only slightly but their properties are
very different. The pH-value of sweet whey can range between 5.2-6.7.
Sour Whey. This can be acid whey, quarg or cottage cheese whey and sour
sweet whey. Acid whey, also known as casein whey, orginates from the
manufacture of casein by means of lactic acid and hydrochloric acid. The
origin of quarg or cottage cheese whey is self-explanatory. Lactic acid
created through natural fermentation gives the whey a high acidity. The pH
value of these types of whey is normally about 4.6. If insufficient care is
given to the cheese whey, it becomes more sour by continued natural
fermentation. Such a process is of course undesirable so that soured (not
sour) whey cannot be considered a natural product.
All types of whey can be spray dried. Each, however, requires its own
handling techniques. Layouts for spray dried whey can vary from the very
simple to the sophisticated. Generally speaking, sweet (cheese) whey is
easier to dry than acid whey. The main operations used for the manufacture of
whey powder are as follows : preheating—concentration—precrystallization—spray drying—after-crystallization—after-drying in a vibro-fluidizer
(accompanied by a final crystallization) cooling in a pneumatic cooling
system—cooling in a vibro-fluidizer (512).
These operations are combined in various ways to handle the different
types of whey. In all there are five important processing layouts involving
spray drying. Four involve crystallization.
(1) Spray Drying without Crystallization Treatment (Ordinary Whey Powder).
Processing consists of a preheating, concentration, spray drying and
pneumatic cooling (figure 15.6(a)). The spray dryer features a standard
pneumatic conveying system similar to that shown in figure 15.2(a) for skim
and whole milk powders. Nozzle atomization is vertical or horizontal (box)
drying chambers can also be used. Pressure nozzles operate around 200
atm.
APPLICATIONS IN THE FOOD INDUSTRY
557
Process Diagram for Ordinary Whey Powder
FRESH WHEY 6% T.S.
(a)
Pre-heating
Concentration
50 % T.S.
Spray-drying
96 % T.S.
Cooling of powder
Bagging-off
ORDINARY
WHEY POWDER
Process Diagram for Non-caking Whey Powder
6 % T.S.
FRESH WHEY
(b)
Pre-heating
Concentration
50 % T.S.
Precrystallization
Spray-drying
86% T.S.
After-crystallization
Fluid-bed drying
96% T.S.
Cooling of powd6r
Bagging-off
NON-CAKING
WHEY POWDER
Figure 15.6. Processing stages in the spray drying of whey.
Ordinary whey powder obtained by this process is very fine, dusty, hygroscopic and therefore caking. Hygroscopicity and caking are influenced by
the type of whey and by local climatic conditions. The hygroscopicity,
caking and all the problems associated with the stickiness of ordinary whey
powder is mainly due to lactose being present in an amorphous glassy state.
In the spray drying of milk products, lactose is in an amorphous state and
is not stable in atmospheric air of normal humidity. The only form of lactose
that is stable to humidity is a-lactose monohydrate. Since the lactose content
of whey powder contains more than 70 % of the total solids in comparison
with 30 % in whole milk the problem of the lactose content in whey powder
is more severe. However, since the solubility of lactose is 17 g/100 cm' H 2 O
at 20°C, it is easy to guide the drying process in such a way that a great part of
558
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
the lactose can be transformed to the stable ce-lactose monohydrate form
during the drying process.
(ii) Spray Drying including Crystalline Treatment. Spray drying with precrystallization. The basic process layout (figure 15.6(a)) is modified by
conducting a precrystallization before spray drying.
During the precrystallization process, it is easy to keep ideal conditions
for crystallization. Viscosity of the concentrate is reasonably low, temperatures may be exactly adjusted and controlled, displacing of used solution
from the surface of crystals may be accelerated by agitation, and the required
amount of suitable crystals of lactose may be ensured by proper seeding.
Furthermore, there is normally time enough for the precrystallization
process so that it is no problem to reach the theoretical degree of crystallization at this stage. Crystallization of all lactose cannot be achieved in one
stage and the product obtained shows only a slight improvement in the
undesirable properties of hygroscopicity, caking tendencies and dustiness.
Spray drying with cooling bed system. The process stages consist of preheating,
concentration, precrystallization, spray drying, and cooling in a vibrofluidizer. When compared with the product described above the product
from the cooling bed system is again improved. The process has the advantage
of recycling fines and the resulting agglomeration reduces the dustiness of
the final powder. The spray dryer layout resembles figures 15.2(b) and 15.5.
Spray dryer with after drying cooling system. This process is more sophisticated, consisting of preheating, concentration, precrystallization, spray
drying, after-drying and cooling in a vibro-fluidizer. The product made by
this process is non-caking and being agglomerates is dustless and free flowing.
The agglomerates tenti to be small and thus the bulk density is relatively high.
The spray dryer layout resembles figure 15.2(c). Whey concentrate is of the
order 50 % T.S.
The low temperature process (non caking whey powder). This process consists
of all operations as in the process above, plus further operations of aftercrystallization that takes place following spray drying (figure 15.6(b)). The
drying process is guided in such a way that in addition to the 70-75 % of the
lactose which can be crystallized in the whey concentrate prior to spray
drying, the remaining lactose can also be converted to the stable crystalline
form. This process has the advantage of being suitable for both sweet cheese
and acid wheys.
The spray dryer is operated to give a 12-14 % moist powder from the
chamber. This is achieved by using a low outlet drying temperature. Within
the moist powder the lactose is still in a state of solution and thus can migrate
to the surface of the lactose crystal and crystallize there. The crystalline time
needed is achieved by holding the moist powder on a conveyor belt (figure
15.7) mounted at the base of the drying chamber. Powder falls from the
APPLICATIONS IN THE FOOD INDUSTRY
559
-
-
Figure 15.7. Spray dryer producing non-caking whey powder (after-crystallization carried out
on conveyor band), (By courtesy of Niro Atomizer.)
560
APPLICATIONS IN THE FOOD INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
belt into a vibro-fluidizer (after drying-cooling) for completion of crystallization and drying.
Non-caking whey powder has the following properties. It is non-caking
and since the hygroscopicity is a minimum the powder is completely stable
in storage. (A non-hygroscopic product does not exist since even if all the
lactose is crystallized the salts and proteins possess some hygroscopicity.)
The powder leaves the chamber as large agglomerates and it is possible to
keep this structure when carrying out the drying process in the mechanically
lenient way of the vibro-fluidizer. The product leaving the vibro-fluidizer
is both dustless and instant. The large agglomerates give a low bulk
density.
Choice of whey process. The choice of process depends upon the type of
whey available, intended market for the whey powder and plant location.
The ability to handle sweet whey and yet produce a non-caking product
enables use of a more simpler layout than if sour whey must be processed.
If the dried powder is intended for dry powder mixes, fiowability is essential
and the process must be equipped with crystallization and cooling stages to
ensure this property is obtained. Local climate should also be considered.
If the climate is too humid, it is advisable to use a process that produces a
less hygroscopic product having low caking tendencies
(d) Fat Enriched Milk (Milk Replacer)
Fat enriched milk is a replacement for whole milk in which the butter fat
has been replaced by a cheaper animal or vegetable fat.
In recent years there has been increasing interest in utilizing skim milk
powder enriched with animal and/or vegetable fats for feeding of calves,
pigs, lambs and other domestic animals. •For calves it has been used for
both breeding and fattening. A special powder composition is used in each
case. A number of products have been manufactured. Feeding trials with
these products have shown that they can be used successfully and economically as whole milk substitutes, particularly in the case of rearing veal
calves.
The first products of this type were manufactured by simple mixing of the
ingredients. This method is still used, but gives numerous problems as the
entire fat content is in the state of free fat (directly extractable by organic
solvent), which adversely effects both the digestibility and shelf life of the
product. The process usually applied today is an emulsifying of the fat in
the skim milk. In this way, the fat is protected by the milk solids and does
not show the disadvantages mentioned above. In general the melted fat is
mixed with preconcentrated skim milk. The mixture is homogenized prior
to spray drying.
Fat enriched skim milk feed for calves usually contains 15-20 % fat. The
fat content of the reconstituted milk will thus be 2-3 %. The composition of
.
561
the marketed fodder varies a great deal according to the purpose of animal
feeding. A calf fodder, for example, is
15-20
1. Fat
50-80
2. Skim milk solid (milk solids non-fat, MSNF)
0-30
3. Dextrose, lactose, possibly whey solids
4. Emulsifying agents (lecithin, mono-glycerides, succro0-2
glycerides, etc.)
0-7
5. Flour
6. Minerals (chlorides, phosphates, bicarbonates of Ca, Na, Mg, Cu) 0-0.5 %
0-0.5
7. Vitamins A, D, E (B, C)
trace
8. Antibiotics (terramycin, aureomycin)
Powder Production by Spray Drying an Emulsion of Skim Milk Concentrate
and Fat. The method produces the so-called premix. A premix contains
up to 60 % fat in the total solids content. This is much higher than is desired
in the finished fodder mixture. Normally a lower fat content mix is produced
by dry mixing to meet the strict quality specifications. A higher fat content
cannot meet such standards.
Pasteurized skim milk with a titratable acidity below 0.19 % lactic acid is
concentrated to 42-45 %. The exact concentrate solids content depends
upon the amount of fat to be added later in the process. Skim milk leaves
the evaporator at about 110°F (45°C) and is preheated to 150-160°F (6570°C). Fat is then added. Fat must be of first class quality with free fatty
acid content less than 0.4 % (as oleic acid) and a very low content of impurities
and water.
Prior to use the fat is stored at temperatures not exceeding 120°F (50°C).
Antioxidants are generally added (13utylhydroxytoluene (BHT), Nardihydroguajaretic acid (NDGA)). If emulsifiers are required, they are added_
at this stage. Lecithin and monoglycerides are usually used. Fat is then
heated to the same temperature as the milk concentrate. Appropriate
amounts of skim milk concentrate and fat are mixed while undergoing
stirring. The ratio between fat and skim milk concentrate is selected so that
the fat content in the powder obtained from the spray drying plant lies
between 40 and 50 %. The mixing of fat and skim milk concentrate can be
carried out either batchwise or continuously. The mixture is homogenized
prior to spray drying. The purpose of homogenizing is to divide the fat
globules and make them more easily digestible for the animals. The aim is a
maximum size of fat globule (3 micron) and a minimum of 90 % less than
1 micron. Homogenizing causes an increase in viscosity and care must be
taken to prevent formation of too viscous a feed which will later cause
atomization and poor drying conditions. Emulsifiers can reduce viscosity
increase, but excess use can cause deposit formation in the spray drying
chamber.
562
APPLICATIONS IN THE FOOD INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
Batch mixing is performed volumetrically but continuous mixing is
mostly adopted nowadays, Milk concentrate and melted fat are delivered
from the evaporator and melted fat tank storage. The milk and fat are best
supplied by a triple piston pump. One piston supplies milk, the second fat,
and the third emulsifier (if required). The proportioning between fat and
skim milk concentrate can be adjusted by changing the stroke of each piston.
The milk and fat is pumped into a combinator for product heating and
mixing, and then to the homogenizer, which also acts as the feed pump to
the spray dryer.
A co-current flow spray dryer with a conical chamber base is used.
Chambers having flat or slightly conical bases are not suitable as the product
is warm and sticky. Use of scrapers create lumps and free fat formation.
Inlet drying air temperature is usually in the range 320-390°F (160-200°C).
A cooling vibro-fluidizer is mounted at the base of the drying chamber.
Ambient or refrigerated air is used. Powder from the cyclones is transported
by a vibrated conveyor to the vibro-fluidizer. The cooled powder is sieved
before packing. Before marketing, the spray dried powder (i.e. premix) is
blended with other powders and additives, e.g. skim milk powder, whey
powder, vitamins, antibiotics (antidiarrhoeatics). The blend depends upon
the purpose to which it is intended, i.e. feeding of calves, pigs, or lambs.
The content of fat in the final product is normally between 15 and 28 Z.
In many countries, the authorities request the addition of a tracer in order
to provide against abuse, and to prevent the product from being introduced
onto the market as ordinary whole milk powder.
(e) Sodium Caseinate
The preparation of sodium caseinate feed for spray drying can be from
either fresh wet casein curd or dried casein (491). The liquifaction of casein
in alkali is performed by either batchwise or (more recently) continuous
methods. In the batchwise method, casein, water and alkali are fed to a
heated vessel equipped with an agitator. The contents of the vessel is pumped
through a colloid mill and back to the vessel. The process usually takes
1-4 hr per batch. In the continuous method, wet curd (produced
continuously) is the raw material. The curd is mixed with water to give the
proper solid content, then passed through a colloid mill and mixer. The
mixer is also supplied by sodium hydroxide and steam (figure 15.8). On
leaving the mixer, the mixture composition is controlled continuously by
pH, temperature and viscosity measuring equipment. The sodium caseinate
solution has a high viscosity and this usually limits solids content in the
feed to the spray dryer to 22-25 Z. This low solids content leads to a low
bulk density product (^ 0.3 g/cm 3 ) with a high occluded air content. To
increase the bulk density and reduce occluded air, the spray dryer is operated
at the highest feed solids the rotary atomizer can handle, and powder leaves
563
WATER
SODI UM
C AS El NA T E
POWDER
1. Feed pump
2. Mixing aggregate
3. Mill
4. Mixer
5. Dosification pump
6. PH meter controller
7. Temperature controller
8. Viscosity meter controller
9. Intermediate tank
10. Feed pump
11. Spray dryer
12. Flash dryer
13. Pneumatic cooling section
Figure 15.8. Flow diagram for the production of spray dried sodium caseinate.
the chamber moist. An after-drying is required, and this is accomplished in a
flash dryer. Heated filtered air enters the horizontal vortex section
tangentially, picks up the moist powder and passes into the vertical section.
Moisture removal is completed and the powder is recovered in cyclones for
normal pneumatic conveying to bagging-off. Pneumatic conveying cools
the powder. Powder recovered from the exhaust spray dryer air is returned
to the central part of the flash dryer. Sodium caseinate produced in this
manner has a powder density of 0.45-0.50 g/cm 3 . Uses of sodium caseinate
include luncheon meats and sausages, coffee/tea whitener, diet foods and
other special foodstuffs.
564
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
(f) Baby Foods
Dried milk based baby foods, that closely resemble breast milk when
reconstituted are sold extensively nowadays. The dried powder is prepared
for packing by spray drying. Preconcentrated skim milk is mixed with
mineral and vitamin. additives and certain animal or vegetable fats in strictly
controlled proportions. Carbohydrates and whey proteins are also used.
The fat chosen is more easily dealt with by the limited digestive power of a
new born baby or young infant than the fats present in normal cows milk.
The mixture is pasteurized carefully, and spray dried. The powder is often
packaged in tins under ultra-violet light to maintain a low bacteria count.
Long shelf life is obtained by packing in an inert atmosphere of nitrogen.
Baby foods can be instantized using the straight-through or rewet systems. The rewet system differs slightly from the layout shown in figure 15.4.
A rotary atomizer replaces the agglomerating tube. The powder is sprayed
with a fine mist of water or solution of required additive. Powder is
moistened to 8-10 % prior to entering the vibro-fluidizer. This form of
wetting gives droplet agglomeration of powder. The agglomeration process
is not so forceful as surface agglomeration, and smaller agglomerates are
formed.
(g) CoffeelTea Whiteners
Whiteners for addition to coffee and tea are widely used as a cream substitute
of excellent keeping quality. Based upon sodium caseinate, corn syrup,
vegetable fat, emulsifying agent, potassium phosphate and sodium silicoaluminate, the feed can be handled using straightforward spray drying
procedures. Whiteners can be instantized successfully using a straightthrough system or by rewetting (involving droplet agglomeration).
(h) Cheese
Melted processed cheese is normally used in spray drying. Formally only
cheeses with imperfect structure were used but nowadays demand for cheese
powder is such that cheeses are produced solely for spray drying. There are
no particular difficulties in spray drying cheese. The cheeses are crushed
and mixed with water to form a smooth cream. This is atomized (atomizer
wheel) in a dryer with a conical chamber base. There are limited chamber
wall deposits due to the high fat content in the cheese. Inlet air temperatures
are of the order 350°F (175°C). Low outlet temperatures are used for
maximum retention of flavour. Cheese powders cannot be handled successfully in a pneumatic conveying system, and a cooling bed is preferred at the
chamber base. For dryers in residential areas, deodorizing equipment
(active carbon) is required to treat the 'exhaust air. Alternatively a semiclosed cycle system can be adopted where part of the exhaust air is used as
combustion air in an indirect air heater. Passage through the heater destroys
the odour in the exhaust air.
APPLICATIONS IN THE FOOD INDUSTRY
565
(i) Ice-cream Mix
Ice-cream is essentially a frozen aqueous emulsion of fat, dairy and non-dairy
solids, sugar, flavourings, stabilizers and colours. Dairy ice-cream mixes
include butter fat, skim milk solids, sucrose or corn syrup solids as main
ingredients. Vegetable fat replaces butter fat, and sodium caseinate replaces
skim milk solids in non-dairy ice-cream. Moisture contents are 1-2 %.
Ice-cream mixes are difficult to spray dry due to high contents of sugar
(up to 30 %). It is usual to dry the mix with only part of the sugar in the
liquid feed. The remaining sugar is added after drying by dry mixing.
Special equipment is available to handle the total sugar content. The
remaining sugar is added to the dryer via a fines-return system. The straightthrough instantizer process is then used. The ice-cream mix is obtained as a
dustless, non-caking powder.
(j) High Fat Powder for Bakery Use (also Butter)
The drying of products containing 60-80 % fat requires overcoming the
problem of easy rupture of the fat protective membrane (usually protein),
that leads to fat release. This rupture is prevented by minimizing the
mechanical handling of the powder, especially while still warm, or increasing
the melting point of the fat. Conditions of lenient handling are obtained
by incorporating a cooling bed system at the base of the chamber.
Alternatively the air in the base of the drying chamber is cooled by secondary
air to prevent melting of chamber powder, and solidifying particle surfaces
prior to mechanical handling. Where cyclones are used as a powder collector,
cool air is often introduced prior to the cyclones to prevent product melting.
The cyclone powder has a quality inferior to that of the chamber powder.
Increasing the melting point of many vegetable and animal fats is practiced,
but for a product such as butter this is not practicable due to loss of flavour
and the strict specifications of what butter may contain.
Studies on spray drying of high fat food products has been reported by
Amundson (330).
(k) Buttermilk
Spray drying sweet and acid buttermilk overcomes the problem of disposing
any excess that is not consumed in liquid form. Buttermilk cannot be
discharged due to effluent problems, or be mixed with skim milk due to
coagulation. Spray dried powder can be used as an ingredient in animal feeds.
Sweet buttermilk presents no difficulties in spray drying. Dryers for skim
milk are used. Buttermilk is concentrated to 45-50 % and dried in cocurrent flow dryers with pneumatic powder conveying. The moisture content
of the powder is around 4 %.
Acid buttermilk presents no difficulties in spray drying but preconcentration
requires special attention. The buttermilk acidity (due to addition of lactic
acid bacteria starter during butter-making) is in the range 4.5 and coagulation
566
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
APPLICATIONS IN THE FOOD INDUSTRY
of casein occurs. The buttermilk viscosity is high and there is danger of
scorching on the heat surfaces of the evaporator. Buttermilk and concentrate
temperatures are kept low (134.5-136°F, 57-58°C). There must be no high
temperatures in the first effect of the evaporator, or pasteurization. The
circulation in the first and second effects is carried out as lower temperatures
result in low capacities. Ordinary evaporators concentrate to 25 %. Special
types with a de-gassing stage concentrate to 36 %. The concentrate is highly
viscous and is pumped (110°F, 43°C) to the spray dryer. (Skim milk dryer
designs are applicable.) Inlet drying temperatures are in the range 350-375°F
(175-190°C). Light deposits occur in the chamber, but use of low outlet
temperatures prevents discolouration. The powder on packing is cream
coloured and consists of small particles. The moisture content is held
around 4 %. The tapped bulk density is 0.77-0.83 g/cm. 3
15.1.2. Eggs
Whole egg, egg yolk and white (albumen) are spray dried commercially (513).
Although egg white and egg yolk powders have very different characteristics,
spray dryers of similar designs can handle both types successfully. However,
for cleaning purposes, it is recommended to have a dryer for each product
group, e.g. one for whites and one for whole eggs/yolks.
The flow diagram for processing eggs is shown in figure 15.9. Eggs are
supplied from cold storage (40-60°F, 5-15°C) unpacked and weighed.
F RH ENTATIDN TANKS
DUALITY CONTROL
FRESH
EGGS
W OLE EGG
CANDLING
STABILIZING
BREAKING
STORAGE
HOMOGENIZING
SPOILED AND
. BROKEN EGGS
COOLING
or
HE AT IN G
SPRAY DRYING
HOLDING TANK
HOLDING TANK
FE ED
TANK
PASTEURIZER
EGG POWDER
Figure 15.9. Piocessing diagram for production of egg powders.
567
Rotten eggs are removed by candling. Dirty eggs are removed to a washing
machine equipped with detergent sprays, then fan dried and returned to the
candling machine. The eggs are broken. This is often done by hand, as the
operation is easy. Presence of an operator allows a further chance to
remove any rotten eggs that failed to be detected during candling. If the egg
whites and yolks are to be separated, a cup and ring device facilitates
separation. A skilled operator can break 500-800 eggs per hour. In spite of
this high rate fully automatic breaking and separating machines have been
developed and are established in the industry. Up to 18 000 eggs per hour
can be handled per machine when separating whites from yolks or when
breaking without separation. All egg shells contain small amounts of whites,
and this is recovered by centrifuging the shells. For processing egg whites,
efficient separation of whites from yolks is of great importance as no yolk
must contaminate the whites. Even a small amount of yolk oil in albumen
greatly reduces the whipping quality. Nevertheless modern machines are
able to achieve limits of not more than 0.02 % yolk oil in egg white, and this is
acceptable.
In the production of whole egg/egg yolk powder, whole eggs/egg yolk
are homogenized, filtered to remove chalazae, membranes and any shell,
and pasteurized. Pasteurization (normally 147-151°F, 64-66°C, holding
time 2-4 min) ensures the desired microbiological requirements that are
usually a total count less than 100 000 col/g absence of B. Coli and
Salmonella.
For whole egg especially, pasteurization conditions are precisely chosen
and maintained to obtain desired powder characteristics without affecting
baking performance. Whole eggs (solids content 25-27 %) or egg yolk
(solids content 45-48 %) are fed to the spray dryer. Co-current flow dryers
with rotary or nozzle atomization are used. The drying air inlet temperatures
range between 290-390°F (145-200°C). Powder moisture content is 2-4 %.
The powders are conveyed pneumatically in ambient air to the packing area.
Powder airborne in the exhaust drying air is recovered in cyclones.
For improving the shelf-life of egg powders, a fermentation process can
be used to remove glucose. Thus discolouration and development of
disagreeable smell during powder storage is avoided by preventing the
so-called `Maillard' reaction,. which proceeds between glucose and proteinamino groups. Nevertheless, fermentation is not very common in the
production of whole egg and egg yolk powders. On the other hand, it is
currently used for egg albumen.
Egg albumen is fermented by means of bacteria, yeast or enzyme (glucoseoxidase) to reduce the content of glucose below 0.1 %. The performance and
conditions of fermentation vary considerably from factory to factory.
This also applies to conditions of pasteurization. Normally stabilizing salts
568
APPLICATIONS IN THE FOOD INDUSTRY
APPLICATIONS OF SPRAY DRYING IN INDUSTRY
are added to prevent heat damage, and temperatures around 133-138°F
(56-59°C) are used for several minutes. Prior to pasteurization, egg albumen
is filtered to remove chalazae, and foam created during fermentation. A 1012 % solids feed is spray dried in co-current flow dryers with either rotary
or nozzle atomization. Drying air inlet temperatures range between 290390°F (145-200°C). Dried egg albumen contains normally 7-9 % moisture.
Pneumatic powder conveying is adopted. Powder airborne in the exhaust
drying air is recovered in a battery of high efficient cyclones or bag filter.
Egg powders are attractive in appearance and fully soluble. They are
packed in plastic bags, plastic lined boxes, tins or cartons. Whole egg/egg
yolk powder is much denser than egg white powder. The lightness of egg
white powder is due to the low solids content in the feed. Denser powder
can be produced if the solids content is increased. Vacuum evaporation can
be adopted, but the most promising technique for future egg white drying
is the use of reverse osmosis where feeds up to 25 % solids appear attainable.
15.2. Beverages, Flavouring Compounds, Meats, Protein from
Vegetable Sources
15.2.1. Beverages
(a) Instant Coffee
The history of instant coffee powder dates back many decades and the product
has been commercially produced during the past fifty years. However,
intensive development did not start until some ten years ago, but during this
fairly short period, the quality of spray dried coffee has been improved and
the product enjoys an extensive consumer market.
In the production of instant coffee, process stages of roasting, granulation
and extraction proceed spray drying. A typical flow diagram is shown in
figure 5.10. The raw coffee on delivery is used immediately or stored in bags.
An additional method is to transport the green coffee to.collecting bins and
silos. Raw coffee for use in the production of instant coffee is first cleaned
to remove wood, leaves and other foreign matter. Cleaning is carried out in a
cyclone system and a set of vibrating sieves. Following cleaning the raw
coffee passes to silo storage. As various types of coffee are used in manufacture,
there is a silo for each type. The basic material for instant coffee production
is a green coffee mixture. Blending equipment ensures uniform mixture of
the desired blend. The following roasting stage is vital to taste, colour and
overall quality of the product. Both continuous and batchwise roasting
techniques are fully developed with varying degrees of automation. Full
automatic control of mixing and roasting is the usual requirement.
Combustion air from gas or fuel burning heaters is used for roasting. Loss in
coffee bean weight during roasting is around 14-20 % (dry matter basis).
569
GREEN COFFEE
BEANS
NOZZLE
ATOMIZER
ASSEMBLY
EXHAUST
AIR
• FINES
TO ATOMIZATION
ZONE
POWDER TO
AIR-CONDITIONED
FACI•OND AREA
1. Pneumatic conveyor
2. Cleaner
3. Pneumatic conveyor
4. Silos
5. Blending scale
6. Roaster
7. Cooling screens
8. Stoner
9. Scale
10. Bucket conveyor
11. Silo for roasted beans
12. Magnetic separator
13. Granulator
14. Weighing unit
15. Crane beam
16. Feed hopper
SPENT GROUNDS
17. Semi-continuous (batch) extraction battery
18. Continuous extractor
19. Water treatment plant
20. Clarifier
21. Cooler
22. Scale tank
23. Storage tank/feed tank
24. Bulk density regulator
25. High pressure unit for feeding extract to
dryer
26. Co-current flow spray dryer (nozzle tower)
27. Cyclones
28. Vibrating cooler
29. Scale
30. Hoppers
Figure 15.10. Flow diagram for production on instant coffee powder by spray drying.
The roasted product is transported to a silo and afterwards to a granulator.
The purpose of grinding is chiefly to obtain a particle size suitable for the
subsequent extraction. This particle size may vary for the different types of
extraction equipment. Principally, the larger the surface of a given amount of
coffee, the faster and/or more thorough will be the extraction. However,
this indicates a very small particle size to be desirable, yet a too finely ground
570
APPLICATIONS OF SPRAY DRYING. IN INDUSTRY
coffee will pack too tightly and cause mechanical difficulties during extractor
operation.
Grinding is controlled to produce granulized roasted coffee particles of
size between 1000-2000 micron with a very small percentage of 'fines'.
The extraction of roasted and ground coffee takes place in three steps;
wetting, solubles extraction, and solubles formation by hydrolysis. The
particles are first wetted and selectively absorb an amount of water from the
extract equal to about double their dry weight. This causes a raise in extract
concentration and gases are evolved from the particles. The soluble matter
in the particles dissolves and as the concentration of solubles in the surrounding extract is lower than the concentration in the solution within the
particle, the solubles diffuse towards the surface. In order to secure the
highest efficiency, the mean concentration differences must be kept as high
as possible. Hence counter-current extraction principles are used. Any
back-mixing in either extract or grounds streams decreases the efficiency.
Fluctuations in the speed of the stream
