Using “Super Cesium” Catalyst to Increase - AIChE

Using “Super Cesium” Catalyst
to Increase
Sulfuric Acid Production
David D. Clark P.E., Atis Vavere, Ph.D. and
John R. Horne
MECS, Inc.
St. Louis, MO
Presented at AIChE Convention
Clearwater, Florida
June 2007
Abstract
Super cesium sulfuric acid catalyst has been used to increase sulfuric acid production at the
same or lower sulfur dioxide emissions. Super cesium catalyst is much more active than
conventional potassium-promoted vanadium catalyst and can operate at temperatures as
low as 735oF. By taking advantage of the low temperature properties of this catalyst in the
final converter pass, the converter gas strength and/or air flow rate can be increased to
make more sulfuric acid production. This paper will describe applications of this catalyst in
several sulfuric acid plants.
Introduction
A current focus within the sulfuric acid industry is to reduce sulfur dioxide emissions to the
atmosphere while maintaining or increasing acid production. This trend is supported by an
industry-wide consolidation of production capabilities and increased emphasis on improving
air quality. The use of conventional potassium-promoted sulfuric acid catalyst has nearly
reached the practical limit regarding minimization of emissions with increasing acid
production rates. The development of an exceptional low temperature catalyst promoted by
cesium (Cs) compounds has expanded the useful operation range of sulfuric acid plants,
allowing for increased acid production rates (with higher gas strengths and greater
volumetric gas flow) while maintaining or reducing SO2 emissions. The MECS “Super
Cesium” catalyst (SCX-2000) has revolutionized this growing industrial effort and helps to
generate improved plant operations in an economical fashion. This paper presents the
capabilities of this novel catalyst and discusses several successful applications of this
technology.
Catalyst Development and Applications
In the contact sulfuric acid process, there is often an interest in reducing the inlet
temperatures to various adiabatic catalyst beds in order to provide more favorable
equilibrium conditions. The addition of cesium (Cs) salts to the conventional alkalivanadium sulfuric acid catalyst formulations has long been known to enhance the low
temperature properties of the catalyst (1). The cesium promoter stabilizes the vanadium +5
oxidation state (V5+) at temperatures below 790oF (420oC) and keeps the active vanadium
species solubilized in the molten salt. In the conventional K-V sulfuric acid catalyst, the
various vanadium compounds begin precipitating from the molten salt at low bed
temperatures, leading to catalyst deactivation (2, 3).
At relatively high operating
temperatures (> 806oF/430oC), the reaction rate is approximately the same for both the
conventional material and the cesium-promoted catalyst. However, if the temperature is
below 780oF (415oC), the conventional catalyst begins to deactivate due to the precipitation
while the stabilized cesium catalyst continues to perform well. Therefore, the useable
temperature range for effective conversion is greatly expanded, providing versatility to the
overall operation.
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MECS has been focused on advancements in Cesium catalyst for many years. A patent
(U.S. Patent No. 4,193,894) was issued in 1980, recognizing the specific low temperature
activity of the Cs-K mixture of vanadium sulfates (4). After continuing developments, the
MECS Cs-110 catalyst was introduced in 1989. The newest MECS cesium-promoted
formulation was developed to maximize the low temperature catalytic activity while
maintaining ring strength and long term stability, and was introduced to the market in 2000.
The SCX-2000 catalyst incorporates a high surface area support for optimum catalyst
utilization and an enhanced cesium/potassium/vanadium catalyst component which
maximizes the catalyst activity.
The six lobed-star shape provides optimum dust
management leading to extended operating periods and minimum bed pressure drop. The
combination of all of these properties leads to an ideal lower bed catalyst.
The
conventional cesium-promoted product (XCs-120) is designed for the harsh environment in
the upper beds while providing the versatility of a cesium-promoted formulation. Therefore,
for increasing acid production while maintaining or reducing SO2 emissions, the use of the
MECS SCX-2000 ring catalyst is the ideal material for use in fourth/fifth beds after Interpass
absorption.
SCX-2000 Catalyst Applications
A classic example of the application of the SCX-2000 “Super Cesium” catalyst is provided by
a 3x1 sulfur burning plant where increasing acid production was desired while maintaining
SO2 emissions at or below 3.5 lbs./STPD. In this case, a full fourth bed of SCX-2000 was
installed and the bed inlet temperature was optimized at 750oF (399oC). In this situation, an
increase in acid production of approximately 19% was possible, relative to circumstance with
a full fourth bed of the standard LP-110 catalyst:
Table 1
SCX-2000 versus LP-110
MAXIMUM POSSIBLE ACID PRODUCTION
SULFUR BURNING PLANT
4TH Bed Catalyst
Optimum
4th Bed
Inlet Temp.
Maximum Acid
Production Rate
SO2
Emissions
SCX-2000 RINGS
750OF/399OC
2675 STPD
3.5 lbs./STPD
LP-110 RINGS
775OF/413OC
2250 STPD
3.5 lbs./STPD
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In the case of this sulfur burning plant, one can examine the other advantages of SCX-2000
catalyst at a given production rate relative to operating the plant with a full bed of standard
LP-110. The following chart shows results which demonstrate the emissions reduction
capabilities of the “Super Cesium” catalyst:
EFFECT OF SCX-2000 CATALYST
SO2 Emissions
(lbs./STPD)
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
Acid Production
Rate = 2550 STPD
Fourth Bed
Loading = 64.5 L/STPD
SCX-2000 (750oF)
LP-110 (750oF)
LP-110 (780oF)
4TH Bed Catalyst (Bed Inlet Temperature)
As can be seen in this illustration, the SCX-2000 catalyst allows the plant to achieve very
low emissions which would not be possible using the LP-110 catalyst under the same
conditions.
Another example of the application of the unique properties of the SCX-2000 “Super
Cesium” catalyst is in a fourth bed installation in a sulfur burning 3x1 plant. In this case, the
attainable 4th bed inlet temperature was relatively low (768oF/409oC), the maximum catalyst
loading was about 54 L/STPD, and the low atmospheric pressure at this site contributed to
the “conversion challenge.” A full bed of the SCX-2000 catalyst rings provided the required
low temperature activity to achieve the required conversion and increase the acid production
rate by over 100 STPD. With conventional K-V catalyst in the fourth bed (at the low inlet
temperature), the SO2 emissions would have been nearly double relative to the SCX-2000
case:
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4th BED CATALYST
SO2 EMISSIONS
ACID PRODUCTION
SCX-2000 “SUPER CESIUM “
395 ppm
1520 STPD
K-V CATALYST RINGS
883 ppm
1405 STPD
(Bed Inlet Temperature = 768OF/409OC)
Benefits of Super Cesium Catalyst: Production and Emissions
1600
1400
ACID PRODUCTION (STPD
or
EMISSIONS (ppm)
1200
1000
800
600
400
200
0
PRODUCTION
STANDARD CATALYST
EMISSIONS
SCX-2000
Again, the operation flexibility afforded by the SCX-2000 “Super Cesium” catalyst allows the
plant operators to select the conditions which provide the needed acid production with
acceptable sulfur dioxide emissions.
A third example of the application of the SCX-2000 catalyst is a spent acid plant with a
relatively low final bed inlet temperature. The focus of this SCX-2000 application was to
reduce SO2 emissions while maintaining acid production. There were some temperature
limitations to Bed # 4 which also leads to the use of the “Super Cesium” catalyst. In this
application, a loading of about 77 L/STPD of the SCX-2000 catalyst was installed in Bed # 4
and the bed inlet temperature was set to 732oF (389oC). Under these conditions, SO2
emissions were reduced to less than 1.0 lb./STPD while maintaining the desired acid
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production rate. The use of conventional K-V catalyst under these conditions would have
resulted in extremely low conversion through Bed # 4, resulting in emissions exceeding
1000 ppm SO2 from this double absorption plant.
Conclusion
There are numerous applications of the newly developed SCX-2000 “Super Cesium” sulfuric
acid catalyst throughout the world. The use of this catalyst in the final bed after Interpass
absorption allows the plant operators to increase acid production while maintaining
emissions or alternatively, significantly reducing SO2 emissions with the desired acid
production rate.
References
(1) Tandy, G. H., J. Appl. Chem. 6, 68 (1956) and the references therein.
(2) Villadsen, J., and Livbjerg, H., Catal. Rev. Sci. Eng., 17, 203 (1978)
(3) Boghosian, S., Fehrmann, R., Bjerrum, N. J., and Papatheodorou, G. N.,
J. Catalysis 119, 121 (1989)
(4) Villadsen, J., Catalyst for Oxidation of Sulfur Dioxide, U.S. Patent Office,
Patent No. 4,193,894 (3/18/1980)
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