5. Process Description In the separation section of the ethylene plant, there are several subsections that the process stream must go through in order to achieve the desired product purity. These subsections include a cracked gas compression system, a caustic wash system, pre-cooling and drying, separation of the C3 and lighter components from C4 and heavier mixtures by way of depropanizer towers, an acetylene hydrogenation reactor, cold train, and a demethanizer tower. The process stream molar flow rate to the separation section is 10,203 kgmol/hr. The process stream from the cracking section is fed through a 3-stage compression system with intercoolers. In the first stage of compression, the process stream pressure is increased from 2.2 bar to 4.6 bar and the temperature rises from 35oC to 98oC. The stream then passes through an air cooler to bring the temperature back down 65oC. Once cooled down, the mixed phase stream is separated in a flash drum where the overhead gas continues to the next stage of compression and the bottoms liquid is then sent to waste. The second stage of compression is similar to the first stage of compression. The process stream is compressed further from 4.3 bar to 9.8 bar. This results in an increased temperature from 65oC to 139oC. The process stream is then cooled through in the second air intercooler to 65oC. Once again, this is a mixed phase stream and requires separation of the gas from the liquid in a flash drum. The overhead gas continues to the caustic wash system and the bottoms liquid is sent to waste. The caustic wash system is fed from the second stage of compression. Here the CO2 and H2S is removed to prevent these poisons from going through distillation and to prevent the poisoning of the acetylene hydrogenation reactor catalyst. Once these two components are removed, the process stream continues to the third and final stage of compression. Here the pressure is increased from 9.4 bar to 20.7 bar and the temperature rises from 65oC to 136oC. The process stream is cooled down in the third air intercooler to 65oC and the mixed phase stream is sent to the flash drum. Here the overhead gas continues on to the dryer and the bottoms liquid stream goes to waste. Since most of the heavy hydrocarbons and all of the liquid water have been removed, a molecular sieve dryer is used to remove any residual water vapor left in the process stream. It is important to remove any residual vapor early on in the process in order to prevent cold temperature sections from getting clogged up. Due to the condensation of water vapor in the dryer, the process stream temperature rises to 86oC. The process stream passes through yet another air cooler to bring down the temperature to 64oC before entering the depropanizer. After the cracked gas compression system, the molar flow rate of the process stream is 9771 kgmol/hr. The C3 and lighter components must be separated out from the C4 and heavier mixtures by going through the two distillation towers, a high pressure depropanizer and a low pressure depropanizer. One tower could have been used, but the reason for having two towers is to enhance the separation efficiency. The stream from the cracked gas compression system is first fed to the high pressure depropanizer tower. The overhead pressure of this tower is 19.0 bar and a reflux ratio is kept to a minimum of 0.4 in order to reduce the condenser and reboiler duties. The bottoms stream of the high pressure depropanizer has a molar flow rate of 71 kgmol/hr, a temperature 60.4oC and a pressure of 29.0 bar. This kettle reboiler uses level 3 steam, which has a temperature of 168oC and a pressure of 3.8 bar. The required amount of steam is 247 kgmol/hr. The process stream then passes through a let-down valve to reduce the pressure from 29.00 bar to 8.00 bar which also reduces the temperature from 60oC to 20oC. This stream serves as the feed to the low pressure depropanizer. The overhead pressure of the low pressure depropanizer tower is 8.0 bar and has a reflux ratio of 6.0. The bottoms stream of the low pressure depropanizer has a temperature of 66.1oC. This kettle reboiler uses 95 kgmol/hr of level 3 steam. This bottoms stream has a molar flow rate of 31 kgmol/hr and is sent to storage. The overhead of the low pressure depropanizer has a molar flow rate of 40 kgmol/hr, a temperature of 10oC, and a pressure of 8.0 bar. This stream then goes through the condenser where the temperature is brought down to 1.5oC. This condenser uses level 3 propylene refrigerant, which is at -46oC and 1.1 bar, to cool down the process stream. Here, the required amount of propylene is 205 kgmol/hr. This overhead stream of the low pressure depropanizer is recompressed from 8.00 bar to 19.00 bar, which results in an increase of temperature from 1.5oC to 52oC, and is fed back to the high pressure depropanizer. The overhead product of the high pressure depropanizer tower has a molar flow rate of 9740 kgmol/hr, a temperature of -25.9oC, and a pressure of 19.00 bar. This process stream goes through the condenser, which brings down the temperature to -42.7oC. The condenser uses level 3 ethylene refrigerant, which has a temperature of -101oC and a pressure of 1.2 bar, to cool down the process stream. Here, the required amount of ethylene is 3,975 kgmol/hr. The overhead from the demethanizer is then fed to the acetylene reactor section. Prior to entering the acetylene reactor, the stream must be heated and compressed. The stream first enters an integrated heat exchanger which heats it from -42.7 oC to 67 oC by simultaneously cooling the process stream exiting the acetylene reactor from 116 oC to 15 oC. The heated process stream then enters a centrifugal compressor which compresses the process stream from 67 oC and 18.7 bar to 119 oC and 36.6 bar. A trim exchanger is used here to control the temperature of the feed to the reactor. The acetylene reactor uses -alumina supported palladium catalyst in order for selective hydrogenation to occur. This process is very precarious because there is an undesirable reaction which can take place converting ethylene back into ethane. One hundred percent of the acetylene that enters the acetylene is reacted to form ethylene. Methyl acetylene and propadiene are both partially reacted to form propylene. The reactions that take place in the acetylene reactor are Desirable: C2H2 + H2 C2H4 C3H4 (Methyl Acetylene) + H2 C3H6 C3H4 (Propadiene) + H2 C3H6 Undesirable: C2H4 + H2 C2H6 After the process stream exits the acetylene reactor and is cooled in the integrated heat exchanger prior to the compressor, it is ready to enter the cold train section of the process. The stream begins at 15 oC and 35.4 bar. A series of plate-fin heat exchangers (commonly referred to as cold boxes) are used for pre-cooling and condensation of the feed into the demethanizer. When temperatures are low enough to require refrigeration, traditional approach temperatures are not economical. The large surface area of cold boxes allows very low approach temperatures. This makes them ideal when cryogenic conditions are necessary. The entire cooling load can not be handled by the cold boxes alone. As a result, refrigerants are used in makeup heat exchangers in order to ensure that the streams reach the desired low temperatures. Four stages of cooling are used. The first three stages are comprised of cold boxes, makeup heat exchangers, and flash drums. The first stage of cooling lowers the process stream temperature down to -43 oC using first a cold box and then by -46 oC propylene refrigerant in a heat exchanger. The liquid is then separated out using a phase separator. The remaining gas is then cooled further in the second stage of cooling. Here it is allowed to reach a temperature of -71 oC by cooling it using a cold box followed by a heat exchanger with -78 oC ethylene refrigerant. The liquid is once again separated out. The remaining gas is then cooled again to a temperature of -96 oC through cooling it in the third cold box followed by heat exchange with -101 oC ethylene refrigerant in the third stage of cooling. A flash drum is used to remove the liquid from the stream. The uncondensed stream receives its final cooling through a cold box alone. It reaches a temperature of -121 oC. Another flash drum is used to remove the rest of the condensed liquid. The cold boxes are cooled using let down non condensable process stream and the expanded overhead from the demethanizer. Lowering the pressure of these streams helps to provide a better temperature driving force for cooling. The liquid streams which have been separated in the cold train section are all fed into the demethanizer. Since the demethanizer is a cryogenic distillation column, the cold temperatures require that the feed stream does not contain water. The demethanizer has 82 trays and the final composition in the bottoms product should not exceed 0.005 mol% of methane. The reflux to the demethanizer is condensed using -101 oC ethylene refrigerant and has a reflux ratio of approximately 2.75. The reboiler of the demethanizer is heated by the condensation of vaporized 6.4 oC propylene refrigerant. The refrigerant is cooled to 4.0 oC and is then fed back to the refrigeration section. This relieves the compressor duty required in the refrigeration section significantly. The condensed stream portion of the stream is then separated using a flash drum and fed to the purification team. The stream consists of C2s, C3s, and trace amounts of methane at 2 oC and 35.8 bar. Figures 2.2 and 2.3 represent the process flow diagrams of the Separation section. Table 2.1 provides flow rates, temperatures and pressures of major streams.
