• •  The process consists of two separate reactor-separator-recycle networks in which one corresponds to the ethylbenzene flow sheet and the other corresponds to styrene flowsheet.

• We first present the flowsheet for ethylbenzene production.

• Benzene (wet) is sent first to an azeotropic distillation unit that separates water and produces dry Benzene. Dry Benzene is required so as to avoid unnecessary reactions in the alkylation reactor as well as damage to the catalyst as alumina can get formed.

• Dry Benzene + Ethylene + Ethyl chloride + AlCl₃ enter the alkylator catalyst.

• The reactor could be a jacketed tower where water is used as a cooling fluid in the jacket to control the reactor temperature.

• The reactor produces two products namely uncondensible gases and the liquid product in which AlCl₃ complex is available. This complex needs to be regenerated and sent back to the alkylator.

• The alkylator product is sent to a cooler which upon cooling to 40^oC separates the aluminium chloride complex stream from the product stream. The other stream from the cooler is the ethylbenzene rich product stream.

• The aluminium chloride stream is partially recycled to the alkylator so as to maintain the required catalyst requirements. The other portion of the AlCl₃ complex is sent to a dealkylator unit in which the feed is heated to 200^oC. By doing so, the polyethylbenzenes formed in the alkylator are converted to benzene and ethylbenzene (cracking reaction).

• The benzene and ethylbenzene are returned to the cooler.

• The delkylator produces a residue product consisting of tar + AlCl₃ mixture.

• From this mixture, AlCl₃ is recovered using water extraction as AlCl₃ is soluble in water. From there AlCl₃ is recovered from the water and returned back to the alkylation reactor.

• The product stream from the cooler consisting of ethylbenzene is mixed with 50 % NaOH to remove acidic impurities. Eventually, after settling waste is eliminated.

• The purified ethylbenzene then enters a stripper that separates ethylbenzene + benzene from the polyalkylbenzenes. The polyalkylbenzenes are sent to a polyalkyl still that separates the benzene + ethylbenzenes from the polyalkylbenzenes (bottom product). The polyalkyl still is operated under vacuum. The polyalkylbenzenes are fed to the dealkylator and the benzene + ethylbenzene rich stream is sent to a heat integrated exchanger that extracts heat from the vent gases and then eventually enters the alkylation reactor.

• The top product from the stripper is ethylbenzene + benzene and it enters a benzene column that separates wet benzene from crude ethylbenzene. The wet benzene is recycled to the azeotropic dryer where it is mixed with fresh wet benzene to enter the azeotropic dryer.

• The crude ethylbenzene is further purified in a fractionator where the bottom product (with benzene) is mixed with the top product of the polyalkyl still. Thereby, the stream enters the heat integrated exchanger.

• The ethylbenzene is further subjected to caustic wash and finally it is sent to a dryer to produce dry ethylbenzene.

• We now move to the dehydrogenation flow sheet.

• The ethylbenzene (dry) is heated with superheated steam to enter the catalytic dehydrogenator. Excess steam is used in this process. The feed pre-heating is carried out using the product vapour stream. The reaction is gas phase catalytic reaction.

• The vapour stream after cooling with the feed stream in a heat integrated exchanger is fed to a quench tower using steam quenching.

• After quenching, partial condensation of the quenched vapors produces three streams one being the vapour vent, the other being water and the third being the organic phase rich with styrene.

• The styrene rich stream is sent first to a benzene column to recover the benzene + toluene and this is sent to a benzene-toluene distillation column to recover benzene. The benzene is sent to the azeotropic distillation unit as a raw-material.

• The bottom product from the benzene column enters an ethylbenzene column which separates ethylbenzene from the styrene stream. The ethylbenzene stream is mixed with the dry ethylbenzene to enter the catalytic dehydrogenator.

• The bottom product from the ethylbenzene column is the styrene enriched stream and this is sent to the finishing column where styrene is further purified from unwanted impurities such as tar. The tar is further batch distilled to recover styrene from the tar. The styrene finishing column also produces styrene product. Both styrene products from batch still and styrene finishing column are mixed and cooled to store as styrene product.

• All three columns namely benzene, ethylbenzene and finishing columns are operated under vacuum.

24.2.3 Uses :

Styrene is mainly used for making plastic toys and model kits. Moreover, housing for machines as well as refrigerator doors and air conditioner cases are made of styrene.