Sidestream Purge Column

The final sidestream column configuration studied in this chapter is the case where there is a very small amount of the intermediate component in the feed. The lightest component is the distillate product. The heaviest component is the bottoms product. The intermediate component builds up in the middle of the column, forming a composition bubble. The objective is to purge off a very small stream from the column at the location where the intermediate composition is fairly large so that there is only a small loss of valuable products. Thus, a high-purity sidestream is not required. 

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We must ensure that all components (reactants., products, and inerts) have a way to leave or be consumed within the process. The consideration of inerts is seldom overlooked. Heavy- inerts can leave the system in the bottoms product from a distillation column. Light inerts can be purged from a gas recycle stream or from a partial condenser on a column. Intermediate inerts must also be removed in some wav, for example in sidestream purges or separate distillation columns. 

Part of the stream is washed countercurrently with a feed sidestream in the vent H2 absorber (9) for benzene recovery. The absorber overhead flows to the hydrogen purification unit (10) where hydrogen purity is increased to 90%+ so it can be recycled to the reactor. The stabilizer (11) removes light ends, mostly methane and ethane, from the flash drum liquid. The bottoms are sent to the benzene column (12) where high-purity benzene is produced overhead. The bottoms stream, containing unreacted toluene and heavier aromatics, is pumped to the recycle column (13). Toluene, C8 aromatics and diphenyl are distilled overhead and recycled to the reactor. A small purge stream prevents the heavy components from building up in the process. 

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