Distillation sequencing

Much work has been carried out to find methods for the synthesis of distillation sequences of simple columns that do not involve heat integration. However, heat integration may have a significant... 

TABLE 5.1 The Number of Possible Distillation Sequences Using Simple Columns... 

Distillation Sequencing Using Columns with More than Two Products... 

Stephanopoulos, G., Linnhoff, B., and Sophos, A., Synthesis of Heat Integrated Distillation Sequences, IChemE Symp. Ser., 74 111, 1982. 

Choose the distillation sequence to minimize the inventory of hazardous material. 

The following steps are necessary in establishing a heat integrated distillation sequence... 

Andrecovich, M. J., and Westerberg, A. W., A Simple S3mthesis Method Based on Utility Bounding for Heat-Integrated Distillation Sequences, A/CAE 7, 31(3) 363, 1985. 

Distillation sequencing. Unless there are constraints severely restricting heat integration, sequencing of simple distillation columns can be carried out in two steps (1) identify the best few nonintegr-ated sequences, and (2) study the heat integration. In most cases, there is no need to solve the problems simultaneously. ... 

Four columns are needed to produce the desired products. Considering the Sharp Distillation Sequencing heuristics, heuristic (/) does not apply, as there is more than one product in this mixture. Fatty acids are moderately corrosive, but none is particularly more so than the others, so heuristic (2) does not apply. The most volatile product, the caproic and capryflc mixture, is a small (10 mol %) fraction of the feed, so heuristic (3) does not apply. The least volatile product, the oleic—stearic acids, is 27% of the feed, but is not nearly as large as the capric—lauric acid product, so heuristic (4) does not apply. The spht between lauric and myristic acids is closest to equimolar (55 45) and is easy. Therefore, by heuristic (5) it should be performed first. The boiling point list implies that the distillate of the first column contains caproic, capryflc, capric, and lauric acids. This stream requires only one further separation, which by heuristic (/) is between the caproic—capryflc acids and capric—lauric acids. 

The most volatile product (myristic acid) is a small fraction of the feed, whereas the least volatile product (oleic—stearic acids) is most of the feed, and the palmitic—oleic acid split has a good relative volatility. The palmitic—oleic acid split therefore is selected by heuristic (4) for the third column. This would also be the separation suggested by heuristic (5). After splitting myristic and palmitic acid, the final distillation sequence is pictured in Figure 1. Detailed simulations of the separation flow sheet confirm that the capital cost of this design is about 7% less than the straightforward direct sequence. 

Fig. 1. Fatty acid distillation sequence (a) sequence generated by ranked heuristics and (b) more expensive direct sequence.

Figure 5 illustrates a typical distillation train in a styrene plant. Benzene and toluene by-products are recovered in the overhead of the benzene—toluene column. The bottoms from the benzene—toluene column are distilled in the ethylbenzene recycle column, where the separation of ethylbenzene and styrene is effected. The ethylbenzene, containing up to 3% styrene, is taken overhead and recycled to the dehydrogenation section. The bottoms, which contain styrene, by-products heavier than styrene, polymers, inhibitor, and up to 1000 ppm ethylbenzene, are pumped to the styrene finishing column. The overhead product from this column is purified styrene. The bottoms are further processed in a residue-finishing system to recover additional styrene from the residue, which consists of heavy by-products, polymers, and inhibitor. The residue is used as fuel. The residue-finishing system can be a flash evaporator or a small distillation column. This distillation sequence is used in the Fina-Badger process and the Dow process. 

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