Basic Processes for Fractionating Ternary Mixtures

There exist three basic column sequences for completely fractionating ternary mixtures by distillation. 

Often the low boiler a is removed first as overhead fraction of coluirm C-1. The binary bottom fraction is fed into colunm C-2 to be fractionated into the pure intermediate boiler b (overhead) and the high boiler c (bottoms). The process depicted in Fig. 11.2-1 is called a-pat/i. 

In the c-path (Fig. 11.2-2), the high boiler c is separated first as bottoms of column C-1. The binary overhead fraction is fed (in vaporous state) into column C-2 to be split into the pure low boiler a (overhead fraction) and the pure intermediate boiler b (bottoms). 

In the a/c-path (Fig. 11.2-3), the ternary feed is fractionated in column C-1 into an overhead fraction, which is free of high boiler c, and a bottom fraction, which is free of low boiler a. Thus, the first separation is performed between the low and high boiling components of the mixture a and c. The intermediate boiler b is present in both fractions produced in column C-1. The two binary fractions of C-1 are fractionated in the subsequent columns C-2 and C-3, respectively. 

The energy demand of distillation processes is a very important quantity since it is the dominant factor for operating costs. Furthermore, a high energy demand also stands for high investment costs since the amount of vapor generated in the reboiler determines the size (in particular the diameter) of the columns (see Chap. 5). 

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