Feasible separation regions for mixtures with S-shaped distillation lines

Abstract

This paper presents an approach for determining feasible separation regions for S-shaped distillation lines. There are three main assumptions: (1) the feed is a mixture of vapor and liquid in equilibrium; (2) the column has a partial or total condenser and a partial or total reboiler; and (3) the border of the feasible separation region is defined by the extreme operating modes of the distillation process. The separation region itself was constructed using a geometric model of the column, while accounting for the shape of the distillation lines. Contrary to well-known cases for C- and U-shaped distillation lines, the border of the feasible separation region for S-shaped lines is formed from a portion of a product composition multitude (i.e., the set of distillate and bottoms composition points for a column with an infinite number of stages operating at total reflux; part of a distillation limit, which demarcates the sloppy splits from the area that is not accessible by distillation; and additional segments). The feasible separation region includes additional product composition areas, which enable the design of distillation columns with fewer theoretical stages, operating under a lower reflux ratio, as demonstrated for a mixture of methyl ethyl ketone, benzene, and toluene.