Extractive distillation is one of the most popular methods for separating azeotropic mixtures. Extractive distillation is widely used in organic synthesis and in isolation of monomers. Over many years, the main attention in developing extractive distillation has been focused on the search for an efficient separating agent. Obviously, without this step, a technology is impossible to design. However, if a chosen extractant has lower volatility than the components of the mixture, the flowsheet of a distillation system is predetermined (Fig. 1a). Other solutions in the industry are virtually absent.Conventional extractive distillation systems are very energy-intensive. Systems with multiple withdrawals are known to significantly decrease the energy consumption for separation [1][2][3][4][5][6][7][8][9][10][11]. However, to date, such systems have been used only in distillation of zeotropic mixtures. At the same time, there are not any technical or thermodynamic constraints on the use of complex columns with multiple withdrawals and feeds and systems with coupled heat and material flows in extractive distillation.The first attempts to estimate the efficiency of complex columns in extractive distillation were made by Brito et al. [12]. However, their proposed variant of a column with side withdrawal does not ensure high product purity. To reach high product quality, which is characteristic of the organic synthesis industry and monomer synthesis, distillation systems with side stripping or rectifying sections should be used. Methods for synthesizing such energy-saving flowsheets for distillation of zeotropic mixtures were developed in detail in our previous works [8,9,13]. The efficiency of the proposed algorithm was confirmed in designing a number of technologies of distillation of zeotropic mixtures [6,[8][9][10][14][15][16][17][18].The purpose of this work is to extend this approach to extractive distillation flowsheet synthesis. The proposed algorithm [8,9,13] uses certain initial approximations as preimage flowsheets; as such a preimage, it is expedient to take the conventional process flowsheet (Fig. 1a). It is only necessary to exclude the operation of collapsing the oriented edge that explicates the extractant recycle. Under the assumption that the extractant is slightly volatile and provides only a change, rather than an inversion, of relative volatilities, the number of variants of distillation of a binary mixture is 1 (Fig. 1a). The operation of transformation u [8,9,13] can give a single extractive distillation system with coupled flows (Fig. 1b).This system is a complex column with a rectifying side section. The bottoms product-extractant-is cooled (the cooler is not shown) and returned to the upper part of the main column. Such a process design provides a significant decrease in the energy consumption for separation [19]. A deeper transformation of the separation system leads to a system involving a complex column with side withdrawal of the product B in the vapor phase below the feed section. However, suc...
