Extractive Distillation Systems Involving Complex Columns with Partially Coupled Heat and Material Flows

Abstract

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...

Application of the partially thermally coupled distillation flowsheets for the extractive distillation of ternary azeotropic mixtures

“…We have developed the algorithm of the extractive distillation flowsheet generation, including PTCDC 53,54 . It is based on the graph algorithm synthesis of the flowsheet which include PTCDC subsystems for zeotropic mixtures distillation 34,43 .…”

“…Earlier, we have identified the feasible ED flowsheets with PTCDC subsystems for the six different types of ternary azeotropic mixture VLE diagrams 53,54 . The aim of this work is to obtain the feasible ED flowsheets with PTCDC subsystems for all types of VLE diagrams of ternary azeotropic mixtures.…”

“…Therefore, the images of the flowsheets with PTCDC subsystems, obtained on the base of schemes P1, P2 and P3, such as Q1.1, Q1.2, F1.1; Q2.1, Q2.2, F2.1; the pre-images. It is provided by the algorithm of generation configurations with PTCDC subsystems described above52,53 . The application of flowsheet P1d and its images is not necessary because the mixture of the components B and C is zeotropic.The flowsheet П4 and its images are infeasible for the separation of mixtures, belonged to VLE diagram 3.1.1-1bD, because the bottom product of the first column (for schemes П4 and Q 4.2) or the liquid flow, directed to the main column from the side column (for schemes Q4.1 and F4.1), is the ternary mixture with the composition located close to the distillation boundary, which connected the points of azeotropes.…”

Criterion of the energy effectiveness of extractive distillation in the partially thermally coupled columns

“…Previously, we developed an algorithm for the synthesis of PCEDS flowsheets (Timoshenko et al, 2005;Timoshenko et al, 2007). The flowsheet in Figure 1a can be represented in accordance with this algorithm as a graph of Figure 2a.…”

Vapor-liquid-liquid phase equilibrium in four-component benzene-heptane-N-methylpyrrolidone-sulfolane system

“…The first group is based on the works of tech nological character on the optimization of operating conditions and apparatus design of extractive distilla tion processes [5][6][7]. The second group includes research works related to the study of phase equilibria, mixing heats, and the interaction mechanisms of the separating agent with the components of separat ing mixtures in order to select the most effective SA [8][9][10][11][12][13][14].…”