Design and comparison of conventional and side-stream extractive distillation sequences for separating the methanol-toluene binary azeotrope with intermediate boiling entrainer

Abstract

The side-stream extractive distillation (SSED) system is attracting widespread attention due to its high eco-efficiency. In the article, three double-column SSED (DC-SSED) sequences and two single-column SSED (SC-SSED) sequences with different side-stream extraction locations are proposed for separating the methanol-toluene azeotrope with intermediate boiling entrainer triethylamine, which are compared with two conventional extractive distillation (CED) processes including direct and indirect CED sequences. The genetic algorithm is employed to optimize seven extractive distillation sequences to minimize the total annual costs (TAC). CO₂ emissions and thermodynamic efficiency of the optimized processes are quantified as well as TAC to calculate the Eco-efficiency Comparison (ECI) Index for eco-efficiency analysis. The results illustrate that two DC-SSED sequences demonstrate the eco-efficiency superiority compared to the optimal CED process (direct CED), and their side-streams are located in the rectifying section and extractive section of the side extractive distillation column, while their ECI indexes reach 97.38% and 97.60%.

Introduction

A large amount of industrial waste liquor of methanol and toluene are often produced in the chemical process of the alkylation of toluene and methanol to manufacture paraxylene due to the limitations of the reaction process (Wang et al., 2019). Methanol and toluene are widely used in chemical, pharmaceutical, textile and energy fields as important organic solvents and raw materials for the production of other high-value products. Therefore, waste liquor of methanol and toluene must be effectively purified to avoid environmental pollution and recover valuable components from the perspective of sustainable development. However, it is impossible to achieve effective separation of methanol and toluene system via conventional distillation method due to the presence and constraint of the binary azeotrope. Extractive distillation can be considered as an efficient separation method to perform the separation of azeotropes system, which has been successfully applied to the separation of many binary azeotropes, such as acetone-methanol (Luyben, 2008a), tetrahydrofuran-water (Ghuge et al., 2017), acetone-chloroform (Luyben, 2008b), ethanol-water (Tututi-Avila et al., 2014), etc.

The first step in designing the feasible extractive distillation separation sequences is to select the feasible entrainer that can break the azeotropic phenomenon and increase the relative volatility of the system. Theoretically, there are three types of entrainer for extractive distillation process, namely heavy entrainer (Rodriguez-Donis et al., 2009), light entrainer (Rodriguez-Donis et al., 2012a), and intermediate boiling entrainer (Rodriguez-Donis et al., 2012b). Compared with the heavy and light entrainer, intermediate boiling entrainer could cause the difficulty of the binary separation since the range of temperatures are bounded by the boiling points of components to be separated. However, it can demonstrate the advantage of no increase in the temperature of system and a small amount of usage (Modla, 2013). Therefore, intermediate boiling entrainer should be considered in the design of extractive distillation processes. However, to our best knowledge, there are a few valuable insights on the extractive distillation process with intermediate boiling entrainer. Doherty and Malone (2001) investigated the extractive distillation process with methyl butyrate as intermediate boiling entrainer for separating the methanol-toluene azeotrope. Modla (2013) designed and optimized three extractive distillation configurations with triethylamine (Et3N) as intermediate boiling entrainer, namely conventional extractive distillation (CED), thermally integrated extractive distillation, and extractive dividing-wall column. As an extension of Modla's work (2013), Wang et al. (2020a) constructed feasible control structures for the CED, thermally integrated extractive distillation and extractive dividing-wall column configurations, respectively, and tested the dynamic performances via introducing the specified feed flowrates and feed compositions disturbances. Luyben (2015) investigated the improved direct CED and indirect CED separation sequences with smaller entrainer flowrates from the aspects of economy and controllability. Ma et al. (2016) investigated the economics and controllability of the CED processes for separating the methanol-toluene system using heavy entrainer aniline and intermediate boiling entrainer Et3N. Huang et al. (2016) investigated two CED sequences including direct and indirect separation sequences and two corresponding extractive dividing-wall column sequences including the wall in the top and the wall in the bottom for separating the methanol-toluene azeotrope with Et3N as entrainer in terms of the economics and controllability.

The study of process intensification is of great significance for seeking substantially smaller, cleaner, safer, and more energy-efficient technologies (Reay et al., 2013; Demirel et al., 2017, 2019a,b). Extractive distillation is no exception. Thus, the design of energy-efficient extractive distillation separation configurations is attracting extensive attention due to the energy-intensive nature of distillation processes. Recently, a novel double-column side-stream extractive distillation (DC-SSED) system was proposed for the separation of three binary azeotropes including ethanol-water, acetone-methanol and heptane-toluene systems, as shown in Fig. 1, which demonstrates the economic and energy efficiency compared to two typical thermal coupled configurations including extractive dividing-wall column and thermally integrated extractive distillation (Tututi-Avila et al., 2017). One side extractive distillation column (SEDC) and one side entrainer recovery column (SERC) are employed to conduct the separation of azeotrope and entrainer recovery, while the SEDC and the SERC are connected by one liquid side-stream extracted from the stripping section (SS) of the SEDC. Simultaneously, a significant phenomenon can be observed that the high-purity entrainer can be produced from the bottom of the SEDC and the SERC, respectively.

Inspired by this work (Tututi-Avila et al., 2017), Ma et al. (2019) and Zhang et al., 2020 established the feasible control structures for the DC-SSED system via taking the separation of the acetone-methanol as an example, respectively. Subsequently, the DC-SSED system has been successfully applied to the separation of other azeotropes. Zhang et al. (2019) investigated the design and control of the DC-SSED system for the separation of the propylene oxide-methanol-water self-entrainer system with water as entrainer. Shi et al. (2020) investigated the multi-objective optimization and effective control of the DC-SSED system with and without heat integration for the separation of the ethyl acetate-ethanol system. Wang et al. (2018a,b) proposed a triple-column SSED separation configuration and conducted a controlled study for separating the ternary multi-azeotropic acetonitrile-benzene-methanol mixture. Based on the work of Wang et al. (2018a,b), Yang et al. (2019a, 2020) constructed the novel control structures without any online composition controllers, which are capable of handling larger feed disturbances. Cui et al. (2020) applied the triple-column SSED separation configuration to effectively achieve the separation of benzene-isopropanol-water heterogeneous ternary multi-azeotropic mixture with ethylene glycol as entrainer. To the best of our knowledge, the entrainer used in the previous studies on the separation of azeotropes via SSED system belong to heavy entrainer, and there are no more efforts to investigate the economic and controllable performance for the energy-efficient SSED with intermediate boiling entrainer, except for Wang et al. (2020b). In this reference, Wang et al. (2020b) proposed and optimized a novel and simple single-column side-stream extractive distillation (SC-SSED) separation configuration, while several robust control structures were established to deal with the specified feed flowrates and feed compositions disturbances. However, the defects of high reflux ratio and high energy consumption can be intuitively observed for the SC-SSED configuration, although the structural design of the SC-SSED system is very ingenious and simple.

As discussed above, despite successful advances in SSED researches, hardly any study has comprehensively investigated the design of multiple SSED sequences with intermediate boiling entrainer, as well as the optimization of the side-stream extraction locations including rectifying section (RS), extractive section (ES) and stripping section (SS). For this purpose, multiple feasible CED and DC-SSED separation sequences with intermediate boiling entrainer Et3N are thoroughly investigated in the present paper, taking the separation of the methanol-toluene binary azeotrope as an example. First of all, the feasible CED and DC-SSED separation sequences are conceptually synthesized on the basis of the thermodynamic criterion. And then, the optimal operating parameters are determined in a bid for the minimum total annual costs (TAC) via genetic algorithm. Correspondingly, CO₂ emissions and thermodynamic efficiency of the optimized sequences are further quantified as well as the minimum TAC to perform the eco-efficiency analysis. The economic and environmental advantages or disadvantages of different DC-SSED separation sequences are emphatically analyzed. Finally, the separation process with the highest eco-efficient is identified.