Extractive distillation is commonly used for the separation of azeotropic or close-boiling mixtures in the chemical industry. During the past decade, the use of ionic liquids (ILs) as entrainers has received considerable attention due to their unique advantages when applied in extractive distillation. This work is devoted to providing an easy-to-read and comprehensive review on the recent progress made by chemical engineers, focusing on the issues of predictive thermodynamic models, structure-property relations, separation mechanisms, and process simulation and optimization. This review spans from the molecular level to the industrial scale, to provide a theoretical insight into the molecular interactions between ILs and the components to be separated. Moreover, a comprehensive database on the vapor–liquid equilibria and activity coefficients at infinite dilution concerning ILs is provided as Supporting Information. Concluding remarks are made on the unsolved scientific issues with respect to this promising special distillation technology. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3312–3329, 2014

Extractive distillation with ionic liquids: A review

Organic solvents are widely used in a variety of industrial sectors. Reclaiming and reusing the solvents may be the most economically and environmentally beneficial option for managing spent solvents. Purifying the solvents to meet reuse specifications can be challenging. For hydrophilic solvents, water must be removed prior to reuse, yet many hydrophilic solvents form hard-to-separate azeotropic mixtures with water. Such mixtures make separation processes energy intensive and cause economic challenges. The membrane processes pervaporation (PV) and vapor permeation (VP) can be less energy intensive than distillation-based processes and have proven to be very effective in removing water from azeotropic mixtures. In PV/VP, separation is based on the solution-diffusion interaction between the dense permselective layer of the membrane and the solvent/water mixture. This review provides a state-of-the-science analysis of materials used as the selective layer(s) of PV/VP membranes in removing water from organic solvents. A variety of membrane materials, such as polymeric, inorganic, mixed matrix, and hybrid, have been reported in the literature. A small subset of these are commercially available and highlighted here: poly(vinyl alcohol), polyimides, amorphous perfluoro polymers, NaA zeolites, chabazite zeolites, T-type zeolites, and hybrid silicas. The typical performance characteristics and operating limits of these membranes are discussed. Solvents targeted by the U.S. Environmental Protection Agency for reclamation are emphasized and ten common solvents are chosen for analysis: acetonitrile, 1-butanol, N,N-dimethyl formamide, ethanol, methanol, methyl isobutyl ketone, methyl tert-butyl ether, tetrahydrofuran, acetone, and 2-propanol.

Review: Membrane Materials for the Removal of Water from Industrial Solvents by Pervaporation and Vapor Permeation.

BACKGROUND
There are three binary azeotropes in the tetrahydrofuran/ethanol/water ternary system at atmospheric pressure. Ternary extractive distillation (ED) processes were studied to separate the ternary azeotropic mixture using two single component solvents and a mixed solvent as entrainer.

RESULT
Based on the sequential iterative optimization procedure, the minimal total annual costs (TACs) of the ternary extractive distillation process using ethylene glycol (EG), dimethyl sulfoxide (DMSO), and the mixture of DMSO and EG as entrainer were obtained. Compared with two single component entrainers DMSO and EG, the TAC of the extractive distillation process with mixed entrainer (60 mol% DMSO + 40 mol% EG) decreases by 6.55% and 31.88%, respectively. Furthermore, the reboiler heat duties of two extractive distillation columns (EDCs) with mixed entrainer were compared with two single component entrainers.

CONCLUSION
The results show that the optimal ED process using mixed entrainer performs better than that using single component entrainers from the perspective of economics. Moreover, there is a tradeoff between the reboiler heat duties of two EDCs, which can be adjusted by changing the entrainer performance in two EDCs. The use of mixed entrainer enhances the flexibility of the tradeoff because it only needs to change the composition of the mixed entrainer. © 2017 Society of Chemical Industry

Optimization of the composition of mixed entrainer for economic extractive distillation process in view of the separation of tetrahydrofuran/ethanol/water ternary azeotrope.

A mixture of organic solvent and ionic liquid (IL) was proposed as entrainer for the ethanol/water separation by extractive distillation. The vapor–liquid equilibrium (VLE) experimental results showed that the relative volatility of ethanol to water is significantly enhanced using the ethylene glycol (EG) and [EMIM]+[Ac]− mixed entrainers when compared to that of the benchmark solvent EG. The nonrandom-two-liquid model parameters were correlated using the measured ternary and quaternary experimental VLE data. On this basis, process simulation was carried out to evaluate the improvement of the proposed mixed entrainers. It was found that for the ethanol/water separation using the mixed entrainers, the overall heat duties on reboilers in the processes without and with heat integration (HI) decrease 11.53% and 10.34%, respectively, when compared to the conventional benchmark solvent EG. Moreover, the presented process intensification method is easily achieved in practice by substituting the proposed mixed en...

Extractive Distillation with a Mixture of Organic Solvent and Ionic Liquid as Entrainer

The use of the mixture of ionic liquid (IL) and solid inorganic salt in place of the single IL as entrainers for extractive distillation, which integrates the advantages of a liquid solvent, that is, IL (easy operation and nonvolatility) and a solid salt (high separation ability) has been proposed in this work. The vapor–liquid equilibrium experiments indicated that the combination of [EMIM]+[Ac]− and KAc is the most promising for the separation of ethanol and water among all of the entrainers investigated. Based on the thermodynamic study, the conceptual process design was developed to evaluate the competitiveness of the suggested entrainers for the separation of ethanol and water. It was determined that the overall heat duty on reboilers in the extractive distillation process using the new mixed entrainers decreases 19.04% compared with the benchmark entrainer [EMIM]+[Ac]−. Moreover, the density functional theory and COSMO-RS model were used to achieve theoretical insights at the molecular level. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2994–3004, 2014

Extractive distillation with the mixture of ionic liquid and solid inorganic salt as entrainers

Isobaric vapor–liquid equilibria were measured for the ternary system ethanol + water + ethylene glycol and its three constituent binary mixtures at 101.3 kPa using a modified Rogalski–Malanoski equilibrium still. The thermodynamic consistency of experimental binary data was checked using the point and area tests. The experimental binary data were then correlated by the Wilson, nonrandom two-liquid (NRTL), and universal quasichemical activity coefficient (UNIQUAC) equations. The ternary vapor–liquid equilibria data were predicted using the binary parameters for the three equations with good accuracy. The selectivity of ethylene glycol as entrainer for the separation of the azeotropic system ethanol + water by extractive distillation was discussed using the NRTL parameters.

Isobaric Vapor–Liquid Equilibria for Ethanol + Water + Ethylene Glycol and Its Constituent Three Binary Systems

Isobaric vapor–liquid equilibria (VLE) were measured for four binary systems containing tetrahydrofuran (THF): THF + water, and three different mixtures of THF + n-alkane (n-pentane, n-hexane, and n-heptane). Measurements were conducted at pressures in the range 40.0–101.3 kPa using a simple, automatic apparatus developed in our laboratory. From the experimental VLE data it was confirmed that the THF + water and THF + n-hexane systems formed minimum-boiling-point azeotropes, whereas the THF + n-pentane and THF + n-heptane systems were non-azeotropic mixtures. Non-random two-liquid (NRTL) parameters were also determined on the basis of the experimental VLE data.

Naoki Kamihama

Papers

Isobaric Vapor–Liquid Equilibria for Ethanol + Water + Ethylene Glycol and Its Constituent Three Binary Systems

Measurement of Isobaric Vapor–Liquid Equilibria for Binary Systems Containing Tetrahydrofuran Using an Automatic Apparatus