Elsevier

Fluid Phase Equilibria

Volume 412, 25 March 2016, Pages 94-100
Fluid Phase Equilibria

Separation of 2-propanol and water azeotropic system using ionic liquids as entrainers

https://doi.org/10.1016/j.fluid.2015.11.034Get rights and content

Abstract

2-propanol and water form an azeotropic mixture of the minimum boiling point at constant pressure. In this work, three ionic liquids (ILs), namely 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]), 1-butyl-3-methylimidazolium acetate ([BMIM][OAc]) and 1-ethyl-3-methylimidazolium bromide ([EMIM][Br]), were used as entrainers to separate the azeotropic mixture by the method of extractive distillation. Isobaric vapor–liquid equilibrium (VLE) data for the ternary systems of 2-propanol + water + [EMIM][OAc], 2-propanol + water + [BMIM][OAc], and 2-propanol + water + [EMIM][Br] were measured at 101.3 kPa. The results demonstrate that the relative volatility of 2-propanol to water is dramatically enhanced with the addition of ILs at areas of 2-propanol molar fraction higher than 0.2. As the amounts of ILs increase, the azeotropic point is pulled up and the azeotropy is even eliminated gradually. The separation effect (namely the effect of ILs on enhancement of the relative volatility) of the three ILs follows the order: [EMIM][OAc] > [BMIM][OAc] > [EMIM][Br]. Moreover, The experimental VLE data were well correlated with the nonrandom two-liquid model (NRTL).

Introduction

2-propanol is an important chemical product and raw material. Due to its excellent physico-chemical properties, 2-propanol finds its use in various areas, such as used as an intermediate or a solvent in pharmaceutical industry [1], applications in biofuels [2], and other engineering applications [3], [4]. Generally, there are two commercial routes to produce 2-propanol, namely direct hydration and indirect hydration of propylene [5]. Therefore, it becomes necessary to separate 2-propanol from water. However, a separation problem occurs because 2-propanol and water form a minimum azeotrope at 2-propanol mass fraction of 87.4%, and the azeotropic temperature is 353.45 K at 101.3 kPa [6].

Due to the inefficiency of conventional distillation for azeotrope separation, it is indispensable to turn to special distillations, such as extractive distillation, pressure swing distillation, azeotropic distillation, salt adding distillation and so on [7]. In this study, the extractive distillation separation method draws our attention, for its high-efficiency separation capacity for binary azeotropes and other close boiling point mixtures. Extractive distillation, by adding a third solvent (namely entrainer) to the separated mixtures, achieves the goal of valid separation [8]. Hence, the selection of entrainers should be paid more attention. Traditional entrainers such as salts [9] and organic solvents [10] have been applied to separate the azeotropic mixture, but they have the disadvantages of corrosivity, pollution to the environment and difficulty in recycling.

In this work, ionic liquids (ILs) are proposed as entrainers because of their attractive advantages, such as non-volatility, non-flammability, high chemical and thermal stability, convenient recycling and tailorable structures [11]. During the past years, the isobaric or isothermal vapor–liquid equilibrium data for 2-propanol and water containing ionic liquids have been reported. Various ionic liquids have been investigated to test their abilities to break the azeotropy, including 1-ethyl-3-methylimidazole tetrafluoroborate [12], 1-butyl-3-methylimidazole tetrafluoroborate [6], 1-butyl-3-methylimidazole chloride [13] and 1-butyl-3-methylimidazole acetate [14]. The ILs [EMIM][OAc], [BMIM][OAc] have also been used to separate the mixture [15], but the experiment conclusion only can be made at the 2-propanol-rich region. In this study, the ILs [EMIM][OAc], [BMIM][OAc], [EMIM][Br] were utilized to separate the azeotropic mixture of 2-propanol and water. The VLE data expanded to water-rich region were measured at 101.3 kPa and were correlated with the non-random two liquid (NRTL) model. By analyzing the results, we tried to find the potential entrainers for the system.

Section snippets

Material

2-propanol (mass fraction ≥ 99.7%) used in this work was provided by Sinopharm Group without further purification. Water used in this work was ultrapure water produced by Water purification system (Direct-Q3UV). The ILs, [EMIM][OAc], [BMIM][OAc], and [EMIM][Br] were synthesized in our lab, and the mass purities of them were determined by Liquid Chromatography. The ILs were dried in a vacuum desiccator at 373 K for 24 h before experiment with the purpose of making the moisture content down to

Experimental data

With the purpose of verifying the reliability of the still used in this work, the binary VLE data of 2-propanol and water were determined at 101.3 kPa. The VLE data, listed in Table 2, were compared with the data calculated by the NRTL model and the data from literature [12]. As shown in Fig. 1, the VLE data of 2-propanol and water system measured in this work agree well with the literature and the calculated data, indicating that the still used in the work is reliable.

As mentioned above, the

Conclusions

The VLE data of the 2-propanol (1) + water (2) + [EMIM][OAc] (3), 2-propanol (1) + water (2) + [BMIM][OAc] (3), 2-propanol (1) + water (2) + [EMIM][Br] (3) ternary systems were measured at 101.3 kPa. All the three ILs produced a salting-out effect on 2-propanol at areas of higher 2-propanol content, enhancing the relative volatility of 2-propanol to water, and could eventually eliminate the azeotropic point with the increase of the amounts of ILs. [EMIM][OAc] has a better separation effect than

Acknowledgment

This work is financially supported by National Science Foundation of China (Project No. 21076126), National Science Foundation of China (Project No. 21576166), and Liaoning Province Science Foundation of China (Project No. 2014020140).

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