Effect of ionic liquids on (vapor + liquid) equilibrium behavior of (water + 2-methyl-2-propanol)

Abstract

Isobaric T, x, y data were reported for ternary systems of {water + 2-methyl-2-propanol (tert-butyl alcohol, TBA) + ionic liquid (IL)} at p = 100 kPa. When the mole fraction of TBA on IL-free basis was fixed at 0.95, measurements were performed at IL mass fractions from 0.6 down to 0.05, in a way of repeated synthesis. The vapor-phase compositions were obtained by analytical methods and the liquid-phase compositions were calculated with the aid of mass balances. Activity coefficients of water and TBA were obtained without the need of a thermodynamic model of the liquid-phase. Six ILs, composed of an anion chosen from [OAc]⁻ or [Cl]⁻, and a cation from [emim]⁺, or [bmim]⁺, or [hmim]⁺, were studied. Relative volatility and activity coefficients were presented in relation with the IL mole fraction, showing the effect of the ILs on a molar basis. The effect of the ILs on relative volatility of TBA to water was depicted by the effect of anions and cations on, respectively, the activity coefficients of water and TBA. The results indicated that, among the six ILs studied, [emim][Cl] has the most significant effect on enhancement of the relative volatility, which reaches a value of 7.2 at an IL mass fraction of 0.58. Another IL, [emim][OAc], has also significant effect, with an appreciable value of 5.2 for the relative volatility when the IL mass fraction is 0.6. Considering the relatively low viscosity and melting point of [emim][OAc], it might be a favorable candidate as solvent for the separation of water and TBA by extractive distillation. Simultaneous correlation by the NRTL model was presented for both systems of (water + ethanol + IL) and (water + TBA + IL), using consistent binary parameters for water and IL.

Introduction

Ionic liquids (ILs) are salts that are liquid at low temperatures (<373 K) which have attracted considerable attention for their potential use as solvent in extractive distillation [1], [2], [3], [4], [5], [6]. The performance of an IL depends large on the behavior of (vapor + liquid) equilibrium (VLE) of the IL-containing mixtures. With the addition of ILs, the components to be separated change their liquid-phase nonidealities and the activity coefficients of these components are affected to a different extent. It is desirable that the changes in activity coefficients result in an increase of relative volatility. Assuming an ideal vapor-phase, the relative volatility of component i to component j, α_i,j, can be related with the ratio of the activity coefficients by ${\alpha }_{i\text{,}j}=({\gamma }_i/{\gamma }_j)·(p_i^{\text{sat}}/p_j^{\text{sat}})$. The vapor pressure ratio, $p_i^{\text{sat}}/p_j^{\text{sat}}$, is insensitive to temperature and provides a natural contribution to the relative volatility [7].

In our previous work [8], [9], [10], we presented an ebulliometer for measurement of T, x, y data at constant pressure, in which the vapor-phase compositions were obtained by analytical methods while the liquid-phase compositions were calculated with the aid of mass balances. The tedious procedure for liquid-phase analysis was avoided, while the activity coefficients of the volatile components were obtained without the need of a thermodynamic model of the liquid-phase. Effects of ILs on the relative volatilities and activity coefficients of (water + 2-propanol) [9] and (water + ethanol) [10] were studied by measurements of VLE data for, respectively, (water + 2-propanol + IL) and (water + ethanol + IL). While the mole fraction of 2-propanol or ethanol calculated on IL-free basis, $x_2^{\prime }$, was kept almost unchanged at 0.95, isobaric T, x, y data were measured at p = 100 kPa and at different IL mass fractions. The measurements provided directly trends of relative volatilities and activity coefficients in relation with IL mass fraction. Effect of the ILs on the relative volatilities was also depicted by their effect on the activity coefficients.

In this work, we have studied the effect of ILs on the phase behavior of {water + 2-methyl-2-propanol (tert-butyl alcohol, TBA)}. VLE data have been measured for systems of {water (1) + TBA (2) + IL (3)}. The ILs that we have used are systematically composed of an anion chosen from [OAc]⁻ or [Cl]⁻, and a cation from [emim]⁺, or [bmim]⁺, or [hmim]⁺. Therefore their were six ILs in this work: 1-ethyl-3-methylimidazolium acetate ([emim][OAc]), 1-butyl-3-methylimidazolium acetate ([bmim][OAc]), 1-hexyl-3-methylimidazolium acetate ([hmim][OAc]), 1-ethyl-3-methylimidazolium chloride ([emim][Cl]), 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), and 1-hexyl-3-methylimidazolium chloride ([hmim][Cl]). So far as we know at the moment, VLE data are not available for mixtures containing water, TBA, and the ILs.