Selection of ionic liquids as entrainers for separation of water and 2-propanol

Abstract

A procedure for experimental evaluation of ionic liquids (ILs) as entrainers was proposed. For selection of ILs which can be potentially used for the separation of the azeotropic mixture of water and 2-propanol by extractive distillation, vapor–liquid equilibria were measured for ternary systems of water + 2-propanol + IL at 100 kPa. A previously presented ebulliometer was modified to improve the reliability of measurement of equilibrium temperature and vapor phase composition. A packed column was adopted in the place of the Cottrell pump tube to enhance the equilibration of temperature and compositions of the arising mixture of vapor and liquid. By use of the modified ebulliometer, the experimental measurement was performed in a way of continuous synthesis, in which analysis of liquid phase composition was avoided. While the mole fraction of 2-propanol calculated on IL-free basis, x ′ 2 , was kept almost unchanged at 0.95, isobaric T, x, y data were measured at different IL mass fractions. Activity coefficients of the volatile components as well as their relative volatility were obtained from the experimental data without the need of a thermodynamic model of the liquid phase. There were seven ILs in our investigation: 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4]), 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF 4]), 1-butyl-3-methylimidazolium dicyanamide ([bmim][N(CN) 2]), 1-ethyl-3-methylimidazolium dicyanamide ([emim][N(CN) 2]), 1-butyl-3-methylimidazolium acetate ([bmim][OAc]), 1-ethyl-3-methylimidazolium acetate ([emim][OAc]), and 1-butyl-3-methylimidazolium chloride ([bmim][Cl]). Results showed that the activity coefficients are mainly decided by the anions and the interactions of the anions with water and 2-propanol are in the same order, namely [Cl] − ≈ [OAc] − > [N(CN) 2] − > [BF 4] −. The similarity in the interactions cancels in some extent the effect of the ILs. While the cation has relatively small effect on the activity coefficients, their interactions with water and 2-propanol show reversed orders, namely [emim] + > [bmim] + for interactions with water and [emim] + < [bmim] + for interactions with 2-propanol. The reversed effect on the activity coefficients results in a consistent effect on the relative volatility. Among the seven ILs studied, the analysis of activity coefficients showed that the ILs with [OAc] − or [Cl] − as the anion, and [emim] + as the cation, have the most significant ability to enhance the relative volatility of the mixture of water and 2-propanol. The experimental data were correlated by the NRTL model.