Predict Thermodynamic Moedls Of Ionic Liquids And Ionic Liquid Intensification For Extractive Distillation

Extractive distillation has been widely applied in chemical industry for the separation of close-boiling or azeotropic systems, where an additional solvent (namely entrainer or separating agent) is required to alter the relative volatility (or selectivity) of the components to be separated. It is evident that selection of a suitable entrainer is very important to effective and efficient design of extractive distillation. Up to date, there are four kinds of entrainers used in extractive distillation, i.e. solid salts, liquid solvents, the mixture of liquid solvents and solid salts, and ionic liquids (ILs). Among others, ILs as the entrainers comprise both advantages of liquid solvents (easy operation) and solid salts (high separation ability), and thus have attracted considerable attention for their potential use in recent years.The previous studies showed that for the separation of nonpolar/nonpolar systems, the ILs with small molecular volume, unbranched group, and sterical shielding effect (or delocalized charge) around anion charge center have a high selectivity at infinite dilution but a low solvent capacity as well, whereas the ILs with a low selectivity at infinite dilution exhibit a high capacity. Thus, it is anticipated that the mixed ionic liquids (MILs) should have both high selectivity and solvent capacity at finite concentration. On the other hand, for the separation of polar/polar systems, e.g. ethanol/water, the ILs with small molecular volume, unbranched group, and no sterical shielding effect (or localized charge) around anion charge center have both high selectivity and solvent capacity. In principle, the salting effect of solid inorganic salts could be stronger than ILs, due to their smaller molecular volumes. But when solid salt is used as entrainer only, dissolution, reuse and transport of solid salt are quite a problem in actual industry. Therefore, in the same way as the traditional combination of ethylene glycol and solid salt that has already been widely used in industry, the mixture of IL and solid salt as entrainer will also comprise the advantages of easy operation and high separation performance. Furthermore, both IL and solid salt are nonvolatile, which facilitates entrainer recovery by simple flash distillation or gas stripping.In this work, n-hexane/1-hexene is taken on as the representative of nonpolar/nonpolar systems. Meanwhile, ethanol/water is taken on as the representative of polar/polar systems. Therefore, the focus of this work is on addressing the cogently interesting issues for extractive distillation with ILs as to (ⅰ) make clear of the specific relationship between separation performance (i.e. selectivity and solvent capacity) and molecular structure in positive ion and negative ion of ILs for the separation of nonpolar/nonpolar and polar/polar systems, respectively, which requires the close integration of theoretically predictive models and experiments. For this purpose, the conductor-like screening model for real solvents which is always written as COSMO-RS model in short was used for evaluating the ILs as entrainers, because as an a priori predictive model, it requires molecular structure as the only information and is independent of experimental data;(ⅱ) intensifying the IL entrainers for the separation of n-hexane/1-hexene by using MILs which can be easily available from chemical markets to make a compromise between selectivity and solvent capacity; and (ⅲ) intensifying the IL entrainers for the separation of ethanol/water by using the mixture of IL and solid inorganic salt to improve the selectivity of ethanol to water. The COSMO-RS model was extended to predict the separation ability of solid inorganic salts with various combinations of cations and anions, and a comparison between predicted results and vapor-liquid equilibrium (VLE) experimental data was made,(iv) The binary interaction parameters of NRTL equation were obtained from the vapor-liquid equilibrium experimental data about the ternary systems of ethanol and water with ionic liquid. The binary interaction parameters were used to predict the concentration of vapor phase and relative volatility and the results agreed well with the experimental data, Which justified the applicability of NRTL equation for the vapor-liquid equilibrium of the systems studied in this paper.