Extractive distillation with ionic liquids as solvents : selection and conceptual process design

In this chapter, a solvent selection methodology for extractive distillation processes is applied to identify promising ionic liquid (IL) solvents for the following separation cases: methylcyclohexane/toluene, 1-hexene/n-hexane and ethanol/water. Thermodynamic and phase stability analysis are done in order to understand the strong interactions between the solutes and the ionic liquids (solvents) and vice versa. The solvent preselection is done with COSMOtherm software (version C2.1 Release 01.11a) and selectivities and relative volatilities are obtained experimentally and compared with the predicted values. Variations in the IL structure (in the cations and anions) and their effect in the solubility and selectivity are theoretically studied and experimentally confirmed. Suitable ILs are selected by experimentation at finite dilution (real solutions). A suitable IL for the separation of 1-hexene from n-hexane yielding a better performance than the conventional solvent N-methyl-2-pyrrolidone (NMP) was not found. Tetracyanoboratebased ILs seem to be promising solvents for the extractive distillation of toluene from methylcyclohexane as a replacement of the conventional solvent NMP. For the separation of ethanol from water, the ILs 1-ethyl-3-methyl-imidazolium acetate and 1-ethyl-3-methylimidazolium dicyanamide (due to its thermal stability) seem to be suitable candidates and possible replacements of ethylene glycol (EG), which is used as a conventional solvent for the separation of this mixture. 2.