Energy-economic analysis of ionic liquids extractive-heat pump distillation process for recovery of ethanol and isopropyl alcohol from wastewater

In the process of synthesis or application of alcohols, water always presents the coexistence relationships with alcohols. The subsequent separation of homogenous mixtures composed of one or more alcohols and water to treat wastewater is needed. In this work, ionic liquids (ILs) single column extractive distillation (SCED) integration with heat pump distillation (HPD) process for recovery of ethanol and isopropyl alcohol (IPA) from wastewater was proposed. The thermodynamic vapor–liquid equilibrium (VLE) was applied to analyze the separation sequences and the abilities for the mixtures using ILs or organic solvent as entrainer. The ILs 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]) was selected as the optimal entrainer considering the viscosity and the selectivity of ethanol to water, and the mechanisms on enhancing VLE was explained via the sigma-profiles. Two alternative separation sequences with two alternative entrainers involving SCED with ethylene glycol (EG), SCED with EG integration with HPD, SCED with [EMIM][DCA] integration with HPD, and double-column extractive distillation (DCED) with EG processes were investigated. This work aimed to compare ILs and organic solvent processes by considering energy and economic aspects. Compared with EG-SCED, EG-SCED-HPD and EG-DCED process, the energy cost of ILs-SCED-HPD can reduce by 57.7%, 28.6%, and 28.3%, respectively. Economic analysis indicted that the total annual cost (TAC) of ILs process was similar with EG process with increase of price of CO2 emission. ILs-SCED-HPD was confirmed as the promising process both in energy and economic perspectives for recovery of ethanol and IPA from wastewater.