Ionic Liquids for Separating 4N-Grade Chemicals: Hierarchical Screening Strategy, Mechanism, and Process Parallel Optimization

The separation of high-value 4N-grade chemicals is of significant industrial importance, notably illustrated by the separation of ethyl propionate from the ethyl propionate/ethanol/water mixture for application in lithium battery electrolytes. In this work, a hierarchical screening strategy for ionic liquids (ILs) is developed to achieve the separation of 4N-grade ethyl propionate. ILs with excellent thermodynamic abilities are preselected at the molecular level, which are then downselected at the process level through a particle swarm optimization algorithm, incorporating thermal decomposition temperature as a critical constraint. A multiprocess and multiparticle parallel optimization method is proposed to accelerate the process optimization. Quantum chemistry calculations and wave function analyses are used to reveal the mechanism of azeotrope separation enhancement by [EMIM][DEP], with a comparative analysis against representative molecular solvents, such as glycerol. A [EMIM][DEP]-based extractive distillation process is proposed, comparing three IL recovery configurations: single-stage Flash, multi-stage Flash, and single-stage Flash integrated with nitrogen stripping. The single- or double-heat pump configurations for [EMIM][DEP] processes are compared. The results indicate that although [EMIM][DEP], [EMIM][SCN], [EMIM][Ac], and [EMIM][DCA] all demonstrate excellent thermodynamic abilities, only [EMIM][DEP] passes the technical feasibility screening. [EMIM][DEP] has superior affinity for ethyl propionate through hydrogen bonding and van der Waals interactions. The [EMIM][DEP]-based extractive distillation process demonstrates superior energy–economic–environmental benefits compared to the glycerol process, with reductions of 41.05% in energy consumption, 42.75% in total annual cost (TAC), and 42.79% in gas emissions. In addition, the energy-saving [EMIM][DEP] process reduces 24.77% in TAC compared with the original process.