Rational Design and Screening of Ionic Liquid Absorbents for Simultaneous and Stepwise Separations of SO2 and CO2 from Flue Gas

In this work, a rational ionic liquid (IL) design and screening strategy, including vapor–liquid equilibrium-based absorption–selectivity–desorption index, simultaneous and stepwise separations, thermodynamic and physical property constraints, and process simulation, is established to remove SO2 and CO2 from flue gas. Based on the UNIFAC method, the vapor–liquid equilibrium composition of flue gas is considered to calculate the absorptivity, selectivity, and desorptivity of two separation processes. Through benchmark solvents, melting points, and viscosity constraints, five optimal ILs are obtained. Finally, optimal ILs with reported ILs 1-ethyl-3-methylimidazolium tetracyanoborate [EMIM]­[TCB] and 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM]­[BF4] are introduced to Aspen Plus. In light of solvent requirements and energy consumption analysis, 1-(2-hydroxyethyl)-imidazolium bis­(trifluoromethylsulfonyl)­amide [C2OHIM]­[Tf2N] and 1-ethylpyridinium dicyanamide [C2PY]­[DCA] are selected as the best IL absorbents, the processes of which save 55.71 and 31.25% solvent and 70.73 and 24.52% energy compared to [EMIM]­[TCB] and [EMIM]­[BF4], respectively. Furthermore, stepwise separation saves at least 50% energy compared to simultaneous separation.