CO₂ stripping from ionic liquid at elevated pressures in gas-liquid membrane contactor

Abstract

In this study, the gas-liquid membrane contactor was considered for regeneration of the room-temperature ionic liquids (RTIL) that can be used as physical solvents for carbon dioxide capture process at elevated pressures. Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) was selected as a membrane material due to its high mass transport characteristics and good mechanical properties. Nine different RTILs, such as [Emim][DCA], [Emim][BF₄], [Emim][DEP], [Bmim][BF₄], [Bmim][Tf₂N], [Hmim][TCB], [P66614][DCA], [P66614][Br] and [P66614][Phos], were used to evaluate the solvent-membrane compatibility. The long-term sorption tests (40+ days) revealed that the solvent-membrane interaction is mainly determined by the liquid surface tension regardless of viscosity and molecular size of RTILs. For instance, [Emim][BF₄] and [Emim][DCA], having the surface tension of 60.3 and 54.0 mN/m, demonstrated a very low affinity to the bulk material of PTMSP (sorption as low as 0.02 g/g; no swelling); while for the next ionic liquid [Bmim][BF₄] with surface tension of 44.4 mN/m, the sorption and swelling of PTMSP was 0.79 g/g and 21%, respectively. The long-term RTIL permeation test (Δp = 40 bar, T = 50°С t > 400 h) confirmed that there is no hydrodynamic flow through PTMSP for [Emim][DCA] and [Emim][BF₄]. The concept of CO₂ stripping from RTIL with the membrane contactor by the pressure (Δp = 10 bar) and temperature (ΔT = 20 °С) swing was proved by using dense PTMSP membrane and [Emim][BF₄]. The overall mass transfer coefficient value was equal to (1.6 − 3.8) × 10⁻³ cm/s with respect to liquid flow rate. By using the resistance-in-series model, it was shown that the membrane resistance contribution to the gas transfer was estimated to be approximately 8%.