Humidity Effects in Supported Ionic Liquid Membranes for Spacecraft CO2 Capture

A supported ionic liquid membrane (SILM) consists of a porous membrane filled with a nonvolatile ionic liquid (IL) sorbent. SILMs are attractive for gas separations as they allow continuous in-line separation and operate without any liquid flows. Instead, there are two gas flows – the feed gas mixture flows over one side of the membrane, where it contacts the supported liquid sorbent, and the liquid then releases the captured target molecule into the gas phase on the other side of the membrane. This continuous, isothermal, one-step absorption-desorption process reduces process complexity and power requirements. However, ionic liquids are hygroscopic -- the presence of water vapor is expected to change the viscosity and surface tension of the IL, affecting its stability within the polymer membrane as well as its gas sorption and transport behavior. Previous investigations focused on the CO2 separation performance of the ionic liquid 1-butyl-3-methylimidazolium acetate in simulated dry cabin atmospheres with the assumption that an upstream desiccant would remove water vapor. Here, we explore the effects of humidity on the mass transfer and separation performance of SILMs using polypropylene and nylon membrane supports. Their performance is compared with analogous liquid-flow IL contactors. Finally, the stability of these membranes is quantified by measuring the bubble-point: the transmembrane pressure at which ionic liquid is pushed out of the pores, causing the SILM to lose its selectivity.