Testing and evaluation of room temperature ionic liquid (RTIL) membranes for gas dehumidification

Membranes studied for gas dehumidification, drying, or dehydration historically included polymers, triethylene glycol, and aqueous LiCl. RTILs are hydroscopic liquids with negligible vapor pressures that form stable supported liquid membranes. This work evaluated the potential of RTIL-membranes as dehumidification membranes. The work studied the dehumidification of both nitrogen and methane. It tested the role of diffusion (RTIL viscosity) and water solubility (RTIL-anion) in water vapor permeation. The procedures also evaluated the performance and stability of both water immiscible and water miscible RTILs from 0% to >90% relative humidity (rH). In contrast to polymer-based membranes that can have water permeabilites that increase with rH, RTIL-membrane water permeances do not change with rH. Water/methane selectivities are constant with rH for the water immiscible RTIL-membranes tested; however, for the water miscible RTIL-membrane the selectivity decreased with increasing rH from 8100 to 5800. Both water miscible and immiscible RTIL-membranes were stable when operated at rHs up to at least 90%. Taken together, this preliminary evaluation of RTIL-membranes for gas dehumidification encourages further research especially in those applications where they are competitive with existing membrane materials such as, the treatment of bio-methane from anaerobic digesters, flue gas dehydration, and building ventilation systems.