Performance and Stability for Room Temperature Ionic Liquid Membranes for the Dehumidification of Methane

The biogas production from organic waste—animal, sewage, landfill material— could be a valuable renewable fuel. Before this biogas—predominately bio-methane— can be converted to fuel, it must be dehumidified. Typically membrane-based gas dehumidification uses hydrophilic polymer membrane materials, such as polydimethylsiloxane (PDMS) and cellulose acetate (CA). The main problems with polymer dehumidification membranes are methane loss and the susceptibility of polymer materials to plasticization by H2O. Room temperature ionic liquid membranes (RTIL- membranes), according to recent literature, are not plasticized by water and have large water/CH4 selectivities. We built on the initial literature RTIL-dehumidification data by looking at a series of imidazolium-based ionic liquids. One membrane based on the trifluoromethanesulfonate [TfO] anion had a water permeance of 2000 GPUs with a H2O/CH4 selectivity of 12,000. With such high water permeances, boundary layer and support structures might significantly contribute to the overall water transport resistance; we, therefore, obtained data using procedures to minimize and model this effect. Of additional interest to the membrane community is our finding that there is an impact on the stability of some of the RTIL-membranes from the water vapor in the feed. Counter to initial expectations, this impact appears as an increase in membrane mechanical stability and H2O/CH4 selectivity with increasing feed gas relative humidity. In an attempt to explain this phenomena we explored the impact of the Feed rH on the capillary forces in the stabilized RTIL