Room Temperature Ionic Liquid (RTIL) Membranes for Biomethane Dehumidification

Renewable fuels are heavily researched due to the increasing price of and resulting pollution from fossil fuels. The production of useable energy from waste—animal, sewage, discarded food, landfill matter would be a valuable renewable fuel production method.  Biogas is a renewable fuel that is produced by anaerobic digestion, composed mainly of methane, carbon dioxide and small percentages of hydrogen sulfide and water vapor. Before this biogas—predominately bio-methane—can be converted to fuel, it must be dehumidified to become useable. 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 [6].  Water vapor does not plasticize room temperature ionic liquid membranes (RTIL-membranes) according to literature data and RTIL-membranes have water/CH4 selectivities > 6000.  In our paper we will build on the initial literature RTIL-dehumidification data by looking at a series of ammonium and imidazolium-based ionic liquids.  One membrane based on the trifluoromethanesulfonate [TfO] anion had a water permeance of 2000 GPUs with a H₂0/CH₄ selectivity of 16000.  The bulk of the RTILs exhibited similar water permeances ranging from ~700-900 GPU. The methane permeances ranged from 0.15 GPU – 0.5 GPU with [emim][TfO] having the lowest average CH₄-permeance.   The data collected was for membranes with approximately 135 mm active film thickness.  Even with these relatively thick active film layers, the water vapor separation performances of the RTIL membranes are on par with current polymer membranes.  Work in our (and other) laboratories, continue on means to reduce the thickness of stability RTILs; thereby, increasing their permeances.  Some of these methods, such as gel-membrane formation, also increase the viscosity of the RTIL which would tend to decrease RTIL-membrane permeances as found for CO₂-permeances.  Data indicates that this permeance relationship with viscosity may not be as strong for H₂0-permeance as it was found for CO₂-permeance.