(640a) Crosslinkable Polyimides with Tunable Membrane Properties for Natural Gas Purification

Cross-linkable membrane materials based on a 6FDA-DAM:DABA (3:2) polyimide backbone are considered for highly aggressive natural gas feeds containing CH₄, CO₂ and H₂S. Three different crosslinking agents were considered: dietheylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TetraEG).   These glycols are all potentially useful for extension to practical asymmetric membrane structures; however, TEG shows the best balance of properties as a cross-linking agent.  Compared to uncrosslinked 6FDA-DAM:DABA (3:2) films, higher permselectivity and membrane stability resulted for crosslinked samples under aggressive feed conditions.  Permeation and gas sorption data were collected at 35 °C, thereby allowing estimation of effects of operating conditions on diffusion coefficients.  In uncrosslinked samples, plasticization occurred at relatively low feed pressures, while the crosslinked films resisted plasticization under more aggressive conditions.   Under mixed gas feed conditions with 20% H₂S, 20% CO₂, and 60% CH₄ the crosslinked samples showed attractive selectivities above 22 and 27 for H₂S/CH₄ and CO₂/CH₄, respectively, for a feed pressure of 62 bar.  Performance is compared between the TEG crosslinked polymer, a propane diol crosslinked variant and cellulose acetate, the most common membrane material for natural gas purification.  Finally, methods for continued development of the TEG polyimide membrane material for aggressive sour gas separations are presented with some results for actual asymmetric hollow fibers.