(699f) Polyimide Membrane Materials Development for Sour Gas Separations

          The acid gas species CO₂ and H₂S are frequently present at unacceptable levels in raw natural gas. Removal of these species to below pipeline specifications is necessary to prevent corrosion of transportation equipment, reduce toxicity, and increase the heating value of natural gas. Recently, membranes have become more widely used in bulk CO₂ separations due to the development of high performance polymers and other materials. However, H₂S separations are still almost exclusively performed using energy inefficient thermal separation techniques – physical or chemical solvents, generally. A limited amount of data currently exists on membrane-based H₂S separations because of the difficulties associated with handling this highly toxic species. Fortunately, the recent completion of a dedicated sour gas testing facility provides us with unique high-concentration H₂S testing capabilities. Sour gas separations data that has been reported in the literature so far focus primarily on rubbery materials. For H₂S/CH₄ separations, these materials perform quite well; however, their CO₂/CH₄ performance lies below that of current state of the art glassy polymers. Whereas 4% of proven reserves in the U.S. are sub-quality due solely to elevated H₂S levels, it is estimated that 25% of proven resources contain unacceptable concentrations of either CO₂ alone or both CO₂ and H₂S. As such, this work focuses of materials development of glassy polymers for simultaneous CO₂ and H₂S removal from raw natural gas. Beginning with an advanced cross-linkable polyimide, 6FDA-DAM:DABA, which has been reported to give excellent CO₂/CH₄ performance, materials development for superior (CO₂+H₂S)/CH₄ performance is currently underway. A rigorous fundamental study of H₂S transport through this glassy base material was conducted, giving insight into possible routes for improving sour gas separation performance. Competition effects were found to be crucial – nearly doubling H₂S/CH₄ selectivity over the ideal selectivity value at high pressures of a binary H₂S/CH₄ mixture. Additionally, polymer-penetrant interactions are prevalent in aggressive sour gas separations. Both H₂S and CO₂ are highly condensable and lead to rapid plasticization of the membranes at moderate partial pressures. To mitigate this effect, thermal annealing or covalent cross-linking of the polymer network can be used to stabilize the membrane. Thermal annealing was investigated in the course of this study and appears to provide small and inadequate gains in penetrant-induced plasticization resistance. While promising, the results of this initial study indicate that H₂S/CH₄ performance in these materials must be improved in order to achieve our overall goal of superior (CO₂+H₂S)/CH₄ performance. To accomplish this, new materials based on 6FDA-DAM:DABA have been synthesized to improve H₂S transport properties and plasticization resistance through favorable polymer-penetrant interactions and covalent cross-linking. The results of this novel study on high-concentration H₂S transport as well as preliminary results using the developed materials will be reported.