Polyimide Membrane Materials Development for Sour Gas Separations

Thursday, October 20, 2011: 2:00 PM
200 D (Minneapolis Convention Center)
Brian E. Kraftschik, JR Johnson, Oguz Karvan and William J. Koros, School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA

          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.

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See more of this Session: Membranes for Gas Separations I
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