Natural gas sweetening—the effect on CO2–CH4 separation after exposing a facilitated transport membrane to hydrogen sulfide and higher hydrocarbons

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The PVAm/PVA blend composite membrane was exposed to synthetic natural gas mixtures containing aggressive gases like hydrogen sulfide (H2S) at a concentration of 1mol% and H2S in combination with the condensable hydrocarbons n-hexane and propane. The effect on the performance of the membrane was studied under different relative humidity conditions and the possible interactions between the membrane and impurities were analyzed. The performance of the membrane was found to be reduced at low humidity conditions due to H2S induced conditioning and sorption of n-hexane both in the polysulfone support and PVAm/PVA selective layer, while at high relative humidity the CO2 facilitated transport helps the membrane to retain its performance to a value very close to that of a fresh membrane. The CO2 facilitated transport is closely related to the membrane swelling. The combined effects of H2S and hydrocarbons are comparable to the effects of H2S alone. Under high humidified conditions, the maximum CO2 permeance loss is 18% and the maximum CO2/CH4 selectivity loss is 16% after two weeks exposure at this harsh and humidified conditions. Our conclusion based on the current investigations, is that the aggressive environment does not introduce permanent damage to the material and the PVAm/PVA blend membrane is keeping its separation performance quite well after the exposure to H2S and hydrocarbons, and may have a potential for being used in natural gas sweetening.