Improving polyimide membrane resistance to carbon dioxide plasticization in natural gas separations
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Polyimide membranes have been widely applied for gas separations due to their attractive permeability, selectivity, and processing characteristics. Their use for natural gas and hydrocarbon separations is limited by plasticization-induced selectivity losses in feeds with significant partial pressures of CO2 and C3+ hydrocarbons. This project focuses on understanding CO2-induced plasticization of polyimide membranes and how it can be controlled by thermal annealing and crosslinking. Covalent and ionic crosslinking are investigated as approaches for suppressing plasticization, while retaining attractive transport properties. A novel covalent crosslinking protocol has been developed, which offers significant advantages over the traditional post-treatment that was initially used. The twostep crosslinking treatment allows for spectroscopic characterization of the reaction yields in the monoesterification and transesterification reactions. These crosslinking reactions occur at temperatures well below the glass transition and no additives are required in the casting solution, making the approach attractive for the eventual production of asymmetric hollow fibers. The ionically crosslinked membranes are not as stable against CO2 plasticization as the covalently crosslinked materials. By varying the ionic crosslinking density, the effects on long-term sorption and permeation at high CO2 pressures were investigated. From STEM images, it does not appear that heterogeneity in the ion distribution is the cause of the membrane plasticization. With covalent crosslinking, the copolymer composition, crosslinking agent, and thermal treatment are important factors in determining the final membrane transport properties. The crosslinking reaction is accompanied by a heat treatment that can also lead to stabilization of aromatic polyimides. These effects were decoupled by systematic variations in the polymer structure and thermal treatment. In a plasticized membrane, the sorption, diffusion, and swelling processes are all interdependent. The key to controlling plasticization is to control the membrane swelling, since this is related to the increase in polymer chain segmental mobility facilitated by the CO2 sorption. Mixed gas separations demonstrate the non-ideal factors that must be accounted for when modeling membrane performance over a wide range of pressures. The separation performance at practically relevant feed conditions is intrinsically better and more stable than the commercial polymeric membranes currently used for natural gas separations.