(8b) Gas Separations Using Polymer Membranes

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This presentation will discuss structural features important in the use of polymers as rate-controlling membranes for gas separations.  In particular, materials having desirable combinations of high permeability and high selectivity based upon solubility selectivity (e.g., butane removal from natural gas, CO₂ separation from H₂ or N₂) or diffusivity selectivity (e.g., CO₂ removal from natural gas) will be presented.  For example, cross-linked poly(ethylene oxide) (XLPEO) polymers, which are flexible, rubbery polymers identified as promising materials to remove polar and acid gases, such as CO₂, from mixtures with light gases, such as H₂.  One member of this family of materials was reported to have a CO₂ permeability coefficient of approximately 500 Barrer and a CO₂/H₂ mixed gas selectivity of 30 at -20C.  Such materials achieve high selectivity based upon their high solubility selectivity favoring CO₂ transport. 

Polymers can also be tailored to achieve high selectivity based upon high diffusivity selectivity.  In this case, highly rigid, glassy polymers with proper free volume element size and size distribution are desirable.  Polyimides with ortho-position functional groups may be solution-processed to form conventional films and membranes.  Such materials can undergo thermal rearrangement to form highly rigid benzoxazole or benzithiazole structures having very high permeability coefficients and high selectivity.  For example, one member of this family was prepared having a CO₂ permeability coefficient of 1610 Barrer and a CO₂/CH₄ selectivity, under mixed gas conditions, of 42-46, depending on the partial pressure of CO₂ in the mixture.  These thermally rearranged (TR) polymers are insoluble in common solvents, giving them good chemical stability, and highly thermally stable, which are important attributes for membranes that would be used in chemically or thermally aggressive environments.