Selective permeation of CO2 and CH4 through kapton polyimide: Effects of penetrant competition and gas‐phase nonideality

Sorption isotherms for pure CO2 and pure CH4 in Kapton H® polymide films at 60°C are reported for pressures up to 20 atm and are analyzed in terms of the dual‐mode sorption model. An experimental scheme for the measurement of steady‐state permeabilities of both pure and mixed gas feeds is described. Permeabilities of Kapton to the individual components at 60°C are presented for a mixture comprised of 32.2% CO2 in CH4 as functions of feed pressure up to 590 psi (absolute). The permeabilities for the individual penetrants in the mixed feed are lower than the respective purecomponent values at the corresponding partial pressures. Furthermore, the permeabilities of both penetrants drop as the feed pressure is increased at constant composition. The dual‐mobility transport model used to analyze the data postulates that the observed pressure and composition dependence of the permeabilities is due to competition between penetrants for a limited microvoid sorption capacity in the glassy polymer. It is demonstrated that in addition to flux depressions due to dual‐mode effects, nonideality of the gas phase must be accounted for to explain the substantial flux depressions observed for the CO2/CH4 mixtured used in this study.