Mixed gas sorption in glassy polymeric membranes: I. CO2/CH4 and n-C4/CH4 mixtures sorption in poly(1-trimethylsilyl-1-propyne) (PTMSP)

Abstract

The aim of this work is to study in detail the sorption of binary gas mixtures containing methane into polymers suitable for membrane separations. A novel measuring procedure has been developed and validated by performing mixed gas sorption tests on the n-C4/CH4 mixture in films of poly(1-trimethylsilyl-1-propyne) (PTMSP), for which literature data are available. Then, individual uptakes of CH4 and CO2 from binary gas mixtures were measured at 35°C and up to 35bar in PTMSP, in the whole gas composition range. The experiments were conducted on a pressure-decay apparatus equipped with a gas chromatographic device. The novel experimental procedure was set up in order to obtain data either at constant partial pressure of one gas, as done by previous authors, or at constant gas composition and variable total pressure. The latter protocol allows to mimic more closely the real membrane separation processes, where only the total pressure of the mixture can be varied arbitrarily.It was observed that the presence of CH4 does not alter significantly the sorption of CO2 and of n-C4 in PTMSP, while the mixed gas solubility of CH4 is lower than the pure gas value at the same CH4 fugacity. In particular, the CH4 solubility coefficient, as well as the mixed gas solubility selectivity are univocal functions of CO2 fugacity (or concentration) and are otherwise independent of the gas phase composition. The real CO2/CH4 solubility-selectivity of PTMSP is similar to the ideal value at low CO2 fugacity but it becomes significantly higher, up to 4.5 times, at 25bar of CO2 fugacity.A quantitative rule can be drawn using data of several binary gas mixtures in glassy polymers, based on which the ratio between actual mixed gas and pure ideal solubility selectivity of CO2 over CH4 is a single, monotonously increasing function of the ratio between the concentration of the two components, c(CO2)/c(CH4), which becomes higher than unity as c(CO2)>c(CH4). In other words, the competition effects depress more significantly the less abundant penetrant in the polymer, that is usually CH4.