Mixed gas sorption in glassy polymeric membranes: II. CO2/CH4 mixtures in a polymer of intrinsic microporosity (PIM-1)

The individual solubility of CH4 and CO2 from binary gas mixtures was measured at 35 °C and up to 35 bar in a polymer of intrinsic microporosity (PIM-1), at different compositions of the gas phase (from 0 to 50 mol% of CO2). The experiments were conducted on a pressure-decay apparatus equipped with a gas chromatograph, allowing a highly flexible measuring procedure. The gas solubility was plotted versus gas phase composition, total pressure, gas fugacity and second gas concentration. The mixed gas solubility of both species, CH4 and CO2, is lower than the pure gas value at the same fugacity, but the reduction of methane solubility due to the presence of CO2 is generally more significant. Such behavior is due to the fact that CO2 has normally higher solubility than methane: indeed the depression of the solubility coefficient with respect to the pure gas value is similar for both gases, when reported at the same concentration of the second gas.The real, mixed gas solubility selectivity is in general higher than the ideal value calculated from pure gas behavior. The ratio between real and ideal solubility selectivity increases with CO2 concentration in the membrane, according to a single mastercurve, reaching a maximum value of 4, and increases also with the ratio between CO2 and CH4 concentration in the membrane. In particular, as in the case of other glassy polymers, the real solubility selectivity of CO2 over CH4 is higher than the ideal value if c(CO2)>c(CH4), and it is lower than the ideal value if the opposite condition holds true. Such behavior occurs because the competition for sorption is normally less effective on the more abundant penetrant in the polymer. A selectivity-solubility performance plot can be drawn for this system.