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

We have explored the effect of temperature, in the range between 25 and 50 °C, on the mixed gas CO2/CH4 sorption in a polymer of intrinsic microporosity (PIM-1), that has a solubility-selectivity higher than diffusion-selectivity for this mixture. The new data obtained in this work at 25 and 50 °c were combined with previously published data obtained at 35 °C to determine the temperature-dependence of mixed gas solubility and solubility-selectivity, a type of information that has not been obtained before in the literature. The data were collected at different total pressures and gas mixture compositions using a pressure decay − gas chromatographic device. The data collected indicate that the sorption of such mixture in PIM-1 is dominated by competition, whose major effect is to reduce the solubility of gases with respect to the pure gas value at same fugacity. The competitive phenomena follow a generalized trend that is not dependent on total gas pressure, composition and gas type but is only a function of the second gas concentration in the polymer and temperature. In particular the competition, expressed as reduction of gas solubility with respect to pure gas value, decreases with the concentration of the second gas, and increases with increasing temperature. Such effects are however generally favourable to separation, with positive deviations of the CO2/CH4 solubility-selectivity from ideal values calculated from pure gas solubility, by factors as high as 4. As a rule of thumb, observed at all temperatures and also in other glassy polymers, the real solubility-selectivity deviates positively from ideal value calculated from pure gas behaviour if the molar content of CO2 in the membrane is higher than that of methane, which is usually the case, unless low CO2 gas fractions are considered. A new type of generalized plot, reporting departures of multicomponent properties from the corresponding pure gas values, has been traced for this system and indicates that solubility selectivity departure is univocally correlated to CH4 solubility departure, independent of the operative conditions (pressure, composition, temperature) explored during the experiments.