Modeling of CO2/CH4 gas mixture permeation and CO2 induced plasticization through an asymmetric cellulose acetate membrane

The target of this study is derivation of a mathematical model for permeability and effective diffusivity of mixed gases in glassy polymeric membranes in the presence of plasticization. Diffusion coefficients for all components were assumed to be a function of the plasticizing component. The partial immobilization model was employed to determine the fraction of mobile sorbed gases. The model accurately predicted the mixed gas permeation behavior of CO2 as a plasticizer and CH4 as a second component through the asymmetric cellulose acetate membrane in the presence of plasticization. The model parameters were calculated by fitting experimental data from the literature. The plasticization parameter (beta) decreased for both CO2 and CH4 with increasing fraction of CH4 in the feed. This means that plasticization of a glassy polymer was suppressed. This decrease was caused by competitive sorption between CO2 and CH4. Indeed, CH4 in the feed acts as an anti-plasticizer. In addition, permeances of the feed gas components were decreased in comparison to those of the pure gases, which might be attributed to reduction of sorption and the occupation of Langmuir sites by the second component. Moreover, the immobilization factor (F) for CO2 and CH4 decreased with increase in CH4 fraction due to reduction of plasticization. D-eff/l for pure CO2 was significantly pressure dependent. However with increasing fraction of CH4 in the feed, this dependency almost disappeared. Finally, the model predicted the decreasing trend of the separation factor for CO2/CH4 mixed gases with pressure accurately. Therefore, the presented model is capable of being a useful tool with which to enhance our knowledge related with permeation behavior of mixed gas systems through glassy polymeric membranes in the presence of plasticization.