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

Abstract

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 (β) 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.