Polydimethylsiloxane (PDMS) is the most commonly used membrane material for the separation of condensable vapors from lighter gases. In this study, a composite PDMS membrane was prepared and its gas permeation properties were investigated at various upstream pressures. A microporous cellulose acetate (CA) support was initially prepared and characterized. Then, PDMS solution, containing crosslinker and catalyst, was cast over the support. Sorption and permeation of C 3 H 8 , CO 2 , CH 4 , and H 2 in the prepared composite membrane were measured. Using sorption and permeation data of gases, diffusion coefficients were calculated based on solution-diffusion mechanism. Similar to other rubbery membranes, the prepared PDMS membrane advantageously exhibited less resistance to permeation of heavier gases, such as C 3 H 8 , compared to the lighter ones, such as CO 2 , CH 4 , and H 2 . This result was attributed to the very high solubility of larger gas molecules in the hydrocarbon-based PDMS membrane in spite of their lower diffusion coefficients relative to smaller molecules. Increasing feed pressure increased permeability, solubility, and diffusion coefficients of the heavier gases while decreased those of the lighter ones. At constant temperature (25-C), empirical linear relations were proposed for permeability, solubility, and diffusion coefficients as a function of transmembrane pressure. C 3 H 8 /gas solubility, diffusivity, and overall selectivities were found to increase with increasing feed pressure. Ideal selectivity values of 9, 30, and 82 for C 3 H 8 over CO 2 , CH 4 , and H 2 , respectively, at an upstream pressure of 8 atm, confirmed the outstanding separation performance of the prepared membrane.
