Gas phase transport, adsorption and surface diffusion in a porous glass membrane

Due to their improved performance and excellent chemical resistance, there is a growing interest in applying inorganic membranes not only for gas separation, but also in combination with heterogeneous-catalysts in membrane reactors. Obviously, a thorough understanding and a quantification of the mass transfer processes are essential for optimal application. The objective of this work is to study the mass transfer through a Vycor glass membrane, which might be applied successfully in membrane reactors. Transient diffusion experiments were carried out and the exchange of two different gases via the membrane was investigated. The observed pressure responses reveal the complicated nature of the transport through Vycor glass. If adsorbable gases are involved there is a strong asymmetry between corresponding exchange experiments. Very complex responses were observed for the exchange experiments with C4H10 and C3H8. To understand the observed behavior, the adsorption capacity of Vycor glass was measured for several gases at ambient temperature. In order to analyze the transient diffusion experiments, a transport model was applied, which is based on the Dusty Gas Model for the quantification of gas phase transport and on the generalized Maxwell–Stefan theory for the description of the transport of adsorbed species. The model describes relatively well the exchange experiments performed with the pairs H2/N2 and He/CO2. It was found that the quality of the theoretical results for experiments with adsorbable gases depends strongly on the reliability of the model applied to quantify the adsorption isotherms.