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Abstract

Increasing attention is currently given to the development and application of inorganic membranes due to their high thermal stability and relatively high chemical stability. There is a great interest in applying inorganic membranes not only for separation but also in combination with chemical reactions. In this context the understanding of mass transfer rates is essential for their successful application and for the development of new membranes with improved performance. The present work is focused on a quantitative study of the mass transfer of inert and adsorbable gas mixtures through tubular porous Vycor glass. Transient diffusion experiments are performed based on substituting an enclosed gas via the membrane by another gas [1]. For this pressure responses between a closed outer volume and an open inner volume are observed with elapsed time for inert-inert, inert-adsorbable and adsorbable-adsorbable gas mixtures. For the observed pressure responses of the adsorbable gas mixtures, obviously a strong asymmetry between two reverse exchange experiments is found instead of the symmetry found for exchanging inert gas mixtures. The observed data are analyzed based on the Dusty Gas Model for the qualification of gas phase diffusion [2] and the generalized Stefan-Maxwell theory for the description of surface diffusion [3]. The model parameters for adsorbable gas mixtures can be determined from measurements for inert-inert and inert-adsorbable gas systems. The calculated results show a systematic disagreement with the observations if the simple competitive Langmuir equation is applied to describe the adsorption equilibria. The competitive adsorption equilibrium is found to have strong influence on the extend of surface diffusion. Thus, reliable competitive adsorption isotherms are required for accurate predictions of the mass transfer. Experimental investigations devoted to determine the required isotherms are carried out using a volumetric method. Surprisingly, the observed amount adsorbed of C₃H₈ in a 1:1 mixture with CO₂ is very similar to that for the single component. Instead, for CO₂ adsorption there is the expected competition in the mixture compared to single component adsorption. Further work is currently focused on quantifying the observed adsorption equilibria and on understanding and analyzing the transient diffusion experiments performed. Thus, this project aims to contribute to understand better surface diffusion of adsorbable gases in porous membranes. References: [1] A. Tuchlenski, P. Uchytil and A. Seidel-Morgenstern, An Experimental Study of Combined Gas Phase and Surface Diffusion in Porous Glass, Journal of Membrane Science, 140 (1998) 165. [2] E. A. Mason, A. P. Malinauskas, Gas Transport in Porous Media: The Dusty Gas Model, Elsevier, Amsterdam, 1983. [3] R. Krishna, Problems and Pitfalls in the Use of the Fick Formulation for Intraparticle Diffusion, Chem. Eng. Sci., 48 (1993) 845.