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Vycor glass membrane; Gas separation; Knudsen and surface diffusion
Abstract:
The presented work is focused on studying gas adsorption and diffusion in micro- and mesoporous media. Additionally, complex phenomena in adsorption and diffusion will be elucidated.
The permeation experiments were performed through tubular porous glass membranes in the following manner: an enclosed gas was exchanged via the membrane by another gas which was applied continuously to the open sweep gas side [1]. The resulting pressure differences between the closed volume and the open sweep gas volume were observed for inert gases and adsorbable gas mixtures. In the presented work the pore size is in the range of Knudsen diffusion. This indicates that only the Knudsen mechanism is controlling rate in case of the nonadsorbable components. When an adsorbable component is involved than strong interactions between molecules inside the pores and their host adsorbent enter adsorption and surface diffusion as additional contributors.
For exchanging two adsorbable gases, C3H8 and CO2, a complex model has been developed and verified. A strong asymmetry between two reverse exchange experiments was due to adsorption effects. It was found that total flux is very sensitive to surface diffusion, which is on the other hand strongly dependent on the concentration of the adsorbate. Because of that, the closed volume of the diffusion cell was enlarged with a variable additional volume in order to examine adsorption isotherms and the diffusion coefficients separately. The model parameters were determined from evaluations of the experiments with pair of one inert and one adsorbable gas. Two different ways of evaluations were performed. From assumption that gas and surface flow do not influence each other (Dusty gas model, Maxwell Stefan theory) surface diffusion coefficient was observed as difference between experimentally obtained total and well known Knudsen diffusion. This approach required adsorption isotherm of high accuracy. In order to ignore this interdependency between the adsorption parameters and the surface diffusion coefficients a lumped two parameter equation [2] was applied. The latter approach is based on relations between experimentally determined relaxation times of permeating gases and their transport coefficients. For adsorbable gases relaxation times partly compensate each other. For a certain size of the enlarged closed volume, the two parameter equations give direct ratio between surface and Knudsen diffusion coefficient.
The behaviour of binary gas mixtures could be predicted based only on single surface diffusion model information which gives great advantage in studying the transport of adsorbable gas mixtures.
[1] Tuchlenski A., Uchytil P., Seidel-Morgenstern A., J. Mem. Sci., 140 (1998) 165-184
[2] Schlünder E.-U., Yang J., and Seidel-Morgenstern A., Catalysis today, In Press (2006)
