Investigation of mass transfer through inorganic membranes with several layers

Suitable porous membranes for application in membrane reactors usually consist of several layers. The selective separation layer is in general deposited on one or several supporting layers having larger pores. For the sake of simplicity the mass transfer through such composites is frequently modelled using integral parameters. However, this simplified description has serious limitations, e.g, it is not capable to quantify the often observed effects of direction dependencies of flow and selectivity. In this paper the mass transfer is studied simultaneously to the production process of asymmetric membranes. Membranes made of different alpha- and gamma -alumina layers were prepared. In a final preparation step a further silica sol-gel layer was deposited. To characterise the membranes SEM, EDX, Si-29-NMR and permeation measurements were applied. After each deposition of a new layer, permeation was studied and analysed in order to determine characteristic parameters of this layer assuming the parameters of the previous layers to be known. The dusty gas model could be applied successfully for the quantification of the mass transfer through the multi-layer alumina membranes. Using the determined model parameters for all alumina layers, internal pressure profiles were simulated for the two possible flow directions. The differences of fluxes for the same total pressure gradients were quantified. The mass transfer through the finally deposited sol-gel layer was found to be more complex. A qualitative description based on the concept of configurational diffusion was performed. All results obtained emphasise the necessity of taking membrane asymmetries properly into account during the design and analysis of membrane reactors. (C) 2001 Elsevier Science B.V. All rights reserved.