Investigation of mass transfer through inorganic membranes with several layers

Suitable porous membranes for application in membrane reactors usually cons ist of several layers. The selective separation layer is in general deposit ed on one or several supporting layers having larger pores. For the sake of simplicity the mass transfer through such composites is frequently modelle d using integral parameters. However, this simplified description has serio us limitations, e.g, it is not capable to quantify the often observed effec ts of direction dependencies of flow and selectivity. In this paper the mas s transfer is studied simultaneously to the production process of asymmetri c membranes. Membranes made of different alpha- and gamma -alumina layers w ere prepared. In a final preparation step a further silica sol-gel layer wa s deposited. To characterise the membranes SEM, EDX, Si-29-NMR and permeati on measurements were applied. After each deposition of a new layer, permeat ion was studied and analysed in order to determine characteristic parameter s of this layer assuming the parameters of the previous layers to be known. The dusty gas model could be applied successfully for the quantification o f the mass transfer through the multi-layer alumina membranes. Using the de termined model parameters for all alumina layers, internal pressure profile s were simulated for the two possible flow directions. The differences of f luxes for the same total pressure gradients were quantified. The mass trans fer through the finally deposited sol-gel layer was found to be more comple x. A qualitative description based on the concept of configurational diffus ion was performed. All results obtained emphasise the necessity of taking m embrane asymmetries properly into account during the design and analysis of membrane reactors. (C) 2001 Elsevier Science B.V. All rights reserved.