Impact of drying on the catalyst profile in supported impregnation catalysts

The impact of drying conditions and system properties on the final catalyst profile in supported impregnation catalysts is studied. A model is develop ed, which accounts for convective flow in the liquid phase, multi-component diffusion of the metal in the liquid phase, metal adsorption on the porous support, and heat transport. Transport of the gas and liquid phase are des cribed by the dusty gas model and Darcy's law, respectively. Transport of c harged particles (dissolved metal and its ion counterpart) in the liquid so lution, i.e., the convective and diffusive ion transport, are modeled by th e Nernst-Plank equation. Metal adsorption on the porous support is modeled by a Langmuir adsorption isotherm. It was shown that in the case of strong adsorption, drying does not affect the final metal profile, In such cases. the profile is mainly determined during impregnation. In the case of weak m etal adsorption, drying strongly impacts the final catalyst distribution. A ccumulation of the metal at the external particle surface (egg-shell profil e) becomes significant with increasing drying rate, since convective flow t owards the surface is the dominant transport mechanism. Egg-shell catalysts are also obtained, if the permeability of the support is very high, or if the liquid solution has low viscosity. If metal back-diffusion is strong, t he metal is transported towards the particle center, leading to uniform or decreasing egg-yolk catalysts. A dimensional analysis of the model equation s showed that the final catalyst profile is determined by three dimensionle ss groups, which describe the relative strength of convection, diffusion, a nd adsorption. Maps were computed that show regions of different catalyst p rofiles, Therefore, knowledge of these three dimensionless groups allows th e prediction of the final catalyst profile. (C) 2001 Elsevier Science Ltd. All rights reserved.