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 developed, 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 described by the dusty gas model and Darcy's law, respectively. Transport of charged particles (dissolved metal and its ion counterpart) in the liquid solution, i.e., the convective and diffusive ion transport, are modeled by the 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 metal adsorption, drying strongly impacts the final catalyst distribution. Accumulation of the metal at the external particle surface (egg-shell profile) becomes significant with increasing drying rate, since convective flow towards 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, the metal is transported towards the particle center, leading to uniform or decreasing egg-yolk catalysts. A dimensional analysis of the model equations showed that the final catalyst profile is determined by three dimensionless groups, which describe the relative strength of convection, diffusion, and adsorption. Maps were computed that show regions of different catalyst profiles, Therefore, knowledge of these three dimensionless groups allows the prediction of the final catalyst profile. (C) 2001 Elsevier Science Ltd. All rights reserved.