(575r) Impact of Crystallization and Film-Breakage on Drying of Supported Catalysts

Supported catalysts are essential components of many industrial processes and applications, ranging from petrochemical and catalytic converters to fuel cells. They are generally required because of their high surface area, reduced amount of active agent, and high thermal stability. The performance of a catalytic process is intimately related to the catalyst preparation. Supported catalysts are usually prepared by impregnation, where a porous support is contacted with a liquid solution that contains the desired metal as a dissolved salt. This step is usually followed by the evaporation of the liquid solvent and that is drying. It is generally believed that the metal profile is controlled by the conditions that are applied during impregnation. However, experimental work has shown that drying may also significantly impact the metal distribution within the support. Therefore, to achieve a desired metal profile we need to understand the impregnation, but we also need to have a fundamental understanding of the drying stage. Controlling the drying conditions can enhance catalyst performance, and minimize the production of useless batches that have to be disposed, or recycled.

In this work we have developed a theoretical model for drying. In this model, we have taken into account heat transfer from the hot air to the wet support, solvent evaporation in the support, convective flow towards the support external surface due to the capillary force, and metal diffusion and metal deposition due to adsorption and crystallization. In general, the convective flow is the main driving force to transport the metal component and the solvent towards the support external surface, while the back-diffusion causes metal to transport towards the support center. An increase in the evaporation rate may greatly enhance the convective flow.

Of particular interest is to compare simulation results and experimental measurements to determine the key parameters used to predict the impregnation and drying processing. Experimental work has been carried out based on a Nickel/Alumina system. In general, the simulations compare well with the experiments except in the region near the surface, where experimental measurements show more significant egg-shell profiles. We believe this is due to the effect of film-breakage. During drying film-breakage occurs when the water content in the support is low (near the final period of drying). Once it occurs the back-diffusion stops so the metal cannot move from the surface back to the center. When the effect of film-breakage is included in the model, more egg-shell profiles can be predicted.

For high metal loading cases, crystallization may become important during drying because the metal concentration is likely to be above its solubility. We have extended our work to high metal concentration systems, and considered the effect of crystallization in our theoretical model. We find that crystallization can greatly enhance the egg-shell profiles by 1) increasing the amount of the metal deposited near the surface, and 2) decreasing the dissolved metal concentration near the surface and thus reducing the back-diffusion. The effects of parameters, such as the metal concentration, coefficient of crystallization rate, and drying temperature on crystallization are also considered. We also examined high metal concentration systems experimentally.