(570y) Belt Drying and Microwave 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 design. There are four main categories of metal profiles, uniform, egg-yolk, egg-shell and egg-white. The choice of the optimal metal profile in the support is determined by the required activity, selectivity, and by other characteristics of the chemical reaction (kinetics, mass transfer). Usually, a procedure of preparation of supported catalysts includes three steps: 1) impregnation, 2) drying, and 3) calcination and reduction. It is generally believed that the metal profile is controlled by the impregnation conditions. However, experiments have shown that drying may also significantly change the metal distribution obtained from impregnation. Therefore, to achieve a desired metal profile we need to understand both impregnation and drying.

In industry, catalysts are usually dried on conveyor belt dryers. During belt drying, the relative humidity (H) and the temperature (T) in the drying air change gradually. To simulate this procedure, 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. First, our study focuses on drying of a single pellet catalyst. The metal distribution after drying shows a significant difference between constant drying conditions (the drying temperature and humidity in the air are held constant during the drying process) and belt drying conditions (the drying temperature and humidity in the air varies during the drying process). Then, we extend our work to triple layer catalysts. The drying conditions in upper layer catalysts are based on the heat and mass transfer between the catalyst particles and the bulk drying air in lower layers.

During microwave drying, the catalyst samples can be heated volumetrically so the hear transport is eliminated and the drying rate increases. Microwaves drying can provide rapid and uniform drying within the porous support, leading to uniform evaporation of the liquid solvent, which most likely results in a uniform metal distribution. We have carried out experiments for both microwave drying and regular drying in an oven, and find that the microwave drying gives us a much more uniform metal distribution. The effect of the energy intensity on the microwave drying process is also considered in this work.