Drying of supported catalysts for low melting point precursors: Impact of metal loading and drying methods on the metal distribution

Abstract

Supported metal catalysts are used in many industrial processes. Experiments have shown that the metal distribution within the support may be greatly affected by the drying conditions. In this work, we have developed a theoretical model to predict the drying process for high metal load conditions; this was accomplished by building upon a model that was established for low metal loadings. A number of additional phenomena must be accounted for under high metal loadings including the effect of metal precursor concentration on the solution density, solution viscosity, water vapor pressure and solution surface tension. The model predictions of the metal distribution are compared with experimental measurements for a Nickel/Alumina catalyst. It is found that the drying mechanisms for low metal loading conditions and high metal loading conditions are quite different. The constant drying rate stage observed for low metal concentrations does not exist for high metal concentrations. For low metal loadings, the adsorption strength and distribution of liquid in the support must be considered in the model in order to get good experimental agreement. For the conditions we considered, an egg-shell profile was observed for low metal loadings, and the egg-shell profile was enhanced with an increase in the initial metal precursor concentration. For a metal precursor with a high concentration and a low melting point, the metal precursor is dissolved or molten in the liquid phase during drying. In this case, adsorption strength and distribution of liquid in the support are not important, and the metal distribution after drying reaches a nearly uniform profile if the initial metal concentration is above a critical value. Both microwave drying and regular oven drying have been examined in this work. During microwave drying, the catalyst samples can be heated volumetrically so microwave drying resulted in a significantly more uniform metal distribution than regular oven drying.