The Effect of Operating Conditions on the Dispersion State of Supported Metal Catalysts: A Model Study

As an important step in the preparation of supported metal catalysts, the drying of impregnated porous medium is studied by a mathematical model including catalyst-scale mass and heat balances and particle-scale population balance. Both the loading profile and the particle size distribution for the precursors are considered. Calculations show that the combined effect of two migration mechanisms, intrapellet diffusion in the centripetal direction and convection in the centrifugal direction, determine the catalyst structure. Both physical and operational parameters are studied: the former characterize the physical preference toward one migration mechanism, and the latter give rise to the driving forces. Four structure regions are found in the operational space. For some systems with strong preferences, certain regions cannot be accessed in practice. The particle size distributions and hence the surface area for each region are also studied. Studies show that a uniform loading profile could be obtained under very low and very high temperatures, but the particle size resulting from high temperatures is much smaller, implying higher activity and lower thermal stability. In the case of high Thiele numbers, use of egg-shell catalyst may reduce thermal deactivation by having less active material in the inner part of the catalyst pellet where the temperature is much higher. This improvement is compromised by smaller particles, which are always formed in the center during the preparation.