Probing internal structures of FCC catalyst particles: Fromparallel bundles to fractals

The phenomena of transport and reaction in the Ractor-Regenerator Cycle of FCC processes rely onthe large surface area available in the porous catalyst particles. Most of this area resides in randomly interconnected sub-micron cavities within the particles. A good understanding of configuration of the pore space is essential for relable modeling of cracking reactions as well as the catalyst deactivation and regeneration. Earlier approaches have utilized mercury porosimetry in conjunction with the Washburn equation to represent internal voids as parallel bundles of cylindrical pores. But the failure of these models to account for important geometric parameters, such as torbuosity, connectivity and morphology expected of the complex porous framework, renders them highly unrealistic. A number of alternative structural configurations have been developed incorporating various degrees of randomness to provide more realistic visualization of the chaotically oriented cavities. Beginning with intersecting versions of the parallel bundles, work has progressed through corrugated cylindrical pores, towards 2-D and 3-D stochastic pore networks and more recently to fractals. In this paper, a qualitative overview of hierarchical developments in pore space representation and quantification will be presented. Emphasis is given to stochastic pore networks and fractal geometrical concepts.