Effective diffusivities in catalyst pellets: new model porous structures and transport simulation techniques

Abstract

Compressed and sintered porous solids are simulated by random-loose aggregates of spheres that are distributed in size and partially overlapped to achieve the required porosity. The resulting porous networks closely capture the morphological details of diffusing channels within granular materials commonly used as catalyst supports. Effective diffusivities in these model solids are calculated by Monte Carlo techniques that allow the probing of representative regions of the void space throughout the Knudsen, transition, and molecular diffusion regimes. Simulated diffusivities and tortuosity factors are in excellent agreement with experimental observations. These simulations also allow the calculation of accurate pore-size distributions and of transition-region diffusivities, previously estimated by simple geometric arguments and by the Bosanquet approximation, respectively. Mean pore radii calculated from surface area ( S) and porosity (Г) data (r p = 2Г A/ S) closely resemble the exact values obtained in our simulations for compressed solids but less so for sintered materials. Our simulations show that tortuosity factors, when properly defined and calculated, are intrinsic properties of porous solids, and identical in the Knudsen and molecular diffusion regimes.