553 MONTE CARLO SIMULATIONS OF EFFECTIVE DIFFUSIVITIES IN THREE-DIMENSIONAL PORE STRUCTURES Sebastian C. Reyes*, Enrique Iglesia*, and Yee C. Chiew** *Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, NJ 08801, **Department of Chemical and, Biochemical Engineering, Rutgers University, Piscataway, NJ 08855. ABSTRACT A hybrid discrete/continuum Monte Carlo technique combining random walk simulations with first passage time (FPT) concepts is developed here in order to estimate diffusion properties of randomly-assembled sintered porous structures. This work combines the creation of realistic porous solid structures with controlled pore size, shape, and tortuosity features with the application of an efficient algorithm for calculating effective diffusivities in all diffusion regimes (Knudsen, transition, and molecular). The hybrid simulation technique consists of creating a "protective" boundary layer surrounding solid surfaces within which discrete random motion simulations are performed while continuum FPT results are used in the remaining pore space. The boundary layer thickness reflects a characteristic length scale, of the order of a few mean free paths, over which the FPT approximation breaks down. This procedure significantly reduces the computations required to cover statistically representative regions of the porous structure, a serious shortcoming in previous studies of gas diffusion through porous solids; it leads to effective diffusivity estimates that are in excellent agreement with experimental measurements. INTRODUCTION Chemical reaction rates within porous materials are often controlled by reactant and product diffusion. Heterogeneous catalysis and non-catalytic gas-solid reactions are two important areas where diffusion plays a crucial role. Many obstacles currently prevent the a-priori estimation of effective diffusivities in porous materials. For example, several important geometrical and topological properties, such as local fluctuations in pore size, shape, and pore space connectivity, cannot be directly measured with available characterization techniques. These limitations have resulted in many simplified structural pore models. They consist mostly of cylindrical capillaries, randomly interconnected, unconnected, or obeying some prescribed topological rules [1]. These models do not rigorously include dead-ends, irregular pore branching, and fluctuating pore dimensions that are typical of porous materials. Therefore, in order to describe experimental data, these models require adjustable parameters, usually in the form of non-rigorous tortuosity factors, thus defeating the purpose of providing an a-priori estimation procedure. The creation of more realistic three-dimensional models of porous solids together with Monte Carlo simulations of diffusion were pioneered by Evans and co-workers ([2] and references therein). Advances on surface and pore volume characterization continue to provide great opportunities for improving the accuracy of the simulated structures. Moreover, increasing computational capabilities allow the incorporation of greater levels of Mat. Res. Soc. Symp. Proc. Vol. 195. ©1990 Materials Research Society