Effective diffusion coefficient in random porous media

Diffusion in porous media is an important issue in many fields of engineering, and the calculation of the effective gas diffusion coefficient as a function of the micro-morphology of porous solids is crucial in the design and analysis of heterogenous catalyst layers. Advances in computer technology have now made it possible to better represent 3-dimensional micro-morphologies in porous solids, and a new method for predicting the effective gas diffusion coefficient of porous media was presented in this paper. The 3-dimensional micro porous media were constructed using a redeveloped program based on ANSYS code, which allowed the control of micro-morphology features including pore sizes, pore orientations, interconnections of pores and agglomerate sizes. Using the program, a series of random-distributed porous solids with different porosity were generated and used in conjunction with finite element solutions of the gas diffusion equation in order to calculate the effective gas diffusivities. Based on the simulation results and observations, a new correlation was proposed showing the relationship between the porosity and the effective diffusion coefficient. A percolation threshold was identified for random-distributed pore media. It was noted that the percolation threshold decreased with an increase of the connectivity of the pore network. The relationship between the effective diffusivity and porosity was strongly nonlinear when the porosity was less than a certain value. A formula relating the effective diffusion coefficient with porosity was proposed. 14 refs., 1 tab., 6 figs.