PREDICTING BINARY FICKIAN DIFFUSIVITIES FROM PURE-COMPONENT FICKIAN DIFFUSIVITIES FOR SURFACE-DIFFUSION

A kinetic theory is derived for predicting binary Fickian diffusivities from pure-component Fickian diffusivities for surface diffusion. The theory is simple to use, but requires data on concentration-dependent pure-component diffusivities and heats of adsorption. The theory also illustrates the origin of the concentration dependence of surface diffusion. The theory has been compared with the experimental data available from the literature on diffusion in zeolites, and the comparison is surprisingly good in view of the simplicity of the theory. It is shown that the cross-term fluxes in the binary diffusion formulation make significant contributions to the total flux, and that the use of single-component diffusion formulation can cause large errors for binary diffusion.