Reactions with an interface Mass and heat transfer effects

The process can be broadly classified as bulk diffusion, Knudsen diffusion, and surface diffusion. The molecular driving force of diffusion is the chemical potential difference created by a local population of a chemical species. Molecules tend to distribute uniformly across the space while migrating among like or unlike molecules. Bulk diffusion is the predominant mechanism when the pressures are high and pore sizes are large. On the other hand, at lower pressures, Knudsen diffusion prevails, when the mean free path of the molecules are larger than the pore size. When the molecules are adsorbed strongly on the pores or the pore sizes are too small, the mechanism of diffusion becomes surface diffusivity. 

Diffusivities in gases Binary gaseous diffusion coefficients are important parameters in the design of reactors for two-phase reactions involving a gas and a liquid or solid (either as catalyst or as reactant). The recommended equation for low pressures is a modified form of the theoretical Chapman-Enskog equation, but a more readily usable equation is (Gilliland, 1934) 

Diffusivities in liquids We restrict the treatment here to binary mixtures of, say, A and B in which A diffuses in B at infinite dilution, that is, at very low concentrations of A in B. Reference may be made to Reid et al. (1987) for correlations at high concentrations. 

All correlations proposed for estimating the diffusivity at infinite dilution are modifications of the original Stokes-Einstein equation 

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