Mass transport by surface and pore diffusion in drying processes

The effective diffusivity of Won clay was obtained by considering the mass transfer mechanisms occurring in the pores and on the surface. The binary friction model was used to evaluate the effective gas diffusivity and the Maxwell-Stefan approach to mass transfer was applied to the surface diffusion. Different expressions for the effective diffusivity for porous solids were used considering parallel diffusion, series diffusion and the Clausius-Mossoti model. Numerical results were compared with exptl. data and showed that the Maxwell-Stefan approach, used earlier for surface diffusion in microporous solids, is also a strong tool for evaluating surface diffusion of adsorbed species as bound water. In the pore diffusivity, when both present, capillary flow showed dominance over gas diffusion. The best results were obtained for the parallel model, considering the occurrence of discontinuous capillary flow.