Coupling between external and internal mass transfer during drying of a porous medium

We present a pore‐network model for drying processes in porous media that couples convective‐diffusive mass transfer over the external surface of a porous medium with mass transport mechanisms within the porous material, namely, the flow through liquid films that form at the pore walls and diffusion of the liquid vapors through the dry pores. We study the effects of the external boundary layer thickness, a film‐based capillary number, and the Peclet number of the purge gas that flows over the porous medium surface on the shape of the drying curves and the overall recovery times. We show that the drying rate remains practically constant, as long as the liquid films span across the entire pore network and provide hydraulic connectivity between the bulk liquid front and the product surface. This condition is satisfied when the drying process is controlled by the external mass transfer over the porous medium surface, rather than mass transport (through films and diffusion) within the porous material, e.g., for large values of the boundary layer thickness and small values of the film‐based capillary number. Our results explain previously reported experimental findings and provide a rigorous explanation of the experimentally reported constant‐rate period.