Pore network modeling of convective drying

Abstract

Drying of porous media is a process of significant scientific and applied interest. It involves several mechanisms at the pore scale that affect the macroscopic behavior of the drying process. These include phase change at the liquid-gas interface, mass and heat transfer by diffusion and convection, capillarity-induced flow through wetting liquid films and the receding of the liquid gas interfaces under combined viscous, capillary and buoyancy forces. In most typical applications, porous materials are subjected to a flow of a purge gas along the external porous surface, which can significantly enhance the recovery process and reduce drying times. The local mass transfer coefficient depends on the mass transfer conditions within the convective layer over the surface as well as on the liquid saturation at the surface. For a realistic solution of the drying problem, mass transfer in the convective layer needs to be solved in conjunction with mass transfer within the porous medium. This paper presents a coupled pore-network model that accomplishes such a solution. The model accounts for isothermal evaporation at the liquid-gas interface, mass transfer by diffusion and flow through liquid films within the porous medium and convective mass transfer through the convective layer over the surface. We study the effect of velocity profiles in the convective layer, the Peclet number of the purge gas and the thickness of the convective layer on the drying curves. Our results show that the drying rate remains practically constant as long as the liquid films span along the pore network and their density at the surface is sufficiently high. Our results explain previously reported experimental findings and provide a rigorous explanation of the constant-rate period.