Simultaneous Vapor and Liquid Diffusion in Partially Wetted Porous Media

In porous materials with pores containing both a gas and a liquid phase each a mixture of several components mass transfer by diffusion may occur simultaneously in both phases. An effective diffusivity is required in order to describe these effects when a homogeneous model is used. Unsteady-state diffusion experiments were performed such that a bed of glass beads partially saturated with glycerol was either loaded by passing wet air or dried by removing water or 2-propanol initially mixed to the glycerol. The effective diffusivity was obtained by applying the equation of unsteady-state diffusion to the observed change of mass. Superposition of the limiting cases of single-phase vapor and liquid diffusion in partially saturated systems yields a simple equation that describes the effective diffusivity as a function of pore saturation and correlates well with the measured values.