A unified network model is formulated for predicting effective Fickian
diffusivities of condensable vapors in porous media where capillary c
ondensation and adsorption-desorption hysteresis occur The model unifi
es the equilibrium theory based on the pore-blocking interpretation of
hysteresis in the interconnected network of pores and the percolation
model of mass transport in the network with randomly interspersed reg
ions for capillary condensation and surface flow. The Bethe network is
used to represent the porous medium, and the effective medium theory
is employed to obtain the effective diffusivity. Using the information
on the connectivity and the positions of the closure points of the hy
steresis loop enclosed by the equilibrium primary adsorption and prima
ry desorption isotherms, the concentration dependence of the effective
diffusivity is predicted. The model is applied to the systems water v
apor-silica gel, water vapor-activated alumina, and literature data. I
t successfully predicts the concentration dependence of the effective
diffusivity in the whole range of relative pressures for systems both
with and without a peak in the diffusivity.