On the modelling conditions of mass transfer in porous media presenting capacitance effects by a dispersion-convection equation for the mobile fluid and a diffusion equation for the stagnant fluid

The modelling of mass transfer in porous media presenting capacitance effects by a dispersion-convection equation for the mobile fluid and a diffusion equation for the stagnant fluid has been shown (Piquemal, 1992) to be erroneous in the general case, because it is assumed that the mean concentration of the flowing fluid equals the point concentration at the boundary of the stagnant fluid. This boundary condition cannot be realized. This paper gives the conditions that allows the use of this kind of model with an acceptable approximation. The problem has been solved in the case of two schematic structures: the first is a cylindrical tube with stagnant pockets in its wall, the second a stratified medium. The characteristic lengths of the mobile and immobile domain must obey a criterion in which the porous medium characteristics and the flow velocity appear.