Diffusion Through a Porous Material

In a steady-state situation when gas flows through a porous material at a low velocity (laminar flow), the following empirical formula, Darcy s model, is valid  

From Darcy s equation we can determine a formula for the counterforce produced by the porous material to the flowing or diffusing component A, If this counterforce is found, it can be added to the diffusion resistance force caused by component B to component A hence the sum of these two forces represents the total diffusion resistance. 

For a porous material the linear momentum equation can be written as 

Equation (4.303) is valid but it is lacking something. The resistance force that applies to the component A has to be found, and not that for the whole mixture. The force applying to the whole mixture is the sum of the 

Assuming that the force is divided along the ratio of the mass flows, Eq. (4.303) gives 

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