Diffusional flow for surface area measurement

The rate of transfer of a diffusing substance through unit cross-sectional area is proportional to the diffusion gradient and is given by Fick s laws of diftoion [40]  

For uni-directional flow into a fixed volume, the increase in concentration with time is given by  

If one face of the powder bed is kept at a constant concentration, i.e. infinite volume source (C = C2 at x = 0), while at the other face the initial concentration (C ifOj at x = L, t = 0) changes, i.e. fixed volume sink, a finite time will pass before steady state conditions are set up and  

The two diffusion constants are not necessarily the same. Absorption into pores can take place during the unsteady state period so that the pore volume in the two regimes may be different Grafdlis of outlet pressure pi against time can be obtained at various fixed inlet pressures P2- These will be asymptotic to a line of slope  

These lines will intersect the line through pi(0) and parallel to the abscissa at time  

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