Theoreticals of the Process

The process can be considered as the flow of the diffusing substance through a membrane. If one face x = 0 of the membrane is kept at a constant concentration Cq and the other x = L, Q, is maintained at 0, and the membrane is initially free from this substance, the equation of diffusion 7.4 applies  

The amount of substance that emerges at time t from the external face, at x=L, is M,  

As the concentration of substance through the membrane varies with space x and with time t, the process is said to be run under transient conditions. 

After infinite time, or rather a time long enough so that the series in Equations 7.35 and 7.36 vanishes, the steady state is attained, and the simple equation is obtained  

For a gas or a vapor, the surface concentrations Cq and Q may not be known but only the gas or vapor pressures Pq and on the two sides of the membrane. The rate of transfer in the steady state is then sometimes written  

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