Concentration polarisation in diffusive membrane separations

Ac steady state the flux equations of component i through each phase are equal. The flux of component i in the boundary layer of the feed is given by 

In many books and articles the membrane phase contribution C/Pj is frequently indicated as 1/Pj (i.e. C/Pj = l/p,v()- 1/pM has the dimension of a mass transfer coefficient while Pj (= Dj.K) is defined as the permeability coefficient. 

This concept of resistances in series can be applied to various processes (see also chapter V6.2 and VI.4.4.1) and the approach is to determine the mass transfer coefficients by means of the semi-empirical relationships given in table VII. 1 (see also ref. [ 1 ]). When the resistances in the boundary layers are small compared to that of the membrane resistance the permeation rate is given by eq. VII - 49. 

Although the driving forces, the. separation principle and the membranes are completely different in elecirodialysis from those in pressure-driven membrane processes, polarisation phenomena may severely aflect the separation efficiency. 

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