EFFECT OF CONVECTIVE BOUNDARY-LAYER ON THE CURRENT EFFICIENCY OF A MEMBRANE BEARING NONUNIFORMLY DISTRIBUTED FIXED CHARGES

The current efficiency eta of a planar membrane bearing nonuniformly d istributed fixed charges is analyzed theoretically. In particular, the effect of the convective boundary layer near the outer boundary of th e membrane is discussed, Two types of fixed charge distribution are co nsidered: a class of linear distribution and a class of exponential di stribution. We show that, if the flow velocity in the membrane phase i s small, increasing the inhomogeneity of fixed charge distribution is capable of raising eta. However, if it is appreciable, the reverse is true. If a boundary is present, eta decreases with the increase in cur rent density the reverse is true if it is absent. The former is due to the fact that the degree of concentration polarization in the boundar y layer increases with current density, Although the existence of a bo undary layer has the effect of reducing eta, the convective flow of fl uid perpendicular to membrane has a positive effect on eta. If a bound ary layer is absent and the fixed charges are distributed homogeneousl y, eta is independent of current density I; if the boundary layer is p resent, eta will vary with I.