Characterizing an electrodialysis reversal pilot plant

Electrodialysis uses ion-exchange membranes to reduce the ionic content of water by applying an electrical field. An electrodialysis reversal (EDR) plant with two electrical stages, each with three hydraulic stages, was characterized over two ranges of salinity by desalting standard aqueous solutions prepared with sodium chloride. The parameters which characterize the working optimum of an EDR pilot plant are the values of the applied voltage used in the electrical stages and the feed water pressure corresponding to the maximum separation percentage and minimum energy consumption. These values were obtained from the surfaces corresponding to separation percentage and consumed power vs. applied voltage and pressure. On the other hand, the flux of ions removed by ED is limited by the concentration polarization at the interfaces between membranes and solutions. Limiting current densities in an EDR stack show critical values where boundary layer effects become important. In order to know the maximum current densities that can be used without polarization of the membranes, the relation between potential applied to the ED stack and current intensities was obtained for different values of feed water pressure. The surfaces corresponding to mean current intensity (I) vs. applied voltage (V) and pressure (P) were obtained, from which and for each value of P, the I-V curves were drawn. From the I-V curves, the limiting currents were obtained by a tangent method whose values were correlated with the values of the product flow.