Current-voltage curves for an electrodialysis reversal pilot plant:: determination of limiting currents

Ion-exchange membranes are used in electrodialysis (ED) for desalination of aqueous electrolyte solutions. The flux of ions removed by this process is limited by the concentration polarization at the interfaces between membranes and solutions. According to the classical theory of current polarization for ion-exchange membranes, the current should increase linearly at low voltage, then increase more slowly and finally reach a "plateau" characterizing the so-called limiting current (I-lim). This paper aims at obtain the limiting current densities for an ED reversal pilot plant. The ED reversal stack in question was characterized over two ranges of salinity by desalting standard aqueous solutions prepared with sodium chloride. With the same fixed working voltage for both electrical stages, a feed water pressure (P) sweep was carried out over the range 0.4 to 1.8 atm. The applied voltage (V) was varied between 30 and 110 V, with steps of 10 V. The current intensity I was read at each stage for every electric voltage and feed pressure value, and the mean value for I was calculated, With these data, two characteristic surfaces (P, VI) for the plant were obtained, from which the I-V curves were drawn for each value of P. From these curves, the limiting current densities (,,,) were obtained by a tangent method. Fitting the i(lim) values vs product flows (Q(p)) allowed us to establish a relationship between both magnitudes: i(lim) = aQ(p)(beta) with beta congruent to 0.5 for the considered salinities. The results are consistent with dimensional analysis characterizing our ED system and allow us to determine the hydrodynamic regime in the studied range of solution flow velocities from the hydraulic resistance factor as a function of the Reynolds number.