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.

1. The characterization of an EDR plant by means of the determination of the working optimum allows us operation under the most suitable conditions with maximum efficiency. 2. This characterization can be affected by the surfaces corresponding to separation percentage and consumed power vs. applied voltage and pressure, whose study reveals the existence of a maximum for the separation percentage and minimum energy consumption, which also correspond to the minimum cost. 3. Limiting current densities in an EDR stack shows 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. To do this, 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. 4. In the case of our pilot plant, Aquamite I, the characterization was accomplished by the use of standard aqueous solutions of NaCl. For the low salinity range (<6000 ppm) and mean salinity range (6000–8000 ppm), the pressure sweep was between 0.4 an 1.8 atm and the optimum voltages were at 60 and 70 V respectively, with a feed water pressure of 1.2 atm in both cases and a cost of $0.17 and $0.25/m3 product water.The limiting currents were over the range [0.91–1.86] A and [1.26–2.66] A, respectively, at 70 V for low salinity and 60 V for mean salinity. 5. The limiting current densities (ilim) were correlated with the values of the product flow (Qp) that enables us to establish the relationship ilim = α Qβ where β ≅ 0.5. p