Current distribution along electrodialysis stacks and its influence on the current-voltage curve: behaviour from near-zero current to limiting plateau

The current maldistribution along the flow path of electrodialysis (ED) units is a well-known phenomenon. However, it has been poorly quantified via experimental tests so far. This work aimed at measuring the current distribution in an ED stack with 40 cm path length, provided with four-segment electrodes. The current-voltage curve (CVC) of each segment was recorded under several operating conditions of inlet concentrations and feed velocity. In the CVC of the first segment, the current increased approaching a saturation, while in that of the remaining segments it passed through a maximum and then settled at near-zero values when high voltage values were applied. The current fraction on the segments decreased from inlet to outlet. In the overall CVC's first region, up to ~50 % of total current flowed through the first quarter of stack. As the voltage increased, the current fraction increased in the first segment, while it decreased in the other ones tending to vanish. At the limiting condition, the relative standard deviation of the current fraction distribution ranged from 40 % to 68 %. This strong non-uniformity in the ED stack, which may represent one stage of an industrial plant, implies that the design of commercial units must be improved.