Using feed-forward voltage-control to increase the ion removal rate during batch electrodialysis desalination of brackish water

Abstract

Batch electrodialysis (ED) desalination, which relies on diluate recirculation to produce a desired product, is often conducted at constant voltage. Here we show that constant-voltage operation under-utilizes membrane area because the applied current is much lower than the limiting current early in a batch cycle. Time-variant voltage-control, targeted at raising the average ratio of applied to limiting current during a batch cycle, can therefore increase the rate of ion-transfer achievable using a fixed membrane area. We designed a feed-forward voltage controller, which provided within −15 to +20% of the desired current, and used it to raise the production rate by up to 37% ± 2%, relative to constant-voltage operation, without exceeding the limiting current density. Furthermore, an analytical prediction of the batch completion times was derived and validated under varying feeds (1500, 2000, and 3000 mg/L), products (200, 300, and 500 mg/L), and flow velocities (4.3, 6.4, and 8.5 cm/s). Supported by experiments, the predictive model indicates that time-variant voltage-control can provide the greatest increase in production rate at high feed-to-product concentration ratios and low flow velocities. This work will assist designers and operators seeking to size, evaluate, and maximize the production performance of new and existing batch ED processes.