Flowpath modification to reduce electrodialysis module size and energy consumption

Abstract

Thermodynamic equipartition of ideal electrodialysis (ED) through the time-varying batch operation that maintains a constant current or instantaneous entropy generation rate has been proposed in the literature to reduce its specific energy consumption at fixed module size. In this study, the optimal current density distribution to minimize the specific energy consumption of ED in the presence of diffusive salt and water fluxes is evaluated. The local current density must increase as the salinity difference between the diluate and concentrate streams widens to reduce the impact of diffusive losses, but maintaining such a current density variation requires adjusting the applied potential continuously. As a practical alternative, we propose and numerically evaluate a novel recirculation flow architecture to improve the energetic performance of steady-state ED while using a conventional constant voltage power source. Like other methods that implement equipartition, maximum savings are obtained at a large baseline system size. Unlike multi-staging, no further savings are observed by adding additional recirculation cell pairs beyond two passes. A modified channel aspect ratio to maintain feed flow velocity in the channels is proposed to keep overall pressure drop and pumping energy similar to the baseline design. Energy (at fixed size) and area (at fixed energy consumption) can be reduced compared to conventional single-stage ED by up to 5% and 25% respectively for seawater brine concentration, and 35% and 55% respectively for seawater desalination.