Limiting concentration during batch electrodialysis process for concentrating high salinity solutions: A theoretical and experimental study

Electrodialysis (ED) has demonstrated favorable concentration performance and good energy-saving potential in the treatment of high salinity solutions. In this work, the study is focused on the investigation of the limiting concentration (maximum concentration) during a batch ED concentration process. A theoretical model was built to describe the coupling of salt and water transport and the limiting concentration. Effects of initial concentration, volume ratio of the diluted to concentrated solution, and current density on performance were investigated with two commonly used LiCl and NaCl solutions. The results showed that the salt concentration had a significant effect on the salt transport number and salt hydration number while the current density exhibited negligible. The limiting concentration could be increased significantly by increasing the current density. However, the trade-off between the limiting concentration and energy consumption should be balanced. Furthermore, the ideal and system concentration characteristic curves were proposed to provide theoretical guidance for the batch concentration process. According to the system curve, the three-stage continuous operation was required for NaCl solution to reach saturation at an initial mass concentration of 3.5% and a current density of 400 A/m2. The proposed methods in this study would be instructive for future design of relative systems such as the liquid desiccant regeneration system.