Electrodialytic Desalination at High Currents – Facilitation of Electroconvection by Synergetic Use of Spacers and Membrane Surface Modification

• Applicant: Professor Dr.-Ing. Matthias Wessling
• Subject Area: Chemical and Thermal Process Engineering ; Mechanical Process Engineering
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 518118073

Electrodialysis is an important membrane-based desalination process to treat salty water resources and produce drinking or process water. Depleted and enriched streams are produced in alternating compartments divided by anion and cation-exchange membranes. In the depletion compartment, strong concentration gradients in the laminar boundary limit the transport of ions, recognized as the limiting current density. Therefore, at a given applied potential, one would want to minimize resistances and maximize the current density by increasing the local turbulence and depolarizing the diffusion boundary layer. In the currently running project, we gained novel and deep insight into the interplay of electroconvection, convection induced by spacers and water splitting at high current densities. Macro- and micro-scale experimental work resulted in an understanding of the intricate details of forced flow hydrodynamics interacting with space-charge induced convection, i.e., electroconvection (EC). Two main challenges became apparent from the implementation of spacers: (1) increasing resistance due to the lack of conductivity of the spacers, (2) decrease in the facilitating effect of electroconvection on ion transport due to the suppression by transverse forced flow along the surface of the membrane. Based on the understanding obtained from the current project, the proposed follow-up project aims to overcome these challenges by tailoring the spacers' conductivity and the membranes' surfaces to fully control electroconvection in the boundary layer. We propose a flow simulation-guided comprehensive methodology to design spacers and membranes in the union to comprehend and utilize electroconvection. To overcome the drawback of increased resistance due to the implementation of spacers, the first means is to fabricate two different ion-conductive spacers: this is realized by either (i) coating of spacers with conductive material or (ii) 3D printing to novel spacers using porous materials. A second means to influence the break-up of the diffusion boundary layer (BL) are 2D-printed or profiled membrane surfaces. The surface modifications trigger and control the evolution of electroconvection in the BL. We hypothesize further that a balanced integration of both means synergistically maximizes the contribution of EC under realistic hydrodynamic conditions. The project will deliver deep insight-based design guidelines for fluid flow control to achieve optimal electrolyte mixing at the lowest cell resistance and suppressed water splitting.

• DFG Programme: Research Grants