On the interaction of electroconvection at a membrane interface with the bulk flow in a spacer-filled feed channel

Abstract

The operational range of electro-driven processes can be extended by (i) reducing diffusion resistances at the membrane/electrolyte interface using spacers and (ii) by electroconvective mixing of the boundary layer at overlimiting current densities. Although many studies have addressed each of these phenomena separately, the details of the interplay of spacer-engineered hydrodynamics and electroconvection are unexplored. In this study, we establish a methodology of a new membrane cell design and the use of a high-speed microparticle tracking velocimetry setup for direct observation and quantification of the 3D velocity field in a spacer-filled electrodialysis channel under process conditions relevant for real electrodialysis applications, i.e., overlimiting currents and elevated Reynolds. We unravel details of the superposition of electroconvective vortices and forced flow velocity with and without an inserted spacer. Overall, the forced flow flattens the vortex structure of electroconvection at increasing Reynolds numbers. In terms of vortex heights, a spacer with intentional dead zones outperforms a commercial spacer that decreases the boundary layer thickness. This study suggests a counter-intuitive spacer design: to exploit electroconvection, an engineered spacer design for maximum use of electroconvection in the far overlimiting region should aim to create intentional regions of low flow velocity.