Analysis of RED/dRED stack performance using a resistances in series model

Abstract

In this study, reverse electrodialysis (RED) and driven RED (dRED) experiments were performed for a wide range of conditions, encompassing different inter-membrane distances (or spacer thicknesses), spacer shadow factors, salinity gradients and flow velocities. A resistances in series (RIS) model was proposed based on average concentrations in the stack chambers and tested to describe voltage-current relationships under the above conditions. Membrane resistances were modeled to be a function of solution concentrations as determined by non-contact resistance (NCR) experiments. The potential of RED and dRED was evaluated for demineralizing SWRO brine. The RIS model showed reasonable agreement with experiments under most conditions, consistent with a picture where membrane and spacer resistances are purely additive. The range of the RED region, where mass transfer is spontaneous, was significantly enhanced on the voltage-current curve upon using the 0.11 mm thick spacer “type C” that was characterized by an open mesh having a low shadow factor of 1.13. A maximum gross power density of −2.45 W/m2 was achieved using 0.9 M and 0.02 M NaCl solutions in the concentrate and dilute streams respectively, the highest reported so far for similar salinity gradients. The process feasibility was examined from the point of view of net power density and operation at short circuit currents. This data can be used for designing electro-membrane stacks and operating them under optimum conditions for minimizing the capital and operating expenditure of a RED/dRED plant.