Modeling cell pair resistance and spacer shadow factors in electro-separation processes

The electrical resistance of the stack is one of the most important parameters that affects the performance of reverse electrodialysis (RED1) and other electro-separation processes. The stack is made up of many cell pairs, each cell pair consisting of alternating anion and cation exchange membranes separated by spacer-filled channels which allow fluid to flow across the membrane surfaces, provide mechanical support and maintain intermembrane distance. The presence of non-conducting mesh in the spacer-filled channels increases the electrical resistance of the cell pairs; this so-called shadow effect can be characterized by the shadow factor (β) of the mesh. The shadow factor has not been determined experimentally so far. Furthermore, the extent of the shadow effect on the flow channels and membranes has not been investigated in much detail and the proposed analytical models in the literature are based only on assumptions. In this paper, we used a non-contact resistance (NCR2) method to measure the resistances of mesh filled spacer channels, membranes and sandwiched unit cells separately in high (1 M) and low (0.1 M) salinity NaCl solutions. The shadow factors of four different spacer mesh samples were determined experimentally. The existing methods of modeling the cell pair resistance were evaluated and the most suitable functional form for the resistances in series (RIS3) model was determined. While the mesh shadow factors were found to be independent of solution concentration, the membrane resistances were found to decrease with an increase in solution concentration. The mesh shadow factor was found to have little influence on the active area of the membranes. The NCR method could expedite the process of down selecting appropriate low resistance spacer meshes and membranes for the RED process. The validated RIS model can be used in techno-economic models for designing electro-separation processes for commercial scale plants.