Concentration polarization controls the performance of practical electrodialysis systems. Because ions selectively permeate the membrane, the concentration of some of the ions in the solution immediately adjacent to the membrane surface becomes significantly depleted compared to the bulk solution concentration. As the voltage across the stack is increased to increase the flux of ions through the membrane, the solution next to the membrane surface becomes increasingly depleted of the permeating ions. Depletion of the salt at the membrane surface means that an increasing fraction of the voltage drop is dissipated in transporting ions across the boundary layer rather than through the membrane. Therefore the energy consumption per unit of salt transported increases significantly. A point can be reached at which the ion concentration at the membrane surface is zero. This represents the maximum transport rate of ions through the boundary layer. The current through the membrane at this point is called the limiting current density, that is, current per unit area of membrane (mA/cm2). Once the limitingcurrent density is reached, any further increase in voltage difference across the membrane will not increase ion transport or current through the membrane. Normally the extra power is dissipated by side reactions, such as dissociation of the water in the cell into ions, and by other effects. Concentration polarization can be partially controlled by circulating the salt solutions at high flow rates through the cell chambers. But even when very turbulent flow is maintained in the cells, significant concentration polarization occurs.
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