Using Temperature Gradients to Increase Selective Transport through Ion Exchange Membranes

Electrodialysis (ED) and Reverse Electrodialysis (RED) are established techniques based on a stack of charge selective membranes, for the separation of ions from brackish water and the power harvesting from salinity gradients respectively. Mass transfer limitations, mostly diffusive, limit the operating windows and recovery rates of these processes. Most ED processes are operated in the so-called Ohmic regime, where the current increases linearly with increasing the applied potential. Above a critical current, the transport becomes limited by diffusion effects and the ‘limiting-current’-regime is entered. Low grade waste heat from industrial sources is widely available and can be used to generate temperature gradients in ED and RED systems. Temperature has a significant influence on the diffusivity of ions in solution, and can yield viscosity changes resulting in a change of separation efficiency. After initial numerical work indicating the possible enhancement of selective transport through a charge selective interface in the presence of a temperature gradient, experiments were conducted using a commercially available, lab-scale electrodialysis stack.

The influence of temperature and temperature gradients on the transport of different ions was measured for both RED and ED, in different operating regimes. For ED, using a 1:1 electrolyte (NaCl), a clear increase in total ion transport was measured when increasing the temperature of either the depleted or enriched stream by 20˚C when compared to having both streams at a lower temperature. The energy required for the ED process was reduced by 9% when heating one of the feed streams. Increasing the temperature in both streams enhanced the total transport even further and reduced the required energy input for the ED process by 15%. However, in contrast to our expectations, no significant change in selective transport was measured for any of the different temperature configurations in the linear, Ohmic regime. The reduced power input is mainly attributed to the increasing diffusivity of the ions at elevated temperatures, and the viscosity of the solutions was found to be of less influence.

In the limiting current regime, where the depletion boundary layer is dictating the overall transport through the stack, the influence of a temperature gradient is more pronounced. We find that the direction of the temperature gradient influences the total transport of ions and the selectivity towards transport. A significant improvement of the total transport is found when the depleted stream is heated by 20 ˚C, when compared to heating the concentrated stream. Additionally, for systems containing a mixture of monovalent (Na⁺) and divalent (Mg²⁺) ions, a change in selectivity was found when applying a temperature gradient. The selectivity favored the separation of divalent ions when the dilute stream was heated when compared to isothermal systems. This is attributed to competitive transport of the mono- and divalent ions that is influenced by their different response on temperature.

The results obtained in these experiments indicate that the application of temperature gradients has a positive influence on the required power for electrodialysis systems and can be of use in enhancing the selective transport between mono- and divalent ions. This is of potential use in industrial systems where heavy metals must be selectively removed and in water softening technologies exchanging divalent for monovalent ions. The utilization of industrial waste heat, available from various processes and at various locations, for the application of a temperature gradient is a promising method for the improvement of overall process efficiency.