Proceedings of ICAD2009 The Fifth International Conference on Axiomatic Design

Cost-effective desalination of saline water, which is more energy efficient than the widely used reverse osmosis (RO) and evaporation processes, is desired to solve the growing freshwater crisis. In recent years, capacitive deionization (CDI), which is based on the principle of electrosorption of ions on charged high surface-area electrodes, has been reported as a promising technology for desalting brackish water, because of its low power requirements. Despite its intrinsic advantages, the low water recovery ratio of CDI, limited plant efficiency and throughput has hindered its development into an industrial process. These problems are a direct consequence of the coupling that exists between the charging and discharging processes. There exist three primary functional requirements for the complete CDI process whereas only two top-level design parameters are utilized to address these. Using axiomatic design principles, we propose a novel capacitive deionization process employing permeating flow discharge (PFD), which adds a new design parameter (solvent drag) thereby decoupling the functional requirements. In the proposed scheme, waste water is permeated through the porous electrodes during the discharging process in contrast to the flow in-between the electrodes employed for the AFD process. The underlying principle is that the rate of removal of ions by solvent drag in PFD is significantly greater than by convection-diffusion in AFD. Using a bench-top CDI module, we show that the proposed solution removes the unavoidable coupling present in the AFD scheme. Furthermore we demonstrate a reduction in the discharge time by a factor of two resulting in an approximately 30 % increase in the throughput of the CDI process