Modeling desalination and energy performance of membrane capacitive deionization



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The fresh water crisis due to population increase, climate change and pollution encourages the development of alternative water sources. A particularly attractive alternative is brackish groundwaters which are abundant and contain less salt than seawater. Capacitive deionization (CDI) is an emerging desalination technique for removing salts from low-salinity brackish water and is scalable and potentially applicable to distributed water treatment systems. By introducing a pair of ion-exchange membranes (IEMs) into conventional CDI to form membrane CDI (MCDI), the desalination rate, salt removal efficiency, and energy efficiency can be improved. This dissertation develops a physics-based two-dimensional process model for depicting ion transport and adsorption dynamics in MCDI, and uses that model to evaluate the effects of cell geometry, cell cycle time, and operating conditions on salt removal efficiency, energy consumption, and selective ion removal. A comprehensive CDI process model incorporating modified Donnan (mD) theory based CDI models was proposed to capture the effects of electrode surface charges, Faradaic reactions, solution pH, and various cell architectures on desalination performance and energy behaviors. A fully coupled two-dimension MCDI process model was proposed by incorporating the hydraulic dispersion effects in the channel and co-ion penetration in the IEM. This model was quantitatively compared to available data and applied to evaluating the differences between MCDI over CDI in rate of desalination and ion adsorption capacity. The effects of cell geometry, cell cycle mode and operating conditions on salt removal efficiency were explored and led to a conclusion that use of an IEM, increasing applied voltage, decreasing flow rate, prolonging electrode length and thickness, reducing dispersivity and channel thickness could enhance salt removal efficiency. An operating approach designed to maximize salt removal efficiency by rapidly switching between adsorption and desorption cycles (cut-off mode) was shown to have significant advantages compared to operating the system to equilibrium. The MCDI process model was applied to evaluating the energy performance of constant voltage (CV) mode MCDI under cut-off and equilibrium cycle modes. Cut-off mode was generally found to achieve higher specific energy consumption (SEC) and thermodynamic energy efficiency (TEE) compared to equilibrium mode. Pump losses were dominant in equilibrium mode especially with long cell length, while external resistive losses were dominant under high water recovery in cut-off mode. Energy storage in the CDI cell during desalination accounted for 20-40% of SEC. TEE decreased with increasing water productivity, and was competitive with RO for desalinating near-fresh water at high water recovery. Overall, constant voltage (CV) mode MCDI was more energy efficient under moderate water recovery compared to constant current (CC) mode MCDI. The MCDI process model was extended to capture ion exclusion effects and to predict ion selectivity in a multi-ion solution. Trade-offs between ion selectivity and ion removal efficiency were evaluated. Overall, MCDI was found to be feasible for partially softening industrial cooling tower blowdown water and slightly softening domestic tap water with moderate water recovery with low energy consumption.

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Membrane Capacitive Deionization, Constant Voltage (CV), Cut-off CV Mode, Desalination Performance, Energy Behaviors