Capacitive deionization (CDI) has become a very attractive desalination technology due to its capability of returning a fraction of the input energy during the regeneration of its adsorbent electrodes. As in any separation technique, analysis of the mass transfer phenomena occurring in this water treatment method is vital to evaluate and extend the performance of a desalination system. This publication proposes a novel method to estimate the net electro-adsorption rate of a CDI cell from a series of low concentration desalination experiments coupled with a one-dimensional electro-adsorption model. In the proposed methodology, a one-dimensional model is presented and two regimes are identified based on the presence or absence of a convection-diffusion layer within the bulk solution, as dictated by the electro-diffusion based Peclet number. For each of these regimes, the net adsorption flux is calculated based on the velocity at which ions are transported toward the electrodes. The proposed model is then solved, first under the assumption of an infinite electrode adsorption capacitance before relaxing this condition, and correlated against the experimental data to assess the global electro-adsorption rate. The analysis in this paper also provides unique physical insight into the macro-scale mass transfer processes that control desalination in CDI.

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although the focus has been placed on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses an electric field to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization and electrodialysis reversal where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow rates. Voltage and electrical current measurements are conducted during the desalination and electrode regeneration processes and used to evaluate the energy recovery ratio. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption efficiency. Two analytical species transport models are developed to estimate the net ionic adsorption rates in a steady-state and nonsaturated porous electrode scenario. Finally, the convective and electrokinetic transport times are compared and their effect on desalination performance is presented. Steady test results for outlet to inlet concentration ratio show a strong dependence on flow rate and concentration independence for dilute solutions. In addition, transient test results indicate that the net electrical energy requirement is dependent on the number of carbon electrode regeneration cycles, which is thought to be due to imperfect regeneration. The energy requirements and adsorption/desorption rate analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the reprocessing of lubricants or spent nuclear fuels in a near future.

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although much of the attention and efforts have focused on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high-surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses electric fields to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization (EDI) and electro-dialysis reversal (EDR) where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow-rates. Voltage and electrical current measurements are conducted during the desalination and porous electrode regeneration processes and used to evaluate the percentage of energy recovery.. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption/desorption efficiencies. Correlation of these measurements with an analytical species transport model provides information about the net ionic adsorption/desorption rates in non-saturated-carbon-electrode scenarios. The results show a strong dependence of the net electrical energy requirements with the number of carbon electrodes regeneration cycles. Finally, a non-dimensional number that compares the convective and electro-kinetic transport times is presented. The energy requirements and adsorption/desorption rates analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the re-process of spent nuclear fuels in a near future.