Theoretical framework for designing a desalination plant based on membrane capacitive deionization

Despite significant progress made in multiple aspects of capacitive deionization (CDI), a rational framework is in need for optimizing the design and operation of a large desalination system based on CDI. In this work, we develop a theoretical framework for guiding the design of a desalination plant based on CDI with ion exchange membranes (i.e. membrane CDI, or MCDI). This framework is established by identifying (1) the practical design constraints, (2) the inter-relationships between different design and operating parameters, (3) a set of independent variables, and (4) the key performance metrics. The proposed design framework reduces the degrees of freedom of the system and facilitates more focused and systematic analysis of the overall performance of an MCDI-based desalination plant. Careful analysis using the proposed design framework suggests the presence of an optimal tradeoff curve that comprises all the possible optima of design and operating conditions with which an MCDI-based desalination plant is the most cost-effective. We also show that the typical practice of using equal flowrates for charging and discharge yields very good performance compared to the optima, as long as water recovery is not too high. Finally, we also briefly explain the implication of this framework on cost-based optimization of the design and operation of an MCDI-based desalination plant.