Advancements in capacitive deionization (CDI): application in desalination plant and design of a novel lab-scale cylindrical flow electrodes device

Abstract

The first part of this study investigates the usage of membrane capacitive deionization (MCDI) as a potential post-treatment process after first-pass seawater reverse osmosis (SWRO). The performance of both one-pass and two-pass MCDI desalination systems demonstrates its ability to produce fresh water that complies with regulation standards of 250 ppm and 25 ppm respectively. Furthermore, the two-pass process has better energy performance than the options of one-pass MCDI or second-pass of RO. However, the conventional MCDI system has several restrictions due to its configuration with fixed electrode. The electrosorption capacity is associated with the fixed amount of electrodes in the system, once the saturation of the electrode is reached, the periodical desorption and regeneration step is needed, this resulted in the intermittent pattern of the purified output flow. In order to overcome the limitations of the conventional MCDI systems, a flowelectrode capacitive deionization (FCDI) system based on a cylindrical module with continuous circulating of flow-electrodes was designed and fabricated. The flow electrodes used in this study consisted of activated carbon (AC) particles or graphene flakes suspended in deionized water. It was found that the flow-electrodes operated under mixed cycle mode showed better salt removal efficiencies than the independent cycle mode. Hence, this design avoided the energy intensive intermittent desalting/regeneration pattern of the conventional CDI procedure. Comparison of the properties of pristine and recycled flow-electrode, both AC and graphene flakes showed that similar salt removal efficiencies were achieved. Furthermore, under the same methodology used for both flow electrodes, graphene flakes showed 30% higher salt removal efficiency than AC flow electrodes. Overall, MCDI has been evaluated as a post first-pass RO salt removal process, and the results of water recovery, salt removal efficiency and energy consumption provided useful information on understanding the potential of using this technology as real option in desalination plant. In addition, by designing a novel configuration of FCDI, it was found that the flow-electrodes operated under mixed cycle mode showed good salt removal efficiencies, and confirmed that this design with FCDI avoided the energy intensive intermittent desalting/regeneration pattern of the conventional CDI approach. This preliminary results suggested that FCDI could be considered as an alternative configuration of capacitive deionization technology.

Date of Award	May 2017
Original language	American English