Modelling of capacitive de-ionization with activated carbon electrodes

This paper presents mathematical modeling of capacitive de-ionization (CDI) system with activated carbon electrodes. In CDI, a voltage is applied across two oppositely placed porous electrodes that results in adsorption of ions from the saline water stream forcing anions to move towards anode and cations to move towards cathode under the effect of electric field, thus producing ion depleted product stream. The ions are stored in the electric double layer in the activated carbon electrodes. When the electrodes capacity is reached, voltage is reduced to zero or inverted releasing the ions from the electrodes to produce a highly concentrated salty stream (brine). A mathematical model of CDI is presented based on its electrical equivalent circuit that describes how the effluent salt concentration varies with respect to time in a CDI cell. The model also predicts the amount of charge captured by the CDI cell and the current in the CDI cell, both as a function of time. The effectiveness of the model is evaluated by comparing its results with the electrosorption experimental results of CDI unit from AQUA Electronic Water Purifier (EWP) [1], [2]. The model also evaluates the performance of the AQUA EWP CDI cell to different operational parameters i.e., the feed total dissolved solids concentration and flow rates. The model results are in good agreement with the experimental results.