Abstract
Estimating and increasing limiting current density (LCD) levels is of fundamental importance for the development of electrodialysis (ED) systems, and it is becoming clear that the use of porous spacers can significantly increase such LCD levels. In this study, a three-dimensional numerical simulation was proposed for evaluating the mass transfer within a porous spacer unit cell and for estimating LCD levels. It was found that our proposed method is effective for estimating the minimum value of an LCD, which is a significant factor related to the safe operation of ED systems. Furthermore, it was found that increasing the minimum effective Sherwood number provides a key to increasing LCD levels. Porous spacer design guidelines were proposed based on the numerical simulation results, after which a new spacer was introduced, designed according to those guidelines. It was found that flow disturbances on the membrane caused by porous spacer structures can lead to increases in effective Sherwood numbers and that LCD levels could be increased by eliminating the flow stagnation behind the structures on the membrane. The LCD of our new spacer was found to be higher than that of the spacers with the highest LCD levels in use at present. Therefore, we can conclude that the proposed design guidelines are effective for increasing LCD levels.
Highlights
Electrodialysis (ED) utilizing ion transport through cation and anion exchange membranes, in which electrically charged ions pass through those membranes due to the influence of electrical potential differences, makes it possible to desalinate water and concentrate target minerals in ionic solutions
We focused on mass transfer in the dilute phase, since this is the phase where dilute concentration polarization occurs and the phase that impacts limiting current density (LCD) levels
The effects of porous spacers on ion mass transfer were investigated in numerical simulations as a means of increasing limiting current densities
Summary
Electrodialysis (ED) utilizing ion transport through cation and anion exchange membranes, in which electrically charged ions pass through those membranes due to the influence of electrical potential differences, makes it possible to desalinate water and concentrate target minerals in ionic solutions. Bai et al [15] investigated the effect of porous spacer structures on the performance for ED desalination by using a three-dimensional (3D) printer to fabricate such spacers According to their studies [14,15], the role of a porous spacer is to use flow mixing to supply ions directly to the boundary layer on a membrane so that the formation of concentration polarization can be suppressed, in comparison to a conventional net spacer [16,17,18]. Balster et al proposed the use of a multi-layer net spacer [19] and a membrane with an integrated spacer [20] Taken together, these studies show that the spacer structure is a matter of importance when working to increase LCD levels, which indicates that a simple method for evaluating the effectiveness of a spacer structure on the LCD is required. We propose a new spacer that can provide improved LCD levels based on a set of numerical simulation results
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