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Energy efficiency enhancement of electromembrane desalination systems by local flow redistribution optimized for the asymmetry of cation/anion diffusivity

Authors
Kim, BumjooChoi, SiwonPham, Van SangKwak, RhokyunHan, Jongyoon
Issue Date
Feb-2017
Publisher
ELSEVIER SCIENCE BV
Keywords
Electrodialysis; Mass transport enhancement; Spacer engineering; Concentration polarization; Ion concentration visualization
Citation
JOURNAL OF MEMBRANE SCIENCE, v.524, pp.280 - 287
Indexed
SCIE
SCOPUS
Journal Title
JOURNAL OF MEMBRANE SCIENCE
Volume
524
Start Page
280
End Page
287
URI
https://scholarworks.bwise.kr/hanyang/handle/2021.sw.hanyang/2770
DOI
10.1016/j.memsci.2016.11.046
ISSN
0376-7388
Abstract
In an electrochemical system, ion transport near ion exchange membranes or electrodes induces inevitable concentration polarization (i.e., formation of diffusion boundaries), which impedes the mass transport and worsens the energy efficiency. To mitigate the effect of concentration polarization (CP), various efforts towards mass transport enhancement employed structures, often called spacers, which promote mixing and modify the flow velocity distribution in the channel. In this work, we employed an electrodialysis (ED) system as a model to investigate the mass transport effects of embedded microstructures, which can redistribute the local flow velocity. We placed a row of cylindrical posts inside the diluate channel and varied the distance from the posts to the membranes. We studied the effect of this post-to-membrane distance on the mass transport by measuring the current-voltage responses and visualizing ion concentration and flow velocity profiles. The study was done through microfluidic ED model experiments and direct numerical simulation based on the coupled Navier-Stokes and Poisson-Nernst-Planck equations. Our results indicate that when the posts are positioned near the center of the channel, the mass transport is enhanced due to the increase in local convection near the concentration boundary layers. More importantly, we discovered that the mass transport is maximized when the location of the posts is slightly off-centered, due to the asymmetry of the cation/anion diffusivity (DNa+ ˂ DCl-). Compared to a system without any structures, the embedded posts can improve both the electrical energy efficiency and the salt removal.
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