Electrodialysis System Essay

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Introduction
Recently, water shortage problem becomes more and more serious in the world [1]. The desire to make a drinking water by treating a ground water, a surface water, a sea water and so on has been increasing. An electrodialysis (ED) is one of the useful methods and has applied to make the drinking water as well as RO and NF membranes [2~5].
ED is an electrical system utilizing ion exchange membranes. Ion exchange membranes have permselectivity similar to RO and NF membranes [6]. It is very important to understand the mechanism of permselectivity of ion exchange membranes to design ED system. The transport number ratio between target ion and standard ion has been treated to discuss the permselectivity of ion exchange membrane in ED system [7]. Recently, it was found that the ratio of transport number of several anions to chloride ion changed with the progress of electrodialysis. In this paper, a mechanism of permselectivity of ions in the electrodialysis system is theoretically discussed to make clear the reason why the transport number ratio changes with the progress of deionization in electodialysis process. In addition, the simple way to simulate ED system is proposed.

2. Theoretical
2.1 Model of electrodialysis system
The ratio of transport number of anion a and chloride ion, PaCl is defined by eq.(1) [7].
Here, ta and tCl denotes the transport number of anion a and Cl ion, respectively. [a]B and [Cl]B are the concentration (eq/m3) of anion a and Cl in the diluted compartment, respectively. [Ja] (eq/m2s) denotes the flux of anion a and[Js] the sum of all ion fluxes through the anion exchange membrane. Thus, PaCl defined by eq.(1) shows the transport number ratio of anion a and Cl at ...

... middle of paper ...

... the transport number ratio depends on only the total electrolyte concentration in the diluted compartment, CB. PaCl is determined by the membrane resistances of ions transport in the system where CB is sufficiently high. On the other hand, in the system where CB is sufficiently low, PaCl is determined by the equivalent conductance of electrolytes in the diluted compartment. Thus, PaCl changes with the progress of electrodialysis, since the electrolyte concentration in the diluted compartment decreases with time. The transport number ratio was analyzed with this model and it was shown that the model explained the experimental results very well.
It was also possible to simulate ED system after the electric resistances of the equivalent circuit were obtained from the analysis of the time course of PaCl. The simulation results agreed with ED data very well.

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