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Ushiku, Japan

The first part of this article describes the program of a one-stage continuous electrodialysis process operating at a constant current mode. The full continuous electrodialysis program is developed and explained definitely by arranging equations systematically with the following supplementary steps. For preventing scale formation in concentrating cells, salt solutions supplied to the desalting cells are also fed to the concentrating cells. Influence of temperature to the performance of the electrodialyzer is taken into account. Pressure drop in the electrodialyzer is evaluated by incorporating the functions of hydrodynamic diameters of desalting and concentrating cells and slots. An electric current screening effect of a spacer is determined by the volume ratio of spacer rods in a desalting and concentrating cell.In the second part of this article, saline water is desalinated with the multi-stage electrodialysis program by operating the process at a constant concentration mode. Changing salt concentration of a feeding solution in each stage incrementally, the performances of the electrodialyzer such as; ion and solution flux across a membrane pair; cell voltage; current density; salt concentration in concentrating cells; energy consumption; water recovery; limiting current density; pressure drop in the cells and slots are computed in each stage. Energy consumption, water recovery, pressure drop and membrane area are computed in the total stages to produce drinking water. © 2012 Elsevier B.V.. Source

A computer simulation program is developed to predict desalinating performance of a feed and bleed electrodialysis process, inputting membrane characteristics, electrodialyzer specifications and electrodialytic conditions. Computing results enable to discuss the phenomena such as influence of cell voltage on current density, ionic fluxes, solution fluxes, current efficiency, ohmic potential and membrane potential, and further the influence of cell voltage and electrolyte concentration on the output of desalted solutions, energy consumption and water recovery. Excepting limiting current density, the performance of an electrodialyzer is scarcely influenced by the standard deviation of the normal distribution of the solution velocity ratio in desalting cells. Energy consumption in a feed and bleed process is larger than that in a batch process for higher feed concentration, and it is less than that in a reverse osmosis process at feed concentration less than one thousand and hundred-odd mg/l. © 2009 Elsevier B.V. All rights reserved. Source

Tanaka Y.,IEM Research
Journal of Membrane Science | Year: 2010

Based on pH measurement in water dissociation experiments, the mechanism of a water dissociation (water splitting) reaction is discussed assuming that the reaction is generated in a reaction layer formed in an ion exchange membrane and the reaction layer extends throughout the space in the membrane. The process is established to estimate the current efficiency for H + and OH - ions in the water dissociation reaction, area and specific electric resistance of the reaction layer, potential gradient and potential difference in the reaction layer and forward reaction rate constant of the water dissociation reaction. The forward reaction rate constant is expressed by the function of potential gradient and electric potential in the reaction layer and the function of the reaction rate characteristic parameter. The water dissociation reaction is influenced by the potential gradient applied to the reaction layer and the electric potential induced by ion exchange groups in the reaction layer. Namely, the intensity of the water dissociation reaction (generation and transport of co-ions in the reaction layer) is decreased by repulsive force between ion exchange groups in the reaction layer and co-ions generated by the water splitting reactions. H + ion current efficiency in a cation exchange membrane is less than OH - ion current efficiency in an anion exchange membrane, because the repulsive force against co-ions in the cation exchange membrane is larger than that in the anion exchange membrane. In spite of the natures of the water dissociation reaction mentioned above, the contribution of H + and OH - ions to an electric current is insufficient and the rest of the electric current is carried by electrolyte ions supplied by accelerated solution convection in a boundary layer. In order to understand the mechanism of the water dissociation reaction, it is not necessary to be based on the concept of the protonation and deprotonation reaction. The influence of the second Wien effect on the water dissociation reaction is negligible because the potential gradient in the reaction layer is low. © 2010 Elsevier B.V. All rights reserved. Source

The overall membrane pair characteristics included in the overall mass transport equation are understandable using the phenomenological equations expressed in the irreversible thermodynamics. In this investigation, the overall membrane pair characteristics (overall transport number , overall solute permeability , overall electro-osmotic permeability and overall hydraulic permeability ) were measured by seawater electrodialysis changing current density, temperature and salt concentration, and it was found that occasionally takes minus value. For understanding the above phenomenon, new concept of the overall concentration reflection coefficient * is introduced from the phenomenological equation. This is the aim of this investigation. * is defined for describing the permselectivity between solutes and water molecules in the electrodialysis system just after an electric current interruption. * is expressed by the function of and . * is generally larger than 1 and is positive, but occasionally * becomes less than 1 and becomes negative. Negative means that ions are transferred with water molecules (solvent) from desalting cells toward concentrating cells just after an electric current interruption, indicating up-hill transport or coupled transport between water molecules and solutes. Copyright © 2012 Yoshinobu Tanaka. Source

Tanaka Y.,IEM Research
Membrane Water Treatment | Year: 2010

The performance of an electrodialyzer for concentrating seawater is predicted by means of a computer simulation, which includes the following five steps; Step 1 mass transport; Step 2 current density distribution; Step 3 cell voltage; Step 4 NaCl concentration in a concentrated solution and energy consumption; Step 5 limiting current density. The program is developed on the basis of the following assumption; (1) Solution leakage and electric current leakage in an electrodialyzer are negligible. (2) Direct current electric resistance of a membrane includes the electric resistance of a boundary layer formed on the desalting surface of the membrane due to concentration polarization. (3) Frequency distribution of solution velocity ratio in desalting cells is equated by the normal distribution. (4) Current density i at x distant from the inlets of desalting cells is approximated by the quadratic equation. (5) Voltage difference between the electrodes at the entrance of desalting cells is equal to the value at the exits. (6) Limiting current density of an electrodialyzer is defined as average current density applied to an electrodialyzer when current density reaches the limit of an ion exchange membrane at the outlet of a desalting cell in which linear velocity and electrolyte concentration are the least. (7) Concentrated solutions are extracted from concentrating cells to the outside of the process. The validity of the computer simulation model is demonstrated by comparing the computed results with the performance of electrodialyzers operating in salt-manufacturing plants. The model makes it possible to discuss optimum specifications and operating conditions of a practical-scale electrodialyzer. Source

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