0815_A computer simulation of ion exchange membrane electrodialysis for concentration




Paper title: "A computer simulation of ion exchange membrane electrodialysis for concentration of seawater"

Published in: Membrane Water Treatment, Vol. 1, No. 1 (2010) 13-37

Author: Yoshinobu Tanaka
IEM research Ibaraki Japan

Because Japan has no NaCl mineral deposits, it has to cover its salt demand by imports and, previously, by salt fields.  Thus the option of table salt recovery from sea water is of special interest. In 1971, all salt field methods are converted into ion exchange membrane methods, which enable the production of nearly 1,000,000 ton/year of edible salt at present.  But still the price for imported salt is lower than the ion exchange produced salt, especially with the currently rising energy prices.  This paper thus seeks further understanding of the electrodialysis process.

In an ion exchange membrane electrodialysis process, cation exchange membranes, anion exchange membranes, desalting cells and concentrating cells are arranged alternately.  An electric current induces ions to electro-migrate through the respective membranes into the concentrating cells.

Concentration polarization on the membrane surface limits current density.  At voltages above the limiting current density water dissociation and scaling are likely to occur.  Thus, concentration polarization limits the ion exchange.

This paper uses the following experimental input as a basis:  membrane characteristics, membrane electric resistance, solution velocity and current density distribution in the electrolyzer, voltage difference between the electrodes, physical properties of the solution, limiting current, solution leakage.

The factors that need to be optimized in order to optimize the ion exchange membrane electrodialysis process are:
- Solution velocity distribution between desalting cells (uniform distribution to ensure a stable process)
- Solution leakage (due to membrane pinholes)
- Electric current leakage (through metal parts of the unit)
- Distance between the membranes (to reduce electrical resistance but not too small to reduce fuid flow resistance)
- Spacer (to increase turbulence to alleviate concentration polarization)
- Simplicity of structure of an electrodialyzer

The developed program incorporates the following phenomena:
- mass transport
- current density distribution
- cell voltage
- NaCl concentration in a concentrated solution and energy consumption
- limiting current density.

The assumptions made are:
- Leakage of both, solution and electric current are negligible.
- Electric resistance of a membrane includes the electric resistance of the concentration polarization boundary layer.
- The frequency of the solution velocity ratio in desalting cells follows the normal distribution.
- Current density i at x distant from the inlets of desalting cells is approximated by a quadratic equation.
- Voltage difference between the electrodes at the entrance of desalting cells is equal to the value at the exits.
- 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.
- Concentrated solutions are extracted from concentrating cells to the outside of the process.

The Author reaches the following conclusions:
- The solution velocity ratio in the desalting cells has little influence on the electrodializer performance.  However, it has an influence on the limiting current density.
- In order to reduce salt manufacturing cost, it is desirable to promote the following research and development:
Integration of fine porous membranes having dense structure into an electrodialyzer being operated at rather lower electric current density.
Development of fine porous membranes without notable increase of electric resistance.
Decrease of solution leakage and electric current leakage in an electrodialyzer.

Contribution to the lab's work:
This paper points at an interesting aspect of sea water desalination.  Japan has not only demand for clean water, but also salt, earlier in world history already considered a precious commodity.  This paper shows that technologies that desalinate water and, at the same time, also recover salt at a table salt quality, have a clear economical advantage.

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