Electrodialysis

The process Electrodialysis is a process in which electrically charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process, illustrated in Figure 15, utilizes an electrodialysis stack built on the filter press principle. The stack consists of 200-400 alternate cationic and anionic membranes between two electrodes; the aqueous feed solution flows through the cells between each pair of membranes. When an electrical potential difference is applied between the two electrodes, positively charged cations in the feed solution move toward the cathode. These ions easily pass through the negatively charged cation exchange membranes, but are retained by the positively charged anion exchange membranes. Similarly, negatively charged anions migrate towards the anode, pass through the anion exchange membrane and are retained by the cation exchange membrane. Because of the arrangement of ion-selective membranes, the migrating ions become concentrated in each alternate cell in the stack. Thus, ions removed from the aqueous feed solution are concentrated into two separate streams.

Applications

Brackish water Brackish water desalination is the largest application of electrodialysis. The competitive technologies are ion exchange for very dilute solutions (below 500 ppm) and reverse osmosis for solutions above 2000 ppm salt. In the 500-2000 ppm range, electrodialysis is almost always the lowest cost process. One advantage of electrodialysis when applied to brackish water desalination is that a large fraction, typically 80-95% of the brackish feed, is recovered as potable water. However, these high recoveries mean that the concentrated brine stream produced is 5-20 times more concentrated than the feed. Precipitation of insoluble salts in the brine can limit the water recovery.

Since the first electrodialysis plants were produced in the early 1950s, several thousand brackish water

Figure 15 Schematic diagram of a plate-and-frame electrodialysis stack. Alternating cation- and anion-permeable membranes are arranged in a stack of up to 100 cell pairs.

electrodialysis plants have been installed around the world. Modern electrodialysis units are generally fully automated and require only periodic operator attention. This has encouraged installation of many small trailer-mounted plants. However, a number of very large plants with production rates of 10 million gal per day or more have also been produced.

The power consumption of an electrodialysis plant is directly proportional to the salt concentration in the feed water, and varies from 4 kWh per 1000 gal (4 MJm~3) for 1000 ppm feed water to 10-15 kWh per 1000 gal (10-15 MJm~3) for 5000 ppm feed water. About one-quarter to one-third of this power is used to drive the feed water recirculation pumps.

Seawater A second major application of elec-trodialysis is the production of table salt by concentration of seawater. This process is only practised in Japan, which has no other domestic salt supply. The process is heavily subsidized by the government. Total production is approximately 1.2 million tons per year of salt, with more than 500 000 m2 of membrane used in the plants.

A flow scheme for one such seawater salt-production plant is shown in Figure 16. A cogeneration power plant produces the power required for the electrodialysis operation, which concentrates the salt in seawater to about 18-20 wt%. Waste steam from

Figure 16 Flow scheme of a typical electrodialysis process used in a seawater salt concentration plant.

the power plant is then used to concentrate the salt further by evaporation.

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