Electrodialysis

The electrodialysis (ED ) (from the Greek dialysis = resolution) is an electrochemically driven membrane process (see Membrane technology ) are used in the ion exchange membranes in combination with an electrical potential difference to separate ionic species from uncharged solvents or impurities.

Principle of operation

In an electrodialysis separator, the space between two electrodes separated by a stack of alternating anion and cation exchange membranes. Each pair of ion exchange membranes forms a separate "cell". These stacks consist of more than two hundred pairs of membranes in technical systems. , A direct electric voltage applied to the electrodes, the anions migrate to the anode. The anions can easily change the positively charged anion- happen, but they are stopped in each case at the nearest negatively charged cation exchange membranes. Because the same (of course with the opposite sign ) happens also with the cations, is the net effect of electrodialysis in an accumulation of salts in the cells with an odd number ( anion exchange / cation exchange membrane ), while depleting the cells with an even number (cation exchanger membrane / anion exchange ) of salt. The solutions with high salt concentration are combined to concentrate, while the low-salt solutions form the dilute.

Because of the large number of cells between the electrodes electrochemical electrode reactions have practically no influence on the energy consumption of electrodialysis separator. Taking into account the energy required for pumping the solutions through the separator units of the energy consumption of electrodialysis separation is proportional to the salt concentration of the input solution. For this reason, the electro-osmosis is at low salt concentrations, for example, more economical than reverse osmosis. Currently, the electrodialysis process competes strongly with reverse osmosis systems for desalination of brackish water. Approximately 350,000 tonnes of edible salt by means of electrodialysis plants are produced annually in Japan. Seawater is thus enriched by about 15 g / L to 200 g / L sodium chloride content, before the remaining water is removed by conventional evaporation. A pilot plant for desalination of sea water in Singapore processed 50 cubic meters of water a day and it takes only 1.5 kilowatt hours per cubic meter electricity. The hitherto most efficient desalination technology, reverse osmosis, costs more than twice as much energy. With decreasing salt concentration, however, the process is inefficient, because the electrical resistance of the water increases. Therefore, the final percentage of salt extracted a continuous Electrodeionisation. Here, take ion exchange resins between the membranes of the ions and transport them further.

Other technically realized applications of electrodialysis affect the wastewater treatment, eg the regeneration of copper deposition baths in the production of integrated circuits. Cheese whey is demineralized by electrodialysis and one dextran desalted using this procedure.

The technical use of electrodialysis processes is limited by the limited selective permeability currently available ion-exchange membranes in high salt concentrations. The process is uneconomical as soon as the current utilization is below 50-60%. This is done in the present ion-selective membranes, when the salt concentration exceeds about 10-15 mol / l.

Electrodialysis with polarity reversal

This type of electrodialysis, short UED of reversible electric dialysis or English: Electrodialysis Reversal and abbreviated as EDR - works with a regular alternation of electric polarity of the membranes. This Although the equipment costs are significantly higher, but by the polarity reversal of the anode to the cathode of internal cleaning of the membranes is achieved. This cleaning effect is obtained, because decreases in the active cathode surface of the membrane of the pH and increases the anode surface. The formation of deposits is suppressed even at slightly supersaturated water. In particular in the production of drinking water from the raw water containing hydrochloric more EDR process is used with success.

Reverse electrodialysis

The reverse electrodialysis process is the reverse, in which a saline solution, and fresh water is passed through a block of alternating cathode and anode exchange membranes. The difference in chemical potential between salt and fresh water creates a voltage difference between each membrane. The total electric potential of the system is the sum of the potential differences of all the diaphragms.