Electrodialysis energy requirements

The concentrates of salt solutions made by electrodialysis of seawater are suited as feed to the evaporators of salt manufacturing plants with considerable savings in overall energy requirements. Other applications also are based on the concentrating effects of electrodialysis, for instance, tenfold increases of concentrations of depleted streams from nickel and copper plating plants are made routinely. 

The total energy required in electrodialysis for the actual desalination process is given by the current passing through the electrodialysis stack multiplied with the total voltage drop encountered between the electrodes ... 

The operation of an electrodialysis unit requires one or more pumps to circulate the diluate, the concentrate, and the electrode rinse solution through the stack. The energy required for pumping these solutions is determined by the volumes of the solutions to be pumped and the pressure drop. It can be expressed by ... 

Electrochemical synthesis, 645 cell types, 648,653,654 energy requirements, 653 fuel cells, 646, 655 overvolfage, 654 reactions, 646 Electrodialysis, 508, 517 equipment, 513, 514 performance, brackish water, 515... 

Figure 2. displays cell potential measured from the electrodialysis cell as a function of operation temperature. As operation temperature increased from 35 to 90°C, the cell potential decreased from 0.68 to 0.41 V at the applied current of 2 A. This is because high temperatures reduce the amount of electrical energy required to concentrate HI molarity from a thermodynamic standpoint and hence electrical energy demand AG for electrodialysis of Hix solution decreases with increasing temperature... 

Electrodialysis has advantages and disadvantages. For instance, the process requires very little energy and can recover highly concentrated solutions. On the other hand, similarly to other membrane processes, electrodialysis membranes are susceptible to fouling and must be regularly replaced. 

The electrodialysis process to make silica aquasols commercial electrodialysis units is technically feasible. The new variable temperature method allows to make very small particle size sols (4-6 nm) and concentrated small particle sols (7 nm) at higher, more efficient temperatures. Electrodialysis remains an attractive technical option for new plant an electrodialysis plant would cost less than a completely new ion exchange resin plant. Electrodialysis is an energy intensive process but it is essentially pollution-free and recovers substantial NaOH values for credit. Location of a plant in a low-cost power area could be attractive. Electrodialysis would also be a technical option in case environmental pressures require further reduction of sodium sulfate level in the silica sol waste of the ion exchange resin process. 

A new application area for membranes is energy production. Reverse electrodialysis and pressure retarded osmosis could provide significant quantities of energy from the mixing of fresh water with seawater or mixing the concentrated brine effluent of desalination plants with seawater. However, these applications will require significant reduction in membrane and module costs. 

Electrodialysis has the ability to concentrate salts to high levels with much less energy consumption than evaporation would require. This capability has been utilized in Japan to make edible salt by recovering NaCl from seawater and concentrating it to 20% before evaporation. The plants there are huge some have greater than 100 000 square meters of membrane. Salt recovered by electrodialysis in Kuwait is the raw material for a chlor-alkali plant there. Electrodialysis has also been used to concentrate salts in reverse osmosis brines [32]. 

Other options available, but of limited significance, involve liquid-liquid extraction, electrolysis, evaporation or electrodialysis. Evaporation can result in zero discharges but is confined to dry sunny climates, where no energy input other than sunlight is required to cause evaporation. 

Removal efficiencies of up to 99% for SO2 and greater than 90% for NO, are claimed for the combined processes. The electrodialysis ceU stack is described as being energy efficient in comparison to electrolytic cells. Condensate quality water is required for make-up (Byszewski, 1992). 

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