Electrodialysis, membrane process

Electrodialysis (membrane process based on ion-selective membranes). 

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

Electrodialysis reversal can produce water of below 5 pS/cm conductivity (sometimes lower) the membranes are essentially impervious to fouling, and the polarity reversal provides an excellent membrane and cell compartment cleaning mechanism. This technology lends itself to an all-membrane approach (triple-membrane process) for the production of high-purity water employing UF/EDR/RO. 

An survey of recent developments in membrane processes, involving reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF), electrodialysis (ED), dialysis (D), pervaporation (Pr), gas permeation (GP), and emulsion liquid membrane (ELM), has been provided by Sirkar (1997). 

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 track-etch membrane can be used in reverse osmosis and electrodialysis separation processes where it consists of a thin metal layer with a thin layer of insulator material on each side. The membrane pore diameters were in the range 0.5-10 nm. 

Electrodialysis (ED) is a membrane process based on the ability of semipermeable membranes to pass select ions in feedwater. A direct ctinem electrical field transports the ions through the membranes, in the basic system, alternating cation- and anion-selcctivc membranes arc placed in an electrical field. An eleclrodialysis stack schematic is shown in Fig. 3. The cation selective membranes permit only the transport of cations anion-selective membranes allow only the transport of anions. 

Electrodialytic method Combines electrokinetic processes with electrodialysis membranes to remove contaminants from wet solids and aqueous solutions. 

The four developed industrial membrane separation processes are microfiltration, ultrafiltration, reverse osmosis, and electrodialysis. These processes are all well established, and the market is served by a number of experienced companies. 

In the United States, electrodialysis was developed primarily for desalination of water, with Ionics being the industry leader. In Japan, Asahi Glass, Asahi Chemical (a different company), and Tokuyama Soda developed the process to concentrate seawater [8], This application of electrodialysis is confined to Japan, which has no domestic salt sources. Electrodialysis membranes concentrate the salt in seawater to about 18-20% solids, after which the brine is further concentrated by evaporation and the salt recovered by crystallization. 

Figure 10.13 Flow scheme of a three-stage electrodialysis plant [25]. Reprinted from A.N. Rogers, Design and Operation of Desalting Systems Based on Membrane Processes, in Synthetic Membrane Processes, G. Belfort (ed.), Academic Press, Copyright 1977, with permission from Elsevier...

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