Electrodialysis principle

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 Reversal. Electro dialysis reversal processes operate on the same principles as ED however, EDR operation reverses system polarity (typically three to four times per hour). This reversal stops the buildup of concentrated solutions on the membrane and thereby reduces the accumulation of inorganic and organic deposition on the membrane surface. EDR systems are similar to ED systems, designed with adequate chamber area to collect both product water and brine. EDR produces water of the same purity as ED. 

Process Description Electrodialysis (ED) is a membrane separation process in which ionic species are separated from water, macrosolutes, and all uncharged solutes. Ions are induced to move by an electrical potential, and separation is facilitated by ion-exchange membranes. Membranes are highly selective, passing either anions or cations and very little else. The principle of ED is shown in Fig. 20-79. 

FIG. 5 Basic operating principles of an electrodialysis process using (A) a couple of monopolar (anionic, a, and cationic, c) membranes or (B) a bipolar membrane. 

Figure 5.2 Schematic diagram illustrating the principle of desalination by electrodialysis in a stack with cation- and anion-exchange membranes in alternating series between two electrodes.

Principles and applications of electrochemical remediation of industrial discharges are presented by Pallav Tatapudi and James M. Fenton. Essentials of direct and indirect oxidation and reduction, membrane processes, electrodialysis, and treatment of gas streams, and of soils, are complemented by discussions of electrode materials, catalysts, and elements of reactor design. 

This review summarizes the monopolar and BPM principles of operation. Moreover, this review presents shortly the main applications in chemical processing, pollution control, and resource recovery, and details the specific applications of electrodialysis with BPMs to food and bio-industries. 

Principle of Electrodialysis with Bipolar Membrane 21.2.3.1 H" and OH Generation... 

Jain SM and Reed PB. Electrodialysis. In Cooney CL and Humphrey AE, Eds. Comprehensive Biotechnology, Vol.2 The principles of Biotechnology Engineering Considerations, Pergamon, New York, 1985, pp. 575-590. 

The basic principle of electrodialysis for desalination is to drive the cations and anions from saline water feeds under the influence of an electric potential gradient through cation- and anion-selective membranes. The electric potential prevents diffusion of oppositely charged ions in the other direction. A schematic of the process is shown in Figure 29.7. In a typical electrodialysis cell to deionize a salt solution, anion- and cation-exchange membranes are arranged alternatively in a... 

FIGURE 5.4 Principles of operation of two types of membrane water desalination units reverse osmosis and electrodialysis. (From Pryde [22], and reprinted courtesy of Cummings Publishing Co.)... 

In principle, the electrodialysis process can be intensified significantly when the applied pulse is sufficiently smaller than this transition time. Experimental results qualitatively support the model predictions. The desalination can be intensified several times, depending on the pulse-pause characteristics. 

A recent innovation by a U.S. manufacturer of ED systems has significantly minimized the need for pretreatment. This innovation, known as electrodialysis reversal (EDR), operates on the same basic principle as the standard electrodialysis unit, except that both the product and the brine cells are identical in construction. At a frequency of 3-4 times/hr, the polarity of the electrodes is reversed and the flows are simultaneously switched by automatic... 

Multi-compartment electrodialysis equipment using ion-selective membrane was introduced in 1940. Figure 17.2 illustrates the working principle of such a cell. In the cell anion-selective membranes (a) alternate with cation-selective membranes (c). Under an applied d-c potential, cations M+ tend to move towards the cathode. These cations are able to permeate the cation-selective membranes but not the anion-selective membranes. Similarly, anions X tend to move towards the... 

---