Electrodialysis examples

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. 

Leading Examples Electrodialysis has its greatest use in removing salts from brackish water, where feed salinity is around 0.05-0.5 percent. For producing high-purity water, ED can economically reduce solute levels to extremely low levels as a hybrid process in combination with an ion-exchange bed. ED is not economical for the produc tion of potable water from seawater. Paradoxically, it is also used for the concentration of seawater from 3.5 to 20 percent salt. The concentration of monovalent ions and selective removal of divalent ions from seawater uses special membranes. This process is unique to Japan, where by law it is used to produce essentially all of its domestic table salt. ED is very widely used for deashing whey, where the desalted product is a useful food additive, especially for baby food. 

Electrodialysis is a process for the separation of an electrolyte from the solvent and is used, for example, in desalination. This process occurs in a system with at least three compartments (in practice, a large number is often used). The terminal compartments contain the electrodes and the middle compartment is separated from the terminal compartments by ion-exchanger membranes, of which one membrane (1) is preferentially permeable for the cations and the other one (2) for the anions. Such a situation occurs when the concentration of the electrolyte in the compartments is less than the concentration of bonded ionic groups in the membrane. During current flow in the direction from membrane 1 to membrane 2, cations pass through membrane 1 in the same direction and anions pass through membrane 2 in the opposite direction. In order for the electrolyte to be accumulated in the central compartment, i.e. between membranes 1 and 2 (it is assumed for simplicity that a uni-univalent electrolyte is involved), the relative flux of the cations with respect to the flux of the solvent, /D +, and the relative flux of the anions with respect to... 

The latter concept implies providing local life support systems for unfriendly environments. By now, Ukrainian scientists and engineers have developed a variety of processes for potable water treatment by adsorption, electrochemical oxidation, electrocoagulation, electro-coprecipitation, electrodialysis, electrofloatation, floatation, membrane techniques etc. Each family must get small units for water purification, air cleaning and removal of hazardous substances from the food as soon as possible, for it may take decades to introduce cleaner production on a national scale. Here, we should follow the example of Western business people who bring with them to Ukraine devices enabling a safe existence in this unfriendly environment. 

Chemical precipitation has traditionally been a popular technique for the removal of heavy metals and other inorganics from wastewater streams. However, a wide variety of other techniques also exist. For example, ion-exchange, reverse osmosis, evaporation, freeze crystallization, electrodialysis, cementation, catalysis, distillation, and activated carbon have all been used for removal of inorganics. 

Atom economy is high. As a reagent, no compounds are needed and consequently none are produced as the electron is immaterial. This results in a greater advantage of electrochemical reactions compared to chemical conversions, namely, an effective contribution to pollution control. The direct ET from the electrode to the substrate avoids the problem of separation and waste treatment of the, frequently, toxic end products of chemical reductions or oxidations. Furthermore, by electrodialysis, organic acids or bases can be regenerated from their salts without the use of, for example, sulfuric acid or... 

In a hydrodynamically free system the flow of solution may be induced by the boundary conditions, as for example when a solution is fed forcibly into an electrodialysis (ED) cell. This type of flow is known as forced convection. The flow may also result from the action of the volume force entering the right-hand side of (1.6a). This is the so-called natural convection, either gravitational, if it results from the component defined by (1.6c), or electroconvection, if it results from the action of the electric force defined by (1.6d). In most practical situations the dimensionless Peclet number Pe, defined by (1.11b), is large. Accordingly, we distinguish between the bulk of the fluid where the solute transport is entirely dominated by convection, and the boundary diffusion layer, where the transport is electro-diffusion-dominated. Sometimes, as a crude qualitative model, the diffusion layer is replaced by a motionless unstirred layer (the Nemst film) with electrodiffusion assumed to be the only transport mechanism in it. The thickness of the unstirred layer is evaluated as the Peclet number-dependent thickness of the diffusion boundary layer. 

The electric membrane or electrodialysis process for removing excess dissolved salts and minerals from water is rapidly increasing in use, both in the United States and abroad. In the United States for example, as of January 1, 1958, there were only two production—i.e., nonexperimental—electric membrane plants in operation. By April 1, 1960, only 21/4 years later, 11 plants with a combined capacity of 350,000 gallons per day were serving almost 10,000 people in Montana, Texas, Alaska, New York State, California, Utah, South Dakota, Arizona, and Illinois. 

Plate-and-frame units have been developed for some small-scale applications, but these units are expensive compared to the alternatives, and leaks through the gaskets required for each plate are a serious problem. Plate-and-frame modules are now only used in electrodialysis and pervaporation systems and in a limited number of reverse osmosis and ultrafiltration applications with highly fouling feeds. An example of one of these reverse osmosis units is shown in Figure 3.39 [111],... 

Transport of ions in an electrodialysis cell, in which the salt solutions in the chambers formed between the ion exchange membranes are very well stirred, is shown in Figure 10.7. In this example, chloride ions migrating to the left easily... 

Example. Electrodialysis has been used to purify day suspensions from electrolyte impurities. It has also been used to prepare homoionic day partide surfaces, although the only form that can be prepared in this manner is that having protons as the exchangeable ions, i.e., H-clays. Figure 4.12 provides a schematic illustration. This can be problematic in that preparing H-form days can lead to dissolution of the clay lattice itself [267]. 

Ionic species in waters can be concentrated, and the water purified at the same time, by electrodialysis. Ion-exchange membranes are employed with an applied electric field in order to force ionic salts to pass from dilute into concentrated solutions. Examples of the use of electrodialysis are in the concentrating of Ni2+ in used nickel plating solutions for recirculation, and in desalination plants in the purification of sea water or well water to acceptable levels to make it fit for drinking. 

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