Electrical separations electrodialysis

Pressure membranes are membranes that are used to separate materials from a fluid by the application of high pressure on the membrane. Thus, pressure membrane filtration is a high pressure filtration. This contrasts with electrodialysis membranes in which the separation is effected by the impression of electricity across electrodes. Filtration is carried out by impressing electricity, therefore, electrodialysis membrane filtration may be called electrical filtration. 

The recovery and purification of the desired product demands a further breakdown of exergy in the sense of mixing the aqueous feed with (pure) solvents (precipitation and extraction), salts (ion exchange), heat (evaporation and solvent recovery), electrical power (electrodialysis), pressure (filtration and membrane separations), or just extra water (gel filtration). This is shown schematically in Fig. 5. 

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. In electro dialysis (ED), the saline solution is placed between two membranes, one permeable to cations only and the other to anions only. A direct electrical current is passed across this system by means of two electrodes, causiag the cations ia the saline solution to move toward the cathode, and the anions to the anode. As shown ia Figure 15, the anions can only leave one compartment ia their travel to the anode, because a membrane separating them from the anode is permeable to them. Cations are both excluded from one compartment and concentrated ia the compartment toward the cathode. This reduces the salt concentration ia some compartments, and iacreases it ia others. Tens to hundreds of such compartments are stacked together ia practical ED plants, lea ding to the creation of alternating compartments of fresh and salt-concentrated water. ED is a continuous-flow process, where saline feed is continuously fed iato all compartments and the product water and concentrated brine flow out of alternate compartments. 

Among electrochemical methods of water purification, one can also list the various electromembrane technologies, electrodialysis in particular. The simplest elec-trodialyzer consists of three compartments separated by semipermeable membranes (usually, cation- and anion-exchange membranes). The water to be purified is supplied to the central (desalination) compartment. In the outer (concentration) compartments, electrodes are set up between which a certain potential difference is applied. Under the effect of the electric field, ions pass througfi the membranes so that the concentration of ionic contaminants in the central compartment decreases. 

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. 

Electrodialysis. Electrodialysis enhances the dialysis process with the aid of an electrical field and ion-selective membranes to separate ionic species from solution. It is used to separate an aqueous electrolyte solution into a concentrated and a dilute solution. Figure 10.15 illustrates... 

As discussed by Pletcher 24, electrodialysis is an electrically driven membrane separation process. The main use of electrodialysis is in the production of drinking water by the desalination of sea-water or brackish water. Another large-scale application is in the production of sodium chloride for table salt, the principal method in Japan, with production exceeding 106 tonne per annum. 

Electrodialysis involves the application of an electric field to a colloidal dispersion that has been placed in a chamber arranged so that one or both electrodes are separated from the dispersion by semi-permeable membrane(s). Typically, dissolved ions can flow through the membrane(s) in response to the electric field gradient, while the dispersed particles (or other species) are restrained within the chamber. 

Electrodialysis — In electrodialysis electrically charged - membranes and an electrical potential difference are used to separate ionic species from an aqueous solution and uncharged components. It refers to an industrial-scale process of electrolyte concentration/depletion due to separation on anion- and cation-exchange membranes under the influence of an electric field. The electrodialysis cell is constructed like a bipolar filter-press electrolyzer, with anion-exchange membranes sandwiched alternately with cation-exchange membranes, see following Figure. 

Electrodialysis. In electrodialysis, separation of an aqueous stream is achieved through the use of synthetic membranes and an electric field. The membrane allows only one type of ions to pass through and may be chosen to remove other ions that move in the opposite direction. Therefore, it produces one stream rich in particular ions and another stream depleted of those ions. The two streams can be recycled or disposed off. This technique is commonly used in the desalination of brackish water. The other uses are in acid mine drainage treatment, the desalting of sewage-plant effluents, and in sulfite-liquor recovery. 

Electrodialysis can be performed with two main cell types multi-membrane cells for dilution-concentration and water dissociation applications, and electrolysis (or electro-electrodialysis [EED]) cells for oxidoreduction reactions. In multimembrane cells, only the membrane transport phenomena intervene, while electrochemical reactions occurring at the electrodes do not interact with the separation process the electrodes are simple electrical terminals immersed in electrolytes allowing the current transfer. The electrolysis cell operates with only one membrane that separates two solutions circulating in each electrode compartment. This application is based on electrode redox reactions, which are electrolysis specific properties. The anode induces oxidations, and reductions occur at the cathode [4]. 

Electrodialysis (ED) is a membrane separation process, which exploits an electrical field as the driving force instead of pressure. Charged compounds are separated by ion-exchange membranes. In the pulp and paper industry, ED is being studied for the... 

Membrane processes are widely used in oil water separation. In general, a membrane is classified into two groups pressure-driven membrane and electrical membrane, known as electrodialysis. The most applicable process for oily wastewater removal is the former type. The pressure-driven membrane applications include microfiltration (MF), ultrafil-tration (UF), nanofiltration (NF), and reverse osmosis (RO). All of them are categorized by the molecular weight or particle size cut-off of the membrane as shown in Table 5. 

Electrodialysis involves the use of a selectively permeable membrane, but the driving force is an electrical potential across the membrane. Electrodialysis is useful for separating inorganic electrolytes from a solution, and can therefore be used to produce freshwater from brackish water or seawater. Electrodialysis typically consists of many cells arranged side by side, in a stack. Figure 9.12 illustrates a two-cell stack. 

Each membrane process requires a driving force for separation to occur. For example, dialysis requires a concentration difference, electrodialysis requires a difference in electric potential, and reverse osmosis requires a pressure difference. 

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