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28-414: In an electrodialysis stack, the dilute (D) feed stream, brine or concentrate (C) stream, and electrode (E) stream are allowed to flow through the appropriate cell compartments formed by the ion-exchange membranes . Under the influence of an electrical potential difference, the negatively charged ions (e.g., chloride ) in the dilute stream migrate toward the positively charged anode . These ions pass through

56-407: A cathode) within the electrodialysis stack. A galvanic potential is supplied as a voltage generated at the electrodes. A typical industrial electrodialysis stack consists of two chambers: a product-water chamber and a concentrate-reject chamber. During stack operation, salts are transferred from the product to the concentrate. As a result, the reject stream is concentrated up while the product stream

84-417: A separate E tank. However, some (e.g.,) have proposed collection of hydrogen gas for use in energy production. Current efficiency is a measure of how effective ions are transported across the ion-exchange membranes for a given applied current. Typically current efficiencies >80% are desirable in commercial stacks to minimize energy operating costs. Low current efficiencies indicate water splitting in

112-485: A time frame. A batch can go through a series of steps in a large manufacturing process to make the final desired product. Batch production is used for many types of manufacturing that may need smaller amounts of production at a time to ensure specific quality standards or changes in the process. This is opposed to large mass production or continuous production methods, where the product or process does not need to be checked or changed as frequently or periodically. In

140-521: Is a concern since scaling can build up on the membranes. However, electrodialysis can support higher concentrations of those foulants than reverse osmosis. In addition, electrodialysis membranes, as they have rectangular shape, can be removed from the stack and cleaned, whereas reverse osmosis membranes can't be cleaned that way because of their spiral geometry. Various chemicals are also available to help prevent scaling. Also, electrodialysis reversal systems seek to minimize scaling by periodically reversing

168-532: Is called cycle time. Each batch may be assigned a lot number . Because batch production involves small batches, it is good for quality control. For example, if there is a mistake in the process, it can be fixed without as much loss compared to mass production. This can also save money by taking less risk for newer plans and products etc. As a result, this allows batch manufacturing to be changed or modified depending on company needs. In certain cases, batch production may require less expensive equipment, thus reducing

196-399: Is desalted. Exemplary applications of ion-exchange membranes utilized in electrodialysis and EDR include seawater desalination, industrial wastewater treatment of highly scaling waters, food and beverage production, and other industrial wastewaters. Batch production Batch production is a method of manufacturing where the products are made as specified groups or amounts, within

224-459: Is more cost-effective for TDS feed concentrations less than 3,000 ppm or when high recoveries of the feed are required. Another important application for electrodialysis is the production of pure water and ultrapure water by electrodeionization (EDI). In EDI, the purifying compartments and sometimes the concentrating compartments of the electrodialysis stack are filled with ion-exchange resin . When fed with low TDS feed (e.g., feed purified by RO),

252-489: Is usually employed to prevent the reduction and/or oxidation of salt ions from the feed into the electrode plates. Depending on the stack configuration, anions and cations from the electrode stream may be transported into the C stream, or anions and cations from the D stream may be transported into the E stream. In each case, this transport is necessary to carry current across the stack and maintain electrically neutral stack solutions. Reactions take place at each electrode. At

280-404: Is very useful for Zero Liquid Discharge treatments, providing a reduction in energy consumption compared to evaporation. In application, electrodialysis systems can be operated as continuous production or batch production processes. In a continuous process, feed is passed through a sufficient number of stacks placed in series to produce the final desired product quality. In batch processes,

308-441: The proton-exchange membranes , that transport H cations , and the anion exchange membranes used in certain alkaline fuel cells to transport OH anions . An ion-exchange membrane is generally made of organic or inorganic polymer with charged (ionic) side groups, such as ion-exchange resins . Anion-exchange membranes contain fixed cationic groups with predominantly mobile anions; because anions are

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336-409: The C stream, prevented from further migration toward the cathode by the positively charged anion-exchange membrane. As a result of the anion and cation migration, electric current flows between the cathode and anode. Only an equal number of anion and cation charge equivalents are transferred from the D stream into the C stream and so the charge balance is maintained in each stream. The overall result of

364-435: The cathode, while at the anode, Small amounts of hydrogen gas are generated at the cathode and small amounts of either oxygen or chlorine gas (depending on composition of the E stream and end ion-exchange membrane arrangement) at the anode. These gases are typically subsequently dissipated as the E stream effluent from each electrode compartment is combined to maintain a neutral pH and discharged or re-circulated to

392-468: The desalination of brackish water or seawater as an alternative to RO for potable water production and seawater concentration for salt production (primarily in Japan ). In normal potable water production without the requirement of high recoveries, reverse osmosis is generally believed to be more cost-effective when total dissolved solids (TDS) are 3,000 parts per million (ppm) or greater, while electrodialysis

420-447: The diluate and/or concentrate streams are re-circulated through the electrodialysis systems until the final product or concentrate quality is achieved. Electrodialysis is usually applied to deionization of aqueous solutions. However, desalting of sparingly conductive aqueous organic and organic solutions is also possible. Some applications of electrodialysis include: The major application of electrodialysis has historically been

448-434: The diluate or concentrate streams, shunt currents between the electrodes, or back- diffusion of ions from the concentrate to the diluate could be occurring. Current efficiency is calculated according to: where Current efficiency is generally a function of feed concentration. As electrodialysis works by transporting salt ions from the diluted channels to the concentrated channels, energy consumption greatly increases as

476-649: The diluted channel. Selective electrodialysis is usually done by employing monovalent anion and/or cation exchange membranes, that only allows migration of monovalent anion or cations, respectively. This is useful when only monovalent ions are required to be separated, reducing electricity consumption and desalination time. For example, this is useful for irrigation water. Monovalent cations usually are particularly (Na, Cl) harmful for crops, whereas most divalent ions (Ca, Mg, SO 2 ) are beneficial nutrients for plants. Therefore, monovalent selective electrodialysis may provide water with an ideal composition for agriculture, reducing

504-424: The electrodialysis process is an ion concentration increase in the concentrate stream with a depletion of ions in the dilute solution feed stream. The E stream is the electrode stream that flows past each electrode in the stack. This stream may consist of the same composition as the feed stream (e.g., sodium chloride ) or may be a separate solution containing a different species (e.g., sodium sulfate ). The E stream

532-522: The feed salt concentration becomes lower, and with fewer ions in solution to carry current, both ion transport and energy efficiency greatly declines. Consequently, comparatively large membrane areas are required to satisfy capacity requirements for low concentration (and sparingly conductive) feed solutions. Innovative systems overcoming the inherent limitations of electrodialysis (and RO) are available; these integrated systems work synergistically, with each sub-system operating in its optimal range, providing

560-447: The feed salt concentration increases. Seawater desalination is usually more energy efficient by reverse osmosis than electrodialysis. However, for water streams with lower salt concentration electrodialysis may be the most energy efficient process. Additionally, water streams with very high salt concentrations, that cannot be separated by reverse osmosis, can be concentrated by electrodialysis up to concentrations near to saturation. This

588-517: The flows of diluate and concentrate and polarity of the electrodes. Ion-exchange membrane An ion-exchange membrane is a semi-permeable membrane that transports certain dissolved ions, while blocking other ions or neutral molecules. Ion-exchange membranes are therefore electrically conductive. They are often used in desalination and chemical recovery applications, moving ions from one solution to another with little passage of water. Important examples of ion-exchange membranes include

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616-604: The least overall operating and capital costs for a particular application. As with RO, electrodialysis systems may require feed pretreatment to remove species that coat, precipitate onto, or otherwise "foul" the surface of the ion-exchange membranes. This fouling decreases the efficiency of the electrodialysis system. Species of concern include calcium and magnesium hardness , suspended solids , silica , and organic compounds. Water softening can be used to remove hardness, and micrometre or multimedia filtration can be used to remove suspended solids. Hardness in particular

644-497: The majority species, most of the conductivity is due to anion transport. The reverse holds for cation-exchange membranes . The so-called heterogeneous ion-exchange membranes have low cost and a thicker composition with higher resistance and a rough surface that can be subject to fouling. Homogeneous membranes are more expensive, but have a thinner composition with lower resistance and a smooth surface, less susceptible to fouling. Homogeneous membrane surfaces can be modified to alter

672-525: The manufacturing batch production process, the machines are in chronological order directly related to the manufacturing process. The batch production method is also used so any temporary changes or modifications can be made to the product if necessary during the manufacturing process. For example, if a product needed a sudden change in material or details changed, it can be done in between batches. As opposed to assembly production or mass production where such changes cannot be easily made. The time between batches

700-745: The membrane permselectivity to protons, monovalent ions, and divalent ions. The selectivity of an ion-exchange membrane is due to Donnan equilibrium and not due to physically blocking or electrostatically excluding specific charged species. The selectivity to the transport of ions of opposite charges is called its permselectivity. Ion-exchange membranes are traditionally used in electrodialysis or diffusion dialysis by means of an electrical potential or concentration gradient, respectively, to selectively transport cationic and anionic species. When applied in an electrodialysis desalination process, anion- and cation-exchange membranes are typically arranged in an alternating pattern between two electrodes (an anode and

728-545: The necessity for minerals fertilization. Electrodialysis has inherent limitations, working best at removing low molecular weight ionic components from a feed stream. Non-charged, higher molecular weight, and less mobile ionic species will not typically be significantly removed. Also, in contrast to RO, electrodialysis becomes less economical when extremely low salt concentrations in the product are required and with sparingly conductive feeds: current density becomes limited and current utilization efficiency typically decreases as

756-441: The positively charged anion-exchange membrane, but are prevented from further migration toward the anode by the negatively charged cation-exchange membrane and therefore stay in the C stream, which becomes concentrated with the anions. The positively charged species (e.g., sodium ) in the D stream migrate toward the negatively charged cathode and pass through the negatively charged cation-exchange membrane. These cations also stay in

784-568: The product can reach very high purity levels (e.g., 18 M Ω -cm). The ion-exchange resins act to retain the ions, allowing these to be transported across the ion-exchange membranes. The main usage of EDI systems are in electronics, pharmaceutical, power generation, and cooling tower applications. Electrodialysis can adapt to intermittent energy input and voltage variations, so it can be easily coupled to renewable electricity sources Selective electrodialysis uses ion selective exchange membranes to concentrate only some ions, whereas other species remain in

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