Electrodialysis process flow

Electrodialysis can be applied to the continuous-flow type of operation needed in industry. Multi-membrane stacks can be built by alternately spacing anionic- and cationic-selective membranes. Among the technical problems associated with the electrodialysis process, concentration polarization is perhaps the most serious (discussed later). Other problems in practical applications include membrane scaling by inorganics in feed solutions as well as membrane fouling by organics. 

Because of the relatively high concentrations of the acid and base as well as the salt solution the limiting current density is in general no problem and a bipolar membrane stack can generally be operated at very high current densities compared to an electrodialysis stack operated in desalination. However, membrane scaling due to precipitation of multivalent ions such as calcium or heavy-metal ions is a severe problem in the base-containing flow stream and must be removed from the feed stream prior to the electrodialysis process with a bipolar membrane. 

Nucleation and buildup or growth of the silica particles during the electrodialysis process were followed by measuring silica concentration and specific surface area of the silica sol as a function of time firom the moment electric current begins to flow through the sol-electrolyte solution. Average equivalent particle diameter of the silica particles and estimated number of silica particles... 

More often the sediment contains carbonate and hydroxide. Therefore, the most common method to remove the sediment is concentrate acidification or current reversal [18]. Electrodialysis reversal (EDR) is an electrodialysis process with periodic reversal of the electric field. As the direction of electric current is switched desalination chambers become concentrating ones and vice versa. Colloidal and organic precipitate accumulates in the desaHnation chamber. As this chamber becomes a concentration one after switching the current direction, the organic or colloidal precipitate is washed away with concentrate flow. 

Industrial cells consist of one or two hundred modules with membrane surfaces that can reach up to one m. These facilities are able to soften brackish water with flow rates spanning from a few hundred up to one thousand m per day, and all with an energy cost of about 1 kWh per m . The precise nature of the electrode reactions taking place in the compartments at both ends of the cell plays no direct role in the electrodialysis process. The intermembrane space has a thickness lower than 1 mm, in order to decrease the ohmic drop. However if the solution rec uires a stronger demineralising effect, then the ohmic drop can be very large because this solution will become poorly conducting. 

Fig. 2.39 Typical laboratory pilot-scale facilities using divided, parallel-plate cells, (a) The major components of a flow system for anodic electrosynthesis. After Reilly Tar and Chemical Corp.) (see also Fig. 6.7) (b) A small laboratory pilot facility based on the MP ElectroCell. Here, an electrodialysis process is being performed. The facility is also used for electrosynthesis by means of simple modification of the cell components and process streams. (Courtesy Electrosynthesis Co., Inc.), (c) A DEM cell for synthesis. (Courtesy Electricity Council Research Centre and University of Strathclyde.)...

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. 

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 membranes in electrodialysis stacks are kept apart by spacers which define the flow channels for the process feed. There are two basic types(3), (a) tortuous path, causing the solution to flow in long narrow channels making several 180° bends between entrance and exit, and typically operating with a channel length-to-width ratio of 100 1 with a cross-flow velocity of 0.3-1.0 m/s (b) sheet flow, with a straight path from entrance to exit ports and a cross-flow velocity of 0.05-0.15 m/s. In both cases the spacer screens are... 

Figure 15.21. Electrodialysis equipment and processes, (a) View of the components of an electrodialysis stack Lacey, 1978). (b) Flow pattern through an electrodialyzer for removal of NaCl from water Ionics Inc.), (c) Electroreduction with the use of an ion exchange diaphragm (d) Flowsketch of a three-stage electrodialysis for treatment of brackish water Rogers, in Belfort, 1984).

Sonin, A.A. and Isaacson, M.S. 1974. Optimization of flow design in forced flow electrochemical systems with special application to electrodialysis. Ind. Eng. Chem. Process Des. Develop. 13, 241-248. 

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...

Power Supply and Process Control Unit. Electrodialysis systems use large amounts of direct current power the rectifier required to convert AC to DC and to control the operation of the system represents a significant portion of a plant s capital cost. A typical voltage drop across a single cell pair is in the range 1 -2 V and the normal current flow is 40 mA/cm2. For a 200-cell-pair stack containing 1 m2 of membrane, the total voltage is about 200-400 V and the current about... 

Water transport in electrodialysis from the diluate to the concentrate process stream can affect the process efficiency significantly. If a convective flux as a result of pressure differences between flow streams can be excluded there are still two sources for the transport of water from the diluate to the concentrate solution. The first one is the result of osmotic-pressure differences between the two solutions, and the second is due to electro-osmosis that results from the coupling of water to the ions being transported through the membrane due to the driving force of an electrical potential. 

The membranes in an electrodialysis cell are separated by spacer gaskets as indicated in Figure 5.3, which shows schematically the design of a so-called sheet flow electrodialysis stack. The spacer gasket consists of a screen that supports the membranes and controls the flow distribution in the cell and a gasket that seals the cell to the outside and also contains the manifolds to distribute the process fluids in... 

Electrodialysis equipment and process design The performance of electrodialysis in practical applications is not only a function of membrane properties but is also determined by the equipment and overall process design. As far as the stack design is concerned there are two major concepts used on a large scale. One is the sheet-flow concept, which is illustrated in Figure 5.3 and the other is the so-called tortuous path concept, which is illustrated in Figure 5.5. 

The second typical technology applied for d. of water is -> electrodialysis. After appropriate pretreatment (as above), the feed solution is pumped through the unit of one or more stacks in series or parallel. The concentrated and depleted process streams leaving the last stack are recycled, or finally collected in storage tanks. The plants operate unidirectionally, as explained, or in reverse polarity mode, i.e., the current polarity is changed at specific time intervals (minutes to hours), and the hydraulic flow streams are reversed simultaneously, thus preventing the precipitation in the brine cells. 

Finally, electrodialysis may be mentioned, a process widely used to de-salt aqueous solutions. An electric field is applied across a stack of alternating cation-exchange and anion-exchange membranes. Ions in the electrolyte solutions between these membranes are transported till they meet a membrane of the same sign, so that electrolyte-rich and electrol)rte-freed solutions are created. The process involves conduction and electro-osmosis. Obviously, irreversible thermod mamics appears very suitable to describe the various flows... 

Desalination of seawater is one of the important applications of membrane processes. There are various ways to produce fresh water such as distillation, electrodialysis, membrane distillation, freezing, membrane bioreactor, and reverse osmosis. Among them, distillation is the most used technique, but RO is becoming more popular in the desalination industry. A flow diagram of a single-stage RO system is shown in Fig. 4. 

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