Electrodialysis design

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. 

Electrodialysis is compatible with most plating baths, and the design size of a unit is dependent upon the rinsewater flow rate and concentration.22... 

The value of /jim is determined by the discontinuity in the dependence of cell current on applied cell voltage which occurs when the interfacial concentration approaches zero. The polarisation parameter is convenient in the design and scale-up of electrodialysis equipment. It can be easily measured in small-scale stacks at a given value of bulk concentration and then used to predict limiting current densities in larger stacks at other concentrations. Most stacks use operating values of the polarisation parameter that are 50-70 per cent of the limiting values. 

In AAC technologies, water is exposed to an AAC material, and metals in the water are adsorbed by the material. AAC systems can be designed and built as stand-alone units or integrated to work efficiently in concert with complementary water treatment systems designed for hydrocarbon removal, pH control, particulate removal, or electrodialysis. AAC systems can tolerate hard water (calcium and magnesium) and high temperatures (up to 200°F) without a decrease in performance. 

T. Mohammadi, A. Moheb, M. Sadrzadeh and A. Razni, Separation of copper ions by electrodialysis using Taguchi experimental design, Desa-linination, 169(1), 2004, 131-139. 

Only one basic design of electrodialysis equipment for demineralization appears to be in use. This is an assembly of alternate cation and anion ion exchange sheets separated by spacers in groups of several hundred clamped together between electrodes. The assembly physically resembles a plate-and-frame filter press. Figures 15.21(a) and (b) show such assemblies, and some dimensional data were stated in Section 15.5, Electrodialysis. 

A process that was no more than a laboratory phenomenon when the Office of Saline Water was authorized in 1952 has been rapidly developed to the point where it is now one of the most economical processes for the conversion of brackish water to fresh. This process, electrodialysis, was selected for the third demonstration plant. It will be located at Webster, S. D. The Bureau of Reclamation Laboratories in Denver, Colo., prepared the specifications for this plant which will be designed to produce 250,000 gallons of fresh water per day. Operating on water containing between 1500 and 1800 parts of salt per million parts of water, this plant will remove more than 1 ton of salt daily to produce product water with less than 500 parts of salt per million parts of water which is required to meet U. S. Public Health Standards for good drinking water. Construction specifications for this plant were available August 1, and sealed bids will be opened on October 4, 1960. 

W., Design, Construction and Field Testing Cost Analyses on the Experimental Electrodialysis Demineralizer for Brackish Waters, Ionics, Inc., Office of Saline Water, Research and Development Progress Report, No. 11, Washington, D. C., 1956. 

Interaction of Technical and Economic Demands in the Design of Large Scale Electrodialysis Demineralizers... 

I n the design of an electrodialysis plant, economic and technical aspects are powerful determinants. In general, economic factors control the quantity of membranes required, and technical considerations govern their arrangement. 

Here, then, is the technical demand in electrodialysis plant design that conditions be fixed so that operation is always near the limiting current. This demand was revealed in experimental studies it is not merely a theoretical nicety, but a practical requirement. Fortunately, on each side of the critical conditions there is some leeway which allows practical designs at small penalties, but it is necessary to change the flow rate from stage to stage downstream in order to maintain limiting current. 

To produce low-cost water, a plant must meet two main conditions a moderate cost per square foot of active area and a relative freedom from expensive supervision and frequent overhaul. These conditions can be met by large cells. In fact, large scale, community-sized electrodialysis plants can allow the freedom of design needed to make desalinization economically feasible. 

This paper reviews the most recent innovations in electrodialysis (ED) modules and/or processes that appear to affect the food and drinks industries in the short-medium term, together with their basic mass transport equations that might help ED unit design or optimization. Future perspectives for ED processing in the food sector are also outlined. 

Bailly, M., Roux-de Balmann, H., Aimar, P., Lutin, F., and Cheryan, M. 2001. Production processes of fermented organic acids targeted around membrane operations Design of the concentration step by conventional electrodialysis. J. Membr. Sci. 191, 129-142. 

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. 

A portion of the electric current can be carried by the stack manifold, bypassing the membrane cell. Modem electrodialysis stack designs generally make losses due to this effect negligible. 

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...

The efficiency of electrodialysis is determined to a large extent by the properties of the membranes. But it is also affected by the process and system design that determine the limiting current density, the current utilization, the concentration polarization and the overall efficiency and costs [20, 21]. 

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... 

The limiting current density is determined by concentration-polarization effects at the membrane surface in the diluate containing compartment that in turn is determined by the diluate concentration, the compartment design, and the feed-flow velocity. Concentration polarization in electrodialysis is also the result of differences in the transport number of ions in the solution and in the membrane. The transport number of a counterion in an ion-exchange membrane is generally close to 1 and that of the co ion close to 0, while in the solution the transport numbers of anion and cations are not very different. 

Current utilization In practical application electrodialysis is affected by incomplete current utilization. The reasons for the incomplete current utilization are poor membrane permselectivity, parallel current through the stack manifold, and water transport by convection and due to osmosis and electro-osmosis. In a well-designed stack with no pressure difference between diluate and the concentrate convective water transport is negligibly low and also the current through the manifold can be neglected. Under these conditions the overall current utilization is given by ... 

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. 

Electrodialysis with Bipolar Membrane System and Process Design... 

The design of an electrodialysis process with bipolar membranes is closely related to that of a conventional electrodialysis desalination process. 

Stack design in bipolar membrane electrodialysis The key component is the stack which in general has a sheet-flow spacer arrangement. The main difference between an electrodialysis desalination stack and a stack with bipolar membranes used for the production of acids and bases is the manifold for the distribution of the different flow streams. As indicated in the schematic diagram in Figure 5.10 a repeating cell unit in a stack with bipolar membranes is composed of a bipolar membrane and a cation- and an anion-exchange membrane and three flow streams in between, that is, a salt... 

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