Electrodialysis system 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. 

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. 

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 gaskets not only separate the membranes but also contain manifolds to distribute the process fluids in the different compartments. The supply ducts for the diluate and the brine are formed by matching holes in the gaskets, the membranes, and the electrode cells. The distance between the membrane sheets, i.e. the cell thickness, should be as small as possible to minimize the electrical resistance. In industrial size electrodialysis stacks membrane distances are typically between 0.5 to 2 mm. A spacer is introduced between the individual membrane sheets both to support the membrane and to help control the feed solution flow distribution. The most serious design problem for an electrodialysis stack is that of assuring uniform flow distribution in the various compartments. In a practical electrodialysis system, 200 to 1000 cation- and anion-exchange membranes are installed in parallel to form an electrodialysis stack with 100 to 500 cell pairs. 

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...

Electrodialysis with Bipolar Membrane System and Process Design... 

For radioactive effluent treatment, the relevant membrane processes are microfiltration, ulfrafiltration (UF), reverse osmosis, electrodialysis, diffusion, and Donnan dialysis and liquid membrane processes and they can be used either alone or in conjunction with any of the conventional processes. The actual process selected would depend on the physical, physicochemical, and radiochemical nature of the effluents. The basic factors which help in the design of an appropriate system are permeate quality, decontamination, and VRFs, disposal methods available for secondary wastes generated, and the permeate. 

AMJ has a completely new design comprising the use of tubular cells. A one-membrane prototype unit was built by AMJ and successfully tested as a nickel recovery system by Special Products. It appears that the tubular design could be adopted to make two-membrane cells for our electrodialysis process. Details are given in Appendix B. 

AMJ, located in Great Neck, NY, sells industrial equipment for electrodialysis and Donnan dialysis. Its special products division has tested successfully a prototype unit designed and built by AMJ for nickel recovery systems. At the time we were working in electrodialysis (1974), AMJ was engaged in further development work with Special Products under a secrecy agreement. 

AMJ designs and sells the AMJ Electro Cell System for plastic preplate etch. This is an electrodialysis operation that requires the reoxidation of the reduced chromium in solutions that contain pure chromic acid. 

The AMJ Electro Cell System is an electrodialysis unit with a tubular design claimed to be free of leaks and more efficient than the conventional rectangular stack design. This is a two-compartment system which according to AMJ can be adapted to make the three-compartment cell required by our sodium silicate electrodialysis process to make colloidal silica. 

Tremendous opportunity exists for hybrid processes consisting solely of membrane processes or a combination of membrane and non-membrane processes. Of the large number of potential combinations, studies of several are reported in the literature including nanofiltration with reverse osmosis [99] nanofiltration with electrodialysis [100] ultrafiltration with nanofiltration and reverse osmosis [101] ultrafiltration with membrane distillation [102] nanofiltration with reverse osmosis and a microfiltration membrane-based sorbent [103] microfiltration with flotation [104] microfiltration and ultrafiltration with ozone and activated carbon adsorption [105] and membrane processes with photocatalysis [106-107]. Despite the activity in this area, a comprehensive approach to designing hybrid systems does not exist future work would benefit from the development of such a design framework. 

This system, which is based upon electrodialysis, has been designated Brandy (boron regeneration d electro-dialysis) and is shown in diagrammatic form in Fig. 2 (ref. 4). 

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