Membranes for electrodialysis

The electncol resistance of ion-exchange membranes lie in the range of 2 - U) Q.cm and the charge density is about 1 - 2 mequiv/g dry polymer. 

The most important application of electrodialysis is the production of potable water from brackish water [82]. Avery special application is the reverse case, the production of salt. In the latter case the concentrate is the product stream whereas in the former case the diluate stream is the product. Moreover, there is an increasing number of industrial applications where ions have to be removed from.a process stream such as demineralisation of whey, deacidification of fruit juices, production of boiler feed water, removal of organic acids from a fermentation broth.lt is even possible to separate amino acids from each other as will be shown below. 

Amino acids contain both a basic and an acidic group and because of this amphoteric character the molecule can be positively or negatively charged depending on the pH of the 

In membrane electrol sis an electrolysis process is combined w ilh a membrane separation process. The classical example is the chlor-alkali process in which sodium chloride is converted into chlorine and caustic soda. Other examples are the electrolytic recovery of (hea 7) metals and the production of acid and base from the corresponding salts. 

---

Linkov VM and Belyakov VN. Novel ceramic membranes for electrodialysis. Sep Purif Technol 2001 25(Suppl. l-3) 57-63. 

Membranes for electrodialysis and polymer electrolyte membrane fuel cell (PEMFC) have electric charges. Most of the nanofiltration membranes also carry negative charges. The content of electric charge in a polymer is given by ion-exchange capacity (meq (milliequivalent)/g of dry polymer). 

S. Tsuneda, K. Saito, H. Mitsuhara, T. Sogo, Novel ion-exchange membranes for electrodialysis prepared by radiation-induced graft polymerization, J. Electrochem. Soc., 1995, 142, 3659-3663. 

G.A. Gutter and H.K. Bishop, Investigation of inorganic ion exchange membranes for electrodialysis application, Office of Saline Water, Research and Development Progress Report, No. 279. 

R.B. Hodgdon, E. Witt and S.S. Alexander, Macroreticular anion exchange membranes for electrodialysis in the presence of surface water foulants, Desalination, 1973, 13, 105-127. 

T. Sata, S. Emori and K. Matsusaki, Thermally responsive novel anion exchange membranes for electrodialysis, J. Chem. Soc., Chem. Commun., 1998, 1303. 

Electrodialysis has advantages and disadvantages. For instance, the process requires very little energy and can recover highly concentrated solutions. On the other hand, similarly to other membrane processes, electrodialysis membranes are susceptible to fouling and must be regularly replaced. 

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... 

An additional interesting application of ion-exchange membranes for the treatment of electroorganic product solutions is the electrodialysis (e.g. [70, 72]). It... 

M. Peri, Hydrophobic solvent type charged membranes for selective electrodialysis, Ph.D. thesis, The Weizmann Institute of Science, Rehovot, Israel, 1980. 

The electric membrane or electrodialysis process for removing excess dissolved salts and minerals from water is rapidly increasing in use, both in the United States and abroad. In the United States for example, as of January 1, 1958, there were only two production—i.e., nonexperimental—electric membrane plants in operation. By April 1, 1960, only 21/4 years later, 11 plants with a combined capacity of 350,000 gallons per day were serving almost 10,000 people in Montana, Texas, Alaska, New York State, California, Utah, South Dakota, Arizona, and Illinois. 

The two water desalination applications described above represent the majority of the market for electrodialysis separation systems. A small application exists in softening water, and recently a market has grown in the food industry to desalt whey and to remove tannic acid from wine and citric acid from fruit juice. A number of other applications exist in wastewater treatment, particularly regeneration of waste acids used in metal pickling operations and removal of heavy metals from electroplating rinse waters [11]. These applications rely on the ability of electrodialysis membranes to separate electrolytes from nonelectrolytes and to separate multivalent from univalent ions. 

M. Seko, H. Miyauchi and J. Omura, Ion Exchange Membrane Application for Electrodialysis, Electroreduction, and Electrohydrodimerisation, in Ion Exchange Membranes, D.S. Flett (ed.), Ellis Horwood Ltd, Chichester, pp. 121-136 (1983). 

FIG. 22-48 Transport mechanisms for separation membranes a) Viscous flow, used in UF and MF. No separation achieved in RO, NF, ED, GAS, or PV (Z ) Knudsen flow used in some gas membranes. Pore diameter < mean free path, (c) Ultramicroporous membrane—precise pore diameter used in gas separation, d) Solution-diffusion used in gas, RO, PV Molecule dissolves in the membrane and diffuses through. Not shown Electrodialysis membranes and metallic membranes for hydrogen. 

The future for electrodialysis-based wastewater treatment processes appears bright. The dilute concentrations of metals in the waste streams do not degrade or foul the cation or anion exchange membranes. The concentrate streams are recirculated to build up their metal content to a level that is useful for further recovery or direct return to the process stream. Ongoing research in the development of cheaper cation exchange membranes, and stable anion exchange and bipolar membranes will allow electrodialysis-based applications to become more competitive with other treatments. 

The first widespread use of polymeric membranes for separation applications dates back to the 1960-70S when cellulose acetate was cast for desalination of sea and brackish waters. Since then many new polymeric membranes came to the market for applications extended to ultrafiltration, miciofiltration, dialysis, electrodialysis and gas separations. So far ultrafiltration has been used in more diverse applications than any other membrane processes. The choice of membrane materials is dictated by the application environments, the separation mechanisms by which they operate and economic considerations. Table 1.4 lists some of the common organic polymeric materials for various membrane processes. They include, in addition to cellulose acetate, polyamides. 

Several examples of cells employing ion-exchange membranes have been given in the section on cell construction. Ion-exchange membranes are also used for electrodialysis. 

Concentration of HI over Hix solution by polymer electrolyte membrane electrodialysis was investigated using galvanodynamic and galvanostatic polarisation method. For this purpose, Hix solution with sub-azeotrope composition (HI L HjO = 1.0 0.5 5.8) was prepared. It was noticed that the electrical energy demand for electrodialysis of Hix solution decreases with increasing temperature. From the experimental results, it is concluded that the system resistance crucially affects the electrodialysis cell overpotential and hence the optimisation of cell assembly as well as the selection of low resistance materials should be carried out in order to obtain high performance electrodialysis cell. 

For electrodialysis, a membrane is placed between electrodes of an otherwise conventional electrolysis cell to carry out standard electrochemical processes. The membrane in this type of application serves to separate the products of the electrode reaction. This is shown in Figure 36-7, p. 426. 

---