Electromembrane

Among electrochemical methods of water purification, one can also list the various electromembrane technologies, electrodialysis in particular. The simplest elec-trodialyzer consists of three compartments separated by semipermeable membranes (usually, cation- and anion-exchange membranes). The water to be purified is supplied to the central (desalination) compartment. In the outer (concentration) compartments, electrodes are set up between which a certain potential difference is applied. Under the effect of the electric field, ions pass througfi the membranes so that the concentration of ionic contaminants in the central compartment decreases. 

Lantagne and Velin [267] have reviewed the application of dialysis, electrodialysis and membrane cell electrolysis for the recovery of waste acids. Because of the new trends governed by environmental pressures, conventional treatment methods based on neutralization and disposal are being questioned. Membrane and electromembrane technologies are considered to be potential energy-efficient substitutes for conventional approaches. Paper mills will focus on the application of ion-exchange membranes namely dialysis, electrodialysis and membrane cell electrolysis for recovery of waste acids. 

T.A. Davis, V. Grebenyuk, O. Grebenyuk in Electromembrane Processes in Membrane Technology in the Chemical Industry (Eds. S.P. Nunes, K.-V. Peinemann), Wiley-VCH, Weinheim, 2001. 

Treatment of selected waste streams by activated carbon, ion exchange, electromembranes, chemical coagulation, sand, and dual and multimedia filtration. 

Nagarale RK, Gohil GS, Shahi VK (2006) Recent developments on ion-exchange membranes and electromembrane processes. Adv Colloid Interface Sci 119 97-130... 

Ahlgren, R. M. 1977. Electromembrane technology for whey processing. In Proceedings, Whey Products Conference 1976. Pub. No. ARS-NE-81, USDA ARS. Eastern Regional Research Center, Philadelphia. 

Rmp Apparent membrane pack electric resistance (O) rk Generic electromembrane surface resistance (flm2)... 

The long-term chemical stability of the electromembranes affects the economics of any ED application and is generally determined by assessing... 

In the same ED cell pair, the overall water transport through the electromembranes from the dilute stream to the concentrate one can be expressed by accounting for electroosmosis (i.e., the migration of water molecules associated with ions, this being proportional to j) and osmosis phenomena ... 

With reference to Figure 9, at the boundary layers adjacent each electromembrane in the diluting or concentrating compartment it is possible to establish the following ion mass balance by accounting for the ion transport corresponding to NP equation both in solution and membrane phases ... 

The main bottlenecks of ED application in the sugar industry were both the short membrane life, especially for the anion-exchange membranes, and the high viscosity of cane- or beet-sugar syrups, the maximum working temperature for the electromembranes being generally less than 40 °C. 

The effectiveness of this ED process increases with temperature, being presently limited by the maximum operating temperature (35°Q of the AMV and CMV electromembranes used (Table II). 

Kikuchi et al. (1995) made use of an ED stack composed of Selemion-CMV and -AMV electromembranes (Table II) to separate almost completely a mixture of amino acids, that is, glutamic acid, methionine, and lysine. In this way, while methionine was not affected by the voltage applied, glutamic acid or lysine was found be transported across the anion- or cation-exchange membranes, respectively. 

Thus, any ED unit design or optimization exercise relies on quite a great number of engineering parameters, such as ion transport numbers in solution (t+ and t ) and electromembranes (t, ) effective solute (ts) and water (tw)... 

The solute and water transport numbers in the electromembranes reported in the literature may be accurate enough to predict the solute concentrations in C and D tanks and may need no extra experimental trials. 

As an example of the application of the aforementioned sequence, Table XYI lists the main engineering parameters necessary to design or optimize ED stacks equipped with AMV and CMV electromembranes (Table II) and committed to the recovery of the sodium salts of some weak monocarboxylic acids of microbial origin (i.e., acetic, propionic, and lactic acid) and of a strong inorganic acid (i.e., chloride acid), as estimated by Fidaleo and Moresi (2005a,b, 2006). 

As the salt molecular mass (MB) increased from 58 to 112 Da, the transport number for Na+ in the corresponding solution tended to increase from 0.4 to 0.6 for the progressively smaller equivalent conductance at infinite dilution (20 ) of acetate, propionate, and lactate ions with respect to that of Cl-. Nevertheless, the current within the electromembranes was almost exclusively carried by the counterions. 

The maximum cell voltage, which varies in the range of 0.8-1.5 V/cell under the current density recommended by the manufacturers, tends to increase with time as the charged groups in the electromembranes vanish with use as a result of their chemicophysical reactions with the feed contaminants. Beyond such potential difference limits, it is generally advisable to replace the membranes to limit the overall electric power consumption. 

So, any attempt to minimize the specific electromembrane costs, especially now that the membrane market is open to new Far East manufacturers (Table II), is expected to reduce significantly the investment and membrane replacement costs. 

In conclusion, a greater knowledge of the effect of the key controlling parameters of this powerful separation technique, as well as improvement in membrane life time of the currently available commercial electromembranes and reduction in their costs, would ensure further growth beyond desalination and salt production and foster ED applications in the food sector, as well as in the chemical, pharmaceutical, and municipal effluent treatment areas. This will of course need extensive R D studies and will highly likely result in hybrid processes combining ED to other separation techniques, such as NF, IE, and so on, so as to shorten present downstream and refining procedures. 

Ahlgren, R.M. 1972. Electromembrane processing of cheese whey. In Industrial Processing with Membranes (R.E. Lacey and S. Loeb, eds), pp. 57-69. Wiley-Interscience, John Wiley Sons, New York. 

Nasr-Allah, A. and Audinos, R. 1994. A novel electromembrane process for recovery of tartaric acid and of an alkaline solution from waste tartrates. In Actes du Colloque of the Congres International sur le Traitement des Effluents Vinicoles , pp. 199-202. Narbonne and Epemay (F), June 20-24, 1994. 

Electromembrane processes such as electrolysis and electrodialysis have experienced a steady growth since they made their first appearance in industrial-scale applications about 50 years ago [1-3], Currently desalination of brackish water and chlorine-alkaline electrolysis are still the dominant applications of these processes. But a number of new applications in the chemical and biochemical industry, in the production of high-quality industrial process water and in the treatment of industrial effluents, have been identified more recently [4]. The development of processes such as continuous electrodeionization and the use of bipolar membranes have further extended the range of application of electromembrane processes far beyond their traditional use in water desalination and chlorine-alkaline production. 

The term electromembrane process is used to describe an entire family of processes that can be quite different in their basic concept and their application. However, they are all based on the same principle, which is the coupling of mass transport with an electrical current through an ion permselective membrane. Electromembrane processes can conveniently be divided into three types (1) Electromembrane separation processes that are used to remove ionic components such as salts or acids and bases from electrolyte solutions due to an externally applied electrical potential gradient. (2) Electromembrane synthesis processes that are used to produce certain compounds such as NaOH, and Cl2 from NaCL due to an externally applied electrical potential and an electrochemical electrode reaction. (3) Eletectromembrane energy conversion processes that are to convert chemical into electrical energy, as in the H2/02 fuel cell. 

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