Membrane separation processes electrodialysis

Process Description Electrodialysis (ED) is a membrane separation process in which ionic species are separated from water, macrosolutes, and all uncharged solutes. Ions are induced to move by an electrical potential, and separation is facilitated by ion-exchange membranes. Membranes are highly selective, passing either anions or cations and very little else. The principle of ED is shown in Fig. 20-79. 

As discussed by Pletcher 24, electrodialysis is an electrically driven membrane separation process. The main use of electrodialysis is in the production of drinking water by the desalination of sea-water or brackish water. Another large-scale application is in the production of sodium chloride for table salt, the principal method in Japan, with production exceeding 106 tonne per annum. 

The four developed industrial membrane separation processes are microfiltration, ultrafiltration, reverse osmosis, and electrodialysis. These processes are all well established, and the market is served by a number of experienced companies. 

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

Refs. [i] Spiegler KS (1980) Salt water purification, 2nd edn. McGraw-Hill, New York [ii] Korngold E, Kock K, Strathmann H (1978) Desalination 24 129 [Hi] McRae WA (1983) Electrodialysis. In Porteous A (ed) Desalination technology developments and practice. Applied Science, London, pp 249-264 [iv] Strathmann H (2002) Membranes and membrane separation processes. In Ullmanns encyclopedia of industrial chemistry, 6th edn. Wiley-VCH [v] Levy J, Morgan J, Brown E (2004) Oxford handbook of dialysis. Oxford University Press... 

Electrodialysis (ED) is a membrane separation process, which exploits an electrical field as the driving force instead of pressure. Charged compounds are separated by ion-exchange membranes. In the pulp and paper industry, ED is being studied for the... 

The traditional membrane separation processes (reverse osmosis, micro-, ultra- and nanofiltration, electrodialysis, perva-poration, etc.), already largely used in many different applications, are today combined with new membrane systems such as CMRs and membrane contactors. Membranes are applied not only in traditional separation processes such as seawater desalination but also in medicine, bioengineering, microelectronics, the life in the space, etc. 

Available techniques for the removal of metal ions include chemical precipitation, ion exchange, evaporation, solvent extraction and a variety of membrane separation processes including reverse osmosis, ultrafiltration and electrodialysis [3]. Each of these methods has its own advantages but all lack the ability of certain electrochemical techniques to produce metal directly in a controlled fashion. 

As any membrane separation process elcctrodialysis is effected by concentration polarization and membrane fouling. The magnitude of concentration polarization is largely determined by the electrical current density, by the cell and particularly spacer design, and by the flow velocities of the diluate and brine solutions [39]. Concentration polarization effects electrodialysis lead to a depletion in the laminar boundary layer at the membrane... 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

Membrane separation processes have been applied to many industrial production systems for the purpose of clarification, concentration, desalting, waste treatment, or product recovery. Broadly speaking, membrane filtration can be classified as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and dialysis or electrodialysis. In this section, the discussion will only cover microfiltration and ultrafiltration, both of which are pressure-driven membrane processes. 

Membrane processes that use ion-exchange membranes and electric potential difference as the driving force for ionic species transport are referred to as electromembrane processes (Strathmann, 2004). The following electro-membrane separation processes (Scheme 5.1) can be distinguished electrodialysis (ED), including variations such as electrodialysis reversal, electro-electrodialysis and bipolar membrane electrodialysis, electrodeionization (EDI), and Donnan dialysis (DD). 

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. 

Advantages to Membrane Separation This subsertion covers the commercially important membrane applications. AU except electrodialysis are pressure driven. All except pervaporation involve no phase change. All tend to be inherently low-energy consumers in the-oiy if not in practice. They operate by a different mechanism than do other separation methods, so they have a unique profile of strengths and weaknesses. In some cases they provide unusual sharpness of separation, but in most cases they perform a separation at lower cost, provide more valuable products, and do so with fewer undesirable side effects than older separations methods. The membrane interposes a new phase between feed and product. It controls the transfer of mass between feed and product. It is a kinetic, not an equihbrium process. In a separation, a membrane will be selective because it passes some components much more rapidly than others. Many membranes are veiy selective. Membrane separations are often simpler than the alternatives. 

The track-etch membrane can be used in reverse osmosis and electrodialysis separation processes where it consists of a thin metal layer with a thin layer of insulator material on each side. The membrane pore diameters were in the range 0.5-10 nm. 

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