Electrodialysis cell pair

To illustrate the concentration polarization phenomenon, we consider an infinitely long electrodialysis cell pair having parallel channels in which the flow is fully developed and laminar. The qualitative behavior of the development of the salt concentration and potential distributions along the channels of a dialysate and concentrate cell pair are shown schematically in Fig. 6.2.3 for the case where the inlet salt concentrations are the same in both channels (Probstein 1972). 

Figure 6.2.3 Development in electrodialysis cell pair of (A) salt concentration, and (B) potential distribution. [After Probstein, R.F. 1972. Desalination Some fluid mechanical problems. Trans. ASME J. Basic Eng. 94, 286—313. With permission.]...

To model the electrodialysis stack, we assume that since there are many cells in a stack the behaviors in different pairs of adjacent dialysate and concentrate channels are the same. If we neglect the potential drop in the electrode cells adjacent to the electrodes as small compared with that in the rest of the system, the potential drop across a channel pair is constant and equal to the total applied voltage divided by the number of channel pairs. The dialysate and concentrate channels are taken to have the same separation 2h (Fig. 6.2.1). Since there is symmetry about the center plane of each channel, we may model the electrodialysis cell pair of Fig. 6.2.1 by one half of the dialysate channel and one half of the adjacent concentrate channel separated by a membrane, as shown in Fig. 6.2.4. For specificity we choose the cation exchange membrane. Both types of membranes are assumed to have the same resistances and thicknesses and to be perfectly selective. To simplify the problem somewhat further, we take the membrane resistance to be small so that the ohmic drop within the membranes may be neglected. 

Figure 10.2 Schematic diagram of a plate-and-frame electrodialysis stack. Alternating cation- and anion-permeable membranes are arranged in a stack of up to 100 cell pairs...

Many plants use a single electrodialysis stack, as shown in Figure 10.12. Manifolding may be used to allow the feed and brine solutions to pass through several cell pairs, but the entire procedure is performed in the single stack. 

Power Supply and Process Control Unit. Electrodialysis systems use large amounts of direct current power the rectifier required to convert AC to DC and to control the operation of the system represents a significant portion of a plant s capital cost. A typical voltage drop across a single cell pair is in the range 1 -2 V and the normal current flow is 40 mA/cm2. For a 200-cell-pair stack containing 1 m2 of membrane, the total voltage is about 200-400 V and the current about... 

The electrod ialysis stack A key element in electrodialysis is the so-called stack, which is a device to hold an array of membranes between the electrodes that the streams being processed are kept separated. A typical electrodialysis stack used in water desalination contains 100-300 cell pairs stacked between the electrodes. The electrode containing cells at both ends of a stack are often rinsed with a separate solution which does not contain Cl- ions to avoid chlorine formation. 

Gering and Scamehorn [86] studied the removal of CdCl2 and CdS04 from water by electrodialysis using a platinum-coated columbium anode and a Hastelloy cathode. Ionics (Watertown, MA) cation and anion exchange membranes were used in the electrodialysis stack. The effective cell pair area, defined as half the area of... 

Stack, and a potential difference sufficient to force current through the stack is applied between the two electrodes placed at each end of the stack. For current to pass between the electrodes, ions must be transported through each of the membranes. By arranging the feeds to the various intermembrane compartments, it is possible to force ionic salts to pass from the dilute stream to the concentrated stream. In this way, a salt can also be split into its acid and base components. By combination of several cell pairs that comprise an anion- and a cation-selective membrane sheets in parallel, a stream concentrated in the original salts may be prepared. This configuration is the common method for industrial use, in which electrodialysis gives broadly the same result as reverse osmosis and has found very similar applications to general water treatment. 

The data in Figure 8.16 show the results of an experiment to determine the values of i jm. Those data were obtained from electrodialysis of a solution of NaCl in a stack containing ten cell pairs.21 The applied voltage, current, and pH of the depleted product water were monitored. A control experiment with one cell pair in the stack was used to determine the voltage drop attributable to the electrodes and rinse streams. The cell-pair resistance, Rcp = (Vapp - V ec) x Acp/I, was observed to increase slightly for moderate increases in the polarization parameter, i/N. [The polarization parameter is defined as the current density, i = l/Acp, divided by the log-mean concentration of the depleting stream, N = (C n - Cout)/ln(Cjn/Cout).]... 

The salt solution is pumped through the dialysate and concentrate channels, with salt removed continuously along the length from the dialysate channel and transferred to the concentrate channel. A dialysate and concentrate channel with the associated membranes are termed a cell pair. A typical electrodialysis stack may have 50 to 300 cell pairs between a single pair of electrodes, and a number of stacks may be used in series to achieve the desired level of salt removal. 

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. 

In the examples following in which the invention is demonstrated in whole or in part by electrodialysis, the electrodialysis unit is a cell pair comprised of a single coated titanium base anode common to two cells in the following sequence ... 

Fig. 7.16 Three flow patterns for electrodialysis cells to reduce salt concentration from 3200 mgdm to 400 mg dm" The relative currents in each stack are shown, (a) Eight pairs of membranes in series in one stack, (b) Three stacks in series, each stack with N pairs of membranes, (c) Batch process with recycle through one stack.

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

A schematic of the production of acid and base by electrodialytic water dissociation is shown in Fig. 20-84. The bipolar membrane is inserted in the ED stack as shown. Salt is fed into the center compartment, and base and acid are produced in the adjacent compartments. The bipolar membrane is placed so that the cations are paired with OH" ions and the anions are paired with H. Neither salt ion penetrates the bipolar membrane. As is true with conventional electrodialysis, many cells may be stacked between the anode and the cathode. 

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