Electrodialysis 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. 

Figure 8.17. Schematic flow-diagram for an electrodialysis stack...

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... 

Figure 15.21. Electrodialysis equipment and processes, (a) View of the components of an electrodialysis stack Lacey, 1978). (b) Flow pattern through an electrodialyzer for removal of NaCl from water Ionics Inc.), (c) Electroreduction with the use of an ion exchange diaphragm (d) Flowsketch of a three-stage electrodialysis for treatment of brackish water Rogers, in Belfort, 1984).

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...

Figure 10.8 Schematic of the concentration gradients adjacent to a single cationic membrane in an electrodialysis stack. The effects of boundary layers that form on each side of the membrane on sodium ion concentrations are shown...

A portion of the electric current can be carried by the stack manifold, bypassing the membrane cell. Modem electrodialysis stack designs generally make losses due to this effect negligible. 

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. 

Figure 10.18 Schematic of the electrodeionization process using mixed-bed ion exchange resin to increase the conduction of the dilute compartments of the electrodialysis stack...

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. 

The membranes in an electrodialysis cell are separated by spacer gaskets as indicated in Figure 5.3, which shows schematically the design of a so-called sheet flow electrodialysis stack. The spacer gasket consists of a screen that supports the membranes and controls the flow distribution in the cell and a gasket that seals the cell to the outside and also contains the manifolds to distribute the process fluids in... 

Figure 5.3 Exploded view of a sheet-flow-type electrodialysis stack arrangement indicating the individual cells and the spacer gaskets containing the manifold for the distribution of the different flow streams.

The concentration polarization occurring in electrodialysis, that is, the concentration profiles at the membrane surface can be calculated by a mass balance taking into account all fluxes in the boundary layer and the hydrodynamic conditions in the flow channel between the membranes. To a first approximation the salt concentration at the membrane surface can be calculated and related to the current density by applying the so-called Nernst film model, which assumes that the bulk solution between the laminar boundary layers has a uniform concentration, whereas the concentration in the boundary layers changes over the thickness of the boundary layer. However, the concentration at the membrane surface and the boundary layer thickness are constant along the flow channel from the cell entrance to the exit. In a practical electrodialysis stack there will be entrance and exit effects and concentration... 

Since the thickness ofthe laminar boundary in an electrodialysis stack is difficult to determine in an independent measurement, the limiting current density in practical application is generally not calculated by Equation 5.18 but by an experimentally determined relation which describes the limiting current density as a function to the feed-flow velocity in the electrodialysis stack [4]. The limiting current density is expressed by ... 

Figure 5.8 Flow scheme of an electrodialysis stack operated in a feed and bleed mode, that is, with partial recycling of the diluate and concentrate solutions.

The total energy required in electrodialysis for the actual desalination process is given by the current passing through the electrodialysis stack multiplied with the total voltage drop encountered between the electrodes ... 

Because of the relatively high concentrations of the acid and base as well as the salt solution the limiting current density is in general no problem and a bipolar membrane stack can generally be operated at very high current densities compared to an electrodialysis stack operated in desalination. However, membrane scaling due to precipitation of multivalent ions such as calcium or heavy-metal ions is a severe problem in the base-containing flow stream and must be removed from the feed stream prior to the electrodialysis process with a bipolar membrane. 

Investment costs in electrodialysis with bipolar membranes Investment costs include nondepreciable items such as land and depreciable items such as the electrodialysis stacks, pumps, electrical equipment, and monitoring and control devices. The investment costs are determined mainly by the required membrane area for a certain plant capacity. The required membrane area for a given capacity plant can be calculated from the current density in a stack that is in electrodialysis with a bipolar membrane not limited by concentration-polarization effects. The required membrane area for a given plant capacity is given by ... 

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... 

Fig. 21. Schematic configuration of an electrodialysis stack used for metal removal.

COLOR FIGURE 21.41 Electrodialysis stacks, (a) Aqualyzer EDC (conventional ED) and (b) aqualyzer EDBP (EDBMs). (Erom Eurodia Co.)... 

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