Electrodialysis sheet-flow

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... 

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.

Electrodialysis equipment and process design The performance of electrodialysis in practical applications is not only a function of membrane properties but is also determined by the equipment and overall process design. As far as the stack design is concerned there are two major concepts used on a large scale. One is the sheet-flow concept, which is illustrated in Figure 5.3 and the other is the so-called tortuous path concept, which is illustrated in Figure 5.5. 

Stack design in bipolar membrane electrodialysis The key component is the stack which in general has a sheet-flow spacer arrangement. The main difference between an electrodialysis desalination stack and a stack with bipolar membranes used for the production of acids and bases is the manifold for the distribution of the different flow streams. As indicated in the schematic diagram in Figure 5.10 a repeating cell unit in a stack with bipolar membranes is composed of a bipolar membrane and a cation- and an anion-exchange membrane and three flow streams in between, that is, a salt... 

As a guide to sizes of existing sheet flow types of electrodialysis plants, Table I compares the membrane area of the FSG plant with those of the next largest commercial plants known to be in commercial operation in early 1961, and which were operating with multiple packs in a press. 

The operation of the FSG plant for over 18 months has given the first opportunity for assessing the characteristics of a fully integrated plant designed for multimillion gallons per day output, using a sheet-flow type of electrodialysis apparatus. 

A large-scale sheet flow type of electrodialysis plant can be operated satisfactorily and reliably when large membranes and large presses are used. 

Fig. 9 Schematic diagram of a) a tortuous-path electrodialysis spacer gasket, and b) a sheet-flow clectrodialysis spacer gasket...

In stack designs employing the sheet-flow principle, a peripheral gasket provides the outer seal and the solution flow is approximately in a straight path from the entrance to the exit ports which are located on opposite sides in the gasket. This is illustrated in Figure 7 a) which shows the schematic diagram of a sheet-flow spacer of an electrodialysis stack. 

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. 

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