Sheet-flow stack

Process Configuration Figure 22-56 shows a basic cell pair. A stack is an assembly of many cell pairs, electrodes, gaskets, ana manifolds needed to supply them. An exploded schematic of a portion of a sheet-flow stack is shown in Fig. 22-60. 

In tortuous-path stacks there is no need for spacer screens as thicker membranes, narrow channels, and plenty of cross-straps are used. On the contrary, in sheet-flow stacks spacers of different geometry and thickness are necessary to prevent membrane contact (that would result in burning through), as well as to induce turbulence in the flowing solution (Kuroda et al., 1983). Spacers generally consist of a sealing frame and a net in the... 

Solution flow velocities in sheet-flow stacks lie typically between 4 and 10 cm/sec, whereas in tortuous-path stacks solution flow velocities of 10 to 30 cm/s are required [42, 43]. Because the higher flow velocities and longer flow paths, higher pressure drops in the order of 2 to 3 bars result in tortuous-path stacks than in sheet-flow systems where pressure drops of 1 to 2 bars occur. 

In sheet-flow stacks, spacer screens almost always are needed because the width between gaskMing devices is ttrach greater than that in the usual tortuous-path stacks. The solution flow in sheet-flow stacks is in approximately a straight path from one or mote entrance ports to an equal number of exit ports as indicate in Fig. 21.2-3. As the solution flows in and around the filaments of the spacer screens, a mixing action is Imparted to the solutions to aid in reducing the thicknesses of difiusional boundary layers at the surfaces of the membranes. 

Solution velocities in sheet-flow stacks are typically in the range of 5-14 cm/s, whereas the velocities in tortuous-path stacks usually are much higher, 30-100 cm/s. The drtq> in hydraulic pressure through a sheet-flow stack, and thus the pumping power, is normally lower than that through a tortuous-path stack because of the lower velocities and shorter path lengths. 

Calculate the pressure drop in a sheet-Qow ED stack and in a tortuous path stacks with and without spacer lire stack contains 100 cell pairs. The simension of one compartment is. length of 1 m. a width of 0.5 m and a height of 1 mm. The tortuous path is divided in 5 sections. The average velocity is 10 cm/s for the sheet-flow stack and 25 cm/s for the tortuous path stack. Density and viscosi ate equal to that of water. Other data are ... 

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 differences in vs, direction changes, and path length make the pressure drop (200-400 kPa) through a tortuous-path stack by far greater than that (20-70 kPa) in a sheet-flow one, as reported by Lacey (1972) and Strathmann (1992). 

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... 

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. 

Figure 2.36 Schematic drawing of a sheet flow design ED stack. Source [52].

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