Tortuous-path spacer

Two basic flow schemes are used tortuous path flow and sheet flow (Fig. 22-59). Tortuous path spacers are cut to provide a long path between inlet and outlet, providing a relatively long residence bme and high velocity past the membrane. The flow channel is open. Sheet flow units have a net spacer separating the membranes. Mass transfer is enhanced either by the spacer or by higher velocity. 

In dilute solutions, the avoidance of local or general polarization is the most crucial task of the cell designer. The so-called tortuous path spacer is utilized in Ionics electric membrane systems (8). A tortuous path spacer consists of one or more flow paths—for example, approximately 1 cm. in width—traversing the face of a mem-... 

Late in 1958, Ionics began field tests of a new tortuous path spacer called the Mark... 

A new commercially available cation membrane, Nepton CR-61 on 4-ounce dynel backing, is 23 mils thick and has an electrical through resistance in 0.01A NaCl of 14 ohms per sq. cm., about two thirds that of a similar membrane 30 mils thick on 9-ounce dynel backing. It has the same electrical resistance as a 9-ounce glass-backed CR-61 membrane, also 30 mils thick, but much better chemical stability to acids and alkalies and better mechanical stability to vibrations. The Mullen burst strength of the 4-ounce membranes is 140 p.s.i., about half that for the 9-ounce dynel, but still sufficient to be useful for field operation. Its stiffness and resistance to bowing, for which no standard measurements are yet available, are sufficient to allow its use in membrane stacks with tortuous path spacers operating at pressures of 60 p.s.i. and flow velocities of 60 cm. per second. 

The allowable current density—normality ratio for electric membrane operation has been approximately doubled by an improved tortuous path spacer with strap turbulence promoters and by operation at higher pressures up to 60 p.s.i. As a result, twice as much water can now be demineralized per square foot of membrane utilized and/or greater demineralization achieved per pass in electric membrane units. One practical result of this development is a new continuous-flow, two-stage single-stack demineralizer which will provide 93% demineralization at a capacity of 5000 gallons per day and 72% demineralization at a capacity of 30,000 gallons per day. These units produce from 67 to 150% more water per unit membrane area than previously used automatic batch-recirculating units and are far simpler in construction and operation. 

Figure 5.5 Schematic drawing illustrating the sheet-flow and a tortuous-path spacer concept.

Figure 8.5 Diagram of a tortuous-path spacer 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... 

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... 

FIG. 6 Schematic diagrams of (A) tortuous-path and (B) sheet-flow ED spacer gaskets. 

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

In normal electrodialysis the spacers are made of nonconductive material. However, ion-conducting spacers are used to receive extremely desalinated water. Depending on design, the fluid between the membranes can move through channels, which form either a tortuous path or a set of parallel paths. 

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. 

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 ... 

FIG. 22-59 Schematic of two ways to pass solution across an ED membrane. Tortuous flow (left) uses a special spacer to force the solution through a narrow, winding path, raising its velocity, mass transfer, and pressure drop. Sheet feed (right) passes the solution across the plate uniformly, with lower pressure drop and mass transfer. Coutiesy Elsevier.)... 

---