This description of module construction will first discuss the overall device and then the individual cell. Commercially available devices are produced in two main configurations: plate-and-frame or spiral wound. The plate-type devices are similar in concept to a plate-and-frame heat exchanger, with multiple fluid compartments sandwiched between a set of endplates (and electrodes) that are held in compression by bolts or threaded rods. The compartments alternate between diluting and concentrating, and are hydraulically in parallel but electrically in series. An exploded view of a typical plate-and-frame EDI device is shown in Figure 1.
Plate-and-frame EDI device
The spiral EDI devices are analogous to a spiral wound membrane element, but with the membrane, resins, and spacers wound spirally around a center electrode rather than a permeate tube. Spiral wound devices must be installed inside a pressure vessel, while plate-and-frame devices incorporate some means of sealing on the individual fluid compartments, essentially making each a pressure vessel. Spiral wound devices are somewhat more difficult to assemble than plate-and-frame units. A typical spiral device is shown in Figure 2.
Spiral-wound EDI device
The cells themselves can be classified at either thin cell or thick cell. Thin-cell devices are those with a spacing of approximately 1.5-3.5 mm between the ion exchange membranes in the diluting compartments, while thick cell devices typically use intermembrane spacing of 8-10 mm. Both plate-and-frame and spiral wound configurations are suitable for either thin or thick cell construction. As will be shown below, thin-cell devices allow the use of intimately mixed anion and cation exchange resins in the product compartments, while thick cells work best with separate regions that contain primarily resins of the same polarity.
All commercial EDI devices use ion exchange resin in the diluting compartments, and therefore require a component to contain the resin. This 'resin spacer' consists of an inlet port, an inlet distributor, the resin compartment, an outlet distributor, and an outlet port. It is necessary to provide a means of sealing the ion exchange membrane against the spacer to form the sides of the resin compartment. Some designs will also include additional ports to allow slurrying the resin in and out of the cell. A typical dilute spacer for a plate-and-frame EDI device is shown in Figure 3.
Dilute spacer from thick cell, layered bed EDI module
All EDI devices will also require flow compartments for the concentrate and electrode streams as well. The two types most commonly used are either a flow-through screen or a resin compartment. The flow-through screen is similar to a sheet-flow electrodialysis spacer. It generally consists of a woven plastic mesh screen (also like an RO feed spacer) that incorporates some sort of sealing mechanism, such as a rubber gasket interpenetrated in the perimeter of the screen.
The use of screen-type concentrate spacers is quite common in EDI devices, as they are fairly inexpensive and relatively easy to fabricate. Their major disadvantage is that they are not conductive. Since the makeup water feeding the concentrate compartments is normally RO permeate (to avoid scaling and fouling), the concentrate stream is not very conductive, in spite of the ions transferring into the concentrate from the diluting compartments. For example, a EDI system fed RO permeate with a conductivity of 5 µS/cm and operating at 90% water recovery would typically have a concentrate outlet conductivity of about 50 µS/cm. This is low enough to limit the amount of current that can be passed through the module (see discussion of resistance, below). Several manufacturers recommend injection of salt into the concentrate to raise the conductivity to 300 µS/cm or more.
An alternative to the use of screen-type spacers for the concentrate and electrode compartments is to use a resin-filled compartment similar to the ones used for the diluting compartments. By employing a conductive filler, the use of salt injection can be avoided. It has been found that injection of a salt solution into a resin-filled concentrate compartment has little effect on module resistance.
Use of the same size compartments for both diluting and concentrating compartments would typically require a concentrate recirculation pump to maintain adequate velocity in the concentrate while limiting the water sent to drain, to provide high water recovery. An alternative approach is to make the concentrate resin compartments thinner than the dilute resin compartments, to avoid the use of a concentrate recirculation pump.
Electrode Reactions and Material Selection
At the cathode, or negatively charged electrode, electrons are transferred from the external circuit to ions in the solution by the following reaction:
H2O+ e-->½ H2 + OH-
Therefore an electrode that is stable in the presence of base and hydrogen is required. The most common cathode material for EDI devices is stainless steel.
At the anode, or positively charged electrode, electrons are transferred from ions in solution to the external circuit by one or more of the following reactions:
½ H2O -> ¼ O2 + H+ + e-
Cl- -> ½ Cl2 + e-
Commonly used anode materials include iridium-coated titanium and platinum-coated titanium.
Gases are evolved by the reactions at both the cathode and anode. These must be removed to prevent masking the surface of the electrode, which would result in a voltage drop and reduce the voltage applied to the cells. Removal of the gases is accomplished by maintaining a flow of water across the surface of the electrodes during operation. This requires the use of a flow compartment adjacent to the electrode. Such compartments could be either gasketed screen-type spacers or resin-filled compartments.
Since copper wire is commonly used to conduct electric current to the electrodes, the junction of the copper wire and the non-copper electrode may be subject to corrosion, particularly if it is damp. If possible, it is best to have a projection of the electrode material that passes through the end plate of the module to an external connection that can be kept clean and dry (Schweitzer).