There are two distinct operating regimes for EDI devices: enhanced transfer and electroregeneration (Ganzi, 1988). In the enhanced transfer regime, the resins within the device remain in the salt forms. In low conductivity solutions the ion exchange resin is orders of magnitude more conductive than the solution, and acts as a medium for transport of ions across the compartments to the surface of the ion exchange membranes. This mode of ion removal is only applicable in devices that allow simultaneous removal of both anions and cations, in order to maintain electroneutrality.
The second operating regime for EDI devices is known as the electroregeneration regime. This regime is characterized by the continuous regeneration of resins by hydrogen and hydroxide ions from the electrically-induced dissociation of water. This dissociation preferentially occurs at bipolar interfaces in the ion-depleting compartment where localized conditions of low solute concentrations are most likely to occur (Simons). The two primary types of bipolar interfaces in EDI devices are resin/resin and resin/membrane. The optimum location for water splitting depends on the configuration of the resin filler. For mixed-bed devices water splitting at both types of interface can result in effective resin regeneration, while in layered bed devices water is dissociated primarily at the resin/membrane interface (Ganzi et. al., 1997).
Regenerating the resins to their H+ and OH- forms allows EDI devices to remove weakly ionized compounds such as carbonic and silicic acids, and to remove weakly ionized organic compounds. This mode of ion removal occurs in all EDI devices that produce ultrapure water. Figure 1 (previous page) is a representation of the process showing two diluting compartments, which illustrates the transport of ions and electrochemical regeneration of ion exchange resins in one type of EDI cell.
To construct a large scale EDI device, many of these cells are assembled together and fed in parallel as shown in Figure 2.
The animation below illustrates the removal of ions and the splitting of water molecules:
There are various types of EDI devices. Due to the nature of the process however, manufacturers to date have taken only two design approaches. These are plate and frame and spiral wound. The plate and frame design was the first to emerge and is similar to a plate and frame filter press or heat exchanger in construction. Alternating diluting and concentrating cells are stacked between the electrodes and sandwiched together with some type of closing mechanism. Increasing the number of cell pairs (one dilute and one concentrate cell) increases the capacity of the unit. The main advantage of this type of construction is the ease of assembly.
In contrast, spiral wound EDI is a bit more complicated and hence more difficult to assemble. We will not attempt to explain the details of spiral EDI construction, however the basic principles of deionization are similar to plate and frame configuration.
Currently, the majority of EDI devices on the market use plate and frame construction. Plate and frame devices can be broken down into two major subsets. These are thin cell and thick cell, designated as such based on the thickness of the diluting compartments. For the purposes of this discussion, thin cell will encompass devices with a diluting cell thickness of 2-3 mm and thick cell will encompass 8-10 mm.