What is electrodialysis


The three industries of power, pharmaceutical and microelectronics have different needs and specifications for high-purity water. This report will describe the use of electrodeionization (EDI) technology for the production of pure water in the power and microelectronics fields. Two of the field installations described are in the power industry and use EDI to produce high-quality water for boiler makeup. The third installation of interest applies EDI technology in the microelectronics industry for microchip fabrication rinse water. Although the EDI technology is being used in two different types of applications, the data presented demonstrates that EDI meets and exceeds the specifications of both the microelectronics and power industries. Because of this, EDI has grown in popularity and continues to be accepted as a standard method of water treatment. Data is presented in this report demonstrating the flexibility, reliability and quality of the continuous EDI process.

EDI process
High-purity water production has traditionally used a combination of membrane separation and ion exchange processes. One well-known membrane separation concept is electrodialysis (ED), which uses an electrical potential to transport and segregate charged aqueous species.

EDI is a further refinement of electrodialysis in that it combines the semi-permeable membrane technology with ion-exchange media to provide a high-efficiency demineralization process. While the fundamental concept is somewhat simple with the basic desalting unit being an ED dilute cell filled with mixed-bed ion-exchange resin, some complex chemical reactions take place within the resin-filled cell. It is these reactions that help to produce the very high purity water required.

When flow enters the resin-filled diluting compartment of an EDI stack, several processes are set in motion. Strong ions are scavenged out of the feed stream by the mixed bed resin. Under the influence of the strong DC field applied across the stack of components, charged ions are pulled off the resin and drawn toward the respective, oppositely charged electrodes, cathode or anode. As these strongly charged species, such as sodium and chloride, migrate toward the ion-exchange membrane, they are continuously removed and transferred into the adjacent concentrating compartments (see Figure 1).

As the strong ions are removed from the dilute process stream, the conductivity becomes quite low. This relatively pure water helps to set the stage for further chemical reactions. The electrical potential splits water at the surface of the resin beads, producing hydrogen and hydroxyl ions. These act as continuous regenerating agents of the ion-exchange resin. These regenerated resins, in turn, act as micro-regions of high or low pH permitting ionization of neutral or weakly ionized aqueous species such as carbon dioxide or silica. Once these species acquire a charge through this ionization process, they become subject to the influence of the strong DC field and are removed from the diluting compartment through the ion-exchange membranes (see Figure 2). The membranes used in EDI stacks are flat sheet, homogeneous, ion exchange membranes which help to provide efficient ion transfer.


Figure 1: EDI Flow Schematic
Figure 2: Ionization Reactions in EDI Diluting Compartments

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