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| EDI Stack | 100 GPM System |
25 GPM System
How EDI Works
Continuous Electrodeionization (EDI) removes ions from water using conventional ion exchange resin. Multiple beds of ion-exchange material are arranged in narrow rectangular spaces (Dilute, or "D" chambers) contained between sheets of ion-exchange membrane. The deionization takes place in this resin bed, much as in conventional mixed-bed ion exchange. Water does not pass through the membranes, only ions do.
Alternating between the D chambers are narrow spaces called the Concentrate, or "C" chambers. Each layer of resin, membrane and associated spacers is called a cell. A group of cells is arranged in series, and electrodes within terminal Electrode or "E" Chambers are located at both ends.
The key difference between EDI and conventional DI systems lies in the method used for resin regeneration. In EDI, regeneration is achieved by using a DC voltage to create a current through the resin bed. Because each bead of resin touches an adjacent bead, the ions can migrate from one bead to the next in the direction of the electrode. The DC voltage then draws cations towards the cathode and anions towards the anode. As the ions reach the membrane they can pass into the C chamber but cannot move any further towards the electrode - they are blocked by the adjoining membrane containing resin possessing the same fixed charge. The voltage involved can be up to 20V per cell.
How to achieve higher purity
The inlet area of the bed is in the unregenerated state. Ions are taken up by the resin and removed by being transferred towards the membrane. There is an additional mechanism occurring within the stack. At the outlet end of the resin bed there are insufficient ions remaining in the water to sustain the applied current. Here the DC electrical energy splits water into H+ and OH-, maintaining the resin in a regenerated state. For removal of weakly charged ions, such as silica, it is essential to use sufficient electric current to maintain the resin in a highly regenerated state.
How concentrated ions are removed
The conductivity of water in the C chamber quickly exceeds 300µS/cm. In order to promote mixing, the water through the C chamber is recirculated by way of a centrifugal pump. This is called the C loop.
To prevent ion concentration reaching the point of precipitation, a small amount of water is continuously bled from the C loop. In addition, make-up water enters the C loop from the EDI feed water manifold. The rate of flow from the C loop is adjustable and determines the recovery of the unit. This water may be recycled back to the inlet of the RO pretreatment stage.
Electrode Flush
A small fraction of the C loop water is used to flush the electrodes (E chambers) and remove any buildup of gases.
Limitations of Current Technology
The specific benefits of the E-Cell product are best explained by first understanding the limitations of older EDI technology. Inherent process limitations in technology render competing EDI products membrane dependent and labor intensive to assemble. Not only are these expensive, but the technology requires a multitude of thin chambers each of which requires resin filling.
Also, initial EDI products were developed to handle only low-flow-rate laboratory applications. Their non-industrial design, inability to be economically scaled up, and other limiting factors have made it difficult for these products to compete with large-flow-rate mixed beds for industrial applications.
E-Cell's competitors have had limited success in overcoming the challenges mentioned above. New membranes have been developed but are not cost effective. Special resin loading techniques to reduce labor have also been developed. However, this same equipment uses a large area of membrane per unit flow capacity. A number of new companies are working on other solutions, but until the introduction of the E-Cell, none had succeeded in developing a cost-effective product.
How E-Cell Works
E-Cell uses the generic process for EDI described in the previous section. A major innovation, however, is E-Cell's standard Stack design which incorporates a wider chamber thickness to reduce the number of cells required, compared with competing technologies. Using a standard Stack design enables high-volume manufacturing, and results in reduced unit costs. Any number of Stacks can be arranged in parallel to handle the required flow.
Extensive research by a team from E-CELL Corporation in Canada, and Asahi Glass Company in Japan, developed a unique design for the E-Cell. This patented approach requires less membrane area than other EDI designs, further reducing the cost of the E-Cell Stack.
Essentially the E-Cell Stack is a rectangular pressure vessel - as can be seen by its robust appearance. The compact design allows for leak-free operation up to maximum design pressure, 100 psi . Every Stack is pressure tested and rated. Significant QC testing at 1.5 times rated pressure, 150 psi has been performed in accordance with ASME guidelines.
The E-Cell Product Family
The E-Cell product is configured in a standard "Stack and Rack" format using a modular building block approach. The basic unit and heart of E-Cell technology is the single Stack. A Stack contains multiple cells each consisting of a D chamber filled with ion-exchange resin between an anion and a cation membrane, with C chambers on both sides. The Stack's end housing consists of an E chamber containing either an anode or cathode, gasket and end plate. Each E-Cell Stack is encapsulated with two aluminum end housings and two aluminum side housings, held together with 28 bolts. Five pipe connections for dilute in, product out, concentrate in, concentrate out and electrolyte out extend past one end housing with quick-connect sanitary fittings to connect to the interstack headers.
Key Stack Specifications
| Parameter | Range |
| Stack Flow | 15 GPM maximum (per stack) |
| Nominal Recovery | 90-95% |
| Temperature | 40-100° ; F (5 to 38° C) |
| Inlet Pressure | 45-100 psi (3.1 to 6.8 Bar) |
| Feed to Product delta P | » 35 psi (2.4 Bar) |
| Voltage (input from rectifier) | 600V DC (maximum) |
| Dimensions | 12" W x 24" H x 17.5" D |
For large flow-rate applications, E-Cell Stacks can be mounted in a modular multi-stack system, complete with the interstack header and flange connections located at one end and protected within the skid constraints.
Because of the “stack” and “rack” capability, E-Cell Systems can be configured to handle flow rates in appropriate increments from 6 GPM to 300 GPM.
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