EDI - electro demineralization - also known as CDI - continuos demineralization - is an evolution of mixed bed resin ion exchange demineralization process. In EDI cations and anions soluble in water are exchanged on a resin with H + and OH- ions, producing demineralized water; the difference with the traditional method is that EDI process is continuous, since the resin is regenerated by means of semipermeable membranes by imposing a DC electric field. It is therefore no longer necessary to interrupt normal operation of the plant and to carry out chemical regeneration with hydrochloric acid and soda. The H + and OH- ions are generated on-site by dissociation of the water molecule:
Pretreated water is fed into compartments filled with resin and delimited by membranes selectively permeable to cations and anions. Electrodes are positioned at the end of these compartments; these allow migration of ions through semipermeable membranes to the adjacent compartments where they are collected as a concentrated solution. This solution is removed by imposing a flow rate, using a small part of the feed flow.
The system is very simple, since it consists essentially of the EDI stack, of a feed pump to increase feed pressure to a compatible level (if needed) and of a current rectifier.
EDI technology takes advantage of the excellent exchange capacity of modern ion exchange resins, but at the same time avoids the disadvantages deriving from them and essentially linked to:
- discontinuity of the cycle and need to perform chemical regeneration;
- need to manage large amounts of dangerous chemicals;
- generation of high amounts of effluents to be neutralized and dispose of.
EDI technology, despite having been developed in theory more than 50 years ago, has found its first commercial application in 1986; it is therefore a relatively young technology, still to be considered fully consolidated. In fact, there are thousands of installations worldwide, primarily in the following markets:
- production of demineralized water for boiler feed;
- production of ultrapure water for pharmaceutical use;
- ultrapure water production in the electronics industry.
We can say that the typical EDI meets the needs of modern industry to manufacture high-quality products with minimal environmental impact and ensuring easy operation.
EDI application and modular concept
EDI technology allows to obtain high purity water with a resistivity greater than 16 MΩcm and silica concentrations below 5 ppb. It requires, however, a good quality of the incoming water (conductivity of less than 50 microS/cm, typically in the order of 10-20 microS/cm). This water supply is in most cases produced by means of a reverse osmosis system: the two technologies are an excellent combination, since even reverse osmosis does not require the use of chemicals.
EDI modules are suitable for modular assembling to manufacture reliable high-capacity systems: it is simple to install multiple modules in parallel to achieve greater capacity.
E-Cell company has been forerunner in the development of EDI technology and makes available through our organization a wide range of products. Sepra provides OEMs and installation companies an adequate technical and logistical support, including assistance in sizing systems and in the definition of the correct pretreatment strategies. We point out again that for the proper functioning of EDI it is essential to ensure an adequate quality of incoming water.
E-Cell has developed EDI modules with different processing capacity to help deal with the construction of systems of different sizes and with different quality requirements:
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Stacks can be supplied as separate components or complete with rectifier and cables.