EDI ultrapure water equipment

The essence of EDI technology

Continuous Electrodeionization (EDl, Electro-deionization or CDl, Continuous Electrodeionization) is a process in which mixed ion exchange resins are used to adsorb anions and cations in the feed water. At the same time, these adsorbed ions are removed through anion and cation exchange membranes under the action of DC voltage. In this process, the ion exchange resin does not need to be regenerated with acid and alkali. This new technology can replace traditional ion exchange (DI) devices and produce ultrapure water with a resistivity of up to 5-18 MQcm.

EDI Technology Advantages

★ Compared with traditional ion exchange (DI), EDI has the following advantages: EDI does not require chemical regeneration, saving acid and alkali. EDI can operate continuously to provide stable water quality. It is easy to operate and manage, and has low labor intensity.

★ Modular assembly is possible.

★ Reduce the building area.

★ Low operating costs.

How EDI works

In the figure, ion exchange membranes are represented by vertical lines and indicate the types of ions they allow to pass through. These ion exchange membranes do not allow water to pass through, so they can isolate fresh water and concentrated water flows. Ion exchange membranes and ion exchange resins work on similar principles and can selectively pass ions. Anion exchange membranes only allow anions to pass through, but not cations; while cation exchange membranes only allow cations to pass through, but not anions. Filling a pair of anion and cation exchange membranes with mixed ion exchange resins forms an EDI unit.

In the figure, ion exchange membranes are represented by vertical lines and indicate the types of ions they allow to pass through. These ion exchange membranes do not allow water to pass through, so they can isolate fresh water and concentrated water flows. Ion exchange membranes and ion exchange resins work on similar principles and can selectively pass ions. Anion exchange membranes only allow anions to pass through, but not cations; while cation exchange membranes only allow cations to pass through, but not anions. Filling a pair of anion and cation exchange membranes with mixed ion exchange resins forms an EDI unit.

The space between the anion and cation exchange membranes occupied by the mixed ion exchange resin is called the fresh water chamber. A certain number of EDI units are arranged together so that the anion exchange membranes and cation exchange membranes are arranged alternately, and special ion exchange resins are added between the ion exchange membranes. The space formed is called the concentrated water chamber. Driven by a given DC voltage, in the fresh water chamber, the anions and cations in the ion exchange resin migrate to the positive and negative electrodes respectively, and enter the concentrated water chamber through the anion and cation exchange membranes. At the same time, the ions in the feed water are adsorbed by the ion exchange resin and occupy the vacancies left by the ion electromigration. In fact, the migration and adsorption of ions occur simultaneously and continuously. Through this process, the ions in the feed water pass through the ion exchange membrane and enter the concentrated water chamber to be removed and become demineralized water.

Negatively charged anions (such as OH-, CI-) are attracted by the positive electrode (+) and pass through the anion exchange membrane into the adjacent concentrated water chamber. After that, these ions continue to migrate toward the positive electrode and encounter the adjacent cation exchange membrane, which does not allow anions to pass through, so these ions are blocked in the concentrated water. Cations in the fresh water stream (such as Na+, H+) are blocked in the concentrated water chamber in a similar way. In the concentrated water chamber, the ions passing through the cation and anion membranes maintain electrical neutrality. The current of the EDI component is proportional to the amount of ion migration. The current consists of two parts, one part comes from the migration of the removed ions, and the other part comes from the migration of H+ and OH- ions produced by the ionization of the water itself.

There is a high voltage gradient in the EDI component, under which water will electrolyze to produce a large amount of H+ and OH-. These H+ and OH- generated in situ have a continuous regeneration effect on the ion exchange resin.

The ion exchange resin in the EDI component can be divided into two parts, one part is called working resin and the other part is called polishing resin. The boundary between the two is called working front. The working resin is responsible for removing most ions, while the polishing resin is responsible for removing ions that are difficult to remove, such as weak electrolytes. The pretreatment of EDI feed water is the primary condition for EDI to achieve its optimal performance and reduce equipment failures. Contaminants in the feed water will have a negative impact on the desalination components, increase maintenance and reduce the life of the membrane components.

Application Areas

Ultrapure water is often used in the microelectronics industry, semiconductor industry, power generation industry, pharmaceuticals, electric power, chemical petroleum, metallurgy, papermaking, automobile and other industries and laboratories. EDI pure water can also be used as pharmaceutical distilled water, food and beverage production water, chemical industry and other process water, as well as other ultrapure water application fields.

Equipment parameters

The operating results of the Super EDI components depend on a variety of operating conditions, including system design parameters, feed water quality, feed water pressure, etc. The following table lists the more typical operating conditions.