EDI electric desalination
Ⅰ、System introduction
EDI(Electrodeionization)is a pure water manufacturing technology combining ion exchange technology, ion exchange membrane technology and ion electromigration technology. It ingeniously combines electrodialysis and ion exchange technology, makes use of high pressure at both ends of electrodes to make charged ions move in water, and cooperate with ion exchange resin and selective resin film to accelerate ion movement removal, so as to achieve the purpose of water purification. EDI equipment system Thus, EDI system here is a pure water manufacturing system.
In the desalination process of EDI, ions are removed by ion exchange membrane under the action of electric field. At the same time, water molecules under the action of an electric field to produce hydrogen and hydroxide ions, these ions to the ion exchange resin continuous regeneration, to keep the ion exchange resin in the best state.
EDI ultrapure water ultrapure water equipment manufacturing history first stage: preprocessing filters -- > -- > Yin Yang bed bed -- > mixed bed in the second stage: preprocessing filter -- > > reverse osmosis mixed bed at this stage: preprocessing filters - > reverse osmosis - > EDI (without ph) since the past few decades, mixed bed ion exchange technology (D) has been as a standard of the preparation process. As it requires periodic regeneration and consumes a large amount of chemicals (acid and alkali) and industrial pure water in the regeneration process, and causes certain environmental problems, it is necessary to develop acid-free and alkali ultra-pure water system. Because traditional ion exchange has been increasingly unable to meet the needs of modern industry and environmental protection, EDI technology, which combines membrane, resin and electrochemical principle, has become a revolution in water treatment technology. The regeneration of its ion-exchange resin uses electrical energy instead of acid and base, so it meets the environmental requirements of the world today.
Ⅱ、Working principle
Electric deionizing (EDI) system is a kind of scientific water treatment technology which purifies the water quality through the selection of ions by exchange membrane and the directional movement of the ions in the water through the diaphragm under the action of direct current electric field. Between a pair of electrodes in an electrodialysis machine, there are usually alternating groups of anion, anion and diaphragm (A and B) to form thick and weak chambers (i.e. cation and anion can pass through the anion). The cations in the fresh chamber water migrate to the negative pole and pass through the positive membrane, and are trapped by the negative membrane in the thick chamber. Anionic migration towards the positive direction of Yin membrane in water, be strong chamber of Yang membrane intercept, such by light chamber on the number of ions in water gradually reduce, become fresh water, and thick the water chamber, due to the ion of Yin and Yang were strong chamber, dielectric ion concentration rising, become thick water, thus achieve the purpose of desalination, purification, concentration and refining.
Ⅲ、The system characteristics
EDI systems have been greatly developed in pharmaceutical, semiconductor, electric power and surface cleaning industries, and are also widely used in wastewater treatment, beverage and microbial industries.
EDI equipment is applied in reverse osmosis system, replacing the traditional mixed bed ion exchange technology (MB-DI) to produce stable ultra-pure water. EDI technology has the following advantages compared with mixed ion exchange technology:
①Water quality stability
②Easy to realize automatic control
③No downtime due to regeneration ④EDI system for chemical regeneration is not required ⑤Low operating cost
⑥Small workshop area
⑦Working principle of NO sewage discharge EDI:
EDI module forms EDI unit by intercharging ion exchange resin between anion/cation exchange membrane. The working principle of EDI is shown in the figure. EDI module divides a certain number of EDI units with grid to form thick water chamber and fresh water chamber. The cathode/anode is set at both ends of the unit group. Driven by direct current, the anion and anion in the water flow through the fresh water chamber pass through the anion exchange membrane and enter into the concentrated water chamber and are removed in the fresh water chamber. EDI equipment generally uses secondary reverse osmosis (RO) pure water as EDI feed water. RO pure water resistivity is generally 40-2 S/cm(25℃). EDI pure water resistivity can be as high as 18 M Ω. Cm (25 ℃), but according to deionized water usage and system configuration Settings, EDI ultrapure water is suitable for the preparation of resistivity requirement in 1-18.2 M Ω. Cm (25 ℃) of pure water.
Ⅳ、System is running
Factors affecting operation
(1)Effect of EDI on influent conductivity. At the same operating current, with the increase of raw water conductivity, the removal rate of weak electrolyte by EDI decreases, and the conductivity of effluent also increases. If the raw water conductivity low ion content is low, and low concentration of ions in the light room of resin and film formed on the surface of the electromotive force gradient is big, causes the water dissociation degree enhancement, limiting current increases, the number of H + and OH - more, make the room filled with light of anion and cation exchange resin regeneration effect is good.
(2)Effect of operating voltage-current. The working current increases and the water quality gets better. However, if the current is increased after increasing to the highest point, due to the excessive amount of H+ and OH- ions generated by water and electricity separation, in addition to being used for regenerating resin, a large number of surplus ions act as current-carrying ions to conduct electricity. At the same time, accumulation and blockage occur during the movement of a large number of current-carrying ions, or even reverse diffusion, resulting in the decline of the water quality.
(3)The influence of turbidity and pollution index (SDI). The water production channel of EDI module is filled with ion-exchange resin. Excessively high turbidity and pollution index will block the channel, resulting in the rise of system pressure difference and the decrease of water production.
(4)Effect of hardness. If the residual hardness of the water in EDI is too high, it will lead to scaling on the film surface of the concentrated water channel, the concentrated water flow rate and the water yield resistivity will decrease. It will affect the water quality of production. In serious cases, concentrated water and pole current channel of the assembly will be blocked, resulting in the destruction of the assembly due to internal heat.
(5)TOC(Total organic carbon). If the content of organic matter in the water is too high, it will cause organic pollution of resin and selective permeable film, which will lead to the rise of system operating voltage and the decline of water quality. It is also easy to form organic colloid in the concentrated water channel and block the channel.
(6)Effect of CO2 in water. The HCO3- generated by CO2 in the influent is a weak electrolyte, which is easy to penetrate the ion-exchange resin layer and cause the water quality to decline.
(7)Effect of total anion content (TEA). High TEA will reduce the water production resistivity of EDI, or the operation current of EDI needs to be improved. However, too high operating current will lead to the increase of system current and the increase of residual chlorine concentration of polar water, which is unfavorable to the life of polar film.
In addition, water inlet temperature, pH value, SiO2 and oxide also affect EDI system operation.
Inlet water quality control
(1)Control of influent conductivity. Strictly control the conductivity in the pretreatment process, make EDI inlet conductivity less than 40 S/cm, can ensure the qualified effluent conductivity and the removal of weak electrolyte.
(2)Operating voltage-current control. The appropriate voltage-current should be selected when the system is operating. At the same time, there is a position of limit voltage-current point on the voltage-current curve of EDI water purification equipment, which is related to inlet water quality, film and resin performance, membrane structure and other factors [4]. In order to generate enough H+ and OH- ions from a certain amount of water and electricity to regenerate a certain amount of ion exchange resin, the voltage-current working point of the selected EDI water purification equipment must be greater than the limit voltage-current point.
(3)Control of influent CO2. The pH can be adjusted by adding alkali before RO to remove CO2 to the maximum extent. Degassing tower and degassing film can also be used to remove CO2.
(4)Control of inlet water hardness. The water intake of RO can be softened and alkali added in combination with CO2 removal. When the salt content of water is high, the first RO or nanofiltration can be added in combination with desalination.
(5)TOC control. In combination with other index requirements, a level RO is added to meet the requirements.
(6)Control of turbidity and pollution index. Turbidity and pollution index are one of the main indexes of inlet water control of RO system. Qualified RO outlet can generally meet the inlet water requirements of EDI.
(7)Control of Fe. The Fe controlling EDI inlet water in operation is less than 0.01mg/L。If the resin has been "poisoned", an acid solution can be used for resuscitation.[1]
(8) EDI System inlet water quality requirements
Based on the above analysis, the water quality requirements of EDI inlet are shown in the table, which can ensure that the effluent quality meets the requirements of high purity water required by semiconductor manufacturing in the electronic industry.
Ⅴ、Application field
1、Semiconductor and electronics industry - Ultra pure water
2、Biological and Pharmaceutical industries - Purified water
3、Power plant - Boiler make-up water
4、Surface coating
5、Consumer goods and cosmetics industries
6、Instead of all kinds of distilled water
7、Other industries that require high water purity
