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Principle and application of electrodialyzer

release time:2019-11-02 17:00:25

First, the working principle
The basic principle of electrodialysis demineralization is to utilize the selective permeability of the ion exchange membrane. The cation exchange membrane only allows the passage of cations, and the anion exchange membrane allows only anions to pass. Under the action of a DC electric field, the ions in the water migrate directionally, and most of the ions in one water migrate to another ion water. Thereby achieving the purpose of desalination of brine.
Second, the scope of application
The electrodialyzer has the advantages of simple process, high salt removal rate, low water production cost, convenient operation and no pollution to the environment. It is widely used in the demineralization of water. The specific application is as follows: seawater and brackish water desalination, according to me The unit's test data can be used to dilute brackish water with a salt content of up to 60 g/l into drinking water to solve drinking water sources in desert areas. Preparation of soft water, (water resistivity is 105 ohms a centimeter), available for low-pressure boiler feed water, does not require salt regeneration, and can save about 20% of coal.
The pre-treatment of deep demineralized water and high-purity water uses electrodialysis-ion exchange method to expand the scope of application of raw water, widely used in electric power, electronics, chemical, pharmaceutical, scientific research and other occasions, reducing water production costs by more than 50%. It saves about 80% of acid and alkali for regeneration by ion exchange method and prolongs the regeneration cycle by more than five times. It is used in the purification of the beverage and food industry to improve the quality of beer and soda, and creates conditions for creating high-quality brand-name products. The electrodialyzer can also be used for chemical separation, concentration and recovery of industrial wastewater treatment.
Third, the structure and assembly method
1. Construction: The electrodialyzer consists of a membrane stack, a polar zone and a compacting device.
(1) Membrane block: It is assembled from a considerable number of film pairs.
Membrane pair: consists of a cation exchange membrane, a separator A (or B); a negative membrane, a separator B (or A).
Ion exchange membrane: It is a key component of electrodialysis unit. The factory uses heterogeneous membrane produced by Shanghai Chemical Plant.
Separator: Divided into thick and fresh water partitions, alternately placed between the yin and yang membranes, keeping a certain interval between the anion and the yang membrane, passing the water flow along the plane of the partition, and passing the current through the plane of the vertical partition. The partition is thicker than 0.9 mm.
(2) The polar region includes an electrode, a pole frame, and a water deflector.
Electrode: used for connecting the power supply, the electrode of our factory is coated with titanium.
Pole frame: placed between the electrode and the membrane to prevent the membrane from sticking to the electrode and supporting it.
(3) Pressing device: It is used to compact the electrodialyzer so that the membrane stack, electrodes and other components form a whole without water leakage.
2. Assembly method: The assembly of the electrodialyzer is represented by “stage” and “segment”, and the membrane stack between a pair of electrodes is called “first stage”. Each membrane in the same direction of water flow is called a "segment." Increasing the number of segments is equivalent to increasing the desalination process, which is to increase the desalination efficiency and increase the membrane logarithm to increase the water treatment capacity.
The assembly mode of the electrodialyzer can be adjusted according to the different requirements of fresh water production and effluent quality. Generally, there are the following assembly forms: one stage; one stage and multiple stages; multiple sections; multiple stages and multiple sections.
 Fourth, auxiliary equipment
In addition to the body, the electrodialyzer must be equipped with auxiliary equipment such as rectifiers, filters, pickling facilities, and water pump meters. For the convenience of users, the factory can provide a full set of auxiliary equipment and instruments, and the auxiliary equipment matching with the electrodialysis unit is listed.
(1) Applicable to the salt content of raw water ≤ 800 mg / liter, if the salt content of raw water is > 800 mg / liter, the model of the flow regulator should be adjusted according to the salt content.
(2) The deionization rate of the electrodialyzer is calculated by using DDS-11A conductivity meter.
(3) When using the concentrated water circulation system, it is necessary to configure the concentrated water tank and the water pump. The concentrated water tank can be used with a plastic water tank, and its volume is equivalent to 40% of the fresh water volume.
Five, matters needing attention
(1) Water pretreatment is one of the factors to ensure the normal operation of the electrodialyzer. Therefore, the necessary pretreatment must be carried out before the water enters the electrodialysis unit to ensure that the raw water quality entering the electrodialyser meets the following indicators.
a: turbidity: ≯ 3 mg / 1; b: total iron content: < 0.3 mg / 1;
c: total manganese content: <0.1 mg / 1; d: color < 15 degrees;
e: Oxygen consumption: <3mg/1 (KMno4) f: Water temperature: 5-40 °C
g: pollution index: <7
(2) When starting, be sure to pass the water first, then turn on the power. When stopping, first cut off the power, then cut off the water, and stop the water from stopping.
(3) The ratio of fresh water flow to concentrated and extreme water flow should be adjusted appropriately. To prevent leakage of concentrated water, the pressure of concentrated water and polar water can be appropriately reduced, generally 0.2×9.8×104Pa.
(4) Depending on the water quality, the electrode should be exchanged after 2-8 hours of operation of the electrodialyzer.
(5) Metal objects are prohibited from being placed on the film stack to prevent short circuits.
Electrodialysis - is the process of separating the electrolyte from water by the selectivity of the ion exchange membrane under the action of a direct current electric field. The principle of electrodialysis desalination is shown in Figure 17-3.
 Electrodialysis desalination principle
  A plurality of positive and negative membranes are alternately arranged in the electrodialyzer to separate into small water chambers. When raw water enters these chambers, the ions in the solution migrate directionally under the action of a direct current electric field. The cation membrane only allows passage of cations while intercepting the anion; the membrane only allows passage of anions and intercepts the cations. As a result, a part of these cells becomes a fresh water chamber containing little ions, and the effluent is called fresh water. The chamber adjacent to the fresh water chamber becomes a concentrated water chamber that collects a large amount of ions, and the effluent is called concentrated water. Thereby the ions are separated and concentrated, and the water is purified.
Electrodialysis principle and prototype
        Electrodialysis (ED) processes can concentrate or deplete charged ions and molecules in aqueous solutions. The principle is that under the action of a direct current electric field, ions are transmitted through the selective ion exchange membrane to obtain a desalted diluent fluid or a concentrate fluid. Main features of electrodialysis technology: A. High selectivity to separated components; B. Low energy consumption and low engineering investment; C. Continuous operation, high degree of automation; D. High water recovery rate;
     E, does not change the nature of the solution due to chemical action and thermal degradation;