Demineralized water treatment equipment for thermal power plants

Total ion exchange, reverse osmosis + ion exchange, secondary reverse osmosis and reverse osmosis + electrical desalination processes can all be used to prepare boiler make-up water, but due to their respective characteristics, different types of boilers require make-up water There are also differences, so each has its scope of application.

1. The pure water requirements of medium pressure boiler feed water and the corresponding water treatment technology

Medium-pressure boiler feed water has certain requirements for ion hardness, conductivity, silica, etc. In order to achieve the required pure water quality, the water treatment process can adopt the full ion exchange method, reverse osmosis + mixed bed method, and two-stage reverse osmosis method. Compared with reverse osmosis + electric desalination method, the following is a comparison of the four processes.

  • In full ion exchange, due to ion exchange regeneration, a large amount of acid and alkali are required, so there is a large amount of waste regeneration liquid discharge, which seriously pollutes the environment; in the environment with increasingly strict environmental protection requirements, its application will be increasingly restricted.
  • In the reverse osmosis + ion exchange process, although reverse osmosis removes a large part of the salt in the influent water, the acid and alkali consumption required for the ion regeneration process is greatly reduced, but there is still waste acid and alkali regeneration liquid discharged, which pollutes the environment. Therefore, its application is relatively limited;
  • In the reverse osmosis + electrical desalination process, the primary reverse osmosis product water often fails to meet the water inlet requirements of the electrical desalination device, so the secondary reverse osmosis product water is required as the inlet water of the electrical desalination device, resulting in a significant investment cost Therefore, its application is relatively limited by investment costs.
  • In the secondary reverse osmosis process, although the conductivity of the secondary reverse osmosis product water is relatively high, it can meet the requirements for the medium pressure boiler feed water quality standard, and there is no acid-base treatment in the entire process. , So that it has the advantages of good operating environment and environmental protection.

2. Secondary reverse osmosis treatment process

This system can be divided into three parts: pretreatment, primary reverse osmosis, and secondary reverse osmosis. The brief process is as follows:
External network raw water ® pretreatment ® primary reverse osmosis ® secondary reverse osmosis ® external pure water supply

1. Pretreatment

The main function of the pretreatment device is to make the raw water from the external network meet the requirements of the reverse osmosis device for the influent water quality.

Main water quality indicators of reverse osmosis device

No.ItemUnitValue
1TurbidityNTU<1
2SDI <4<4
3Residual chlorinemg/l<0.1

The traditional pretreatment of the reverse osmosis device is a multi-media filter and an activated carbon filter. The medium in the filter, such as quartz sand, is used to intercept suspended solids and colloidal impurities in the water to reduce the turbidity of the effluent; Activated carbon absorbs residual chlorine, organic matter, etc. in the incoming water, so that the quality of the pretreated water produced meets the influent water quality requirements of the reverse osmosis device.
The filtration accuracy of the ultrafiltration membrane is much better than that of the traditional quartz sand, so the ultrafiltration device has the advantages of low water turbidity and so on. However, the ultrafiltration membrane has the advantage of high filtration accuracy, which causes the ultrafiltration membrane to be easily contaminated, and the filtration performance is not easy to recover, which limits its application. At present, with the emergence of the recoilable ultrafiltration device, the contaminated ultrafiltration membrane The performance is easy to recover, so that the ultrafiltration device can be widely used in the pretreatment of reverse osmosis devices. The following table compares the two pretreatment processes.

Comparison of two pretreatment processes

With its excellent water quality, the ultrafiltration device effectively protects the subsequent reverse osmosis membrane from being polluted; thereby ensuring the stable operation of the reverse osmosis equipment and prolonging the service life of the reverse osmosis equipment.

2. Primary reverse osmosis

The reverse osmosis device is the core part of the system. The water treated by the reverse osmosis device can remove most of the inorganic salts, organic matter, microorganisms, bacteria, etc.
The reverse osmosis device is composed of multiple reverse osmosis membrane elements according to different water requirements; the reverse osmosis membrane element is composed of multiple reverse osmosis membranes; the reverse osmosis membrane commonly used in the industry is a polyamide composite membrane. The similar membrane is composed of three layers of materials , the surface layer is a polyamide membrane, the surface pore size is about 10A, and the thickness is about 2000A. This ultra-thin separation layer is the core layer of the reverse osmosis membrane, and a large part of the raw water enters the water. The interception of salts, bacteria, organics, etc. is mainly completed by the separation layer; the middle layer is a highly water-permeable polysulfone microporous membrane with a pore size of about 150A, which is used to support the surface film; the bottom layer is a non-woven fabric, which is the entire The diaphragm provides high strength.
The amide composite membrane element has a large membrane area and relatively large water flux, and has a salt rejection rate of more than 99% for NaCl, CaCl2, MgCl2 and other salts; the reverse osmosis device composed of such elements generally has a salt rejection rate Achieve over 98%.

3. Secondary reverse osmosis

When the primary reverse osmosis product water cannot meet the terminal water quality requirements, the secondary reverse osmosis can be used for further treatment.
Since the reverse osmosis membrane cannot remove the carbon dioxide dissolved in water, a small amount of NaOH is usually added to the inlet water of the secondary reverse osmosis device to adjust the pH value, so that the carbon dioxide is converted into bicarbonate, which is removed by the reverse osmosis membrane to reduce The conductivity of the terminal outlet water.
The secondary RO water produced can meet the requirements of the following table:
Water quality of secondary RO product water

No.ItemUnitValue
1Total hardnessμmol/l≈0
2Conductivityμs/cm≤5
3Silicaμg/l≤100

Note: Water quality with raw water conductivity ≤ 3000μs/cm.
It can be seen from the above table that the water quality of the secondary reverse osmosis product can fully meet the requirements of the medium pressure boiler for the make-up water quality.

3. Application examples

1. 2×75m3/h pure water station of a Sichuan limited company

Raw water: surface water, conductivity 620-710μs/cm;
Treatment process: pretreatment + secondary reverse osmosis
Usage status:
The turbidity of the pretreatment effluent is <1NTU, SDI <4, and residual chlorine <0.1PPm, which meets the requirements of the reverse osmosis device;
The performance of the first-stage reverse osmosis device: the conductivity of the produced water is 10-12μs/cm, and the salt rejection rate is >98%;
Performance of the secondary reverse osmosis device: the conductivity of the produced water is 1.1-1.9μs/cm.
The performance of the entire system fully meets the user’s requirements for water production.

2. 80m3/h pure water station of a chemical company in Inner Mongolia

Raw water: groundwater, conductivity 550-570μs/cm;
Treatment process: pretreatment + secondary reverse osmosis
Usage status:
The turbidity of the pretreatment effluent is <1NTU, SDI <4, and residual chlorine <0.1PPm, which meets the requirements of the reverse osmosis device;
The performance of the first-stage reverse osmosis device: the conductivity of the produced water is 9-11μs/cm, and the salt rejection rate is >98%;
Performance of the secondary reverse osmosis device: the conductivity of the produced water is 0.9-1.6μs/cm.
The performance of the entire system fully meets the user’s requirements for water production.

4. Conclusion

Because the two-stage reverse osmosis process has the advantages of high desalination rate, high degree of automation, and environmental protection, and its water quality can meet the requirements of boiler make-up water, the application of two-stage reverse osmosis technology in the production of medium-pressure boiler make-up water is bound to increase Wider.

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