The shortage of fresh water resources is gradually becoming a global problem restricting the sustainable development of human society. Seawater desalination, as an important means that is expected to alleviate the crisis of freshwater resources, has made great progress in the past half century, and has developed a number of key technologies such as multi-stage flash evaporation, low-temperature multi-effect distillation, and reverse osmosis membrane. At the same time, it has driven the development of key materials and processes required for equipment construction.
Seawater desalination technology has also gradually developed many methods such as freezing method, gas hydrate method, permeable membrane method and ion exchange method from the original single distillation method.
1 Development status of key technologies for seawater desalination
1.1 Distillation
The principle of distillation is simple. It is the earliest seawater desalination technology used. It uses seawater heating, evaporation, and condensation to remove the harmful salts and impurities in seawater to obtain fresh water.
1.1.1 Multi-effect distillation
The multi-effect distillation method connects a series of distillers in series, and the heat source steam (generally the waste heat of the plant) enters the seawater that transfers heat to the first-stage distillation chamber from the first stage. Under the action of negative pressure in the distillation chamber, the boiling point of seawater decreases, which makes it easy to be heated and gasified by low-grade waste heat steam. The vaporized seawater steam can be used as the heat source for the second stage distillation. The multi-stage distillation chambers in series make the waste heat of the heat source be fully utilized, producing distilled water much larger than the heat source steam, so it can be used in large-scale seawater desalination projects. The low-temperature multi-effect distillation method can use the low-grade waste heat of the plant (the heat source steam temperature is low At 70 ℃), and is widely used. The Shougang Jingtang Seawater Desalination Project is a typical representative of low-grade waste heat utilization in the steel production process. Its principle is shown in Figure 2. Using the low-grade waste heat of the steel process as a heat source, the hot steam from the steel plant enters the heat exchange tube in the first-stage distillation chamber from the left-hand pipe (red). The seawater spraying device on the roof of the chamber sprays the seawater to spray, thereby increasing the contact area between the water mist and the heat exchange tube. The water mist absorbs the heat of the heat exchange tube and vaporizes into water vapor, condenses into the distilled water pipe, and the concentrated brine flows into the collection tank. The remaining steam carries the waste heat into the next-stage heat exchange tube as the heat source of the second stage. The heat of the steam from the steel plant is fully utilized through the seven-stage distillation chamber. The temperature of the steam drops from about 70 ° C in the first stage to 45 in the seventh stage. Around ℃. The air ejection and steam ejection device at the seventh stage distillation chamber in Figure 2 creates a negative pressure in the entire equipment, thereby reducing the boiling point of water and accelerating the gasification process. At the same time, the negative pressure is used as hot steam in the entire system from the first stage. The driving force to level seven.
1.1.2 Multi-stage flash evaporation method
In the multi-stage flash evaporation method, seawater is preheated and then introduced into the flash chamber. The pressure in the flash chamber is lower than the saturated vapor pressure of the hot seawater. Therefore, the hot seawater enters the flash chamber and evaporates quickly, condensing the steam to obtain fresh water. Because of its safety and reliability, most oil-producing countries in the Middle East adopt this method and combine it with power plants. For example, the UAE has built the world's largest power plant-multi-stage flash seawater desalination equipment. The biggest advantage of this method is that the heat transfer surface and the evaporation surface of the entire container are separated from each other during desalination of seawater, thereby preventing the formation of scale. In addition, the multi-stage flash method also has the advantages of mature process, low maintenance, low raw water requirements, long service life, good effluent quality, and high thermal efficiency, so it has become the most widely used process at present. The disadvantage is that the power consumption is large. The desalination water output of this method accounts for more than 70% of the world's total output, and the energy consumption has been maintained above 10kWh / m3.
1.2 Freezing method
When seawater freezes, the salt content is excluded from the ice crystals. After washing, separating, and melting the ice crystals, fresh water can be obtained, which is the freezing method. The freezing process mainly includes the formation, washing, separation, and melting of ice crystals. According to the different ways of ice crystal formation, it can be divided into natural freezing method and artificial freezing method, and artificial freezing method can be divided into direct freezing method and indirect freezing method. However, the indirect freezing method has low heat transfer efficiency and requires a large heat transfer area, which limits its use.
1.2.1 Direct refrigerant freezing method
Use n-butane, which is insoluble in water and has a boiling point close to the freezing point of seawater, as a refrigerant, and mix it with pre-cooled seawater into the freezer. In the case where the pressure is slightly lower than atmospheric pressure, n-butane vaporization absorbs heat to maintain the temperature in the freezing chamber at about -3 ° C, and the seawater freezes and freezes.
The n-butane vapor is compressed by the compressor to more than 1 atmosphere, and enters the melter to directly contact with the ice. The n-butane vapor is liquefied, and the ice melts to form water. The n-butane and water are incompatible with each other, and are separated by density difference. The product is released and n-butane is recycled in the process. Butane freezing method is convenient and reliable, and it is widely used in current large and medium-sized seawater desalination plants. However, due to the use of butane, the system must be tightly sealed. Otherwise, the leakage of refrigerant will cause local accumulation of refrigerant, which will bring hidden safety risks and increase investment costs. In addition, although butane and water are not miscible, if the removal is incomplete, water will inevitably contain a small amount of butane and be contaminated.
1.2.2 Vacuum evaporative direct freezing method
Vacuum evaporative direct freezing method is a method that utilizes the triple point principle of water. Gas, liquid and solid three phases coexist near the triple point of water. If seawater is controlled near the triple point, evaporation and freezing of seawater will proceed simultaneously, and ice and steam will be melted and condensed separately to obtain fresh water. The key technology of vacuum evaporative direct freezing method is how to remove the generated steam, which can be divided into vacuum freezing vapor compression method and vacuum freezing vapor absorption method according to the way of vapor removal. Application of key materials for seawater desalination equipment The selection of materials for seawater desalination equipment requires comprehensive consideration of multiple factors such as cost, specific strength, process performance, and stability in the medium. Due to the harsh service environment, materials have always been an important factor restricting the development of equipment.
2 Application status of key materials for seawater desalination equipment
2.1 Key materials of distillation equipment
The evaporator is the core part of the distillation desalination equipment. The main components include the shell, internals (tube sheet support plate, spray system, internal piping) and heat exchange tubes.
2.1.1 Shell and inner parts
When selecting materials for the shell and internal parts, it is necessary to focus on the ability of the material to resist seawater corrosion. Studies have shown that the main factors of seawater corrosion are dissolved oxygen concentration, pH value and average salinity. Under the combined action of the three, pitting corrosion, crevice corrosion and stress corrosion of stress-bearing components of metal materials will result, resulting in material failure.
2.1.2 Heat exchange tube group
During the service of the heat exchange tube group, it must withstand the spray of hot seawater and the erosion of hot salt mist. When selecting materials, the thermal conductivity and corrosion resistance are mainly considered, and the strength is a secondary factor. At present, it mainly uses three kinds of materials: titanium and titanium alloy, copper alloy and aluminum alloy. Titanium alloy has excellent corrosion resistance, heat transfer efficiency is lower than copper alloy, and the cost is higher. Therefore, most of the rows of heat exchange tubes on the top of the heat exchange tube group use titanium alloy, and their use of spray erosion resistance can make the tube group reach a maintenance-free reliability level. In multi-stage flash evaporation equipment, if an oxidizing bactericide is injected to remove bacteria in seawater, all titanium tubes are required.
2.2 Selection of key materials for membrane equipment
The material selection of the shell and pipeline of the membrane method equipment is similar to that of the distillation method. Because the membrane method equipment requires a high-pressure pump to generate a driving force for the solution to enter from the reverse osmosis membrane side to the other side, the selection of the high-pressure pump material becomes a key link. In 1979, the first large-scale membrane process equipment was built in Saudi Arabia. Its high-pressure pump used 316L, which caused severe crevice corrosion during operation. Two membrane process equipments built in the Middle East in 1986 were changed to 317L, which also caused corrosion problems of different degrees. Subsequently, 904L and 2205 stainless steels were gradually used, operating stably in most cases and occasionally experiencing corrosion problems. In 1991, the Fujairah membrane process equipment in the UAE used 254SMO super austenitic stainless steel as the main material of the high-pressure pump. It was very stable during service, so 254SMO gradually became the primary choice for high-pressure pumps for large-scale membrane process equipment. In the late 20th century, due to its excellent corrosion resistance and low cost, 2507 stainless steel replaced 254SMO as the mainstream material for high-pressure pumps.
3 Summary
After more than half a century of development, the three technologies of low-temperature multi-effect distillation, multi-stage flash evaporation and membrane permeation have gradually emerged from many seawater desalination technologies, becoming a widely used and mature technology. The maturity of technology is inseparable from the appearance of new corrosion-resistant materials and the reduction of costs. The material selection of the evaporator shell and internal parts gradually changed from 316L stainless steel to duplex stainless steel. The heat exchange tube group formed a titanium alloy-copper alloy matching mode. The material selection of the high pressure pump gradually changed from ordinary stainless steel to super austenitic stainless steel.