Application of titanium tube in seawater desalination
1、 Chemical composition and mechanical properties of titanium tube
According to ASTM B338/ASMESB338, the chemical composition of Gr2 titanium pipe is as follows:
Ti: margin; In the impurity content, N ≤ 0.03%; C≤0.08%； H≤0.015%； Fe≤0.30%； O≤0.25%
Single impurity of other elements ≤ 0.1% (possible non-human elements include aluminum, alum, tin, chromium, molybdenum, niobium, zirconium, hafnium, bismuth, palladium, copper, silicon, cobalt and thallium); Total impurities of other elements ≤ 0.4%.
Mechanical property: tensile strength ≥ 345Mpa; Yield strength ≥ 275Mpa; Elongation ≥ 20%
2、 The main characteristics of titanium tube for seawater desalination:
1. Low density, high specific strength (strength/specific gravity)
The density of titanium metal is 4.51g/cm3, which is higher than aluminum but lower than steel, copper and nickel. However, the specific strength is at the top of metal, 3 times that of stainless steel and 1.3 times that of aluminum alloy;
2. Good corrosion resistance
Titanium is a very active metal, its equilibrium potential is very low, and its thermodynamic corrosion tendency in the medium is large. But in fact, titanium is very stable in many media, for example, titanium is corrosion resistant in oxidizing, neutral and weak reducing media. This is because titanium has a great affinity with oxygen. In the air or oxygen containing medium, a dense, strong adhesion and inert oxide film is formed on the titanium surface, which protects the titanium matrix from corrosion. Even due to mechanical wear, it will recover or regenerate quickly. This indicates that titanium is a metal with strong passivation tendency. The oxide film of titanium keeps this characteristic when the medium temperature is below 315 ℃.
3. Good heat exchange performance
Although the thermal conductivity of titanium metal is lower than that of carbon steel and copper, due to its excellent corrosion resistance, its wall thickness can be greatly reduced. In addition, the heat transfer mode between the surface and steam is drop condensation, which reduces the heat group. The thermal resistance can also be reduced without scaling on the surface, which significantly improves the heat transfer performance of titanium.
4. Good heat resistance
The new titanium alloy can be used for a long time at 600 ℃ or higher.
5. Good chemical resistance to low temperature
Low temperature titanium alloys represented by titanium alloys TA7 (Ti-5Al-2.5Sn), TC4 (Ti-6Al-4V) and Ti-2.5Zr-1.5Mo increase their strength with decreasing temperature, but their plasticity changes little.
6. The tensile strength is close to its yield strength
This property of titanium shows that its yield strength ratio (tensile strength/yield strength) is high, indicating that the plastic deformation of titanium metal is poor during forming. Due to the large ratio of yield limit to elastic modulus of titanium, the resilience of titanium during molding is large.
3、Titanium tubes in seawater desalination equipment will gradually replace the original copper alloy tubes. Compared with the two, titanium tubes have the following advantages:
1. Under the same operating conditions, the titanium pipe has thinner wall thickness and less pipe consumption. Generally, the equal wall thickness of copper alloy pipe is 0.9mm-1.2mm; Titanium tube shall be used instead. Thin wall tube with wall thickness of 0.5mm can be used in places with low corrosivity.
2. Titanium tube has good thermal conductivity. The thermal conductivity of titanium is 17W/(m · k), that of aluminum brass is lOOW/(m · k), and that of 70/30 white copper is 29W/(m · k). The thermal conductivity of titanium is low. However, with thin-walled titanium tubes, although the thermal conductivity is worse than that of aluminum brass, it is equivalent to that of 90/10 white copper and better than that of 70/30 white copper.
3. The use of titanium tube is more cost-effective. The price of titanium tube can compete with that of copper alloy tube. Because of the low density of titanium and the same wall thickness, the quality of titanium tube with the same length is only 50% of that of copper alloy tube. When the wall thickness of titanium tube is 50% of that of copper alloy tube, the quality of titanium tube with the same heat transfer area is only 1/4 of that of copper alloy tube. It can be seen that titanium tube is competitive in price.
4. The service life of titanium tube is longer. As sea water is often mixed with sediment and sea creatures, they attach to the heat transfer tubes and tube ends, which will erode copper alloy tubes, and copper alloys will also be corroded by Br - in sea water. This problem will not occur with titanium tubes, especially when oxygen has to be injected to kill bacteria in seawater, it is necessary to use titanium tubes with good corrosion resistance.
Application of Titanium Materials in Seawater Desalination
Application of Titanium Materials in Seawater Desalination At 15:47 on October 28, 2009, the Muiti Stage Flash (abbreviated as MSF) seawater desalination technology played a leading role. Its production capacity is 56% of the total output of seawater desalination in the world. In the world, 77.3% of the seawater desalination units with a scale of more than 4000t/d are MSF units. At present, the MSF unit with large single unit capacity ZUI is produced by Italian SIR/EU-TECO company, with output of 36000t/d, and installed in Italy.
From the current market share, distillation and reverse osmosis are the main types. The reverse osmosis desalination unit consumes less energy, and the shipment volume has increased rapidly in recent years. Among ships going to sea, distillation desalination units are the leading products, and there are many types, including multistage flash, boiling distillation and compressed air distillation, as well as tube type and plate type. The multi-stage flash generation technology is now relatively mature, and its heat source is mainly boiler steam. The compressed air distillation method requires an additional steam compression device, which is suitable for occasions where the steam supply is inconvenient and there is no low-grade heat available. Boiling distillation has incomparable characteristics compared with other methods, such as simple and reliable equipment, no need for pretreatment of raw water, and higher effluent quality. Although the plate distillation method has high evaporation efficiency, the fluid channel between plates is easily blocked by solid particle deposition, which is not suitable for distillation units where the tail gas is directly heated.
Titanium is SX material for heat exchanger of seawater desalination equipment
Titanium has strong corrosion resistance to chlorine and is the SX material of heat exchanger in seawater desalination equipment. With the rapid development of petrochemical, electric power and other industries in coastal areas, using seawater as industrial cooling medium instead of increasingly scarce fresh water can save a lot of fresh water resources and obtain significant economic benefits. However, due to the strong corrosivity of seawater, when ordinary carbon steel or stainless steel pipes are used, seawater as a cooling medium will cause serious corrosion to the pipes, significantly reducing the service life of the heat exchanger. In this way, not only the replacement times of equipment are increased, but also the economic benefits are reduced due to too frequent downtime caused by equipment failure. In order to solve this problem, it is necessary to upgrade the materials of pipes, which are mainly titanium pipes.
As far as materials are concerned, ZUI is widely used for industrial pure titanium (ASTM Grade 2). Grade 7 or Grade 12 is used for seawater heaters with higher temperatures. Grade 16 (Ti-0.5% Pd) has higher corrosion resistance, but its cost is higher. Facts show that Grade 1, Grade 2 and other industrial pure titanium have special resistance to stress corrosion in natural water, seawater and various chlorides. In titanium seawater desalination equipment with seawater flow rate of 3-5m/s, the biological fouling phenomenon is slight, and the fouling coefficient of titanium heat exchanger is about 0.99-0.95.
The industrial pure titanium TA1 seamless tube is selected as the condenser tube of the flash evaporator and the heat exchange tube of the brine heater, and the tube sheet is made of TA1+16MnR+316L double-sided composite steel plate. Because titanium is soft and light, corrosion resistant, and high strength, it is a good seawater corrosion resistant material, and its reliability is high. The use of titanium clad steel plate can reduce the use of titanium, meet the use requirements and reduce the cost of the device.
Northwest Nonferrous Metals Research Institute, Beijing Nonferrous Metals Research Institute and other units have also developed a series of corrosion resistant titanium alloys for offshore engineering, such as Ti75, Ti31 and Ti631.
Application of Titanium Plate Heat Exchanger in Seawater Desalination Abroad
At present, the main seawater desalination methods are multi-stage flash evaporation (MSF), reverse osmosis (SWRO), multiple effect evaporation (MED) and pressurized steam distillation (VC), while the methods applicable to large-scale seawater desalination are SWRO, MSFT and MED.
Commercial and military aviation titanium accounts for 50% of the total demand in the European market, which is relatively stable. However, industrial titanium and other fields are not stable. Chemical industry, electric power, desalination and others account for 28%, heat exchanger accounts for 11%, marine industry accounts for 8%, and armor industry accounts for 3%.
In foreign countries, the market of titanium heat exchanger is also very considerable. The United States has developed titanium heat exchangers, titanium condensers, and titanium evaporators for titanium production platforms in the fields of marine oil and gas, natural gas, and seawater desalination. The copper and nickel alloy tubes of all heat exchangers have been replaced by titanium tubes in Delaware Refinery of the United States, with a total length of more than 300km. The titanium condenser on the top of the desulfurization separation tower has been operating for 15 years without corrosion.
In 1999, Japan used 2100 tons of titanium to manufacture heat exchangers, which is estimated to reach 5000 tons in 2009. The characteristics of titanium application in Japan are still civil. Chemical industry, electric power and seawater desalination are the main areas of Japan's titanium market.
Ti-0.5Ni-0.5Ru Ti-0.005Pd-0.13Cr Ti-0.4Ni-0.016Pd-0.025Ru-0.13Cr and other new corrosion resistant alloys have been introduced to the market. Ti-0.05Pd-0.3Co has crevice corrosion resistance and is also used in heat exchangers.
In terms of development and application of thin-walled titanium welded pipe, Japan has established a mass production system of strip coils for seawater desalination and titanium thin-walled welded pipe for power plant condenser, and developed the production technology of thin-walled welded pipe accordingly. The condensers of Hitachi, Mitsubishi and Toshiba power plants use 0.5mm thick titanium welded pipes, and the seawater desalination plants produced by Mitsubishi, Kawasaki, Hitachi, Mitsui and Kobe Steel use 0.5-0.7mm thick titanium welded pipes.
It is reported that 60000 titanium tubes are used in the marine water desalination plant with a daily output of 100 tons. From 1967 to 1994, in nearly 30 years, 52 sets of condensers for atomic level thermal power generation and 7 sets of seawater desalination equipment were produced, and 11000 tons of titanium welded pipes were used.
