Application of Titanium Materials in Seawater Desalination Equipment Freshwater accounts for approximately 3% of the Earth's water resources. Obtaining new freshwater resources through seawater desalination has become a major trend in the world's water usage in the future. Currently, seawater desalination has become the main way for water-scarce regions like the Middle East to obtain water sources. 1. The Situation of Seawater Desalination in the World By 1993, there were a total of 9,014 seawater desalination devices in 5,738 regions around the world, with a total capacity of 1.624×10⁷ m³/d. The total capacity in the Middle East region alone reached 8.91×10⁶ m³/d, accounting for 55%, and that in the United States was 2.37×10⁶ m³/d, accounting for 15%. As early as the 1950s, the method of seawater desalination had been used to produce freshwater. The main methods of seawater desalination are: ① Evaporation methods: Multi-stage flash distillation, single-stage flash distillation, vertical multiple-effect distillation, horizontal multiple-effect distillation, submerged tube method, vapor compression method; ② Membrane methods: Electrodialysis method, reverse osmosis method; ③ Composite method. Among them, the evaporation method accounts for 60%, the reverse osmosis method accounts for 33%, and the electrodialysis method accounts for 5.5%. 2. The Application of Titanium in Seawater Desalination Equipment

2.1 Heat-conducting Tubes in Seawater Desalination Equipment The original heat-conducting tubes of seawater desalination equipment mainly used copper alloy tubes. Due to many deficiencies of copper alloy tubes, they have now been replaced by titanium tubes with high reliability and requiring no maintenance. (1) Wall Thickness of Titanium Tubes The wall thickness of the heat-conducting tubes is determined by the operating conditions, tube sheet material, the construction ability of the tube expansion operation, and the welding technology at the tube ends. Since the diameter of the heat-conducting tubes is small and the requirement for strength is not high, in actual use, tubes with relatively thin wall thickness are adopted. Generally, the wall thickness of copper alloy tubes is 0.9mm - 1.2mm; when replaced by titanium tubes, in places with less corrosion, thin-walled welded tubes with a wall thickness of 0.3mm can be used. (2) Thermal Conductivity of Titanium Tubes Due to different materials of the heat-conducting tubes, the thermal conductivity is also different. For example, the thermal conductivity of titanium is 17W/(m·K), that of aluminum brass is 100W/(m·K), that of 90/10 cupronickel is 47W/(m·K), and that of 70/30 cupronickel is 29W/(m·K). Therefore, the thermal conductivity effect of the heat-conducting tubes can be controlled by changing the wall thickness. Among the above materials, titanium has the smallest thermal conductivity. If thin-walled titanium welded tubes are used, although the thermal conductivity is worse than that of aluminum brass, it is equivalent to that of 90/10 cupronickel and better than that of 70/30 cupronickel. (3) Economy of Titanium Tubes The unit mass price of titanium tubes is 2 to 6 times higher than that of copper alloy tubes. However, considering the performance-price ratio, the price of titanium tubes can compete with that of copper alloy tubes. Due to the low density of titanium, when the wall thickness is the same, the mass of a titanium tube of the same length is only 50% of that of a copper alloy tube. When the wall thickness of the titanium tube is 50% of that of the copper alloy tube, the mass of the titanium tube with the same heat transfer area is only 1/4 of that of the copper alloy tube. According to the current price level, the overall price of using thin-walled titanium welded tubes is the same as that of aluminum-copper tubes and is even cheaper than that of cupronickel tubes. It can be seen that titanium tubes are competitive in terms of price. 2.2 The Development and Application of Thin-walled Titanium Welded Tubes in Japan The successful development of titanium strip rolling technology has become the basis for the mass production of titanium welded tubes. In the 1960s, titanium wires were used in the production of caustic soda by the mercury method in Japan. In the early 1990s, to prevent pollution, the caustic soda production process was improved. With the adoption of the diaphragm method, more than 700 tons of titanium strips were applied. Taking this as an opportunity, Japan researched and developed the technology for the continuous production of hot-rolled and cold-rolled titanium strip coils, established a mass production system for strip coils used in thin-walled titanium welded tubes for seawater desalination and power station condensers, and correspondingly developed the production technology of thin-walled welded tubes. The power station condensers produced by Hitachi, Mitsubishi, and Toshiba use titanium welded tubes with a thickness of 0.5mm. The seawater desalination devices produced by companies such as Mitsubishi, Kawasaki, Hitachi, Mitsui, and Kobe Steel use titanium welded tubes with a thickness of 0.5mm - 0.7mm. Mainly applied in power stations, titanium welded tubes have been widely used as heat transfer tubes in seawater desalination, ironmaking, shipbuilding, petroleum refining, chemical engineering and other fields. By 1983, within 16 years, Japan produced 4,038 tons of thin-walled titanium welded tubes for seawater desalination equipment around the world, and no damage due to seawater corrosion has occurred so far. (1) Ventilation Condensers and Jet Compressors The real seawater desalination equipment in Japan was a 2,650 t/d seawater desalination equipment built by Matsushima Carbon Mining Co., Ltd. in 1967. For the heat transfer tubes and tube sheets of the ventilation condensers and jet compressors of this device, due to the corrosion of Br⁻ in seawater, copper alloys could not be used. After replacing them with titanium, no failures caused by corrosion have occurred. (2) Heat Release Section Condensers The multi-stage flash condensers use seawater as the cooling water to cool the water vapor generated in each flash chamber. Since seawater often contains sediment and marine organisms, they adhere to the inside and ends of the heat transfer tubes and erode the copper alloy tubes. Therefore, almost all the heat transfer condensers of MSF (Multi-Stage Flash) type seawater desalination equipment now use titanium tubes. Especially when it is necessary to inject oxygen to kill the bacteria in seawater, titanium tubes with good corrosion resistance are more required. (3) Heat Recovery Section Condensers The heat transfer area of the heat recovery section condensers is relatively large. Due to economic reasons, copper alloy tubes are usually used currently, and titanium tubes are only used in special cases, such as when the medium containing pollutants like ammonia or hydrogen sulfide severely corrodes the copper alloy. In 1977, for the 3,600 t/d MSF type desalination device exported to Germany, since it was an auxiliary device of ammonia, copper alloy could not be used, and titanium was adopted; due to the corrosion of hydrogen sulfide, the aluminum-brass tubes of the 3,120 t/d MSP type desalination equipment in Peru corroded after one year of use, and finally all the heat transfer tubes were replaced with titanium tubes. It is reported that a seawater desalination device with a daily output of hundreds of tons uses up to 60,000 titanium tubes. From 1967 to 1994, within nearly 30 years, a total of 52 sets of condensers for atomic-level thermal power generation and 7 sets of seawater desalination equipment were produced, using a total of 11,000 tons of titanium welded tubes. 3. Problems Needing Attention in Use (1) Galvanic Corrosion Titanium has a relatively positive potential in seawater. When it comes into contact with other metals, it can promote the corrosion of other metals. The prevention methods include using titanium for both the heat transfer tubes and the tube sheets, or using the method of sacrificial anodes. Above 80°C, to prevent hydrogen absorption, an Fe-90%Ni alloy is used as the sacrificial anode; below 80°C, coated or rubber-lined steel plates are used. (2) Crevice Corrosion Titanium tubes are installed on titanium tube sheets by the tube expansion method. Crevice corrosion may occur in seawater at 100°C with a pH value of 8. However, in actual water chambers, copper alloys are used. Even if the seawater temperature reaches 120°C, crevice corrosion will not occur. In reality, to improve the reliability of the equipment, when used above 100°C, tube end welding is often used to prevent crevice corrosion. (3) Hydrogen Absorption In seawater above 80°C, titanium may absorb hydrogen; when cathodic protection is applied, overprotection will cause hydrogen absorption. If an Fe-9%Ni alloy is used as the sacrificial anode plate, hydrogen absorption by titanium will not occur. (4) Vibration Since the wall of the titanium tube is thin, when replacing the copper alloy tube, attention should also be paid to the damage caused by tube vibration. This problem can be solved by using a smaller interval between the tube support plates than that of the copper alloy tubes.