Application of High-Pressure Reactors in Supercritical Water-Related Experiments

　　Supercritical water is the state of water under conditions of high temperature (374°C) and high pressure (22.1 MPa). This supercritical water possesses many unique characteristics. For instance, it exhibits extremely strong oxidizing properties. When a reaction medium requiring treatment is introduced into supercritical water and oxygen is subsequently dissolved into it, its oxidizing power surpasses that of substances such as potassium permanganate. Moreover, many substances can be burned in supercritical water. Additionally, supercritical water can dissolve a wide range of substances that are insoluble under conventional conditions, and after dissolution, the volume of these substances significantly shrinks. Fourth, supercritical water exhibits exceptionally strong catalytic effects, dramatically accelerating the rate of catalytic reactions.

　　The numerous advantages of supercritical water have prompted researchers to devote tremendous effort and dedication to its study. Since supercritical water must be generated under specific conditions, and given that it can slowly corrode nearly all metallic materials—including gold—this poses even higher demands on the selection of materials for high-pressure reactors. Among the materials most commonly considered for applications involving supercritical water are titanium (or titanium alloys) and nickel-based alloys. A wealth of literature, both domestically and internationally, has reported on this topic. Generally speaking, titanium alloys exhibit superior corrosion resistance, and in the fabrication of high-pressure reactors, they are often used as linings. However, at high temperatures (above 230°C), the pressure-bearing capacity of titanium alloys begins to weaken or even disappear entirely. To maintain adequate pressure resistance at elevated temperatures, it is necessary to line the reactor with a material that retains good pressure-bearing performance under such conditions. Moreover, welding titanium alloys is highly specialized: under normal ambient conditions, they cannot be effectively welded to other materials at all. Instead, welding must be carried out in an environment filled with inert gas, placing stringent requirements on both welding conditions and the technical expertise of the personnel involved, thereby increasing the manufacturing costs of the equipment. Of course, nickel-based alloys are also frequently used in the construction of high-pressure reactors for supercritical water research—particularly in reaction systems involving strongly alkaline supercritical water. Studies have shown that such reaction systems exert virtually no corrosive effect on high-pressure reactors made from nickel-based alloys.