Supercritical water corrosion problems with

Supercritical water oxidation poses a unique corrosion problem that has not been experienced in other chemical processes. Development of new materials is needed to address the problem. The material needs to withstand a chemically harsh environment along with the high temperature and pressure conditions. Even if some current materials are available for... 

Residence time for supercritical water oxidation systems may be as short as several minutes at temperatures of 600 to 650°C. More than 99.9 percent conversion of EPA priority pollutants such as chlorinated solvents has been achieved in a pilot-scale plant with retention time less than 5 minutes. The system is limited to treatment of liquid wastes or solids less than 200 microns in diameter. Char formation during reaction may impact the oxidation time of the organics, while separation of inorganic salts during the process may be a problem. Typical materials for the reactor are Hastelloy C-276 and Iconel 625 (high nickel alloys), which can withstand high temperatures and pressmes and the corrosive conditions. 

A way to further minimize corrosion is by adding base to the feed or reactor, so dial acids formed during the oxidation reaction are immediately neutralized. However, one must then deal with the resulting salts. Whether formed during reaction or already contained in the feed, salts will quickly precipitate in supercritical water. As these salts tend to adhere to and accumulate on the reactor walls and other surfaces within the reactor, they can inhibit and ultimately block process flow unless they are removed or their accumulation is controlled. Nonsalt solids (e.g., metal oxides, grit), by contrast, have little tendency to stick to process surfaces but can be a problem with respect to erosion and system pressure control. Methods that have been developed to manage and/or minimize the impact of corrosion, salt precipitation/accumulation, and solids handling are discussed in Sections 6.5 and 6.6. 

Corrosion can be a problem, especially if a chlorine-containing compound is decomposed. This may require that the reactor be made of titanium. The corrosion can be reduced by the use of sodium carbonate.207 Use of this system at 380°C reduced the content of polychlorinated biphenyls in a sample from 20 mg/L to less than 0.5 /tg/L. Trichloroethylene, 1,1,1-trichloroethane, and trichloroacetic acid were destroyed with 99.96% efficiency at 450°C for 60 ss using 1.5% hydrogen peroxide plus sodium bicarbonate.208 Sodium nitrate or sodium nitrite could be used in place of the hydrogen peroxide. Nitrates, ammonium hydroxide, and amines, all are converted to nitrogen at 350-360 C. Emulsions of petroleum, water, and solids can be broken by heating to 350oC. Supercritical water has been used to recover 2,4-diamino-toluene from distillation residues from the manufacture of 2,4-toluenediisocyanate.209... 

Recommendation (Demo I) GA-2. Before construction of a full-scale supercritical water oxidation (SCWO) system, additional evaluations of constraction materials and fabrication techniques will be necessary because corrosion and plugging prevent continuous operation with the present design. If the new construction materials do not solve these problems, then alternative SCWO reactor designs should be investigated. 

Finding (Pueblo) GA-7. Corrosion remains a serious operating problem with the GATS supercritical water oxidation system. Failure to shut down in time to replace a perforated reactor liner could result in rapid corrosion of the high-pressure reactor shell. 

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