Destroying Toxic Wastes
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Hazardous or toxic organic compounds can be oxidized into soluable gases and acids at high temperatures
and pressures in a process called supercritical water oxidation (SCWO). A typical flow diagram of the a
process is pictured here.  |
Nickel and titanium alloys face the challenges of the supercritical water oxidation
market
Nickel magazine, Jun. 01 -- Throughout the world, efforts are under way to reduce the amount
of waste that is incinerated or relegated to landfill sites, and one technology that promises to make a
profound difference in this area is supercritical water oxidation (SCWO).
In the SCWO process, hazardous or toxic organic contaminants in aqueous wastes are rapidly oxidized at
temperatures of about 600ºC, pressures of 24 to 30 megapascals, a pH range of 2 to 12, and oxygen
concentration levels ranging from parts-per-million up to percentage levels. Under these conditions, water
acts like a dense gas, becoming very soluble to organic substances and gases like oxygen and nitrogen. Many
organic compounds are completely oxidized in single-phase reactions to carbon dioxide, water and various
acids.
At a special session during the annual meeting of NACE International in Houston, Texas, U.S.A. delegates from five nations, including Germany, China, Japan, France and the U.S., discussed the merits of various materials that could be used to construct these waste-destroying reactors. One thing is clear: while the designers of SCWO reactors are confident about the technology's potential, they are nonetheless concerned about corrosion rates inside their equipment.
Increasingly, the solution to this corrosion problem would seem to rest with alloys of nickel and/or titanium.
Some work has been done to determine corrosion rates of titanium alloys under the conditions described, though the technical literature contains conflicting results. "Most people see good material performance, but there are examples of severe corrosion," said Dr. Bryce Mitton, a senior researcher at the Massachusetts Institute of Technology. "Stainless steel S31603, on the other hand, may be acceptable [in some applications], but because of stress corrosion cracking, it will probably not be acceptable for highly corrosive feeds."
According to Nikolaos Boukis and Peter Kritzer of the Institut für Technische Chemie, Karlsruhe, Germany, existing nickel alloys, such as alloy N06625, are sufficient for reactor construction. Based on their studies, N06625 performs well in the high-temperature section of the reactor.
However, in the so-called temperature transition section of the reactor, corrosion rates are high for both nickel and titanium alloys. To deal with this problem, some have suggested coating the nickel alloys with gold and platinum.
Nickel depletion, or "de-alloying," is another concern, and additionally it results in nickel entering the effluent.
One way to control corrosion rates inside of SCWO reactors is to blend feed materials to adjust the ever-critical pH. This can also be controlled by adding chemicals, such as sodium sulphate. Sensors are being developed (at Pennsylvania State University, for example) that will allow operators to monitor and adjust the pH, and, significantly, these sensors are being constructed of nickel-containing stainless steels.
The market for SCWO reactors is growing as nations seek to destroy stockpiled wastes. There are some 23,000 tonnes of chemical agents presently being stored at eight sites throughout the continental United States alone. And Japan foresees decomposing organic wastes from nuclear power plants, such as gloves, plastic bottles and cation exchange resins, in SCWO reactors. In one such case in Japan, N06625 was selected as a material of construction and a titanium-platinum alloy was selected as a liner material.
Diagram: BRYCE MITTON, MASSACHUSETTS INSTITUTE OF TECHNOLOGY
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Please see the following NACE Papers, which are available from www.nace.org |
