Supercritical water oxidation

Supercritical water oxidation or SCWO is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point. Under these conditions water becomes a fluid with unique properties that can be used to advantage in the destruction of hazardous wastes such as PCBs. The fluid has a density between that of water vapor and liquid at standard conditions, and exhibits high gas-like diffusion rates along with high liquid-like collision rates. In addition, solubility behavior is reversed so that chlorinated hydrocarbons become soluble in the water, allowing single-phase reaction of aqueous waste with a dissolved oxidizer. The reversed solubility also causes salts to precipitate out of solution, meaning they can be treated using conventional methods for solid-waste residuals. Efficient oxidation reactions occur at low temperature (400-650 °C) with reduced NOx production.

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SCWO can be classified as green chemistry or as a Clean Technology but care should be taken because of high pressures and temperatures.

A unique addition to the world of supercritical water (SCW) oxidation is generating high-pressure flames inside the SCW medium. The pioneer works on high-pressure Supercritical Water Flames were carried out by professor EU Franck at the German University of Karlsruhe in the late 80s. The works were mainly aimed at anticipating conditions which would cause spontaneous generation of non-desirable flames in the flameless SCW oxidation process. These flames would cause instabilities to the system and its components. ETH Zurich pursued the investigation of hydrothermal flames in continuously operated reactors. The rising needs for waste treatment and destruction methods motivated a Japanese Group in the Ebara Corporation to explore SCW flames as an environmental tool.

Currently few academic research groups continue to explore the environmental potential of SCW flames.

Commercial applications

The development of supercritical water oxidation is still limited. Several companies in the United States are trying to commercialize supercritical reactors to destroy hazardous wastes, but due to resource limitations they are unable to address the fundamental issues involved in the process. Partial oxidation has found some success, however, in the thermal depolymerization of non-hazardous agricultural wastes, recycling them into heating oil and other petroleum-like products. Chemical mechanisms and chemical kinetics at high pressure, the effects of halogens, mixing of the fuel and oxidizer, mass transport, heat transfer, and the existence and structure of supercritical flames are only a few of the phenomena that need to be clarified if the technology is to be applied to more demanding applications.

In Japan a few commercial applications exist, among them one unit for treatment of halogenated waste built by Organo. In Korea two commercial size units have been built by Hanwha.

In Europe, Chematur Engineering AB of Sweden has commercialized the SCWO technology for treatment of spent chemical catalysts to recover the precious metal, the AquaCat process. The unit has been built for Johnson Matthey in the UK. It is the only commercial SCWO unit in Europe and with its capacity of 3 m³/h it is the largest SCWO unit in the world. Presently Chematur are actively trying to commercialize the SCWO process for treatment of sludge, e.g. deinking sludge and sewage sludge. Many long duration trials on these applications have been made and thanks to the high destruction efficiency of 99.9%+ the solid residue after the SCWO process is well suited for recycling. In case of deinking sludge as paper filler and in case of sewage sludge as phosphorus and coagulant.

See also

• Wet oxidation
• Incineration

Super Critical Fluids International (SCFI) who are based in Ireland recently acquired the AquaCritox SCWO process developed by Chematur Engineering of Sweden. SCFI now claim to have significantly improved on the processing capacity and energy recovery of the AquaCritox process. SCFI currently target the sewage sludge market

• High Pressure Processes Group (Spain)
• Clean Technology Group (UK)
• FZK Karlsruhe (Germany)
• ETH Zurich, Transport Processes and Reactions Laboratory (Switzerland)

• UCL (London, UK) Clean Materials technology group working on Continuous Hydrothermal Flow Synthesis