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Review date: July 24, 2003
PNNL-SA-27883

Supercritical Water Oxidation/Synthesis
Related publications, More applications

Oxidation

Hydrothermal flow reactor for studying kinetics and pathways at temperatures up to 600°C

In the supercritical oxidation process a solution containing the hazardous component is heated to near-critical or supercritical conditions in the presence of appropriate oxidizers. Since the critical point of water is relatively high (374°C) waste components can be "burned" to simple byproducts. Under hydrothermal processing conditions the main components of the waste are converted to three harmless byproducts; carbon dioxide, nitrogen and water. Significant control over the reaction can be obtained by varying the temperature or pressure of the supercritical or near-critical solution. Supercritical water exists at conditions above its critical temperature (374°C) and pressure (217 atm); its properties are highly dependent upon pressure when the system is near the critical point. In supercritical fluids, the density, dielectric constant, and viscosity, as well as other properties, can be varied over a wide range by manipulating pressure. This offers added flexibility for controlling the reaction pathways and reaction rates. Often the most difficult components to destroy in a waste stream are the nitrate/nitrites. Earlier work at PNNL has demonstrated that nitrate is effectively destroyed using ammonium ion as the reductant.

The advantages of the supercritical water oxidation over more conventional disposal techniques are

1. the "combustion" processes occurring under supercritical water conditions are known to be highly efficient at converting the waste to harmless byproducts (>99.9%)
2. it is a closed system that has no emissions to the atmosphere
3. the system is easily controlled and can be shut down quickly
4. the required equipment is relatively simple so that the commercial-scale unit is small enough for local installations.

Synthesis

Dysprosium hydroxyfluoride fibers produces at 450°C, 450 bar.

Dy/W/O platelets formed at 450°C, 530 bar.

We have also used supercritical water chemistry for the synthesis of nanofibers and nanoparticles of different morphologies for use as novel optical materials and as high-strength materials.