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    Review date: July 24, 2003


    Supercritical Water Oxidation/Synthesis
    Related publications, More applications

    Supercritical water is now widely recognized for its capacity to destroy toxic or hazardous materials. Supercritical water is also an interesting medium for chemical synthesis. We have extensive capabilities and experience in working with supercritical water solutions. At PNNL there are various facilities available for bench- to pilot-scale studies of supercritical water systems. For example there is bench-top flow reactor (up to 600°C) that can be used to measure kinetics and pathways for both oxidation and synthesis reactions. In situ spectroscopic techniques include NMR, FTIR, and X-ray absorption or scattering. For instance, infrared spectroscopy is a fast, elegant method of making in situ measurements of reaction rates at elevated temperatures and pressures. This technique combined with the off-line assay allows us to quickly assess the feasibility of a conversion by supercritical methods.


    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.


    Hydrothermal flow reactor for studying kinetics and pathways at temperatures up to 600&degr;C
    Dysprosium hydroxyfluoride fibers produces at 450°C, 450 bar.

    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.




    For information about supercritical fluid capabilities at PNNL,
    please contact Clement Yonker, at (509) 372-4748,