Fundamental Chemistry And Thermodynamics Of Hydrothermal Oxidation Processes

Hydrothermal oxidation (HTO) is a promising technology for the treatment of aqueous-fluid hazardous and mixed waste streams. Waste streams identified as likely candidates for treatment by this technology are primarily aqueous fluids containing hazardous organic compounds, and often containing inorganic compounds including radioisotopes (mixed wastes). These wastes are difficult and expensive to treat by conventional technologies (e.g. incineration) due to their high water content; in addition, incineration can lead to concerns related to stack releases. An especially attractive potential advantage of HTO over conventional treatment methods is the total containment of all reaction products within the overall system. The potential application of hydrothermal oxidation (HTO) technology for the treatment of DOE hazardous or mixed wastes has been uncertain due to concerns about safe and efficient operation of the technology. In principle, aqueous DOE wastes, including hazardous an d mixed waste, can be treated with this technology. Oxidation reactions are carried out in the aqueous phase at high temperatures ({approx}600 C), effectively converting organic waste constituents to nonhazardous materials (e.g., CO2). Inorganic materials which become insoluble in supercritical water may precipitate as scales adhering to components of the reactor, limiting reactor availability and necessitating frequent cleaning of the system. Also, most hazardous organic compounds contain heteroatoms (other than carbon, hydrogen, and oxygen). These heteroatoms, including halides (F, Cl, Br, I), sulfur, phosphorus, and some nitrogen groups, form strong mineral acids on oxidation of the organic compounds, resulting in a solution having low pH and high oxidation potential. This combination, in conjunction with the high temperatures and high fluid densities attained in both the heating and cooling regions of an HTO reactor, can lead to corrosion of structural materials (usually metal s) anticipated for use in HTO reactor construction. Methods have been suggested for mitigating the problems arising from the production of mineral acids and insoluble solids in HTO processes (Barnes et al., 1993). Previous work in this Laboratory centered on the problems arising from the presence of corrosive or insoluble inorganic compounds in HTO fluids (Simonson et al., 1993, 1994, 1995). However, significant gaps in our knowledge of process chemistry remained at the initiation of this project. It was not possible to determine accurately the properties of coexisting fluid phases; the solubilities of radioactive components of mixed wastes were unknown at high temperatures; and molecular level understanding of interparticle interactions was needed for reliable extrapolation of phenomenological equations for solution behavior beyond the range of experimental results. The present project was undertaken to address these deficiencies. The project was undertaken to provide fundamental information needed to support deployment decisions related to HTO technology, and no innovations in the technology per se were anticipated. Rather, the innovations of this project involved applying new or existing experimental and modeling approaches to studies of aqueous inorganic reactions and properties under the rigorous anticipated HTO operating conditions. This work was made possible in part through the support of researchers at ORNL and the University of Tennessee, Knoxville, by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences of the Department of Energy. This support has allowed significant, unique experimental and 2 computational resources to be developed for studies of aqueous solution chemistry at high temperatures and pressures