Chemical destruction using supercritical water

In this chapter, the progress that has been made in this area will be reviewed. First, however, the nature of supercritical fluids in general will be introduced and then the properties of supercritical water in particular discussed. In the final section, corrosion problems faced by those wishing to establish processes are examined. 

When two molecules approach each other in a fluid, at a temperature where their relative speed is likely to be low, their mutually attractive forces will bring about a temporary association between them. If there is a sufficient density of molecules, there is the possibility of condensation to a liquid. On the other hand, if the temperature and the probable relative speeds are high, the attractive force will be too weak to have more than a slight effect on the molecular velocities, and condensation cannot occur however high the molecular density. It is therefore reasonable to expect, on the basis of molecular behaviour, that for every substance there is a temperature below which condensation to a liquid (and evaporation to a gas) is possible, but above which these processes cannot occur. 

In recent years, fluids have been exploited above their critical temperatures and pressures and the term supercritical fluids has been coined to describe these media. The greatest advantages of supercritical fluids are typically not too far (say 10-50 K) above their critical temperatures. Nitrogen gas in a cylinder is a fluid, but is not usually considered as a supercritical fluid, but more often described by an older term as a permanent gas. Sometimes substances are exploited a little below the critical temperature in the liquid region where the behaviour is similar to that of a supercritical fluid. For this reason some people prefer the term near-cri- 

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Superheated and supercritical water are used in several applications. Supercritical water is most often used in the destruction of organic wastes, including some chemical warfare agents, as an alternative to incineration (Katritzky et al., 1996 Sherman et al., 1998). Recent reports describe the use of both forms as a solvent and as a reactant in synthetic chemistry (Katritzky et al., 1996 An et al., 1997). Some of the reactions investigated include metal-mediated alkyne cyclizations, Pd-catalyzed al-kene arylations, aldol reactions, the Fischer indole synthesis, and hydrolysis reactions. Waterborne coatings and the destruction of wastes in supercritical water are fully... 

The destruction of hazardous chemical wastes by oxidation in supercritical water is a promising new technology which has several advantages over conventional methods of toxic chemical waste disposal. Although the feasibility of the supercritical water oxidation process has been demonstrated, there is little kinetic information available on the underlying reaction mechanisms. We have recently determined the oxidation kinetics of several model compounds in supercritical water, and now report on our results of the oxidation of methanol, a conunon industrial solvent, in supercritical water. Globd kinetic expressions are presented and our attempts to model the reaction using a free-radical mechanism with 56 elementary reactions are discussed. The inability of the elementary reaction model to represent oxidation in supercritical water is demonstrated and future model modifications are discussed. 

A chemical destruction method that has been used for the treatment of PCBs in contaminated dielectric liquids or soil is based on the reaction of a polyethylene glycol/potassium hydroxide mixture with PCBs (De Filippis et al. 1997). This method can be used successfully for the destruction of higher chlorinated PCBs with an efficiency of >99%, but was found to be unsuitable for the treatment of di- and trichlorobiphenyls due to low destruction efficiencies (Sabata et al. 1993). Irradiation of PCBs in isooctane and transformer oil by y-radiation resulted in degradation of PCBs to less chlorinated PCBs and PCB-solvent adducts (Arbon et al. 1996). Supercritical fluid technology has shown promise as a method for extraction of PCBs from soils, coupled with supercritical water oxidation of the extracted PCBs (Tavlarides 1993,1998a). Hofelt and Shea (1997) demonstrated the use of semipermeable membrane devices to accumulate PCBs from New Bedford Harbor, Massachusetts water. Another method showing... 

The use of supercritical fluids as alternatives to organic solvents is revolutionising a huge number of important science areas (24). Scientific applications vary from established processes, such as the decaffeination of coffee and the extraction and synthesis of active compounds, to the destruction of toxic waste in supercritical water, the production of nanoparticles and new materials, to novel emerging clean technologies for chemical reactions and extraction. 

Chapter 6 presents estimations of thermochemical properties of intermediates, transition states and products important to destruction of the aromatic ring in the phenyl radical + O2 reaction system. We have employed both DFT and high-level ab initio methods to analyze the substituent effects on a number of chemical reactions and processes involving alkyl and peroxyl radicals. Partially based on the results obtained in the vinyl system, high-pressure-limit kinetic parameters are obtained using canonical Transition State Theory. An elementary reaction mechanism is constructed to model experimental data obtained in a combustor at 1 atm, and in high-pressure turbine systems (5-20 atm), as well as in supercritical water [31]. 

If, on the other hand, none of the existing and available technologies can be used as is—for example, if substantial research and development resources would be needed to adapt them to the destruction of nonstockpile neutralents— the committee recommends that NSCMP, as part of track two, should invest first in chemical oxidation and wet-air/02 oxidation. Only if these technologies cannot be adapted easily does the committee recommend that the Army consider investing R D resources in supercritical water oxidation (batch mode).i ... 

Water becomes supercritical above 374°C and 220 atm, becoming less polar. This allows homogenization with non-polar organic materials, thus making them available for chemical reactions. An important potential use is the rapid destruction of toxic organic compounds and waste by oxygen.20... 

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