Supercritical water oxidation types

L. B. Kriksunov, D. D. Macdonald Corrosion in Supercritical Water Oxidation Systems A Phenomenological Analysis, J. Electrochem. Soc. 142, 1995, 4069. X.Y. Zhou, S.N. Lvov, X.J. Wei, L.G. Benning, D.D. Macdonald, Quantitative Evaluation of General Corrosion of Type 304 Stainless Steel in Subcritical and Supercritical Aqueous Solutions via Electrochemical Noise Analysis , Corrosion Science, 44 (2002) 841. 

Table 10.5 provides performance data regarding the SCWO process. Typical destruction efficiencies (DEs) for a number of compounds are also summarized in Table 10.5, which indicates that the DE could be affected by various parameters such as temperature, pressure, reaction time, oxidant type, and feed concentration. Feed concentrations can slightly increase the DE in supercritical oxidation processes. For SCWO, the oxidation rates appear to be first order and zero order with respect to the reactant and oxygen concentration, respectively. Depending upon reaction conditions and reactants involved, the rate of oxidation varies considerably. Pressure is another factor that can affect the oxidation rate in supercritical water. At a given temperature, pressure variations directly affect the properties of water, and in turn change the reactant concentrations. Furthermore, the properties of water are strong functions of temperature and pressure near its critical point. 

Another very important green chemistry solvent is supercritical water (SCW) [14], Water under supercritical conditions is an extremely powerful oxidizing and cleansing agent that has been proven remarkably promising as a soil decontaminant by efficiently degrading persistent organic toxic wastes that are difficult to eliminate from polluted soils, and in the treatment of several types of industrial wastes such as textile and cellulose wastewater [2],... 

The Research Centre of Karlsruhe conducted tests with a view to finding materials capable of withstanding the oxidizing, corrosive conditions involved in the use of supercritical water. The materials examined included various alloys such as austenite, AISl type 316 SS austenitic CrNi stainless steel and several Ni-based alloys (viz. Inconel 625, Hastelloy C-276, Nicrofer 5923, Nicrofer 6025 and Haynes alloy 214) that were used to fabricate pressure tubes for exposure to supercritical water containing 0.5-5 mol O /kg and/or 0.05-0.5 mol HCI/kg over a period of 150 h (a preliminary step on this line) [190]. 

Even after employing methods to selectively remove especially toxic species from chemical waste, we will continue to have to dispose of quantities of chemical waste. While many types of waste can be dealt with by incineration, often on site, some types of waste will demand chemical treatment to render them safe. Oxidation is very important in this context and apart for hydrogen peroxide and wet air, the use of supercritical water offers some exciting possibilities for the total oxidation of chemical waste. Chapter 15 deals with this powerful technique including a discussion of the remarkable properties of supercritical liquids as well as consideration of engineering aspects of the technology such as corrosion and plant design. 

Surfactants and Colloids in Supercritical Fluids Because very few nonvolatile molecules are soluble in CO2, many types of hydrophilic or lipophilic species may be dispersed in the form of polymer latexes (e.g., polystyrene), microemulsions, macroemulsions, and inorganic suspensions of metals and metal oxides (Shah et al., op. cit.). The environmentally benign, nontoxic, and nonflammable fluids water and CO2 are the two most abundant and inexpensive solvents on earth. Fluorocarbon and hydrocarbon-based surfactants have been used to form reverse micelles, water-in-C02... 

Catalytic oxidation in a two-phase water-C02 medium has been investigated for the synthesis of adipic acid from cyclohexene [2b]. The substrate and products were dissolved in the supercritical fluid while the oxidant (e.g. NaI04) resided in the aqueous phase. The catalyst RUO2 was oxidized to RUO4 in the aqueous phase which in turn oxidized the substrate, presumably at the liquid-supercritical interface. This is an example of a Type Ilia process. Unfortunately, catalyst efficiency was fairly low (five catalytic cycles), probably due to deactivation by formation of carbonates in the aqueous phase. 

This brief survey begins in Sec. II with studies of the aggregation behavior of the anionic surfactant AOT (sodium bis-2-ethylhexyI sulfosuccinate) and of nonionic pol-y(ethylene oxide) alkyl ethers in supercritical fluid ethane and compressed liquid propane. One- and two-phase reverse micelle systems are formed in which the volume of the oil component greatly exceeds the volume of water. In Sec. Ill we continue with investigations into three-component systems of AOT, compressed liquid propane, and water. These microemulsion systems are of the classical Winsor type that contain water and oil in relatively equal amounts. We next examine the effect of the alkane carbon number of the oil on surfactant phase behavior in Sec. IV. Unusual reversals of phase behavior occur in alkanes lighter than hexane in both reverse micelle and Winsor systems. Unusual phase behavior, together with pressure-driven phase transitions, can be explained and modeled by a modest extension of existing theories of surfactant phase behavior. Finally, Sec. V describes efforts to create surfactants suitable for use in supercritical CO2, and applications of surfactants in supercritical fluids are covered in Sec. VI. 

The common surfactants discussed repeatedly here and elsewhere are in general unsuitable for water-supercritical carbon dioxide emulsions of all types (including microemulsions). Selections are made, to start with, on the basis of the water/ carbon dioxide solubility behavior. A surfactant used in some of the initial studies is ammonium carboxylate perfluoropolyether [24,25]. Subsequently, a variety of triblock co-polymers, e.g. poly(propylene oxide-b-ethylene oxide-b-propylene... 

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