Oxidations in supercritical water

Supercritical water as a reaction medium has been investigated sinee the early 1980s, in particular in studies by the research groups of Modell, Antal and Tester. Due to the severe conditions, focus was mainly set on degradation reactions. Supercritical water oxidation of waste compounds can be seen as a further 

Methane Methanol Oxides of Mn, Co, Pd, Mo, Cr, V, Cu 25-250 1-69 Continuous, homogeneous or heterogeneous catalyst CO2 mixed with perfluorinated compounds (expanded liquid) 139 

Propylene O2 Propylene glycol Cul-Cu20-Mn02 with supports 100-250 100-500 Continuous reactor H2O in feed S — 95% 141 

Methanol O2 Dimethyl ether, Formaldehyde methyl formate Fe203, Fe203/Si02, Fe203-Mo03 200-330 83-124 Continuous reactor Ymax = 81%(DME) Ymax = 95%(FA) Ymax = 69% (MF) 143 

Benzene O2/H2/N2O Phenol Noble metal on Ti02 or V-silicalite, Pd/TS-1 20-100 10-300 Batch reactor Y = 0.11 % 144 

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The main advantage of using catalysts in SCWO processes is to reduce the operating temperature. A complete review of catalytic oxidation in supercritical water can be found in reference [8]. 

Anitescu, G., Zhang, Z., and Tavlarides, L., Kinetic study of methanol oxidation in supercritical water, Indust. Eng. Chem. Res., 38(6), 2231-2237, 1999. 

Reviewed previous SCWO research with model pollutants and demonstrated that phenolic compounds are the model pollutants studied most extensively under SCWO conditions Studied supercritical water oxidation of aqueous waste Explored reaction pathways in SCWO of phenol Studied catalytic oxidation in supercritical water Explored metal oxides as catalysts in SCWO Studied decomposition of municipal sludge by SCWO Investigated the SCWO kinetics, products, and pathways for CH3- and CHO-substituted phenols Determined oxidation rates of common organic compounds in SCWO... 

Gopalan, S. and Savage, P.E., Reaction mechanism for phenol oxidation in supercritical water,. Phys. Chem., 98, 2646, 1994. 

Steeper, R.R., Methane and methanol oxidation in supercritical water chemical kinetics and hydrothermal flame studies, Sandia Rep., Sand96-8208.UC-1409, 1-150, 1996. 

Thornton, T.D. and Savage, RE., Kinetics of phenol oxidation in supercritical water, AIChE ]., 38, 321, 1992a. 

Holgate HR, Webley PA, Tester JW. Carbon monoxide oxidation in supercritical water the effects of heat transfer and the water-gas shift reaction on observed kinetics. Energy Fuels 1992 6 586-597. 

Tester JW, Welby PA, Holgate HR. Revised global kinetic measurements of methanol oxidation in supercritical water. Ind Eng Chem Res 1993 32 236-239. 

Webley PA, Tester JW. Fundamental kinetics of methane oxidation in supercritical water. Energy Fuels 1991 5 411 -19. 

Tester JW, Webley PA, Holgate HR. Revised global kinetic measurements of methanol oxidation in supercritical water. Ind Eng Ch R 1993 32(l) 236-239 Helling RK, Tester JW. Fundamental kinetics and mechanisms of hydrogen oxidation in supercritical water. Combust Sci Technol 1993 88(5-6) 369-397. 

Thornton TD, Savage PE. Phenol oxidation in supercritical water. J Supercrit Fluids 1990 3 240-248. 

Li R, Savage PE, Szmukler D. 2-Chlorophenol oxidation in supercritical water global kinetics and reaction products. AIChE J 1993 39(1) 178 187. 

Savage PE, Smith MA. Kinetics of acetic acid oxidation in supercritical water. Environ Sci Technol 1995 29 216-221. 

Fig. 5.8 Examples of oxidative water treatment technologies used in industry, research and development [adapted from FIGAWA (1997), and supplemented by novel methods]. The numbers 1 to 9 refer to the generalized reaction sequences presented in Figure 5-9. a) Oxidation at elevated temperatures between 220°C < T <300°C or supercritical water oxidation at AT >374°C, Ap >221 bar (221000 kPa) (cf Chapter 1) b) oxidation in the presence of bimetallics Fe°/Ni° or Zn°/Ni° (Cheng and Wu, 2001) or heterogeneous oxidation in supercritical water catalyzed by metals Me = Cu, Ag, Au/Ag-alloy c) Fenton reaction at pH <5 d) photo-assisted Fenton reaction, irradiation in the UV-B/VIS range e) the mixture of oxidants O3/H2O2 is called PEROXONE f) ozonation using solid-bed catalysts with conditioned activated carbon (AC) g) vacuum-UV photolysis of water.

Fundamental Kinetics of Methanol Oxidation in Supercritical Water... 

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. 

Application of Equation 8 to oxidation in supercritical water at 550 0 gives a reaction rate an order of magnitude lower than that observed and a conversion of 9.4% compared to an observed conversion of 94%. 

Complete oxidation in supercritical water occurs at a high rate usually, complete conversion to carbon dioxide is achieved in the reaction time of about 1 min for most organics. New applications of supercritical water are emerging where only partial oxidation is carried out and oxygenated hydrocarbons are produced. Here, the reaction time has to be limited to the order of seconds. 

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