Fundamental kinetics of methane oxidation in supercritical water


Fundamental kinetics of methane oxidation in supercritical water...

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Energy & Fuels 1991,5, 411-419

411

Articles Fundamental Kinetics of Methane Oxidation in Supercritical Water Paul A. Webley and Jefferson W. Tester* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received August 22, 1990. Revised Manuscript Received January 24, 1991

The oxidation kinetics of methane in supercritical water were determined in an isothermal, plug flow reactor over the temperature range 560-650 "C at 245.8 bar. The oxidation rate was found to be first order in methane concentration and 2/3-orderin oxygen concentration. The activation energy over the temperature range 560-650 OC was 42.8 f 4.3 kcal/mol. A pressure-corrected elementary reaction model for gas-phase combustion was applied to the oxidation of methane in supercritical water. Reaction path analysis identified several key reactions but the model underpredicted the methane conversion by a factor of 5 and predicted a first-order activation energy of 66 kcal/mol over the temperature range 560-650 "C. Variation of the rate constants for a few important reactions within their uncertainty limits resulted in reasonable agreement with experimental data. A more detailed examination of elementary reactions in supercritical water is therefore warranted.

Introduction Supercritical water oxidation is an advanced technology for the destruction of hazardous chemical and is also currently being investigated as a method for treating spacecraft wastewater for recycle.3 Pure water is considered supercritical if ita temperature and pressure exceed 374.2 O C and 221 bar, respectively. In the critical and near-critical region, the density is a strong function of both temperature and pressure, leading to large changes in the physical properties particularly the solvation behaviors4 Inorganic salts are insoluble6 in supercritical water while organics and gases are completely miscible.6 In supercritid water oxidation, organics, air, and water are mixed at temperatures typically above 400 "C and pressures of 250 bar or more. Oxidation is initiated spontaneously and the heat of combustion is released within the fluid resulting in a rise in temperature to 5504350 O C . Organics are oxidized rapidly with conversions in excess of 99.99% at reactor residence times of approximately 1min. Heteroatoms (such as chlorine) are converted to acids (HC1) which can be precipitated out of solution as salts (NaC1) by adding a base to the feed.' Under proper operating conditions of temperature and pressure, oxidation of organics to carbon dioxide and molecular nitrogen is complete without the formation of noxious byproducts such as NO, compounds.8 Kinetic information is now available for the oxidation of carbon monoxide? and ammonia" in supercritical water. Methane was selected for this study as it is a simple organic whose oxidation kinetics in the gas phase have been well studied. In addition, methane is frequently an intermediate in the oxidation of higher organics and represents the rate-limiting step in the overall oxidation to carbon dioxide and water. In a previous in-

vestigation of methane oxidation in supercritical water in our laboratory,'2 temperature measurement problems were discovered in a subsequent review of experimental procedure~.'~As reaction temperatures were not known accurately, this partially invalidates the earlier data. The data presented in this paper were obtained in an improved version of the plug flow reactor and are considered to be more reliable.

Experimental Section Experimental Apparatus. The reactor used to obtain the data for methane oxidation was 4.71 m of 0.635 cm 0.d. X 0.171 (1) Staszak, C. N.; Malinowski, K. C.; Killilea, W. R. Enuiron. h o g . 1987, 6(2), 39. (2) Modell, M. United States Patent No. 4,338,199, 1982. (3) Hong, G.T.; Fowler, P. K. 'Supercritical Water Oxidation of Urine and Fecea", Final Report to NASA, Contract NAS2-12176,MODAR, Inc., Natick MA 1987. (4) Franck, E. U. Acre Appl. Chem. 1970,24(13), 13. (5) Martynova, 0.I. In High Temperature, High Pressure Electrochemistry in Aqueous Solutions, January 7-12,1973, The University of Surrey, England; Jones, D. de G., Staehle, R. W., Chairmen; National Association of Corrosion of Engineers: Houston, TX, 1976; p 131. (6) Connolly, J. J. Chem. Eng. Data 1966, 11, 13. (7) Thomason, T. B.;Modell, M. Hazard. Waste 1984, 1(4), 453. (8)Timberlake, S. H.; Hong, G. T.; Simson, M.; Modell, M. SAE Technical Paper Series No. 820872; 12th Intersociety Conference on Environmental Systems, San Diego, CA, July 19-21,1982. (9) Helling, R. K.; Teeter, J. W. Energy Fuels 1987, I , 417. (10)Webley, P. A.; Tester, J. W. Supercritical Fluid Science and Technology; ACS Symposium Series 406; American Chemical Society: Washington, DC, 1989; p 269. (11) Webley, P.A.; Tester, J. W. SAE Technical Paper Series No. 901333; 20th Intersociety Conference on Environmental Systems, Williamsburg, VA, July 9-12, 1990. (12) Webley, P.A.; Tester, J. W. SAE Technical Paper Series No. 881039; 18th Intersociety Conference on Environmental Systems, San Francisco, CA, 1988. (13) Webley, P.A. Fundamental Oxidation Kinetics of Simple Compounds in Supercritical Water. Ph.D. Thesis in the Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, 1989.

0887-0624/91/2505-0411$02.50/0@ 1991 American Chemical Society

412 Energy & Fuels, Vol. 5, No. 3,1991

Webley and Tester

Table I. Experimental Data for Methane Oxidation in Supercritical Water (Pressure = 245.8 bar) [CH,Io,' [OZlO,e av rate, mass T,"C 7:s X,b '3% mol/L mol/L Re Sc%f mol/(L-s) In M In klh balance: '3% 600 7.7 51.7 1.67 X 3.24 X 1.94 0.90 1.12 X lo-' 1.61 -2.36 98.5 600 11.6 60.0 1.70 X 3.16 X 1.86 0.94 8.79 X loa 1.48 -2.54 97.5 95.1 580 8.0 31.8 1.79 X 3.45 X 1.92 0.79 7.12 X loa 0.83 -3.05 92.5 560 8.4 21.6 1.91 X 3.60 X 1.89 0.66 4.91 X 0.27 -3.54 7.3 61.2 -2.05 94.1 630 1.70 X 3.09 X 1.82 0.95 1.43 X lo-' 1.99 91.7 7.5 56.6 1.79 X 3.24 X 1.81 0.93 1.35 X lo-' 1.79 -2.20 615 581 8.0 32.1 2.49 X 0.82 9.99 X loa 0.89 -3.03 96.8 3.44 X 1.38 581 6.2 22.7 3.00 X 2.73 X 0.91 0.72 1.10 X lo-' 0.91 -3.18 97.9 8.0 30.3 3.24 X 3.29 X 1.01 0.84 1.23 X lo-' 0.92 -3.10 90.9 580 580 6.2 19.7 4.12 X 2.54 X 0.62 0.70 1.31 X lo-' 0.86 -3.34 96.9 92.1 580 8.1 37.2 1.05 X 3.43 X 3.27 0.93 4.82 X loa 0.97 -2.86 580 8.1 28.1 1.12 X 2.52 X 2.25 0.89 3.89 X loa 0.85 -3.20 86.0 581 8.2 24.4 1.11 X 1.80 X 1.62 0.88 3.30 X loa 0.93 -3.37 88.8 581 10.6 23.3 1.43 X loq3 1.30 X 0.91 0.86 3.14 X loa 0.91 -3.69 90.4 652 7.1 0.0 4.20 x 10-3 6.82 x 104 0.00 0.00