Energy & Fuels 1991,5, 411-419
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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 90% a t 60% conversion). These observations are consistent with carbon monoxide being the intermediate in the overall oxidation to carbon dioxide. A trace amount of hydrogen (1000 10 25 19 14.5 0+0+M 02 +M 0.0 -1.8 200-4000 1.3 25 20 14.2 0.0 23.6 700-1000 CO + HOp COZ + OH 3 25 21 18.0 0.0 129.5 300-900 CO2 + M = CO + 0 + M 2 33 22 13.7 106.9 1000-2000 0.6 CHI CHS + H 1.5 39 23 13.6 56.9 CHI + O4 = CHS+ HOz 0.0 no data 5 25 24 4.4 CH4 + H CHS + HZ 3.0 8.8 640-820 1.5 25 25 9.0 CH4 + 0 = CHS + OH 1.5 8.6 300-2250 25 1.3 7.2 26 1.8 2.8 300-1500 CH4 + OH CHS + H20 0.1 25 27 11.3 18.6 0.0 CHI + HOz CHS + HzO2 no data 5 25 28 13.9 CHS + 0 HCHO + H 0.0 0.0 300-2000 2 25 29 12.9 0.0 CHS + OH = HCHO + H2 0.0 300 3 25 30 8.3 1.8 3.0 HCHO + H = CHO + H2 260-480 3 25 13.3 HCHO + 0 = CHO + OH 0.0 290-750 2 31 3.1 25 9.5 32 HCHO + OH = CHO + H2O 1.2 300-1600 -0.4 3 25 14.3 0.0 2 33 CHO + H CO + H2 0.0 298 25 13.5 34 CHO + 0 = CO + OH 0.0 0.0 no data 3 25 13.5 35 no data CHO + 0 = CO2 + H 0.0 0.0 3 25 13.7 0.0 0.0 3 36 no data CHO + OH CO + H2O 25 12.5 37 CHO + 0 2 = CO + HOz 0.0 0.0 300-500 10 25 16.0 38 CHO + M = CO + H + M -0.1 20.3 no data 5 25 13.4 no data 0.0 4.0 5 25 HpO2 + H H2O + OH 39 4.0 3 25 HZ02 + 0 OH + HO2 7.0 40 2.0 no data 2 13.3 41 HCHO + 0 2 = HOZ + CHO 0.0 720-820 38.9 25 12.3 no data 3 11.7 25 42 0.0 HCHO + H02 H202 + CHO 3.9 no data 2.9 5 22.5 25 43 CH4 + CHO CHS + HCHO 2 44 13.6 0.7 HCHO = H + CHO 1000-3000 89.6 40 16.3 CHSOH = CHS + OH 0.0 200-3000 91.8 10 26 45 44.9 13.3 0.0 no data CHSOH + 02 = CHiOH + HO 10 26 46 12.7 0.0 260-803 1.3 1.3 41 47 CHsOH + OH CHlOH + H2O 12.2 0.0 300-1000 2.0 CHSOH + 0 = CHZOH + OH 2.3 26 48 7.3 200-2000 3 CHgOH + H = CHZOH + H2 2.1 4.9 49 26 12.7 4 50 300-1000 0.0 CHSOH + H = CHg + H20 5.3 42 350-2100 1.5 3.3 2 7.2 51 CHgOH + CHS CHIOH + CH4 26 11.0 12.6 52 CHSOH + HO2 = CH2OH + Hi02 0.0 no data 10 26 CH2OH HCHO + H 14.8 0.0 300-2000 1.3 29.6 41 53 12.1 2 54 0.0 300 0.0 CHZOH + 02 = HCHO + H02 26 700-2500 CHS + 0 2 = CHSO + 0 0.0 29.0 13.4 3 25 55 13.2 56 1030-1115 CHS + H02 = CHSO + OH 0.0 0.0 3 25 300-2000 CHSO HCHO + H 0.0 25.4 1.3 14.2 41 57 5 58 300-630 2.6 0.0 CHsO + 0 2 = HCHO + H02 10.8 25 300 CHSO + H HCHO + Hz 0.0 0.0 13.3 3 25 59 0.0 0.0 no data 12.8 5 25 60 CHSO + 0 HCHO + OH no data 0.0 0.0 13.3 5 25 61 CH.0 + OH = HCHO + HoO CHiO + HOP = HCHO + H202 0.0 0.0 10 no data 11.5 25 62 0.0 455 10 11.2 25 CHSO + CH, = CHSOH + CHS 8.8 63 25 323-408 0.0 3 CH.0 + HCHO CHqOH + CHO 3.0 11.0 64 300-420 0.0 11.8 13.2 25 65 CHiO + CO = CH, + boz 3 0.0 300 CH90 + CHO = CH,OH + CO 0.0 3 14.0 25 66 Olog A = log of the preexponential factor. b = parameter in the expression k = AP exp(-E,,/RT). cE,, = activation energy, kcal/mol. * T (K)= temperature range over which the rate was measured or estimated. .UF = uncertainty factor. Estimated uncertainty of the rate constant. no.
where the rate parameters A, b, and E, have been evaluated at 250 bar. Those unimolecular reactions for which the collision partner M has been explicitly omitted are in the high-pressure limit and are written ri = ATb exp(-E,,/RT)[i]
(11)
The first test of the model was to determine if the predicted methane conversions from the elementary reaction model were in agreement with the experimentally observed values. Figure 3 shows a comparison of the conversion of methane calculated from the elementary reaction model with the experimental conversion. At 650
Webley and Tester
418 Energy & Fuels, Vol. 5, No. 3, 1991 OH la)s
C h O H Z CYOH
O9
i
1
0
I
/
!I
I
2
4
6
8
lime (5)
Figure 8. Profiie of major free radicals in the reactor predicted by the elementary reaction model. T = 650 O C , P = 245.8 bar.
OC, a conversion of only 17% was predicted, in contrast to a measured conversion of 62%. The predicted effect of temperature on methane oxidation kinetics is shown in the Arrhenius plot, Figure 2. The model underpredicts the experimental data over the measured temperature range. The first-order rate constant is approximately 2 orders of magnitude too low. The model predicts an activation energy of 66 kcal/mol over the temperature range 560-660 OC. This is 23 kcal/mol higher than the experimental value. The products of methane conversion calculated by the model are carbon monoxide and carbon dioxide with a trace of methanol. At 560 OC, the model predicts that 42% of the oxidized methane appears as COz increasing to 92% at 640 OC. At 640 OC, the experimental value for COz ‘selectivity” is 94%, in good agreement with the elementary reaction model. The model predicts almost no hydrogen formation. While this was experimentally verified for methane oxidation, a similarly formulated model was unable to predict hydrogen formation which was experimentally observed for the oxidation of carbon monoxide in supercritical water.’ Figure 8 shows the profile of the important free radicals OH, HOz, CH3, and hydrogen peroxide (HzOz)as calculated by the model for methane oxidation at 650 “C. A steady state is reached quickly with [OH], = 6 X lo4 mol/L, [HO,], = 2.8 X lo4 mol/L, and [CH,], = 4.5 X lo4 mol/L. At steady-state, free-radical chain initiation occurs through the reactions HzO + 02 = OH + HOz (12) CHI + 02 = CH3 + HOz (13) CH4 + M = CH3 + H + M (14) Reaction path analysis indicates that reaction 13 is the main chain initiation step with a rate of approximately 1 X mol/(L.s) at 640 “C. A reaction path analysis was conducted to determine the major reaction channel followed by carbon in the oxidation from methane to carbon dioxide. Figure 9 shows the dominant pathways for a simulation at 650 OC and 250 bar. We see that the initial hydrogen abstraction reaction by OH is very fast with a rate of mol/&$ and establishes equilibrium quickly. The rate-controlling steps are the subsequent reactions of the CH3 radical to either CH30 or CH30H. A t 650 “C, approximately 90% of the methyl radicals are consumed by the reverse reaction with water to give methane. Of the remaining 10% that proceed further along the reaction path, 60% react to give CH30 and 40% to CH30H. This branching ratio is subject to
OH/
f
HP45U
I
Figure 9. Freeradical reaction pathways for methane oxidation from elementary reaction model. T = 650 O C , P = 250 bar, M = third body.
considerable uncertaintyz7and is a major source of error in the current model. A first-order sensitivity analysis revealed that the most important reaction in determining the methane conversion is CH3 + HOz = CH30 + OH (15) Variation of the rate constant of this reaction by a factor of 5 leads to complete methane conversion to carbon dioxide at 650 “C. Figure 9 shows that methanol formation and oxidation provides a pathway for CH3 removal. Consequently, a small amount of methanol (
