Environ. Sci. Technol. 1988, 22, 1319-1324
Wehry, E. L. In Analytical Photochemistry and Photochemical Analysis; Fitzgerald, J. M., Ed.; Marcel Dekker: New York, NY, 1971; p 173. Falk, H. L.; Markul, I.; Kotin, P. A M A Arch. Ind. Health 1956, 13, 13. Wehry, E. L.; Mamantov, G.; Garrison, A. A.; Yokley, R. A.; Engelbach, R. J. In Polynuclear Aromatic Hydro-
carbons: Chemistry, Characterization and Carcinogenesis; Cooke, M., Dennis, A. J., Eds.; Battelle: Columbus, OH, 1986;p 985. Received for review February 12,1988. Accepted M a y 13,1988. This work was supported by U.S. Department of Energy, Grant No. 80 EV10449.
Oxidation of Simple Compounds and Mixtures in Supercritical Water: Carbon Monoxide, Ammonia, and Ethanol Richard K. Heilingt and Jefferson W. Tester" Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139
w Supercritical water is an ideal environment in which to oxidize and destroy dilute, aqueous, hazardous organic materials, but fundamental reaction kinetics in this medium are not known. The goals of this study were to determine the oxidative reaction kinetics of dilute carbon monoxide, ammonia, and ethanol both individually and in mixtures in supercritical water. An isothermal, plug-flow reactor made of Inconel 625 was operated between 400 and 540 "C, at pressures around 24.6 MPa (3550 psig), and with 6-13-9 residence times. Empirical Arrhenius parameters and orders of reaction were determined in global expressions for the direct oxidation of carbon monoxide in supercritical water and for the water-gas shift reaction. The activation energy for the first-order oxidation of ethanol in supercritical water was 340 kJ/mol, comparable to the carbon-carbon bond energy of 370 kJ/mol. Ethanol can also decompose or pyrolyze without oxygen in supercritical water and does not significantly alter the oxidation rates of ammonia or carbon monoxide. Below 540 "C, ammonia did not oxidize at a measurable rate. Introduction
A fluid is considered supercritical if its temperature and pressure both exceed their critical values. Fluids near their critical point exhibit a strong functionality of fluid density with temperature and pressure which causes pronounced changes in their solvent character. These properties have led to many applications of supercritical fluids as solvent extraction media (1). The use of supercritical carbon dioxide to extract hazardous organics from either wastewater or contaminated soil was noted in a review by Groves et al. (2). Supercritical fluids, and in particular supercritical water, can also be used as a medium for oxidative and thermal reactions to destroy hazardous wastes ( 3 ) . Under supercritical conditions [temperatures above 374 "C (705 O F ) and pressures above 22.1 MPa (3200 psia)] water behaves like a dense gas with a high solubility of organics ( 4 ) . It is also completely miscible in all proportions with oxygen (5) and shows a low solubility and dissociation of inorganics, particularly ionic salts (6, 7). The changes in solubility are due in part to the reduction in hydrogen bonding, since hydrogen bonding is a short-range force and has a diminished influence as the density of water decreases in the critical region. The solvation properties make supercritical water an excellent medium for oxidation of wastes, since organics and oxygen can be intimately mixed in a single, homogeneous phase, and *Author to whom correspondence should be sent. Present address: Dow Chemical U.S.A.. Western Division. Pittsburg, CA 94565. 0013-936X/88/0922-1319$01.50/0
inorganics can be readily removed from solutions by precipitation. Model1 et al. (8) used a continuous flow system to demonstrate the high destruction efficiency of the supercritical water oxidation process for toxic wastes. Oxidations were conducted with less than l-min residence time, with reactor outlet temperatures above 550 "C. The five feed mixtures included 27 000-33 000 ppm organic carbon from different combinations of DDT, methyl ethyl ketone, l,l,l-trichloroethane, benzene, PCB 1234, PCB 1254,2,4-dinitrotoluene, o-xylene, hexachlorocyclopentadiene, and nine other compounds. No chlorinated dibenzo-p-dioxanswere detected in any effluents, and the organic chloride destruction efficiencies exceeded 99.99% (greater efficiencies could not be resolved given the detection limits of their analytical instruments). The destruction efficiency for total organic carbon (TOC) exceeded 99.97%, so that essentially all the carbon is gaseous or inorganic at normal conditions. The high destruction efficiencies are believed to be due largely to the complete solubilization of the organics and their oxidation byproducts, which enables them to be oxidized completely to carbon dioxide rather than form chars or tars. Organic nitrogen was converted to molecular nitrogen or N 2 0 at temperatures above 500 "C with no formation of hazardous NO or NO, (9). The oxidation of other waste material in supercritical water has been demonstrated (10-13). These studies used flow reactors with residence times of 4 min or less and temperatures greater than 400 "C. Price (12) eliminated 88-93% of the liquid TOC, although the depressurized gas phase contained up to 11% residual carbon monoxide. Urea was completely oxidized in supercritical water, which produced no NO, compounds, but did leave significant amounts of ammonia in the depressurized effluent (11). Destruction of ammonia from a urea source was achieved to 99.9% at a nominal reaction temperature of -630 "C, residence times of 1 min, and with the addition of ethanol to promote the reaction in a heated-wall, laminar flow reactor (14). Timberlake et al. (11) suggested that cooxidation increases the reaction of ammonia either by generating free radicals from the decomposition of the organic or by providing local hot regions from the exothermic organic oxidation to stimulate ammonia oxidation. Cunningham et al. (10) demonstrated the applicability of this process to complete destruction of biopharmaceutical waste. Oxidative reaction kinetics in supercritical water were investigated for phenol, a common industrial waste, and acetic acid, a characteristic byproduct of conventional wet oxidation (13). Analogous to carbon monoxide, ammonia oxidizes slower than other nitrogen-containing species and is the rate-
0 1988 American Chemical Society
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Environ. Sci. Technol., Vol. 22, No. 11, 1988
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temperalure or Pressure indicator
0 high - pressure mfiller
tee
Figure 1. Schematic of experimental apparatus.
limiting reaction in the conversion of organic nitrogen to molecular nitrogen in gas-phase combustion, wet oxidation, and oxidation in supercritical water. The kinetics of ammonia oxidation in the gas phase have been well-studied and reviewed (15). Wightman (13) indicated that 20% conversion of ammonia could be achieved in supercritical water at 400 "C, which is contrary to other observations, where temperatures below 500 "C were not sufficient to oxidize ammonia from urea (11). Ammonia is also a stable product in the pyrolysis of aromatic amines in supercritical water a t 450 " C for 48 h (16). Although the efficient destruction of organic material has been demonstrated in the supercritical water oxidation process, the reaction kinetics and mechanisms are unknown. This work determined the oxidative reaction kinetics of simple molecules in supercritical water under well-defined conditions. Carbon monoxide and ammonia were selected as they are the simplest reactive carbon and nitrogen compounds and have been well-studied in the uncatalyzed and catalyzed gas-phase reactions. Their slow oxidation rates limit the overall conversion of organic nitrogen and organic carbon from complex molecules to molecular nitrogen and carbon dioxide. Ethanol was selected as a rapidly reacting hydrocarbon known to accelerate the oxidation of ammonia in supercritical water, although the detailed mechanism of this acceleration process has not been verified (14). The oxidation and water-gas shift kinetics of carbon monoxide in supercritical water are presented in more detail elsewhere (15,17). This paper presents investigations of the oxidation kinetics of ethanol and ammonia in supercritical water, and also oxidation of mixtures of ammonia or carbon monoxide with added ethanol in supercritical water. Experimental Section A tubular reactor system waa designed to produce well-characterized data for the quantitative determination of reaction kinetics in supercritical water, as described in detail elsewhere (15) and shown in Figure 1. Analysis of kinetics was simplified by the reactor being both iso1320
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thermal and in radially well-mixed, one-dimensional plug flow. The reactor consisted of 4.24 m of 0.635-cm 0.d. X 0.211-cm i.d. (1/4 X 0.083 in.) Inconel 625 tubing immersed in a fluidized-bed sand bath for temperature control. The 625 alloy was selected for its high strength and corrosion resistance. The reactor was maintained a t steady experimental conditions for a t least 20 min prior to collecting kinetics data. Dilute inlet concentrations of combustibles (ammonia, carbon monoxide, and ethanol) and oxidant (oxygen) were used in this system. Dilute feeds were prepared by dissolving reagents in room-temperature water in 1-L agitated autoclaves. Two feed solutions (one containing a combustible and the other containing oxygen) were pressurized and heated separately to reaction conditions in less than 10 s through a narrow (0.108-cm i.d.) Hastalloy C276 tubing and mixed at the reactor inlet. This avoided the operating difficulty of controlling the composition of the feed by mixing a large flow of water with a low flow of compressed oxygen or combustible, although since the system used four independent pump heads, there was still an uncertainty of -2% in the feed rate from one saturator. Use of dilute reagents also limited the maximum possible adiabatic temperature rise of the reactions to less than 2 "C. The reactor effluent was cooled quickly in a heat exchanger and depressurized. A 316SS tube packed with glass beads was used to effectively separate the product into measured gas and liquid flows. The composition of the gas phase was determined by gas chromatography with thermal conductivity detection (Perkin-Elmer Sigma lb). Ethanol in the liquid effluent was also measured by gas chromatography using a flame ionization detector. Ammonia in the liquid phase was measured with an ion-specific electrode (HNU ammonia electrode). Reactor conditions for these experiments ranged in temperature from 397 to 541 "c at nominally constant pressure of 24.6 MPa (3550 psig) and mass flow rate (1.67 X kg/s or 10 g/min). These yield typical Reynolds numbers in the reactor of 3100 f 400 (indicative of radially well-mixed flow) and residence times between 6 and 14 s.
Table I. Ranges of Conditions for Oxidation Experiments
co temp, O C density, kg/m3 inlet concn of combustible, mol/L inlet concn of oxygen, mol/L
444-540 78-160 0.75-3.9 -0, 0.59-4.2
combustible EtOH NH,/EtOH
3"
25, 397-541 78-169 1.71-4.57 3.19-6.99
The upper temperature limit was the operational limit of the sand bath. Experiments were also conducted at 25 "C to verify the absence of measurable reaction at room temperature. The ranges on inlet concentrations for carbon monoxide, ammonia, ethanol, and oxygen in these experiments are listed in Table I. The mole fraction of water was in all cases greater than 0.999, and oxygen was usually in excess. The feed concentration of ethanol was 0.1 vol % (NTP),which resulted in a range of inlet concentrations of (6.7-7.9) X lo4 mol/L due to the change in fluid density with temperature, from 160 kg/m3 at 400 "C to 78 kg/m3 at 541 "C. The measured quantities in all experiments included the system temperatures and pressures, gas and liquid flow rates, gas-phase composition, and specific concentrations in the liquid phase. The calculated quantities from the data were the reactor residence time, concentrations in the inlet and outlet, conversion of reactant, an apparent first-order rate constant, average reaction rate, and the solubilities in the saturators. The residence times were calculated from the observed flow rates by assuming the fluid density in the reactor was that of water. The mole fractions of species in the reactor effluent were calculated from the measured gas-phase composition by assuming phase equilibrium between the gas and liquid products. This was an important assumption for carbon dioxide, which is very soluble in water, and was not used for ammonia, the concentration of which was measured in the liquid phase of the effluent. The reactor inlet concentrations were calculated both from the measured outlet concentrations by assuming closure of the mass balance and from the known Henry's law solubility of the gases as a function of temperature and pressure in the 1-L autoclaves. These methods did not differ by more than 10%. A detailed discussion of the estimated error in this analysis is presented elsewhere (15).
Experimental Results Carbon Monoxide. The form of the global expression for oxidation reactions in supercritical water was assumed as d[CI --- A ex~(-~,/~T)[Cl"[Ozl~[HzoI~ (1) dt where A = preexponential factor [(L/mol)a+b+c-ls-l I, [CI = concentration of combustible species (CO, NH3, EtOH) (mol/L), E, = activation energy (kJ/mol), R = universal gas constant (8.31 J/mol K), T = absolute temperature (K), and a, b, c = reaction orders. For some cases, reaction orders were set to specific values (such as zero or one) before regression of the other parameters. The Arrhenius temperature dependence of a reaction was determined by plotting the natural log of the apparent first-order rate constant h* (s-l), calculated from the measured rate with a = 1and b = c = 0 in eq 1, versus the reciprocal absolute temperature. The slope yields an apparent activation energy, and the intercept yields the preexponential constant. Although determining the reaction order for water ("c") could be of value in highlighting
484-541 78-92 0.67-0.79 -0, 2.2-3.3
540 78 2.45/0.67 3.31
CO/EtOH 484, 520 92,83 1.4/0.79; 1.3/0.71 3.8, 4.1
-1 .o*
-i -2.0
-5.aI
-5.5
0.06125
0.06120
O.Ob130
0.06135
0.06140
0 Oh145
0 06150
1/T(K)
Flgure 2. Arrhenius plot for first-order oxidation of carbon monoxide in supercritical water at 24.5 MPa.
a mechanism path, it was not possible to vary the water density at constant temperature sufficiently with this apparatus to accurately estimate this exponent, so it was set equal to zero in our analysis. Oxidation of carbon monoxide in supercritical water can occur by two global reaction pathways: a direct oxidation pathway, represented as
and a water-gas shift pathway, or Fifty-nine experiments with carbon monoxide and supercritical water confirmed that both these pathways occur, as reported in detail elsewhere (15, 17). In our experiments, production of hydrogen demonstrated the presence of the water-gas shift, which accounted for 75% of the carbon dioxide production during oxidation at 400 "C, decreasing to -20% as the reaction temperature increases to 540 "C. Carbon dioxide is the thermodynamically favored product when oxygen is present, and equilibrium was not reached in these experiments. Water is also the thermodynamically favored product when oxygen is present, which indicates there is a kinetics limitation for oxidizing the hydrogen produced by the water-gas shift to water. These results showed that reactions can occur both between solutes (CO and 0,) and between a solute and the solvent (CO and HzO). Arrhenius plots for these data are shown in Figures 2 and 3. A Gauss-Newton nonlinear optimization technique (18) was used to determine the "best fit" equation for the global oxidation rate for carbon monoxide in supercritical water as
-
d[COI dt
-
107.25A0.53
exp[ (-120
f
7.7) / R T ][co]1.014*0.09[O,l 0.03*0.04
(4) Environ. Sci. Technol., Vol. 22,
No. 11, 1988 1321
and for the water-gas shift as d[COI --= dt 101.60*3.57 exp[(-62.9 f 8.6)/RT] [CO]o.568*o.107 (5) where the stated uncertainties are at 95% confidence levels for the fitted parameters (about twice as large as the standard deviation). The oxygen concentration dependence term is not statistically significant in the empirical oxidation expression. The key differences between the two rate expressionswas that the water-gas shift rate has about half the activation energy, a much lower preexponential term, and an order with respect to carbon monoxide 44% lower than the global oxidation rate expression. The two rate expressions and data sets overlap at low temperatures (400-450 "C), but they diverge significantly at increasing temperatures, so that above 500 "C the difference in the two pathways is clear. Neither empirical rate expression has an explicit dependence on oxygen concentration, although the presence of oxygen allows the more highly activated reaction rate (eq 4) to be favored. Ethanol. The direct oxidation of ethanol in supercritical water was investigated in a series of exploratory experiments. Because no experiments were conducted where the concentrations of ethanol or oxygen were varied significantly, the reaction was assumed to be first order in ethanol and zero in oxygen, which is consistent with the observed global rate law for carbon monoxide oxidation in supercritical water (eq 4). A first-order dependence in organic concentration is also consistent with many organic oxidation reactions (19). An Arrhenius plot is shown in Figure 4 for four ethanol oxidation experiments and other experiments with ethanol. The reaction is clearly highly activated, going from a conversion of 16% at 484 "C to 99.5% a t 541 "C, and the In k* versus 1/T data are reasonably linear. A least-squares linear regression of the four oxidation data yielded a rate expression as d[EtOH] dt 1021.82*2.7 exp[(-340 f 41 kJ)/RTI [EtOH] (6) where the uncertainties are given as standard deviations because of the limited number of data points. The rate expressed in eq 6 is for the disappearance of ethanol, or the conversion of ethanol to any product. The closure of the mass balance for carbon shown in Table I1 is adequate, being between 93.5 and 105.8%. Ethanol was converted primarily to either carbon monoxide or carbon dioxide, with more carbon dioxide formed at higher temperatures, consistent with the carbon monoxide oxidation experiments. Only one other liquid product of reaction was identified, acetaldehyde, which accounted for 1-10% of the carbon in the feed. Since the water-gas shift reaction of carbon monoxide with water is an important path in the production of carbon dioxide, reactions of water with ethanol (hydrolysis) and also the thermal decomposition (pyrolysis) of ethanol could be important paths in the destruction of ethanol in supercritical water. One experiment was performed to investigate this possibility, at 519.5 "C and with the same ethanol concentration as in the similar oxidation test, but with no oxygen. The observed 18% conversion of ethanol by a hydrolysis or pyrolysis path was significant, but much lower than the 90% conversion observed at the same temperature when oxygen was present. Insufficient gas was evolved during the experiment to obtain a reliable sample for analysis of the gas-phase composition. From this experiment, we estimate that less than 4% of the 1322
Environ. Sci. Technol., Vol. 22, No. 11, 1988
\
I
I
I,
'"a,
41
I
k'~lO(6.5)~xp(-ll2,OOOlRl)
*\
-41
-5
0 0 120
O.OdlZ5
0.~190
0.06135
,I
O.Obl40
0.06145
0 0 I50
I/l(K)
Figure 3. Arrhenius plot for first-order water-gas shift reaction in supercritical water at 24.5 MPa. 0 0.
I
4 Feed C o m p o s i t i o n :
o
E t O H + 02
I
1i
-4.0 -4.5 l/T(K)
Figure 4. Arrhenius plot for first-order oxidation of ethanol in supercritical water at 24.5 MPa. Table 11. Distribution of Carbon and Closure of Carbon Balance in Experiments with Ethanol
feed EtOH
+ O2
EtOH only EtOH +
o2+ co
EtOH
+ NH,
C fed," temp, mol, percentage of inlet C asb+ ' C min X lo4 EtOH AA CO C02 total 484 541 520 499.5 519.5 519.5
1.70 1.67 1.68 1.69 1.65 3.13
83.8 0.23 9.44 71.0 81.6 6.77
NA 0.51 4.34 88.6 0.91 20.2 84.4 105.7 10.0 25.0 60.1 104.6 12.0 3.73 8.95 95.7 9.0 NA NA 90.6 4.41 33.9 53.4 98.4
484 540.5
3.12 1.67
48.0 3.41 32.1 16.0 1.60 1.8 16.1 35.9
99.6 55.4d
"The feed rate uncertainty is approximately f3% for all experiments except those with CO, in which it is f 5 % . *Species abbreviations: EtOH, ethanol; AA, acetaldehyde; CO, carbon monoxide; COz, carbon dioxide; 02,oxygen; NH,, ammonia. cThe analytical uncertainty in the concentrations for each species are approximately 12% for EtOH, 110% for AA, 17% for CO and COz,and 1 8 % for EtOH at 484 "C. We believe an undetermined amount of COz was trapped in the liquid product as ammonium carbonate.
ethanol could have pyrolyzed in the preheaters during the oxidation experiments. The oxidation of ethanol in supercritical water is more highly activated than the carbon monoxide oxidation or water-gas shift, and it also contains a significant portion
of the total disappearance that is due to a strictly thermal reaction or a direct reaction with water. Three tests were conducted in which more than one combustible was fed to the reactor. The first was a simultaneous oxidation of ethanol and ammonia, where the ethanol did not affect the reaction of ammonia. The other two experiments were simultaneous oxidations of ethanol and carbon monoxide, at 484 and 520 "C. The destruction of ethanol at 520 " C was not statistically significantly affected by the presence of added carbon monoxide: the experimental conversion of carbon monoxide decreased from 64 to 51%. The conversion of carbon monoxide in this cooxidation experiment was calculated by assuming the product distribution from the oxidation of ethanol was the same with or without carbon monoxide as a feed, since carbon dioxide is a product of oxidation of both carbon monoxide and ethanol. In the lower temperature experiment (484 "C), the conversion of ethanol was reduced from 17 to 11% and the conversion of carbon monoxide decreased from 40% (estimated from eq 2) to -33% when the two were oxidized together. These experiments suggest there is a small effect of competition between parallel reactions. Oxidation of carbon monoxide is inhibited by the presence of other hydrocarbons in gas-phase combustion due to competition for hydroxyl (OH) radicals between the hydrocarbons and carbon monoxide. For example, the presence of methane, ethane, and ethylene in combustion at a total concentration 20% of that of carbon monoxide reduced the fuel-lean, gas-phase combustion rate of the wet carbon monoxide by an order of magnitude from that predicted by global rate expressions (20). Ammonia. Measurable but small conversions of ammonia in our work were only observed at 540 O C . No experiments below 525 "C produced a conversion that was statistically significantly different from zero. Difficulty in reaching and controlling high sand bath temperatures limited operation to temperatures below 550 "C with the present apparatus. The lower temperatures resulted in such low conversions of ammonia that kinetic parameters for ammonia oxidation in supercritical water could not be quantitatively determined. Two experiments conducted at 540 O C were at essentially identical conditions, except that ethanol at a mass concentration equal to 73% of the ammonia was added to the feed in one of the experiments. These experiments were to investigate the possible influence of cooxidation of an organic with ammonia. Both experiments exhibited a small conversion, approximately 5 f 5%, where the uncertainty is at the 95% confidence level. There is no statistically significant difference in the conversion of ammonia, despite the fact that greater than 95% of the ethanol was reacted in the experiment with ethanol. Similarly, ammonia had little effect on the rate of disappearance of ethanol, as the total conversion of ethanol was 98.4% at 540 "C with ammonia present and 99.5% at 541 O C in an ethanol oxidation experiment without ammonia. The lack of an effecf of ethanol on the oxidation of ammonia suggests that the observations in the previous work were due to local hot spots rather than free radicals. Much higher reaction temperatures (at or above 600 "C) and/or longer residence times than pcasible in our apparatus will be required to study the kinetics of homogeneous ammonia oxidation (with or without organics) in supercritical water.
Discussion The predictions of carbon monoxide conversion in supercritical water using known, global gas-phase rate expressions were typically off by an order of magnitude or more. Furthermore, these global expressions failed to
predict the importance of the water-gas shift, as none showed any significant production of hydrogen (17). An elementary reaction model was also used to predict rates of carbon monoxide oxidation in supercritical water. The model by Westbrook et al. (21) includes 21 elementary reactions and yields accurate predictions of oxidation rates for wet carbon monoxide under a wide range of combustion conditions. The elementary model predicted conversions of carbon monoxide to carbon dioxide slightly better than the best global model, typically being within a factor of 2 of the experimentally determined conversion. Unfortunately, the elementary model also did not predict any significant formation of hydrogen. Predictions were lower than observed values of hydrogen production by a factor of 50. In theory, the elementary reaction model should have accounted for hydrogen production at least as accurately as it did for carbon monoxide and dioxide. Modeling of the carbon monoxide data is described in more detail elsewhere (17). The residence time required to reach a 10% conversion of ammonia by oxidation in supercritical water was calculated from known, global rate expressions (22-26). These calculations were done for reactor conditions of both 400 and 600 "C and 27.6 MPa. The calculated residence times ranged between 1.7 and 125 s at 400 "C and 0.003 and 1.1s at 600 OC. Although they showed wide variations, they had a much narrower range than similar calculations for the carbon monoxide system. All the global expressions predict that the reaction should have taken place to a measurable extent over the conditions studied experimentally. The global expressions do not include an explicit dependence on the concentration of water, but still predict the high rates due to the higher concentrations of reactants possible in the supercritical fluid than in the gas phase. The oxidation kinetics for ammonia in supercritical water were also modeled with a network of elementary reactions. This model was based on that of Dean, Hardy, and Lyon (29, but with some added reactions for the H/O reaction set and carbon monoxide reactions, yielding a total of 43 elementary reactions (15). The conversion of ammonia at 541°C was predicted to be 0.0014%, compared to 5.9 f 5.6% observed experimentally. Reaction temperatures of 750, 830, and 870 "C were required in the simulation to yield conversions in 10 s of 4.2,4.7, and 95%, respectively. The low reactivity of ammonia in supercritical water predicted with the elementary model is qualitatively consistent with the experimental results, although the predicted rates are probably low, since temperatures between 550 and 600 OC seem likely from the experiments to be conditions that will exhibit measurable conversion. Global expressions have not been reported for the gasphase oxidation of many simple organic compounds, including ethanol. Even early researchers of these combustion reactions recognized the importance of radical reactions in controlling the overall rate and did not attempt to determine global parameters (19). Global activation energies have been determined for the oxidation of similar alcohols at 400-1000 OC and near-atmospheric pressures and are 188 and 163 kJ/mol for methanol and 1-butanol, respectively. These are much lower than the experimentally observed 340 kJ/mol for the oxidation of ethanol in supercritical water, which implies that the key mechanistic steps in the supercritical water oxidation environment are much different than those in gas-phase combustion. The experimental activation energy is close to the binding energy of a carbon-carbon bond, which is about 370 kJ/mol. The failure of existing models to predict the reaction rates in supercritical water leads to speculation on the Environ. Sci. Technol., Vol. 22, No. 11, 1988
1323
effect of the solvent on the reaction rates and mechanisms. Supercritical fluids as solvents may also form loosely ordered structures on a molecular level about solute molecules (28). Association on a molecular scale in supercritical water may be due to the finite level of hydrogen bonding still present at reaction conditions. Raman spectral evidence suggests that 5-8% of water molecules at 500 “C are still in a weakly associated state with neighboring molecules (29). A “cage” of water molecules associated around solute molecules could stabilize the existence of radicals or transition states, prevent solute radicals from reacting with other solutes, or both. Such a structure was postulated to explain the observed oxidation kinetics of carbon monoxide in supercritical water. Qualitatively, a cage structure would yield lower rates for direct oxidation and higher rates for solute-solvent reactions, such as the water-gas shift reaction ( I 7).
Conclusions Supercritical water is an excellent environment in which to oxidize hazardous organic material, since the high solubility of both organics and oxygen in the fluid allow essentially complete oxidation with no char formation at temperatures much less than incineration. The unique effects of the water as a solvent on the reactants and radicals makes predictions of the oxidation rates of carbon monoxide and ammonia invalid from either global rate expressions or elementary reaction models based on lowpressure, combustion-like reaction conditions. Empirical expressions for the oxidation of carbon monoxide and ethanol in supercritical water are presented. Both carbon monoxide and ethanol partially oxidize without molecular oxygen present in supercritical water. In cooxidation studies, ethanol did not significantly alter the oxidation rates of ammonia or carbon monoxide under the conditions studied. Ammonia is very difficult to oxidize below 540 “C at residence times of 6-13 s. We currently plan to extend the temperature range of our apparatus to 700 “C for future ammonia oxidation studies. Acknowledgments Helpful discussions and review of our work were provided by A. F. Sarofim, M. Modell, H. P. Meissner, and G. A. Huff. Registry No. CO, 630-08-0; NH3, 7664-41-7;EtOH, 64-17-5; HzO, 7732-18-5.
Literature Cited (1) Paulaitis, M. E.; Krukonis, V. J.; Kurnik, R. T.; Reid, R. C. Rev. Chem. Eng. 1983, 1(2), 181. (2) Groves, F. R., Jr.; Brady, B.; Knopf, F. C. CRC Crit. Rev. Environ. Control 1985, 15(3), 237. (3) Josephson, J. Enuiron. Sci. Technol. 1982, 16, 548.4. (4) Connolly, J. F. J. Chem. Eng. Data 1966, 11, 13. (5) Pray, C. M.; Schweickert, C. E.; Minnich, B. H. Znd. Eng. Chem. 1952,44(5), 1146. (6) Marshall, W. L. In High Temperature, High Pressure Electrochemistry in Aqueous Solutions, January 7-12, 1973, The University of Surry, England; Jones, D. de G., Staehle, R. W., Chairmen; National Association of Corrosion
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