Ind. Eng. Chem. Res. 2003, 42, 285-289
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KINETICS, CATALYSIS, AND REACTION ENGINEERING Supercritical Water Oxidation of Nitrobenzene Guangming Zhang and Inez Hua* School of Civil Engineering, Purdue University, 1284 Civil Engineering Building, West Lafayette, Indiana 47907-1284
Nitrobenzene was oxidized in supercritical water in a plug-flow reactor. The experimental conditions included a temperature range from 378 to 600 °C, a pressure range from 250 to 346 atm, and residence times between 80 and 180 s. The disappearance of nitrobenzene during supercritical water oxidation exhibited a global rate law that was 1.04 ( 0.11 order in nitrobenzene, 0.49 ( 0.16 in oxygen, and 0.07 ( 0.10 in water. The activation energy was 36 000 ( 5000 J mol-1, and the Arrhenius preexponential factor was 34.9 ( 10.2 s-1. When nitrobenzene decomposed, greater than 90% of the organic nitrogen was converted to inorganic species, mainly elemental nitrogen, and greater than 60% of the organic carbon was transformed into carbon monoxide (CO) and carbon dioxide (CO2), with a high selectivity toward CO2. Nearly complete (>95%) mineralization was achieved at T ) 600 °C, P ) 346 atm, and a residence time of 160 s. Introduction Supercritical water oxidation (SCWO) is a promising technique for the destruction of hazardous compounds. Refractory compounds can be mineralized within short periods.1-6 Reaction conditions include temperatures between 400 and 650 °C and approximately 250 atm of pressure. Organic compounds and oxygen are both much more soluble under these conditions.7-10 Thus, a single phase containing oxygen, organic substrate, and water exists, and oxidation of the organic substrate is complete within a few minutes.3,11 The physical properties of supercritical water, including density, viscosity, and dielectric constant, can be varied by modifying the pressure and temperature to enhance specific chemical reactions. High destruction efficiencies within minutes have been demonstrated for a variety of organic compounds, including phenols, benzenes, and polychlorinated biphenyls.2,12-17 Detailed kinetic and mechanistic information is available for simple hydrocarbons or oxygenated hydrocarbons. However, nitrogen-containing organic compounds have not been widely studied. Understanding the kinetics and mechanisms of these compounds is essential because many important organic pollutants contain nitrogen and nitrogen from the parent compound may undergo speciation to a number of byproducts. Earlier studies have reported the SCWO of quinoline, benzonitrile, aniline, dinitrotoluene, and pyridine.6,8,18,19 Nitrobenzene is a highly toxic and widely used compound.20 The kinetics and mechanisms of nitrobenzene destruction in supercritical water in the absence of oxygen have been studied.21 In the absence of oxygen, * Corresponding author. Tel: (765) 494-2409. Fax: (765) 496-1988. E-mail:
[email protected].
the activation energy was 68 000 ( 9000 J mol-1, and the primary products were benzene and nitrite. Very little of the organic carbon was transformed into carbon monoxide and carbon dioxide. The presence of oxygen enhanced the nitrobenzene removal ratios at T ) 440 °C and water density ) 0.25 g m L-1. However, information about the kinetics and products of nitrobenzene destruction in the presence of oxygen is very limited. The goal of this study was to assess the performance of SCWO for nitrobenzene destruction and mineralization. More specifically, the decomposition kinetics and the fate of nitrogen and carbon are examined, in addition to the conditions necessary to completely mineralize the organic carbon. Experimental Methods The oxidation reactions were conducted in a 15 mL isothermal, isobaric flow-through reactor. The reaction streams were prepared by dissolving oxygen in deaerated and deionized water in one tank and an aqueous solution of nitrobenzene in another tank blanketed with helium. Two Eldex model 100-B metering pumps drew the streams and pressurized them to the desired operating pressure with the aid of a back-pressure regulator (GO Inc. model BP66). Each stream was preheated in a 50 cm length × 0.63 cm o.d. (Hastelloy C-276) tubing to the desired operating temperature. The preheated streams were then mixed at the entrance of the reactor (6.3 mm o.d. Hastelloy C-276). The length of the reactor ) 55 cm, and i.d. ) 0.42 cm. Both preheaters and the reactor were coiled within a fluidized sand bath (Techne Inc. model SBL-2) equipped with a temperature controller (Omega model MCS3910AKC). The effluent of the reactor was cooled in a water bath and depressurized to the ambient pressure. The effluent was separated into gas and liquid phases
10.1021/ie010479j CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002
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Table 1. Summary of Nitrobenzene Oxidation Experiments (Experiments in Duplicate at Each Set of Conditions)a expt run no.
T (°C)
P (atm)
τ (s)
nitrobenzene (mM)
O2 (mM)
H2O (M)
χ
1 2 3 4 5 6 7 8 9 10 11
378 400 400 400 396 398 398 480 550 550 600
250 250 250 250 250 300 346 250 250 346 346
80 120 180 80 80 80 80 120 60 80 160
3.36 1.25 1.25 1.25 1.35 3.00 3.55 0.72 0.59 0.90 0.75
50.0 12.4 8.3 37.2 10.0 44.6 52.9 7.2 11.7 8.9 20
24.9 9.26 9.26 9.26 9.99 22.2 26.3 5.35 4.37 6.66 5.50
0.36 0.61 0.72 0.60 0.61 0.76 0.78 0.81 0.82 0.92 0.99
a
The concentration of water varies with reactor conditions.35
and was collected for analysis. These experiments were performed at temperatures between 378 and 600 °C and pressures between 250 and 346 atm. For each experiment, the temperature and pressure were maintained constant. Reactor residence times ranged from 80 to 180 s. All kinetics experiments were performed at least twice. Nitrobenzene (Aldrich) was analyzed by using a gas chromatograph (Hewlett-Packard 5890A) equipped with a flame ionization detector (GC-FID) and a DB624 column. The temperature program was as follows: the initial temperature was held at 60 °C for 1 min and then increased to 250 °C at 20 °C min-1 and maintained there for 1 min. Prior to the analysis, the aqueous samples were extracted with hexane (Aldrich). Benzene, phenol, and aniline (Aldrich) were also measured with the same equipment and procedures. Another GC equipped with a total thermal conductivity detector and a Porapak Q column was used for the separation and detection of carbon monoxide, carbon dioxide, and nitrogen gas. For this analysis, the initial oven temperature was held at 25 °C for 2 min and then increased to 100 °C at 7 °C min-1 and maintained there for 2 min. Quantification of nitrite and nitrate concentrations was carried out with an ion chromatograph (Dionex DX-300) equipped with an ASI 11 column and an eluent (flow rate ) 2 mL min-1) of 10 mM each of sodium carbonate and sodium bicarbonate. Results and Discussion Global Kinetics of Nitrobenzene. Table 1 lists the results of kinetic studies for nitrobenzene. The complex nature of SCWO and the reactor configuration precluded a simple fitting of nitrobenzene degradation into a zero, first, or second reaction order model. Instead, the global Arrhenius equation was employed to describe the nitrobenzene disappearance:
rate ) A exp(-Ea/RT)[C6H5NO2]a[O2]b[H2O]c
Figure 1. Estimated and measured nitrobenzene conversion during SCWO in the presence of oxygen. Experimental conditions are listed in Table 1.
present in large excess, their concentrations could be taken to be approximately equal to their initial concentrations. For a constant-volume, plug-flow reactor, eq 1 leads to22
(1 - χ)1-a - 1 ) (a - 1)A exp(-Ea/RT)[C6H5NO2]a-1 [O2]0b[H2O]0cτ
ln(1 - χ) ) -A exp(-Ea/RT)[O2]0b[H2O]0cτ when a ) 1 (3) where χ ) nitrobenzene conversion ratio ) 1 - [nitrobenzene]effluent/[nitrobenzene]initial; [C6H5NO2]0, [O2]0, and [H2O]0 are the initial concentrations of nitrobenzene, oxygen, and water, respectively; and τ is the hydraulic residence time of nitrobenzene. Note that Reynolds number calculations indicated that the reactor flow conditions were not fully turbulent. Thus, there is some error introduced into the plug-flow reactor analysis because of differences in how the reactants are dispersed. Optimized values for the parameters A, Ea, a, b, and c were obtained by applying a nonlinear regression to data in Table 1. The method was to minimize the sum of the squares of the difference between predicted χ values using eq 2 or 3 and the measured χ values. Data from experiment 11 were not used because the extended residence time (160 s) was to facilitate mineralization and nitrobenzene destruction was complete before 160 s. Equation 2 yielded a ) 1.04 ( 0.11. Therefore, eq 3 was used to do the regression with the a value from eq 2. The resulting values lead to the global rate law (eq 4) for nitrobenzene oxidation. The uncertainties are for
(1)
where A ) Arrhenius parameter, Ea ) activation energy, T ) temperature, and a, b, and c are the reaction orders of nitrobenzene, oxygen, and water, respectively. The rate is expressed in mol L-1 s-1, concentration in mol L-1, and Ea in J mol-1. The objectives of the global kinetics analysis were to determine A, Ea, a, b, and c in the above power rate equation. Because oxygen and water were always
when a * 1 (2)
(
rate ) (34.9 ( 10.2) exp -
36000 ( 5000 RT
)
[C6H5NO2]1.04(0.11[O2]0.49(0.16[H2O]0.07(0.10 (4) the 95% confidence intervals. Figure 1 shows the calculated χ values using eq 4 versus the measured values. The error bars on the calculated values represent the range of estimation.
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The SCWO of nitrobenzene depends strongly on the concentration of nitrobenzene and moderately on the concentration of oxygen. These observations agree with the SCWO of phenols.2 Lee and Park showed that, in the absence of oxygen, Ea was 65 000 J mol-1 and the reaction order for nitrobenzene was 0.39.21 Therefore, the addition of oxygen significantly accelerated the rate by lowering the activation energy. Another difference was the increased order with respect to nitrobenzene, which might result from different mechanisms in the absence or presence of oxygen. In the absence of oxygen, hydrolysis might be an important pathway for nitrobenzene transformation, whereas a large excess of oxygen will favor oxidation. The concentration of water exerted little influence on the kinetics in this system, as demonstrated by eq 4. Product Analyses. Common products observed during SCWO of aromatic compounds include CO, CO2, H2O, and dimers when the operation is not optimized.1,17,21,23,24 The formation of short-chain organic acids has been reported when reactor residence times are short (6 s).25 These acids should be destroyed when the residence time is on the scale of minutes.8 Similar products during treatment of phenols by SCWO and by incineration suggest that the two thermal oxidation processes are closely related.26 The combustion products of nitrobenzene include carbon, CO, CO2, H2O, and NOx.20 Elemental carbon is an unlikely product of SCWO and has not been reported. At T ) 440 °C and P ) 365 atm, SCWO of nitrobenzene in the absence of oxygen yielded aniline, phenol, benzonitrile, dibenzofuran, and nitrogen gas.21 In this study, the products of nitrobenzene destruction in the presence of oxygen were identified and quantified. Four sets of reactor conditions were selected to study the products; they are represented by experimental conditions 1, 4, 10, and 11 in Table 1. Temperature, pressure, residence time, reactant concentrations, and nitrobenzene conversion ratios varied considerably and therefore represent a wide range of possible experimental conditions. The following inorganic byproducts were detected: nitrogen (N2), nitrite (NO2), nitrate (NO3), carbon dioxide (CO2), and carbon monoxide (CO). No organic byproducts were identified, although two unidentified high molecular weight compounds were detected in the liquid phase from experiments 1 and 4. The high temperatures at which the reactor operated during experiments 10 and 11 enabled the degradation of these compounds to below detectable levels. Less than 10% of the nitrogen in nitrobenzene was converted to nitrite and nitrate under all conditions (Figure 2). The conversion to nitrite and nitrate is calculated as the NOx yield ) ([NO2] + [NO3])/∆[nitrobenzene], where ∆[nitrobenzene] is the difference between the initial and final nitrobenzene concentrations. Most of the nitrogen was converted to N2 (Table 2), and NO2- generally was detected at higher concentrations than NO3-. These observations can be explained as follows. C6H5-NO2 is a weak bond27 (dissociation energy of 143 kJ mol-1) and can be easily ruptured:
C6H5-NO2 f •NO2 + •C6H5
(5)
In the oxidative environment of a SCW reactor, the 2 free radical further reacts to form NO2 and N2. The reaction temperatures were not high enough to oxidize N2, which accumulated.
•NO
Figure 2. Nitrate and nitrite concentration (mM) and yield of NOx after SCWO of nitrobenzene. The yield is calculated as yield ) ([NO2] + [NO3])/∆[nitrobenzene] and is a dimensionless number. Experimental conditions are included in Table 1. Table 2. Yields of N2, CO, and CO2 for Selected Experimentsa expt run no. 1 4 10 11 a
N2
CO 10-1
2.73 × 5.59 × 10-1 8.24 × 10-1 8.69 × 10-1
10-3
8.71 × 6.46 × 10-3 2.48 × 10-3 2.31 × 10-3
CO2 2.13 × 10-1 4.09 × 10-1 7.29 × 10-1 9.43 × 10-1
Specific conditions for each experiment are listed in Table 1.
Although both homolytic and heterolytic pathways have been observed during SCWO of aromatic compounds,2 it has been found that water could not retain its ionic properties when the density was less than 0.4 g mL-1 and homolytic (free-radical) mechanisms dominated.28 As a result, the homolytic pathway should predominate under the conditions of this study. The initial step of the homolytic mechanism is the breakdown of water:29
H2O f H• + •OH
(6)
H• may react with •C6H5 (eq 5) to form benzene, a reaction which results in significant accumulation of benzene in the absence of oxygen.21 Another possible branch reaction is the combination of •C6H5 and •OH to form phenol. However, neither benzene nor phenol was detected under any of the experimental conditions in this study. Furthermore, it is unlikely that phenol formed and then was destroyed in our system because complete destruction of phenol within 80 s is unlikely during SCWO.24 A more plausible explanation for the lack of aromatic byproducts is that, with a large excess of oxygen available, •C6H5 does not follow a simple freeradical addition pathway but is transformed via ring opening. The thermal degradation of the phenyl radical can produce •C6H430 •
C6H5 f •C6H4 + H•
(7)
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Figure 3. Degradation and mineralization of nitrobenzene during SCWO. The conversion ratios are defined as follows: organic nitrogen conversion ratio ) ([N2] + [NO2] + [NO3])/∆[nitrobenzene]; organic carbon conversion ratio ) ([CO] + [CO2])/∆[nitrobenzene] × 6; and the nitrobenzene conversion ratio ) χ ) 1 - [nitrobenzene]effluent/[nitrobenzene]initial. Experimental conditions are included in Table 1.
very good: most of the inorganic carbon was present as CO2. Nearly complete conversion (>95%) of the organic carbon to CO or CO2 was achieved only at 600 °C and 346 atm at a residence time of 160 s. Effects of Temperature, Pressure, and Oxygen Concentration. High temperatures should accelerate nitrobenzene removal according to eq 1, which is supported by data in Table 1. Mineralization is also enhanced at high temperatures; operating at over 600 °C allows for total mineralization for most organic compounds. At higher temperatures, the water viscosity and the dielectric constant are significantly lower,32 allowing better mixing of the reactants. A decrease in the viscosity will enhance reaction rate constants of diffusion-controlled reactions, and many free-radical reactions (which are postulated to occur during the SCWO of nitrobenzene) are diffusion-controlled. The water density is also impacted by temperature and, in turn, modifies the concentrations of all present species. Higher pressure also enhances nitrobenzene oxidation, though to a lesser degree than temperature (see Table 1, conditions 5-7). The principal effect of pressure is enhancement of the following free-radical reactions:33 •
OH + •OH T H2O2
or the β scission of •C6H5 results in n-C4H3• and acetylene: •
C6H5 f HCtCCHdCHCHdCH
•
•
(8)
HCtCCHdCHCHdCH f HCtCCHdCH• + C2H2 (9) The acetylene reacts with OH• to form methyl radicals:
C2H2 + OH• f CH3• + CO
(10)
Further oxidation transforms CH3• and CO into CO2. Finally, the large excess of oxygen also precluded the formation of aniline in our studies, although aniline has been observed in the absence of oxygen.21 An ideal process for the destruction of organic compounds would completely convert the organics to carbon dioxide, water, and inorganic salts (mineralization). Thus, nitrobenzene would ideally react as follows:
C6H5NO2 + 6.25O2 f 6CO2 + 2.5H2O + 0.5N2
(11)
In our studies, N2, CO, and CO2 were detected as gasphase products at the conclusion of selected experiments. Table 2 reports the yields of each gas under different operating conditions. The yields are defined as follows: N2 yield ) [N2]/∆[nitrobenzene]; CO yield ) [CO]/∆[nitrobenzene] × 6; and CO2 yield ) [CO2]/∆[nitrobenzene] × 6. The sum of the yields of CO and CO2 constitutes the organic carbon conversion ratio, and the sum of the N2 and NOx- yields constitutes the organic nitrogen conversion ratio. These two ratios represent the extent of mineralization of nitrobenzene during SCWO and were compared to the nitrobenzene conversion ratio (χ) in Figure 3. The organic nitrogen conversion ratios were very good under all conditions; greater than 85% of the nitrobenzene that was destroyed during SCWO yielded N2 or NOx-. The organic carbon conversion ratios were much lower at conditions 1, 4, and 10, although the selectivity toward CO2 was
H + O2 f •HO2
(13)
HO2 + H2O f H2 + •OH
(14)
•
•
(12)
These reactions are likely to occur in a SCWO reactor operated under the conditions of this study. The changes of •H and •OH concentrations impact the nitrobenzene degradation as discussed before (eqs 6-10), while •HO2 and H2O2 may also react with nitrobenzene and the intermediates. Note also that, as the density of SCW increases at higher pressures, so too will concentrations, which results in a higher oxidation rate. Additionally, it is possible for the activation energy to vary with pressure. A detailed treatment of this subject can be found elsewhere.34 With respect to oxygen concentrations, it is evident that an oxidant (such as oxygen) is necessary for complete organic destruction, given the results in the absence of oxygen.21 The large excess of oxygen also eliminates the formation of benzene, aniline, and phenol. Mineralization and the selectivity toward CO2 are favored by high oxygen concentrations (Table 1). It has been found that, for monosubstituted phenols, the total organic carbon reduction rate was only 10-65% of the rate of the reactant disappearance.24,35 In this study, the greater selectivity toward CO2 relative to CO resulted from the excess of oxygen and the longer residence time. Conclusions The disappearance of nitrobenzene during SCWO exhibited a global rate law that was 1.04 ( 0.11 orders in nitrobenzene, 0.49 ( 0.16 orders in oxygen, and 0.07 ( 0.10 orders in water. The activation energy was 36 000 ( 5000 J mol-1, and the Arrhenius preexponential factor was 34.9 ( 10.2 s-1. The mineralization to inorganic carbon and inorganic nitrogen was good under the experimental conditions. When nitrobenzene decomposed, greater than 90% of the organic nitrogen was
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converted to inorganic nitrogen species, mainly nitrogen. Nitrite and nitrate accounted for less than 10% of the original nitrogen in nitrobenzene. Overall, 67-95% of the organic carbon was transformed into CO and CO2 with a high selectivity toward CO2. Nearly complete mineralization was achieved at 600 °C, 346 atm, and a residence time of 160 s. Higher temperature, pressure, oxygen concentration, and longer residence times enhance ntirobenzene destruction. Acknowledgment The authors acknowledge financial support from the Showalter Trust Foundation (Award No. 1219992277) and generous help from Dr. Changhe Xiao and Mr. Tom Cooper of Purdue University. Literature Cited (1) Crain, N.; Tebbal, S.; Li, L.; Gloyna, E. F. Kinetics and reaction pathways of pyridine oxidation in supercritical water. Ind. Eng. Chem. Res. 1993, 32, 2259-2268. (2) Ding, Z. Y.; Aki, S. N.; Abraham, M. A. Catalytic supercritical water oxidation of aromatic compounds: phenol conversion and product selectivity. Environ. Sci. Technol. 1995, 29, 27482754. (3) Ding, Z. Y.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic oxidation in supercritical water. Ind. Eng. Chem. Res. 1996, 35, 3257-3279. (4) Gopalan, S.; Savage, P. A reaction network model for phenol oxidation in supercritical water. AIChE J. 1995, 41, 1864-1871. (5) Goto, M.; Nada, T.; Kodama, A.; Hirose, T. Kinetic analysis for destruction of municipal sewage sludge and alcohol distillery wastewater by supercritical water oxidation. Ind. Eng. Chem. Res. 1999, 38, 1863-1865. (6) Aki, S.; Abraham, M. A. Catalytic supercritical water oxidation of pyridine: comparison of catalysts. Ind. Eng. Chem. Res. 1999, 38, 358-367. (7) Shaw, R. W.; Brill, T. B.; Clifford, A. A.; Eckert, C. A.; Franck, E. U. Supercritical water: a medium for chemistry. Chem. Eng. News 1991, 69, 26-39. (8) Schmieder, H.; Abel, J. Supercritical water oxidation: state of the art. Chem. Eng. Technol. 1999, 22, 903-908. (9) Savage, P. E. Organic chemical reactions in supercritical water. Chem. Rev. 1999, 99, 603-621. (10) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at supercritical conditions: applications and fundamentals. AIChE J. 1995, 41, 1723-1778. (11) Ekart, M. P.; Bennett, K.; Ekart, S. M.; Gurdial, G. S.; Liotta, C. L.; Eckert, C. A. Cosolvent interactions in supercritical fluid solutions. AIChE J. 1993, 39, 235-248. (12) Aki, S.; Ding, Z.; Abraham, M. A. Catalytic supercritical water oxidation: stability of Cr2O3 catalyst. AIChE J. 1996, 42, 1995-2004. (13) Boock, L. T.; Lamarca, C.; Klein, M. T. Hydrolysis and oxidation in supercritical water. Endeavour 1993, 17, 180. (14) Chang, K. C.; Li, L.; Gloyna, E. F. Supercritical water oxidation of acetic acid by potassium permanganate. J. Hazard. Mater. 1993, 33, 51-62. (15) Hatakeda, K.; Ikushima, Y.; Sato, O.; Aizawa, T.; Saito, N. Supercritical water oxidation of polychlorinated biphenyls using hydrogen peroxide. Chem. Eng. Sci. 1999, 54, 3079-3084.
(16) Huang, Y.; Wang, H. P.; Li, C. T.; Chien, Y. C. Minimization of cobalt nuclide emissions in supercritical water oxidation of spent resin. Chemoshpere 2000, 40, 347-349. (17) Li, R.; Savage, P. E.; Szmukler, D. 2-Chlorophenol oxidation in supercritical water: global kinetics and reaction products. AIChE J. 1993, 39, 178-187. (18) Li, L.; Gloyna, E. F.; Sawicki, J. E. Treatability of DNT process wastewater by supercritical water oxidation. Water Environ. Res. 1993, 65, 250-254. (19) Tiffany, D. M.; Houser, T. J.; McCarville, M. E.; Houghton, M. E. Am. Chem. Soc., Div. Fuel. Chem. 1984, 29, 56-64. (20) Fire, F. L. Chemical data notebook series #119: nitrobenzene. Fire Eng. 1996, 76-79. (21) Lee, D. S.; Park, S. D. Decomposition of nitrobenzene in supercritical water. J. Hazard. Mater. 1996, 51, 67-76. (22) Martino, C. J.; Savage, P. E.; Kasiborski, J. Kinetics and products from o-cresol oxidation in supercritical water. Ind. Eng. Chem. Res. 1995, 34, 1941-1951. (23) Yang, H. H.; Eckert, C. A. Homogeneous catalysis in the oxidation of p-chlorophenol in supercritical water. Ind. Eng. Chem. Res. 1988, 27, 2009-2013. (24) Martino, C. J.; Savage, P. E. Total organic carbon disappearance kinetics for the supercritical water oxidation of monosubstituted phenols. Environ. Sci. Technol. 1999, 33, 1911-1915. (25) Holgate, H. R.; Meyer, J. C.; Tester, J. W. Glucose hydrolysis and oxidation in supercritical water. AIChE J. 1995, 41, 637-648. (26) Thornton, T. D.; LaDue, D. E.; Savage, P. E. Phenol oxidation in supercritical water: formation of dibenzofuran, dibenzo-p-dioxin, and related compounds. Environ. Sci. Technol. 1991, 25, 1507-1510. (27) Dean, J. A. Lange’s Handbook of Chemistry, 14th ed.; McGraw-Hill Inc.: New York, 1994. (28) Antal, M. J.; Brittain, A.; DeAlmeida, C.; Ramayya, S.; Roy, J. C. Heterolysis and Homolysis in Supercritical Water. In ACS Symposium Series; Squire, T. G., Paulaitis, M. E., Eds.; American Chemistry Society: Washington, DC, 1987; Vol. 329, p 302. (29) Williams, G. H. Homolytic aromatic substitution; Pergamon Press Ltd.: New York, 1960. (30) Kern, R. D.; Xie, K.; Chen, H. Combust. Sci. Technol. 1992, 85, 77-82. (31) Matubayasi, N.; Wakai, C.; Nakahara, M. Structural study of supercritical water I. Nuclear magnetic resonance spectroscopy. J. Chem. Phys. 1997, 107, 9133-9140. (32) Holgate, H. R.; Tester, J. W. Oxidation of hydrogen and carbon monoxide in sub- and supercritical water: reaction kinetics, pathways, and water-density effects. 2. Elementary reaction modeling. J. Phys. Chem. 1994, 98, 810-822. (33) Martino, C.; Savage, P. E. Supercritical water oxidation kinetics and pathways for ethylphenols, hydroxyacetophenones, and other monosubstituted phenols. Ind. Eng. Chem. Res. 1999, 38, 1775-1783. (34) Chialvo, A.; Cummings, P. T. Solvation Effects on Kinetic Rate Constant of Reactions in Supercritical Solvents. AIChE J. 1998, 44, 667-680. (35) Haar, L.; Gallagher, J. S.; Kell, G. S. NBS/NRC Steam Tables; Hemisphere Publishing Corp.: Washington, DC, 1984.
Resubmitted for review September 13, 2002 Revised manuscript received November 14, 2002 Accepted November 18, 2002 IE010479J
