Heterogeneous Oxidation Kinetics of Acetic Acid in Supercritical Water

Department of Environmental Planning, Graduate School of. Environmental Studies, Seoul National University,. Shinlimdong san 56-1, Kwanak Ku, Seoul ...
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Environ. Sci. Technol. 1996, 30, 3487-3492

Heterogeneous Oxidation Kinetics of Acetic Acid in Supercritical Water DONG SOO LEE* Department of Environmental Planning, Graduate School of Environmental Studies, Seoul National University, Shinlimdong san 56-1, Kwanak Ku, Seoul 151-742, Korea

The potential catalytic effect by reactor surface has been noted in a number of studies of supercritical water oxidation (SCWO) of organic compounds. In the present study, acetic acid was tested to investigate the catalytic effect of the reactor surface on its SCWO kinetics. Batch experiments were carried out over the temperature range of 400-460 °C at a supercritical water (SCW) density of 0.25 g/mL. The acetic acid oxidation kinetics increased with the surface to volume ratio and was affected by the oxidation state of the reactor surface. A power law rate equation was determined as follows: r ) 109.8(0.7 exp[-(148.5 ( 9.4)/RT][CH3COOH]1.23(0.03[O2]0.23(0.01. Also, the observed kinetics was consistent with a proposed heterogeneous rate model assuming a rate control by the surface reaction between adsorbed oxygen and unadsorbed acetic acid. The results of this study show that the destruction performance by SCWO may significantly vary depending upon the type and state of reactor material. Therefore, potential heterogeneous characteristics of any reaction of interest should be sufficiently taken into consideration to maintain the desired performance of the SCWO process.

Introduction Supercritical water oxidation (SCWO) is a thermal treatment process that is expected to possess a complete destruction capability for hazardous organic wastes. In water above its critical point (374.2 °C, 221 bar), various organic compounds (1-4) and oxygen (5) freely dissolve, and the organicoxygen-SCW mixtures may form a homogeneous phase. With this solvating property, supercritical water can serve as an excellent reaction medium for the complete oxidation of hazardous organic substances. Early SCWO works reported high destruction efficiencies for a wide variety of organic compounds (6-9), but later emphasis shifted to the investigation of the detailed kinetics and mechanisms (10-17). SCWO kinetics was often described by the power law model, but as exemplified in the kinetics of SCWO of ammonia (15), the power law rate expression may not be adequate to describe heterogeneous characteristics. Homogeneous or heterogeneous catalytic reaction mecha* Fax: 82-2-886-2361; e-mail: [email protected].

S0013-936X(96)00134-4 CCC: $12.00

 1996 American Chemical Society

nisms were suggested in a few previous studies (11, 12, 18). The main objective of the present work was to investigate the heterogeneous characteristics of SCWO and to develop a rate expression that could properly describe the observed kinetics. Acetic acid was selected as the model compound since it has been known as a refractory or a rate-controlling byproduct in the SCWO of organic compounds (18, 19). Furthermore, the potential catalytic effect of the reactor wall on the SCWO of acetic acid was suspected. As pointed out by Meyer et al. (20), wide variations of Arrhenius parameters and reaction orders reported in previous works on this compound might be due to a potential catalytic effect of the reactor materials and of the surface area to volume ratio.

Experimental Method Reactor System. The batch SCWO reactor system shown in Figure 1 consisted of a reactor vessel assembly, a wrist action shaker, a fluidized sand bath with a temperature controller, a water bath, a pneumatic device for transferring the reactor vessel assembly, and a temperature meter. The reactor vessel assembly consisted of a U-shaped tubular vessel, a type-K thermocouple with 0.16 cm (1/16 in.) o.d., two shut-off valves (HIP, 15-11AF1), two head plugs, and high-pressure fittings (Swagelok). The tubular vessel was made of SS316 tubing of 1.27 cm (1/2 in.) o.d. with 0.3 cm wall thickness with an internal volume of 20 mL. The tip of the thermocouple was positioned 1 cm above the bottom in the vessel. During a test, the top portion of the reactor vessel stayed above the fluidized sand level. Hence, a pair of SS316 head plugs were inserted to fill up the top portion of the reactor internal space, one with a 0.16 cm diameter vertical hole through it for thermocouple penetration. The other plug had an inclined hole of the same diameter for the introduction or discharge of reaction fluids. Experimental Procedures. In each test, the temperature in the sand bath was raised to 30-50 °C ((3 °C) above the reaction temperature. Prior to the introduction of acetic acid solution, the reactor vessel was purged with oxygen, and then 5 mL of acetic acid solution was introduced to ensure that supercritical water conditions were achieved at the reaction temperature range (400-500 °C). Oxygen was introduced subsequently, typically at 200% of the stoichiometric demand. Both the shut-off valves were closed, and the reactor vessel assembly was attached to the arm of the wrist action shaker. The reactor vessel was submerged into the sand bath while being vigorously shaken. The reaction temperature reached a desired value within 40-45 s and thereafter stayed within (2 °C of the desired temperature. After a preselected reaction time, the reactor vessel assembly was withdrawn from the sand bath and quenched in the water bath. The quenching was completed within 15 s. Leaking occurred on rare occasions during the test. Hence, the batch test was judged valid only if the recovery of the aqueous phase was greater than 95% (4.75 mL). Aqueous phase was forced with helium into a vial and was either analyzed immediately or stored at 4 °C until the analyses. Analytical Methods. The acetic acid concentration in aqueous phase samples was determined by gas chromatography (Shimadzu, GC-9A) with FID and a wide bore fused

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FIGURE 1. Experimental apparatus: the batch supercritical water oxidation system.

capillary column (Hewelett-Packard, HP-FFAP, 0.53 mm i.d. × 10 m, 1.0-µm film thickness) at 90 °C. An external standard technique was used for quantification with at least four calibration standards. The peak area calculation was made by using a digital integrator (Shimadzu, GR2 AX). Acetic acid conversion was calculated as 1.0 - [CH3COOH]out/[CH3COOH]in where [CH3COOH]in and [CH3COOH]out denote the initial and final concentrations, respectively. A calibration standard was injected every 20 injections so that the calibration curve prepared at the beginning of the analyses could be validated within (10%. A reference standard (0.025 mol/L) was injected every 10 injections. Also, duplicate injections were made every five samples to keep the relative percent difference within (10%. X-ray diffraction (XRD) analysis with monochromator was performed to characterize the surface of the reactor and SS316 chips. The samples were rinsed with distilleddeionized water and air-dried prior to the analysis. The XRD operating conditions included a 2θ range from 25 to 100°, θ position of 3°, scanning speed of 10°/min, divergence slit of 1°, receiving slit of 4°, and scattering slit of 4 mm.

Results and Discussion Effect of Reactor Surface. To evaluate the potential surface effect, three sets of experiments were conducted at 400, 450, and 500 °C. Each set of the experiments was started with a new reactor vessel. The observed acetic acid conversions were then correlated with the corresponding number of times the reactor vessel was used. As shown in Figure 2, the conversion of acetic acid initially increased with the number but leveled off later at 400 and 450 °C, although this trend was less obvious at 500 °C. To examine the run-to-run variability, 15 replicate experiments were conducted by using a reactor vessel that was previously exposed to the SCWO conditions at 500 °C for 2 h. The relative standard deviations in the replicate runs were 8.6%

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FIGURE 2. Increase in the acetic acid conversion with the number of times of reactor vessel use ([CH3COOH]in ) 0.0125 gmol/L, [O2] ) 0.05 gmol/L, density of SCW ) 0.25 g/mL).

and 2.6% at 400 and 450 °C, respectively. Therefore, the increases in the conversion presented in Figure 2 clearly exceeded the run-to-run variabilities. This result suggested that the reactor surface might affect the acetic acid oxidation kinetics in supercritical water. Additional experiments were conducted by introducing into the reactor rectangular SS316 chips (0.5 cm × 0.5 cm × 0.02 cm), which were previously exposed to the SCWO conditions at 500 °C for 2 h. As shown in Figure 3, the conversion of acetic acid was gradually increased from 0.43 to 0.69 as the surface area to volume ratio increased from 10.8 to 27 cm-1, indicating that the SCWO of acetic acid was affected by the reactor surface. Hence, the reaction was at least partially heterogeneous. XRD analysis indicated that oxides of Fe and Cr were formed on the surface of the SS316 chips and the reactor vessel during SCWO, as reported in previous studies (15, 21). Since it is well-known that metal oxides catalyze various oxidation reactions (22), the surface effect observed in the

FIGURE 3. Effect of surface area to volume ratio on SCWO of acetic acid.

present work might be attributed to the catalysis of the metal oxides to a varying degree. Once the reactor surface was changed to a certain extent, the surface effect would stabilize, and further use of the reactor would not enhance the acetic acid conversion. Also, the surface effect would level off earlier at higher temperature probably because the surface change occurred faster. If both a homogeneous mechanism and a heterogeneous mechanism are operative, it would be of interest to estimate the significance of the heterogeneous mechanism. We may do so by assuming that, in the first test at 400 °C, the oxide layer would just start to form. Hence, the surface effect should be lowest when compared to all the tests that followed. If the rate constant of the first run is assumed to be that of the homogeneous reaction, the ratio of the rate constant of the fifth test to that of the first test would provide a conservative estimate of the relative significance of the heterogeneous mechanism. Based on the power law model (r ) k[CH3COOH]1.23[O2]0.23), which was developed later, the fifth rate constant (0.024 (L/gmol)0.46 s-1) was eight times higher than the first one (0.0030 (L/gmol)0.46 s-1). Therefore, it was concluded that once the surface effect was stabilized, the acetic acid oxidation in SCW proceeded predominantly via the heterogeneous mechanism. Power Law Model. Even though the heterogeneous mechanism might be predominant, determining a power law model rate equation was informative for the selection of a proper catalytic model to be determined later. To minimize the varying surface effect of the reactor, a single reactor vessel with the fully stabilized surface effect was used throughout the kinetics tests. The dependency on the acetic acid concentration was investigated by varying the initial concentration of acetic acid from 8.33 × 10-4 to 4.17 × 10-2 gmol/L at 400 °C at the fixed initial oxygen concentration of 0.174 gmol/L and the retention time of 270 s. Since the effect of supercritical water density has been reported negligible in previous studies (16, 18), the density was kept constant at 0.25 g/mL in all experiments. As shown in Figure 4, the conversion of acetic acid increased with its initial concentration, which implied that the reaction order with respect to acetic acid was not unity. For a constant volume batch reactor, the reaction rate can be related with the reactant concentration as:

r)-

d[CH3COOH] ) k[CH3COOH]m[O2]n dt

where m and n are the reaction orders with respect to acetic

FIGURE 4. Effect of initial concentration of acetic acid on its oxidation in SCW (density of SCW ) 0.25 g/mL, [O2] ) 0.174 gmol/L).

FIGURE 5. Determination of reaction order with respect to acetic acid (density of SCW ) 0.25 g/mL, [O2] ) 0.174 gmol/L, T ) 400 °C).

acid and oxygen, respectively. Because oxygen was present in great excess, the rate expression simplifies to give

r ) k′[CH3COOH]m

(k′ ) k[O2]n)

ln r ) m ln[CH3COOH] + ln k′ A plot of ln r versus ln[CH3COOH] would give a straight line with slope m and intercept ln k′. The mean concentrations were used in the differential kinetic analysis. As indicated in Figure 5, m is 1.23 ( 0.03 where the error is at the 95% confidence limit. The same m value and error range were obtained from the integral reactor analysis, which was conducted to assure the validity of the differential reactor assumption. Similarly, the effect of the oxygen concentration was investigated by varying the initial oxygen concentration from 0.0125 to 0.125 gmol/L at the constant initial acetic acid concentration of 0.0125 gmol/L at 400 °C. Figure 6 indicated that the acetic acid conversion increased with the oxygen concentration. Since the reaction order with acetic acid was determined to be 1.23, the rate expression is

r ) k[CH3COOH]1.23[O2]n ln r - 1.23[CH3COOH] ) n ln [O2] + ln k By similar analysis, the 95% confidence interval of the reaction order with respect to oxygen was determined to be 0.23 ( 0.01 as shown in Figure 7. For the later selection of a proper catalytic model, the reaction orders with acetic

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TABLE 1

Experimental Conditions and Resultsa sample temp RT ID no. (°C) (s) [CH3COOH] in [O2] in

FIGURE 6. Effect of oxygen on acetic acid conversion in SCW (density of SCW ) 0.25 g/mL, [CH3COOH]in ) 0.0125 gmol/L).

FIGURE 7. Determination of reaction order with respect to oxygen (density of SCW ) 0.25 g/mL, [CH3COOH]in ) 0.0125 gmol/L).

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

400 400 400 400 400 440 440 440 400 400 400 400 400 420 420 420 420 420 440 440 460 400 400 400 400 400 420 420 440 440 460

60 150 270 390 510 60 150 270 60 150 270 390 510 60 150 270 390 510 60 150 60 60 150 270 390 510 60 150 60 150 60

0.00417 0.00417 0.00417 0.00417 0.00417 0.00417 0.00417 0.00417 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0125 0.0417 0.0417 0.0417 0.0417 0.0417 0.0417 0.0417 0.0417 0.0417 0.0417

0.0167 0.0167 0.0167 0.0167 0.0167 0.0167 0.0167 0.0167 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.167 0.167 0.167 0.167 0.167 0.167 0.167 0.167 0.167 0.167

conversion

k (L/gmol)0.46 s-1

0.197 0.368 0.457 0.579 0.622 0.493 0.772 0.897 0.158 0.374 0.565 0.671 0.715 0.315 0.554 0.750 0.840 0.899 0.584 0.855 0.778 0.244 0.393 0.652 0.791 0.842 0.445 0.815 0.622 0.890 0.818

0.034 0.030 0.023 0.023 0.020 0.115 0.114 0.110 0.016 0.018 0.019 0.019 0.017 0.037 0.034 0.035 0.035 0.036 0.092 0.096 0.175 0.015 0.011 0.015 0.016 0.015 0.034 0.046 0.060 0.066 0.118

a Supercritical water density was kept constant at 0.25 g/mL in all tests. RT and [ ] denote the reaction time and molar concentration, respectively.

acid and oxygen that were statistically higher than 1.0 and 0.0, respectively, were noteworthy. This rate expression (r ) k[CH3COOH]1.23[O2]0.23) was then used to estimate the temperature effect based on the oxidation tests conducted over the temperature range of 400-460 °C. The experimental conditions and results are listed in Table 1. From an Arrhenius plot in Figure 8, the activation energy and pre-exponential factor were determined to be 148.5 ( 9.4 kJ/gmol and 109.8(0.7 (L/gmol)0.46 s-1, respectively. An activation energy that is significantly less than the typical O-H or C-H bond dissociation energy is consistent with the catalytic nature of the reaction. When compared with the previous works (20, 23, 24), the rate data of the present work agree within a factor of 4 if rate equations of the first order with acetic acid are enforced. The spread in the rate constants might be attributable to the experimental errors as well as ignoring the effects of O2 and H2O by enforcing the first-order rate equations or to the different material and type of reactors used in these works. In Meyer et al.’s work (20), a continuous reactor made of Inconel 625 was used whereas in Frish’s (23) and Savage’s works (24) batch reactors of SS316 were used. Catalytic Model. Since the heterogeneous nature was observed for the SCWO of acetic acid, the use of a catalytic model may be adequate for the kinetics description. The Langmuir-Hinshelwood-Hougen-Watson (LHHW) approach may be useful when an actual mechanism is not thoroughly investigated. According to the LHHW model

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FIGURE 8. Arrhenius plot for SCWO of acetic acid at 400-460 °C.

(25) for an irreversible bimolecular reaction, a rate expression of positive reaction orders with both reactants can occur on occasions such as rate control by surface reaction (between an adsorbed and a fluid phase reactants or between two adsorbed reactants) and by the desorption of products. As one possible explanation for the observed kinetics, a catalytic model assuming a rate control by surface reaction was proposed in the present work. While the adsorption of oxygen onto transition metal oxides has been observed (22, 26), the adsorbability of acetic acid is not well known. If oxygen is solely adsorbed, an Eley-Rideal mechanism (27) may be assumed wherein the reaction sequence involves two important steps: fast

TABLE 2

Summary Data for Catalytic Model Evaluationa temp (°C)

[CH3COOH]out

RT (s)

[O2]t

conversion

r, rate [gmol/L s-1)

[CH3COOH]out[O2]t/r

notes

10-5

400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400 400

0 30 60 105 150 0 60 105 150 210 0 60 150 210 270 310 360

0.00417 0.00368 0.00334 0.00296 0.00263 0.01250 0.01052 0.00925 0.00782 0.00656 0.04167 0.03148 0.02528 0.01891 0.01452 0.01404 0.01131

0.01667 0.01569 0.01502 0.01425 0.01360 0.05000 0.04604 0.04350 0.04065 0.03813 0.16667 0.14630 0.13389 0.12115 0.11237 0.11141 0.10594

0.000 0.117 0.197 0.290 0.368 0.000 0.158 0.260 0.374 0.475 0.000 0.244 0.393 0.546 0.652 0.663 0.729

1.12 × 1.04 × 10-5 9.63 × 10-6 8.45 × 10-6 7.27 × 10-6 3.69 × 10-5 3.22 × 10-5 2.87 × 10-5 2.51 × 10-5 2.04 × 10-5 1.47 × 10-4 1.15 × 10-4 9.29 × 10-5 7.82 × 10-5 6.34 × 10-5 6.09 × 10-5 4.86 × 10-5

6.195 5.539 5.214 4.988 4.930 16.917 15.036 14.036 12.663 12.273 47.214 40.014 36.419 29.311 25.749 25.691 24.658

at 400 °C, 1/kso ) 246 ( 22.3; 1/kKoxSo ) 1.98 ( 1.90; Kox ) (246 ( 22.3)/(1.98 ( 1.90) ) 124.2 ( 127.8

420 420 420 420 420 420 420 420

0 30 60 150 0 30 60 150

0.01250 0.01024 0.00857 0.00558 0.04167 0.03158 0.02314 0.00773

0.05000 0.04548 0.04214 0.03615 0.16667 0.14649 0.12962 0.09879

0.000 0.181 0.315 0.554 0.000 0.242 0.445 0.815

8.11 × 10-5 6.55 × 10-5 5.17 × 10-5 3.06 × 10-5 3.86 × 10-4 3.07 × 10-4 2.49 × 10-4 8.88 × 10-5

7.709 7.105 6.990 6.586 17.999 15.065 12.058 8.599

at 420 °C, 1/kso ) 77.9 ( 19.7; 1/kKoxSo ) 3.30 ( 2.0; Kox ) (77.9 ( 19.7)/(3.30 ( 2.0) ) 23.6 ( 19.5

440 440 440 440 440 440 440 440 440

0 30 60 0 30 60 0 30 60

0.00417 0.00300 0.00211 0.01250 0.00825 0.00520 0.04167 0.02667 0.01574

0.01667 0.01433 0.01256 0.05000 0.04150 0.03540 0.16667 0.13667 0.11481

0.000 0.280 0.493 0.000 0.340 0.584 0.000 0.360 0.622

4.48 × 10-5 3.40 × 10-5 2.61 × 10-5 1.53 × 10-4 1.20 × 10-4 8.65 × 10-5 5.55 × 10-4 4.35 × 10-4 3.05 × 10-4

1.550 1.263 1.018 4.098 2.854 2.129 12.503 8.386 5.924

at 440 °C, 1/kso ) 64.8 ( 11.4; 1/kKoxSo ) 0.17 ( 0.96; Kox ) (64.8 ( 11.4)/(0.17 ( 0.96) ) 381.2 ( 2129.2

a

[ ] denotes the molar concentration. The error ranges denote 95% confidence intervals.

adsorption of oxygen and rate-controlling surface reaction. Then, the overall rate equation is

r ) k[CH3COOH](O2S) where (O2S) and k denote the adsorbed oxygen and the rate constant of the surface reaction, respectively. With (O2S) ) KoxSo[O2]/(1 + Kox[O2]), the rate equation leads to the following expression:

r)

kKoxSo[CH3COOH][O2] 1 + Kox[O2]

where Kox and So are the equilibrium constant of the oxygen adsorption and the total active sites, respectively. Taking the inverse of and multiplying both sides by [CH3COOH][O2] gives

[O2] [CH3COOH][O2] 1 + ) r kKoxSo kSo The total oxygen concentration ([O2]t ) [O2] + [(O2S)]) could approximate [O2] since (O2S) might be assumed negligible as compared to [O2] when oxygen was supplied in an excessive quantity. If the Eley-Rideal assumption appropriately describes the kinetics, a plot of [CH3COOH][O2]t/r versus [O2]t would give a straight line with a slope of 1/kSo and an intercept of 1/kKoxSo. The data in Figure 7 (collected originally to investigate the effect of the oxygen concentration at 400

FIGURE 9. Test plot to evaluate the proposed Eley-Rideal model at 400 °C.

°C) were used to construct this plot. As shown in Figure 9, a straight line with excellent linearity was obtained, supporting the proposed Eley-Rideal model. This analysis was extended to the data in the temperature range of 400440 °C and is summarized in Table 2. The reaction rate (r) was directly estimated by the graphical differentiation method (28). Figure 10 demonstrates that the oxidation kinetics were consistent with this catalytic model. Also, this catalytic model predicted the acetic acid conversion better than the power law model. However, large uncertainties of Kox values and its temperature dependency are

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FIGURE 10. Evaluation of the proposed Eley-Rideal model at 400, 420, and 440 °C.

notable. The heat of adsorption as determined by van’t Hoff relation was 87 ( 440 kJ/mol at thhe 95% confidence level. Due to the uncertainties and because other catalytic models have not been evaluated, the proposed catalytic model should be regarded as just one possible explanation. Author-Supplied Registry Numbers: Acetic acid, 6419-7; oxygen, 7782-44-7; water, 7732-18-5.

Acknowledgments Financial support for this work was provided by Korea Science and Engineering Foundation (Contract KOSEF 9231000-006-2). The author would like to thank C. Koo and K. S. Park for their experimental efforts and Dr. J. Y. Yoon at Ajou University for his valuable comments.

Literature Cited (1) (2) (3) (4)

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(7) Modell, M.; Gaudet, G. C.; Simson, M.; Hong, G. T.; Bieman, K. Solid Wastes Manage. 1982, August. (8) Price, C. M. S.M. Thesis, Massachusetts Institute of Technology, 1981. (9) Modell, M. Detoxification and Disposal of Hazardous Organic Chemicals By Processing in Supercritical Water; Final report to the Department of Army on Contract No. DAMD 17-80-C-0078; Modar, Inc.: Natick, MA, 1987. (10) Helling, R. K. Sc.D. Dissertation, Massachusetts Institute of Technology, 1986. (11) Helling, R. K.; Tester, J. W. Energy Fuels 1987, 1 (5), 417. (12) Yang, H. H.; Eckert, C. A. Ind. Eng. Chem. Res. 1988, 27, 2009. (13) Rofer, C. K.; Streit, G. E. Kinetics and Mechanism of Methane Oxidation in Supercritical Water; Report LA-11439-MS; Los Alamos National Laboratory: Los Alamos, NM, 1988; DOE/HWP64. (14) Li, R.; Thornton, T. D.; Savage, P. E. Environ. Sci. Technol. 1992, 26 (12), 2388. (15) Webley, P. A.; Tester, J. W.; Holgate, H. R. Ind. Eng. Chem. Res. 1991, 30 (8), 1745. (16) Lee, D. S.; Gloyna, E. F. Environ. Sci. Technol. 1992, 26 (8), 1587. (17) Li, R.; Savage, P. E.; Szmukler, D. AIChE J. 1993, 39, 178. (18) Lee, D. S. Ph.D. Dissertation, The University of Texas at Austin, 1990. (19) Crain, N.; Tebbal S.; Li, L.; Gloyna, E. F. Ind. Eng. Chem. Res. 1993, 32 (10), 2259. (20) Meyer, J. C.; Marrone, P. A.; Tester, J. W. AIChE J. 1995, 41 (9), 2108. (21) Mathews, C. F.; Gloyna, E. F. Corrosion Behavior of Three HighGrade Alloys in Supercritical Water Oxidation Environments; CRWR Report 234; The University of Texas at Austin: Austin, TX, 1992. (22) Kung, H. H. Transition Metal Oxides; Elsevier Inc.: New York, 1989. (23) Frisch, M. A. M.S. Thesis, The University of Texas at Austin, 1992. (24) Savage, P. E.; Smith M. A. Environ. Sci. Technol. 1995, 29 (1), 216. (25) Carberry, J. J. Chemical and Catalytic Reaction Engineering, McGraw-Hill: New York, 1976. (26) Iwamoto, M.; Yoda, Y.; Tamazoe, N.; Seiyama, T. J. Phys. Chem. 1978, 82, 2564. (27) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGrawHill: New York, 1980. (28) Levenspiel, O. Chemical Reaction Engineering; Wiley: New York, 1972.

Received for review February 13, 1996. Revised manuscript received July 22, 1996. Accepted August 1, 1996.X ES9601346 X

Abstract published in Advance ACS Abstracts, October 15, 1996.