Kinetics of the Catalytic Oxidation of Phenol over Manganese Oxide in

A kinetic analysis was made for the phenol disappearance rate in catalytic oxidation of phenol over MnO2 in supercritical water at a fixed temperature...
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Ind. Eng. Chem. Res. 1999, 38, 4183-4188

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Kinetics of the Catalytic Oxidation of Phenol over Manganese Oxide in Supercritical Water Yoshito Oshima,* Kengo Tomita, and Seiichiro Koda Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Tokyo 113-8656, Japan

A kinetic analysis was made for the phenol disappearance rate in catalytic oxidation of phenol over MnO2 in supercritical water at a fixed temperature of 425 °C and pressures between 22.7 and 27.2 MPa. The nonsupported MnO2 catalyst possessed a strong activity for promoting phenol oxidation, though the overall reaction rate was appreciably influenced by internal mass-transfer resistance. From the kinetic analysis on the reaction rate of the phenol disappearance, the global rate expression of the surface reaction was obtained, where the reaction orders with respect to phenol, oxygen, and water were almost unity, 0.7, and -2.0, respectively. A Langmuir-type mechanism, in which phenol and oxygen adsorbed on the catalytic sites and water adsorbed on the same site to inhibit the phenol and oxygen adsorption, was proposed to explain the reaction orders for phenol, oxygen, and water. Introduction Supercritical water oxidation (SCWO) is a promising technique for the treatment of hazardous wastewater. The complete oxidation of organic compounds in supercritical water has received much attention, not only because of higher reaction rates compared to those of the conventional “wet oxidation” process but also because of its unique physical and chemical properties as a solvent. Though most organic compounds are found to be oxidized to H2O and CO2 within a reasonable residence time in supercritical water, the previous kinetic studies suggest that, in the case of such a chemically stable compound as phenol, the reaction rate at 400-450 °C is not high enough to achieve complete destruction within a reasonable residence time.1-4 To increase the reaction rate and to suppress the formation of byproducts, the oxidation reaction needs to be operated under higher temperatures, though operating at lower temperature is frequently desirable for industrial processes. Recently, heterogeneous catalytic oxidation in supercritical water has received attention as a new approach to solve this problem. This concept is based on the catalytic wet air oxidation, which has been proven to give a greater destruction efficiency than the conventional noncatalytic wet air oxidation for a wide variety of organic compounds.5 Because the reaction temperature of SCWO is higher than that of wet air oxidation, the enhancement of the reaction rate as well as the complete oxidation to CO2 is expected in the catalytic SCWO of organic compounds. In fact, several researchers have reported that, in catalytic SCWO of phenol, supported or nonsupported metal oxides, such as copper oxide and zinc oxide supported by a steam-treated porous cement,6 MnO2/CeO2 and V2O5,7 and MnO2/CuO on amorphous Al2O3 (CARULITE 150),8 exhibit high activities for the complete oxidation of phenol to H2O and CO2. Though the presence of these catalysts has been demonstrated to promote the oxidation reaction, * Corresponding author. Tel.: +81-3-5841-7352. Fax: +813-5841-7279. E-mail: [email protected].

much more kinetic information is necessary to establish this technique for a novel engineering process. In this paper, a kinetic analysis of the catalytic SCWO of phenol over MnO2 is provided. We mainly focus on the global rate expression of phenol disappearance and discuss the influence of mass transfer on the reaction rate, though the information on the products and the disappearance rate of the intermediates are also important to evaluate catalysis in supercritical water oxidation. Experimental Section The experiments were performed using a tubular flow reactor. Figure 1 is a schematic drawing of the apparatus used in this study. As a source of oxygen, we supplied H2O2, which stoichiometrically pyrolyzed into molecular O2 and water while flowing through the preheat line. It was confirmed elsewhere9 that the concentration of nondissociated H2O2 should be negligibly low at the entrance of the reactor. The aqueous solutions of phenol and H2O2 were pumped separately to the desired pressure using high-pressure HPLC pumps (Tosoh CCPD) and preheated while flowing in their respective preheat line. The flow rate of each solution was controlled by a HPLC pump, ranging from 1.5 to 8 cm3/min at an ambient condition. An electrically heated fluidized sand bath was used for heating the reactor (SUS-316 tubing of 0.4 cm i.d. and 1.5 cm length) and the preheat lines (Hastelloy C276 tubing of 0.108 cm i.d. and 5.4 m length for the decomposition of H2O2 to O2 and tubing of 0.108 cm i.d. and 1.8 m length for the phenol supply). The two flows were mixed just before the inlet to the reactor and let into the catalyst bed. Sintered Hastelloy porous disks (5 µm porosity, GKN Sinter Metals) were placed at both ends of the reactor to confine the catalyst within the reactor. The temperature of the fluid in the tubing was monitored directly using a thermocouple at the entrance of the reactor. The fluid emitted from the reactor was promptly cooled by external cooling-water flow, depressurized using a backpressure regulating valve, and separated to gaseous and liquid parts in a gas-liquid separator. The quantitative

10.1021/ie9902939 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/14/1999

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Figure 1. Schematic drawing of the continuous-flow, fixed-bed reactor system: (1) H2O2 feed tank, (2) phenol feed tank, (3) feed pump, (4) fluidized sand bath, (5) preheat line, (6) fixed-bed reactor, (7) heat exchanger, (8) backpressure regulator, (9) gas/ liquid separator.

analysis of the liquid sample was performed using a HPLC equipped with a Jasco Finepak SIL C18S column. The mobile phase was acetonitrile and water (3:4 by volume), and the UV absorbance was monitored at a wavelength of 210 nm. The powdered MnO2 catalyst was prepared by grinding the commercial pellets (Wako Chemicals), followed by separation into three particle size ranges (0.5-1.0, 0.25-0.50, and 0.18-0.25 mm). A total of 0.19 g of catalyst was loaded into the reactor and was used for the oxidation reaction without any additional treatment. The surface areas of the different catalyst size fraction were measured by the BET method to be 13-19 m2/g, independent of the average particle size. The phenol oxidation experiments were conducted at a fixed temperature of 425 °C and pressures between 22.7 and 27.2 MPa. The phenol concentration at the reactor entrance was in the range from 7.9 × 10-5 to 2.0 × 10-4 mol/dm3, and the initial concentration of O2 was always at least 36 times greater than that of phenol. Results and Discussion Effect of Particle Size on the Reaction Rate. The temporal profiles of the phenol conversion with MnO2 catalyst of different particle sizes at 425 °C and 24.7 MPa are shown in Figure 2. The contact time (t) in this figure is defined as eq 1. As for the product distribution,

τ) catalyst bulk volume fluid volumetric flow rate at the reactor entrance (1) the major product was CO2 whose selectivity was higher than 70% in most cases, and the formation of such compounds as CO, dibenzofuran, p-benzoquinone, phenoxyphenols, and 2,2′-biphenol was also observed as minor products. The carbon balance was always close to unity. Ding et al. reported that, by using Al2O3-supported MnO2/CeO2 catalyst, almost 100% conversion of phenol is achieved at 390 °C and 5.4 s of residence time.7 The paper of Zhang and Savage shows that a commercial catalyst, CARULITE 150 (45-60% MnO2, 1-3% CuO, supported on Al2O3), effectively promotes the complete

Figure 2. Time profile of phenol conversion with MnO2 of three different particle sizes at 425 °C and 24.7 MPa. Initial concentrations of reactants are 1.4 × 10-4 (mol/L) for phenol and 7.4 × 10-3 (mol/L) for oxygen, respectively (particle size: 2, 0.18-0.25 mm; 9, 0.25-0.50 mm; b, 0.50-1.0 mm).

oxidation of phenol.8 The experimental results in Figure 2 show that, in the case of the small particle size, over 90% of phenol was converted within 0.1 s or less at 425 °C, 24.7 MPa, which suggests that nonsupported MnO2 catalyst also possesses a strong activity for promoting phenol oxidation. Figure 2 also shows that the catalyst with a smaller particle size gave larger phenol conversion at the same contact time. The effect of the particle size on the reaction rate is likely due to the mass-transfer resistance. Thus, the influence of mass transfer on the reaction rate for each catalyst size was assessed. At first, the effect of external mass transfer on the reaction rate was evaluated quantitatively by estimating the reactant concentration on the catalyst surface. The mass balance in the laminar film at the outer surface of the catalyst gives the following relationship between the reaction rate per unit mass of the catalyst (-rA) and the reactant concentration at the outer surface of the individual catalyst particle (Cs):

-rA ) kcam(Cb - Cs)

(2)

kc is the mass-transfer coefficient, am is the external surface area per unit mass of the catalyst, and Cb is the concentration of the reactant in the bulk fluid. If the difference between Cb and Cs is much smaller than Cb, the external mass-transfer resistance is considered to be negligible. The procedure for the evaluation of kc is briefly explained below. The correlation between kc and the Sherwood number, Sh, is

Sh ) kcdp/Dm,ph

(3)

where Dm,ph and dp designate the mutual-diffusion coefficient of phenol in supercritical water and the average diameter of the catalyst particle, respectively. Because the value of Dm,ph has not been reported previously, we estimated it on assumption that the ratio of Dm,ph to the self-diffusion coefficient of water, Dm,w, is inversely proportional to the ratio of the LennardJones radius of phenol (σph) to that of water (σw). That is

Dm,ph σw ) Dm,w σph

(4)

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The apparent reaction rate per unit catalyst mass is described as eq 6, where k is the intrinsic rate constant

apparent reaction rate ) ηkCs

(6)

of the surface reaction. When eqs 2 and 6 are combined, the value of ηk for each run can be obtained experimentally. The effectiveness factor is a function of the Thiele modulus (φ), given by eqs 7 and 8 for a spherical particle, where Fp designates the bulk density of the

φ) Figure 3. Test for the first-order kinetics in the catalytic SCWO of phenol with MnO2 at 425 °C and 24.7 MPa (particle size: 0.180.25 mm). Table 1. Effect of Mass-Transfer Resistance on the Reaction Rate in the SCWO of Phenol over MnO2 Catalyst external mass transfer

internal mass transfer

mass-transfer particle coefficient, kc (Cb - Cs)/ Thiele effectiveness size (mm) Cb (%) (cm/s) modulus, φ factor, η 0.18-0.25 0.25-0.50 0.50-1.0

0.7-1.4 0.5-1.1 0.4-0.8

4.4-5.2 8.2-10 17-21

3.0-4.3 9.0-12 29-39

0.22-0.30 0.08-0.11 0.03-0.04

Dm,w was calculated using the experimental correlation,10 and the Lennard-Jones radii for phenol and water adopted here were 5.22 × 10-8 and 2.641 × 10-8 cm, respectively.11,12 The estimated value of Dm,ph, for example, was 1.35 × 10-3 cm2/s at 425 °C and 24.7 MPa. For the estimation of Sh, the following correlation13 with the Reynolds number (Re) and the Schmidt number (Sc) was adopted.

Sh ) 2 + 1.1Sc1/3Re0.6

(5)

Using the value of µ as 2.83 × 10-4 cm/s in the Steam Tables,14 the Reynolds number was estimated to range from 25 (u ) 2.6 cm/s) to 100 (u ) 10.6 cm/s) for the smallest catalyst size. The results are summarized in Table 1. The values of (Cb - Cs)/Cb were about 4-5% for the small-size (0.18-0.25 mm), 8-10% for the mid-size (0.25-0.50 mm), and 17-21% for the large-size (0.50-1.0 mm) catalyst. From these results, it is suggested that the external mass-transfer resistance was not important for the small-size catalyst, whereas, in the case of the largesize catalyst, the external mass-transfer resistance was not negligible under the reaction conditions adopted in this study. The effect of internal mass transfer was assessed by the effectiveness factor (η), which accounts for the influence of intraparticle diffusion on the rate. To assess the internal diffusion, we need to know the intrinsic reaction order (n) with respect to phenol. Using the experimental data in Figure 2, ln(1 - X), where X designates the phenol conversion, was plotted against the residence time, whose result is shown in Figure 3. The linear relationship between ln(1 - X) and the residence time suggests that the apparent reaction order of phenol was close enough to be regarded as unity. In the following calculation, therefore, the intrinsic reaction order with respect to phenol was assumed to be unity, because n equals unity when the apparent reaction order for phenol (1/2(n + 1)) is unity.

η)

(

x

dp 6

kFp De

1 1 1 φ tanh(3φ) 3φ

(7)

)

(8)

catalyst. The effective diffusivity, De, is estimated from the molecular diffusion coefficient, Dm,ph, and the Knudsen diffusion coefficient, DKN,ph, as eq 9, where  and τp

De )

1  τp 1/DKN,ph + 1/Dm,ph

(9)

are the void fraction and the tortuosity factor of the catalyst, respectively. The values of Fp (4.2-4.5 g/cm3) and  (0.10-0.15) were obtained from the results of BET measurement, and the tortuosity was approximated as τp ) 2 for all of the catalysts used in this work, based on Wheeler’s parallel-pore model.15 When the value of ηk is combined with eqs 7 and 8, the values of the Thiele modulus and effectiveness factor for each catalyst can be calculated, the results of which are displayed in Table 1. The low value of the effectiveness factor, even when the smallest catalyst was used, suggests that the rate was strongly affected by the internal mass transfer. It is noticeable, however, that nonsupported MnO2 catalyst possessed such a high activity that the conversion of phenol reached almost 100% within a very short contact time despite the strong mass-transfer limitation. In theory, it is preferable to use a catalyst with a smaller particle size than 0.18 mm to eliminate such a mass-transfer effect on the apparent reaction rate. It was impossible, however, to use MnO2 catalyst with a particle size smaller than 0.18 mm because it resulted in a large pressure drop across the catalyst bed at reaction conditions. To minimize the mass-transfer effect within experimental limitations, the following kinetic analysis was made using the smallest catalyst size fraction, where we consider that appreciable corrections to the obtained kinetic data on the basis of diffusion limitation are required. Kinetics of the Surface Reaction. To evaluate the rate constant for the surface reaction, 26 independent data at a constant temperature of 425 °C were taken under various initial concentrations of the reactants and fluid densities, which are summarized in Table 2. Because the discussion in the previous section suggests that the external mass-transfer resistance was not important for small-size catalysts Cs was approximated to be equal to the phenol concentration in the bulk fluid. Thus, the solution of eq 6 is simply given as

1 - X ) exp(-ηkFpt)

(10)

where X designates the observed conversion of phenol

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Table 2. Summary of the Experimental Data over MnO2 Catalyst at 425 °C (Particle Size of the Catalyst: 0.18-0.25 mm) pressure (MPa)

density (g/cm3)

[phenol]0 (mmol/dm3)

[O2]0 (mmol/dm3)

contact time (s)

conversion

effectiveness factor, η

rate constant (cm3/g of catalyst/s)

24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 24.7 23.7 22.7 26.7 25.7 23.2 25.2 27.2 26.2

0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.124 0.115 0.107 0.144 0.134 0.111 0.129 0.150 0.139

0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.10 0.12 0.16 0.13 0.12 0.09 0.08 0.13 0.16 0.17 0.13 0.12 0.16 0.15 0.12 0.14 0.17 0.15

7.20 7.20 7.20 7.20 7.20 7.20 7.20 7.20 10.79 9.89 8.64 9.59 10.07 11.19 11.51 9.59 8.40 8.22 5.76 5.36 7.20 6.68 5.56 6.44 7.48 6.94

0.055 0.110 0.083 0.041 0.033 0.028 0.024 0.021 0.041 0.041 0.033 0.037 0.033 0.037 0.033 0.028 0.028 0.024 0.051 0.048 0.064 0.059 0.049 0.057 0.067 0.062

0.66 0.88 0.81 0.62 0.51 0.40 0.39 0.35 0.69 0.64 0.48 0.60 0.52 0.60 0.57 0.47 0.44 0.38 0.67 0.67 0.66 0.66 0.68 0.67 0.66 0.66

0.29 0.30 0.28 0.25 0.27 0.29 0.27 0.27 0.22 0.24 0.29 0.24 0.26 0.24 0.23 0.25 0.27 0.28 0.28 0.28 0.29 0.29 0.28 0.28 0.29 0.29

16.0 15.0 16.6 21.0 19.2 15.5 18.1 18.1 30.2 24.1 16.1 24.0 19.9 24.8 25.3 21.7 18.2 16.5 18.1 19.4 13.6 14.9 19.3 15.9 12.7 14.0

at the contact time of τ. Using eqs 7, 8, and 10, the rate constant, k, as well as the effectiveness factor, η, at each reaction condition was determined, the results of which are also listed in Table 2. It is suggested from Table 2 that the rate constant is a function of the initial concentrations of O2 and water. Therefore, we assumed that the surface reaction obeys a power-law rate expression, as described in eq 11,

surface reaction rate ) k′[phenol][O2]0β[H2O]0γ (11) where β and γ are the reaction orders of oxygen and water, respectively, and k′ is the intrinsic rate constant of the surface reaction. Because O2 was always present in large excess of the stoichiometric requirement and the concentration of water was also extremely higher than that of phenol, the changes of the concentration of O2 and water due to the reaction were negligible. In this analysis, therefore, the concentrations of O2 and water at the catalyst surface were regarded to be same as those in the bulk fluid. With arrangement of eq 11, the relationship between the first-order rate constant, k, and the concentrations of O2 and water is given as eq 12. The fitting of eq 12 to

ln k ) ln k′ + β ln [O2]0 + γ ln[H2O]0

(12)

the experimental data shown in Table 2 led to the reaction orders of β ) 0.74 ((0.22) for oxygen and γ ) -1.98 ((0.69) for water. The numbers in the parentheses represent the 95% confidence interval. The comparison of conversions between the predicted values and experimental results are shown in Figure 4, where eq 14 can also nicely reproduce the experimental data. From these analyses, it is concluded that the apparent rate of the phenol disappearance is almost proportional to the phenol concentration and that the reaction orders with respect to oxygen and water are about 0.7 and -2.0, respectively. In the homogeneous SCWO of phenol, such a large negative dependence on the water concentration has not

Figure 4. Comparison of the phenol conversion between experimental data and predicted values by the power-law rate expression of the surface reaction.

been reported. Also, the value of the reaction order with respect to oxygen for the heterogeneous catalytic oxidation is slightly different from the reported values for the homogeneous SCWO. These phenomenological facts are explainable if a Langmuir-type reaction mechanism is applied to the heterogeneous oxidation reaction. Suppose that phenol and oxygen adsorb on the same catalytic site on the catalyst surface and that the oxidation reaction takes place between the absorbed phenol and oxygen. In this model, water is supposed to adsorb on the same site and reduce the fraction of sites on which phenol and oxygen can adsorb. The overall rate of phenol disappearance is written as eq 13. In eq 13, K

-

d[phenol] ) dt k′′KphKO2[phenol][O2] (1 + Kph[phenol] + KO2[O2] + Kw[H2O])2

(13)

designates the adsorption equilibrium constant for each component, none of which have been reported previously under similar reaction conditions. Based on this equation, the apparent reaction order of phenol may vary in

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model, where water inhibits the adsorption of phenol and oxygen. Summary and Conclusions

Figure 5. Comparison of the phenol conversion between experimental data and predicted values by the Langmuir-type reaction model.

the range of zero to unity depending on the values of Kph and [phenol]. Taking the observation into account that the apparent reaction order of phenol was very close to unity, Kph[phenol] is negligibly small compared to other terms in the denominator of eq 13. A nonlinear regression analysis was attempted to fit the phenol conversions from experimental results to eq 13, which led to k′′Kph ) 9.4 × 103, KO2 ) 4.7 × 102, and Kw ) 1.1, respectively. Because the initial concentrations of oxygen and water were not varied widely enough to determine three equilibrium constants in eq 13, the parameters have large uncertainties (for example, 95% confidence interval of Kw ) (1.8). The predicted conversion of phenol with the values shown above, however, agrees fairly well with the experimental phenol conversion, as shown in Figure 5. It should be noticed that the observed dependence on oxygen and water can also be explained by a different but similar Langmuir-type mechanism. In this model, phenol and oxygen adsorb on the different catalytic sites on the catalyst surface, and water is supposed to adsorb on both sites. The overall rate of phenol disappearance in this case is written as eq 14. Based on this equation, -

d[phenol] ) dt k′′KphKO2[phenol][O2]

(1 + Kph[phenol] + Kw,1[H2O])(1 + KO2[O2] + Kw,2[H2O])

(14) the apparent reaction order of phenol may also vary in the range of zero to unity depending on the values of Kph and [phenol]. Taking the observation into account that the apparent reaction order of phenol was very close to unity, Kph[phenol] is negligibly small compared to unity and/or Kw,1[H2O]. Similarly, the apparent reaction order of oxygen is expected to be in the ranges from zero to unity. In the case that Kw,2[H2O] > KO2[O2] . 1, the apparent order for oxygen could be close to unity, whereas the apparent order for water could be close to -2. At the moment, it is impossible to judge which model is more plausible because the initial concentrations of oxygen and water were not varied widely enough in our experiments to discriminate these two models. However, it is concluded that the reaction orders, including the negative dependence on the water concentration, are qualitatively explainable by a Langmuir-type reaction

A kinetic analysis was made for the phenol disappearance rate in catalytic oxidation of phenol over manganese oxide in supercritical water at a fixed temperature of 425 °C and pressures between 22.7 and 27.2 MPa. The experimental results show that nonsupported MnO2 catalyst actively promotes the oxidation of phenol. From an assessment of the effect of masstransfer resistance on the apparent reaction rate, it is found that the rates under the reaction conditions investigated here were strongly influenced by internal mass transfer. The surface reaction rate of the phenol disappearance is almost proportional to the phenol concentration, and the reaction orders with respect to oxygen and water are about 0.7 and -2.0, respectively. The reaction orders that were obtained, including the negative dependence on the water concentration, were satisfactorily explained by a Langmuir-type reaction model, where water inhibits the adsorption of phenol and oxygen. Further improvements in the design of catalyst and reaction environments will be necessary to suppress the mass-transfer limitation as well as the inhibition by water adsorption on the catalyst. Nomenclature am ) surface area per unit catalyst mass [cm2/g of catalyst] Cb ) concentration in the bulk fluid [mol/cm3] Cs ) concentration at the outer surface of the catalyst particle [mol/cm3] De ) effective diffusion coefficient [cm2/s] DKN ) Knudsen diffusion coefficient [cm2/s] Dm,ph ) mutual-diffusion coefficient of phenol in water [cm2/ s] Dm,w ) self-diffusion coefficient of water [cm2/s] dp ) average diameter of the catalyst particle [cm] Ki ) absorption equilibrium constant of i k ) rate constant kc ) mass-transfer coefficient [cm/s] rA ) reaction rate per unit catalyst mass [mol/g of catalyst/ s] Re ) Reynolds number Sh ) Sherwood number Sc ) Schmidt number u ) superficial velocity [cm/s] X ) conversion  ) void fraction φ ) Thiele modulus η ) effectiveness factor µ ) fluid viscosity [g/cm/s] F ) fluid density [g/cm3] Fp ) bulk density of the catalyst [g/cm3] σi ) Lennard-Jones radius of i [cm] τ ) contact time [s] τp ) tortuosity factor

Acknowledgment This work has been partly supported by “Research for the Future” Program by the Japan Society for the Promotion of Science (96P00401), which is greatly appreciated. Literature Cited (1) Savage, P. E.; Gopalan, S. A Reaction Network Model for Phenol Oxidation in Supercritical Water. AIChE J. 1995, 41, 1864.

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(2) Krajnc, M.; Levec, J. On the Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1996, 42, 1977. (3) Koo, M.; Lee, W. K.; Lee, C. H. New Reactor System for Supercritical Water Oxidation and Its Application on Phenol Destruction. Chem. Eng. Sci. 1997, 52, 1201. (4) Oshima, Y.; Hori, K.; Toda, M.; Chommanad, T.; Koda, S. Phenol Oxidation Kinetics in Supercritical Water. J. Supercrit. Fluids 1998, 13, 241. (5) Ding, Y. Z.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 3257. (6) Krajnc, M.; Levec, J. Catalytic oxidation of Toxic Organics in Supercritical Water. Appl. Catal. B 1994, 3, L101. (7) Ding, Y. Z.; Aki, S. N. V.; Abraham, M. A. Catalytic Supercritical Water Oxidation: Phenol Conversion and Product Selectivity. Environ. Sci. Technol. 1995, 29, 2748. (8) Zhang, X.; Savage, P. E. Fast Catalytic Oxidation of Phenol in Supercritical Water. Catal. Today 1998, 40, 333. (9) Thammanayakatip, C.; Oshima, Y.; Koda, S. Inhibition Effect in Supercritical Water Oxidation of Hydroquinone. Ind. Eng. Chem. Res. 1998, 37, 2061.

(10) Lamb, W. J.; Hoffman, G. A.; Jonas, J. Self-diffusion in Compressed Supercritical Water. J. Chem. Phys. 1981, 74, 6875. (11) Lai, C.-C.; Tan, C.-S. Measurement of Molecular Diffusion Coefficients in Supercritical Carbon Dioxide Using a Coated Capillary Column. Ind. Eng. Chem. Res. 1995, 34, 674. (12) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. (13) Wakao, N.; Kaguei, S. Heat and Mass Transfer in Packed Beds; Gordon and Breach: New York, 1982. (14) Haar, L.; Gallagher, J. S.; Kell, G. S. NBS/NRC Steam Tables; Hemisphere: Washington, DC, 1984. (15) Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; Massachusetts Institute of Technology Press: Cambridge, MA, 1970.

Received for review April 26, 1999 Revised manuscript received August 16, 1999 Accepted August 18, 1999 IE9902939