CeO2 Catalyst

Ind. Eng. Chem. Res. 1998, 37, 1707-1716

1707

KINETICS, CATALYSIS, AND REACTION ENGINEERING Supercritical Water Oxidation of NH3 over a MnO2/CeO2 Catalyst Zhong Yi Ding, Lixiong Li, Daniel Wade, and Earnest F. Gloyna* Environmental and Water Resources Engineering Program, Department of Civil Engineering, The University of Texas at Austin, Austin, Texas 78712

Catalytic oxidation of ammonia in supercritical water (SCW) was studied using a continuousflow, packed-bed reactor at temperatures ranging from 410 to 470 °C, a nominal pressure of 27.6 MPa, and reactor residence times of less than 1 s. The kinetics and catalyst performance of MnO2/CeO2 for oxidation of ammonia in SCW was evaluated. In this reaction environment, ammonia was predominantly converted into molecular nitrogen (N2), and the rate of ammonia conversion was enhanced by MnO2/CeO2. For example, 40% of the ammonia was converted when using the MnO2/CeO2 catalyst at a temperature of 450 °C and a reactor residence time of 0.8 s. It was reported that, without a catalyst, essentially no ammonia conversion was observed below 525 °C (Helling, R. K.; Tester, J. W. Environ. Sci. Technol. 1988, 22 (11), 1319) and 10% of the ammonia was converted at a temperature of 680 °C, a pressure of 24.6 MPa, and a reactor residence time of 10 s (Webley, P. A.; Tester, J. W.; Holgate, H. R. Ind. Eng. Chem. Res. 1991, 30 (8), 1745). Kinetic models developed for the gas-phase catalytic oxidation of ammonia were adopted and proven to be adequate for catalytic oxidation of ammonia in supercritical water. The best-fit global rate expression for catalytic supercritical water oxidation of ammonia by MnO2/CeO2 was obtained as follows: r ) 1.14 × 1014 exp(-189 kJ/mol/RT) [NH3]0.63[O2]0.71. The BET surface area and X-ray diffraction analyses of the exposed catalyst indicated a significant reduction of surface area and changes in the crystalline structure of the catalyst. Introduction Supercritical water oxidation (SCWO) is a novel and effective means of transforming hazardous organic compounds to innocuous products such as carbon dioxide and water. Water, when subjected to conditions above its critical point (374 °C and 22.1 MPa), becomes an excellent solvent for organic compounds and oxygen (Himmelblau, 1960; Connolly, 1966; Japas and Franck, 1985). As such, an intrinsic reaction environment can be created for the oxidation of organic compounds in supercritical water (SCW). In SCWO processes, highdestruction efficiencies (>99.99%) for most organic compounds can be achieved within 1 min of reaction time and at a temperature of about 500 °C (Modell, 1989; Tester et al., 1992). However, because some reaction intermediates, such as ammonia and acetic acid, are relatively stable (Helling and Tester, 1988; Webley et al., 1991; Li et al., 1991; Lee and Gloyna, 1992), the conversion of these refractory transformation compounds is one of the critical factors to improve the SCWO process effectiveness. Further elimination of these compounds requires the employment of either higher temperatures, longer reactor residence times, catalysts, or combinations thereof. The effectiveness of several catalysts for the oxidation of selected compounds in SCW has been reviewed (Ding et al., 1996). For example, using a batch reactor setup and MnO2/CeO2 as the catalyst, 99% of the acetic acid was converted at a relatively low temperature of 427 * To whom correspondence is addressed. Tel.: 512-471-5928. Fax: 512-471-1720. E-mail: [email protected].

°C, water density of 0.3 g/mL, and residence time of 5 min (Frisch, 1992). Similarly, increased catalytic oxidation rates were observed for benzene, V2O5/Al2O3, and MnO2/CeO2 (Ding et al., 1995a); acetic acid, TiO2 (Frisch, 1995); phenol, CuO/ZnO (Krajnc and Levec, 1994), and V2O5/Al2O3 and MnO2/CeO2 (Ding et al., 1995a,b); dichlorobenzene, V2O5/Al2O3 (Jin et al., 1992; Ding et al., 1995a), and MnO2/CeO2 (Ding et al., 1995a); and chlorophenol, CuF2BO4, and MnCl2 (Yang and Eckert, 1988). However, no data have been reported describing the effectiveness of metal oxide catalysts for the oxidation of ammonia in SCW. Ammonia transformation and destruction have been noted as an important issue in various hydrothermal oxidation applications. First, ammonia may be produced during oxidation of nitrogen-containing organic compounds in subcritical water (Ottengraf and Lotents, 1978; Imamura et al., 1982; Ito et al., 1989) and SCW (Killilea et al., 1992; Shanableh and Glayna, 1991). Second, ammonia is relatively stable in SCWO environments when oxygen is used as the oxidant and even at temperatures above 600 °C (Helling and Tester, 1988; Webley et al., 1991). Without using catalysts, the ammonia conversion was reported to be near zero with temperatures below 640 °C, and about 10% at a temperature of 680 °C, pressure of 24.8 MPa, and a residence time of 10 s (Webley et al., 1991). In the same study, when Inconel beads were packed in the same reactor, the ammonia conversion increased 4-fold. The catalytic transformation of ammonia using composite catalysts Co3O4/BiO(OH), Co3O4/CeO2, and Mn2O3/CeO2 in wet air oxidation (WAO) was reported to be fairly

S0888-5885(97)00934-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/15/1998

1708 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998

Figure 1. Schematic of the continuous-flow, fixed-bed reactor system.

effective (Imamura et al., 1985). The highest conversion (∼50%) was attained using a Mn/Ce catalyst at 263 °C and 1 h of contact time. Furthermore, MnO2/CeO2 not only exhibited high activity in WAO and gas-phase oxidation of ammonia, the catalyst was relatively stable in SCWO environments involving the destruction of aromatic compounds (Ding et al., 1995a) and acetic acid (Frisch, 1992). Enhanced ammonia oxidation rates in SCW were observed when using nitrate salts as the oxidant (Dell’Orco et al., 1997). For reactions between NaNO3 and NH4OH at 500 °C and 30 MPa, ammonia conversion reached about 40% at a reactor residence time of 10 s. The major products of these reactions were nitrogen and nitrous oxide. In another study involving ammonia oxidation by nitric acid, the optimum HNO3/NH3 ratio was found to be 0.65 where complete ammonia conversion was achieved in 30 s at 500 °C (Proesmans et al., 1995). Such a nitrate-based reaction system may find special hydrothermal oxidation applications. Specifically, ammonia nitrate is an effective oxidizer for oxidizing refractory organic compounds such as acetic acid, methanol, and phenol (Proesmans et al., 1995). The objectives of this study were to demonstrate the effectiveness of the MnO2/CeO2 catalyst for ammonia oxidation in SCW, develop mechanistic and global kinetic models, and evaluate the catalyst performance. Experimental Section Catalytic oxidation of ammonia by MnO2/CeO2 in SCW was studied using a continuous-flow, packed-bed

reactor at temperatures ranging from 410 to 470 °C, pressure of 27.6 MPa, and reactor residence times of less than 1 s. This section provides detailed descriptions of (1) materials and preparation, (2) apparatus and procedures, and (3) sample analyses. Materials and Preparation. Ammonia feed solutions were prepared daily by dissolving a known amount of anhydrous ammonia (Scott Specialty Gases, 99.9% purity) in a small beaker containing distilled and deionized (DDI) water. This concentrated ammonia solution was then transferred into the feed tank followed by appropriate dilutions for the desired feed concentration. To monitor the influent ammonia concentration, feed samples were collected for analysis at the beginning and end of each experiment. The MnO2/CeO2, as received from Corus Chemical Co., was crushed, sieved (40-45 mesh), washed with DDI water to remove fine particles, and dried overnight at 104 °C. This narrow-sized catalyst was then packed into a tubular reactor. The reactor was made of a piece of stainless steel (SS) 316 tube with dimensions of 30.5cm (1 ft-) long, 0.635-cm (0.25 in.-) outside diameter, and 0.124-cm (0.049 in.-) wall thickness. Catalyst particles were supported at the bottom of the reactor by a sintered metal disk (Hastelloy C-276, 10-µm pore size). At the beginning of each test, about 3.4 g of the catalyst were packed in the reactor. Apparatus and Procedures. The continuous-flow, packed-bed reactor system was a laboratory-scale unit with a feed capacity of 30 g/min. As shown in Figure 1, the major components of this reactor system included a feed pump, high-pressure oxygen supply, heat ex-

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1709 Table 1. Summary of Experimental Conditions and Results for Ammonia Oxidation in Supercritical Water Using the MnO2/CeO2 Catalyst run no.

reaction temp. (°C)

catalyst weight (g)

residence time (s)

NH3 feed conc. × 10-3 (mol/L)

O2 feed conc. × 10-2 (mol/L)

water conc. conv. (mol/L)

NH3 (%)

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

450 450 450 450 448 448 448 448 470 449 449 449 449 429 430 428 410 409 409 411 408 410 450 450 450 450 450

0 0 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4 3.4

0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.55 0.49 0.55 0.55 0.55 0.55 0.63 0.63 0.63 0.80 0.80 0.80 0.80 0.80 0.80 0.55 0.55 0.55 0.55 0.55

0.86 6.13 8.53 8.53 8.60 8.60 8.60 8.60 7.73 0.67 3.75 11.8 15.4 8.21 8.21 8.21 10.4 10.4 10.4 6.29 6.29 6.94 5.01 5.01 1.72 1.72 1.72

3.09 2.23 1.73 1.73 2.32 2.90 1.14 0.61 1.59 1.21 1.16 1.15 1.16 1.36 1.98 3.24 1.81 2.44 4.10 2.66 1.56 3.83 5.63 4.48 4.52 3.89 5.19

7.11 7.10 7.10 7.10 7.10 7.10 7.10 7.10 6.38 7.11 7.10 7.09 7.08 8.21 8.21 8.21 10.43 10.43 10.43 10.43 10.43 10.43 7.10 7.10 7.10 7.10 7.10

2.7 0.0 30.0 29.8 29.8 36.0 26.7 18.8 46.0 74.4 46.6 34.2 33.8 11.7 20.2 23.2 10.4 13.8 22.3 20.4 4.6 24.0 93.7 84.8 87.8 85.9 93.0

changer, preheater, reactor heater, packed-bed reactor, back-pressure regulator, gas-liquid separator, and monitors for temperature and pressure. Thermocouples were located at the reaction zone, preheater, heater, and sampling ports. Pressure gauges were located before and after the reaction zones. All wetted parts of the system were made of SS 316 stock. Oxygen was supplied from a 5000-psi accumulator. Two metering valves were used in series to control the oxygen flow. A mass flowmeter was calibrated against a bubble meter and was used to monitor the oxygen flow. For a typical test, the preheater and reactor, respectively, were heated initially to 450 and 200 °C. Then, oxygen and DDI water were introduced. To minimize the pressure across the catalyst bed, the system pressure was gradually increased to 27.6 MPa by adjusting the back-pressure regulator so that the reactor environment was maintained under steam or SCW conditions when the reactor temperature was increased. The reactor inlet and outlet temperatures were monitored by two thermocouples; the average of these two temperatures was defined as the reaction temperature. A steady state was assumed when the difference between these two temperatures was within (1 °C. Pressure readings were taken at both the preheater inlet and the reactor outlet. The difference of these two indictors was typically within (0.7 MPa (100 psi). After the desired operating condition was reached, the pump was switched from DDI water to the ammonia feed solution. At this time, a sample of the feed solution was collected. Ammonia was detected in the gas-liquid separator within 10 min after introducing the feed solution. After 10 min more, the influent and effluent flows of the reactor system were considered to have reached a steady state. After an additional 10 min (i.e., a total of 30 min after introducing the feed solution), a gaseous effluent sample was taken from the top of the gas-liquid separator with the aid of a syringe or a gas collection bag. Similarly, a liquid effluent sample was taken from

NO3(mg/L)

nitrogen balance

4.4 6.6 2.2 0.7 9.4 2.0 3.8 2.5 5.2 5.2 7.8 3.3 1.8 1.4 1.8 2.1 3.1 3.7

1.13 1.08 1.01 1.06 1.09 1.63 1.16 1.05 1.15 1.14 1.15 1.19 1.18 1.23 1.22

the bottom of the separator. For each test, multiple samples (2 or 3) were taken during a period of 60 min. Sample Analyses. The gas samples were analyzed for selected nitrogen species (N2 and NOx). The liquid samples were analyzed for ammonia, nitrate, and nitrite. A gas chromatograph equipped with a thermal conductivity detector and a packed column (60/80 mesh molecular sieve) was used for nitrogen analyses; helium served as the carrier gas; the column temperature was maintained at 68 °C; the analytical error for nitrogen in the gas phase was within (5%. Periodically, the gaseous samples were analyzed for NOx (Dragger-tube detectors, Lab Safety Supply YB-4643). The NOx detection limits for these tubes ranged from 0.5 to 10 ppm. Ammonia concentrations in aqueous feed and effluent solutions were determined by using an ion-specific membrane electrode with a digital ionalyzer (Orien Research model 701A). The analytical error of ammonia concentration was (10%. An ion chromatograph was used to analyze nitrate and nitrite in the liquid effluent samples. The physical characteristics of both new and used catalysts were evaluated through the use of X-ray diffraction (Philip PW 1729 diffractometer) and BET surface area measurements (Quantachrom Autosorp1). The BET surface area calculations were based on the three-point method using liquid nitrogen. Results and Discussion The oxidation of ammonia in SCW was investigated at temperatures ranging from 410 to 470 °C and at a nominal pressure of 27.6 MPa (4000 psi). A total of 25 catalyst tests and two baseline (without loading the catalyst) tests were performed with ammonia concentrations ranging from 100 to 2000 mg/L and molar ratios of oxygen-to-ammonia ranged from 0.7 to 18.0. Experimental results derived from these experiments are summarized in Table 1. Each data entry is an average

1710 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998

of values from at least two samples. Without the presence of a catalyst, the conversion of ammonia was negligible at a temperature of 450 °C even with excess oxygen. When the MnO2/CeO2 was used, the ammonia conversion reached as high as 96%, indicating the catalytic effect of the MnO2/CeO2. Results from this study are discussed under three topics: (1) product distribution, (2) kinetic aspects, and (3) catalyst performance associated with SCWO of ammonia by MnO2/ CeO2. Product Distribution. Identification and quantification of reaction intermediates and products provided some insight into reaction pathways for catalytic oxidation of ammonia in SCW. First, only trace amounts of NO and NO2 were detected in some gaseous effluent samples using the Dragger tubes, and in most cases, the NOx concentration in the effluent was below the detection limit of the Dragger tubes. Second, previous reports indicated that nitrous oxide (N2O) was a minor product in either WAO (Imamura et al., 1985) or SCWO of ammonia (Webley et al., 1991; Killilea et al., 1992). Although N2O was not analyzed in this study, the ammonia oxidation may have followed a pathway involving the N2O intermediate which may have been in turn consumed, as shown by the following reactions:

NH3 + (cat.) f N2O + (cat.) f N2 + 1/2O2

(1)

NH3 + (cat.) f N2O + (cat.) + NH3 f N2 + H2O (2) Third, the concentration of NO3- in all liquid effluents was in the range of 2-8 mg/L. Increasing the ratio of oxygen to ammonia resulted in a slight increase in the NO3- concentration. No nitrite was found in the liquid effluents. Since oxygen molecules activated on the catalyst surface are much more active than OH• or HO2• radicals (i.e., little ammonia oxidation occurred at 450 °C without a catalyst), the impact of these radicals on ammonia oxidation is neglected. As shown in Table 1, the nitrogen balance (Nout - Nin)/ Nin, ranged from 1.0 to 1.2. The only exception was test no. 9, in which the lowest ammonia feed concentration was used. Since the gas-phase analysis was not on-line, it was assumed that atmospheric nitrogen contamination occurred when handling these gas samples. Since the amount of nitrogen derived from the liquid (NO3-) and gaseous (NOx) species was negligible as compared to the converted nitrogen, it can be concluded that almost all the nitrogen from ammonia formed molecular nitrogen and possibly small amounts of nitrous oxide. Kinetic Aspects. Gas-phase, catalytic oxidation is an important industrial process and has been wellstudied. Golodets (1983) proposed three parallel pathways for gas-phase, catalytic oxidation of ammonia.

2NH3 + 1.5O2 f N2 + 3H2O

(3)

2NH3 + 2O2 f N2O + 3H2O

(4)

2NH3 + 2.5O2 f 2NO + 3H2O

(5)

The selectivity of three nitrogen products depends on both reaction temperature and type of catalysts used. For metal oxide catalysts (MnO2, Co2O3, CuO, and NiO) and reaction temperatures ranging from 200 to 450 °C, the primary end products are N2 and N2O. The catalytic

oxidation reactions take place between the gas phase of ammonia and adsorbed oxygen on the catalyst surface. The proposed reaction mechanism involving adsorption, surface reaction, and desorption is described below: k1

O2 + * 98 O2* f 2O* k2

NH3 + O* 98 NH* + H2O k3

NH* + O* 98 HNO** k4

NH* + HNO** 98 N2 + H2O + 3* k5

HNO** + HNO** 98 N2O + H2O + 4*

(6) (7) (8) (9) (10)

In these elemental reactions, the active site on the catalyst surface is represented by “*”. The rate-controlling steps involve adsorption of oxygen on an active site (eq 6) and surface reaction with gaseous ammonia (eq 7). Therefore, the rate of ammonia destruction can be presented as follows:

r ) 1/2k2[NH3]ΘO2 ΘO2 )

k1[O2] k1[O2] + γk2[NH3]

γ ) γN2SN2 + γN2OSN2O

(11) (12) (13)

where [NH3] and [O2] are the concentrations of ammonia and oxygen, ΘO2 is the fraction of the catalyst surface covered by oxygen molecules, γ is the stoichiometric coefficient for oxygen (γN2 ) 3/4 and γN2O ) 1), SN2 and SN2O are the selectivity of N2 formation (eq 3) and N2O formation (eq 4), and k1 and k2 are reaction rate constants (eqs 6 and 7). Detailed discussion on these parameters are provided in the original work by Golodets (1983). In the SCWO environment, the adsorption of water onto active sites may have a significant effect on the overall ammonia oxidation rate. Two equilibria involving adsorption of water onto catalyst surfaces may exist. The adsorption equilibria involving vacant active sites (*) and oxidized active sites (O*) are shown in eqs 14 and 15, respectively.

H2O + (*) f (H2O*)

(14)

H2O + (O*) f (O*,H2O)

(15)

The adsorption equilibrium constants (b1 and b2) for both reactions between water and catalyst surfaces can be quantified as

b1 )

b2 )

Θ(*,H2O) [H2O]ΘV Θ(O*,H2O) [H2O]ΘO2

(16)

(17)

where ΘV is the fraction of vacant site surfaces, Θ(*,H2O)

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1711

Figure 2. Test of mechanistic model for SCWO of ammonia over a MnO2/CeO2 catalyst.

Figure 3. Arrehenus plot of k1 and k2 in the mechanistic model.

is the fraction of vacant sites covered by water, and Θ(O*,H2O) is the fraction of oxidized vacant sites covered by water. The surface fraction of catalyst covered by oxygen (ΘO2) in the SCWO environment can be expressed as

ΘO2 )

k1[O2] k1[O2](1 + b2[H2O]) + γk2[NH3](1 + b1[H2O]) (18)

By substituting eq 18 into eq 11, the rate of catalytic SCWO of ammonia becomes

r)

k1k2[NH3][O2] 1 2 k1[O2](1 + b2[H2O]) + γk2[NH3](1 + b1[H2O]) (19)

and eq 19 can be further rearranged into

[O2] [O2] +c )a r [NH3]

(20)

where a ) 2(1 + b2[H2O])/k2 and c ) 2γ(1 + b1[H2O])/ k1. If nitrogen is the predominant product, it can be assumed that the selectivity of nitrogen formation is 1, (i.e., γ ) γN2SN2 ) (3/4)(1) ) 3/4. Since the reactor residence time was relatively short (i.e., ∆t < 1 s), r values approximated by ∆[NH3]/∆t should be acceptable for use in eq 20 even at high ammonia conversions. Plots of [O2]/r versus [O2]/[NH3] for three reaction temperatures (410, 430, and 450 °C) are shown in Figure 2. The destruction rate of ammonia was estimated by using the concentration change over the calculated reactor residence time. For a given temperature, a and c were first obtained through linear regression of eq 20 using the experimental data. By using trial-and-error methods, both b1 and b2 were found to be very small, near-zero values. Similar observations on the effect of water were also reported in other reaction systems (Watzenberger and Emig, 1992). Therefore, both b1 and b2 were assumed to be zero. The reaction rate constants k1 and k2 were calculated from k1 ) 2γ/c and k2 ) 2/a, respectively. The Arrhenius plots for the rate constants k1 and k2 are shown in

Figure 4. Mechanistic model prediction for SCWO of ammonia over a MnO2/CeO2 catalyst. Table 2. Summary of Correlation Parameters for the Mechanistic Model temp. (°C)

a (s)

b1 (L/mol)

b2 (L/mol)

c (s)

k1 (1/s)

k2 (1/s)

410 430 450

1.30 0.977 0.543

∼0 ∼0 ∼0

∼0 ∼0 ∼0

10.6 6.44 2.03

0.142 0.233 0.739

1.54 2.05 3.68

Figure 3. Both reaction rate constants are given below.

k1 ) 9.69 × 1011 exp(-168 kJ/mol/RT)

(21)

k2 ) 9.82 × 106 exp(-89.5 kJ/mol/RT)

(22)

A comparison of experimental data with calculated results for the reaction rate r is shown in Figure 4. With the exception of 2 out of 27 data points, the calculated rates represented the experimental data well. The two data points showing relatively large deviations correspond to experiments conducted at low ratios of oxygen to ammonia (i.e., data derived from runs no. 11 and no. 12 as shown in Table 1). In both of these cases, the ammonia concentrations were relatively high and the ratio of oxygen to ammonia was close to the stoichiometric value (0.75). Under the low-oxygen conditions, other reacting species, such as hydroxyl radicals, may have contributed to the conversion of ammonia. The activation energies for k1 and k2 are further compared with those reported for gas-phase catalytic oxidation of ammoina (Il’Chenko and Golodets, 1975)

1712 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 3. Comparison of Activation Energies for the Mechanistic Model temp. range (°C)

catalyst

Ea1 (kJ/mol)

Ea2 (kJ/mol)

ref

135-155 410-470 680

MnO2 MnO2/CeO2

125 168 244

71.1 89.5 271

Il’Chenko and Golodets, 1975 this work Webley et al., 1991

and oxidation of ammonia in SCW without using catalysts (Webley et al., 1991). As shown in Table 3, the values of Ea1 and Ea2 for catalytic oxidation of ammonia in SCW over MnO2/CeO2 are about 30% higher than those obtained for gas-phase catalytic oxidation of ammonia over MnO2, respectively. Therefore, the values of Ea1 remain larger than those of Ea2 for both of these reaction systems. Webley et al. (1991) used the same mechanistic model to correlate their experimental data for noncatalytic oxidation of ammonia in SCW. The values of Ea1 and Ea2 thus obtained appear to be higher and the value of Ea1 becomes smaller than that of Ea2. As emphasized by Webley et al. (1991), these values of Ea1 and Ea2 are not physically meaningful but rather represent useful correlating parameters for the catalytic model. However, the changes of Ea1/Ea2 ratios in all of the three systems may have some physical implications, suggesting the importance of activated sites for enhancing the rate of ammonia oxidation. The destruction of ammonia can also be represented by a global kinetic model expressed as

r ) k[NH3]x[O2]y[H2O]z

(23)

where k is the overall reaction rate constant, and x, y, and z are reaction orders for ammonia, oxygen, and water, respectively. Since the two denominator terms k1[O2] and γk2[NH3], eq 17, are in the same order of magnitude to each other (for example, the terms k1[O2] and γk2[NH3] are equal to 0.19 and 0.28, respectively, for run no. 2), the values of x and y, respectively, should be less than 1 and greater than zero. Furthermore, since the water concentration has a negative effect on the global reaction rate, the value of z should be less than zero and greater than -1. An initial analysis of experimental data was made to narrow the ranges of these parameters. The values of x and y were first estimated by using the following equation:

X ) 1 - {1 - k(1 - x)[NH3]x-1[O2]yt}1/(1-x) for x * 1 (24) where X is the ammonia conversion and [O2] is the initial oxygen concentration which is assumed to be constant. Nonlinear regression of experimental data was performed by varying z values ranging from -1.0 to 0.0. However, the z value did not affect the estimates of x and y, which were about 0.6 and 0.7, respectively. The lack of sensitivity to the reaction order for water may be the result of relative high and constant water concentrations. Also, as indicated by the mechanistic model, the effect of water appeared to be negligible in this reaction system. On the basis of this result, the z value was assumed to be zero for the global kinetic model. As shown in eq 24, the global rate expression for catalytic oxidation of ammonia by MnO2/CeO2 in

Figure 5. Global model prediction of for SCWO of ammonia over a MnO2/CeO2 catalyst.

SCW was obtained with a 95% confidence level.

r ) 1014.1(1.6 × exp(-189 ( 18 kJ/mol/RT)[NH3]0.63(0.08[O2]0.71(0.13 (25) As shown in Figure 5, eq 24 predicted the experimental conversion with reasonable success. The experimental data can be further interpreted through the effects of process parameters, mainly oxygen concentration, ratio of oxygen-to-ammonia, and reaction temperature. Excess oxygen is essential to achieve high destruction of ammonia at all reaction temperatures. Twelve tests with oxygen ranging from 0% to 400% excess of the stoichiometric demand were run at a fixed ammonia concentration of 8.6 × 10-3 g mol/L and a residence time of 0.5 s. An increase in the oxygen concentration improved the ammonia conversion. The effect of the ratio of oxygen-to-ammonia was also studied by varying the concentration of ammonia at a oxygen concentration of 0.011 g mol/L, temperature of 450 °C, and residence time of 0.5 s. The conversion of ammonia increased dramatically by an increased molar ratio of oxygen-to-ammonia, especially at low ammonia concentrations. These MnO2/CeO2 experimental results suggest that oxygen concentration, especially the ratio of oxygen-to-ammonia, is an important parameter in the oxidation of ammonia in SCW. As discussed later, excess oxygen increases the regeneration rate of an active site and thereby enhances the rate of ammonia conversion. Comparisons of ammonia conversion and conversion rates under catalytic and noncatalytic conditions are summarized in Table 4. As reported by Webley et al. (1991), less than 10% of the ammonia was converted at 680 °C in the noncatalytic SCWO process. By using the MnO2/CeO2 catalyst, 10% of ammonia was converted at a reaction temperature of 410 °C and a reaction time of less than 1 s. At 450 °C, the rate of ammonia conversion using the MnO2/CeO2 catalyst increased nearly 600 times. Even at a reduced temperature of 410 °C, the rate of catalytic ammonia conversion was at least 50 times faster as compared to that of a noncatalytic

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1713

Figure 6. X-ray diffraction patterns of fresh, calcined, and used MnO2/CeO2 catalysts. Table 4. Comparison of Catalytic and Noncatalytic Rates for Ammonia Oxidation in Supercritical Water rate equation (r) (M/s)

catalyst

none k[NH3]0.63[O2]0.71 MnO2/CeO2 k ) 1014.1 exp(-189/RT) k[NH3]0.76[O2]0.2 none k ) 106.5 exp(-157/RT) k[NH3] Inconel wire k ) 100.1 exp(-29.7/RT)

temp. pres. (°C) (MPa)

time (s)

[NH3] × [O2] × [H2O] NH3 rate × 10-3 (M) 10-2 (M) (M) conv. (%) 10-5 (M/s)

450 448 410 680

27.6 27.6 27.6 246

0.55 0.55 0.80 10.8

6.13 8.60 10.4 4.71

2.23 2.90 1.81 26.0

680 680

246 246

15.7 15.7

4.37 4.13

15.3 10.6

conversion rate at a temperature of 680 °C. This significant decrease in the SCWO temperature requirement may result in cost reduction. Catalyst Performance in the SCWO Environment. The change in catalyst texture and composition were evaluated by comparing X-ray diffraction analyses of both fresh and used catalysts. As indicated in Figure 6, the catalyst appeared to have changed after exposure to SCWO environments. The amorphous state of the fresh catalysts appeared to exhibit a more crystalline state after use. Also, Mn2O3 was present in the used catalysts. This finding was consistent with previous observations (Frisch, 1992). In addition, phase diagrams of MnO2-Mn2O3 (Klingsberg and Roy, 1960) indicated the potential of Mn2O3 formation under similar experimental conditions. These changes in oxide phase and surface morphology may create an orientational face that might be in favor of the transition-state formation (eqs 6 and 7). As summarized in Table 5, experimental results show that the catalyst activity increased slightly with increasing exposure time. Therefore, an effort was made to verify if this increase of the catalyst activity was due to the

7.10 7.10 10.4

ref

∼0 36.0 10.4 7.2

∼0 563 135 3.15

this work this work this work Webley et al., 1991

42.5 32.4

11.9 8.53

Webley et al., 1991

phase change. On the basis of the fact that MnO2 can be converted to Mn2O3 at 550 °C in air (Klingsberg and Roy, 1960), a batch of catalyst was prepared by calcining fresh MnO2/CeO2 at 550 °C in air for 50 h. This calcined catalyst was then tested for oxidation of ammonia under conditions similar to those using the untreated MnO2/ CeO2 catalyst. The X-ray diffraction patterns of the untreated (fresh) catalyst, the calcined catalyst, and the exposed catalyst are compared in Figure 6. Although the extent of the phase change in the calcined catalyst was small compared to that of the untreated catalyst, the calcined catalyst increased the ammonia conversion from 32% to 41%, as shown in Table 5. In addition, no significant change in catalyst activity was observed in experiments conducted at 450 °C over a period of 3-4 h using either the fresh catalyst or the calcined catalyst. Furthermore, no catalyst transformation occurred when the catalyst was heated in air at the reaction temperature. Imamura et al. (1985) found that the activity of a physical mixture of Mn2O3 and CeO2 in wet air oxidation was lower than that of coprecipitated Mn2O3/CeO2. This observation suggest that there may exist a synergetic

1714 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Table 5. Catalyst Performance at Different Experimental Conditions NH3 conc. × 10-3 (mol/L)

O2 conc. × 10-2 (mol/L)

NO3(mg/L)

nitrogen balance

41.8 40.6

BDLa BDL

0.97 1.04

17.3 36.7

2.2

1.12 1.21

run no.

reaction temp. (°C)

catalyst weight (g)

exposure time (min)

391 397

450 450

3.48 3.48

45 190

Fresh Catalyst Tested at a Fixed Condition 8.53 1.73 7.10 8.53 1.73 7.10

33.2 32.1

4241 4247

452 451

3.4 3.4

30 220

Baked Catalyst Tested at a Fixed Condition 7.61 1.18 7.10 7.61 1.24 7.10

3301 3302

448 449

3.48 3.48

Fresh Catalyst Tested at Variable Conditionsb 30 7.10 1.19 7.10 360 7.10 1.25 7.10

water conc. (mol/L)

NH3 conv. (%)

a BDL ) below detected limit. b Temperature changed from 450 °C f 430 °C f 410 °C f 450 °C while O concentration ranged from 2 100% to 300% excess and the run time was about 1.5 h at each temperature.

Table 6. Changes in Catalyst Properties before and after Oxidation of Ammonia in Supercritical Water catalyst test condition exposure time crystallinity oxide state/phase (X-ray) physical strength moisture content (wt %, ambient) BET surface area (m2/g)

fresh

calcined

used

used

amorphous β-MnO2, CeO2 hard 11.0 198.0

550 °C, air, 50 h amorphous β-MnO2, Mn2O3 hard 5.0 100.0

SCWO 0.5 h partially crystallized β-MnO2, Mn2O3, CeO2 soft 1.1 13.5

SCWO 8h more crystallized β-MnO2, Mn2O3, CeO2 soft 1.3 14.8

N/A

mechanism between the manganese and cerium oxides. The promotion of catalytic activity and the oxygen storage capacity by CeO2 is well-identified (Sayle et al., 1994; Marecot et al., 1994; Kalakkad et al., 1992; Ihara et al., 1990; Hepburn and Gandhi, 1992; Herrmann et al., 1989; Li et al., 1989). The synergetic mechanism may be explained by the following chain of reactions:

MnO2 f Mn2O3 + O*

(26)

Mn2O3 + CeO2 f MnO2 + Ce2O3

(27)

Ce2O3 + 1/2O2 f CeO2

(28)

The existence of Mn2O3 is largely dependent on the oxygen partial pressure and reaction temperature. At low oxygen partial pressures, the reaction described in eq 27 may not be as fast as that shown in eq 26. This explanation supports the evidence, reported by Frisch (1992), that at certain temperatures and acetic acid concentrations, excess oxygen was necessary to maintain the MnO2 form. Since the concentration of oxygen in the current study was relatively low, regeneration steps may have become inefficient, and this explains the presence of Mn2O3 in the used catalyst. The coexistence of MnO2-Mn2O3 might explain the increased oxidation rate of ammonia. Normally, the adsorbed oxygen on manganese oxide, eq 6, would be expected to participate in the oxidation reaction (Fierro et al., 1986; Novakova, 1971). Additional oxygen generated, eq 26, may be more active and easier to access. Furthermore, the oxygen transfer from CeO2 to Mn2O3, eq 27, may further enhance these chain reactions. The change of catalyst properties during ammonia oxidation in SCW are summarized in Table 6. Besides the aforementioned change of the oxide phase, the catalyst surface area (BET) was reduced after being exposed to the SCWO environments. This surface area reduction occurred during the initial exposure (i.e., within the first half-hour). As given in Table 6, the BET surface area of the catalyst reduced from an initial value of 198 to 13.5 m2/g after the first half-hour exposure. The catalyst surface area remained relatively constant

after the catalyst was exposed to SCWO environments from 0.5 to 8 h. Therefore, the observed catalyst activity appeared not to be related to changes in the catalyst surface area. Catalyst properties, such as strength and durability, are also important factors for SCWO applications. In gas-phase reactions, where most commercial catalysts have been used, the nature of active sites is more critical than the physical strength of the catalyst. In SCWO applications, mechanical durability of a catalyst is also critical since physical changes in catalysts may result from exposure to SCW and SCWO environments. For example, in this study the Mn2O3/CeO2 catalyst became softer and easier to break after exposure to the SCWO environment. The development and application of catalysts in SCWO environments have been reported elsewhere (Ding et al., 1996). Summary and Conclusions Catalytic oxidation of ammonia in SCW was studied using a laboratory-scale, fixed-bed reactor system at temperatures ranging from 410 to 470 °C and a nominal pressure of 27.6 MPa. The ammonia feed concentration ranged from 1 to 16 mM. Oxygen was provided in excess stoichiometric requirements. Under these conditions, the conversion of ammonia to nitrogen was enhanced by a commercially available MnO2/CeO2 catalyst. With this catalyst, the rate of ammonia conversion increased about 2 orders of magnitude as compared to that of noncatalytic oxidation. Both mechanistic and global kinetic models were established and validated for oxidation of ammonia in SCW catalyzed by MnO2/CeO2. The activation energy associated with the global kinetic model was 189 kJ/ mol. The overall reaction order was 1.34 and could be approximated by a pseudo-first-order reaction model. Exposure of the MnO2/CeO2 catalyst to the SCWO environment resulted in structural (physical and chemical) changes. A 90% reduction in the catalyst surface area was observed. Also, changes of the oxidation state and crystalline composition of the catalyst were detected. These changes in the catalyst occurred within

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1715

the first half-hour of the exposure. However, the catalyst activity, as characterized by the ammonia conversion, showed a slight increase with time. Catalyst type and reaction environments are likely to be important factors for further improving the rate of ammonia conversion. Acknowledgment Financial support for this study was provided by Advanced Technology Program (State of Texas Higher Education Coordinating Board, Project No. 003658-354), Eco Waste Technologies, Inc. (Austin, TX), and The University of Texas at Austin (Separations Research Program and the Bettie M. Smith Chair in Environmental Health Engineering). The authors express their gratitude to Anna Iwasinska for her kind assistance in sample analyses. Nomenclature a ) constant, 2(1 + b2[H2O])/k2 b ) adsorption equilibrium constant between water and catalyst surface, ΘH2O/([H2O]Θv) c ) constant, 2γ(1 + b1[H2O])/k1 k ) the overall reaction rate constant (M1-x-y-z/s) k1,6 ) the elemental reaction rate constants defined by eqs 6-10 r ) the reaction rate (M/s) t ) time (s) x ) the reaction order for ammonia y ) the reaction order for oxygen z ) the reaction order for water A ) preexponential factor (M1-x-y-z/s) Ea ) activation energy (kJ/mol) SN2 ) the selectivity of N2 formation (eq 3) SN2O ) the selectivity of N2O formation (eq 4) R ) the gas constant T ) temperature (°C or K) X ) conversion γ ) the stoichiometric coefficient for oxygen (γN2 ) 3/4 and γN2O ) 1) Θ(*,H2O) ) the fraction of the vacant active sites covered by water molecules Θ(O*,H2O) ) the fraction of the oxidized active sites covered by water molecules ΘO2 ) the fraction of the catalyst surface covered by oxygen molecules ΘV ) the fraction of the catalyst surface which is vacant [ ] ) the concentration (M ) mol/L)

Literature Cited Connolly, J. F. Solubility of Hydrocarbons in Water Near the Critical Solution Temperature. J. Chem. Eng. Data, 1966, 11 (3), 13. Dell’Orco, P. C.; Gloyna, E. F.; Buelow, S. J. Reactions of Nitrate Salts with Ammonia in Supercritical Water. Ind. Eng. Chem. Res. 1997, 36, 6 (7), 2547. Ding, Z. Y.; Aki, S. N. V. K.; Abraham, M. A. Catalytic Supercritical Water Oxidation: An Approach for Complete Destruction of Aromatic Compounds; ACS Symposium Series; Hutchenson, K. W., Foster, N. R., Eds.; American Chemical Society: Washington, DC, 1995a; Vol. 608, p 232. Ding, Z. Y.; Aki, S. N. V. K.; Abraham, M. A. Catalytic Supercritical Water Oxidation: Phenol Conversion and Product Selectivity. Environ. Sci. Technol. 1995b, 29, 2748. Ding Z. Y.; Frisch, M. A.; Li, L.; Gloyna, E. F. Catalytic Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 5 (1), 3257. Fierro, J. L. G.; Garcia, de la Banda; Juan, F. Chemisorption of Probe Molecules on Metal Oxides. Catal. Rev. 1986, 28, 265.

Frisch, M. A. Supercritical Water Oxidation of Acetic Acid Catalyzed by CeO2/MnO2. M.S. Thesis, The University of Texas at Austin, Austin, TX, 1992. Frisch, M. A. Catalyzed Supercritical Water Oxidation of Acetic Acid: Kinetics and Anatase Transformation. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 1995. Golodets, G. I. Heterogeneous Catalytic Reactions Involving Molecular Oxygen. Studies in Surface Science and Catalysis 15; Elsevier Science Publishers B.V.: New York, 1983. Helling, R. K.; Tester, J. W. Oxidation of Simple Compounds and Mixtures in Supercritical Water: Carbon Monoxide, Ammonia, and Ethanol. Environ. Sci. Technol. 1988, 22 (11), 1319. Hepburn, J. S.; Gandhi, H. S. Relationship Between Catalyst Hydrocarbon Conversion Efficiency and Oxygen Storage Capacity. SAE Technol. Pap. Ser. 920831, 1992, 1-7 (Published by SAE, Warrendale, PA). Herrmann, J. M.; Ramaroson, E.; Tempere, J. F.; Guilleux, M. F. Semiconductivity Study of Ceria-Supported Nickel Related to Its Methanation Catalytic Activity. Appl. Catal. 1989, 53 (2-3), 117. Himmelblau, D. M. Solubilities of Inert Gases in Waters0 °C to Near the Critical Point of Water. J. Chem. Eng. Data 1960, 5 (1), 10. Ihara, K.; Murakami, H.; Ohkubo, K. Improvement of Three-Way Catalyst Performance by Optimizing Ceria Impregnation. SAE Trans. 1990, 99 (4), 1085. Il’Chenko, N. I.; Golodets, G. I. Catalytic Oxidation of AmmoniasI. Reaction Kinetics and Mechanism. J. Catal. 1975, 39, 57. Imamura, S.; Kinunaka, H.; Kawabata, N. The Wet Oxidation of Organic Compounds Catalyzed by Co-Bi Complex Oxide. Bull. Chem. Soc. Jpn. 1982, 55, 3679. Imamura, S.; Dol, A.; Ishido, S. Wet Oxidation of Ammonia Catalyzed by Cerium-Based Composite Oxides. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24 (1), 75. Imamura, S.; Nakamura, M.; Kawabata, N.; Yoshida, J. I. Wet Oxidation of Poly(ethylene glycol) Catalyzed by ManganeseCerium Composite Oxide. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25 (1), 34. Ito, M. M.; Akita, K.; Inoue, H. Wet Oxidation of Oxygen- and Nitrogen-Containing Organic Compounds Catalyzed by Cobalt(III) Oxide. Ind. Eng. Chem. Res. 1989, 28 (7), 894. Japas, M. L.; Franck, E. U. High-Pressure Phase Equilibria and PVT-Data of the Water-Oxygen System including Water-Air to 673 K and 250 MPa. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1268. Jin, L.; Ding, Z. Y.; Abraham, M. A. Catalytic Supercritical Water Oxidation of 1,4-Dichlorobenzene. Chem. Eng. Sci. 1992, 47, 2659. Kalakkad, D.; Datye, A. K.; Robota, H. Interaction of Platinum and Ceria Probed by Transmission Electron Microscopy and Catalytic Reactivity. Appl. Catal. B: Environ. 1992, 1 (3), 191. Killilea, W. R.; Swallow, K. C.; Hong, G. T. The Fate of Nitrogen in Supercritical Water Oxidation. J. Super. Fluids 1992, 5, 72. Klingsberg, C.; Roy, R. Solid-Solid and Solid-Vapor Reactions and a New Phase in the System Mn-O. J. Am. Ceram. Soc. 1960, 43, 620. Krajnc, M.; Levec, J. Catalytic Oxidation of Toxic Organics in Supercritical Water. Appl. Catal. B: Environ. 1994, 3, 101. Krietemeyer, S.; Wagner, T. Supercritical Water Oxidation; EPA/ 504/S-92/006; Risk Reduction Engineering Laboratory: Cincinnati, OH, Sept. 1992. Lee, D.-S.; Gloyna, E. F. Hydrolysis and Oxidation of Acetamide in Supercritical Water. Environ. Sci. Technol. 1992, 26, 1587. Li, J.; Li, F.; Li, X.; Luo, L. Studies of the Surface Behavior and Catalytic Properties of Pt-Ce-gamma-Al2O3. J. Less-Common Met. 1989, 148 (1-2), 405. Li, L.; Chen, P.; Gloyna, E. F. Generalized Kinetic Model for Wet Oxidation of Organic Compounds. AIChE J. 1991, 37, 1687. Marecot, P.; Pirault, L.; Mabilon, G.; Prigent, M.; Barbier, J. Influence of the Redox Properties of Ceria on the Preparation of Three-Way Automotive Platinum-Rhodium/Alumina-Ceria Catalysts. Appl. Catal. B: Environ. 1994, 5 (1-2), 57. Modell, M. Supercritical-Water Oxidation. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1989. Novakova, J. Isotopic Exchange of Oxygen 18O between the Gaseous Phase and Oxide Catalysts. Catal. Rev. 1971, 4, 78.

1716 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 Ottengraf, S. P. P.; Lotents, J. P. Heat Treatment and Low Pressure Oxidation of Aqueous Sewage Sludge. Water Res. 1978, 12 (3), 171. Proesmans, P. I.; Luan, L.; Buelow, S. J. Use of Reclaimed Ammonium Nitrate as an Oxidizer for the Hydorthermal Oxidaiton of Organic Wastes. Final Report LA-UR-95-4138; Los Alamos National Laboratory: Los Alamos, NM, 1995. Sayle, T. X. T.; Parker, S. C.; Catlow, C. R. A. Role of Oxygen Vacancies on Ceria Surfaces in the Oxidation of Carbon Monoxide. Surf. Sci. 1994, 316 (3), 329. Shanableh, A.; Glayna, E. F. Supercritical Water Oxidations Wastewater and Sludges. Water Sci. Technol. 1991, 23, 389. Tester, J. W.; Holgate, H. R.; Armellini, F. J.; Webley, P. A.; Killilea, W. R.; Hong, G. T.; Barner, H. E. Supercritical Water Oxidation Technology: A Review of Process Development and Fundamental Research; ACS Symposium Series; Tedder, D. W., Pohland, F. G., Eds.; American Chemical Society: Washington, DC, 1992; Vol. 518.

Watzenberger, O.; Emig, G. The Influence of Water on the Oxygehydrogenation of Isobutyric Acid over Heteropolyacid Catalysis. In Studies in Surface Science and Catalysis; Ruiz. P., Delmon, B., Eds.; Elsevier Science and Catalysis: New York, 1992; Vol. 72, p 71. Webley, P. A.; Tester, J. W.; Holgate, H. R. Oxidation Kinetics of Ammonia and Ammonia-Methanol Mixtures in Supercritical Water in the Temperature Range 530-700 °C at 246 bar. Ind. Eng. Chem. Res. 1991, 30 (8), 1745. Yang, H. H.; Eckert, C. A. Homogeneous Catalysis in the Oxidation of p-Chlorophenol in Supercritical Water. Ind. Eng. Chem. Res. 1988, 27, 2009.

Received for review December 30, 1997 Revised manuscript received February 25, 1998 Accepted February 28, 1998 IE9709345