358
Ind. Eng. Chem. Res. 1999, 38, 358-367
Catalytic Supercritical Water Oxidation of Pyridine: Comparison of Catalysts Sudhir Aki and Martin A. Abraham* Department of Chemical and Environmental Engineering, The University of Toledo, Toledo, Ohio 43606
The catalytic supercritical water oxidation (CSCWO) of pyridine in the presence of several catalysts such as Pt/γ-Al2O3, MnO2/γ-Al2O3, and MnO2/CeO2 was studied. Experiments conducted with R-Al2O3 and γ-Al2O3 confirmed the inertness of these catalysts toward SCWO of pyridine. Complete conversions were obtained at 370 °C in the presence of platinum catalyst. Ammonia and nitrogen oxides were not observed. Platinum catalyst favored the formation of nitrate ions and nitrous oxide whereas the MnO2 catalysts favored the formation of nitrogen and nitrate ions. The rate of pyridine oxidation was modeled assuming power-law kinetics. The results indicate that the two manganese catalysts performed similarly. The power-law analysis indicates that the rate of oxidation is first order with respect to pyridine on the manganese catalyst whereas it was found to be second order for the platinum catalyst. Two different reaction mechanisms are proposed to describe the experimental observations. Introduction Supercritical water oxidation (SCWO) is an emerging technology that has been developed to treat hazardous wastewater streams. During the SCWO process, organic compounds are oxidized by an oxidizing agent such as oxygen or hydrogen peroxide, in the presence of water above its critical point (374 °C and 22.13 MPa). Generally, temperatures greater than 500 °C and pressures in excess of 25 MPa are required to convert aqueous organic wastes into innocuous products. There is no formation of NOx or SO2, and in situ neutralization of the produced acid gases can be achieved.1 One major limitation of this process is the formation of condensation products at temperatures less than 500 °C. The addition of a catalyst can increase the yields of the complete oxidation products and decrease the process temperature required. Earlier work in our laboratory showed that complete oxidation products can be achieved at temperatures as low as 390 °C and a pressure of 24 MPa by using MnO2/CeO2 catalyst.2 The effect of catalyst on the SCWO of several model compounds (such as phenol, benzene, and 1,3-dichlorobenzene) has previously been reported.3-9 The addition of the catalyst led to the complete destruction of the model compounds in a temperature range between 380 and 450 °C. The yields of the complete oxidation products increased in the presence of the catalyst. Most previous work has focused on the destruction of chlorinated compounds. However, several other classes of compounds present equally important environmental challenges. As an example, N-heterocyclic compounds are present in numerous applications, including fiber production, insulation manufacture, agriculture, pharmaceuticals, and military applications. Previous results with SCWO have indicated that these compounds can be difficult to oxidize and may produce ammonia as a stable intermediate. However, the reaction rate of ammonia in supercritical water increased by 600 times with the addition of MnO2/CeO2 catalyst.10 Pyridine was selected as a model compound to verify the efficacy of the SCWO process in treating N-heterocyclic compounds. Pyridine and its derivatives have
recently received attention because of their presence in the environment. Pyridine and its derivatives were extensively used as solvents in the synthesis of a wide range of agricultural chemicals, textiles, rubber chemicals, and pharmaceuticals. They are present in the vicinity of industrial and agricultural activities as a result of processing of synthetic fuels and coal tars, pesticide application, and through a variety of chemicals manufacturing activities.11 In addition, pyridine is a toxic compound classified as a priority pollutant by the Environmental Protection Agency. Aqueous wastes contaminated with pyridine have been treated using several different methods, including incineration, catalytic combustion, biological oxidation, carbon adsorption, and photocatalytic degradation.12-20 The combustion of pyridine was studied in the temperature range of 675 and 775 °C.21 The nitrogen products present in the reactor effluent include N2, N2O, NOx, and HCN. These results confirm the presence of undesirable products, such as NOx and HCN. Catalytic incineration of pyridine on several catalysts, including Pt, CuCr2O4, NiCr2O4, and CuO supported on γ-Al2O3, has also been reported.22-24 At temperatures between 240 and 550 °C, products from the reaction were carbon dioxide, water, nitrogen, and NOx, with the yield of NOx increasing as the temperature increased. They also reported the formation of hydrogen cyanide and complete conversion of pyridine. The yield of NOx decreased when metal oxide catalysts were used instead of Pt catalyst. Crain et al.25 studied the SCWO of pyridine in a temperature range of 425-527 °C and found the conversion to increase from 0.03 at 426 °C to 0.68 at 527 °C at a residence time of 10 s. The products identified during SCWO of pyridine include ammonia, dimethylamine, and several carboxylic acids such as formic acid, acetic acid, glutaconic acid, and oxalic acid. Wightman26 also studied SCWO of pyridine in a flow system and reported a conversion of around 16% at 400 °C and a pressure of 40.82 MPa. In these experiments, the residence time was fixed at 29 s and an oxygen concentration of 100% excess was used. Katritzky and Bar-
10.1021/ie980485o CCC: $18.00 © 1999 American Chemical Society Published on Web 12/23/1998
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 359
cock27 found that pyridine was unreactive in the absence of oxygen, even in SCW. They also reported that formic acid was effective in catalyzing the hydrolysis of pyridine. In our previous work, we reported the effect of MnO2/CeO2 catalyst on the SCWO of pyridine.28 These results were obtained in a batch reactor system and under the influence of mass-transfer limitations, and therefore the rate expressions obtained in this work should be used with caution. These results indicate that aqueous wastes contaminated with pyridine and its derivatives can be treated using several methods. Most of these methods have been developed for a specific application and may not be generally useful. For example, if the concentration of the organic pollutant is less than 250 ppm, a treatment process based on photocatalysis or biological oxidation can be used. These processes may not be effective in the presence of other pollutants. Other treatment alternatives (incineration and catalytic combustion) are limited by the generation of undesirable partial oxidation products such as hydrogen cyanide and NOx. Clearly there is a need for the development of new processes to treat aqueous wastes contaminated with N-heterocyclic compounds. The process should not yield undesirable partial oxidation products. Within this paper, we show that catalytic supercritical water oxidation (CSCWO) is one process that can be used to treat these wastes. Experimental Section The experiments were conducted in a packed-bed flow reactor system. The main components of this system are the two high-pressure metering pumps, gas compressor, reactor, electric furnaces, high-pressure separation system, backpressure regulator, and mass flowmeter. LDC Analytical 2396-89 and Eldex model A-30-S metering pumps were used to feed water and the pyridine solution to the reactor. The oxidant gas (breathing air or a Ar/O2 mixture) was compressed from 7 to 27 MPa and delivered to the reactor using a Haskel model AG62 air-driven gas compressor. Reactor tubes of several different lengths made of stainless steel 316 were used. The details of the reactor geometry are described later. A backpressure regulator (GO Inc., BP-66) was used to depressurize gas to atmospheric pressure. The mass flow rate of this low-pressure gas was measured using an Omega model FMA-870-V mass flowmeter. The gas was analyzed using a Nicolet Impact Fourier transform infrared analyzer. The details of the experimental system and the experimental methods are described in detail elsewhere.29 A Nicolet Impact 400D Fourier transform infrared analyzer equipped with a gas cell was used to analyze effluent gas from the reactor. An ultramini gas cell made by Infrared Analysis Inc. with a fixed path length of 2.4 m and a volume of 120 cm3 was used for this purpose. This instrument was calibrated on a regular basis for carbon dioxide, carbon monoxide, and nitrous oxide using the peak areas of the respective spectral bands. The standard spectra for the nitrogen oxides, ammonia, and hydrogen cyanide were also available. The detection limit for all of these gases was found as 1 ppm (by volume) in air. Liquid samples were analyzed by a Hewlett-Packard 5890 gas chromatograph equipped with a flame ionization detector. A 10 m long HP-17 (cross-linked 50% phenyl methyl silicone) capillary column was used. An
Table 1. Characteristics of the Catalysts
catalyst Pt/γ-Al2O3 MnO2/CeO2 MnO2/γ-Al2O3 γ-Al2O3 R-Al2O3
active metal surface loading, area, F, 2 3 wt % m /g g/cm 200 150 150 100 0.4
1.33 1 3.5 N/A N/A
0.5 65-75 8-10 N/A N/A
manufacturer
0.38 0.38 0.38 0.38 N/A
Sigma-Aldrich Carus Chemical Co. Carus Chemical Co. Strem Chemical Co. Strem Chemical Co.
internal standard method was used for the analysis, using 1,4-dioxane as the standard. The detector was maintained at 250 °C, and the injection port was maintained at 235 °C. The oven was maintained at 40 °C for 3 min. It was then heated at a rate of 10 °C/min to a final temperature of 225 °C and held at that final temperature for 5 min. The gas chromatograph was operated in a split mode with a split ratio of 20:1. Helium was used as the carrier gas. A Cole-Parmer nitrate ion selective electrode was used for nitrate ion analysis. The electrode was calibrated using standard solutions. The ionic strength of the samples was adjusted using a 2 M (NH4)2SO4 solution. A Perkin-Elmer Plasma II emission spectrometer was used to analyze platinum, iron, nickel, chromium, and aluminum ions in the reactor effluent. The instrument was calibrated using standard solutions purchased from Fisher Scientific. The reactor effluent samples were analyzed on a regular basis. All chemicals were used as received. The reactant pyridine (99.9% HPLC grade) was purchased from Sigma-Aldrich Chemical Co. The characteristics as reported by the manufacturer of the catalysts used in the present study are given in Table 1. Catalysts were used as received. Smaller particles in the required particle size distribution were obtained by crushing the larger particles using a ceramic mortar and pestle. Results Chemical oxidation of organic compounds occurs at very high temperatures and in the presence of excess oxygen. Many metals are converted to metal oxides at these conditions and may become unstable in the presence of water at high concentration, whereas noble metals can resist oxidation at these extreme conditions and remain active. Also, noble metal catalysts provide high specific activity, they can be supported in a highly dispersed form, and they require very small amounts of the active metal. Previous results also showed that MnO2/CeO2 catalyst was an effective oxidation catalyst in supercritical water.3 Therefore, the effect of these catalysts, specifically Pt/γ-Al2O3, MnO2/CeO2, MnO2/γAl2O3, γ-Al2O3, and R-Al2O3, on the supercritical water oxidation of pyridine was studied. Previously we reported the effect of mass-transfer limitations on the SCWO of pyridine in the presence of Pt/γ-Al2O3 catalyst.30 The absence of external masstransfer limitations and the presence of internal masstransfer limitations were confirmed experimentally. However, the effectiveness factor was calculated to be greater than 0.9 for catalyst particles smaller than 0.2 mm. The results presented in the present work were obtained in the presence of particles of 0.2 mm or less; thus, the kinetics reported in this work are close to the intrinsic values. Even with catalyst particles of this small diameter, the pressure drop across the bed was less than 0.6 MPa, or less than 3% of the operating pressure of 24.2 MPa.
360 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 Table 2. Details of the Reactor and the Amount of Catalyst Used during the CSCWO of Pyridine particle diameter, mm
length, cm
2.2 N/A 0.2 0.165 0.211 0.09 0.165 0.165 0.165
23.5 34 14 29.2 29.2 29.5 17 10 10
R-Al2O3 empty tube γ-Al2O3 MnO2/γ-Al2O3, 1 MnO2/γ-Al2O3, 2 MnO2/CeO2 Pt/γ-Al2O3, 1 Pt/γ-Al2O3, 2 Pt/γ-Al2O3, 3
reactor specifications o.d., cm 2.54 0.3175 0.95 0.95 0.95 0.95 0.3175 0.3175 0.9525
i.d., cm 1.27 0.1016 0.635 0.635 0.635 0.635 0.2286 0.2286 0.3175
weight of catalyst catalyst, g R-Al2O3, g 0 0 3.5 0.2558 7.6520 1.2636 2.0807 0.5187 0.1014
30 0 0 16.7963 0 15.0945 0 1.8733 4.9725
The details of the reactor geometry and the amount of catalyst used are given in Table 2. In most of the experiments the active catalyst was diluted using R-Al2O3. The required amounts of the active catalyst and R-Al2O3 were thoroughly mixed in a separate glass vial before loading into the reactor. The effect of the process variables such as temperature and pyridine and oxygen concentrations on the rate of pyridine oxidation was studied. The conversion of pyridine was defined as
X)
FPyr,i - FPyr FPyr,i
(1) Figure 1. Effect of temperature and catalyst on the SCWO of pyridine. P ∼ 24.2 MPa; CO2 ∼ 0.1 mol/L; CPyr ∼ 0.85 mmol/L.
Multiple samples were taken at each experimental condition, and eq 1 was used to calculate the conversion of pyridine. This information was used to estimate the pure error variance from which the standard deviation of the experimental conversion was obtained. The deviation in the conversion was estimated to be within (0.03. The observed products were quantified and reported in terms of yield of the identified product, where the yield was defined as
yCO2 ) yN2O )
FCO2 5FPyr,i
0.5FN2O FPyr,i
(2)
(3)
The carbon and nitrogen balances are estimated using the following expressions:
Cbalance ) FCO2 + FCO + FCH3OH + 2FCH3COOH + 3FC3H6O + 5FPyr 5FPyr,i (4) Nbalance )
FNO3- + FNO2- + 2FN2O + FPyr FPyr,i
(5)
Effect of Temperature. The effect of temperature and various catalysts on the SCWO of pyridine is shown in Figure 1. The amount of catalyst indicated in Figure 1 corresponds to the actual weight of the active metal. All data were obtained at a pressure of 24.2 MPa, a pyridine concentration of 0.85 mmol/L, and an oxygen concentration of 0.1 mol/L. Note that at CPyr ) 0.85 mmol/L the stoichiometric oxygen concentration is 0.006 mol/L; thus, these experiments were performed at very high excess oxygen. The conversion increased with an increase in temperature in all cases. Maximum conversion was obtained in the presence of platinum catalyst. For ex-
ample, at 380 °C, the conversion of pyridine was 0.55 in the presence of Pt/γ-Al2O3, whereas it was 0.4 in the presence of MnO2 catalyst and no conversion was observed with R-Al2O3. In the presence of 0.5 mg (0.1014 g of total weight) of platinum catalyst the conversion of pyridine increased from 0.2 to 0.95 as the temperature was increased from 365 to 400 °C. In the absence of the catalyst, that is, in the presence of R-Al2O3, temperatures greater than 460 °C were required to obtain conversion greater than 0.9. In the presence of MnO2 catalyst, conversion above 0.85 was obtained at 420 °C. The catalyst support did not have any effect on the performance of the MnO2 catalyst as evidenced by the similar results for MnO2/γ-Al2O3 and MnO2/CeO2 catalysts. These results indicate that R-Al2O3 can be assumed to be inert toward SCWO of pyridine at the conditions of these experiments. The only gas-phase products observed were carbon dioxide, carbon monoxide, and nitrous oxide. The yields of these products increased with an increase in temperature. Carbon monoxide was not observed in the presence of Pt/γ-Al2O3 and MnO2/γ-Al2O3 catalysts; it was observed in the other two cases. R-Al2O3 catalyst yielded the maximum amounts of carbon monoxide, the yield of which increased with an increase in the temperature. The maximum yield of carbon dioxide was obtained in the presence of Pt catalyst. Small amounts of liquidphase organic products were observed in all cases and included methanol, acetone, and acetic acid. Nitrogen-containing products included nitrous oxide as well as nitrate and nitrite ions. The yield of nitrate ions was greater than that of nitrite ions. The nitrogen atom in the pyridine molecule was converted predominantly to nitrate in the presence of Pt catalyst. Product Distribution. In all cases, carbon dioxide was the most consistent and the dominant product observed. The absence of the catalyst led to the formation of several partial oxidation products. Carbon monoxide was present during homogeneous oxidation and in the presence of MnO2/CeO2 catalyst. In the presence of the platinum catalyst carbon monoxide was not
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 361
Figure 2. Effect of pyridine concentration and catalyst on the SCWO of pyridine: P ∼ 24.2 MPa; CO2 ∼ 0.1 mol/L; T ) 380 °C.
Figure 3. Effect of pyridine concentration and excess oxygen on conversion in the presence of Pt/γ-Al2O3 (dp ∼ 0.165 mm) and MnO2/CeO2 (dp ∼ 0.09 mm) at P ) 24.2 MPa.
observed. The carbon balance was in general above 0.9. The yields of nitrogen products were found to be dependent on the type of catalyst used. Nitrous oxide and nitrate ions were the major products during the oxidation using platinum catalyst. Nitrogen was present in trace levels, and in general the nitrogen balance was approximately 0.95. The addition of MnO2 catalysts decreased the yield of nitrous oxide. The major nitrogen containing products identified in this case are nitrate and nitrite ions. The nitrogen balance for the experiments using MnO2 catalyst was approximately 0.6. We assume that the remaining nitrogen was present as molecular nitrogen, because no other nitrogen-containing compounds were observed in either the liquid or vapor phases. The results from this work confirm the absence of undesirable products such as ammonia, nitrogen oxides, and hydrogen cyanide. The absence of these products separates SCWO from other treatment alternatives. As indicated earlier, incineration and wet air oxidation of N-heterocyclic compounds leads to the formation of ammonia, nitrogen oxides, and hydrogen cyanide. The absence of nitrogen oxides during SCWO is not surprising as the oxidation was carried out at much lower temperatures. At lower temperatures thermodynamic equilibrium favors the formation of nitrogen and nitrous oxide, with the selectivity to nitrogen oxides increasing with an increase in the temperature.31 The complete oxidation of ammonia during CSCWO of pyridine was reported.10,32 In both these reports, MnO2/CeO2 catalyst was used and the results indicate that ammonia can be oxidized effectively below 440 °C. The products from this reaction include nitrogen, nitrous oxide, and nitrate and nitrite ions. Also platinum is the catalyst of choice for the oxidation of ammonia. It is used in the commercial production of nitric acid from ammonia. Accordingly, the selectivity to nitrate ions was found to be greater than the selectivity of the nitrous oxide and nitrogen in the presence of platinum catalyst. Effect of Pyridine Concentration. The effect of pyridine concentration on the conversion of pyridine in the presence of several catalysts is shown in Figure 2. These results were obtained at 24.2 MPa pressure and 380 °C. The oxygen concentration was maintained at a constant level of approximately 0.1 mol/L. The results were obtained at very high excess oxygen. The effect of pyridine concentration on the conversion was dependent on the type of catalyst used. The concentration did not
have any effect on the conversion of pyridine in the presence of MnO2/γ-Al2O3 catalyst, suggesting that the rate of pyridine oxidation is first order with respect to pyridine. A small effect of concentration was obtained at low pyridine levels using MnO2/CeO2 catalyst. At higher concentration, the conversion of pyridine remained constant. These results again suggest that the rate of pyridine oxidation was close to first order with respect to pyridine. Comparable conversion was obtained in using both of these two catalysts. Conversion of pyridine was dependent on the pyridine concentration during oxidation in the presence of Pt/γAl2O3 catalyst. The conversion increased from 0.45 to 0.77 as the pyridine concentration was increased from 0.35 to 1.3 mmol/L, suggesting that the order of the reaction in pyridine concentration is greater than 1. Similar trends were observed at other temperatures. Higher conversions were obtained with Pt catalyst than with manganese catalyst even though a smaller amount of platinum catalyst was used. Effect of Oxygen Concentration. The effect of excess oxygen concentration on the SCWO of pyridine in the presence of Pt/γ-Al2O3 (at two different pyridine concentrations) and MnO2/CeO2 catalysts is shown in Figure 3. The effect of oxygen concentration in the presence of these two catalysts differed significantly. In both cases the excess oxygen was varied between 200 and 4500%. In the presence of MnO2/CeO2 catalyst the conversion of pyridine increased with an increase in the excess oxygen. The conversion of pyridine at 400 °C and at a pyridine concentration of 0.5 mmol/L increased from 0.4 to 0.77 as the excess oxygen was increased from 1000 to 2500%. An increase in the excess oxygen led to an increase in the yield of carbon dioxide and decreased the yield of carbon monoxide. The reaction led to very low yields of nitrous oxide. Neither ammonia nor nitrogen oxides were observed at all conditions. These results are consistent with the reaction mechanisms proposed for the oxidation of organic compounds in the presence of MnO2 catalyst. This catalyst belongs to a group of catalysts that do not have a metal-oxygen double bond. These catalysts adsorb considerable amounts of oxygen, and the adsorbed oxygen is the main active species for oxidizing the organic reactant.3 For complete oxidation processes, adsorbed oxygen is far more reactive than the lattice oxygen, as it can form an active species such as O3-.3 An increase in the concentration of oxygen in the fluid phase leads to an increase in the amount of adsorbed oxygen, accounting for the
362 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999
observed increase in the pyridine conversion with an increase in the excess oxygen. The effect of excess oxygen on the conversion of pyridine in the presence of Pt/γ-Al2O3 catalyst differed significantly from that of MnO2/CeO2 catalyst, as shown in Figure 3. The conversion of pyridine increased rapidly with an increase in the excess oxygen from 100 to 1000%. A further increase in the excess oxygen led to a decrease in the conversion of pyridine. The conversion of pyridine did not change significantly above 2500% excess oxygen. The yields of carbon dioxide and nitrous oxide also followed similar trends. Comparable results were observed at other temperatures. The carbon and nitrogen balances were in general greater than 0.9. The major nitrogen products in this case are nitrate ions and nitrous oxide. The presence of nitrogen at trace levels was determined by conducting experiments using an O2/ Ar gas mixture as the oxidant instead of air. One possible reason for the decrease in the pyridine conversion at high excess oxygen is the formation of undesired platinum oxide. At higher excess oxygen, the platinum metal may be reacting with oxygen to form platinum oxide. Platinum oxide should be less reactive than platinum itself; thus, very high levels of excess oxygen could lead to a decrease in the pyridine conversion. Regardless of the mechanism for inhibition by high oxygen concentration, these data indicate that our use of power-law kinetics should be limited to oxygen loading less than 1000% excess. Discussion The rate of reaction was assumed to follow simple power-law kinetics. The density of the supercritical fluid mixture was used to estimate the residence time, volumetric flow rate, and concentration of the reactants at the process conditions. The Peng-Robinson equation of state was used to estimate the density of the fluid at the reaction conditions. The constants in the PengRobinson equation of state were estimated for the water-pyridine-air mixture using van der Waals mixing rules. Model Development. The one-dimensional mass balance equation for pyridine in a packed-bed reactor under plug flow conditions is given as
-u
dCPyr + r′Pyr,observedFb ) 0 dz
(6)
This equation was developed assuming that the reactor was operated at isothermal and steady-state conditions and that there are no pressure gradients across the catalyst bed. It is common to assume Langmuir-Hinshelwood kinetics for a heterogeneous reaction. However, this requires certain assumptions about the underlying reaction mechanism that cannot be justified by our experimental data. Thus, we choose to model the observed rate of reaction, -r′Pyr,observed, in the form of power-law kinetics, providing
-r′Pyr,observed ) kCPyrmCO2n
(7)
The rate constant k is dependent on temperature, which was described using the Arrhenius law
k ) A exp(-Ea/RT)
(8)
Combining eqs 6-8 and on integration, we obtain
CPyr ) [CPyr,i1-m - (1 - m)A × exp(-Ea/RT)CO2nzFbu-1]1/(1-m); for m * 1 (9) and
CPyr ) CPyr,i exp[-A exp(-Ea/RT)CO2nFbzu-1]; for m ) 1 (10) In this derivation, it was assumed that the dispersion, DPyr-S, is negligible and the oxygen concentration was constant at the initial value. This is a fairly good assumption, because the experiments were conducted at high flow rates and high excess oxygen (>300%). These equations can be further simplified by expressing the concentration of pyridine in terms of conversion as
[
X ) 1 - 1 - (1 - m)A ×
( )
exp
]
-Ea m-1 C nC Wv-1 RT O2 Pyr,i
1/(1-m)
; for m * 1 (11)
and
(
( )
X ) 1 - exp -A exp
)
-Ea C nWv-1 ; for m ) 1 RT O2 (12)
Equations 9-12 provide a means of correlating the dependence of conversion on the temperature and pyridine and oxygen concentrations. Data taken under a wide range of experimental conditions were used to estimate the kinetic parameters A and Ea and the reaction orders m and n. A multivariable nonlinear regression technique was used to evaluate the constants. The best-fit values were obtained by minimizing the sum of squares error n
[XExptl - XPred]2 ∑ i)1
(13)
where XExptl refers to experimental measurement of conversion and XPred refers to the conversion obtained from solution of eq 11 or eq 12. The standard deviation, s, for a sample of N data points and p parameters can be obtained from
s)
x
[XExptl - XPred]2 N-p
(14)
and approaches zero as the quality of the model improves. Computations were performed using Matlab (MathWorks Inc.,), which has built-in algorithms to perform a nonlinear least-squares data fit by the Gauss-Newton method. The program also calculates the residuals and the Jacobian matrix at the solution. It has algorithms to estimate the 95% confidence interval on each parameter and on the predicted responses. A Matlab code was written to evaluate the parameters using these algorithms and the experimental data. The problem was initialized at randomly selected values, and then the best-fit values for the parameters as found by the Matlab program were used as the solution to the problem, if the solution converged. The best-fit values
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 363 Table 3. Summary of Pyridine Conversion from Homogeneous SCWO
Figure 4. Comparison between experimental and predicted conversion during the SCWO of pyridine in the presence of no packing, R-Al2O3, and γ-Al2O3. The rate expression developed in the present study was used to predict the results reported by Crain et al.25
were assumed to be the global minimum for the problem only if the same solution was obtained for different initial values. The predicted values and residuals and the Jacobian matrix were used to calculate the confidence intervals on the best-fit values and the predicted responses. Analysis of r- and γ-Al2O3 Packing. Results were obtained at three temperatures, four pyridine concentrations, and three oxygen concentrations using R-Al2O3 particles as the reactor packing. In all, 21 data points were obtained. These results were used to estimate the rate parameters according to eq 11, providing a rate expression for the SCWO of pyridine in the presence of R-Al2O3 particles
-r′Pyr,observed )
( 47.5 (-227.61 )C 8.314T
1014.32(3.6 exp
0.42(0.18 CO20.73(0.28 Pyr
(15) The standard deviation was found as 0.088, indicating the good quality of the model fit to the experimental data. This rate expression was used to predict the conversion and provides good agreement with the experimental data as shown in Figure 4. Houser et al.21 reported homogeneous oxidation of pyridine and found that the rate of pyridine oxidation was half-order in pyridine and first-order in oxygen. They also reported that the activation energy and the preexponential factor for this reaction were 226 ( 16.3 kJ/mol and 1014.9 ( 0.9 L/(mol s), respectively. These results compare well with the kinetic parameters obtained in the present work as shown in eq 15 and provide one piece of evidence that R-Al2O3 particles are inert toward the SCWO of pyridine. A second means of showing that R-Al2O3 was inert was accomplished by comparing the results from R-Al2O3 with those obtained in the absence of any packing material. For this purpose, experiments were performed in a 34 cm long SS-316 reactor without any packing. This gave a residence time of 1 s. The results are given in Table 3. The effect of pyridine concentration was obtained at a temperature of 460 °C. The conversion was less than 0.1 at all of the conditions studied, substantially less than that obtained in the presence of R-Al2O3 particles. However, the empty tube was run with a residence time of only 1 s, compared to the residence time of 14 s for R-Al2O3 particles. Despite the differences in residence times, eq 15 was used to compare all of the
T, °C
packing
CPyr, mmol/L
CO2, mol/L
τ, s
X
461.5 459.8 460.3 396 421 440 420 421 440 460
none none none γ-Al2O3 γ-Al2O3 γ-Al2O3 R-Al2O3 R-Al2O3 R-Al2O3 R-Al2O3
2.45 3.73 9.34 2.92 2.86 3.12 2.06 1.83 1.98 2.33
6.39 × 10-2 7.27 × 10-2 8.60 × 10-2 1.40 × 10-1 1.40 × 10-1 1.34 × 10-1 1.66 × 10-1 8.48 × 10-2 7.30 × 10-2 6.60 × 10-2
1.04 1.00 1.21 2.38 2.30 2.39 15.02 13.40 14.42 14.21
0.06 0.04 0.08 0.03 0.04 0.07 0.18 0.05 0.56 0.65
results as shown in Figure 4 and adequately predicted the results obtained with the empty tube. This result confirms the inertness of R-Al2O3 particles toward the SCWO of pyridine. Experiments were also performed in the presence of γ-Al2O3 to verify its inertness toward the SCWO of pyridine. A residence time of approximately 2.5 s was obtained with a 14 cm long reactor. These data are also summarized in Table 3. The conversion increased from 0.03 to 0.07 as the temperature was increased from 400 to 440 °C. These values were comparable to that observed with the empty tube; the slightly higher levels in this case may be attributed to the longer residence time. Regardless, this low conversion is within the limits of our experimental error. As shown in Figure 4, these results were also predicted well by eq 15, confirming the inertness of γ-Al2O3. Finally the results obtained in the present study were compared with those reported by Crain et al.,25 who studied the SCWO of pyridine in a plug-flow reactor at temperatures between 426 and 527 °C and residence times between 2 and 11 s. In these experiments, the oxygen to pyridine molar ratio was varied between 1 and 2. These authors were unable to obtain convergence using a model similar to eq 11; however, they suggested that the first-order model (eq 12) provided an adequate fit to their data. Equation 15 was used to predict the conversion using the data reported by Crain et al.,25 and the comparison between the experimental and predicted conversion is shown in Figure 4. When the model of eq 15 was compared with the experimental data of Crain et al.,25 eq 15 consistently underpredicted the conversion. This may in part be due to the assumptions in the derivation of eq 15, where it was assumed that the oxygen concentration is constant; this is not true for the experiments reported by Crain et al.25 However, an alternative explanation is the literature suggestion that Hastelloy C-276 walls may have a catalytic effect on the reaction rate,10 especially at higher temperatures. The total reactor wall surface from Crain’s experiments was much higher (305-871 cm2) than those used in the current work (21.67 and 46.85 cm2). Catalysis by the reactor wall could account for the higher conversions observed by Crain et al.25 Stability of the Catalysts. One potential drawback of the addition of a heterogeneous catalyst is the stability of the catalyst at these extreme conditions. Two different types of experiments were conducted to verify the stability of the catalysts used in the present study. The catalyst can be considered stable if the conversion of pyridine does not change with time on stream and also if the conversion of pyridine at a given set of process conditions does not change when repeated after a known time interval.
364 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 Table 4. Stability of the Catalyst during the SCWO of Pyridine (P ∼ 24.2 MPa; CO2 ∼ 0.1 mol/L) T, °C
CPyr, mmol/L
W, g of catalyst
X
t, h
expt no.
0 13 58.5 102 9 67
1 2 4 8 1 5
390 390 390 390 380 380
2.7 2.7 0.8 0.8 0.5 0.5
Pt/γ-Al2O3 2.0801 0.89 2.0801 0.92 0.1014 0.76 0.1014 0.81 0.1014 0.51 0.1014 0.49
400 400
0.55 0.55
MnO2/CeO2 1.2636 0.77 1.2636 0.77
16.5 23.2
1 2
400 400 390 390 390 390 390 390
1.08 1.08 0.8 0.8 0.8 0.8 0.8 0.8
MnO2/γ-Al2O3 7.6522 0.68 7.6522 0.34 7.6522 0.67 7.6522 0.33 7.6522 0.39 7.6522 0.32 7.6522 0.32 7.6522 0.33
0 19 4 30.8 31.2 32.1 32.7 33.3
2 3 1 2 2 2 2 2
Figure 5. Stability of the catalysts used for the SCWO of pyridine at T ∼ 400 °C and P ∼ 24.2 MPa. MnO2/CeO2 catalyst: W ) 0.882 g; τ ) 3.3 s; dp ) 0.09 mm. MnO2/γ-Al2O3 catalyst: W ) 0.588 g; τ ) 3.3 s; dp ) 0.165 mm.
The stability of the Pt/γ-Al2O3 catalyst was confirmed by repeating certain data points after a known time interval as given in Table 4. Three selected data points were repeated at various time intervals. These results were obtained from different experiments. The experiment number, given in Table 4, corresponds to the number of times the reactor was started and stopped. The conversion of pyridine did not change in any case. For example, the conversion of pyridine did not change when the datum point taken at 380 °C and 0.5 mmol/L was repeated after 57 h. These results were obtained in experiments 1 and 5 (i.e., after four shutdowns and startups of the reactor system). Similar results were obtained at other conditions. These results confirm the stability of the Pt/γ-Al2O3 catalyst. The stability of the MnO2/CeO2 catalyst was confirmed by monitoring the conversion of pyridine as a function of time on stream at a set of process conditions and also repeating a datum point after a known time. The conversion of pyridine at 400 °C and 0.55 mmol/L was monitored as a function of time on stream as shown in Figure 5. The conversion did not change in 16.5 h, confirming the stability of this catalyst during the length of a typical experiment. In addition, results shown in Table 4 indicate that the conversion did not change after a complete shutdown and startup cycle. Along with the conversion, the yields of the various products remained constant. These two individual tests confirmed the stability of this particular catalyst. These two types of experiments were conducted to verify the stability of the MnO2/γ-Al2O3 catalyst. The
conversion of pyridine at 400 °C and a pyridine concentration of 1.5 mmol/L was obtained as a function of time on stream. These results are shown in Figure 5. The conversion did not change significantly in 15.5 h, indicating that this catalyst is stable within a single run. However, after the reactor was shut down and restarted at identical conditions, the conversion changed as shown in Table 4. For example, the conversion at 400 °C and 1.08 mmol/L, decreased from 0.68 to 0.34 when repeated after 19 h. These points were obtained in experiments 2 and 3, between which the system was stopped and started again. When fresh catalyst was used (experiment 1), the conversion at 400 °C and at a pyridine concentration of 0.35 mmol/L was 0.85. That is, the conversion of pyridine decreased from 0.85 to 0.34 within three experiments. On the other hand, the conversion did not change within an experimental run. The conversion of pyridine at 390 °C and a pyridine concentration of 0.8 mmol/L remained constant at approximately 0.35 for 2 h. These data were obtained in experiment 2, and the conversion was lower than that obtained in the first experiment. Therefore, it can be concluded that the catalyst was becoming deactivated during either the startup or shutdown process. Note that the results in Figures 1 and 2 were obtained from experiment 1, that is, before the catalyst was deactivated. The catalyst bed was removed from the reactor after completion of this series of experiments. The catalyst particles did not come out of the reactor freely. Instead, lumps of catalyst particles were observed. This suggests some type of catalyst agglomeration might be taking place in the reactor, which decreases the amount of active catalyst for each successive experiment. This would lead to a decrease in the conversion of pyridine with each successive experiment. To provide an engineering description of deactivation, the deactivation of MnO2/γ-Al2O3 catalyst was modeled assuming power-law dependence
-da/dσ ) kday
(16)
where a is the activity of the catalyst for a given experiment number, σ. Assuming an unit initial activity and integrating eq 16 yield
a ) (1 + (y - 1)kd(σ - 1))1/(1-y)
(17)
The rate of pyridine oxidation in the presence of catalyst deactivation is given as
-r′Pyr,observed ) -dFPyr,i/dW ) akCPyrmCO2n (18) Analysis of the data obtained from the fresh MnO2/γAl2O3 catalyst (see Figure 2) revealed that the reaction was first order in pyridine (m ) 1). Data for MnO2/CeO2, a stable catalyst for which we obtained oxygen concentration dependence data (see eq 23), indicate that oxidation of pyridine over MnO2 was also first order with respect to oxygen (n ) 1). With these simplifications, integration of eq 18 provides
(
X ) 1 - exp -akCO2
)
WCPyr,i FPyr,i
(19)
The kinetic parameters and the activity for this catalyst were estimated using eqs 17 and 19. Data were obtained
Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999 365
at four temperatures, four pyridine concentrations, and three catalyst weights and include multiple experimental cycles. In all, 35 data points were used to estimate the constants present in eqs 17 and 19. The rate of pyridine oxidation in the presence of MnO2/γ-Al2O3 catalyst is described as
-r′Pyr,observed )
( 25.11 (-190.85 )C 8.314T
a × 1011.66(1.96 exp
PyrCO2
where a ) (1 + 1.675(σ - 1))-0.653
(20) (21)
The standard deviation was estimated to be 0.072. Equation 19 was used to predict the conversion using these parameters and was compared with the experimental values. As shown in Figure 6, a good comparison was obtained between the experimental and predicted conversion. It is important to note that the same catalyst support was used for the manganese and platinum catalysts, but Pt/γ-Al2O3 was found to be stable over the course of all experiments, as shown in Table 4. However, in the case of platinum, the active catalyst was substantially diluted with R-Al2O3 particles, which may have prevented the agglomeration of the catalyst particles in the reactor. As shown in Table 2, the active catalyst made up only 2 wt % of the total catalyst packing. Also, it has been reported that oxidation conditions can be used to redisperse platinum metal on the support.34 The dispersion of platinum metal in the presence of oxygen increases with an increase in the temperature from 400 to 600 °C. The redispersion of Pt metal on the support may have helped to maintain the catalyst activity at a constant level, even while the catalyst support underwent a phase transition. This may account for the observed stability of the Pt/γ-Al2O3 catalyst. Comparison of Kinetics for Stable Catalysts. The effect of temperature and reactant concentration along with the weight of catalyst on the CSCWO of pyridine was studied. These results were used along with eq 11 to estimate the kinetic parameters for each catalyst by minimizing the sum of squares error. For the platinum catalyst 80 data points (three catalyst weights, five temperatures, and a wide range of pyridine and oxygen concentrations) were obtained and the rate of oxidation of pyridine in the presence of this catalyst was best described by
-r′Pyr,observed ) 1025.3(6.7 × -287.26 ( 75.61 exp CPyr2.25(0.18CO20.43(0.28 (22) 8.314T
(
)
The standard deviation was 0.088, and these kinetic parameters were used along with eq 11 to predict the conversion of pyridine. The predicted conversion was compared with the experimental conversion, and a good agreement was obtained as shown in Figure 7. The rate of pyridine oxidation in the presence of MnO2/CeO2 catalyst was obtained as
-r′Pyr,observed ) 1014.25(2.5 × -196.42 ( 27.8 CPyr1.12(0.13CO21.14(0.23 (23) exp 8.314T
(
)
This equation was obtained using 74 data points (four temperatures, four pyridine concentrations, and three
Figure 6. Parity plot comparing the model of eqs 20 and 21 with experimental data for the CSCWO of pyridine on MnO2/γ-Al2O3 catalyst.
Figure 7. Parity plot comparing model prediction with experimental data for the CSCWO of pyridine on Pt/γ-Al2O3 catalyst.
Figure 8. Parity plot comparing model equation with experimental data for the CSCWO of pyridine on MnO2/CeO2 catalyst.
oxygen concentrations), and the standard deviation was estimated as 0.073. The kinetic parameters given in eq 23 were used to estimate conversion of pyridine at the experimental conditions. The predicted conversion thus estimated was compared with experimental conversion. As shown in Figure 8, a good comparison was obtained. Equation 22 indicates that the rate of pyridine oxidation in the presence of platinum catalyst is second order in pyridine, whereas it was found to be close to first order in the presence of MnO2/CeO2 catalyst. Accordingly, the activation energy in the presence of platinum catalyst was found to be greater than that for the MnO2/ CeO2 catalyst. For a second-order reaction the rate of oxidation increases with an increase in the organic concentration; therefore, the platinum catalyst will be more effective than the manganese catalyst in treating
366 Ind. Eng. Chem. Res., Vol. 38, No. 2, 1999
wastes with higher concentrations of organic contaminants. The activation energy for the platinum catalyst indicates that the rate of oxidation increases rapidly with a small increase in the temperature. As the rate of pyridine oxidation increases rapidly in the presence of platinum catalyst with a small increase in the temperature, the reaction may become mass-transferlimited at temperatures lower than that of MnO2/CeO2. The absence of intermediate oxidation products limited our ability to predict a reaction pathway for the CSCWO of pyridine. However, these results are consistent with those obtained during the photocatalytic oxidation of pyridine. It was suggested that the intermediates formed during the photocatalytic oxidation are more reactive than pyridine itself.13 It was reported that the pyridine is first converted to 2-hydroxypyridine, which is further oxidized to form complete oxidation products.13,25 Ding3 suggested the formation of hydroxy compounds as the first step during the CSCWO of benzene and phenol. Therefore, 2-hydroxypyridine can be assumed to be the first product formed during the CSCWO of pyridine in the presence of MnO2/CeO2 catalyst. This compound is further oxidized on the catalyst surface to form aromatic ring-opening products that are oxidized to carbon dioxide, nitrogen, and nitrous oxide. These results indicate that the oxidation of aromatic organic compounds with different functional groups follows similar reaction pathways. Also the reaction pathways suggested by Ding3 could be extended to describe the SCWO of pyridine in the presence of MnO2/CeO2 catalyst. This mechanism is consistent with the first-order reaction that was found to describe the oxidation of pyridine in the presence of MnO2/CeO2 catalyst. The kinetic parameters (eq 22) suggest that the SCWO of pyridine in the presence of Pt/γ-Al2O3 catalyst proceeds through a mechanism that is different from the one proposed for the MnO2/CeO2 catalyst. The second order dependence on pyridine suggests the formation of dimerization products on the catalyst surface prior to the complete oxidation of pyridine. Primet et al.35 reported the formation of 2,2′-dipyridyl during the chemisorption of pyridine on Pt/γ-Al2O3 catalyst. A mechanism that involves the formation of a dimerization product on the catalyst surface as the first step is consistent with both our results and previous data. The formation of dimerization product requires two pyridine molecules to be adsorbed on the catalyst surface, consistent with the second-order dependence on pyridine concentration obtained from our power-law analysis. This product then reacts with either adsorbed oxygen or oxygen present in the bulk phase to form ringopening products, which are then oxidized to form complete oxidation products. Conclusions The addition of the heterogeneous catalyst increased the efficiency of the SCWO process in oxidizing pyridine. Results confirmed the inertness of R-Al2O3 and γ-Al2O3 toward the oxidation of pyridine. Among the catalysts studied, the platinum catalyst was found to be the most effective catalyst. Complete conversion of pyridine can be obtained at temperatures as low as 370 °C. No partial oxidation products were observed. The platinum catalyst favored the formation of nitrous oxide and nitrate ions as the nitrogen-containing products. Nitrogen and nitrate ions were the preferred products in the presence of the manganese catalyst.
The stability of the catalysts was verified experimentally. The results indicate that the MnO2/γ-Al2O3 catalyst was unstable at these extreme conditions as a result of agglomeration of the γ-Al2O3 particles. This was not a problem with Pt//γ-Al2O3, presumably because of the high diluent concentration and the ability of platinum to redistribute on the support during oxidation. The deactivation of MnO2/γ-Al2O3 catalyst was modeled using a power-law deactivation model. Results were modeled assuming power-law kinetics. The order of the reaction and the activation energy were dependent on the type of catalyst used. The rate of pyridine oxidation on Pt/γ-Al2O3 catalyst was described by a second-order reaction, whereas it was found as first order in the presence of manganese catalyst. These observations reveal that the reaction mechanism for the oxidation of pyridine in SCW was also dependent on the type of catalyst. The redox mechanism was able to describe the oxidation of pyridine on MnO2 catalyst. However, the formation of a dimerization product on the catalyst surface as the first step was proposed to describe the second-order behavior of the pyridine oxidation in the presence of platinum catalyst. The proposed mechanisms are similar to those used to describe catalytic oxidation in the gas phase, indicating the potential to extrapolate gas-phase catalytic data to supercritical conditions. Acknowledgment This research has been supported by the National Science Foundation under Grant No. CTS-9796060. Nomenclature a ) activity of the catalyst A ) preexponential factor, (mol/g of catalyst s)(L/mol)m+n CO2 ) concentration of oxygen, mol/cm3 CPyr ) concentration of pyridine, mol/cm3 CPyr,i ) initial concentration of pyridine, mol/cm3 DPyr-S ) dispersion coefficient of pyridine in solvent, cm2/s Ea ) activation energy, kJ/mol F ) molar flow rate, mol/s k ) rate constant, (mol/g of catalyst s)(L/mol)m+n kd ) deactivation rate constant N ) number of data points m ) order of the reaction with respect to pyridine n ) order of the reaction with respect to oxygen p ) number of parameters R ) gas constant, 8.314 J/(mol K) -r′Pyr,observed ) observed rate of reaction, mol/g of catalyst s s ) standard deviation T ) temperature, K u ) superficial velocity, cm/s v ) volumetric flow rate of the fluid at reaction conditions, cm3/s W ) weight of the catalyst, g X ) conversion of pyridine yi ) yield of product i z ) length of the catalyst bed, cm Fb ) bulk density of the catalyst, g/cm3 σ ) experiment number
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Received for review July 27, 1998 Revised manuscript received November 2, 1998 Accepted November 9, 1998 IE980485O
