Kinetics of the Complete Combustion of Dilute Propane and Toluene

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Energy & Fuels 2005, 19, 54-63

Kinetics of the Complete Combustion of Dilute Propane and Toluene over Iron-Doped ZrO2 Catalyst V. R. Choudhary,*,†,‡ G. M. Deshmukh,‡ and D. P. Mishra‡ Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India, and Dr. Babasaheb Ambedkar Technological University, Lonere (Raigad) 402103, India Received May 1, 2004. Revised Manuscript Received August 20, 2004

The kinetics of the complete combustion of propane and toluene at very low concentrations in air (0.45 and 0.3 mol % in air, respectively) over iron-doped ZrO2 (cubic) catalyst (Fe/Zr ) 0.25) at different temperatures (598-723 K) in kinetic control regime have been investigated. The combustion rate data could be fitted very well to both the power-law and redox (Mars-Van Krevelen) models. However, the redox model provided a better fit to the kinetic data for the propane combustion. It also showed a better fit to the toluene combustion data at the lower temperatures (623 K). The reaction order (with respect to the hydrocarbon), apparent activation energy, and frequency factor (from the power-law model) for the propane combustion were 0.96 (average), 21.16 kcal/mol, and 4.67 × 105 mol g-1 h-1 kPa-n, respectively, and those for the toluene combustion were 0.77 (average), 26.08 kcal/mol, and 1.48 × 107 mol g-1 h-1 kPa-n, respectively.

1. Introduction Propane is less reactive than butanes and, hence, the concentration of the former is much higher in the unburned hydrocarbons present in the exhaust of liquefied petroleum gas (LPG)-fueled engines. Also, among the aromatic hydrocarbons, toluene is a commonly used solvent in chemical and processing industries. Direct release of the hydrocarbons in the atmosphere is hazardous, because of their toxic nature, malodor, and/ or photochemical smog resulting from their photochemical reaction by sunlight with nitrogen oxides (NOx) and other airborne chemicals. Hence, the hydrocarbon emissions need to be controlled by their complete combustion to harmless or less-harmful products (e.g., water (H2O) and carbon dioxide (CO2)). Several studies have been reported in the literature on the catalytic combustion of propane over supported noble-metal catalysts,1-6 Mn3O4,7 Cr2O3 and Co3O4Cr2O3,8 SiO2-fiber-supported oxides of cobalt, nickel, manganese, and chromium9 and manganese-, cobalt-, chromium-, iron-, and nickel-doped ZrO210 and also on * Author to whom correspondence should be addressed. Telephone: +91-20-5890765. Fax: +91-20- 5893041/5893355. E-mail: vrc@ ems.ncl.res.in/[email protected]. † National Chemical University. ‡ Dr. Babasaheb Ambedkar Technological University. (1) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Appl. Catal., A 2002, 234, 1-23. (2) Ishikawa, A.; Komai, S.; Satsuma, A.; Hattori, T.; Murakami, Y. Appl. Catal., A 1994, 110, 61-66. (3) Krauns, C.; Barelko, V.; Fabre, G.; Tredicce, J.; Krinsky, V. Catal. Lett. 2001, 72, 161-165. (4) Wampler, F. B.; Clark, D. W.; Gaines, F. A. Combust. Sci. Technol. 1976, 14, 25-31. (5) Bruno, C.; Walsh, P. M.; Santavicca, D. A.; Sinha, N.; Yaw, Y.; Bracco, F. V. Combust. Sci. Technol. 1983, 31, 43-74. (6) Neyestanaki, A. K.; Narendra, K.; Lindfors, L. Fuel 1995, 74, 690-696. (7) Baldi, M.; Finocchio, E.; Milella, F.; Busca, G. Appl. Catal., B 1998, 16, 43-51. (8) Ravi, P.; Kennedy, L. A.; Eli, R. Combust. Sci. Technol. 1980, 22, 271-280.

the catalytic combustion of toluene over supported noble metals,11-17 supported cobalt, copper, iron, and manganese oxides,18 CuNaHY and CuY,19,20 Fe2O3-supported gold, silver, and copper catalysts,21,22 and copper, magnesium, and aluminum hydrotalcities.23 However, detailed kinetic studies that have been reported on the combustion of propane and toluene are very few (viz., the combustion of propane over Pt/Al2O3,24 Pt/ZrO2,25 and La0.66Sr0.34Ni0.3Co0.7O3 pervoskite oxide26 and the combustion of toluene over Pt/Al2O327,28). Over the Pt/ (9) Neyestanaki, A. K.; Lindfors, L. E. Combust. Sci. Technol. 1995, 110, 303-320. (10) Choudhary, V. R.; Banerjee, S.; Pataskar, S. G. Submitted to Appl. Catal., A. (11) Paulis, M.; Gandia, L. M.; Gil, A.; Sambeth, J.; Odriozola, J. A.; Montes, M. Appl. Catal., B 2000, 26, 37-46. (12) Noordally, E.; Richmond, J. R.; Tahir, S. F. Catal. Today 1993, 17, 359-366. (13) Paulis, M.; Peyrard, H.; Montes, M. J. Catal. 2001, 199, 3040. (14) Hua, W.; Gao, Z. Catal. Lett. 1996, 42, 209-212. (15) O’Malley, A.; Hondett, B. K. Stud. Surf. Sci. Catal. 1997, 110, 1137-1144. (16) Gina, P.; Patricio, R.; Aljandra, F.; Fierro, J. L. G. Bol. Soc. Chil. Quim. 2000, 45, 213-218. (17) Pina, M. P.; Irusta, S.; Menendez, M.; Santamaria, J.; Hughes, R.; Boag, N. Ind. Eng. Chem. Res. 1997, 36, 4557-4566. (18) Larrson, P. O.; Berggren, H.; Andersson, A.; Augustsson, O. Catal. Today 1997, 35, 137-144. (19) Antunes, A. P.; Ribeiro, M. F.; Silva, J. M.; Ribeiro, F. R.; Magnoux, P.; Guisnet, M. Appl. Catal., B 2001, 33, 149-164. (20) Antunes, A. P.; Silva, J. M.; Ribeiro, M. F.; Ribeiroa, F. R.; Magnoux, P.; Guisnet, M. Stud. Surf. Sci. Catal. 2001, 135, 50375044. (21) Salvatore, S.; Simona, M.; Carmelo, C.; Signorino, G. Catal. Commun. 2001, 2, 229-232. (22) Minico, S.; Scire, S.; Crisafulli, C.; Maggiore, R.; Galvagno, S. Appl. Catal., B 2000, 28, 245-251. (23) Kovanda, F.; Jiratova, K.; Rymes, J.; Kolousek, D. Appl. Clay Sci. 2001, 18, 71-80. (24) Ma, L.; Trimn, D. L.; Jiang, C. Appl. Catal., A 1996, 138, 275283. (25) Yazawa, Y.; Kagi, N.; Komai, S.-I.; Satsuma, A.; Murakami, Y.; Hattori, T. Catal. Lett. 2001, 72, 157-160. (26) Song, K. S.; Klvana, D.; Kirchnerova, J. Appl. Catal., A 2001, 213, 113-121. (27) Mazzarino, I.; Barresi, I. I. Catal. Today 1993, 17, 335-348.

10.1021/ef0498871 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/09/2004

Combustion of Dilute Propane and Toluene

Energy & Fuels, Vol. 19, No. 1, 2005 55

Al2O3 catalyst,24 the propane combustion could be described well by the Langmuir-Hinshelwood model. However, for the La0.66Sr0.34Ni0.3Co0.7O3 catalyst, the best fit for the propane combustion data was obtained with the Mars-van Krevelen kinetic model.26 For the toluene combustion over a Pt/Al2O3 catalyst,27,28 the Mars-van Krevelen kinetic model could be fitted to the combustion kinetic data. The present work was undertaken with the objective of studying the kinetics of the combustion of propane and toluene at low concentration (0.45 and 0.3 mol % in air, respectively) over the iron-doped ZrO2 (cubic) catalyst at different temperatures (598-723 K). Efforts have been made to fit the combustion rate data to power-law, redox (Mars-van Krevelen), and EleyRideal rate models. 2. Experimental Section The iron-doped ZrO2 (cubic) catalyst was prepared by coprecipitating mixed zirconium and iron hydroxides from a mixed aqueous solution of zirconyl nitrate and ferric acetate, with a Fe/Zr ratio of 0.25. Tetramethylammonium hydroxide (TMAOH, 25%) was used as the precipitating agent, under vigorous stirring at room temperature (300 K) and a pH of 8, washing (with deionized water) and drying (at 382 K for 2 h) the resulting precipitate and then calcining it in air at 773 K for 8 h. The calcined mass was powdered, palletized without binder and crushed to particles with a size of 100-120 mesh. The iron-doped ZrO2 catalyst is characterized, in regard to its surface area, by the single-point Brunauer-Emmett-Teller (BET) method, using a Monosorb surface area analyzer (Quantachrome Corp., USA), and, in regard to its crystalline phases, via X-ray diffractometry (XRD) (using a model PW/1730 X-ray generator (Philips, Eindhoven, The Netherlands) with CuKR radiation). The catalyst is also characterized by its temperature-programmed reduction (TPR) by H2 from 373 to 873 K, with a linear heating rate of 10 K/min in a flow of H2-argon (3.7 mol % in H2) mixture (space velocity of 30 600 cm3 g-1 h-1) in a quartz reactor (inner diameter of i.d. ) 4 mm) that had a low dead volume. The hydrogen consumed in the TPR of the catalyst is measured quantitatively with a thermal conductivity detector (TCD). Before the TPR, the catalyst is pretreated in a flow of nitrogen (30 mL/min) at 873 K for 1 h. The catalytic combustion of propane (0.45 mol % in air) and toluene (0.3 mol % toluene in air) over the iron-doped ZrO2 catalyst was conducted at atmospheric pressure in a continuous fixed-bed quartz microreactor (i.d. ) 10 mm) containing 0.1 g of catalyst (particle size of 100-120 mesh) mixed uniformly with 0.4 g of R-alumina particles (particle size of 100-120 mesh) at different temperatures (598-723 K) and space velocities (25 000-100 000 cm3 g-1 h-1). The space velocity was measured at a temperature of 273 K and a pressure of 1 atm. The diluted gaseous toluene reactant feed was obtained by mixing an air stream, which was saturated with the reactant at a temperature of an ice bath (ice-water slurry) (274 K), with another air stream such that a desired concentration of the reactant in air could be obtained. The flow rates of both the air streams were controlled using digital differential pressure flow controllers (the air used was zero grade, free from moisture and hydrocarbons). The reaction temperature was measured/controlled by a chromel-alumel thermocouple that was located in the catalyst bed, as shown in Figure 1. The products (after cooling to the room temperature) were analyzed by an online gas chromatograph with thermal conductivity and flame ionization detectors, using a model SE-30 column for the analysis of unreacted hydrocarbon and the partial oxidation products and a Poropack-Q column (28) Ordonez, S.; Bello, L.; Sastre, H.; Rosal, R.; Diez, F. V. Appl. Catal., B 2002, 38, 139-149.

Figure 1. Schematic diagram of the quartz reactor used in the catalytic combustion of propane and toluene.

Figure 2. X-ray diffraction (XRD) spectra of the iron-doped ZrO2 (Fe/Zr ) 0.25) catalyst. (with TCD) for the analysis of carbon monoxide (CO) and CO2. No formation of CO or other partial oxidation products (oxygenates) in the combustion of propane and toluene under the reaction conditions used in the present work was detected.

3. Results and Discussion 3.1. Catalyst Characterization. The surface area of the catalyst was 102 m2/g. The catalyst has also been characterized using X-ray diffractometry (XRD) (Figure 2) and TPR (Figure 3). The XRD data (Figure 2) show the presence of only the cubic ZrO2 phase in the catalyst. The absence of ferric oxide phase indicates the complete doping of iron in the ZrO2 of the catalyst. The amount of H2 consumed in the TPR analysis (Figure 3) indicated that the degree of catalyst reduction, according to the reactions

Fe2O3 + H2 f 2FeO + H2O

(1)

FeO + H2 f Fe + H2O

(2)

is 20.6% for the reduction of Fe2O3 to FeO (reaction 1) and 6.8% for the reduction of Fe2O3 to Fe (reactions 1

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Choudhary et al.

Figure 3. Temperature-programmed reduction (TPR), by H2, of the iron-doped ZrO2 (Fe/Zr ) 0.25) catalyst.

and 2). The observed small degree of catalyst reduction further confirms the doping of iron in the ZrO2. The catalyst reduction occurs in two stages, as indicated by the two TPR peaks: the large TPR peak at 673 K is followed by the small TPR peak at 873 K. The first TPR peak is expected because of reduction of the ferric oxide close to the surface of the catalyst. However, the second TPR peak (at the higher temperature) is attributed most probably to the hydrogen consumed in the reaction with subsurface lattice oxygen, which is migrated at the higher temperatures from the interior to the surface of the catalyst. 3.2. Fittings of Kinetic Data. The kinetic data for the complete combustion of propane (0.45 mol % in air) and toluene (0.3 mol % in air),

C3H8 + 5O2 f 3CO2 + 4H2O

(3)

C6H5CH3 + 9O2 f 7CO2 + 4H2O

(4)

over the iron-doped ZrO2 catalyst at different temperatures are presented in Figures 4 and 5, respectively.

The ratio of weight of catalyst to flow rate of hydrocarbon (W/F) is based on the hydrocarbon in the feed. To avoid the formation of hot spots in the catalytic bed, the catalyst (0.1 g) was diluted with the inert solid particles (diluent/catalyst weight ratio of 4), so that the heattransfer area of the catalyst bed is increased. When the catalyst (0.1 g) without dilution was used, a higher conversion was observed in all cases. An increase in the diluent/catalyst ratio from 4 to 6 had no significant effect on the conversion. An increase in the catalyst particle size from 100-120 mesh to 60-80 mesh size had no significant effect on the conversion (at the highest temperature) in both cases, indicating no significant effect of both the external (film diffusional and pore diffusional) mass transfer on the combustion in both the cases. To be on the safe side, the catalyst with the smaller particle size was used for collecting the kinetic data. The curves of X versus W/F (see Figures 4 and 5) were fitted to the following expression:

X ) a + b(W/F) + c(W/F)2 + d(W/F)3

(5)

via the linear regression method. When differentiated, with respect to the W/F value of eq 5, we get

dX ) b + 2c(W/F) + 3d(W/F)2 d(W/F)

(6)

where dX/d(W/F) is the rate of combustion. The kinetic rate data (reaction rate at different partial pressures of the hydrocarbons) have been obtained from eq 6 and the fractional conversion at the respective W/F values (see Figures 4 and 5). The oxygen in the reaction mixture was in large excess and, hence, the variation in its concentration due to the combustion was quite small in all cases. Efforts were made to fit the kinetic data to the powerlaw, redox (Mars-van Krevelen), and Eley-Rideal rate models, as follows.

Figure 4. Plots of conversion versus W/F for the combustion of propane at different temperatures.



Figure 5. Plots of conversion versus W/F for the combustion of toluene at different temperatures.

Figure 6. Plots of ln r vs ln P for the combustion of propane at different temperatures.

3.2.1. Power-Law Rate Model. The kinetic data for the combustion of propane and toluene could be fitted very well to the power-law mode:30

r ) kP nA

(7)

where r is the combustion rate, k the apparent rate constant, PA the partial pressure of propane or toluene, and n the reaction order (with respect to propane or toluene). The plots of ln r versus ln PA in both cases were linear for the combustion of propane at 623 and 673 K and for that of toluene at all temperatures (29) Golodets, G. J. Heterogeneous Catalytic Reactions Involving Molecular Oxygen; Elsevier: Amsterdam, 1983; pp 126-140. (30) Hermia, J.; Vigneron, S. Catal. Today 1993, 17, 349-358.

(Figures 6 and 7). The kinetic parameters (k and n) of the power-law model for both cases, along with the residual sum of squares (RSS) and the mean residual sum of squares (MRSS), are given in Table 1. For the combustion of propane and toluene, the average reaction order n is determined to be 0.95 and 0.77, respectively. The activation energy for the combustion of the propane or toluene has been obtained from the temperature dependence of k (Figure 8), according to the Arrhenius equation:

(

k ) A exp -

E RT

)

(8)

where A is the frequency factor, E the activation energy,

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Choudhary et al.

Figure 7. Plots of ln r vs ln P for the combustion of toluene at different temperatures. Table 1. Kinetic Parameters of the Power-Law Model for the Combustion of Propane and Toluene temp, T (K)

rate constant, k (mol g-1 h-1 kPa-n)

623 673 723

0.018 0.064 0.190

598 623 648 673

0.0046 0.011 0.032 0.042

a RSS ) ∑[r(observed) - r(estimated)]2. freedom.

b

residual sum of squares, RSSa

mean residual sum of squares, MRSSb

Propane Combustion 0.93 1.04 0.90

2.62 × 10-9 2.03 × 10-6 3.75 × 10-4

2.91 × 10-10 2.26 × 10-7 4.16 × 10-5

Toluene Combustion 0.72 0.72 0.89 0.73

1.49 × 10-8 1.85 × 10-7 1.51 × 10-7 7.76 × 10-7

1.87 × 10-9 2.32 × 10-8 1.88 × 10-8 9.71 × 10-8

reaction order, n

MRSS ) ∑[r(observed) - r(estimated)]2/(n - 1), where n represents the degrees of

Figure 8. Arrhenius plots (ln k versus 1/T) for the combustion of propane and toluene (for the power-law model).



Figure 9. Plots of robserved vs restimated (power-law model) for the combustion of propane at (0) 623 K, (4) 673 K, and (O) 723 K.

Figure 10. Plots of robserved vs restimated (power-law model) for the combustion of toluene at (0) 598 K, (4) 623 K, (O) 648 K, and (]) 673 K.

R the gas constant, and T the temperature. The values of the apparent activation energy E and frequency factor A for the combustion of propane and toluene are given in Table 2. A comparison of the results in Table 2 showed that the values of both E and A are higher for the combustion of toluene. Table 2. Arrhenius Parameters (Obtained from the Power-Law Model) for the Combustion of Propane and Toluene reaction

activation energy, E (kcal/mol)

frequency factor, A (mol g-1 h-1 kPa-n)

propane combustion toluene combustion

21.16 26.08

4.67 × 105 1.48 × 107

The plots of robserved versus restimated (from the powerlaw model) in Figures 9 and 10, for the combustion of propane and toluene, respectively, clearly shows a good fit of the kinetic data to the power-law model (see eq 7) for the toluene combustion at all temperatures and also for the propane combustion at the lower temperatures (623 and 673 K). The values of E and A for the combustion of propane and toluene (obtained in the present work) are compared with those reported earlier for the different catalysts2,7,26-28 in Table 3. 3.2.2. Redox (Mars-van Krevelen) Rate Model. The redox model of Mars-van Krevelen for catalytic hydro-

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Table 3. Comparison of the Arrhenius Parameters for the Combustion of Propane and Toluene over Different Catalysts sample run no.

catalyst

activation energy (kcal/mol)

1 2 3

Fe-doped ZrO2 Mn3O4 La0.66Sr0.34Ni0.3Co0.7O3 perovskite


4 5 6 7 8

LaCoO3/LaMnO3 La0.66Sr0.34Ni0.3Co0.7O3 perovskite Fe-doped ZrO2 Pt on γ-alumina Pt/Al2O3

Toluene Combustion 9.45-17.7 17.27 26.08 17.42 17.26

frequency factor 4.67 × 105

1.51 × 104 1.48 × 107 1.51 × 104

Figure 11. Redox model plots (PO2/r versus PO2/Ppropane) for the combustion of propane.

Figure 12. Redox model plots (PO2/r versus PO2/Ptoluene) for the combustion of toluene.

reference present work Baldi et al.7 Song et al.26 Ishikawa et al.2 Song et al.26 present work Ordonez et al.28 Mazzarino and Barresi27



Table 4. Kinetic Parameters of the Redox (Mars-van Krevelen) Model for the Combustion of Propane and Toluene rate constant (mol g-1 h-1 kPa-1)

temp, T (K)

k1

k2



623 673 723

0.032 0.054 0.078


2.54 × 10-9 1.15 × 10-6 2.15 × 10-4

2.54 × 10-10 1.15 × 10-7 2.15 × 10-5

598 623 648 673

0.0099 0.0134 0.0313 0.040

Toluene Combustion 0.0057 0.0177 0.0441 0.080

6.21 × 10-9 4.56 × 10-8 6.28 × 10-7 1.03 × 10-5

6.21 × 10-10 4.56 × 10-9 6.28 × 10-8 1.03 × 10-6

Figure 13. Arrhenius plots (ln k1 or ln k2 vs 1/T) for the combustion of propane (for the redox model).

carbon oxidation reactions29 involves the following two irreversible steps, operating in the cyclic manner: first, the reaction of hydrocarbon with the lattice oxygen of the catalyst leading to the formation of oxidation products and the reduction of the metal oxide in the catalyst, and second, the reoxidation of the reduced metal oxide by the oxygen present in the feed. The overall rate of the hydrocarbon oxidation can be expressed by the redox expression

r)

k1k2PAPO2 k1PO2 + k2γPA

(9)

or

PO2 r

)

( )( ) ( ) γ 1 PO2 + k2 PA k1

(10)

where k1 is the rate constant for the reoxidation of the catalyst, k2 the rate constant for the oxidation of hydrocarbon by the lattice oxygen, and γ the stoichiometric coefficient for oxygen in the combustion (γ ) 5 and 9, for the combustion of propane and toluene, respectively).

Linear plots of PO2/r versus PO2/PA (where PA is the partial pressure of hydrocarbon) for the combustion of propane and toluene, which are shown in Figures 11 and 12, respectively, indicate that the combustion of propane or toluene follows the redox mechanism. Values of the rate constants (k1 and k2), along with the RSS and MRSS values at the different temperatures for the combustion of propane or toluene are given in Table 4. Figures 13 and 14 show the temperature dependence of the rate constants k1 and k2 of the redox model, according to the Arrehenius equation, for the combustion of propane and toluene, respectively. The values of the Arrhenius parameters (activation energy and frequency factor) for the combustion of propane and toluene, obtained from the linear Arrhenius plots (Figures 13 and 14) are given in Table 5. Table 5. Arrhenius Parameters (Obtained from the Redox (Mars-van Krevelen) Model) for the Combustion of Propane and Toluene activation energy (kcal/mol) reaction

E1

E2

propane combustion toluene combustion

7.98 16.12

23.91 28.45

frequency factor (mol g-1 h-1 kPa-1) A1

A2

2.037 × 101 4.51 × 106 7.17 × 103 1.53 × 108

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Choudhary et al.

Figure 14. Arrhenius plots (ln k1 or ln k2 vs 1/T) for the combustion of toluene (for the redox model).

Figure 15. Plots of robserved versus restimated (redox model) for the combustion of propane at (0) 623 K, (4) 673 K, and (O) 723 K.

Similar to that observed for the power-law model, for the redox model, the E and A values for the toluene combustion also are observed to be larger (see Tables 2 and 4). The plots of robserved vs restimated (from the redox model, eq 8) in Figures 15 and 16, for the combustion of propane and toluene, respectively, clearly show a very good fit of the kinetic data to the redox model. However, a comparison of the RSS and MRSS data for the redox model (Table 4) with that for the power-law model (Table 1) indicates that, although the redox model provides a better fit to the kinetic data for the combustion of propane, the power-law model gave a better fit

to the toluene combustion data at the higher temperature (>648 K). 3.2.3. Eley-Rideal Rate Model. When the propane or toluene combustion rate data were fitted to the EleyRideal model,

r)

krKO2Po2PA 1 + Ko2Po2

(11)

both the reaction rate constant (kr) and oxygen adsorption constant (KO2) had negative values, at all temperatures (Table 6). Hence, in neither case does the combustion follow the Eley-Rideal mechanism.



Figure 16. Plots of robserved versus restimated (redox model) for the combustion of toluene at (0) 598 K, (4) 623 K, (O) 648 K, and (]) 673 K. Table 6. Kinetic Parameters of the Eley-Riedal Model (r ) krKO2PAPO2/(1 + KO2PO2)) for the Combustion of Propane and Toluene rate constant temp, T (K)

Kr (mol g-1 h-1 kPa-1)

KO2 (kPa-1)



623 673 723

-0.0014 -0.0063 -0.0033

Propane Combustion -0.046 -0.056 -0.049

2.52 × 10-9 2.05 × 10-6 2.39 × 10-4

2.52 × 10-10 2.05 × 10-7 2.39 × 10-5

598 623 648 673

-0.0000814 -0.0002 -0.00095 -0.00045

Toluene Combustion -0.0495 -0.0495 -0.0489 -0.0498

1.54 × 10-8 2.11 × 10-7 2.29 × 10-7 2.02 × 10-6

1.71 × 10-9 2.34 × 10-8 2.55 × 10-8 2.02 × 10-7

4. Conclusion The rate data for the complete combustion of propane and toluene (at very low concentration in air) over the iron-doped ZrO2 (cubic) catalyst could be fitted well to both the power-law and redox (Mars-van Krevelen) models. However, the redox model provides a better fit to the rate data for the combustion of propane and toluene (at the lower temperatures). Nomenclature

kr ) reaction rate constant (mol g-1 h-1 kPa-1) KO2 ) oxygen adsorption constant (kPa-1) n ) reaction order, with respect to hydrocarbon PA ) partial pressure of hydrocarbon (kPa) PO2 ) partial pressure of oxygen (kPa) R ) gas constant (cal g-1 mol-1) r ) combustion rate (mol g-1 h-1) T ) temperature (K) W ) weight of catalyst (g) X ) fractional conversion Greek Letters

Notations A ) frequency factor (mol g h kPa ) E ) activation energy (kcal/mol) F ) flow rate of hydrocarbon (mol/h) k ) apparent rate constant (mol g-1 h-1 kPa-n) k1 ) rate constant for reoxidation of catalyst (mol g-1 h-1 kPa-1) k2 ) rate constant for the oxidation of hydrocarbon by lattice oxygen (mol g-1 h-1 kPa-1) -1

-1

-n

γ ) stoichiometric coefficient of oxygen in the combustion Subscripts A ) propane or toluene O2 ) oxygen EF0498871

Kinetics of the Complete Combustion of Dilute Propane and Toluene

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