Energy & Fuels 1992, 6, 189-194
189
Experimental Study and Kinetic Modeling of Higher Hydrocarbons Oxidation in a Jet-Stirred Flow Reactor Corinne BalGs-Gugret, Michel Cathonnet,* Jean-Claude Boettner, and Franqoise Gaillard Laboratoire de Combustion et Systdmes RBactifs-C.N.R.S., 1 C, Avenue de la Recherche Scientifique, 45071 Orleans Cedex 2, France Received July 30, 1991. Revised Manuscript Received December 11, 1991
The oxidation of n-decane and n-propylcyclohexane was studied in a jet-stirred flow reactor in the temperature range 873-1033 K at atmospheric pressure. The concentrations of molecular species were measured at different extents of reaction by gas chromatography. The main hydrocarbon intermediates formed in both hydrocarbons oxidation were ethylene, propene, methane, l,&butadiene, and ethane. In the oxidation of n-decane, 1-butene was also formed in significant concentration and several other unsaturated hydrocarbons were detected as minor products: 1-pentene, 1-hexene, 1-heptene, 1-octene, and 1-nonene. In the oxidation of n-propylcyclohexane, 1,3-pentadiene, cyclopentadiene, pentene, benzene, methylcyclohexene, and cyclohexene were found as minor products. On the basis of the experimental observation of a low concentration level for large hydrocarbon intermediates, quasi-global chemical kinetic reaction mechanisms were developed to reproduce the experimental data.
Introduction Commercial liquid fuels are complex mixtures of a great number of hydrocarbons and detailed chemical kinetic modeling of their combustion would need to know their precise composition, and the collection of an enormous body of chemical kinetic data which are at present unavailable. Nevertheless, we showed recently1 that the oxidation of a commercial kerosene TRO (Jet Al) can be represented by a kinetic model involving an equivalent mixture of a small number of pure hydrocarbons from Cg to c11. In this respect, we have started a series of experiments on the oxidation of liquid hydrocarbon fuels in a jet-stirred flow reactor. These studies represent an extension to higher hydrocarbons of current work on light alkanes and alkenes oxidation,H The jet-stirred reactor technique has the advantage to permit the description of the evolution of the chemical species in a large range of reaction extent including the induction period. The combustion of higher alkanes was the subject of several studies performed in recent years. This combustion involves the formation of a great n h b e r of intermediates. Detailed descriptions of the hydrocarbon intermediates formed in the oxidation of n- and isooctane in a turbulent flow reactor' and in laminar f l a m e ~ ~have 9 ~ been published. More recently, Delfau et al.l0 analyzed the structure of a rich laminar premixed n-decane flame. The present paper reports the results of an analytical study of n-decane and n-propylcyclohexane oxidation in a jet-stirred flow reactor. On the basis of the experimental results, a quasi-global mechanism was developed to describe the evolution of intermediate species during the course of the reaction. Experimental Study Experimental Device. The jet-stirred flow reactor used in this study is similar to the one used in previous studies on smaller hydrocarbons2+ and was adapted for the study of liquid fuels.'J' It is a sphere of 4 cm diameter made of fused silica, where stirring
* Author to whom correspondence should be addressed.
Table I. n-Decane Oxidation (Initial Fuel Mole Fraction, 10-9 mean carbon equivaresidence balance, lence mixture no. ratio time, s temp, K % 1 0.2 0.1 922 105.5 923 105.4 0.14 924 99.7 0.16 923 94.1 0.18 925 92 0.20 924 87 0.22 3 0.2 0.1 1027 104 0.14 1032 110 1030 105 0.16 0.18 1034 85 0.20 1038 88 0.22 1039 92 5 1 0.1 1033 99.4 0.14 1031 96 1032 92.3 0.16 0.18 1033 93 0.20 1035 93 0.22 1038 93 1028 99.4 7 1.5 0.1 1031 100.4 0.14 1035 94 0.16 0.18 1034 87 0.20 1036 94 0.22 1035 93 is achieved by four turbulent jets of the incoming gases, and heated by an electrical resistance system. ~~~
(1)GuBret, C.; Cathonnet, M.; Boettner, J. C.; Gaillard, F. TwentyThird Symposium (International) on Combustion (Proceedings);The Combustion Institute: Pittsburgh, 1990; pp 211-216. (2) Dagaut, P.; Cathonnet, M.; Boettner, J. C.; Gaillard, F. Combust. Sci. Technol. 1987,56, 23-63. (3)Dagaut, P.; Cathonnet, M.; Boettner, J. C.; Gaillard, F. Combust. Flame 1988, 71, 295-312. (4) Dagaut, P.; Cathonnet, M.; Boettner, J. C. J. Phys. Chem. 1988, 92,661-671. ( 5 ) Chakir, A,; Cathonnet, M.; Boettner, J. C.; Gaillard, F. TwentySecond Symposium (International) on Combustion (Proceedings);The Combustion Institute: Pittsburgh, 1988; pp 873-981. (6)Chakir, A.;Cathonnet, M.; Boettner, J. C.; Gaillard, F. Combust. Sci. Technol. 1989,65, 207-230. (7)Dryer, F. L.; Brezinsky, K. Combust. Sci. Technol. 1986, 45, 200-212.
Q887-0624/ 92/25Q6-Q189$03.00/ 0 0 1992 American Chemical Society
Bales-Gueret et al.
190 Energy & Fuels, Vol. 6, No. 2, 1992 Table 11. Propylcyclohexane Oxidation (Initial Fuel Mole Fraction, mean carbon equivaresidence balance, lence mixture no. ratio time. s temD. K %
1
1.2E-003
-
j
08880
1.OE-003
I
N-Decane Ethylene Propene 1-Butene
I
8.OE-004
~~~
2
3
6
9
0.2
0.2
1
1.5
0.1 0.14 0.16 0.18 0.20 0.22 0.08 0.1 0.14 0.16 0.18 0.20 0.22 0.08 0.1 0.14 0.16 0.18 0.2 0.22 0.08 0.1 0.14 0.16 0.20 0.22
922 921 923 921 925 926 1020 1021 1020 1022 1023 1026 1029 970 972 973 973 975 976 975 1019 1021 1024 1023 1024 1026
100 100 100 100 96.7 94.4 105.5 100 101.4 100.8 98.7 98.1 98.1 91.1 91.1 98.9 96.7 96.7 99.6 98 90 96.7 91.1 93.3 97.8 98.9
Before its introduction into the reactor, the liquid fuel is atomized through a small annular orifice in a nitrogen flow and prevaporized in a separate vaporizer at a temperature maintained near 473 K. The preheated oxygen flow diluted by nitrogen is mixed with the vaporized fuel at the entrance of the injector. The residence time of the mixture in the injector is more than 1order of magnitude lower than the residence time in the reactor, which prevents the occurrence of reactions in the injector. The flow rates of oxygen and nitrogen are measured and controlled by rotameters, and the fuel is delivered a t a precise flow rate by a Gilson injection pump. The pressure of the liquid in the injection pump is adjusted and controlled by a f i e metering valve to ensure a constant liquid flow rate. The temperature inside the reactor is registered by an uncoated 0.16 mm diameter chromel-alumel thermocouple which is moved along a whole diameter to check the reactor homogeneity. Corrections for radiations losses are not necessary since the reactor wall temperature is nearly identical with that of the gases. Several factors limit the temperature rise due to reaction: high dilution of the fuel, heat losses to the walls, and relatively low reaction extents. When the mean residence time in the reactor is varied, the power of the several heaters is carefully adjusted in order to minimize the temperature gradient through the reactor and to maintain it a t 10 K or less. Tables I and I1 give the initial composition of some of the mixtures studied with the measured temperatures a t different mean residence times. Chemical samples are extracted from the reactor at low pressure (2-20 Torr) through a sonic quartz probe and stored at 20 Torr in glass bottles. They are analyzed by gas chromatography. When necessary, the identification of unknown compounds is made on an ion trap detector (Finnigan) connected with the chromatograph. To prevent condensation or adsorption, the sample storing system and all the connections with the chromatographs are free of grease and heated up to 350 K. Several chromatographs were used for complete analysis of the samples. 02,N2, CO, COPwere separated on a Carbosieve SI1 packed column, hydrocarbons up to C9 on a 50-m PLOT A1203 (8)Axelsson, E.; Rosengreen, L. G. Combust. Flame 1985,62,91-93. (9) Axelsson, E.; Rosengreen, L. G. Combust. Flame 1986,&4,229-232. (10) Delfau, J. L.; Bouhria, M.; Reuillon, M.; Sanogo, 0.;Akrich, R.;
Vovelle, C. Twenty-Third Symposium (International) on Combustion (Proceedings); The Combustion Institute: Pittsburgh, 1990; pp 1567-1572. (11) GuCret, C. Th&e de Doctorat, University of Orleans, 1989.
G V
E
6.OE-004
-0 4 . O E - 0 0 4 Q1
I
2.OE-004
O.OE+OOO
8.00
0.05
0.10
0.15
0.20
0 . 25
M e a n R e s i d e n c e T i m e (s)
6.OE-005 5.OE-005
-
1-He 1-oc 1-NO
5 4.OE-005 z=0 3.OE-005
0 2.OE-005 Q1
=E
1 .OE-005
0.OE+000
5
M e a n R e s i d e n c e T i m e (s)
Figure 1. Experimental concentration profiles in n-decane oxidation (mixture 1 in Table I).
(Chrompack) capillmy column and higher hydrocarbons on a 25-m
CP Si1 5 CB (Chrompack) capillary column. The experiments were performed at atmospheric pressure, in the temperature range 873-1033 K for an initial fuel concentration of 0.1 %, at equivalence ratios ranging from 0.2 to 1.5. By varying the mean residence time in the reactor it was possible to describe the evolution of the reaction from low conversion to the formation of the final products. The accuracies of the temperature measurements and of the sampling technique have been discussed elsewhere.2d The carbon deficit was found to be lower than 10-15% as shown in Tables I and 11. The only difference from previous studies performed with gaseous fuels was the occurrence, in some conditions, of small slow temperature oscillations generated by the liquid pump. n-Decane (99% pure) and n-propylcyclohexane (99% pure) were provided by Aldrich, while N2 and O2 (99.995% pure) were provided by Air Liquide. Experimental Results. The main intermediates formed during the oxidation of n-decane were carbon monoxide, ethylene, propene, methane, 1-butene, and 1,3-butadiene. Among all these intermediates, ethylene was the predominant species until carbon monoxide accumulation. Several other hydrocarbons were also formed as intermediates but in much lower concentration: acetylene and ethane, and also a-olefins (1-pentene, 1-hexene, 1-heptene, 1-octene, and 1-nonene) which accumulate during the early stage of the reaction, their concentrations reaching rapidly a maximum. Figure 1 shows an example of the concentration profiles of major products and large hydrocarbon intermediates a t low reaction extent. In propylcyclohexane oxidation, the same intermediate hydrocarbons as in n-decane oxidation were found to be the major products. However, the minor products were different: they involved linear and cyclic alkenes (methylcyclohexene, cyclohexene, pentenes), dienes (cyclopentadiene, 1,3-pentadiene, propadiene, but also C6 and C7 dienes), and alkynes (acetylene and propyne). An example of major and minor products distribution is shown in Figure 2. The general observation of this experimental study was that in both fuels oxidation, hydrocarbons lower than C5 were the dominant intermediate products (Figures 1and 2). Even during the early part of the induction period, their molar yield was always
Energy & Fuels, Vol. 6, No. 2, 1992 191
Hydrocarbons Oxidation in a Jet-Stirred Flow Reactor
-
1.2E-003
P.C.Hexane Ethylene Propene Methane 1.3--8utadiene
-
-
1 .OE-003
+h+9-4
'0 8 . O E - 0 0 4 &= V
6.OE-004
0 4.OE-004 01
I
2.OE-004
0'oE+00?3.00
0.10
0.05
0.15
0.20
Mean Residence T i m e
0.25 (5)
k = A P exp(-E/R!I')(cm3, mol, s, kcal).
Benzene 1.3-Pentadiene Cyclopentadiene
W 0 4.OE-005
I
O.OE+OOO
5
0.
Mean Residence T i m e ( s )
Figure 2. Experimental concentration profiles in propylcyclohexane oxidation (mixture 1 in Table 11).
higher than 60%, and at a slightly greater reaction extent (50% of fuel consumption) and before the accumulation of carbon monoxide, it exceeded 90%. Another point revealed by our experimental results was on carbon monoxide formation during the oxidation of both fuels. Carbon monoxide formed during n-decane oxidation accumulates at the end of induction period characterizedby initial fuel consumption, as shown previously for the oxidation of other alkanes? The present study shows that in propylcyclohexane oxidation, carbon monoxide begins to accumulate sooner. This fact was also noticed in the high temperature oxidation of cyclohexane.12
Reaction Mechanism Formulation of the Kinetic Model. The first propagation step in the oxidation of alkanes is the formation of an alkyl radical by H abstraction from the fuel. The second step is the decomposition of this large radical to form alkenes and smaller radicals. These chemical steps are followed by the oxidation of the intermediates into carbon monoxide and water, the heat release occurring during the oxidation of carbon monoxide to carbon dioxide.' Several chemical mechanisms were developed to model the high-temperature oxidation of C, and C8 alkanes, including quasi-global schemes13or detailed mechanisms.14 Warnatz showed that the prediction of flame velocities of alkanes from C3to C8does not necessitate the development of a detailed mechanism for the decomposition of the primary alkyl radicals since this step is not a rate-limiting (12)Gulati, S.K.;Walker, R. W. J. Chem. Soc. Faraday Trans. 1989, 85,1799-1812.
(13)Warnatz, J. Twentieth Symposium (International)on Combustion (Proceedings);The Combustion Institute: Pittsburgh, 1985; pp 845-856.
+
-
-CCCH
Table 111. Reactions Consuming n -Decanea reaction A n E 1.10 x 1015 0.00 70.0 CIOHZZ = C5Hll + C5Hll 3.23 x 1013 0.00 29.1 C5H11 = C2H4 + n-C3H7 0.00 49.0 2.52 X CloHzz+ Oz = p-CloHzl + HOz c ~ +~oz= H ~ ~- c ~~+ H ~ OH~ ~ ~1.60 x 1014 0.00 48.0 CloHzz+ OH = p-Cl,,Hzl + HzO 8.58 X lo9 1.05 1.81 C10Hz2 + OH = s-CloHz1 + HzO 1.04 X 10" 1.25 0.7 5.65 x 107 2.00 7.7 C1oHzz + H = P-GoH21 + Hz 7.20 x 107 2.00 5.0 C10Hzz + H = s-C&IZl + Hz c ~ +~o =H~ -~ c ~+~ OH ~ H ~1.00 ~ x 1014 0.00 7.85 2.24 x 1014 0.00 5.2 CloHzz + 0 = S-CloHzl + OH CloHzz HOz = p-ClOHZ1+ H202 1.12 X 1013 0.00 19.4 Cl0HZ2+ HOz = s-CloHzl + HzOz 2.69 X 1013 0.00 17.0 CloHz2+ CH3 = p-CloHzl + CHI 3.90 X 10l2 0.00 11.6 9.60 X 10" 0.00 9.5 CloH22 + CH, = s-CloH21 + CH4 1.00 x 1014 0.00 28.0 p - and s-CloHzl products
(14)Axelaaon, E.;Brezinsky, K.; Dryer, F. L.; Pitz, W. J.; Westbrook, C. K. Twenty-First Symposium (International)on Combustion (Proceedings); The Combustion Institute: Pittsburgh, 1986; pp 783-793.
process. The kinetic scheme built to reproduce the present experimental data is based on the observation that hydrocarbons with more than four carbon atoms are only minor intermediate products. Using the quasi-steady state hypothesis for these species, the decomposition reaction of the large radical parent to the initial fuel was represented by a single global step forming the small intermediate hydrocarbons. This approach was not only used by Warnatz13 to model the combustion of alkanes up to C8 but also by Jackson and Laurendeau15 to reproduce experimental data on benzene combustion. n -Decane Oxidation. The first steps of n-decane oxidation are initiation reactions by molecular decomposition, the most probable being C10H22 = C5Hll + C5Hll (1) or by reaction with molecular oxygen (p = primary; 8 = secondary): ClOH22 + 0
2
= p-C10H21+ HO2
(2)
CioH22 + 0 2 s-CloH,i + HO2 (3) The rate constant of reaction 1was initially taken from Axelason et al.14 but a better agreement between computed and experimental profiles of n-decane was obtained using the expression quoted in Table 111. For reactions 2 and 3, the rate constants of ref 14 were used in the computation. H abstraction from n-decane by a radical is an important propagation step which forms a primary or a secondary n-decyl radical CloHz2+ X = p - or s-CloHzl + XH
X being OH, H, 0, H02, or CH, radicals. It is generally assumed that the rate constants of these reactions are only dependent on the type of the C-H bond broken and not on the size of the molecule. Thus we have taken for these reaction steps the rate coefficients given in ref 14 and applied the additivity rule.14 The resulting rate coefficients are shown in Table 111. For the decomposition of n-decyl radicals, we have assumed a global step forming directly C4 and lower hydrocarbons and H atoms. From the experimental concentration profiles of the main hydrocarbons measured at low conversion, we have deduced the following global expression CloHzl 0.5C4H8+ 0.12C4H, + 0.78C3H6+ 0.08C2H6 + 2.41CzH4 + 0.2CH4 0.68H
-
(15) Jackson, M. C.; Laurendeau,N. M.Energy Fuel 1987,1,405-411.
192 Energy & Fuels, Vol. 6, No. 2, 1992 6.OE-003 ooooo N - d e c a n e DDDOO AAAAA
Ethylene
co
0v000c02
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0.05
0.10
0.15
0.20
0.25
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0.OE+000
0.30
0.00
0.05
0.10
0.15
0.20
0.25
Mean Residence Time
M e a n R e s i d e n c e T i m e (s)
0.30
(5)
1.6E-004
4.OE-004
o o o o o Pro e n e
OODDO AAAAA
1 --$7utene 1.3 B u t a d i e n e V V V V O Ethane OQOOO
1.3 B u t a d i e n e Ethane
AAAAA
3.OE-004
1.2E-004 S
0 c
.-0 c
IF 0
u
e 2.OE-004
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-
01 0
W
0
3E
I
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m
.
1
0
0.15
0.20
0.25
4.OE-005
O.OE+OOO
0.
0 .OO
Mean Residence Time ( s )
Figure 3. Oxidation of n-decane in lean mixture (mixture 3 in Table I). Lines, simulation; points, experiment.
which was used for both primary and secondary radicals. The other reaction steps used in the computation are the submechanism of C4hydrocarbons oxidation. It is a detailed mechanism including the reactions of C4and lower hydrocarbons formed in the conditions of the present study, as well as the CO-H2 subsystem. This mechanism has been validated in previous studies on lower hydrocarbons oxidation (including ethene, propane, propene, n-butane and 1-butene) and has been presented elsewhere.2+ The rate constant expressions used for the oxidation of higher species are shown in Table 111. The rate constant of the global decomposition step of n-decyl radical was estimated from the rate constant of octyl radicals decomp~sition.'~But the computation is very sensitive to the value attributed to this rate constant, and the value reported in Table I11 gave the best agreement with experiments. The kinetic data relative to the submechanism of lower species oxidation which have been detailed in previous papers2+ were used in the present computation without modifications and are not reported here. This submechanism consists of 360 reactions plus their reverse and 56 speciesm5 The computation was performed using a perfectly-stirred reactor computer program which has been presented previou~ly.~-~ This code solves the equations of conservation of ma~sfor each chemical species, the temperature used for the computation being the experimental temperature, so the energy equation is not needed. Figures 3-5 show some examples of comparison between computation and experiments for different equivalence ratios. As shown in this figure, the prediction of the concentration of the main products by the model is generally acceptable, the major disagreement being those concerning carbon monoxide and carbon dioxide concentrations at low reaction yield.
0.05
0.10
0.15
0.25
0.20
0.
M e a n R e s i d e n c e T i m e (s)
Figure 4. Oxidation of n-decane in stoichiometric mixture (mixture 4 in Table I). Lines, simulation; points, experiment. 2.OE-003
N-Decane Ethylene Pro e n e +++++ M e t E o n e
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0.05
0.10
0.15
0.20
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Mean Residence T i m e
DOODD AAAAA
0.
(5)
1,3-EutadIene Ethene 0
0
0
0
0
8.OE-005
4.OE-005
O.OE+OOO
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Figure 5. Oxidation of n-decane in rich mixture (mixture 7 in Table I). Lines, simulation; points, experiment.
Propylcyclohexane Oxidation. The literature body devoted to cyclic saturated hydrocarbons oxidation is by far less abundant than the one devoted to n-alkanes oxidation. The most studied cycloalkane is cyclohexane, but almost exclusively under 500 0C.12J6-19 Gulati and
Hydrocarbons Oxidation in a Jet-Stirred Flow Reactor Table IV. Reactions Consuming n -Prowlcuclohexaneo A n E 4.50 X 1013 0.00 60.0 1.00 x 1014 0.00 30.2 1.26 X 1Ol1 0.00 49.0 1.40 X 1013 0.00 48.0 2.00 X 10" 0.00 46.0 4.29 X lo9 1.05 1.81 9.10 X lo9 1.25 0.7 4.00 X 10" 0.00 0.44 2.81 x 107 2.00 7.7 6.30 X lo7 2.00 5.0 7.3 1.26 x 1014 0.00 5.01 X 1013 0.00 7.85 1.96 X 10" 0.00 5.2 1.00 X 1Ol6 0.00 3.28 5.61 X loll 0.00 19.4 2.35 X 10l2 0.00 17.0 1.00 x 1013 0.00 14.4 1.95 X 10" 0.00 11.6 8.40 X 1OI2 0.00 9.5 1.00 x 10" 0.00 7.9 1.00 x 107 0.00 15.0
Energy & Fuels, Vol. 6, No. 2,1992 193 ooooo N - P . C y c l o h e x a n e o o o o o Ethylene 00000
COZ
-al 0
I
1.OE-063
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0.00 0.05 0.10
0.15 0.20 0.25 0.30
M e a n R e s i d e n c e T i m e (s)
= AT" exp(-E/RT) (cm3,mol, s, kcal).
WaIkerl2showed that, at 480 "C, the main primary products are the conjugated olefin (cyclohexene) and an unsaturated aldehyde (presumed to be formed by decomposition of an epoxide). Concerning substituted cyclanes, the only results available are from pyrolysis studies in flow reactorss23 or in shock tubeR.= They have established that the initiation step consists of the breaking of the C-C bond of the lateral chain to form the cyclohexyl radical. In the present study, we have considered two initiation steps, a thermal decomposition forming the cyclohexyl radical CgHla = CeHll+ n-C3H7 (4) and a bimolecular reaction with O2forming the primary, secondary, and tertiary propylcyclohexyl radicals: (5a, 5b, 5c) CgHla + 02 p-, S-, or t-C9H1, + HOP
0.00 0.05
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1.2E-003 o o o o o Ethylene A A A A A Prapenw 00000 CO
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2.OE-004 00000 M e t h a n e
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z!
(16) Guedj, R.; Julien, J. Bull. SOC.Chim. Fr. 1964, 1501. (17) Zeelenberg, A. P.; De Bruijn, H. W. Combust. Flame, 1965, 9, 281-295. (18) Bonner, B. H.; Tipper, C. F. H. Combust. Flame 1965,9,317-327. (19) Grif'fiths,J.; Skirrow, G.; Tipper, C. F. H. Combust. Flame 1968, 12,443-446. (20) Beju~,M.; Veaely, V.; Leclercq, P. A.; Rijks, J. A. Znd. Eng. Chem. Prod. Res. Deu. 1979, 18, 135-142. (21) Satanova,R. B.; Nametkin, N. B.; Zimmermann, G. Kinet. Katal. 1979, 22, 291. (22) hibike, P. S.; Susu, A. A.; Ogunye,A. F. Thermochim.Acta 1981, 47, 1-14. (23) Zychlinsky, W.; Bach, G.; Glauch, B.; Zimmermann, G. J. F. Prakt. Chem. Band 1983,32466, (24) Taang, W. J. Phys. Chem. 1972, 76, 143. (25) Atkinson, R. Int. J. Chem. Kinet. 1987, 19, 799-828.
0.15
Figure 6. Oxidation of propylcyclohexane in lean mixture (mixture 3 in Table 11). Lines, simulation; points, experiment.
The rate constant coefficients used for reaction 4 and 5 are reported in Table IV. H abstraction reactions from propylcyclohexane by a radical are important chain propagation steps in the oxidation of this fuel according to the general scheme: CgH18 + X = p-, S-,or t-CgH17 + XH Abstraction of an H atom from the alkyl chain forms a primary or a secondary propylcyclohexyl radical, while radical attack on a carbon from the cycle forms a secondary or a tertiary propylcyclohexyl. In a compilation on the reactions of OH radical with cycloalkanes, A t k i n ~ o nproposed ~~ an additivity rule with correction factors taking into account the environment of
0.10
Mean Residence T i m e (s)
5.OE-005 5.OE-OO!
0.OE+000
0
0.
M e a n R e s i d e n c e T i m e (s)
Figure 7. Oxidation of propylcyclohexane in stoichiometric mixture (mixture 6 in Table 11). Lines, simulation; points, experiment.
the C-H bond broken. Using this additivity rule, we have computed the rate constants of the reactions propylcyclohexane + OH. The values obtained are very close to those derived from the rate expressions of Axelsson et al.14
194 Energy & Fuels, Vol. 6, No. 2, 1992
-
0.3C4He + 0.57C3H6 + 0.08CzHe + 1.42C&, + 0.4CH4 + 2.69CO + 4.02H This assumption of direct CO formation by a high molecular mass intermediate is in agreement with the statement of Gulati and Walker that, in cyclohexane oxidation, CO is formed by the following sequence: cyclohexyl radical + O2 cyclohexylperoxy radical cyclohexylperoxy radical 1,4-cyclohexane oxide 1,4-cyclohexane oxide hex-5-en-l-al- CO
CgH17 + 02
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Bales-Gueret et al.
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- co
The computation was very sensitive to the value chosen for the rate constant of the global reaction of propylcyclohexyl radical (Table IV). The submechanism of small hydrocarbons oxidation was the same as the one used in n-decane oxidation, since the major reaction products are identical. Examples of comparison between computation and experiment are given in Figures 6-8 for different equivalence ratios. The figures show that the computation fit reasonably the experiments, the major disagreement concerning CO concentration profiles.
0)
=E
5.OE-005
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Mean
-
0.20
0.25
0.
Residence T i m e ( s )
Figure 8. Oxidation of propylcyclohexane in rich mixture (mixture 9 in Table 11). Lines, simulation;points, experiment.
for alkanes. This is in agreement with Gulati and WalkerW2deduction that the methylene group in cyclohexane is indistinguishable from those in alkanes. Therefore, we have used for the reactions of propylcyclohexane with OH, H and CH,, the same rate constants as those proposed in ref 14 for the rate coefficients of alkanes with these radicals (Table IV). For the reactions of propylcyclohexyl radicals, the same assumption of a global step was made as for n-decyl radical. However, the experimental observation that in propylcyclohexane oxidation carbon monoxide was formed earlier than in n-decane oxidation led to include the formation of carbon monoxide in the primary reaction steps. From the experimental concentration profiles measured at low conversion for carbon monoxide and the main hydrocarbons, we have deduced the following kinetic expression for the reactions of p - , s-, or t-propylcyclohexyl radicals:
Discussion This study showed that, although the oxidation of linear and cyclic higher alkanes involves the formation of a great number of intermediates, a large majority of them are hydrocarbons lower than Cg. These light hydrocarbons are formed directly from the large alkyl radical parent to the initial fuel, or by oxidation of large hydrocarbon intermediates. But the oxidation of these large hydrocarbons is so rapid that their concentration always remains at a low level. The oxidation of the light hydrocarbon intermediates leads to the formation of carbon monoxide, but it was shown here that in cycloalkanes oxidation this product appears earlier than in linear alkanes. This is an indication of another source, the oxidation of a primary product for the formation of CO in cycloalkanes oxidation. The quasi-global mechanisms developed from these experimental observations and from previous studies on lower alkanes and alkenes oxidation predicts reasonably the present experimental data. Acknowledgment. This research was supported by D.R.E.T. (Ministhe de la DBfense), contract no. 88/055. Registry No. n-Decane, 124-18-5; n-propylchclohexane, 1678-92-8; 1,3-butadiene,106-99-0;ethylene, 74-85-1; propene, 115-07-1;methane, 74-82-8; ethane, 74-84-0; l-butene, 106-989.
