Energy Fuels 2010, 24, 3817–3830 Published on Web 06/28/2010
: DOI:10.1021/ef100253z
Reassessment of the Kinetic Influence of Toluene on n-Alkane Pyrolysis Frederic Lannuzel,†,‡ Roda Bounaceur,† Raymond Michels,‡ Gerard Scacchi,† and Paul-Marie Marquaire*,† † Laboratoire R eactions et G enie des Proc ed es, LRGP CNRS-UPR 3349, Nancy University, ENSIC, BP 20451, 54001 Nancy, France, and ‡G2R CNRS-UMR 7566, Nancy University, BP 236, 54501 Vandoeuvre-l es-Nancy, France
Received March 5, 2010. Revised Manuscript Received June 10, 2010
The inhibition effect of toluene on the kinetics of n-alkane pyrolysis has been well-known for a long time. However, most studies were performed at high-temperature-low-pressure conditions. The present study investigates a wider range of experimental pressures and temperatures (from 0.001 to 700 bar and from 350 to 600 °C). To account for those, a kinetic model based on free-radical reactions was developed. This model was tested against available literature data for the low-pressure range and against new experiments for the high-pressure range. Whatever the temperature and pressure, it arises that toluene has indeed an inhibitive effect on the pyrolysis of n-octane. This effect is explained by the formation of benzyl radicals stabilized by resonance, via hydrogen-transfer reactions, that leads to new termination reactions. However, this inhibition will be significantly modulated as a function of the pressure, temperature, and reaction progress, from strong to very weak. Our paper describes the mechanistic reasons for this change in the extent of the inhibition effect and proposes an integrated model for the kinetic effects of monoaromatic hydrocarbons on n-alkanes during pyrolysis.
pyrolysis of aliphatic compounds1-6 and aromatic compounds.7-12 However, the chemical and kinetic behavior of these compounds in hydrocarbon mixtures remains poorly understood. Work on the interactions of hydrocarbons in mixtures relates to toluene, tetralin, and their inhibiting effect on alkane pyrolysis.13-18 Burkle-Vitzthum et al.15 highlighted the inhibition role of alkyl aromatics with a side chain comprising more than four carbon atoms on n-alkane pyrolysis, whereas these compounds were initially expected to act as accelerators.19 The purpose of this work is to reassess the kinetic effects of toluene on n-alkane pyrolysis in a broad range of pressures and temperatures (from 0.001 to 700 bar and from 350 to 600 °C). For low-pressure conditions, the abundant available literature was investigated, while for high-pressure conditions, new experiments were conducted. Radical chemical mechanisms based
1. Introduction Hydrocarbon pyrolysis is of concern in a large variety of research fields, such as coal liquefaction, petroleum refining, thermal evolution of crude oils in sedimentary basins, heavyoil recovery, oil-shale retort, and thermal cracking of jet fuels. As a result, the individual thermal decomposition of hydrocarbons is a continuous subject of publications. Until now, a great number of these works were related to the study of the *To whom correspondence should be addressed. E-mail: paul-marie.
[email protected]. (1) Fabuss, B. M.; Smith, J. O.; Lait, R. I.; Fabuss, M. A.; Satterfield, C. N. Kinetics of thermal cracking of paraffinic and naphthenic fuels at elevated pressures. Ind. Eng. Chem. Process Des. Dev. 1964, 3 (1), 33–37. (2) Domine, F. Kinetics of hexane pyrolysis at very high pressures. 1. Experimental study. Energy Fuels 1989, 3 (1), 89–96. (3) Behar, F.; Vandenbroucke, M. Experimental determination of the rate constants of the n-C25 thermal cracking at 120, 400, and 800 bar: Implications for high-pressure/high-temperature prospects. Energy Fuels 1996, 10 (4), 932–940. (4) Yu, J.; Eser, S. Kinetics of supercritical-phase thermal decomposition of C10-C14 normal alkanes and their mixtures. Ind. Eng. Chem. Res. 1997, 36 (3), 585–591. (5) Dahm, K. D.; Virk, P. S.; Bounaceur, R.; Battin-Leclerc, F.; Marquaire, P. M.; Fournet, R.; Daniau, E.; Bouchez, M. Experimental and modelling investigation of the thermal decomposition of n-dodecane. J. Anal. Appl. Pyrolysis 2004, 71 (2), 865–881. (6) Herbinet, O.; Marquaire, P. M.; Battin-Leclerc, F.; Fournet, R. Thermal decomposition of n-dodecane: Experiments and kinetic modeling. J. Anal. Appl. Pyrolysis 2007, 78 (2), 419–429. (7) Blades, H.; Blades, A. T.; Steacie, E. W. R. The kinetics of the pyrolysis of toluene. Can. J. Chem. 1954, 32, 298–311. (8) Burkle-Vitzthum, V.; Michels, R.; Scacchi, G.; Marquaire, P. M. Mechanistic modeling of the thermal cracking of decylbenzene. Application to the prediction of its thermal stability at geological temperatures. Ind. Eng. Chem. Res. 2003, 42 (23), 5791–5808. (9) Leininger, J. P.; Lorant, F.; Minot, C.; Behar, F. Mechanisms of 1-methylnaphthalene pyrolysis in a batch reactor. Energy Fuels 2006, 20 (6), 2518–2530. (10) Pamidimukkala, K. M.; Kern, R. D.; Patel, M. R.; Wei, H. C.; Kiefer, J. H. High-temperature pyrolysis of toluene. J. Phys. Chem. A 1987, 91 (8), 2148–2154. (11) Poutsma, M. L. Fundamental reactions of free radicals relevant to pyrolysis reactions. J. Anal. Appl. Pyrolysis 2000, 54 (1), 5–35. r 2010 American Chemical Society
(12) Savage, P. E. Mechanisms and kinetics models for hydrocarbon pyrolysis. J. Anal. Appl. Pyrolysis 2000, 54 (1), 109–126. (13) Bounaceur, R.; Scacchi, G.; Marquaire, P. M.; Domine, F.; Brevart, O.; Dessort, D.; Pradier, B. Inhibiting effect of tetralin on the pyrolytic decomposition of hexadecane. Comparison with toluene. Ind. Eng. Chem. Res. 2002, 41 (19), 4689–4701. (14) Burkle-Vitzthum, V.; Michels, R.; Bounaceur, R.; Marquaire, P. M.; Scacchi, G. Experimental study and modeling of the role of hydronaphthalenics on the thermal stability of hydrocarbons under laboratory and geological conditions. Ind. Eng. Chem. Res. 2005, 44 (24), 8972–8987. (15) Burkle-Vitzthum, V.; Michels, R.; Scacchi, G.; Marquaire, P. M.; Dessort, D.; Pradier, B.; Brevart, O. Kinetic effect of alkylaromatics on the thermal stability of hydrocarbons under geological conditions. Org. Geochem. 2004, 35 (1), 3–31. (16) Khorasheh, F.; Gray, M. R. High-pressure thermal cracking of n-hexadecane in tetralin. Energy Fuels 1993, 7 (6), 960–967. (17) Khorasheh, F.; Gray, M. R. High-pressure thermal cracking of n-hexadecane in aromatic solvents. Ind. Eng. Chem. Res. 1993, 32 (9), 1864–1876. (18) Taylor, H. S.; John, J.; Smith, O. The reactions of methyl radicals with benzene, toluene, diphenyl methane and propylene. J. Chem. Phys. 1940, 8, 543. (19) Domine, F.; Dessort, D.; Brevart, O. Towards a new method of geochemical kinetic modelling: Implications for the stability of crude oils. Org. Geochem. 1998, 28 (9-10), 597–612.
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Figure 1. Evolution of the ratio of the initial pyrolysis rate of neopentane in the presence of toluene to the initial pyrolysis rate of pure neopentane as a function of the toluene/neopentane ratio at 512 °C and 33 mbar (according to Baronnet21).
on elementary reactions were considered and allowed to construct a general kinetic model able to describe the behavior of the toluene-n-alkane system in a large range of reaction conditions.
2.2. Previous High-Pressure Studies. The available literature on the pyrolysis of the toluene-alkane binary mixture is fairly scarce. Khorasheh et al.16 pyrolyzed a toluenehexadecane mixture (5% alkane) at 420 °C and 139 bar within a tubular reactor. They highlighted a moderate inhibition of hexadecane pyrolysis by toluene. On the basis of the experimental data of Khorasheh et al.16 and using the theoretical study of the acceleration or inhibition mechanisms of chain reactions by Niclause et al.,23 Bounaceur24 proposed reaction mechanisms of alkane pyrolysis in the presence of toluene. This allowed for a theoretical study on the influence of various factors on the inhibiting behavior of toluene on n-alkanes. For instance, Bounaceur24 performed simulations of a mixture of n-octane and 5, 20, and 70 mol % of toluene at 380 °C and 700 bar with a residence time of 4 days. Figure 3 shows the conversion of n-octane in these different mixtures. The model predicts that toluene considerably reduces the conversion of n-octane. The effect increases with the concentration of toluene. 2.3. Definition of the “Inhibition Factor” (IF). To measure the inhibition/acceleration effect of a compound (the additive) on the thermal degradation of a co-reactant, Bounaceur et al.13 as well as Burkle-Vitzthum et al.15 defined the IF as follows: conversion of reactant without additive IF ¼ conversion of reactant with additive
2. Literature Overview 2.1. Previous Low-Pressure Studies. Taylor et al. were the first authors to highlight the inhibiting role of toluene on the cracking of alkanes.18 Szwarc studied the pyrolysis of ethylbenzene and used toluene as a carrier gas to stop the chain reaction induced by the CH3• radical (reaction 1) at temperatures from 615 to 745 °C and pressures of a few millibars.20 C6 H5 CH2 CH3 f C6 H5 CH2 • þ CH3 •
ð1Þ
C6 H5 CH3 þ CH3 • f C6 H5 CH2 • þ CH4
ð2Þ
With the concentration of this compound being very important in the medium (toluene/ethylbenzene = 50:1), the CH3• radicals are converted into CH4 according to reaction 2. The rate of formation of CH4 is then directly correlated to the decomposition rate of ethylbenzene according to reaction 1, with the rates of reactions 1 and 2 being equal. This reaction known as the “toluene carrier technique” has been thereafter extensively applied. Baronnet21 observed in experiments at 512 °C that the addition of toluene very strongly decreased the decomposition rate of neopentane (Figure 1). Razafinarivo22 also studied the influence of toluene on n-octane pyrolysis under inert gas at a total pressure of 1300 mbar (1.3 mbar of n-octane and a molar toluene/octane ratio of 0.9) at 450 °C. The results for two of the principal reaction products are presented in Figure 2. As expected, the presence of toluene induces a strong inhibition of the products formation by a factor higher than 3.
IF values greater than 1 indicate an inhibition effect, whereas IF values below 1 indicate an acceleration effect, on the decomposition rate of the co-reactant. Thus, in the case of Figure 3, obtained by simulations, the IF of the n-octane-toluene mixture varies from about 4 to 38 when the proportion of toluene rises from 5 to 70 mol % at 380 °C and 700 bar. Burkle-Vitzthum et al.14,15 measured and compared the IF values for several mixtures between 300 and 400 °C at 700 bar.
(20) Szwarc, M. The C-C bond energy in ethylbenzene. J. Chem. Phys. 1949, 17 (5), 431–435. (21) Baronnet, F. La Pyrolyse du Neopentane, son Inhibition et son Autoinhibition; Faculte des Sciences: Nancy, France, 1970. (22) Razafinarivo, N. Etude cinetique de la pyrolyse du n-octane induite par un hydroperoxyde. Application a l’Evolution Thermique des P etroles dans les Gisements; Institut National Polytechnique de Lorraine (INPL): Nancy, France, 2006.
(23) Niclause, M.; Martin, R.; Baronnet, F.; Scacchi, G. Etude theorique d’un mecanisme d’acceleration ou d’inhibition de reactions en chaıˆ nes de decomposition. Rev. Inst. Fr. Pet. 1978, 21, 1724–1760. (24) Bounaceur, R. Mod elisation Cin etique de l’Evolution Thermique des P etroles dans les Gisements; Institut National Polytechnique de Lorraine (INPL): Nancy, France, 2001.
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Figure 2. Comparison of the evolutions of the molar fraction of two of the pyrolysis products for pure n-octane and the binary mixture n-octane/toluene (1:0.9) at 450 °C and 1.3 mbar of n-octane diluted in nitrogen, with a total pressure of 1300 mbar (according to Razafinarivo22).
Figure 3. Simulated conversion evolution of n-C8 according to the residence time at 380 °C and 700 bar (according to Bounaceur24). Table 1. Critical Pressures and Temperatures for n-Octane, Toluene, and Argon
(i) The IF value for the 20 mol % decylbenzene-80 mol % hexadecane mixture is about 2.5. (ii) The IF value for the 20 mol % tetralin-80 mol % hexadecane is about 5. Behavior of the IF for decylbenzene and tetralin as a function of the temperature and mixture composition can be found from Burkle-Vitzthum et al.14,15 These authors demonstrate that IF values may evolve by two orders of magnitude with the temperature in a non-linear fashion. In the 300-400 °C and 700 bar conditions, tetralin is a more powerful inhibitor for hexadecane cracking than decylbenzene, but at T = 200 °C, decylbenzene is a much greater inhibitor. The IF cannot be predicted without construction of the detailed reaction mechanisms.
toluene n-octane argon
critical pressure (bar)
critical temperature (°C)
41 24.8 48.7
318 295 -122
the autoclaves were rapidly (5 min) cooled to room temperature in a water heat exchanger, so that the cooling time was negligible relative to the heating time. For each experimental condition, three samples were used for quantitation as well as reproducibility check and one sample was used for product identification. Details of the confined pyrolysis procedure can be found in Landais et al.25 and Michels et al.26,27 3.3. Identification of Products. Gold cells were pierced, cut into pieces, placed into a vial containing hexane, and then extracted in an ultrasonic bath for 1 h. Compounds were identified by gas chromatography-mass spectrometry [HP 5890 series II gas chromatograph (GC) coupled to a HP 5971 mass spectrometer] using a 60 m DB, 5 J&W Scientific, 0.25 mm inner diameter, 0.1 mm
3. Experimental Section 3.1. Samples. n-Octane (purity 99%) and toluene (purity 99.5%) were obtained from Aldrich and Fluka, respectively, and used as received. 3.2. Confined Pyrolysis Procedure. Pyrolysis was carried out in gold cells (40 mm length, 5 mm inner diameter, and 0.5 mm thick). Gold tubes were sealed at one end, then filled with 30 mg of sample under a helium atmosphere (purity 99.9999%) to avoid the presence of oxygen, and then arc-welded at the other end under a refrigerated nitrogen flow in order not to damage hydrocarbons. The gold cells were loaded in stainless-steel autoclaves and pressurized by a fluid up to 700 bar for temperatures between 330 to 450 °C from 24 h to 1 month. At the end of pyrolysis,
(25) Landais, P.; Michels, R.; Poty, B. Pyrolysis of organic matter in coldseal pressure autoclaves. Experimental approach and applications. J. Anal. Appl. Pyrolysis 1989, 16, 103–115. (26) Michels, R.; Landais, P. Artificial coalification: Comparison of confined pyrolysis and hydrous pyrolysis. Fuel 1994, 73 (11), 1691–1696. (27) Michels, R.; Landais, P.; Philp, R. P.; Torkelson, B. E. Influence of pressure and the presence of water on the evolution of the residual kerogen during confined, hydrous, and high-pressure hydrous pyrolysis of Woodford shale. Energy Fuels 1995, 9 (2), 204–215.
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Figure 4. Prediction of the state of phase of a ternary mixture of n-C8/toluene/argon as a function of the temperature and pressure.
film, fused silica column. The temperature program was 60160 °C at 15 °C/min followed by heating to 300 °C at 3 °C/min. 3.4. Quantitation of Products. Gold cells were pierced in a vacuum line maintained at 250 °C (apparatus and procedure are described by Gerard and Landais28). An aliquot of 0.5 mL was sampled online and injected through a heated transfer line into a HP 5890 series II GC with a 60 m DB, 5 J&W Scientific, 0.32 mm inner diameter, 0.45 μm film, fused silica column connected to a flame ionization detector (FID). The temperature program was 0 °C for 1 min followed by an increase of 6 °C/min up to 300 °C. Compounds were quantitated by calibration of the FID using commercially available standards. 3.5. Phase State of the n-Octane/Toluene/Argon Mixture. Before experimentally studying the pyrolysis of the n-octanetoluene mixture, it was necessary to verify the phase state of the system. Indeed, all reactants need to be in the same homogeneous phase, so that homogeneous kinetic formalism can apply. If it was not the case, both co-reactants could be in two different phases and contact would be limited to the interface. In our experimental conditions, n-octane, toluene, and argon, given their critical pressures and temperatures (Table 1), are in the same supercritical state. To test this over the experimental range, we used the PPR78 model, developed by Jaubert and Mutelet,29 allowing us to predict the phase state of our ternary mixture. Figure 4 presents the predictions of this model according to the temperature and pressure for a mixture constituted in equal quantity of toluene and octane and various molar proportions of argon. The inside of the various envelopes of phase corresponds to a biphasic state, and the entire zone outside of these envelopes indicates a single phase. The model shows that, for our experimental temperatures (T>350 °C), the mixture n-octane/toluene/argon is in a single phase regardless of the argon proportion and pressure.
Figure 5. Evolution of the partial pressure of n-pentane (a pyrolysis product of n-octane) as a function of the pyrolysis time for the n-octane/toluene and n-octane/benzene molar mixtures at 350 °C and 700 bar.
was carried out at 350 °C and 700 bar for a mixture of 10% toluene in n-octane. The kinetic effect is followed by the formation of n-pentane, one of the main products of n-octane pyrolysis. In these experimental conditions, toluene does not show an important kinetic effect on the pyrolysis of n-octane; the inhibition factors are of the order of 1.2 at most (Figure 5). n-Octane was also pyrolyzed in the mixture with benzene, a compound for which the kinetic effect on n-octane is expected to be negligible. The effects of toluene and benzene on n-octane pyrolysis are very similar (Figure 5). In another series of experiments, the influence of the abundance of toluene on the cracking of n-octane at 700 bar was studied. Bounaceur13 showed by simulation that increasing amounts of toluene should induce a significant increase in the inhibition effect in these conditions. Figure 6 presents the evolution of the partial pressure of n-butane as a function of the pyrolysis time for various molar ratios of toluene at 10, 20, 30, 50, and 90%. An inhibition of the pyrolysis of n-octane by
4. Results 4.1. Influence of the Presence of Toluene on the Conversion of n-Octane at High Pressures. The first series of experiments (28) Gerard, L.; Elie, M.; Landais, P. Analysis of confined pyrolysis effluents by thermodesorption-multidimensional gas chromatography. J. Anal. Appl. Pyrolysis 1994, 29 (2), 137–152. (29) Jaubert, J. N.; Mutelet, F. VLE predictions with the PengRobinson equation of state and temperature dependent kij calculated through a group contribution method. Fluid Phase Equilib. 2004, 224 (2), 285–304.
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5.1. Pure n-Octane Cracking Mechanism. 5.1.1. Mechanism Construction. The pyrolysis of n-octane can be described as for the other n-alkanes by a free-radical mechanism. At low conversion, the elementary reactions include initiation, hydrogen transfer, radical decomposition by β-scission, addition to double bonds, and termination. To limit the number of reactions and calculation time, a lumped mechanism by reaction type has been used. This approach was developed by Bounaceur et al.30,31 Two types of radicals play a part in the pyrolysis mechanism of saturated hydrocarbons: radicals that decompose by monomolecular reactions and radicals that react by bimolecular reactions. To represent the homogeneous pyrolysis of an organic substance referred to as μH, Goldfinger et al.32 proposed the following nomenclature: (i) a radical that reacts by monomolecular reactions is named μ• and (ii) a radical that reacts by bimolecular reactions is named β•. Using this symbolic representation “β and μ” as well as the elementary reactions presented previously, it is then possible to write the primary radical mechanism describing the pyrolysis of a n-alkane (μH). initiation : μH f free radicals ðtype βÞ
Figure 6. Partial pressure of n-butane (a pyrolysis product of n-octane) as a function of the pyrolysis time for pure n-octane and various compositions of the n-octane/toluene mixture at 350 °C and 700 bar.
toluene is observed but remains very low, with IF ranging between 1 and 1.4. On the contrary to what was predicted by the model by Bounaceur,24 our experimental results indicate that the amount of toluene has a very weak influence on the inhibition effect. Our experimental results conducted at a pressure of 700 bar, thus, do not present the inhibition effect of toluene on the thermal decomposition of n-octane as predicted by the literature. 4.2. Detailed Study of the Pyrolysis Products. 4.2.1. Pure n-Octane Pyrolysis. Major pyrolysis products of n-octane are distributed into three types (Figure 7): linear alkanes from C1 to C6 (called alkanes-minus, alkanes lighter than the reactant), branched alkanes from C9, and higher homologues (called alkanes-plus). 4.2.2. Pure Toluene Pyrolysis. The main product of pure toluene pyrolysis is benzene. Other reaction products are methane, C2 compounds, biaromatics, xylenes, and trimethylbenzenes (Figure 8). Isomers of biaromatics were not structurally identified and were grouped for quantitation. 4.2.3. n-Octane/Toluene Mixture Pyrolysis. The C1-C6 compounds are similar in proportion to those generated during the pyrolysis of pure octane. However, in contrast to the pyrolysis of pure toluene, benzene was not detected. The majority of the compounds having a greater molecular weight than toluene correspond to products of cross-reactions between n-octane and toluene. They correspond to a homologous series of C4-C9 alkylbenzenes and methylalkylbenzenes. Compounds derived from each of the two reactants are also found in lesser proportions: undecane, dodecane derived from n-octane and toluene and methylbiphenyl- and methylbenzylphenyl-type compounds derived from toluene (Figure 9).
propagation I :
terminations :
μ• f alkene þ β• β• þ μH f alkane-minus þ μ• ðalkane-minus ¼ βHÞ μ• þ μ• f products β• þ β• f products β• þ μ• f products
A secondary propagation can be written as propagation II :
alkene þ μ• f heavy radical• heavy radical• þ μH f alkane-plus þ μ•
The mechanism of n-octane pyrolysis is thus described by 91 elementary reactions, including 21 molecules and 27 radicals. It is presented in Table A1 in the Appendix. To each reaction is associated a kinetic constant [of kinetic parameters A and Ea (in mol, cm3, s, and cal)] as well as, if necessary, an adjustment factor (noted ka/ke). The kinetic parameters used are those proposed by the software EXGAS.33 5.1.2. Validation of the Model. The proposed model of n-octane pyrolysis is validated by a comparison to experimental data (Figure 10). There is a good agreement between the measurements of the remaining reactants after pyrolysis of n-octane and the calculated values. Good agreement for reaction products was also obtained. This suggests that our model takes into account major reactions and can thus be considered as validated. The same results with similar modeling (30) Bounaceur, R.; Warth, V.; Glaude, P. A.; Battin-Leclerc, F.; Scacchi, G.; C^ ome, G.-M.; Faravelli, T.; Ranzi, E. Chemical lumping of mechanisms generated by computer. Application to the modelling of normal butane oxidation. J. Chim. Phys. Phys.-Chim. Biol. 1996, 93, 1472–1491. (31) Bounaceur, R.; Warth, V.; Marquaire, P. M.; Scacchi, G.; Domine, F.; Dessort, D.; Pradier, B.; Brevart, O. Modeling of hydrocarbons pyrolysis at low temperature. Automatic generation of free radicals mechanisms. J. Anal. Appl. Pyrolysis 2002, 64 (1), 103–122. (32) Goldfinger, P.; Letort, M.; Niclause, M. Volume commemoratif Victor Henri: Contribution a l’etude de la structure moleculaire. Desoer, Li ege 1947-1948, 283. (33) Warth, V.; Stef, N.; Glaude, P. A.; Battin-Leclerc, F.; Scacchi, G.; C^ ome, G. M. Computer-aided derivation of gas-phase oxidation mechanisms: Application to the modeling of the oxidation of n-butane. Combust. Flame 1998, 114 (1-2), 81–102.
5. Mechanism Construction The pyrolysis mechanism of the n-octane/toluene mixture is constructed by taking into account the entire pyrolysis mechanisms of the two pure compounds and adding the various crossreactions between the major molecules and/or major radicals produced from each reactant. These coupling reactions must be written in a systematic way, as in the case of pure compounds, by considering successively the various types of elementary reactions. 3821
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Figure 7. Chromatogram of the products obtained after the confined pyrolysis of n-C8 pyrolysis at 350 °C and 700 bar for 120 h.
Figure 8. Chromatogram of the pyrolysis products of pure toluene at 450 °C and 700 bar for 2 h.
were obtained on hexadecane pyrolyzed at similar experimental conditions.15 5.2. Pure Toluene Cracking Mechanism. Toluene pyrolysis was studied by Lannuzel et al.34 between 350 and 400 °C under a pressure of 700 bar, and a detailed kinetic model consisting of 30 free-radical reactions was developed to describe the thermal cracking of toluene at low conversion. The mechanism includes 13 molecules and 8 radicals. The nomenclature used for the writing of the reactions as well as for the mechanism is presented in Table A2 in the Appendix. The main reactions taken into account in our model for high-pressure pure toluene pyrolysis are as follows.
These reactions are also called “reverse radical disproportionation” (RRD) reactions.11,35,36 The cyclohexadienyl radicals
(34) Lannuzel, F.; Bounaceur, R.; Michels, R.; Scacchi, G.; Marquaire, P. M. An extended mechanism including high pressure conditions (700 bar) for toluene pyrolysis. J. Anal. Appl. Pyrolysis 2010, 87 (2), 236–247.
(35) Benson, S. W. On the reaction between ethylene and cyclopentene, a radical mechanism. Int. J. Chem. Kinet. 1980, 12 (10), 755–760. (36) Poutsma, M. L. Free-radical thermolysis and hydrogenolysis of model hydrocarbons relevant to processing of coal. Energy Fuels 1990, 4 (2), 113–131.
5.2.1. Bimolecular Initiations.
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Figure 9. Chromatogram of the products obtained after pyrolysis at 450 °C and 700 bar for 3 h of the toluene/n-octane mixture (0.9:1).
[with H in ipso (a) or R (b) position] lead by β-scission to the formation of a methyl or hydrogen radical and a benzene or toluene molecule. Another possibility is the opening of the aromatic ring, which should lead to the formation of C2H2, but this product is not observed in the experiments of Lannuzel et al.,34 and they conclude that this reaction is insignificant at low temperatures ( 5, and the reaction mechanism works as classical “μH and YH”. When the conversion increases, the produced alkenes increase, consequently leading to more addition reactions. Thus, the benzyl radical concentration (Y•) decreases as well as the IF. (ii) At low temperature and high pressure (350 °C and 700 bar), the radical chain carrier μ• is dominant ([μ•] . [β•]) and toluene should act as an inhibitor on the pyrolysis of n-octane. However, the highpressure conditions favor the radical addition reactions. At very low conversion (X < 10-4), inhibition is observed (IF = 2.5) because the alkene concentrations are too low to quench most of benzyl radicals. When conversion increases, the produced alkenes increase while they react with the benzyl radical, leading to a significant decrease of inhibition. At conversion close to 1%, IF is about 1.2 and tends toward 1 with further progress.
A new propagation appears, noted IV, implying the new radical R• and giving the following stoichiometry: YH þ alkene ¼ RH This typical mechanism “μH and YH improved” can be schematized as in Figure 14. Using the mechanism, the evolution of the IF at 350 °C and 700 bar and at 500 °C and 0.03 bar as a function of conversion is calculated in Figure 15 for a n-C8/toluene mixture 3827
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7. Conclusion
Table A1. Continued
At very low pressure (a few millibars), toluene inhibits the pyrolysis of alkanes regardless of the temperature (350500 °C). Its action mechanism of type “μH and YH” was clearly identified. The objective of this work aimed at checking if this compound kept the same behavior at high pressure (several hundreds of bars). Our experiments, carried out between 350 and 450 °C for pressures of 100 and 700 bar, did not show a significant inhibition under these conditions. The analysis of the products of pyrolysis of n-C8/toluene mixtures allowed us to develop a mechanism and elucidate the difference in toluene reactivity between the high and low pressures. Thus, the absence of inhibition at 350 °C at high pressures is explained by an addition reaction of the benzyl radicals on alkenes, negligible at low pressures. This new reaction implies that the inhibiting behavior of toluene evolves with temperature, pressure, and reaction progress. Inhibition will decrease with reaction progress and a pressure increase. Our study reveals that the inhibition effect of toluene on alkane pyrolysis can vary drastically as a function of reaction conditions but can be predicted by our kinetic model. A further study will explore, by simulation, the influence of toluene on alkane pyrolysis in geological conditions, i.e., at low temperatures (200 °C) and high pressures (100-1000 bar). These results should enable us to predict the influence of methyl aromatics on the thermal stability of oils in geological reservoirs.
reactions Recombinations CH3• þ CH3• f C2H6 C2H5• þ C2H5• f C4H10 C3H7• þ C3H7• f C6H14 C4H9• þ C4H9• f n-C8 C5H11• þ C5H11• f C10H22 C6H13• þ C6H13• f C12H26 μ8• þ μ8• f C16H34 CH3• þ C2H5• f C3H8 CH3• þ C3H7• f C4H10 CH3• þ C4H9• f C5H12 CH3• þ C5H11• f C6H14 CH3• þ C6H13• f C7H16 CH3• þ μ8• f C9H20 C2H5• þ C3H7• f C5H12 C2H5• þ C4H9• f C6H14 C2H5• þ C5H11• f C7H16 C2H5• þ C6H13• f n-C8 C2H5• þ μ8• f C10H22 C3H7• þ C4H9• f C7H16 C3H7• þ C5H11• f n-C8 C3H7• þ C6H13• f C9H20 C3H7• þ μ8• f C11H24 C4H9• þ C5H11• f C9H20 C4H9• þ C6H13• f C10H22 C4H9• þ μ8• f C12H26 C5H11• þ C6H13• f C11H24 C5H11• þ μ8• f C13H28 C6H13• þ μ8• f C14H30
27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
A
E
3.00 1013 1.00 1013 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 1.00 1013 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011 5.00 1011
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Secondary Mechanism
Acknowledgment. This work was supported by TOTAL Exploration and Production (Pau, France).
Appendix Table A1. n-Octane Cracking Mechanisma reactions
A
55 56 57 58 59 60
Additions μ8• þ C2H4-A f C10H21• μ8• þ C3H6-A f C11H23• μ8• þ C4H8-A f C12H25• μ8• þ C5H10-A f C13H27• μ8• þ C6H12-A f C14H29• μ8• þ C7H14-A f C15H31•
4.00 1011 4.00 1011 4.00 1011 4.00 1011 4.00 1011 4.00 1011
8000 8000 8000 8000 8000 8000
61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85
Decompositions C10H21• f C3H7• þ C7H14-A C10H21• f C4H9• þ C6H12-A C10H21• f C5H11• þ C5H10-A C10H21• f CH3• þ C9H18-A C11H23• f C4H9• þ C7H14-A C11H23• f C3H7• þ C8H16-A C11H23• f C5H11• þ C6H12-A C11H23• f CH3• þ C10H20-A C12H25• f C4H9• þ C8H16-A C12H25• f C5H11• þ C7H14-A C12H25• f C3H7• þ C9H18-A C12H25• f CH3• þ C11H22-A C13H27• f C5H11• þ C8H16-A C13H27• f C4H9• þ C9H18-A C13H27• f C3H7• þ C10H20-A C13H27• f CH3• þ C12H24-A C14H29• f C5H11• þ C9H18-A C14H29• f C6H13• þ C8H16-A C14H29• f C4H9• þ C10H20-A C14H29• f C3H7• þ C11H22-A C14H29• f CH3• þ C13H26-A C15H31• f C5H11• þ C10H20-A C15H31• f C4H9• þ C11H22-A C15H31• f C3H7• þ C12H24-A C15H31• f CH3• þ C14H28-A
4.00 1013 4.00 1013 4.00 1013 4.00 1013 4.00 1013 4.00 1013 4.00 1013 4.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013 3.00 1013
28700 28700 28700 31000 28700 28700 28700 31000 28700 28700 28700 31000 28700 28700 28700 31000 28700 28700 28700 28700 31000 28700 28700 28700 31000
86 87 88 89 90 91
Metathesis n-C8 þ C10H21• f μ8• þ C10H22 n-C8 þ C11H23• f μ8• þ C11H24 n-C8 þ C12H25• f μ8• þ C12H26 n-C8 þ C13H27• f μ8• þ C13H28 n-C8 þ C14H29• f μ8• þ C14H30 n-C8 þ C15H31• f μ8• þ C15H32
4.00 1011 4.00 1011 4.00 1011 4.00 1011 4.00 1011 4.00 1011
12200 12200 12200 12200 12200 12200
E
Primary Mechanism 1 2 3 4
Initiations n-C8 f C4H9• þ C4H9• n-C8 f C3H7• þ C5H11• n-C8 f C2H5• þ C6H13• n-C8 f CH3• þ C7H15•
1.10 1017 1.10 1017 1.10 1017 1.10 1017
83408 83837 83796 85674
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Decompositions μ8• f CH3• þ C7H14-A μ8• f C2H5• þ C6H12-A μ8• f C3H7• þ C5H10-A μ8• f C4H9• þ C4H8-A μ8• f C5H11• þ C3H6-A μ8• f C6H13• þ C2H4-A C6H13• f CH3• þ C5H10-A C6H13• f C2H5• þ C4H8-A C6H13• f C3H7• þ C3H6-A C6H13• f C4H9• þ C2H4-A C5H11• f CH3• þ C4H8-A C5H11• f C2H5• þ C3H6-A C5H11• f C3H7• þ C2H4-A C4H9• f CH3• þ C3H6-A C4H9• f C2H5• þ C2H4-A C3H7• f CH3• þ C2H4-A
2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013
31000 28700 28700 28700 28700 28700 31000 28700 28700 28700 31000 28700 28700 31000 28700 31000
21 22 23 24 25 26
Metathesis n-C8 þ CH3• f μ8• þ CH4 n-C8 þ C2H5• f μ8• þ C2H6 n-C8 þ C3H7• f μ8• þ C3H8 n-C8 þ C4H9• f μ8• þ C4H10 n-C8 þ C5H11• f μ8• þ C5H12 n-C8 þ C6H13• f μ8• þ C6H14
2.00 10 2.00 1011 2.00 1011 2.00 1011 2.00 1011 2.00 1011 11
9600 11200 11200 11200 11200 11200
a
3828
Units: mol, cm3, s, and cal.
Energy Fuels 2010, 24, 3817–3830
: DOI:10.1021/ef100253z
Lannuzel et al.
Table A2. Toluene Cracking Mechanisma reactions
A
n
E
Unimolecular Initiation 1 2 3 4
toluene f benzyl• þ H• benzyl• þ H• f toluene toluene f C6H5• þ CH3• C6H5• þ CH3• f toluene
3.10 1015 2.59 1014 1.00 1017 1.39 1013
0 0 0 0
89200 0 97000 0
5 6
Bimolecular Initiation toluene þ toluene f benzyl• þ C6H6CH3a• toluene þ toluene f benzyl• þ C6H6CH3b•
2.50 1014 2.50 1014
0 0
71300 68800
7 8 9
Ipso Additions toluene þ H• f C6H6 þ CH3• toluene þ CH3• f xylene þ H• xylene þ CH3• f trimethylbenzene þ H•
1.20 1014 5.00 1012 3.00 1012
0 0 0
8100 15940 15940
10 11 12 13 14
Metathesis on a Benzylic H Atom 1.20 1014 toluene þ H• f benzyl• þ H2 4.00 1011 toluene þ CH3• f benzy•l þ CH4 4.00 1012 toluene þ C3H5V• f benzyl• þ C3H6V 7.90 1013 toluene þ C6H5• f benzyl• þ C6H6 7.90 1013 toluene þ C6H4CH3• f benzyl• þ toluene
0 0 0 0 0
8400 11100 8000 12000 12000
15 16
Metathesis on a Phenylic H Atom 6.00 108 toluene þ H• f C6H4CH3• þ H2 2.00 1012 toluene þ CH3• f C6H4CH3• þ CH4
1 0
16800 15000
17 18 19 20 21
C6H6CH3• Decomposition C6H6CH3a• f 2C2H2T þ C3H5V• C6H6CH3b• f 3C2H2T þ CH3• C6H6CH3a• f 2C2H2T þ C3H4 þ H• C6H6CH3a• f toluene þ H• C6H6CH3b• f C6H6 þ CH3•
2.00 1013 2.00 1013 2.00 1013 2.00 1013 2.00 1013
0 0 0 0 0
21700 21700 21700 28700 28700
22 23 24
Benzyl Additions on Aromatic Ring 2.00 1012 benzyl• þ C6H6 f benzylphenyl þ H• benzyl• þ toluene f benzylphenyl þ H• 2.00 1012 benzyl• þ xylene f benzylphenyl þ H• 2.00 1012
0 0 0
23000 23000 23000
25 26 27 28 29 30
2 benzyl• f bibenzyl benzyl• þ CH3• f etC6H5 benzyl• þ C3H5V• f benzC3H5V C6H4CH3• þ H• f toluene C6H4CH3• þ CH3• f xylene CH3• þ CH3• f C2H6
Terminations
a
2.50 1011 5.00 1012 5.00 1012 1.00 1014 1.00 1013 3.00 1013
0.4 0 0 0 0 0
0 0 0 0 0 0
cf. ref 34.
Table A3. Cross-reactions of the n-Octane/Toluene Mixture Pyrolysis reactions
A
n
E
0.32 120 1.20 1011 1.20 1011 1.20 1011 1.20 1011 1.20 1011 1.20 1011 1.20 1011 1.20 1011 1.20 1011 1.20 1011 1.20 1011 120 120 120 120 120 120 120 120 120
3.3 3.3 0 0 0 0 0 0 0 0 0 0 0 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3 3.3
8559 18170 13400 13400 13400 13400 13400 13400 13400 13400 13400 13400 13400 18170 18170 18170 18170 18170 18170 18170 18170 18170
F
Metathesis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
toluene þ μ8• f benzyl• þ n-C8 n-C8 þ benzyl• f μ8• þ toluene toluene þ C2H5• f benzyl• þ C2H6 toluene þ C3H7• f benzyl• þ C3H8 toluene þ C4H9• f benzyl• þ C4H10 toluene þ C5H11• f benzyl• þ C5H12 toluene þ C6H13• f benzyl• þ C6H14 toluene þ C10H21• f benzyl• þ C10H22 toluene þ C10H21• f benzyl• þ C10H22 toluene þ C12H25• f benzyl• þ C12H26 toluene þ C13H27• f benzyl• þ C13H28 toluene þ C14H29• f benzyl• þ C14H30 toluene þ C15H31• f benzyl• þ C15H32 benzyl• þ C2H6 f toluene þ C2H5• benzyl• þ C3H8 f toluene þ C3H7• benzyl• þ C4H10 f toluene þ C4H9• benzyl• þ C5H12 f toluene þ C5H11• benzyl• þ C6H14 f toluene þ C6H13• benzyl• þ C7H16 f toluene þ C7H15• benzyl• þ C10H22 f toluene þ C10H21• benzyl• þ C11H24 f toluene þ C11H23• benzyl• þ C12H26 f toluene þ C12H25•
3829
3.5
Energy Fuels 2010, 24, 3817–3830
: DOI:10.1021/ef100253z
Lannuzel et al. Table A3. Continued
reactions
A
n
E
120 120 120
3.3 3.3 3.3
18170 18170 18170
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3.3 0 0
6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 14000 8559 11175 3880
23 24 25
benzyl• þ C13H28 f toluene þ C13H27• benzyl• þ C14H30 f toluene þ C14H29• benzyl• þ C15H32 f toluene þ C15H31•
25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
Reactions on Alkenes 2.00 1011 C2H4 þ benzyl• f benzylcenyl• 2.00 1011 C3H6 þ benzyl• f benzylcenyl• 2.00 1011 C4H8 þ benzyl• f benzylcenyl• 2.00 1011 C5H10 þ benzyl• f benzylcenyl• 2.00 1011 C6H12 þ benzyl• f benzylcenyl• 2.00 1011 C7H14 þ benzyl• f benzylcenyl• 2.00 1011 C8H16 þ benzyl• f benzylcenyl• 2.00 1011 C9H18 þ benzyl• f benzylcenyl• 2.00 1011 C10H20 þ benzyl• f benzylcenyl• 2.00 1011 C11H22 þ benzyl• f benzylcenyl• 2.00 1011 C12H24 þ benzyl• f benzylcenyl• 2.00 1011 C13H26 þ benzyl• f benzylcenyl• 2.00 1011 C14H28 þ benzyl• f benzylcenyl• 2.00 1011 C15H30 þ benzyl• f benzylcenyl• 2.00 1011 C16H32 þ benzyl• f benzylcenyl• 3.20 1012 benzylcenyl• þ n-C8 f benzylcene þ μ8• benzylcenyl• þ toluene f benzylcene þ benzyl• 0.32 1.60 1012 n-C8 þ H• f μ8• þ H2 2.65 1011 n-C8 þ C6H5• f μ8• þ C6H6
44 45 46 47 48 49 50 51
benzyl• þ μ8• f nonylbenzene benzyl• þ C7H15• f octylbenzene benzyl• þ C6H13• f heptylbenzene benzyl• þ C5H11• f hexylbenzene benzyl• þ C4H9• f pentylbenzene benzyl• þ C3H7• f butylbenzene benzyl• þ C2H5• f proylbenzene benzyl• þ CH3• f ethylbenzene
F
Terminations 1.60 1011 1.60 1011 1.60 1011 1.60 1011 1.60 1011 1.60 1011 1.60 1011 1.60 1011
3830
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
