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Energy & Fuels 2006, 20, 2518-2530
Mechanisms of 1-Methylnaphthalene Pyrolysis in a Batch Reactor Jean-Philippe Leininger,*,†,‡ Franc¸ ois Lorant,† Christian Minot,‡ and Franc¸ oise Behar† Institut Franc¸ ais du Pe´ trole, DiVision Ge´ ologie et Ge´ ochimie, 1-4 AVenue de Bois-Pre´ au, 92852 Rueil-Malmaison, France, and Laboratoire de Chimie The´ orique, Case 137, UniVersite´ Pierre et Marie Curie, 75252 Paris Cedex 05, France ReceiVed March 2, 2006. ReVised Manuscript ReceiVed July 21, 2006
1-Methylnaphthalene (1-MNa) thermal decomposition was studied in a batch reactor (gold tube) submitted to a constant pressure (100 atm), at various temperatures from 380 to 450 °C and for residence times in the range 1-72 h. Pyrolysis effluents were recovered by two successive solvent extractions: first in n-pentane and then in dichloromethane. Light compounds and gaseous effluents were identified and quantified by gas chromatography, while the amount of heavier compounds was determined by weight. Study at low conversion of 1-MNa thermal cracking, below 2%, showed that the rate of decomposition of 1-MNa was accelerated by the formation of dimethylbinaphthalenes. 1-MNa was mostly converted into naphthalene, methylated dimers, methane, and hydrogen gas. Above 10% conversion, the secondary products were dimethylnaphthalenes, 2-methylnaphthalene, and heavier dehydrogenated polyaromatics. The global conversion (10-80% conversion) of 1-methylnaphthalene was correctly modeled by a first-order law with respect to the reactant, with rate constants depending on temperature according to the Arrhenius law. Correspondingly, the apparent kinetic parameters derived from the experimental data were E ) 47.4 kcal mol-1 and A ) 109.9 s-1. These results were in good agreement with those obtained on heavier methylated polyaromatics. Hence, data generated from this work supported the assumption that there may exist one general kinetic model accounting for the thermal cracking of methylated polyaromatics in HP/HT reservoir conditions. Therefore, a reaction pathway for the thermal decomposition of methylated polyaromatics was elaborated on the basis of the free-radical mechanisms proposed previously in the literature and explaining the formation of the main products.
Introduction The main purpose of this work is related to the study of the natural process of thermal cracking of oils in sedimentary basins at temperatures between 150 and 200 °C and pressures ranging from 20 to 100 MPa (in high-pressure/high-temperature natural reservoirs). The thermal stability of aromatic compounds is a key parameter to better understand the thermal evolution of oils which is controlled by the kinetics of cracking reactions. Aromatic compounds are ubiquitous in crude oils and represent 20-45 wt % of the total hydrocarbons in crude oils.1 The aromatic fraction is composed of about 45 wt % of di-, tri-, and tetraaromatic, essentially mono-, di-, and trimethylated,1 compounds and is then representative of part of the aromatic compounds in crude oils. Those methylated aromatic compounds are used with their isomers as maturity indicators in source rock extracts and oils.2-5 A better understanding of aromatic compound thermal cracking would allow a better prediction of oil thermal cracking via basin modeling,6 especially in hightemperature/high-pressure reservoirs.7 Indeed, methylated poly* To whom correspondence should be addressed. E-mail:
[email protected]. † Institut Franc ¸ ais du Pe´trole. ‡ Laboratoire de Chimie The ´ orique. (1) Tissot, B. P.; Welte, D. H. Petroleum Formation and Occurrence, 2nd ed.; Springer-Verlag: Berlin, 1984. (2) Radke, M.; Welte, D. H.; Willsch, H. Geochim. Cosmochim. Acta 1982, 46, 1-10. (3) Radke, M.; Welte, D. H.; Willsch, H.; Leythaeuser, D.; Teichmu¨ller M. Geochim. Cosmochim. Acta 1982, 46, 1831-1841. (4) Alexander, R.; Kagi, R. I.; Rowland, S. J.; Sheppard, P. N.; Chirila, T. V. Geochim. Cosmochim. Acta 1985, 49, 385-395. (5) Garrigues, P.; de Sury, R.; Angelin, M. L.; Bellocq, J.; Oudin, J. L.; Ewald, M. Geochim. Cosmochim. Acta 1988, 52, 375-384.
aromatics could be a source of late methane generation8 at HP/ HT reservoir conditions. To elucidate the complex thermal reactivity of this chemical class, pyrolysis experiments of model compounds representative of the main aromatic structures are performed. A radical kinetic scheme derived from these experiments points out the nature and occurrence of the chemical processes involved in natural oils. Although methylated polyaromatics are ubiquitous in crude oils, this chemical family has received little attention and requires enhanced experimental study to ascertain the chemical mechanism involved during thermal cracking at low temperature. For instance, 1- and 2-methylnaphthalene were pyrolyzed at high temperature (600-1000 °C)9 for which the cracking mechanisms can be extremely different from those occurring at reservoir conditions (T < 200 °C). Furthermore, the authors mainly focused on methane formation and did not address complete mass balances. In an other work by Behar et al.,10 the thermal decomposition of 9-methylphenanthrene was studied at ∼400 °C. However, the main objective of this work was the development of a stoichiometric reaction-based kinetic scheme and the calculation of bulk kinetic parameters. The elementary processes occurring during the thermal degradation of 9-methylphenanthrene were not elucidated. Smith and Savage pyrolyzed (6) Behar, F.; Ungerer, P.; Kressmann, S.; Rudkiewicz, J. L. ReV. Inst. Fr. Pet. 1991, 46, 151-181 (7) Vandenbroucke, M.; Behar, F.; Rudkiewicz, J. L. Org. Geochem. 1999, 30, 1105-1125. (8) Lorant, F.; Behar, F.; Vandenbroucke, M.; McKinney, D. E.; Tang, Y. Energy Fuels 2000, 14, 1143-1155. (9) Gra¨ber, W.-D.; Hu¨ttinger, K. J. Fuel 1982, 61, 505-509. (10) Behar, F.; Budzinski, H.; Vandenbroucke, M.; Tang, Y. Energy Fuels 1999, 13 (2), 471-481.
10.1021/ef0600964 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006
1-Methylnaphthalene Pyrolysis
3-methylanthracene isomers in neat conditions.11 The authors concluded that pyrolysis proceeded through three reaction pathways, demethylation, methyl addition, and hydrogenation. Key elementary reactions were determined for methylanthracene pyrolysis, but a complete kinetic scheme was not suggested. The formation of gaseous effluents and benzene-insoluble compounds were observed but not identified or quantified. In another study, Smith and Savage12 derived a kinetic scheme accounting for 1-methylpyrene thermal degradation. Only demethylation and methyl addition pathways were observed. However, the authors did not identify and quantify gaseous effluents and the heavy products. So their kinetic scheme was validated only on pyrene, methylpyrene, and dimethylpyrene yields and not on hydrogen or heavy-product yields. In conclusion, experimental pyrolyses conducted on methylated aromatics mainly focused on the cleavage of the arylalkyl bond. The identification and quantification of the gaseous effluents and heavy products were still missing. This lack of experimental data could have distorted the mechanistic interpretation and the reaction pathways proposed. For this reason, we propose, in this work, to (i) quantify and identify these products and estimate the impact of their formation on the global degradation of methylated aromatic compounds and (ii) update reaction pathways on the basis of experimental results. For those purposes the following strategy has been conducted: Methylated aromatic compounds with three and four aromatic rings received much more attention than methylnaphthalenes, although the latter are in greater amounts in crude oils1. So 1-MNa was chosen to compare its thermal cracking with that of heavier methylated polyaromatics. As 1-MNa is more reactive than 2-MNa,13 1-MNa was preferred to 2-MNa for the evaluation of the beginning of the methylnaphthalene thermal-cracking window and the estimation of what proportion of “isomerization” from 1-MNa to 2-MNa may occur. The global conversion of 1-MNa was measured for each pyrolysis experiment, and the pyrolysis products, especially the gaseous and heavy products, were identified and quantified. To estimate at best the impact of the formation of gaseous and heavy products on the whole reaction scheme, it was necessary to discriminate primary from secondary products to elucidate the main primary elementary processes involved. Consequently, an extensive experimental work was done at low 1-MNa conversion (
