Ind. Eng. Chem. Res. 1998, 37, 4601-4608
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Thermal Decomposition of Jet Fuel Model Compounds under Near-Critical and Supercritical Conditions. 2. Decalin and Tetralin Jian Yu‡ and Semih Eser*,†,‡ Department of Energy and Geo-Environmental Engineering, 154 Hosler Building, and Laboratory for Hydrocarbon Process Chemistry, 209 Academic Projects Building, The Pennsylvania State University, University Park, Pennsylvania 16802
Thermal decomposition of two jet fuel model compounds, Decalin and Tetralin, was studied under near-critical and supercritical conditions. Under high-pressure supercritical conditions, the thermal decomposition of both compounds was dominated by isomerization reactions. This is different from the results obtained under low-pressure and high-temperature conditions where cracking reactions (for Decalin) or dehydrogenation reactions (for Tetralin) dominate. The major liquid products from Decalin were spiro[4,5]decane, 1-butylcyclohexene, 1-methylcyclohexene, 1-methylhydrindan, two octalins, and toluene. The cis-decalin was converted to trans-decalin as the reaction proceeded. The main liquid products from Tetralin were 1-methylindan and naphthalene with the former being the dominant product that was favored at high pressures. Introduction Recently, we have reported on the thermal decomposition of C10-C14 n-alkanes under near-critical and supercritical conditions in relation to future jet fuel thermal stability problems.1,2 In this article, we present the results from thermal reactions of Decalin and Tetralin which are typical components found in a coalderived jet fuel.3 A companion paper gives the results from the thermal decomposition of n-butylbenzene and n-butylcyclohexane.4 Previous Work. As a model compound for naphthenic feedstocks for alkene production, Decalin has been pyrolyzed at high temperatures (>700 °C) and low pressures ( trans-decalin (Figure 3). It seems that an increase in the pressure shifts equilibrium toward trans-decalin. Figure 4 shows that although the concentration of Decalin does not change with pressure, the ratio of trans-decalins to cis-decalin increases with the increasing pressure. Although the reactivities of cis- and trans-decalins differ significantly, the yields (based on 100 mol of the reactant converted) of major products from the thermal decomposition of the three Decalin samples showed no significant differences. This means that the thermal decomposition of cis- and trans-decalins proceeds by the same mechanism. Reaction Mechanisms for the Thermal Decomposition of Decalin. The major products obtained from the thermal decomposition of Decalin can be explained by the reactions shown in Scheme 1. The initiation occurs by homolytic cleavage of the carboncarbon bond shared by both rings to form a biradical (eq 1).7 This biradical could undergo further reactions to form two small radicals and one or two unsaturated products, depending on the number of decomposition steps. The radicals produced from the initiation reactions can abstract hydrogen atoms from Decalin to form three decalyl radicals (eq 2), of which most are 9-decalyl radicals because the formation of a tertiary radical requires a lower activation energy than that of a secondary radical. The 9-decalyl radical could decompose by β-scission to form a 1-cyclohexenylbutyl radical, which could either abstract a hydrogen atom from Decalin to form 1-butylcyclohexene (eq 3a) or undergo ring closure and subsequent hydrogen abstraction to form spiro[4,5]decane (eq 3b). The 9-decalyl radical could also decompose at the C1-C2 bond to form 1-propyl-2-methylenecyclohexane upon hydrogen abstraction (eq 4), or eject a hydrogen atom to form 1,2,3,4,5,6,7,8-octahydronaphthalene (eq 5). The formation of 1-methylcyclohexene (also propylene) can be explained by a concerted molecular mechanism proposed by Miller,23 as shown in the last reaction in eq 4. The 1-decalyl radical could undergo the reactions shown in eqs 6-9 to form 1-methylhydrindan, cyclodecene, 3-butylcyclohexene, and 2,3,4,5,6,7,8,10-octahydronaphtha-
Scheme 2. Reactions for the Formation of C7H10 Compounds
lene while the 2-decalyl radical could undergo the reactions shown in eqs 10 and 11 to form butenylcyclohexane and 1-methyl-2-allylcyclohexane. The n-butylbenzene can be formed from 1-butylcyclohexene or 3-butylcyclohexene through a series of dehydrogenation reactions. Toluene could be derived from some C7H10 intermediates, such as 3-methylenecyclohexene, 2-methyl-1,3-cyclohexadiene, and 1-methyl-1,3cyclohexadiene, via dehydrogenation reactions. The formation of 3-methylenecyclohexene can be explained by an intramolecular isomerization, followed by β-scission, as shown in eqs 12 and 13 in Scheme 2. The 2-methyl-1,3-cyclohexadiene and 1-methyl-1,3-cyclohexadiene could be formed from 3-methylenecyclohexene through the reactions shown in eqs 14 and 15.
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4605 Scheme 3. Reactions for the Formation of High-Molecular-Weight Compounds
Some toluene could also be converted from 1-methylcyclohexene via a series of dehydrogenation reactions. This route for toluene formation, however, is less important as evidenced in the n-butylcyclohexane study. It was found that 1-methylcyclohexene was the major product while toluene was formed in a small amount in the thermal decomposition of n-butylcyclohexane.4 The formation of three sets of high-molecular-weight compounds can be attributed to the addition reactions between decalyl radicals and ethylene, propylene, or 3-methylenecyclohexene. Scheme 3 gives some examples of these addition reactions. As the pressure increases, radical addition reactions become important. This increasing importance of the addition reactions with increasing pressure explains the lower yields of ethylene and propylene at higher pressures (Table 2). The formation of other minor compounds can be explained by further reactions of the major products. Under high-pressure supercritical conditions, the formation of spiro[4,5]decane is more favorable than that of 1-butylcyclohexene (Figure 1), indicating that high pressures favor the addition reactions over hydrogen abstraction reactions. The 1-methylcyclohexene is formed by a molecular mechanism (the last reaction in eq 4 of Scheme 1) which is unfavorable under high pressures (because it is a unimolecular reaction). Therefore, the yield of 1-methylcyclohexene decreases with the increasing pressure (Figure 1). Kinetics of Decalin Thermal Decomposition. The rate constants for the thermal decomposition of Decalin (mixture of cis- and trans-decalins) were obtained by assuming a first-order kinetics.9,24 Figure 5 shows the first-order plots, ln[1/(1 - x)] versus time, where x is the fractional conversion of Decalin. A fixed loading ratio of 0.36 was used in the experiments with the glass tube reactor. The first-order rate constants obtained at 425, 450, and 475 °C are 0.0238, 0.132, and 0.669 h-1, respectively. From the Arrhenius law, the following kinetic parameters were obtained: apparent activation energy Ea ) 69 kcal/mol and preexponential factor A ) 1020.1 h-1. The activation energy obtained in this work is higher than a value of 52 kcal/mol obtained from the steam cracking of trans-decalin at 700-850 °C.7 It, however, compares well with a value of 64 kcal/mol reported by Fabuss and co-workers9 for the first-order decomposition of Decalin (mixture of cisand trans-decalins) at similar temperature and pressure conditions. Product Distributions from the Thermal Decomposition of Tetralin. Thermal stressing of Tetralin was conducted at 425-474 °C for different times. The effects of pressure were studied at 450 °C for 30 min. Analysis of the original Tetralin sample revealed that it contained about 1.3 wt % impurities, including 1-tetralone, 1-tetralol, naphthalene, 1,2-dihydronaph-
Figure 5. First-order plots for the thermal decomposition of Decalin. Table 3. Comparison of Gaseous Product Distributions between Supercritical and Subcritical Conditions from Tetralin in a Tubing Bomb Reactor at 450 °C for 300 min product yield, mol/100 mol of reactant converted hydrogen methane ethane ethylene propane propylene butane butene
supercritical, Pr ) 1.17
subcritical, Pr ) 0.64
35.8 2.8 1.7 0.4 1.7 0.6 0.4 0.1
87.8 3.6 4.0 1.1 2.0 1.3 0.08 0.04
thalene, 1-methylindan, cis- and trans-decalins, ethylbenzene, toluene, benzene, and an unidentified compound which eluted between cis-decalin and 1-methylindan from the GC column. As the reactions proceeded, cis-decalin was isomerized to trans-decalin, and 1,2dihydronaphthalene, benzene, and the unidentified compound remained unchanged, while the yields of 1-tetralone and 1-tetralol decreased and the yields of all other compounds presented above increased. The most abundant product from thermal decomposition of Tetralin under the conditions used was 1-methylindan, followed by naphthalene, n-butylbenzene, and 2-methylindan. Also appearing, but in lower yields, were n-propylbenzene, ethylbenzene, and toluene. The yields of the reaction products changed with pressure, temperature, and conversion. Table 3 shows a comparison of gaseous product distributions between supercritical and subcritical conditions from the thermal decomposition of Tetralin in the tubing bomb reactor at 450 °C for 300 min. The gaseous products included hydrogen, methane, ethane, ethylene, propane, propylene, butane, and butene. Hydrogen was the dominant gaseous product under both conditions. The formation of gaseous products was suppressed under high-pressure supercritical conditions. Figure 6 shows the product distributions as a function of pressure from the thermal decomposition of Tetralin at 450 °C for 30 min. The yields of 1-methylindan and naphthalene were adjusted to account for the feed impurities. As the pressure increases, the yield of 1-methylindan increases while those of naphthalene, n-butylbenzene, and 2-methylindan decrease. In the pressure range examined, the major products were 1-methylindan and naphthalene and the yield of 1-methylindan was always higher than that of naphthalene. For example, as Pr increased from 0.64 to 1.91, the yield of 1-methylindan increased from 49.8 to 77.1 mol while
4606 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998
Figure 6. Product distributions as a function of the initial reduced pressure from Tetralin at 450 °C for 30 min.
that of naphthalene decreased from 36.7 to 19.2 mol, for 100 mol of the reactant converted, indicating that, under the conditions used, the major reactions were ring isomerization and dehydrogenation. Figure 7 shows the product distributions as a function of temperature for similar conversions (5.0, 5.3, and 5.6% at 425, 450, and 474 °C for 360, 90, and 25 min, respectively). The yield of 1-methylindan decreases while that of 2-methylindan increases with an increase in temperature. The yields of naphthalene and nbutylbenzene do not change significantly with temperature. Figure 8 shows the product distributions as a function of conversion from the thermal decomposition of Tetralin at 450 °C with a loading ratio of 0.36 (Pr ) 1.17). The yield of 1-methylindan increases slightly and that of naphthalene decreases with the increasing conversion. The yields of n-butylbenzene and 2-methylindan do not change significantly with the conversion. Reaction Mechanisms for the Thermal Decomposition of Tetralin. The product distributions obtained from the thermal decomposition of Tetralin can be rationalized by the reactions shown in Scheme 4.7,11,13,25 The source of initiating radicals is not clear and the trace impurities present in original Tetralin may be crucial.11 Penninger13 proposed that the initiation can occur by a reactor-wall-catalyzed cleavage of the R carbon-hydrogen bond to form a 1-tetralyl radical and a hydrogen atom. Poutsma,11 however, suggested that the molecular disproportionation of 1,2-dihydronaphthalene, which is usually present in Tetralin as a trace impurity, may be the major initiation reaction. The formation of 1-methylindan and 2-methylindan can be explained by eqs 20 and 21.25 Naphthalene could
Figure 7. Product distributions as a function of temperature from the thermal decomposition of Tetralin.
Figure 8. Product distributions as a function of the conversion from Tetralin at 450 °C.
be formed by a series of dehydrogenation reactions of the 1-tetralyl radical (eq 22).7,13 Poutsma,11 however,
Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4607 Scheme 4. Reaction Mechanism for the Thermal Decomposition of Tetralin
Figure 9. First-order plots for the thermal decomposition of Tetralin.
h-1, respectively. From the Arrhenius law, the following kinetic parameters were obtained for Tetralin thermal decomposition: apparent activation energy Ea ) 58 kcal/ mol and preexponential factor A ) 1016.1 h-1. It appears that Tetralin is the most stable compound among jet fuel model compounds studied in this and previous work.1,2,4 The following gives approximate relative values of the first-order rate constants for the decomposition of five C10 compounds at 450 °C: 46, 20, 8, 4, and 1 for n-butylbenzene, n-decane, n-butylcyclohexane, Decalin, and Tetralin, respectively. indicated that at low-temperature and high-pressure conditions, 1,2-dihydronaphthalene in eq 22 is mainly formed by disproportionation of the 1-tetralyl radical, as shown in eq 23. It should be mentioned that under high-pressure supercritical conditions, the interconversion between the 1-tetralyl radical and the 2-tetralyl radical (eq 24) could also occur.11 The formation of n-butylbenzene can be explained by the hydrogen atom addition to Tetralin, followed by decomposition and subsequent hydrogen abstraction (eq 25).7,13 The product distributions are affected by pressure, as shown in Figure 6. Increasing the pressure increases the yield of 1-methylindan because the formation of the cyclohexadienyl intermediate from the 2-tetralyl radical is favorable at high pressures. The yields of naphthalene and 2-methylindan decrease with the increasing pressure because dehydrogenation and β-scission reactions are suppressed at high pressures. The yield of n-butylbenzene decreases as the pressure increases because of the decreased concentration of hydrogen atoms with the increasing pressure (because of the depressed dehydrogenation reactions at high pressures). Both 1-methylindan and 2-methylindan are derived from the 2-tetralyl radical (eqs 20 and 21). In general, bond-breaking reactions (such as the first reaction in eq 21) have higher activation energies than bondmaking reactions (such as the first reaction in eq 20). For two parallel reactions, high temperatures favor the reaction with a higher activation energy. Therefore, the yield of 2-methylindan would be higher while the yield of 1-methylindan would be lower at higher temperatures, as shown in Figure 7. Kinetics of Tetralin Thermal Decomposition. Kinetic data were obtained from the thermal decomposition of Tetralin in the glass tube reactor at three different temperatures. The rate constants were determined by assuming a first-order kinetics. Figure 9 shows the first-order plots for the thermal decomposition of Tetralin. The first-order rate constants obtained at 425, 450, and 474 °C are 0.00900, 0.0370, and 0.139
Conclusions The thermal decomposition of Decalin under highpressure supercritical conditions produced spiro[4,5]decane, 1-butylcyclohexene, 1-methylcyclohexene, 1-methylhydrindan, two octalins, and toluene as the major liquid products. The formation of significant amounts of isomerization reaction products indicates that the reaction mechanism under the conditions used is different from that under low-pressure and high-temperature conditions which result in light gases, C6-C8 cycloalkenes, and C6-C8 cycloalkanes as major products. The product distributions from trans- and cisdecalins were very similar, although the former exhibited higher stability than the latter. The interconversion of the two Decalins was also observed. Tetralin pyrolysis under near-critical and supercritical conditions led to 1-methylindan and naphthalene as predominant products. High pressures favored the formation of 1-methylindan while low pressures promoted the production of naphthalene. In the pressure range examined, the yield of 1-methylindan was always higher than that of naphthalene, indicating that isomerization is favored over dehydrogenation under the conditions used. Among jet fuel model compounds studied in this and previous work,1,2,4 the most stable one is Tetralin, followed by Decalin, n-butylcyclohexane, n-decane, ndodecane, n-tetradecane, and n-butylbenzene. It seems that Tetralin and Decalin are good components for advanced jet fuels. In addition to their high thermal stability, these compounds can also inhibit the degradation of other reactive compounds because of their good hydrogen-donor ability. Acknowledgment This work was supported by the Air Force Wright Laboratory/Aero Propulsion and Power Directorate, Wright-Patterson Air Force Base. We thank Prof.
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Harold H. Schobert of Penn State for his support. We thank Mr. Douglas A. Smith for the fabrication of the glass tubes and Mr. Ron Copenhaver for the fabrication of the tubing bombs. We also thank the reviewers for their helpful comments. Literature Cited (1) Yu, J.; Eser, S. Thermal Decomposition of C10-C14 Normal Alkanes in Near-Critical and Supercritical Regions: Product Distributions and Reaction Mechanisms. Ind. Eng. Chem. Res. 1997, 36, 574. (2) Yu, J.; Eser, S. Kinetics of Supercritical-Phase Thermal Decomposition of C10-C14 Normal Alkanes and Their Mixtures. Ind. Eng. Chem. Res. 1997, 36, 585. (3) Lai, W.-C.; Song, C. Temperature-Programmed Retention Indices for GC and GC-MS Analysis of Coal- and PetroleumDerived Liquid Fuels. Fuel 1995, 74, 1436. (4) Yu, J.; Eser, S. Thermal Decomposition of Jet Fuel Model Compounds under Near-Critical and Supercritical Conditions. 1. n-Butylbenzene and n-Butylcyclohexane. Ind. Eng. Chem. Res. 1998, 37, 4591. (5) Bredael, P.; Rietvelde, D. Pyrolysis of Hydronaphthalenes. 2. Pyrolysis of cis-Decalin. Fuel 1979, 58, 215. (6) Virk, P. S.; Korosi, A.; Woebcke, H. N. Pyrolysis of Unsubstituted Mono-, Di-, and Tricycloalkanes. Adv. Chem. Ser. 1979, 183, 67. (7) Hillebrand, W.; Hodek, W.; Kolling, G. Steam Cracking of Coal-Derived Oils and Model Compounds. 1. Cracking of Tetralin and t-Decalin. Fuel 1984, 63, 756. (8) Billaud, F.; Chaverot, P.; Freund, E. Cracking of Decalin and Tetralin in the Presence of Mixtures of n-Decane and Steam at about 810 °C. J. Anal. Appl. Pyrolysis 1987, 11, 39. (9) Fabuss, B. M.; Kafesjian, R.; Smith, J. O.; Satterfield, C. N. Thermal Decomposition Rates of Saturated Cyclic Hydrocarbons. Ind. Eng. Chem. Process Des. Dev. 1964, 3, 248. (10) Shabtai, J.; Ramakrishnan, R.; Oblad, A. G. Hydropyrolysis of Model Compounds. Adv. Chem. Ser. 1979, 183, 297. (11) Poutsma, M. L. Free-Radical Thermolysis and Hydrogenolysis of Model Hydrocarbons Relevant to Processing of Coal. Energy Fuels 1990, 4, 113. (12) Hooper, R. J.; Battaerd, H. A. J.; Evans, D. G. Thermal Dissociation of Tetralin between 300 and 450 °C. Fuel 1979, 58, 132.
(13) Penninger, J. M. L. New Aspects of the Mechanism for the Thermal Hydrocracking of Indan and Tetralin. Int. J. Chem. Kinet. 1982, 14, 761. (14) de Vlieger, J. J.; Kieboom, A. P. G.; van Bekkum, H. Behaviour of Tetralin in Coal Liquefaction. Examination in LongRun Batch-Autoclave Experiments. Fuel 1984, 63, 334. (15) Bredael, P.; Vinh, T. H. Pyrolysis of Hydronaphthalenes. 1. Pyrolysis of Tetralin, 1,2-Dihydronaphthalene and 2-Methylindene. Fuel 1979, 58, 211. (16) Grigorieva, E. N.; Panchenko, S. S.; Fedorova, T. L.; Korobkov, V. Y.; Kagan, D. N.; Kaletchitz, I. V. Tetralin Pyrolysis under H2 Pressure, between 350 and 510 °C. Fuel Process. Technol. 1994, 38, 85. (17) Edwards, T.; Zabarnick, S. Supercritical Fuel Deposition Mechanisms. Ind. Eng. Chem. Res. 1993, 32, 3117. (18) Mallinson, R. G.; Chao, K. C.; Greenkorn, R. A. Reactions of Three Double Ring Heteroaromatic Model Coal Compounds in Excess Tetralin. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1980, 25 (4), 120. (19) Daubert, T. E.; Danner, R. P. Physical and Thermodynamic Properties of Pure Chemicals; Design Institute for Physical Property Data, AIChE: Washington, DC, 1992. (20) Yu, J. Thermal Decomposition of Hydrocarbons under Near-Critical and Supercritical Conditions. Ph.D. Thesis, The Pennsylvania State University, University Park, PA, 1996. (21) Soave, G. Equilibrium Constants from a Modified RedlichKwong Equation of State. Chem. Eng. Sci. 1972, 27, 1197. (22) TRC Thermodynamic TablessHydrocarbons; Thermodynamic Research Center: The Texas A & M University System, College Station, TX, 1995. (23) Miller, D. B. Higher Alpha, Omega-Dienes in Paraffin and Olefin Pyrolyzates. Ind. Eng. Chem. Prod. Res. Dev. 1963, 2, 220. (24) Steacie, E. W. R. Atomic and Free Radical Reactions; Reinhold: New York, 1946. (25) Franz, J. A.; Camaioni, D. M. Fragmentations and Rearrangments of Free Radical Intermediates during Hydroliquefaction of Coals in Hydrogen Donor Media. Fuel 1980, 59, 803
Received for review May 18, 1998 Revised manuscript received September 29, 1998 Accepted October 2, 1998 IE980302Y
