Supercritical Thermal Cracking of - American Chemical Society

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Energy & Fuels 2008, 22, 3960–3969

Supercritical Thermal Cracking of N-Dodecane in Presence of Several Initiative Additives: Products Distribution and Kinetics Guozhu Liu, Yongjin Han, Li Wang, Xiangwen Zhang,* and Zhentao Mi Key Laboratory of Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, P.R. China ReceiVed May 11, 2008. ReVised Manuscript ReceiVed August 8, 2008

Supercritical initiative-thermal cracking of a jet fuel model compound, n-dodecane, was studied in presence of several initiative additives, such as1-nitropropane (NP), triethylamine (TEA), and 3,6,9-triethyl-3,6,9-trimethyl1,4,7-triperoxonane (TEMPO) in view of improving heat sink of jet fuel. It was found that remarkable promoting effect of the initiative additives on the cracking rates, compared with the thermal cracking of pure n-dodecane, were observed up to 20-150% in the following order: NP > TEMPO > TEA. Comparisons of products distributions from the thermal cracking of n-dodecane with and without initiators indicated that initiators type had a slight effect on the gas products selectivity, but a non-negligible effect on liquid products distributions. Apparent first-order kinetics was used to describe the supercritical initiative-thermal cracking of n-dodecane, and the apparent cracking activation energy of pure n-dodecane were 256.56 kJ/mol, which decreased to 185.80 kJ/mol by NP, 196.05 kJ/mol by TEMPO, and 242.83 kJ/mol by TEA. Attempts were also made to explain the observed experimental results with proposed reaction mechanisms for the thermal cracking of pure initiators.

Introduction As aircraft flight speed increases to supersonic and hypersonic regimes, aerodynamic heating becomes increasingly severe and thus one of the crucial issues in the development of hypersonic aircrafts is thermal managements of the vehicles and engines. Currently, a lot of studies are being conducted to develop an active cooling system, called regenerative cooling, that uses liquid hydrocarbon fuel in an advanced aircraft not only as an energy source but also a primary coolant to absorb heat from various aircraft components such as engine lubricating oil, hydraulic fluid, environmental control system, electrical system, and air frame.1-3 It is generally accepted that conventional liquid hydrocarbon fuels may offer required cooling capacity because of their significant sensible heat sink capacities and endothermic reactions resulting from chemical decompositions at that high temperature.1-3 In a practical view of obtaining more endothermic heat and improving the cooling capability, high cracking degree of hydrocarbon fuel was essential considering that the sensible heat sinks capacities changed slightly for different hydrocarbon fuels. It was well-known that thermal cracking of hydrocarbons generally took place at very higher temperature to occur at useful rate. Therefore, it was necessary to develop novel methods to increase the cracking reaction rates of jet fuel and thus to provide necessary endotherm at high speed flight. Catalytic cracking of jet fuel in presence of a zeolite catalyst was one of important choices to obtain a high conversion with * To whom correspondence should be addressed. Tel./Fax: 86 22 2789 2340. E-mail: [email protected]. (1) Petley, D. H.; Jones, S. C. Thermal management for a Mach 5 cruise aircraft using endothermic fuel. J. Aircraft 1992, 29 (3), 384–389. (2) Huang, H.; Spadaccini, L. J.; Sobel, D. R. Fuel-cooled thermal management for advanced aeroengines. J. Eng. Gas Turbines Power 2004, 126 (2), 284–293. (3) Edwards, T. Cracking and deposition behavior of supercritical hydrocarbon aviation fuels. Combust. Sci. Technol. 2006, 178, 307–334.

a carbenium ion mechanism. Unfortunately, catalytic cracking had several fatal drawbacks: the catalyst must be applied in very thin layers to reduce the impact of catalyst in heat transfer, and zeolite catalyst deactivated rapidly by accumulation of coke and exhibited high selectivity for coke and low selectivity for olefins compared with thermal cracking. This made the design, and construction of the reactor very complex, and brought many difficulties for regeneration of catalyst.2 Some researchers noted that thermal cracking of a hydrocarbon molecule proceeded via chain reaction of free radicals generated from C-C bond cleavage, in which free radical generation was a typical rate-limited step of this process.4 Therefore, addition of an initiator that rapidly produced a great amount of free radicals due to lower bond dissociation energy (BDE) than C-C bond, would result in an increase in the overall rate of thermal cracking without changing radical reaction mechanism. Chang et al.5-7 proposed that the addition of some free radical initiators into the oils, such as ditertbutyl peroxide (DTBP), is helpful in generating free radicals at lower temperature and thus improve s the reaction rate dramatically. They investigated a series of initiative additives including sulfur dioxide, hydrogen peroxide, acetone, DTBP, and nitromethane to examine their effects on the cracking rates of oil and bitumen model compound 1-phenyldodecane and found that the effects of initiative additives depended on the properties of additives and reactants; the hydrothermal cracking conversion of the (4) Watanabe, M.; Tsukagoshi, M.; Hirakoso, H.; Adschiri, T.; Arai, K. Kinetics and product distribution of n-hexadecane pyrolysis. AIChE J. 2000, 46, 843–856. (5) Chang, J.; Fan, L.; Fujimoto, K. Enhancement Effect of Free Radical Initiator on Hydro-thermal Cracking of Heavy Oil and Model Compound. Energy Fuels 1999, 13, 1107–1108. (6) Chang, J.; Tsubaki, N.; Fujimoto, K. Elemental sulfur as an effective promoter for the catalytic hydrocracking of Arabian vacuum residue. Fuel 2001, 80 (11), 1639–1643. (7) Chang, J.; Fu, Y.; Shibata, Y.; Yoshimoto, M.; Fujimoto, K.; Tsubaki, N. Promotional effect of oxidation pretreatment on hydro-thermal cracking of Canadian oil sand bitumen. Fuel 2005, 84 (12-13), 1661–1663.

10.1021/ef800323d CCC: $40.75  2008 American Chemical Society Published on Web 09/18/2008

Supercritical InitiatiVe-Thermal Cracking of n-Dodecane

model compound was remarkably improved from 9.8% to about 40% by 1 wt % DTBP. Wickham et al.8 investigated the promoting effect of an initiator in the thermal cracking of heptane under supercritical condition, and observed that only 2 wt % initiator accelerated the cracking rate of heptane up to 200% with the similar reaction mechanism and product distribution as the thermal cracking under supercritical conditions (773.15 K, 6.8 MPa). Wickham et al.9,10 further applied laboratory bench-scale and pilot-scale calorimetric tests to directly measure heat sink capacity of JP-7 and model compound n-decane with or without initiator, and found that the initiator improved the heat sink (from 25 to 550 °C under 550 psi) from 766 to 849 Btu/lb for n-decane, from 732 Btu/lb to 791 Btu/lb for JP-7 fuel at liquid hourly space velocity (LHSV) of 1000 h-1. Those work confirmed the feasibility of applying initiator as an effective additive to improve heat sink of jet fuel under supercritical condition. Wang et al.11-14 also studied thermal cracking of heptane initiated by triethylamine (TEA) and tributylamine (TBA) in a flowing reactor at the temperature of 550-650 °C under atmospheric pressure and supercritical conditions. They indicated that TEA and TBA are effective in promoting the cracking rate of heptane and ascribed the accelerating effects to the initiative release of ethyl radical by TEA from the scission of the C-N bond based on GC-MS and molecular simulation results. Recently, Zhao et al.15 studied initiative cracking of n-heptane diluted with steam in BDE of 180-260 kJ/mol were considered as good initiators for initiated cracking of n-heptane and nitroethane was the best in the used initiators. In conclusion, initiative-thermal cracking is a promising resolution to improve thermal cracking rates of jet fuel and gained abroad attentions because of their good solubility and service performances. Unfortunately, uncertainties still exist in the design of active cooling process whether the initiator has a negative influence on the thermal stability of jet fuel during the thermal stressing, which is another essential issue in the application of initiators. Up to now, little effort has been made to investigate both the thermal oxidation and pyrolytic stability of jet fuel in the presence of initiators, which may result in the heat transfer resistance to the active cooling system, and dispersion in the fuel plugs filters or nozzles.16 As the first step of a series work on the thermal stability of the initiative-thermal cracking of jet fuel, further studies are still necessary to provide more information on the conversion and product distribution of jet fuel cracking in presence of (8) Wickham, D. T.; Engel, J. R.; Hitch, B. D.; Karpuk, M. E. J. Initiators for Endothermic Fuels. J. Propul. Power 2001, 1253–1257. (9) Wickham, D. T.; Engel, J. R.; Hitch, B. D. Additives to Increase Fuel Heat Sink Capacity in a Fuel/Air Heat Exchanger. Am. Inst. Aeronaut. Astronaut. 2002, 3872–3882. (10) Wickham, D. T.; Engel, J. R.; Rooney, S. Additives to Improve Fuel Heat Sink Capacity in Air/Fuel Heat Exchangers. J. Propul. Power 2008, 24 (1), 1253–1257. (11) Wang, Z.; Lin, R. S.; Fang, W. J.; Li, G.; Guo, Y. S.; Qin, Z. W. Triethylamine as an initiator for cracking of heptane. Energy 2006, 31, 2773– 2790. (12) Wang, Z.; Lin, R.; Guo, Y.; Li, G.; Fang, W.; Qin, Z. Tributylamine as an initiator for cracking of heptane. Energy ConVers. Manage. 2008, 49 (6), 1584–1594. (13) Wang, Z.; Guo, Y.; Lin, R. Effect of triethylamine on the cracking of heptane under a supercritical condition and the kinetic study on the cracking of heptane. Energy ConVers. Manage. 2008, 49 (8), 2095–2099. (14) Wang, Z.; Lin, R. Theoretical study on the reaction route for the major liquid product from pyrolysis of triethylamine. J. Anal. Appl. Pyrolysis 2008, 81, 205–210. (15) Zhao, R.; Wang, X.; Gao, J.; Gao, J. Production of light alkenes by initiated cracking of heavy hydrocarbons. Petrochem. Technol. 2007, 36, 1110–1113. (16) Huang, H.; Spadaccini, L. J. Coke removal in fuel-cooled thermal management systems. Ind. Eng. Chem. Res. 2005, 44 (2), 267–278.

Energy & Fuels, Vol. 22, No. 6, 2008 3961 Scheme 1. Structure Formulation of TEMPO

initiative additives. Jet fuels include hundreds of aliphatic and aromatic hydrocarbons, among which major components are normal or branched alkanes, cycloalkanes, and aromatics. n-Dodecane (Dod), a major component of some petroleumderived jet fuels (RP-3, JP-7, JP-8) is one of the surrogates usually chosen for normal alkanes.17,18 The fuel system in an advanced aircraft is generally operated under pressure of 3.4-6.9 MPa above 400 °C because of the increasing thermal management requirement.3 The critical temperature and pressure of jet fuels is typically 370-400 °C and 2.5 MPa, and thus jet fuel is typically supercritical fluids in those conditions.3,19 Reaction mechanisms under supercritical conditions can often show dramatic differences from lower pressure conditions.20 Therefore, all the experimental runs of this work were carried out using Dod as a jet fuel model compound above 420 °C and initial pressure of 3.11-4.30 MPa (Pr ) 1.72-2.37), which ensures similar working conditions of the experimental runs as the future thermal environments of jet fuels. In the literature, nitromethane, DTBP, and TEA were used in the thermal cracking of hydrocarbons as the initiators because of the lower BDE of the C-N and O-O bonds. It was a pity that the DTBP initiator was not suitable for hydrocarbon fuels because of its little solubility and nonagitation process. Likewise, it was also impossible to use nitromethane because of its lower flash point and thermal instability. A cyclic triperoxy initiator 3,6,9-Triethyl-3,6,9-trimethyl-1,4,7-triperoxonane (TEMPO, see Scheme 1), compared with DTBP, has better thermal stability and solubility in the hydrocarbons up to 40 wt %. 1-Nitropropane (NP) with higher flash point and stability than nitromethane was also selected as an initiator. Therefore, in this work, we choose TEA, NP, and TEMPO as the initiative additives for the supercritical thermal cracking of Dod. The objective of this work was to investigate the supercritical initiative-thermal cracking of a jet fuel model compound, n-dodecane(Dod) in presence of TEA, 1-nitropropane (NP), and 3,6,9-triethyl-3,6,9-trimethyl-1,4,7-triperoxonane (TEMPO) at 420-455 °C under initial pressure from 1.70 to 2.40 MPa (calculated from the SRK equation21). The Dod conversions and gas and liquid product distributions with and without various initiators were compared to show the influence of the initiators. (17) Edwards, T.; Maurice, L. Q. Surrogate mixtures to represent complex aviation and rocket fuels. J. Propul. Power 2001, 17 (2), 461– 466. (18) 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, 419–429. (19) Yu, J.; Eser, S. Determination of Critical Properties of Some Jet Fuels. Ind. Eng. Chem. Res. 1995, 34, 404–409. (20) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at supercritical conditions: applications and fundamentals. AIChE J. 1995, 41 (7), 1723–1778. (21) 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 (3), 574–584.

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Table 1. Conditions for Thermal Cracking of n-Dodecane with and without Initiators params

values

T (°C) initial pressure (MPa) reaction time (min) reactor volume (mL) loading ratio additive concentration (wt %)

420-455 3.11-4.30 12-80 33 0.303 0-4

Attempts were also made to provide the first-order apparent cracking kinetics and possible cracking mechanisms. Experimental Section Materials. Dod with 99.5% purity, was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). TEA with 99.0% purity was purchased from Tianjin Chemical Reagent Company. NP with 98.0% purity was provided by Alfa Aesar. A cyclic trifunctional peroxide initiator TEMPO (41% solution in aromatics free mineral spirit) was purchased from Aacros Organics. All the chemicals were used as received. Thermal Cracking Experiments. The thermal cracking experiments were conducted in a 316 stainless steel tubing bomb reactor, as described by Yu and Eser21-25 and Song et al.26,27 The volume of the reactor body is about 25 mL, and the total volume of the reactor system is about 33 mL. After special passivation treatment, 10 mL of the sample was loaded into the reactor. The reactor was then sealed and purged with UHP-grade nitrogen (99.999%, Tianjin Special Gas Company, Tianjin, China) to remove oxygen and the final head space nitrogen pressure was maintained. A self-designed electronic oven with deviation of (2.5 °C was used to heat the reactor. Before an experiment, the oven was preheated to the desired temperature. The reactor was then plunged into the oven. The heatup period for the tubing bomb to reach 420-455 °C was about 2-4 min. Table 1 summarized reaction conditions for thermal cracking of n-dodecane with and without initiators. It should be mentioned that the average reaction temperature for a tubing bomb experiment was slightly lower than the assigned temperature because part of the top stem was not immersed in the oven. After completion of each run, the reactor was quickly taken out and quenched in cool water. Analysis. The liquid products were first weighed and then analyzed quantitatively by a Hewlett-Packard 4890 gas chromatograph (GC) equipped with a flame ionization detector (FID) and a PONA capillary column (50 m × 0.20 mm × 0.50 µm, Zhongke Kaidi Chemical Technolgy Co., Ltd., Lanzhou, China). The column temperature was programmed from 70 to 280 at 7 °C/min with an initial isothermal period of 1 min at 70 °C and a final isothermal period of 10 min at 280 °C. The liquid products were identified by Agilent 6890N/5975 inert GC-MSD equipped with a HP-5 MS column (30 m × 0.25 mm × 0.25 µm). High pure helium (99.999%) was used as a carrier gas with a flow rate of 1 mL/min. The column temperature was first kept at 60 °C for 3 min, and then programmed to 200 at 5 °C/min, and to 280 at 10 °C/min with a final isothermal (22) 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. (23) Yu, J.; Eser, S. Supercritical-phase thermal decomposition of binary mixtures of jet fuel model compounds. Fuel 2000, 79 (7), 759–768. (24) 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 (12), 4591–4600. (25) Yu, J.; Eser, S. Thermal decomposition of jet fuel model compounds under near-critical and supercritical conditions. 2. Decalin and Tetralin. Ind. Eng. Chem. Res. 1998, 37 (12), 4601–4608. (26) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Degradation Studies of a Coal-Derived Aviation Jet Feuls. Energy Fuels 1993, 7, 234– 243. (27) Song, C.; Lai, W. C.; Schobert, H. H. Condensed-phase pyrolysis of n-tetradecane at elevated pressures for long duration. Product distribution and reaction mechanisms. Ind. Eng. Chem. Res. 1994, 33 (3), 534–547.

Figure 1. Typical total ion chromatography of liquid cracking products of n-dodecane with NP (a) propene, (b) butane, (c) pentene, (d) pentane, (e) cyclopentane, (f) hexane, (g) hexane, (h) 2-hexene, (i) 2-hexene(Z), (j) 2-methyl-1-pentene, (k) 2,4-dimethylpetane, (l) 3-methylhexane, (m) 1-heptene, (n) heptane, (o) 2-heptene, (p) 3-heptene, (q) 2,3-dimethyl2pentene, (r) 2,3-dimethyl-2-pentene, (s) 1-nitropropane, (t) 1-octene, (u) octane, (v) 3-octene, (w) 1-nonene, (x) nonane, (y) 3-nonene, (z) 2-nonene, (A) 1-decene, (B) decane, (C) 5-decene, (D) 1-undecene, (E) undecane, (F) tridecane, (G) tetridecane, (H) tetradecane, (I) 5-ethyldecane, (J) 3,4dimethyloctane, (K) 3-methyl-5-propylnonane, (L) 6-methyltridecane, (M) 5-methyloctadecane.

period of 10 min at 280 °C. The transfer line temperature was 280 °C and Quadrupole MSD temperature was kept at 150 °C. The mass range and the scan rate were 50-400 amu and 5.12 scans/s. Figure 1 presents a typical total ion chromatography (TIC) of liquid products of Dod cracking. Both the primary and the secondary products were found. The primary products include n-alkanes from C5 to C11 and 1-alkenes from C5 to C11. The secondary products include 2-alkenes, n-C13, and C14 normal and branch alkanes. Analysis errors in the liquid compositions were less than 1.5%. The gaseous products were collected by the water displacement method and analyzed quantitatively using a SP-3420 GC equipped with two different columns and detectors. One Al2O3/S capillary column (50 m × 0.53 mm × 0.50 µm) from Zhongke Kaidi Chemical Technolgy Co., Ltd. (Lanzhou, China) was used to determine the yields of gases with a FID. The temperature of column with the FID was kept at 80 °C. The gaseous hydrocarbon products were identified and quantified by using standard gas mixtures. The other stainless steel column packed with C-2000 PLOT capillary column from Zhongke Kaidi Chemical Technolgy Co., Ltd. (Lanzhou, China) was used to determine the yields of H2 with a thermal conductivity detector (TCD). Uncertainties associated to the measurement of gas composition were less than 1.0%. Gas mass could be calculated from average molecular weight. Errors in the mass balance of reactant and products before and after reaction were less than 1.5%.


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The conversion was defined as the mole fraction of the reactant converted. Selectivity of liquid-phase reaction products was expressed as the number of moles obtained per 100 moles of the reactant converted. The gas products yield was defined as the mass fraction of the reactant converted. Selectivity of gaseous products was expressed as the mole number of certain compound per 100 moles gas products. Reproducibility tests showed that the relative uncertainties for the conversion and the yields of major products, with the exception of the C3 and C4 products dissolved in liquid products, were lower than 2.5%. The relative uncertainties for n-propane and 1-propylene were 10% because the GC peaks for the two compounds were not completely separated. In this paper, to describe the promoting effects of initiators, we used the following two items: conversion improvement was defined as percent difference between Dod conversion with and without initiators, and the performance enhancement is the ratio of conversion improvement with pure Dod conversion.

Results and Discussion Conversion of Dod. Figure 2 presents the effect of 2wt% initiators (TEA, TEMPO, and NP) on the cracking conversion of Dod at 420-455 °C. The conversion of Dod at 420 °C (Pr ) 1.72) only reached 17.0% after 80 min. In contrast, the conversion sharply increased to 20, 30, and 32.5% (almost 100% performance enhancement) by adding only 2 wt % TEA, TEMPO, and NP, respectively (see Figure 2a). At 440 °C (Pr ) 2.10), performance enhancement of 150% was obtained with 2% NP, 100% with 2% TEMPO, and only 50% with 2% TEA after 20 min (see Figure 2b). It was clear that for all the experiments initiators had a significant acceleration effect of 20-150% on the thermal cracking rate of Dod in the following order: NP > TEMPO > TEA. This remarkable acceleration effect could result from the following facts: initiators decomposed rapidly at high temperature and produced a great amount of free radical, which could abstract hydrogen from Dod and initialized chain reactions at the initial stage besides the conventional initiation step of C-C bond cleavage in the thermal cracking. Consequently, the conversion improvement should be kept constant after the completed decomposition of initiators below the temperature, at which C-C bond cleavage rate was distinct. In fact we also observed those results in this work, for example at 440 °C in this work, it was possible for the initiators to completely decomposed into free radicals that enhances thermal cracking rates in a very short time, and thus the conversion improvement (about 20%) by NP changed slightly with the reaction time, taking the experimental errors into consideration. From the above results and discussions, it was concluded that there exists a maximum cracking rate or reactant conversion enhancements for given initiator and concentration, which is helpful for the screening and applications of initiators. Temperature is another important factor influencing both the initiator decomposition rate and C-C cleavage rate, and thus the promoting effects of initiators. At lower temperature of 420 °C, both thermal cracking conversion of Dod and homolytic cracking conversions of initiators were very slow at 20 min, which leads to a conversion improvement of only 7% by 2% NP. However, conversion improvement increased significantly with the reaction time because of incensement of homolytic cracking conversion of initiators and reached a maximum of about 15% after 80 min (see Figure 2a). With the increasing reaction temperature, it was possible for the initiators to achieve complete cracking in a very short time, whereas the thermal cracking rate of Dod was slower, resulting in the observed conversion improvement being kept almost constant with the extension of reaction time. As shown in Figure 2b, when the temperature rised to 440 °C, conversion improvement of ca.

Figure 2. Effects of initiator additives on cracking conversion of n-dodecane under supercritical conditions ((a) T ) 420 °C, Pr ) 1.72; (b) T ) 440 °C, Pr ) 2.10; (c) T ) 455 °C, Pr ) 2.38. 9, Pure Dod; b, Dod + 2 wt % NP; 2, Dod + 2 wt % TEMPO;1, Dod + 2 wt % TEA).

18% almost kept constant in 80 min reaction. However, when the temperature was raised to 455 °C, the conversion improvement decreased from 22 to 12% with time changing from 20 to 80 min (see Figure 2c). The reasonable explanation may be the fact that C-C cleavage rate at high temperature was high enough for a distinct thermal cracking of Dod. As evidence, it was observed that in a given cracking period of 20 min, the Dod conversion was so low at 400 °C that could not be detected, but it increased from 2.5% at 420 °C, even to 22% at 455 °C, which may considerably depress the promoting effect of initiative additives. Wickham et al.9,10also reported the similar results in their supercritical cracking experiments of JP-7 and Norpar-12 with 4 wt % initiator. They found that the chemical heat sink improvement attributed to the cracking of JP-7 and Norpar-12 at 475 °C was very small below 10 Btu/lb; it increased to ca. 60 and 100 Btu/lb at 550 °C, but decreased to ca. 20 and 60 Btu/lb at 600 °C. Wang et al.11 also observed the


Liu et al.

Figure 4. Effects of initiator additives on gaseous produce distribution of Dod cracking (T ) 420 °C, Pr ) 1.72, 35 min).

Figure 3. Effects of initiator additives on gaseous produce yields of n-dodecane cracking under supercritical conditions ((a) T ) 420 °C, Pr ) 1.72; (b) T ) 440 °C, Pr ) 2.10; (c) T ) 455 °C, Pr ) 2.38. 9, pure Dod; b, Dod + 2 wt % NP; 2, Dod + 2 wt % TEMPO;1, Dod + 2 wt % TEA).

similar phenomena in the heptane cracking with 4% TEA using a flowing reactor. In their results conversion improvement was only 8% at 550 °C; it increased to 12% at 600 °C, and then decreased to 8% at 650 °C. In conclusion, pronounced promoting effects of initiative additives on the thermal cracking rates of n-alkanes were also observed in this work. Because of this promoting effect, the reaction temperature could be lowered by adding an initiator, while keeping the same conversion level. Furthermore, temperature is also an important factor influencing the promoting effects. Gas Product Distribution. Generally, variations trends of gaseous products yields from the initiative-thermal cracking of Dod with reaction time and temperature (see Figure 3) were similar as the conversions shown in Figure 2. It was evident that gaseous products yields increased sharply by 50-150% because of conversion enhancement by initiative additives. However, at the given conversion, the gaseous products yields changed considerably for thermal cracking and initiative-thermal cracking. For example, gaseous products yields of Dod thermal cracking for 80 min at 440 °C reached about 150 mg/100 g (conversion is only 36%), which was higher than 90 mg/100 g

(conversion is 40%) for initiative cracking by NP for 35 min (see Figure 2b and Figure 3b). This result might be attributed to the secondary cracking of primary cracking products, such as normal n-C8, n-C9, etc., into smaller molecules given enough reaction time that they were verified by the gaseous product distributions in the following sections. For the thermal cracking of the pure Dod, the primary products were C1-C10 n-alkanes and C2-C11 1-alkenes; the secondary products were 2-alkenes, n-C11, n-C13, and C14-C20 normal and branched alkanes; and the relative yields of the primary and secondary products were dependent upon the reaction conditions.21 Figure 4 sketched the gaseous product distributions of Dod decomposition with or without initiators at T ) 420 °C, Pr ) 1.72, 35 min. The major gaseous products from both thermal and thermal-initiative cracking of Dod were hydrogen, methane, ethane, propane, propene, and butylene. Only a small quantity of butane, pentane, and pentylene were observed in the gas phase because most of them dissolve in the liquid phase. Among all the gaseous products, the yield of methane was always lower than those of ethane and propane, and the yield of ethylene was always lower than that of propylene. Yu and Eser21 also observed the similar results and ascribed the lowest yields of ethylene to the particular reaction mechanism under the supercritical conditions. Examination of Figure 4 revealed that there was only a slight change in the products distribution, indicating that the addition of initiators only increases the conversion but does not remarkably change the reaction mechanism but C-C bonds hemolytic cleavage as an initiation step. It was also noted that the initiator type also influenced the gaseous composition. For the thermal cracking of the pure Dod or with TEA and TEMPO, ethylene selectivities were higher than that of Dod with NP. In contrast, propane selectivity of Dod with NP became highest among pure Dod or with initiators. This is probably a consequence of fact that the elimination of NO2• from chain offers more possibility to form propane. Likewise, Dod with TEA produces more ethane than other initiators. We noted that the conversion of Dod with 2 wt % NP (at 440 °C for 35 min) is 41.2%, which is close to pure Dod conversion of 44.8% for 80 min, and 42.2% with 2 wt % TEMPO for 50 min, and 43.05% with 2 wt % TEA for 65 min. In this respect, we further compared their gaseous product distributions to show the effect of additives on the selectivity on several major gaseous products, as shown in Figure 5. In general, either pure Dod or with initiators had almost the same gaseous product distribution under the similar conversion level, except there were slight differences in the selectivities of several products, which showed that the addition of initiator only slightly



Figure 5. Comparison of gaseous product selectivity of Dod cracking at closing conversion level (T ) 440 °C, Pr ) 2.10; Dod conversion is 41.2% with 2 wt % NP for 35 min, 42.2% with 2 wt % TEMPO for 50 min, and 43.05% with 2 wt % TEA for 65 min, pure Dod conversion is 44.8% for 80 min).

Figure 7. Liquid product distribution of n-dodecane cracking with different initiator additives under supercritical conditions (T ) 440 °C, Pr ) 2.10. 9, Dod conversion of 41.2% with 2 wt % NP for 35 min; b, 42.2% with 2 wt % TEMPO for 50 min; 2, 43.05% with 2 wt % TEA for 65 min; 1, pure Dod conversion of 44.8% for 80 min).

Figure 6. Effects of initiator additives on gaseous produce distribution of n-dodecane cracking under supercritical conditions (T ) 420 °C, Pr ) 1.72, 35 min. 9, Dod; b, Dod + NP; 2: Dod + TEMPO; 1: Dod +TEA).

changed the gas product distribution and supported the fact that initiative-thermal cracking and thermal cracking occurred in the similar mechanism of free radical reactions. The results in Figure 5 also indicated that the selectivity to methane, ethane, and propane for the pure Dod is higher but to hydrogen, ethylene, and propylene, and butylenes is lower than those for Dod with various initiators. This result also verified the above assumptions that some primary alkane products, such as n-C9, n-C10, et al., would further decompose into smaller molecules in a longer reaction time, which leads to the improvement in the selectivity to hydrogen and olefins.

As shown before, the initiator type also slightly affected the selectivity to gas products. For instance, the propylene selectivity was improved from 10 to about 15% by the addition of NP, ethylene selectivity was improved by TEA, butylenes by TEMPO, as shown in Figure 5. Wang et al.12 also reported that cracking of n-heptanes with TBA produced more C4 compounds in contrast with TEA, which further confirms the effect of initiators types on the products distributions. Liquid Product Distribution. Figure 6 shows the carbon number distributions of n-alkane and 1-alkene in the liquid phase products from the thermal and initiative cracking of n-C12 with different initiators. For the Dod with or without initiators, product yield decreased with the increasing carbon number due to increased decomposition tendencies of large radicals (see panels a and b in Figure 6), which was also observed in the reports of Yu and Eser21 and Watanabe et al.4 It was also interesting to observe an increase in C6-C11 n-alkane yields (Figure 6a) and a decrease in 1-alkene yields (Figure 6b) with the addition of initiators. One possible explanation was that initiators might increase the rate for bimolecular hydrogen abstraction reaction and decrease that for unimolecular radical decomposition, the yields of higher n-alkanes would be expected to increase and those of 1-alkenes and light n-alkanes would decrease. Another possible explanation was that the conversion enhancement by initiators led to the increase in C6-C11 n-alkane yields and the decrease in 1-alkene yields, as reported by Yu and Eser.21 We also compared the liquid product distributions at the similar conversion level to show the effect of additives on the liquid product selectivity. Figure 7a presented the production of n-alkanes distributions from the thermal cracking of Dod with and without initiators. It was found that at the similar conversions the selectivities of n-alkanes for pure Dod (in a longer


Figure 8. Effect of initiator concentration on n-dodecane conversion ((a) T ) 420 °C, Pr ) 1.72, 35 min. b, Dod +NP; 2, Dod +TEMPO; 1, Dod +TEA).

reaction time) were higher than those for the Dod with initiators. The formation of 1-alkenes was favored with initiators (at shorter reaction time), as seen in Figure 7b. This could be ascribed to the following facts that bimolecular reactions such as hydrogen abstraction and radical addition were favored over unimolecular radical decomposition reactions in the presence of a great amount of the free radicals produced by the initiators. This meant that the larger primary radicals formed from the β-scission of the parent radicals would tend to undergo radical addition and then further decomposition. This would result in the formation of significant amounts of lower n-alkenes. Therefore, at the similar conversion level, the alkene selectivities benefit from the addition of initiators. Initiator Concentration. Initiator concentration is another important factor influencing the acceleration effect. Figure 8 illuminates the effect of TEA, NP, TEMPO addition amount

Liu et al.

on the thermal cracking of Dod at 420 °C, Pr ) 1.72, and 35 min. It was clear that the Dod conversion dramatically improved from ca. 8 to 12.5% with the addition of 1 wt % NP, and then up to 16.5% with 2 wt %. However, further raising the initiator concentration led to only a slight conversion improvement. Wickham et al.,8 Wang et al.,11 and Zhao et al.15 observed the similar trends in the heptane cracking with 0.5-2 wt %. This might result from the fact that higher free radical concentration improved the possibility of radical termination which led to the lower efficiency, and that abstracted hydrogen and β-scission reaction become the major rate-limiting steps after the rapid decompositions of the initiators. In conclusion, with the increasing additive amount, acceleration effect of imitators increased and kept almost stable at addition amounts of 2-4 wt % because the initiators only accelerated the C-C bond cleavage rate of Dod. Therefore, it is unnecessary to add a large amount of initiative additive into the reactant to get higher conversion. Reaction Kinetics. According to Yu and Eser,22 and Wickham et al.,9,10 the following simple first-order kinetics for the thermal cracking of straight-chain alkanes were used to determine the corresponding rate constants 1 1 k ) ln t 1-X

(1)

where X was the conversion of Dod, k was the apparent firstorder rate constant (h-1), and t was the reaction time (h). For three different temperatures (420, 440, and 455 °C), the rate constants were determined from the method of least-squares by plotting ln[1/(1 - X)] as a function of time. Figure 9 shows the relationship between ln[1/(1 - X)] and time for the thermal decomposition of Dod. Table 2 shows the calculated rate constants. According to the first-order rate constants shown in Table 2, the apparent activation energies (Ea, kJ/mol) and

Figure 9. Plots of ln[1/(1 - X)] versus time for initiative-thermal cracking of n-dodecane with initiators ((a) Dod; (b) Dod + NP; (c) Dod + TEMPO; (d) Dod +TEA. 2, 420 °C; b, 440 °C; 9, 455 °C).



Table 2. Kinetic Parameters for Initiative-Thermal Cracking of n-Dodecane with Initiators rate constant, h-1 reactant

693.15 K

713.15 K

728.15 K

Eaa

log Ab

Dod Dod + NP Dod + TEMPO Dod + TEA

0.144 0.305 0.264 0.168

0.438 0.774 0.654 0.51

1.266 1.432 1.362 1.284

258.56 185.80 196.05 242.83

18.63 13.82 14.19 17.51

a E , apparent activation energy in kJ/mol. a in h-1.

b

A, preexponential factor

Figure 10. Arrhenius plots for initiative-thermal cracking of n-dodecane with initiators (9, Dod; b, Dod + NP; 2, Dod + TEMPO; 1, Dod + TEA).

preexponential factors (A, h-1) could be determined using the following Arrhenius law

( )

-Ea (2) k ) Aexp RT Figure 10 shows the Arrhenius plots for the Dod with and without initiators. The apparent activation energies and preexponential factors obtained from the Arrhenius plots were also shown in Table 2. In the previous literatures, several researchers reported thermal cracking active energy of Dod fall within the range of 245-275 kJ/mol, wherein Yu and Eser,22 and Wickham et al.9,10 obtained the apparent active energy of 264.6 and 268.8 kJ/mol (under supercritical conditions), respectively. The activation energies for the thermal cracking of pure Dod obtained in this work was 256.56 kJ/mol, which was in good agreement with the above literature values.

The apparent thermal-cracking activation energy of n-Dod was reduced sharply to 185.80 kJ/mol by NP, to 196.05 kJ/mol by TEMPO, and 242.83 kJ/mol by TEA, respectively. Wickham et al.9,10 obtained an activation energy of 163.8 kJ/mol for the thermal cracking of Dod with initiators. Those results were consistent with the initiative-thermal cracking mechanism of paraffin by which the initiator was a lower energy source of free radical species. Pure n-Dod Cracking Mechanism. According to the product distributions and the changes in product composition with reaction conditions, Yu and Eser19 proposed a modified free radical chain reactions mechanism to describe the thermal decomposition of n-alkanes under near-critical and supercritical conditions. In their reaction mechanism, cracking reaction began from homolytic C-C bond cleavage to produce two primary radicals (eq 3). These radicals then abstracted hydrogen atoms from the surrounding reactant molecules to form various parent radicals (eq 4), of which most were secondary radicals. The parent radical isomerized and then decomposed by β-scission to form a 1-alkene and a lower primary radical (eq 5). This lower primary radical could undergo one of the following reactions: hydrogen abstraction from the reactant molecule (eq 6), decomposition to form a 1-alkene and a smaller primary radical (eq 7-8), and addition to the terminal carbon of a 1-alkene or radical (eq 9-12) to give a higher radical or products with a fairly low conversion. The reaction mechanism was briefly shown as follows n-C12H26 f RI1 • + R1j •

(3)

R1i • + n-C12H26 f n-CiH2i+2 + n-C12Hx25 •

(4)

n-C12Hx25 • f n - C12Hy25 • f 1 - CkH2k + R1n • (|y - x| g 4; 2 e k e m - 1; n e m - 2) (5) R1n • + n-C12H26 f n-CnH2n + Rmx • 1 • R1n • f C2H4 + Rn-2

(3 e n e 5)

1 • R1n • f Rxn • f 1 - CkR2k + Rn-k

(n g 6; x g 5)

x • R1n • + 1-CkH2k f Rn-k

Rm1 •

x + 1 - CkH2k f Rm-k •

b • Rmx • + 1 - CkH2k f Rm-k

(x * 1)

R1n • + Rmx • f products

(6) (7) (8) (9) (10) (11) (12)

Figure 11. Chromatography of liquid cracking products from NP ((a) propylene; (b) propane; (c) acetonitrile; (d) propanal; (e) nitropropane).


Liu et al.

Cracking Mechanism of n-Dod with Initiators. Wickham et al.8 provided a general initiative cracking mechanism as follows: the first step of the n-alkanes cracking in the presence of initiators was the hemolytic cleavage of the bond (such as O-O bond) within the initiator molecules to form initiator radicals, and then initiator radical abstracted hydrogen from n-alkane to produce hydrocarbon radicals, which undergo the same reaction steps as the thermal cracking. This simple mechanism well-explained the significant acceleration effect and the product distributions. For the initiative-thermal cracking of n-alkanes with TEA and TBA, Wang et al.11-14 studied the reaction mechanism by GC-MS and ascribed the promoting effect on the faster decomposition of TEA and TBA to produce a great amount of ethyl and butyl radicals, which then abstracted the hydrogen of reactant to form parent radicals. The further cracking of the parent radical yields small olefins and lower primary radicals, which also undergo the three reactions described above (eqs 6-12). The thermal decompositions of aliphatic nitro compounds have been extensively investigated over the past few decades. It is generally accepted that there are two major routes for the thermal decomposition of 1-nitropropane: C-N bond fission and HONO elimination.28 On the basis of the experimental results of pure NP cracking, we believed that under our conditions, C-N bond fission primarily took place as the first step with the following mechanism (eqs 13-23):

Figure 12. Chromatography of gas cracking products from NP ((a) ethylene; (b) propene; (c) propane; (d) butane).

work of Eyler et al.29 and Dubnikova et al.30 and the experimental results in Figure 12, we provided the following cracking mechanism of TEMPO, as shown in eqs 27-36

C3H7NO2 f C3H7 • + NO2 •

(13)

C3H7NO2 f C3H7O + NO

(14)

C3H7O f C3H6O + H•

(15)

C3H7 • f C3H6 + H•

(16)

C3H7 • + H • f C3H8

•C2H5 f C2H4 + H•

(27)

(17)

•C2H5 + •H f C2H6

(28)

C3H7 • f C2H4 + CH3 •

(18)

•C2H5 + •C2H5 f C4H10

(29)

C3H7NO2 + H • f C3H7NO + HO•

(19)

C3H7NO + H • f C3H6N + H2O

(20)

NO2 • + H • f NO + HO•

(21)

C3H7 • + HO • f C3H6 + H2O

(22)

•C2H5 + •OH f C2H5OH

(31)

C3H7 • + CH3 • f C4H10

(23)

•C2H5 + •CH3 f C3H8

(33)

•C2H5 + C3H8 f •C3H7 + C2H6

(34)

•C3H7 f C3H6 + H•

(35)

•H + •CH3 f CH4

(36)

To obtain further evidence of the mechanism, both gas and liquid products of the thermal cracking of pure NP were analyzed using GC-MS. In the liquid products of NP decomposition, propylene, propane, acetonitrile, and propanal were detected, as shown in Figure 11. In the gas products of NP decomposition, several compounds, including ethylene, propylene, propane, propanal, and butane, were detected, wherein the content of propylene, the most abundant product, exceeded 90%, as shown in Figure 12. The elementary reactions producing propylene formation involved propyl radical, implying propyl radical possibly abstracted the hydrogen of Dod to form parent radicals during the initiation phase of Dod thermal cracking, rather than eq 4 for pure Dod. The further cracking of the parent radical yields small olefins and lower primary radicals, which also undergoes the three reactions described above (eqs 6-12). The thermal decomposition of TEMPO was also carried out to show the possible cracking mechanism. According to recent

To obtain further evidence of the mechanism, both gas and liquid products of the thermal cracking of pure TEMPO were analyzed using GC-MS. As shown in Figure 13, in the liquid products of TEMPO decomposition, butane, methyl acetate, 2-butanone, and ethyl acetate were detected and the most abundant product was 2-butanone. In the gas products of TEMPO decomposition, several compounds, including methane, ethane, ethylene, propane, propylene, and butane, were detected, as shown in Figure 14. In this sense, the biradical produced from TEMPO decom-

(28) Zhang, Y.-X.; Bauer, S. H. Gas-Phase Pyrolyses of 2-nitropropane and 2-nitropropanol: shock-tube kinetics. J. Phys. Chem. A 2000, 104, 1207– 1216.

(29) Eyler, G. N.; Mateo, C. M.; Alvarez, E. E.; Canizo, A. I. Thermal decomposition reaction of acetone triperoxide in toluene solution. J. Org. Chem. 2000, 65, 2319–2321.



Figure 13. Chromatography of liquid cracking products from TEMPO ((a) propane; (b) butylene; (c) ethyl alcohol; (d) butane, (e) 2-butanone; (f) ethyl acetate).

Figure 14. Chromatography of gas cracking products from TEMPO ((a) methane; (b) ethane; (c) ethylene; (d) propane, (e) propylene; (f) butane).

position play an important role in the initiation phase of Dod thermal cracking. Conclusions Supercritical initiative-thermal cracking of jet fuel model compound Dod was studied in presence of TEA, NP, and TEMPO at 420-455 °C under initial pressure from 3.11 to 4.30 MPa. The observed promoting effect of initiative additives on the Dod conversion reaches 20-150% in the interesting conditions with the following order: NP > TEMPO > TEA. Both initiator decomposition rate and Dod thermal cracking rates (30) Dubnikova, F.; Kosloff, R.; Almog, J.; Zeiri, Y.; Boese, R.; Itzhaky, H.; Alt, A.; Keinan, E. Decomposition of triacetone triperoxide is an entropic explosion. J. Am. Chem. Soc. 2005, 127, 1146–1159.

were strongly dependent on the reaction temperature, which leads to gradual decreases of performance enhancement by initiators. With the increasing initiator concentration from 0 to 2%, Dod conversion increased remarkably and then kept stable at the addition amounts of 2-4 wt %. In general, initiators type had a slight effect on the gas products selectivity, but a non-negligible effect on liquid products distributions, indicating the effect of initiators on the reaction mechanism. Apparent first-order kinetics was used to describe supercritical thermal-initiative cracking of Dod, wherein apparent cracking active energy of Dod was 256.56 kJ/mol, 185.80 kJ/mol with NP, 196.05 kJ/mol with TEMPO, and 242.83 kJ/mol with TEA, which agrees well with the literature results. To explain the observed results, we also proposed thermal-initiative cracking mechanisms of pure initiators. Now we are carrying out experiments on the thermal stability of Dod and RP-3 jet fuel in presence of initiative additives, which will provide some novel information on the coke formation and deposition behavior for the supercritical initiativethermal cracking of jet fuel. Acknowledgment. Financial support by the China Postdoctoral Science Foundation (20060400191), Innovative Research Project of Petrochina Co. Ltd, and the Programme of Introducing Talents of Discipline to Universities (B06006) is gratefully acknowledged. Grateful thanks are also given to the reviewers of this manuscript for their academic and helpful suggestions. EF800323D

Supercritical Thermal Cracking of - American Chemical Society

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