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Energy & Fuels 2001, 15, 714-723
Relationship between the Formation of Aromatic Compounds and Solid Deposition during Thermal Degradation of Jet Fuels in the Pyrolytic Regime John M. Andre´sen,* James J. Strohm, Lu Sun, and Chunshan Song* The Energy Institute and Department of Energy & Geo-Environmental Engineering, The Pennsylvania State University, 209 Academic Projects Bldg., University Park, Pennsylvania 16802 Received November 8, 2000. Revised Manuscript Received February 8, 2001
The formation of pyrolytic solid deposit, or coke, in the fuel line can be detrimental to the operation of high-speed aircraft. Yet, the formation of coke from the fuel has not been well characterized. The present study has investigated the relationship between the formation of aromatic compounds and solid deposition for three candidates for high-thermal-stability jet fuels at 482 °C (900 °F) with stressing periods up to 2 h. The fuels include one coal-derived (JP-8C), one paraffinic petroleum-derived (JP-8P), and one naphthenic petroleum-derived (DA/HT LCO). The DA/HT LCO, an extensively hydrotreated light cycle oil where virtually all aromatics have been hydrogenated to cycloalkanes, suppressed the solid deposition to a greater extent than that of the more paraffinic petroleum-derived JP-8P and showed a comparable low solid deposition to that of the coal-derived fuel JP-8C. Both GC/MS and solution-state 13C NMR analysis on the stressed fuels confirmed that the paraffinic content is most likely to crack under thermal stress, while cycloalkane structures are more thermally stable. Solution-state 13C NMR and HPLC investigations of the overall structure of the stressed liquids indicate that the solid deposition is a function of the rise in the aromatic content and also the amount and rate of development of the nonprotonated aromatic carbons, giving mostly 2 to 4 rings aromatics. Furthermore, solid-state 13C NMR was used to follow the development of the aromatic structure in the corresponding solid deposit as a function of the buildup of aromatic compounds in the stressed liquid fuel.
Introduction Solid deposition is considered to be a growing problem for the jet fuels on-board due to the drive for aircraft operating at higher speed reaching high Mach numbers.1,2 The solid deposition in jet fuels is first associated with dissolved oxygen in the autoxidative regime from around 150 to 350 °C.3 The effect of oxygen can be halted by different approaches, including antioxidants additives.4 Second, after the consumption of the oxygen, the jet fuel can form solid deposits in the pyrolytic regime, i.e., coke, when heated to temperatures above 400 °C.5 As the flight speed is increased, the fuel is expected to experience temperatures up to 482 °C (900 °F) or higher when it also is functioning as the main coolant for the different electronic and mechanical parts of the jet plane.6 Even though fairly short residence times are expected at such elevated temperatures, in the range of seconds to a few minutes, paraffinic petroleum(1) Edwards, T.; Zabarnick, S. Ind. Eng. Chem. Res. 1993, 32, 31173122. (2) Edwards, T. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 2000, 45 (3), 436-439. (3) Hazlett, R. N. Thermal Oxidative Stability of Aviation Turbine Fuels; ASTM: Philadelphia, PA, 1991. (4) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier Science Pub.: Amsterdam, The Netherlands, 1993; pp 121-157. (5) Taylor, P. H.; Rubey, W. A. Energy Fuels 1988, 2, 723-728. (6) Edwards, T. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1996, 41 (2), 481-487.
derived jet fuels have shown a rapid degradation leading to a high degree of solid deposition or coking in the pyrolytic regime.7,8 The main reason for coke formation in paraffin-based fuels has been associated with the poor thermal stability of long-chain alkanes.9 The cracking products from long-chain alkanes associated with conventional petroleum-derived jet fuels were found to form C1-C4 gases and cyclo-alkenes that further developed different aromatic compounds before coke was observed. The nature of coke formed under thermal stressing of jet fuels has mainly been studied by its optical appearance and focused on the catalytic effect of the surface of the fuel line.10 Only a few studies have used techniques such as FT-IR, temperature-programmed oxidation, and CP-MAS NMR to characterize coke formation.11,12 Therefore, there is still a lack of information about the transformation of hydrocarbons in jet fuel liquids over to polyaromatic solids. Nevertheless, the correlation between the formation of aromatic com(7) Song, C.; Eser, S.; Schobert, H. H.; Hatcher, P. G. Energy Fuels 1993, 7, 234-243.ap (8) Andre´sen, J. M.; Strohm, J. J.; Song, C. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1998, 43 (3), 412-414. (9) Song, C.; Lai, W.-C.; Schobert, H. H. Ind. Eng. Chem. Res. 1994, 33, 534-547 and 548-557. (10) Altin, O.; Eser, S. Ind. Eng. Chem. Res. 2000, 39, 642-645. (11) Eser, S. Carbon 1996, 34, 539-547. (12) Song, C.; Peng, Y.; Jiang, H.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1992, 37 (2), 484-492.
10.1021/ef000256q CCC: $20.00 © 2001 American Chemical Society Published on Web 03/16/2001
Thermal Degradation of Jet Fuels in Pyrolytic Regime
pounds in jet fuels stressed under pyrolytic conditions and the structure of the resultant solid deposition can provide insight into the development of high thermal performance jet fuels with designed ability to suppress coke formation. Accordingly, this study has followed the pyrolytic, i.e., noncatalytic, formation of aromatic compounds in one coal-derived and two petroleum-derived jet fuels at 482 °C (900 °F) with stressing periods up to 2 h and correlated the findings on the liquids with the structural characterization of the corresponding coke that was formed. A wide range of analytical techniques was used in this work, including GC/MS, solution-state 13C NMR, HPLC, and solid-state 13C NMR. While previous studies have generally focused either on thermally stressed liquids or resultant cokes, the present study has combined the above techniques to characterize both the stressed jet fuels and the related cokes to follow the pathway of solid deposition for jet fuels under pyrolytic conditions. Both GC/MS and solution-state 13C NMR analysis were used on the thermally stressed fuels to follow the different rates of thermal cracking of the long-chain alkanes and cycloalkane structures as well as the subsequent formation of aromatic compounds. The GC/MS approach was useful to monitor the thermal decomposition changes for individual model compounds for fuels,7,13 while the solution-state 13C NMR is a good complement to the GC/ MS technique, especially for complex liquids from thermally stressed fuels.14,15 The NMR technique can follow the changes in distribution of the main hydrocarbon groups in the fuel during thermal stressing, particularly the formation of aromatic carbon. However, both the above techniques do have some limitations in the case of following the development of aromatic ring systems. Therefore, the use of HPLC can add valuable information about the development of aromatic ring systems in thermally stressed fuels and related liquids, since HPLC can detect aromatic compounds containing up to 8-10 condensed rings.16,17 Furthermore, solidstate 13C NMR was used to follow the development of the aromatic structure in the corresponding solid deposit as a function of the buildup of aromatic compounds in the stressed liquid fuel. The quantitative single-pulse excitation technique (SPE) used in this work has been previously proved to detect virtually all the carbon in carbonaceous systems such as condensed as coal tar pitches, anthracites coals, chars, and cokes.18-20 Hence, this work characterizes the path from liquid compounds to coke for different jet fuels in the pyrolytic regime on the basis of their average structures, as determined by a battery of analytical techniques conducted on both the stressed liquids and their solid counterparts. (13) Lai, W. C.; Song, C. Fuel 1995, 74, 1436-1451. (14) Mckinney, D. E.; Bortiatynski, J. M.; Hatcher, P. G. Energy Fuels 1993, 7, 578-581. (15) Strohm, J. J.; Andresen, J. M.; Song, C. Prepr.-Pap.sAm. Chem. Soc., Div. Pet. Chem. 2000, 45 (3), 465-469. (16) Barman, B. N.; Cebolla, V. L.; Membrado, L. Crit. Rev. Anal. Chem. 2000, 30, 75-120. (17) Mckinney, D. E.; Clifford, D. J.; Hou, L.; Bogdan, M. R.; Hatcher, P. G. Energy Fuels 1995, 9, 90-96. (18) Andre´sen, J. M.; Martı´n, Y.; Moinelo, S. R.; Maroto-Valer, M. M.; Snape, C. E. Carbon 1998, 36, 1043-1050. (19) Maroto-Valer, M. M.; Andre´sen, J. M.; Schobert, H. H. Prepr.Pap.sAm. Chem. Soc., Div. Pet. Chem. 1999, 44 (3), 675-679. (20) Maroto-Valer, M. M.; Andre´sen, J. M.; Rocha, J. D.; Snape, C. E. Fuel 1996, 75, 1721-1726.
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Experimental Section Samples. The coal-derived jet fuel, JP-8C, was obtained by hydrotreating and hydrocracking a coal tar.7 The paraffinic petroleum-derived jet fuel, JP-8P, was similar to the commercial jet fuel Jet A-1. The naphthenic petroleum-derived jet fuel (DA/HT LCO) was produced by British Petroleum through three successive stages: (i) severe hydrotreatment of a light cycle oil (LCO) at 350 °C, (ii) dearomatizing (335 °C), and (iii) further hydrotreatment at 320 °C.8 These fuel samples were stored in a refrigerator before test in the small batch reactors. Pyrolysis of Samples. About 5 mL of the jet fuels was stressed at 482 °C (900 °F) in a 25 mL microautoclave under an initial pressure of 100 psi N2 in a fluidized sandbath. After reaction, the microautoclaves were rinsed of sand and quenched by water for about 5 s for instant cooling. When the microautoclaves were cooled to ambient temperature, cleaned for sand, and dried, it was checked to ensure that no products had leaked during stressing. The gas phase was then released from the microautoclaves and collected in special gas bags for immediate analysis (not reported here), and the weight of the gaseous products was determined from the weight decrease of the microautoclave. The liquid fraction was filtered using a 0.2 µm filter and stored for analysis. The tube and the stem of the microautoclave were washed in pentane until a clear color and dried. The pentane washes were also filtered using a 0.2 µm filter to accumulate any solids. The solid deposit was determined from the weight gain of the tube and the stem of the microautoclave and the amount of solid deposition filtered from the stressed jet fuel solution and pentane washes. The liquid fraction was obtained by difference. The weight of the clean and empty microautoclaves was determined before and after each experiment and stayed within a confidence range of (0.005 g. The solid deposit was physically recovered, vacuum-dried at 80 °C, and weighed. In general, the recovered solid was slightly less than estimated by weight difference, because 100% recovery was practically difficult. Typical sample loading of a microreactor was 4 g, giving an error in the measurements of (0.005/4 × 100 ) (0.1 wt %. Analysis of Liquids and Solids. The GC-MS was performed on a Hewlett-Packard 5890 Series II GC coupled with a HP 5971A MS detector. The column used was a slightly polar J&W DB-5 coated with 5% phenyl-95% methyl polysiloxane and it was heated from 40 to 290 °C with a heating rate of 3 and 6 °C min-1. Around 90-95% of the peak area was investigated, accounting for around 200-300 peaks in each case. The peak areas of n-C8 to n-C18 and some aromatic compounds were calibrated with standards, and the calibrated peak areas were used to semiquantitatively determine the volume % of the different components in the samples. The solution-state 13C NMR was performed on a Bruker AMX360 with a field of 8.4 T equivalent to a 90 MHz resonance frequency for 13C. Approximately 0.2 mL was diluted in 1 mL CDCl3 and charged into a 5 mm tube. The gated decoupling pulse sequence with a 7 s pulse delay was found to give a quantitative spectrum. The high-pressure liquid chromatography (HPLC) was conducted on a Waters 600E Pump/ Controller system coupled with a Waters 991M Photodiode UV-VIS Array Detector, using a wavelength range of 240 to 450 nm and a gradient elution method with n-hexane and methylene chloride as solvents.21 The column used was a Hypersil PAH-2 which is based on tetrachlorophthalimidopropyl-bonded silica.21 The solid-state 13C NMR experiments were conducted on a Chemagnetics M-100 with a field of 2.4 T and a spinning speed of about 3.5 kHz. For the cross polarization (CP) measurements around 10k-20k scans were accumulated using 1 ms contact time and 1 s recycle delay. For the quantitative single-pulse excitation experiments, SPE, about (21) Saini, A. K.; Song, C. Prepr.-Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (3), 796-800.
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Table 1. Changes in Gaseous Products and Solid Deposits for the Three Jet Fuels Investigated, JP-8P, JP-8C, and DA/HT LCO, Stressed at 482 °C (900 °F) for up to 2 Hours JP-8P
JP-8C
DA/HT LCO
time/ min
gas/ wt %
solids/ wt %
gas/ wt %
solids/ wt %
gas/ wt %
solids/ wt %
15 30 45 60 90 120
4.4 10.1 17.6 24.2 35.1 35.6
