Energy & Fuels 2009, 23, 2997–3001
2997
Coking of Model Hydrocarbon Fuels under Supercritical Condition Wenjie Xie, Wenjun Fang,* Dan Li, Yan Xing, Yongsheng Guo, and Ruisen Lin Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed December 26, 2008. ReVised Manuscript ReceiVed April 10, 2009
Coking of three model compounds of hydrocarbon fuelsn-heptane, cyclohexane, and tricyclo[5.2.1.02.6]decane (JP-10)sduring their thermal cracking processes under supercritical condition (873.15 K, 4.1 MPa) has been investigated. The product distributions of the thermal cracking are analyzed by gas chromatography-mass spectrometry (GC-MS). The morphology and microstructures of the cokes are characterized by the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). The results show that chemical structures play important roles in the thermal stability and coking property of the fuels. The thermal cracking conversion of n-heptane is highest, and the coke yield of JP-10 is highest under the same conditions. It is interestingly observed that the morphologies of the cokes produced from the thermal cracking of three fuels are quite different, which from n-heptane, cyclohexane, and JP-10 are in the forms of carbon nanofilaments, carbon nanotubes, and irregular carbon particles, respectively.
1. Introduction With the progress of high-speed aircrafts, the problem of cooling walls of the engine becomes more and more important. When an aircraft is flying at Mach 6, the temperature of its engine wall can reach 1650 K.1,2 The fuel will be required to serve as both propellant and coolant3 for the high heat loads and weight limitation of the aircraft. The fuel circulating around the composite or metallic material walls can not only cool down the engine wall but also recover the waste heat.4,5 However, the formation of solid cokes is considered to be a worse problem when hydrocarbon fuels are operated at high temperatures. The depositing cokes can reduce heat transfer efficiency, constrict fuel flow system, plug fuel nozzle or filter, and potentially lead to engine shut down. Therefore, particular attentions should be paid to the thermal cracking and coking of any hydrocarbon fuels.6-11 Under supersonic or hypersonic speeds, a hydrocarbon fuel might be exposed to supercritical temperature and pressure.12 * Corresponding author: Phone: +086-571-87952371; fax: +086-57187951895; e-mail:
[email protected]. (1) Dahm, K. D.; Virk, P. S.; Bounaceur, R.; Battin-Leclerc, F.; Marquaire, P. M.; Fournet, R.; Daniau, E.; Bouchez, M. J. Anal. Appl. Pyrolysis 2004, 71, 865–881. (2) Petley, D. H.; Jones, S. C. J. Aircraft 1992, 3, 384–389. (3) Natarajan, S.; Randolph, T. W. J. Supercrit. Fluid 1997, 10, 65–70. (4) Falempin, F.; Bouchez, M.; Salmon, T.; Lespade, P.; Avrashkov, V. An Innovation Technology for Fuel-cooled Composite Materials Structure. American Institute of Aeronautics and Astronautics: 2001; AIAA-2001-1880. (5) Huang, H.; Sobel, D. R.; Spadaccini, L. J. Endothermic Heat-sink of Jet Fuel for Scramjet Cooling. American Institute of Aeronautics and Astronautics: 2002; AIAA-2002-3871. (6) Widegren, J. A.; Bruno, T. J. Ind. Eng. Chem. Res. 2008, 47, 4342– 4348. (7) Edwards, T. Combust. Sci. Technol. 2006, 178, 307–334. (8) Andre’sen, J. M.; Strohm, J. J.; Sun, L.; Song, C. Energy Fuels 2001, 15, 714–723. (9) Altin, O.; Eser, S. Ind. Eng. Chem. Res. 2001, 40, 589–595. (10) Eser, S. Carbon 1996, 34, 539–547. (11) Wohlwend, K.; Maurice, L. Q.; Edwards, T.; Striebich, R. C.; Vangsness, M.; Hill, A. S. J. Propul. Power 2001, 17, 1258–1262. (12) Edwards, T. USAF Supercritical Hydrocarbon Fuels Interests. American Institute of Aeronautics and Astronautics: 1993; AIAA-19930807.
To develop advanced fuels, it is important to understand the properties of the thermal cracking and coking of the fuels under supercritical conditions. However, the composition and chemical structure are essential to the thermal stabilities of the fuels.13,14 In this work, n-heptane, cyclohexane, and JP-10 are chosen as model compounds15,16 to study mainly the coking property of the thermal cracking of paraffin, cycloparaffin, and multicycloparaffin under supercritical conditions. JP-10, tricyclo[5.2.1.02.6]decane or exo-tetrahydrodicyclopentadiene, is a highenergy density hydrocarbon fuel. It takes advantage of the increased energy storage available through its strained cyclic geometry, and it is presently served as the missile fuel by the Navy and the Air Force in the USA.17 The morphology and microstructures of the cokes produced in the thermal cracking processes are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and differential scanning calorimeter (DSC). The mechanism of the coke formation is discussed from the analyses of the composition and morphology of the cokes. 2. Experimental Section 2.1. Materials. n-Heptane (mass fraction g99.0%) and cyclohexane (g99.5%) were supplied by Shanghai Chemical Corporation, China. The sample of JP-10 was obtained from Liming Research Institute of Chemical Industry, China. The predominant components of the JP-10 sample were determined by a Hewlett-Packard 6890/ 5973 GC-MS. Exo-tricyclo[5.2.1.02.6]decane, trans-decahydronaphthalene, adamantane, and endo-tricyclo[5.2.1.02.6]decane were (13) Roan, M. A.; Boehman, A. L. Energy Fuels 2004, 18, 835–843. (14) Balster, L. M.; Corporan, E.; DeWitt, M. J.; Edwards, J. T.; Ervin, J. S.; Graham, J. L.; Lee, S. Y.; Pal, S.; Phelps, D. K.; Rudnick, L. R.; Santoro, R. J.; Schobert, H. H.; Shafer, L. M.; Striebich, R. C.; West, Z. J.; Wilson, G. R.; Woodward, R.; Zabarnick, S. Fuel Process. Technol. 2008, 89, 364–378. (15) Aribike, D. S.; Susu, A. A. Thermochim. Acta 1988, 127, 259– 273. (16) Billaud, F.; Duret, M.; Elyahyaoui, K.; Baronnet, F. Ind. Eng. Chem. Res. 1991, 30, 1469–1478. (17) Rao, P. N.; Kunzru, D. J. Anal. Appl. Pyrolysis 2006, 76, 154– 160.
10.1021/ef8011323 CCC: $40.75 2009 American Chemical Society Published on Web 05/12/2009
2998
Energy & Fuels, Vol. 23, 2009 Table 1. Characterized Properties of the Model Fuels
Xie et al. Table 2. Product Yields (Mass %) of the Thermal Cracking at 873.15 K and 4 MPa of Three Model Fuels n-heptane product yield/%
a
Reference 18. b Reference 19.
Figure 1. A schematic of experimental apparatus: 1. feed; 2. pump; 3. buffer storage; 4. needle valves; 5. manometers; 6. reaction furnace; 7. condenser; 8. backpressure control valve; 9. separating funnel; 10. pressure control valve; 11. nitrogen; 12. thermal couples; 13. proportional controllers; 14. gas chromatography.
detected with relative mass contents of 96.85, 1.72, 1.19, and 0.23%, respectively. Some physical properties18,19 of the model fuels are listed in Table 1, where F is the density, η is the viscosity, Tb is the normal boiling temperature, and Tc and pc are the critical temperature and pressure. 2.2. Apparatus and Procedure. The schematic diagram of the experimental apparatus is shown in Figure 1. The reaction zone was 700 mm in length. The tubular reactor was heated in a furnace controlled by a proportional-controller. The thermal cracking of the model fuels under supercritical condition was performed in a commercial 316 L stainless steel tube (3 mm outer diameter, 1.5 mm inner diameter) reactor. A 1900 mm long tube was made into a spiral coil (35 mm outer diameter) with preheating zone of 1000 mm and reaction zone of 900 mm. A backpressure control valve was used to control the pressure. Before each measurement, the tube reactor was heated up to 873.15 K and was kept at that wall-temperature for 2 h, and then N2 was blown into the reactor to exhaust the inside air. When the system reached the set temperature, the fuel was pumped into the reactor at the flow rate of 5 mL min-1 under atmospheric pressure. When the fuel began to flow out, the system pressure was slowly adjusted to 4.1 MPa with the pressure control valve. The thermal cracking of each fuel was performed for 30 min. The effluent was quenched by passing through the water condenser. The reactor was cooled to room temperature under nitrogen atmosphere. The amount of cokes was calculated from mass conservation, which was obtained from the mass difference between the hydrocarbon feed and the liquid effluent and gaseous products. 2.3. Analysis of Products. The products from the thermal cracking were collected and separated at room temperature and pressure. The gaseous products were analyzed quantitatively using a gas chromatography (GC) equipped with a capillary column and an FID detector. The liquid products were analyzed by gas (18) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids, 5th ed.; McGraw-Hill: 2001; pp 513-515. (19) Bruno, T. J.; Huber, M. L.; Laesecke, A.; Lemmon, E. W.; Perkins, R. A. Natl. Inst. Stand. Technol. 2006, 41–42.
cyclohexane product
JP-10
yield/%
product
yield/%
methane ethane ethene propane
0.85 2.17 2.50 1.18
methane ethane ethene propane
0.01 0.02 0.05 0.01
0.01 0.04 0.02 0.55
propene
2.28
propene
0.03
butene hexene
3.06 2.49
butene hexene
0.10 0.33
cokes
0.92
cyclohexene benzene cokes
1.03 0.24 0.35
methane ethene propene 5-methyl-2,3-dihydro1H-indene cis-bicyclo[3.3.0] oct-2-ene 1,3-cyclopentadiene cyclopentene/ cyclopentane toluene cokes
0.33 1.25 0.33 0.10 1.83
chromatography-mass spectrometry (GC-MS) using a HewlettPackard (HP) 6890 GC connected with a HP 5973 mass-selective detector. SEM images were taken with a field-emission scanning electron microscope (FEI SIRION-100, GENENIS-4000, Netherlands). TEM images were taken with a JEM-200CX (JEOL, Japan) instrument using an operating voltage of 160 kV. The differential scanning calorimeter (DSC, NETZSCH STA 409 PC/PG) was used to analyze the thermal decomposition process of the solid cokes with a heating rate of 10 K min-1 in air. The phase purity of the products was characterized by XRD using a Thermo X-ray diffractometer (Bruker, Germany) with monochromatized Cu KR radiation (λ ) 1.5405 Å).
3. Results and Discussion 3.1. Distributions of Products. The compositions of the products, collected at room temperature and atmospheric pressure, from the thermal cracking at 873.15 K and 4.1 MPa of three model fuels were determined with GC-MS. The mass conversions of n-heptane, cyclohexane, and JP-10 were observed to be 15.46, 2.18, and 4.47%, respectively. The yields (mass %) of the main hydrocarbon products are listed in Table 2. The product distributions are similar to those reported in the literature.15-17,20-22 The conversion of thermal cracking of a fuel is effected by its structure, properties, and the average residence time. The Reynolds number of n-heptane, cyclohexane, or JP-10 under the experimental condition is estimated to be 2550, 2740, or 3330, respectively. The values indicate that the investigated fuels at 873.15 K and 4.1 MPa should be in turbulence states or nearturbulence states. The boundary layer of a fuel in the tube reactor is thin, and the radial velocity gradient is very low. The average residence time of n-heptane, cyclohexane, or JP-10 can then be estimated to be 1.64, 1.21, or 1.94 s, respectively. Because the thermal stability of an alkane is lower than that of a cycloalkane,8 n-heptane has the highest conversion among these three fuels. The activation energy of ring-opening reaction of cyclohexane is higher than that of JP-10,22 and the average residence time is lower than that of JP-10. Therefore, the conversion of cyclohexane is lower than that of JP-10 or n-heptane. Different amounts of cokes are obtained with the mass yields of 0.92, 0.35, and 1.83% from thermal cracking of n-heptane, cyclohexane, and JP-10, respectively. The formation of cokes results from the precursors produced simultaneously from (20) Orme, J. P.; Curran, H. J.; Simmie, J. M. J. Phys. Chem. A 2006, 110, 114–131. (21) Xing, Y.; Fang, W.; Xie, W.; Guo, Y.; Lin, R. Ind. Eng. Chem. Res. 2008, 47, 10034–10040. (22) Herbinet, O.; Sirjean, B.; Bounaceur, R.; Fournet, R.; Battin-Leclerc, F.; Scacchi, G.; Marquaire, P. J. Phys. Chem. A 2006, 110, 11298–11314.
Coking of Hydrocarbon Fuels
thermal cracking of the fuels. The active materials such as Fe, Ni, and Cr9 in the stainless steel can enhance coking. The precursors, 1,3-cyclopentadiene, toluene, and 5-methyl-2,3dihydro-1H-indene, produced from thermal cracking of JP-10, have strong tendencies for forming cokes.23 Polymerization and cyclization of alkenes from the higher conversion of n-heptane produce precursors and induce coking finally.23-25 The coke value of cyclohexane is lowest due to its lowest conversion among three fuels. From the amounts of cokes and the conversion of thermal cracking, the selectivity of cokes for n-heptane, cyclohexane, or JP-10 is calculated to be 5.95, 16.13, or 41.03%. The results are in agreement with the coking tendency order:23 polycyclic alkane > cycloalkane > alkane. 3.2. Characterization of Cokes. After each experiment, the reaction tube was cut into small segments, and the cokes deposited on the inner wall were observed by SEM. The collected cokes were then analyzed by TEM, DSC, and XRD. The SEM images of the cokes on the reactor surface from the thermal cracking of three model fuels at 873.15 K and 4.1 MPa are shown in Figure 2. It indicates that the cokes from n-heptane and cyclohexane are mainly composed of filamentous carbons (panels a and b, respectively) and that the cokes from JP-10 (panel c) are mainly composed of carbon particles. Figure 3 gives the TEM micrographs of the three kinds of cokes. As shown in Figure 3a, most of the cokes from the thermal cracking of n-heptane are carbon nanofilaments and a little are carbon particles. The carbon nanofilaments are between 70 and 150 nm in the diameter and between 1 and 10 µm in length. Another apparent feature is that metal particles at the tips of the carbon nanofilaments can be found. As shown in Figure 3b, however, the cokes from the thermal cracking of cyclohexane are carbon nanotubes. The carbon nanotubes are approximately 50 nm in diameter, 1∼10 µm in length, and metal particles can also be found at their tips. The cokes from JP-10 are quite different in the morphology. Many irregular carbon particles are found with the size range from 500 nm to 4 µm. Only a little carbon nanofibers can be found (Figure 3d). Figure 4 shows the XRD patterns of the cokes from three model fuels. The characteristic peaks of the cokes appear at 2θ ) 26.4, 35.6, 43.8, 44.8, and 51.0°. The peak at 2θ ) 26.4° is the characteristic peak of graphite, which indicates that all the three kinds of cokes contain the products with the graphite structure.26 The graphitization degree of the cokes decreases in the order: cyclohexane > n- heptane > JP-10. The peaks at 2θ ) 35.6, 43.8, 44.8 and 51.0° are due to the cokes contain Fe3O4, Fe, Cr, and iron carbides, respectively. It implies that Fe3O4, Fe, Cr, and iron carbides have catalytic activities for the formation of carbon nanofilaments and carbon nanotubes.27,28 Fe3O4 were detected in the cokes even though the system was purged with N2 to remove the air, which indicates that a small amount of Fe3O4 from the reaction between the iron and oxygen in the air was still remained on the surface of stainless steel tube. The comparison of the DSC curves for three kinds of cokes is given in Figure 5. It can be seen that the exothermic peaks of three cokes appeared at a bit of different temperatures. (23) Kopinke, F. D.; Zimmermann, G.; Reyniers, G. C.; Froment, G. F. Ind. Eng. Chem. Res. 1993, 32, 56–61. (24) Roan, M. A.; Boehman, A. L. Energy Fuels 2004, 18, 835–843. (25) Ervin, J. S.; Ward, T. A.; Williams, T. F.; Bento, J. Energy Fuels 2003, 17, 577–586. (26) Sajitha, E. P.; Prasad, V.; Subramanyam, S. V.; Eto, S.; Takai, K.; Enoki, T. Carbon 2004, 42, 2815–2820. (27) Blanchard, J.; Hassani, H. O.; Abatzoglou, N.; Jankhah, S.; Gitzhofer, F. Chem. Eng. J. 2008, 143, 186–194. (28) Boehm, H. P. Carbon 1973, 11, 583–586.
Energy & Fuels, Vol. 23, 2009 2999
Figure 2. SEM images of coke samples obtained from the thermal cracking of three model fuels: (a) n-heptane, (b) cyclohexane, (c) JP10.
These results imply that different types of carbon are contained in the cokes from n-heptane, cyclohexane, and JP10.29 The oxidation temperature of the cokes increases in the order: TJP-10 < Tn-heptane < Tcyclohexane. It indicates that the oxidative stability of the cokes increases in the order: JP-10 < n-heptane < cyclohexane. In addition, a second exothermic (29) Chang, C. W.; Tseng, J. M.; Horng, J. J.; Shu, C. M. Compos. Sci. Technol. 2008, 68, 2954–2959.
3000
Energy & Fuels, Vol. 23, 2009
Xie et al.
Figure 3. TEM images of coke samples obtained from the thermal cracking of three model fuels: (a) n-heptane, (b) cyclohexane, (c, d) JP-10.
Figure 5. DSC curves of the coke samples obtained from thermal cracking of three model fuels: (a) n-heptane, (b) cyclohexane, (c) JP10.
from the thermal cracking of JP-10, which is in agreement with the TEM observations. 3.3. Mechanism of Coking. The cokes from thermal cracking of fuels can be divided into the catalytic and noncatalytic cokes. When the precursors are adsorbed onto the active surface, the filament or bar-shaped cokes are obtained by dehydrogenation and catalysis processes. When the physical adsorption occurs, the precursors undergo dehydrogenation deeply and produce particles or amorphous cokes. Several mechanisms30-32 have been proposed to describe the formation of carbon nanofilaments and carbon nanotubes. Carbon diffuses into nanometer-scale catalyst particles and can precipitate out with a graphitic structure when the solubility limit of the carbon within the particles is reached. Graphite, carbon filaments, or carbon nanotubes can form depending on the size of the catalyst particles. During the formation process of carbon nanofibers or carbon nanotubes, the original catalyst particles will either remain fixed to the substrate (called root growth) or detach from the surface and remain encapsulated within the opposite end (called tip growth). Fe and Cr are the main components of the stainless steel. Fe, Cr, Fe3O4, and iron carbides have catalytic activities for the formation of carbon nanofilaments and carbon nanotubes. On the basis of the above mechanism and the experimental results (Figures 3 and 4), it is known that the coking precursors from thermal cracking of n-heptane and cyclohexane in the stagnant layer were adsorbed on active surface of the reactor, producing carbon atom clusters after dehydrogenation reaction. Carbon atom clusters dissolve in the active particles, diffuse through the particles, and deposit there regularly, which results in the formation of carbon nanofilaments and carbon nanotubes via the tip growth mechanism. Hence, catalyst particles are observed to be lifted from the surface of the reactor and come into the cokes.
peak of the cokes from JP-10 appears at 873.15 K (close to the oxidation temperature of the cokes from cyclohexane). This indicates that there are two kinds of cokes in the deposits
(30) Sinnott, S. B.; Andrews, R.; Qian, D.; Rao, A. M.; Mao, Z.; Dickey, E. C.; Derbyshire, F. Chem. Phys. Lett. 1999, 315, 25–30. (31) Zhang, X. F.; Cao, A.; Wei, B. Q.; Li, Y. H.; Wei, J. Q.; Xu, C. L.; Wu, D. H. Chem. Phys. Lett. 2002, 362, 285–290. (32) Deck, C. P.; Vecchio, K. Carbon 2005, 43, 2608–2617.
Figure 4. XRD patterns of coke samples obtained from thermal cracking of three model fuels: (a) n-heptane, (b) cyclohexane, (c) JP-10.
Coking of Hydrocarbon Fuels
However, polymerization, cyclization, and dehydrogenation reactions of the coking precursors can also produce carbon particles. The polymerization and cyclization reactions of the hydrogen-deficient products from the thermal cracking of JP10 in the stagnant layer might occur easily. One part of coking precursors is adsorbed on the active surface and forms carbon filaments, the other part produces carbon particles. These carbon particles can cover the active surface and inhibit the adsorption of coking precursors on it, which lead to stop the growing of the carbon filaments. As a result, the noncatalytic cokes increase with the reaction time going. Many irregular carbon particles and a few carbon nanofilaments are observed in the cokes from thermal cracking of JP-10. 4. Conclusion Coking properties of n-heptane, cyclohexane, and JP-10 during their thermal cracking processes under supercritical condition have been investigated. The chemical structure plays
Energy & Fuels, Vol. 23, 2009 3001
an essential role in determining the coking property. The cokes obtained from the three model fuels are quite different in morphology. The cokes from n-heptane, cyclohexane, and JP10 are mainly in the forms of carbon nanofilaments, carbon nanotubes, and irregular carbon particles, respectively. Fe, Cr, Fe3O4, and iron carbides have catalytic activities for the formation of carbon nanofilaments and carbon nanotubes. They were formed by the tip growth mechanism. The polymerization and cyclization reactions of the hydrogen-deficient products from the thermal cracking of JP-10 occur easily, and many irregular carbon particles are observed in the cokes. Acknowledgment. The authors are grateful for the financial supports from the National Natural Science Foundation of China (No. 20573096) and the Natural Science Foundation of Zhejiang Province of China (No. Y404329). EF8011323
