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Ind. Eng. Chem. Res. 2008, 47, 10034–10040
GENERAL RESEARCH Thermal Cracking of JP-10 under Pressure Yan Xing, Wenjun Fang,* Wenjie Xie, Yongsheng Guo, and Ruisen Lin Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China
Thermal cracking of a high density hydrocarbon fuel, JP-10 (exo-tetrahydrodicyclopentadiene), was studied on a batch reactor under different pressures. The effluent was cooled and collected at room temperature and atmospheric pressure. The gaseous and liquid components were quantitatively determined by gas chromatography and gas chromatography-mass spectrometry, respectively. The conversion of JP-10 has relatively low value at atmospheric pressure and increases under pressure. With an increase of the pressure, the relative content of ethene or propene decreases and that of methane, ethane, or propane increases simultaneously. In the liquid products, cyclopentane, cyclopentene, 1,3-cyclopentadiene, and cis-bicyclo[3.3.0]oct-2-ene are found to be major components. Substituted cyclopentene, benzene, toluene, and naphthalene are also observed under high pressures and temperatures. A probable mechanism of the thermal cracking of JP-10 is proposed to explain the product distribution. The process of isomerization might be dominating for liquid product formation during the thermal cracking under elevated pressure. 1. Introduction The aerodynamic heating brings a significant heat load for the thermal management system, and an active cooling system must be needed for supersonic and hypersonic aircrafts. Hydrocarbon fuels used as coolants can offer the required cooling capacity without the problems such as a required large container volume, high cost, logistics, and operational safety which are associated with cryogenic fuels.1 Hydrocarbon fuels flowing through the high temperature department of an aircraft absorb excess heat and crack into methane, ethane, ethene, propane, propene, and other small molecules. Products with low molecular weight introduced into the combustion chamber can improve the combustion efficiency. Under supersonic or hypersonic speeds, the hydrocarbon fuel is exposed to supercritical conditions.1 To develop advanced aviation fuels, it is important to understand thermal cracking processes of hydrocarbon fuels under various pressures and temperatures.2-9 JP-10, tricycle[5.2.1.02.6]decane or exo-tetrahydrodicyclopentadiene, is one of the high-energy density hydrocarbon fuels. It takes advantage of the increased energy storage available through its strained cyclic geometry and is presently serving as missile fuel by the Navy and the Air Force in the U.S.A.10 Although the studies on thermal or catalytic cracking of JP-10 can be found in the literature,11-14 information is unavailable on the cracking under nearcritical and supercritical conditions. The objectives of the present work are mainly to investigate the thermal stability of JP-10 under elevate pressures and to explore the effects of temperature and pressure on product distributions. A probable mechanism is proposed to give a visual clue to the thermal cracking of JP-10 to explain the product distribution.
Table 1. Physical Properties of JP-10 Tba (K) TCb (K) pCb (MPa) JP-10 457.15 a
678
3.7
Fa (g cm-3)
ηa (mPa s)
0.9336 (298.15K) 3.087 (298.15K)
b
This work. Reference 15.
predominant components of the JP-10 sample were determined by a Hewlett-Packard 6890/5973 gas chromatographmass spectrometry (GC-MS).exo-tricycle[5.2.1.02.6]decane, trans-decahydronaphthalene, adamantane, and endo-tricycle[5.2.1.02.6]decane were detected with relative contents of 96.85%, 1.72%, 1.19%, and 0.23%, respectively. Some physical properties such as the boiling point (Tb), the critical temperature (TC), the critical pressure (pC),15 the density (F), and the viscosity (η) of JP-10 are listed in Table 1. 2.2. Apparatus. A schematic diagram of the experimental apparatus is shown in Figure 1. The thermal cracking of JP10 under nearcritical and supercritical pressures was performed in a tubular stainless-steel reactor. The feed was injected into the reactor at the required flow rate using a dosing pump. It was preheated to 300 °C in a furnace. The tubular reactor was heated in a two-zone furnace controlled by separate proportional-controllers. A backpressure controlling valve was used to control the pressure.
2. Experimental Details 2.1. Materials. The sample of JP-10 was obtained from Liming Research Institute of Chemical Industry, China. The * To whom correspondence should be addressed. E-mail: fwjun@ zju.edu.cn. Tel.: +86 571 8795 2371. Fax: +86 571 8795 1895.
Figure 1. A schematic of experimental apparatus: (1) feed; (2) pump; (3) buffer storage; (4) needle valve; (5) manometer; (6) preheater; (7) reaction furnace; (8) condenser; (9) backpressure control valve; (10) separating funnel; (11) pressure control valve; (12) nitrogen; (13) thermal couple; (14) proportional controller; (15) gas chromatography.
10.1021/ie801128f CCC: $40.75 2008 American Chemical Society Published on Web 11/20/2008
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10035 Table 2. Residence Time (tR) for the Thermal Cracking of JP-10 under Different Pressures and Temperatures
Figure 2. Conversion for thermal cracking of JP-10 under different temperatures and pressures.
2.3. Procedure. Before each run, the tubular reactor was purged by N2 to wipe the air inside, and the system pressure was adjusted to a certain value with the pressure control valve. Then, JP-10 was pumped into the reactor at the flow rate of 2 mL min-1 for thermal cracking. The reaction effluent was quenched by passing through the water condenser, and then
p (MPa)
550 °C
570 °C
tR (s) 590 °C
610 °C
630 °C
0.1 1.8 2.7 3.2 3.8
0.53 11.3 17.1 21.1 26.4
0.51 10.8 16.3 20.1 24.9
0.50 10.5 15.7 19.2 23.7
0.49 10.2 15.1 18.4 22.6
0.48 9.9 14.6 17.7
exposed to room temperature and atmospheric pressure. The effluent was separated by a gas-liquid separator. The volume of the gaseous phase products was quantified by the water displacement method and the liquid residue was collected with conical flask and weighed. The gaseous products were analyzed quantitatively using gas chromatography (GC) equipped with a capillary column and an FID detector. The GC was programmed from 50 to 180 °C at a rate of 5 °C min-1 with an initial isothermal period of 5 min. The liquid products were analyzed by GC-MS using a Hewlett-Packard (HP) 6890 GC connected with a HP 5973 mass selective detector. The gas chromatography was equipped with a DB-17 column and programmed from 60 to 180 °C at a rate of 10 °C min-1 with an initial isothermal period of 5 min.
Figure 3. Molar fraction of gaseous products under different temperatures and pressures.
10036 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Figure 4. Molar fraction of gaseous products under different pressures.
propane increases with increasing the pressure and that of ethene or propene deceases simultaneously. The selectivity of alkene in the gaseous products, selectivity )
Figure 5. Selectivity of alkene in gaseous products under different temperatures and pressures.
3. Results and Discussion 3.1. Conversion of JP-10. Thermal cracking of JP-10 has been studied under pressures from 0.1 to 3.8 MPa at temperatures from 550 to ca. 630 °C. Each residence time is given in Table 2. Different values of the conversion, defined as initial mass(g)- final mass(g) 100 initial mass(g) for the thermal cracking of JP-10 under various temperatures and pressures are shown in Figure 2. It can be observed that the conversion has relatively low value at atmospheric pressure and increases rapidly before 1.8 MPa, especially at high temperatures. It changes slowly when the pressure is higher than 1.8 MPa. Under the same reaction temperature, increasing pressure is disadvantage for the thermal cracking process, but the residence time is an advantage factor for the increase of the conversion. The shortest residence time at 0.1 MPa determines the lowest conversion. The slow increase of conversion under pressures is the result of competition between the two opposite effects of higher pressure and longer residence time. 3.2. Selectivity of Gaseous Products. The major components in the gaseous products are methane, ethane, propane, ethene, and propene. The detailed changes of molar fraction for these components are compared in Figure 3. The relative contents for each component under various pressures are shown in Figure 4. It is found that the relative content of methane, ethane, or conversion )
alkenes of gaseous products (mol) 100 all gaseous products (mol)
versus the reaction temperature at each pressure is shown in Figure 5. It follows that the selectivity of alkene under atmospheric pressure is obviously different from that under elevated pressure. The selectivity of alkene increases with the rise of the temperature under atmospheric pressure, while it exhibits converse phenomenon with the pressure increasing. The small molecules in gaseous products are mainly produced by the β-cracking of the parent radicals and their secondary radicals. With increase of the pressure, the distance between molecules becomes shorter and the reaction probability of two molecules increases. Free radicals can abstract hydrogen easily and convert into saturated alkanes. This finally induces the selectivity of alkene in the gaseous products to decrease. The endothermic requirement means that the cracking products are unsaturated hydrocarbons (olefins) such as ethene, rather than more stable, thermodynamically favored saturated products such as methane and ethane (and coke).13 It suggests that the higher the alkene ratio is in the gaseous products, the more endothermic capability the fuel has. But the experimental facts show that the alkene ratio increases slowly with increasing temperature at 0.1 MPa; and it decreases when the pressure is higher than 1.8 MPa. Therefore, elevated pressure suppresses the formation of alkene, and the decrease of the alkene content may reduce the endothermic and combustion properties of hydrocarbon fuels. 3.3. Liquid Product Distributions. The detailed liquid products for the thermal cracking of JP-10 determined by GC-MS are listed in Tables A1-A3 in the Appendix. The major components of the liquid products are cyclopentene, cyclopentane, 1, 3-cyclopentadiene, cis-bicyclo[3.3.0]oct-2-ene, 3-cyclopentylcyclopentene, 1(3)-ethenyl-cyclopentene, benzene, and toluene. Distributions of these liquid products obtained at different temperatures and pressures are shown in Figures 6 and 7. The mass fractions of cyclopentene/cyclopentane, 1,3-cyclopentadiene, cis-bicyclo[3.3.0] oct-2-ene, 3-cyclopentylcyclopentene, and 1(3)-ethenyl-cyclopentene increase with the rise of temperature. This indicates that temperature is beneficial to the thermal cracking of JP-10. Figure 7 shows that the mass fractions of the products increase rapidly just before 1.8 MPa, and then the changes become slight when the pressure is higher
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10037
Figure 6. Content for liquid components produced from thermal cracking of JP-10 at different temperatures.
Figure 7. Content for liquid components produced from thermal cracking of JP-10 at different pressures (610 °C).
Figure 8. Biradicals (8a) and single radicals (8b) obtained from JP-10.
than 1.8 MPa at the same temperature. This phenomenon is consistent with the changes of conversion versus pressure (Figure 2). 3.4. Hypothetical Mechanism. JP-10 is a polycyclic compound. The breaking of one C-C bond maybe produce a biradical. The possible biradicals from JP-10 are listed in Figure 8a. JP-10 molecules can abstract hydrogen atoms from these biradicals and produce several single radicals which are listed in Figure 8b. The mechanism proposed by Olivier14 can explain the formation of cyclopentadiene, benzene, 3-cyclopentyl-
cyclopentene, and small molecules in gaseous products for the thermal cracking of JP-10 under atmospheric pressure. Different from the thermal cracking of JP-10 under atmospheric pressure, 1, 5-hexadiene, which was detected as one of the major products,14,16 is absent under elevated pressures, and 1(3)-ethenyl-cyclopentene and cis-bicyclo[3.3.0]oct-2-ene are detected under relatively high pressures. Although the thermal cracking of JP-10 under elevated pressures also belongs to biradical initiations (Figure 8a) and the processes are similar to those under the atmospheric pressure, the product distributions under elevated pressures should be explained by a developing mechanism. The hypothetical decomposition processes are proposed in Figures 8 and 9 to explain the formation of cisbicyclo[3.3.0]oct-2-ene, 1(3)-ethenyl-cyclopentene, indene, and naphthalene, which are found during the thermal cracking of JP-10 under elevated pressures. The biradicals from JP-10 can absorb hydrogen atoms and change to single radicals. The possible decomposition processes of these single radicals are listed in eq 1-5 in Figure 9. The radicals mostly pass though β-scission decomposition to produce smaller molecules and second radicals. The second radicals react continuously to produce final products. It is clear that the smallest molecules produced from the thermal cracking are almost ethene or propene. Benzene, toluene, cyclopentadiene, and substituted cyclopentadiene are mainly from the scission of radicals listed in routes 1-5; cis-bicyclo[3.3.0]oct-2-ene can be obtained from route 1; 1(3)-ethenyl-cyclopentene is produced through route 6 (Figure 9). Indene and naphthalene are obtained from the isomerization of radicals and the detailed processes are listed in routes 6-7 in Figure 9. It can be considered that isomerization is significant in the formation of the liquid products with relatively large molecular weights during the thermal cracking of JP-10 under elevated pressures. Although pressure is a disadvantage factor for thermal cracking according to thermodynamics, the inhibition of pressure on thermal cracking leads to the formation of the abnormal liquid products such as indene, naphthalene, etc. This phenomenon may represent the important effects of pressure on the liquid product distribution. 4. Conclusion The thermal cracking of JP-10, a high density hydrocarbon fuel, has been studied under elevated pressures, and the hypothetical mechanism is proposed. The major gaseous components in the products are methane, ethane, propane, ethene, and propene. With increasing pressure, the relative
Figure 9. Hypothetical mechanism for thermal cracking of JP-10 under elevated pressures.
10038 Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008
Ind. Eng. Chem. Res., Vol. 47, No. 24, 2008 10039
content of ethene or propene decreases, but the content of methane, ethane, or propane increases. The major liquid products are found to be cyclopentane, cyclopentene, 1,3-cyclopentadiene, cis-bicyclo[3.3.0]oct-2-ene, 3-cyclopentylcyclopentene, 1(3)ethenyl-cyclopentene, benzene, and toluene. A biradical mechanism is developed to explain the distribution of cracking products. The process of isomerization is predominant during the thermal cracking under elevated pressures. Acknowledgment The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 20573096) and the Natural Science Foundation of Zhejiang Province of China (No. Y404329). Appendix
Table A1. Components in the Liquid Effluent for Thermal Cracking of JP-10 at 1.8 MPa and Different Temperatures content (%) composition
550 °C
exo-tricyclo 96.81 [5.2.1.02.6] decane naphthalene, decahydro-, 1.70 trans 4,7-methano-1H-indene, 0.25 octahydroadamantane 1.24 cyclopentene/ cyclopentane 1,3-cyclopentadiene 1(3)-ethenyl-cyclopentene cis-bicyclo[3.3.0]oct-2-ene 3-cyclopentylcyclopentene benzene toluene 5-methyl-2,3-dihydro-1H-indene 1-methyl-cyclopentene ethylbenzene 1-propynylbenzene indene naphthalene 1-methyl1,3-cyclopentadiene 1,2,3,4-tetrahydro-naphthalene 1,3-cyclohexadiene 1,3-cycloheptadiene
570 °C
590 °C
610 °C
630 °C
96.49
92.49
88.75
81.71
1.74
1.84
1.82
1.52
0.27
0.28
0.30
0.26
1.31 0.19
1.69 1.47
1.70 3.54
2.05 4.22
0.69 0.41 0.36 0.74
0.60 0.77 0.48 1.15 0.35 0.34 0.20
0.60 1.26 0.95 1.22 1.28 1.32 0.20 0.45 0.36 0.12 0.31 0.26 0.47 0.24 0.28 0.92
Table A2. Components in the Liquid Effluent for Thermal Cracking of JP-10 at 2.7 MPa and Different Temperatures content (%) composition
550 °C
exo-tricyclo 96.58 [5.2.1.02.6] decane naphthalene, decahydro-, 1.87 trans 4,7-methano0.27 1H-indene, octahydroadamantane 1.28 cyclopentene/ cyclopentane 1,3-cyclopentadiene 1-methyl-cyclopentene 1(3)-ethenyl-cyclopentene cis-bicyclo[3.3.0]oct-2-ene 3-cyclopentylcyclopentene benzene toluene ethylbenzene 1-propynylbenzene indene 5-methyl-2,3-dihydro-1H-indene (1-methyl-2-cyclopropen-1-yl) benzene naphthalene
570 °C
590 °C
610 °C
630 °C
96.45
92.27
89.26
83.55
1.86
2.00
1.66
1.59
0.28
0.43
0.47
0.55
1.31 0.11
1.80 1.20
1.79 3.05
1.95 3.88
0.41 0.13 0.43 0.45 0.88
0.61 0.27 0.50 0.69 1.15 0.29 0.27
1.21 0.46 1.00 0.69 1.66 0.84 0.78 0.19 0.22 0.37 0.18 0.24 0.19
Table A3. Components in the Liquid Effluent for Thermal Cracking of JP-10 at 610 °C and Different Pressures content (%) composition
0.1 MPa 1.8 MPa 2.7 MPa 3.2 MPa 3.9 MPa
exo-tricyclo 96.53 [5.2.1.02.6] decane naphthalene, decahydro-, 1.87 trans 4,7-methano0.29 1H-indene, octahydroadamantane 1.31 cyclopentene/ cyclopentane 1,3-cyclopentadiene 1(3)-ethenyl-cyclopentene cis-bicyclo[3.3.0]oct-2-ene benzene toluene 3-cyclopentylcyclopentene 5-methyl-2,3-dihydro-1H-indene indene 1-methyl-cyclopentene 4,7-methano-1H-indene,3a,4,7,7a-t 2-methylbicyclo[4.3.0]non-1(6)-ene naphthalene
88.75
89.26
85.60
84.10
1.82
1.66
2.22
2.07
0.30
0.47
0.27
0.43
1.70 3.54
1.79 3.05
2.46 3.20
2.73 3.72
0.60 0.77 0.48 0.35 0.34 1.15 0.20
0.61 0.50 0.69 0.29 0.27 1.15
0.72 0.57 0.97 0.53 0.53 1.32 0.15 0.14 0.15 0.57 0.20
0.91 0.77 1.08 0.57 0.57 1.53 0.31 0.18 0.24 0.42 0.20 0.17
0.27
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ReceiVed for reView July 22, 2008 ReVised manuscript receiVed September 22, 2008 Accepted October 16, 2008 IE801128F
