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Thermal cracking of JP-10: Kinetics and product distribution
J. Anal. Appl. Pyrolysis 76 (2006) 154–160 www.elsevier.com/locate/jaap
Thermal cracking of JP-10: Kinetics and product distribution P. Nageswara Rao, Deepak Kunzru * Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 08016, India Received 9 May 2005; accepted 7 October 2005 Available online 21 November 2005
Abstract The product distribution and kinetics of thermal cracking of JP-10 (exo-tetrahydrodicyclopentadiene) were investigated in an annular tubular reactor at atmospheric pressure, in the temperature range of 903–968 K. No inerts were added with the feed. The major products were methane, ethylene, propylene, cyclopentene, cyclopentadiene, benzene and toluene. The pre-exponential factor, activation energy and overall reaction order for thermal cracking of JP-10 were determined by non-linear regression analysis to be 2.4 1013 (m3/mol)0.1 s1, 256.2 kJ/mol and 1.1, respectively. # 2005 Elsevier B.V. All rights reserved. Keywords: JP-10; Exo-tetrahydrodicyclopentadiene; Thermal cracking; Pyrolysis; Kinetics
1. Introduction In the last decade great emphasis has been given to airbreathing propulsion for high Mach number applications. Aircraft designed for hypersonic flight must incorporate active cooling in the propulsion system, because at high Mach numbers, the sensible heating of the fuel cannot provide sufficient heat sink to cool the supersonic vehicles. Therefore, endothermic reactions, such as thermal cracking, catalytic cracking or catalytic gasification are needed to augment the cooling capacity of the fuel [1]. Moreover, practical air-breathing pulse detonation engines (PDE) will most probably be based on storable liquid hydrocarbon fuels such as JP-10 or Jet A. However, without fuel modifications, these liquid fuels are not suitable for use in PDE. One possibility of fuel modification is to thermally crack the fuel [2]. JP-10 (exo-tetrahydrodicyclopentadiene), a synthetic fuel, is commonly used in missile applications. Due to its strained cyclic structure, it has a high volumetric energy density and is suited for volume limited applications. Very meager information is available in the published literature on the thermal cracking of JP-10. Cooper and Shepherd [2] examined the thermal and catalytic decomposition of JP-10 at 500 8C. They
* Corresponding author. Tel.: +91 512 2597193 E-mail address: [email protected] (D. Kunzru). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2005.10.003
compared the results with and without catalyst, and at 500 8C reported a conversion of 3.51% in thermal cracking, whereas it was relatively higher in catalytic cracking (34% with HY type zeolite). They did not measure the kinetics or the gaseous product distribution. Davidson et al. [3] developed a high speed UV absorption spectrograph to identify the initial products formed during the thermal decomposition of JP-10 in the temperature range of 1100–1700 K. Based on the product distribution, they proposed that the first step in the decomposition was the breaking of C–C bond to form cyclopentene and other products. Assuming first-order kinetics, the activation energy for the decomposition of JP-10 was calculated to be 72,500 cal/mol. Green and Anderson [4] studied the decomposition of JP-10 in a micro-flow-tube reactor over the temperature range of 298–1300 K on a millisecond time scale. They found that decomposition set in at 840 K and was complete by 1000 K. At high temperatures, the only significant product identified by them in the mass range above 50 amu was benzene. Wohlwend et al. [5] studied the thermal stability of JP10 under supercritical conditions (34 atm and a temperature range of 200–650 8C). The experiments were conducted at a feed flow rate of 0.5 ml/min, corresponding to a residence time of 1.8 s at 200 8C. They found that JP-10 started to fragment at 450 8C and decomposed readily at 600 8C. The major decomposition products observed were cyclopentene and cyclopentadiene. Other products identified included benzene, toluene, naphthalene, substituted cyclopentene and substituted cyclopentadiene. The product yields were not quantified.
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Nomenclature AC CA CA0 E FA0 k k0 k0 l n PE Q0 rA R T TE T* V VE XA
cross- sectional area of reactor, m2 concentration of JP-10, mol/m3 inlet concentration of JP-10, mol/m3 activation energy, kJ/mol inlet molar flow rate of JP-10, mol/s rate constant of pyrolysis reaction, s1 or (m3/ mol)0.1s1 pre-exponential factor, s1 or (m3/mol)0.1s1 reparameterized pre-exponential factor, defined in Eq. (11) axial coordinate, m reaction rate reference pressure, Pa inlet volumetric flow rate, m3/s rate of reaction, mol/m3 s gas constant, J/mol/K reactor temperature, K equivalent reactor temperature, K average temperature used for reparameterization, K volume of reactor, m3 equivalent reactor volume, m3 conversion of JP-10
Greek Letters e expansion factor t space time, s n stoichiometric coefficient, moles of total product /mol of JP-10 cracked
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Striebich and Lawrence [6] also studied the decomposition of JP-10 under supercritical conditions at 34 atm. The residence time in their study varied from 5 to 1 s as the reaction temperature was increased from 100 to 600 8C. For this range of temperature and residence time, the decomposition of JP-10 was less than 10%, even at 600 8C. Considering the lack of information available on the high temperature chemistry of JP-10, the objective of this study was to investigate effect of temperature and residence time on the conversion and product distribution during the thermal cracking of JP-10 at atmospheric pressure. Another objective was to determine the kinetics of cracking. This investigation is part of an overall study to determine the effect of pressure on the product distribution and kinetics of thermal decomposition of JP-10. 2. Experimental Pyrolysis of JP-10 was conducted in an annular tubular reactor. A schematic diagram of the experimental set-up is shown in Fig. 1. The feed was injected into the reactor at the required flow rate using a dosing pump. No inerts were added to the JP-10 feed. The reactor (7.7 mm i.d. 11.3 mm o.d. 800 mm long) was made of Inconel and heated in a three-zone furnace. The temperature of each zone was controlled by separate proportional controllers. The axial temperature profile was measured using a chromel-alumel thermocouple, manually driven in an Inconel thermowell tube (3.2 mm o.d.) attached to the inlet end of the reactor. No separate preheater was used and the first part of the reactor served as the preheater. To passivate the walls of the reactor, prior to each run, the reactor was presulfided by passing water containing 200 ppm of CS2. Due to this passivation, the wall
Fig. 1. Experimental set-up.
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effects are expected to be negligible [7]. A mercury manometer with one leg connected to system and the other leg open to the atmosphere was used to measure the pressure difference between the system and the surroundings. The reactor effluent was quenched by passing through two condensers, connected in series. A water-glycerol mixture at 268 K was used as the coolant and circulated through the condensers using a refrigerated circulator. The condensed liquid hydrocarbon products and any unconverted reactant were collected in separating flasks attached to the condensers. Nitrogen, injected after the first separating funnel, served as an internal standard in the subsequent chromatographic analysis. The nitrogen flow rate was maintained at the desired value by using a mass flow controller. The noncondensables passed through a gas sampling valve and were then vented. During the course of a run, the condensed liquid as well as the composition of the liquid and gaseous products was measured at regular intervals. Moreover, the axial temperature profile was measured for each run. After completion of the run, the reactor was flushed with helium for 15 min, and then the reactor was decoked with air. The condensed liquid hydrocarbon products were analyzed on a capillary column (Petrocol DH column, 0.25 mm i.d., 100 m long). n-Octane was used as the internal standard. The noncondensables, which mainly consisted of C1–C5 hydrocarbons, were analyzed using three columns, namely Durapak, Porapak Q and Carbosphere. The separation of methane and nitrogen was carried out on a Carbosphere column using a thermal conductivity detector, with helium as the carrier. Determination of C1–C5 hydrocarbons was performed on a 3 m long Durapak column using flame ionization detector. As the separation of ethane and ethylene was not possible on the Durapak column, Porapak Q column was used to separate methane, ethane and ethylene. 3. Results and discussion
volume that, at a chosen equivalent temperature, TE, and reference pressure PE, would give the same conversion as the actual reactor with its temperature and pressure profile. For a first-order reaction with negligible pressure drop in the reactor, VE can be expressed as [8],
VE ¼
ZV
TE E 1 1 dV exp R T TE T
(1)
0
where E is the activation energy for the reaction and T is the actual reactor temperature and is function of reactor length. Since the activation energy is not known a priori, the procedure for determining VE involves trial and error, and is discussed in Section 3.4.1. The equivalent temperature, TE, was taken to be the temperature in the central portion of the reactor whereas T was measured experimentally at different axial positions. The space time was then evaluated from the calculated VE as t¼
VE Q0
(2)
3.2. Effect of temperature and space time on conversion The variation of conversion of JP-10 with space time at different temperatures is shown in Fig. 2. Due to the axial temperature profile, a single reaction temperature for a run is not defined and the temperature indicated for each run refers to the reference temperature. At each temperature, conversion increased with space time, tending to level off at higher space times. Conversion increased significantly with temperature. For instance, at a space time of 2.5 s, the conversion was 14.4% at 903 K and increased to approximately 55% at 968 K. There is no previously reported data at these conditions.
In this study, the effect of space time and temperature on the conversion and product yields in JP-10 pyrolysis at atmospheric pressure was investigated. No inerts were added with the feed. The experiments covered the following range of variables: temperature, 903–968 K, and JP-10 flow rate, 0.4–2.3 g/min, and thus a space time of 0.68–6.4 s. JP-10 was obtained from DMSRDE, Kanpur. The purity, as determined from GC analysis, was 96.4 wt%. 3.1. Calculation of space time Analysis of the experimental data in any pyrolysis study is rendered difficult due to the axial temperature profile that exists in the reactor. The concept of equivalent reactor volume was used to calculate the space time for data representation and also as one of the methods for determination of kinetic parameters. A nonisothermal reactor can be represented as a pseudoisothermal reactor, by using the concept of equivalent reactor volume. The equivalent reactor volume, VE, is defined as the
Fig. 2. Variation of conversion of JP-10 with space time (temperature, K (4) 903; (~) 923; (&) 948; (& ) 968). Space time calculated from the temperature profile, activation energy of 62.4 kcal/mol and the flow rate of JP-10, which was varied from 0.4 to 2.3 g/min.
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Table 1 Product yields during pyrolysis of JP-10 fuel Temperature (K) Space time (s)
968 0.7
Yields of products, wt% feed Methane Ethylene Ethane Propylene Propane 1-Butene 2- Butene + isobutylene + 1,3-butadiene Cyclopentene Cyclopentadiene Other non-condensables Benzene Toluene P1(unidentified) Mass balance Conversion (%)
0.6 2.9 0.6 2.8 0.1 0.1 0.2 1.6 4.6 Traces 1.6 1.3 1.2 100.5 27.1
968 3.3 2.5 7.6 2.0 7.6 0.3 0.3 1.0 2.3 11.7 0.12 6.2 4.2 0.9 97.9 61.1
3.3. Product yields The effect of temperature and space time on the product yields (wt% feed) and selectivities of various liquid and gaseous products was investigated. The products obtained from thermal cracking were a mixture of condensables and noncondensables. Minor amount of coke formation was observed at 948 and 968 K, especially at high conversions. Most of the gaseous products were identified. Due to experimental limitations, the yield of hydrogen was measured for only a few runs and varied between 0.2 and 0.3 wt% of feed. The mass balance closure for all the runs was within 5% and the detailed product distribution for some runs is given in Table 1. The percent yield of a product is defined as the kg of product formed per kg of JP-10 fed to the reactor. In the pyrolysis of JP-10, the major gaseous products were ethylene, propylene, cyclopentene and cyclopentadiene. Other gaseous products included methane, ethane, propane, 1-butene, other C4s and some C5 compounds. Acetylene was present in trace amounts only. Cyclopentene and cyclopentadiene were present in the liquid products also. The combined yields of ethylene, propylene, cyclopentene and cyclopentadiene varied from 4.3 to 29.2 wt% as the conversion varied from 11.2% to 61.1%. The number of liquid products formed during the thermal pyrolysis of JP-10 was as many as 40 at high conversions, and around 15 at lower conversions. The major liquid products included benzene and toluene. Naphthalene and dicyclopentadiene were also formed in minor amounts (<1.0 wt% feed). The other liquid products could not be identified. The variation in the yields of methane, ethylene, propylene, cyclopentene, cyclopentadiene and benzene with conversion at different temperatures is shown in Fig. 3. The yield of methane and benzene increased monotonically with conversion, whereas the yields of ethylene, propylene, cyclopentadiene and cyclopentene tend to level off at high conversions. These plots also show that at a particular conversion, the yields were not significantly affected by reaction temperature. The variation in the yield of toluene with conversion (not shown) was similar to
948 0.8 0.3 1.3 0.3 1.5 0.02 0.03 0.1 1.0 2.2 0.02 0.7 0.7 1.0 100.5 15.4
948 5.3 2.5 6.7 2.0 7.8 0.3 0.3 1.0 2.0 9.1 0.18 1.8 2.4 0.7 97.6 50.7
923 1.3 0.2 1.2 0.2 1.3 0.02 0.02 0.1 1.0 2.3 0.01 0.6 0.6 0.8 100.4 11.2
923 6.4 1.5 4.8 1.4 5.5 0.2 0.2 0.6 2.0 6.6 0.19 1.8 1.9 0.9 99.1 36.7
903 2.5 0.2 0.3 0.8 0.8 0.02 0.01 0.04 1.2 2.5 0.02 0.4 0.4 0.4 100.4 14.4
903 6.4 0.6 1.9 0.6 2.0 0.10 0.05 0.2 1.3 3.6 0.04 0.8 0.8 0.8 99.4 21.1
that of benzene, the only difference being that the toluene yields were lower. These results show that the stable species (methane, benzene and toluene) once formed do not crack further at these conditions. The trends of benzene and toluene yields are in agreement with the results reported by Wohlwend et al. [5] who studied the decomposition of JP-10 at a constant residence time and reported the benzene and toluene yields in terms of ‘percent yield relative to parent response’. At a temperature of 650 8C, the benzene and toluene yields reported by them were 6.5 and 4.5, respectively. It is of interest to determine the primary products formed during thermal cracking of JP-10. Primary products will have a nonzero initial selectivity whereas the selectivity of secondary products is initially zero. Selectivity is defined as the number of moles of product formed per mole of JP-10 cracked. Therefore, by extrapolating the selectivity versus conversion plots to zero conversion one can determine whether a product obtained is primary or secondary. Based on such plots, the main primary products were determined to be methane, ethylene, ethane, propylene, cyclopentene, cyclopentadiene, benzene, toluene and an unidentified product (P1). The variation of selectivity of methane, ethylene and benzene with conversion is shown in Fig. 4. As shown in Fig. 4, at high conversions, the methane selectivity increased with an increase in conversion and leveled off at lower conversions, leading to an initial selectivity value of 0.18 mol/mol of JP-10 cracked. The ethylene selectivity increased marginally with conversion. The initial ethylene selectivity was 0.5 mol/mol JP-10 cracked. Propylene selectivity followed the same trend as ethylene. It can be observed from Fig. 4 that benzene selectivity increased almost linearly with conversion, with an initial selectivity of around 0.09 mol/ mol of JP-10 cracked. For a fixed conversion, temperature did not have a significant effect on the selectivities of most of the liquid products. The initial selectivities of the main primary products are shown in Table 2. A carbon balance shows that approximately 69.3% of the carbon in JP-10 is accounted for by these products. (For this calculation, P1 was assumed to be a C7 compound).
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Fig. 3. Variation of product yields with conversion (a) methane or benzene (b) ethylene (c) propylene (d) cyclopentene or cyclopentadiene. (temperature, K: (4) 903; (~) 923; (&) 948; (&) 968).
3.4. Kinetic analysis The overall kinetics of thermal cracking of JP-10 was determined both by the equivalent reactor volume method as well as non-linear regression analysis. In the equivalent reaction volume method, the overall decomposition of JP-10 was assumed to be first order, whereas from non-linear regression analysis the reaction order was also determined.
3.4.1. Equivalent reactor volume method The performance equation for an isothermal plug flow reactor can be expressed as VE t¼ ¼ CA0 Q0
X ZAf
dXA rA
(3)
0
For a first-order reaction, rA ¼ kCA
(4)
Table 2 Initial selectivities of main primary products
Fig. 4. Variation of selectivity of methane, ethylene and benzene with conversion (temperature, K: (4) 903; (~) 923; (&) 948; (&) 968).
Primary product
Initial selectivity (mol/mol of JP-10 cracked)
Methane Ethane Ethylene Propylene Cyclopentene Cyclopentadiene Benzene Toluene P1
0.18 0.11 0.50 0.38 0.19 0.37 0.09 0.06 0.09
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159
and at constant pressure, CA ¼ CA0
1 XA 1 þ eXA
(5)
where e is the expansion factor and represents the relative change in the volume of the reaction mixture from zero to 100% conversion and includes the effect of diluents. For a reaction A ! vP and no inerts added in the feed, e ¼ ðv 1Þ
(6)
Substituting Eqs. (4) and (5) in Eq. (3), we obtain
kt ¼
X ZAf
1 þ eXA dXA 1 XA
(7)
Fig. 5. Determination of overall first order rate constants at different temperatures (temperature, K: (4) 903; (~) 923; (&) 948; (&) 968).
(8)
3.4.2. Non-linear regression analysis For this analysis, the assumption of first-order kinetics was relaxed and the pre-exponential factor, activation energy and overall reaction order were determined using non-linear regression. Making a mass balance on a differential element of a non-isothermal plug flow reactor, we obtain
0
On integration, Eq. (7) gives the following expression: ð1 þ eÞln
1 eXAf ¼ kt ð1 XAf Þ
In the pseudoisothermal approach, the non-isothermal data is reduced to isothermal conditions by calculating the equivalent reactor volume. Since the activation energy is not known, this method involves trial and error. An activation energy is guessed, the corresponding VE for a given reference temperature determined from Eq. (1), and the rate constant calculated from Eq. (8). This is done for different reference temperatures, and from an Arrhenius plot of the rate constant, a new value of E is obtained. If this not equal to the guessed value, the entire procedure is repeated. A rapid convergence of this procedure thus requires a good initial estimate of the activation energy. In the present study, for each run, the axial temperature profile was measured at 5 cm. intervals. An initial guess of 55 kcal/mol for the activation energy was tried. This was later determined to be close to the actual value of 62.4 kcal/mol. Since all the products could not be identified, the experimental value of v was not available. Based on the experimentally determined selectivities and published information on pure hydrocarbons of this carbon number range, v was assumed to be 3 in this conversion range. For this analysis, first-order kinetics was assumed for the thermal cracking of JP-10. Davidson et at. [3] also correlated their data on JP-10 pyrolysis by using first order kinetics for the overall reaction. The rate constants at various reference temperatures (TE) were calculated from the slopes of the plots of the LHS of Eq. (8) versus space time and are shown in Fig. 5. The plots for all the temperatures were linear for the whole range of conversion. The rate constants at different temperatures were calculated from the slopes of these plots and are tabulated in Table 3. The pre-exponential factor and activation energy, as determined from an Arrhenius plot (Fig. 6), were 5.7 1013 s1 and 261.1 kJ/mol (62.4 kcal/ mol), respectively.
FA0 dXA E ¼ k0 exp CAn RT AC dl
(9)
where CA is given by Eq. (5). The optimal values of k0, E and P n were determined by minimizing the objective function N1 ðXAcalc XAexp Þ2 for the
Table 3 Rate constants at different temperatures for thermal cracking of JP-10 Temperature (K)
Rate constant (s1)
903 923 948 968
0.047 0.093 0.238 0.480
Fig. 6. Arrhenius plot for pyrolysis of JP-10.
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the calculated conversions were in very good agreement with the experimental values for the whole range of conversion studied (11–61%). 4. Conclusions
Fig. 7. Calculated conversion from non-linear regression versus experimental conversion.
N experimental runs by varying k0, E and n. Due to the strong correlation between k0 and E, a reparameterization was necessary. After reparameterization Eq. (9) becomes, dXAcalc AC E 1 1 ¼ k exp CAn R T T dl FA0 0
(10)
where k0 ¼ k0 exp
In the temperature range of 903–968 K and space time from 0.7–6.4 s, the conversion of JP-10 during thermal cracking varies in the range of 10.4–61.1%. The major products are methane, ethylene, propylene, cyclopentene, cyclopentadiene, benzene and toluene. The overall decomposition of JP-10 in this conversion and temperature range can be represented by an overall reaction order of 1.1 with a pre-exponential factor of 2.4 1013 (m3/mol)0.1 s1 and an activation energy of 256.2 kJ/mol (61.0 kcal/mol). Acknowledgements The financial support provided by Defense Research and Development Laboratory (DRDL), Hyderabad, for this study is gratefully acknowledged. We are thankful to Defense Materials and Stores Research and Development Establishment (DMSRDE), Kanpur for supplying the JP-10. References
E RT
(11)
T* is the average temperature for all the runs and was taken to be 935 K. XAcalc was obtained by numerically integrating Eq. (10) using a fourth order Runge-Kutta–Gill algorithm. For each run, the measured axial temperature profile was fitted to a sixth order polynomial expression for use in the numerical integration. The objective function was minimized by using the genetic algorithm. The values of k0 , E/R and n were constrained in the range of 0.1–3.0, 20000–40000 and 0.1–2.0, respectively. Based on the minimization, the pre-exponential factor, activation energy and overall order were determined to be 2.4 1013 (m3/mol)0.1 s1, 256.2 kJ/mol (61.0 kcal/mol) and 1.1, respectively. As shown in Fig. 7, using these parameters,
[1] D.T. Wickham, J.R. Engel, B.D. Hitch, M.E. Karpuk, J. Propulsion Power 17 (2001) 1253–1257. [2] M. Cooper, J.E. Shepherd, in: Proceedings of the 39th AIAA/ASME/SAE/ ASEE Joint Propulsion Conference and Exhibit, paper no. AIAA-20034867, Huntsville, AL., July 20–23, 2003. [3] D.F. Davidson, D.C. Horning, M.A. Oehlschlaeger, R.K. Hanson, in: Proceedings of the 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, paper no. AIAA-2001-3887, Salt Lake City, UT, July 8–11, 2001. [4] R.J. Green, S.L. Anderson, in: Proceedings of the 13th ONR Propulsion Meeting, Minneapolis, MN, August 10–12, 2000, pp. 271–276. [5] K. Wohlwend, L.Q. Maurice, T. Edwards, R.C. Striebich, M. Vangsness, A.S. Hill, J. Propulsion Power 17 (2001) 1258. [6] R.C. Striebich, J. Lawrence, J. Anal. Appl. Pyrolysis 70 (2003) 339. [7] G.F. Froment, B.O. Van De Steene, P.J. Vanden Berghe, A.G. Goossens, AICHE J. 23 (1977) 93. [8] P.S. Van Damme, G.F. Froment, Ind. Eng. Chem. Proc. Des. Dev. 20 (1981) 366.
Thermal cracking of JP-10: Kinetics and product distribution P. Nageswara Rao, Deepak Kunzru * Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 08016, India Received 9 May 2005; accepted 7 October 2005 Available online 21 November 2005
Abstract The product distribution and kinetics of thermal cracking of JP-10 (exo-tetrahydrodicyclopentadiene) were investigated in an annular tubular reactor at atmospheric pressure, in the temperature range of 903–968 K. No inerts were added with the feed. The major products were methane, ethylene, propylene, cyclopentene, cyclopentadiene, benzene and toluene. The pre-exponential factor, activation energy and overall reaction order for thermal cracking of JP-10 were determined by non-linear regression analysis to be 2.4 1013 (m3/mol)0.1 s1, 256.2 kJ/mol and 1.1, respectively. # 2005 Elsevier B.V. All rights reserved. Keywords: JP-10; Exo-tetrahydrodicyclopentadiene; Thermal cracking; Pyrolysis; Kinetics
1. Introduction In the last decade great emphasis has been given to airbreathing propulsion for high Mach number applications. Aircraft designed for hypersonic flight must incorporate active cooling in the propulsion system, because at high Mach numbers, the sensible heating of the fuel cannot provide sufficient heat sink to cool the supersonic vehicles. Therefore, endothermic reactions, such as thermal cracking, catalytic cracking or catalytic gasification are needed to augment the cooling capacity of the fuel [1]. Moreover, practical air-breathing pulse detonation engines (PDE) will most probably be based on storable liquid hydrocarbon fuels such as JP-10 or Jet A. However, without fuel modifications, these liquid fuels are not suitable for use in PDE. One possibility of fuel modification is to thermally crack the fuel [2]. JP-10 (exo-tetrahydrodicyclopentadiene), a synthetic fuel, is commonly used in missile applications. Due to its strained cyclic structure, it has a high volumetric energy density and is suited for volume limited applications. Very meager information is available in the published literature on the thermal cracking of JP-10. Cooper and Shepherd [2] examined the thermal and catalytic decomposition of JP-10 at 500 8C. They
* Corresponding author. Tel.: +91 512 2597193 E-mail address: [email protected] (D. Kunzru). 0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jaap.2005.10.003
compared the results with and without catalyst, and at 500 8C reported a conversion of 3.51% in thermal cracking, whereas it was relatively higher in catalytic cracking (34% with HY type zeolite). They did not measure the kinetics or the gaseous product distribution. Davidson et al. [3] developed a high speed UV absorption spectrograph to identify the initial products formed during the thermal decomposition of JP-10 in the temperature range of 1100–1700 K. Based on the product distribution, they proposed that the first step in the decomposition was the breaking of C–C bond to form cyclopentene and other products. Assuming first-order kinetics, the activation energy for the decomposition of JP-10 was calculated to be 72,500 cal/mol. Green and Anderson [4] studied the decomposition of JP-10 in a micro-flow-tube reactor over the temperature range of 298–1300 K on a millisecond time scale. They found that decomposition set in at 840 K and was complete by 1000 K. At high temperatures, the only significant product identified by them in the mass range above 50 amu was benzene. Wohlwend et al. [5] studied the thermal stability of JP10 under supercritical conditions (34 atm and a temperature range of 200–650 8C). The experiments were conducted at a feed flow rate of 0.5 ml/min, corresponding to a residence time of 1.8 s at 200 8C. They found that JP-10 started to fragment at 450 8C and decomposed readily at 600 8C. The major decomposition products observed were cyclopentene and cyclopentadiene. Other products identified included benzene, toluene, naphthalene, substituted cyclopentene and substituted cyclopentadiene. The product yields were not quantified.
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Nomenclature AC CA CA0 E FA0 k k0 k0 l n PE Q0 rA R T TE T* V VE XA
cross- sectional area of reactor, m2 concentration of JP-10, mol/m3 inlet concentration of JP-10, mol/m3 activation energy, kJ/mol inlet molar flow rate of JP-10, mol/s rate constant of pyrolysis reaction, s1 or (m3/ mol)0.1s1 pre-exponential factor, s1 or (m3/mol)0.1s1 reparameterized pre-exponential factor, defined in Eq. (11) axial coordinate, m reaction rate reference pressure, Pa inlet volumetric flow rate, m3/s rate of reaction, mol/m3 s gas constant, J/mol/K reactor temperature, K equivalent reactor temperature, K average temperature used for reparameterization, K volume of reactor, m3 equivalent reactor volume, m3 conversion of JP-10
Greek Letters e expansion factor t space time, s n stoichiometric coefficient, moles of total product /mol of JP-10 cracked
155
Striebich and Lawrence [6] also studied the decomposition of JP-10 under supercritical conditions at 34 atm. The residence time in their study varied from 5 to 1 s as the reaction temperature was increased from 100 to 600 8C. For this range of temperature and residence time, the decomposition of JP-10 was less than 10%, even at 600 8C. Considering the lack of information available on the high temperature chemistry of JP-10, the objective of this study was to investigate effect of temperature and residence time on the conversion and product distribution during the thermal cracking of JP-10 at atmospheric pressure. Another objective was to determine the kinetics of cracking. This investigation is part of an overall study to determine the effect of pressure on the product distribution and kinetics of thermal decomposition of JP-10. 2. Experimental Pyrolysis of JP-10 was conducted in an annular tubular reactor. A schematic diagram of the experimental set-up is shown in Fig. 1. The feed was injected into the reactor at the required flow rate using a dosing pump. No inerts were added to the JP-10 feed. The reactor (7.7 mm i.d. 11.3 mm o.d. 800 mm long) was made of Inconel and heated in a three-zone furnace. The temperature of each zone was controlled by separate proportional controllers. The axial temperature profile was measured using a chromel-alumel thermocouple, manually driven in an Inconel thermowell tube (3.2 mm o.d.) attached to the inlet end of the reactor. No separate preheater was used and the first part of the reactor served as the preheater. To passivate the walls of the reactor, prior to each run, the reactor was presulfided by passing water containing 200 ppm of CS2. Due to this passivation, the wall
Fig. 1. Experimental set-up.
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effects are expected to be negligible [7]. A mercury manometer with one leg connected to system and the other leg open to the atmosphere was used to measure the pressure difference between the system and the surroundings. The reactor effluent was quenched by passing through two condensers, connected in series. A water-glycerol mixture at 268 K was used as the coolant and circulated through the condensers using a refrigerated circulator. The condensed liquid hydrocarbon products and any unconverted reactant were collected in separating flasks attached to the condensers. Nitrogen, injected after the first separating funnel, served as an internal standard in the subsequent chromatographic analysis. The nitrogen flow rate was maintained at the desired value by using a mass flow controller. The noncondensables passed through a gas sampling valve and were then vented. During the course of a run, the condensed liquid as well as the composition of the liquid and gaseous products was measured at regular intervals. Moreover, the axial temperature profile was measured for each run. After completion of the run, the reactor was flushed with helium for 15 min, and then the reactor was decoked with air. The condensed liquid hydrocarbon products were analyzed on a capillary column (Petrocol DH column, 0.25 mm i.d., 100 m long). n-Octane was used as the internal standard. The noncondensables, which mainly consisted of C1–C5 hydrocarbons, were analyzed using three columns, namely Durapak, Porapak Q and Carbosphere. The separation of methane and nitrogen was carried out on a Carbosphere column using a thermal conductivity detector, with helium as the carrier. Determination of C1–C5 hydrocarbons was performed on a 3 m long Durapak column using flame ionization detector. As the separation of ethane and ethylene was not possible on the Durapak column, Porapak Q column was used to separate methane, ethane and ethylene. 3. Results and discussion
volume that, at a chosen equivalent temperature, TE, and reference pressure PE, would give the same conversion as the actual reactor with its temperature and pressure profile. For a first-order reaction with negligible pressure drop in the reactor, VE can be expressed as [8],
VE ¼
ZV
TE E 1 1 dV exp R T TE T
(1)
0
where E is the activation energy for the reaction and T is the actual reactor temperature and is function of reactor length. Since the activation energy is not known a priori, the procedure for determining VE involves trial and error, and is discussed in Section 3.4.1. The equivalent temperature, TE, was taken to be the temperature in the central portion of the reactor whereas T was measured experimentally at different axial positions. The space time was then evaluated from the calculated VE as t¼
VE Q0
(2)
3.2. Effect of temperature and space time on conversion The variation of conversion of JP-10 with space time at different temperatures is shown in Fig. 2. Due to the axial temperature profile, a single reaction temperature for a run is not defined and the temperature indicated for each run refers to the reference temperature. At each temperature, conversion increased with space time, tending to level off at higher space times. Conversion increased significantly with temperature. For instance, at a space time of 2.5 s, the conversion was 14.4% at 903 K and increased to approximately 55% at 968 K. There is no previously reported data at these conditions.
In this study, the effect of space time and temperature on the conversion and product yields in JP-10 pyrolysis at atmospheric pressure was investigated. No inerts were added with the feed. The experiments covered the following range of variables: temperature, 903–968 K, and JP-10 flow rate, 0.4–2.3 g/min, and thus a space time of 0.68–6.4 s. JP-10 was obtained from DMSRDE, Kanpur. The purity, as determined from GC analysis, was 96.4 wt%. 3.1. Calculation of space time Analysis of the experimental data in any pyrolysis study is rendered difficult due to the axial temperature profile that exists in the reactor. The concept of equivalent reactor volume was used to calculate the space time for data representation and also as one of the methods for determination of kinetic parameters. A nonisothermal reactor can be represented as a pseudoisothermal reactor, by using the concept of equivalent reactor volume. The equivalent reactor volume, VE, is defined as the
Fig. 2. Variation of conversion of JP-10 with space time (temperature, K (4) 903; (~) 923; (&) 948; (& ) 968). Space time calculated from the temperature profile, activation energy of 62.4 kcal/mol and the flow rate of JP-10, which was varied from 0.4 to 2.3 g/min.
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Table 1 Product yields during pyrolysis of JP-10 fuel Temperature (K) Space time (s)
968 0.7
Yields of products, wt% feed Methane Ethylene Ethane Propylene Propane 1-Butene 2- Butene + isobutylene + 1,3-butadiene Cyclopentene Cyclopentadiene Other non-condensables Benzene Toluene P1(unidentified) Mass balance Conversion (%)
0.6 2.9 0.6 2.8 0.1 0.1 0.2 1.6 4.6 Traces 1.6 1.3 1.2 100.5 27.1
968 3.3 2.5 7.6 2.0 7.6 0.3 0.3 1.0 2.3 11.7 0.12 6.2 4.2 0.9 97.9 61.1
3.3. Product yields The effect of temperature and space time on the product yields (wt% feed) and selectivities of various liquid and gaseous products was investigated. The products obtained from thermal cracking were a mixture of condensables and noncondensables. Minor amount of coke formation was observed at 948 and 968 K, especially at high conversions. Most of the gaseous products were identified. Due to experimental limitations, the yield of hydrogen was measured for only a few runs and varied between 0.2 and 0.3 wt% of feed. The mass balance closure for all the runs was within 5% and the detailed product distribution for some runs is given in Table 1. The percent yield of a product is defined as the kg of product formed per kg of JP-10 fed to the reactor. In the pyrolysis of JP-10, the major gaseous products were ethylene, propylene, cyclopentene and cyclopentadiene. Other gaseous products included methane, ethane, propane, 1-butene, other C4s and some C5 compounds. Acetylene was present in trace amounts only. Cyclopentene and cyclopentadiene were present in the liquid products also. The combined yields of ethylene, propylene, cyclopentene and cyclopentadiene varied from 4.3 to 29.2 wt% as the conversion varied from 11.2% to 61.1%. The number of liquid products formed during the thermal pyrolysis of JP-10 was as many as 40 at high conversions, and around 15 at lower conversions. The major liquid products included benzene and toluene. Naphthalene and dicyclopentadiene were also formed in minor amounts (<1.0 wt% feed). The other liquid products could not be identified. The variation in the yields of methane, ethylene, propylene, cyclopentene, cyclopentadiene and benzene with conversion at different temperatures is shown in Fig. 3. The yield of methane and benzene increased monotonically with conversion, whereas the yields of ethylene, propylene, cyclopentadiene and cyclopentene tend to level off at high conversions. These plots also show that at a particular conversion, the yields were not significantly affected by reaction temperature. The variation in the yield of toluene with conversion (not shown) was similar to
948 0.8 0.3 1.3 0.3 1.5 0.02 0.03 0.1 1.0 2.2 0.02 0.7 0.7 1.0 100.5 15.4
948 5.3 2.5 6.7 2.0 7.8 0.3 0.3 1.0 2.0 9.1 0.18 1.8 2.4 0.7 97.6 50.7
923 1.3 0.2 1.2 0.2 1.3 0.02 0.02 0.1 1.0 2.3 0.01 0.6 0.6 0.8 100.4 11.2
923 6.4 1.5 4.8 1.4 5.5 0.2 0.2 0.6 2.0 6.6 0.19 1.8 1.9 0.9 99.1 36.7
903 2.5 0.2 0.3 0.8 0.8 0.02 0.01 0.04 1.2 2.5 0.02 0.4 0.4 0.4 100.4 14.4
903 6.4 0.6 1.9 0.6 2.0 0.10 0.05 0.2 1.3 3.6 0.04 0.8 0.8 0.8 99.4 21.1
that of benzene, the only difference being that the toluene yields were lower. These results show that the stable species (methane, benzene and toluene) once formed do not crack further at these conditions. The trends of benzene and toluene yields are in agreement with the results reported by Wohlwend et al. [5] who studied the decomposition of JP-10 at a constant residence time and reported the benzene and toluene yields in terms of ‘percent yield relative to parent response’. At a temperature of 650 8C, the benzene and toluene yields reported by them were 6.5 and 4.5, respectively. It is of interest to determine the primary products formed during thermal cracking of JP-10. Primary products will have a nonzero initial selectivity whereas the selectivity of secondary products is initially zero. Selectivity is defined as the number of moles of product formed per mole of JP-10 cracked. Therefore, by extrapolating the selectivity versus conversion plots to zero conversion one can determine whether a product obtained is primary or secondary. Based on such plots, the main primary products were determined to be methane, ethylene, ethane, propylene, cyclopentene, cyclopentadiene, benzene, toluene and an unidentified product (P1). The variation of selectivity of methane, ethylene and benzene with conversion is shown in Fig. 4. As shown in Fig. 4, at high conversions, the methane selectivity increased with an increase in conversion and leveled off at lower conversions, leading to an initial selectivity value of 0.18 mol/mol of JP-10 cracked. The ethylene selectivity increased marginally with conversion. The initial ethylene selectivity was 0.5 mol/mol JP-10 cracked. Propylene selectivity followed the same trend as ethylene. It can be observed from Fig. 4 that benzene selectivity increased almost linearly with conversion, with an initial selectivity of around 0.09 mol/ mol of JP-10 cracked. For a fixed conversion, temperature did not have a significant effect on the selectivities of most of the liquid products. The initial selectivities of the main primary products are shown in Table 2. A carbon balance shows that approximately 69.3% of the carbon in JP-10 is accounted for by these products. (For this calculation, P1 was assumed to be a C7 compound).
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Fig. 3. Variation of product yields with conversion (a) methane or benzene (b) ethylene (c) propylene (d) cyclopentene or cyclopentadiene. (temperature, K: (4) 903; (~) 923; (&) 948; (&) 968).
3.4. Kinetic analysis The overall kinetics of thermal cracking of JP-10 was determined both by the equivalent reactor volume method as well as non-linear regression analysis. In the equivalent reaction volume method, the overall decomposition of JP-10 was assumed to be first order, whereas from non-linear regression analysis the reaction order was also determined.
3.4.1. Equivalent reactor volume method The performance equation for an isothermal plug flow reactor can be expressed as VE t¼ ¼ CA0 Q0
X ZAf
dXA rA
(3)
0
For a first-order reaction, rA ¼ kCA
(4)
Table 2 Initial selectivities of main primary products
Fig. 4. Variation of selectivity of methane, ethylene and benzene with conversion (temperature, K: (4) 903; (~) 923; (&) 948; (&) 968).
Primary product
Initial selectivity (mol/mol of JP-10 cracked)
Methane Ethane Ethylene Propylene Cyclopentene Cyclopentadiene Benzene Toluene P1
0.18 0.11 0.50 0.38 0.19 0.37 0.09 0.06 0.09
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and at constant pressure, CA ¼ CA0
1 XA 1 þ eXA
(5)
where e is the expansion factor and represents the relative change in the volume of the reaction mixture from zero to 100% conversion and includes the effect of diluents. For a reaction A ! vP and no inerts added in the feed, e ¼ ðv 1Þ
(6)
Substituting Eqs. (4) and (5) in Eq. (3), we obtain
kt ¼
X ZAf
1 þ eXA dXA 1 XA
(7)
Fig. 5. Determination of overall first order rate constants at different temperatures (temperature, K: (4) 903; (~) 923; (&) 948; (&) 968).
(8)
3.4.2. Non-linear regression analysis For this analysis, the assumption of first-order kinetics was relaxed and the pre-exponential factor, activation energy and overall reaction order were determined using non-linear regression. Making a mass balance on a differential element of a non-isothermal plug flow reactor, we obtain
0
On integration, Eq. (7) gives the following expression: ð1 þ eÞln
1 eXAf ¼ kt ð1 XAf Þ
In the pseudoisothermal approach, the non-isothermal data is reduced to isothermal conditions by calculating the equivalent reactor volume. Since the activation energy is not known, this method involves trial and error. An activation energy is guessed, the corresponding VE for a given reference temperature determined from Eq. (1), and the rate constant calculated from Eq. (8). This is done for different reference temperatures, and from an Arrhenius plot of the rate constant, a new value of E is obtained. If this not equal to the guessed value, the entire procedure is repeated. A rapid convergence of this procedure thus requires a good initial estimate of the activation energy. In the present study, for each run, the axial temperature profile was measured at 5 cm. intervals. An initial guess of 55 kcal/mol for the activation energy was tried. This was later determined to be close to the actual value of 62.4 kcal/mol. Since all the products could not be identified, the experimental value of v was not available. Based on the experimentally determined selectivities and published information on pure hydrocarbons of this carbon number range, v was assumed to be 3 in this conversion range. For this analysis, first-order kinetics was assumed for the thermal cracking of JP-10. Davidson et at. [3] also correlated their data on JP-10 pyrolysis by using first order kinetics for the overall reaction. The rate constants at various reference temperatures (TE) were calculated from the slopes of the plots of the LHS of Eq. (8) versus space time and are shown in Fig. 5. The plots for all the temperatures were linear for the whole range of conversion. The rate constants at different temperatures were calculated from the slopes of these plots and are tabulated in Table 3. The pre-exponential factor and activation energy, as determined from an Arrhenius plot (Fig. 6), were 5.7 1013 s1 and 261.1 kJ/mol (62.4 kcal/ mol), respectively.
FA0 dXA E ¼ k0 exp CAn RT AC dl
(9)
where CA is given by Eq. (5). The optimal values of k0, E and P n were determined by minimizing the objective function N1 ðXAcalc XAexp Þ2 for the
Table 3 Rate constants at different temperatures for thermal cracking of JP-10 Temperature (K)
Rate constant (s1)
903 923 948 968
0.047 0.093 0.238 0.480
Fig. 6. Arrhenius plot for pyrolysis of JP-10.
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the calculated conversions were in very good agreement with the experimental values for the whole range of conversion studied (11–61%). 4. Conclusions
Fig. 7. Calculated conversion from non-linear regression versus experimental conversion.
N experimental runs by varying k0, E and n. Due to the strong correlation between k0 and E, a reparameterization was necessary. After reparameterization Eq. (9) becomes, dXAcalc AC E 1 1 ¼ k exp CAn R T T dl FA0 0
(10)
where k0 ¼ k0 exp
In the temperature range of 903–968 K and space time from 0.7–6.4 s, the conversion of JP-10 during thermal cracking varies in the range of 10.4–61.1%. The major products are methane, ethylene, propylene, cyclopentene, cyclopentadiene, benzene and toluene. The overall decomposition of JP-10 in this conversion and temperature range can be represented by an overall reaction order of 1.1 with a pre-exponential factor of 2.4 1013 (m3/mol)0.1 s1 and an activation energy of 256.2 kJ/mol (61.0 kcal/mol). Acknowledgements The financial support provided by Defense Research and Development Laboratory (DRDL), Hyderabad, for this study is gratefully acknowledged. We are thankful to Defense Materials and Stores Research and Development Establishment (DMSRDE), Kanpur for supplying the JP-10. References
E RT
(11)
T* is the average temperature for all the runs and was taken to be 935 K. XAcalc was obtained by numerically integrating Eq. (10) using a fourth order Runge-Kutta–Gill algorithm. For each run, the measured axial temperature profile was fitted to a sixth order polynomial expression for use in the numerical integration. The objective function was minimized by using the genetic algorithm. The values of k0 , E/R and n were constrained in the range of 0.1–3.0, 20000–40000 and 0.1–2.0, respectively. Based on the minimization, the pre-exponential factor, activation energy and overall order were determined to be 2.4 1013 (m3/mol)0.1 s1, 256.2 kJ/mol (61.0 kcal/mol) and 1.1, respectively. As shown in Fig. 7, using these parameters,
[1] D.T. Wickham, J.R. Engel, B.D. Hitch, M.E. Karpuk, J. Propulsion Power 17 (2001) 1253–1257. [2] M. Cooper, J.E. Shepherd, in: Proceedings of the 39th AIAA/ASME/SAE/ ASEE Joint Propulsion Conference and Exhibit, paper no. AIAA-20034867, Huntsville, AL., July 20–23, 2003. [3] D.F. Davidson, D.C. Horning, M.A. Oehlschlaeger, R.K. Hanson, in: Proceedings of the 37th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, paper no. AIAA-2001-3887, Salt Lake City, UT, July 8–11, 2001. [4] R.J. Green, S.L. Anderson, in: Proceedings of the 13th ONR Propulsion Meeting, Minneapolis, MN, August 10–12, 2000, pp. 271–276. [5] K. Wohlwend, L.Q. Maurice, T. Edwards, R.C. Striebich, M. Vangsness, A.S. Hill, J. Propulsion Power 17 (2001) 1258. [6] R.C. Striebich, J. Lawrence, J. Anal. Appl. Pyrolysis 70 (2003) 339. [7] G.F. Froment, B.O. Van De Steene, P.J. Vanden Berghe, A.G. Goossens, AICHE J. 23 (1977) 93. [8] P.S. Van Damme, G.F. Froment, Ind. Eng. Chem. Proc. Des. Dev. 20 (1981) 366.