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Kinetics and Product Distributions for Thermal Cracking of a Kerosene-Based Aviation Fuel Yan Xing, Wenjie Xie, Wenjun Fang,* Yongsheng Guo, and Ruisen Lin Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, People’s Republic of China ReceiVed April 13, 2009. ReVised Manuscript ReceiVed June 19, 2009
Thermal cracking reactions of a kerosene-based aviation fuel were carried out in a constant-volume reactor, and the kinetics were investigated at different temperatures from 663 to 703 K. The gaseous and liquid products collected from the efflux cooled to room temperature at atmospheric pressure were determined by gas chromatography and gas chromatography-mass spectrometry. The major hydrocarbon components in the gaseous phase were methane, ethane, ethene, propane, and propene; methane had the highest content among them. In the liquid effluence, the contents of components with carbon atom number higher than C11 decreased while those with lower carbon atom number clearly increased with the temperature or reaction time increasing. The gas yield ratios increased with the reaction temperature or time increasing. The thermal cracking kinetic process was correlated well by the pseudo first-order kinetic equation. The correlation followed that the rate constants were in the range 0.163 × 10-2 h-1 at 663 K to 2.809 × 10-2 h-1 at 703 K. The Arrhenius parameters were then derived with the values of the apparent activation energy Ea of 280 ( 6.5 kJ mol-1 and the logarithm of the pre-exponential factor, log A ) 19.3 ( 0.5. The values were in reasonable agreements with those for the first-order decomposition of the chain alkanes such as n-dodecane.
1. Introduction In advanced aircrafts, liquid propellants are designed to be used as coolants to absorb heat from various aircraft components such as engine lubricating oil, electrical system, environmental control system, and air frame.1 The performances of hydrocarbon fuels operated at high temperatures are affected by the thermal cracking process and the formation of solid deposits. Studies have been reported on the thermal cracking for several model fuels including C7∼C25 n-alkanes, cycloalkanes, aromatic compounds, and their mixtures.2-9 It has been shown that the products from the thermal cracking of hydrocarbon mixtures are more complicated than those from pure compounds. As an example, C12∼C20 alkyl cyclohexanes with normal and branched side chains can be founded in the mixture of n-C12/n-butylcyclohexane.3 Although the model fuels have advantages of their simple and known compositions, they can not accurately represent the properties of real fuels. The thermal cracking kinetics of some real fuels, such as Jet A and RP-1, were recently introduced.10,11 However, in the case of such complex mixtures, there are a series of reactions, and the rate constants and the * Corresponding author: Phone:+86 571 87952371; fax: +86 571 8795 1895; e-mail:
[email protected]. (1) Edwards, T. AIAA-1993-0807. (2) Yu, J.; Eser, S. Ind. Eng. Chem. Res. 1997, 36 (3), 585–591. (3) Yu, J.; Eser, S. Fuel 2000, 79, 759–768. (4) Mickae¨l, S.; Bruno, R.; Fre´de´ric, S. AIAA-2005-3402. (5) Daniau, E.; Sicard, M. AIAA-2005-3404. (6) Pant, K. K.; Kunzru, D. J. Anal. Appl. Pyrolysis 1996, 36, 103– 120. (7) Wu, G. Z.; Katsumura, Y.; Matsuura, C.; Ishigure, K. Ind. Eng. Chem. Res. 1996, 35, 4747–4754. (8) Khorasheh, F.; Gray, M. R. Ind. Eng. Chem. Res. 1993, 32, 1853– 1863. (9) Behar, F.; Vandenbroucke, M. Energy Fuels 1996, 10, 932–940. (10) Andersen, P. C.; Bruno, T. J. Ind. Eng. Chem. Res. 2005, 44, 1670– 1676. (11) Widegren, J. A.; Bruno, T. J. Ind. Eng. Chem. Res. 2008, 47, 4342– 4348.
Table 1. Characterized Properties of the Kerosene-Based Aviation Fuel property
value
M F (298.15 K)/g cm-3 Tb (101.3 kPa)/K Tfp/K IBP/K FBP/K η/mPa s Tc/K pc/MPa
154.6 0.79328 453.90 317.95 429 547 1.764 682.04 2.16
Arrhenius parameters for thermal cracking are not easily determined. More attention should be paid to the practical hydrocarbon fuels, especially to the thermal cracking kinetics under pressure. The hydrocarbon fuel investigated in the present work is a kerosene-based aviation fuel refined by deep dearomatization, which has high thermal-oxidation stability and is a potential candidate for the development of an endothermic hydrocarbon fuel.12 This paper focuses on its thermal cracking kinetics. The thermal cracking is performed with the static method in a stainless-steel reactor. Although fuels under the flow state are relatively close to actual conditions, there are significant advantages of researching with the static method. The reaction time in the static reactor can be controlled easily. The quantities of material and the total volume of the gaseous products can be obtained accurately. In this work, the extent of reaction expressed as gas yield ratios as a function of time is determined at each temperature. The decomposition rate constants and the corresponding Arrhenius parameters are then calculated. The product distributions are analyzed by gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). (12) Li, D.; Fang, W. J.; Xing, Y.; Guo, Y. S.; Lin, R. S. Fuel 2008, 87, 3286–3291.
10.1021/ef9003297 CCC: $40.75 2009 American Chemical Society Published on Web 07/14/2009
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Figure 2. Gas yield ratios of the aviation fuel under thermal cracking at different temperatures.
Figure 1. Apparatus for thermal cracking of the kerosene-based aviation fuel: 1. furnace; 2. reactor; 3. temperature measurement tube; 4. liquid phase tube; 5. reactor cover; 6. needle valve; 7. manometer; 8. thermal couple; 9. safety valve; 10. fixed bolts. Table 2. Kinetic Parameters for Thermal Cracking of the Aviation Fuel T/K
663
673
683
693
703
p/MPa k × 10-2/ h-1 R2
2.0 0.16 0.9935
2.7 0.33 0.9865
3.0 0.76 0.9900
3.4 1.54 0.9992
3.9 2.81 0.9995
2. Experimental Section 2.1. Materials. The sample investigated was a refined kerosenebased aviation fuel with further dearomatization from a military jet fuel. The relative contents of n-paraffin, iso-paraffin, cycloparaffin, and aromatics were determined in mass fraction to be 13.62, 41.00, 45.11, and 0.27%, respectively. The initial and final boiling points (IBP and FBP) of the hydrocarbon fuel are 156 and 274 °C, respectively. Some physical properties are listed in Table 1. M is the average relative molecular mass, F is the density, Tb is the normal boiling point, Tfp is the flash point, η is viscosity, and Tc and pc are the critical temperature and pressure. The density was measured by an Anton Paar DMA55 vibrating tube digital densimeter. The normal boiling point was determined by an inclined ebulliometer. The value of M was estimated from the GC-MS analysis results. Tc and pc were calculated with the Riazi-Daubert method.7 The predominant compositions were detected by a Hewlett-Packard 6890/5973 GC-MS. 2.2. Apparatus and Procedure. A schematic diagram of the static stainless-steel reactor with a total volume of 200 mL is shown in Figure 1. The maximum operation temperature and pressure for the reactor are 773 K and 30 MPa, respectively. The thermal cracking reactions of the kerosene-based aviation fuel were performed under near-critical and supercritical pressures. The fuel with a definite mass of 50 g was injected into the reactor and heated in the furnace controlled by a proportional-controller. The pressure of the system was measured by a manometer. Before each measurement, the aviation fuel was injected into the reactor, and it was purged with N2 to wipe the air in the reactor. Then, the reactor was heated by the furnace to the seated temperature. After a given reaction time, the reactor was removed from the furnace and cooled down quickly to the room temperature. The volume of the gaseous phase products was quantified by the water displacement method. The liquid residue was taken out by a syringe and then weighed and kept. To minimize the systematical error, the dead volume of the reactor should be measured. The gas yield ratio was then calculated from the difference between the
material and its liquid residue. The gaseous products were analyzed quantitatively using a GC equipped with a 50 m KCl/Al2O3 PLOT column and an FID detector. The GC was programmed from 323 to 453 K at a rate of 5 K min-1 with an initial isothermal period of 5 min. The liquid products were separated and detected by a Hewlett-Packard (HP) 6890 GC equipped with a DB-17 column and a HP 5973 mass-selective detector. The gas chromatography was programmed from 333 to 453 K at a rate of 10 K min-1 with an initial isothermal period of 5 min.
3. Results and Discussion 3.1. The Gas Yield Ratio. An aviation fuel is a complicated mixture with hundreds of hydrocarbons. The components from the thermal cracking of the aviation fuel are more complex than the material. The light products from the decomposition reactions caused pressure increase in the reactor. The pressure when the reactor just reached each setting temperature is given in Table 2. After each reaction, the reactor contained a pressurized mixture of vapor and liquid even when it was cooled to room temperature. The nearly colorless liquid was changed to be pale yellow at the lower temperatures and was dark brown at higher temperatures. However, no detectable cokes were observed. To minimize the effects of the time required for preheating and cooling the reactor on the kinetic data, the decomposition under every temperature and the result analysis of the fuel were also performed at the zero reaction time, when the reactor is just heated to the operating temperature, then immediately quenched. Because of the complex compositions, it is difficult to distinguish the actual consumption or formation of any individual component. As an approximation, the mass difference between the material before and after its thermal cracking collected at room temperature and atmospheric pressure is supposed to be the total value of the gaseous products. The gas yield ratio can then be defined as Gas yield ratio )
Initial mass(g) - Final mass(g) Initial mass(g)
(1)
The gas yield ratios of the thermal cracking of the aviation fuel under various conditions are shown in Figure 2. These results reflect the conversions of the thermal cracking to a certain extent. It can be clearly seen from Figure 2 that the cracking severity of the aviation fuel increased with the reaction temperature or time increasing. These time- or temperaturedependent results can be treated as the kinetic data for the thermal cracking of such a complex system.
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1 1 k ) ln t 1-x
Figure 3. Pseudo first-order plots for thermal cracking of the aviation fuel at different temperatures.
where x is the gas yield ratio and k is the apparent first-order rate constant. Their values can be obtained from the thermal cracking measurements under different reaction conditions. Plots of ln[1/(1 - x)] against time for the thermal cracking of the aviation fuel at five temperatures (663, 673, 683, 693, and 703 K) are shown in Figure 3. The rate constants are determined by the least-squares method and are listed in Table 2, along with the linearly dependent coefficient (R2). These suggest that the first-order kinetic assumption should be suitable for the system. According to the rate constants at different temperatures, the apparent activation energy, Ea, can be determined using the Arrhenius law ln k ) ln A -
Figure 4. Arrhenius plots for rate constants of thermal cracking of the aviation fuel.
3.2. Rate Constants and Activation Energy. A simplifying assumption is necessary in order to more accurately describe the thermal cracking of the aviation fuel. The pseudo first-order rate constant is used to describe the bulk behavior of the complex system.11,13,14 The rate constant can be determined by the following expression:
(2)
Ea RT
(3)
where A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature. Figure 4 shows the Arrhenius plots for the aviation fuel. The logarithm of the pre-exponential factor, log A ) 19.3 ( 0.5. The apparent activation energy is 280 ( 6.5 kJ mol-1, which is similar to that of 260 kJ mol-1 for the thermal cracking of n-dodecane.2 For the first-order decomposition of chain alkanes (C7∼C25),15,16 the apparent activation energy usually falls in the range of 231-273 kJ mol-1. These results indicate that the gas yield ratio can represent the kinetic process of the thermal cracking of the aviation fuel. 3.3. Gaseous Products. The gaseous hydrocarbon products from the decomposition of the aviation fuelssuch as methane, ethane, ethene, propane, propene, and components with C4 and C4+ carbon atomsswere detected by GC. As an example, the relative contents of the gaseous products from thermal cracking at 703 K are shown in Figure 5a. It can be clearly seen that the major products in the gaseous phase are the alkanes and methane has the highest concentration. The distributions of gaseous products with the increase of temperature are presented in Figure 5b. It is shown that the contents of ethene and propene decrease with the temperature increasing and those of methane, ethane, and propane increase simultaneously. The high content of methane in the gaseous phase is ascribed to the composition and structure of the compositions in the aviation fuel, which is
Figure 5. Distribution of gaseous products from thermal cracking of the aviation fuel at 703 K (a), and distribution of gaseous products at 1 h reaction time (b).
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Figure 6. Effects of reaction temperature on the liquid product distributions at 1 h reaction time (a), and effects of reaction time on the liquid product distributions at 693 K (b).
a kerosene-based fuel with large contents of branched paraffin and substituted cycloalkanes. The thermal cracking of these branched alkanes occurs easily between the bonds at the side chain of the molecule. The more branches the hydrocarbon molecule has, the more side chains are there in this molecule. Methyl branched alkanes are easier to produce methane. With the increases of temperature and pressure during the reaction procedure, the probabilities of intermolecular collisions increase. These enhance the probability of an intermolecular hydrogen transfer reaction and result in the high content of saturated hydrocarbon in the gaseous phase. Furthermore, polymerizations between alkene molecules can occur; the content of ethene in the gaseous products decreases with the increase of temperature. 3.4. Liquid Phase Products. The liquid samples after thermal cracking under different conditions were detected by GC-MS. Since a large number of components can be found in an aviation fuel, the characterization of the thermal decomposition samples is not a simple issue. It is unnecessary to do a detailed identification of each product for the kinetic analysis of the fuel. The effects of temperature and reaction time on the major liquid products classified by carbon number are summarized in Figures 6a and 6b. The relative contents of compounds with carbon number from 7 to 16 are more than 80% (by mass) of the total liquids obtained from the thermal cracking procedure. The temperature and reaction time have effects on the liquid product distributions. The content of components with higher carbon number (>C11) decreases with the increase of the temperature or reaction time and that of components with lower number (
