Thermal Decomposition Kinetics of the Thermally Stable Jet Fuels JP

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Thermal Decomposition Kinetics of the Thermally Stable Jet Fuels JP-7, JP-TS and JP-900 Raina V. Gough, Jason A. Widegren, and Thomas J. Bruno* NIST Boulder, Boulder, Colorado 80305, United States S Supporting Information *

ABSTRACT: The thermal decomposition kinetics of JP-7, JP-TS and JP-900 were studied, motivated by the need of the hypersonic vehicle community for a fuel that has a high degree of thermal stability. Decomposition reactions were performed at 375, 400, 425 and 450 °C in stainless-steel ampule reactors. In all cases, the pressure before decomposition was 34.5 MPa (5000 psi). Decomposition as a function of time at each temperature was quantified by analyzing the thermally stressed liquid phase using gas chromatography. These results were used to determine global first-order rate constants that approximate the overall decomposition rate of of each fuel. For JP-7, these first-order rate constants ranged from 1.79 × 10−5 s−1 at 375 °C to 3.02 × 10−4 s−1 at 450 °C. For JP-TS, the rate constants had values between 1.74 × 10−5 s−1 at 375 °C to 2.70 × 10−4 s−1 at 450 °C. For JP900, the rate constants ranged from 1.03 × 10−5 s−1 at 375 °C to 3.60 × 10−4 s−1 at 450 °C. At all temperatures studied, these three fuels have similar rate constants for thermal decomposition; with only one exception, the values of k′ are identical within the combined uncertainty. The rate constants for the decomposition of RP-2, a fuel being considered as a replacement fuel for hypersonic vehicles, are similar in the temperature range studied. Considering the time needed for 1% of the sample to decompose (t0.01), we find that required instrument residence times range from 16 min at 375 °C to 30 s at 450 °C. The rate constants measured here, as well as the Arrhenius parameters that we calculate, can be used to design and plan physical property measurements at additional temperatures.

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INTRODUCTION The development of hypersonic aircraft resulted in fuel requirements more stringent than were necessary for nonhypersonic aircraft. Hypersonic aircraft fuel serves the dual role as a heat sink (for aircraft surfaces, hydraulic systems, climate control systems) and as a propellant. Fuel that comes into contact with hot metal surfaces can experience thermal stress and undergo pyrolysis reactions that change the fuel chemistry as well as produce particulate matter. The recognition of these effects in the 1950s lead to research hydrocarbon based propellants with enhanced thermal stability. JP-7 (MIL-DTL38219),1 a highly processed hydrocarbon-based kerosene fraction, was developed in the 1950s to meet the requirements. JP-7 has a relatively low volatility and is thermally stable up to 287 °C.2 The high temperatures encountered during Mach 3 flight made JP-7 an appropriate fuel for hypersonic vehicles. Cracking reactions were found to generate relatively low molecular mass molecules when the fluid was used as a coolant prior to being burned in the engines. Although JP-7 meets the operational demands for supersonic and hypersonic aircraft, it is no longer produced due to the relatively low demand and the fact that no refinery has maintained the capability. Another fuel with enhanced thermal stability (and a low freezing and cloud point) developed for the U2 program was JP-TS (MIL-DTL25524),3 a fluid that is currently in production albeit by only two refiners. This fluid is also used in the newer TR-1 aircraft. The hypersonic vehicle community is considering the use of several formulations of rocket kerosene, including an ultralow sulfur version of Rocket Propellant 2, or RP-2 (MIL-DTL25576D),4 as a replacement for JP-7. The need to use rocket engines multiple times led to reformulations of the kerosene This article not subject to U.S. Copyright. Published 2014 by the American Chemical Society

component of liquid rocket propellants. This work led to the development of RP-2, an ultralow sulfur rocket kerosene.5 RP-2 has a more stringent density and volatility range and much lower sulfur, olefin and aromatic content than those of the common turbine aviation fuels. Another approach was to blend a liquid derived from bituminous coal tar with and light cycle oil. The resulting mixture, JP-900, is treated by use of high temperature and pressure hydroprocessing. The fluid has high chemical stability up to 480 °C (900 °F, leading to the name of the prototype: JP-900).6 Efforts are also underway to develop additives for jet propellant JP-8 and missile propellant JP-10 to meet hypersonic flight requirements and therefore to potentially replace JP-7 and JP-TS.7 The National Institute of Standards and Technology (NIST) is investigating the thermophysical properties of several fuels. The multiple thermophysical properties that are being measured include equilibrium properties of the fluid (such as density, vapor pressure, volatility and heat capacity) and transport properties (such as thermal conductivity and viscosity).8−16 The goal of the thermophysical property measurements is the development of equation of state models to describe these properties.17−20 Models such as these are critical to all design phases. Specifically, when property measurements are planned at high temperatures, it is valuable to know Arrhenius parameters for the overall thermal decomposition because these can be used to predict rates of decomposition at other temperatures. Received: February 5, 2014 Revised: April 2, 2014 Published: April 3, 2014 3036

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The Arrhenius parameters are determined from the relationship between the decomposition rate constant and temperature. For complex mixtures such as fuels, we approximate the decomposition reaction as first order, which is a significant simplification. Nevertheless, the decomposition rate constants (and Arrhenius parameters that we derive from them) will yield information that can be used to place residence time constraints on future high temperature measurements. These data are also valuable for engine design. In this paper, we report the thermal decomposition kinetics of JP-7, JP-TS and JP-900 using a method that we have previously used to study kerosene-based fuels,21−24 organic Rankine cycle fluids25 and surrogate fuel components.26,27 By use of this method, the fuel samples are thermally stressed in small stainless-steel reactors, and decomposition of the fluid is monitored with gas chromatography (GC). Specifically, the emergent suite of light decomposition products carried in the liquid phase is analyzed. We use these data to calculate global first-order rate constant values that approximate the overall decomposition rate of a given fuel at a given temperature. Rate constants for thermal decomposition were determined at four temperatures between 375 and 450 °C. These rate constants were then used to derive Arrhenius parameters that allow the prediction of rate constants at additional temperatures. We then compare the decomposition kinetics to that of RP-2, a thermally stable fuel that is being considered as drop-in replacement.

(2)

[B]t = [B]∞ (1 − e−k ′ t )

(3)

Once k′ was determined at each temperature, the half-life, t0.5 and t0.01 (the time required for 1% of each fuel to decompose) were calculated using eq 4: ln

[A]t = −k′t [A]0

(4)

where [A]t is the concentration of reactants remaining at time t and [A]0 is the initial concentration of reactants. Finally, the temperature dependence of the rate constants were used to evaluate the parameters of the Arrhenius equation (eq 5):

⎛ −E ⎞ k′ = A exp⎜ a ⎟ ⎝ RT ⎠

(5)

where Ea is the activation energy of decomposition, A is the pre-exponential term, T is the temperature (in K) and R is the gas constant.

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EXPERIMENTAL SECTION

Chemicals and Reagents. Reagent-grade acetone, n-hexane and n-dodecane were used as solvents. These solvents were obtained from a commercial source and used as received. All solvents had stated purities of higher than 99%, a value consistent with our routine chromatographic analyses.30 The JP-7, JP-TS and JP-900 fuels studied here were obtained from the Air Force Research Laboratory (Fuels Branch, Wright Patterson Air Force Base). All three fuels were used as received. None of the fuels contained dye and all were clear and colorless. We used gas chromatography/mass spectrometry (GC−MS) to determine the composition of each of the three fuel samples prior to heating. Our sample of JP-7 consists mainly of C10 through C14 nalkanes, with some branched alkanes. Our sample of JP-TS consists mainly of C9 through C14 normal and branched alkanes, with some substituted benzenes as well. Our sample of JP-900 had a very different composition, because it is largely derived from coal oil. JP-900 contained very few normal or branched linear alkanes and instead mostly substituted cyclohexanes, fully hydrogenated indenes and naphthalenes, and other bicyclic and tricyclic compounds. This is consistent with previous studies of the composition of JP-900.31 Apparatus. The apparatus used for the fuel decomposition reactions has been described previously21,22,24,26,32−34 and is based on our work with larger scale reactors.35−39 A detailed description of the stainless steel thermostated block and stainless steel reaction cells used to perform the decomposition reactions is contained in the Supporting Information for the convenience of the reader. Decomposition Reactions. The procedure used to fill the reactors and perform the decomposition reactions was essentially identical to the one used for previous thermal stability studies on RP-2 and others. A detailed description of the filling procedure and reaction conditions is available elsewhere in the Supporting Information. In summary, we used an equation of state for n-dodecane40 to calculate the mass of fuel needed to achieve an initial pressure of 34.5 MPa for each reaction temperature and cell volume. This procedure was designed to limit differences in the initial pressure for all of the decomposition reactions. This is a reasonable assumption because, despite the compositional complexity of jet fuels, models derived from the properties of n-dodecane have been successfully used to approximate the physical properties of kerosene-based fuels.40,41 Thermal decomposition of the fuel that was only minimally degassed (one freeze−pump−thaw cycle) in order to maintain most volatile components. This more closely mimics the conditions under which a fuel is typically used, and it is known that autoxidation reactions

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THEORY The thermal decomposition of fuels such as JP-7, JP-TS and JP900 is very complex. Each fuel contains a large number of components, each compound may decompose by multiple reaction pathways, each decomposition pathway may yield multiple products and the initial decomposition products likely decompose further to other species. In addition, the decomposition rate of each individual component can be affected when in a mixture with other species.28 Because of the complexity of these fluids, simplifying assumptions are helpful to gain insight into the overall thermal stability of a fuel. In this paper, we made two such assumptions: first, we treat the overall reaction as that of a simple first-order reaction (sometimes referred to as a pseudo-first-order reaction). Additionally, we assume that the measured suite of light decomposition products carried in the liquid phase is representative of all of the decomposition products. The rate constants that we measure and derive here, therefore, are “global” first-order rate constants for the overall thermal decomposition of the fuel. The assumption of pseudo-first-order conditions is justified because the residence time in an actual engine is very short and secondary reactions, although possible, will not dominate the product suite. For each of the three fuels studied here, we monitored the formation of light decomposition products as a function of time, t, during the thermal decomposition reactions. At four temperatures, data were collected at four different reaction times with at least three measurements at each of at least four reaction times. Values for the first-order rate constant, k′, for the overall decomposition reaction were determined by fitting these experimental data to a first-order kinetic rate law, given in eqs 1−3, where [B]t is the product concentration at time t and [B]∞ is the product concentration at t = ∞:29 A→B

−d[A] d[B] = = k′t dt dt

(1) 3037

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caused by dissolved oxygen are relatively unimportant during fuel decomposition at temperatures higher than 250 to 300 °C.42 Fuel samples were heated at temperatures of 375, 400, 425, or 450 °C for between 10 min to 24 h. High temperature thermal reactions required much less decomposition time, as expected. We previously estimated26 that the thermal equilibration (warm-up) time for a sample cell is approximately 2 min at a reaction temperature of 450 °C. Therefore, times shorter than 10 min were not used in order to avoid introducing errors due to this thermal equilibration. Determination of Liquid Phase Decomposition Products by Gas Chromatography. The formation of low molecular mass decomposition products caused the pressure inside the reaction cells to increase during and after thermal decomposition. Even after the thermally stressed fuel samples were cooled to room temperature, the reactors contained a pressurized mixture of both vapor and liquid. When collecting samples for GC analysis, it was important to minimize both flash and evaporative losses from the fluid samples. Therefore, we have designed a collection procedure to minimize the loss of light compounds from the sample. A short piece of stainless steel tubing was attached to the valve and the other end was placed in a chilled glass vial. When the valve was slowly opened, a portion of the reacted fuel was expelled into the glass vial. Any liquid remaining in the cell was then transferred by syringe to the chilled glass vial. In this study, the gas phase products were not specifically collected and analyzed, with the exception of whatever volatile species were dissolved in the liquid phase. We recognize that this is a slight limitation that might introduce some uncertainty, however we are unable to quantitate the separate gaseous phase on a practical basis when the ampule reactors are used; the samples are simply too small. We have in the past applied a gas liquid separator to achieve a qualitative analysis of the separate gaseous phase (to determine the catalytic or thermal nature of the decomposition).44 When, in other research, larger quantities of thermally stressed fluids were produced in reactors, a separate quantitative analysis was indeed possible.45−47 The collection vial described above was crimp sealed with a polytetrafluoroethylene-sandwiched silicone septum closure, and the mass of the liquid sample was measured. Then, the thermally stressed fuel sample was immediately diluted with a known mass of ndodecane. This solvent was chosen because it does not interfere with the chromatographic analysis of the emergent suite of decomposition products used here to determine the rate constant values. The resulting n-dodecane solution typically had a concentration of 5% (mass/mass) reacted fuel. Aliquots (3 μL) of sample were injected by use of an automatic sampler into a gas chromatograph equipped with a flame ionization detector (FID). Nitrogen gas (research-grade) was used as the carrier gas and the makeup gas. The injection inlet was maintained at 300 °C at all times, and samples were separated on a 30 m capillary column ((5% phenyl)-methylpolysiloxane, 0.1 μm coating). The following temperature program was used: a 4 min isothermal separation at 80 °C, followed by a 20 °C/min ramp to 280 °C.The decomposition products were identified using GC−MS. The column and temperature program were to the same as those described above for the GC-FID analysis. Mass spectra from 40 to 550 relative molecular mass (RMM) units were collected for each peak and species were identified using the NIST/EPA/NIH mass spectral database with guidance from retention indices.48,49 Decomposition of JP-7, JP-TS and JP-900 was quantified by measuring the total increase in chromatographic peak area of the emergent product suite. The retention times for which peak integration was performed varied for each fuel; we used the area between 1.50 and 4.20 min for JP-7, between 1.50 and 2.65 min for JPTS and between 1.50 and 2.70 min for JP-900. These particular time periods were chosen because the chromatograms of each unheated fuel contained a small number of only very small peaks in these regions, thus the selections were made to minimize uncertainty. Following thermal stress, however, a much larger suite of decomposition products was observed to elute in these respective regions (see Figure 1).

Figure 1. Initial portion of the gas chromatograms for JP-7, JP-TS and JP-900 (as received) and then for samples of each that had been thermally stressed at 450 °C for 30 min. The emergent product suite that was used for the kinetic analysis of each fuel is boxed. The peak area contained in these retention time ranges was used to analyze the decomposition kinetics. The raw peak area was corrected for the n-dodecane dilution by multiplying by the appropriate dilution factor. The peak area was also corrected to account for drifts in detector response over the course of our project. This was done by analysis of an aliquot of a stock solution (consisting of n-pentane and n-hexane in n-dodecane) along with each set of samples of thermally stressed fuel. Using these corrected peak area values as [B]t, the data for each temperature (peak area vs reaction time) were fit to eq 3 with a nonlinear least-squares program. Because of secondary decomposition reactions that may be occurring, as well as the long reaction times needed at the lower temperatures, it was not possible to experimentally determine a value for [B]∞. We treated both [B]∞ and k′ as floating variables when fitting the kinetic data, as is recommended.29 The use of peak area for this kinetic analysis is possible because all products being formed are hydrocarbon species and we are using a FID for detection. For hydrocarbon species, the relative sensitivity of this type of detector (on the basis of moles of carbon−hydrogen bonds) varies by only a few percent.50 Therefore, it is unnecessary to calibrate the detector for each individual compound because this would not be expected to significantly change the values of the derived rate constants we report here.

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RESULTS AND DISCUSSION After each fuel was thermally stressed, we made qualitative observations of fluid thermal stability, namely the change in color and the development of a pressurized vapor phase due to the formation of light decomposition products. After heating, the fluid was typically clear to pale yellow, although samples heated for longer times or at higher temperatures were more likely to be darker in color. Particulate matter was not observed in any of the heated fluids, even in the case of the most thermally stressed samples. The formation of low molecular mass reaction products was seen in all samples, regardless of the temperatures or reaction times used here. In most cases, these gaseous or light products caused a portion of the thermally stressed sample to be expelled from the ampule when the valve was opened during sample extraction. For each of the three fuels studied here, GC−MS was used to identify the products of a 90 min decomposition reaction at 425 °C. 3038

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Tables 1, 2 and 3 list the most abundant compounds of the emergent suite defined earlier (based on % of total peak area)

formed during the decomposition of JP-7, JP-TS and JP-900, respectively. To obtain this list, the time periods defined above (1.50−4.20 min for JP-7, 1.50−2.65 min for JP-TS and 1.50− 2.70 min for JP-900) were integrated and the following % area cutoffs were used: 1.0% in the case of JP-7 and JP-TS products and 2.5% in the case of JP-900 products. For all thermally decomposed fuel samples, the solvent peak was subtracted prior to peak integration, as were any peaks that were also present in the fuel as received. The decomposition products of JP-7 and JP-TS were mainly light (with 9 or fewer carbons) linear, slightly branched or monocyclic hydrocarbons. The only exception was small amounts of xylenes that formed during JP-7 decomposition. Thermally stressed JP-900 also contained light (carbon numbers ≤9) linear or slightly branched olefinic hydrocarbons, but also several cyclohexane species that had lost some or all of their substituents. In the case of all three fuels studied, we deduce from the emergent product suites that the cracking occurring during heating is mainly thermal and that a catalytic cracking mechanism, if present, plays a negligible role.51 Quantitative determination of the thermal stability of each fuel was based on the chromatographic analysis of the light compounds in the emergent suite present in the thermally stressed liquid phase, as described above. Shown in Figure 1 is the early part of the chromatograms obtained for thermally stressed JP-7, JP-TS and JP-900. The unstressed fuels are shown for comparison. The emergent product suite used for the kinetic analysis of the thermal stability of each fuel is boxed in Figure 1. Some heavier (longer retention time) decomposition products were formed as well, but these are not considered in the current treatment because we are unable to sufficiently resolve them from the background. Figure 2a,b,c shows the kinetic data (the area of the product suite plotted against time in seconds) for the decomposition at 450 °C of JP-7, JP-TS and JP-900, respectively. The value of k′ was determined from the nonlinear fit (eq 3) to the peak area against time data (the fit is shown as a solid line in each portion of Figure 2). For JP-7 at 450 °C, k′ = 3.02 × 10−4 s−1 with an uncertainty of 9.4 × 10−5 s−1, for JP-TS at 450 °C, k′ = 2.70 × 10−4 s−1 with an uncertainty of 9.8 × 10−5 s−1 and for JP-900 at 450 °C, k′ = 3.60 × 10−4 s−1 with an uncertainty of 1.46 × 10−4 s−1. The value of the rate constant k′ was determined for each fuel at each temperature and then values for t0.5 and t0.01 were calculated from each k′ using eq 4. The decomposition rate constants for JP-7, JP-TS and JP-900 at all four temperatures, along with values of t0.5 and t0.01, are presented in Tables 4, 5 and 6, respectively. Also given in these tables is the uncertainty for each k′ value, which is the standard error in the coefficient of the nonlinear fit. The values of t0.01 for all fuels show that an instrument residence time of approximately 10 min may be acceptable for property measurements at 375 °C. However, at 450 °C, residence times longer than 1.0 min may be unacceptable for the thermal stability of the fuels. At every temperature studied, the rate constants for JP-7, JPTS and JP-900 are indistinguishable within their combined uncertainties, with the exception of the k′ value of JP-900 at 425 °C. At this temperature, the rate constant for decomposition of JP-900 is lower than that of the other fuels. From the data shown in Tables 4 through 6, we conclude that the thermal stabilities of JP-7 and JP-TS do not differ significantly over the temperature range studied (375 to 450 °C), at least with respect to the formation of low molecular

Table 1. Summary of JP-7 Thermal Decomposition Products Formed after 90 min at 425 °C compound

% of total area

propane n-butane n-pentane 2-methylpentane n-hexane methylcyclopentane 3-methylhexane n-heptane methylcyclohexane 2-methylheptane 3-ethylhexane 1,2-dimethylcyclohexane n-octane 1,3-dimethylcyclohexane 4-methyloctane 3-methyloctane xylene n-nonane 1-ethyl-2-methylcyclohexane

4.53 6.77 12.81 2.62 11.40 1.30 6.45 11.89 2.77 3.27 4.07 2.69 9.70 2.00 3.36 1.67 2.81 7.78 2.12

Table 2. Summary of JP-TS Thermal Decomposition Products Formed after 90 min at 425 °C compound

% of total area

propane n-butane isobutane 2-methylpentane n-hexane 3-ethylhexane n-heptane methylcyclohexane 2-methylheptane n-octane

5.40 9.28 28.63 5.84 14.93 7.36 14.92 2.74 3.70 7.19

Table 3. Summary of JP-900 Thermal Decomposition Products Formed after 90 min at 425 °C compound

% of total area

propane n-butane n-pentane 2-methoxy-2-methylpropane n-hexane methylcyclopentane cyclohexane cyclohexene methylcyclohexane 4-methylcyclohexene 1-methylcyclohexene 1,4-dimethylcyclohexane n-octane 1,2-dimethylcyclohexane 1,3-dimethylcyclohexane 1,3-dimethyl-1-cyclohexene

6.67 8.29 7.84 3.91 6.21 2.97 9.64 10.78 7.53 5.68 10.61 3.30 5.16 2.98 3.00 5.42

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Table 6. Kinetic Data for the Thermal Decomposition of JP900 T (°C) 375 400 425 450

k (s−1) 1.03 1.90 7.54 3.60

× × × ×

uncertainty in k (s−1) −05

10 10−05 10−05 10−04

1.08 9.90 3.83 1.46

× × × ×

10−06 10−06 10−05 10−04

t0.5 (h)

t0.01 (min)

18.7 10.1 2.55 0.53

16.3 8.83 2.22 0.47

times and temperature regimes) applied to these fuels in the future can be the same or similar. To enable the estimation of the rates of thermal decomposition at other temperatures and also to determine the activation energy, Ea, of the decomposition reaction, an Arrhenius plot was constructed for each fuel. Figure 3 shows an

Figure 3. Arrhenius plot of the first-order rate constants for the formation of light decomposition products of JP-7, JP-TS and JP-900. This figure demonstrates that the three fuels decompose at similar rates over this temperature range and have similar temperature dependence. The error bars show the standard uncertainty in each individual k′ value, derived from the exponential fit to the data (eq 3). The solid line is a linear fit to the data for JP-7. The solid squares represent the first-order rate constants previously measured for the thermal decomposition of RP-2.22

Figure 2. Plot of the corrected peak area of the emergent suite of light, liquid phase products of (a) JP-7, (b) JP-TS and (c) JP-TS decomposition as a function of time. These data were all obtained at 450 °C. The first-order rate constant for thermal decomposition was determined from the nonlinear fit to the data (solid line). Note that although the scale of the x-axis remains constant, the y-axis scale varies by more than a factor of 5 for the different fuels.

Arrhenius plot (ln k′ vs 1/T) for JP-7, JP-TS and JP-900. The solid line seen in Figure 3 is a linear regression to the JP-7 data, which yields Arrhenius parameters of A = 6.18 × 107 s−1 and Ea = 156 kJ·mol−1. The uncertainty in Ea, determined from the standard error in the slope of the regression, is 24 kJ·mol−1. This activation energy of JP-7 is very similar to a previously published value of Ea = 157 kJ·mol−152 despite much higher temperatures present during this measurement (623−1200 K). A linear regression to the data for JP-TS (fit not shown) yields similar Arrhenius parameters of A = 4.80 × 108 s−1 and Ea = 169 (±49) kJ·mol−1. A linear regression to the data for JP-900 (fit not shown) yields Arrhenius parameters of A = 6.51 × 109 s−1 and Ea = 185 (±23) kJ·mol−1. The relatively large uncertainties in the Ea values reflect the nonlinearity of the Arrhenius plots for the decomposition of these fluids over the range of temperatures studied. For comparison, the decomposition rate constants for the kerosene-based rocket propellant RP-2 were measured previously in our laboratory using this experimental protocol.22 Because this fuel is being considered as a drop-in replacement for thermally stable turbine fuels, a comparison of k′ values is informative. Previous work22 determined the following values

Table 4. Kinetic Data for the Thermal Decomposition of JP7 T (°C) 375 400 425 450

k (s−1) 1.79 4.57 2.22 3.02

× × × ×

uncertainty in k (s−1) −05

10 10−05 10−04 10−04

6.7 2.25 1.33 9.4

× × × ×

−06

10 10−05 10−04 10−05

t0.5 (h)

t0.01 (min)

10.8 4.21 0.87 0.64

9.36 3.67 0.75 0.55

Table 5. Kinetic Data for the Thermal Decomposition of JPTS T (°C) 375 400 425 450

k (s−1) 1.74 1.83 2.37 2.70

× × × ×

uncertainty in k (s−1) −5

10 10−05 10−04 10−04

1.33 6.1 4.3 9.8

× × × ×

−05

10 10−06 10−05 10−05

t0.5 (h)

t0.01 (min)

11.1 10.5 0.81 0.71

9.63 9.15 0.71 0.62

mass, liquid phase decomposition products. JP-900 is perhaps slightly more thermally stable. These results suggest that measurement protocols and conditions (such as residence 3040

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of k′ for RP-2 decomposition: 1.33 × 10−5 (±3.0 × 10−6) at 375 °C, 9.28 × 10−5 (±2.01 × 10−5) at 400 °C, 1.33 × 10−4 (±3.3 × 10−5) at 425 °C, and 5.47 × 10−4 (±8.0 × 10−5) at 450 °C. To facilitate the comparison, these values of k′ are plotted in Figure 3 along with the k′ values of JP-7 and JP-TS measured experimentally in this study. In most cases, the measured rate constants for RP-2 are the same, within combined uncertainty, as those of the three fuels studied here. The exception is at 400 °C, at which point the measured rate constant of RP-2 exceeds that of any of the three fuels studied here. A comparison of the activation energies of the three fuels studied here with that of RP-2 (Ea = 180 (±30) kJ·mol−1) yields consistent results; the Ea values of all four fuels are identical within their combined uncertainties.

CONCLUSIONS We have measured the thermal decomposition kinetic parameters of three jet fuels being considered for hypersonic vehicles: JP-7, JP-TS and JP-900. These fuels were developed to be highly thermally stable, yet the rate constants of thermal decomposition were not previously measured. We report firstorder rate constants for thermal decomposition of these fuels from 375−450 °C and an initial pressure of 34.5 MPa. In terms of the formation of light, liquid phase decomposition products, we have found no significant difference between the thermal stability of JP-7, JP-TS and JP-900 under these experimental conditions. In general, the thermal decomposition of the three fuels studied here (JP-7, JP-TS and JP-900) would be expected to occur at generally the same rate as the thermal decomposition of the rocket kerosene fuel RP-2. In terms of thermal stability, it appears that RP-2 may be a suitable drop-in replacement for JP-7, JP-TS or JP-900. These results are useful for planning chemical or physical property measurements at high pressures and temperatures. Our results suggest that similar measurement protocols (residence times and temperature regimes) may be applied to these three fuels as well as to RP-2. ASSOCIATED CONTENT

S Supporting Information *

Detailed description of the apparatus used for the thermal stability measurements and decomposition reactions. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*T. J. Bruno. E-mail: [email protected]. Tel: 303.497.5158. Fax: 303.497.6682. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. J. Timothy Edwards at the Air Force Research Laboratory/Wright Patterson Air Force Base for supplying the three fuels studied here. A National Academy of Sciences/ National Research Council postdoctoral fellowship is gratefully acknowledged by R.V.G. 3041

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