Comparison of JP-8 and JP-8+ 100 with the Advanced Distillation

An additive that is used to increase operating temperatures for heat sink applications is the +100 additive that is added to JP-8. As part of a large ...
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Comparison of JP‑8 and JP-8+100 with the Advanced Distillation Curve Approach Thomas J. Bruno,* Kathryn R. Abel, and Jennifer R. Riggs Thermophysical Properties Division, National Institute of Standards and Technology (NIST), 325 Broadway, Boulder, Colorado 80305, United States S Supporting Information *

ABSTRACT: Additive packages are used with aviation kerosenes to achieve performance enhancements, such as higher or lower operating temperatures or increased lubricity. An additive that is used to increase operating temperatures for heat sink applications is the +100 additive that is added to JP-8. As part of a large effort on the characterization of JP-8, we measured the effect of the +100 additive on the volatility of JP-8 by use of the composition-explicit or advanced distillation curve approach. We found that the vaporization temperatures increased modestly in the early stages of the distillation but the difference disappeared after the 70% distillate fraction. We tracked selected components of the additive through the distillation curve and related the concentration to the temperature data grid. This allowed us to determine that the heaviest components of the additive are concentrated late in the distillation curve and a significant quantity in the recovered residue. Still incomplete is our understanding of the role and disposition of the oligomer component of the +100 additive. However, some preliminary viscosity and density measurements are provided to suggest what the fate of this component might be.

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bulk maximum temperature of 55 °C (historically specified in English units, from 325 to 425 °F).9,10 The +100 additive package, typically used to treat JP-8 at 256 mg/L (256 ppm, mass/vol) in the bulk JP-8 fuel, consists of four main functional components.11,12 An antioxidant, butylated hydroxytoluene (2,6-di-tert-butyl-4-methylphenol, CAS number 128-37-0) comprises approximately 25 ppm (mass/ mass) of the final mixture. This is a commonly used antioxidant in food products. Phenolic compounds, such as this, function as chain-breaking antioxidants, preventing the formation of soluble gums and insoluble particulates. A metal deactivator (MDA), typically N,N-disalicylidene-1,2-propanediamine [α,α′(1-methylethylenediimino)-diortho-cresol, CAS number 94-917] composes approximately 2 ppm (mass/mass). MDAs suppress the catalytic effect of some metals that are wetted by the fuel. A detergent/dispersant, typically the proprietary compound designated as 8Q405, composes approximately 100 ppm (mass/mass). The purpose of the dispersant is to minimize the accumulation of coke deposits and to clean coke deposits that form. The dispersant is an oligmer with a molecular mass in excess of 10 000.12 The solvent or carrier [at approximately 129 ppm (mass/mass)] is typically a heavy aromatic naphtha solvent that contains methylnaphthalenes, indenes, naphthalene, biphenyl, toluene, and xylenes. There has been a great deal of recent interest in further developing the requisite knowledge base to allow for the expanded adoption of JP-8 under the One Fuel Forward paradigm. This work is concurrent with similar efforts with rocket kerosenes and with fuel for hypersonic vehicles and has required a comprehensive research program that has included

INTRODUCTION

While the main purpose of aviation kerosene is a fuel for gas turbine engines, in many aircraft, the fuel serves the additional role as a heat sink. Especially in high-performance aircraft, the fuel serves as the primary coolant for the hydraulics, environmental subsystems, and engine. In some applications, leading edge surfaces are also cooled by fuel circulation. The result of fuel usage for such non-combustion tasks is the exposure of the fuel to high temperatures for what may be significant times. This exposure can cause the fuel to degrade and foul heat exchangers and other critical aircraft components. Because of these issues, there is considerable interest in developing fuels that are more resistant to thermal stress. The main aviation fuel used in the United States military is JP-8, a fluid similar to the commercial aviation kerosene Jet-A-1 (which is itself similar to Jet-A and Jet-A-1 but with a supplemental additive package). The standard additive package for JP-8 consists of an icing inhibitor, a combined corrosion inhibitor and lubricity enhancer (typically a single component), and an antistatic additive. This fuel is intended not only for use in aircraft but also in all ground mobile diesel applications under the “One Fuel Forward” concept. Thus, U.S. military consumption of JP-8 is high, accounting for approximately 50% of the military energy budget.1 Research on the modification of this commodity fuel for high heat sink application began with the Thermal Management Working Group [Wright-Patterson Air Force Base (WPAFB)], which, in 1990, recommended the development of a high thermal stability version of JP-8.2,3 After nearly a decade of development, the supplemental additive package was formulated to improve heat sink capability by 50%.4−7 This additive also has been found to decrease the tendency of soot formation relative to JP-8.8 The fuel treated with this additive is called JP-8+100, indicative of the increased This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society

Received: July 2, 2012 Revised: August 1, 2012 Published: August 3, 2012 5843

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Moreover, because there are proprietary aspects of the additive, we focus more detail on components that have been reported in the literature.

studies of combustion, detailed kinetics mechanisms, and thermophysical properties. Much of this work has required the adoption of surrogate mixtures, because dealing with the real fuels is too complex.13−16 The surrogate mixtures are then used to model the thermophysical properties and detailed reaction kinetics. Recent research at the National Institute of Standards and Technology (NIST) as part of this program has included the measurement and modeling of thermophysical properties of Jet-A, JP-8, JP-7, RP-1, RP-2, and potential synthetic replacements. In the course of these efforts, we have had to take explicit account of some additive packages. Indeed, the +100 additive package has been used in thermal decomposition work performed on RP-1.17−19 In this paper, we present measurements on the volatility of a real, flight line sample of JP-8 and that fluid with the +100 additive at the concentration given by the specification cited above. The purpose of this work was to determine the effect, if any, of the additive package on the volatility of JP-8, information that is critical in refining thermophysical property models that will be used to represent the properties of JP-8. We used for this purpose the composition-explicit or advanced distillation curve (ADC) method. This technique allows for the measurement of the volatility of the complex fluid and, in addition, allows for the tracking of specific components through the distillation curve. Because of the similarity of the distillation process with the droplet combustion process, the ability to track additive components through the distillation curve is very instructive. ADC Measurement. In earlier work, we described a method and apparatus for an ADC (or composition-explicit) measurement that is especially applicable to the characterization of fuels.20−29 This method is a significant improvement over current approaches, featuring (1) a composition-explicit data channel for each distillate fraction (for both qualitative and quantitative analyses), (2) temperature measurements that are true thermodynamic state points that can be modeled with an equation of state, (3) temperature, volume, and pressure measurements of low uncertainty suitable for equation of state development, (4) consistency with a century of historical data, (5) assessment of the energy content of each distillate fraction, (6) trace chemical analysis of each distillate fraction, and (7) corrosivity assessment of each distillate fraction. The composition-explicit data channel allows for the temperature data grid (the temperature−volume fraction curve) to be directly related to the distillate composition. Moreover, the advantage offered by this approach is the ability to model the distillation curve resulting from our metrology with equationof-state-based models. Such thermodynamic model development is simply impossible with the classical approach to distillation curve measurement or with any of the other techniques that are used to assess fuel volatility or vapor liquid equilibrium. We have applied this metrology to gasolines,27,30−33 diesel fuels,32,34−39 aviation fuels,15,16,21,28,40−46 rocket propellants,21,44,47,48 and crude oils,49,50 and herein, we apply it to compare the volatility of JP-8 and JP-8+100. We emphasize that our purpose in examining JP-8, with and without the +100 additive, was to assess the overall effect on the volatility, with the ultimate goal of modeling the thermophysical properties of Jet-A/JP-8 in a robust fashion that can take explicit account of additives. In doing so, it was not necessary to assess the contribution of all functional components of the +100 additive; only the major components that might affect thermophysical properties were considered.

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EXPERIMENTAL SECTION

The JP-8 used in this work was obtained from the Propulsion Directorate of the Air Force Research Laboratory (AFRL) at WPAFB in Ohio. This fluid was obtained from the flight line at WPAFB and contained the full additive package of an icing inhibitor, a corrosion inhibitor/lubricity enhancer, and an antistatic additive. This fluid was stored at 15 °C to preserve any volatile components. No phase separation was observed as a result of this storage procedure. The JP-8 was examined by gas chromatography (30 m capillary column of 5% phenyl−95% dimethylpolysiloxane, having a thickness of 1 μm, flame ionization detection and mass spectrometric detection, a column temperature of 45 °C for 2 min, 50 °C/min to 200 °C, and 200 °C for 4 min, head isobaric head pressure of 50 kPa, and split ratio of 50:1).51,52 After each temperature program, the column was maintained at 250 °C for several minutes to ensure cleanup. The detailed analysis of this fluid was presented previously.45 The +100 additive package was obtained from the Propulsion Directorate of the AFRL at WPAFB. The additive, a viscous amber liquid with a kerosene-like odor, was examined by gas chromatography (30 m capillary column of 5% phenyl−95% dimethylpolysiloxane, having a thickness of 1 μm, flame ionization detection and mass spectrometric detection, column temperature of 45 °C for 2 min, 50 °C/min to 200 °C, and 200 °C for 4 min, head isobaric head pressure of 50 kPa, and split ratio of 50:1). After each temperature program, the column was maintained at 250 °C for several minutes to ensure cleanup. The analysis results of this fluid were consistent with the literature. The JP-8+100 used in this work was prepared from the flight line JP-8 and the +100 additive by the addition of 256 mg/L additive to the base JP-8 fuel. We note that this is approximately 320 ppm (mass/ mass). The JP-8+100 mixture was prepared as a stock solution to eliminate any variations in preparation; all measurements with JP8+100 were performed with the same stock solution. This procedure also ensures that the comparisons drawn between JP-8 and JP-8+100 are not affected by the large variability of differing batches of JP-8. Only one batch of JP-8 was used, and the only sample of JP-8+100 considered in this work was the one prepared from that batch. The comment above requires elaboration, because recent work on the compositional variability of Jet-A and JP-8 has shown a surprisingly large variability in the volatility of turbine kerosene.53 Indeed, we have found that an individual Jet-A sample can have a lower volatility than an individual sample of JP-5, which is specifically formulated to minimize volatility. Thus, the overall variability of turbine kerosene will greatly overshadow the volatility variation caused by the addition of the +100 additive. The effect on an individual batch, especially as might be prepared at the flight line by splash blending, is nonetheless reflected in the work presented herein. n-Hexane used as a solvent in this work was obtained from a commercial supplier and was analyzed by gas chromatography (30 m capillary column of 5% phenyl−95% dimethylpolysiloxane, having a thickness of 1 μm, temperature program from 50 to 170 °C, at 5 °C/ min, head isobaric head pressure of 50 kPa, and split ratio of 50:1) using flame ionization detection and mass spectrometric detection. These analyses revealed the purity to be approximately 99.9%, and the fluid was used without further purification. The method and apparatus for the distillation curve measurement has been reviewed in a number of sources; therefore, an additional general description will not be provided here. The required volume of fluid for the distillation curve measurement (in each case, 200 mL) was placed into the boiling flask with a 200 mL volumetric pipet and an automatic pipetter. The thermocouples were then inserted into the proper locations to monitor Tk, the temperature in the fluid, and Th, the temperature at the bottom of the takeoff position in the distillation head. Enclosure heating was then commenced with a four-step program based on a previously measured distillation curve. This 5844

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program was designed to impose a heating profile on the enclosure that led the fluid temperature by approximately 20 °C. Volume measurements were made in the level-stabilized receiver, and sample aliquots were collected at the receiver adapter hammock. In the course of this work, we performed between four and six complete distillation curve measurements for each of the fluid samples. Because the measurements of the distillation curves were performed at ambient atmospheric pressure (measured with an electronic barometer), temperature readings were corrected for what should be obtained at standard atmospheric pressure (1 atm = 101.325 kPa). This adjustment was performed with the modified Sydney Young equation, in which the constant term was assigned a value of 0.000 109.54−57 This value corresponds to a carbon chain of 12. In the chemical analysis of the aviation fuel samples (see above) as well as in previous work on aviation turbine fuel, it was found that n-dodecane can indeed represent the fluid as a very rough surrogate. The magnitude of the correction is of course dependent upon the extent of departure from standard atmospheric pressure. The location of the laboratory in which the measurements reported herein were performed is approximately 1650 m above sea level, resulting in a typical temperature adjustment of 7−8 °C. The actual measured temperatures are easily recovered from the Sydney Young equation at each measured atmospheric pressure.

carefully observed. Direct observation through the bore scope ports allowed for the measurement of the onset of boiling behavior for each fluid. Typically, during the earlier stages of a measurement, the first bubbles will appear intermittently and are rather small. These bubbles cease if the stirrer is stopped momentarily. The temperature at which this is observed is called the onset temperature and is the result of vaporization of air and light components. It is used primarily as a diagnostic. Sustained bubbling, which occurs subsequent to onset, is characterized by larger, more vigorous bubbles and is still observed when the stirring is briefly stopped. This temperature is also used as a diagnostic, along with the onset, and is not always reported or interpreted. Finally, vapor is observed to rise into the distillation head, causing an immediate response on the Th thermocouple. This temperature has been shown to be the initial boiling temperature (IBT) of the fluid. Furthermore, this temperature is of low uncertainty (0.3 °C) and thermodynamically consistent, and it can therefore be modeled theoretically with an equation of state. In Table 2, we present the average vapor rise temperatures for JP-8 and JP-8+100. Each temperature was measured 6

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RESULTS AND DISCUSSION Composition of the +100 Additive. The major results of the chemical analysis of the +100 additive are summarized in Table 1. As we mentioned earlier, we will focus on the

Table 2. Summary of the Initial Behavior of JP-8 and JP8+100a

Table 1. Components Found in the +100 Additive by Use of the GC−MS Method Described in the Texta

observed temperature (°C)

JP-8 83.90 kPa

JP-8+100 82.39 kPa

vapor rise

183.0

184.8

a

The vapor rise temperature is that at which vapor is observed to rise into the distillation head, considered to be the initial boiling point of the fluid. These temperatures have been adjusted to 1 atm with the Sydney Young equation; the experimental atmospheric pressures are provided to allow for recovery of the actual measured temperatures. The uncertainties are discussed in the text.

active components butylated hydroxytoluene carrier components indene x-methylindene (1-methyl-3-cyclopropen-1-yl) benzene 1,2-dihydronaphthalene naphthalene x,y-dimethyl-1H-indene 1-methylene-2-butenyl benzene 2-methylnaphthalene 1-methylnaphthalene biphenyl 1-ethylnaphthalene 2-ethylnaphthalene x,y-dimethylnaphthalene

times, and the expanded uncertainty in temperature was 0.3 °C. The ambient pressure was also recorded and used to correct the temperatures to atmospheric pressure by use of the modified Sydney Young equation. The repeatability of the pressure measurement (assessed by logging a pressure measurement every 15 s for the duration of a typical distillation) was 0.001 kPa. We note a slight increase in the vapor rise temperature of approximately 2 °C upon the addition of the +100 additive. This increase is consistent with the addition of a small quantity of a heavy additive. Most of the constituents of the +100 additive have boiling temperatures in excess of the initial boiling temperature of the base JP-8; thus, this effect at the beginning of vaporization is understandable. Distillation Curves. During the measurement of the distillation curves, both the kettle and head temperatures were recorded. The ambient pressure was also recorded and used to correct the temperatures to atmospheric pressure using the modified Sydney Young equation. Each curve was measured 6 times, and the expanded uncertainty in the temperature was 0.3 °C. The uncertainty in the volume measurement that is used to obtain the distillate volume fraction was 0.05 mL in each case. As with the measurement of the IBT, the repeatability of the pressure measurement (assessed by logging a pressure measurement every 15 s for the duration of a typical distillation) was 0.001 kPa. Head and kettle temperatures, as well as the measured atmospheric pressure, are presented as a function of the distillate cut for a representative measurement for each fluid in Table 3. The curves are plotted in Figure 1,

a

Of these, the butylated hydroxytoluene is an active ingredient (the antioxidant), while the remaining compounds are part of the carrier matrix. These components are listed in order of gas chromatographic retention time (see the method details in the text). Where the substitution in ambiguous, the isomerization is indicated by x-, y-, etc.

constituents that have been previously reported in the literature and not on those that are legally protected. We note that the antioxidant, butylated hydroxytolune (BHT), is found, in addition to many components of the naphtha carrier solvent. The MDA (N,N-disalycylidene-1,2-propanediamine, present at 2 ppm, mass/mass) is not a major constituent and is not listed. The dispersant (the oligomer 8Q405) is also not listed. This proprietary additive, described in general terms in the Introduction, is too heavy for analysis by our gas chromatography−mass spectrometry (GC−MS) method. Initial Boiling Temperatures. During the initial heating of each sample during the distillation, the behavior of the fluid was 5845

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where the initial boiling temperatures are indicated on the y axis as a hatch mark.

increase in vaporization temperatures is consistent with the addition of small concentrations of heavier additives to a base fluid. We further note that the boiling temperatures of most of the carrier constituents and the antioxidant are somewhat higher than the constituents of the base JP-8. It is thus somewhat surprising that the curves converge at higher distillate volume fractions. As we will show below when discussing the composition channel information, these heavier constituents are indeed concentrated in later distillate fractions. One possible explanation would be the gradual cross-linking (with a rising temperature in the distillation) of the smaller oligomer species present in the dispersant fraction. In an effort to test this hypothesis, we subjected the pure +100 additive to the same temperature profile as was applied to the JP-8+100 mixture during the distillation. This effectively imposed a temperature ramp that brought the additive to above 250 °C in approximately 45 min, thus providing a thermal stress to the additive. The resulting fluid was then removed from the apparatus and cooled to ambient conditions. We noted a slight darkening and a marked increase in viscosity after this treatment. We then measured the density and viscosity of the additive at 30 °C with a commercial viscodensimeter58 and compared the result to the same measurements made on an unstressed sample. The results of these measurements are provided in Table 4. We noted that, subsequent to thermal

Table 3. Representative Distillation Curve Data (Given as the Average of Four Separate Distillation Curves) for JP-8 and JP-8+100a JP-8 83.90 kPa

JP-8+100 82.39 kPa

distillate volume fraction (%)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

185.8 188.1 190.4 192.8 195.3 198.0 200.4 203.4 206.0 209.4 213.0 216.4 220.8 225.4 229.9 235.1 241.0 248.3

174.7 178.9 182.3 184.8 188.4 186.0 191.9 194.6 194.1 196.0 196.1 197.8 204.5 206.8 216.4 220.8 227.2 229.2

187.5 189.8 192.1 194.4 196.9 199.6 201.8 204.6 207.8 211.2 214.4 217.8 221.9 225.4 230.3 235.8 241.6 246.9

176.0 179.6 182.8 185.0 187.6 190.3 193.0 195.3 199.0 203.2 206.1 209.7 214.9 218.2 224.1 230.4 235.0 241.4

Table 4. Measured Viscosity and Density for the +100 Additive as Received and after the Additive Is Subjected to the Thermal Stress of the Distillation Temperature Profilea

a

The uncertainties are discussed in the text. These temperatures have been adjusted to 1 atm with the Sydney Young equation; the experimental atmospheric pressures are provided to allow for recovery of the actual measure temperatures.

a

sample

viscosity (mPa s)

density (g/mL)

+100, as received +100, thermally stressed

8.316 (0.001) 10.532 (0.001)

0.9471 (0.0001) 0.9466 (0.0001)

The expanded uncertainty is provided in parentheses.

stress, the viscosity of the additive has increased by over 21%, while the density has remained unchanged. One potential cause of this behavior is the onset of cross-linking among the oligomers.59 This would have the effect of removing some fraction of the lower molecular mass oligomers from participation in vapor liquid equilibrium; they would simply become too heavy. This test is by no means definitive, and efforts to further explore the changes caused by thermal stress are underway. The observation of the dispersant throughout the distillation furnishes a possible explanation of the lower soot formation observed with the use of JP-8+100.8 It appears that not only does this component remain in the fluid through the distillation, but it polymerized to a somewhat larger species. Finally, we can examine the distillation curves expressed in both Tk and Th (see Table 3), and we note no convergence of temperatures in any part of the curve. This indicates that the +100 additive does not cause any apparent azeotropic behavior when added to JP-8. Compositional Channel Information. As discussed above, the composition-explicit data channel of the ADC allows for tracking of specific components through the distillation curve, whereby their concentrations can be related to the temperature data grid. One can sample and examine the individual fractions as they emerge from the condenser, as discussed in the Introduction. For each distillate fraction, 7 μL samples were withdrawn at the sampling hammock and dissolved in a weighed volume of hexane. While one can, in principle, track all of the components identified in Table 1, we

Figure 1. Distillation curves of JP-8 and JP-8+100. Here, we present Tk, the temperature measured directly in the fluid, adjusted to standard atmospheric pressure with the modified Sydney Young equation. The tick marks on the y axis represent the initial boiling point of each mixture. The uncertainties, discussed in the text, are smaller than the plotting symbols.

We note that the addition of the +100 additive has a modest but noticeable effect on the volatility profile of JP-8. The additive causes an increase in the vaporization temperatures of approximately 2−3 °C from the start of vaporization until the 70% distillate volume fraction. This increase is well within the experimental uncertainty. Subsequent to this fraction, the elevation of distillation temperatures in JP-8+100 is much smaller, approaching the experimental uncertainty. This slight 5846

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will focus only on the more important components and those components that will have influence on the volatility. Thus, we do not necessarily have to track the MDA through the distillation curve to understand the influence on the volatility; the concentration is too low for its effect to be experimentally observed with the ADC method. Moreover, as a practical matter, we must also focus on components that are distinct from the base composition of JP-8. In this respect, it matters little to the overall JP-8 volatility if the additive slightly increased the naphthalene concentration in the final JP-8+100 mixture. Ideally, it is best if the component of interest is baseline-resolved in the chromatographic analysis, to clearly separate the +100 constituent from those of the base fuel. Given that the naphtha carrier shares many components that are present in low levels in JP-8, this idealized case cannot be realized. We instead chose constituents that suffer the minimum interference from the base fluid. On the basis of these considerations, we chose to track four constituents of the +100 additive and relate them to the temperature data grid. These include the antioxidant, BHT, and three components of the carrier, 1-methylnaphthalene, 2-methylnaphthalene, and biphenyl. These components could be sufficiently resolved from the base JP-8 fluid to allow for quantitative analysis to be performed. BHT and biphenyl are not found in the base JP-8, and the chromatographic separation provides nearly baseline resolution of these peaks. The methylnaphthalene isomers are in fact present at trace concentrations in the base fluid; therefore, an adjustment was made (on the basis of the area counts from a separate chromatographic analysis of JP-8) to partially compensate. Some basic information on these four additive components is provided in Table S1 of the Supporting Information.60,61 The samples were analyzed with a gas chromatographic mass spectrometric method (30 m capillary column of 5% phenyl− 95% dimethylpolysiloxane, having a thickness of 1 μm, temperature program of 45 °C for 2 min, from 50 to 200 °C, at 5 °C/min, head isobaric head pressure of 50 kPa, and split ratio of 50:1). The mass spectrometer was operated in selected ion monitoring (SIM) mode: for BHT, ions at m/z 205 and 220 were measured; for biphenyl, ions at m/z 76 and 154 were measured; and for the methylnaphthalenes, ions at m/z 115 and 142 were measured. Calibration was performed by the external standard method, in which four solutions each of BHT, biphenyl, and the methylnaphthalenes were prepared in hexane. While we could identify two very close but separate chromatographic peaks for 1- and 2-methylnaphthalene when the mass spectrometer was operated in scanning mode, these were observed to coalesce into one peak in SIM mode. Expanding the second temperature ramp to 5 °C/min provided nearly baseline resolution and the ability to determine each isomer separately. The concentrations of these tracked components are presented as histograms in Figures 2−5. We observe in Figure 2 that the concentration of BHT begins at a relatively low level and increases gradually as the distillation proceeds, although appreciable BHT is found early in the curve. This component has the highest boiling temperature of all of the additives tracked, and we note that the highest concentration is found in the residue recovered in the boiling flask after the distillation. This is consistent because the boiling temperature of BHT is nearly 20 °C higher than what was observed for the 90% distillate volume fraction of JP-8+100. We note that there appears to be significantly more BHT found in the distillate

Figure 2. Histogram representation of the measured distillate concentration in percent (mass/mass) of BHT as a function of the distillate fraction. The uncertainty is discussed in the text.

cuts than would be anticipated by the typical concentrations that are reported present in the additive as received. The analysis discussed above in fact revealed the presence of 7.8% (mass/mass) BHT in the additive. Upon addition to JP-8 in the specified proportion, one would expect to observe a BHT concentration of approximately 0.0324% (mass/mass), assuming a density of 0.79 g/mL for JP-8. In Figure 3, we observe a similar trend in the histogram for biphenyl. The concentrations that are observed, even early in

Figure 3. Histogram representation of the measured distillate concentration in percent (mass/mass) of biphenyl as a function of the distillate fraction. The uncertainty is discussed in the text.

the distillation curve, are somewhat higher than that of BHT. This is due to the lower boiling temperature of this constituent. We also observe that the concentration reaches a maximum at the 90% distillate volume fraction and that the concentration is actually lower in the recovered residue. This is consistent with the boiling temperature; there is only a 7 °C difference between the temperature of the 90% distillate volume fraction and the boiling temperature of biphenyl. In Figures 4 and 5, the histograms for the methylnaphthalenes show an even more pronounced concentration effect. Here, with the tracked constituents boiling 5 °C lower than the 90% distillate volume fraction, we observe a higher concentration of the methylnaphthalenes through the curve and a relatively low concentration in the recovered residue. A somewhat unusual feature of the uncertainty bars in many of the histograms shows that, as the concentration increases, in many cases, the uncertainty likewise increases. This is counter to what is typically encountered in chromatographic analysis, where the uncertainty is usually higher when the constituent is at a lower concentration. Here, the increasing uncertainty is 5847

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high temperatures (albeit with possible cross-linking), provides a potential explanation of the decreased soot formation observed with JP-8+100.

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ASSOCIATED CONTENT

S Supporting Information *

Data on the chemicals studied in this work (Table S1). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 4. Histogram representation of the measured distillate concentration in percent (mass/mass) of 2-methylnaphthalene as a function of the distillate fraction. The uncertainty is discussed in the text.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the Air Force Office of Scientific Research (MIPR F1ATA06004G004). Kathryn R. Abel and Jennifer R. Riggs acknowledge the financial support of a NIST Summer Undergraduate Research Fellowship and a NIST Professional Research Experience Program (PREP) Fellowship. We acknowledge the assistance of our colleague Dr. Tara Fortin for performing the viscosity and density measurements on the neat and thermally stressed +100 additive. We acknowledge very useful discussions with Dr. J. Tim Edwards of the Air Force Research Laboratory at WPAFB and Dr. Peter Hseih of NIST.

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REFERENCES

(1) Karbuz, S. U.S. Military energy consumptionFacts and figures. Energy Bulletin; Post Carbon Institute: Santa Rosa, CA, 2007; http:// www.energybulletin.net/node/29925. (2) Heneghan, S. P.; Harrison, W. E. JP-8+100: The development of high thermal stability jet fuel. Proceedings of the 6th International Conference on Stability and Handling of Liquid Fuels; Vancouver, British Columbia, Canada, Oct 13−17, 1997; pp 271−352. (3) Harrison, W. E. Aircraft Thermal Management: Report of the Joint WRDC/ASD Thermal Management Working Group; Wright Research and Development Center (WRDC): Wright-Patterson Air Force Base, OH, 1990; WRDC-TR-90-2021. (4) Heneghan, S. P.; Zabarnick, S.; Ballal, D. R.; Harrison, W. E. JP8+100: The development of high thermal stability jet fuel. Trans. ASME 1996, 118, 170−179. (5) Ervin, J. S.; Zabarnick, S.; Binns, G.; Dieterle, G.; Davis, D. Investigation of the use of JP-8+100 with cold flow enhancer additives as a low cost replacement for JPTS. Energy Fuels 1999, 13, 1246−1251. (6) Grinstead, B.; Zabarnick, S. Studies of jet fuel thermal stability oxidation and additives using an isothermal oxidation apparatus equipped with oxygen sensor. Energy Fuels 1999, 13, 751−760. (7) Jones, E. G.; Balster, L. M.; Balster, W. J. Quantitative evaluation of jet fuel fouling and the effect of additives. Energy Fuels 1995, 9, 906−912. (8) Edwards, J. T.; Harrison, W.; Zabarnick, S.; DeWitt, M.; Bentz, C. Proceeding of the 40th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit; Ft. Lauderdale, FL, July 11−14, 2004. (9) Wright, B. E.; Witzig, W. L. Methods for reducing fouling deposit formation in jet engines. U.S. Patent 5,621,154, April 15, 1997. (10) Wright, B. E.; Witzig, W. L.; Goliaszwdki, E.; Carey, W. S.; Peltier, J. H. Methods and compositions for reducing fouling deposit formation in jet engines. U.S. Patent 5,596,130, Jan 21, 1997. (11) Muhammad, F.; Brooks, J. D.; Riviere, J. E. Comparative mixture effects of JP-8(100) additives on the dermal absorption and disposition of jet fuel hydrocarbons in different membrane model systems. Toxicol. Lett. 2004, 150, 351−365.

Figure 5. Histogram representation of the measured distillate concentration in percent (mass/mass) of 1-methylnaphthalene as a function of the distillate fraction. The uncertainty is discussed in the text.

caused by peak broadening and the resulting decline in resolution. Area delineation of the peak becomes somewhat less precise. It is often possible to compensate for peak broadening with adjustments to the method; however, in the case of a complex fluid, such as a fuel, with many surrounding peaks, this is not practical in single-dimensional chromatography.

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CONCLUSION In this paper, we have reported volatility measurements made with the ADC method that study the effect of the +100 additive to the military aviation kerosene JP-8. We found that this additive causes a slight increase of vaporization temperatures, especially early in the distillation curve. After the 70% distillate volume fraction, the additive is found to make little difference to the volatility. This is somewhat surprising because most of the constituents of the additive boil at temperatures above those of the constituents of JP-8. Indeed, the composition channel of the ADC shows that, of the constituents that we tracked through the curve, the highest concentrations are to be found late in the vaporization and, in some cases, in the recovered residue. Thermally stressing the neat additive suggests a possible explanation: cross-linking of some of the lower molecular mass oligomer constituents of the dispersant. Such cross-linking has the potential of removing the lower oligomers from participation in vapor liquid equilibrium. The presence of the dispersant throughout the distillation, even at 5848

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