The Laboratory Characterization of Jet Fuel Vapor and Liquid - Energy

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Energy & Fuels 2003, 17, 216-224

The Laboratory Characterization of Jet Fuel Vapor and Liquid James E. Woodrow Center for Environmental Sciences and Engineering, University of Nevada, Reno, Nevada 89557 Received June 24, 2002

Jet fuel (Jet A, Jet A1) was characterized using two gas chromatographic techniques. Headspace gas chromatography (HS-GC) was used to determine component partial pressures and total vapor pressures by measuring vapor densities in equilibrium with the liquid fuel. Component partial pressures and total vapor pressures were also derived from analysis of the neat jet fuel liquid by determining liquid component mole fraction and using this result with Raoult’s law to calculate vapor pressure. Measurements of some of the fuel vapor samples were made at 40, 50, and 60 °C and at vapor volume-to-liquid volume (V/L) ratios of 274 and 1.2, representing, respectively, a nearly empty (2.9 kg/m3) and a half-filled (364 kg/m3) center wing tank (CWT) in a Boeing 747100 series aircraft. The temperatures and V/L ratios were chosen to cover the range of conditions that could exist in the CWT under some aircraft operations. Measurements of other fuel vapor samples were made at the individual fuel flash point temperatures (V/L ) 1.2). Characterization of the liquid fuels was done by simple injections of the neat liquids onto a temperatureprogrammed gas chromatograph. Results from the liquid analyses were used to calculate fuel vapor properties for comparison with the HS-GC results. Results from both characterization methods were used to predict fuel flammability by calculating fuel-to-air (F/A) mass ratios for jet fuels under different use conditions. These F/A ratio results were compared with ratios determined for the fuels at their flash points.

Introduction Concerns over jet fuel safety call for a greater understanding of the composition of jet fuels and their behavior under typical use conditions. One approach toward characterizing jet fuel is to subject the fuel to speciation by identifying the 100s of hydrocarbon components in the fuel. However, this approach is tedious and it is much too easy to overlook fuel components. Besides, fuel safety concerns are primarily focused on fuel flammability, which is related to vapor composition and concentration. So, it is not necessary to speciate the fuel, since vapor density and pressure can be reasonably estimated with gas chromatographic (GC) characterization of the fuel by using relatively few reference hydrocarbons. This approach effectively reduces the fuels from hundreds of individual hydrocarbon components to a manageable number of “components”, each consisting of a hydrocarbon component group characterized by its reference hydrocarbon. This report describes the merits of the GC approach to the characterization of commercial jet fuels and discusses the analytical results in terms of fuel behavior under actual use conditions.1-5 Important goals of this study included providing technical information about the properties of jet fuel and its vapor in the center wing (1) Woodrow, J. E.; Seiber, J. N. The Laboratory Characterization of Jet Fuel Vapor Under Simulated Flight Conditions. Final report to the National Transportation Safety Board (Order No. NTSB12-970255), November, 1997. www.galcit.caltech.edu/EDL/projects/JetA/ documents.html. Exhibit 20H.

tank (CWT) of a Boeing 747-100 series aircraft under typical use conditions and specifically addressing the question of fuel flammability under certain flight conditions. As described below, these goals were achieved by modeling the CWT with a sealed laboratory headspace vial. This approach was justified by the fact that, in both the vial and CWT, fuel vapor behaved as an ideal gas, since hydrostatic pressures remained well below critical valuessthe CWT was vented to the atmosphere and internal vial pressures did not exceed 1.5 atm. An important consequence was the equivalent liquid-vapor distribution of fuel components at equal temperature for both cases, resulting in the same vapor pressure values. In the study discussed below, the nearly empty tank scenario (2.9 kg/m3 [∼190 L]) was selected to represent the CWT status at the end of a nonstop international flight that began with a full tank (52164 L). This was the case for the TWA Flight 800 aircraft that began its international flight at Athens, Greece, and was terminated at the JFK Airport in New York. The CWT was (2) Woodrow, J. E. The Laboratory Characterization of ARCO Jet Fuel Vapor and Liquid. Final report to the National Transportation Safety Board, June, 2000a. www.galcit.caltech.edu/EDL/projects/JetA/ documents.html. Exhibit 20R. (3) Woodrow, J. E. Determination of Fuel/Air Mass Ratios for Jet Fuels at their Flash Point Temperatures. Final report to the Federal Aviation Administration, December, 2000b. (4) Woodrow, J. E.; Seiber, J. N. J. Chromatogr. 1988, 455, 53-65. (5) Woodrow, J. E.; Seiber, J. N. Evaluation of a Method for Determining Vapor Pressures of Petroleum Mixtures by Headspace Gas Chromatography. Final report to the California Air Resources Board (Contract no. A6-178-32), September, 1989.

10.1021/ef020140p CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002

Jet Fuel Vapor and Liquid

Energy & Fuels, Vol. 17, No. 1, 2003 217

Figure 1. Typical headspace gas chromatogram of jet fuel vapor showing standard retentions (carbon nos. 5-12) and subsections (vertical lines).

used for this flight, which left the tank with only about 190 L of fuel (vapor volume-to-liquid volume [V/L] ratio ) 274). After the international flight, the aircraft was quickly turned around for another flight that used the outboard fuel tanks. The CWT was not refueled. On July 17, 1996, after leaving the JFK Airport and while approaching 14000 ft, the aircraft broke up over the Atlantic Ocean due to an explosion. The present study was part of an extensive investigation into the cause of the accident.6 The investigation focused on the presence of fuel vapors in the CWT and found that the probable cause of the accident was an explosion resulting from ignition of the flammable fuel/air mixture in the CWT. The laboratory headspace test temperatures of 40, 50, and 60 °C (see below) were selected to approximate the range of probable temperatures in the Flight 800 CWT that would result in flammable fuel/air mixtures for a nearly empty tank (2.9 kg/m3; V/L ) 274). Experimental Procedures The National Transportation Safety Board (NTSB) and the Federal Aviation Administration (FAA) supplied the jet fuels characterized in this study. Not all of the fuel samples were identified as to the formulators, but all were selected to represent a range of domestic and foreign commercial fuels. The fuel sources varied, where some of the fuels were obtained directly from refueling trucks and fuel depots, while others were obtained from aircraft fuel tanks and had undergone some weathering (i.e., differential losses of volatile fractions). One series of experimental fuels, provided by the NTSB, was (6) NTSB. Aircraft Accident Report. National Transportation Safety Board (NTSB/AAR-00/03) August 23, 2000. www.ntsb.gov.

specially formulated by the Atlantic Richfield Company (ARCO) to vary the vapor pressures. Headspace (Vapor) Method. Into separate chilled 22 mL glass headspace vials (Perkin-Elmer, Norwalk, CT) were placed 0.08 and 10 mL of chilled liquid fuel samples, and the vials were immediately sealed with Teflon-lined septa in crimped aluminum caps. These volumes of fuel represented vapor volume-to-liquid volume (V/L) ratios of 274 and 1.2, respectively. The sealed samples were placed in an HS-40 autosampler and injector (Perkin-Elmer), where they were thermostated at 40, 50, and 60 °C (V/L ) 1.2, 274), or at the fuel flash point (V/L ) 1.2), for at least 30 min. After the samples were thermostated, the HS-40 automatically punctured the septa with a hollow sampling needle, the vials were pressurized to about 1,500 mbar with helium, the equilibrated vapor was sampled for 0.01 min, the resulting vapor aliquot was injected onto a 60 m × 0.32 mm (i.d.) DB-1 fused silica open tubular (FSOT) capillary column (J&W Scientific, Folsom, CA), the injected vapor aliquot was chromatographed at a column carrier gas (helium) flow rate of about 3 mL/min, and the chromatographed vapor was detected by a flame ionization detector. The column was held at 100 °C for 4 min, after which time it was programmed at 2°/min to140 °C, where it was held for 1 min. The fuel samples were evaluated using a mixed hydrocarbon standard, which consisted of an equal volume mix of the normal alkanes pentane (C5) through dodecane (C12). Into separate chilled headspace vials were placed 1, 0.5, 0.25, and 0.1 µL of the mixed standard and the sealed vials were processed in the same way as for the fuel samples. Using the gas chromatographic retention times of the hydrocarbon standards, the fuel vapor chromatograms were divided into eight subsections, each of which was approximately centered about the retention time of a hydrocarbon reference (Figure 1). The vapor mass density for the completely volatilized

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Figure 2. Typical gas chromatogram of liquid jet fuel showing standard retentions (carbon nos. 5-20) and subsections (vertical lines). pentane was used to generate a standard curve (vapor mass density vs GC peak area). The peak areas in each subsection were summed and treated as a single peak in the pentane vapor density regression equation to calculate subsection vapor densities, which were used to calculate subsection partial pressures. Liquid Fuel Method. Each of the fuel samples was injected as the neat liquid (0.1-0.2 µL) onto a 60 m × 0.32 mm (i.d.) DB-1 FSOT capillary column (J&W Scientific), and each hydrocarbon component was monitored using a flame ionization detector installed in a Hewlett-Packard model 5890 Series II gas chromatograph. All samples were automatically injected using a computer-controlled enhanced autoinjector with a nanoliter adapter installed (Agilent, San Fernando, CA). The capillary column was held at 50 °C for four minutes, and then programmed at a rate of 1 °C/min to 250 °C, where it was held for 10 min. Starting with eicosane (C20), and working down in carbon number, a mixed hydrocarbon standard (pentane through eicosane) was prepared by weighing each component as it was added to the mixture. The mixed standard was chromatographed under the same conditions used for the liquid fuel samples. On the bases of elution times for the reference hydrocarbons, each fuel chromatogram was divided into sixteen subsections, with each subsection centered approxi-

mately on its respective reference hydrocarbon (Figure 2). By injecting different amounts of the standard mixture, a regression equation was generated for each fuel subsection (mass injected vs GC peak area).

Analysis Methods Headspace (Vapor) Method. The method used in this study for characterizing jet fuel vapor was derived from an earlier method for the determination of vapor pressures for fuels and crude oils.4,5 In the earlier method, vapor mass density standard curves for each subsection were generated from the respective subsection reference hydrocarbons. The vapor mass density for a particular subsection was calculated using that subsection’s standard curve. The present study differs from the previous work by using a single standard curve, based on pentane, for all subsections rather than using a separate standard for each one. The selection of pentane assured that the highest vapor density (∼3.6 µg/mL) at the lowest test temperature (40 °C) would still be far from vapor saturation (∼3.2 mg/mL), and result in a consistently linear and representative standard curve.2 The HS-40 vapor autosampler was not able to sample vapor in a predictable way for a hydrocarbon near vapor saturation. This became a problem only for reference hydrocarbons >C9 and not for the



jet fuel samples, whose individual components remained well below saturation. The use of a single standard curve based on a particular hydrocarbon was justified, since the same mass density for any hydrocarbon gave the same number of moles of carbon in the vapor. Response of the GC flame detector was directly proportional to the vapor molar density of carbon. This approach simplified data handling and gave more consistent results. V/L ) 1.2 and 274. Each fuel subsection GC area was used in the pentane reference standard regression equation to obtain subsection vapor mass density (g/L). Then, the partial pressure Pi corresponding to each subsection was obtained from the ideal gas law and the molecular weight of the n-alkane reference for each subsection. That is,

were calculated using the Wagner equation,7 which represents the vapor pressure behavior of substances over the entire liquid range and takes the following form:

Pi ) (n/V)iRT

Pi ) γiXiPi°

∑P

i

mi/(mw)i ) ni

(3)

where i ) 5-20. Then, the subsection mole fraction Xi was given by

Xi ) ni/

∑n

i

(4)

These liquid mole fraction values can be used to calculate fuel vapor pressure at any given temperature if the saturation vapor pressure for each subsection reference hydrocarbon is known. The saturation vapor pressures at each temperature

(6)

For these calculations, it was assumed that the activity coefficient γi for each component was unitysa reasonable assumption within experimental error. By using the liquid subsection partial pressures Pi, derived from the subsection mole fractions as discussed above, the vapor composition above each liquid fuel was estimated at 40, 50, and 60 °C or at the fuel flash points. This was done by using a rearranged form of the ideal gas law equation:

(n/V)i ) Pi/(RT)

(2)

No correction for real gas behavior was necessary since component partial pressures remained far below the critical pressures. Liquid Fuel Method. While the headspace method will allow the direct measure of fuel vapor density at vapor-liquid equilibrium, it would be useful to have a method that could be used to estimate the equilibrium under any set of conditions. This can be accomplished if the composition of the liquid fuel is known. What follows is a description of the theoretical basis for the liquid fuel method and how the theoretical relationships can be used with the GC analytical results to describe the characteristics of jet fuel at vapor-liquid equilibrium. For the V/L ) 1.2 case (half-filled vial [fuel tank]), it was assumed that fuel component vapor density would be at saturation and that composition of the liquid fuel would not have changed significantly after reaching vapor-liquid equilibrium. This assumption was used to estimate fuel component vapor density from the liquid fuel characterization. For the V/L ) 274 case (nearly empty vial [fuel tank]), where liquid and vapor composition are quite different compared to the V/L ) 1.2 case, liquid-vapor component distribution coefficients were derived from the liquid characterization results and used to estimate component vapor density. V/L ) 1.2. As was done in the HS-GC vapor method, each subsection summed GC peak area (5-20) in the liquid method was treated as an individual compound and was used in the appropriate subsection reference standard regression equation to calculate subsection mass mi. Each mass was then divided by the molecular weight of the reference hydrocarbon (mw)i, giving the number of moles ni from which subsection mole fraction was derived. That is,

(5)

where Pi° (i ) 5-20) is the vapor pressure (bar); Pc and Tc are the critical pressure and temperature, respectively, of the hydrocarbon; T is the test temperature; and τ ) (1 - T/Tc). The terms a, b, c, and d were taken from Appendix A, Section D, in ref 7. Each subsection partial pressure (Pi, i ) 5-20) was then obtained from the product of the subsection saturation pressure Pi° and subsection liquid mole fraction Xi using Raoult’s law:

(1)

where i ) 5-12, (n/V)i is the vapor molar density (mol/L), R is the gas constant (83.136 L‚mbar/K‚mol), and T is absolute temperature. Also, (n/V)i ) (g/L)/(mw)i, where (g/L) is the vapor mass density obtained from the pentane standard curve and (mw)i is the molecular weight of the subsection reference hydrocarbon. The total vapor pressure P5-12 for the fuel sample was, then, just simply a summation of the individual partial pressures:

P5-12 )

ln Pi° ) ln Pc + (Tc/T)(aτ + bτ1.5 + cτ2.5 + dτ5)

(7)

where (n/V)i is subsection molar density in the vapor (mol/L), R is the gas constant (83.136 L‚mbar/K‚mol), and T is absolute temperature. This subsection molar density was then used to calculate total vapor mass density ([g/L]5-20):

[g/L]5-20 )

∑[g/L]

i

(8)

[g/L]i ) (n/V)i[mw]i

(9)

where

V/L ) 274. As stated above, the liquid and vapor composition for the nearly empty vial (fuel tank) will be significantly different from the compositions for a half-filled vial (fuel tank). This is due to some depletion of the liquid fuel of the more volatile components and their under-saturation in the vapor, leading to measurably lower total vapor pressures for fuels where V/L ) 274. Because of this, an approach different from the one described in the previous section is needed to estimate vapor composition. Starting with the liquid characterization results and calculating the liquid and vapor molar densities at saturation, liquid-vapor distribution coefficients were calculated for the fuel components. These distribution coefficients were then used to estimate vapor composition and component partial pressures for the nearly empty vial (fuel tank). An important assumption is that the distribution coefficients were the same for V/L ) 274 as for V/L ) 1.2. Using the data from the liquid characterization, the molar concentration in the liquid (i.e., mol/L) was calculated for each subsection carbon number. This was simply done by dividing the subsection mass by the reference hydrocarbon molecular weight (eq 3), and then dividing the result by the total volume of sample injected into the analytical instrument. As described above, the subsection partial pressures were calculated using each subsection liquid mole fraction and saturation vapor pressure, calculated using the Wagner relationship (eq 5). These partial pressures were then converted to their equivalent vapor concentrations (mol/L) using the ideal gas law equation (eq 7). (7) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids, 5th ed.; McGraw-Hill: New York, 2000; pp 7.5, A.47-A.60.

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The resulting liquid and vapor concentration data were used in the following expression:

CVi ) C°Li/(κi + [V/L])

(10)

where i ) 5-20, CVi is the component concentration in the vapor (mol/L), C°Li is the initial component concentration in the liquid (mol/L), κi is the hydrocarbon liquid-vapor distribution coefficient for each component, and V/L is the vapor volume-to-liquid volume ratio ()274) that would occur in the headspace vial for the nearly empty fuel tank scenario.8 Equation 10 shows that the vapor concentration (CVi) of a hydrocarbon component in the headspace vial depends not only on κi but also on V/L. For a given κi, CVi will decrease (increase) as V/L is increased (decreased). This expression can be derived in the following way. At vapor-liquid equilibrium, the following relationship holds:

C°LiL ) CViV + CLiL

(11)

CLi ) κiCVi

(12)

C°LiL ) CViV + κiCViL

(13)

Since

then

Dividing by L and rearranging leads to eq 10. Before eq 10 could be used to calculate component concentration in the vapor CVi for V/L ) 274, the component distribution coefficients κi needed to be estimated. Using Cvi and C°Li for V/L ) 1, κi was calculated for each subsection carbon number:

κi ) (C°Li - [CVi(V/L)])/CVi

(14)

To obtain κi, this expression corrects C°Li for component losses when the liquid reaches equilibrium with the vapor (see eq 12, where CLi ) C°Li - [CVi(V/L)]). Thus, given C°Li, κi, and V/L ) 274, CVi was then calculated for each hydrocarbon subsection for a nearly empty vial (fuel tank) using eq 10. The distribution coefficient is not a fundamental property or constant, but, as shown above, is dependent on liquid and vapor concentrations, as determined by the temperaturedependent vapor-liquid distribution of fuel components.

Analysis Methods Results An important objective of this study was to use the described fuel characterization methods to determine component partial pressures and total vapor pressures of samples of jet fuel representative of the type of fuel used in commercial aviation. A further objective was to establish the relative distribution of fuel components in the vapor. These objectives also included a description of the fuel conditions that could exist in the CWT of a Boeing 747-100 series aircraft when the fuel tank contains about 3 kg/m3 fuel (V/L ) 274), a nearly empty tank (the full tank ≈800 kg/m3). Headspace (Vapor) Method. This method was used for both the half-filled (V/L ) 1.2) and nearly empty tank (V/L ) 274) scenarios to determine fuel vapor pressures and compositions at 40, 50, and 60 °C and also at individual fuel flash points (V/L ) 1.2). The average coefficient of variation (CV) using the headspace method (8) Ioffe, B. V.; Vitenberg, A. G. Headspace Analysis and Related Methods in Gas Chromatography; John Wiley & Sons: New York, 1984; p 26 (Translated from the Russian by I. A. Mamantov).

to measure vapor pressure was about 3.9% (range, 0.37-11%; med, 3.4%; n ) 76). This compares well with the average value of 3.4% obtained in an earlier study (range, 0.36-15%; med., 2.2%; n ) 34).4 The wide range in the CV values was due primarily to some variability in fuel vapor sampling by the analytical instrumentation. This variability was caused by the relatively short sampling time of 0.01 min, which required a rapid open/ close cycle of the electromechanical solenoid valves. Longer sampling times were possible which lessened sampling variability. But, there was concern regarding the possible saturation of the detector, especially for the more volatile samples at 60 °C. Tables 1-3 summarize the results at 40, 50, and 60 °C for a series of commercial fuels, including a fuel obtained from the Reno/Tahoe International Airport (fuel no. 1). These tables contain the subsection carbon number partial pressure and total vapor pressure of each fuel, along with the total vapor mass density. Samples 1-4 were taken fresh from fuel storage tanks/ trucks, whereas samples 5-11 were taken from the CWT of a 747-100 aircraft and had undergone some weathering (differential loss of lighter components). Based on the results for fuels 1-4 (V/L ) 1.2), unweathered saturation vapor pressures for commercial jet fuels were on average 8.3, 13.3, and 19.2 mbar at 40, 50, and 60 °C, respectively. These values compared reasonably well (0-5.7%) with the values 8.8 (40 °C), 13.3 (50 °C), and 19.7 mbar (60 °C) measured by investigators at the California Institute of Technology for commercial jet fuel obtained from the Los Angeles International Airport (LAX).9 Vapor pressures for the LAX fuels were measured using electromechanical pressure gauges after the fuels had been purged of dissolved gases. Fuel samples 5-11 were the result of a single fuel weathered in a nearly empty (V/L ) 274) CWT of a 747100 aircraft, leading to vapor pressures significantly lower than pressures for the unweathered fuels. To generate the test flight samples, fuel was obtained from the outboard wing tank of a 747 aircraft that arrived from an international flight, and about 1360 kg was loaded into a fuel truck. Approximately 360 kg was offloaded from the truck to purge the fuel line on the truck, and about 190 L was then pumped into the CWT of the test 747-100 series aircraft. The fuel remained on board for all of the test flights, during which time different combinations of three environmental control system (ECS) packsslocated directly below the CWTswere operated to cool the crew/passenger cabins. Fuel weathering occurred as a result of fuel tank venting through stringer vents as the aircraft gained altitude. The lighter fuel components would, of course, be preferentially vented causing a differential change in fuel composition. Sample 5 was taken from the CWT after the fuel tank had been loaded with about 190 L of fuel and before the test flights. The series of decreasing vapor pressures represented an increasing number of flight cycles where the fuel was allowed to vent at altitude (∼10000-14600 ft) during each cycle. As a result, subsection partial (9) Shepherd, J. E.; Krok, J. C.; Lee, J. J. Jet A Explosion Experiments: Laboratory Testing. Report prepared for the National Transportation Safety Board (Order NTSB 12-97-SP-0127), June 26, 1997. www.galcit.caltech.edu/EDL/projects/JetA/documents.html. Exhibit 20D.



Table 1. Headspace GC Results for Test Jet Fuels (Jet A, Jet A1) at 40 °C V/L ) 1.2: subsection partial pressure, mbar 7 8 9 10

fuela

5

6

1 (Reno) 2 3 4 5 6 7 8 9 10 11

0.678 1.06 0.962 0.943 0.179 0.023 0.024 0.016 0.009 0.005 0.005

0.343 0.827 0.818 0.791 0.435 0.056 0.046 0.032 0.019 0.014 0.008

0.934 2.22 2.20 2.18 0.888 0.350 0.283 0.230 0.177 0.140 0.097

1.83 1.89 1.88 1.77 1.15 0.773 0.689 0.625 0.536 0.480 0.404

2.17 1.20 1.19 1.15 1.25 1.09 1.05 1.01 0.990 0.962 0.919

1.51 0.753 0.745 0.735 1.13 1.12 1.13 1.12 1.14 1.16 1.18

11

12

total pressure (mbar)b

vapor mass density (g/m3)

0.600 0.407 0.397 0.418 0.535 0.528 0.542 0.543 0.583 0.591 0.626

0.139 0.150 0.149 0.147 0.153 0.160 0.163 0.161 0.169 0.163 0.176

8.21 ( 0.25 8.51 ( 0.08 8.34 ( 0.11 8.13 ( 0.28 5.72 ( 0.24 4.09 ( 0.27 3.93 ( 0.18 3.73 ( 0.27 3.62 ( 0.13 3.51 ( 0.07 3.41 ( 0.09

38.1 36.0 35.4 34.6 26.9 20.6 20.0 19.2 18.8 18.4 18.1

11

12



0.491 0.369 0.385 0.396 0.516 0.513 0.615 0.559 0.687 0.664 0.712

0.121 0.132 0.150 0.155 0.181 0.184 0.205 0.213 0.238 0.238 0.245

6.07 ( 0.48 5.66 ( 0.10 5.72 ( 0.20 5.44 ( 0.02 4.37 ( 0.41 3.31 ( 0.17 3.43 ( 0.08 3.43 ( 0.33 3.48 ( 0.10 3.28 ( 0.10 3.25 ( 0.14

29.3 25.5 25.9 24.7 21.3 17.0 17.8 17.8 18.4 17.5 17.5

V/L ) 274:

b

subsection partial pressure, mbar 7 8 9 10

fuela

5

6

1 (Reno) 2 3 4 5 6 7 8 9 10 11

0.125 0.124 0.120 0.112 0.048 0.006 0.006 0.005 0.002 c c

0.159 0.400 0.398 0.368 0.204 0.030 0.026 0.018 0.012 0.009 0.004

0.623 1.51 1.52 1.42 0.615 0.238 0.200 0.178 0.133 0.113 0.076

1.52 1.47 1.46 1.36 0.859 0.573 0.542 0.539 0.438 0.397 0.324

1.80 1.00 1.01 0.968 1.00 0.850 0.874 0.905 0.873 0.824 0.790

1.24 0.655 0.678 0.658 0.950 0.919 0.963 1.01 1.09 1.04 1.10

a Samples 1-4 are unweathered commercial fuels; samples 5-11 were derived from a commercial fuel subjected to in-flight weathering. Average ((SD) of three determinations. c No peaks detected.

Table 2. Headspace GC Results for Test Jet Fuels (Jet A, Jet A1) at 50 °C V/L ) 1.2: fuela

5

6

1 (Reno) 2 3 4 5 6 7 8 9 10 11

0.945 1.24 1.15 1.17 0.276 0.034 0.038 0.027 0.014 0.011 0.010

0.531 1.16 1.16 1.16 0.701 0.090 0.078 0.052 0.032 0.024 0.013

Subsection Partial Pressure, mbar 7 8 9 10 1.49 3.4 3.41 3.50 1.54 0.558 0.487 0.384 0.288 0.248 0.159

3.06 3.09 3.10 3.02 1.62 1.33 1.28 1.13 0.954 0.889 0.699

3.78 2.00 2.01 2.03 2.51 2.03 2.11 1.97 1.90 1.92 1.72

2.63 1.28 1.29 1.31 2.47 2.29 2.39 2.28 2.30 2.41 2.30

11

12



0.990 0.748 0.742 0.770 1.16 1.13 1.20 1.15 1.21 1.26 1.25

0.219 0.243 0.223 0.255 0.337 0.318 0.353 0.340 0.342 0.348 0.351

13.6 ( 0.5 13.2 ( 0.2 13.1 ( 0.3 13.2 ( 0.3 10.6 ( 0.1 7.79 ( 0.21 7.94 ( 0.27 7.32 ( 0.17 7.04 ( 0.23 7.10 ( 0.08 6.50 ( 0.26

62.0 55.2 54.9 55.5 49.6 38.5 39.6 36.8 35.7 36.3 33.5

11

12



0.864 0.675 0.672 0.669 0.906 1.02 1.09 1.16 1.15 1.27 1.30

0.197 0.210 0.228 0.231 0.275 0.369 0.401 0.418 0.406 0.441 0.455

10.1 ( 1.0 8.69 ( 0.12 8.54 ( 0.28 8.34 ( 0.21 8.01 ( 0.56 6.43 ( 0.33 6.27 ( 0.32 6.66 ( 0.46 5.95 ( 0.08 6.19 ( 0.12 5.91 ( 0.20

47.7 38.7 38.1 37.3 38.2 32.4 31.9 34.0 30.8 32.2 31.0

V/L ) 274:

b

fuela

5

6

1 (Reno) 2 3 4 5 6 7 8 9 10 11

0.121 0.124 0.115 0.111 0.050 0.007 0.007 0.005 - -c - -c - -c

0.183 0.473 0.461 0.446 0.281 0.038 0.031 0.023 0.013 0.012 0.004

subsection partial pressure, mbar 7 8 9 10 0.994 2.08 2.04 2.03 0.961 0.366 0.287 0.263 0.185 0.158 0.108

2.35 2.33 2.28 2.15 1.60 1.01 0.878 0.896 0.693 0.642 0.532

3.09 1.67 1.64 1.60 1.95 1.63 1.57 1.70 1.49 1.51 1.40

2.26 1.13 1.10 1.10 1.99 1.99 2.01 2.20 2.01 2.16 2.12

a Samples 1-4 are unweathered commercial fuels; samples 5-11 were derived from a commercial fuel subjected to in-flight weathering. Average ((SD) of three determinations. c No peaks detected.

pressures decreased for the lighter vapor components (C9). Flammability under Flight Conditions. Despite compositional changes in the fuel due to weathering and

handling, will the fuel still be flammable under certain flight conditions? To address this question, we used the vapor mass density data for the fuel samples weathered in aircraft fuel tanks (samples 5-11) to calculate fuelto-air (F/A) mass ratios and fuel mole fractions in air

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Table 3. Headspace GC Results for Test Jet Fuels (Jet A, Jet A1) at 60 °C V/L ) 1.2: fuela

5

6

1 (Reno) 2 3 4 5 6 7 8 9 10 11

1.06 1.57 1.46 1.49 0.325 0.046 0.049 0.034 0.024 0.019 0.014

0.648 1.64 1.63 1.62 0.884 0.132 0.115 0.079 0.048 0.032 0.021


4.58 4.47 4.46 4.42 2.84 1.97 1.79 1.62 1.38 1.22 1.00

5.82 2.98 2.98 2.96 3.44 3.08 3.05 2.91 2.77 2.66 2.49

4.28 1.98 1.96 1.97 3.41 3.47 3.48 3.31 3.37 3.40 3.35

11

12



1.64 1.18 1.13 1.17 1.68 1.79 1.77 1.76 1.79 1.85 1.85

0.385 0.399 0.397 0.391 0.481 0.523 0.515 0.504 0.540 0.532 0.534

20.3 (1.2 19.1 ( 0.7 18.8 ( 0.6 18.8 ( 0.8 15.1 ( 0.8 11.8 ( 0.4 11.4 ( 0.2 10.8 ( 0.4 10.3 ( 0.3 10.0 ( 0.3 9.50 ( 0.40

91.1 78.2 77.4 77.3 68.5 57.0 55.5 52.5 50.9 49.8 47.4

11

12



1.38 1.03 1.04 1.08 1.35 1.38 1.54 1.66 1.79 2.07 1.92

0.375 0.385 0.391 0.400 0.417 0.418 0.533 0.596 0.663 0.756 0.722

14.9 ( 0.9 12.3 ( 0.2 12.2 ( 0.4 12.1 ( 0.4 10.7 ( 0.9 8.83 ( 0.96 9.04 ( 0.91 9.14 ( 0.32 8.59 ( 0.85 9.33 ( 0.01 8.44 ( 0.76

69.1 54.0 53.6 53.3 50.2 43.1 44.6 45.5 43.3 47.4 43.1

V/L ) 274:

b

fuela

5

6

1 (Reno) 2 3 4 5 6 7 8 9 10 11

0.159 0.133 0.126 0.116 0.055 0.008 0.010 0.005 0.005 0.005 0.005

0.232 0.581 0.566 0.559 0.294 0.047 0.040 0.032 0.016 0.014 0.011


3.36 3.26 3.22 3.16 2.00 1.38 1.26 1.17 0.916 0.886 0.715

4.69 2.47 2.45 2.42 2.69 2.38 2.39 2.36 2.10 2.20 1.96

3.55 1.72 1.72 1.73 2.78 2.75 2.88 2.98 2.86 3.18 2.95

a Samples 1-4 are unweathered commercial fuels; samples 5-11 were derived from a commercial fuel subjected to in-flight weathering. Average ((SD) of three determinations.

at sea level and at 14,000 feet for a nearly empty tank (V/L ) 274). This latter altitude is where the TWA Flight 800 explosion occurred in the nearly empty CWT. Table 4 summarizes the calculated results for these fuel samples at 40, 50, and 60 °C. Inspection of the data indicates that, compared with a lower flammability limit of about 0.038 F/A mass ratio or 0.009 mole fraction,10 the weathered fuel samples that were at 50 °C not only exceeded these values but were well within the flammability range for the 14000 ft altitude. Results for the Reno fuel (no. 1) were included to show that an unweathered fuel would exceed the flammability limit at an even lower temperature at 14000 ft. The results in this table for the weathered samples compared well with measured F/A mass ratios (0.048-0.054) and fuel mole fractions (0.010-0.012) for fuel vapor samples taken from the CWT of a 747-100 aircraft during flight tests at 14000 ft.11 Prior to the test flight, the aircraft was allowed to remain on the tarmac with all of the ECS packs in operation. This resulted in elevated ullage temperatures within the CWT of as much as 63 °C on the ground and 53 °C at 14000 ft.6 Correlation of F/A Ratio with Flash Point. Related to the issue regarding F/A mass ratio and fuel flammability is the F/A mass ratio at the fuel flash point. Table 5 lists the flash point temperatures for 10 commercial fuels supplied by the FAA. Also listed are the air mass densities and fuel vapor mass densities, (10) Shepherd, J. E.; Nuyt, C. D.; Lee, J. J. Flash Point and Chemical Composition of Aviation Kerosene (Jet A). Explosion Dynamics Report FM99-4; California Institute of Technology: Pasadena, May 26, 2000. www.galcit.caltech.edu/EDL/projects/JetA/documents.html. Exhibit 20S. (11) Sagebiel, J. C. Sampling and Analysis of Vapors from the Center Wing Tank of a Test Boeing 747-100 Aircraft. Final Report to the National Transportation Safety Board, November, 1997.

Table 4. Fuel/Air Mass Ratios and Fuel Mole Fractions for Test Flight Samples at V/L ) 274 fuel/air mass ratio 40°

50°

60°

sample

0 fta

14 kftb

0 fta

14 kftb

0 fta

14 kftb

5 6 7 8 9 10 11 1 (Reno)

0.019 0.015 0.016 0.016 0.016 0.016 0.016 0.026

0.033 0.026 0.028 0.028 0.028 0.028 0.028 0.045

0.035 0.030 0.029 0.031 0.028 0.029 0.028 0.044

0.060 0.052 0.050 0.054 0.048 0.050 0.048 0.076

0.047 0.041 0.042 0.043 0.041 0.045 0.041 0.065

0.081 0.071 0.073 0.074 0.071 0.078 0.071 0.112

fuel mole fraction 40°

50°

60°

sample

0 ftc

14 kftd

0 ftc

14 kftd

0 ftc

14 kftd

5 6 7 8 9 10 11 1 (Reno)

0.004 0.003 0.003 0.003 0.003 0.003 0.003 0.006

0.007 0.005 0.005 0.005 0.005 0.005 0.005 0.010

0.008 0.006 0.006 0.006 0.006 0.006 0.006 0.010

0.014 0.010 0.010 0.010 0.010 0.010 0.010 0.017

0.010 0.009 0.009 0.009 0.008 0.009 0.008 0.015

0.017 0.016 0.016 0.016 0.014 0.016 0.014 0.026

a Atmospheric mass density (dry air): 1127.4 g/m3, 40°C; 1092.4 g/m3, 50°C; 1059.6 g/m3, 60°C. b Mass ratios at 14 kft were determined by multiplying the ratios at sea level by 1 atm/0.578 atm. c Air molar density: 39.1 mol/m3, 40°C; 37.9 mol/m3, 50°C; 36.7 mol/m3, 60°C. Molar densities were determined from the average molecular weight of air (∼28.84 g/mol) and the mass densities of air at the various temperatures. d Fuel mole fractions at 14 kft were determined by multiplying the fractions at sea level by 1 atm/0.578 atm.

determined at the respective fuel flash points using the HS-GC method. The F/A mass ratios for the fuels at their flash points fell in the range 0.041-0.047, depend-



Table 5. Fuel/Air Mass Ratios Derived from Data Obtained Using the HS-GC (Vapor) Method fuel designation

flash point (°C)

air density (g/m3)a

fuel vapor density,g/m3

fuel/air mass ratio

FAA-1 FAA-2 FAA-3 FAA-4 FAA-5 FAA-6 FAA-7 FAA-8 FAA-9 FAA-10

49.1 52.0 57.5 41.1 46.5 37.5 54.1 59.0 43.5 51.6

1095.8 1085.7 1066.0 1123.8 1106.2 1134.6 1079.0 1062.8 1113.1 1085.7

45.4 46.6 47.1 49.1 47.4 51.6 45.2 44.0 52.6 49.7

0.041 0.043 0.044 0.044 0.043 0.045 0.042 0.041 0.047 0.046

a Derived from the following expression: Density (g/m3) ) -1.50328 + 3.53342 × 105(1/T), where T is the absolute temperature.

Table 6. Comparison of Commercial Jet Fuel Vapor Pressures Derived from the HS-GC (Vapor) and Liquid Methods

Table 7. Comparison of Fuel Flash Point Fuel-to-air (F/A) Mass Ratios Derived from the HS-GC (Vapor) and Liquid Methods fuel designation

flash point, °C

FAA-1 FAA-2 FAA-3 FAA-4 FAA-5 FAA-6 FAA-7 FAA-8 FAA-9 FAA-10

49.1 52.0 57.5 41.1 46.5 37.5 54.1 59.0 43.5 51.6

F/A ratios at the flash point HS-GCa liquid-GC 0.041 0.043 0.044 0.044 0.043 0.045 0.042 0.041 0.047 0.046

%∆b

0.044 0.043 0.044 0.044 0.043 0.044 0.044 0.044 0.045 0.044

7.3 0.0 0.0 0.0 0.0 2.2 4.8 7.3 4.2 4.3

a Taken from Table 5. b Absolute differences with respect to the HS-GC method.

Table 8. Measured and Calculated Vapor Pressures for Jet Fuels at V/L ) 274 vapor pressure, mbar

total vapor pressure, mbar fuel

temperature °Ca

HS-GC

liquid-GC

%∆b

3c “ “ FAA-1 FAA-2 FAA-3 FAA-4 FAA-5 FAA-6 FAA-7 FAA-8 FAA-9 FAA-10

40 50 60 49.1 52.0 57.5 41.1 46.5 37.5 54.1 59.0 43.5 51.6

8.34 13.1 18.8 9.62 10.6 10.0 11.2 10.4 11.9 9.95 9.18 11.7 11.6

7.91 12.6 19.4 10.4 10.7 10.0 11.4 10.6 11.6 10.7 9.68 11.3 10.8

5.2 3.8 3.2 8.1 0.9 0.0 1.8 1.9 2.5 7.5 5.4 3.4 6.9

a Flash point temperatures for the FAA fuels. b Absolute differences with respect to the HS-GC method. c See Tables 1-3.

ing on composition. The average CV value for the F/A determinations was 5.6-6.6%. This was based on an uncertainty of about (0.5-1 °C in flash point measurement, plus the uncertainty inherent in the headspace method. A comparison of the F/A results with the F/A mass ratios in the CWT at 14000 ft (Table 4) clearly shows that the unweathered Reno fuel at 40 °C and all of the fuels at 50 °C met or exceeded the flash point F/A mass ratios. Liquid Fuel Method. As discussed above, the liquid fuel chromatograms were divided into sixteen subsections, each of which was represented by a normal alkane reference (Figure 2). However, for direct comparison with the headspace method results, much of the liquid characterization results discussed below represent subsections C5-C12 only. The average CV value derived from the liquid characterization method for commercial grade fuels was about 1.9% (range, 0.28-7.4%; med, 1.4%; n ) 24), compared to 3.9% for the headspace method. Half-Filled Tank (V/L ) 1.2). Table 6 compares the total fuel vapor pressures, along with absolute percent differences (|%∆| ) 100[(H - L)/H], H ) headspace, L ) liquid), derived from the headspace (HS-GC) and liquid characterizations of one NTSB and ten FAA fuels. Differences between the headspace and liquid characterization results fell in the range 0-8.1%, with an average difference of 3.9% and a median of 3.4%. For eleven out of the thirteen comparisons, vapor pressures

sample 3a ARCO-1 ARCO-2 ARCO-3 ARCO-4 ARCO-5 ARCO-6 ARCO-7

method

40 °C

50 °C

60 °C

HS-GCb calculatedc HS-GC calculated HS-GC calculated HS-GC calculated HS-GC calculated HS-GC calculated HS-GC calculated HS-GC calculated

5.72 6.09 12.7 14.7 3.08 3.48 2.19 2.18 1.66 1.57 1.40 1.39 1.11 1.00 0.940 0.819

8.54 9.17 19.4 20.9 5.04 5.62 3.46 3.72 2.62 2.77 2.18 2.44 1.71 1.82 1.49 1.52

12.2 13.4 27.3 28.8 8.35 8.88 6.19 6.24 5.02 4.83 4.35 4.28 3.29 3.35 2.94 2.85

a See Tables 1-3. b Measured using the headspace method at V/L ) 274 (0.08 mL fuel in 22 mL vial). c Calculated using the headspace equation,8CVi ) C°Li/(κi + [V/L]), with liquid method data.

from the two methods were not significantly different (95% confidence interval). The exceptions were fuels FAA-1 and -7, which had the largest %∆ values. Further comparison is made in Table 7 of the F/A mass ratios at fuel flash point for the ten FAA fuels. The average CV value for the F/A results using the liquid characterization method was 3.3-4.3%, compared to 5.6-6.6% for the headspace method. This was based on an uncertainty of about (0.5-1 °C in flash point measurements, plus the uncertainty inherent in the liquid characterization method. Absolute differences for the Table 7 values fell in the range 0-7.3%, with an average of about 3.0% and a median of 3.2%. None of the F/A ratios from the two methods were significantly different (95% confidence interval). Nearly Empty Tank (V/L ) 274). Table 8 compares headspace (HS-GC) vapor pressures for one NTSB fuel and seven ARCO fuels at V/L ) 274 (2.9 kg/m3) with calculated pressures derived using the headspace equation (eq 10), after estimating the component distribution coefficients κi with liquid method data and eq 14. The ARCO fuels were specially formulated to vary vapor pressure. The two sets of data compare reasonably well. The average absolute difference (with respect to the HSGC method) was 6.6% (range: 0.46-15.7%), with a median difference of 6.4%.

224


The CV values for vapor pressure determination using the headspace and liquid methods increased for the less volatile fuels in Table 8. For example, the CV values increased from about 3.9% (headspace) and 1.9% (liquid) for commercial fuel no. 3 to about 7% (headspace and liquid) for specially formulated fuel ARCO-7. A similar trend was observed in an earlier study4 where the CV values for the headspace method increased from about 0.54-1.3% for light crude and gasoline (vapor pressures ≈ 260-589 mbar), to 2.4% for medium-light crude (vapor pressure ≈ 30 mbar), to about 25% for heavy crude (vapor pressure ≈ 0.4 mbar), all at 38 °C. This kind of behavior in the current study affected the statistical comparison of the vapor pressure values from the two methods. For example, a difference greater than about 6.5% (with respect to the headspace method) was significant for fuel no. 3, whereas the 13% difference for ARCO-7 at 40 °C was not significant. The greater CV values for the less volatile fuels was probably due to the increased significance of hydrocarbon contaminants inadvertently introduced to the samples or carried over from previous GC runs. Summary and Conclusions The headspace gas chromatography (HS-GC) method showed good accuracy and precision when used to characterize vapor-liquid equilibria of jet fuels under simulated flight conditions in the laboratory. It was not necessary to know the exact composition of the jet fuels, but these complex mixtures could be approximated with n-alkane reference standards whose GC retention times spanned the chromatogram envelopes of the fuels. Of critical importance is the fact that the HS-GC method will respond only to hydrocarbons and will be unaffected by non-hydrocarbon constituents, such as dissolved air, water, etc. Total vapor pressure of the fuels declined with weathering when the fuels were exposed to typical flight conditions in the CWT. The ability of the HS-GC method to measure fuel component properties showed that these changes in fuel properties were due primarily to changes in fuel composition through the loss of the more volatile components (C9). Although weathered fuel had lower total vapor pressures, partial pressures of the higher molecular weight components

Woodrow

were greater than pressures for the same components in unweathered fuel under the same conditions. This partly off-set the effects of losses of the more volatile components by generating enough vapor mass at 50 °C and 14000 ft to maintain flammability, as was indicated by calculations of fuel/air mass ratio and fuel mole fraction in air. Within the limits of experimental uncertainty, the liquid method gave results that were essentially equivalent to the results from the HS-GC method for both the half-filled (V/L ) 1.2[364 kg/m3]) and nearly empty tank (V/L ) 274[2.9 kg/m3]) scenarios. Compared to the HSGC method, the liquid characterization method showed consistently better precision. But the primary advantage of the liquid method over the HS-GC method is the ability to characterize jet fuel vapor-liquid equilibria under any specified set of conditions without additional laboratory measurements. All that is needed here are the saturation vapor pressures of the reference hydrocarbons at the specified conditions. Such information can be readily calculated from data available in handbooks. The increased uncertainty in vapor pressure determination for the least volatile ARCO fuel compared to a commercial fuel suggests a practical vapor pressure limit for the application of the headspace and liquid methods. For good precision (CV < 5%), vapor pressures >1-2 mbar at the lowest test temperature are recommended. If vapor pressures are much less than 1 mbar at the lowest temperature, an alternative approach would be to evaluate the hydrocarbon mixture at elevated temperaturesswhere good precision is possibles and then to use a Clausius-Clapeyron type relationship to estimate vapor pressure at the lower temperature by extrapolation. However, the uncertainty in this approach would need to be evaluated and compared with the direct measurement of vapor pressure for low volatility mixtures. Acknowledgment. The author gratefully acknowledges the sponsorship of the National Transportation Safety Board and the Federal Aviation Administration, and the many fruitful and helpful discussions with professor Joseph E. Shepherd (Graduate Aeronautical Laboratories, California Institute of Technology). EF020140P

The Laboratory Characterization of Jet Fuel Vapor and Liquid - Energy

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