Article pubs.acs.org/EF
Composition-Explicit Distillation Curves of Alternative Turbine Fuels R. V. Gough and T. J. Bruno* National Institute of Standards and Technology (NIST), 325 Broadway, Boulder, Colorado 80305, United States ABSTRACT: In recent years, environmental considerations, the potential for supply disruptions, and rising fuel prices have led to the development of turbine fuels produced from non-petroleum feedstocks. To determine the suitability of an alternative turbine fuel, it is important to characterize the fuel properties and assess the degree of departure of the alternative fuel characteristics from those of petroleum-derived fuels. One very important property to use for this purpose is the volatility, as expressed by the distillation curve. In this paper, we present advanced distillation curve measurements of three prototype alternative turbine fuels and compare the distillation curve, composition, and combustion enthalpy to those of the petroleumderived turbine fuels JP-8 and JP-10. We studied a hydrotreated fuel derived from chicken fat, a fuel composed of hydrogenated pinene dimers derived from turpentine, and a gas−liquid fuel produced from natural gas via the Fischer−Tropsch process. We found that the distillation curves of the chicken-fat-derived fuel and the gas−liquid turbine fuel were similar to those of JP-8, deviating the most at high distillate volume fractions. The chicken-fat-derived fuel deviated by at most 17 °C from the distillation curve of JP-8, and the gas−liquid turbine fuel deviated by at most 36 °C. The hydrotreated turpentine dimer fuel was much less volatile than JP-10, a fuel with which the turpentine dimer fuel shares some structural similarities. The shape of the distillation curves of these two fuels was similar, however. The major components of all fuels were determined, and these were used to calculate the enthalpy of combustion for several distillate volume fractions of the alternative turbine fuels. The gas−liquid turbine fuel was most similar to petroleum-derived fuel in its energy content, and the turpentine dimer fuel had a high volumetric enthalpy of combustion value similar to that of JP-10.
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INTRODUCTION The development of alternative aviation turbine fuels is of interest because of local and global environmental considerations, the potential for supply disruptions, and rising fuel prices. Turbine fuels produced from non-petroleum sources are under investigation by many research groups, with current efforts focusing on fuels produced from natural gas via the Fischer− Tropsch process and fuels derived from renewable, sustainable feedstocks. For alternative turbine fuels to enhance or replace petroleum-derived fuels, it is desirable that they be fungible and meet current requirements for fuels, such as ASTM D16551 and ATSM D7566.2 For this to occur, the alternative turbine fuel and the petroleum-derived fuel should have compatible properties (such as densities, viscosities, cloud point and cold-flow properties, and combustion enthalpies). A particularly important property for fuel replacement design and proper engine performance is fuel volatility.1,3 Indeed, the volatility of any complex fluid is a critical specification that is always provided for design purposes. The volatility is crucial for engine operation and design, and unlike many fuel properties, it is very sensitive to compositional variability. Moreover, this property, if measured properly, can be related to fundamental theory and the development of an equation of state. The availability of such an equation for a complex fluid permits calculation of numerous properties that are used in simulations required for design and even operational properties that describe fuel levels and flow rates.4−9 Moreover, such properties are important for the processing of feedstocks, from crude oils and resourced oils to renewable, biomass-based oils.10−13 Volatility is best measured by the distillation (or boiling) curve, a graphical depiction of the boiling temperature of a fluid or fluid mixture plotted against the volume fraction distilled. This article not subject to U.S. Copyright. Published 2012 by the American Chemical Society
The distillation curve is one of the most important and informative properties that can be measured for a complex fluid mixture, providing the only practical avenue to assess vapor− liquid equilibrium of a complex fluid. The standard test method, ASTM D86, provides the usual approach to measurement of the distillation curve, yielding an initial and final boiling point and temperatures at intermediate distillate fractions.14 This method has several major drawbacks, however, including large uncertainties in measured temperatures and little theoretical significance.15 The advanced distillation curve (ADC) method, a technique that has been described in detail previously,16−18 provides (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) consistency with a century of historical data, and (4) an assessment of the energy content of each distillate fraction.15,19−22 In this paper, we apply the ADC method to compare samples of prototype turbine fuels produced from different feedstocks. Three turbine fuels were studied: a gas−liquid fuel synthesized from natural gas in a Fischer−Tropsch process using an iron catalyst (GTL), a fuel derived from hydroprocessed chicken fat (HCF), and a hydrogenated fuel derived from the dimerized components of crude turpentine (TDF).23 We compare the ADC results of these three alternative turbine fuels to those of the petroleum-derived turbine fuels that they are intended to replace or supplement (JP-8 in the case of the HCF and GTL fuels and Received: October 16, 2012 Revised: November 23, 2012 Published: December 3, 2012 294
dx.doi.org/10.1021/ef3016848 | Energy Fuels 2013, 27, 294−302
Energy & Fuels
Article
the proper locations to monitor Tk, the temperature of the fluid in the kettle, and Th, the temperature of the vapor at the bottom of the takeoff position in the distillation head. Enclosure heating was then commenced with a three-step program based on a previously measured distillation curve. Volume measurements were made in a level-stabilized receiver, and sample aliquots were collected at the sampling adapter hammock. The uncertainty in the volume measurement that was used to obtain the distillate volume fraction was 0.05 mL in each case. The typical experimental atmospheric pressure for the ADC measurements presented herein was approximately 83 kPa. The expanded uncertainty in the individual pressure measurements is 0.003 kPa. Because the measurements of the distillation curves were performed at ambient atmospheric pressure at 1650 m above sea level, temperature readings (Tk and Th) were adjusted for what should be obtained at standard atmospheric pressure (1 atm = 101.325 kPa) for all data presented in this paper. This adjustment was performed with the modified Sydney Young equation, in which the (dimensionless) constant term was assigned a value of 0.000 109,31−34 corresponding to a n-alkane carbon chain of 12.35 This constant was determined by use of experimental data.15 Some of the fuels studied here may be better represented by a hydrocarbon chain length longer than 12; however, no data are available to produce an improved correlation. We chose to use the constant term associated with n-dodecane rather than use a predicted value for the constant term (for a larger chain), so that we are consistent with previous measurements of petroleum-derived turbine fuel. We note, however, that, for a carbon chain of 18, the predicted constant would be 0.000 095. Use of this value (in place of that of ndodecane) would lower the presented temperatures uniformly by 1.2 °C, and the shapes of the distillation curves would be unchanged. For the measurements presented here, a typical temperature adjustment was approximately 8 °C. The measured temperature data can be obtained using the Sydney Young equation. To provide the composition channel information to accompany the temperature data grid, sample aliquots were withdrawn for selected distillate volume fractions. To accomplish this, aliquots of approximately 7 μL of emergent fluid were withdrawn from the sampling hammock in the receiver adapter with a blunt-tipped chromatographic syringe and added to a crimp-sealed vial containing a known mass (approximately 1 mL) of n-hexane solvent. A sample was withdrawn from the first drop of fluid to emerge from the condenser and then at each of 19 additional volume fractions of distillate (5, 10, 15%, etc.), for a total of 20 sample aliquots. Each distillate volume aliquot underwent two analyses. First, each fraction was subjected to chemical analysis and peak identification by GC−MS. The aliquots were also analyzed with GC−FID with external standards of octane to determine the concentrations of all of the compounds. In both cases, aliquots (1 μL) from the crimp-sealed vials of each sample were injected with an automatic liquid sampler. For both analyses, the temperature program was identical to those described above for the neat turbine fuel analyses. High-purity nitrogen was used as the carrier gas for the GC−FID. After determination of the concentrations of major components, an enthalpy of combustion analysis that has been described in detail previously22 was performed on the distillate fractions.
JP-10 in the case of the TDF fuel). JP-10 is a high-density, high volumetric enthalpy aviation turbine fuel that is essentially a pure component (consisting of 97% exotetrahydrodicyclopentadiene24) and is used primarily in cruise missiles. The TDF fuel possesses high energy density as well, because of the polycyclic structures contained in the components of natural turpentine as well as the dimerized species. Thermophysical and thermochemical comparisons are valuable in determining the applicability of alternative turbine fuels as extenders, blends, or replacements for petroleum-derived turbine fuels. Additionally, the metrology and measurements discussed in this paper are important for developing an equationof-state-based thermodynamic model for alternative fuels.
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EXPERIMENTAL SECTION
n-Hexane used as a solvent in this work was obtained from a commercial supplier and was analyzed with gas chromatography (GC) with flame ionization detection (FID) and/or mass spectrometric (MS) detection.25−27 n-Hexane was injected with a syringe into a split/ splitless injector set with a 50:1 split ratio. The injector was operated at a temperature of 325 °C and a constant head pressure of 69 kPa (10 psig). A 30 m capillary column of 5% phenyl−95% dimethylpolysiloxane, having a thickness of 1 μm, was used with a temperature-ramping program from 50 to 170 °C at a heating rate of 5 °C/min. These analyses revealed the purity to be greater than 99%, and the solvent was used without further purification. The alternative turbine fuels used in this work were obtained from a commercial supplier (the GTL fuel), from National Aeronautics and Space Administration (NASA) Glenn Research Center (the HCF fuel), and from China Lake Naval Air Warfare Center Weapons Division (NAWCWD) (the TDF fuel). The JP-8 and JP-10 fuel samples used for comparison to the alternative fuel samples were both obtained from the Fuels Branch of the Air Force Research Laboratory (AFRL, Wright Patterson Air Force Base). The measurements of JP-8 were previously published,28 and the measurements of JP-10 were published by Bruno et al.24 The JP-10 sample consisted of 96.5% exotetrahydrodicyclopentadiene, 2.5% endotetrahydrodicyclopentadiene, and 1.0% adamantine (all mass percent values).24 The alternative aviation turbine fuel samples were stored in tightly sealed plastic bottles, and care was taken to minimize exposure to the atmosphere to limit oxidation, evaporation of the more volatile components, or uptake of moisture. The samples were not physically or chemically dried. The GTL and TDF fuels were pale yellow in color, and the HCF fuel was clear. The turbine fuel samples were subjected to chemical analysis with GC−MS before the measurement of the distillation curve. The GTL and HCF fuels were analyzed with a 30 m capillary column of 5% phenyl− 95% dimethylpolysiloxane, with a thickness of 1 μm. The TDF fuel components were found to be better separated by a 30 m capillary column of (50% cyanopropyl)-methylpolysiloxane, with a thickness of 1 μm. In all cases, neat fuel samples were injected with a syringe into a split/splitless injector set with a 50:1 split ratio. The injector was operated at a temperature of 325 °C and a constant head pressure of 34.5 kPa (5.00 psig). Different temperature programs were used for the analysis of each fuel to optimize the separation of components. The following temperature programs were used: for the GTL fuel, 40 °C for 4 min, temperature ramping at 10 °C/min to 200 °C, and then maintained at 200 °C for 10 min; for the TDF fuel, 70 °C for 2 min, temperature ramping at 2 °C/min to 150 °C, and then 150 °C for 18 min; and for the HCF fuel, 40 °C for 4 min, temperature ramping at 10 °C/min to 200 °C, and then 200 °C for 20 min. Mass spectra were collected for each peak from 15 to 550 relative molecular mass (RMM) units. Peaks were identified with guidance from the NIST/EPA/NIH mass spectral database and also on the basis of retention indices.25,27 The method and apparatus for ADC measurements have been reviewed in detail elsewhere,15,19−22,29,30 and thus, only a limited description is provided herein. For each distillation curve measurement, 200 mL of each fuel was placed into the boiling flask with a volumetric pipet. Two thermally tempered J-type thermocouples were inserted into
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RESULTS AND DISCUSSION Chemical Composition of Three Alternative Turbine Samples. In parts a and b of Table 1, we present a listing of the major compounds found in the GTL and HCF fuels, respectively. These compounds are listed in order of retention time (RT). In the case of these two fluids, we include only compounds found to be in excess of 1.5% of the total uncorrected area. Use of this area cutoff results in the identification of 35 and 28% of the total uncorrected area in GTL and HCF fuels, respectively. In the past, we have found that neglecting minor (
