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Jun 9, 2015 - variables in play simultaneously) makes it difficult at best to propose strategic changes to engine and fuel design by use of convention...
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Application of the Advanced Distillation Curve Method to Fuels for Advanced Combustion Engine Gasolines Jessica L. Burger,† Nico Schneider,‡ and Thomas J. Bruno*,† †

Applied Chemicals and Materials Division, National Institute of Standards and Technology, Boulder, Colorado 80305, United States Ruhr-Universität Bochum, 44801 Bochum, Germany

‡

S Supporting Information *

ABSTRACT: Incremental but fundamental changes are currently being made to fuel composition and combustion strategies to diversify energy feedstocks, decrease pollution, and increase engine efficiency. The increase in parameter space (by having many variables in play simultaneously) makes it difficult at best to propose strategic changes to engine and fuel design by use of conventional build-and-test methodology. To make changes in the most time- and cost-effective manner, it is imperative that new computational tools and surrogate fuels are developed. Currently, sets of fuels are being characterized by industry groups, such as the Coordinating Research Council (CRC) and other entities, so that researchers in different laboratories have access to fuels with consistent properties. In this work, six gasolines (FACE A, C, F, G, I, and J) are characterized by the advanced distillation curve (ADC) method to determine the composition and enthalpy of combustion in various distillate volume fractions. Tracking the composition and enthalpy of distillate fractions provides valuable information for determining structure property relationships, and moreover, it provides the basis for the development of equations of state that can describe the thermodynamic properties of these complex mixtures and lead to development of surrogate fuels composed of major hydrocarbon classes found in target fuels.

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INTRODUCTION Incremental but fundamental changes are currently being made to fuel composition and combustion strategies, enabling engines to accept diversified energy feedstocks, decrease pollution, and increase efficiency. To make changes in the most time- and cost-effective manner, the design and development of surrogate fuels is of utmost importance to computational studies, experimental design, and thermophysical property modeling.1−3 Surrogate blends with a limited number of components can more readily provide insight into property effects on mixing, vaporization, and combustion, which lead to improvements in engine efficiency, decreased emissions, and enhanced performance.4−9 Indeed, both theoretical and experimental studies of surrogates (frequently blends of components, such as n-heptane and isooctane) have been conducted comparing the properties of proposed surrogates to those of gasoline.10−18 In addition, surrogate fuels have value as time invariant references, by allowing for direct comparisons of results from different laboratories using the same surrogate fuel. This approach avoids the temporal differences in the composition of individual refinery streams, which are blended together to make finished fuels, because commercial fuels and even reference fuels tend to vary over time.3 Despite this variability, one of the best options for researchers looking for fuel consistency is the use of reference fuel sets characterized by industry groups, such as the Coordinating Research Council (CRC) Fuels for Advanced Combustion Engines (FACE) group.19 The FACE group is composed of volunteers from industry, government, and academia, who work to recommend sets of reference fuels. These reference fuels are composed mostly of refinery stream This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society

products and span wide property ranges in chemistry, volatility, and ignition quality for both gasoline- and diesel-based fuels. A matrix of four properties, including research octane number (RON) (70, 85, and 95), octane sensitivity [the RON minus the motor octane number (MON)] (≤2 and ∼10), aromatic content (5 and 35 vol %), and n-paraffin content (5 and 25 vol %), was used to develop these fuels.20 While some of these factors are conflicting and trade-offs had to be made, 10 fuels were developed while maintaining a statistically sound matrix. The matrix was built on the chosen properties because they are of primary importance to the performance of gasoline-fueled engines.20 For example, RON and octane sensitivity are measures of autoignition quality, and aromatic and paraffinic composition are measures of the composition of a fuel; therefore, these parameters determine the fuel vaporization, heat release, and activity of pollutants.21,20 In this work, a subset of six reference gasolines (FACE A, C, F, G, I, and J) were characterized by the advanced distillation curve (ADC) method to determine the composition and enthalpy in various distillate volume fractions, providing the basis for thermophysical property modeling.22−24 Tracking the composition and enthalpy of distillate fractions provides valuable information for elucidating structure property relationships, and moreover, this approach provides the basis for the development of equations of state.25,26 Received: April 8, 2015 Revised: June 9, 2015

A

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relaxation delay). A sweep width of 12 019.23 Hz (from −4 to 16 ppm) was used. After 64 scans, the spectra had signal-to-noise ratios of about 1 × 104. Quantitative 13C spectra were obtained by use of inverse-gated waltz-16 proton decoupling and a relaxation agent (see above). An acquisition time of 0.909 s, a relaxation delay of 10.0 s, and a sweep width of 36 057.69 Hz (from −20 to 220 ppm) were used. After 512 scans, the spectra had signal-to-noise ratios of about 2000. The effectiveness of these parameters for producing quantitative 13C spectra was verified previously by collecting spectra for three test compounds under the same conditions.43 For each neat fuel, we also obtained 13C DEPT-90 and 13C DEPT135 spectra. For the distortionless enhancement by polarization transfer (DEPT) experiments, a coupling constant (JC−H) of 140 Hz was used, as recommended for hydrocarbon fuels with both aromatic and aliphatic components.44 A sweep width of 29 761.9 Hz (from −10 to 190 ppm) was used. Other acquisition parameters for the DEPT experiments included an acquisition time of 1.10 s, a relaxation delay of 2.0 s, and a total of 1024 scans. The DEPT spectra were used to determine the number of hydrogen atoms bonded to each type of carbon; that is, they were used to identify 13C peaks and not to quantitate the different types of carbon.

EXPERIMENTAL SECTION

Materials. The gasolines used in this work were obtained from a non-commercial collaborator. The fuels were stored in tightly sealed containers at room temperature. No phase separation was observed as a result of the storage conditions. These fuels were used without any purification or modification. The composition of neat FACE gasolines was investigated using a gas chromatographic (GC) method (30 m capillary column of 5% phenyl/95% dimethyl polysiloxane having a thickness of 1 μm, with the column temperature held at 40 °C for 3 min, then increased at 1 °C/min to 45 °C, and finally increased at 15 °C/min to 300 °C) with both flame ionization detection (FID) and mass spectrometric (MS) detection in separate analyses. Samples were injected with a syringe into a split/splitless injector set with a 100:1 split ratio. The injector was operated at a temperature of 325 °C and with a constant head pressure of 55.2 kPa (8 psig). 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.27,28 ADC Measurements and Sampling. Previous papers have described the ADC apparatus and procedure in detail;29−31 therefore, only a short explanation is provided below. For each distillation curve measurement, 200 mL each of FACE gasoline was placed into a boiling flask at atmospheric pressure. To prevent the loss of volatile compounds and distillate, thermally stable high-vacuum grease was used on all glass connections. The thermocouples were positioned to record (a) the temperature of the fluid in the kettle (Tk) and (b) the temperature of the vapor at the bottom of the takeoff position in the distillation head (Th). In terms of significance, Tk is a thermodynamically consistent bubble point temperature, while Th approximates what might be obtained from the classical distillation measurement procedure.32−35 Enclosure heating was then commenced with a model-predictive temperature controller.36 The heating program used leads the distillation curve by approximately 20 °C but had a similar shape. Volume measurements were made in a level-stabilized receiver, and sample aliquots were collected at the receiver adapter hammock.30 Because distillation curves were measured at ambient atmospheric pressure (measured with an electronic barometer), temperature readings were adjusted using the modified Sydney Young equation for what would be obtained at standard atmospheric pressure (1 atm = 101.325 kPa). For this equation, a constant term corresponding to a carbon chain of eight was used (0.000 119).37−39 The composition of each distillate volume fraction of fuel was also studied by a GC method (30 m capillary column of 5% phenyl/95% dimethyl polysiloxane, with a thickness of 0.25 μm) with MS detection and FID.40 The GC−MS analysis of all samples was performed with helium carrier gas at 55.2 kPa (8 psig), and column temperature programming was given above. MS was used with the aid of the NIST/ EPA mass spectral database following column separation to provide compositional information by identification of peaks in the resulting chromatogram.27,14 Analysis of the Neat Fuels by Nuclear Magnetic Resonance (NMR) Spectroscopy. A commercial 600 MHz NMR spectrometer with a cryoprobe, operated at 150.9 MHz for 13C, was used to obtain quantitative 1H and 13C spectra.41 Samples for 1H NMR spectroscopy were prepared by dissolving 10 μL of the fuel sample in 1 mL of acetone-d6; this NMR solvent contained 0.05% of the chemical shift reference tetramethylsilane (TMS). Samples for 13C NMR spectroscopy were prepared by mixing 0.5 mL of the fuel with 0.5 mL of chloroform-d; this NMR solvent contained 1.5% by mass (0.06 M) of the relaxation agent chromium(III) acetylacetonate [Cr(acac)3]. Therefore, the final concentration of Cr(acac)3 in the NMR sample was 0.03 M, which is comparable to concentrations conventionally used.42 The samples were maintained at 25 °C for all of the NMR measurements. 1H NMR spectra were referenced to the TMS peak at 0.0 ppm, and 13C NMR spectra were referenced to the solvent peak at 77.0 ppm. Quantitative 1H NMR spectra were obtained with a 30° flip angle and a long interpulse delay (10.0 s acquisition time and 10.0 s

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RESULTS AND DISCUSSION Initial Boiling Temperatures. Careful observation of each sample in the distillation flask during initial heating allowed us Table 1. Summary of the Average Observed Boiling Behavior of the FACE Fuelsa observed boiling behavior (°C)

FACE A 84.2 kPa

FACE C 82.6 kPa

FACE F 83.6 kPa

sustained vapor rise observed boiling behavior (°C)

52.7 56.0 FACE G 83.5 kPa

54.7 60.1 FACE I 83.4 kPa

55.6 62.0 FACE J 83.1 kPa

sustained vapor rise

52.7 58.6

56.8 63.0

57.2 64.2

a

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

to determine the onset of boiling for each fuel (reported with Tk). To establish initial boiling behavior, we recorded the onset of bubbling, the temperature at which bubbling is sustained, and the temperature at which the vapor rises into the distillation head. It has previously been demonstrated that this last temperature is the initial boiling temperature (IBT) (that is, an approximation of the bubble point temperature at ambient pressure) of the FACE gasoline.25,26 This measurement is noteworthy because it can be modeled with an equation of state and is the only point at which the temperature, pressure, and liquid composition are known. Vapor rise is accompanied by a sharp increase in Th, and is, therefore, far less subjective to ascertain than the onset of bubbling and sustained bubbling and, thus, the least uncertain of the initial temperatures observed. Experience with previous mixtures, including n-alkane standard mixtures that were prepared gravimetrically, indicates that the uncertainty in the onset of the bubbling temperature is approximately 3 °C and the uncertainty in the vapor rise temperature is approximately 0.3 °C.32 In Table 1, we present the initial temperature observations for the fuel samples. B

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Table 2. Representative Distillation Curve Data (Given as the Average of Three Distillation Curves) for FACE Fuels. The uncertainties are discussed in the text. These temperatures have been adjusted to 1 atm with the modified Sydney Young equation; the average experimental atmospheric pressures are provided to allow recovery of the actual measure temperatures FACE A 84.2 kPa distillate volume fraction (%) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Tk (°C)

FACE C 82.6 kPa

FACE F 83.6 kPa

Th (°C)

Tk (°C)

Th (°C)

Tk (°C)

Th (°C)

69.0 62.2 75.4 70.1 80.0 76.2 85.3 81.9 89.3 86.3 92.4 89.9 94.6 92.4 96.5 94.7 97.9 96.4 99.0 98.3 100.0 99.9 100.8 101.3 101.6 102.7 102.4 104.9 103.2 106.9 103.7 108.8 105.1 111.1 FACE G 83.5 kPa

68.7 72.6 76.7 80.6 84.1 88.0 91.5 94.5 97.4 100.2 102.0 104.1 107.0 109.5 112.2 116.0 122.3

63.4 69.0 73.5 77.7 81.7 86.0 89.8 93.2 96.2 99.0 101.3 103.8 105.7 107.8 110.6 113.8 118.1

69.2 72.5 74.9 78.0 80.9 84.1 87.3 90.4 93.5 96.8 99.8 102.8 105.8 108.5 111.2 114.4 118.3

65.0 69.0 71.6 75.5 78.6 82.0 85.4 88.6 91.8 94.9 98.1 101.1 103.8 106.7 109.4 112.6 116.7

FACE I 83.4 kPa

Figure 1. Distillation curves for gasolines as measured by the ADC method. The uncertainties are discussed in the text. The hash marks on the y axis are IBT (vapor rise temperature).

FACE J 83.1 kPa

distillate volume fraction (%)

Tk (°C)

Th (°C)

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

70.6 76.8 83.2 90.2 97.1 105.2 113.4 121.0 129.7 138.0 145.0 153.4 160.3 166.0 170.6 174.4 177.7

59.3 66.0 72.3 79.9 84.9 93.0 100.8 107.9 116.0 126.3 133.5 145.1 153.7 161.8 167.8 172.5 176.3

78.9 82.9 85.6 87.4 88.9 90.2 91.2 92.3 93.2 94.2 95.0 95.8 96.8 97.7 98.5 99.6 102.0

72.4 79.0 83.0 85.0 87.0 88.5 90.0 91.6 93.2 95.3 96.9 98.4 101.1 103.0 104.8 106.7 111.0

98.9 106.5 109.4 111.3 112.9 114.5 116.4 118.4 121.2 124.3 128.2 133.4 140.0 146.5 153.1 158.3 163.9

84.2 100.5 104.0 106.2 107.9 110.0 111.8 114.0 116.7 119.5 122.2 127.3 133.5 141.3 150.5 157.6 166.2

Figure 2. Energy content, presented as the composite enthalpy of combustion, −ΔHc (kJ/mol), as a function of the distillate volume fraction for FACE fuels. The uncertainties are discussed in the text. Lines are drawn to guide the eyes of the viewer and do not represent a fit.

Figure 3. Energy content, presented as the composite enthalpy of combustion, −ΔHc (kJ/mL), as a function of the distillate volume fraction for FACE fuels. The uncertainties are discussed in the text. Lines are drawn to guide the eyes of the viewer and do not represent a fit.

Distillation Curves. The temperatures at both the kettle position and the head position were recorded throughout the measurement of the distillation curves (Tk and Th, respectively) at set distillate volume fractions. The ambient atmospheric pressure was also logged, and the modified Sydney Young equation was used to adjust the temperatures to their atmospheric pressure equivalent. The uncertainty in temperature measurements, Tk, was approximately 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. Average kettle and head temperatures as well as the average measured atmospheric pressure are reported at each distillate volume fraction for FACE fuels in Table 2. These data are also

represented graphically in Figure 1. The IBT is indicated as a hatch mark on the temperature axis. The Th and Tk distillation curves were compared, and azeotropic behavior was not observed for any of the gasolines.45 The gasolines analyzed had many components with very similar boiling points, and it was necessary to slow the enclosure heating from the rate that was more commonly used for similar fluids. This caused the (well-understood) radiative heat transfer from the glass C

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(242.3) (248.4) (240.2) (247.7) (232.6) (233.7) 4845.5 4968.4 4804.8 4953.4 4651.2 4674.6 (240.0) (244.1) (234.1) (245.1) (229.0) (228.2)

The uncertainties are discussed in the text and are provided in parentheses. a

70

4800.1 4881.4 4682.5 4902.1 4579.1 4565.0 (237.9) (236.1) (224.6) (238.4) (226.8) (227.1)

60

4757.2 4721.9 4492.7 4767.3 4536.8 4541.4 4668.8 (233.4) 4560.1 (228.0) 4279.6(214.0) 4586.0 (229.3) 4464.1 (223.2) 4517.5 (225.9)

50 40

4597.6 (229.9) 4343.8 (217.2) 4065 (203.3) 4180.1 (209.0) 4415.5 (220.8) 4504.2 (225.2) (219.1) (203.6) (192.8) (189.0) (213.8) (224.0)

30

4381.1 4072.2 3856.2 3780.8 4276.5 4480.7 (201.3) (193.7) (184.7) (175.5) (210.9) (221.5) 4025.5 3874.8 3693.7 3509.2 4218.0 4430.5

20 10

3371.3 (168.6) 3688.1(184.4) 3531.4 (176.6) 3296.1 (164.8) 3965.4 (198.3) 4034.7 (201.7) (179.8) (180.1) (167.7) (162.0) (187.8) (174.0)

0.03

3595.9 3688.1 3354.8 3240.5 3755.0 3480.3 A C F G I J FACE FACE FACE FACE FACE FACE

apparatus components to the Th thermocouple. Therefore, Th was frequently very similar to Tk, and in some cases, the temperature is higher than Tk. Despite this, Th for the distillation curves generally fall in the ranges observed when the gasolines were characterized by ASTM D86. Additionally, the distillation curves of Tk match the shapes of the distillation curves determined by ASTM D86. Differences in the measurements given by the ADC method and ASTM D86 have been discussed in prior work.22−24 It should be noted again that differences between ASTM D86 and Th may be due to differences in insulation during distillation, as discussed above, and differences in altitude between laboratories where distillation curves were measured. While the shapes of the distillation curves are similar, the ADC method provides thermophysically consistent measurements that cannot be obtained from the ASTM D86 method. Distillate Fraction Composition and Energy Content. We were able to sample and analyze the individual fractions of condensed vapor as they appeared from the condenser, as stated above in the Experimental Section. Following the analytical procedure described in the Experimental Section, 7 μL samples were collected in autosampler vials containing a known mass of n-tetradecane. Chemical analyses of each fraction were performed by GC−MS and GC−FID. Calibration for GC−FID was performed by the external standard method, in which four solutions of known concentrations of octane (prepared gravimetrically) were also prepared in n-tetradecane. The chromatographic parameters were the same as stated in the Experimental Section. For each fluid, the mass spectrometer was operated in scan mode and baseline resolution was achieved. As we have demonstrated previously, it is possible to add thermochemical information to the distillation curve when the composition channel of data is used to provide quantitative analysis on specific distillate fractions.30,34,35 This is performed by calculating a composite enthalpy of combustion based on the enthalpy of combustion of individual (pure) components of a distillate fraction and the measured mole fractions of those components. The major components of the neat fuels are listed in Table S1 of the Supporting Information and agree with previous GC−FID data (because of the use of GC−FID, the

distillate volume fraction (%)

Figure 4. Energy content, presented as the composite enthalpy of combustion, −ΔHc (kJ/g), as a function of the distillate volume fraction for FACE fuels. The uncertainties are discussed in the text. Lines are drawn to guide the eyes of the viewer and do not represent a fit.

composite enthalpy of combustion (kJ/mol)

Table 3. Energy Content, Presented as the Composite Enthalpy of Combustion, −ΔHc (kJ/mol), as a Function of the Distillate Fraction for FACE Fuelsa

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Table 4. Energy Content on a Mass Basis, Presented as the Composite Enthalpy of Combustion, −ΔHc (kJ/g), as a Function of the Distillate Fraction for FACE Fuelsa composite enthalpy of combustion (kJ/g) distillate volume fraction (%) FACE FACE FACE FACE FACE FACE a

A C F G I J

0.03 44.9 44.8 44.5 44.6 44.8 45.2

(2.2) (2.2) (2.2) (2.2) (2.2) (2.3)

10 45.1 44.8 44.5 44.6 44.7 44.6

(2.3) (2.2) (2.2) (2.2) (2.2) (2.2)

20 44.7 44.7 44.4 44.3 44.5 44.1

30

(2.2) (2.2) (2.2) (2.2) (2.2) (2.2)

44.6 44.7 44.3 44.1 44.5 44.0

40

(2.2) (2.2) (2.2) (2.2) (2.2) (2.2)

44.5 44.5 44.2 43.8 44.4 43.8

(2.2) (2.2) (2.2) (2.2) (2.2) (2.2)

50 44.5 44.5 44.2 43.5 44.4 43.7

60

(2.2) (2.2) (2.2) (2.2) (2.2) (2.2)

44.4 44.4 44.1 42.9 44.4 43.4

70

(2.2) (2.2) (2.2) (2.1) (2.2) (2.2)

44.4 44.3 44.0 41.8 44.4 42.7

80

(2.2) (2.2) (2.2) (2.1) (2.2) (2.1)

44.4 44.2 43.9 41.2 44.4 41.9

(2.2) (2.2) (2.2) (2.1) (2.2) (2.1)

The uncertainties are discussed in the text and are provided in parentheses.

Table 5. Energy Content on a Volume Basis, Presented as the Composite Enthalpy of Combustion, −ΔHc (kJ/mL), as a Function of the Distillate Fraction for FACE Fuelsa composite enthalpy of combustion (kJ/mL) distillate volume fraction (%) FACE FACE FACE FACE FACE FACE a

A C F G I J

0.03 28.5 28.6 29.6 29.4 28.9 28.1

(1.4) (1.4) (1.5) (1.5) (1.4) (1.4)

10 34.7 28.7 29.9 29.5 29.4 29.7

(1.4) (1.4) (1.5) (1.5) (1.5) (1.5)

20 33.8 29.1 30.3 30.3 29.9 30.9

30

(1.5) (1.5) (1.5) (1.5) (1.5) (1.5)

34.2 29.0 30.6 30.9 30.0 31.2

40

(1.5) (1.4) (1.5) (1.5) (1.5) (1.6)

35.7 30.1 30.9 31.6 30.3 31.5

(1.5) (1.5) (1.5) (1.6) (1.5) (1.6)

50 35.7 30.5 31.1 32.5 30.5 31.7

60

(1.5) (1.5) (1.6) (1.6) (1.5) (1.6)

35.0 30.9 31.3 33.6 30.6 32.1

70

(1.5) (1.5) (1.6) (1.7) (1.5) (1.6)

35.3 31.3 31.5 35.3 30.7 32.9

80

(1.6) (1.6) (1.6) (1.8) (1.5) (1.6)

35.9 31.5 31.9 36.1 30.8 34.7

(1.6) (1.6) (1.6) (1.8) (1.5) (1.7)

The uncertainties are discussed in the text and are provided in parentheses.

Table 6. Table of the Hydrocarbon Family Types Resulting from the ASTM D2789 Analysis Performed on the Neat Samples of FACE Fuels sample FACE FACE FACE FACE FACE FACE

A C F G I J

paraffins (%)

monocycloparaffins (%)

dicycloparaffins (%)

alkylbenzenes (%)

indanes and tetralins (%)

naphthalene (%)

76.6 72.4 59.9 28.6 71.6 38.0

21.5 25.1 33.3 20.5 25.5 13.8

0.7 0.7 1.0 1.8 0.8 0.5

1.1 1.5 5.1 45.5 2.1 44.6

0.0 0.3 0.5 3.5 0.0 3.2

0.0 0.0 0.2 0.1 0.0 0.0

the distillate fraction as the concentration of heavier paraffins and aromatics increases in the later distillate fractions and roughly corresponds to the variation in distillation curves reported above of the gasoline samples. As shown in Tables 4 and 5, the enthalpies by mass and volume for all distillate cuts and fuels show similar behavior because of the similar densities of all distillate fractions; however, FACE A has a somewhat higher enthalpy at distillate fractions in the middle of the distillation curve because of a very slight shift toward denser compounds in these cuts. Hydrocarbon Classification. The analysis of distillate composition may be further enhanced by the use of a mass spectrometric classification technique, similar to ASTM D2789, which gives the percentage of the sample found in various hydrocarbon family types51,52 The ASTM D2789 analysis, developed specifically for low-olefinic gasoline, categorizes the hydrocarbon family types by ion fragments. Consequently, a direct, quantitative comparison to other classification methods is not always possible. A more subtle difficulty is that, with the ASTM D2789 method, only characteristic ion fragments are used to classify hydrocarbon type, meaning that, if ion peaks are produced that are not part of the classification system, the fragments are not included in the calculations. In addition, a molecule may produce multiple characteristic ions and contribute to more than one hydrocarbon type. In addition, this method significantly overestimates cycloparaffins. This has

area percent is very close to the percent concentration of the neat fuel).20,46 The enthalpy of combustion of the individual (pure) components is taken from a reliable database compilation.47 Uncertainty in this calculation has been attributed to a number of sources,30,34,35 including (1) the neglect of the enthalpy of mixing, (2) the uncertainty in the individual (pure component) enthalpy of combustion as tabulated in the database, (3) the uncertainty in the measured mole fraction, (4) the uncertainty posed by very closely related isomers that cannot be resolved by the analytical protocol, (5) the uncertainty introduced by neglecting components present at very low concentrations (that is, uncertainty associated with the chosen area cutoff), (6) the uncertainty introduced by a complete misidentification of a component, (7) the uncertainty in quantitation introduced by eluting peaks that are poorly resolved, and (8) the uncertainty introduced when experimental data for the pure component enthalpy of combustion are unavailable (and the Cardozo equivalent chain model must be used).47 On the basis of the uncertainty sources listed above and the samples being investigated, a 5% uncertainty is claimed for the molar enthalpy calculations reported in this work.48−50 Figures 2−4 show the enthalpy of combustion as a function of the distillate fraction for each of the gasoline samples (provided in multiple units for completeness). The enthalpies with their uncertainties are provided in Tables 3−5. The molar enthalpy of combustion (Table 3 and Figure 2) increases with E

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Figure 5. Plots of the hydrocarbon family types resulting from the moiety family analysis performed on FACE fuels. The uncertainty is discussed in the text.

been noticed previously in test solutions containing only nparaffins. The procedures, uncertainty, and potential difficulties of this method have been discussed previously.34 This analysis was applied to the FACE gasolines, even though some samples

contain olefins (in particular, FACE F and I). Table 6 gives the percentages of the neat fuel found in each hydrocarbon family type. The sum of the paraffin fraction (paraffins + cycloparaffins) and the aromatic fraction generally agrees with what F

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Table 7. Comparison of the Integral Values for 1H Spectral Regions for All of the Neat FACE Fuels ν (ppm)

proton type

10.7−7.4 7.4−6.2

polyaromatic CH monoaromatic CH olefinic CH olefinic CH2 α-to-aromatic CH2 α-to-aromatic CH3 paraffinic CH2 paraffinic CH3

6.2−5.1 5.1−4.3 4.3−2.4d 2.4−2.1 2.0−1.02 1.02−0.2

FACE A (mol % 1H)

FACE C (mol % 1H)

FACE Fa (mol % 1H)

FACE G (mol % 1H)

FACE I (mol % 1H)

FACE J (mol % 1H)

relative uncertaintyb (%)

c 0.22

c 2.14

c 2.80

0.30 9.71

c 0.52

0.04 9.66

4.1 1.9

c c 1.63

c c 1.24

0.63 1.25 1.49

0.86 0.34 4.34

0.40 0.74 1.35

0.07 c 3.72

4.4 2.7 24.1

0.25

3.72

3.64

15.27

0.45

13.82

1.9

27.33 70.57

20.57 72.33

29.35 59.48

31.93 37.26

35.66 60.89

37.46 35.22

1.2 1.6

a

These are the averaged values from four separate spectra of the same lot of fuel. bThis is the relative combined standard uncertainty in the integral values. cThe integral value was ≤0.02%, which is the detection limit. dThe water signal at ∼2.8 ppm was excluded from the integral.

Table 8. Comparison of the Integral Values for 13C Spectral Regions for All of the Neat FACE Fuels ν (ppm)

carbon type

170−131.2 131.2−115.5 115.5−100 70.0−45.0 45.0−32.7 32.7−30.8 30.8−28.5

quaternary aromatic aromatic CH olefin paraffinic CH paraffinic CH and CH2 chain γ-CH2, β to aromatic CH2 chain δ-CH2, α to aromatic naphthenes, aromatic attached ethyl CH2 cycloparaffin CH2 chain β-CH2, α to ring CH3 α to ring CH3 aromatic-attached ethyl CH3 chain α-CH3 branched-chain CH3

28.5−25.0 25.0−21.9 21.9−17.6 17.6−14.7 14.7−12.3 12.3−0.0

FACE A (mol % 13C)

FACE C (mol % 13C)

FACE Fa (mol % 13C)

FACE G (mol % 13C)

FACE I (mol % 13C)

FACE J (mol % 13C)

relative uncertaintyb (%)

0.07 0.28 c 4.05 8.42 6.49 22.73

1.17 1.65 c 4.29 8.23 6.94 18.80

3.47 5.15 1.47 3.14 7.68 5.60 16.78

11.69 18.65 0.03 0.08 0.87 6.61 11.89

1.17 0.82 1.02 1.67 12.81 7.48 15.25

9.53 18.15 c c 7.64 6.55 8.07

15.3 10.2 25.4 19.1 4.5 2.2 2.1

10.65 16.27 14.24 2.70 7.46 6.62

9.39 19.95 11.43 3.35 10.33 4.46

21.23 10.99 12.36 2.53 4.66 4.94

12.95 9.77 15.31 2.83 5.78 3.52

8.09 19.73 13.00 1.99 10.91 6.06

3.43 14.72 13.62 1.51 13.45 3.33

2.1 3.3 4.2 3.8 2.4 6.4

a These are the averaged values from three separate spectra of the same lot of fuel. bThis is the relative combined standard uncertainty in the integral values. cThe integral value was ≤0.02%, which is the detection limit.

has been reported previously for FACE fuels.20 Figure 5 shows the changes in the percentage of the hydrocarbon family through the distillations of FACE fuels. As expected, the percentage of aromatics increases through the distillation in FACE samples where aromatics were present, and a high concentration of alkylbenzenes, as present in FACE G and J, shifts the distillation curve to higher temperatures in the later distillate fractions. Analysis of the Neat Fuels by NMR Spectroscopy. The relative amounts of various types of hydrogen and carbon were determined by the integration of spectral regions following literature methods.46,53 Some minor changes in reported integral regions were made for jet fuel samples, as reported previously, and the same changes were used here.43 Three sources of uncertainty were considered for the peak integrals reported herein: incomplete relaxation and residual nuclear Overhauser effects (NOEs), which are significant only for the 13C NMR spectra, repeatability in the distillation, and baseline drift. The magnitude of the influence of these uncertainties has been assessed previously.43 The estimated values for these sources of uncertainty were added in quadrature to arrive at the combined standard uncertainties that are reported.

Table 7 shows a comparison of the integral values for 1H NMR spectral regions for all of the neat FACE fuels. The integrals in Table 1 have not been corrected for the relative number of hydrogen atoms per carbon. That is, the integrals in Table 1 still reflect the fact that a paraffinic CH3 group has 3 times the signal intensity that an aromatic CH will have. Another caveat for the data in Table 1 is the obvious lack of a category for paraffinic CH, because peaks for this type of proton are not well separated from the other spectral regions. From the 13C DEPT spectra, it is clear that paraffinic CH exists in all of the FACE fuels. The CH peaks are expected to be mostly subsumed into the large paraffinic CH2 integral,28 where their relative effect is minimized. They do, however, contribute significantly to the cycloparaffin region, causing the amount of cycloparaffins to be overestimated. Table 8 shows a comparison of the integral values for the 13C NMR spectral regions for all of the neat FACE fuels. The repeatability of the 13C NMR measurement is not as good as the 1H measurement, because baseline drift is more important for the 13C NMR spectra (which have relatively low signal-tonoise ratios), and there is some uncertainty because of relaxation and residual NOEs. Consequently, the integral values for the 13C NMR spectral regions have larger uncertainties. G

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Notes

CONCLUSION The objective of this study was to characterize a subset of FACE gasolines by the ADC method to determine composition and enthalpy of the neat fuels and distillate fractions. Volatility studies of complex fluids provide detailed insight into the nature of fluids because volatility changes markedly with composition compared to other thermophysical properties. These volatility measurements provided the basis for the development of equations of state and the development of surrogate fuels. Thermophysical property measurements are required to develop blends of a limited number of compounds that can adequately approximate the composition, ignition quality, and volatility characteristics of real-world fuels containing hundreds to thousands of compounds. On the basis of the above results, it can be concluded that the gasolines studied here are mainly composed of light hydrocarbon compounds (all lighter than decane), with the greatest percentage of their composition being n-paraffins and isoparaffins. While the ratios of paraffins and isoparaffins varies (as did the inclusion of other components, such as aromatics, olefins, and cycloparaffins), the similarity in compositions lead to gasolines with vapor-rise temperatures that were less than 10 °C apart. In addition, we see distillation curves that are very close to each other in temperature but with distinct shapes that correspond to the formulation of each FACE gasoline in the matrix and are dependent upon the concentrations of components, such as olefins and aromatics. The shape of the distillation curve is instructive because the front end (lowtemperature region) of the distillation curve of gasoline (up to approximately 70 °C) is used to assess and optimize ease of starting and the potential for hot weather vapor lock in engines. The midrange of the gasoline curve (up to a temperature of approximately 100 °C) is used to assess and optimize cold weather performance, the operational readiness of a hot engine, and the acceleration behavior of a hot engine under load. In addition, the top range of the distillation curve is used to assess and optimize fuel economy in a hot engine.54,55 The molar enthalpy of combustion increases with the distillate fraction as the concentration of longer (C8 and C9) paraffins and aromatics (when present in the neat fuel) increases in the later distillate fractions and roughly corresponds to the variation in distillation curves. The enthalpies by mass and volume for all distillate cuts and fuels are very similar. A comparison of a number of gasoline samples allows us to better understand properties of fuel blends, which can lead to a more efficient development of equations of state. Future work will use the data presented here and additional data to determine compositional and energetic variability to develop a methodology to model the fuel blends and surrogates.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Jessica L. Burger acknowledges the PREP postdoctoral associateship program support for research performed at National Institute of Standards and Technology (NIST), Boulder, CO, for this work. Nico Schneider acknowledges the Ruhr University Research School PLUS, funded by Germany’s Excellence Initiative (DFG GSC 98/3) for support for this research. The collaboration of King Abdullah University of Science and Technology Clean Combustion Research Center for arranging access to the samples is gratefully acknowledged.

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ASSOCIATED CONTENT

* Supporting Information S

Listing of major components found in neat samples of FACE fuels by GC−MS and GC−FID (Table S1) (PDF). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b00749.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: 303-497-5158. Fax: 303-497-6682. E-mail: [email protected]. H

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