Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Density and Speed of Sound Measurements of Surrogate Diesel Fuels Tara J. Fortin*
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National Institute of Standards and Technology, Material Measurement Laboratory, Applied Chemicals and Materials Division, 325 Broadway, Boulder, Colorado 80305-3328, United States ABSTRACT: The densities and speeds of sound of four surrogate diesel fuels of varying compositional accuracy were measured at atmospheric pressure over the combined temperature range 278.15−343.15 K. Measurement results for the surrogate fuels were compared to results obtained for the target fuel as well as to predicted values. For both density and sound speed, the least compositionally accurate four-component surrogate was the most dissimilar to the target fuel, with average absolute deviations (AADs) of 3.3 and 2.1%, respectively, while the most compositionally accurate ninecomponent surrogate was the most similar, with AADs of 0.8 and 0.4%, respectively. However, the relative behavior of the other two surrogates implies that greater complexity is not always required in a surrogate; the minimal number of components required for a given surrogate fuel will ultimately be determined by the specific research goals it is meant to achieve.
1. INTRODUCTION The desire for energy stability, in terms of both fuel costs and sources, in addition to concerns about the environmental impact of emissions, are driving simultaneous advancements in both engine design and commercial fuels. The computational co-optimization of new engine technologies and evolving fuels promises significant savings in terms of both cost and time over traditional build-and-test methodologies.1 Since the composition of a typical commercial fuel is far too complex to model exactly, the development of surrogate fuels has been the focus of much fuel-related research.2−24 A surrogate fuel is a simplified mixture, made up of a select number of pure compounds (typically 10 or fewer) that are selected to capture select properties and performance characteristics of a target fuel. The surrogate fuels studied in this work were developed as part of a large coordinated research effort under the auspices of the Coordinating Research Council (CRC) Advanced Vehicles, Fuels, and Lubricants (AVFL) technical committee. The initial phase of research focused on the development of a novel approach for the formulation of a surrogate fuel that would be representative of real-world diesel fuels. The approach is described only briefly here since it has been previously described in detail elsewhere.25 First, a target fuel was identified. Initially, two fuels were chosen, but only one of those was retained as the target in subsequent research after it was deemed to be more representative of a typical North American commercial diesel fuel. That fuel was a grade no. 2-D ultralow sulfur (S15) diesel emissions certification fuel, and it is referred to herein as CFA. Next, four fuel properties (i.e., composition, ignition quality, volatility, and density) were selected in an effort to match the vaporization, mixing, and combustion processes of the target fuel. Surrogate palette compounds were This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
then selected. This selection process requires balancing the desire for compounds that are actually found in commercial diesel fuel with the need to consider their cost and available purity, as well as their ability to be accurately modeled. Each surrogate fuel’s composition was determined by optimizing the aforementioned properties via use of a thermophysical property regression model developed at the National Institute of Standards and Technology (NIST).26−31 The resulting recipe was then used to blend together the pure palette compounds to formulate the surrogate fuel. Surrogates formulated during the initial phase performed well in tests, validating the overall methodology.25 Still, subsequent work sought to further improve the match to target fuels, as well as to better understand the minimal compositional accuracy required for a surrogate fuel to adequately emulate the behavior of its target fuel.32 Consequently, four surrogate fuels of increasing compositional accuracy were formulated and characterized.32 The first, designated V1, was an eight-component surrogate fuel formulated during the initial study. Using this as a baseline, three additional surrogate fuels were developed. Two lower-accuracy, lower-cost surrogates, designated V0a and V0b, contained four and five components, respectively, and one, higher-accuracy surrogate, designated V2, contained nine components. These four surrogates were the fuels measured in this work. Previously published work largely focused on the characterization of these four surrogate fuels in terms of the four selected target-fuel properties, particularly composition and Received: March 29, 2018 Accepted: August 15, 2018
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DOI: 10.1021/acs.jced.8b00264 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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volatility.25,32,33 Because fuel properties are often interdependent,34 a properly formulated surrogate fuel should be able to accurately represent target-fuel properties besides those that were specifically selected for replication.25 The work described herein tests that assumption by extending the temperature range of density comparisons and by introducing a previously unreported property, speed of sound. Along with the isentropic bulk modulus and viscosity, the speed of sound of a fuel is critical to an engine’s injection timing, with implications for engine performance and NOx emissions.35−38 In this work, we report measurements of density and speed of sound at atmospheric pressure and over the range 278.15−343.15 K for the four above-described surrogate diesel fuels, as well as the target CFA. Adiabatic compressibilities (i.e., the inverse of the bulk modulus) have also been calculated and are reported here for completeness.
samples, the initial compositions of the blended surrogate fuels were also verified using GCxGC-FID and the results reported by Mueller et al.32 Upon receipt of the surrogate fuels, their compositions were verified using gas chromatography (GC) with a combination of mass spectrometry (MS) and FID. The GC-MS and GC-FID analyses results33 were consistent with the GCxGC-FID results32 and are summarized in Table 2. Table 2. Composition of Surrogate Fuels compound V0a 2,2,4,4,6,8,8-heptamethylnonane n-hexadecane 1-methylnaphthalene trans-decahydronaphthalene V0b33 2,2,4,4,6,8,8-heptamethylnonane n-octadecane 1,2,3,4-tetrahydronaphthalene 1-methylnaphthalene 1,2,4-trimethylbenzene V133 2,2,4,4,6,8,8-heptamethylnonane n-octadecane 1-methylnaphthalene 1,2,3,4-tetrahydronaphthalene 1,2,4-trimethylbenzene trans-decahydronaphthalene n-butylcyclohexane n-hexadecane V233 1,3,5-triisopropylbenzene n-octadecane n-butylcyclohexane 1,3,5-triisopropylcyclohexane 1,2,3,4-tetrahydronaphthalene 1-methylnaphthalene 2-methylheptadecane perhydrophenanthrene n-eicosane
2. MATERIALS AND METHODS 2.1. Fuel Samples. The five samples measured in this work were provided to the participants of projects AVFL-18 and AVFL-18a of the CRC Fuels for Advanced Combustion Engines (FACE) working group.25,32 The four surrogate fuel samples were prepared gravimetrically from palette compounds following the completion of purity, sulfur, and derived cetane number (DCN) analyses, silica-gel treatment (SGT), and the introduction of antioxidant (AO) and lubricity improver (LI) additives.32 Additional details concerning the preparation of the four diesel fuel surrogates can be found in the literature.25,32 Previously reported information regarding commercial suppliers and stated purities for all source chemicals has been summarized in Table 1 for reference; additional details can be found in Mueller et al.32 Table 1. Chemical Information for Surrogate Fuel Components compound
suppliera
purityb
n-butylcyclohexane trans-decahydronaphthalene n-eicosane 2,2,4,4,6,8,8-heptamethylnonane n-hexadecane 2-methylheptadecane 1-methylnaphthalene n-octadecane perhydrophenanthrene 1,2,3,4-tetrahydronaphthalene 1,3,5-triisopropylbenzene 1,3,5-triisopropylcyclohexane 1,2,4-trimethylbenzene
TCI America TCI America Sigma-Aldrich Fisher Scientific Fisher Scientific Eastern Sources Eastern Sources Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Sigma-Aldrich Fisher Scientific
>0.99 >0.98 0.99 0.98 0.99 ≥0.98 ≥0.98 0.99 ≥0.98 0.99 ≥0.98 ≥0.98 0.98
CAS no.
xa
4390-04-9 544-76-3 90-12-0 493-02-7
42.0 32.2 15.3 10.5
4390-04-9 593-45-3 119-64-2 90-12-0 95-63-6
32.8 32.1 14.7 12.3 8.1
4390-04-9 593-45-3 90-12-0 119-64-2 95-63-6 493-02-7 1678-93-9 544-76-3
35.1 27.3 10.9 10.8 4.8 4.1 3.8 3.2
717-74-8 593-45-3 1678-93-9 34387-60-5 119-64-2 90-12-0 1560-89-0 5743-97-5 112-95-8
16.6 15.2 14.8 12.8 12.0 10.8 10.2 6.4 1.2
33
a
Percent mass fraction.
Surrogate fuels V0a and V0b both contain compounds that have been employed in previous studies using simple surrogates, but the addition of n-octadecane to V0b greatly improves that fuel’s heavy-end distillation behavior relative to that of the target fuel.32 As was previously mentioned, surrogate fuel V1 was developed as part of an earlier study using the same target fuel.25 Surrogate fuel V2 contains five new compounds to better match the compositional characteristics of CFA.32 Alternatively, the surrogate fuel samples can be compared according to their hydrocarbon classifications. Burger et al.33 previously reported the results of a classification method based on ASTM method D-278939 where hydrocarbons are classified as paraffins (P), monocycloparaffins (MCP), dicycloparaffins (DCP), alkylbenzenes (AB), indanes and tetralins (I&T), or naphthalenes (N) based on their mass spectral fragmentation patterns. Although this method is specifically intended for low olefinic gasoline and has significant limitations and associated uncertainties,40 it can still be an effective tool for exploring general trends and differences among related fluids. Additionally,
a
In order to describe materials and experimental procedures adequately, it is occasionally necessary to identify commercial products by manufacturers’ names or labels. In no instance does such identification imply endorsement by the National Institute of Standards and Technology, nor does it imply that the particular product or equipment is necessarily the best available for the purpose. b Stated purity in mass fraction.
The composition of the target fuel was previously determined using two-dimensional gas chromatography with flame ionization detection (GCxGC-FID); the results have been reported by Mueller et al.32 Predictably, CFA is a highly complex mixture containing on the order of 5000 individual compounds and isomers.32 Prior to the distribution of the B
DOI: 10.1021/acs.jced.8b00264 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 1. Schematic representation of the hydrocarbon classification of surrogate fuel samples based on GCxGG-FID analysis.32 Samples have been classified into eight families: normal paraffins (NP), branched paraffins (BP), monocycloparaffins (MCP), dicycloparaffins (DCP), polycycloparaffins (PCP), alkylbenzenes (AB), indanes and tetralins (I&T), and naphthalenes (N). Numbers shown represent the measured mass fractions for each of the eight families.
Mueller et al.32 previously reported results of a more accurate and detailed hydrocarbon classification using GCxGC-FID; those results have been summarized schematically in Figure 1. With this method, paraffins have been further separated into normal paraffins (NP) and branched paraffins (BP), and cycloparaffins containing more than two rings (i.e., polycycloparaffins (PCP)) have been separately quantified. Both methods effectively illustrate the differences among the four surrogate fuel samples. For example, Figure 1 clearly shows that the surrogate V0a is the only one of the four lacking any significant amount of alkylbenzenes or indanes and tetralins, while V0b is the only surrogate that does not contain any cycloparaffins. 2.2. Experimental Methods. Density and sound speed were simultaneously measured over the combined temperature range from 278.15 to 343.15 K and at ambient pressure (∼83 kPa in Boulder, CO) using a commercial analyzer. The speed of sound (w) is determined by measuring the time required for ∼3 MHz sound pulses to travel between two transducers located on the opposite sides of the sound speed cell. The density (ρ) is derived from the resonant frequency of a samplefilled U-shaped tube as it vibrates in an electromagnetic field. In this instrument, the vibrating tube is constructed of borosilicate glass. Both measurement cells are housed in a thermostated copper block whose temperature is controlled between 278.15 and 374.15 K via the use of thermoelectric Peltier elements and an integrated Pt-100 resistance thermometer. The apparatus constants in the instrument’s working equations
are determined by means of an adjustment procedure where air and water are measured at 293.15, 313.15, and 333.15 K. In between adjustment procedures, the instrument’s performance is regularly verified by measuring water and toluene standard reference material (SRM) 211d every 5 K from 343.15 to 278.15 K. In this work, verification measurements resulted in average absolute deviations (AADs) from reference values of ≤0.006% and ≤0.05% for density and speed of sound, respectively. Additional details about the density and sound speed analyzer can be found in Fortin et al.41 and Laesecke et al.42 For measurements, approximately 3 mL of sample liquid were injected into the instrument using a disposable syringe and a programmed scan was performed from 373.15 K to the minimum temperature in 5 K increments. For V0a and CFA, the minimum temperature was 278.15 K. For V0b, V1, and V2, the formation of crystals of solidified fuel made it impossible to perform measurements below 283.15 K. It should be noted that, for all but V2, these observations are consistent with the cloud points reported by Mueller et al.32 for these samples. Prior to injection, each sample was preheated to help expel any entrained gases and avoid gas bubble formation in the measurement cells at higher temperatures. At least four measurement scans were performed for each of the fuels with a fresh aliquot of sample fluid injected into the instrument prior to the start of each scan. To avoid cross-contamination, the measurement cells were thoroughly cleaned and dried in between each fuel sample. Finally, it should be noted that two identical instruments were C
DOI: 10.1021/acs.jced.8b00264 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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Table 3. Measured Densities (ρ̅) and Expanded Uncertainties (U(ρ̅)) for Five Fuel Samples at Ambient Pressurea V0a T (K) 343.15 338.15 333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15 288.15 283.15 278.15 T (K) 343.15 338.15 333.15 328.15 323.15 318.15 313.15 308.15 303.15 298.15 293.15 288.15 283.15 278.15
ρ̅ (kg·m−3) 783.21 786.70 790.19 793.67 797.15 800.62 804.08 807.54 811.00 814.45 817.90 821.34 824.78 828.20
t95b 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.036 −3
ρ̅ (kg·m ) 816.46 820.15 823.83 827.50 831.17 834.83 838.49 842.15 845.81 849.45 853.10 856.75 860.40
V0b U(ρ̅) (kg·m−3) 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 V2 t95b 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037
ρ̅ (kg·m−3) 802.06 805.62 809.16 812.70 816.24 819.77 823.29 826.82 830.34 833.85 837.36 840.87
t95b 2.035 2.035 2.035 2.035 2.035 2.035 2.035 2.035 2.035 2.035 2.035 2.035
V1 U(ρ̅) (kg·m−3) 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60
ρ̅ (kg·m−3) 793.60 797.14 800.67 804.20 807.72 811.24 814.75 818.25 821.76 825.25 828.74 832.21
t95b 2.037 2.037 2.037 2.037 2.036 2.037 2.037 2.037 2.037 2.036 2.036 2.036
U(ρ̅) (kg·m−3) 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60
CFA −3
U(ρ̅) (kg·m ) 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60
−3
ρ̅ (kg·m ) 810.06 813.65 817.23 820.81 824.38 827.95 831.52 835.08 838.64 842.19 845.75 849.30 852.86 856.41
t95b 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037 2.037
U(ρ̅) (kg·m−3) 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60 0.60
The ambient pressure during measurements was ∼83 kPa. bThe coverage factor from the t-distribution for each corresponding degree of freedom and a 95% level of confidence.
a
fuels range from an overall minimum of 783.21 kg·m−3 to an overall maximum of 860.40 kg·m−3, with an overall spread of approximately 4%. The observed trend in densities for the surrogate fuels is V0a < V1 < V0b < V2. Also included in Figure 2 are the measured densities for CFA, which range from 810.06 to 856.41 kg·m−3, placing the target fuel between V0b and V2. The top half of Figure 3 more clearly shows the deviations between the four surrogate fuels and the target fuel, plotted here as percent deviation as a function of temperature. The measured densities for all four surrogates are within approximately 3% of CFA, exceeding the stated goal of matching the target fuel density within 5%.32 V0a exhibits the largest deviations with an average absolute deviation (AAD) of 3.3%, followed by V1 and V0b, with AADs of 2.0 and 1.0%, respectively. V2 has densities that most closely match CFA, with an AAD of 0.8%. This close match between V2 and CFA is actually expected; one of the selection criteria for the five new compounds found in the V2 palette was that they have more representative densities.32 In contrast, the significantly better agreement observed for V0b relative to that observed for V0a is perhaps initially surprising given that the two fuels differ by only a single component. However, upon closer inspection, V0b contains three different compounds, including two new classes of compounds, relative to V0a (Table 2). Specifically, the n-hexadecane in V0a has been replaced with a longer-chain normal paraffin (NP), n-octadecane, and the dicycloparaffin
run in tandem for this work; V0a and V1 were measured in one instrument and V0b, V2, and CFA were measured in the other.
3. RESULTS AND DISCUSSION 3.1. Density. Density measurement results for the four surrogate fuels and CFA are presented in Table 3. Tabulated densities are an average (ρ̅) of results from four to five separate temperature scans. Expanded uncertainty estimates (U(ρ̅)) are calculated according to the expression U (ρ ̅ ) = t 95(dfρ ) ·u(ρ ̅ )
(1)
where t95(dfρ) is the coverage factor taken from the t-distribution for dfρ degrees of freedom and 95% confidence level43 and u(ρ̅) is the combined standard uncertainty for the density measurements. The combined standard uncertainty is estimated using a comprehensive approach analogous to one described in detail in previous publications.42,44 It includes the standard deviation of the average, as well as contributions from the instrument resolution and uncertainties in instrument calibration, temperature, and pressure. The corresponding values of t95(dfρ) are included in Table 3 for clarity. For this work, the resulting expanded uncertainties are 0.6 kg·m−3, corresponding to relative expanded uncertainties of approximately 0.07−0.08%. The density results reported in Table 3 are plotted as a function of temperature in Figure 2. Densities for the four surrogate D
DOI: 10.1021/acs.jced.8b00264 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
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exhibited the closest match (within
