Density, Viscosity, Speed of Sound, Surface Tension, and Flash

Chemistry Department, United States Naval Academy, 572M Holloway Road ... (2) Algal HRD has been tested as a 50 % mixture with petroleum F76 diesel in...
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Density, Viscosity, Speed of Sound, Surface Tension, and Flash Point of Binary Mixtures of n‑Hexadecane and 2,2,4,4,6,8,8Heptamethylnonane and of Algal-Based Hydrotreated Renewable Diesel Dianne J. Luning Prak,*,† Paul C. Trulove,† and Jim S. Cowart‡ †

Chemistry Department, United States Naval Academy, 572M Holloway Road Annapolis, Maryland 21402, United States Mechanical Engineering Department, U.S. Naval Academy, 590 Holloway Road Annapolis, Maryland 21402, United States

‡

ABSTRACT: The physical properties of algal hydrotreated renewal diesel (algal HRD) and several binary mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane were measured in this work. The density and viscosity were measured at temperatures ranging from (293.15 to 393.15) K and the pure components values fell within the range of previously reported values. Speed of sound data were measured at temperatures ranging from (293.15 to 323.15) K and ranged from (1170 to 1360) m·s−1. The bulk modulus was calculated from the density and speed of sound data, and its values ranged from (1050 to 1425) MPa. For the pure n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane and their mixtures, the flash point values ranged from (367 to 408) K and the surface tension values ranged from (24.1 to 27.3) mN·m−1. The values of viscosity and flash point (360 K) for the algal HRD did not fall within the range of values for the mixtures of n-hexadecane and 2,2,4,4,6,8,8heptamethylnonane, but the values of density, speed of sound, bulk modulus, and surface tension (26.0 mN·m−1) did fall within the range of values measured for the mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane. These data are important for future modeling of these renewable fuels.

■

INTRODUCTION The development and testing of alternative fuels have become a focus of industrialized nations that are striving to reduce their dependence on foreign oil sources. One alternative fuel of particular interest is hydrotreated renewable diesel derived from algae (algal HRD). Algal HRD is produced by extracting the lipids from algae and reacting the triglycerides with hydrogen over a catalyst to remove the oxygen and to saturate the double bonds.1 This process produces a mixture containing predominantly linear and branched chained hydrocarbons.2 Algal HRD has been tested as a 50 % mixture with petroleum F76 diesel in warships.3−5 In addition to warship testing, laboratory engine testing of pure algal HRD has been conducted6,7 and numerical modeling efforts strive to understand the combustion process of these fuels. As with many fuels, modeling and combustion efforts can be simplified if a surrogate mixture can be found that matches the algal fuel’s properties. One potential surrogate that contains both a linear and a branched hydrocarbon is a mixture of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane. Successful use of a surrogate in modeling efforts requires physical property information. In this work, the density, viscosity, surface tension, speed of sound, and flash point were measured for binary mixtures of nhexadecane and 2,2,4,4,6,8,8-heptamethylnonane. These properties are important for the modeling the droplet formation and This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society

combustion of fuels in an engine. Density and viscosity values of mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane have been reported at 298.15 K.8 The current study expands the temperature range for the density and viscosity measurements of these mixtures and also measures speed of sound, surface tension, and flashpoint. From the speed of sound and density measurements, bulk modulus was calculated. These measurements are compared with those of algal HRD to determine if any of these mixtures could potentially serve as a surrogate for algal HRD in combustion and numerical modeling experiments. The development of surrogate mixtures using physical properties has been conducted for aviation fuels, coal-derived liquid fuel, and petroleum diesel fuel.9−11

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MATERIALS AND METHODS Mixtures of n-hexadecane (Aldrich, 0.99 mol fraction purity) and 2,2,4,4,6,8,8-heptamethyl nonane (Aldrich, 0.98 mol fraction purity) were prepared using pipettes at 293.15 K. Chemicals were used as received from the supplier (Table 1). The mole fraction was determined using volume, density, and molar mass. The error in mole fraction of 2,2,4,4,6,8,8Received: October 24, 2012 Accepted: March 13, 2013 Published: March 21, 2013 920

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manufacturer. Duplicate samples of each individual liquid or liquid mixture were measured at seven temperatures between (293.15 and 323.15) K, and these duplicates were used to determine the precision of the measurement. A Kruss DS100 axisymmetric drop shape analyzer was used to measure the surface tension of the algal HRD, the organic liquids, and their mixtures. In this analyzer, a droplet of the organic phase is formed in the air. The system takes an image of the droplet, enlarges it, and analyzes the droplet shape by fitting it with the Young-LaPlace equation using the densities of the organic phase and air.13,14 Over 15 measurements were taken for three droplets of each liquid. The flash point of the samples was measured using a Setaflash Series 8 closed cup flash point tester models 82000-0 (Stanhope-Seta) under temperature ramping setting. Per the manufacturer’s literature, this flash point model conforms to ASTM D3828 (gas ignition option), ASTM D1655 (gas ignition option), ASTM D3278, ASTM D7236, and ASTM E502.

Table 1. Sample Information chemical name

source

mole fraction purity

analysis methoda

n-hexadecane 2,2,4,4,6,8,8heptamethylnonane

Aldrich Aldrich

> 0.99 > 0.98

GC GC

a

Gas−liquid chromatography.

heptamethylnonane was determined by propagating the error in volume and density using methods described by Skoog et al. (2004)12 and was determined to be less than 0.001, except for the lowest mole fraction of 2,2,4,4,6,8,8-heptamethylnonane, which had an error of 0.003. The mole fraction error was more sensitive to errors in volume than in errors in density. The algal-based HRD (lot number 12011-04302-000) was provided by Naval Fuels and Lubricants Cross Function Team at Patuxent River, Maryland (PAX River). The algae-based HRD was produced by Solazyme and refined by Honeywell UOP. Solazyme uses proprietary algae grown in environmentally controlled reaction vessels. Honeywell UOP utilizes a two stage process that first removes the heteroatoms and saturates the double bonds using a propriety catalyst and then isomerizes and selectively cracks the resulting products to increase the distribution of components. An SVM 3000 Stabinger Viscometer (Anton Paar) was used to measure the density and viscosity of the algal HRD, the organic liquids, and their mixtures, while a DSA 5000 Density and Sound Analyzer (Anton Paar) was used to measure the speed of sound and also density. The accuracy of the SVM 3000 Stabinger Viscometer was tested with a certified viscosity reference standard (Standard S3, Cannon Instrument Company) and recleaned and retested if the density deviated by more than 0.1 % from the reference values between (806 and 862) kg·m−3 and if the viscosity deviated by more than 1 % from the reference values between (4.673 and 1.159) mm2·s−1. Duplicate samples of each individual liquid or liquid mixture were measured at five temperatures between (293.15 and 373.15) K, and these duplicates were used to determine the precision of the measurement. The DSA 5000 analyzer was checked before use with degassed distilled water and recalibrated if it failed the check, as specified by the

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RESULTS Density. The density values for the n-hexadecane, 2,2,4,4,6,8,8-heptamethylnonane, and their mixtures are given in Table 2 for both measurement systems as a function of the mole fraction of 2,2,4,4,6,8,8-heptamethylnonane (x1). The measured values using the DSA 5000 for n-hexadecane fall within the range of values reported by researchers at various temperatures as shown in Figure 1. The value at 298.15 K, 770.2 ± 0.2 kg·m−3, falls within the range of 13 values summarized by Fermeglia and Torriano8 including 770.7,15 770.90,16 769.81,17 770.08,8 and 770.30 kg·m−3.18 The density values of n-hexadecane from the SM3000 can be interpolated to 298.15 K to give 770.1 ± 0.3 kg·m−3, which also falls within the range of values summarized by Fermeglia and Torriano.8 The measured density value for 2,2,4,4,6,8,8-heptamethylnonane at 298.15 K using the DSA 5000 analyzer, 781.2 ± 0.2 kg·m−3, agrees with the value of 781.23 kg·m−3 reported by Fermeglia and Torriano.8 The interpolated value for the density of 2,2,4,4,6,8,8-heptamethylnonane at 298.15 K using the SM3000 analyzer, 781.1 ± 0.3 kg·m−3, also agrees with the value reported by Fermeglia and Torriano.8

Table 2. Experimental Values of Density ρ (kg·m−3) for Algal HRD and the System of 2,2,4,4,6,8,8-Heptamethylnonane (1) in n-Hexadecane (2) from T = (293 to 374) Ka x1 T/K

device

293.15

DSA 5000 SVM 3000 DSA 5000 DSA 5000 DSA 5000 DSA 5000 SVM 3000 DSA 5000 DSA 5000 SVM 3000 SVM 3000 SVM 3000

298.15 303.15 308.15 313.15 318.15 323.15 333.15 353.15 373.15

Algal HRD

0.000

0.2023 ± 0.0006

± 1.3

773.6 773.6 770.2 766.7 763.3 759.8 759.7 756.3 752.9 745.8 731.8 718.0

775.5 775.5 772.1 768.7 765.3 761.9 761.7 758.5 755.1 748.0 734.1 720.3

776.1 776.6 772.8 769.5 766.2 762.8 762.7 759.4 756.0 748.8 734.8 720.8

± ± ± ±

1.0 1.0 1.0 0.7

± 0.5 ± 0.3

0.3532 ± 0.0002 777.0 777.1 773.7 770.3 766.9 763.5 763.5 760.1 756.7 749.9 736.1 722.4

± 0.4 ± ± ± ±

0.4 0.4 0.4 0.5

± 0.5 ± 0.5

0.5035 ± 0.0004 778.8 778.7 775.5 772.1 768.7 765.4 765.2 762.0 758.3 ± 1.3 751.6 738 724.3

0.6348 ± 0.0002 780.2 780.4 776.8 773.4 770.1 766.7 767.0 763.3 759.9 753.4 739.7 726.1

± 0.9 ± ± ± ±

0.9 0.9 1.0 1.0

± 1.0 ± 1.0

0.8146 ± 0.0005 782.2 782.1 779.0 775.7 772.3 769.0 768.7 765.6 762.4 754.2 741.5 728.0

± 0.5

± 0.3 ± 0.3 ± 0.5

1.0000 784.6 784.5 781.2 777.8 774.5 771.1 771.0 767.8 764.4 757.4 743.8 730.3

a

x1 is the mole fraction of 2,2,4,4,6,8,8-heptamethylnonane in the (2,2,4,4,6,8,8-heptamethylnonane + n-hexadecane) mixture. Standard uncertainties u are u(T) = 0.01 K, u(x1) ≤ 0.0006, and combined expanded uncertainties Uc are Uc(ρ) = 0.3 kg·m−3 for the SVM 3000 and Uc(ρ) ≤ 0.2 kg·m−3 for the DSA 5000 unless otherwise indicated (level of confidence = 0.95, k ≈ 2). 921

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Table 3. Parameters for eq 1, ρ/kg·m−3 = Ax1 + B, that Correlates Density ρ (kg·m−3) to Mole Fraction of 2,2,4,4,6,8,8-Heptamethyl nonane (x1) in n-Hexadecane at T = (293 to 373) Ka T/K

instrument

293.15

DSA 5000 SVM 3000 DSA 5000 DSA 5000 DSA 5000 DSA 5000 SVM 3000 DSA 5000 DSA 5000 SVM 3000 SVM 3000 SVM 3000

298.15 303.15 308.15 313.15

Figure 1. Density (kg·m−3) of n-hexadecane: ■, this study from DSA5000; □, ref 21; Δ, ref 17; ○, ref 15. Error bars, which are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2), are smaller than symbols.

318.15 323.15 333.15 353.15 373.15

For the mixtures, the density increased linearly as mole fraction of 2,2,4,4,6,8,8-heptamethylnonane increased as shown in Figure 2 for select temperatures. A similar trend was found

a

A 10.9 10.9 11.1 11.2 11.3 11.4 11.4 11.5 11.6 11.3 12.1 12.4

± ± ± ± ± ± ± ± ± ± ± ±

B 0.6 0.6 0.5 0.5 0.4 0.4 0.5 0.4 0.6 1.3 0.5 0.5

773.4 773.4 769.9 766.5 763.1 759.6 759.6 756.2 752.7 745.8 731.8 718.0

± ± ± ± ± ± ± ± ± ± ± ±

0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.3 0.8 0.2 0.3

The error represents the 95 % confidence interval.

has an error of 2 % is in agreement with the data presented herein at the 95 % confidence interval. The viscosity values for each compound or mixture decrease as temperature increases. An empirical equation in the form20 η /mPa·s = A exp(BT −1/K−1)

was used to fit the viscosity data. The fits were good with R2 values greater than 0.99, and the coefficients of eq 2 are given in Table 5. All mixtures tested have the same dependence on temperature with B values around 1860 K. The equation for pure hexadecane predicts a viscosity of 3.06 ± 0.04 mPa·s at 298.15 K, which is in agreement with the literature values of (3.024818 and 3.08118) mPa·s at the same temperature. Figure 4 shows that the viscosity values for hexadecane and those found in the literature at various temperatures. At each temperature, as the mole fraction of 2,2,4,4,6,8,8heptamethylnonane (x1) increases, the viscosity decreases slightly, then levels off around a mole fraction of 0.3, then increases to its highest level at x1 = 1. This trend is similar to the trend found by Fermeglia and Torriano,8 and is shown in Figure 5 for their data at 298.15 K and the data measured herein at 293.15 K. The viscosity values for the algal HRD are also given in Table 4. The viscosity values of the algal HRD are slightly less than those of the 2,2,4,4,6,8,8-heptamethylnonane and hexadecane mixtures at all temperatures tested. In some cases, the level of uncertainty suggests an overlap of the viscosity of the algal HRD with some of the mixtures. The temperature dependence of the fitting parameter B in eq 2 for algal HRD, 1960 K, is larger than that of the mixtures, but the values of both the A and B parameters for the algal HRD fall within the 95 % confidence intervals for the mixtures of nhexadecane and 2,2,4,4,6,8,8-heptamethylnonane as listed in Table 5. Speed of Sound. The speed of sound for the n-hexadecane, 2,2,4,4,6,8,8-heptamethylnonane, and their mixtures are given in Table 6 as a function of the mole fraction of 2,2,4,4,6,8,8heptamethylnonane (x1). The speed of sound decreases with increasing temperature (Table 6) and is linear over the temperature range studied. For n-hexadecane, the speed of sound measurements are the same as those recently measured by Outcalt et al.21 within the 95 % confidence interval (Figure 6). Early measurements were lower and may have been

Figure 2. Density (kg·m−3) of (2,2,4,4,6,8,8 heptamethylnonane (x1) + n-hexadecane) mixtures at ■, 298.15 K, this study; □, 298.15 K, ref 8; ▲, 308.15 K, this study; Δ, 323.15 K, this study; ●, 333.15 K, this study; ○, 353.15 K, this study; ×, 373.15 K, this study. Error bars, which are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2), are smaller than symbols. Lines shown are linear fits to eq 1 with the coefficients in Table 3.

by Fermeglia and Torriano8 whose data are also shown in Figure 2 for 298.15 K. Density and mole fraction data were fit to ρ /kg·m−3 = Ax1 + B

(2)

(1)

In this equation x1 is the mole fraction of 2,2,4,4,6,8,8heptamethylnonane and A and B are fitting parameters. The fitting parameters are given in Table 3. In general A increased slightly from (10.9 to 12.4) kg·m3·x1−1 as temperature increased from (293.15 to 373.15) K. The density values for the algal HRD are also given in Table 2. The variability of the measurements for the fuel is larger than for the mixtures of pure components. The density of the algal HRD falls within the values measured for the 2,2,4,4,6,8,8heptamethylnonane and hexadecane mixtures. Viscosity. The dynamic and kinematic viscosity values for the n-hexadecane, 2,2,4,4,6,8,8-heptamethylnonane, and their mixtures are given in Table 4 as a function of the mole fraction of 2,2,4,4,6,8,8-heptamethylnonane (x1). For 2,2,4,4,6,8,8heptamethylnonane, the values reported by previous researchers are shown in Figure 3. The data from Canet et al.,19 which 922

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Table 4. Experimental Values of Dynamic Viscosity η (mPa·s) and Kinematic Viscosity ν (mm·s−1) for Algal HRD and the System of 2,2,4,4,6,8,8-Heptamethylnonane (1) in N-hexadecane (2) from T = (293 to 373) Ka x1 T/K

Algal HRD

0.000

0.2023 ± 0.0006

0.3532 ± 0.0002

0.5035 ± 0.0004

0.6348 ± 0.0002

0.8146 ± 0.0005

1.0000

3.40 4.37 2.15 2.81 1.47 1.97 1.08 1.47 0.83(7) 1.16

3.46 4.47 ± 0.05 2.23 2.92 1.55 2.08 1.14(6) 1.57 ± 0.05 0.88(6) 1.24

3.42 4.41 2.21 2.90 1.54 2.06 1.13(9) 1.55 0.88(2) 1.22

3.45 ± 0.06 4.43 ± 0.07 2.23 2.92 ± 0.06 1.56 2.07 1.15(2) 1.56 0.89(2) 1.23

3.45 4.42 2.23 2.91 1.55 2.06 1.14(7) 1.55 0.88(7) 1.22

3.51 ± 0.07 4.50 ± 0.08 2.27 2.96 ± 0.06 1.58 2.09 ± 0.05 1.16(5) 1.57 0.90(0) 1.24

3.57 ± 0.05 4.56 ± 0.07 2.30 2.99 ± 0.05 1.60 2.11 1.17(4) 1.58 0.90(5) 1.24

3.70 4.72 2.37 3.08 1.63 2.16 1.19(7) 1.61 0.91(8) 1.26

η/mPa·s ν/mm·s−1 η/mPa·s ν/mm·s−1 η/mPa·s ν/mm·s−1 η/mPa·s ν/mm·s−1 η/mPa·s ν/mm·s−1

293.15 313.15 333.15 353.15 373.15 a

x1 is the mole fraction of 2,2,4,4,6,8,8-heptamethylnonane in the (2,2,4,4,6,8,8-heptamethylnonane + n-hexadecane) mixture. Standard uncertainties u are u(T) = 0.01 K, u(x1) ≤ 0.0006, and combined expanded uncertainties Uc are Uc(η) ≤ 0.04 mPa·s and Uc(ν) ≤ 0.04 mm·s−1 unless otherwise indicated (level of confidence = 0.95, k ≈ 2).

Figure 4. Viscosity (mPa·s) of n-hexadecane ■, this study; □, ref 15; ◊, ref 8; +, ref 17. The error bars are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2).

Figure 3. Viscosity (mPa·s) of 2,2,4,4,6,8,8-heptamethylnonane ■, this study; □, ref 19; ◊, ref 8; +, ref 33. The error bars are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2).

Table 5. Parameters for eq 2, η/mPa·s = A exp(BT‑1/K‑1), that Relates Dynamic Viscosity η of Algal HRD and Mixtures of 2,2,4,4,6,8,8-Heptamethylnonane (x1) in n-Hexadecane to Temperature for T = (293 to 373) Ka 103·A

x1 0.000 0.202 ± 0.003 0.3532 ± 0.0006 0.5035 ± 0.0007 0.6348 ± 0.0003 0.8146 ± 0.0006 1.0000 Algal HRD a

5.9 6.0 6.2 6.0 6.0 5.8 5.4 4.2

(4.1, (4.2, (4.4, (4.2, (4.3, (4.2, (3.9, (3.3,

8.5) 8.6) 8.7) 8.5) 8.4) 8.0) 7.7) 5.3)

R2

B 1860 1850 1850 1860 1860 1880 1900 1960

(1740, (1740, (1740, (1740, (1750, (1770, (1790, (1880,

1980) 1970) 1960) 1960) 1960) 1980) 2020) 12040)

0.998 0.999 0.999 0.999 0.999 0.999 0.999 0.998

Figure 5. Viscosity (mPa·s) of (2,2,4,4,6,8,8 heptamethylnonane (x1) + n-hexadecane) mixtures: □, current study at 293.15 K and ■ from ref 8 at 295.13 K. The error bars are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2).

Values in parentheses represent 95 % confidence interval.

obtained from compounds of a different purity.15,22 Outcalt et al.21 fit a second order polynomial to their speed of sound data (c) for n-hexadecane at atmospheric pressure over the range of (290 to 350) K:

thylnonane in the mixture increases as shown at 293.15 K in Figure 7. The isentropic bulk modulus, K, was calculated at each temperature and ambient pressure from the speed of sound (c) and density (ρ) by23

c /m·s−1 = 2804.158 − 6.077077(T /K) + 0.00038984(T /K)2

(3)

This polynomial successfully predicts the data reported herein for n-hexadecane as shown in Figure 6 as a line. The speed of sound decreases as the percentage of 2,2,4,4,6,8,8-heptame-

K /Pa = (c 2/m 2·s−2)(ρ /kg·m−3) 923

(4)

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Table 6. Experimental Values of Speed of Sound Measurements c (m·s−1), and Calculated Isentropic Bulk Modulus K (MPa) for Algal HRD and the System of 2,2,4,4,6,8,8-Heptamethylnonane (1) in n-Hexadecane (2) from T = (293 K to 323) Ka x1 T/K 293.15 298.15 303.15 318.15 313.15 318.15 323.15

c/m·s−1 K/MPa c/m·s−1 K/MPa c/m·s−1 K/MPa c/m·s−1 K/MPa c/m·s−1 K/MPa c/m·s−1 K/MPa c/m·s−1 K/MPa

Algal HRD

0.0000

0.2023 ± 0.0006

0.3532 ± 0.0002

0.5035 ± 0.0004

0.6348 ± 0.0002

0.8146 ± 0.0005

1.0000

1340.9 1395 ± 1322.2 1351 ± 1303.6 1308 ± 1285.1 1265 ± 1266.8 1224 1248.7 1184 1230.7 1145

1357.0 1425 1338.2 1379 1319.5 1335 1301.1 1292 1282.8 1250 1264.7 1210 1246.8 1170

1341.9 1396 1323.3 1352 1304.9 1309 1286.6 1267 1268.5 1226 1250.5 1186 1232.7 1147

1331.3 1377 1312.8 1333 1294.4 1291 1276.2 1249 1258.1 1208 1240.2 1169 1222.4 1131

1320.5 ± 0.7 1358 1302.0 ± 0.8 1315 1283.6 ± 0.7 1272 1265.4 ± 0.7 1231 1247.3 ± 0.8 1191 1229.4 ± 0.7 1152 1211.7 ± 0.7 1113 ± 2

1310.5 ± 0.9 1340 ± 2 1292.1 ± 0.9 1297 ± 2 1273.7 ± 1.0 1255 ± 2 1255.4 ± 1.0 1214 ± 2 1237.3 ± 0.9 1174 ± 2 1219.4 ± 0.9 1135 ± 2 1201.6 ± 0.9 1097 ± 2

1299.9 1322 1281.2 1279 1262.7 1237 1244.3 1196 1226.2 1156 1208.3 ± 0.7 1118 1190.5 ± 0.7 1080

1285.9 1297 1267.2 1254 1248.6 1213 1230.1 1172 1211.9 1133 1193.9 1094 1176.0 1057

2 2 2 2

a

x1 is the mole fraction of 2,2,4,4,6,8,8-heptamethylnonane in the (2,2,4,4,6,8,8-heptamethylnonane + n-hexadecane) mixture. Standard uncertainties u are u(T) = 0.01 K, u(x1) ≤ 0.0006, and combined expanded uncertainties Uc are Uc(c) ≤ 0.6 m·s−1 and Uc(K) ≤ 1 MPa unless otherwise indicated (level of confidence = 0.95, k ≈ 2). The density values from the DSA 5000 were used to calculate bulk modulus.

values are higher than those of Rolling and Vogt22 measured in 1960, as shown in Figure 8.

Figure 6. Speed of sound (m·s−1) of hexadecane: □, this study; ●, ref 22; Δ, ref 15; ×, ref 21; line is the second-order fit from ref 21. Error bars, which are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2), are smaller than symbols.

Figure 8. Bulk modulus of hexadecane: ○, this study; ■, ref 22; +, ref 21. Error bars, which are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2), are smaller than symbols.

The speed of sound and isentropic bulk modulus for the algal HRD are also given in Table 6. The speed of sound and isentropic bulk modulus of the algal HRD fall within the values measured for the 2,2,4,4,6,8,8-heptamethylnonane and hexadecane mixtures at all temperatures tested. Flash Point and Surface Tension. The surface tension and flash point values for the n-hexadecane, 2,2,4,4,6,8,8heptamethylnonane, and their mixtures are given in Table 7 as a function of the mole fraction of 2,2,4,4,6,8,8-heptamethylnonane (x1). The surface tension values of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane are shown with literature values in Figure 9. A linear regression of surface tension versus temperature data found in the literature24−28 for n-hexadecane (Figure 9) predicts that the value of surface tension at 294.3 K should be 27.4 mN·m−1, which agrees with the data reported herein of 27.3 ± 0.2 mN·m−1. For 2,2,4,4,6,8,8-heptamethylnonane, a linear regression of surface tension versus temperature data found in the literature27 (Figure 9) predicts that the value of surface tension at 294.3 K should be 24.2 mN·m−1, which agrees with the data reported herein of 24.1 ± 0.2 mN·m−1. The flashpoint value of 408 ± 2 K for hexadecane agrees with

Figure 7. Speed of sound (m·s−1) of (2,2,4,4,6,8,8 heptamethylnonane (x1) + n-hexadecane) mixtures at 293.15 K. Error bars, which are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2), are smaller than symbols.

These values are given in Table 6. For n-hexadecane, the bulk modulus values match are the same as those recently measured by Outcalt et al.21 within the 95 % confidence interval. These 924

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Table 7. Experimental Values of Flash Point and Surface Tension for Algal HRD and the System of 2,2,4,4,6,8,8Heptamethylnonane (1) in n-Hexadecane (2)a x1 flash point/K surface tension/mN·m−1

Algal HRD

0.0000

0.2023 ± 0.0006

0.3532 ± 0.0002

0.5035 ± 0.0004

0.6348 ± 0.0002

0.8146 ± 0.0005

1.0000

360 26.0

408 27.3

392 26.4

384 25.9

378 25.4

374 25.1

370 24.7

367 24.1

a

x1 is the mole fraction of 2,2,4,4,6,8,8-heptamethylnonane in the (2,2,4,4,6,8,8-heptamethylnonane + n-hexadecane) mixture. Standard uncertainties u are u(x1) ≤ 0.0006, and combined expanded uncertainties Uc are Uc (flash point) = 2 K and Uc (surface tension) ≤ 0.2 mN·m−1 (level of confidence = 0.95, k ≈ 2). Surface tension measurements were taken at room temperature, 294 ± 1 K.

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CONCLUSION In this work, the physical properties of two component mixture of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane were measured and compared with those values for algal hydrotreated renewable diesel fuel. Many of the measurements of the pure components and mixtures fell within values reported in the literature. Comparison of the data for the algal HRD with those of the mixtures reveals that the values of viscosity and flash point (360 K) for the algal HRD fell below the range of values for the mixtures of n-hexadecane and 2,2,4,4,6,8,8heptamethylnonane. The values of density, speed of sound, bulk modulus, and surface tension (26.0 ± 0.2 mN·m−1) of the algal HRD did fall within the range of values measured for the mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane. Since many of the physical properties are similar between the algal HRD and two-component mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane, one of these mixtures may serve as a reasonable surrogate in numerical modeling efforts for the combustion of algal HRD.

Figure 9. Surface tension of hexadecane: ■, this study; ▲, ref 24; +, ref 25; Δ, ref 26; □, ref 27; ×, ref 28. For 2,2,4,4,6,8,8heptamethylnonane: ●, this study; ○, ref 27. Error bars, which are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2), are smaller than symbols. The lines are linear regressions of the data found in the literature for each compound.

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the reported values of 407 ± 2 K29 and 408 K,30 and the flashpoint value of 367 ± 2 K for 2,2,4,4,6,8,8-heptamethylnonane agrees with the reported values of 368 K31 and 369 K.32 For the mixtures, as the mole fraction of 2,2,4,4,6,8,8heptamethylnonane (x1) increases, the surface tension and flash point decrease (Figure 10). The flash point and surface

AUTHOR INFORMATION

Corresponding Author

*Tel.: (410) 293-6339. Fax: (410) 293-2218. E-mail: prak@ usna.edu. Funding

This work was funded by the Office of Naval Research. Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 10. Flash point, ■, and surface tension, □, of (2,2,4,4,6,8,8heptamethylnonane (x1) + n-hexadecane) mixtures. The error bars are the combined expanded uncertainties with 0.95 level of confidence (k ≈ 2).

tension of the algal HRD are also given in Table 7. The flash point of the algal HRD, 360 K, is lower than the values measured for the 2,2,4,4,6,8,8-heptamethylnonane and nhexadecane mixtures. The surface tension of the algal HRD, 26.0 mN·m−1, falls within the values measured for the 2,2,4,4,6,8,8-heptamethylnonane and hexadecane mixtures. 925

dx.doi.org/10.1021/je301337d | J. Chem. Eng. Data 2013, 58, 920−926

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