Fuel Heat of Vaporization Values Measured with Vacuum

Apr 8, 2014 - The fuel heat of vaporization (ΔHvap) can be an important factor for internal combustion engine performance. Heat is required to evapor...
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Fuel Heat of Vaporization Values Measured with Vacuum Thermogravimetric Analysis Method Guangci Zhou, Stephen Roby,* Tao Wei, and Nay Yee Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94801, United States ABSTRACT: A fuel heat of vaporization (ΔHvap) is a fundamental thermodynamic property and an important parameter for internal combustion engine design. The ΔHvap value is measured with a newly developed vacuum thermogravimetric analysis (VTGA) method. The technique is applicable to jet and diesel range fuels. The measured values for the selected components in the fuels agree with the literature values within a range of ±5%. VTGA is a quick, cost-effective, and accurate method to measure the ΔHvap of renewable fuels. The ΔHvap values for binary mixtures of fuels can be simply estimated from the ΔHvap values and mix ratio of the parents. boiling point are often accurate to within a few percent.8,9 Tables of enthalpies of vaporization have been published, summarizing the available data but largely for pure compounds.9,10 Unfortunately, commercial fuels are far from pure compounds, often containing hundreds of components, many with unknown structures. The complexity of conventional petroleum-derived or renewable jet and diesel fuels makes modeling prohibitively complex. Widegren and Bruno11 developed an experimental method capable of accurately measuring heats of vaporization for complex mixtures, such as biodiesel [fatty acid methyl esters (FAMEs) derived from vegetable or animal fats]. Their gas saturation method passes a carrier gas through a series of temperature-controlled “saturators” impregnated with the liquid of interest. The carrier gas enters a tared adsorber, where the components are trapped. The carrier gas continues on into the next adsorber, and the process is repeated. After sufficient time has passed to ensure adequate vaporization, the flow is stopped, the adsorbers are removed, and the contents are analyzed by gas chromatography or gas chromatography− mass spectrometry. When the total volume of carrier gas used, recovered mass of each volatile component, and molar mass of each component are measured, the vapor pressures can be calculated. Widegren and Bruno used up to 18 separate collectors per experiment, all in a temperature-controlled environment. While their method returned accurate data, it was slow, taking up to 34 days per experiment, not including the analysis time. Thermogravimetric analysis (TGA) monitors the chemical and physical changes of a sample when heated under controlled conditions. In the TGA experiment, a few milligrams of material are placed in a sample boat and suspended from a microbalance inside an oven. The temperature is raised or lowered at a carefully controlled rate. The heating rate can be a simple linear change with time or can be quite complex with several step changes. The weight loss of the sample is plotted as

1. INTRODUCTION The fuel heat of vaporization (ΔHvap) can be an important factor for internal combustion engine performance. Heat is required to evaporate the fuel. In spark-ignited engines, cold intake air, particularly when combined with a cold engine start, will not support complete evaporation of fuels with high heats of vaporization, such as methanol. At low temperatures and high relative humidity, ice may form in the carburetor. The icing prevents proper engine operation.1 In compression-ignited (diesel) engines, fuel volatility can impact engine startup, engine warm-up time, and engine smoke.2,3 Tsujimura et al. found that increasing the volatility of a diesel fuel can reduce soot emissions and widen the operating range of the engine for ultralow emissions.4 Allen et al. have suggested that vapor pressure and boiling point, both related to the heat of vaporization of a liquid, could become important quality control properties of biofuels for compression-ignited engines.5 Modeling of combustion processes has become popular in recent years. Several software packages are designed to model the chemical kinetics and physical transport phenomena needed to sustain efficient combustion of fuels in internal combustion engines. The heat of vaporization of the fuel is one factor in the modeling. Allen et al. have noted that critical temperature and enthalpy of vaporization data for biodiesel components is limited. These data can be used in calculating the temperature reduction in the combustion reaction zone, where hot engine gases supply the necessary heat to vaporize a fuel before ignition.6 The heat of vaporization of a liquid can be determined by several methods, each with its own advantages and difficulties.7−9 Chickos et al.7 reported on a method to estimate heats of vaporization for hydrocarbons and simple derivatives. Their assumption was that the heat of vaporization is a group property related to the total of structural factors, including the number of quaternary carbons. Common functional groups were assigned contributions, which added to the total heat of vaporization. While estimates by Chickos et al. were reasonable, the method required detailed structural information not generally available for commercial fuels. Dependent upon the model used, the estimated heats of vaporization at or near the © 2014 American Chemical Society

Received: December 17, 2013 Revised: April 7, 2014 Published: April 8, 2014 3138

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Table 1. Chemical Composition of Selected Vegetable Oils and Animal Fats for FAMEa oil or fat

reference

canola coconut Jatropha palm soybean tallow (beef) boiling point of methyl ester (°C)

15 15 16 15 15 15 15

lauric (%, C12:0)

myristic (%, C14:0)

44−51

13−18.5 0.6−2.4

266

3−6 295

palmitic (%, C16:0)

stearic (%, C18:0)

oleic (%, C18:1)

linoleic (%, C18:2)

4.5 7.5−10.5 14.7 32−46.3 2.3−11 25−37 415

1−2 1−3 6.9 4−6.3 2.4−6 14−29 442

55−63 5−8.2 42.4 37−53 22−30.8 26−50 218

20−31 1−2.6 35.2 6−12 49−53 1−2.5 215

lenolenic (%, C18:3) 9−10

2−10.5 109

a

C12:0 equates to a 12 carbon long chain with no carbon−carbon double bonds. C18:3 indicates an 18 carbon long chain with 3 carbon−carbon double bonds.

Figure 1. (a) TGA weight loss curve of 1-nonanol at 760 Torr. (b) TGA weight loss curves of 1-nonanol at reduced pressures.

Clausius−Clapeyron equation relates the boiling point of a liquid to its heat of vaporization. The derivation of the Clausius−Clapeyron equation from thermodynamic principles is stated in Physical Chemistry.6 The equation is usually written as

a function of either the temperature or time. The atmosphere inside the oven can be inert (for example, nitrogen) or oxidative (for example, oxygen or air). In vacuum thermogravimetric analysis (VTGA), the procedure is conducted at reduced pressure and often in a non-reactive atmosphere, thus suppressing undesirable side reactions. VTGA has been applied to development of specialty lubricants and greases for earth satellites. In this case, VTGA measured the volatility of the lubricant and lubricant additives to ensure that both stayed where desired and did not physically change in the harsh high-vacuum environment.12 In other applications, Ashby et al. used high-vacuum TGA to study the gas evolved from air-sensitive materials,13 while Huang and associates developed a VTGA method to evaluate the distillation profiles of heavy crude oils.14 In the experiments described here, the thermogravimetric analyzer is equipped with a vacuum pump and a large ballast tank to operate at reduced pressures. The ballast tank provides sufficient system volume that small variations in the efficiency of the vacuum pump are insignificant. The reduced pressure allows the system to measure the boiling point of liquid fuels to a fraction of an atmosphere. Therefore, a new VTGA method was developed for measuring the heat of vaporization of real fuels, including jet and diesel range traditional petroleum fuels, renewable fuels, and biodiesels.

ln(P /Po) = −ΔH vap/RT + C

(1)

where P is the system pressure at which the boiling point is measured, Po is the standard pressure, R is the gas constant, and T is the boiling point temperature in kelvin. By determining the boiling points of a liquid at several pressures, the ΔHvap can be calculated from the slope of the ln(P) versus 1/T plot. ΔH vap = −slope X (8.314/1000) kJ/mol

(2)

The ΔHvap is often reported at atmospheric pressure. However, ΔHvap is temperature-dependent, and above a certain critical temperature Tc, ΔHvap disappears because the liquid phase can no longer exist. Nevertheless, for a limited temperature range, ΔHvap is nearly constant.

3. MATERIALS AND INSTRUMENT SETUP 3.1. Materials. The renewable fuels selected for study included commercially available biodiesels and several proprietary experimental jet range hydrocarbon mixtures. The biodiesels were FAMEs made by transesterifying a lipid with methanol. Properties of typical vegetable oils and animal fats used to manufacture similar FAMEs are shown in Table 1. The diesel used to blend with the biodiesels was a commercial ultralow-sulfur fuel.

2. PRINCIPLE The heat, or enthalpy, of vaporization (ΔHvap) is the amount of energy required to convert a quantity of a liquid into a gas. The 3139

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3.2. Unit and Equipment Setup. A commercial thermogravimetric analyzer was used for the measurement. It is capable of measuring initial sample mass to 0.1 mg and heating with constant rates from room temperature to 350 °C. To make precise and accurate measurements of boiling points at different pressures on liquid fuels become possible, the system was equipped with the following: (1) An oil-free vacuum pump that can reach 100 Torr was used to create the vacuum and minimize contamination that a conventional oil vacuum pump might produce. In turn, the fuel vapors would contaminate the oil of the conventional pump, shortening the life of the oil and increasing maintenance requirements. A digital pressure readout was used to measure the pressure. A 500 mL ballast volume and a needle valve were installed in the system to control the pressure in the system to ±0.1 Torr. (2) A universal crimper press was employed to seal the aluminum TGA pans with a lid containing a 0.05 mm laser-drilled hole in the center. The lid suppresses spatter. The hole is big enough to prevent self-pressurization inside the pan but small enough to restrict diffusion out of the pan. 3.3. Operation and Measurement. A total of 5−10 mg of the sample is loaded into an aluminum pan. The pan is sealed with a lid that has a 50 μm laser-drilled hole at the center and then loaded onto the TGA instrument. The system pressure is gradually reduced and then maintained at the desired setting with an oil-free vacuum pump. Once the test chamber reaches pressure equilibrium, the heating is started at a constant rate. The boiling point is taken at the onset of weight loss, which is the intersection of the initial weight line with the tangent of the isothermal weight loss slope. Figure 1 illustrates the calculation of the boiling point from a TGA curve and the change in the boiling point with pressure. Table 2 lists a set of boiling point values measured at different pressures with VTGA. Figure 2 shows a plot of the ln(P) versus 1/T

Table 3. Method Precision Determination sample name heat of vaporization (ΔHvap) (kJ/mol)

standard deviation

boiling point (°C)

762.0 625.0 508.1 373.3 228.4

199.1 190.5 182.1 170.8 154.2

tallow biodiesel (B100)

57.1 53.3

75.0 79.7

55.6 55.3 1.4

78.4 77.7 2.4

4.2. Method Accuracy. Six pure chemicals with a range of total carbon numbers from 7 to 16, which are in the jet-diesel range fuels, were selected for determination of the method accuracy. The results are listed in Table 4 along with their literature values for comparison. 4.3. ΔHvap Values of Renewable Fuels Measured with VTGA Method. Six biodiesels (B100) made from six different stocks, including canola oil, coconut oil, Jatropha oil, palm oil, soybean oil, and tallow, were measured for their ΔHvap values with the new method. Table 5 lists the results in kilojoule per mole (kJ/mol) and British thermal units per pound (BTU/lb), which is another unit commonly used in the petroleum industry. The variations in ΔHvap values of these biodiesels are significant, ranging from 39.8 to 83.7 kJ/mol. This demonstrates that the ΔHvap measurements can be used as a quality control test for biodiesel fuels. Also, the large variations in ΔHvap values could impact the diesel engine design for using biodiesel fuels. The difference in heat of vaporization values of various biodiesel samples is largely a reflection of the difference in the intermolecular forces of various samples. The intermolecular force is determined by a number of factors, such as the molecular size (hydrocarbon chain length), the molecular shape (branches), and the degree of saturation (number of double bonds). For example, the coconut FAME has the lowest heat of vaporization of about 40 kJ/mol and is the richest by far in low-molecular-weight components, lauric (C12:0) and myrisitic (C14:0) esters. The canola, Jatropha, and soy FAMEs have a heat of vaporization of about 60 kJ/mol and contain the highest concentrations of multiple unsaturated chains. Tallow and palm FAMEs have the highest ΔHvap at approximately 80 kJ/mol. Both contain high concentrations of C16:0 and C18:0 methyl esters. The ΔHvap values for several commercial biodiesel fuels are given in Table 5. ΔHvap values for selected jet and diesel range renewable fuels are shown in Table 6.

Table 2. VTGA Measurement Data for a R-8 Renewable Jet Fuel pressure (Torr)

first run second run third run average

1nonanol

data for a renewable jet fuel. The calculated ΔHvap value for the renewable jet fuel is 45.1 kJ/mol.

5. DISCUSSION 5.1. Boiling Point and Normal Boiling Point. The boiling point (Tb) or normal boiling point (Tnb) is one of the important properties of fuels for not only quality control but also internal combustion engine modeling. On the basis of the interception (A) and slope (B) values of the linear-plotted ln(P) = A − B/T, one is able to predict the normal boiling point at 1 atm, the boiling point at a given pressure, or the vapor pressure at a given temperature. Table 7 below lists the interception (A), slope (B), and VTGA-measured normal boiling point (Tnb) values in kelvin for the six B100 biodiesels. The Tnb values well match the ones reported by Yuan et al. and Goodrum.20,21 5.2. Estimation of ΔHvap Values of Renewable Fuel Blends. Biodiesel blends can be treated as near-ideal solutions14 and so can renewable fuel blends. Because ΔHvap

Figure 2. Clausius−Clapeyron plot for a R-8-grade renewable jet fuel.

4. RESULTS 4.1. Method Precision. One pure chemical, 1-nonanol (97%, Acros Organics), and one biodiesel fuel, tallow biodiesel (B100), were selected for determination of the method precision. Each sample was tested 3 times, and the results are listed in Table 3. 3140

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Table 4. Method Accuracy Determination heat of vaporization (ΔHvap, kJ/mol)

a

number of carbons

sample description

measured value

literature value

RPDa

C7 C8 C9 C12 C16 C16

n-heptane, CH3(CH2)5CH3, 98%, Fisher Scientific isooctane, CH3C(CH3)2CH2CH(CH3)2, 99%, Fisher Scientific 1-nonanol, CH3(CH2)8OH, 97%, Acros Organics n-dodecane, CH3(CH2)10CH3, 99+%, Aldrich 1-hexadecanol, CH3(CH2)15OH, 96%, Acros Organics n-hexadecane, CH3(CH2)14CH3, 99%, Acros Organics

36.6 35.4 55.6 50.9 59.5 51.4

36.617 34.417 54.417 49.618 59.119 51.219

0.0 −2.9 −2.2 −2.6 −0.7 −0.4

Relative percent difference (RPD) = 100 × (literature value − measured value)/(literature value).

Table 5. Biodiesel (B100) ΔHvap Value

For a given binary, ΔHvap,1 and ΔHvap,2 are fixed known values. The only variable in eq 5 is the volume fraction or mix ratio. A palm FAME B100 was selected to be blended with a petroleum diesel because it has the largest difference in ΔHvap values among those reported here. The blend ratios were 25, 50, and 75%, in volume-per-volume percentage (v/v %), respectively. The comparison results listed in Table 8 show a good agreement between the calculated and measured values.

heat of vaporization (ΔHvap) biodiesel (B100)

kJ/mol

BTU/lb

canola biodiesel coconut biodiesel Jatropha biodiesel palm biodiesel soybean biodiesel tallow biodiesel

65.1 39.8 64.6 83.7 65.7 78.4

101.4 77.8 93.5 128.1 96.7 119.1

Table 8. ΔHvap Versus Mix Ratio for Palm Biodiesel Blends

Table 6. ΔHvap Values of Selected Renewable Fuels renewable fuels diesel range

renewable renewable renewable renewable renewable renewable

heat of vaporization (ΔHvap, kJ/mol)

heat of vaporization (ΔHvap, kJ/mol) fuel fuel fuel fuel fuel fuel

1 2 3 4 5 6

blend name

60.9 48.1 48.9 67.5 57.5 56.3

palm B0 (a petroleum diesel) palm B25 palm B50 palm B75 palm B100

Table 7. Intercept (A), Slope (B), and Measured Normal Boiling Point (Tnb) Values for Selected B100 Biodiesels biodiesel (B100)

intercept (A)

slope (B)

measured normal boiling point (Tnb in K)

canola biodiesel coconut biodiesel Jatropha biodiesel palm biodiesel soybean biodiesel tallow biodiesel

12.6 8.9

7826 4782

621.1 535.2

12.5

7771

620.5

16.7 12.8

10071 7902

600.5 618.9

15.9

9586

606.0

∑ XiΔHvap,i

RPDa

0

48.2

not applicable

not applicable

25 50 75 100

59.4 63.6 76.2 83.7

57.1 66.0 74.8 not applicable

3.9 −3.7 1.9 not applicable

6. CONCLUSION VTGA is a quick and accurate method to measure ΔHvap of renewable fuels. VTGA ΔHvap results for single known components agree with literature values within a range of ±5%. The ΔHvap values for binary mixtures of fuels can be simply estimated from the ΔHvap values and mix ratio of the parents.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 510-242-1273. Fax: 510-242-1792. E-mail: [email protected].

(3)

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. C. Munson, Dr. W. Cannella, Dr. T. Smagala, K. DeFabio, D. Gingell, R. Joves, M. Mukundan, and Dr. A. Maria for their valuable support, discussions, and contributions.

(4)

■

where Vi is the volume fraction of the ith component in the blend. For a binary blend, eq 4 can be simply rewritten as ΔH vap,binary blend≈V1ΔH vap,1+V2ΔH vap,2

calculated

Relative percent difference (RPD) = 100 × (measured value − calculated Value)/(calculated value).

where Xi is the molar fraction of the ith component in the blend, ΔHvap,i is the ΔHvap value of the ith component in the blend, and ΔHvap,blend is the ΔHvap value of the blend. Equation 3 can be restated as eq 4 because the density of every component in a real fuel blend is similar such that ΔH vap,blend≈∑ Vi ΔH vap, i

measured

a

is a fundamental thermodynamic property, eq 3 describes an ideal mixed blend ΔH vap,blend =

FAME mix ratio (v/v %)

(5) 3141

NOMENCLATURE ΔHvap = heat of vaporization B100 = pure form of biodiesel FAME = fatty acid methyl ester dx.doi.org/10.1021/ef402491p | Energy Fuels 2014, 28, 3138−3142

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TGA = thermogravimetric analysis VTGA = vacuum thermogravimetric analysis

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REFERENCES

(1) Automotive Handbook, 4th ed.; Robert Bosch GmbH: Stuttgart, Germany, 1996. (2) Caterpillar Oil and Gas Resource Center. Volatility and Engine Operation; http://www.catoilandgasinfo.com (accessed Aug 2013). (3) Goodrum, J. W. Volatility and boiling points of biodiesel from vegetable oils and tallow. Biomass Bioenergy 2002, 22, 205−211. (4) Tsujimura, T.; Goto, S. Study on improvement of combustion and effect of fuel property in advanced diesel engine. Proceedings of the SAE 2010 World Congress; Detroit, MI, April 13−15, 2010. (5) Allen, C. M.; Toulson, E.; Hung, D. L. S.; Schock, H. J.; Miller, D.; Lee, T. Ignition characteristics of diesel and canola biodiesel sprays in the low-temperature combustion regime. Energy Fuels 2011, 25 (7), 2896−2908. (6) Alberty, R. A. Physical Chemistry, 7th ed.; John Wiley and Sons: Hoboken, NJ, 1987; pp 187−189. (7) Chickos, J. S.; Hesse, D. G.; Liebman, J. F.; Panshin, S. Y. Estimations of the heats of vaporization of simple hydrocarbon derivatives at 298 K. J. Org. Chem. 1988, 53, 3424−3429. (8) Rechsteiner, C. E. Heat of vaporization. In Handbook of Chemical Property Estimation Methods; Lyman, W. J., Reehl, W. F., Rosenblatt, D. H., Eds.; McGraw-Hill: New York, 1990; Chapter 13. (9) Chickos, J. S.; Acree, W. E. Enthalpies of vaporization of organic and organometallic compounds 1880−2002. J. Phys. Chem. Ref. Data 2003, 32, 519. (10) Lange’s Handbook of Chemistry, 14th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1992; Section 6, pp 41−68. (11) Widegren, J. A.; Bruno, T. J. Vapor pressure measurements on saturated biodiesel fuel esters by the concatenated gas saturation method. Fuel 2011, 90 (5), 1833−1839. (12) Gschwender, L. J.; Snyder, C. E.; Massey, M.; Peterangelo, S. Improved liquid/grease lubricants for space mechanisms. Lubr. Eng. 2000, 12, 25−31. (13) Ashby, E. C.; Claudy, P.; Bousquet, J.; Etienne, J. High vacuum DTA−TGA instrumentation for air-sensitive compounds. J. Chem. Educ. 1975, 52 (9), 618−20. (14) Huang, H.; Wang, K.; Wang, S.; Klein, M. T.; Calkins, W. H. A novel method for the determination of the boiling range of liquid fuels by thermogravimetric analysis. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 1995, 40 (3), 485−491. (15) Knothe, G.; Dunn, R.; Bagby, M. Biodiesel: The use of vegetable oils and their derivatives as alternative diesel fuels. In Fuels and Chemicals from Biomass; Saha, B. C., Woodward, J., Eds.; American Chemical Society (ACS): Washington, D.C., 1997; ACS Symposium Series, Vol. 666, Chapter 10, pp 172−208. (16) Chatakanonda, P.; Sriroth, K.; Vaysse, L.; Liangprayoon, S. Fatty acid composition and properties of Jatropha seed oil and its methyl ester. Proceedings of the 43rd Kasetsart University Annual Conference; Kasetsart University, Bangkok, Thailand, Feb 1−4, 2005. (17) Thermodynamics Research Center, National Institute of Standards and Technology (NIST). http://trc.nist.gov (accessed Aug 2013). (18) CRC Handbook of Chemistry and Physics, 68th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1987; p C-675. (19) Yaws, C. L. Yaws’ Handbook of Thermodynamic and Physical Properties of Chemical Compounds; Knovel: New York, 2003; http:// app.knovel.com/hotlink/toc/id:kpYHTPPCC4/yaws-handbookthermodynamic. (20) Yuan, W.; Hansen, A. C.; Zhang, C. Vapor pressure and normal boiling point predictions for pure methyl esters and biodiesel fuels. Fuel 2005, 84, 943−950. (21) Goodrum, J. W.; Geller, D. P.; Lee, S. A. Rapid measurement of boiling points and vapor pressure of binary mixtures of short-chain triglycerides by TGA method. Thermochim. Acta 1998, 311, 71−79.

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