794
Energy & Fuels 2009, 23, 794–798
Measurements on Vapor Pressure and Thermal Conductivity for Pseudo-binary Systems of a Hydrocarbon Fuel with Ethylene and Diethylene Glycol Dimethyl Ethers Dan Li, Wenjun Fang,* Wenjie Xie, Yan Xing, Yongsheng Guo, and Ruisen Lin Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed August 27, 2008. ReVised Manuscript ReceiVed NoVember 12, 2008
Vapor pressures for mixtures of a hydrocarbon fuel with ethylene glycol dimethyl ether (EGDME) and diethylene glycol dimethyl ether (DEGDME) were measured by comparative ebulliometry with inclined ebulliometers. Experimental data of vapor pressures and equilibrium temperatures for each mixture were correlated by the Antoine equation. The pseudo-binary vapor-liquid equilibrium (VLE) data were calculated with a non-random two liquid (NRTL) model. Prediction of the vapor pressures for the hydrocarbon fuel from the American Petroleum Institute (API) equations gave relatively satisfactory results. Furthermore, thermal conductivities for the pseudo-binary systems were measured at 273.15 and 298.15 K by means of a relative transient calorimeter. The data were compared to those calculated from the Jamieson equation. These results could provide important information on the development of oxygenate-blended fuels.
1. Introduction Jet fuels are complicated mixtures of hydrocarbons produced from the refining of crude oils. In addition to combustion, one fuel plays another important role in modern aircrafts: it serves as a heat sink for cooling component systems.1,2 During these thermal processes, the oxygenates have been added recently into the fuels to reduce pollution problems.3-5 Ethylene glycol dimethyl ether (EGDME) and diethylene glycol dimethyl ether (DEGDME) are good candidates because they have high oxygen contents (35.6% for EGDME and 35.8% for DEGDME) and have good solubility in hydrocarbon fuels. As new additives in hydrocarbon fuels, they have significantly improved brakespecific energy consumption and exhaust emissions.6,7 However, it should be noted that some basic properties, such as volatility and thermal conductivity, related to combustion and heat transfer may change with the addition of an oxygenated compound. The volatility property, characterized by vapor pressure, determines the evaporative emission of a fuel and the performance during engine startup.8,9 Thermal conductivity is an important thermophysical property, and its values are needed in heat-transfer calculation, especially for dealing with the * To whom correspondence should be addressed. Telephone: +86-57187952371. Fax: +86-571-87951895. E-mail:
[email protected]. (1) Maurice, L. Q.; Lander, H.; Edwards, T.; Harrison, W. E., III. Fuel 2001, 80, 747–756. (2) Brown, S. P.; Frederick, R. A., Jr. J. Propul. Power 2008, 24, 206– 212. (3) Gramajo de Doz, M. B.; Bonatti, C. M.; Solimo, H. N. Energy Fuels 2004, 18, 334–337. (4) Huang, Z.; Lu, H.; Jiang, D.; Zeng, K.; Liu, B.; Zhang, J.; Wang, X. Energy Fuels 2005, 19, 403–410. (5) de Menezes, E. W.; da Silva, R.; Catalun˜a, R.; Ortega, R. J. C. Fuel 2006, 85, 815–822. (6) Bertoli, C.; Giacomo, N. D.; Beatrice, C. SAE Tech. Pap. 972972. (7) Nabi, N.; Chowdhury, W. J. Teknol. 2006, 44, 1–12. (8) French, R.; Malone, P. Fluid Phase Equilib. 2005, 228-229, 27– 40. (9) Yuan, W.; Hansen, A. C.; Zhang, Q. Fuel 2005, 84, 943–950.
convective heat transfer of fluids.10-12 Therefore, the data of vapor pressure and thermal conductivity are indispensable and required for the development and operation of hydrocarbon fuels. In this work, the bubble point vapor pressures and the thermal conductivities for mixtures of a kerosene-based jet fuel with EGDME and DEGDME are investigated. The vapor pressures were determined by the comparative ebulliometry from measurements on the boiling temperatures of the sample and the reference material under the same pressure. The thermal conductivities were determined by means of a relative transient calorimeter. The aim of this work is mainly to provide reference information on the choice of oxygenated additives and the thermal design for new hydrocarbon fuels. 2. Experimental Section 2.1. Materials and Characterization. The hydrocarbon fuel investigated was a refined kerosene-based fuel. The predominant compounds were analyzed by Hewlett-Packard 6890/5973 gas chromatography/mass spectrometry (GC/MS). The experimental conditions were as follows: column, HP-5MS (30 m × 0.25 mm × 0.25 µm) elastic quartz capillary column; carrier gas, helium, 1.0 mL min-1 (flow rate); split ratio, 1:50; sample, 0.1 mL; solvent delay, 2 min; temperature program, 333.15 K (initial temperature), 2 min (initial hold), 10 K min-1 (rate), 553.15 K (final temperature), 2 min (final hold); mass spectrometer scanning range, 35-450 amu. It was observed that the fuel was a complex mixture of hydrocarbons mainly with the carbon number ranging from C7 to C25. Some fundamental properties were determined for characterization of the fuel. The density was measured by an Anton Paar DMA55 vibrating tube digital densimeter. The normal boiling point Tb was determined by an inclined ebulliometer. The initial and final boiling points (IBP and FBP) were determined according to American Society for Testing and Materials (ASTM) D 86. The flash points were measured by an SYD-261 closed-cup flash point (10) Wu, J.; Liu, Z.; Jin, X.; Pan, J. J. Chem. Eng. Data 2005, 50, 102– 104. (11) Lei, Q.; Lin, R.; Ni, D. J. Chem. Eng. Data 1997, 42, 971–974. (12) Lei, Q.; Hou, Y.; Lin, R. Chem. Eng. Sci. 1997, 52, 1243–1251.
10.1021/ef8007163 CCC: $40.75 2009 American Chemical Society Published on Web 12/31/2008
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Table 1. Characterized Properties of the Hydrocarbon Fuel property
value
acidity (mg of KOH g-1) sulfur (wt %) copper corrosion (373.15 K, 2 h) viscosity (293.15 K) (mm2 s-1) viscosity (253.15 K) (mm2 s-1) F (293.15 K) (g cm-3) Tb (101.3 kPa) (K) flash point (K) IBP (K) FBP (K) M (g mol-1) 4vapHm (kJ mol-1) relative contents (wt %) n-paraffin iso-paraffin cycloparaffin aromatics
0.004 0.062 e1 1.540 3.535 0.7933 451.25 317.55 429.15 547.15 154.6 39.35 15.45 36.02 47.38 1.15
Table 2. Properties of EGDME and DEGDME
a
From ref 12. b From ref 13. c From ref 14. d From ref 15. e From ref
16.
analyzer (Changji Instrument Co., Ltd., Shanghai, China), which was operated according to the Chinese standard test method, GB/T 261, in reference to the standard test method, ASTM D93. The physical properties are listed in Table 1. Both EGDME and DEGDME were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The purities were checked by Agilent 1100 GC with EGDME of 99.7% and DEGDME of 98.5%, respectively. The determined physical properties together with the literature values13-17 of EGDME and DEGDME are listed in Table 2. Absolute methanol (g99.5%), ethanol (g99.8%), n-pentanol (g95.0%), toluene (g99.5%), n-hexane (g97.0%), ethyl acetate (g99.5%), and carbon tetrachloride (g99.5%) were supplied by Shanghai Chemical Corporation, China. The pseudo-binary mixtures with different compositions of the hydrocarbon fuel and EGDME or DEGDME were prepared directly by weight with a Mettler balance, having a precision of (0.0001 g. 2.2. Vapor Pressure Measurements. An apparatus, with two same inclined ebulliometers made of quartz glass, of comparative ebulliometry designed on the basis of the principle of the quasistatic method was used to measure the vapor pressures. The schematic diagram of the apparatus has been described in detail previously.18 The bubble point temperatures of each sample with the prepared composition and a reference material (ethanol) in two separate inclined ebulliometers were measured under the same pressure. The equilibrium pressure could be calculated from the boiling temperature of ethanol and its well-known pressure-tem(13) Pal, A.; Sharma, S. J. Chem. Eng. Data 1998, 43, 532–536. (14) Alzawa, K.; Kato, M. J. Chem. Eng. Data 1991, 36, 159–161. (15) Wallace, W. J.; Mathews, A. L. J. Chem. Eng. Data 1964, 9, 267– 268. (16) Steele, W. V.; Chirico, R. D.; Knipmeyer, S. E.; Nguyen, A.; Smith, N. K. J. Chem. Eng. Data 1996, 41, 1285–1302. (17) Mansson, M. J. Chem. Thermodyn. 1969, 1, 141–151. (18) Sun, H.; Fang, W.; Guo, Y.; Lin, R. Fuel 2005, 84, 825–831.
Figure 1. Schematic diagram of the bridge circuit of the apparatus.
perature behavior. The temperatures inside the two ebulliometers were measured within the uncertainty of 0.01 K with two fourlead 25 Ω platinum resistance thermometers (WZPB-2, Yunnan Instrument Factory, China) and a digital multimeter (Keithley 195A). The thermometers were calibrated by the Standard Bureau of Zhejiang Province, China. 2.3. Thermal Conductivity Measurements. Measurements on the thermal conductivities for mixtures of the hydrocarbon fuel with EGDME or DEGDME were performed with an uncertainty of less than 2.0% by means of a relative transient calorimeter.19,20 The bridge circuit for measuring is shown schematically in Figure 1. A glass-coated bead thermistor immersed in the liquid sample is used as both a temperature sensor and a heating source. The temperaturedependent thermistor has a negative temperature coefficient of resistance. It is connected to the circuit of an unbalanced bridge. R1 and R2 are standard resistances, with 1000 Ω each. R3 is an adjustable resistor, and RT is the resistance of the glass-coated bead thermistor. The thermal conductivity cell is cylindrical in form, 100 mm in length, and 15 mm in inner diameter. It is placed in a circulated ethanol bath (HAAKE C immersion circulator, Germany), which maintains the temperature stable to (0.01 K. A Fluke multimeter (Model 8520A) is used to indicate the balance of the bridge. All data acquisitions and instrument controls can be operated by a computer. The liquid sample of 4 mL was added into the thermal conductivity cell. After 30 min, a stable temperature was achieved. A stable current of 0.03 mA then flowed through the thermistor bridge. The resistance of the potentiometer R3 was adjusted to balance the current across the bridge, and a heating current of 0.5 mA was made to flow through the thermistor. As a result, the bridge balance was destroyed and the resistance of the thermistor decreased with its temperature increasing. The voltages of the unbalanced bridge (∆V) were recorded for 5 s. The data from 1.5 to 2.5 s were used to calculate the slope of the voltage against time (dV/dt) of the unbalanced bridge. The short time was kept to avoid convection, which might undermine the measurement accuracy. The instrument was calibrated at 273.15 and 298.15 K using seven reference liquids: methanol, ethanol, n-pentanol, toluene, n-hexane, ethyl acetate, and carbon tetrachloride. The literature values11,19-23 of thermal conductivities used in the calibration were given in Table 3. All measurements were performed at atmospheric pressure. Each sample was measured for 10 times at the given temperature, and the average value was reported.
3. Results and Discussion 3.1. Vapor Pressures and Phase Lines. The bubble point vapor pressures of the pseudo-binary mixtures, EGDME + hydrocarbon fuel and DEGDME + hydrocarbon fuel, have been (19) Wang, C.; Yang, M. Thermochim. Acta 1995, 255, 365–370. (20) Yang, M.; Wang, C.; Zhang, Z. Heat Mass Transfer 2001, 37, 507– 509. (21) Raal, D.; Rijsdijk, R. L. J. Chem. Eng. Data 1981, 26, 351–359.
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Table 3. Reference Liquids and the Values of Thermal Conductivities Used in the Calibration 273.15 K 298.15 K
λ (W m-1 K-1) a
methanol
ethanol
n-pentanol
toluene
n-hexane
ethyl acetate
carbon tetrachloride
0.206a 0.1972e
0.1774a 0.1695d
0.1587a 0.1572e
0.1385b 0.1311b
0.1288c 0.1217c
0.1526d 0.1445d
0.1072d 0.1025e
From ref 21. b From ref 22. c From ref 23. d From ref 11. e From ref 19.
Figure 2. Comparison of vapor pressures with reference data24,25 for ethanol and EGDME.
measured from 8 to 102 kPa for the compositions from 0 to 100%. Comparisons of the experimental data (pexp) with the reference values (pref)24,25 of ethanol and EGDME with satisfactory agreements are shown in Figure 2, where the deviation is defined as ∆p/p ) (pref - pexp) × 100%/pexp
(1)
The vapor pressure data for each sample are fitted to the Antoine equation ln p ) A - B/(T + C) (2) where p is the vapor pressure in kPa, T is the equilibrium temperature in K, and A, B, and C are constants. The average relative deviation (ARD) n
ARD )
∑ |(p
cal - pexp)/pexp × 100%|k/n
(3)
Figure 3. Equilibrium temperature (T)-composition (x1, y1) phase diagrams for pseudo-binary systems of (3a) EGDME (1) + fuel (2) and (3b) DEGDME (1) + fuel (2).
k)1
is given for each sample, where n is the number of the experimental data points. The average initial enthalpy of vaporization, ∆vapHm, for each mixture at the experimental temperature range is obtained from the linear regression of ln p against 1/T on the basis of the Clapeyron-Clausius equation. The temperature ranges of the measurements and the correlation results are summarized in Table 4. The experimental data were correlated by the non-random two liquid (NRTL) model26 of liquid-phase activity coefficients
[( [(
ln γ1 ) x22 τ21
G21 x1 + x2G21
ln γ2 ) x12 τ12
G12 x2 + x1G12
) )
2
2
+ +
τ12G12 (x2 + x1G12)2 τ21G21 (x1 + x2G21)2
] ]
(4)
(5)
(22) Nieto de Castro, C. A.; Li, S. F. Y.; Nagashima, A.; Trengove, R. D.; Wakeham, W. A. J. Phys. Chem. Ref. Data 1986, 15, 1073–1086. (23) Brykov, V. P.; Mukhamedzyanov, G. Kh.; Usmanov, A. G. J. Eng. Phys. Thermophys. 1970, 18, 62–65. (24) Atik, Z. J. Chem. Thermodyn. 2008, 40, 467–470. (25) Cabezas, J. L.; Beltra´n, S.; Coca, J. J. Chem. Eng. Data 1991, 36, 184–188. (26) Tochigi, K.; Takahara, H.; Shiga, Y.; Kawase, Y. Fluid Phase Equilib. 2007, 260, 65–69.
Figure 4. Comparison of vapor pressures predicted from API equations with experimental data for the hydrocarbon fuel.
where G12 ) exp(-R12τ12) and G21 ) exp(-R21τ21). The correlation was performed on the minimization of the following objective function: OF )
∑ (1 - p
2 cal/pexp)
(6)
with R12 ) R21 ) 0.47, τ12 ) 0.43, and τ21 ) 0.07 for the (EGDME + fuel) system and R12 ) R21 ) 0.47, τ12 ) 0.51,
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Energy & Fuels, Vol. 23, 2009 797
Table 4. Correlation Results of Vapor Pressure Data by the Antoine Equation for Mixtures of EGDME (1) + Fuel (2) and DEGDME (1) + Fuel (2) x1
experimental data points
experimental temperature range (K)
Aa
0.0000 0.0435 0.0793 0.1008 0.2032 0.3001 0.3931 0.5283 0.7201 0.8728 1.0000
16 16 16 16 17 18 17 19 18 17 18
382–452 345–437 333–426 329–418 321–397 310–383 306–378 299–371 305–364 304–361 304–358
EGDME (1) + Fuel (2) 11.621 9.295 8.521 8.151 7.648 9.069 10.471 11.592 12.463 13.896 14.740
0.0000 0.0509 0.1009 0.2232 0.3067 0.4396 0.6334 0.7998 1.0000
16 17 17 15 15 18 17 16 16
382–452 376–450 371–447 368–441 372–439 364–436 367–435 365–433 371–434
DEGDME (1) + Fuel (2) 11.621 10.122 10.607 11.032 11.117 11.864 14.982 15.331 15.354
a
Ba
Ca
4vapHm (kJ mol-1)
ARD (%)
2285.269 1348.869 970.328 814.299 542.651 889.551 1379.037 1799.587 2088.763 2733.844 3227.926
-127.406 -149.037 -177.587 -189.837 -218.691 -182.432 -141.396 -112.224 -97.703 -65.441 -38.836
39.35 29.58 29.10 28.60 30.64 33.81 33.77 34.30 34.76 35.29 34.47
1.95 1.78 0.96 2.17 1.34 1.29 1.42 0.93 0.44 0.07 0.60
2285.269 1503.647 1700.775 1803.306 1800.092 2138.996 4028.523 4171.219 4073.399
-127.406 -177.747 -163.061 -160.017 -161.585 -140.873 -45.712 -43.878 -54.517
39.35 38.73 39.53 40.66 41.74 42.13 42.67 43.78 45.38
1.95 1.90 0.96 1.36 0.99 1.18 0.99 0.80 0.44
ln p (kPa) ) A - B/(T (K) + C).
and τ21 ) 0.22 for the (DEGDME + fuel) system, at 100 kPa. The ARDs of the pressures for two systems are 1.25 and 0.85%, respectively. On the basis of eqs 2, 4, and 5, along with the corresponding properties of the “pure” components, the mole fraction of the vapor phase yi is estimated by the following equation: yi ) xiγipsi /p
(7)
where p is the equilibrium pressure of the binary system and psi is the vapor pressure of “pure” component i. The thermodynamic consistency27 of the vapor-liquid equilibrium (VLE) data has been checked by means of the area test I)
∫
1
0
[ln(γ1/γ2)]dx1
(8)
From the correlation results, the temperature-composition lines of T-(x, y) under several pressures can be obtained. The pseudo-binary phase diagrams of (EGDME + fuel) and (DEGDME + fuel) are plotted in Figure 3. The dotted lines represent the values calculated from the NRTL model. The equilibrium data clearly show positive deviation of the pressure from Raoult’s law. The maximum temperature departure for (EGDME + fuel) is about 43 K at 100 kPa, and that for (DEGDME + fuel) is about 9.1 K at 100 kPa. It indicates that the effect on the volatility of the hydrocarbon fuel by adding EGDME is higher than that by adding DEGDME. 3.2. Estimation of Vapor Pressure for the Fuel from American Petroleum Institute (API) Equations. Because of complicated compositions, measurements or estimations of the vapor pressure and the VLE property of a hydrocarbon fuel are generally difficult. In this section, it is attempted to use the API equations,28 which are usually used in petroleum and chemical industries for estimating the vapor pressures of pure hydrocarbons and narrow-boiling petroleum fractions. The forms of the API equations are introduced in the Appendix. (27) Prausnitz, J. M.; Lichtenthaler, R. N.; Azevedo, E. G. Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd ed.; Prentice-Hall: Englewood Cliffs, NJ, 2005. (28) American Petroleum Institute (API). API Technical Data Books Petroleum Refining, 4th ed.; API: Washington, D.C., 1982.
Figure 5. Plots of the thermal conductivity λ versus dV/dt for seven reference liquids in Table 3.
With the API equations, the vapor pressures of the hydrocarbon fuel at different temperatures can be predicted only from its density and normal boiling point. The predicted values compared to the experimental data are shown in Figure 4, with the mean deviation of 1.96 kPa and the ARD of 5.6%. It might be possible to further predict the VLE data and flash points for fuel systems with complicated components from a combination of an appropriate model and the predicted vapor pressures. 3.3. Thermal Conductivity. To establish accurate measurements on the thermal conductivity (λ), calibration experiments were performed for seven reference liquids: methanol, ethanol, n-pentanol, toluene, n-hexane, ethyl acetate, and carbon tetrachloride. Figure 5 shows a typical plot of λ versus the slope of the determined voltage against time (dV/dt). The work equations at 273.15 and 298.15 K of the apparatus are obtained as follows: 273.15 K : λ ) -334.48dV/dt + 473.81 (R2 ) 0.9985)
(9)
298.15 K : λ ) -212.01dV/dt + 494.69 (R2 ) 0.9987) (10) The uncertainty of the experimental results is estimated to be within 2.0%, and the reproducibility of the data is better than 1.0%. The thermal conductivities of the mixtures of (EGDME + fuel) and (DEGDME + fuel) have been measured by the
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log p ) (3000.538X - 6.761560)/(43X - 0.987672) 0.8752041 (A1) for X > 0.0022 (p < 0.27 kPa) log p ) (2663.129X - 5.994296)/(95.76X - 0.972546) 0.8752041 (A2) for 0.0013 e X e 0.0022 (0.27 < p e 101.3 kPa) log p ) (2770.085X - 6.412631)/(36X - 0.989679) 0.8752041 (A3) for X < 0.0013 (p > 101.3 kPa) X ) (Tb′/T - 0.00051606Tb′)/(748.1 - 0.3861Tb′) (A4) Figure 6. Experimental and calculated thermal conductivities from the Jamieson equation with R ) 1 for the mixtures of EGDME (1) + fuel (2) and DEGDME (1) + fuel (2).
calibrated apparatus. The experimental data are presented in Figure 6. The thermal conductivity of the blended fuel increases with the increasing content of EGDME or DEGDME. The calculated results from the Jamieson equation29 λm ) W1λ1 + W2λ2 - R(λ1 - λ2)(1 - W11⁄2)W2
(11)
with R ) 1 are also shown in Figure 6 for comparison. λm, λ1, and λ2 (λ2 > λ1) are the thermal conductivities of the binary mixture and two components, respectively. W is the mass fraction, and R is a variable parameter. The maximum deviation of the predicted values from the experimental data is 2.4%, and the average deviation is 0.9%. 4. Conclusions Bubble point vapor pressures for the pseudo-binary systems, EGDME + and DEGDME + hydrocarbon fuel, were presented by comparative ebulliometry with inclined ebulliometers. Thermal conductivities of the mixtures over the entire composition range were measured by a relative thermistor-based method. The phase diagrams can be obtained from the correlation on the experimental results with the Antoine equation and the NRTL model. The experimental data of thermal conductivity were correlated by the Jamieson equation. The addition of EGDME or DEGDME has a critical effect on the vapor pressure or the thermal conductivity of the fuel systems. The mixtures appear with positive deviations of the vapor pressures from Raoult’s law. EGDME or DEGDME can improve the ability of heat transfer of the fuel systems. Acknowledgment. The authors are grateful for the financial support from the National Natural Science Foundation of China (20573096) and the Natural Science Foundation of Zhejiang Province, China (Y404329).
Appendix The universal equations for estimating the vapor pressures of pure hydrocarbons and narrow-boiling petroleum fractions presented by the API are as follows: (29) Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids, 5th ed.; McGraw-Hill: New York, 2000.
Tb′ ) Tb - 1.39f(K - 12) log(p/101.3)
{
0 f ) (Tb - 366.5) ⁄ 111.1 1.0
Tb < 368 K 368 < Tb < 473 K Tb > 473 K
K ) 1.216 × (Tb1/3/S)
(A5) (A6) (A7)
where p is the vapor pressure, Tb′ is the normal boiling point corrected to K ) 12, Tb is the normal boiling point, T is the absolute temperature, f is the correction factor, K is the Watson characterization factor, and S is the relative specific gravity. Nomenclature A, B, and C ) Antoine constants ARD ) average relative deviation f ) correction factor K ) Watson characterization factor IBP ) initial boiling point FBP ) final boiling point M ) relative molecular mass n ) number of data points p ) vapor pressure, kPa R ) gas constant (8.314 J mol-1 K-1) S ) relative specific gravity T ) equilibrium temperature, K Tb ) normal boiling temperature, K W ) mass fraction x ) mole fraction of the liquid phase y ) mole fraction of the vapor phase ∆vapHm ) enthalpy of vaporization, kJ mol-1 dV/dt ) slope of the voltage against time of the unbalanced bridge, mV s-1 Greek Letters R12, R21, τ12, and τ21 ) NRTL parameters λ ) thermal conductivity, W m-1 K-1 F ) density, g cm-3 R ) variable parameter in the Jamieson equation Subscripts cal ) calculated value exp ) experimental value ref ) reference value i and j ) components i and j EF8007163
