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Energy & Fuels 2007, 21, 1188-1192
Volatility of Blended Fuel of Biodiesel and Ethanol Yongsheng Guo,* Jing Zhong, Yan Xing, Dan Li, and Ruisen Lin Department of Chemistry, College of Science, Zhejiang UniVersity, Zheda Road 38, Hangzhou 310027, People’s Republic of China ReceiVed NoVember 17, 2006. ReVised Manuscript ReceiVed January 20, 2007
The transesterification of sunflower seed oil was carried out in supercritical ethanol without using any catalyst. Ethyl esters of vegetable oils have several outstanding advantages because ethanol is derived from agricultural products and is renewable and biologically less objectionable in the environment. On the other hand, because of lower volatility, ignition delay and combustion problems could occur when ethyl esters are used. In the present work, ethanol is added to enhance the vapor pressure of biodiesel. The bubble-point pressures of mixtures of biodiesel and ethanol as a function of temperature were measured by comparative ebulliometry with inclined ebulliometers. Experimental data of vapor pressures and equilibrium temperatures were correlated by the Antoine equation. The bubble-point lines of pressure versus the composition at different temperatures and the temperature versus the composition at different pressures were obtained. It is found that the mixtures of biodiesel and ethanol have visible positive deviations from Raoult’s law. The addition of ethanol has a critical effect on the vapor pressure of fuels.
1. Introduction Biodiesel, an alternative diesel fuel, is made from renewable biological sources such as vegetable oils and animal fats. It is biodegradable and nontoxic and has low emission profiles and so is environmentally beneficial.1-3 One hundred years ago, Rudolf Diesel tested vegetable oil as fuel for his engine. With the advent of cheap petroleum, appropriate crude oil fractions were refined to serve as fuel, and diesel fuels and diesel engines evolved together. Recently, because of increases in crude oil prices, limited resources of fossil oil, and environmental concerns, there has been a renewed focus on biodiesel fuel.3,4 Continued and increasing use of petroleum will intensify local air pollution and magnify the global warming problems caused by CO2. Biodiesel fuel has the potential to reduce the level of pollutants and the level of potential or probable carcinogens. Biodiesel is an efficient, clean, 100% natural energy alternative to petroleum fuels. Due to the great molecular similarities of biodiesel to paraffinic diesel fuel compounds, this alternative fuel has a chance of fulfilling the demands that diesel engine makes of its fuel. Essentially, no engine modifications are required to substitute biodiesel for diesel fuel to maintain the engine performance. Ethyl esters of vegetable oils have several outstanding advantages among other new-renewable and clean engine fuel alternatives because ethanol is derived from agricultural products * Corresponding author tel.: +86 571 87952371; fax: +86 571 87951895; e-mail:
[email protected]. (1) Fangrui, M.; Milford, A. H. Biodiesel Production: A Review. Bioresour. Technol. 1999, 70, 1-15. (2) Cao, W.; Han, H.; Zhang, J. Preparation of Biodiesel from Soybean Oil Using Supercritical Methanol and Co-Solvent. Fuel 2005, 84, 347351. (3) Demirbas, A. Biodiesel Fuels from Vegetable Oils via Catalytic and Non-Catalytic Supercritical Alcohol Transesterifications and Other Methods: A Survey. Energy ConVers. Manage. 2003, 44, 2093-2109. (4) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Technical Aspects of Biodiesel Production by TransesterificationsA Review. Renewable Sustainable Energy ReV. 2006, 10, 248-268.
and is renewable and biologically less objectionable in the environment. On the other hand, because of lower volatility, ignition delay and combustion problems could occur when ethyl esters are used.5,6 In the present work, ethanol is added to enhance the vapor pressure of biodiesel. The bubble-point pressures of mixtures of biodiesel and ethanol as a function of the temperature were measured by the comparative ebulliometry using inclined ebulliometers set up in this laboratory.7,8 The bubble-point lines of pressure versus the composition at different temperatures and the temperature versus the composition at different pressures were obtained. The results may provide important information for the development of renewable energy sources. 2. Experimental Section 2.1. Materials and Characterization. The sunflower seed oil used in this study was supplied from Inner Mongolia. Absolute ethanol with a purity better than 99.8% as claimed by the supplier, Sinopharm Chemical Reagent Company, was used without further distillation. 2.2. Supercritical Ethanol Transesterification Method. The supercritical ethanol transesterification system employed in this work is shown in Figure 1. All the runs of transesterification were performed in a 200 mL cylindrical autoclave made of stainless steel in which the pressure and temperature were monitored in real time, covering up to 33 MPa and 550 °C, respectively. In a typical run, the autoclave was charged with a given amount of sunflower seed (5) Tate, R. E.; Watts, K. C.; Allen, C. A. W.; Wilkie, K. I. The Viscosities of Three Biodiesel Fuels at Temperatures up to 300 °C. Fuel 2006, 85, 1010-1015. (6) Yuan, W.; Hansen, A. C.; Zhang, Q. Vapor Pressure and Normal Boiling Point Predictions for Pure Methyl Esters and Biodiesel Fuels. Fuel 2005, 943-950. (7) Sun, H.; Fang, W.; Guo, Y.; Lin, R. Investigation of Bubble-Point Vapor Pressures for Mixtures of an Endothermic Hydrocarbon Fuel with Ethanol. Fuel 2005, 84, 825-831. (8) Wang, Z.; Fang, W.; Lin, R.; Guo, Y.; Zhou, X. Volatility of Blended Fuel of Endothermic Hydrocarbon Fuel and Triethylamine. Fuel 2006, 85, 1794-1797.
10.1021/ef0605807 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/28/2007
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Figure 1. Supercritical ethanol transesterification system: (1) autoclave, (2) electrical furnace, (3) raw material entrance valve, (4) temperature control monitor, (5) pressure gauge, (6) product exit valve, and (7) product collecting vessel.
Figure 3. Comparison of the vapor pressure data for ethanol.
Figure 2. GC/MS analysis results of biodiesel. Table 1. The Physicochemical Properties of Biodiesel ∆Hc (kJ
g-1)
40.45
Tb (°C)
Tf (°C)
335
98
F20 (g
cm-3)
0.873
υ40
(mm2 s-1) 7.443
Figure 4. Vapor pressure at different ethanol contents (WEtOH). M 304
oil and liquid ethanol with a molar ratio of 1:20; the larger amount of ethanol was used to shift the reaction equilibrium to the right side and produce more ethyl esters, the proposed product.9-12 The predominant composition range of biodiesel was analyzed by a Hewlett-Packard 6890/5973 gas chromatography-mass spectrometry (GC/MS) machine. The GC/MS analysis results are shown in Figure 2. The C16-C18 fatty acid ethyl esters are the major components. The physicochemical properties of biodiesel are listed in Table 1. 2.3. Ebulliometric Method. The whole set of the apparatus was designed and constructed on the basis of the principle of the quasistatic method, which mainly consisted of inclined ebulliometers with pumplike stirrers, magnetic stirring systems, a pressure control and measurement system, and a temperature control and measurement system.7 (9) Demirbas, A. Biodiesel from Vegetable Oils via Transesterification in Supercritical Methanol. Energy ConVers Manage. 2002, 43, 2349-2356. (10) Saka; S.; Kusdiana, D. Biodiesel Fuel from Grapeseed Oil as Prepared in Supercritical Methanol. Fuel 2001, 80, 225-231. (11) Kusdiana, D.; Saka, S. Effects of Water on Biodiesel Fuel Production by Supercritical Methanol Treatment. Bioresour. Technol. 2004, 91, 289295. (12) Soumanou, M. M.; Bornscheuer, U. T. Improvement in LipaseCatalyzed Synthesis of Fatty Acid Methyl Esters from Sunflower Oil. Enzyme Microb. Technol. 2003, 33, 97-103.
The inclined structure can reduce the promoted height of the liquid, and the stirring function is useful for circulation of the solution. As a result, the ebulliometer reduces the heating power to achieve the aim of reducing the reflux ratio and liquid holdup factor effectively. Therefore, this method can enable one to keep the compositions of the fluid hardly changed during the procedure of vapor pressure measurements for multicomponent systems such as the fuels, complex mixtures of hydrocarbons of varying volatility.13 The bubble-point temperatures of a sample and a reference material (ethanol) in two separate ebulliometers were measured under the same pressure, which avoided the necessity of measuring the equilibrium pressure directly with a mercury manometer because the pressure could be calculated from the boiling temperature of ethanol and its well-known pressure-temperature behavior. The temperatures inside the two ebulliometers were measured with two standard platinum resistance thermometers connected to Keithley 195A digital multimeters. Vapor pressures at various temperatures were measured over the equilibrium pressure range from about 14 to 101.3 kPa.
3. Results and Discussion Bubble-point vapor pressures at various temperatures for 15 mixtures with different compositions of biodiesel and ethanol (13) Lei, Y.; Li, H.; Zhu, L.; Han, S. Isobaric Vapor-Liquid Equilibria of the Binary Mixtures 2-Methyl-3-buten-2-ol + Ethanol and 2-Methyl-3buten-2-ol + 1-Butanol. Fluid Phase Equilib. 2003, 206, 87-94.
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Table 2. Bubble-Point Vapor Pressure Data for Biodiesel + Ethanol Systems T/K
P/kPa
T/K
P/kPa
T/K
P/kPa
T/K
P/kPa
T/K
P/kPa
WETOH% ) 1.0000 320.04 25.47 323.40 30.01 326.87 35.47 329.26 39.61 331.60 44.14 333.33 47.74 335.28 52.14 337.36 57.20 339.35 62.27 345.26 80.00 346.66 84.93 348.38 91.07 350.82 100.40
WETOH% ) 0.8999 322.60 28.67 326.51 34.58 328.88 38.66 331.46 43.65 333.61 48.04 335.32 51.89 337.01 55.93 338.38 59.64 339.79 63.47 343.08 73.75 347.30 86.81 348.13 89.85 348.98 92.90 350.94 100.60
WETOH% ) 0.7999 314.72 19.05 314.77 19.08 319.46 24.26 319.49 24.27 322.90 28.78 322.95 28.79 324.77 31.50 324.81 31.52 327.98 36.69 330.68 41.61 332.98 46.25 335.01 50.67 335.03 50.69 342.07 67.74 342.45 68.33
WETOH% ) 0.7001 309.79 14.53 315.16 19.28 319.99 24.64 323.74 29.62 326.39 33.62 329.03 38.06 331.22 42.11 333.27 46.26 335.13 50.28 336.76 54.04 338.27 57.74 339.57 60.91 344.44 75.22 345.33 77.99 345.94 80.05
WETOH% ) 0.6002 310.07 14.56 315.18 19.06 319.60 23.85 322.74 27.88 325.40 31.69 327.92 35.69 330.08 39.46 332.13 43.35 334.07 47.34 335.72 50.95 337.22 54.46 338.63 57.89 341.29 64.90 342.41 68.11 343.59 71.56 344.90 75.65 346.15 79.67 347.34 83.64 351.85 100.34
WETOH% ) 0.5000 311.79 15.52 316.59 20.07 320.73 24.72 323.35 28.10 326.46 32.62 329.12 36.95 331.44 41.13 333.54 45.22 335.45 49.30 337.27 53.45 338.82 57.18 340.08 60.37 341.30 63.63 342.77 67.72 343.94 71.13 345.16 74.85 346.07 77.69 347.29 81.66 348.55 86.05
WETOH% ) 0.4000 312.30 15.92 317.36 20.38 321.21 24.71 324.61 29.16 327.62 33.65 330.27 38.02 332.68 42.44 334.85 46.77 336.67 50.70 338.54 54.97 340.19 59.02 341.66 62.84 342.92 66.25 344.31 70.035
WETOH% ) 0.3000 311.01 14.18 316.63 18.98 321.33 23.97 325.15 28.86 328.42 33.69 331.33 38.54 333.85 43.08 335.80 46.94 337.79 51.22 339.67 55.49 344.45 68.12 345.79 72.38 348.44 80.55 349.59 84.28 351.93 92.65
WETOH% ) 0.2000 313.37 14.85 320.30 21.00 323.85 24.91 327.48 29.58 330.43 33.94 333.74 39.57 336.12 43.93 338.56 48.61 340.31 52.55 341.86 56.40 343.46 60.47 344.84 64.55 346.27 68.60 347.71 72.98 351.77 85.90 353.27 90.10 354.39 93.99 356.09 100.50
WETOH% ) 0.1078 319.27 16.76 324.62 21.84 330.05 27.22 332.99 31.05 336.21 35.14 339.39 39.88 341.69 43.92 344.34 48.74 345.93 52.23 347.57 56.03 350.09 60.49 351.46 64.32 352.88 68.02 353.84 71.19 363.12 99.65
WETOH% ) 0.0799 324.51 17.19 328.75 21.51 332.41 25.57 335.61 29.35 339.01 33.90 341.74 38.03 345.14 43.42 347.62 48.21 350.18 54.04 352.28 58.98 356.72 70.05 358.65 74.89 359.91 79.30 364.69 94.36
WETOH% ) 0.0600 325.32 16.45 330.15 20.67 333.56 24.23 337.23 28.39 340.03 32.23 343.38 36.66 345.79 40.59 348.65 45.87 351.08 50.86 353.91 56.31 356.02 61.30 358.21 66.91 363.58 81.24 364.56 84.54 366.06 89.65 367.78 94.31 369.41 99.63
WETOH% ) 0.0402 330.60 16.60 335.32 21.44 339.26 25.68 342.77 30.09 346.65 34.73 349.67 38.72 352.06 42.45 356.71 51.88 362.12 62.63 363.58 67.69 365.94 72.87 368.20 78.16
WETOH% ) 0.0201 338.76 15.54 344.23 20.20 360.85 39.56 362.81 43.90 366.38 48.05 368.97 52.11 371.19 57.48 374.72 63.07 376.92 67.64 379.64 71.74 381.61 75.94 382.83 78.99 392.91 98.60
WETOH% ) 0.0000 363.98 18.43 370.74 20.87 388.19 23.31 392.93 25.41 400.30 27.88 423.70 33.69 440.21 38.54 454.88 43.08 466.87 46.94 479.76 51.22 492.21 55.49 505.32 60.12 516.13 64.03 527.22 68.12 538.56 72.38 548.60 76.22 559.78 80.55 569.29 84.28 578.98 88.13 590.21 92.65 608.26 100.00
were measured; the amount of each charged mixture is 100 mL. The experimental data are listed in Table 2, where W is the mass fraction, T is the bubble-point temperature, and P is the bubble-point vapor pressure. The vapor pressure data for ethanol are compared with the literature data14 in Figure 3. They are in reasonable agreement.
Figure 4 gives the change of vapor pressures for several mixtures with different compositions of biodiesel and ethanol against the temperature. The “pure” biodiesel has a lower vapor pressure, which can result in ignition delay and combustion problems, but the mixture with a low composition of ethanol has considerably high values. Therefore, the addition of ethanol
(14) Diogo, H. P.; Santos, R.; Nunes, P. M.; Minas da Piedade, M. E. Ebulliometric Apparatus for the Measurement of Enthalpies of Vaporization. Thermochim. Acta 1995, 249, 113-120.
(15) Lepori, L.; Matteoli, E. Excess Volumes of (Tetrachloromethane + an Alkanol or + a Cyclic Ether) at 298.15 K. J. Chem. Thermodyn. 1986, 18, 13-19.
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Energy & Fuels, Vol. 21, No. 2, 2007 1191
Figure 5. Bubble-point vapor pressure against composition at several temperatures (left) and departures of equilibrium pressure from the linear addition values (right).
Figure 6. Bubble-point temperature against composition at several pressures (left) and departures of equilibrium temperature from the linear addition values (right). Table 3. Correlation Results of Vapor Pressure by Antoine Equation for Biodiesel + Ethanol Systems Antoine equation coefficients WEtOH
datum points
1.0000 0.8999 0.7999 0.7001 0.6002 0.5000 0.4000 0.3000 0.2000 0.1078 0.0799 0.0600 0.0402 0.0201 0.0000
13 14 15 15 19 19 14 15 18 15 14 17 12 13 21
temperature range (K) 320-351 322-351 314-343 309-346 310-352 311-349 312-345 311-352 313-357 319-364 324-365 325-370 330-369 339-393 363-609
A
B × 10-3
C
AAD (kPa)
ARD (%)
16.750 14.789 12.474 16.650 16.460 15.809 17.116 17.083 16.795 16.736 14.419 14.976 10.254 7.911 7.016
3.679 2.618 1.692 3.633 3.536 3.191 3.968 3.953 3.815 4.188 2.719 3.082 1.072 0.498 1.437
47.800 93.750 137.220 49.782 53.511 67.560 35.823 37.070 42.878 18.219 89.426 72.234 186.526 242.658 12.861
0.03 0.14 0.12 0.02 0.01 0.03 0.06 0.07 0.29 0.32 0.29 0.21 0.58 0.53 0.18
0.06 0.24 0.31 0.04 0.03 0.08 0.17 0.14 0.47 0.64 0.66 0.36 1.33 1.03 0.61
had a critical effect on the vapor pressure and the phase equilibrium behaviors of the biodiesel. These results may be very useful in the development of environmentally friendly alternative fuels.
A nonlinear regression method is used to fit the vapor pressure data to Antoine’s equation. Table 3 gives the Antoine constants, together with the errors given by the average absolute deviation (AAD) and average relative deviation (ARD).7
1192 Energy & Fuels, Vol. 21, No. 2, 2007
From the correlation results, the bubble-point lines of P-WEtOH at several temperatures and T-WEtOH at several pressures are shown in Figures 5 and 6, along with the departures of the equilibrium pressure or temperature from the corresponding linear addition values. Clearly, the mixtures of biodiesel and ethanol have visible positive deviations on the values of pressures from Raoult’s law and visible negative deviations on the values of temperatures. The pressure departure increases with increasing temperature, and the maximum pressure departure ∆P among the six temperatures is about 90 kPa at 365 K, and the maximum temperature departure is about -230 K at 100 kPa. The positive deviation from Raoult’s low suggests that hydrogen-bonded polymers are broken down to monomers by addition to ethyl esters.15
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4. Conclusion The transesterification of sunflower seed oil was carried out in supercritical ethanol without using any catalyst. Bubble-point vapor pressures for the mixtures of biodiesel and ethanol were measured by comparative ebulliometry with inclined ebulliometers. The bubble lines of equilibrium pressure or temperature versus the composition can be obtained from the correlation on the experimental results. The addition of ethanol has a critical effect on the vapor pressure of fuels. The mixtures of biodiesel and ethanol appear with very large positive deviations from Raoult’s law. Acknowledgment. The authors are grateful to the National Natural Science Foundation of China (No. 20403015) EF0605807
