Energy Fuels 2010, 24, 5708–5716 Published on Web 09/10/2010
: DOI:10.1021/ef100382f
High-Pressure Viscosity of Biodiesel from Soybean, Canola, and Coconut Oils )
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Andrew M. Duncan,† Azita Ahosseini,† Reece McHenry,† Christopher D. Depcik,‡,§ Susan M. Stagg-Williams,*,†,‡, and Aaron M. Scurto*,†,‡, )
† Department of Chemical and Petroleum Engineering, ‡DOT-KS Transportation Research Institute (TRI), §Department of Mechanical Engineering, and Center for Environmentally Beneficial Catalysis, University of Kansas, 1530 West 15th Street, 4132 Learned Hall, Lawrence, Kansas 66045
Received March 29, 2010. Revised Manuscript Received July 2, 2010
Current and future injector designs for diesel engines approach pressures of greater than 100 MPa. However, the high-pressure physical properties, such as viscosity, of biologically derived diesel fuel (biodiesel) are nearly absent in the literature. This study focuses on the viscosity of biodiesel samples, fatty acid methyl esters (FAMEs), derived from soybean oil, soybean oil from Vistive soybeans, canola oil, recycled canola oil that has been used in cooking and frying, and coconut oil from 283.15 to 373.15 K and pressures up to 131 MPa. Petroleum-derived diesel (ultra-low sulfur, number 2 diesel) has also been investigated to compare to the biodiesel samples. The viscosity of the samples increases linearly with pressure until approximately 35 MPa, followed by a higher order response to pressure. Except for coconutoil-derived biodiesel, the biodiesel samples have viscosities that are greater than petroleum-derived diesel at both ambient and elevated pressures. However, at lower temperatures and high pressures, the diesel and biodiesel samples become more similar. The viscosity of the biodiesel samples with pressure can increase nearly 300% over the pressure range investigated over their respective ambient-pressure viscosity; number 2 diesel increases up to ∼400% over similar pressures. The biodiesel samples at 283.15 K were found to experience pressure-induced cloud points (solid-liquid equilibrium) from 70 to 100 MPa, which significantly increases their viscosity. The Tait-Litovitz equation was found to correlate the data very well over the large range of both temperature and pressure.
affect properties, such as cloud point, viscosity, density, oxidative stability, etc.2,3 Current fuel injection systems for compression-ignition (diesel) engines reach pressures approximately 1600 and 1400 bar for passenger car and commercial vehicle systems, respectively.4 Future systems are targeting even higher pressures in excess of 2000 bar while also expanding the possible number of injections per cycle from five to nine.5 Under these pressures, the liquid viscosity of the fuel can increase up to approximately 10 times atmospheric levels.6 Upon injection of the fuel in the cylinder, the large depressurization of the fuel results in a significant gradient of the viscous properties of the fluid. A proper understanding of these values is therefore required for modeling the discharge coefficient and flow rate through the injector. These parameters are key in ensuring the proper rate shaping of the fuel injection spray to optimize performance while minimizing emissions.7 Generally speaking, biodiesel and biodiesel blends from a range of feedstocks are considered to have lower emissions of
1. Introduction Biodiesel, as an alternative fuel, has undergone significant research, development, and large-scale distribution over the past few years. Biodiesel is a mixture of fatty acid esters and is commonly used as an alternative to or blended with number 2 petroleum diesel. The use of biodiesel as an alternative fuel has been driven by the fact that, in comparison to petroleumbased diesel, biodiesel is domestically produced and renewable, has low sulfur and aromatic contents, has low particulate emissions, has high energy content, and can be used in conventional compression-ignition (diesel) engines with relatively little or no modification.1 Typical biodiesel is produced via the transesterification of fats and oils with an alcohol, such as methanol. The composition of the fat or oil feedstock determines the fatty acid methyl ester (FAME) profile and ultimately the chemical and physical properties of the resulting biodiesel. For example, biodiesel produced from soybean oil typically contains methyl ester profiles rich in oleate (C18:1) and linoleate (C18:2), while biodiesel from coconut oil is rich in laurate (C12:0) and myristerate (C14:0). These differences in compositions can
(3) Ramos, M.; Fernandez, C.; Casas, A.; Rodriguez, L.; Perez, A. Influence of fatty acid composition of raw materials on biodiesel properties. Bioresour. Technol. 2009, 100 (1), 261–268. (4) Bosch, R. Bosch Automotive Handbook, 7th ed.; Robert Bosch GmbH: Stuttgart, Germany, 2007. (5) Tomishima, H.; Matsumoto, T.; Okl, M.; Nagata, K. The advanced diesel common rail system for achieving a good balance between ecology and economy. SAE Tech. Pap. 2008-28-0017, 2008. (6) Giannadakis, E.; Gavaises, M.; Theodorakakos, A. The influence of variable fuel properties in high-pressure diesel injectors. SAE Tech. Pap. 2009-01-0832, 2009. (7) Schmidt, D. P.; Corradini, M. L. The internal flow of diesel fuel injector nozzles: A review. Int. J. Engine Res. 2001, 2 (1), 1–22.
*To whom correspondence should be addressed. Telephone: (785) 864-4947. Fax: (785) 864-4967. E-mail:
[email protected] (S.M.W.);
[email protected] (A.M.S.). (1) Graboski, M.; McCormick, R. Combustion of fat and vegetable oil derived fuels in diesel engines. Prog. Energy Combust. Sci. 1998, 24 (2), 125–164. (2) Rodrigues, J.; Cardoso, F.; Lachter, E.; Estev~ao, L.; Lima, E.; Nascimento, R. Correlating chemical structure and physical properties of vegetable oil esters. J. Am. Oil Chem. Soc. 2006, 83 (4), 353–357. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 5708–5716
: DOI:10.1021/ef100382f
Duncan et al.
carbon monoxide (CO), total hydrocarbons (THC), and particulate matter (PM) than standard diesel fuel but equal or slightly higher levels of nitrogen oxides (NOx).8 In some cases, recalibration of engines based on blending of biodiesel with other fuels has resulted in decreasing NOx concentrations.9,10 Part of the change in NOx relates to the inadvertent advancing of injection timing caused by the rapid transfer of the pressure wave from the fuel injection pump to the fuel injector, causing it to open earlier. This pressure wave is a function of the properties of the fuel. Choi et al. attributed the change in fuel injection timing to the higher viscosity of biodiesel;11 however, others conclude that viscosity does not affect timing and the bulk modulus, which effects the speed of sound as the main factor.12-15 In any case, viscosity does affect the pressure dynamics within the fuel lines and during the spray breakup process inside the cylinder. This is a concern for engine manufacturers while designing the fuel injection systems for these engines under the range of potential pressure and temperature conditions (up to 3500 bar and 273.15373.15 K).16,17 Schaschke and co-workers18 have demonstrated a new high-pressure viscosity apparatus that uses a falling sinker to measure the viscosity of oils and free fatty acids to 150 MPa. This method requires very accurate knowledge of the high-pressure density of the liquid sample to compute the viscosity, which is often unknown and must be estimated. Very recently, they report on the viscosity of biodiesel from one sample of sunflower oil and one sample of waste cooking oil at temperatures below e293.15 K and pressures to
