Energy Fuels 2010, 24, 664–672 Published on Web 11/13/2009
: DOI:10.1021/ef900818s
Estimating the Viscosity of Vegetable Oil and Biodiesel Fuels K. Anand, Avishek Ranjan, and Pramod S. Mehta* Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai-600036, India Received July 30, 2009. Revised Manuscript Received September 28, 2009
Among the various alternative fuels, vegetable-oil-based fuels have been attracting greater attention as a promising alternative to fossil diesel fuel in compression ignition (CI) engines. Fuel viscosity has a definite effect on fuel injection, spray development, and combustion processes of CI engines; hence, the viscosity estimation of new candidate fuels is significant. This paper discusses the methodologies used to estimate the viscosities of vegetable oil and biodiesel fuels, based on their fatty acid composition. While the methyl ester composition of biodiesel is directly related to the fatty acid composition of the oil, a basis for determining the triglyceride composition to estimate the straight vegetable oil viscosity is elucidated in the paper. The proposed methodologies are validated over a wide range of available viscosity data for vegetable oils and biodiesel fuels of varying composition and for varying temperatures. A comparison of the estimated viscosities with the measured values for 13 vegetable oils and 14 biodiesel samples shows an agreement within a predicted error of 10%.
data on engine specific fuel consumption, brake thermal efficiency, and nitric oxide emissions with biodiesel fuel operation.5-16 These variations may be due to changes in engine type, engine operating conditions, and the type of vegetable oil that is used to produce biodiesel fuel. However, it is strongly believed that the changes in fuel composition are the major cause for variations in engine performance and emission characteristics operated on biodiesel fuels. Graboski and McCormick17 reported wide variations in the measured properties of the same biodiesel fuel, because of changes in its fatty acid composition. Allen18 observed significant differences in viscosities of different biodiesel fuels and opined that it may be a cause for variations in the performance of biodiesel engines. These observations concerning fuel variability factors and their effects on engine performance require characterization of the biodiesel fuels based on their origin and composition. More so, a methodology to estimate fatty acids properties can facilitate the production of genetically modified vegetable oils meeting the characteristics of a designer’s fuel for compression ignition engines.19
1. Introduction The concerns on depleting fossil fuel reserves, pollution due to conventional fuels, and stringent emission norms are the driving forces for research on alternative fuels. Vegetable-oilbased fuels seem to have become an attractive alternative for diesel fuel, because of their better ignition quality, comparable energy content, higher density, greater safety (due to their higher flash point), nontoxic character of their exhaust emissions, almost-zero sulfur content, cleaner burning, and renewable nature.1,2 Straight vegetable oils (SVOs) are primarily mixtures of triglycerides that contain three fatty acids and one glycerol. Fatty acids vary in their carbon chain length and number of double bonds. The fatty acids usually found in vegetable oils are palmitic (C 16:0), stearic (C 18:0), oleic (C 18:1), linoleic (C 18:2), and linolenic (C 18:3). Triglycerides are bulky molecules with a high molecular mass, which contributes to the much higher viscosity of vegetable oils, compared to methyl esters. The higher viscosity, lower volatility, and the poor oxidative stability due to the polyunsaturated character of the raw vegetable oils restrict their use in diesel engines.3 Therefore, a transesterified fuel (biodiesel) has found wider acceptability without any engine modifications.4 It is observed that there are wide variations in the reported
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*Author to whom correspondence should be addressed. Tel.: þ91 44 22574670. Fax: þ91 44 22574652. E-mail:
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Energy Fuels 2010, 24, 664–672
: DOI:10.1021/ef900818s
Anand et al.
Table 1. Summary of the Empirical Coefficients for the Viscosity Correlations of Vegetable Oilsa Empirical Coefficients (see eq 1) vegetable oils investigated
crambe rapeseed corn soybean milkweed lesquerlla coconut
26-vegetable oilsa 26-vegetable oilsa
dependent variable/ range of applicability (°C)
T, 23.9-110 T, 23.9-110 T, 23.9-110 T, 23.9-110 T, 23.9-110 T, 23.9-110 T, 37.8 -110
T, 0-100 T and IV/SV, 0-100
A
absolute error (%)
B
C
Data Taken from Noureddini et al. 2.028 926.59 -24.462 6210.21 -19.56 5225.44 -20.06 5323.74 -20.385 5403.03 6.612 -6288.77 1.095 -2251.54
-154.27 -0.02667 -0.01994 -0.02063 -0.02095 1780000 917780
0.07 0.07 1.32 1.44 1.91 1.65 0.12
Data Taken from Dutt and Prasad -0.6298 273.66 -1.4 þ 0.25 (IV/SV) 500-375 (IV/SV)
88.61 140-85 (IV/SV)
14.5 13.0
a Almond, butter fat, coconut, cod liver, corn, cottonseed, groundnut, cottonseed (hydrogenated), rapeseed, soybean, lard, lardolein, linseed, neatsfoot, mustard, olive, palm, palm kernel, palm olein, rapeseed, raw perilla, refined whale, sardine, soybean, sunflower, tallow.
Noureddini et al.23 and Dutt and Prasad24 proposed a simple form of eq 1 that was comprised of only the first two terms, which include the empirical constants A, B, and C. Regression analysis of the measured viscosity for seven vegetable oils that was performed by Noureddini et al.23 yielded values of the empirical coefficients individually with an estimation error of
