An empirical approach in predicting biodiesel viscosity at various temperatures

A thermodynamic model is proposed for the determination of kinematic viscosities of saturated fatty acid methyl esters (FAMEs) of various chain lengths at different temperatures. The linearity of the natural logarithm of viscosity-carbon number, plot is limited to a narrow carbon number range. The predicted viscosities of FAMEs of C_12:0-C_18:0, which are commonly found in vegetable oils and used as biodiesels, agree well with the experimental values. The highest difference is 0.354 cSt (5.60%), for methyl stearate at 40 °C. When the proposed method for viscosity calculation of saturated FAMEs are used in combination with the methods for viscosities of biodiesel the mixtures, the predicted viscosities agree well with the values reported in the literatures and the measured values. The differences between the predicted viscosities and those reported in the literatures (at 40 °C) are 1.08 to 8.56% (for eight different vegetable oil methyl esters). The differences between the predicted viscosities and the measured values for coconut methyl esters, at 25, 40 and 50 °C are 9.20, 5.53 and 5.57%, respectively. The differences are slightly higher than those of palm oil methyl esters (4.48, 2.06 and 2.48%, respectively).The proposed method can also be applied to predict the viscosities of free fatty acids and it is speculated it may be applied to other homologous series as well.