VEGETABLE OIL-BASED FUELS, ADDITIVES AND COPRODUCTS
Location: Bio-oils Research Unit
Title: Kinematic viscosity of fatty acid methyl esters: Prediction, calculated viscosity contribution of esters with unavailable data, and carbon-oxygen equivalents
Submitted to: Fuel
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 27, 2011
Publication Date: July 12, 2011
Citation: Knothe, G.H., Steidley, K.R. 2011. Kinematic viscosity of fatty acid methyl esters: Prediction, calculated viscosity contribution of esters with unavailable data, and carbon-oxygen equivalents. Fuel. 90:3217-3224.
Interpretive Summary: Biodiesel is an alternative to diesel fuel obtained from petroleum. It is produced from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils by means of a chemical reaction called transesterification. A major reason for producing biodiesel is that biodiesel is thinner (i.e., flows more easily) than vegetable oil or the other feedstocks. This leads to reduced engine problems when using biodiesel compared to its feedstocks. The physical property associated with this is known as viscosity. In this work, a method is described for determining the viscosity of components of biodiesel for which no data are otherwise available. Using these data, the viscosity of biodiesel can be predicted. The viscosity of biodiesel components is also compared to that of components in diesel fuel from petroleum. A concept for better comparison of such materials is developed. These results lead to a better understanding of biodiesel performance. Therefore, the results are of interest to biodiesel researchers and producers and, thus, indirectly affect other parties interested in this fuel.
Many properties of biodiesel, the mono-alkyl esters of vegetable oils, animal fats or other triacylglycerol-containing feedstocks, are largely determined by its major components, the fatty acid alkyl esters. Therefore, information on the properties of individual components and their interaction is essential to understanding and predicting the properties of biodiesel fuels. Viscosity, which affects flow and combustion of a fuel, is such a property. In previous literature, the effect of the structure of fatty esters on viscosity was discussed. However, these data are largely confined esters with an even number of carbon atoms in the chain and that are liquid at 40EC. To gain a better understanding of kinematic viscosity, this work additionally reports data on esters with an odd number of carbons in the fatty acid chain and some unsaturated fatty esters. Furthermore, the kinematic viscosity of some biodiesel fuels is affected by components that are solids at 40EC. A method based on polynomial regression for determining the calculated viscosity contribution (CVC) of esters that are solid at 40EC (saturated esters in the C20-C24 range) or esters which are not available in pure form is presented as these values are essential for predicting the kinematic viscosity of mixtures containing such esters. Similarly, kinematic viscosity values can be obtained for esters that are liquid but for which no viscosity values in the pure form are available. The kinematic viscosity data of esters are compared to those of aliphatic hydrocarbons in the C6 to C18 range and those of dimethyl diesters. The increase of kinematic viscosity with an increasing number of CH2 groups in the chain is non-linear and depends on the terminal functional groups, chain length and double bonds. To illustrate this effect, carbon-oxygen equivalents (COE) are used in which the number of carbon and oxygen atoms are added. A straightforward equation, taking into account only the amounts and kinematic viscosity values of the individual neat components, suffices to predict the viscosity of mixtures of fatty esters (biodiesel) at a given temperature.