Submitted to: Fuel
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/7/2007
Publication Date: 3/7/2007
Citation: Knothe, G.H., Steidley, K.R. 2007. Kinematic viscosity of biodiesel components (fatty acid alkyl esters) and related compounds at low temperatures. Fuel. 86:2560-2567. Interpretive Summary: Biodiesel is an alternative diesel fuel derived from vegetable oils such as soybean oil or other sources such as animal fats and waste frying oils. Fatty acids, in the form of triacylglycerols (triglycerides) are the major components of fats and oils. Biodiesel is made by a chemical reaction of the vegetable oil or animal fat with chemical compounds called alcohols. The resulting materials are also known as fatty acid alkyl esters. These fatty acid alkyl esters have different properties which affect the overall properties of a biodiesel fuel. An important fuel property is viscosity, which is the thickness of a liquid. This property affects the performance of biodiesel in a diesel engine. This work describes how the thickness of various fatty acid alkyl esters and biodiesel depends on the temperature, especially low temperatures. This information is important because biodiesel is known to have some performance problems at low temperatures as they occur in colder climates. This information ultimately is also important for determining which fatty alkyl esters make a better biodiesel fuel and can be used in designing an improved fatty acid composition of vegetable oils and biodiesel.
Technical Abstract: Biodiesel, defined as the mono-alkyl esters of vegetable oils and animal fats, has undergone rapid development and acceptance recently. Kinematic viscosity is one of the fuel properties specified in biodiesel standards, with 40 deg C being the temperature at which this property is to be determined and ranges of acceptable kinematic viscosity given. While data on kinematic viscosity of biodiesel and related materials at higher temperatures are available in the literature, this work reports on the kinematic viscosity of biodiesel and a variety of fatty acid alkyl esters at temperatures from 40 deg C down to -10 deg C in increments of 5 deg C using the appropriately modified standard reference method ASTM D445. Investigating the low-temperature viscosity of biodiesel and its components is important because of the problems associated with the use of biodiesel under these conditions. Such data can also aid in developing biodiesel fuels optimized for fatty ester composition. Compounds tested included a variety of saturated, monounsaturated, diunsaturated and triunsaturated fatty esters, methyl ricinoleate, in which the OH group leads to a significant increase in viscosity as well as triolein, as well as some fatty alcohols and alkanes. Esters of oleic acid have the highest viscosity of all biodiesel components that are liquids at low temperatures. The behavior of blends of biodiesel and some fatty esters with a low-sulfur diesel fuel was also investigated. An index termed here the low-temperature viscosity ratio (LTVR) using data at 0 deg C and 40 deg C (divide viscosity value at 0 deg C by viscosity value at 40 deg C) was used to evaluate individual compounds but also mixtures by their low-temperature viscosity behavior.