Thermal Degradation of Fatty Acids in Biodiesel Production by Supercritical Methanol

Abstract

In this study, thermal degradation of the fatty acids of sunflower seed oil during biodiesel production by supercritical methanol method was investigated. The term biodiesel means the monoalkyl esters of long chain fatty acids made from biolipids such as vegetable oils, animal fats, tall oil or algae oils. The oils and fats contain polyunsaturated fatty acid chains that their double bonds have highly chemical reactivity. The thermal degradation occurs on the double bonds of unsaturated aliphatic carbons chains in fatty acids. Oxidation of biodiesel is results in the formation of hydroperoxides. The formation of the hydroperoxide follows a well known peroxidation chain mechanism. The olefinic unsaturated fatty acid oxidation is a multi-step reaction process where primary products decompose and chemically interact with each other to form numerous secondary oxidation products. The content of linoleic acid in the sunflower seed oil is 72.4%, while the linoleic acid account for 62.5% of the total fatty acids in the biodiesel. The proportion of the linoleic acid is lower in the biodiesel obtained by supercritical methanol transesterfication method. The viscosity of biodiesel increases with increasing of thermal degradation degree due to the trans isomer formation on double bonds. The decomposition of biodiesel and its corresponding fatty acids linearly increases from 293 K to 625 K. The densities of biodiesel fuels decreased linearly with temperature from 293 K to 575 K. The combustion heat of biodiesel partially decreases with increasing of thermal degradation degree.