Transesterification of heated rapeseed oil for extending diesel fuel

Fatty acid methyl esters are well established as an alternative fuel called “biodiesel.” For economic reasons, used frying oil is an interesting alternative feedstock for biodiesel production. The chemical changes that occur during heating of rapeseed oil, especially the formation of polymers, were investigated. Heated rapeseed oil samples were transesterified with methanol and analyzed by size-exclusion chromatography. During heating, the amount of polymers in the starting oil increased up to 15 wt%, but only up to 5 wt% in the transesterified samples. So during transesterification, dimeric and trimeric triglycerides in the starting oil were mainly converted into monomeric and dimeric fatty acid methyl esters. The amount of polymeric fatty acid methyl esters had a negative influence on fuel characteristics. After 6 h of heating, the amount of Conradson carbon residue and after 16 h the viscosity exceeded that of the existing specifications for biodiesel. Therefore, the amount of polymers in waste oil is a good indicator for the suitability for biodiesel production. Presented in part at the 89th Annual Meeting, American Oil Chemists’ Society, Chicago, IL, May 1998.