Solidification behavior of acylglycerols in fatty acid methyl esters and effects on the cold flow properties of biodiesel

The formation of precipitates in biodiesel (comprising fatty acid methyl esters [FAMEs] obtained from plant oils) can lead to the clogging of fuel filters. Such precipitates are often caused by the solidification of acylglycerols (monoacylglycerols [MAGs], diacylglycerols [DAGs], and triacylglycerols [TAGs]) that have higher melting points than FAMEs. Based on our prior study on the solidification behavior of MAG/FAME binary mixtures, the present work investigated the behavior of various DAGs and TAGs combined with FAMEs. Differential scanning calorimetry was used to clarify the effects of acylglycerols on the cold-flow properties of biodiesel. When DAGs and TAGs were added to FAMEs, the liquidus temperatures (above which the mixtures were completely liquid) increased steeply even at low concentrations. This same behavior was observed previously in trials with MAGs, indicating that all acylglycerols readily precipitate in combination with FAMEs. However, thermodynamic analyses established that the reasons for such precipitation were different for different compounds. MAGs precipitates because they contain two hydroxyl groups and therefore have a low affinity for FAMEs. In contrast, TAGs precipitates as a result of their high enthalpies of fusion (which in turn are caused by high molecular weights), while both factors affect the precipitation of DAGs. A non-solid-solution thermodynamic model that assumes a eutectic system was found to accurately predict the liquidus temperatures of binary and multicomponent mixtures containing various acylglycerols with FAMEs