Understanding the Thermal Behaviour of Blends of Biodiesel and Diesel: Phase Behaviour of Binary Mixtures of Alkanes and FAMEs

The study of the phase behaviour of binary mixtures of fatty acid methyl esters (FAME) and alkanes are here studied to understand the behaviour of the mineral diesel/biodiesel blends. Six binary mixtures of the most common saturated methyl esters (methyl stearate or methyl palmitate) with a saturated alkane (hexadecane, octadecane or eicosane) were evaluated by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and optical microscopy. The binary mixtures studied show a more complex phase behaviour than previously reported, which it is shown to be dependent of the size difference between the alkyl chain length of the esters and the alkanes. It was found that in mixtures with equal alkyl chain length a co-crystal is formed, and when the alkane chain length is larger than in the FAME by two methylene groups the formation of a solid-solution increasing significantly the solid phase stability is observed. The results and conclusions derived from the phase behaviour of this set of binary mixtures, between alkanes and methyl esters, can be used as a model for the interpretation of the cloud and pour points increase in biodiesel blends (in special the ones with a rich fraction of methyl palmitate or methyl stearate when combined with mineral diesel with high content of octadecane or eicosane respectively).