Prediction of solidification behavior of biodiesel containing monoacylglycerols above the solubility limit

Abstract

Monoacylglycerols (MAGs) are typical impurities in biodiesel (fatty acid methyl esters, FAMEs) and are often the cause of solid precipitation because of their high melting points. In this study, the liquidus temperature of biodiesel, below which solidification of biodiesel components can occur, was measured by differential scanning calorimetry or visual observation, and was predicted by thermodynamic models. First, the solubility limit of MAGs, defined as the total MAG content above which MAGs can solidify before FAMEs, was found to be about 0.25 wt% for coconut methyl esters and about 0.5 wt% for palm methyl esters and rapeseed methyl esters. For biodiesel containing MAGs above the solubility limit, the compound formation (CF) model showed good agreement with the experimentally determined liquidus temperatures. This thermodynamic model assumed different types of MAGs solidifying simultaneously while forming molecular compounds. However, within the range of the total MAG content of actual biodiesel (typically less than 0.7 wt%), the number of fitting parameters in the CF model was excessive. This led to the use of a simplified version of the CF model with only one parameter, which still fitted the experimental results well. One parameter value was determined for biodiesel from one feedstock, allowing the liquidus temperature of biodiesel from a known feedstock to be predicted based only on the total MAG content, even for biodiesel containing diacylglycerols in addition to MAGs.