Effect of low-level impurities on low-temperature performance properties of biodiesel

Biodiesel, a renewable fuel consisting of fatty acid methyl esters and made from lipid feedstocks, has presented persistent cold weather operability problems that are not predicted using the standards tests common in the petroleum refining industry. These problems have been referred to as "precipitate formation above cloud point" and are known to be caused by minor impurities. This study investigates the fundamental causes of this issue. To this end, water, steryl glucosides (SG), and saturated monoglycerides (SMGs) were spiked into 100% biodiesel (B100) from various feedstock sources. Only SMGs were found to have a significant effect on the cloud point (CP) and final melting temperature (FMT) in four B100 samples with a range of CP. A large difference between FMT and CP indicates that a metastable phase of SMG forms initially and can transform into a more stable, less soluble polymorph over time or upon heating. This occurred for SMG content above approximately 0.2 to 0.3 wt%. For more saturated B100, a large FMT-CP difference was only observed at slower heating rates, suggesting a slower rate of phase transformation. beta-monostearin solubility (the most stable phase) in B100 was measured as a function of temperature. CP measurements suggest the metastable phase is as much as 10 times more soluble than the beta phase. Differential scanning calorimetry experiments suggest that the metastable phase is a hydrated alpha-gel. SMGs at concentrations above 0.24 wt% caused failure of the cold soak filtration test (ASTM D7501); however, at higher water concentrations (similar to 1200 ppm), the effect of SMGs was significantly reduced. Addition of SGs had no effect on cold soak filterability.