Effect of Saturated Monoglyceride Polymorphism on Low-Temperature Performance of Biodiesel

To investigate precipitates above the cloud point (CP) in biodiesel, three saturated monoglycerides (SMGs), monomyristin, monopalmitin, and monostearin, were spiked into distilled soy and animal fat-derived B100. It was shown that above a threshold or eutectic concentration the SMGs significantly raise the CP of B100. A comparison to published data suggests that commercial B100 has SMG content in the same range as the eutectic point. SMGs have an even greater impact on the final melting temperature (FMT, as measured when the sample is heated) at concentrations above the eutectic point. These results were verified and visualized using a controlled temperature stage microscope. It was shown that the FMT was highly dependent on the rate of heating. It is hypothesized that a lower melting point crystalline form of the SMG forms upon rapid cooling and then transforms into a more stable, higher melting point crystalline form when slowly heated or held at constant temperature. The CP and FMT results of this study were compared to an ideal solution thermodynamic model. The model was able to provide reasonable prediction of the eutectic point but was less successful at predicting CP and FMT above the eutectic.