Effects of High-Melting Methyl Esters on Crystallization Properties of Fatty Acid Methyl Ester Mixtures

摘要

Biodiesel is a renewable alternative diesel fuel made from vegetable oils and animal fats. The most common forms of biodiesel in the U.S. are fatly acid methyl esters (FAME) made from soybean oil, canola oil, used cooking oils, waste greases, and tallow. Cold-flow properties of biodiesel depend on the crystallization properties of high-melting FAME in the mixture. For soybean oil FAME, the saturated FAME (SFAME) species have melting points (MP) that are more than 45°C higher than the unsaturated FAME (UFAME) species. The present study evaluated the use of equations from freezing point depression theory to model crystallization onset temperatures (Tj) for binary mixtures of SFAME (MeCIO to MeC20) and UFAME (MeC18:l and MeC18:2). Melting and crystallization properties were determined by differential scanning calorimetry (DSC) heating and cooling curve analyses. Results showed that the DSC scan rate did not significantly affect Onset temperatures or peak enthalpies for analyses of pure FAME. Hysteresis effects were observed for pure FAME where freezing points (FP) from DSC cooling curves were at slightly lower temperature than melting points (MP) from melting curves. For independent crystallization of SFAME from binary mixtures with UFAME, the effects of changes in heat capacity (A.C_P) as the mixture temperature decreased below the standard MP of the SFAME were negligible. Calculated T_f values were greater than FP from DSC analysis of mixtures, leading to the conclusion that binary SFAME/UFAME mixtures deviate from ideal solution behavior at low temperatures.