Thermal decomposition of ethanol-based biodiesel: Mechanism, kinetics, and effect on viscosity and cold flow property

Thermal decomposition of the ethanol-based biodiesel (FAEEs) was evaluated in batch reactors by thermal exposure at 250-425 °C for durations from 3 to 63 min, with and without the presence of ethanol. The results of GC analysis show that FAEEs were relatively stable at 250 and 275 °C, and stability reduced as temperature and heating time increased. Major decomposition reactions consisted of isomerization, polymerization, and pyrolysis reactions to form isomers, dimers/polymers, smaller chain FAEEs, hydrocarbons, and carboxylic acids the latter of which are not generated in the decomposition of methanol-based biodiesel (FAMEs). This suggests that when applying the sub/supercritical ethanol technology to produce FAEEs, the reaction temperatures must be modest to avoid generating acids which increases the acid value of the final product. A three-lump model was used to predict concentrations of compounds in the FAEEs stressed at 250-325 °C. The decomposition degree of the FAEEs biodiesel was simulated by using first order one-step reaction models (reversible and irreversible), and results show that the reversible model performed better than the irreversible model except for data of 425 °C. The data show that FAEEs are less stable and decompose more completely than FAMEs. The presence of ethanol was shown to reduce the decomposition. Dynamic viscosity was measured, and differential scanning calorimetry (DSC) was used to determine the crystallization onset temperatures to represent cold flow properties. The values are significantly influenced by the polymerization and pyrolysis reactions.