In this study, we demonstrate that under sub/supercritical transesterification (SCTE) conditions and with use of acid catalysts, a relatively high ethanol-based biodiesel (FAEE) yield can be achieved. We first studied FAEE thermal decomposition reactions and the effect on fuel viscosity and cold flow properties, and then evaluated the SCTE reaction kinetics and phase behavior during the SCTE reaction process.
In the FAEE thermal stability study, prepared FAEE was thermally stressed in batch reactors at 250-425 oC for reaction times ranging from 3 to 63 min, with and without the presence of ethanol. Biodiesel samples were collected and analyzed by GC-FID and GC-MS. The results show that FAEE were stable at 250 and 275 oC, and stability reduced as temperature and heating time increased. The decomposition reactions consist of isomerization and pyrolysis to form isomers, smaller chain FAEEs, hydrocarbons, and organic acids. Compared with methanol-based biodiesel (FAME) thermal decomposition from our previous study, FAEE decomposition at high temperature generated much more organic acids. It also shows that FAEE is much less stable at high temperatures than FAME. Kinetics of FAEE biodiesel decomposition was simulated using both reversible and irreversible first order reaction models, and results show that the reversible model was superior to the irreversible one. Dynamic viscosity was measured by a micro viscometer, and cold flow properties were characterized by differential scanning calorimetry (DSC). The crystallization onset temperature determined by DSC correlates with cold flow properties. The influence of FAEE thermal decomposition reactions on those properties are discussed.
Based on the FAEE thermal stressing experiments, the reaction temperature and residence time applied to the SCTE experiments were selected to minimize FAEE decomposition. A visualization system was designed to observe phase behavior of the reaction system at different conditions, and it shows that at pressures of 200 bar for the temperature range studied the ethanol-oil binary system formed a homogeneous phase. Accordingly the reaction pressure was set to 200 bar for all runs. The effect of different additives (water, acetic acid, sulfuric acid) on the FAEE yield were studied. A small amount of sulfuric acid (i.e. 0.1 wt%) can significantly improve the FAEE production yield. Acetic acid also acted as a catalyst. The presence of water benefited the yield, which means lower cost hydrated ethanol is an ideal choice in terms of both saving production cost and increasing FAEE yield. The kinetics of FAEE synthesis at SCTE conditions with and without acids was simulated using one-step and three-step models. The reaction rate constants and activation energies for selected models were determined and compared with literature data.
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