Evaluation of optimal activity coefficient models for modeling and simulation of liquid-liquid equilibrium of biodiesel + glycerol + alcohol systems

Biodiesel is considered a good alternative for diesel engines, given its characteristics, for being non-toxic and for being a more environmental-friendly fuel. A liquid-liquid equilibrium (LLE), comprising biodiesel + glycerol + alcohol, occurs during the production of biodiesel. The use of reliable thermodynamics models is essential to describe biodiesel + glycerol + alcohol systems as to design and analyze the process and its equipment. In this work, experimental data of 34 different systems (27 for biodiesel + glycerol + alcohol; and 7 for methyl oleate + glycerol + methanol) were used for parameter estimation and to evaluate four of the main activity coefficient models (ASOG, UNIFAC, UNIFAC-LLE and UNIFAC-Dortmund) that could be used with biodiesel systems. Additionally, binary parameters for glycerol/methanol, methanol/methyl oleate and glycerol/methyl oleate have been estimated by NRTL and UNIQUAC models. Root mean square deviations (RMSD) have been used to compare experimental and simulated data. RMSD analysis showed that the UNIFAC-Dortmund model was the model that best represented the biodiesel liquid-liquid equilibrium. Best representation of the system followed the order: UNIFAC-Dortmund > ASOG > UNIFAC-LLE > UNIFAC > UNIQUAC > NRTL.