Liquid-Liquid Phase Equilibrium Of Biodiesel Systems

As an emerging renewable energy, biodiesel has attracted extensive research. At present, research on biodiesel focused on development of new catalysts such as enzyme catalyst and solid acid catalyst and optimization of reaction conditions. There are little reports on phase equilibrium data of biodiesel systems which are important to equipment design of biodiesel plants. Separation limits are determined by liquid-liquid phase equilibrium (LLE) data, based on which proper extractants are chosen. LLE data is also the basis of chemical process simulation for new systems. The lack of LLE data for biodiesel systems limits the development of separator design and process simulation.Determination of phase equilibrium data and thermodynamic calculation model is the theoretical basis for the separation process. Cloud point turbidity and gas chromatography (GC) are widely used in test of phase equilibrium data. GC obtained better development for its high accuracy and repeatability. In this paper, high temperature GC (HT-GC) is used because soybean oil and rapeseed oil in the components have extremely high boiling points.Using self-designed constant temperature oscillator, LLE data of biodiesel systems (methyl palmitate + methanol + water, methyl palmitate + ethanol + water, methyl soyate + methanol + glycerol, methyl soyate +methanol + soybean oil, rapeseed oil methyl ester + methanol + glycerol and rapeseed oil methyl ester + methanol +rapeseed oil at different temperatures are tested and presented in triangular phase diagrams. The triangular phase diagrams show that oil is not soluble in methanol which limits the transesterification rate. Addition of proper cosolvent can solve the problem.For the wide use of LLE data, NRTL and UNIQUAC models are used to regress the experiment data through chemical process simulation software Aspen Plus. Binary interaction parameters are obtained and RMSDs are calculated to check the precision of regression. The results show that these parameters can be used to predict phase behavior of ternary systems. And these parameters expand the use scope of LLE data for biodiesel systems.Biodiesel system made from transesterification is a complex system. After making adjustments to NRTL model parameters, phase behavior of quaternary system (soybean oil + methyl soyate + methanol + glycerol) is forecasted using Aspen Plus. Results show that binary interaction parameters of components in ternary systems can be used in quaternary systems. With these binary interaction parameters, phase behavior of quaternary systems for biodiesel made from transesterification can be known through Aspen Plus. According to this, more realistic prodiction will be made.