Liquid-Liquid Phase Equilibrium Of Fatty Acid Ethyl Ester-Ethanol-Glycerol Systems

Biodiesel is a very important renewable energy,and its industrial production process has attracted unprecedented attention in recent years.The design of separation devices during the production process needs adequate liquid-liquid phase equilibrium(LLE)data.Thus,it is very significant to obtain LLE data and thermodynamic model parameters among various components in biodiesel system.In this paper,the phase equilibrium of fatty acid ethyl ester-ethanol-glycerol systems during the production of biodiesel using ethanol as ester-exchange agent was investigated to provide basis for the biodiesel separation.The experimental operation of fatty acid ethyl ester systems is conducted by constant temperature shaking-standing method under various temperatures(30-70?),including ethyl oleate-ethanol-glycerol,ethyl linoleate-ethanol-glycerol,ethyl palmitate-ethanol-glycerol,ethyl stearate-ethanol-glycerol,ethyl oleate-ethanol-glycerol(alkaline condition),ethyl linoleate-ethanol-glycerol(alkaline condition),ethyl palmitate-ethanol-glycerol(alkaline condition),ethyl stearate-ethanol-glycerol(alkaline condition).The phase equilibrium data of fatty acid ethyl ester systems are measured by high temperature gas chromatography and then depicted in the phase diagrams.The phase diagrams show that the mutual solubility of fatty acid ethyl ester and glycerol is very small,which increases when ethanol proportion or temperature rises.Under alkaline condition,the mutual solubility of fatty acid ethyl ester and glycerol is larger and more sensitive to variations of ethanol proportion and temperature.Based on the 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 under the regression of the whole temperature range,NRTL model and UNIQUAC model agree well with the phase equilibrium behaviors of ethyl oleate-ethanol-glycerol,ethyl linoleate-ethanol-glycerol,ethyl palmitate-ethanol-glycerol,ethyl linoleate-ethanol-glycerol(alkaline condition),and ethyl stearate-ethanol-glycerol(alkaline condition);but only NRTL model is suitable for ethyl stearate-ethanol-glycerol,ethyl oleate-ethanol-glycerol(alkaline condition),and ethyl palmitate-ethanol-glycerol(alkaline condition).Biodiesel made from oil crops through transesterification is a mixture consisting of different fatty acid ethyl esters.To apply the above binary interaction parameters to mixed fatty acid ethyl ester system,some adjustments are made for NRTL and UNIQUAC parameters,and then phase behavior of mixed fatty acid ethyl ester system is forecasted using Aspen Plus.The comparison of forecasted results and experimental results shows that NRTL model is more suitable for forecasting the phase equilibrium of mixed fatty acid ethyl ester system.