Removal Of Saturated Fatty Acid Mrthyl Esters From Biofuel By Freezing Crystallization

Fatty acid esters can effectively improve the lubricating properties of low sulfur diesel oil.The properties of the fatty acid esters antiwear agent are related to the composition.The lubrication effect of saturated fatty acid ester is worse than that of unsaturated fatty acid ester.The content(quality)of saturated fatty acid ester in fatty acid esters anti-wear agent can not be higher than 2.5%.This paper studied the removal of saturated fatty acid methyl esters from mixed fatty acid methyl esters,using frozen crystallization separation technology which depending on the different melting point of each fatty acid methyl ester.First,the step cold curves and binary melt phase diagrams of methyl oleate-methyl stearate and methyl oleate-palmitic acid methyl ester were determined by thermal analysis.On the binary melt phase diagram of methyl oleate and methyl stearate/ palmitate,the liquidus and the solidus are very close together.When the mass fraction of methyl oleate in the mixture is less than 0.8,the two-phase composition is more sensitive to equilibrium temperature.When the mass fraction of methyl oleate is more than 0.8,the liquidus and the solidus almost completely overlap,and the composition of the mixture has a great influence on the melting temperature.Two crystallization systems,namely suspension crystallization and recirculating cooling crystallization,were constructed.In the suspension crystallization system,the effects of crystallization temperature,stirring speed,raw material composition,freezing time,solvent type and dosage on the separation efficiency were investigated.The crystallization temperature has a great influence on the separation effect.A melt liquad with low content of saturated fatty acid methyl ester but low yield can be got at low crystallization temperature.The addition of solvent can reduce the viscosity of the system and increase the yield melt liquid,but decrease the removal rate of saturated fatty acid methyl ester.When the raw materials with the content of methyl palmitate less than 1.3% was frozen at-5 ° C for 2 h,the methyl stearate content in the melt can decrease to 1.2% or less,and the total content of saturated fatty acid methyl ester can decrease to 2.5% which can meet the requirements of anti-wear agent.When the content of palmitic acid methyl ester more than 1.3% in the raw materials,the materials must be frozen at-20 ? in order to get products meeting the requirements of anti-wear.In the recirculating cooling crystallization system,the influence of the equipment configuration and operating conditions on heat transfer and separation is mainly investigated.The recirculating cooling crystallization system set the material heat exchange and crystallization in two different devices.During the heat exchange process,changing the structure of the heat exchanger,increasing the flow rate of the feed liquid and the coolant can improve the heat exchange perfermance,reduce the heat transfer temperature difference between the coolant and the liquid.During crystallization process,setting the baffle,increasing a diameter of the crystallizer can reduce the influence of material flow on the crystal system disturbance and increase the width of metastable zone.The mass transfer in the crystallizer can be improved by micro-disturbance.When the temperature of the external temperature of the crystallizer was-5 ?,the temperature of the coolant into the heat exchanger was-10 ?,the flow rate of the coolant was 5.92 m / min,the flow rate of material was 2.65 m / s,the temperature of the liquid in the crystallizer could be maintained at-7.5 ?,a melt which the content of methyl stearate can achieve 0.15% and saturated fatty acid methyl ester can achieve 2.5% can be obtained and the yield of melt can achieve 82.92%.