Kinetics Of Cascade Transesterification Of Triglycerides In Supercritical Methanol

The research was carried out in a batch-type reactor and heated beyond supercritical temperature and pressure of methanol. The effects of temperature, mass ratio of moisture to soybean oil on transesterification of triglycerides(TG), driglycerides(DG) and monoglycerides(MG), and on the yield of fatty acid methyl esters(ME) were investigated. Kinetics equation of the transesterification of triglycerides and methanol was established. The main research works and results of the paper were summarized as following:(1)A set of continuous operation of supercritical transesterification equipment was independently designed and built. The equipment consists of units, which is raw material conveying unit, raw material preheating unit and reaction unit. The critical unit is a high temperature and high pressure kettle which has a heating system, automatic temperature control system and Mechanical agitation system, thus the reactor temperature and pressure inside the kettle be precisely controlled, Mechanical agitation system can make the reactants mixing and prevent local overheating. Because of the raw material preheating unit, the zero time of reaction could be controlled exactly, which is the basic of obtaining credible kinetic data.(2)A set of separation, analysis and calculation method of sample was also established. The biodiesel samples were analyzed by gas chromatograph after vacuum distillation, centrifugal liquid separation and dryness. The method is established to calculate the mass fraction of TG, DG, MG and ME, through which we can get the extent of transesterification.(3) Experiments were carried out at250℃and9.0MPa,using of greatly excess methanol and soybean oil or oil-water mixture, in which mass fraction of moisture is from2%～10%. Experimental results show that the presence of moisture can speed up the transesterification rate of TG, DG and MG, but with the increment of moisture content in the mixture rate increase is not obvious; the presence of moisture also speed up yield of ME, but the impact on the final yield is not very obvious.(4) Experiments were also carried out at9.0MPa and different temperatures, which are beyond supercritical temperature of methanol, using of greatly excess methanol and soybean oil or oil-moisture mixture, in which mass fraction of moisture is2%. It was found that transesterification reaction can happen without the presence of catalyst in supercritical methanol, and the presence of moisture can speed up the transesterification rate of TG, DG and MG, then speeding up the formation of ME; Triglycerides can be completely converted to the corresponding DG and ME, but DG and MG can not be completely converted, both respectively maintaining at a certain balance concentration finally, which of DG was significantly less than that of MG.(5)Under the experimental conditions, supercritical methanol transesterification of TG is irreversible first order reaction, transesterification of DG or MG is reversible reaction. The reaction kinetics equation of transesterification of TG is established. Reaction pressure is at9MPa, kinetic constants are obtained at different temperature of different moisture content respectively, which fit the Arrhnius equation very well. Compared with the pre-exponential factor of2%moisture content, that of0%is smaller, but its activation energy is bigger. Thus kinetic constant of2%moisture content would be greater than that of the0%. So the presence of moisture can speed up the transesterification rate of TG.Duo to the full analysis of the ester sample during the reaction, the whole reaction course can be well investigated and a good understanding of the transesterification in methanol supercritical condition, the variation of the ester phase composition under different reaction conditions can be obtained. It is found that triglycerides can be more soon fully transformed into the corresponding diglyceride and fatty acid methyl esters, driglycerides and monoglycerides into fatty acid methyl ester reaction is reversible, but slow rate, making the final product contains a certain amount of driglycerides and monoglycerides. The information of dynamics of transesterification obtained from this paper can provides the basis of more complete data for industrial design and application of the process.This study results shows via supercritical methanol transesterification method the ratio of moisture in the raw material is not strictly limited, the presence of moisture will promote the transesterification reaction, which greatly reduces the requirement of raw materials, unrefined raw materials can directly produce biodiesel.Therefore, supercritical methanol transesterification of biodiesel is a viable process method.