Kinetics Of Transesterification For Synthesis Of Biodiesel In Supercritical Methanol

Energy is great important for the development of a country.Because of the limited deposit of petroleum and the environmental pollution caused by a heavy use of petroleum used brings to the world,the exploitation of a new reproducible green energy is imperative.Biodiesel,aliphatic methyl-esters made from renewable biological sources such as vegetable oils(or fats)and alcohol via transesterificaion,is an alternative diesel fuel of environment friendly because of its great combustion energy and low emission.Acid and alkaline catalyzing methods were widely used in the traditional production of biodiesel.Many disadvantages exists in the acid and alkaline catalyzing process,such as the two-phase condition of oil and methanol,great purity demanding of the raw materials,complicated separation and purification of the products.In supercritical methanol,oil could be dissolved and methanol could decompose to CH₃O^-to catalyze transesterification.So,supercritical methanol technology is chosen to synthesize biodiesel in this paper.The effects of reaction temperature and pressure on the transesterification were investigated and kinetics for the transesterification of soybean and methanol was obtained.The main works in the paper were summarized as following:(1)A set of continuous operating supercritical reaction equipment was established, which is composed of liquid transportation unit,pre-heating unit and reaction unit.The critical unit is a kettle which could endure high temperature and high pressure up to 40MPa.Because of the preheating unit,the zero time of reaction could be controlled exactly,which is necessary for obtaining credible kinetic data.(2)A set of separation and analyzing method of biodiesel products was also established.The products was analyzed by GC after distillation,liquid separation and dryness.(3)The transesterification reaction was studied under such condition:temperature 250～280℃,pressure 9～15MPa,molar ratio of methanol to oil is 42:1.The conversion rate of glyceride in this supercritical reaction could exceed 99%,which could also reach 70%in 1～2 hours.The reaction rate was affected obviously by both temperature and pressure,it increased when either temperature increased or pressure decreased.(4)Under experiment conditions,the transesterification reaction of soybean oil in supercritical methanol is an irreversible first order reation.The relationship between temperature and kinetic constant fitted the Arrhnius equation very well.in supercritical reaction condition,the pressure played important role in reaction rate constant,which decreased when pressure increased,due to the decrease of activation energy.The effect of temperature and pressure on transesterification reaction constant could be expressed as follows: lnk'=-5.29047P+87.05861+22942.58146P-399998.928/RT(5)The kinetic model of transesterification in supercritical methanol was established as follows: -dC_TG/dt=k'C_TGor x_TG=1-exp(-k'/RT)The reaction results of transesterification vegetable oil in supercritical methanol could be forcated well via the above equation.Supercritical method could make oil and methanol become one phase so as to elevate the reaction rate greatly.The reaction could proceed without catalyst due to the CH₃O^- generated from methanol under supercritical condition,which could simplify the separation and purification process with not serious demanding of raw materials.So, biodiesel production via supercritical methanol method is feasible for industrialization.