Study On The Process Of Synthesis Biodiesel By Supercritical (Near-critical) Methanol And Enzyme-Catalysed Method

With the shortage of fossil fuels and the price increasing of engine fuels, energy crisis isbecoming a global problem. The development of renewable engery, such as biomass engery,solar power, terrestrial heat, has been growing rapidly in recent years and obtained muchattention by many government. Biodiesel(BD), as for its good combustibility, renewableresource, environmental friendly, has become the focus on enterprise and research agents.With the aim of exploitation biodiesel process, the feasibility, process conditon, operationparameters of supercritical(subcritial) methanol method and biocatalyst method is analyzedand discussed. The co-solvent and a little amount of alkali is added to the reaction mixture toimprove the process of supercritical method. The co-solvent of supercritical carbon dioxide isadded to the reaction mixture in biocatalyst method, trying to synthesis of biodiesel withsupercritical carbon dioxie. The activity of enzyme in supercritical carbon dioxide is alsodiscussed. The main result is as follows:(1) The experiment is carried out in the 250mL batch vessel with soybean oil andsupercritical methanol method. The operation parameters, reaction temperature and time,molar ratio of methanol to oil are analyzed for the yield of BD. The optimal process isobtained: with an optimal temperature of 350℃, molar ratio of methanol to oil of 42, a 95.4%conversion of soybean oil is observed in 10 min at a reaction pressure of 21MPa. With theresult of experiment, the reaction kinetics is also discussed.(2) The co-solvent hexane and carbon dioxide are added to the reaction mixture in order todecrease the operating parameters. With 2.5wt% hexane, temperature of 300℃, methanol tooil ratio of 42, a 85.5% conversion is observed in 30 min, while a 62.2% conversion isobserved without hexane in the same conditon; with less carbon dioxide, temperature of 300℃, methanol to oil ratio of 42, a 91.6% conversion is observed in 20 min, while a 51.4%conversion is observed without carbon dioxide in the same conditon.(3) The supercritical method is improved with a little catalyst in the reaction mixture, withaim of decreasing the reaction temperature. With only 0.1wt% potassium hydroxide(KOH),methanol to oil ratio of 24, 2MPa, temperature of 160℃,a 96.5% conversion is observed in10min. Compared with the supercritical methanol method, the reaction temperature is largelydecreased by 100℃and the use of methaol is decreased by 50%. Compared with theconventionl method, the catalyst is 10% and the reaction time is greatly shortened to 10min. (4) With the aim of developing a mild and small methanol process of synthesis biodiesel, afree enzyme catalyst is selected as the catalyst and the feasibility and process condition isdisscussed with biocatalyst method. The operation parameters, reaction temperature, reactiontime, molar ratio of methanol to oil, the style of adding methaol, water content in the reactionmixture, co-solvent, are discussed for the yield of BD.The optimal process is observed:tremperature of 40℃, methanol to oil ratio of 3, 10wt%enzyme, 30vol%water, 20vol%hexane, a 54.7% conversion is observed in 26h with a high impeller speed.(5) The enzyme catalyzed reaction, lipase catalyzed transesterification in supercriticalcarbon dioxide, is carried out with carbon dioxide as co-solvent instead of conventionalorganic solvent. The results indicate that: the free enzyme has activity with only some waterin it. So it causes the losss of activity in the reaction for the lower pH with carbonic acid inthe presence of large amount of carbon dioxide. But it exhibits good activity with the pureSCCO₂. As for the immobilized lipase Novozym435, it represents high activity in SCCO₂.These result could be the foundation of following reasearch on this field.