| Petrochemical energy depletion will be a serious threat to energy security and ecological environment; therefore it has become inevitable to seek new renewable and environmental friendly energy. Among them, biodiesel is proved to be the most promising energy because its physical and chemical properties are similar to fossil diesel; furthermore it is biodegradable, non-toxic and has good combustion performance. Its main component is a series of fatty acid alkyl ester produced by transesterification of vegetable oil or animal fat and methanol.Supercritical methanol method is a new preparation process for biodiesel process without catalyst, advantages of which are no special requirements of raw materials, high reaction rate and high conversion, easy separation of products, etc. However, seldom discussion from the aspect of thermodynamics was reported. The process of solid catalysis has solved the problems of homogeneous catalysis like separate difficulty, sewage pollution and so on. However, the reaction rate of this method is lower and reaction time is long. In this paper, supercritical methanol method was conducted to prepare biodiesel and the results of the experiment were analyzed and discussed from thermodynamics aspect. In addition, synthesis of biodiesel in carbon dioxide expanded system was investigated, and the effects of various factors were discussed.1. The experiments were carried out in 250ml batch autoclave with soybean oil and supercritical methanol. The effects of operation parameters like the molar ratio of methanol to oil, temperature and pressure on the methyl ester yield were investigated. The results showed that the yield of methyl ester was less than 50% even after 90min at low molar ratio. There was a significant increase of the methyl ester yield from 260℃to 300℃. The ester yield increased from 51% to 90.9% as the pressure increased from 8.5MPa to 15.5MPa at high temperatures. Obviously, promoting the pressure could shorten the reaction time and improve the methyl ester yield. However the effect of pressure on the ester yield was not positive at low temperatures.2. The critical parameters and solubility parameters of reaction mixture were calculated, and the process of supercritical experiments was analyzed from the thermodynamic aspect. The result indicated that the reaction rate changed with the change of phase states. The moleculars of soybean oil and the methanol will have more opportunies to contact with each other when the two solubility parameters were close. Therefore the reaction rate was improved.3. Synthesis of biodiesel was carried out in the carbon dioxide expanded liquid with sodium hydrogen sulfate and potassium phosphate as catalysts. Effects of the molar ratio of methanol to oil, reaction temperature, reaction pressure, catalyst content and reaction time on the yield of biodiesel were respectively investigated. The conclusions are as following:(1) In sodium hydrogen sulfate catalysis, a yield of 33% was achieved when the reaction was performed with a 9:1 molar ratio of methanol to soybean oil and sodium hydrogen sulfate content of 5wt% at 90℃for 6h, while the yield could reach 81% after adding carbon dioxide (lOMPa) at the same conditions. To obtain the same yield,3wt% catalyst was used in this experiment as well as 9wt% catalyst in the non-CO2 catalysis experiments. The yield of 45.2% was obtained in 12h in the non-CO2 catalysis experiment while the yield was 61.8% in 4h in this experiment.(2) The result of potassium phosphate catalysis indicated that the highest yield of 79.1% was achieved when the reaction was performed with a 6:1 molar ratio of methanol to soybean oil and potassium phosphate content of 4wt% at 60℃,5MPa for 90min.
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