Process Enhancement Of Biodiesel Production By Transesterification In Supercritical Methanol With Catalyst

Biodiesel is a renewable, new environmental-friendly biomass fuel, which is an ideal substitute for petroleum diesel. In recent years, the competition in the field of new energy technology focuses on biodiesel. Biodiesel is a series of mono-alkyl ester of fatty acids, produced by transesterification of vegetable oil or animal fat and methanol.Supercritical transesterification process is a new non-catalytic technique for biodiesel production. Due to the advantages such as faster reaction rate, higher conversion and simplier purification, supercritical transesterification process has attracted considerable attention. However, there are also disadvantages for this process, especially the higher temperature and pressure.Aiming at the disadvantages of supercritical transesterification, the feasibility and process conditions of supercritical/subcritical methanol method with solid catalysts were presented and discussed in this paper. The thermodynamic and the kinetics of transesterification were also calculated. In addition, biodiesel production by transesterification in carbon dioxide expanded-liquids was studied. It mainly concludes these contents as follows:1. In order to improve the operation conditions of supercritical transesterification technique, in this work, the transesterifcation of soybean oil in supercritical/subcritical methanol with a small amount of catalysts of calcined sodium silicate and tri-potassium phosphate was studied. The variables affecting the fatty acid methyl ester (FAME) yield during the transesterification reaction, such as the reaction temperature, the molar ratio of methanol to soybean oil, the reaction time, the catalyst content and the recycle of catalyst, were investigated.(1) A yield of 95.6% was achieved when the reaction was performed with a 36:1 molar ratio of methanol to soybean oil and calcined sodium silicate content of 0.5wt% at 220℃for 30min. The stability of calcined sodium silicate was discussed at optimal condition. Using the catalyst after three times, the methyl ester yield is above 90%, after four times, the methyl ester yield decreases to 79.1%.(2) A yield of 95.6% was achieved when the reaction was carried out with a 24:1 molar ratio of methanol to soybean oil and tri-potassium phosphate content of lwt% at 220℃for 30min. The stability of tri-potassium phosphate was discussed at optimal condition. Using the catalyst after two times, the methyl ester yield is above 90%, after four times, the methyl ester yield decreases to 71.2%.2. Based on the experimental data of supercritical/subcritical transesterification with tri-potassium phosphate and calcined sodium silicate, the kinetics of transesterification was studied. The results show that the order of transesterification with tri-potassium phosphate was 1.5, and the activation energy was 17.27kJ/mol. Moreover, the order of transesterification with calcined sodium silicate was 1, and the activation energy was 1.80kJ/mol.3. The critical parameters of soybean oil and palm kernel oil were calculated by the Lydersen, Joback, C-G methods. The results show that the calculated values by C-G method have consistency with the literature values. Then the critical parameters of soybean oil-methanol mixture system were calculated by C-G method combined with L-B style mixture rule.4. Biodiesel production by the catalytic transesterification with tri-potassium phosphate and sulfuric acid in the carbon dioxide expanded-liquids was investigated. The primary results indicate that:in the process with tri-potassium phosphate, due to the higher acidity of mixture system after adding carbon dioxide, the catalytic activity of tri-potassium phosphate was weakened. However, the higher yields of biodiesel were obtained by the transesterification in the carbon dioxide expanded-liquids with sulfuric acid. The reaction rate increased observably by adding carbon dioxide into the reaction system.