| With the depleting of fossil fuel resources and environmental protection, biodiesel was gaining more and more attention as an cleaning,safety and renewable alternative fuel for diesel engines. The cost problem of material was the primary factor that restricts the development of biodiesel line.So searching for cheap material and using it to synthesize biodiesel is one of direction to research.Waste grease, because of its cheapness and extensive source, was suitable for synthesize biodiesel.At the mean time, it was able to resolve the price problem, alleviate the energy crisis, and deal with the food security and environment pollution. This study using frying waste grease and methanol as the material to synthesize biodiesel by supercritical method. The feasibility, technological condtions, operation parameters were also analyzed and discussed in the research. The co-solvent and a little amount of alkali catalyst were added into the reaction system to improve the process of supercritical tranesterfication.The main results of this study were as follows:(1)Comparing the physical and chemical indicators with rapeseed grease, frying waste grease has higher acid value and water content. So it is not appropriate to synthesize biodiesel by base-catalysis transesterification directly. It has low material requirement to synthesize biodiesel by supercritical transesterification method. Therefore, this method could reduce the energy consumption of pretreating material.(2)The experiment was carried out in the 250ml batch vessel with waste frying grease by supercritical transesterification method. The effiction of operation parameters as reaction temperature, pressure, molar ratio of methanol to oil and time were analyzed for the yield of biodiesel. The optimal process is obtained with a temperature of 340℃, pressure at 12MPa, molar ratio of methanol to grease at 42:1 and reacting time for 10 minites. An 86.25% yield was observed in this condition.(3)As an excellent co-solvent, cyclohexane could improve the cosolvency of methanol and waste frying grease. With 0.15 (molar ratio of methanol to cyclohexane) cyclohexane, temperature at 300℃, molar ratio of methanol to grease at 42:1, reaction pressure at 12MPa, yield could reach 90.25% at 10 min, while a 78.3% conversion was observed at the same condition without cyclohexane. The results showed that:the supercritical transesterification method was significantly improved with a little amount cyclohexane and could get higher yield at a low operation condition.(4)Added a little NaOH to the reaction system, the optimum technological conditions became to:1wt%NaOH, temperature at 240℃, pressure at 8MPa, molar ratio of methanol to grease at 28:1.89.78% yield could obtain after 10 minutes. Comparing with the reaction system without NaOH, the temperature, pressure and molar ratio of methanol to grease could greatly reduce to get the same yield, which would cut the cost and establish the foundation for industrial production of biodiesel by supercritical transesterification method.(5)A dynamic model was described through the reasonable hypothesis.According to the calculated results, we got the reaction ordel was 0.5, reaction rate constant k1= 0.2004 (l/mol)1/2-min-1.The reaction rate equation was rA=0.2004.CA0.5 under the temperature of 340℃, pressure at 12MPa, molar ratio of methanol to grease at 42:1.(6)According the GB testing methods and requirements ues GC and GC-MS methods on the test product performance tests:the physical and chemical performance products reached GB/T20828-2007 and ASTM 6751-03-S500 standards;the main composition of the production were methyl palmitate, acid methyl ester, octdecenoic acid methyl ester, methyl stearate,and so on.According to the GB standard test and GC-MS test, the physical and chemical property of biodiesel obtained in this study reached the standards of GB/T20828-2007 and ASTM 6751-03-S500. The main components of the biodiesel include methyl palmitate, acid methyl ester, octdecenoic acid methyl ester, methyl stearate, etc. |
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