| Biodiesel,as a kind of renewable,clean and green fuel,can become the replacement of petro-diesel.Expanded use of biodiesel would enormously contribute to environmental protection,energy-saving and rural development.However,the biodiesel production cost is very high,while the international oil price is very low,which hinders the development of biodiesel industry.Therefore,how to decrease the biodiesel production cost so as to improve the competitiveness of biodiesel,is the key problem we have to solve.The high cost of the feedstocks of biodiesel is a major impediment to the commercialization of biodiesel.Therefore,selecting an inexpensive raw material source is important if biodiesel is to become economically viable.Furthermore,the other aspects of the production cost must be further reduced by improving the downstream production process in order to make biodiesel products more competitive relative to fossil fuels.At present,homogeneous base-catalyzed methanolysis is the predominant technique used for the industrial production of biodiesel due to its many advantages,such as faster reaction rate,more moderate reaction conditions,and no corrosion to equipment compared to acid catalysis.However,water and free fatty acids have significant,negative effects on the homogeneous base-catalyzed transesterification process.So improve the water and acid tolerances of the transesterification reaction system,and inhibite the hydrolysis and saponification of glycerin esters during the transesterification,which can reduce the requirement for the feedstocks thereby lowering the production cost.In addition,the methanol and oil phases are immiscible,and the mass transfer between the two phases limits the reaction rate initially.Therefore,some new methods for improving the mutual solubility of the reaction system,such as the use of co-solvents,have been widely researched.Co-solvents can improve the mass transfer between the phases present in the transesterification process,thus high biodiesel yield can be achieved in small reaction times,even at room temperature.This research developed new intensified reaction processes for biodiesel production by homogeneous base-catalyzed transesterification,which effectively improved the catalytic activity of homogeneous base catalyst,enhanced the mass transfer process.Thus the purpose of reducing energy consumption and production cost can be achieved.The study objects and results are as follows:1.A new transesterification process for biodiesel production,bentonite-enhanced NaOH-catalyzed transesterification,has been developed.The influence of bentonite on the NaOH-catalyzed methanolysis of soybean oil and the accompanying intensifying mechanisms were investigated.It was found that an appropriate introduction of bentonite can promote the methanolysis.By the rapidly removal of the water from the system,bentonite enhances the transformation of NaOH to the catalytically active methoxide species.Further,the main side reactions,hydrolysis and saponification of glycerin esters,are significantly inhibited.2.Response surface methodology central composite design was applied to optimize the process parameters of the bentonite-enhanced NaOH-catalyzed transesterification.The maximum FAME yield of 98.56%was achieved under the optimized conditions of 7.47:1 methanol/oil molar ratio,0.77 wt%concentration of NaOH,1.76 wt%amount of bentonite and 57.8 ℃ reaction temperature.This study also examined the kinetics and evaluated the thermodynamic activation parameters for the bentonite-enhanced transesterification reaction,and the following results were obtained:the transesterification can be well described using the pseudo-first-order kinetic model,the rate constants at 40～65 0C were 0.0850～0.2082 min-1,the activation energy(Ea)was 31.03 kJ·mol-1,△H=28.33 kJ·mol-1,△S,=-0.18 kJ·mol-,K-1 and △G=83.30～87.69 kJ·mol-1.3.Diethyl ether was added in the bentonite-enhanced NaOH-catalyzed transesterification reaction system to further intensify transesterification.The intensifying mechanism of diethyl ether was investigated,which indicated that the introduction of diethyl ether can improve the mutual solubility between methanol and oil,so the mixing efficiency and mass transfer between the two phases were extremely enhanced.Furthermore,the reaction temperature was greatly reduced,which further inhibited the hydrolysis and saponification of glycerin esters.4.Response surface methodology was applied to optimize the main process parameters of the bentonite and diethyl ether-enhanced NaOH-catalyzed transesterification.The maximum FAME yield of 98.35%was achieved under the optimized conditions of 0.56:1 of DEE/methanol molar ratio,1.07 wt%concentration of NaOH and 5.65:1 methanol/oil molar ratio.The kinetics and thermodynamic analysis for this process was also carried out.The new transesterification process also can be well described using the pseudo-first-order kinetic model,the rate constants at 25～35 ℃ were 0.2053～0.2724 min-1,the activation energy for the process was 23.73 kJ·mol-1,and the thermodynamic analysis reveals that △H =21.21 kJ·mol-1,△S =-0.19 kJ-mol-1·K-1 and △G=76.96～78.65 kJ·mol-1.5.A new biodiesel purification method;i.e.,adsorption refining by using bentonite,has been developed.The kinetics of the biodiesel purification process was studied.The adsorption of glycerin,monoglycerides and diglycerides on bentonite could all be well fitted with the pseudo-second-order kinetic model.The activation energies of the three adsorption processes were 27.1,28.2 and 31.8 kJ·mol-1,respectively,which indicated that these processes are physical adsorption.6.The purification of the by-product glycerin of the bentonite-enhanced NaOH-catalyzed transesterification was also studied.The results showed that the maximum purified glycerin yield and glycerin content could reach 94.5%and 98%,respectively,under the optimized conditions of 1:1 methanol/crude glycerin volume ratio,4.5 solution pH value,1.5 wt%addition amount of activated carbon,70 ℃ decoloring temperature and 30 min decoloring time. |
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