Characteristics Of Transesterification Catalyzed By Dolomite Based Calcium Heterogeneous Basic Catalyst

Transesterification is a major technique of biodiesel production, in which the role of the catalyst is crucial. Traditional homogeneous catalyst has strong catalytic property. However, it suffers from many defects, such as can not be reused, hard to separate from the product, produce large amounts of waste water. With the continuous development, homogeneous catalyst has gradually been replaced by heterogeneous catalyst. CaO is a representative catalyst of the transesterification reaction, due to the high catalytic activity and excellent performance. However, pure CaO or CaO modified by a variety of means will lead to higher costs. Therefore, we need to broaden the scope of the base solid catalyst, looking for cheaper and highly efficient resource.Dolomite is the natural mineral substance and abundantly distributed in China. It is dominantly composed of CaMg(CO3)2, and can be decomposed into CaO and MgO by calcination. If can be used in the transesterification reaction, it will greatly reduce the cost, with very promising future. Therefore, we propose to explore the possibility of dolomite as the heterogeneous catalyst, which may lay the theoretical foundation for future applications.In order to solve the research problems, such as the basic thermodynamic data is imperfect, the weak performance of CaO and the pyrolysis mechanism is not clear during application, we carried out the following work.1. Firstly, we used the group contribution method and Zhao empirical formula to estimate the basic thermodynamic data of each component of the three-step continuous reaction between the two main ingredients for the peanut oil (triolein and tripalmitin, TGs) and the methanol. Each parameter varied with temperature. The enthalpy and entropy of two kinds of glycerides manifested a consistent variation. The entropy change of each step was less than zero, indicating that both were entropy reduction process. The third step was more difficult to carry out in the reaction. Gibbs free energy change value has increased as the temperature rises, the equilibrium constant reduced. It means that the reversible reaction was carried out more and more incomplete. Rasing temperature is not conducive to a spontaneous reaction.2. The catalyst was prepared by calcination, and the working parameters were optimized, what’s more, the catalytic mechanism was determined. The major components of the peanut oil are the unsaturated fatty acids, where the saponification value, the acid value and the mean molecular weight are 190.39 mg KOH/g,0.44 mg KOH/g and 884.44 g/mol. The catalyst had good reusability, which is even better than pure CaO. The experimental result showed that in the eighth time, the glycerol yield of D-800 still up to 92.11%. Combing the results of TG and XRD, the calcinations temperature should be more than 750℃. Under tha raction condition,4% of the mass fraction of the catalyst,12 of the methanol to oil molar ratio, transesterification time 2 h and temperature 64℃, the glycerol yield of D-800 and D-750 are 95.17% and 96.15%, respectively. For basic performance testing, the indicators matched well with the requirements of ASTM D6751.3. The pyrolysis characteristic of biodiesel was investigated by the thermogravimetric coupled with the Fourier transform infrared spectroscopy (TG-FTIR), where the composition of the released gas was determined. Further, the model free approach of the Vyazovkin method, OFW method and the Avrami theory were used to calculate the activation energy and the reaction order, respectively. The mass loss segment existed from 554 K to 773 K during the biodiesel pyrolysis process and the accompanied mass loss percentage was about 87.59%. During the pyrolysis, the gas released mainly included CO2、H2O℃CH4 and other organic compounds. The gases showed consistent regularity but with different density. Meanwhile, as temperature heating rate increased, the decomposition process shifted to the higher temperature zone. Calculations of the kinetic parameters demonstrated the multistage and the complicated characteristics of the biodiesel pyrolysis. In the different conversion rate range, kinetic parameters varied widely, but there was small difference between the results of two algorithms, further validating the reliability of the algorithm.