Because of its excellent renewability and carbon emission reduction effect,biodiesel has become an important direction of energy transformation in the field of transportation.Due to the influence of raw material source,preparation technology,storage environment and other factors,biodiesel contains alkali/alkaline earth metals,insoluble soap formed by alkali/alkaline earth metals and combustion products of abrasive solid impurities deposited in the engine,resulting in mechanical wear on metal components,which seriously restricted the development and commercial application of biodiesel.Therefore,this essay focuses on the study of the occurrence form and mechanism of alkaline/alkaline earth metals in biodiesel,which provides strong data support for the regulation of the occurrence form of alkaline/alkaline earth metals in biodiesel.In this essay,based on the different solubility of alkali/alkaline earth metals in different solvents,a step-by-step extraction method was established.Eight methods(H2O-CH3COONH4-HCl;H2O-NH4Cl-HCl(85℃water bath 1h;Ultrasonic 1h;Stir at room temperature for 1h;Stir at 60℃for 1h)were carried out,from which the best method and step extraction system were selected.According to the system,Gaussian16W software was used to analyze the mechanism of complex ion exchange types involved in chemical reaction processes.Based on a complete step-by-step extraction system,the effects of different temperature,oxidation degree and additives on the occurrence forms of alkali/alkaline earth metals in biodiesel were investigated,and the changes of oxidation stability and lubrication properties of biodiesel before and after step-by-step extraction under different conditions were studied.The results are as follows:(1)The stage extraction of H2O-NH4Cl-HCl(1h stirring at room temperature)had the lowest uncertainty and the highest form recovery ratio,indicating that the selected stage extraction system could obtain the complete occurrence form of alkali/alkaline earth metals in biodiesel.Based on this extraction system,four types of alkaline/alkaline earth metals in biodiesel were separated and identified,which are water-soluble,ion-exchange,acid-soluble and insoluble.At the same time,under the extraction system,when the stirring rate was 20r/min and the extraction solution concentration was 1mol/L,the extraction amount of alkali/alkaline earth metal in biodiesel was the highest.(2)The energy of sodium butyrate,potassium acetate,calcium propionate and magnesium formate reacted with NH4Cl was the highest,indicating that the binding interaction between the reactants was the strongest,and the reaction stability was high.According to the combined results,NH4Cl is more effective as an organic intermediate extraction solvent,which is more likely to react with alkali/alkaline earth metals in the form of organic acids.(3)The largest grinding diameter of biodiesel extracted at 35℃and the longest induction period of biodiesel extracted at 45℃were obtained.Before and after stepwise extraction,the oxidation stability and lubrication properties of the biodiesel added with propyl gallate were enhanced. |