脂肪酸甲酯添加比例和分子构型对混配柴油系统低温流动性影响的模拟研究

Simulation study on the influences of adding proportion and molecular conformation of fatty acid methyl ester to the low temperature fluidity of mixed diesel system

生物柴油是一种优质的清洁能源,与石化柴油混配能很好地改善柴油的低温流动性。为探讨其降凝作用机理,本文以硬脂酸甲酯、油酸甲酯,亚油酸甲酯、亚麻酸甲酯等4种常见的生物柴油组成分子作为添加剂,以石化柴油的组成分子正十六烷为柴油主体,构建不同混配比例的模拟柴油系统,利用Materials Studio软件,对系统的凝点以及降温凝固过程中系统微观结构变化进行了分子动力学模拟计算。采用热容、原子自扩散系数、链头尾距以及系统溶解度参数综合判断了系统的凝点,发现,系统的凝点随添加剂的添加比例以及含双键的数目的增加而呈下降趋势。利用对原子间径向分布函数的分析,根据降温过程中十六烷链微观结构的变化,探讨了添加剂中双键以及甲酯基对柴油系统低温流动性的影响。初步降凝机理为:双键的类块状结构以及甲酯基的类球状结构使得附近的十六烷分子在低温时仍能保持一定的弯曲度,从而影响了十六烷分子的有序排列,阻滞了蜡化作用。

The biodiesel is a kind of high quality clean fuels, and it can be blended in petroleum diesel to significantly improve the low temperature fluidity of diesel. To study the mechanism of pour point depression, this work performs molecular dynamics simulation, by using Materials Studio software, to calculate the solidifying points and microstructure changes during the cooling and solidification process of the modeled diesel systems which were built at different mix proportion, by using n-hexadecane as the main body of petroleum diesel and four kinds of common biodiesel ingredients i.e. methyl stearate, methyl oleate, methyl linoleate and methyl linolenate as additives. The solidifying point of each system was determined comprehensively with four physical quantities such as the heat capacity, atomic self-diffusion coefficient, distance from head to tail of chain, and system solubility parameter. It was found that the solidifying point of the system declines with the increase of the adding proportion of additives and the number of double bonds. According to the change of the microstructure of hexadecane during cooling process analyzed by the radial distribution function between the related atoms, this paper discusses the effect of the double bond and carbomethoxy in additives on low temperature fluidity of the modeled diesel system. Preliminary mechanism of pour point depression is that, the block type structure of the double bond and sphere type structure of the carbomethoxy make hexadecane molecules still keep the chain with certain bending degree at lower temperature, which weakens the orderly arrangement of hexadecane molecules, and retards the system in wax.