利用热力学相平衡分析生物柴油晶体析出规律

Crystal precipitation law of biodiesel based on thermodynamic phase equilibrium

生物柴油主要是由饱和脂肪酸甲酯和不饱和脂肪酸甲酯组成的混合溶液,从热力学角度分析,生物柴油的蜡晶析出是溶液由液相向固相转变的相平衡过程。该文建立将液相组成作为理想溶液且固相成分不互溶的理想溶液模型(模型一)和基于活度系数模型与正规溶液理论的正规溶液模型(模型二),分别计算了蜡晶的析出温度,并且通过模型二计算出在给定的温度下的析蜡量与石蜡的组成。两模型在计算析蜡点的温度精确性方面还是比较理想的误差在5K左右。研究发现石蜡沉积量与饱和脂肪酸酯的量成一定的比例关系,而且石蜡沉积并不是按照脂肪酸酯熔点高低的顺序进行析出,析出的石蜡中同时包含了低熔点脂肪酸酯。固液平衡常数是表征物质在溶液中析出能力的决定性因素,在一定温度下,不饱和脂肪酸酯的固液平衡常数一般在0.1以下,而饱和脂肪酸酯的固液平衡常数却大得多。该研究阐明的晶体析出规律,可为优化生物柴油低温流动性技术措施,推动生物柴油在低温环境下的应用提供参考。

Biodiesel, as a renewable alternative fuel with the virtue of clean burning, can be directly used in a diesel engine without any mechanic modification. However, biodiesel is easily crystallized at a low temperature, which limits the application of engines fueled with biodiesel, especially in severe cold circumstances. Biodiesel is a mixture solution, and mainly contains of saturated and unsaturated fatty acid methyl esters. According to thermodynamics, the process of wax precipitation is a thermodynamic equilibrium from liquid phase to solid phase. The equilibrium relationship between the solid phase and the liquid phase doesn’t keep constant. As long as one factor of the system changes, the phase diagram will follow changing. Two different kinds of models have been built to depict the crystal precipitation. Model 1, which is called ideal solution model, looks at liquid composition as the ideal solution and solid phase composition as not mutually soluble. Model 2 is called a regular solution model based on activity coefficient model and the regular solution theory. The wax crystal precipitation temperature can be calculated by both models respectively. The quantity of paraffin wax and the composition of wax at the given temperature can also be worked out by Model 2. The accuracy of both models in calculating the wax precipitating temperature is good, and the deviation is within 4 K. With the increasing content of saturated fatty acid methyl esters （SFAME）, the wax crystallized more easily, and the quantity of paraffin wax were proportional to the total amount of saturated fatty acid esters. By comparing and analyzing the characteristic parameters of the intermediate process during the solid-liquid equilibrium of one biodiesel solution at different temperatures, such as the compositions of liquid or solid phase and the solid-liquid equilibrium constants, it was found that the solid-liquid equilibrium constant is a crucial factor to indicate the precipitation ability of a substance in a biodiesel solution. At a given temperature, the solid-liquid equilibrium constant of SFAME is much higher than the value of unsaturated fatty acid methyl esters （UFAME）. The lower the temperature is, the higher the solid-liquid equilibrium constant of FAME is. Thus the FAME is easier to be precipitated. However, the unsaturated fatty acid esters with a low melting point have also appeared in the wax, so the wax doesn’t precipitate with the sequence of the melting point of fatty acid esters. The researching crystal precipitation of biodiesel would play an important role in optimizing its flow properties of low temperature and broadening its utilization.