基于差示扫描量热法和热力学模型的生物柴油结晶行为分析

棕榈酸甲酯(C16:0)、硬脂酸甲酯(C18:0)和油酸甲酯(C18:1)是生物柴油的主要组成部分。为了深入探究生物柴油的结晶行为,该文基于差示扫描量热法(differential scanning calorimetry,DSC)分析了这3种脂肪酸酯的物性参数,研究发现饱和脂肪酸甲酯C16:0和C18:0的熔点和熔化焓远远高出不饱和脂肪酸甲酯C18:1的值,C16:0和C18:0的熔点分别为301.57、310.92 K,C18:1的熔点为255.01 K。对脂肪酸酯组成的二元溶液进行DSC扫描,DSC曲线出现了2个放热峰,并且溶液的结晶点要低于首先析出的饱和脂肪酸酯纯物质时的熔点;随着饱和脂肪酸酯质量分数的增加溶液的结晶点温度也相应提高。将生物柴油当作由多元脂肪酸甲酯的混合溶液时,C16:0和C18:0等饱和脂肪酸甲酯作为溶质,C18:1等不饱和脂肪酸甲酯作为溶剂,建立了热力学模型计算溶液的结晶点温度。将脂肪酸甲酯的混合溶液近似为理想溶液时对此模型进一步简化,并利用简化模型计算得到4种生物柴油的结晶温度,与实测值进行比较得到了很好的验证效果。该研究可为优化生物柴油低温流动性的技术措施提供参考。

Crystallization behavior of biodiesel based on differential scanning calorimetry and thermodynamic model

Abstract: Biodiesel, as a renewable alternative fuel, is easily crystallized at low temperature, which limits the application of engines fueled with biodiesel. Biodiesel is mainly composed of methyl palmitate (C16:0), methyl stearate (C18:0) and methyl oleate (C18:1). The properties of fuel are closely related to the properties of its compositions, and the fuel properties will change with different mole fraction ratios of some composition. So, it is very important to research the thermal parameters of different compositions. Differential scanning calorimetry (DSC) is a thermo analytical technique by which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. The result of a DSC experiment is a curve of heat flux related to temperature or time. Through this curve, it is easy to determine transition temperatures and enthalpies, which makes DSC a valuable tool in producing phase diagrams for various chemical systems. Based on the differential scanning calorimeter, thermal parameters of the three methyl esters have been analyzed. The melting point and fusion enthalpy of saturated fatty acid esters (C16:0 and C18:0) are much higher than the ones of unsaturated fatty acid ester (C18:1), the melting points of C16:0, C18:0 and C18:1 are 301.57, 310.92 and 255.01 K, respectively. The binary solution of fatty acid methyl esters has also been scanned, where each curve exhibits two distinct exothermic peaks. These phenomena can be explained in that the high melting point saturated ester precipitated first and the low melting point unsaturated ester precipitated later. Moreover, the crystallization temperature of these solutions is lower than the melting point temperature of the pure saturated fatty acid esters, which precipitated earliest in the solution. It is also clear that the solution crystallization temperature rises accordingly, because the increase of the mass fraction of saturated fatty acid esters. Supposing that biodiesel is a solution combined with different fatty acid methyl esters, the saturated fatty acid esters C16:0 and C18:0 are treated as solutes and the unsaturated fatty acid ester C18:1 is played as solvent, thus the thermodynamic model for calculating the crystallization temperature of the solution has been established. While predicting the crystallization temperature, the solution of fatty acid esters has been dealt with further as an ideal solution; because it just looks at liquid composition as the ideal solution and solid phase composition as not mutually soluble, the model has been simplified. The crystallization temperature of the four different kinds of biodiesel has been calculated by this simplified model, and also has acquired effective verification as compared with the experimental data.