Crystal precipitation law of biodiesel based on thermodynamic phase equilibrium

Abstract: 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.