Study On Crystallization Process Of Palm Oil Methyl Ester At Low Temperature

Biodiesel has become an excellent substitute for traditional fossil diesel due to its good combustion behavior and characteristic of sustainable and environment-friendly. The vigorous development of biodiesel industry is important to alleviate the energy crisis and improve ecological environment. At the present stage of China, main raw materials to make biodiesel fuels are woody plant oil such as palm oil. Palm oil biodiesel （PME） has poor cold flow property which is easy to crystallize at low temperature. Crystallization has effect on the normal use of PME. In this paper, the influence factors of crystallization behavior and crystal morphology were studied to explore the crystallization mechanism of PME which could provide theoretical basis to improve cold flow property of PME.Palm oil biodiesel which accord with national standard was prepared. The chemical compositions of biodiesel were analyzed by GC-MS. The result showed that PME was composed of three elements, carbon and hydrogen and oxygen. The main composition of PME was fatty acid methyl esters （FAME） with C10～C24 which including saturated fatty acid methyl ester （SFAME） and unsaturated fatty acid methyl ester （UFAME） with fraction of 40.13% and 59.57%. The essential component of SFAME was C16:0 and C18:0 with fraction of 31.04% and 6.64%. UFAME was mainly composed of C16:1～C20:1, C18:2 and C18:3. The mass fractions of C16:1-C20:1, C18:2 and C18:3 were 44.54%,14.44% and 0.59% respectively. Petroleum diesel was composed of carbon and hydrogen.-10PD were mainly composed of n-alkane with 61.26%.The crystallization process of PME in the low temperature condition was observed by cryomicroscope. The research showed temperature, cooling rate, blending with petroleum diesel and adding low temperature flow improver had important influences on the crystallization process of PME. With temperature decreasing, crystal grew and formed three-dimensional network structure. The number of crystal particles would decrease with low cooling rate. The volume of particles enlarged at the same time, crystal was regular polygon thin sheet. When cooling rate was high, the number of crystal particles increased, crystal existed in the form of aggregate structure. Blending with petroleum diesel and adding low temperature flow improver could inhibited the nucleation of PME and the growth of crystal particles.According to the results of crystallization experiment, crystallization mechanism of PME was analyzed. PME could be considered as pseudobinary solution using high-melting-point SFAME as solute and low-melting-point UFAME as solvent. The crystallization process of PME was formation of supersaturated solution, nucleation of SFAME and growth of crystal. There were two kinds of mechanisms for the crystal growth of PME:equilibrium growth and fractal growth. Under equilibrium state, the crystal habit was determined by the growth rate of each crystal face, the shape of crystal particle was regular. If the system departed from equilibrium, fractal growth phenomena was observed, the crystal particle became aggregate structure.C_18:0 was used as model compounds of SFAME to predict the theoretical habit of PME crystal under equilibrium state according to the BFDH and AE model. The theoretical crystal habit of PME predicted by BFDH and AE model was hexahedron sheet crystal with the developing facet{002}. The results was in agreement with the crystal morphology obtained by experiment. The result by AE model was better than that by BFDH model, as the influence of intermolecular interactions was taken into account. C18 was used as model compounds of n-alkane to predict the theoretical habit of diesel according to the BFDH and AE model. The theoretical shapes of diesel under equilibrium state by BFDH and AE model was hexahedron sheet crystal with the developing facet {0 0 1}. Based on the DLA model, the fractal crystal growth of PME was simulated with MATLAB software. The simulation results was in good accordance with the crystal morphology obtained by experiment.