Solidification Characteristics Of Biodiesel At Low Temperature

Biodiesel is a green renewable energy.Compared with petrodiesel,biodiesel has obvious advantages and biodiesel industry has experienced fast development in recent years.However,biodiesel also has some problems such as the poor cold flow property.The study on flow properties of biodiesel at low temperature has just started at the present time.Researches focus on the influencing factors and improved methods for cold flow properties of biodiesel,however,the crystallization behavior of wax at low temperature is little studied which is crucial for the cold flow properties.Based on the crystallization behavior of fatty acid methyl ester at low temperature,the solidification characteristics of biodiesel at low temperature are studied in this work.The main contents of the study are as follows: The wax appearance temperature of biodiesel,the amount and composition of wax precipitated from biodiesel at different temperatures are measured.The crystallization rate constant and viscoelastic properties during cooling gelling and isothermal gelling process are investigated.The wax crystal morphology of biodiesel during cooling process was also studied.Through the above research,the following conclusions can be obtained:Biodiesel with higher composition of saturated fatty acid methyl ester(FAME)especially the long chain saturated FAME has higher cloud point and exhibits worse cold flow properties.When the temperature is just lower than the crystallization temperature of biodiesel,the composition of precipitated crystal is mainly the saturated FAME with long carbon number.With the further decrease of temperature,the concentration of saturated FAME with high carbon number in precipitated crystal becomes lower and the concentration of saturated FAME with low carbon number in precipitated crystal becomes larger,however,the precipitated proportion of all of saturated FAME increase so the amount of precipitated crystal is increased with the decrease of temperature.These precipitated crystals connect with each other by van der Waals force and form by a three dimensional network structure full of entrapped oil so the viscosity of biodiesel gel increases sharply,meanwhile,the obvious viscoelastic characteristics can be observed for biodiesel gel.In addition to the cooling gelling process,biodiesel also shows obvious isothermal gelling characteristics due to the growth and recrystallization of crystals.The isothermal gelling process has important influence on the storage and transport of biodiesels.Meanwhile,the aging process of biodiesel at lower temperature is much faster than at higher temperature.When the cooling rate is 0.1,0.5 and 1.0?/min,the gelation point of waste oil biodiesel is 8.4,7.5 and 6?,respectively.The gelation point temperature as well as the storage modulus during cooling and entire aging process decrease with the increase of cooling rate.The critical strain and storage modulus in linear region of biodiesel gels at different temperatures were obtained by studying the yielding process of gelled biodiesels using controlled stress rheometer.With the decrease of temperature,the storage modulus in linear region increases and the critical strain decreases.The storage modulus in linear region and the critical strain appear a power-law relationship with amount of precipitated crystal.Then the microstructure fractal dimension of biodiesel gels and parameter ? can be obtained based on Wu-Morbidelli scaling model.The results indicate that the ? value fell within a range between 0.18 and 0.40.The ? values for biodiesel gels suggested that the gels are in the transition regime and the contributions of interfloc and intrafloc links are comparable.The fractal dimensions of biodiesel gels increase from about 2.1 to 2.7 with decrease in temperature.It is suggested that the microstructure of the gelled biodiesel is porous and loose at relatively high temperatures and the structure is relatively compact at relatively low temperatures.