Research On The Cold Flow Property And Its Improvement Of Biodiesel

Biodiesel, as a new generation of green, environmentally friendly and renewable biomass fuel, has received more and more attention and reported by a large number of literatures. It is mainly because that the biodiesel is non-toxic, small pollution and with low sulfur content, which is in line with the idea of sustainable development and the concept of circular economy. A lot of raw materials in the preparation of biodiesel can be chosen, including the edible or inedible vegetable oil, animal oil as well as waste cooking oil. Therefore, the biodiesel we obtained are various, leading to some inconvenience on studying its performance. However, they also have many things in common, such as, most of them are obtained through ester exchange between crude oil and methanol with the catalyst (mostly alkali such as sodium hydroxide or acid, like sulfuric acid). Fatty acid methyl esters are the main components of biodiesel prepared, and glycerol is the by-product. The biggest disadvantage of biodiesel in its popularization and application process is its poor low temperature flow performance. The reason is that the fatty acid methyl ester is easy to crystallize in low temperature environment, blocking the engine nozzle, affecting the normal work of the engine. In order to find out the factors affecting the cold flow property and its improvement of biodiesel, we do the work mainly from the following aspects in this paper:(1) We prepared five different kinds of vegetable oil, like soybean oil, corn oil, peanut oil, rapeseed oil, sunflower oil as the raw material, through the methyl ester exchange between them and method to produce five different biodiesels. Then, The effects of the molar ratio of methanol to oil, the amount of catalyst, reaction time and temperature on the yield of biodiesel were explored. We tested and screened out the rapeseed biodiesel with the best low temperature fluidity and the peanut biodiesel with the worst one as the research object of studying the effect of fatty acid esters on the cold flow property of biodiesel.(2) By GC-MS analysis, we determined the kinds and their content of fatty acid methyl esters from the peanut and rapeseed biodiesel. Then we classified the fatty acid methyl esters: a. Saturated straight chain fatty acid methyl ester; b. Unsaturated fatty acid methyl ester (containing only one double bond); c. Fatty acid ester with more than one double bond. Besides, the effects of the branched chain fatty acid ester (isopropyl stearate was selected in this paper) on the low-temperature fluidity of biodiesel were also within the scope of our research. It is worthy of noting that in addition to the branched chain fatty acid ester, the rest of the fatty acid methyl esters were all from rapeseed and peanut biodiesel.(3) In order to improve the cold flow property of biodiesel, we took the rapeseed oil as raw material oil, and methanol was replaceed by isopropyl alcohol to prepare the branched rapeseed biodiesel, and then we also explored the optimal conditions in preparing the branched biodiesel. In addition, we adopted the methanol mixing method and solvent dilution method (the diesel from direct coal liquefaction, i.e. DDCL was selected as the solvent in this experiment) to improve the preparing condition of branched biodiesel. Results showed that:(1) Taking the rapeseed biodiesel as an example, the optimal condition through alkali catalyzed ester exchange reaction is that molar ratio of methanol to oil 7:1; catalyst dosage 0.6 wt%, reaction time 1.25 h, reaction temperature 65?, and the corresponding yield of biodiesel was 96.8%. In addition that the oxidation stability of the prepared biodiesel is slightly somewhat poor, the rest of the performances is basically in accordance with the relevant standards.(2) The more short carbon chain fatty acid esters (Cm:0, m?16), more content of unsaturated fatty acid methyl esters (Cm:1, m?20) and more fatty acid esters with bigger unsaturation were in biodiesel, the better the cold flow properties of biodiesel would be. Besides, fatty acid esters with branched chains can also effectively improve the low temperature flow properties of biodiesel.(3) For the branched rapeseed biodiesel, the optimum conditions for molar ratio of the alcohol to oil is 8:1; catalyst dosage is 1 wt%; the reaction time was 2 h and the reaction temperature is 80? And the corresponding maximum yield of biodiesel was 29.4%. And branched biodiesel prepared from the mixture of isopropyl alcohol and methanol as well as the mixture of isopropyl alcohol and DDCL is much better in cold flow property than the biodiesel prepared by isopropyl alcohol only. In fact, the cold flow property of the biodiesel obtained from DDCL dilution method was the best. When the mass fraction of DDCL was 40%, the cloud point, cold filter plugging point and pour point of the prepared branched biodiesel could be as low as-7?,-21? and-29?, respectively.