| With the increasing depletion of oil resources, the deterioration of environmental pollution and the acceleration of motor vehicles diesel, all countries in the world have accelerated the development of alternative fuels. In this situation, environmental friendly and the technology of renewable biofuels arises at the historic moment. For it is made from renewable resource such as vegetable oil or animal fats, biodiesel is a kind of clean new fuel which can take place of fossil fuel. So it has great potential and wide prospect of market. At present, the method of making biodiesel with acid or alkali catalyst is used widely in industry. But this method has many shortcomings, such as the higher requirements of raw materials, the longer reaction time, the complexity technology of post-treatment, the limited life of the catalysts, the difficulty of separation and so on. On the other hand, biodiesel is chiefly composed of long chain fatty acid methyl esters with unsaturated carbon-carbon double bond, so it is easily oxidized during physical holding of the stock. Furthermore, biodiesel is so easy crystallized and gelatinated at low temperatures that its use is consumedly limited. Therefore, the research of new preparation methods of biodiesel. the oxidative stability and the low-temperature fluidity of biodiesel is of vital significance for practical application of biodiesel.In the present work, the two-step method of sub-critical water and supercritical methanol with no catalysts was used to prepare biodiesel with Jatropha curcas.L seed oil as raw material. Firstly, the Jatropha curcas.L seed oil is hydrolysed into fatty acids in sub-critical water. SecondLly, the fatty acids are esterified to fatty acid methyl esters in supercritical methanol. Some factors, such as reaction temperature, reaction time, reaction pressure and the ratio of raw materials, which may have effects on the preparation of biodiesel in the two step reaction have been investigated. The best conditions that the Jatropha curcas.L seed oil is hydrolyzed in sub-critical water are temperature 290℃, oil-water volume ratio 1:3, reaction time 40min. With these tested conditions, the yield of fatty acids is 98.9%. The most appropriate conditions for esterification of fatty acids in supercritical methanol are reaction temperature 290℃, reaction time 30min, volume ratio of to methanol 1:2. With these tested conditions, the yield of fatty acid methyl esters is 99.02%. At the same time, performance index of Jatropha curcas.L seed oil and its biodiesel were measured and studied. The kinetics of the hydrolysis reaction in sub-critical water and esterification reaction in supercritical methanol were also studied. The results show that the hydrolysis reaction order is 0.7778; the activation energy is 55.34kJ/mol. The esterification reaction order is 1.45 and the activation energy is 66.79 kJ/mol. The reaction mechanisms that are autocatalysis and nucleophilic reaction of hydrolysis in subcritical water and esterification in supercritical methanol were proposed based on the experiments.The oxidative stability of the Jatropha curcas.L seed oil biodiesel were studied in this work. Some factors, such as temperature, storage time, methanol content, Jatropha curcas.L seed oil content, metal Cu and Fe,0#diesel content etc., which may have effects on the oxidative stability of biodiesel have been described in detail. The results indicate that temperature and storage time are most prominent in these factors. For example, induction period time of fresh biodiesel is 4.38h, but 4 months later, it falls to 1.63h. If changing temperature from 110℃to 80℃or 140℃, the induction period time of fresh biodiesel will be 19.81h and 0.42h. At the same time, 10 kinds of common antioxidants also have been studied and measured with regard to the effect on oxidative stability of biodiesel with the same content. This demonstrates that the efficacy of these common antioxidants is obvious, the antioxidant ability with the same additive amount (1000ppm) being different. For example, the induction period time of new biodiesel is 1.53h, but its value changes into 16.97h,14.06h,8.77h and 2.42h with adding gallic acid, tert-butylhydroquinone, butylated hydroxyanisole and ascorbic acid as antioxidants of 1000ppm concentration respectively. Seven kinds of antioxidants were produced, including methyl 3,4,5-trihydroxybenzoate, ethyl gallate, propyl gallate, isopropyl gallate, n-butyl gallate, isobutyl gallate, tert-butyl gallate and so on. These antioxidant additives are beneficial to the oxidative stability of Jatropha curcas.L. Its induction period time has achieved 6h of national standard. The autocatalysis chain reaction mechanism of biodiesel oxidation and the cutting-off chain reaction mechanism by antioxidant was proposed on the basis of experiments. At last, methods of improving the low temperature fluidity of biodiesel have been studied and isopropyl oleate and tebelon have been produced in this work. The results demonstrate that the solidification point of isopropyl oleate and tebelon is-25℃and-28℃, their cold filter plugging point is-15℃and-20℃. These are enormously lower than the solidification point and the cold filter plugging point of biodiesel. The low temperature fluidity of isopropyl oleatev and tebelon has been studied when they were blended with biodiesel. The results show that the low temperature fluidity of biodiesel has been well modified by this means.Biodiesel synthesis with supercritical two-step method can greatly shorten reaction time and production flow, increase efficiency, improve biodiesel quality, and simplify the treatment processes without any catalysts. Utilizing the methods developed in this work to produce biodiesel and studying its performance index may have very important practical significance on the large-scale development of biodiesel. |
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