小桐子生物柴油的超临界两步法制备及其抗氧化耐低温性的研究

【作者】 李法社；

【导师】 王华；

【作者基本信息】 昆明理工大学 ， 冶金能源工程， 2011， 博士

【摘要】 随着石油资源的日益枯竭、环境污染的加重和车辆柴油化趋势的加快,世界各国都加快了对替代石化燃料的开发步伐。在这种形势下,环保、可再生的生物燃料技术应运而生。生物柴油是利用植物油脂或动物油脂等可再生资源制取出来的可以替代石化燃料的清洁新型燃料,具有巨大的潜力和广阔的市场前景。目前,工业上大多采用酸或碱催化法制取生物柴油,但此方法存在对原料要求较高、反应时间较长、后处理工艺较为复杂、催化剂使用寿命有限制、产物分离困难等缺点。另一方面由于生物柴油主要成分多是含有碳碳双键的不饱和长链脂肪酸甲酯,储存中极易发生氧化。此外生物柴油低温结晶和凝胶化限制生物柴油在低温时的应用。因此研究生物柴油的制备新方法及其氧化稳定性能和低温流动性能对生物柴油的实际应用具有非常重要的意义。本文以小桐子油为原料,不添加任何催化剂,采用亚临界水—超临界甲醇两步法制备生物柴油,系统地研究了第一步的小桐子油在亚临界水中水解反应和第二步其脂肪酸在超临界甲醇中酯化反应。论文系统研究了各步反应中反应温度、反应时间、反应压力及原料配比等因素对制备生物柴油的影响。得出小桐子油在亚临界水中水解反应的最佳条件为：反应温度290℃,油水体积比1：3,反应时间40min,转化率达到98.9%；小桐子油脂肪酸在超临界甲醇中酯化反应的最佳条件为：反应温度290℃,反应时间30min,脂肪酸与甲醇体积比1：2,转化率达到99.02%。对小桐子油及其生物柴油的性能指标进行了测定。对水解反应和酯化反应进行了动力学研究,确定出各反应的动力学参数,其中亚临界水解反应的反应级数为0.78,活化能为55.34KJ/mol；超临界酯化反应的反应级数为1.45,活化能为66.79kJ/mol。在试验的基础上提出了亚临界水解反应和超临界酯化反应的白催化反应机理和亲核反应机理。本文对以小桐子油为原料制备的生物柴油的氧化稳定性能进行了系统的研究。具体分析研究了温度、储放时间、甲醇含量、小桐子油含量、金属和0#柴油添加量等因素对小桐子生物柴油氧化稳定性能的影响,其中温度和储放时间影响较大,如新制备生物柴油的诱导期时间为4.38h,存放4个月后其诱导期时间变为1.63h,若测试温度从110℃分别改为80℃和140℃,其诱导期时间变为19.81h和0.42h。对十种常用抗氧化剂以及其添加量对小桐子生物柴油氧化稳定性能进行了系统的研究,生物柴油放置一段时间后的诱导期为1.53h,若加入1000ppm没食子酸、叔丁基对苯二酚、叔丁基羟基茴香醚和抗坏血酸等后其诱导期时间变为16.97h、14.06h、8.77h和2.42h,效果和差别均较为明显。制备了没食子酸酯类抗氧化剂,主要有没食子酸甲酯、没食子酸乙酯、没食子酸丙酯、没食子酸异丙酯、没食子酸丁酯、没食子酸异丁酯和没食子酸叔丁酯等七种,在添加量1000ppm时,其对小桐子生物柴油的抗氧化效果均极佳,其诱导期时间均能达到国家标准6h。提出了生物柴油氧化是自催化的基链反应机理,认为生物柴油通过与氧反应生成过氧化物和直接生成烷基自由基两条途径发生链式氧化反应。提出抗氧化剂切断生物柴油氧化链式反应的抗氧化机理,认为抗氧化剂与过氧自由基反应,破坏自由基,生成氢过氧化物和抗氧化剂自由基,抗氧化剂自由基继续与自由基反应,生成氢过氧化物,从而切断生物柴油氧化链,达到抗氧化的目的。本文最后研究了改进生物柴油的低温流动性能的方法,制备了油酸异丙酯与油酸异丁酯。试验研究结果表明,油酸异丙酯和油酸异丁酯的凝点温度分别达到-25℃和-28℃,冷滤点温度也分别达到了-15℃和-20℃,大大低于生物柴油的凝点和冷滤点。研究了其和生物柴油混合时的低温流动性能,很好地改善了生物柴油的低温性能指标。应用本文中制备生物柴油的工艺方法制备生物柴油可以大大缩短反应时间、不使用催化剂、提高转化率、缩短流程、简化后处理工序、提高生物柴油质量等,利用本文中制备的抗氧化剂和油酸异丙酯及油酸异丁酯可以改善生物柴油的氧化稳定性和低温流动性,对大规模地发展生物柴油具有十分重要的实际意义。更多还原

【Abstract】 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.更多还原