Simulated Study On Production Of Biodiesel Via The Organic Base-Catalyzed Transesterification Method

The increasing demands and the decreasing reserves of the fossil energy spur the increasing interests and concerns on renewable energies. Biodiesel, derived from vegetable oils or animal fats, is a biodegradable, renewable, nontoxic alternative fuel with high cetane value and free of sulfur and aromatic hydrocarbons. Its character makes itself a cleaner burning fuel than petroleum diesel with reduced emissions of CO₂, SOx, CO and aromatic compounds. Biodiesel is mainly produced by the acid or base-catalyzed transesterification reaction of vegetable oils or fats with short chain alcohols, typically methanol or ethanol. However, these catalytic systems have some technological problems, such as low reaction rate and corrosion for the acid system, and emulsification for the basic one, which results in the difficulty in separation of biodiesel from glycerin. To solve these problems, solid base and guanidine-catalyzed transesterification methods as well as supercritical transesterification method are developed. The solid base catalysts and guanidine are expensive, and the transesterification reaction of oil in supercritical methanol, although can be carried out in a quite short reaction time, requires high temperature and pressure. In order to take the advantages of the supercritical transesterification reaction as well as decrease the requirement for high temperature and pressure, we develop a method of transesterification reaction, which is carried out in sub-critical methanol with adding cheap and easily separable organic base as the catalyst.In order to simplify the analysis work, for complex crude oils and fats and biodiesel, we chose triavetin as a model material to investigate the above-proposed reaction. Firstly, the non-catalytic transesterification of triavetin in supercritical methanol was studied. It was found that higher yield of products was obtained with higher mole ratio of methanol (ethanol) to triavetin, at higher reaction temperature, and with higher filling level of the reactor. On the other hand, water and free acid concentration in the feed had negative effects on the reaction. The proposed mechanism of supercritical transesterification may be two simultaneous pathways. One is that methanol directly attacks the carbon atom of carbonyl group. The other isthat the dissociation and protonation of methanol cause alcoholysis. This is a first order reaction and the kinetic equation can be express as r=-d[uTG]/dt=K[uTG].Then, we examined the effects of adding various organic bases on the transesterification of triavetin and methanol conducted in sub-critical methanol. It was found that most of the organic base could catalyze the above reaction, but the catalytic activity was different. Higher relative basicity of organic amines exhibited higher activity. For isomeric compounds, the catalytic activity of the amine with branched chains was higher than the one with linear chains. For the homologues, the amine possessing fewer carbon atoms showed higher catalytic activity. Among the amines we examined, triethanolamine, triethylamine, tertiary-butylamine, and isopropylamine exhibited reasonable high catalytic activities.Subsequently, we investigated the transesterification of triavetin and methanol in sub-critical methanol at the aid of triethanolamine. It was found that the yield of the product increased with the increase of the molar ratio of methanol to triavetin and the increase of the amount of triethanolamine. On the other hand, the yield reached a maximum with the increase of the temperature at the temperature range of 160 to 210 °C. The kinetic equations of triethanolamine-catalyzed transesterification are r = -KCa at sub-critical temperature and r A=-KCA2 at low temperature, respectively.Finally, we examined the transesterification of triavetin and methanol in sub-critical methanol at the aid of triethylamine. The yield of the product increased with the increase of the content of triethylamine and the temperature. It was found that the introduction of propylene epoxide in the reaction system could dramatically enhance the yield of the product. However, hardly did the transesterification reaction produce, when only propylene epoxide was added.