Kinetics of Ethyl Ester Production from Soybean and Sunflower Oils Catalyzed by Sodium Ethoxide
Abstract
The present paper reports the kinetics of soybean and sunflower oils’ ethanolysis. The transesterification reaction was carried out using a molar ratio of ethanol to oil of 9:1 and 1.0 wt% of sodium ethoxide as catalyst under stirring of 400 rpm. The reactions were performed in a stirred batch reactor at three different temperatures (308.15, 323.15 and 338.15 K) over a period of 120 min. The concentration of compounds was analyzed by High-Performance Size Exclusion Chromatography (HPSEC). The kinetic model assumed that ethanolysis occurs in a sequence of three reversible steps with the production of di- and monoacylglycerols as intermediate components. Based on the modeling approach it was possible to determine the rate constants of reaction and activation energies for the transesterification process of soybean and sunflower oils. Despite the phase splitting, no mass transfer control was observed and the proposed mathematical model fitted well the experimental data.
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Introduction
Biodiesel, a clean renewable fuel, is considered as the best candidate for a diesel fuel substitution because it can be used in any compression ignition engine without the need for modification [1]. Ethyl esters (ethyl biodiesel) are produced from triacylglycerols, which can react with ethanol in the presence of a catalyst, usually an alkali or acid catalyst, in a process known as transesterification. The transesterification reaction results in the production of three moles of ethyl esters (EE) and one mole of glycerol (GL) for each mole of triacylglycerol (TG), requiring three moles of alcohol. The reaction occurs as a sequence of three steps with intermediate formation of diacylglycerols (DG) and monoacylglycerols (MG) [2].
Most of the biodiesel produced in the world today is derived from soybean oil, however, all vegetable oils or triacylglycerols can be converted into biodiesel. Factors such as geography, climate and economics determine the vegetable oil of greater interest for potential use in biofuels. Thus, in the United States, for example, soybean oil is regarded as the main raw material and in some tropical countries, it is palm oil. The most common vegetable oils, whose raw materials are abundant in Brazil, are soybean, corn, peanuts, cotton, babassu and palm [3.4].
Methanol is most often used as alcohol in the biodiesel synthesis because of its suitable physical and chemical properties and low cost, but it is usually derived from fossil sources. The production of ethyl esters, rather than methyl esters, is of considerable interest because the ethyl ester based biodiesel is an entirely agricultural fuel [5,6].
Fatty acid esters are broadly available in nature and have been widely used as high–value fine chemicals in the food, cosmetic, pharmaceutical and rubber industries. Currently, fatty acid esters (biodiesel) are being considered as a promising substitute for conventional diesel fuels, due to environmental and economic problems related to the use of conventional fuels [7,10].
There are several variables, such as molar ratio of alcohol to vegetable oil, catalyst type, temperature, and presence of impurities, among others, that affect the transesterification process and should be investigated [11,12].
Conclusion
The experimental data of concentration versus time showed that the production rate of ethyl esters started with a sudden surge followed by a lower production rate when the reaction approaches equilibrium, and the concentration profiles did not followed the sigmoidal behavior observed in the methanolysis.
The reaction rate constants, the corresponding activation energies, and pre-exponential factors reproduced very satisfactorily the process studied, showing that the developed kinetic model can be used to describe the ethanolysis process of soybean and sunflower oils adequately.