Parâmetros cinéticos de reações de transesterificação metanólica de óleos vegetais determinados por análise termogravimétrica

Abstract

With the depletion of natural reserves, a need to replace energy sources that are derived from petroleum for something more sustainable emerges and biofuels appear on a global scenario. Among biofuels, biodiesel has been gaining prominence in the economic, social and environmental scopes. For a biodiesel production, a transesterification or esterification of vegetable oils or fats with short chain alcohols takes place in the presence of a catalyst. The oil conversion into biodiesel is analyzed by the most common methods like chromatography. This work uses a thermogravimetric analysis as a differential method in the determination of the conversion of oil to methyl esters, applying mathematical modeling to evaluate the kinetic parameters. The methyl esters were obtained by transesterification of almond oil from macaúba and soybean oil, all types of catalysts, at various temperatures. By means of the thermogravimetric analysis, it was possible to verify the efficiency in the reading to obtain the conversion of the oils to esters, by observing two well defined processes: the first one at 210 ºC related to the esters formed and the second at a maximum of 365ºC related to residual triglyceride. With these results, the dispersion factor analyzed was 0.9992, confirming the observed behavior and the efficacy of the technique. After obtaining the ester content, the mathematical modeling was applied to validate the method, using kinetic parameters such as speed coefficient and activation energy. For this, two pseudo-homogeneous models were used, one of first order and another of second order, of which the two models achieved satisfactory results. It was then evaluated the use of the catalysts Potassium Methoxide, Sulfuric Acid (H2SO4), p-toluenesulfonic acid (p-TS) and the ionic liquid 1-methylimidazole bisulfite [BMI] [HSO4], according to the applied temperatures, being 65ºC, 55ºC and 45ºC for p-toluenesulphonic acid (p-TS), 45ºC, 35°C and 25°C for potassium methoxide and 95°C for Sulfuric Acid (H2SO4). As a result, the ionic liquid was the only one that did not achieve the expected effect as a catalyst. The ionic liquid used in the reaction was obtained by synthesis of 1-methylimidazole bromide [BMI] [Br], also synthesized with Sulfuric Acid (H2SO4), for 4 hours refluxing and stirring in an ice bath, where the efficacy of the synthesis was proven in the analyzes of infrared and Nuclear Magnetic Resonance (1H NMR). Acidity, iodine, peroxide, density, Infrared with Fourier Transform (FTIR), and thermogravimetric analyzes were performed, which indicated the state of conservation of the oils used, the quality and the content of the resulting ester, according to the standards established in the legislation.