Nova proposta experimental de síntese do biodiesel: avaliação da adição invertida dos reagentes na metanólise alcalina do óleo de soja a 25,0 °C

Abstract

The possible depletion of fossil fuels, the increase accumulation of CO2, and the growing energy demand have aroused interest in alternative sources of green energy. In this sense, biodiesel, a biofuel from renewable sources, has been used as an alternative to replace petroleum diesel. Research involving its synthesis is constantly advancing with the aim of optimizing more and more the process variables. Most of the studies employ a conventional synthesis methodology where the reactants are mixed with the direct addition of the catalyst/alcohol solution over the oil, under constant stirring and heating. This work sought to investigate and optimize an alternative, unprecedented proposal for the synthesis of methyl biodiesel, in which the gradual addition of soybean oil is carried out over the catalyst/methanol solution (inverted procedure), changing the sequence and speed of addition of the reagents. compared to the conventional methodology. The experiments were promoted at 25.0 °C for a lower energy consumption. The study was conducted using a factorial design with 4 factors and 2 levels (24

) aiming at the evaluation of the following reaction conditions: type of catalyst (NaOH and NaOCH3); reaction time (40 and 60 minutes); stirring speed (400 and 600 rpm); and oil addition time in the catalyst/methanol mixture in relation to reaction time (percentage relative time, tR) (60 and 80%). After planning, studies were carried out to achieve greater conversions based on the evaluation of the effect of stirring speed (200, 300, 400, 500 and 600 rpm), tR (20, 40, 60 and 80%) and synthesis in two successive steps of transesterification, additionally, a comparison was made between the conventional and proposed synthesis procedures. The planning indicated that the tR and the time of synthesis were the main factors of influence in the conversion for the studied system. Conversions around 86% were obtained by applying tR of 60% for 60 minutes and 400 rpm with the two catalysts used (NaOH and NaOCH3). The post-planning study demonstrated that the strategy of slow addition of oil over the catalyst/alcohol solution achieved higher conversion averages at room temperature and low stirring speeds (up to 300 rpm) compared to the conventional method. The best condition used was performing the biodiesel synthesis in two transesterification steps of 60 minutes, tR of 20%, stirring speed of 300 rpm and NaOH as catalyst, resulting in a conversion of 96.6%. A high conversion of triglycerides was obtained using the proposed experimental procedure under milder conditions and with lower energy consumption when compared to conventional syntheses under heating. This result makes the new experimental procedure attractive for industrial application, considering that minimal changes in the manufacturing process will be necessary, which can even be compensated in the long term by the lower energy demand. The analysis of acidity index, specific mass, kinematic viscosity and ester content indicated the quality of the resulting biodiesel within the standards established by Brazilian legislation.