Utilization of Static Mixers in the Oil Transesterification Reaction for Biodiesel Production

Biodiesel is a low-emission, highly degradable fuel, obtained by transesterification of a vegetable oil or an animal fat with an alcohol, usually methanol. Because oil and alcohol form two separate phases, mixing is crucial in the biodiesel synthesis reaction, in order to provide a fine dispersion of alcohol in oil, and therefore to favour mass-transfer phenomena. Most transesterification studies focused on mechanically-agitated systems. Conversely, only a few works tested the utilization of static mixers (SM) for the generation of the alcohol/oil dispersion, despite the fact that SM generally require less power input than mechanical agitation at equivalent performance; besides, thanks to the absence of moving parts, SM are characterized by minimum maintenance requirements. In this study, the utilization of Sulzer Chemtech 15 mm SMV^® static mixers for the KOH-catalysed transesterification of sunflower oil was studied by means of batch tests conducted in an experimental ring equipped with a 22 L reactor. Oil and methanol (containing the dissolved KOH) were initially loaded into two tanks, whose headspace was connected to a line of compressed gas. The two streams were mixed in a tee connection and then fed to the reactor by gas displacement. The methanol:oil molar ratio was set to 6, the temperature to 60 °C, and the KOH concentration to 0.8% of the initial oil mass. The effect of SM number (0-5) and superficial velocity (0.4-2.5 m/s) was investigated; the results were compared to those of analogous tests performed in batch conditions with only mechanical agitation, at different rotational speeds. The test conducted with one single SM at a 1.3 m/s superficial velocity (Re = 1490) resulted in a profile of sunflower oil conversion versus time equivalent to that obtained in the best-performing tests with mechanical agitation, characterized by the presence of two Rushton turbines operated at a rotational speed of at least 400 rpm (Remixing >= 2870). In these tests, the equilibrium conversion was equal to 93-96%, and the time for the attainment of a conversion equal to 90% of the equilibrium value (t₉₀) was equal to about 2.5 minutes. The tests conducted, with 1 SM, at superficial velocities higher than 1.3 m/s provided the same reaction profile as the test at 1.3 m/s. This observation, in agreement with an evaluation of the reaction and transport characteristic times, indicates that the test at 1.3 m/s was affected by a negligible mass transfer limitations (kinetically controlled condition). Conversely, a test conducted with 1 SM at a superficial velocity of 0.4 m/s resulted to be characterized by a significant mass-transfer resistance. In the SM tests conducted at 1.3 m/s, the utilization of more than one SM did not lead to any significant increase of the reaction rate. In order to evaluate the actual contribution of the SM section in the generation of the methanol/oil dispersion, a further group of tests was conducted, with the same system for loading oil and methanol, in the absence of SM. The test conducted at a velocity in the empty SM section equal to 1.3 m/s (corresponding to 2.9 m/s in the piping) led to a 65% increase of t₉₀. This result indicates on the one hand that the single SM provides a relevant contribution to the generation of the dispersion, but on the other that contribution of the simple “T” junction of the loading system is not negligible. Lastly, the mixing energy required for the generation of the methanol/oil dispersion with SM and with mechanical agitation was evaluated. In the SM test at 1.3 m/s, the specific energy requirement evaluated at an oil conversion equal to 90% of the equilibrium value (e₉₀) resulted equal to 15 J/kg_biodiesel. In the batch tests with mechanical agitation with two Rushton turbines, in order to attain a similar energetic performance it was necessary to reduce the rotational speed to 250 rpm, which led to a 15-20 % increase of t₉₀. These results show that, with both mechanical agitation and static mixing, the mixing energy for biodiesel production can be reduced to very low values by means of a careful evaluation of the optimal rotational speed or SM superficial velocity. On the whole, this study suggest that static mixing can be effectively applied to oil transesterification processes for biodiesel production.