Energetic optimization of vegetable oil transesterification for biodiesel production

This study was aimed at minimizing the energy consumption required to induce the alcohol-oil dispersion in the transesterification of sunflower oil with methanol. The tests were conducted in a 20-L temperature-controlled reactor provided with two 4-bladed Rushton turbines. The reactions were performed at 60 °C in homogeneous basic catalysis (KOH = 0.8% of oil mass). In the first part of the study we investigated the effect of agitation speed (100-700 rpm) on the profile of biodiesel mass fraction versus time: in each test, oil and methanol were separately loaded in the reactor in the absence of mixing, and the reaction was subsequently monitored at constant agitation speed. At an agitation speed of 400 rpm or higher no mass-transfer limitation was observed (kinetically controlled reaction), and a 0.85 biodiesel mass fraction was obtained in 4 minutes; conversely, at an agitation of 200 rpm or lower significant mass-transfer limitations were observed. In the second part of the study, we tested the effectiveness of static mixers (SM) in the reduction of energy consumption: in each test, oil and methanol were loaded from two pressurized vessels to the agitated reactor by means of a line containing a number of SM (type SMV, manufactured by Sulzer) variable between 1 and 5 (superficial velocity in the SM = 1.3 m/s), and the reaction was subsequently monitored at a constant agitation speed (100 rpm). Independently of the number of SM, the profiles of biodiesel mass fraction versus time coincided with those obtained in the first part of the study at 400 rpm (kinetically controlled reaction). The employment of 1 SM allowed a 94% reduction of energy consumption in comparison with the tests operated at 400 rpm in the absence of SM.