Produção contínua de ésteres metílicos de ácidos graxos a partir do óleo de soja degomado com carbonato de dimetila em condições pressurizadas

Abstract

The continued growth in biodiesel production has encouraged the exploration of new approaches. One of these approaches is the interesterification route, using dimethyl carbonate (DMC) as a reagent, which can be obtained by environmentally friendly methods. The soybean production chain is well established and degummed soybean oil (DSO), a partially refined raw material, has a lower cost than the refined oil, being a good alternative to reduce the biodiesel production cost. This study investigated the continuous process using pressurized conditions for the reaction between DMC and DSO, aiming at the production of fatty acid methyl esters (FAME). The reaction system was analyzed with and without a heterogeneous catalyst, specifically the mixed oxide derived from hydrotalcite in a molar ratio of Mg:Al close to 3:1. The influence of non-catalytic process variables was studied, including reaction temperature (258 to 342 °C), mass ratio DMC:DSO (0.48:1 to 5.52:1) and pressure (6.6 to 23.4 MPa), at different residence times, on the yield of fatty acid methyl esters from the reaction. The temperature presented a more notable influence than the other variables, and the optimization of the response surface equation indicates that it would be possible to obtain a FAME yield of 90.5% at 325 °C, 17 MPa, 3 gDMC/gDSO in the residence time of 10 min. The study of the kinetic profiles confirmed the distinct importance of temperature in the process, and the increase in the free fatty acid content in the raw material, through the addition of oleic acid to DSO, allowed an increase in the FAME yield in the non-catalytic process. Mg:Al hydrotalcite in the molar ratio 3:1, the mixed oxide derived from it and the rehydrated hydrotalcite were synthesized and characterized. The mixed oxide had the highest specific area and was also what enabled the highest yield of esters in the supercritical reaction of DMC and DSO. The process demonstrated stability from the third residence time onwards, and from that time on, small levels of leached Mg were quantified. The most suitable temperature for the catalytic process was 275 °C, the particle size smaller than 1.7 mm and the mass of 5 g of catalyst in the bed. The thermal decomposition of the products was influenced by the increase in temperature and in residence time. The condition with the highest yield for the uncatalyzed process was 325 °C, 1.5 gDMC/gDSO, 20 MPa and residence time of 10 min. For the catalyzed process, the condition was 5 g of Mg:Al mixed oxide in a molar ratio close to 3:1, with particle size between 1.4-1.7 mm, at 275 °C, 3.0 gDMC/gDSO , 15 MPa, in the residence time of 20 min. In both, esters yield was close to 90%, and the combination of catalyst with supercritical conditions allowed expressive FAME yields, with a reduction in process temperature and thermal decomposition, even with a slight increase in residence time.