The use of co-solvent improves the mass transfer between the phases present in the transesterification process. Therefore, high biodiesel yield can be achieved in small reaction times, even at room temperature. The most effective co-solvents were DEE and THF. The first one can be considered as an excellent low-polar solvent that can increase the mutual solubility of oil and methanol. The THF originated similar conversions to the obtained with DEE, but the recuperation of this co-solvent was more difficult in comparison to that of DEE.
After studying the effect of catalyst type and concentration, methanol/oil molar ratio, methanol/co-solvent molar ratio, agitation rate and reaction temperature, the highest methyl ester yields, 97–98%, were achieved with 0.7 wt% KOH, 9:1 as methanol/oil molar ratio, 1:1 as co-solvent/methanol molar ratio, 700 rpm and 30 °C. Transesterification reaction followed a kinetic model of pseudo-first order and the rate constants at several temperatures were determined. Also, activation energy and the variations of enthalpy and entropy for the system of reaction were determined by means of application of Arrhenius and Eyring Polanyi equations, as a useful tool for future works. Finally, all samples of biodiesel were characterized and the ones with the highest ester contents showed only limitation on water content.