Sonochemical techniques to increase the efficiency of methyl esters production from non-food oils

Industrially practiced transesterification for biodiesel production can accept a restricted range of feedstocks, with free fatty acids (FFA) concentration and moisture content lower than 0.5 and 0.2%wt, respectively. Refined oils usually match this requirement but their use increases biodiesel production costs, besides being in competition with food production. Non-food raw oils may require several pretreatment steps before proceeding with the transesterification. The search for high efficiency transformation methods is therefore a key issue. In this work, FFA esterification, catalysed by ion exchange resins, and the homogeneously catalysed transesterification to produce fatty acids methyl esters are studied using single and combined sonochemical techniques such as ultrasound and microwaves. The results show that microwaves are able to enhance FFA esterification activity, allowing to achieve 90% of conversion in 2 hours rather than 4 hours, required by the traditional method. US increases tremendously transesterification conversions, yielding methyl esters above 96.5%wt within 30 minutes, while more than 2 hours are usually required with the traditional method. Moreover, much lower reagents and catalyst amount are requires when ultrasound are applied. The positive effect of microwaves is attributable to the re-orientation of the methanol dipole leading to the formation of a methanol-oil/emulsion with very high exchange area between the phases. The benefits brought by the use of ultrasound may be ascribed to both chemical and physical effects generated by acoustic cavitation.