Conversion of refined and waste oils by ultrasound-assisted heterogeneous catalysis #30

Abstract

Biodiesel synthesized with solid acid catalysts tolerates water and free fatty acids in the feedstock and can be easily separated versus alkaline homogeneous catalysts. Furthermore, corrosion rates are lower. Nevertheless, they are more than 4,000 times less efficient than basic catalysts. Ultrasound accelerates the conversion of oils to biodiesel in homogenous systems by a factor of 300 times (a few seconds of sonication) and reduces the time required by the acid-catalyzed heterogeneous reaction to less than 1 h. Cavitation bubbles form on solid particles, but at the same time, the particles shield the ultrasonic waves so they do not penetrate as deeply into the liquid. Multiple transducers and designing the reactor can distribute the acoustic cavitation better. The literature focuses on classical sonochemical reactors for heterogeneous biodiesel production (baths and horns), whereas several optimized designs are available for homogeneous mixtures. We review the literature on heterogeneously catalyzed systems and include alternative reactor configurations for liquid mixtures that researchers have not yet adopted for solid catalyst systems.We report the effect of various alcohols and the combined action of ultrasound and microwaves to transesterify oil to biodiesel. Also, we discuss several catalyst/ultrasound systems that accelerate the reaction rates.