Ultrasound-Assisted Biodiesel Synthesis: A Mechanistic Insight

Abstract

Use of ultrasound in intensification of biodiesel synthesis process is well-known. However, most of the published literature in this area has focused on results than rationale—in that the exact physical mechanism of the ultrasound-induced enhancement of the biodiesel synthesis has remained unexplored. The research in our group has tried to fulfil this crucial knowledge gap. In this chapter, we have provided an overview and analysis of our studies in establishment of the physical mechanism of ultrasound-assisted biodiesel synthesis. This essentially means identification of the links between physical and chemical effects of ultrasound and cavitation, and the basic chemistry of biodiesel synthesis. The physical effect of cavitation and ultrasound is generation of intense microturbulence in the medium, while the chemical effect is generation of highly reactive radicals through thermal dissociation of the gas and vapor molecules entrapped in the bubble. The basic approach in our research has been concurrent analysis of the experimental results and simulations of cavitation bubble dynamics. We have treated diverse biodiesel synthesis processes that employ edible, non-edible and mixed non-edible feedstocks of oil, both base and acid catalysts in homogeneous form and heterogeneous base catalysts. Our analysis has essentially established that physical effects of ultrasound and cavitation have greater contribution to enhancement and intensification of the transesterification process for biodiesel synthesis. This is essentially manifested through generation of strong emulsion and elimination of mass transfer barriers in the process. However, for heterogeneous catalyzed systems, the mass transfer still remains the rate controlling step, despite intense microconvection generated by sonication.