Mechanistic and kinetic investigations in ultrasound assisted acid catalyzed biodiesel synthesis

This paper attempts to discern physical mechanism and kinetic aspects of ultrasound irradiation on transesterification of soybean oil with methanol using sulfuric acid as catalyst with approach of coupling experimental results with simulations of cavitation bubble dynamics. Kinetic constants as well as activation energies of transesterification reaction were determined at different alcohol to oil molar ratios and reaction temperatures. The results of this study reveal interesting features of inter–relation between mechanics of ultrasound/cavitation, and the intrinsic behavior (represented by specific rate constant) of the transesterification reaction. The beneficial effect of ultrasound irradiation on transesterification is of physical nature. Several anomalies of ultrasound assisted acid catalyzed transesterification (as compared to conventional system) are: occurrence of reaction at low temperature of 15 °C (albeit with low conversions of 13.45% and 10.2% for alcohol to oil molar ratios of 6:1 and 12:1, respectively); despite higher activation energy, higher rate constant at low alcohol to oil molar ratio of 6:1; and minima shown by reaction rate constant with temperature. The major physical effect of sonication is fine emulsification that generates enormous interfacial area for reaction that overwhelms the effect of specific rate constant. As revealed by simulations, physical effects of cavitation (viz. micro-convection and shock waves) are more pronounced at low temperature, which is primary cause leading to these anomalies.

► Mechanistic investigation of ultrasound enhanced acid catalyzed transesterification.
► Approach of coupling experiments with simulations of cavitation bubble dynamics.
► Anomalies of ultrasound assisted transesterification reaction observed.
► Physical effects of ultrasound and cavitation generate enormous interfacial area.
► Difference in cavitation bubble dynamics in oil and methanol responsible for anomalies.