Transesterification of Vegetable Oils under Ultrasonic Irradiation

Ultrasound Technology Influence of ultrasound on transesterification reaction is of purely physical nature. Formation of fine emulsion between oil and alcohol due to microturbulence generated by cavitation bubbles generates enormous interfacial area, which accelerates the reaction.

Ultrasound is the process of propagation of the compression waves with frequencies above the range of human hearing. Ultrasound frequency ranges from 20 kHz to l0 MHz, with associated acoustic wavelengths in liquids of about 100- 0.15 mm. These wavelengths are not on the scale of molecular dimensions. Instead, the chemical effects of ultrasound derive from several nonlinear acoustic phenomena, of which cavitation is the most important.

Acoustic cavitation is the formation, growth, and implosive collapse of bubbles in a liquid irradiated with sound or ultrasound. When sound passes through a liquid, it consists of expansion (negative pressure) waves and compression (positive pressure) waves. These cause bubbles (which are filled with both solvent and solute vapour and with previously dissolved gases) to grow and recompress.

Under proper conditions, acoustic cavitation can lead to implosive compression in such cavities. Such implosive bubble collapse produces intense local heating, high pressures, and very short life-times. Cavitation is an extraordinary method of concentrating the diffused energy of sound into a chemically useable form.

Ultrasonication provides the mechanical energy for mixing and the required activation energy for initiating the transesterification reaction.

Low-frequency ultrasonic irradiation is useful tool for emulsification of immiscible liquids. The collapse of the cavitation bubbles disrupts the phase boundary and causes emulsification, by ultrasonic jets that impinge one liquid on another.