Ultrasonics or ultrasound refers to sound waves beyond the audible range of the human ear. The normal human hearing range is 16-16,000 cycles per second. The accepted terminology for one cycle per second is the Hertz (or Hz), and hence the hearing range is expressed as 16 Hz to 16kHz. Ultrasound is normally considered to lie approximately in the range of 15kHz to 10 MHz, that is, 15 x 103 to 10000 x 103 cycles per second, with acoustic wavelengths of 10 to 0.01 cm. Like any sound wave, ultrasound is propagated through a medium in alternating cycles of compression and stretching or rarefaction. These produce certain effects in the medium that can be usefully exploited. One such application is in the field of synthetic organic chemistry, first reported by Richards and Loomis (1927) and designated sonochemistry. The most appealing feature of sonochemistry is its ability to enhance reaction rates, often to remarkably high levels under environmentally benign conditions. Despite this potential, economic considerations have precluded the use of sonochemical processes. It is noteworthy, however, that a change in perspective appears to be emerging, as evidenced by the fact that a pilot plant is currently being funded by a French company to sonochemically oxidize cyclohexanol to cyclohexanone, and developmental work is underway in Germany to produce 4 tons of Grignard reagent per year (Ondrey et al., 1996). A number of books and reviews covering mostly the chemical aspects of sonochemistry have appeared over the years, for example, Suslick, 1988, 198, 1990a,b; Ley and Low, 1989; Mason, 1986, 1990a,b, 1991; Mason and Lorime 1989; Price, 1992; Bremner, 1994; Low, 1995; Luche, 1998. A recent review Thompson and Doraiswamy (1999) covers both the chemical and engineering aspects of sonochemistry and another by Keil and Swamy (1999) examines the present state of our understanding of sonoreactor design. Sonochemical enhancement of reaction rates is caused by a phenomenon called cavitation. Therefore, we largely confine the treatment in this chapter to the chemical and reaction engineering (scale-up) aspects of cavitation and its associated effects (see Shah et al., 1999, for a detailed treatment). An alternative means of achieving the same result is by mimicking the ultrasonic effect by inducing “hydrodynamic cavitation.”
