CHEMICAL APPLICATIONS FOR ULTRASONIC WAVES 145 Go$ Compressed oir • Siren Stonding woves Liquid Influent mist Cyclone precipitotor Figure 4.--Sonic precipitator for mists. For a given particle size distribution, however, the frequency must be maintained within relatively narrow limits for large effects to be obtained. Several pilot plant sonic precipitators (8, 27) have been constructed with air driven sirens as the sound sources. Figure 4 is a diagram of a unit designed for the precipitation of sulfuric acid mists. In the standing wave field the droplets increase to a size of the order of 10 microns. A cyclone precipitator is then used to complete the precipitation. With sirens operating at a frequency in the range 1000 to 4000 cycles/sec. and driven by a 10 horsepower compressor, units of this type have been re- ported to be capable of handling several thousand cubic feet of mist ladden air per minute (27). Sonic precipitators of similar type also have been used for carbon and other solid particles. Such precipitators have several disadvantages compared to conventional electrostatic precipitators. These include large power consumption, costly maintenance, corrosion problems in the sirens and erratic per- formance with changes in the particle size distribution in the influent. These are some of the reasons why sonic precipitators have not yet found wide use. Sonic precipitators should be considered, however, for systems where other methods of precipitation are not satisfactory, e.g., materials which constitute an explosion hazzard if precipitated electrostatically or which will not acquire the desired electrical charge in an electrostatic precipitator.

146 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 5.--Degassing of water with low intensity ultrasonic waves at a frequency of 800 kc./sec. Degassing of Liquids The formation of cavitation bubbles within a liquid is contingent on the presence of dissolved gases within the liquid. The growth of the cavitation bubbles leads to the degassing of the liquid. Evidence of this degassing effect can be seen in terms of the air bubbles shown in Fig. 5. The water within the vessel is being subjected to relatively low intensity ultrasonic waves (1 watt/cm2.) at a frequency of 800 kc./sec. Degassing effects are even more pronounced if the pressure of the gas above the liquid is reduced so as to help prevent the redissolution of the gas in the liquid. The degassing of liquids with sonic or ultrasonic waves on a large scale is entirely feasible. Such a technique should offer particular advantage in the case of viscous liquids which are difficult to degas. The removal of dissolved gases from molten metals (39) and molten glass (22) with ultra- sonic waves has been reported. Relatively little acoustical power is required and equipment is available for continuous flow processing. Mass and Heat Transfer l•ithin Fluids The rates of many physical and chemical processes involving two or more phases are limited by diffusion in the fluid phases despite extensive me- chanical agitation. Usually a boundary layer persists in the fluid phases at