Sonication heat generation

Figure 10-3. Sonicator or sonic cell disruptor showing the sonic wave generator in the background and the titanium transducing element in the foreground. Needle tips capable of operating in small test tubes may be interchanged with the larger tip (arrow) shown here. (Courtesy of Heat Systems-Ultrasonics, Inc., Plainview, N.Y.)...

Under real drying conditions heat generated within the material volume is transported away by conduction and convection so the final temperature tends to equilibrium determined by the heat balance. The equilibrium temperature for the bulk of the material is the order of few degrees (Zayas and Pento, 1975 Strumillo and Kudra, 1989 Brun and Boucher, 1957), which justifies moisture evaporation due to thermal effects of sound irradiation to be neglected. This obviously favors sonic drying as a method for processing heat-sensitive materials. 

Soxhlet extraction (EPA SW-846 3540) is a very efficient extraction process that is commonly used for semivolatile petroleum constituents. In the method, the solvent is heated and refluxed (recirculated) through the soil sample continuously for 16 hours, or overnight. This method generates a relatively large volume of extract that needs to be concentrated. Thus, it is more appropriate for semivolatile constituents than for volatile constituents. Sonication extraction (EPA SW-846 3550) can also be used for semivolatile compounds, and as the name suggests, involves the use of sound waves to enhance analyte transfer from sample to solvent. Sonication is a faster technique than Soxhlet extraction and can require less solvent. 

In 1916, Hartmann discovered (Ref 7) that intense noise is generated from shallow cylindrical cavities when these are impinged by sonic gas jets, owing to resonant oscillations. Later it was shown that a very slender Hartmann whistle, with a length to diameter ratio of 30, exhibits intense heating at the closed end. Since that time, further improvements have taken place, principally in the incorporation of tapered or stepped cavities (Ref 7) culminating in the flueric match (see Fig 1)... 

These three quantities are not totally independent. The W2/Wx ratio depends on the generator and gives its energetic efficiency. The WK/W2 ratio depends on the efficiency of the coupling between the emitter and the sonicated medium. Due to different acoustic impedance, not all of the energy output from the transducer is transmitted to the medium, and part is reflected at the emitter/medium interface [1] and this is degraded into heat in the transducer. This will depend of course on the nature of the irradiated medium (density, viscosity, gas content,...) and on the experimental conditions (temperature, external pressure), but it will also depend on the mass and almost certainly on the geometry of the reactor. 

Since a large amount of heat is generated in the sonication process, the sonication vial containing the material should be immersed in a cooling bath (temperature approx 4°C). 

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