Cavitation ultrasonic activity

There exists a wide distribution of the cavitational activity in the sonoreactor with the maximum intensity observed just near the transducer (for the standard arrangements such as ultrasonic hom/bath). The intensity varies both axially as well as in the radial direction with decreasing trends as we go away from the transducer (in axial direction) and away from the axis passing through the center... 

At times the net rates of chemical/physical processing achieved using ultrasonic irradiations are not sufficient so as to prompt towards industrial scale operation of sonochemical reactors. This is even more important due to the possibility of uneven distribution of the cavitational activity in the large scale reactors as discussed... 

Sivakumar M, Towata A, Yasui K, Tuziuti T, Iida Y (2005) Ultrasonic cavitational activation a simple and feasible route for the direct conversion of zinc acetate to highly monodispersed ZnO. Chem Lett 35(1) 60—61... 

Lindstrom O (1955) Physico-chemical aspects of chemically active ultrasonic cavitation in aqueous solutions. J Acoust Soc Am 27(4) 654-671... 

The uses of inorganic metal compounds and rare gases to probe the conditions of cavitation collapse have become some of the most important methods available in fundamental ultrasonics. Quantitative determination of collapse temperatures and pressures, and qualitative determination of fundamental aspects of the nature of the cavitation field have been achieved, largely through SL spectroscopic methods. The presence of salts has a marked influence on properties on the acoustic systems, such as the extent of coalescence and bubble size, and the sonochemical activity and SL intensity. 

Procedure Switch on the ultrasonic cleaning tank filled with water and watch for the position of cavitation activity through the circular rings formed on the surface of water. Hold the Al-foil on the circular ring for 2-3 min and remove. Watch the foil against the light or through an overhead projector for the holes created on it as a result of cavitation. 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

Recently, it has been shown that ultrasonic agitation during hydrogenation reactions over skeletal nickel can slow catalyst deactivation [122-124], Furthermore, ultrasonic waves can also significantly increase the reaction rate and selectivity of these reactions [123,124], Cavitations form in the liquid reaction medium because of the ultrasonic agitation, and subsequently collapse with intense localized temperature and pressure. It is these extreme conditions that affect the chemical reactions. Various reactions have been tested over skeletal catalysts, including xylose to xylitol, citral to citronellal and citronellol, cinnamaldehyde to benzenepropanol, and the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. Ultrasound supported catalysis has been known for some time and is not peculiar to skeletal catalysts [125] however, research with skeletal catalysts is relatively recent and an active area. 

Colloidal potassium has recently been proved as a more active reducer than the metal that has been conventionally powdered by shaking it in hot octane (Luche et al. 1984, Chou and You 1987, Wang et al. 1994). To prepare colloidal potassium, a piece of this metal in dry toluene or xylene under an argon atmosphere is submitted to ultrasonic irradiation at ca. 10°C. A silvery blue color rapidly develops, and in a few minutes the metal disappears. A common cleaning bath (e.g., Sono-clean, 35 kHz) filled with water and crushed ice can be used. A very fine suspension of potassium is thus obtained, which settles very slowly on standing. The same method did not work in THF (Luche et al. 1984). Ultrasonic waves interact with the metal by their cavitational effects. These effects are closely related to the physical constants of the medium, such as vapor pressure, viscosity, and surface tension (Sehgal et al. 1982). All of these factors have to be taken into account when one chooses a metal to be ultrasonically dispersed in a given solvent. 

The ultrasonic irradiation of a solution induces acoustic cavitation, a transient process that promotes chemical activity. Acoustic cavitation is generated by the growth of preexisting nuclei during the alternating expansion and compression cycles of ultrasonic waves. For example, in aqueous liquid, temperatures as high as 4300 K and pressures over 1000 atm are estimated to exist within... 

The effects of ultrasonic irradiation on photochemical reactions have been also reported. In those papers, effects of cavitation were demonstrated. Cavitation means the process in which micro bubbles, which are formed within a liquid during the rarefaction cycle of the acoustic wave, undergo violent collapse during the compression cycle of the wave.5) The dissociation of water to radicals is an example of these effects. Since activated chemical species such as free radicals have high reactivity, chemical reactions proceed. In other words, this phenomenon is a chemical effect of ultrasonic waves. 

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