Cavitation mechanism, chemical processing

Since 1945 an increasing understanding of the phenomenon of cavitation has developed coupled with significant developments in electronic circuitry and transducer design (i. e. devices which convert electrical to mechanical signals and vice versa). As a result of this there has been a rapid expansion in the application of power ultrasound to chemical processes, a subject which has become known as Sonochemistry . 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

When deciding on the type of the reactor required for a particular chemical or physical transformation, the first question that needs to be addresses is whether the cavitation enhancement is the result of an improved mechanical process (due to enhanced mixing). If this is the case, then cavitation pretreatment of a slurry may be all that is required before the system is subjected to conventional type transformation scheme and the scale up of the pretreatment vessel would be a relatively simpler task. 

The paper gives an overview of effects occurring or acoustic treatment of dissolved and molten polymers. Emphasis is made on acoustic cavitation discovered recently not only in low-viscous fluids but also in molten polymers. Major guidelines have been specified for practical utilization of acoustic treatment of flowable polymers in molding intensification of extrusion processes, reduction in thickness of produced films, directed mechanical destruction, chemical activation of melts, etc. Efficiency of overlapping high-frequency vibrations in molding of molten thermoplastics is discussed in terms of power consumption. 

A scale-up of a sonochemical process is usually required in order to treat commercially viable quantities of a solution. It is now becoming apparent that higher ultrasonic frequencies present perhaps the best way to scale up a process. The energy required to cavitate water is provided by the transducer in the form of mechanical waves. How does this energy ultimately dissociate the water molecule and produce chemical reactions Fig. 5 shows that for the lower ultrasonic frequencies (i.e., 20 kHz), the water itself cannot support... 

The fact that US influences the mechanism of chemical reactions via the action of highly reactive radicals such as OH- and H- formed during solvent sonolysis is well known (see Chapter 7). Solvents sensitive to thermolysis or sonolysis (e.g. dimethylformamide [158], dimethylsulphoxide [159]) decompose slowly in the presence of intense US. Thus, radical species formed by cavitation are highly reactive and may participate as activators or enhancers in the electrode process [160]. In fast, qt/asr-reversible or irreversible systems, however, the only effect of US is to enhance mass transport without any direct effect on the rate of simple electron transfer processes. 

In the process of sonication, electrical energy is converted into mechanical energy. The sound waves produce cavitation (the process of formation and collapse of microscopic bubbles) which leads to great mechanical shear forces. Particles are hammered to form smaller particles, and the increased energy of the system can promote chemical reactions. Ultrasonic chemistry has recently been reviewed [34]. Most research in sonichemistry has been carried out in ultrasonic cleaning baths. Operations on scale can be carried out by using commercial flow cells [35],... 

Up to a few years ago the majority of applications and developments involved ultrasonic frequencies in the MHz range for noninvasive analysis. A number of uses have been developed for the measurement of such factors as the degree of emulsification or extent of particle distribution within dispersions. More recently the interest of food technologists has turned to the use of power ultrasound in processing. In this case the mechanical and chemical effects of cavitation are important and applications are very wide ranging (Table 1). 

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