Cavitational Reactors

Petrier and Luche, 1987, Lindley and Mason, 1987, Thompson and Doraiswamy, 

Exact quantification of the cavitation, i.e. collapse intensity as a function of different operating and geometric properties is lacking. 

Lack of suitable design strategies linking the theoretically available information on cavity dynamics with the experimentally observed results. 

The existing information is available mainly on the laboratory scale, which would give very large scaleup ratios and hence lack confidence and certainty. 

Local existence of the cavitation phenomena just near the irradiating surface in the case of ultrasonic transducers and wide distribution of the energy dissipation patterns in the reactor due to uneven energy dissipation. 

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Hydrodynamic cavitation reactors have been investigated for more than a decade now in the UDCT Department of Bombay University [63-66]. When applied to some industrially relevant reactions, the hydrodynamically created cavitation appeared to dehver on average an order of magnitude higher cavitation yields than the acoustic cavitation. In addition, the processing volumes could be up to about 100 times larger than in the conventional sonochemical reactors. So far, there is no information about the industrial applications of the hydrodynamic cavitation reactors, although some concepts have already been patented [67]. 

Pandit and co-workers have shown that scale-up may be possible on a more rational basis if cavitation is employed, and some data have been reported by Pandit and Mohalkar (1996), Mohalkar et al. (1999), Senthil et al. (1999), and Cains et al. (1998). A variety of reactors can be used, viz. the liquid whistle reactor, the Branson sonochemical reactor, the Pote reactor, etc. The principal factors affecting the efficiency of a hydrodynamic cavitation reactor are irreversible loss in pressure head and turbulence and friction losses in the reaction rates. 

Gogate PR, Shirgaonkar IZ, Sivakumar M, Senthilkumar P, Vichare NP, Pandit AB (2001) Cavitation reactors Efficiency assessment using a model reaction. AIChE J 47 2526-2538... 

Theory of Cavitation and Design Aspects of Cavitational Reactors... 

Overall, it can be summarized that, use of multiple frequency irradiations based on the use of multiple transducers gives much higher cavitational activity in the reactor and hence enhanced results. It is also recommended that a combination of low frequency irradiation (typically 20 kHz) with other frequencies in the range of 50-200 kHz should be used for obtaining maximum benefits from the cavitational reactors. 

Gogate PR, Pandit AB (2000) Engineering design methods for cavitation reactors I Sonochemical reactors. AIChE J 46 372-379... 

Moholkar et al. [11] studied the effect of operating parameters, viz. recovery pressure and time of recovery in the case of hydrodynamic cavitation reactors and the frequency and intensity of irradiation in the case of acoustic cavitation reactors, on the cavity behavior. From their study, it can be seen that the increase in the frequency of irradiation and reduction in the time of the pressure recovery result in an increment in the lifetime of the cavity, whereas amplitude of cavity oscillations increases with an increase in the intensity of ultrasonic irradiation and the recovery pressure and the rate of pressure recovery. Thus, it can be said that the intensity of ultrasound in the case of acoustic cavitation and the recovery pressure in the case of hydrodynamic cavitation are analogous to each other. Similarly, the frequency of the ultrasound and the time or rate of pressure recovery, are analogous to each other. Thus, it is clear that hydrodynamic cavitation can also be used for carrying out so called sonochemical transformations and the desired/sufficient cavitation intensities can be obtained using proper geometric and operating conditions. 

It is always important to choose an optimum design configuration of the hydrodynamic cavitation reactor so as to maximize the cavitational effects and achieve cost effective operation. In this section, we will discuss available reactor configurations and give some guidelines, based on theoretical analysis coupled with experimental results, for selection of optimum design and operating parameters for hydrodynamic cavitation reactors. 

From the above discussion about various hydrodynamic cavitation reactors, it can be easily concluded that the orifice plate set-up offers maximum flexibility and can also be operated at relatively larger scales of operation. It should be also noted that the scale-up of such reactors is relatively easier as the efficiency of the pump increases with an increase in size (flow rate and discharge pressure) which will necessarily result into higher energy efficiencies. 

Guidelines for Selection of Hydrodynamic Cavitation Reactor Configurations... 

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