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Appendix_D-_Cavitation_A_technology_on_the_horizon.pdf

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Cavitation: A technology on the horizon

Parag R. Gogate1, Rajiv K. Tayal2 and Aniruddha B. Pandit1,*

1 2

An overview of the application of cavitation phenomenon for the intensification of chemical/physical processing applications has been presented here, discussing the causes for the observed enhancement and highlighting some of the typical examples. The important conside- rations required for efficient scale-up of the cavitatio- nal reactors and subsequent industrial applications have been depicted based on the work carried out as a result of sponsored projects at the Institute of Chemi- cal Technology, Mumbai. Overall, it appears that the combined efforts of physicists, chemists and chemical engineers are required to effectively use cavitational reactors for industrial applications. Some recommen- dations for further work to be carried out in this area have also been mentioned, which should allow the ex- ploitation of this technology on an industrial scale.

Keywords: Acoustic cavitation, chemical processing, hydrodynamic cavitation, novel reactors, process intensi- fication.

CAVITATION can be in general defined as the generation,

subsequent growth and collapse of cavities resulting in

18 3 very high energy densities of the order of 1 to 10 kW/m .

Cavitation can occur at millions of locations in a reactor simultaneously and generate conditions of very high tem- peratures and pressures (few thousand atmospheres pres- sure and few thousand Kelvin temperature) locally, with the overall environment being that of ambient conditions. Thus, chemical reactions requiring stringent conditions can be effectively carried out using cavitation at ambient conditions. Moreover, free radicals are generated in the process due to the dissociation of vapours trapped in the cavitating bubbles, which results in either intensification of the chemical reactions or in the propagation of certain unexpected reactions. Cavitation also results in the gen- eration of local turbulence and liquid micro-circulation (acoustic streaming) in the reactor, enhancing the rates of transport processes.

The four principle types of cavitation and their causes can be summarized as follows:

Acoustic cavitation

In this case, pressure variations in the liquid are effected using sound waves, usually ultrasound (16 kHz–100 MHz).

Chemical Engineering Department, Institute of Chemical Technology, Matunga, Mumbai 400 019, India Department of Science and Technology, Technology Bhavan, New Mehrauli Road, New Delhi 110 016, India

RESEARCH ACCOUNT

The chemical changes associated with the cavitation indu- ced by the passage of sound waves are commonly termed as sonochemistry.

Hydrodynamic cavitation

Cavitation is produced by pressure variations, which is obtained using the geometry of the system creating velocity variation. For example, based on the geometry of the sys- tem, the interchange of pressure and kinetic energy can be achieved resulting in the generation of cavities as in the case of flow through orifice, venturi, etc.

Optic cavitation

This is produced by photons of high intensity light (laser) rupturing the liquid continuum.

Particle cavitation

This is produced by a beam of elementary particles, e.g. a neutron beam rupturing a liquid, as in the case of a bubble chamber.

Applications of cavitation phenomenon

Among the various modes of generating cavitation given above, acoustic and hydrodynamic cavitations have been of academic and industrial interest due to the ease of operation and generation of the required intensities of cavitational conditions suitable for different physical and chemical transformations. It is worthwhile to overview different applications where cavitation can be used efficiently before discussing the design and scale-up aspects for these two types of cavitation in detail.

Chemical synthesis

In order to understand the way in which cavitational col- lapse can affect chemical transformations, one must con- sider the possible effects of this collapse in different systems. In the case of homogeneous liquid phase reactions, there are two major effects. First, the cavity that is formed is unlikely to enclose a vacuum (in the form of void) – it will almost certainly contain vapour of the liquid medium

*For correspondence. (e-mail: abp@udct.org)

CURRENT SCIENCE, VOL. 91, NO. 1, 10 JULY 2006

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