Chemistry Reference
In-Depth Information
cavitation results in much higher cavitational intensity and is expected to control the
chemical processing applications and reactions involving stringent conditions.
Distribution of the cavitational activity in large-scale reactors, especially sonochemical
reactors, is a very important issue considering the net overall effects in any processing
applications. Theoretical prediction of cavitational activity in terms of the pressure field
gives an efficient way of designing cavitational reactors with uniform activity. Numerical
simulations can be used to characterize the ultrasonic field propagation and to obtain the
spatial distribution of the mechanical effects. Based on theoretical analysis, one can obtain
the pressure field distribution in any new sonochemical reactor with different geometries
and operating conditions, which can aid in optimization for maximum/uniform cavitational
activity. Modelling studies can be extended to the quantification of other useful parameters,
such as the distribution of temperature, mass transfer coefficient and so on, which can be
controlling parameters depending on the type of application. A detailed discussion of the
distribution of the cavitational activity and the effects of different operating and geometric
parameters, with guidelines for designing reactors based on theoretical simulations, is
beyond the scope of the present work, and readers are requested to refer to existing
literature on this subject [23].
7.5 Optimization of Operating Parameters in Cavitational Reactors
The intensification obtained due to the cavitation phenomena for any physical or
chemical processing application are strongly dependent on the operating and geometric
parameters of the equipment. The important parameters in the case of sonochemical
reactors are the frequency and intensity of irradiation and the geometrical arrangement
of the transducers, the geometry of the reactor and the liquid phase physicochemical
properties. The important parameters in the case of hydrodynamic cavitation reactors are
the inlet pressure, cavitation number, design of the cavitation chamber and liquid-phase
physicochemical properties. Based on a critical overview of the literature illustrations
related to modelling of cavitational reactors [24-27], some guidelines for the selection of
the optimum set of operating parameters are presented in this section.
7.5.1 Sonochemical Reactors
The frequency of ultrasonic equipment is usually fixed and cannot be varied over a wide
range as the maximum transfer efficiency is obtained only when the transducer is driven at
its resonating frequency. Generally, it has been observed that increasing the frequency of
irradiations to an optimum value (in the range of 200-500 kHz depending on the type of
application) results in better conversions, especially for chemical processing applications.
However, the power requirement is higher for inception of cavitation events in a high-
frequency operation. The use of multiple-frequency operation can be considered an
efficient alternative to the drawbacks associated with single-frequency high-power
operation, especially when higher cavitational intensities are required. It has been reported
that the cavitational intensity and active volume for a combination of frequencies are
higher than those obtained for single-frequency operation [28-30]. It is thus recommended
that
the a combination of low-frequency irradiation (typically 20 kHz) with other
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