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activate the solid catalyst and increase the material mass transfer to the surface through the
disruption of the interfacial boundary layers, as well as dislodging the material occupying
the active sites. Collapse on the surface, particularly of powders, produces enough energy
to cause fragmentation (even for finely divided metals). Thus, in this situation cavitational
effects increase the surface area for a reaction and provide additional activation through
efficient material mixing and enhanced mass transport.
In heterogeneous liquid/liquid reactions, cavitational collapse at or near the interface
will cause disruption and mixing, resulting in the formation of very fine emulsions.
When very fine emulsions are formed, the surface area available for the reaction between
the two phases is significantly increased, thus increasing the rates of reaction. This is very
beneficial, particularly in the case of phase-transfer-catalysed reactions or in biphasic
systems.
It is very important to understand the exact mechanism of the expected intensification of
chemical processing applications using the cavitational effects. When deciding on whether
to select cavitational reactors as a replacement for conventional reactors in a particular
chemical or physical transformation, the first question that needs to be addressed is whether
the cavitational enhancement is a result of an improved mechanical process (due to
enhanced mixing). If so, cavitation pre-treatment of a slurry may be all that is required
before the system is subjected to a conventional transformation scheme, and the scale-up of
the pre-treatment vessel will be a relatively simple task. If, however, the effect is truly
based on the cavitation chemistry, cavitation must be provided during the transformation
itself, either in a continuous manner or in suitable pulsed operation. The scientific database
from the laboratory study provides the most effective parametric window and gives the
cause-effect relationships between the operating parameters, the observed cavitational
effects and the expected levels of PI. An important step in laboratory-scale investigations is
to try to understand the mechanisms of interaction from the observed phenomena, so that
the desired cavitation field can be created on a larger scale in order to promote similar
interactions. The medium physicochemical properties are very important in deciding
whether or not cavitational effects can be beneficial and determining the associated costs
for the expected intensification. For example, high-viscosity media with low vapour
pressure will require greater energy to generate cavitation, whereas the presence of
entrained or evolved gases will facilitate cavitation, as will the presence or generation
of solid particles. Overall, it is crucial that a proper analysis in terms of system parameters,
the expected mechanism of the cavitational effects and the degree of PI balanced against
economics is made before the selection of cavitational reactors is recommended for the
specific transformation.
7.3 Reactor Configurations
7.3.1 Sonochemical Reactors
Sonochemical reactors are based on the principle of generation of cavitation by the use of
ultrasound, described commonly as 'acoustic cavitation'. In the case of acoustic cavitation,
the pressure variations in the liquid are effected using high-frequency sound waves, with
frequencies in the range of 16 kHz-5MHz. If a sufficiently large negative pressure is
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