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reactions [36]. However, it should be noted that an optimum salt concentration can
exist beyond which the cavitational activity decreases. Wall et al. [37] have reported
that the optimum concentration is usually in the range 1-2M, and above this optimum
the sonoluminescence intensity can be observed to reduce drastically.
7.6.2 Use of a Combination of Cavitation and Other Processes
Intensification can be achieved using a combination of cavitation and other processes with
similar intensifying mechanisms that yield synergistic effects [38]. Combination with
advanced oxidation process will be beneficial only for those chemical synthesis applica-
tions in which free radical attack is the governing mechanism. For reactions governed by a
pyrolysis-type mechanism, the use of process-intensifying parameters that result in an
overall increase in the cavitational intensity, such as solid particles, sparging of gases and
so on, or a combination with microwave irradiations is recommended.
The observed synergistic effects of combining photocatalytic oxidation with cavita-
tioncanpossiblybeattributedtothemechanical effects of cavitation. The turbulence
and acoustic streaming generatedinthereactorincrease the transport of the reactants
and products at the catalyst surface and in the solution. Also, continuous cleaning and
sweeping of the TiO
2
surface due to acoustic microstreaming allows for access to more
active catalyst sites at any given point. There is also a possibility that the catalyst surface
area is increased by fragmentation or pitting of the catalyst. Cavitation-induced
turbulence enhances the rates of desorption of intermediate products from the catalyst
active sites and helps in continuous cleaning of the catalyst surface. It should be noted
that in situations where the adsorption of reactants at specific sites is the rate-controlling
step, ultrasound will play a profound role, due to a substantial increase in the number of
active sites and the increased surface area available due to fragmentation of the catalyst
agglomerates under the action of turbulence generated by acoustic streaming and an
increase in the diffusional rates of the reactants. Toma et al. [39] have given a brief
overview of different studies pertaining to the effects of ultrasound on photochemical
reactions, concentrating on the chemistry aspects; that is, the mechanisms and pathways
of different chemical reactions. Details regarding the operating and design strategies for
maximizing the synergistic effects obtained using sonophotocatalytic reactors have been
described by Gogate and Pandit [40].
The combination of hydrodynamic cavitation reactors and photochemical or photo-
catalytic reactors has not been much investigated, though some instances of a sequential
combination of hydrodynamic cavitation and photocatalytic oxidation have been reported
for waste water treatment applications. A commercial process (CAV-OX process [41]) that
employs ultraviolet radiation, hydrodynamic cavitation and hydrogen peroxide has been
reported to oxidize organic compounds present in water at ppm-level concentrations to
nondetectable levels. This system has been used with success for the effective degradation
of volatile organic compounds - primarily trichloroethane, benzene, toluene, ethyl benzene
and xylene. The removal efficiency of contaminants is reported to range from 20 to 99%.
Hydrogen peroxide is added to the contaminated ground water, which is then pumped
through a cavitation nozzle and then subjected to ultraviolet radiation. Hydrodynamic
cavitation produced in a nozzle is used to generate additional hydroxyl radicals, which help
in increasing the rate of degradation. Moreover, the use of cavitating conditions and the UV
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