Chemistry Reference
In-Depth Information
applied to the liquid such that the average distance between the molecules exceeds the
critical molecular distance required to hold the liquid intact, cavities or voids are created.
Subsequent compression and rarefaction cycles of the sound waves cause the cavity to
expand, reach a maximum cavity size (the magnitude depends on the operating conditions)
and then collapse, releasing a large amount of energy [2].
A common sonochemical reactor design is the ultrasonic horn, which is most frequently
used for sonochemical research at the laboratory scale [7]. Ultrasonic horns are typically
immersion-type transducers and their most important advantage is that they are capable of
delivering large amounts of power directly to the reaction mixture, which can also be
regulated by varying the amplitude delivered to the transducer, resulting in variable power
dissipation into the system. Ultrasonic streaming from the tip of the probe may be powerful
enough to provide efficient bulk mixing. Most modern units are provided with a pulse
facility that allows the operator to sonicate reactions repeatedly for fractions of a second.
This gives adequate time for cooling between sonic pulses. With such systems, the probe
(horn) can be tuned to give optimum performance, which is important for the reproduc-
ibility of results and the optimization of power dissipation.
Apart from the conventional designs, some modifications aimed at increasing the
cavitationally active volume have been reported. Horst et al. [8] have reported a novel
modification that uses high-intensity ultrasound from a concentrator horn in combination
with the conventional stirred reactor with a flow loop (Figure 7.1). It has been shown that
the concept of a conical funnel fits the demands for nearly perfect radiation effectiveness
and good reaction management. The approach of using a combination of sonochemical
reactors with the conventional reactor might fit well in terms of easy acceptability in
industrial operations. Another design, the Telsonic horn, used by Dahlem et al. [9], uses
(1) reactor (0.5 dm
3
) with sonotrode
(2) stirred tank (2.5 dm
3
)
(3) flow loop pump
(4) closing pump
(5) cryostal
(6) gas flow meter
T thermo couple
P manometer
gas exit
P
20 kHz
T
M
(2)
(5)
(1)
gas feed
(0 - 1 dm
3
/h)
P
(6)
(3)
(4)
liquid feed
(0 - 0.6 dm
3
/h, 2.1 mol/dm
3
R-CI)
liquid product (2.0 mol/dm
3
Grignard)
Figure 7.1 Flow chart for the integration of conical configuration sonochemical reactors with
conventional reactors. Reprinted from [ref 8]
#
1996, with permission from Elsevier.
Search WWH ::
Custom Search