As outlined above, there is considerable scientific evidence demonstrating
the suitability of ultrasonic system to enhance extraction
yields in short extraction times using clean or green solvents. This
implies that, while the technology could exhibit great promise, it
will have to be carefully developed and scaled up for large-scale commercial
applications. In general, the terms ultrasound power,
ultrasonic intensity or acoustic energy density are commonly used
as the primary design parameter, as they account for the energy entering
the extraction system.
There are two main types of transducer commonly used for ultrasonic
applications: piezoelectric and magnetostrictive. Various
types of transducers has been discussed by other authors but are
not considered here in detail [35,36]. Placement of the transducer
plays a vital role in determining extraction yields. It is important
for extraction efficiency, process intensification and energy losses.
Transducers can be placed on either side of the extraction vessel,
so ultrasonic waves will be transmitted through the outer wall of
the extraction vessel. Both bath and probe-type ultrasonic equipment
are commercially available for extraction purposes operating
at various frequencies. The major advantage of this arrangement is
that transducers are not in direct contact with the sample, but significant
losses of acoustic energy occur to vessel and surroundings
(Fig. 2a, b). Multiple transducers can be used in a flow-through
system for large-scale extraction and possibly to develop continuous
extraction system (Fig. 2d). When transducers are in direct
contact with the sample in the presence of a suitable solvent, extraction
efficiency is enhanced, whilst minimizing acoustic energy
losses.