4.2. Influence of ultrasound on supercritical fluid extraction
The application of ultrasound during supercritical fluid extraction
affects both the kinetics and the yield. Balachandran et al.
(2006) studied the application of ultrasound in the supercritical
extraction of pungent compounds from ginger using a commercial
probe system externally attached to the extractor. The nominal
power used was 300W but, due to the ultrasonic transducer being
outside the extractor, the reflection and adsorption of the acoustic
wave decreased the actual intensity received by the samples. However,
they found an important increase in yield when ultrasound
was applied. The particle size could be an important factor to take
into account, in fact, the effects of ultrasound increased when the
particles were smaller. Thus, for a particle size of 4 mm, the yield
was 30% higher in experiments carried out with ultrasound application
compared to non-ultrasonic experiments. Similar behavior
was found by Riera et al. (2004) working on almond oil extraction.
They used a system where the ultrasonic transducer was inside the
reactor and found that ultrasound had a greater influence on the
yield at the lowest particle size tested (3–4 mm compared to 9–
10 mm), achieving an increase of 20% in oil recovery. It is likely
that the highest surface area to volume ratio favors the action of
ultrasound, pointing to an influence of ultrasound on the external
resistance. From microscopic pictures (field emission scanning
electron microscopy, FESEM), Balachandran et al. (2006) observed
that the structure of the material treated with ultrasound showed
cellular damage that could favor the removal of the cell contents.
This fact could indicate that the effect of ultrasound could also
be located in the internal resistance to mass transfer.