FFA conversion to methyl esters is faster with ultrasound (US)
compared to conventional mechanical stirring at all of the experimental conditions tested (oil, acidity, temperature). At 63C, FFA
conversion in canola oil exceeds 90% (Fig. 3) after 6 h. It only
approaches about 80% in tobacco seed oil after the same amount
of time. Oleic acid conversion reaches about 70%. US accelerates
the reaction rates considerably more at lower temperatures (Figs. 4
and 5) and the benefits are also more evident at shorter reaction
times: at 20C the FFA conversion is close to that measured for
the test run at 40C. FFA conversion was about 45% for all three
systems with US after one hour. It took two hours to reach 45%
conversion for the oleic acid–methanol and the tobacco seed oil
systems and 90 min for the canola oil system.
US accelerates the FFA esterification by increasing the mass
transfer due to acoustic cavitation. FFA esterification can be
regarded as a ‘‘dual-heterogeneous system’’ consisting of an
oil-methanol liquid–liquid system and an oil-methanol-catalyst
liquid–solid system. US has been reported to be beneficial to both
kinds of heterogeneous systems[27]. Concerning the liquid–liquid
methanol-oil immiscible system, US forms a very fine emulsion of
two immiscible liquids that are more stable than those generated
by conventional stirring[30]. As a consequence, the surface area
available for the reaction between the two phases is significantly
increased and, therefore, also the reaction rate is expected to
increase [29]