Ultrasound increases the conversion rate of free fatty acids
(FFA) with respect to conventional mechanical stirring. In particular, de-acidification rates are much higher for oleic acid and tobacco seed oil and the accelerated rate may be due to the low mass
transfer in these fluids that have a higher viscosity. The lower
the temperature, the higher is the difference between the reaction
rate between US and mechanical stirring: the conversion of FFA in
the tobacco seed oil at 20C is three times higher than the conventional approach at the same temperature. FFA esterification can be
characterized as a doubly-heterogeneous system with two phase
boundaries: the catalyst-liquid boundary and the one between
the methanol that surrounds the catalyst particle and the oil.
Acoustic cavitation acts to either shrink the boundary layer or to
allow the bulk reactants to penetrate to the reacting surface more
quickly: mass transfer coefficients are several fold higher with US
compared to mechanical stirring