The microbubbles, hence, grow over a few cycles to reach an unstable size and then collapse in the successive compression cycles
[10,11]. This collapse can increase the interfacial area of the reactants and enhance mass transfer[12]. Because ultrasound irradiation influences the pressure variation in the liquid medium, it
induces the generation of a huge amount of microbubbles to form
an emulsion phase. When these microbubbles collapse, the interfacial area of reactants boundary is largely increased[13]. The US
reactors have been further designed for improvement[14,15]and
their applications are widely studied [16]. Table 1summarizes
some research studies into biodiesel production using US batch
reactors. Many types of feedstock have been investigated and their
optimal operating conditions are also summarized in this table. It
can be seen that a high methyl ester yield can be achieved at a lower reaction temperature[17–21], alcohol to oil molar ratio [22]and
reaction time than those achieved by conventional biodiesel production[23,24]. The lower reaction temperature and reaction time
give further advantages in terms of energy required, while the lower oil to methanol molar ratio creates benefits in terms of the cost
for product purification and product separation