The schematic diagram of the MS–US flow reactor used in this experiment is shown in Fig. 1. It can be divided into three main parts, the reactant tank, the generator and the reactor, as can be seen in Fig. 2. A 6 L cylindrical reactant tank was used to preheat the raw materials, oil and methanol, before feeding them into the reactor (Fig. 2a). The preheat temperature and the feed flow rate were fixed at about 45 °C and 55 mL/min (corresponding to residence time of 4 min), respectively, and correspond to the best conditions reported by Cintas et al. [27]. Stirrer speed in the preheat tank was about 160 rpm. The NaOH catalyst was fed into the tank after the reactant temperature (palm oil and methanol) was constant. The experiment was carried out by circulating all products and reactants from the US flow reactor outlet into the reactant tank. Samples were taken at the reactor outlet. The generator unit (Fig. 2b) was used for transducer power generation and for controlling the motor speed of the horizontal stirrer inside the reactor. The rectangular shape (2.5 × 2.5 × 30 cm) stainless steel (316) reactor is shown in Fig. 2c. There were two different transducer frequencies used in the experiments (20 and 50 kHz) and the location of transducers was fixed along the length of the reactor, as shown in Fig. 1. The frequency of the transducers anchored to the reactor wall on opposite sides was the same. The generator provided a power of 50 W per transducer, hence, the total power of this reactor was 800 W when operating with all 16 transducers. This reactor can be operated in 3 modes; the mechanical stirred mode using the horizontal stirring rod inside the reactor with the maximum speed of 25 rpm (MS reactor), ultrasound irradiation (US reactor) and the integration of mechanical stirring and ultrasound irradiation (MS–US reactor).