The feedstock (i. refined and bleached palm oil and, ii. WCO)
was poured into the first tank (1) and heated over the reaction
temperature, at 60C; then 75% by volume of methoxide solution
was added to the oil and premixed for 30 s, to obtain a homogeneous mixture, at a reaction temperature of 55C. The sodium
methoxide solution was prepared by mixing NaOH with methanol
under stirring at room temperature for several hours, until the
entire solid phase was dissolved. This mixture was sucked by the
rotating generator of HC (2), at a flow rate of 390 L/h, and subjected
to controlled cavitation and discharged into the stirred collector
tank (3). The rotor-stator type HC reactor is a commercially available unit designed by E-PIC S.r.l. (Turin, Italy). It can achieve controlled HC by forcing fluids through its rotating cylinder, which is
equipped with a 7.5 kW electric engine and absorbed power is constantly monitored. During high-speed rotation, rotor channels are
periodically aligned with stator channels. The processed liquid is
accelerated in the radial direction in the cavitation chamber and
is subjected to a pressure wave, when flowing through the free
channels, which results in cavitation. Shockwaves are produced
when the cavitation bubbles collapse and the reagents are mixed
on a micron scale, increasing the surface contact area between
the methanolic methoxide solution and the oil phase. The maximized mass transfer and alcohol vapour generated in the hot spots
strongly accelerate the transesterification reaction. The mixture is
maintained at the same temperature for 15 min under moderate
stirring in the discharge tank (3). Glycerol is then finally allowed
to separate from the crude biodiesel and unreacted oil, via the
action of gravity, in the same tank in 30 min. After glycerol separation, crude biodiesel and the mixture of mono- and diglycerides are
transferred to the premixing tank (1) and stirred for 30 s with the
remaining 25% by volume of methoxide solution, at the reaction
temp. of 55C. The mixture is again sucked by the HC reactor, at
the same flow rate of 390 L/h, and discharged into the second tank
(3), where it is maintained at the same temperature for an additional 15 min under moderate stirring. After 30 min, two clean layers of glycerol and biodiesel are separated in the tank and collected
from the bottom