Rapid and efficient esterification of oleic acid by continuous liquid-phase plasma discharge

Conventional esterification of fats/waste oils with high free fatty acids is time-consuming and energy intensive. In this study, a novel liquid-phase plasma discharge (LPPD) reactor was designed and evaluated for rapid esterification to convert oleic acid to biodiesel. Three LPPD operating parameters, i.e., feeding flowrate, methanol to oleic acid molar ratio (M/O), and sulfuric acid to oleic acid weight ratio (A/O), were all evaluated at five levels (1.2, 1.8, 2.4, 3.0, and 3.6 mL s(-1) for flowrate; 4, 6, 8, 10, and 12 for M/O; and 0, 1, 2, 3, and 4 % w/w for A/O) using a fractional factorial experiment design. For each experiment, three treatment times (2, 6, and 10 min) were also tested. The optimum operating parameters for the LPPD were 2.4 mL s(-1) for flowrate, 10 for M/O, and 2.0 % w/w for A/O, respectively, with the maximum conversion rate of 78 %. The power and applied voltage were consumed at 340 W and 2.12 kV during the 2-min treatment. The methyl oleate samples produced in this LPPD process were identified by GC-MS (gas-chromatography mass spectrometry) and FTIR (Fourier transform infrared spectroscopy) to confirm the esterification pathway.