Continuous Production of Biodiesel from Soybean Oil Using Supercritical Methanol in a Vertical Tubular Reactor: I. Phase Holdup and Distribution of Intermediate Product along the Axial Direction

Production of biodiesel with supercritical methanol is a green synthesis process. A study was carried out in a vertical tubular reactor with a length of 3700 mm and a diameter of 20 mm at 275-375 °C, 15 MPa, and molar ratio of methanol to soybean oil of 40?1. The phase holdup, intermediate product, yield and axial distribution of methyl ester (ME) were investigated. Methanol and oil were mixed non-uniformly due to the formation of biodiesel and difference in their densities, even when the reaction system was in the supercritical state. From top to bottom, the phase holdup of methanol increased and that of oil decreased. As temperature increased, the concentrations of monoglyceride and diglyceride decreased gradually and the ME yield increased. When the temperature reached 300 °C, the critical temperature of the system, the ME yield was 50%. Further increase in temperature led to a sharp increase of ME yield. However, at 375 °C after 1200 s of reaction time, the decomposition rate of ME was greater than its formation rate, reducing the ME yield.