(316b) Supercritical Production of Biodiesel Using Methanol and Ethanol in Batch and Continuous Reactors

Abstract: Recent experimental studies on non-catalytic transesterification of vegetable oils have shown that high reaction rates and conversions, which justify the commercial application of this process, can be obtained if the operating conditions are above the critical pressure and temperature of the alcohol. Even though it is generally agreed that temperatures above 550 K and high alcohol/oil ratios are required to obtain high conversions and high reaction rates, there is disagreement among the different authors regarding the operating pressure, the assumed phase conditions, the effect of the use of co-solvents and the reason for the sudden increase of the rate of reaction with temperature. In the present work, the reactor phase transitions are directly observed in a double windowed cylindrical reactor and the conversion to ethyl or methyl esters is measured.

The evolution of the phase behavior with temperature, during the non-catalytic transesterification reaction, with and without propane as a co-solvent, was observed in a see-through, two window cylindrical reactor, at controlled reactor global densities and initial methanol / oil molar ratios. The reactor global density (ñg= defined as the total mass charged into the reactor divided by the reactor volume) is a key factor from a phase equilibrium engineering point of view. A high load global density (ñg > 0.7 g/cm3) produced a steep increase in pressure when the liquid phase completely filled the volume of the reactor. At smaller global densities (ñg < 0.5 g/cm3) the liquid-vapor (LV) equilibrium conditions prevailed up to 563K.

Different oils have been used in the batch transesterification studies: Raphanus sativus L. oil (Forage turnip oil) extracted by cold press, commercial soybean oil, and crude sunflower oil. The optimization of the process conditions was carried out based on a statistical design of experiments where the key process variables were studied over different ranges to obtain a reliable model for the efficiency of the reaction as a function of reactants residence time, temperature, pressure and molar ratio of alcohol/oil for the different oils and alcohols studied. The present results confirm preliminary studies that indicate that high conversions can be obtained at pressures of 10-15 Mpa at temperatures between 570 and 600 K using a molar ratio of 39. From direct observations and the modeling of the phase behavior, a better understanding of the supercritical alcohol transesterification process is obtained as well as the confirmation of the phase equilibrium predictions based on the GCA-EOS model. The optimum conditions obtained by statistical analysis in the batch reactors has been studied in a bench scale continuous reactor to obtain more reliable information between conversion and reactants residence time.

*Corresponding author, email: ebrignole@plapiqui.edu.ar