A Comparative Study of Homogeneous Base Catalysts and Process Conditions for High Biodiesel Conversions from High Free Fatty Acid Rapeseed Oil Using a Mesoscale Oscillatory Baffled Reactor

Homogenous base catalysis of biodiesel production from feedstock that contains high free fatty acids (FFA) content requires a pre-treatment step to reduce the FFA level. Furthermore, drying is required when the water content is above 0.05 wt%. These extra steps add to the costs of biodiesel production from high FFA and water feedstocks. Identification of conditions that could allow for direct base-catalysed methanolysis of such low quality feedstock would be advantageous. This study investigated the comparative catalytic activities of alkali-metal hydroxides and methoxides as homogeneous base catalysts for transesterification of rapeseed oil containing high levels of FFA and water. The investigation was carried out using a mesoscale oscillatory baffled reactor in a continuous mode, to allow rapid screening of conditions. The oscillatory mixing conditions in the reactor ensured that the reactions were kinetically controlled. The operating conditions investigated were: methanol to rapeseed oil molar ratios (4:1-13:1), catalyst loadings (0.5-2 wt %), FFA (0.061wt %) and water 0.05wt% (contents of the rapeseed oil, reaction temperature (40-60 ℃), and residence times (3-16min). Design of experiments (DoE) was used to optimise the reaction conditions. Complete conversions (99%) were achieved over a wide range of reaction conditions. The NaOCH₃ catalyst exhibited a considerably higher turnover frequency than the NaOHat all the reaction conditions. These findings demonstrate that there are process conditions for continuous production of biodiesel that lead to high conversions, even from low quality oil feedstocks, containing up to 5wt% FFA and 3wt% water.