Experimental Investigation on the Supercritical Rapeseed Methanolysis for Biofuel Production: Effects of the Operating Conditions on the Bio-oil Viscosity

The aim of this work is to optimize the supercritical liquefaction process of rapeseeds in order to produce a bio-oil with low viscosity. Reaction parameters, such as reaction temperature, residence time, and solvent to biomass ratio, were studied. Response surface methodology (RSM) based on central composite design (CCD) was used to determine the optimum operating conditions to minimize the bio-oil viscosity. The low bio-oil viscosity of 5.90 mPa.s was obtained at the optimal operating conditions of 280°C, 40 min, and methanol/biomass mass ratio of 5.5/1, at pressure within the batch reactor of 124.59 bars. At these optimal conditions, the bio-oil yield was high and reached around 80wt%, while its heating value was about 38.36MJ/kg. It was proved that the reaction temperature and methanol/biomass ratio were the most influencing parameters on bio-oil viscosity according to the ANOVA results. The predicted values from the RSM model was in good agreement with the experimental results. The GC-MS analysis showed that the bio-oil is mainly composed of methyl esters, which are the main components of biodiesel. This study revealed the complete supercritical transesterification of lipid into alkyl esters resulting in a low amount of triglycerides, monoglycerides, diglycerides, and glycerin, identified by GC-FID. The results will provide useful guidance for predicting other physical properties of bio-oil following a similar methodology to that used in this work. In addition, bio-oil could be used for biodiesel fuel production but after hydrodeoxygenation treatment.