Effect of stirring speed on characteristics of biofuel in catalytic hydrogenation of non-oxygenated bio-oil

Co-pyrolysis of corncobs and polypropylene plastic with equal feeds at low heating rate has succeeded in producing non-oxygenated fraction of bio-oil by spontaneous separation from its oxygenated fraction with the former fraction being higher than the latter. This fraction has higher heating value of 47 MJ/kg and kinematic viscosity of 2.1 cStokes measured at 40°C, which are similar to those of commercial diesel fuel. However, vinyl content of the bio-oil is still 5.4% in comparison of 0% in the diesel fuel, which is unfavourable for its combustion in diesel machine due to its propensity to smoke formation and reduced cetane number. Hydrogenation of the non-oxygenated bio-oil, therefore, will result in biofuel. The present work aims of reducing the vinyl content by catalytic hydrogenation of the non-oxygenated bio-oil conducted in a 300 mL stirred tank reactor using Ni/γ-Al2O3 catalyst in the bio-oil. The reactor was equipped with an impeller of the downflow pitched-blade turbine type. The reactor pressure was set at 8 bar and stirrer speed was varied 350, 650 and 800 rpm. High shear in liquid phase induced by stirrer speed affects hydrogen dispersion and solubility in bio-oil. The present work investigated the effects of the speed on characteristics of bio-oil by FTIR, H-NMR and LC-MS spectroscopies and Saybolt viscometer featuring carbon chains, vinyl content, branching index, molecular weight distribution and viscosity and compared with those of the commercial diesel fuel to verify their similarities. Results show that among different stirrer speeds in hydrogenation reaction, vinyl content in biofuels disappeared as the reaction was operated at 650 and 850 rpm. Among bio-oil and biofuels, there were much different in terms of branching indices and HHVs. However, branching indices of bio-oil and biofuels were still higher, approximately four folds of that of diesel fuel. Among all samples (bio-oil, biofuels and diesel fuel), there were no significant difference in terms of HHV values. In order for biofuel to approach molecular distribution of diesel fuel, hydrogenation should have been operated at stirrer speed in between 650 and 800 rpm.