Effect of impeller rotational speed on biofuel characteristics in non-oxygenated bio-oil hydrogenation reaction using gas self-inducing impeller

The non-oxygenated fraction of bio-oil obtained from co-pyrolysis of biomass-polypropylene plastic contains alkenes which may cause high ignition delay time and instability when compared to commercial diesel fuel. A hydrogenation reaction is needed to upgrade the characteristics of bio-oil to be close to those of commercial diesel fuel. The present work is aimed to investigate the effect of stirring rotational speed of the impeller in a hydrogenation reactor on the characteristics of biofuels in terms of vinyl content, branching index, viscosity, and heating value. The speed of the stirrer was varied 200 to 500 rpm with an increment of 100 rpm. Within the reactor, hydrogen gas was recirculated from the gas phase above the liquid phase through a line of the hollow impeller shaft into the liquid phase by utilizing pressure difference between the gas phase and liquid phase in the wake of impeller blades. The chemical analysis related to the characteristic were FTIR, GC-MS, and H-NMR spectroscopies. A catalyst of Ni/Al2O3 was used to form hydrogen radicals from hydrogen gas. It was found that increasing the stirring rotational speed from 200 to 400 rpm reduces vinyl content in the bio-oil, and on contrary, increasing further to 500 rpm increases the vinyl content expectedly due to the formation of vortex around the impeller shaft which induces the gas into the impeller. In terms of HHV and viscosity values, the biofuel produced by the hydrogenation at a rotational speed of 400 rpm presents similar values to those of commercial diesel. Meanwhile, biofuel produced by the hydrogenation at a rotational speed of 200 rpm has a branching index closest to that of commercial diesel.