Hydrotreatment of high oleic sunflower oil: Optimization of diesel fraction yield

Abstract

The optimization of the hydrotreatment of high oleic sunflower oil towards green diesel fraction yield (C15-C20) under Pd/Al2O3 catalyst was investigated. The catalytic hydrotreatment was carried out in a 250mL batch reactor (Parr 4570 HP/HT) for 4 hours, performing regular purges of gas from reactor head space. Optimization was based on three operative parameters: catalyst loading (0.5-1.25%), reaction temperature (325-375°C) and pressure of H2 (60-120bar), using a central composite design and a response surface methodology. When reactor was operated at 350ºC, 100 H2 bar and using 1% catalyst, a 98% conversion to hydrocarbons was achieved and 84% of these hydrocarbons matched the chain length corresponding to the diesel fuel range. Under identical temperature and H2 pressure, but decreasing the catalyst concentration to 0.5%, the hydrocarbon yield decreased to 84%, while the diesel fraction raised to 97%. Under 1% catalyst loading but operating at a lower temperature and H2 pressure (300°C and 60bar, respectively), the hydrocarbon yield decreased significantly to 50%, while all the hydrocarbons had chain lengths inside the diesel fuel range. Results demonstrated that high temperature and pressure favored the complete conversion to hydrocarbons, but also cracking was promoted with negative effect on the percentage of the diesel fraction. Conversely, when reactor was operated at milder reaction conditions, the conversion to hydrocarbons diminished, but the proportion corresponding to a diesel fuel increased. Optimization permitted to define the more convenient conditions in order to achieve both high conversions and the highest diesel fractions content.