Multi-objective optimization of simultaneous oxidative desulfurization and denitrogenation of model fuel

Petroleum fractions are accompanied with sulfur and nitrogen compounds, and nitrogen compounds tend to compete for the catalyst active sites, reducing desulfurization efficiency. They can be removed by oxidative desulfurization and denitrogenation, wherein the sulfur and nitrogen compounds are converted into corresponding oxides with higher polarity than their parent forms, thus favoring their extractive or adsorptive removal from the oil phase. The introduction of ultrasound into the oxidation system can significantly improve the process efficiency and oxidation kinetics through strong cavitation phenomena involving physical and chemical effects. In this work, ultrasound-assisted simultaneous oxidation of pyridine and dibenzothiophene (DBT) contained in a model fuel was performed. Response surface methodology (RSM) was employed to study the effect of six parameters, viz., catalyst dose (5–12.5 g/L), reaction time (5–60 min), ultrasonic amplitude (10–60 %), volume of hydrogen peroxide (0.5–2 mL), volume of acetic acid (0.5–2 mL) and reaction temperature (40–60 °C) on the conversion of DBT and pyridine. A face-centered central composite design was employed as the RSM tool, and the effect of the parameters and their interactions on the two responses was investigated. The two responses were simultaneously optimized via desirability functions and overlaying individual contour plots. The results revealed that sonication significantly enhanced the conversion of both compounds. All six factors significantly influenced the oxidation of pyridine, while DBT conversion was mainly affected by four factors. The desirability for the process under consideration ranged from 0.85 to 0.89, and at the optimum conditions, 100 % conversion of pyridine and 46.06 % conversion of DBT were obtained. Two-staged oxidation of the fuel at optimum conditions resulted in 100 and 93.7 % conversion of pyridine and DBT, respectively.