Kinetic study for hydrodesulfurization processes of diesel fuels over CoMo-based catalyst

Diesel is an important fuel for transportation vehicles and is to be produced in compliance with Euro V specifications. The sulfur content diesel fuels is limited to be maximum of 10 ppm and deep desulfurization process is required to reduce the sulfur content of the diesel feed. Refineries process various kinds of crude oils containing high amounts of nitrogen, aromatics, metals, and sulfur compounds, with complex structures. In addition, the quality of the crude oil is also being declining; the refineries operate at the commercial units including hydrodesulfurization (HDS) processes at more severe conditions. Several changes are also implemented by the refineries such as using more active catalysts, increasing hydrogen consumption, operating temperature and pressure, improving the reactors and understanding reactor and reaction modeling. However, severe operating conditions cause rapid catalyst deactivation and decrease the catalyst life. To optimize the operating conditions and maximize the catalyst performance, HDS kinetic models are important for reviewing the catalyst activity and comparing the activities of the catalysts in the case of selecting and evaluating new catalysts i.e., catalyst screening. Therefore, it is necessary to understand the nature of HDS process for diesel feeds with respect to different operating conditions. Apparent HDS kinetics applied for industrial real feeds depend significantly on feed properties having variety of different sulfur compounds, nature of the hydrocarbon matrix, operating conditions and type of catalysts. Therefore, kinetic parameters determined for a study is specific to the related process. The purpose of this study is to investigate apparent kinetic model for diesel HDS reaction using commercial CoMo/Al2O3 catalyst based on sulfur distribution data. In the first part of the work, different characterization methods are applied to commercial CoMo/Al2O3 catalyst. In the second part of the work, HDS performance tests were conducted at different operating conditions such as temperature, pressure and H2/oil ratio and LHSV. A pilot plant is designed, constructed and started-up during the course of the work to carry out the performance tests with continuous, safe, 24/7 unattended operations. The diesel feed and the obtained diesel products were analyzed with extensive methods to investigate the changes in the product properties and catalyst activity. In the third part of the work, apparent HDS kinetic models are derived with power law model based on total sulfur of the feed and the products as a reference point to compare with literature. In the fourth part of the work, apparent HDS kinetic model was investigated using the sulfur distribution of the feed and the liquid products by dividing into several sub-fractions using the sulfur distribution data from CNS-Simdis analysis. Then, power law model based on the sulfur contents of the related sub-fractions was applied to determine the apparent reaction rate order and activation energy of each sub-fraction. In the fifth part of the work, the feed and the products were separated physically into several fractions with distillation and the properties of the fractions were analyzed. Power law model was applied to each fraction to compare with the results obtained in the third part of the work.