Enhancing Ultradeep Hydrodesulfurization of Coker Diesel through Adsorptive Predenitrogenation

The petroleum refining industry places significant challenge in the production of ultralow-sulfur diesel (ULSD) from various middle distillates with high nitrogen concentration in an energy-efficient and cost-effective way to meet strict environmental regulations as coexisting nitrogen compounds significantly inhibit the ultradeep hydrodesulfurization (HDS). Among all of the approaches reported in the literature for this challenge, a combination of adsorptive denitrogenation (ADN) and HDS has attracted great attention. This study focuses on ultradeep HDS of coker diesel (CD) through a synergistic approach combining ADN over a carbon-based adsorbent and the current HDS process. The study found that the predenitrogenation significantly enhanced the HDS reactivity of CD and greatly reduced the start of run temperature for producing ULSD by even 20 °C, depending on the denitrogenation depth. It results in dominantly decreasing energy and H₂ consumption in the HDS process, and increasing the lifetime of the catalyst. Furthermore, the predenitrogenation prior to HDS significantly improved the quality of the hydrodesulfurized product by decreasing aromatic content and density, increasing the cetane index, and improving the color and stability of the product as the HDS process for producing ULSD can be conducted at a much lower temperature. The study demonstrated the combination of ADN and HDS for producing ULSD from CD on a pilot unit scale, and the advantages and disadvantages of this combination were discussed in comparison with the conventional HDS process. In addition, it was found that there is a good linear relationship between the logarithm of the product sulfur concentration and the HDS reaction temperature, which can be used conveniently to predict the start-of-run temperature to achieve different sulfur levels.