(81c) Modeling and Simulation of Diesel Hydrotreaters for ULSD Production: Selection of an Optimal Catalyst Composition

The required specifications for ultra-low sulfur diesel (ULSD) have driven refiners towards high performance hydrotreating catalyst that can efficiently remove difficult sulfur down to 15 ppm or lower in the final product. To select the best catalyst composition for the design of grassroots facilities or the revamp of existing ones, engineers need to account for the type of feed and the optimal operating conditions to reach hydrodesulfurization (HDS) and cetane number improvement targets.

This work focuses on the development of a flexible model for commercial ULSD hydrotreating units that predicts product properties, yields and hydrogen consumption for different catalyst compositions. The model uses a species set that contains the necessary structural information required to characterize different types of sulfur species as well as to distinguish among mono-/poly-aromatics, naphthenes and naphtheno-aromatics to model relevant reactions. The proposed reaction set accounts for desulfurization pathways by hydrogenolysis and hydrogenation, saturation of poly-aromatic, mono-aromatic and olefin species, cracking, and inhibition effects by nitrogen impurities in the feed.

Experimental data from the literature for CoMo and NiMo catalysts at different levels of desulfurization and aromatic saturation with temperature, pressure, and LHSV were used to determine two sets of kinetic parameters that represent each type of catalyst. Case studies were developed to assess the performance of each individual catalyst and combinations of catalysts in commercial ULSD hydrotreaters under different scenarios covering HDS and cetane improvement with limited hydrogen consumption, increasing amounts of light cycle oil in a straight-run gas oil feedstock blend, and low to high pressure operation from start of run to end of run conditions. For each scenario, an optimal catalyst composition was found, constrained to a final product sulfur specification of 15 ppm.