PTQ Q4 2026 Issue

Overcoming refractory nitrogen challenges in ULSD hydrotreating

Catalyst design approach for diesel feedstocks containing carbazoles and other refractory nitrogen compounds that inhibit deep sulphur removal

Xavier E. Ruiz Maldonado Topsoe North America

U ltra-low sulphur diesel (ULSD) production has become increasingly demanding as refiners are required to meet very low product sulphur specifications while processing feedstocks with increasing compositional com- plexity. In many ULSD units, sulphur removal is no longer constrained solely by the intrinsic reactivity of sulphur spe- cies. Refractory nitrogen compounds, aromatics, and cracked components increasingly influence catalyst activity, volume swell, deactivation rate, cycle length, and ultimately the eco- nomics of ULSD production. Among the most troublesome molecules in diesel hydro- processing are carbazoles and alkyl-substituted carbazoles. These nitrogen-containing polyaromatics are particularly challenging because of their strong inhibiting effect on hydrodesulphurisation (HDS) activity. Their impact becomes especially pronounced in units processing straight-run die- sel blended with light cycle oil (LCO) or heavy cycle oil (HCO) from the fluid catalytic cracking unit (FCC) or other cracked streams (thermal coker effluents), and in units operating at limited hydrogen partial pressure. Under these conditions, the catalyst must simultaneously provide high HDS and hydrodenitrogenation (HDN) activity to efficiently convert the difficult refractory sulphur molecules that ultimately determine ULSD sulphur compliance. As product sulphur approaches ultra-low levels, the remaining sulphur compounds become increasingly difficult to remove through the direct desulphurisation route alone. Their complex ring structures hinder accessibility to active sites, making hydrogenation of aromatic and nitrogen-con- taining species increasingly important for continued sulphur removal. In other words, the lower the sulphur target, the more critical it becomes for the catalyst to maintain effective hydrogenation functionality. Refiners face constant pressure to enhance profitability by increasing the final boiling point, incorporating more cracked feedstocks, and processing more opportunity crudes. While these strategies can improve margins, they also create a much more severe catalyst processing environment. From a catalyst standpoint, the challenge is to maintain sufficient activity over a sustained cycle length that justifies operating cost, including catalyst cost, while still meeting the current diesel fuel specifications. Catalysts optimised primarily for sulphur removal can lose

effectiveness when feeds contain increasingly refractory nitrogen compounds, while catalysts with stronger nitro- gen-handling capability do not always deliver the same sulphur removal efficiency throughout the run. This combi - nation of demands in the ULSD market requires a catalyst that simultaneously delivers HDS and HDN performance. For example, the proprietary TK-594 HyBRIM catalyst for ULSD service is built around highly dispersed and homo- geneous nanoscale trimetallic Cobalt-Nickel-Molybdenum (CoNiMo) sulphide structures, designed to combine key strengths traditionally associated with both Cobalt- Molybdenum (CoMo) and Nickel-Molybdenum (NiMo) systems. This provides a catalyst architecture capable of maintaining strong sulphur removal performance while pre- serving HDN activity and reactor effectiveness, even when the feed contains high amounts of challenging nitrogen species. The result is not simply higher overall activity, but a more robust catalytic response across the full operating window of ULSD hydrotreating. Why carbazoles matter Producing ULSD on specification may appear straightfor - ward when viewed only in terms of bulk sulphur removal. However, in practice ULSD hydrotreaters must process feeds containing sulphur species with a wide range of reac- tivities, often together with refractory nitrogen compounds and aromatics that compete for accessibility to active sites and available hydrogen. The easiest sulphur compounds are typically removed in the upper portion of the reactor bed. As sulphur concen- tration declines, the remaining sulphur species become progressively more refractory as shown in Figure 1 . At this stage, the catalyst is no longer converting reactive average sulphur molecules, but the most refractory fraction in an increasingly competitive adsorption environment. The chal- lenge becomes even more severe when nitrogen compounds are present, since they adsorb strongly on active sites and suppress the pathways required for deep sulphur removal. For many ULSD units, the true bottleneck may therefore not be the average HDS activity, but the ability to consistently remove the last concentration of the most refractory sulphur compounds under nitrogen-inhibited conditions. This is why catalysts that perform well in relatively clean straight-run

PTQ Q4 2026