PTQ Q4 2025 Issue

Advancing catalytic performance in hydrotreating: Part 2

Review of chemistry and catalysis determining performance in high-pressure distillate hydrotreating, HC-PT, and renewables distillate co-processing applications

Andrea Battiston Ketjen

P art 2 of ‘Advancing catalytic performance in hydro- treating’ continues from Part 1, which was published in PTQ Catalysis 2025 . 1 Part 1 examined key reactions and factors limiting performance in low- and medium-pres- sure distillate hydrotreating and FCC-PT applications when processing conventional (fossil) feedstock, and presented new catalytic solutions to improve operating flexibility and maximise profitability. Part 2 follows the same structure and approach, shift- ing the focus to high-pressure distillate and hydrocracking pretreat (HC-PT) operations when treating conventional feedstock, as well as distillate applications where renewa- ble feedstocks are co-processed. High-pressure distillate hydrotreating For a thorough analysis of the kinetic and thermodynamic factors at play in hydrotreating, including the role of the dif- ferent reaction pathways, the reader is referred to Part 1 of this article, particularly the section ‘Hydrotreaters perfor- mance and operating regimes’. The first consideration regarding high-pressure distillate hydrotreating operations is that, thanks to the higher par- tial hydrogen pressure (PPH2), typically above 50 bar / 725 psig, operating conditions limited by thermodynamics for the hydrogenation (HYD) reactions are rarely encountered, and only late in the operating cycle. At the same time, the tendency of poly-nuclear aromatics (PNAs) to condense on the catalyst surface and form coke is reduced, bringing a lesser burden to catalyst stability. This allows for the selec- tion and application of catalysts with higher HYD activity in all parts of the reactor. A high-pressure distillate hydrotreater can be described as consisting of three main operating zones under the guard section, as depicted in Figure 1 . Zone 1, located at the reactor top, is the section of the reactor under the guard bed where catalyst activity is typ- ically limited by the inhibitory effect of refractory nitrogen and PNAs. The effect is particularly strong when process- ing heavy or cracked feedstock and on highly hydrogenat- ing catalysts, like NiMo catalysts, but is mitigated by high PPH2. The latter allows for NiMo catalysts to be applied also in Zone 1. CoMo catalysts, which are less hydrogenat- ing and thus less prone to coke formation, are adopted in

Guard Removes particulates & poisons Starts saturating olens Zone 1 Saturates olens & removes easy S (DDS) Starts converting N & PNAs to Di-/Mono-Aromatics (HYD) Zone 2 Removes hard S (HYD) Converts N & PNAs to Di-/ Mono-Aromatics (HYD) Zone 3 (Nitrogen-free zone) Boosts removal of residual hard S (HYD) Saturates residual PNAs including part of Mono-Aromatics (HYD)

Log S, N

Guard

Zone 1

Zone 2

S < 10 ppm

Zone 3

Zone 1 only in exceptional cases when the feedstock is par- ticularly rich in refractory nitrogen (total N >500-600 ppm). Zone 2 is the part of the reactor where the inhibition by nitrogen and PNAs is sufficiently low to allow the con - version of refractory sulphur. In Zone 2, PPH2 remains relatively high, favouring HYD reactions. In high-pressure hydrotreaters, starting from Zone 2, NiMo catalysts with higher HYD activity than those applied in Zone 1 are used. These catalysts enable a faster removal of residual nitrogen and refractory sulphur, and more aromatics saturation. The same NiMo catalysts applied in Zone 2 can also be applied in Zone 3, which is defined as the bottom part of the reactor where nitrogen species have been completely removed, and where kinetics is fully controlled by the catalyst’s HYD activity. Zone 3 is regularly encountered in high-pressure distillate hydrotreaters. The absence of nitrogen species allows for an acceleration in the removal of hard sulphur to meet the ultra-low sulphur diesel (ULSD) target, as well as for a deeper saturation of residual aromat- ics (HDA), boosting volume swell. The presence of Zone 3 Figure 1 Reactor zones and function of HYD activity in high-pressure distillate hydrotreaters (PPH2 > 50 bar / 725 psig)

PTQ Q4 2025