Catalysis 2025 Issue

route for HDS at low pressure or when inhibition by nitrogen and PNAs is severe. HYD, HDN, and HDA become increas- ingly favoured at higher pressure. The green region is the safest from an operational perspective, leading to the highest volume swell and hydrogen consumption. Also depending on nitrogen and PNAs inhibition, the green region allows applying both DDS- and HYD-selective catalysts, with HYD catalysts being more effective and thus preferred at higher pressure and when hydrogen consumption is not a limitation. v In the intermediate (yellow) region, the reaction rates of HDN, HYD, and HDA begin to slow down due to limi- tations on the hydrogenation steps imposed by the ther- modynamic equilibrium. When operating in this regime, either DDS- or HYD-selective catalysts could potentially be applied. However, DDS-selective catalysts are usually preferred in view of the risk of slipping into the red region, where a high HYD-selective catalyst is not suited. w In the red region, which has the highest temper- ature-to-hydrogen pressure ratio, all hydrogenation- assisted reaction routes (HYD, HDN, and HDA) are hin- dered. In this region, the rates of sulphur and nitrogen removal are significantly lower, and HDS proceeds almost exclusively via the DDS route. In this regime, dehydrogena- tion and condensation reactions are thermodynamically favoured, and coking and deactivation can occur if a cat- alyst with too high selectivity for hydrogenation is applied. Operating in this regime requires caution and the use of a highly DDS-selective catalyst. As commercial hydrotreaters are characterised by an increasing temperature profile and a decreasing pressure profile when moving from the reactor top (inlet) to the bot - tom (outlet) (Figure 1), the bottom section of the reactor is more likely to be exposed to thermodynamic limitation for hydrogenation (red region). This is often the case for moderate pressure hydrotreaters processing difficult feed - stocks. The top section of the reactor, in contrast, is typi- cally affected by nitrogen and PNAs inhibition. A specific function of the quenches, by decreasing the temperature locally in the reactor, is to increase the fraction of the cat- alyst operating in the green region, providing a larger win- dow for exploiting the hydrogenation reaction pathway for HDS, HDN, and HDA. Catalyst design and performance The performance of a catalyst can be optimised by mod- ifying the properties of the support and the active metal phase. These include the active metal phase’s accessibility for the reactants, composition, morphology, and interac- tion with the support. Catalysts with a strong interaction between the metals and the support are often referred to as Type I, while those with a weak interaction are called Type II. The strength of the interaction is determined by the manufacturing procedure. The morphology of the metal slabs and their interaction with the support determine the DDS or HYD character of a catalyst. This, in turn, influences the catalyst’s activity and stability depending on the operating regime of the hydrotreater, as discussed in the previous section. High DDS selectivity, which leads to lower nitrogen and PNAs

Red region

DDS

out

HYD

Green region

DDS

Reactor PPH (bar)

catalysts to operate in synergy, turn the operating potential of a hydrotreater into actual performance. In conventional (fossil) hydrotreating, catalysts are selected to achieve two primary objectives: controlling poisons (such as sodium, silicon, iron, arsenic, nickel and vanadium) using a guard bed; and optimising hydrodes- ulphurisation (HDS), hydrodenitrogenation (HDN), and hydrodearomatisation (HDA) throughout the cycle with the main catalyst system. The HDS reaction proceeds via two pathways: direct des- ulphurisation (DDS) and hydrogenation (HYD). The DDS pathway primarily removes easy sulphur species, is usually the fastest route at low hydrogen pressure (< ca. 30 bar/450 psig), and is not affected by nitrogen and aromatics inhibition or by thermodynamic equilibrium for hydrogenation. In con- trast, the HYD pathway removes refractory sulphur species and is favoured at higher hydrogen pressure, but it is also severely affected by nitrogen inhibition and thermodynamic equilibrium. Note that the last step in the HYD reaction path- way is a DDS step, so HYD also requires the presence of DDS active sites in the catalyst to be able to proceed. HDN and HDA proceed via a hydrogenation step like HYD. Refractory (basic) nitrogen, in particular, inhibits the HYD and HDA reactions. The ability to remove refractory nitrogen allows for boosting HDS and HDA, resulting in a significant operational advantage. So, HDN activity is key, especially in distillate hydrotreating operations at medium and high pressure. HYD is potentially the fastest HDS reaction path- way. However, it is only effective if PPH₂ is sufficiently high (> ca. 35 bar/500 psig), inhibition by refractory nitrogen and polynuclear aromatics (PNAs) is not too high, and the proper combination of PPH₂ and temperature is met. Based on these observations, three operating regimes (regions) can be identified for a hydrotreating reactor (see Figure 1 ). The effectiveness of different reaction routes and the type of catalysts applied varies across these regions:  In the green operating region, characterised by a low temperature-to-hydrogen pressure ratio, all reactions (DDS, HYD, HDN, and HDA) are effective. DDS is the dominant Figure 1 Operating regions in a hydrotreater as a function of the operating H₂ pressure and temperature

Catalysis 2025