Catalysis 2025 Issue

reaction inhibitor accumulation (coke precursors, organic nitrogen) on its surface. These adsorbed species serve to moderate catalyst activity when reactive feedstock com- ponents are added to the unit. Following catalyst break-in precautions helps ensure the catalyst stays on a path to stable performance and deactivation rate. As shown in Figure 3 , catalyst deactivation by coke in a hydroprocessing unit is a path function. The path is initi- ated when a reactive feedstock (containing hydrogen defi - cient and/or other reactive components) is combined with a hyperactive catalyst (free from adsorbed reaction inhibi- tors). When both elements are present, high reaction rates can cause hydrogen consumption at such a high rate that replacement of the consumed hydrogen is too slow to keep up with reaction requirements. The resulting hydrogen shortage causes ppH₂ to fall and falling ppH₂, in turn, causes hydrogen dissolution into the feedstock and hydrogen diffusion into catalyst pores to fall as well. The result is hydrogen depletion at the active site where it is needed to support reactions. Exothermic hydro- genation reactions simultaneously cause reactor tempera- ture to increase, driving reaction rates higher and adding even more hydrogen demand to the already limited supply. Low ppH₂ and high temperature favour dehydrogenation, oligomerisation, and condensation reactions instead of the desired hydrogenation reactions. Those reactions can generate coke precursors, which strongly adsorb on catalytic surfaces. Since the adsorp- tion of coke precursors inhibits catalyst activity, equilibrium

conditions will eventually be restored. However, substan- tial activity will be lost in the meantime, which is virtually impossible to recover. It is best to avoid starting conditions that lead down the coke deactivation pathway. Formation pathways While not a cracked feedstock per se, the hydrogen- deficient bonds and reactive oxygen heteroatoms in HEFA feedstock cause it to behave strikingly similar to cracked feedstock processed in a fossil hydrotreater. Both feed- stocks consume high amounts of hydrogen and generate high heats of reaction. Both are prone to coke precursor generation when reaction rates are too high. A main difference compared to cracked feeds is that HEFA feedstocks are generally free of aromatics. Polyaromatic condensation is a main cause of coke precursor formation with cracked feedstocks. Coke precursor formation with HEFA feeds more commonly proceeds via a paraffinic olefin oligomerisation pathway. It is worth noting that there are renewable unit feedstocks rich in aromatics, such as tall oil. Units processing this type of feedstock follow a coke precur- sor formation pathway similar to units that process cracked fossil oil feedstock. HEFA units face most of the same constraints as fossil oil hydrotreaters regarding the introduction and processing of reactive feedstocks. Similar precautions apply to protect the catalyst by breaking it in prior to exposure to a reactive feed, but there are differences in how break-in is executed in HEFA units. Since all HEFA feedstock has high reactivity,

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Catalysis 2025