Avoid poisoning in hydroprocessing operations
Advances in arsenic guards in a tailored grading system lead to predictable catalysts deactivation, optimal cycle length control, and fewer mid-cycle catalysts change-outs
Xavier E. Ruiz Maldonado and Mohamed Khalil Topsoe North America Christian Frederik Weise Topsoe R&D
A s refineries increasingly process heavier, opportunity crudes and more complex blends, trace contaminants such as arsenic (As) may impact hydroprocessing unit reliability in terms of catalyst life and throughput. Graded bed catalysts were first introduced to mitigate pressure drop and protect downstream hydroprocessing catalysts from particulate and chemical contaminants. Early implementations demonstrated that properly engineered grading systems could significantly reduce contaminant deposition and improve catalyst utilisation by monitoring feed contaminants and continued analysis of spent catalysts of grading materials. Since their initial industrial deployment in the late 1970s, grading materials have evolved to address specific poisons and fouling mechanisms encountered in modern refinery feeds. As refinery feedstocks have become heavier and more contaminated, grading materials have been required to address not only mechanical fouling and pressure drop con - trol, but also chemical poisoning mechanisms that directly impact downstream catalyst performance. This evolution has driven the development of grading systems with tai - lored physical properties and chemically active components designed to manage increasingly severe contaminant loads. The introduction of active grading in the industry was a breakthrough that transformed how refiners manage feed contamination and mitigate pressure drop. Since then, Topsoe has installed approximately 10,000 grading sys - tems worldwide, incorporating learned lessons from dec - ades of field data and operating experience and applying them across a wide range of process conditions. This includes grading with various optimised recipes and geometries to address pressure drop control and contam - inant mitigation across hydroprocessing units. Different particle shapes and compositions are formulated based on expected fouling severity, contaminant type, and reactor hydrodynamics, allowing grading systems to be tailored to specific feedstock and operating conditions. Arsenic impact A key challenge in modern refineries is protecting active hydroprocessing catalysts from poisons in the feed. Arsenic is widely recognised as one of the most severe and irrevers - ible catalyst poisons in hydroprocessing units. Even trace
concentrations in the range of 20-1,000 wt ppb can lead to accelerated loss of hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) activity by selectively deacti - vating nickel-promoted active sites. Unlike fouling or coke-related deactivation mechanisms, arsenic poisoning permanently affects the hydrogenation functionality, which cannot be recovered through conven - tional regeneration procedures. As a result, arsenic exposure shortens unit cycle length and limits operational flexibility, particularly in units processing opportunity crudes or blended feedstocks. Arsenic is a more severe poison than other com - mon contaminants in hydroprocessing units, such as silicon (Si), phosphorus (P), nickel (Ni), iron (Fe), or vanadium (V). Arsenic is present in a variety of crude oils, with ele - vated concentrations most commonly associated with heavy crudes and oil sands-derived feeds from countries and regions such as Canada, South America, California, Iran, Venezuela, and Russia. Typical arsenic concentrations reported for selected crudes are summarised in Table 1 . The highest arsenic concentrations encountered in North American refining are predominantly associated with Canadian oil sands-derived crudes, such as Bow River, Cold Lake and Syncrude. In the US, elevated arsenic levels are pri - marily observed in heavy California crudes, while most other domestic crudes exhibit low to moderate arsenic concentra - tions that may still contribute to catalyst deactivation when processed as part of complex crude blends. Trapping arsenic Arsenic enters hydroprocessing units primarily in the form
Typical arsenic levels of different crudes
Crude
Arsenic content, wt ppb
Mars/Thunder Horse (US Gulf of Mexico)
10-50
Iranian Heavy Ural Crude Stratfjord Crude
17 37
<10
Bow River Cold Lake
500-600 100-400
Canadian syncrude California Heavy
>250
63-1,100
Table 1
41
PTQ Q2 2026
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