drop, preserving flow distri - bution and ensuring that the arsenic guard operates under conditions favourable for uni- form uptake. Together, these factors contribute to predict- able deactivation behaviour, improved cycle length control, and reduced need for mid-cycle corrective actions in high-arse- nic diesel hydrotreating service. Conclusion Arsenic contamination remains
200 400 600 800 1000 1200 1400 1600 Distance (um)
Figure 5 SEM images of TK-51
structural parameters. The analysis confirms that arsenic is present as crystalline nickel arsenide phases, with both NiAs and the more arsenic-rich NiAs₂ detected. More importantly, the coexistence of NiAs₂ and NiAs is consistent with advanced arsenidation and near-saturation of accessible nickel, supporting the observed high nickel utilisation and uniform arsenic penetration observed by SEM/EDS. This behaviour highlights the effectiveness of the optimised pore architecture and enhanced accessible nickel surface area in mitigating surface-limited uptake, even under diesel-range feed conditions. Overall, these results confirm the high volumetric arse - nic capacity of TK-51 and demonstrate that arsenic uptake in hydroprocessing catalysts is governed by the interplay between feed-dependent diffusion characteristics and nickel surface accessibility. In this case, the optimised catalyst design enables full utilisation of the active phase, supporting robust performance even under severe diesel hydrotreating conditions. The performance observed in this commercial application highlights several practical considerations for arsenic guard system design and placement in DHTs. At the applied operating severity and liquid hourly space velocity, arsenic transport to the guard catalyst is governed by a combina- tion of molecular diffusion and pore accessibility rather than external mass transfer limitations. Under these conditions, guard catalysts with insufficient internal connectivity or low effective nickel surface area tend to develop surface-bi- ased loading profiles, resulting in premature saturation and underutilisation of the catalyst volume. The combination of elevated arsenic concentration and relatively moderate space velocity in this unit favoured deep arsenic penetration when sufficient internal diffusion path - ways were available. The observed uniform arsenic distribu- tion across the catalyst cross-section indicates that internal mass transport was not rate-limiting, enabling effective uti- lisation of the available nickel surface area throughout the particle volume. This behaviour contrasts with diffusion-lim- ited regimes more commonly encountered in higher-boiling feeds or higher space velocity applications, where arsenic uptake is often restricted to the particle’s outer shell. From a loading perspective, the results emphasise the importance of integrating arsenic guards as part of a tailored grading system rather than as a standalone layer. Proper upstream grading minimises particulate fouling and pressure
one of the most severe and irreversible causes of hydro- processing catalyst deactivation, with even trace concen- trations capable of significantly reducing HDS and HDN activity and shortening unit cycle length. As refineries con - tinue to process heavier opportunity crudes and increas- ingly complex feed blends, effective arsenic management can be a critical element of reliable and economical hydro- processing operation. Effective arsenic guard design requires a fundamental understanding of arsenic-active site interactions combined with diffusion-controlled catalyst engineering. Maximising accessible nickel surface area while ensuring uniform inter- nal arsenic penetration is essential to achieving high volu- metric arsenic utilisation and stable long-term performance. By optimising pore architecture and maximising accessi- ble nickel surface area throughout the particle volume, mod- ern arsenic traps maximise uniform arsenic penetration and minimise surface-limited ‘hammock’ loading profiles that restrict effective capacity utilisation. Commercial experience demonstrates that the applica- tion of a properly designed arsenic guard system can signif- icantly reduce catalyst deactivation rates, extend unit cycle length, and provide refiners with greater operational flexi - bility and improved profitability when processing high-ar - senic feedstocks. Further reading 1 E. Furimsky, F. E. Massoth, Catal. Today, 52 (1999) pp. 381-495. 2 A. Puig-Molina, L. Pleth Nielsen, A. Molenbroek, K. Herbst, Catal. Lett., 92 (1-2) (2004) pp. 83-91. 3 P. Jensen, A. Kjekshus, T. Skansen, Acta Chem. Scand. , 20 (1966) pp. 403-414. Xavier E. Ruiz Maldonado is a Technical Service Manager at Topsoe North America with more than 18 years of experience in hydrotreating and hydrocracking technical assistance across the Americas and European regions. Maldonado holds an MSc in chemical engineering from Simon Bolivar University in Venezuela and a petroleum studies degree from the French Institute of Petroleum. Email: xerm@topsoe.com. Mohamed Khalil is a Senior Technical Service Engineer at Topsoe North America. Khalil holds a BSc degree in chemical engineering from The University of Houston (US). Email: mokk@topsoe.com Christian Frederik Weise is a Principal Scientist at Topsoe R&D with more than 12 years of experience in the development of hydroprocess- ing catalyst, with focus on grading and guard materials for both renew- able and fossil fuel applications. Weise holds a PhD in organic chemistry from University of Leipzig, Germany. Email: chfw@topsoe.com
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