Oct-2024

Handling complex hydrocarbon molecules

Growing interest in processing heavy oils with high nitrogen content has created a need for pretreatment catalysts with higher HDN and HDS activity.

Xavier E Ruiz Maldonado and Ryan Jesina, Topsoe Inc, USA
Michal Lutecki Topsoe A/S, Denmark

Article Summary

To help refiners remove harmful environmental pollutants while optimising system stability and profitability, a matrix of hydrocracking pretreat, mild hydrocracking, and FCC pretreat (or gasoil) catalysts with the maximum accessible particle surface area is required to maximise catalyst activity. This can be seen in refineries across the US, where processing units are challenged with a wide range of operating conditions.

These operational parameters are comprised of varying hydrogen partial pressures and diverse feedstock types, including vacuum gasoil (VGO), heavy cracked feedstock, FCC slurry, and synthetic crude-derived feedstock. Refiners must also have the flexibility to meet a wide array of product specifications, spanning both high and low hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) conversion levels, maximum polynuclear aromatics (PNA) saturation/volume swell, and middle distillate conversion. Tailored solutions are needed to meet each facility’s unique pain points that must be overcome in the optimisation of refinery processes, product specifications, and economic performance.

Enhancing hydrogenation activity
In the 2000s, research¹,² and development revealed the presence of basal plane CoMo and NiMo sulphided slabs that shed light on the final step of the hydrogenation route/mechanism. This was revealed using advanced, scanning tunnelling electron microscopes. This advancement resulted in the production of more resilient heterogeneous catalysts for all hydroprocessing applications.

The discovery of active sites for HDS were later designated by Topsoe as BRIM sites. Since then, BRIM technology has been refined and utilised to introduce the first generation of NiMo and CoMo BRIM catalysts for the hydroprocessing industry. Topsoe’s research teams and specialists have strived to advance formulation optimisations to enhance the hydrogenation activity of NiMo BRIM catalysts to further increase catalytic performance. This involved utilising an improved alumina support combined with advanced metals impregnation techniques, which resulted in Topsoe’s HyBRIM generation of catalysts.

The advanced metals impregnation techniques for HyBRIM catalysts result in a higher degree of active metals utilisation when compared to BRIM catalysts. This is due to the combined effect of lower stacking and highly dispersed MoS₂ nano-slabs, as shown in Figure 1. In other words, HyBRIM technology can disperse a higher number of nano-slabs due to a reduced interaction between the metal slab and the carrier, which increases HDS activity via both the direct and the hydrogenation routes.

Heavy gasoil diesel applications
Following the full development of HyBRIM NiMo catalysts, research was conducted to extend this technology to CoMo catalysts. This effort yielded significant results, leading to the development of Topsoe’s new generation of CoMo HyBRIM catalysts, which are suitable not only for heavy gasoil applications but also for diesel applications. An overview of gasoil catalyst development and launch year is shown in Figure 2.

Removal of non-refractive and highly refractive sulphur compounds
The effectiveness of a catalyst to remove both non-refractive and highly refractive sulphur compounds throughout the reactor determines overall HDS activity. However, to maximise a catalyst’s performance, it is crucial to optimise its utilisation of pre-hydrogenation (HYD) and direct desulphurisation (DDS) pathways. The HYD route is well known to be highly inhibited by very complex feed nitrogen compounds, especially basic nitrogen species. Therefore, NiMo catalysts do not become highly active for HDS reactions until the nitrogen concentration reaches an extremely low level. At a lower nitrogen concentration, NiMo catalysts can potentially be less inhibited and more effective at removing the most refractive sulphur compounds. Higher hydrogen partial pressure in the unit creates a kinetically favourable environment and enhances conditions for nitrogen removal while initiating deep desulphurisation more rapidly.

Catalyst researchers and providers have extensively researched HYD and DDS mechanism routes, resulting in a deep understanding of the two different reaction pathways and their relationship with metals stack frequency and size. This understanding has led to the trimetallic TK-594 HyBRIM catalyst system for industrial units targeting high HDS levels while maintaining HDN activity for FCC pretreat applications.

The system contains highly and homogenously dispersed nano NiCoMo sulphided slab structures, allowing optimal reactor volume utilisation at all stages of sulphur removal, independent of feedstock nitrogen level. The HDS and HDN activities achieved by TK-594 HyBRIM are comparable to pure CoMo and pure NiMo catalyst types, embodying the best performance attributes from each catalyst type, resulting in a higher degree of unit stability while achieving desired product targets and specifications.

Optimising DDS and HYD routes
Figure 3 shows the results of calculated MoS₂ edge dispersion via transmission electron microscopy (TEM). S1 and S2 represent the MoS₂ edge dispersion of CoMo BRIM and trimetallic TK-594 HyBRIM catalysts, respectively. Edge dispersion is reflected in slab length (size) and the number of layers (stacking). The HYD route is maximised for short slabs and extended stacking degree of MoS₂, while the DDS route is the most effective when short slabs and low stacking of MoS₂ are present. The TK-594 HyBRIM formulation (S2) shows slightly shorter slabs on average and is better described as a catalyst with hybrid properties, making it efficient for both HYD and DDS routes. It is important to highlight that excessive stacking may lead to a penalty on dispersion and consequent reduced activity.

Figure 3 also shows the elemental distribution of the catalysts. TK-594 HyBRIM (S2) shows highly dispersed Mo, Co, and Ni metals. Homogeneously uniform dispersion of Ni is especially important for high hydrogenation activity required for deep desulphurisation and elevated aromatic saturation. The CoMo BRIM catalyst (S1) shows equally uniform Mo dispersion. However, Co dispersion is slightly less uniform, with small clusters of Co sulphide present to a small extent.

In general, high metal dispersion is an important consideration in catalyst selection, as it is not the metal content itself that makes the catalysts highly active but the correct phase distribution in the form of CoNiMoS structures and the accessibility of these metals to promote desulphurisation.