Catalysis 2024 Issue

Hybrid catalyst loading reduces fill cost and carbon footprint

Using rejuvenated catalyst in a hybrid load significantly lowers fill cost and CO2 footprint while providing the performance advantages of latest generation fresh catalyst

Steve Mayo Eurecat

W hen faced with the task of selecting catalysts for an upcoming hydrotreater turnaround, refiners usually consider replacement with either fresh catalyst or regenerated/rejuvenated catalyst. When the anticipated feedstock, treating severity, and cycle length requirements are unchanged from the current cycle, refill - ing the catalyst with regenerated or rejuvenated catalyst is a good option. Regeneration of Type I catalysts and rejuvenation of Type II catalysts typically restores >90% of original fresh catalyst activity – generally adequate for replicating the performance of a prior cycle with the same fresh catalyst. If, on the other hand, the upcoming cycle offers an oppor- tunity to capture higher margins by increasing throughput, processing a more challenging feedstock, or stretching cycle length (for instance, to meet a planned turnaround), replacing the catalyst with the latest generation fresh cat- alyst may be justified. The latest generation fresh catalyst will typically deliver 15% or higher activity than the previous generation. That extra catalyst activity can be translated into higher throughput, longer cycle length, lower product sulphur/nitrogen or the capability to process a more difficult feedstock. Hybrid loads There is a third option for catalyst selection, which is less often considered – the hybrid catalyst load. A hybrid cata- lyst load utilises a combination of prior generation rejuve- nated catalyst and latest generation fresh catalyst to reach an activity level indistinguishable from a full load of latest generation fresh catalyst. The use of rejuvenated catalyst in a hybrid load significantly reduces both the catalyst fill

cost and its CO₂ footprint while simultaneously provid - ing all the performance advantages of latest generation fresh catalyst. At first glance, one might expect a hybrid catalyst load to have an activity level proportional to the fraction of rejuve- nated and fresh catalyst in the loading multiplied by their respective activities. A hybrid load is able to outperform the arithmetic average activity of the constituent catalysts by exploiting differences in sulphur reactivity and reaction kinetics as a function of the catalyst’s position in the reactor. A variety of sulphur types are present in most hydro- treater feedstocks, but the type and content can vary sig- nificantly, depending on feedstock source and type. Figure 1 shows some of the sulphur types as well as their reactivi- ties. Mercaptans, sulphides, and disulphides typically have extremely high reactivity. Thiophenic sulphur compounds are less reactive and become progressively less so when connected to one or two benzene rings. The least reactive sulphur compounds are dibenzothiophenes with steric hindrance from one or more alkyl groups adjacent to the thiophenic sulphur. The difference in reactivity between most reactive and least reactive sulphur species is massive – more than 100x. The least reactive sulphur molecules clearly benefit from, and in many cases require, high catalyst activity to increase the removal rate, but what about the most reactive sulphur molecules? In a unit designed to remove sulphur with low reactivity, the most reactive sulphur hardly benefits from higher catalyst activity. The highly reactive sulphur compo- nents of a feedstock are quickly hydrotreated at the top of such a unit in a small fraction of the unit’s overall catalyst volume.

High reactivity sulphur Low catalyst activity needed

Low reactivity sulphur High catalyst activity needed

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Figure 1 Sulphur types and their reactivities

Catalysis 2024