Sep-2024

Hybrid (fresh and regenerated) catalyst loading reduces fill cost and carbon footprint (RI 2024)

When faced with the task of selecting catalysts for an upcoming hydrotreater turnaround, refiners usually consider replacement with either fresh catalyst or regenerated/rejuvenated catalyst.

Siddharth Sagar
Eurecat

Article Summary

When the anticipated feedstock, treating severity, and cycle length requirements are unchanged from the current cycle, refilling 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 opportunity to capture higher margins by increasing throughput, processing a more challenging feedstock, or stretching cycle length (for instance, to meet a planned turnaround), then replacing the catalyst with the latest generation fresh catalyst 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 catalyst load utilises a combination of prior generation rejuvenated 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 providing all the performance advantages of a latest generation fresh catalyst.

At first glance, one might expect a hybrid catalyst load to have an activity level proportional to the fraction of rejuvenated and fresh catalyst in the loading multiplied by their respective activities. The magic of a hybrid load outperforming the arithmetic average activity of the constituent catalysts is achieved by exploiting differences in sulphur reactivity and reaction kinetics as function of the catalyst’s position in the reactor.

A variety of sulphur types are present in most hydrotreater feedstocks, but the type and content can vary significantly depending on feedstock source and type. Figure 1 shows some of the sulphur types as well as their reactivities. 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 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 components of a feedstock are quickly hydrotreated at the top of such a unit in a small fraction of the unit’s overall catalyst volume. The rate of removal for these compounds is more generally limited by the rate of mass transfer of reactants into and reaction products out of the catalyst’s pore structure. The catalyst’s intrinsic activity is seldom a limiting factor for removal of these species. This aspect of desulphurisation behaviour can be exploited in a hybrid load by employing a lower cost rejuvenated catalyst in that part of the reactor where intrinsic catalyst activity does not limit a unit’s overall desulphurisation performance.

Maximising intrinsic activity
The observed activity of a unit using a hybrid catalyst loading is higher than the calculated value from arithmetically averaging the relative activity of the constituent catalysts. As shown in Figure 2, the arithmetic average activity expected from filling 30% of a reactor’s volume with a prior generation rejuvenated catalyst (RVA 90) and 70% with a latest generation fresh catalyst (RVA 120) is RVA 111. However, the observed activity for that same catalyst system will be RVA 120 – identical to the activity expected from filling the reactor with 100% latest generation fresh catalyst. Desulphurisation in the highly reactive sulphur regime at the top of the reactor is not limited by intrinsic catalyst activity. The higher intrinsic activity of a latest generation fresh catalyst is mostly lost in this regime such that a prior generation rejuvenated catalyst performs at essentially the same level.

These sulphur reaction regimes are present in all ultra-low sulphur diesel (ULSD) hydrotreaters and many FCC feed pretreatment (FCC-PT) hydrotreaters. Those are ideal units to exploit the benefits of hybrid loading. In both applications, the feedstock to be processed contains a full range of sulphur molecules from high to low reactivity. In ULSD units, since the product sulphur must be below 10 ppm, it is necessary to remove almost all the lowest reactivity sulphur compounds. In an FCC-PT unit, the product sulphur requirement is generally higher, such that some or all the lowest reactivity sulphur can remain untreated. The operating conditions of both applications will generate a reaction regime where the highest reactivity sulphur molecules are not limited by intrinsic catalyst activity. Hybrid loading techniques will be effective in these units to gain the performance expected from a full load of high-activity catalyst plus the cost savings expected from reusing catalyst.

Saving with hybrid Loads
A main driver for most refiners to consider hybrid loads is the savings in catalyst expenditures, typically one of their largest controllable costs. Figure 3 shows the characteristics of a hybrid load compared to a conventional load of fresh catalyst. In terms of observed activity and expected cycle length, there is no difference. The hybrid load delivers 100% of the activity and 100% of the cycle length expected from a full fresh catalyst load. The cost of the hybrid load is significantly less. The actual difference can vary depending on the price of molybdenum, cobalt and/or nickel at the time of purchase. However, a typical savings level is 50% for that fraction of the reactor loaded with rejuvenated catalyst. The reduction in cost will be even greater if the refiner supplies the catalyst to be rejuvenated from their own inventory of previously used catalyst. In that case, the only costs incurred are for regeneration and rejuvenation treatments applied to their spent catalyst.

Hybrid loads are a pain-free and well-proven method to control catalyst fill costs while still capturing the performance benefits of fresh catalyst. The fill cost savings are especially large when a refiner reuses catalyst they already own. The performance of a hybrid load is virtually indistinguishable from that of a full load of fresh catalyst. Finally, hybrid loading promotes circular economies and responsible catalyst management – an increasingly important consideration for many refiners.

This short article originally appeared in the 2024 Refining India Newspaper, which you can VIEW HERE