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

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Article Summary

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. 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), 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 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. 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 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 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 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 the 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, which is 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 required to remove almost all the lowest reactivity sulphur compounds.

In an FCC-PT unit, the product sulphur requirement is generally higher, so 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.

Loading rejuvenated catalyst
Since high intrinsic catalyst activity is not necessary to remove high reactivity sulphur molecules, and since those same molecules react at the top of the catalyst bed, how can the catalyst loading be optimised? The upper part of the reactor, where desulphurisation of high reactivity sulphur dominates, is not a reaction regime where highly active catalysts will be utilised to their full potential. Since the performance benefit of fresh catalyst cannot be fully utilised in this reaction regime, its cost-effectiveness is compromised. A more cost-effective approach is to load a less expensive catalyst in this regime.
A rejuvenated catalyst is a smart choice for use at the top of a reactor since it has sufficient activity for the reaction regime and is significantly less expensive than fresh catalyst. A rejuvenated catalyst within one to two generations of the latest generation will have a suitable activity level to perform well. Figure 3 shows the performance gain expected from Type I and Type II rejuvenated catalysts relative to the activity of their original fresh state.

Rejuvenation boosts the activity of Type II catalysts to at least 90% of the original fresh level, and it typically boosts the activity of Type I catalysts by 15% or more above the level of the original fresh. The increased activity for Type I rejuvenated catalysts, well above the original fresh activity, is a consequence of the rejuvenation process transforming a Type I active phase to a more active Type II active phase morphology. The enhanced activity of Type I rejuvenated catalysts significantly expands the available volume of used catalyst suitable for use in hybrid loads.

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