Catalysis 2024 Issue

While the quantity of highly reactive sulphur for a given feed can be measured using sulphur speciation tech- niques, the position in the catalyst bed where desulphur- isation shifts from a largely mass transfer limited regime to a largely activity limited regime is difficult to predict precisely. In addition, the exact volume and position of the two reaction regimes will shift somewhat with variations in feedstock or operating conditions. Fortunately, it is not necessary to precisely determine at what point in the reac- tor desulphurisation becomes activity limited. Limiting the volume of fresh catalyst displaced by rejuvenated catalyst will slightly reduce the cost-effectiveness, but it guarantees performance will be as expected. The volume of rejuve - nated catalyst used in subsequent cycles can be increased as experience with hybrid loading increases. Industrial experience has shown that at least 25% of the catalyst volume at the top of the reactor can be filled with rejuvenated catalyst without a negative impact on unit performance. This is especially true when the activity difference between the rejuvenated catalyst and fresh cat- alyst is relatively small – less than 25%. The smaller the difference in relative activity between the two catalysts, the less chance that overall unit performance will be adversely affected by the displacement of fresh catalyst with rejuve- nated catalyst. For instance, rejuvenated catalyst from a previous generation will typically have 20-25% lower activ - ity than latest generation fresh catalyst. That difference in activity would allow 30% or more of the reactor volume to be filled with rejuvenated catalyst without reducing overall unit performance. A common question about hybrid loads is who will be responsible for providing technical support for the unit. Refiners with a large, experienced technical staff may pre - fer to design and support a unit using a hybrid load on their own. Catalyst manufacturers are usually willing to support hybrid loads when they are involved in the process of cata- lyst selection and design of the catalyst loading diagram. It should be noted that often it is the refiner who must initiate the dialogue with a fresh catalyst manufacturer about their interest in considering options for hybrid loads. The catalyst manufacturer may not propose an option for hybrid catalyst loading unless the refiner proactively expresses interest. Savings 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 4 shows the characteris- tics 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 reduc- tion in cost will be even greater if the refiner supplies the




TYPE II Rejuvenated


TYPE I Rejuvenated


TYPE I Fresh


TYPE I Regenerated

* RVA dierences between fresh Type I and fresh Type II catalysts vary

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. While the activity of regenerated Type I catalyst would, in most cases, be too low to use in a hybrid load, rejuvenation of that same catalyst renders it suitable for use. When com- pared to the activity of regenerated catalyst, both Type I and Type II catalysts benefit greatly from rejuvenation. This activity gain expands the fraction of total reactor volume in which they can be deployed in a hybrid load without impacting overall unit performance. Importance of pilot plant testing Determining the catalyst volume where sulphur removal is dominated by desulphurisation of high reactivity sulphur molecules is difficult without extensive pilot plant testing. Figure 3 Performance gain expected from Type I and Type II rejuvenated catalysts relative to activity of original fresh state






100% 100% 85% / 80%* 80% * if reusing own catalyst

100% 100% 100% 100%

Observed RVA Relative cycle length Relative ll cost Relative CO footprint

Figure 4 Characteristics of a hybrid load compared to a conventional load of fresh catalyst


Catalysis 2024

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