Catalysis 2024 Issue

30% PRIOR GENERATION REJUVENATED

100% LATEST GENERATION FRESH

100% PRIOR GENERATION REJUVENATED

100% PRIOR GENERATION FRESH

70% LATEST GENERATION FRESH

Calculated RVA Observed RVA

100 100

90 90

120 120

111 120

Figure 2 The arithmetic average activity expected from filling 30% of a reactor’s volume

The rate of removal for these compounds is more gener- ally 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 load- ing 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 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 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 gener - ation 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 applica - tions, 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 sul- phur 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 sav- ings 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 cat- alysts will be utilised to their full potential. Since the per- formance 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 reac - tion regime and is significantly less expensive than fresh catalyst. A rejuvenated catalyst within one to two gener- ations 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 rela - tive 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

Catalysis 2024