Increased activity in FCC pretreat
Advances in metal chelation techniques deliver higher performance catalysts for FCC pretreat. The process of optimising a catalyst is no easy task.
CHARLES OLSEN and BRIAN WATKINS
Advanced Refining Technologies
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A few of the keys to success are designing the proper pore size distribution into the catalyst, optimising the metals composition and maximising utilisation of the metals. Extensive work has been completed to define the optimum pore architecture of catalysts for processing heavy feeds.
Figure 1 compares the relative activity of different catalyst preparations as a function of increasing catalyst pore size. The pore size and surface area of a catalyst are dependent variables, meaning that as the pore size increases, the surface area decreases, and thus the number of active sites decreases. On the left side of Figure 1, the catalysts have a high surface area, but the pores are too small to allow large VGO molecules access to the active sites. On the right side of the figure, the pores are very large and allow easy access, but the surface area (and thus the number of active sites) is quite low. In both cases, the activity is low. The optimum support has a pore size distribution which minimises diffusion constraints and maximises the number of accessible active sites.
Another important parameter for catalyst performance is the composition of active metals. Figure 2 shows the relationship between the relative activity of a catalyst and the active metals loading. As expected, activity increases with increasing level of metals, but there is a limit. The figure indicates that there is an optimum metals loading beyond which adding more metals actually hurts catalyst performance. Catalyst activity decreases when excess metals are added because they begin to block the pores.
Yet another critical aspect is the utilisation of metals, or maximising the number of active sites. Current generation catalysts typically rely on the formation of Type II active sites during catalyst activation. Type II sites are known to provide significantly higher activity at the same metals loading as catalysts containing Type I active sites. An effective way to maximise the concentration of Type II active sites is through the use of chelate technology during the metals impregnation step. ART has been steadily improving the application of chelates with its DX technology.
The latest catalysts developed via this optimisation process are Advanced Refining Technologies (ART) 586DX and 486DX catalysts which are new catalysts designed for maximum performance in FCC pretreat applications. 586DX is a NiMo catalyst with significantly improved HDS and HDN activity. In FCC pretreat applications, this catalyst can provide superior nitrogen removal as well as aromatic and polynuclear aromatic (PNA) saturation in order to provide significant FCC feed upgrading for increased FCC yields. Figure 3 compares the activity of 586DX and AT575. The figure shows that 586DX has nearly 20% higher HDN activity and 15% higher HDS activity compared to AT575. 586DX has a large pore size distribution, similar to AT575, which gives the catalyst exceptional stability on heavy feeds.
It benefits from the latest advancement in DX technology. It has lower loss on ignition than conventional chelated catalysts and requires no special handling or start-up procedures. In just a short time since its commercialisation, 586DX has been installed in four FCC pretreat units. The performance has been excellent with as much as 10°F improvement in HDS activity over the previous catalyst. 586DX has already been selected for another cycle in some of these units.
Another recent advancement to ART’s FCC pretreat catalyst technology is 486DX. This is a CoMo catalyst that has outstanding HDS activity coupled with high HDN activity. Figure 4 shows a comparison of the activity of several generations of ART HDS catalysts for FCC pretreat along with 486DX. The results from side-by-side testing clearly show that 486DX exhibits a significant improvement in performance over the prior generation. There has been continuous improvement in activity with each generation since the introduction of ApART in 2004. Most recently, 486DX has been shown to have a 20% HDS advantage as well as a 10% improvement for HDN over AT795.
In lower pressure applications, 486DX continues to maintain its advantage over previous generations, including AT795 (see Figure 5). Figures 4 and 5 demonstrate the robustness of 486DX performance and indicate that it is an effective catalyst over a wide range of conditions.
Both the hydrotreating catalyst system and the operating strategy for the pretreater are critical to providing the highest quality feed for the FCC. Driving the hydrotreater to remove nitrogen and PNAs improves FCC product value when targeting gasoline production, but this needs to be balanced against the increased costs of higher hydrogen consumption and shorter cycle length that result from this mode of operation. To address this need, ART utilises the ApART catalyst system for FCC pretreatment. This technology is designed to provide increases in HDS conversion while at the same time providing significant upgrading of FCC feedstock quality and increasing yields. In essence, an ApART catalyst system is a staged bed of high activity NiMo and CoMo catalysts where the relative quantities of each catalyst can be optimised to meet each refiner’s goals and constraints. ART has continued to develop a better understanding of the reactions and kinetics involved in FCC pretreating and, through its relationship with Grace Refining Technologies, a detailed understanding of the effects of hydrotreating on FCC unit performance.
The appropriate choice of an FCC pretreat catalyst system must, in addition to other potentially important considerations such as feed metals removal, represent an optimisation of sulphur removal capability as well as HDN and saturation activity. The flexibility of the ApART system offers the potential to provide maximum HDS activity, and thus lowest FCC gasoline sulphur content, while also providing a maximum in FCC unit conversion at constant coke operation.
The addition of 586DX and 486DX to the ApART system portfolio enhances the flexibility and performance of these systems for FCC pretreat. They expand the capability to significantly reduce required SOR temperatures for both HDS and HDN. The optimised system of 586DX/486DX offers refiners an enhanced ability to generate lower sulphur FCC products as well as still delivering the benefits of nitrogen removal and polyaromatic saturation. Figure 6 shows a comparison of an AT575/AT795 system with the new 586DX/486DX system. There is a clear and significant improvement in both HDS and HDN activity with the ApART system.
The additional HDS activity combined with maximum nitrogen removal and aromatic saturation allows refiners to utilise 586DX as a standalone catalyst for maximum upgrade in refinery markets demanding increased yields. Refiners can also choose to utilise 486DX as a standalone catalyst in order to minimise hydrogen consumption while still achieving excellent sulphur removal for meeting Tier III sulphur regulations. Combining these two catalysts in an ApART catalyst system is ideal for hydrotreaters that need to operate with controlled or minimised hydrogen consumption yet still require low sulphur while maintaining yield gains in their FCC. These units are able to benefit from a lower start of run temperature as well as being able to gain some additional improvements that are not often achieved in a system of 100% NiMo or CoMo.
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