Rosann Schiller, Yuying Shu, Ann Benoit and Rick Wormsbecher, Grace Davison Catalyst Technology, discuss strategies for catalytically maintaining delta coke through regenerator with FCC units processing “clean feeds”
Severe FCC feed hydrotreating improves feed “crackability” while providing other measurable benefits to FCC units constrained by NOx and SOx limits (i.e., decreased sulfur and nitrogen content). However, relative to VGO feeds, important changes in operating conditions must be considered when processing hydrotreated feeds. For this type of “clean feed” processing, a refiner can benefit from a new catalyst system developed for maximum activity and coke selectivity.
This maximum activity catalyst technology was discussed at a previous Grace Davison Refining Technologies Seminar. A presentation, “Alcyon™: New Maximum Activity FCC Catalyst,” (Schiller, et al) presented hydrotreated FCC feed cases for clean feed operations requiring maximum delta coke. The authors showed how the enhanced activity of Alcyon™ alleviates circulation constraints while improved coke selectivity provides increased conversion.
A moderate activity formulation of the Alcyon™ technology maximizes gasoline pool barrels while maintaining wet gas compressor load. Commercial trial observations graphically showed how this technology delivered additional delta coke at constant catalyst additions, in addition to improved coke selectivity and enhanced gasoline selectivity.
As previously noted, severe hydrotreating of FCC feedstock is one means to reducing sulfur and nitrogen while increasing hydrogen content. Typical ranges of feed hydrogen content are 11.8% (aromatic feeds) to 13.5% (paraffinic feeds). For a constant coke operation, the conversion level after hydrotreating can be calculated from the hydrogen balance. If the feed is hydrotreated, the hydrogen balance requires that conversion must increase for the same coke yield. For example, at 5 wt% coke in fresh feed, a hydrotreated feed (13.0 wt% hydrogen) will result in an 80 wt% conversion, while a VGO feed (12.5 wt% hydrogen) will result in a 70 wt% conversion.
More bonds are broken at higher conversion as measured by molar expansion (mol/mol basis). At constant coke, more bonds need to be broken to maintain the hydrogen balance after a change in feedstock quality. For example, at a constant coke of 3 wt%, a VGO feed shows a molar expansion of about 3.7 compared to a molar expansion of about 4.425 for a higher conversion hydrotreated feed. For reference, a molar expansion of 4.0 means that three bonds are broken in one feed molecule to produce four product molecules. At high conversion, more moles of product will need to be compressed, leading to concerns that the wet gas compressor may begin to limit the process.
Changes in delta coke
Contributions to delta coke change after hydrotreating (i.e., “clean” FCC feed). A significantly higher percentage of the delta coke needs to be supplied by the FCC catalyst with clean feed processing. This is because the delta coke contribution from feed carbon and contaminants have been significantly reduced after hydrotreating, relative to the amount of delta coke contributed by feed carbon and contaminants seen in VGO feeds.
In consideration of the shift in delta coke sources (i.e., catalytic) observed after hydrotreating, the Alcyon™ catalyst activity should provide the right amount of delta coke. Burning torch oil or recycling slurry to provide delta coke is detrimental to the operation. In addition, the decreased regenerator temperatures seen in units processing hydrotreated feeds lead to several operational concerns, including shifting sources of delta coke, increased circulation rates and potential emissions problems.
The Alcyon™ FCC catalyst technology has been developed for applications that require maximum activity and delta coke. At constant surface area or unit cell size (UCS), Alcyon™ is more active than a traditional catalyst; requiring less C/O to achieve conversion. Conventional routes to improved catalytic activity are increasing surface area and/or rare earth to increase the total number of active sites. The proprietary modification of Grace Davison’s USY zeolite increases activity per unit of surface area (constant total active sites) by enhancing the surface concentration of reactants. The cracking rate (R) is proportional to the number of active sites (N) and the surface concentration of the hydrocarbons on the catalyst (θ):
R ~ N × θ
Wet gas compressor constraint
In one case where the FCC operator used a competitive benchmark technology to process hydrotreated feed, a wet gas compressor constraint became a concern. If ROT is reduced in this case, conversion targets cannot be achieved. However, application of Alcyon™ can maintain conversion at reduced ROT. This catalyst technology will also provide high delta coke but improved coke selectivity, while also delivering excellent gas selectivity, creating room against the compressor constraint.
The catalyst’s high coke selectivity and activity was demonstrated in another case comparing a competitive catalyst. At a constant coke of 3.5 wt%, the Alcyon™ catalyst was about 2.0 wt% higher in conversion C/O ratio that was almost 1.25 units lower than the competitive catalyst. The catalyst also delivers more activity, yet better coke selectivity, showing a higher conversion of about 2 wt% compared to the benchmark catalyst. Even with higher conversion, the catalyst minimizes the load on the wet gas compressor, reducing the wet gas yield by as much as 5% compared to a competitive catalyst in one demonstrated case, in addition to the advantages shown in Table 1 and Table 2.
Table 1. At constant conversion, the higher activity Alcyon™ reduces dry gas, increases gasoline selectivity and makes lower coke.
|Total C4 olefins||5.9||6.2|
Table 2. At constant activity, Alcyon™ increases conversion, maintains dry gas, increases gasoline selectivity and decreases bottoms.
|Total C4 olefins||5.9||6.2|
Increased hydrotreater severity
In the FCC, Alcyon™ will achieve conversion targets even at reduced ROT. In one commercial application where a refiner increased the severity of the FCC feed hydrotreater, new heat balance requirements had to be satisfied while remaining within circulation constraints and maintaining yield selectivity. In the trial objectives with Alcyon™, MAT increased by two units (2.0) at constant catalyst additions; regenerator temperature and circulation was maintained; improved coke selectivity was realized in spite of a higher delta coke (relative to base catalyst); and product yield was maintained. By delivering additional delta coke, the catalyst satisfies heat requirements at lower C/O yet still yields higher conversion.
At constant activity and C/O ratios ranging from 6.0 to 8.0, Alcyon™ delivers better selectivity when compared with a base catalyst, yielding slightly higher gasoline, even at lower C/O. At constant coke, Alcyon™ was also shown to enhance gasoline yield, despite higher delta coke.
For gasoline pool maximization, the Alcyon™ G has the formulation flexibility to moderate activity for less severe operation. Some of the benefits include enhanced butylenes selectivity and maintained or reduced wet gas. At constant LPG, the new formulation has been shown to maximize gasoline pool barrels.
Editor’s Note: This article was based on a presentation from the Grace Davison Refining Technologies 2010 Houston Seminar: “Alcyon™: New Maximum Activity FCC Catalyst.” Further elaboration can be obtained by contacting the primary author, Rosann Schiller (email@example.com). This article is a re-publication of the Feature article previously published in Vol. I, No. 6 of Refinery Operations newsletter.