Continued gains in FCC pretreating: part I
Case studies show where users are achieving significant gains in system capability, flexibility and economics. Also discussed are catalyst system loadings that reduce the need for low-activity demetallisation catalysts traditionally required to process heavier feeds
Desiree J De Haan, Andy Shivaram and Kevin D Carlson
Criterion Catalysts & Technologies
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Within the context of the drive for clean fuels, shifting product patterns and heavier feeds, Criterion’s proprietary Ascent catalysts are being increasingly used for their ability to:
— Show greater stability in more severe operations at higher temperatures and lower pressures
— Be employed flexibly to select the optimum fluid catalytic cracking (FCC) pretreat operating strategy to maximise FCC margins
— Process heavier feeds with more contaminants
— Desulphurise, denitrify and saturate to a high degree using lower amounts of hydrogen
— Produce ultra-low sulphur diesel (ULSD) and low-sulphur gasoline
— Work effectively and in conjunction with mild hydrocracking catalysts in the more severe operating regimes employed for mild hydrocracking.
Clean fuels success
In complex and integrated refineries, meeting stringent gasoline and ULSD specifications has greatly increased the impact of process unit interactions. This move to clean fuels, continued increases in crude pricing as well as shifts in demand between motor gasoline and diesel have required competitive refiners to re-evaluate how to best utilise both available refining assets and limited capital budgets.
In vacuum gas oil (VGO) processing, increases in hydroprocessing performance have been achieved with the Ascent family of catalysts. In today’s refining environment of increasing feed difficulty and contracting profit margins, these improvements in catalytic performance have allowed refiners to review how to best utilise their refining assets. Refiners who are using the Ascent technology to pretreat the feed for FCC operations are realising significant benefits, from extended periods between unit shutdowns, increases in refinery yields to the capability to process more difficult feeds.
FCC in a growing middle distillate market
FCC continues to serve an important role in the refining industry, with an estimated 350–400 FCCUs in operation today. Improvements in technology and catalysis have increased the conversion level of VGO feedstocks into more valuable middle distillates, gasoline and olefin products. Ongoing market changes in pricing and product demands have raised questions over how best FCC assets can be utilised.
While FCC yields vary based on feed quality and process conditions, typical operations yield 50–60 vol% gasoline and 15–20 vol% light cycle oil (LCO) from a VGO feed. Prior to widespread clean fuels introduction, similar pricing for both gasoline and distillate products provided strong incentives for FCC conversion, producing both gasoline and distillate from the low-value fuel oil pool. Continued demand growth for low-sulphur diesel, however, has increased the low sulphur diesel margins, thereby making gasoline production less desirable in some markets.
Globally, both increased coker capacity and increased processing of heavier crudes have resulted in the higher production of lower value, difficult VGOs more suited to the FCC pretreat/FCC complex than a hydrocracking unit due to the high level of feed contamination.
In addition to reviewing how to best optimise FCC operations, Criterion is working with various refiners to increase their middle distillate yield from the FCC pretreat/FCC complex by increasing VGO conversion in the FCC pretreat (PT) unit through modified operating strategies or catalyst systems.
Need for higher activity and stability in pretreatment
Success in the pretreatment of FCC feed is a function of both catalytic performance and catalyst stability. Since being introduced to the market in 2006, Ascent catalysts have been selected globally in over 50 applications involving VGO treatment. Commercial success has been measured by improved FCC operations, longer catalyst cycle lives and the ability to process more difficult feeds. Ascent catalysts are manufactured by a process providing enhanced promoter metals utilisation through an optimised catalyst physical structure designed to better tolerate the contaminant metals present in heavy gas oil feeds.
Ascent DN-3551 is the most active FCC PT nickel molybdenum (NiMo) catalyst. It is well suited to high-severity FCC PT operations, providing high contaminant metals tolerance while performing a high degree of denitrifi-cation (HDN), desulphurisation (HDS) and aromatic saturation (HDA) — reactions critical in FCC feed quality upgrading (to be discussed in more detail in part II of this two-part article). Ascent’s DN-3551 NiMo catalyst is designed to achieve very high FCC feed PT to help meet tighter specifications on downstream FCCU operation and product slate (see Case study 2). It can be supplied in an oxide (non-presulphurised) form and provides the advantage of being able to employ conventional handling and start-up procedures and conventional ex-situ regeneration for reuse in the same or lower severity service. The Ascent manufacturing technology produces inherently stronger catalysts that improve recovery during regeneration.
Ascent DC-2551 features similar performance, stability and regenerability to Ascent DN-3551. It is well suited to severe FCC PT operations that value a high desulphurisation performance while reducing the operational hydrogen requirement (to be discussed in more detail in part II). Its design promotes increased HDN and aromatics saturation compared to other cobalt molybdenum (CoMo) catalysts and provides high contaminant metals (nickel, vanadium, silicon) tolerance. Due to its high HDN and aromatic saturation capability, it is able to overcome the FCC conversion loss typically associated with CoMo catalyst application.
The use of either of these two catalysts, separately or in a combined system, can be tailored to the FCCU objectives of margin improvement. The FCC PT unit can be operated for targeted desulphurisation, denitrification, maximum polynuclear aromatics (PNA) saturation or to effect some mild hydrocracking.
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