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Aug-2017

Leveraging hydrocracking to upgrade low value streams

Flexible design of hydrocracking catalyst systems enables maximum conversion of lower value feed streams into more valuable liquid products.

ADRIENNE VAN KOOPEREN
Criterion Catalysts & Technologies / Zeolyst International

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Article Summary

The global refining market continues to be challenging as oil prices remain low and economic drivers push refiners to process heavier, more difficult and sourer crudes. These opportunity crudes generate a higher percentage of the distressed streams which are either blended into lower value dispositions or, more recently, serve as feed streams to other conversion units. Today, upgrading these difficult streams into lighter, more valuable liquid products becomes paramount under the current conditions as low value outlets disappear, especially in light of the recent International Maritime Organisation (IMO) decision on bunker fuel quality.

The following case studies offer an exploration of refineries that leverage the flexibility of hydrocracker pretreat catalyst systems to upgrade low value, highly processed material via hydrocracking without sacrificing run length, product quality, or margin by disposition of these streams as low valued exports. These streams include, but are not limited to, products from thermal cracking, deasphalting units, catalytic cracking, coking, lube extraction, or ebullated bed residue upgraders. The three case studies described here demonstrate the different operating challenges and catalyst strategies employed to upgrade a broad range of very difficult feedstocks into premium quality clean fuel products.

The final case study focuses on the Shell Scotford refinery to illustrate the evolution of pretreat design and operating strategies that address the increasingly demanding cycle objectives for converting synthetic feeds derived from tar sand bitumen via hydrocracking. We continue working together to identify opportunities to improve unit performance, and solutions to achieve the ultimate goal of minimising or eliminating low valued stream exports by upgrading these materials into high value, clean fuels.

Challenges of converting heavy, low value streams via hydrocracking
Upgrading heavy, distressed streams via hydrocracking to lighter, high value products presents a number of problems from both operational and catalytic perspectives. These range from a variety of issues related to processing significant amounts of cracked material, such as temperature or heat release management, fouling and contaminate mitigation, and hydrogen availability/consumption constraints. Although hydrocrackers have always leveraged multi-tiered catalytic systems, significant advances in catalyst technology and improvements in manufacturing techniques over the last decade have addressed a number of issues related to hydrocracking highly aromatic, refractory feeds and thus enabled refiners to progress from upgrading conventional type feeds to include conversion of these more challenging, lower value streams.

The catalytic approach to upgrading high severity feedstocks via hydrocracking focuses on balancing the performance of a five-part pretreat system to address the specific needs of each refiner. Figure 1 illustrates the multi-tiered hydrocracker pretreat catalytic system.

Grading: foulant and particulate abatement
The top layer of the multi-tiered catalyst system always consists of grading material tailored by size, shape, and activity to provide deep-bed filtration which mitigates pressure drop (DP) build-up due to particulates. The filtration design uses higher voidage, larger particle size materials at the top with subsequent layers of decreasing size and various shape to capture inert solids (for instance iron, scale, solids) or reactive species and thereby protect the lower, active catalyst beds from foulants and remove trace metals. Activity grading also alleviates fouling potential from highly reactive compounds such as diolefins. Proper design of grading and demetallisation catalyst is key to ensuring cycle length and mitigating unplanned outages due to pressure drop or poisoning.

Demetallisation: metal contaminant protection and removal
Metals contamination acts as a primary driver for deactivation of pretreat catalysts. Common contaminants in coker and other high end point feeds, such as nickel (Ni) and vanadium (V), deactivate catalysts by plugging pore mouths, while crude-indigenous contaminants, like arsenic (As), deactivate Ni-containing catalysts by attacking the active nickel species to form nickel arsenide, and impact catalyst activity significantly even at relatively low concentrations.

Designing the appropriate level of demetallisation capacity for a specific process requires clear understanding of feedstock sources and the associated contaminant levels as different trap materials are optimised uniquely with balanced pore size, surface area, and activity to remove specific metals from particular feedstocks. For example, MaxTrap[Ni,V] is an optimised material to remove metals common in high boiling point molecules, such as iron, potassium, sodium, nickel, and vanadium with moderate hydrodenitrogenation (HDN)/hydrodesulphurisation (HDS) activity, while MaxTrap[As] targets the high uptake of arsenic specifically. Demetallisation catalysts also possess various degrees of heteroatom removal activity and act as the initial activity grading layer above the higher activity hydrogenation catalysts.

Conventional type NiMo: balanced heteroatom removal and hydrogenation activity
Application of the highest activity catalyst is neither required, nor recommended, in the top of the hydrocracker pretreat reactor as the molecules that undergo conversion in this regime are very reactive. Loading highly active catalyst directly below the grading/demetallisation layers creates the potential for accelerated deactivation and coke induced pressure drop build from large saturation exotherms. Utilising a balanced HDN/HDS activity nickel molybdenum (NiMo) catalyst between the demetallisation and higher activity catalyst beds provides more than sufficient activity to convert easier sulphur and nitrogen species, and facilitates enhanced robustness against metals poisoning for additional demetallisation capacity due to the underlying pore structure. Criterion’s Ascent catalyst line contains both Type I and Type II active sites to provide improved activity over conventional Type I hydrotreating catalysts with the added benefit of enhanced stability. The optimised blend of active sites provides excellent activity transition and mitigates excessive heat release by not over-saturating cracked materials in the top bed. 

Type II NiMo: deep heteroatom removal, maximum HDN, and hydrogenation activity
Deep heteroatom removal requires the highest hydrogenation activity to saturate aromatic rings in refractory molecules that contain imbedded heteroatoms (for instance, sulphur and nitrogen). This is increasingly important as refiners upgrade more highly processed, aromatic streams in the hydrocracker. The Centera platform represents Criterion’s latest advance in pretreat catalyst expertise, which is derived largely from the proprietary catalyst manufacturing technologies. The unique formulation and production methods of Centera catalysts provide higher dispersion of promoter sites to enable further sulphidation vs conventional Type II catalysts which, in turn, increases the quality of Type II active phases directionally. The higher dispersion and more complete sulphiding of the active sites thus leads to enhanced heteroatom removal activities and superior aromatics saturation.


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