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Apr-2014

Optimising hydroprocessing 
catalyst systems

Flexibility in catalyst technology and processing tactics optimises hydrocracker and ULSD operations to meet diesel and gasoil production targets

WOODY SHIFLETT, CHARLES OLSEN and DAN TORCHIA, Advanced Refining Technologies
DAVID BROSSARD, Chevron Lummus Global
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Article Summary
Growth in the output of refined products is driven strongly by demand for clean diesel and new regulations on gasoils between now and 2020. Notably, annualised global growth in demand for diesel and gasoil is predicted to be about 2%, outpacing gasoline at 1.3%, with the estimated gasoline/diesel gasoil ratio dropping from 0.85 in 2012 to 0.81 in 2020.1 In addition to continued growth in Asia and continuing recovery elsewhere, standards covering motor vehicle mileage as well as ethanol mandates in North America and emerging regulatory restrictions on marine fuels add further momentum to this trend. Bunker fuel regulations that will be effective from 2015 and require 0.1% sulphur limits in emission control areas (ECA) in North America and northern Europe will underlie a shift in demand to diesel/gasoil products with an expected concurrent boost in diesel prices, primarily due to quality requirements. Meanwhile, refining capacity additions will outstrip global demand in the 2015-2020 period and will continue to pressure refining margins. In short, it will be a period of opportunity for refiners that have the flexibility in their hydrocracking capabilities, especially if a robust ULSD hydrotreater is available that can marry their catalyst system needs and operational responses to changing economic scenarios.

In early 2013, Advanced Refining Technologies (ART) and Chevron Lummus Global (CLG) announced an agreement in which ART has exclusive rights to sell CLG’s hydrocracking and lubes hydroprocessing catalysts to petroleum refiners worldwide for unit refills. The outcome of this agreement streamlines hydroprocessing catalyst supply and improves technical service for refining customers by establishing ART as the single point of contact for all their hydroprocessing catalyst needs. In this context, this article addresses approaches to catalyst technology and processing tactics to optimise hydrocracker and ULSD unit operation in order to meet the need for diesel and gasoil production in the future.

Molecular management and hydroprocessing units
Of all diesel boiling range materials, FCC light cycle oil (LCO) stands out as one of the lowest value feedstock materials. It is usually the most difficult to manage operationally in a hydroprocessing unit, largely due to its combination of olefins and the refractory nature of the LCO. It has the highest demand for hydrogen to produce a clean diesel or even 0.1 wt% sulphur marine gasoil, and offers heat release management challenges when processed at higher fractions in a hydroprocessing unit feed. Provided there is adequate hydrogen supply, LCO is sometimes best processed in the hydrocracker along with other feeds such as atmospheric or vacuum gasoils) (AGO or VGO). In addition, LCOs have variable quality: depending on the refinery configuration, the LCO may be produced from an FCC with a 
feed pre-treater and consequently may contain fairly modest levels of sulphur and nitrogen. Although they appear to be ‘easier’ feeds 
due to their lower levels of 
contaminants, the remaining impurities are also among the toughest to treat.

Coker gasoils can be processed either in the hydrocracker or the ULSD unit subject to individual unit capacities and infrastructure limitations such as hydrogen availability, pressure, endpoint, and impurities. Heavy coker gasoils (boiling well above the diesel range, >975ºF, 525°C) are best sent to a hydrocracker although they can present challenges to processing in significant quantities even in modern, robustly designed hydrocrackers. A hydrocracker originally designed or revamped for VGO service is a most suitable outlet. This offers more potential to maximise diesel yields, especially in recycle flow configurations and at higher pressures. Light coker gasoils, on the other hand, are readily processed to ULSD in a diesel hydrotreater provided there is adequate hydrogen partial pressure and the unit has an appropriately tailored catalyst system to remove contaminants and provide the required sulphur conversion.

Straight run (SR) gasoils present the least challenging processing constraints and can be fed to either the hydrocracker or ULSD unit, although the ULSD unit is typically the preferred outlet. Exceptions include cases where the SR feeds are needed as ‘diluent’ components to aid in managing limitations to hydrogen consumption, and heat release issues in hydrocrackers designed for more paraffinic and naphthenic feeds.

Processing tactics are balanced between these considerations of feedstock molecular management and the designs, limitations and strategic intent of the unit in the refining scheme. Hydrocrackers have traditionally been designed to pump hydrogen into the feedstock to convert heavier, higher-boiling materials into more valuable products, while capitalising upon aromatics saturation to increase volume swell as well as product value parameters (density, cetane, smoke point, and so on). Until recently, ULSD has been a secondary priority and generally not even a consideration in the original design of the majority of hydrocracking units operating today. With a robust ULSD unit in the refinery, this order of priorities need not only be overridden but can be augmented by the latest catalyst systems for hydrocracking that have been designed for maximum desulphurisation (HDS) activity as well as fundamental nitrogen removal (HDN), hydrocracking and saturation.

The versatile hydrocracker and catalyst system flexibility
While several hydrocracker configurations are in current use, two dominate the landscape, especially when addressing clean fuels production: single-stage, once-through (SSOT) configurations and two-stage, recycle (TSREC) configurations.2 A perspective over a decade of application is shown in Figure 1.

The SSOT configuration is both simple and versatile, and represents the simplest option when unconverted oil (UCO) has high value as either a lube plant feed or a FCC feed. This configuration dominates the low conversion market (>70% of that market). The SSOT process configuration is shown in Figure 2.

Catalyst system optimisation for the SSOT is often influenced strongly by the desired outlet for the UCO it produces: lube plant feed will favour higher viscosity index (VI), aromatics saturation and HDS, while FCC feed will favour HDN, removal of polynuclear aromatics and HDS.

Balancing with needs are the light product drivers: ULSD or the less demanding 0.1 wt% sulphur marine fuel. If ULSD production is a target, and cannot be produced within the SSOT unit’s constraints, it is critical to factor in and model the effect of this pre-processed component as feed to the ULSD unit. It will clearly include more difficult, sterically hindered sulphur compounds for HDS in the ULSD unit.
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