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Mar-2011

Beyond ULSD

Developments in catalyst and internals technologies exploit the flexibility of
the ULSD unit. The economic incentive to produce diesel is substantial in today’s refining environment.

David Casey, Salvatore P Torrisi, Lawrence Kraus and John Smegal
Criterion Catalysts & Technologies
Yvonne Lucas and Aris Macris
Shell Global Solutions

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Article Summary

While ultra low-sulphur diesel (ULSD) margins over heating oil and fuel oil have fallen off from their record highs, they are still encouraging refiners to maximise their diesel yield. Although the distillate hydrotreater (DHT) has traditionally not been a significant source of profitability within a refinery, the recent situation has changed due to a series of new business drivers, including the distillate supply/demand balance, cost of crude and clean fuels legislation. The global distillate market is expected to remain tight over the next few years, which will continue to encourage refiners to favour diesel production over gasoline. Thus, incentives to increase distillate volume will remain high, with additional volume coming from feeds that are traditionally destined for gasoline, heating oil or fuel oil product pools. Although redirecting heavy naphtha is a way to generate additional diesel barrels, a greater incentive exists to increase the endpoint of diesel streams or upgrade more FCC light cycle oil (LCO) from the heating or fuel oil pool into on-road diesel.

Since 2005, when Euro 4 ULSD specifications became effective, the value of diesel over lower quality dispositions has increased by a factor of 3–5 times (see Figure 1). Thus, there is considerable economic incentive to upgrade heavy diesel molecules or convert LCO into ULSD product now and for the foreseeable future.

 The technical challenge, however, will be to maximise this volume while simultaneously meeting the increased fuel quality requirements, particularly the specifications for sulphur, boiling point, density, cetane and cold flow properties. The good news, and the subject of this article, is that ULSD can act as an enabler for further upgrading.

ULSD chemistry: good match with diesel upgrading
There are two key elements of ULSD that make diesel upgrading easier when compared with the older low-sulphur diesel (LSD) operation: a richer hydrogenation environment and a cleaner product that is devoid of most sulphur and nitrogen species. Both of these factors promote aromatic 
saturation, ring opening, isomerisation and cracking reactions.

This improved operating environment can then be leveraged in a number of different ways to capture a range of upgrading opportunities (see Figure 2). There are two key enablers for implementing these upgrading opportunities: high-activity ULSD catalysts and reactor process technology, both of which are required to implement the additional chemistries.

First key enabler: ULSD catalyst portfolio
A strong ULSD catalyst portfolio is a key enabler for upgrading diesel quality beyond the sulphur specifications. A robust, flexible and high-performance ULSD catalyst portfolio provides four key elements that enable upgrading reactions:
•    Maximises activity to reduce the volume of catalyst necessary to achieve ULSD targets, freeing up reactor volume for other upgrading catalyst system options
•    Provides a range of CoMo 
and NiMo catalysts to control hydrosulphurisation (HDS), hydrodenitrogenation (HDN) and ASAT in feed preparation for upgrading catalyst systems
•    Offers the flexibility to modify hydrogen consumption to offset hydrogen requirements associated with additional upgrading requirements
•    Provides a flexible set of products that can be presulphurised, activated and regenerated to meet unit process requirements, as well providing attractive multi-cycle economics.

Criterion’s ULSD portfolio added a third generation of catalysts in 2008–2009, representing advances in Ascent and Centera technologies. These two developments, Ascent Plus and Centera, provide a sizable increase in ULSD performance in both CoMo and NiMo catalysts compared with the first- and second-generation catalyst technologies (see Figure 3).

These catalysts enable the production of ULSD in a reactor volume that is only 60–75% (~10–15°C more activity) of that required for the first-generation ULSD products. The Centera products in particular provide an opportunity to reduce ULSD catalyst requirements into a smaller volume, freeing up space to utilise other upgrading catalysts in the same reactor system. Both the CoMo DC-2618 at a lower operating pressure and NiMo DN-3630 at a higher operating pressure offer a gain in activity (see Figure 4).

Refiners who designed their ULSD units with first-generation products can take advantage of the additional activity to increase run length or upgrade more diesel, by processing tougher feeds or just more barrels. This additional upgrading can be accomplished by reducing the ULSD catalyst volume in a multi-bed reactor, thus freeing up space for upgrading catalysts in the latter beds (see Figure 5). This article will discuss the latter subject of generating more diesel, upgrading poor-quality feeds or producing a higher-value diesel product.


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