Beyond ULSD

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. Complicated further by the sulphur paradox, senior refinery managers are asking themselves how they can maintain efficiency, throughput and margins in this challenging environment.

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 fluid catalytic cracking light cycle oil (FCC LCO) from the heating or fuel oil pool into on-road diesel. For example, since 2005, when Euro 4 ULSD specifications took effect in Europe, the value of diesel over lower quality dispositions has increased by a factor of three to five times.

With more stringent legislations cropping up all over the globe, refiners have considerable economic incentive to upgrade heavy diesel molecules or convert LCO to ULSD product now and for the foreseeable future.

The challenge will be to adopt more innovative ways of thinking about meeting long-term product standards and emissions legislation in order to maximise diesel volume. The good news is that refiners can optimise operations and meet these requirements through the multi-platform Shell Global Solutions Pentagon Model.

The Pentagon Model can help refiners move “beyond ULSD” to improve cetane, reduce aromatics, upgrade cold flow properties, improve density, reduce heavy diesel end point, process biodiesel components and shift conversion between naphtha, kerosene and diesel. By employing the Pentagon Model with advanced unit designs, customised catalyst solutions and process configurations from the Sulphur Technology Platform, refiners are able to not only survive but thrive in this volatile environment.

A strong ULSD catalyst portfolio

A strong ULSD catalyst portfolio is a key enabler for upgrading diesel quality while meeting sulphur specifications. A robust and flexible portfolio maximises activity to reduce the volume of catalyst necessary to achieve ULSD targets. This frees up reactor volume for other upgrading catalyst system options. The catalyst portfolio provides a range of nickel molybdenum (NiMo) and cobalt molybdenum (CoMo) catalysts to control hydrodesulphurisation (HDS), hydrodenitrification (HDN) and aromatic saturation (ASAT) thus improving the quality of the feed flowing to the catalyst systems.

Offering the flexibility to modify hydrogen consumption and offset hydrogen requirements associated with additional upgrading requirements, the ULSD catalyst portfolio also provides a flexible set of products that can be presulphurised, activated and regenerated to meet unit process and operations requirements.

Criterion Catalysts & Technologies’ portfolio is extensive, reflecting the continuous innovation and commitment to catalysis research and development across all hydroprocessing applications. The latest products allow ULSD to occur in a reactor volume that is only 60-75 per cent (~10-15°C more activity) of first generation ULSD products in the market.

The CENTERA^® products in particular provide refiners with a great opportunity to reduce the amount of catalyst needed to make ULSD, freeing up space to utilise other upgrading catalysts in the same reactor system. Both the CoMo DC-2618 at lower operating pressure and the NiMo DN-3630 at higher operating pressures offer considerable activity gains.

Reactor internals technology

Implementing the additional chemistries requires proper control over the reaction environment, which in many cases requires much finer control over gas and liquid flows as well as reaction temperatures. In fact, if proper mixing and redistribution does not occur, there can be a detrimental impact on the catalytic environment, which can reduce product quality or degrade yields.

Shell Global Solutions’ reactor internals technology provides advanced vapour and liquid distribution and optimised mixing of vapour and liquids. Two of the most widely known are the high dispersion distributor tray (HD-tray) and ultra flat quench (UFQ) assembly, both of which are responsible for ensuring maximum contact of liquid and gas with the catalyst and uniform temperature control of both phases separately, as a result of excellent mixing performance regardless of quench load.

Shell Global Solutions HD-trays have been able to substantially improve performance of units by distributing gas and liquid more widely over the catalyst bed. The robustness of these distribution trays combined with a high tolerance against tray tilt and a boltless, weldless and ergonomic design, have made these trays a desirable choice for refiners in application of the second pentagon (revamps) and for new units.

Over 1500 trays have been supplied to customers in the last 10 years. Besides the HD-tray, the UFQ has been an integral component for higher severity applications like ULSD and hydrocracking units, which have a large heat release and need of uniform temperature control from bed to bed.

After implementing a strong ULSD catalyst portfolio and reactor internals technology, refiners looking to implement additional chemistries can choose from various types of upgrading that are enabled by catalyst and reactor internals technologies; to operate in the “beyond ULSD” mode. These include: ASAT, selective ring opening (SRO), “mild” hydrocracking (MHC) and cold flow improvement via catalytic dewaxing (CDW)

Aromatic Saturation (ASAT)

Refiners can choose from two catalyst plus process ASAT solution options, depending on the degree of aromatic saturation required.

Single Stage – Enhanced Aromatic Saturation (EAS)

EAS involves utilising a bed or two of a multi-bed ULSD unit for ASAT. In this scenario, all or most of the HDS and HDN reactions occur in the lead beds of reactor.

The low levels of organic sulphur and organic nitrogen in the latter beds of the reactor create an environment that is favourable for ASAT to occur. Diesel is then processed in the latter beds to meet cetane, density, and aromatics content targets for ULSD product specifications. This allows a portion of the reactor to be operated in a temperature range that is favourable for ASAT, maximising ASAT and the product property improvements associated with it throughout the catalyst cycle.

Two-Stage - Deep Aromatic Saturation (DAS)
The requirement for the first stage operation of the two-stage ASAT technology, indicated as DAS for deep aromatic saturation, is to prepare ULSD feedstock for the second stage noble metal catalyst system. Since the noble metal catalyst can be poisoned by H2S and NH3, these by-products are removed in an inter-stage stripper, and the stripped liquid is recombined with clean treat gas to complete the aromatic saturation reactions.

An advantage of two-stage ASAT units is that they can be designed for low to moderate pressure operation. The major disadvantages are that they are more complex than single stage operations and are hydrogen intensive.

Selective Ring Opening (SRO)

SRO can provide improved diesel product quality (density, aromatics, cetane) with a distillate selective reaction pathway due to the clean nature of the ULSD product. Catalyst acidity is carefully controlled to avoid full hydrocracking, which would result in excessive distillate yield shift to naphtha.

Shell Global Solutions and Criterion Catalysts & Technologies offer a wide range of solutions including naphtha hydrotreaters to process straight run naphtha and coker naphtha, distillate hydrotreaters, and sulphur recovery units - all of which form part of Shell Global Solutions’ overall Sulphur Technology Platform.

Utilising SRO for cetane enhancement will result in some shift of distillate to naphtha. The multi-functional catalysts used in the SRO system permit some naphthenic molecules to “exit” the reversible aromatic saturation loop via conversion, increasing the overall conversion. SRO catalyst systems can also deliver the required density, aromatics and cetane at ~10 per cent lower overall H₂ consumption than ASAT

“Mild” Hydrocracking (MHC)

With sufficient operating pressure and hydrogen availability, MHC of diesel feed components to gasoline can provide both an opportunity to upgrade the quality of the diesel product as well as an operating flexibility to capture any shifts in diesel and gasoline margins. MHC is different from full hydrocracking in that conversion objectives are much lower. From the product diesel perspective, this is a selective way to remove the worst molecules and generate a much higher quality product. Significant incremental improvement in aromatics, density, and cetane can be achieved with ~10 per cent naphtha production.

Cold Flow Improvement via Catalytic Dewaxing (CDW)

At low temperatures, products with “waxy” components start to crystallise and affect the flow characteristics of the product. To avoid problems of fuel supply to an engine or lubricating problems under low temperature conditions, diesel fuels and lube oils often have stringent specifications on flow properties at low temperature. Implementation of CDW provides feedstock flexibility and enables refiners to achieve cold flow property specifications, avoid expensive cold flow improvement additives and reduce kerosene blending.

Summary

In addition to the catalyst being an enabler for going “beyond ULSD”, reactor hardware and process technology improvements enable one or more upgrading chemistries to occur in a single stage, moderate pressure system, making this a cost-effective unit for producing clean distillate products. These combined catalyst and process options provide opportunities to improve refinery economics by capturing the margins for high quality ULSD over lower value fuels. Possible choices include processing of heavier and dirtier feeds, handling additional LCO, and providing crude slate flexibility.

By investing in the Sulphur Technology Platform, refiners can reach “beyond ULSD” to mitigate the effects of the sulphur paradox and capture opportunities to upgrade diesel quality (or modify yield profiles) without settling for the economic constraints of regular ULSD operations. These opportunities range from simple catalyst changes to capital project upgrades at any stage of the Pentagon Model. These fundamental examples clearly demonstrate how refiners can implement solutions to build flexibility into ULSD assets, driving opportunities to improve profitability and thrive in any economic environment.