Beyond ULSD

With the safe, reliable production of ultra-low-sulphur diesel (ULSD) now increasingly commonplace, Nick Flinn, Projects Commercial Manager, CRI/Criterion Catalyst Co. Ltd, is working with refiners to leverage the ability of their distillate hydrotreaters to upgrade diesel quality further and to capture additional margin.

“The days of producing just ULSD in the distillate unit are over,” he says. “The ability to produce diesel containing 10‑ppm sulphur is no longer a differentiator in most markets. Progressive refiners are pushing this asset so that it can produce even better products and consume more difficult feedstocks while still achieving the very stringent specifications that the market mandates.”

Although the distillate hydrotreater has not traditionally been a significant source of profit in a refinery, the situation has altered in recent years owing to changes in the distillate supply and demand balance, crude costs and clean fuels legislation.

The technical challenge centres on how to maximise diesel volumes while meeting specifications for sulphur, boiling point, density, cetane and cold flow properties. Fortunately, the ULSD process provides an ideal platform for upgrading diesel quality further. It offers a rich hydrogenation environment and a product that is free from most sulphur and nitrogen species: factors that promote aromatic saturation, ring opening, isomerisation and cracking reactions.

“This means that there is a range of upgrading opportunities,” says Flinn. “For instance, we can improve cetane number or reduce density, improve cold flow or reduce aromatics. In fact, the distillate hydrotreater can be debottlenecked relatively easily.”

Key to this are the latest-generation catalysts. “There have been some incredible developments in catalyst technology,” he explains. “Typically, only 50–75% of the previous catalyst amount is required with the latest generation of products, so that freed volume provides opportunities for refiners. We could simply fill it with more catalyst in order to process more barrels or more-difficult feedstocks, but the latest thinking on catalysis chemistry has unlocked valuable opportunities for upgrading.”

Another key enabler is advanced reactor internals such as ultra-flat quench interbed internals. These are extremely flat, so the catalyst volume in the reactor can be expanded. Quenches have typically been used in higher severity operations such as hydrocracking, but they are increasingly seen in the ULSD unit to control the temperature, optimise the gas/liquid mixing and minimise the catalyst deactivation.

One of the techniques that the recent advances in catalysts and reactor internals facilitate is single-stage, enhanced aromatic saturation. This is a drop-in solution that can be used to improve both density and cetane number.

It involves using a catalyst bed or two in a multi-bed ULSD unit. All or most of the hydrodesulphurisation and hydrodenitrification reactions occur in the lead beds of reactor. The low levels of organic sulphur and nitrogen in the later beds of the reactor create an environment that is favourable for aromatic saturation to occur. The ULSD is then processed in these beds to meet cetane, density and aromatic content targets.

Flinn explains: “Enhanced aromatic saturation works in a slightly different temperature regime to ULSD production, so, with good quench technology and a good catalyst, we can take the bottom part of the bed and reduce the temperature. That enables it to operate in the aromatic sweet spot where we can convert almost all of the di-aromatics and some of the mono-aromatics, which is a more difficult reaction to facilitate.”

For specifications that require extremely low aromatic levels, two-stage aromatic saturation may be more appropriate. The first stage prepares a ULSD feedstock for the second-stage noble metal catalyst
system. As the noble metal catalyst can be poisoned by hydrogen sulphide and ammonia, these by-products are removed in an inter-stage stripper, and the stripped liquid is recombined with clean treating gas to complete the aromatic saturation reactions.

The advantage of this configuration is deeper saturation and an improved yield profile. “Noble metal systems result in a very high level of aromatic saturation, even for mono-aromatics,” says Flinn.“The amount of light cycle oil upgrading that can be achieved through this type of operation is limited only by the aromatic precursors in the feed and the unit’s hydrogen supply.”

He cites the example of a refiner that wanted to increase ULSD production volumes and maximise the light cycle oil it feeds into the diesel pool across the entire cycle.

Towards the end of the run, the temperature regime moved outside the aromatic saturation sweet spot and throughput had to be reduced to maintain the cetane number. So a moderate revamp, including the addition of Shell Global Solutions High Dispersion trays and ultra-flat quench internals, enabled the refiner to treat its entire light cycle oil slate. The initial investment was small but it enhanced the refiner’s bottom line by $3 million a year.

Capturing Market Opportunities

Solutions involving aromatic saturation, ring opening, isomerisation and cracking reactions can significantly improve overall refinery economics by leveraging the critical ULSD unit to provide the flexibility to upgrade heavier feeds to make more diesel; maximise light cycle oil for the on-road diesel pool; and provide the ability to process more difficult crudes. But, counsels Flinn, such solutions do not come off the shelf: “These are highly customised options that require the catalyst and hardware providers to work closely with the refiner to design a solution that is optimised to meet specific objectives.

“With the right catalyst and process technology improvements tailored to the application, refiners can move beyond ULSD to improve overall unit economics and capture the flexible margin that is available today,” Flinn concludes.