Optimising the sulphur balance

Product specifications continue to tighten around the globe and force refiners to search for cost-effective strategies to reduce the sulphur content of their products.

In Western Europe, North America and Japan, the allowable sulphur content of diesel has decreased from 500 to 10–15 ppm in recent years, and emerging economies such as India and China are converging on similar specifications. And, even if a refiner’s local market does not mandate ultra-low-sulphur products, its export markets are likely to; many Middle Eastern refiners have had to retool their facilities because they provide significant product volumes to European markets.

This trend is not restricted to road transport fuels. Marine fuels specifications are also becoming more stringent; a mandated staged reduction in the allowable sulphur content of maritime fuels will mean a drop from 1.5% to less than 0.5% by 2020.

“In most cases, solutions need to be customised to the refiner’s specific hardware configuration and business needs and objectives,” says Yvonne Lucas, Licensing Technology Manager, Shell Global Solutions International BV. “Moreover, numerous process facilities are likely to be affected, so an integrated, well-reasoned plan is essential.”

Take a refiner that needs to reduce the sulphur levels in its diesel from 50 to 10 ppm. This will usually require a revamp of the hydrodesulphurisation unit, which will involve replacing the existing reactor internals with state-of-the-art reactor internals for improved catalyst utilisation and using a more active catalyst.

“However,” says Lucas, “the move from 50- to 10-ppm sulphur requires twice the original catalyst volume, so an additional reactor will often be required; the higher concentration of hydrogen sulphide in the reactor section will necessitate the wash-water facilities being upgraded; and a high-pressure amine system in the recycle gas will be essential to limit the additional catalyst volume. As a result, the amine treating and sulphur recovery units will also need to be checked and possibly revamped.”

Often, the changes in feedstock slate exacerbate these issues. With the diminishing supplies of sweet crude, more and more refineries are having to process heavier, dirtier and sourer feed-stocks. The high acidity of sour crudes can lead to corrosion problems during processing if not carefully managed.

Processing heavy crudes also places special demands on catalysts because these oils contain higher concentrations of harmful metals such as nickel and vanadium, and their high nitrogen content can poison the catalyst’s active sites and possibly result in loss of activity. “A refiner processing such a crude may also need to revamp its high-vacuum unit to cut deeper into the barrel; deep-flash technology could be especially valuable here,” says Lucas. “The organisation would need to improve or expand its residue upgrading facilities because the additional sulphur requires greater bottoms management.”

She continues, “The post-treatment units would probably also require a revamp to deal with the inevitable increase in hydrogen sulphide production. Moving from 1–2% sulphur in crude requires the capacity of the gas/liquid treating and sulphur recovery facilities to be doubled.”

The trend towards ultra-low-sulphur fuels seems destined to continue until it is a global reality. Fortunately, there is a wide range of technologies and process configurations that refiners can adopt and tailor to their own business situation. These range from short-term options that may also help to increase margins through to long-term options: no-regret, phased-investment solutions that aim to keep the facility well positioned for even more stringent specifications.

“This is a complex area that requires complex responses, but there are also significant opportunities,” says Lucas. “The smart refiners are finding ways to build in the flexibility to process a greater range of crudes or to future proof their operations against further product specifications by enhancing their capabilities."

Minimising emissions from the refinery complex

Producing the clean fuels that society is calling for is only part of the story. A robust strategy is also required to manage the sulphur that was removed from the crude oil during processing.

This is predominantly in the form of hydrogen sulphide and it is, of course, unacceptable to emit this to atmosphere or to send it to an incinerator and emit the resulting sulphur dioxide to atmosphere. To comply with strict environmental regulations, refiners must convert the sulphur species either to a by-product that can be used, such as elemental sulphur, or to a non-hazardous waste.

Compliance will be increasingly challenging for refiners, but before making new investments or wholesale operational changes, Pervaiz Nasir, Regional Manager Gas/Liquid Treating and Sulphur Processes, Shell Global Solutions (US) Inc., urges operators to consider whether they are getting the maximum use out of their existing refinery amine treating system.

Amine hygiene can be a good place to start, he says. “Most gas streams contain some contaminants that degrade the amine over time and form, for example, organic acids. These organic acids tie up the amine molecule and, as a result, the amine will be less effective.”

To restore the system to its full capacity, these contaminants need to be removed. Nasir also advises refiners to confirm that the treating solvent, the amine molecule and the amount of amine circulating solution are those best suited to picking up the acid gas components.

From that base, refiners could consider enhancing the system capacity by changing the solvent during a turnaround. Many of the new amine solvents available today are more energy-efficient and have higher capacity. Nasir cites the example of a plant in the USA that was using diethanolamine as the solvent. Its inlet gas rates were set to increase by up to 35%, so Shell Global Solutions staff studied the facility and recommended changing the refining process to the Sulfinol®-X process.

“The plant was able to boost its capacity by 30% and save energy for regeneration without making any hardware changes,” says Nasir. “It also reduced the circulation rate owing to the high capacity of the Sulfinol-X process and the smaller heat of regeneration requirements.”

Optimising the amine system was also a priority for another refiner when it was adding a new hydrotreater. It required amine for the recycle hydrogen scrubber in the new unit, and wanted the same amine for use in other parts of the refinery. The facility still needed to meet the sulphur specifications on the low-pressure fuel system.

After a thorough evaluation, the refiner switched from a monoethanolamine process to the Shell Global Solutions ADIP^®-M process, which has an acid-aided regeneration additive to keep the hydrogen sulphide level in the fuel gas low. “This helped to dramatically increase the sulphur handling capacity from 167 to 290 tonnes a day,” says Nasir. “It also meant that amine could be supplied to the new hydrotreater recycle gas scrubber and the low fuel gas hydrogen sulphide specifications could be met without adding a whole new unit.”

Conventional Claus, Super-Claus and Euro-Claus units can take a refinery up to about 99.3% sulphur recovery. However, if a refinery needs to go beyond this, it would have to add tail-gas treating, as the tail gas coming from the sulphur plant contains many sulphur species.

Tail-gas treating converts the various sulphur species in the tail gas to hydrogen sulphide, which is then picked up by an amine system and recycled to the front end of the Claus unit. The SCOT^® Shell Claus offgas treatment unit is a low-sulphur, low-temperature tail-gas-treating unit that enables refiners to achieve sulphur recovery levels of over 99.9%.

“Emissions regulations are only going to become tighter,” Nasir concludes. “And the traditional approach to managing sulphur with point solutions is likely to be insufficient. An integrated approach that takes into account the hydroprocessing and conversion processes, and the gas-treating and absorption solutions can help to optimise the sulphur balance across an entire refinery complex.”