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A step change in hydrocracker performance
How Shell Scotford refinery rose to the dual challenge of shifting to a more difficult feed while also increasing capacity.
The magnitude of the capacity increase is striking, but what is interesting is that many of the improve ments made, especially those implemented before 2002, are the result of relatively minor and evolutionary equipment modifications.
Increasing hydrocracking capacity can have a profound effect on a refinery’s economics. Operators that are interested in the possible extent of a hydrocracker revamp are advised to look at Shell’s Scotford refinery’s ScotMods project, says Wayne Munsterman, ScotMods Process Design Lead, Shell Canada Ltd. This was a remarkable initiative in which the project team unlocked a performance step change in response to compelling business objectives.This was by no means the first debottlenecking exercise on the Scotford hydrocrackers. “There had been a series of successful revamps before this one that had taken the capacity to 170% of design, so you could be forgiven for thinking that we had exhausted all of our options,” says Munsterman. “But, because the technology keeps evolving, we were able to increase the capacity further to 189% of design.”
The hydrocrackers were originally designed to process a relatively light and sweet synthetic gas oil feed: hydrotreated coker gas oil imported from Canadian synthetic crude producers Syncrude and Suncor. However, when Shell’s Athabasca Oil Sands Project came online in 2002, the refinery needed to find a way to process sour, heavy VGO from the Athabasca Oil Sands upgrader. It also needed to do this at an increased rate to match the upgrader’s output.
“This was a substantial challenge,” reflects Munsterman, who was on the revamp team. “The new hydrocracker feed was much heavier and more sour than the previous one. It also had much higher levels of nitrogen and a much higher total acid number (TAN ).”
The new oil-sands-derived feedstock is substantially more difficult to process than the former feedstock.
|Previous feed||New feed|
|Feed rate (% of original design)||170||189|
|Feed origin:||Synthetic hydrotreated coker gas oil||Sour VGO ex Athabasca bitumen|
|- Specific gravity||0.92||0.97|
|- Sulphur (w%)||0.22||3.50|
|- Nitrogen (ppmw)||900||1700|
|- TAN (mg KOH/g)||0.5 (max)||4.5|
The Athabasca Oil Sands Project extracts bitumen from oil sands in Alberta, Canada, and supplies sour, heavy VGO to Scotford’s two hydrocrackers, which are the refinery’s principal conversion units. These are highconversion, two-stage units (each with two reactors in series, with recycle to the second reactor) with fully independent high-pressure reaction sections and a common fractionation section.
As well as meeting the feed and capacity constraints, the technologists charged with responsibility for the revamp also had to ensure that the hydrocracker could achieve a minimum cycle length of 18 months while designing for optimal gasoline/diesel flexibility
The challenge for the hydrocracker was clear: a significantly higher intake of a sourer, heavier feedstock that also contained higher levels of nitrogen and had a higher TAN.
These technical challenges were met with a combination of catalyst, process and engineering solutions through the combined efforts of Shell Canada, Shell Global Solutions and Criterion.
“We faced numerous technical issues related to both the changes in the feed quality and the increased intake,” says Munsterman. “For instance, the increase in feed sulphur was dramatic. The new feed contained more than 10 times as much sulphur as the old one, which created several potential processing difficulties.”
The project team opted to introduce an integrated highpressure amine absorber upstream of the recycle gas compressor to remove hydrogen sulphide from the recycle gas. This reduced the potential pressure drop across the reactor circuit and lowered the power requirements for the recycle gas compressor, which enabled the existing compressor to be retained with only a moderate upgrade to the turbine to provide the incremental power requirements.
Careful attention had to be paid to engineering the modifications to the existing cold, high-pressure separator (CHPS) owing to the increased flow and the water separation issues this would introduce. To minimise fouling, several measures were taken, including filtration, optional hydrogen injection into the feed and intermittent water washing for chlorides.
Because of the new feed’s distinctiveness, the choice of catalyst was a very important decision. It needed to provide the required yield pattern, have high stability and minimise lightgas production and hydrogen consumption. The project teams conducted extensive process research and catalyst testing with the new feedstock to provide data that was crucial for the decision-making process. After a thorough comparison, the choice finally fell on the application of a set of Criterion and Zeolyst catalysts that delivered the best combination of activity, stability, yields and product properties.The lead reactors, originally loaded only with hydrotreating catalyst, were converted to a stacked bed design with two different catalysts: treating catalyst in the top three beds and a
nitrogen-tolerant cracking catalyst in the bottom bed. A full load of cracking catalyst was installed in the second reactors. As part of the previous debottlenecking exercises, the catalyst utilisation of the reactors was maximised by replacing the original interbed distribution trays with Shell HD trays. These
proven internals provide excellent gas–liquid distribution over the entire catalyst bed and permit increased catalyst loading by eliminating the need for inert distribution rings.
Following the hydrocracker’s complete conversion to the new feed and operation, test runs confirmed that all the revamp objectives had been met. “The product yields and properties all met or exceeded the target values,” Munsterman elaborates. “And, even at the new expanded capacity and with the more difficult feed, we achieved a 24-month cycle length for the initial operating cycle."
The scotford hydrocracker's capacity evolution
Scotford has a history of continuously increasing its capacity. The ScotMods project (see main article) took the capacity to 189% of design in 2005 (technical constraints revented further increases without major capital expenditure).
Even before the ScotMods revamp discussed in the main article, Shell Scotford’s technologists had established a long tradition of finding ways to squeeze more value out of the hydrocracker. Since it was commissioned in 1984, they had worked with Shell Global Solutions and Criterion on several programmes that, by 2005, had created a total 89% capacity increase.
“The magnitude of the capacity increase is striking, but what is interesting is that many of the improvements made, especially those implemented before 2002, are the result of relatively minor and evolutionary equipment modifications,” says Brian Denniss, Business Opportunity Manager, Scotford Manufacturing, Shell Canada Ltd.
These include, for instance, upgrading the feed pump system and feed piping, and modifying the make-up gas compressors. In addition, Shell HD trays were introduced in some sections of the reactors, and specialised internals were fitted in the high-pressure separator. Some second reactor beds were combined to increase the overall catalyst volume, and an improved filtration system was installed in the feed system. A real-time optimiser to run the hydrocracker on the basis of economics and scheduling targets was also instituted.
“At each and every turnaround, we look for investment opportunities to improve our capabilities in some way. Over the years, this has helped us to increase capacity, handle a more difficult feedstock, save energy and improve the profitability of the unit,” explains Denniss.
Shell Scotford’s approach is therefore aligned with the Shell Global Solutions Pentagon Model. The cash generated by these operational improvement projects (which are Pentagon I initiatives) helped to fund a larger revamp that achieved a step change in operational and financial performance (a Pentagon II project). The result: a thriving, pacesetting enterprise.