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Optimising enterprise economics
How a catalyst selection study for Pernis refinery’s hydrocracker also benefited the neighbouring Moerdijk petrochemical plant.
The scope of a hydrocracking catalyst selection study is usually confined to improving the unit’s kerosene and gas oil output, the level of conversion and the cycle length.
A recent study at Shell’s Pernis refinery in the Netherlands, however, had another level of complexity because the unit’s technologists were concerned about the potential for adverse affects on Shell’s nearby Moerdijk petrochemicals facility.
Pernis’ 9,000-t/d hydrocracker is a once-through, single-stage, stacked-bed unit.
It generates a product slate that includes Jet A1 and ultra-low-sulphur diesel.
It also yields hydrowax (unconverted oil) that can be sent to the refinery’s fluidised catalytic cracker but is preferentially routed to the 3,800-t/d steam cracker at Moerdijk, which is 35 km southeast of Pernis.
Jeroen Groenhagen, Senior Technologist, Shell Pernis, was responsible for the catalyst selection study and, later, for loading and starting up the hydrocracker after the new catalyst package’s installation.
He says, “We wanted to evaluate installing a completely new cracking catalyst from Criterion Catalysts & Technologies called Z-FX10, which uses a special zeolite technology designed to increase middle distillate yield, product quality and catalyst stability.”
However, the hydrogen content of the hydrowax was a particularly important parameter for Moerdijk because it has a big impact on the ethylene and pitch yields, and the furnace run length.
If the new catalyst had altered the hydrowax’s hydrogen content, this might have adversely affected Moerdijk’s economics.
“The lower the hydrogen content in the hydrowax, the more easily the furnaces in the steam cracker coke up,” Groenhagen explains.
“Even the slightest deterioration in hydrowax quality can have a huge impact on ethylene yield, furnace run length and coke make.
We really needed to understand that parameter’s behaviour over the run length of the catalyst and what it would mean in margin or dollar terms.
“Because market economics change all the time and there is such a wide range of catalysts available, it is very important that one makes a detailed study of what package to select,” he continues.
“Are any changes to product specifications anticipated?
What does the feed look like?
Are any market changes expected?
There is a lot of complexity owing to the number of variables and the uncertainty associated with those variables.”
Early engagement with the numerous stakeholders, such as the Moerdijk steam cracker technologists and process and catalyst experts from Shell Global Solutions and Criterion, was crucial, Groenhagen says, because much depended on their cooperation.
In addition, as this was the catalyst’s first commercial application, he was keen to verify how it would perform in the Pernis hydrocracker and with Pernis’ specific feed, which is relatively heavy with high nitrogen and metal contents.
A full pilot-plant testing programme was commissioned at the Shell Technology Centre in Amsterdam, the Netherlands, to compare the performance of the novel cracking catalyst with the existing catalyst.
Shell and Criterion often recommend pilot plant tests whenever there is anything non-standard about an application, such as an unusual feed or a novel catalyst.
Taking such an integrated approach added substantial complexity to the catalyst selection process, but it also delivered enormous value for the overall enterprise.
“This was important because it gave us confidence that the catalyst package would deliver what it was designed to,” says Groenhagen.
“It also verified the quality of hydrowax that we would be sending to Moerdijk.”
The results of the pilot plant tests fed in to the design of the catalyst package.
In the pretreatment section, where the impurities that could poison the sensitive cracking catalyst are removed, Criterion designed a combination of hydrodemetallisation, hydrodenitrogenation and hydrodesulphurisation catalysts.
Criterion carefully tuned the package.
Performance is continuously monitored by site staff and Shell Global Solutions technologists to help ensure that the unit achieves the targeted cycle length and quality.
Nine months into the cycle, the new catalyst had performed very well.
It has helped Pernis to maximise its middle distillate yield: it now makes more kerosene and diesel than before, and less naphtha while operating at a similar conversion severity.
Crucially, the petrochemical plant has also benefited.
The catalyst selection study enabled Pernis’ technologists to optimise the hydrowax quality specifically for Moerdijk’s processes, which has helped to enhance ethylene yield and maximise the furnace run length.
“Taking such an integrated approach added substantial complexity to the catalyst selection process, but it also delivered enormous value for the overall enterprise,” Groenhagen concludes.
“We have calculated that it has enhanced the combined economics of Pernis and Moerdijk by some $5 million a year.”