amorphous- and zeolite-based catalyst systems in the second stage. For the second stage, an amorphous cata- lyst solution typically starts at 12-15°C higher than a zeolite-based solution (as indicated by T ref ). To get a 3-4 wt% higher naphtha yield, an increase of 11°C is required for the amorphous solution, whereas an increase of only 6°C is required for a zeolite-based solution. Since the con- ventional, amorphous catalyst system already operates at a higher temperature than the zeolite system, a further increase in temperature to enhance naphtha yield pushes it into the zone where HPNA formation starts. This is indi- cated by the red zone in the figure, which is an area of unfa - vourable HPNA content. Operations overview During catalyst loading and start-up, Topsoe provided on- site technical service, working with the refiner to preempt possible challenges. Given that the turnaround happened amidst a raging Covid-19 pandemic, providing on-site tech- nical service required additional planning and cooperation. After starting up the HCU with Topsoe catalysts in 2021, a performance test run was conducted at the design con- version. The unit was stabilised prior to the test run and during the evaluation period. The test run was performed successfully, and start-of-run guarantees for Topsoe cata- lysts were met. One year into the cycle, the benefits of initial cooperation and continuous engagement thereafter have demonstrated the following benefits: The unit is limited by close heat integration. For example, if there is high radial temperature in the first-stage hydro - cracking reactor, first-stage conversion becomes limited. In the current cycle, there is good flow distribution, which has helped improve the performance of the entire unit. HPNA content in recycle oil is under control. This comes from balancing hydrocracking activity between the stages, allowing increased conversion in the second stage while staying below the aromatic saturation equilibrium cross- over temperature. Performance monitoring is profit oriented. During a period in 2022 when the market for naphtha as petro- chemical feedstock was more favourable than the market for middle distillates, heavy naphtha yield was increased by shifting hydrocracking conversion from the first stage to the second stage. As the effects of Covid-19 related lockdown eased up and market demand for transport fuels recovered, the refiner decided to conduct a test run at 110% throughput. Catalyst performance remained in line with expectations at the higher throughput. Demonstrated performance T opsoe has been working in close collaboration with the tech service team from the refinery, providing hydrocrack - ing expert services since the onset. In addition to providing 24/7 on-site support during catalyst loading and start-up, Topsoe has tested feed, product, and intermediate samples from the HCU in its R&D facility at Ravnholm, Denmark, analysing for metals, HPNA content, and so on. Test
Tref + 26˚C
Tref + 15˚C
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Base -1.0–1.5% Base
Max MD mode
Maximising Naphtha mode
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Topsoe
WABT Conventional
WABT Topsoe
Figure 11 Representation of flexibility in product slate for two different catalyst systems
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Figure 12 Middle distillate yield since start-up. Middle distillate yield is lower when the unit is in naphtha mode
Improve middle distillate product properties Optimise second-stage conditions to control HPNA make by: a. Optimising hydrocracking activity to overcome the limit of aromatic saturation equilibrium b. Maintaining temperatures lower than aromatic satura- tion equilibrium Flexible product slate – shift product yield to max naph- tha seasonally Flexibility in operation – allow shifting conversion to the second stage when the first stage is limited . Figure 11 shows the naphtha-yield response of
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2nd stage WABT (represented as BASE+X˚C) Max MD mode Maximising naphtha mode
Total naphtha
Figure 13 Increase in naphtha yield during naphtha-mode operation. The baseline is the naphtha yield in mid- distillate mode
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PTQ Q3 2022
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