December 2022

Special Focus: Catalysts

Partners maximize profitability during the pandemic

The COVID-19 pandemic can be viewed as one of the most challenging periods in the history of the oil and gas sector. Refineries worldwide faced difficult times operating at their turndown capacities or even temporarily shut down units at the refinery.

Zhan, B.-Z. , Advanced Refining Technologies (ART)/ Chevron Lummus Global (CLG); Fritz, J. , Chevron Lummus Global; Somers, D. , Advanced Refining Technologies

The COVID-19 pandemic can be viewed as one of the most challenging periods in the history of the oil and gas sector. Refineries worldwide faced difficult times operating at their turndown capacities or even temporarily shut down units at the refinery. These unforeseen circumstances have put significant pressure on refinery volumes and margins to survive in this challenging market environment, forcing refiners to improvise. Unprecedented quarantines and lockdowns imposed due to COVID-19 had a serious effect on fuel demand and oil prices.

Since hydrocrackers are extremely flexible, they play a vital role in optimizing refinery economics. Hydrocrackers convert the bottom of the barrel, upgrading feedstocks into high-value fuels, lubricants and chemicals, which boost the value of refinery product slates. On top of adapting to cyclical market demands and the more stringent environment IMO regulations in 2020, refiners needed to act swiftly to remain competitive. The recent decline in demand forced refineries to run their hydrocrackers more economically by changing to new operational strategies to meet market demand.

Preemraff Lysekil case study

Preemraff Lysekil is a conversion refinery consisting of an atmospheric crude distillation unit (CDU), a vacuum crude distillation unit (VDU), a visbreaking unit (VBU), a mild hydrocracking unit (HCU), a fluid catalytic cracking unit (FCCU), a hydrodesulfurization unit (HDS), a catalytic reforming unit (CRU), an isocracking unit (ISO), and LPG units in addition to their hydrotreating units ( FIG. 1 ).

FIG. 1. Refining process at Preemraff Lysekil.

The product range varies from LPG to heavy fuel oil. One diesel product is called MK 1 Diesel, which is mainly applicable to the Swedish market, and has a specification on aromatics that must be < 5 vol%.

The Chevron Lummus Global (CLG)-designed Preemraff single-stage liquid recycle (SSREC) unit design ( FIG. 2 ) and the ART-designed catalyst system enabled the Preemraff refinery to adapt to changing market conditions swiftly during the COVID-19 pandemic to maximize profitability while continuing to operate safely within design limitations. This article will discuss how that optimization of the SSREC hydrocracking operation gives refiners opportunities to improve performance in these dynamic circumstances. With CLG as the process licensor and ART as the technical service provider and catalyst vendor, the two companies’ combined design and catalyst expertise created an opportunity to optimize the unit operation and maximize profitability.

FIG. 2. Simplified Preemraff ISO unita SSREC configuration.

The CLG-designed ISO unit a was designed in 2001 and built in 2004 to tackle the new environmental legislation in the EU, which set the maximum sulfur content in gasoline and diesel to 10 ppm. The unit was originally designed to process 385 m 3 /hr of a mixture of heavy atmospheric and vacuum gasoils in a once-through configuration with 50 vol% conversion to make a range of products, including full-range naphtha, kerosene, diesel and unconverted oil (UCO) for FCC feed. The unit design also included an integrated 165-m 3 /hr distillate unit b to make low aromatic content, full boiling range diesel. Unit turndown was targeted to achieve operation at 60 vol% of the minimum design case in each section. The unit operation was subsequently changed to a single-stage recycle mode using the existing lineups for startup circulation to improve middle distillate selectivity.

The UCO from the hydrocracker is used as feed to the FCCU. At first, the plan was to use a UCO flow corresponding to 50 vol% conversion of the maximum hydrocracker feed capacity. This amount of UCO proved to be challenging to process in the FCCU because of the low coke production, so the demand for UCO dropped. This made it possible to use other catalysts in the hydrocracking unit and change the unit operation from single-stage once-through (SSOT) to SSREC to increase the conversion. By only changing the catalysts provided by ART, the unit increased conversion rates and no other investment was required.

Before the 2020 pandemic, the unit ran with high throughput at a typical conversion of about 63 vol%. During the pandemic, demand for fuels dropped, so the unit was run at minimum throughput with about 85 vol% conversion to meet the contractual volumes of diesel. In this period, the FCCU was shut down due to economics. To reduce costs even further, there was a focus on minimizing the consumption of VGO. Together with ART, Preemraff optimized the operation so that conversion could stay continuously close to the 85 vol%–90 vol% range.

Another challenge during the last couple of years has been the new IMO legislation for sulfur content in bunker fuels that has required refineries to change the crude slate from high-sulfur crudes (mainly Urals) to mainly low-sulfur crudes (Johan Sverdrup, Gullfaks and Oseberg). For the hydrocracker, this meant a reduction in sulfur content in the VGO from about 1.5 wt% to 0.5 wt% S, which was made possible through close cooperation between ART/CLG technical services and Preemraff operations.

An improved catalyst system for Preemraff

Starting from the cogel-based catalysts a in the 1960s, scientists at Chevron’s Richmond Technology Center (RTC), its global headquarters for innovation, have developed and continue to expand the family of catalysts a . This effort has been in response to the changing demand of refiners—including Chevron—to be able to process more difficult feeds, produce more and better quality desired liquid products, operate longer on each charge of catalysts, and deliver superior heavy polynuclear aromatics (HPNAs) management by chemical means.

Chevron scientists have utilized varying hydrogenation metals (from base metal to noble metals), and a combination of amorphous silica-alumina (ASA) and crystalline silica-alumina (i.e. zeolites) to customize the catalysts a for each application. 1–4

Optimization of the hydrocracking unit catalyst system, cycle-over-cycle, helped Preemraff increase flexibility further during the pandemic and maximize conversion. The previous cycle showed a surplus of activity to increase the mid-distillate yield cycle-over-cycle. As a result, a more selective system was designed for the current cycle, which enabled the client to use the extra flexibility to increase conversion further without jeopardizing the yields during the pandemic.

The luxury of the versatile and flexible SSREC design and the catalyst system enabled Preemraff to run economically during the pandemic, allowing them to continue running by adjusting feedrates to meet decreased market demand. Additionally, leveraging the catalyst system design, Preemraff operations worked with the author’s company to fine-tune operations to maximize conversion and reduce fresh feedrate due to changing market demand. Close cooperation between ART/CLG technical services and the Preemraff operations team was crucial to operate safely within design limitations and transfer to the new mode of operation.

The transitions to different modes of operation are shown in FIGS. 3 and 4 (note the significant changes in recycle and fresh feed intake).

FIG. 3. Mode of operation pre-pandemic and pandemic period limiting fresh feed intake to the bare minimum during the COVID-19 pandemic.
FIG. 4. Hydrocracking conversion levels, pandemic and pre-pandemic.

Prior to and during the pandemic

Pre-pandemic (prior to 2020), Preemraff focused on maximizing fresh feed intake and conversion while minimizing recycle, achieving maximum mid-distillate yields and securing the minimum UCO amount (130 m 3 /hr) to feed the FCCU. After 2020, minimizing fresh feed intake and maximizing conversion by maximizing unconverted oil recycle simultaneously was required to achieve maximum mid-distillate yields. Proper heat management was required to maintain sufficient heat in the first and second reactors to keep conversion going during the transition while changing modes of operation stayed within design limitations.

Another challenge the client faced was running low-sulfur North Sea crudes while maintaining sufficient heat in both reactors to ensure ongoing conversion. The challenge was to reduce fresh feed intake while running these low-sulfur North Sea crudes and simultaneously increasing unconverted oil recycle, which significantly affects the heat of reaction in the different beds in the first and second reactors. Consequently, heat management of the whole unit has changed significantly, running in this new mode of operation (e.g., running low-sulfur North Sea crudes). While reducing feedrate, special care was taken to keep conversion going until minimum design requirements were met, running all equipment downstream.

During and after the COVID-19 pandemic ( FIG. 4 ), the refinery minimized fresh feed intake and maximized recycle conversion to achieve maximum mid-distillate yields, running much more profitably. During this period, the FCCU was shut down due to economic reasons, so no UCO feed was required to feed the FCCU. Beginning in 2021, the FCCU was started up again and a minimal UCO amount was required to meet the FCCU requirements running at low capacity. Surplus activity in the 2016–2019 cycle resulted in further improvement to a more mid-distillate selective catalyst system.

In FIG. 5 , four distinct segmented periods are selected to demonstrate the operational requirements through the pandemic.

FIG. 5. Four distinct modes of operation visualized vs. the Base Case (filtered data selected).

Pre-pandemic (2016–2019):

  1. 2018: Conversion increased to utilize surplus activity left until end of the run
  2. 2019: Reactor 2 catalyst system with a more mid-distillate selective system during an intermediate shutdown

Pandemic (2020-onwards):

  1. 2020–2021: FCCU shut down during pandemic
  2. Opportunity to run the hydrocracker at maximum conversion running at a bare minimum fresh feed intake to continue to meet contractual obligations
  3. Opportunity to convert maximum UCO required as no UCO was required to feed FCCU
  4. Opportunity running different low-sulfur crude slate during the pandemic to meet IMO bunker fuel specifications
  5. 2021-present: FCCU online again; minimum UCO amount required to feed the FCCU.

These four distinct modes of operation selected (segmented data) vs. a Base Case are visualized in FIGS. 6 and 7 : ~320/65 m 3 /hr (fresh feedrate/UCO recycle rate, hereafter). Minimum UCO-flow requirement to the FCCU = 130 m 3 /hr.

FIG. 6. Operational modes of four distinct cases running at different fresh feed and recycle rates at different hydrocracking conversion levels vs. a Base Case.
FIG. 7. Operational modes of four distinct cases running at different hydrocracking conversion levels and mid-distillate gain vs. a Base Case.

Pre-pandemic: Base Case: ~320/65 m 3 /hr; fresh feedrate at 320 m 3 /hr and UCO recycle rate at 65 m 3 /hr recycle; UCO flow requirement to FCCU at 130 m 3 /hr.

  • Case 1: ~350/25 m 3 /hr; to maximize fresh feed intake with the same UCO flow to the FCCU (i.e., 130 m 3 /hr)
  • Case 2: ~325/35 m 3 /hr; Reactor 2 replaced with a more mid-distillate selective catalyst with the same UCO flow to the FCCU (i.e., 130 m 3 /hr)


  • Case 3: ~200/150 m 3 /hr; FCCU down, maximize hydrocracking conversion as UCO required to feed the FCC is not needed
  • Case 4: 200/150 m 3 /hr; FCCU online with reduced UCO flow to the FCCU at ~85 m 3 /hr.

The pre-pandemic mode of operation maximized conversion by increasing fresh feed intake while still meeting the minimal UCO feed requirement. Pre-pandemic, ART helped Preemraff to replace a second-reactor catalyst earlier than the planned turnaround. The company advised the client to optimize the temperature profile to make optimal use of the more selective catalyst system in Reactor 2, achieving additional mid-distillate gain of > 4 wt% on a fresh feed basis.

During the pandemic, the FCCU was temporarily shut down. The mode of operation was to reduce fresh feed intake to the minimum while maximizing conversion and still meeting client obligations. During the most recent cycle, which occurred during the pandemic, performance improved further by increasing conversion, enabling the unit to increase middle-distillate output by reducing fresh feed intake and running low-sulfur North Sea crudes. FIG. 7 shows that Case 3 runs at the highest conversion around 90 vol% with a mid-distillate gain of > 27 wt% on a fresh feed basis vs. the Base Case when the FCCU was down. Case 4 shows a mid-distillate gain of ~9 wt% on a fresh feed basis at a reduced conversion level of 58 vol% when the FCCU was up again, meeting the minimum UCO requirement to feed the FCCU.

Operational challenges during the pandemic

During the transition between the different modes of operation, proper heat management is required to always keep sufficient heat in both the first and second reactors to meet the desired conversion. Reducing fresh feed intake and increasing UCO recycling reduced the heat of reaction in the first reactor. Additionally, simultaneously changing to low-sulfur North Sea crudes further reduced the heat generation. A smooth transfer between the different modes of operation requires an in-depth understanding of the SSREC CLG design and ART catalysts system to transfer smoothly between the different modes of operation and respond to the reduced market demand.

Besides changes in the inlet temperature of Reactor 1 posted by reduced fresh feed and high UCO recycling, high conversion leads to the risk of HPNA accumulation and undesired secondary cracking. Only catalysts with excellent hydrogenation activity can meet this type of challenge. One of the key features of the proprietary catalysts a is the uniformly dispersed cracking and hydrogenation metals components. The linear relationship of middle distillate yield vs. hydrocracking conversion in FIG. 8 is another piece of evidence. No signs of overcracking and HPNA accumulation are observed in the conversion range from 50% to > 90%.

FIG. 8. Summary of middle distillate selectivity vs. conversion on fresh feed of all cases.


CLG’s SSREC design in combination with ART’s designed catalyst system has shown to be extremely flexible to adapt to continuously changing market circumstances by allowing the unit to be run in different modes of operation over the two cycles. Pre-pandemic, Preemraff maximized fresh feed intake, running high conversion mode while maximizing mid-distillate yield. During the pandemic, Preemraff minimized fresh feed intake to the minimum. The company ran at the highest conversion to convert maximum UCO into mid-distillates while running spot market low-sulfur North Sea crudes. Using the SSREC design combined with the catalyst system while running the low-sulfur North Sea crudes, Preemraff met the IMO bunker fuel specifications to run most profitably during the pandemic.

During the transition between the different modes of operations, the more mid-distillate selective catalyst system showed no sign of HPNA accumulation while increasing conversion level. The key to a successful change in the mode of operation was continuous two-way communication and collaboration between Preemraff’s operational team and the authors’ companies’ technical service. Unit design flexibility and access to the companies’ technical services enabled Preemraff to optimize unit performance within design limits and ensure the unit always runs safely within design limits. HP


The authors would like to express their sincere gratitude for the close cooperation and support provided by the Preemraff team in the process of writing this article. Special thanks to Mats Hörnfelt and Magnus Hernelind for their time and efforts.


a Chevron Lummus Global’s ISOCRACKING®
b Chevron Lummus Global’s ISOFINISHING®


  1. Zhan, B.-Z., L. Jiao, H. Ryu, W. Shiflett and T. Maesen, “Shepherding hydrocracking profitability,” Hydrocarbon Engineering, March 2020.
  2. Mukherjee, U., A. J. Dahlberg, J. Mayer and A. Kemoun, “Maximizing hydrocracker performance using ISOFLEX technology,” NPRA Annual Meeting, March 2005.
  3. Papon, J. M., J. Parekh, H. A. Yoon and A. J. Dahlberg, “Premcor heavy oil upgrade project,” NPRA Annual Meeting, March 2002.
  4. Zhan, B.-Z., T. Maesen, J. Parekh and D. Torchia, “Don’t be medieval, make more diesel,” Hydrocarbon Engineering, November 2013.

The Authors

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