From thermal maldistribution to cycle length extension

How Tüpraş enhanced the performance of a distillate hydroprocessing unit by cost effectively cracking more of the heavier components to diesel.

Ersev Dağ, Elif Kızlap and Enes Cındır, Tüpraş
Mbugua Gitau, Shell Catalysts & Technologies

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Article Summary

Tüpraş is one of Turkey’s largest industrial organisations, maintaining its competitive position by applying leading-edge technologies and finding innovative ways to unlock performance improvements across its refining network. Tüpraş can trace its roots back to 1955, when the Batman refinery began domestic oil refining operations in Turkey.

Under the umbrella of Tüpraş since 1984, oil refining has continued to grow at four refineries located in Izmit, Izmir, Kırıkkale, and the original Batman site. Today, Tüpraş is the 30th largest refining company in the world and the seventh largest in Europe, operating some of the most complex assets in the Mediterranean region. With a total refining capacity of 30 million tonnes, Tüpraş has a 53% total share of the Turkish petroleum products market.

Tüpraş has an ambitious strategic energy transition plan, part of which is the goal to become carbon neutral by 2050. This refiner plans to dedicate a large proportion of its investments to the production of new energy sources, such as sustainable aviation fuel (SAF) and green hydrogen. Under the plan, refining will continue to contribute significantly to earnings, and refining operations will be run more profitably to fund the transition.

So, when Shell Catalysts & Technologies’ ongoing monitoring identified high radial temperature differentials in a distillate hydroprocessing unit (DHP) at the 5 Mt/y Kırıkkale facility, the refiner took immediate action. Thermal maldistribution is a common issue in fixed-bed reactors that can lead to reduced cycle lengths and severely affect a refiner’s profitability. It can often be a major challenge to resolve.

About Kirikkale refinery
Kırıkkale refinery is in the Central Anatolia Region of Turkey, near Ankara (see Figure 1). A variety of crudes arrive by pipeline from the Ceyhan terminal on the Mediterranean coast. For example, in 2021, Tüpraş purchased 21 different types of crude oil from 11 countries, including Turkey, in gravities ranging from 20 to 47 API, mostly with high sulphur content (see Figure 2).

The refinery, which has a 5.4 million t/y capacity and  a Nelson complexity index of 6.32, was commissioned in 1986. Since then, it has met the demand for petroleum products in Central Anatolia, eastern Mediterranean, and eastern Black Sea regions. Over the years, the refinery has developed to mid-level complexity, by Mediterranean standards, with the addition of a hydrocracker complex in 1993 and, subsequently, isomerisation, diesel desulphurisation, and CCR reformer units.

Identifying the issue
The 4,500 m3/d DHP unit at Kırıkkale is the facility’s most valued asset. Commissioned in 2008, it is operated to maximise diesel production from variable heavy feeds. Turkey is a net importer of diesel, so the product holds regional value, and a shorter cycle length would have a major economic impact on the refinery.

The DHP unit had been running since a turnaround in 2015, following a catalyst change, but retaining the original internals from the initial commissioning. Using the instrumentation available on the Tüpraş reactor, Shell Catalysts & Technologies was able to track the unit’s long-term performance using its proprietary CatCheck* monitoring tool, which is a knowledge management system that enables Shell’s experts to help the unit technologists by building a picture of the unit and the way that it is performing.

By studying the information gathered and comparing it with an extensive process database, CatCheck can help with the technologists’ efforts to control the conditions of their unit by continuously optimising performance or quickly troubleshooting any process problems. During this cycle, an issue with gas and liquid distribution was identified. The unit comprised pretreat, cracking, and post-treat catalyst sections, and the problem involved a high radial temperature differential (DT) measured across the cracking bed. The cracking reactor is shown in Figure 3.

Figure 4 illustrates a comparison of the radial to axial DT ratio in the lower catalyst bed of the cracking reactor. For the run starting in 2015 (plotted in blue), inconsistencies are seen throughout the cycle, compared with the cycle starting in 2018 (plotted in red). The high outlying values, as seen in the 2015 cycle, can necessitate unit shutdown due to maximum operating temperature limits being reached.

The measured thermal maldistribution suggested suboptimal catalyst utilisation and poor vapour-liquid distribution throughout the cracking bed, which has the important role of shifting the feed’s higher T-95 value into line with the lower T-95 specified for a diesel product. The DHP reactor takes a three-component feed of heavy diesel, light vacuum oil, and light diesel from a crude distillation unit. By cracking more of the heavier components, a bigger cut of the heavier fractions can be economically converted to diesel.

In a DHP reactor cracking bed, temperatures naturally increase axially, from the top of the catalyst bed to the bottom, due to the exothermicity of the reactions occurring. At the same time, there should be an even radial temperature distribution at any given depth within the bed. The hotspots identified suggested channelling and uneven catalyst wetting, causing over-cracking, which, in turn, reduces diesel yield. Poor catalyst utilisation can also mean running at higher temperatures, which drives up energy costs and shortens cycle life.

Working on the solution
The refinery’s process engineers worked with Shell Catalysts & Technologies to investigate the cause. Analysis and interpretation of video footage taken during the 2015 turnaround helped them to identify flaws in the existing internals. This provided a compelling case for changing the internals at the next turnaround. In 2018, the refinery removed the existing hardware, installed Shell’s latest-generation internals and optimised the catalyst system.

Reactor internals from Shell Catalysts & Technologies include Shell HD trays and ultra-flat quench (UFQ) interbed devices. Shell HD trays (see Figures 5 and 6) provide high uniformity of vapour–liquid distribution, thus addressing the issue of poor catalyst utilisation, which can lead to undesirable radial temperature maldistribution.
Shell UFQ interbed internals (see Figure 7) achieve ultra-uniform temperature distribution in the reactor by promoting uniform gas–liquid quench mixing and redistribution between catalyst beds. These conditions are imperative for optimum catalyst utilisation in the bed below. The UFQ internals integrate fully with the HD trays and help to reduce the radial temperature gradient, typically by a factor of three to five, compared with a conventional mixer.

Results and value added
The results have been dramatic. In February 2022, at the end of a stable four-year cycle, the catalyst was changed, and operational strategy has altered such that further four-year cycles are planned for the future rather than the traditional three-year cycle.

Although the unit at Kırıkkale was designed for 4,500 m3/d, Tüpraş has increased that throughput by 5% to 4,725 m3/d. The new feed rate was sustained throughout the cycle, as seen in Figure 8, and the Covid pandemic had minimal effect on feed rate and operations.

Additionally, the new internals have improved operational flexibility for both the DHP unit and the crude distillation unit in terms of heavy diesel distillation. With good internals, lower differential pressure was observed, which also led to reduced compressed charge gas cycling.

Tüpraş views the internals changeout project as a complete success. A key part of this was the excellent preparation and clear definition of the field operations required before the work took place. Because of this preliminary communication, Tüpraş did not encounter any unforeseen problems during the installation period.
By defining the problem and quickly taking action to solve the issues, the partnership led to a successful project that improved reactor stability and safety on the site.

Against a backdrop of extensive fieldwork, the project shows how excellent planning and cooperation within a trusted partnership led to a smooth shutdown with minimal downtime.

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