Capturing value through integrated improvements at the FCC-PT–FCC complex
Introduction: the importance of fcc-pt–fcc unit integration. As they strive to remain competitive in the face of changing market dynamics, many refiners are looking for ways to extract even more value from their units.
Kevin Carlson and Todd Foshee
Shell Catalysts & Technologies
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However, as the pressure on margins and utilisation intensifies, it is no longer enough for technologists to consider their units in isolation, especially when it comes to the fluidised catalytic cracking pretreatment (FCC-PT) and fluidised catalytic cracking (FCC) units.
Although these units are often in different operating groups, have different hardware, experience different issues and require different expertise, there is growing realisation across the industry that they should be considered as an integrated complex. This is because the quality of the feeds the FCC-PT unit provides can have a major impact on the performance of the FCC unit. It affects the FCC unit’s coking tendency and feed crackability and conversion; removes metals poisons such as nickel (Ni) and vanadium (V) from the feed; and reduces the sulphur oxide emissions from both the FCC regenerator and the product slate.
Consequently, by running these two units as a single, optimised operation, operators can improve the overall refinery’s profitability, extend their units’ cycle lives and improve their reliability. In this paper, we examine some of the key aspects to consider.
How the fcc-pt unit improves feed quality to the fcc unit
In many refinery operations, the FCC unit is the primary conversion engine for vacuum gas oil boiling range materials as it yields smaller, lower boiling molecules, particularly gasoline, distillates and liquefied petroleum gas, that have a higher value as transportation fuels and petrochemical feedstocks.
The primary desired FCC reaction is acid site catalysed cracking where beta scission of paraffinic C–C bonds primarily forms lower-molecular weight olefins. Other FCC reactions that contribute to the yield slate include isomerisation, alkylation, cyclisation, dehydrogenation, hydrogen transfer and thermal cracking reactions: the dehydrogenation and thermal cracking reactions being the least desirable.
Hydrotreating the feed in an FCC-PT unit can help to reduce less desirable FCC reactions and significantly increase the value that the FCC unit can unlock. This is because, as shown in Figure 1, the FCC-PT unit:
• removes sulphur, which helps to reduce the amount of sulphur species in FCC products and sulphur oxide emissions from the regenerator;
• removes nitrogen, which improves conversion (because nitrogen is a temporary poison to the acid site cracking) and can reduce the nitrogen oxide emissions from the regenerator;
• captures metals, which lowers the fresh catalyst make-up rate, improves conversion and reduces dry gas formation; and
• saturates aromatic species, which reduces the aromatic content of FCC products and improves conversion or, at the same conversion rate, reduces the air blower demand because of lower coke make.
Essentially, the goal of the FCC-PT unit is to decrease the constraints that the FCC unit faces. To capture maximum value, it is vital to consider the operation of the integrated complex, rather than just of one unit in isolation.
To develop the ultimate integrated solution, Shell Catalysts & Technologies leverages a variety of tools, techniques, technologies and tactics to help refiners achieve their objectives. These include:
• FCC-PT catalysts. We will work with you to design a catalyst system that is optimised to your feeds, operational constraints and strategic objectives.
For more on this, see Section 4.
• Licensed technologies. Our extensive portfolio includes FCC hardware such as feed nozzles, riser internals, cyclones and third-stage separator technologies, as well as technologies for the FCC-PT unit such as reactor internals.
For more on our licensed FCC hardware, see Section 5.
• Advanced modelling to drive technology improvements with Shell’s FCC kinetic model (SHARC).
For more on this, see Section 6.
An FCC-PT unit enables the economical processing of difficult feeds that contain not only sulphur, nitrogen and aromatics, but also catalyst poisons and foulants. The catalyst system must be designed to manage bed fouling and catalyst deactivation from metal poisoning (especially nickel and vanadium) and other poisons (for example, arsenic (As) and silicon (Si)) while providing a main bed catalyst tailored to provide the required performance.
For most FCC-PT units, as shown in Figure 2, the catalyst system combines a top layer of grading catalyst to protect from foulants, a layer of high-performance demetallisation catalysts to mitigate the impact of catalyst feed poisons, and a high-activity FCC-PT main bed catalyst (or a combination of catalysts) tailored to meet the unit’s performance goals.
Mitigate fouling with the OptiTrap portfolio of grading catalysts
The value in an FCC-PT unit is the reduction of nitrogen and aromatics, which drives FCC conversion gains and sulphur reduction to help meet environmental requirements. However, the push to upgrade challenging feeds can lead to feed contaminant fouling, which can reduce performance and shorten cycle life.
Fouling is a common and troublesome problem in hydroprocessing units, and most refineries are seeing more fouling because of the trend towards processing increasingly difficult feeds.
Fouling causes unwanted pressure drop increases that limit the performance of many hydroprocessing units. The consequences can include short run lengths, unplanned downtime with associated costs, unused catalyst activity, increased maintenance requirements and lost revenue.
Top-bed grading helps to mitigate fouling. The OptiTrap catalysts in Shell Catalysts & Technologies’ SENTRY* line feature various shapes and sizes of bed grading that are used to spread out the foulant (see Figure 3). This process is designed so that no layer of bed grading becomes overwhelmed or underutilised. Our technology, which is based on cold-flow research, enables us to evaluate the size and shape of bed- grading materials to tailor the solution to a specific unit’s characteristics and to extend its run length.
Because the fouling challenge is highly complex and depends on the type of feed, the upstream processing and the unit operating conditions, we also offer additional services and technologies, including:
• root-cause analysis methodologies;
• feed particulate characterisation;
• pilot plant grading optimisation;
• a deep-bed filter system approach to grading materials; and
• reactor internals solutions.
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