Question
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Besides improved catalyst systems, what advances in reactor internals are improving efficiency and throughput while also mitigating the effect of fouling and catalyst poisons?
Mar-2023
Answers
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Andrew Layton, KBC (A Yokogawa Company), andew.layton@kbc.global
Since 1990, flow distribution has become increasingly important as product quality moves to ppm levels. Distributor tray design has changed to mitigate the impact of levelness and flow rate issues using better chimney design. Adding spray nozzles to the trickle bed flows has improved coverage. Distributor trays often lose effectiveness over time due to thermal cycling, poor gasketing, poor construction, and few sites following optimum checking procedures during changeout. This situation has led to poor performance and activity losses exceeding 50%, often a bigger effect than changing to a better catalyst.
The design of the beds has also been improved to optimise mass velocity and bed height. For example, increasing bed depth will cause some loss of good distribution as no bed is perfectly loaded, and the impact worsens as the bed gets longer. The bed loading procedures have improved with the advent of faster, more effective dense loading machines.
New unit designs take turnaround and loading needs into account, including improved distributor tray access and, sometimes, more direct access to each bed rather than top loading only. This is also a better option from a safety viewpoint. In addition, the newer units no longer use interbed dump tubes, which created maldistribution issues. A variety of approaches are now used to mitigate fouling issues in the reactor, including:
• Separate foulant collection trays
• Multiple bed grading with differing sizes, higher surface area, high metals capacity
• Scale traps, or surface area enhancers, can be justified in severe cases if they increase surface area in the right spot with modified design and size, depending on the foulant
• Bypass devices, which alleviate pressure drop by permitting partial bypass as pressure drop builds.Note that tackling a fouling problem in the reactor is a mitigation instead of a solution to the root cause. To improve operations, the following upstream factors should be examined:
• Analyse and size-check foulants
• Modify procedures to minimise foulant movement
• Upgrade construction materials
• Optimise filter design and focus on the right streams to filter, but not necessarily all
• Minimise tankage feed
• Solve upstream corrosion issues, usually at the primary fractionator
• Improve desalter designs
• Careful use of chemicals and consideration of downstream effects
• Collectively, this forms part of the unit’s KPI monitoring to track reliability.Mar-2023
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