Slug catcher debottleneck protects downstream processing (TIA)
Effective separation of oil and gas from different impurities in upstream operations is a matter of paramount importance. All downstream activities and the processing equipment’s service life depend on the purity level obtained in these first steps.
Head of Sales & Technology – Upstream Components, Sulzer
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When three slug catchers at a major oilfield were struggling to effectively process crude oil, Sulzer Chemtech promptly created a designed solution to restore peak performance. The revamp was completed quickly during a planned downtime period and minimised future regular maintenance.
Located between the outlets of pipelines and processing units, slug catchers are the first stage in upstream processing trains and plants. The fundamental purpose of such large vessels is to hold the slugs temporarily and then direct them into downstream equipment and facilities at a rate at which the separated gas and liquid stream can be properly handled. In addition, they support degassing and bulk solid removal from the liquid phase as well as the delivery of a more even supply of liquid to downstream facilities. This in turn prevents any overload in the plant’s gas/liquid handling capacity, especially during pigging operations.
Key components of slug catchers are mist eliminators. These may be pushed to their limits by increasing the unit’s capacity, because of the arrival of a liquid slug or gas surge, or when fouling materials accumulate on them. In these situations, the slug catcher’s demisting devices may be exposed to fouling as well as corrosion and become inefficient. As a result, downstream facilities may be exposed to high liquid and solid entrainment in the gas outlet stream.
When liquid droplets get away from separators
One of the world’s top 10 oil producers was experiencing these issues in three identical high pressure slug catchers at one of its oilfields. The liquid carry-over from the slug catchers was impacting downstream separation processes such as the company’s amine unit. Simultaneously, the entrained solid particles were causing fouling of demisting mesh pads and other downstream components.
To minimise the impact of these negative effects, the company had to drain liquids being accumulated in a downstream scrubber every 13 hours. In addition, the overall production capacity had to be reduced to minimise liquid carry-over and solid particle entrainment.
The oil producer contacted Sulzer to develop a suitable solution that would address these issues and debottleneck its operations. In particular, the company wanted a revamped system to keep the liquid carry-over from the slug catchers equal to or below 0.1 USgal/MMSCF (14 lt/MMNm3).
Designing mist elimination
Sulzer’s specialised engineers began by conducting computational fluid dynamics (CFD) simulations. These would support root cause analysis (RCA) and identify key improvements in the slug catchers’ design that could lead to enhanced separation performance.
This investigation revealed that the demisting capabilities of the existing wire mesh pads could be improved by using additional pieces of equipment, one of them being a T-shaped, half-open inlet device. While other items commonly installed in such vessels might be based on vane packs, Sulzer’s CFD study revealed that two perforated inlet distribution baffles, coupled with a horizontal flow axial cyclone deck at the outlet nozzle of each slug catcher, would provide a better demisting performance.
While vane packs work properly under normal atmospheric conditions, they are generally not recommended for hydrocarbon processing at high pressures, such as in the slug catchers we were investigating, which operate at 72 bar (464 psi). In these situations, they tend to feature lower separation efficiencies. Conversely, axial cyclones are ideal as they can withstand these harsh operating conditions better.
The ideal horizontal flow axial cyclone was identified to be the Shell VersiSwirl, which is a high-capacity system able to remove all liquid droplets while withstanding fouling. To further increase the slug catchers’ performance, Sulzer’s team suggested equipping the top section of the second baffle with its Mellachevron vane pack pre-conditioner. This supports demisting operations by coalescing the liquid droplets to form larger aggregates that are easier to capture.
Happy with the design suggestions proposed by Sulzer’s engineering team, the oil company asked the specialists to plan the installation of the three demisting devices. Key requirements for this phase of the project were the completion of the revamp within the plant shutdown period, two weeks for each slug catcher, and the avoidance of any welding activity on the vessels.
As the slug catchers operate at high pressure, welding would require post-welding heat treatment and vessel rectification. These would not only result in prolonged downtime but would also need large volumes of water for hydrotesting that would not be easily accessible to the plant due to its remote location. To address this issue, Sulzer’s engineers utilised support expansion rings that would not require any welding and could be installed quickly.
After the first slug catcher was successfully upgraded, the downstream scrubber needed to be drained only once a week rather than every 13 hours. In addition, the performance tests, conducted by an external independent laboratory, showed that the total carry-over was way below 0.1 USgal/MMSCF, exceeding the expectations of the oil company.
Fully satisfied with the results obtained, it asked Sulzer to repeat the same revamping activities on the remaining two slug catchers. After completion, draining intervals further lengthened, requiring interventions only every one or two months.
The revamp project conducted for this oil producer showed comprehensive capabilities as a full-service provider of mass transfer and separation solutions and specialised field services.
This short case study originally appeared in PTQ's Technology In Action feature - Q3 2020 issue.
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