Scrubber Design Requires a Balance of Criteria vs. Application
Malfunctioning gas scrubbers can lead to a host of problems for operators, including damage to downstream equipment, unplanned shutdowns, expensive repairs after startup, and safety concerns. The root of several scrubber issues can be traced back to the design phase, but an expert said that these issues are constantly disregarded by operators relying on criteria-based methodologies.
In a presentation hosted by the SPE Separations Technology Technical Section, Elizabeth Morillo discussed the ways in which an inadequate design and sizing methodology can negatively affect scrubber performance. She is a senior process engineer at TechnipFMC.
Morillo said a systematic and holistic approach to scrubber design should include relevant process data, including maximum, minimum, and normal process conditions. Operators should also take into consideration different scenarios—such as lockdown, startup, and shutdown—as well as performance requirements and the limitations of the of the project when designing scrubbers
“We are in the real world,” she said. “We need to know how much it’s going to cost, considering the weight and space that we can afford. But all of this has to be validated by the fundamentals of phase separation. If you try to push a design and force something, you’re going to find yourself in trouble sooner rather than later.”
Morillo focused on case studies involving malfunctioning scrubbers, as well as the steps FMC took to improve their performance. Fig. 1 shows the diagram of a scrubber from an unnamed operator, which comprises a mister device in vertical orientation. Its maximum operating pressure is 118.9 psig, maximum gas volume flow is 121.7 MMscf/D, and maximum gas and liquid densities are 0.525 and 42.13 lb/ft3, respectively. The inner diameter of the vessel is 75½ in.
The vessel had a performance requirement of 0.1 gal/MMscf as the maximum allowable liquid carryover rate, but high levels of carryover were observed when the vessel operated at the higher end of the flow rate envelope. This caused crude oil contamination in the downstream compressors.
FMC was brought on to find solutions for improving the scrubber’s performance, focusing primarily on the design. Morillo said that, while the layout of the upstream piping was not too bad, 65% of the total liquid entering the vessel was entrained droplets, meaning that the bulk separation was poor. She said that the liquid section is often overlooked when evaluating scrubber design, but it can often be a critical indicator of potential issues.
“It can be important,” Morillo said. “It can affect the separation performance because, if the gas is coming into the scrubber at high velocities, it can pick up the liquid on the surface and it could affect the separation efficiency.”
The FMC team followed its analysis by running the scrubber design through a computational fluid dynamics (CFD) simulation. Morillo said this simulation showed chaotic flow, and with the inlet velocities greater than 78 ft/sec, the team determined that the vessel had significant gas maldistribution. Only 62% of the demister had inlet velocities within the range of requirements.
Morillo said these statistics should not be construed as an indictment on this specific design. Rather, it was an indication that this design was inadequate for the given application.
“Should we say that this type of scrubber design should be banned? I would say the answer is no. If you use this type of design for the right application, this could work. The thing here was that it was not the right application for this type of design. Unfortunately, they have too much liquid coming in, the maldistribution inside the vessel made the demister only partially work,” Morillo said.
She said the optimal strategy for fixing the scrubber’s issues would have been to increase the inner diameter of the inlet pipe from 18 in. to 20 in., a change she said would reduce the percentage of liquid entrained in gas from 65% to 45%. Changing the flow from vertical to horizontal by modifying the elevation of the outlet nozzle and adjusting the positioning of the vane pack would also help improve flow distribution inside the vessel. Adding an additional separation stage would also help solve the issue of liquid overloading of the vane pack.
Running the new design through CFD analysis, Morillo said the scrubber still showed elements of chaotic flow, but the velocities would decrease to between 52 and 78 ft/sec. Adding a vane pack after the inlet device would also help evenly distribute the chaotic flow across the vessel area. She said that 99% of the demister would have velocities within the requirements.
However, because it was too late to introduce a new scrubber design into the facility, Morillo said FMC focused on improving the flow distribution inside the existing design. This required adding a separation stage and installing a more efficient demister. They also added a vane pack agglomerator between the inlet device and the demister and replaced the original vane pack with a cyclonic demister. She said these changes helped the scrubber operate more efficiently.
“This introduced an improvement in the flow distribution inside this vessel,” she said. “Of course, inlet velocities are going to be the same, but we have a more uniform and evenly distributed gas flow that is going to fit the demisting device.”
For more information, read “Underperforming Gas Scrubbers: How to Fix Them and How to Avoid Them” by E. Morillo, V. van Asperen, and S. Baaren, written as part of OGF’s Savvy Separator series.
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01 August 2018