NGL Recovery - A New Concept for an Old Scrubber
Gas scrubbers are designed to protect process equipment, such as compressors, dehydration towers, pipelines and molecular sieves from liquids. The efficiency of gas scrubbers for separating natural gas liquids (NGLs) is critical to the recovery of most of the entrained NGL before the gas is routed downstream to pipelines.
We present a specific case study in Southeast Asia where offshore production was delivering 300 MMscf/d of gas to an onshore gas processing facility. The gas was chilled in a mechanical refrigeration unit, and liquids were separated in a vertical scrubber equipped with a simple inlet device and mesh pad assembly. The scrubber was designed to handle 450 MMSCFD. Due to an inefficient scrubber design, the facility was experiencing NGL liquid carryover from the scrubber into the pipelines with the original design basis. The scrubber thereby showed significant amount of liquid carryover of 1,550 BPD of natural gas liquids into the gas pipelines.
Computational fluid dynamics modeling of the scenario clearly showed loss in separation efficiency and NGL carryover from the scrubber to the downstream pipeline, documenting that the existing scrubber was not suitable for such high gas loading and increased gas volume.
Customer approached Schlumberger to retrofit scrubber with a new design and resolve the existing liquid carryover issues which led to revenue loss. We have experience in desiging novel internals such as axial cyclonic inlet (ACI) in these kind of scrubber vessels. These have been tested and optimized in the laboratory using model fluids and further tested in a high-pressure real fluid testing loop. Test results show that ACI provides the required gas/liquid separation efficiency under the higher gas loading and thereby increases the overall separation efficiency to 99.9+%.
The retrofit scrubber was designed to process 450 -500 Mmscfd of gas and increase the NGL production by 8540 BPD showing high ROI for the customer. The scrubber with the ACI shows consistently higher separation efficiency even under turndown or lower inlet gas flow conditions.
We present details of the retrofit scrubber design, importance of using high efficiency separation internals, CFD anlaysis and present test results verifying the liquid carryover before and after implementation of the new inlet section.