Natural-Gas-Liquids Recovery—Retrofit Breathes New Life Into Old Scrubber
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An onshore gas-processing facility in Southeast Asia currently receives rich wet gas from an offshore production unit. Because of an inefficient scrubber design, the gas-processing facility was experiencing natural-gas-liquid (NGL) carryover of 1,550 B/D from the scrubber into the pipelines. A retrofit scrubber was designed to increase the NGL production by 8,540 B/D. This paper presents details of the retrofit scrubber design and shows the importance of using high-efficiency separation internals.
Challenges With the Existing Scrubber
The condensate scrubber vessel was originally designed to handle 300 MMscf/D of gas. The condensate scrubber had an inlet vane and pack internals along with a mesh pad for final demisting. The customer determined that the scrubber was designed incorrectly because, since the plant startup, it was experiencing large NGL carryover from the scrubber into the gas lines. Because of this inefficient scrubber vessel, the facility was losing approximately 1,550 B/D of NGL.
The existing scrubber design was analyzed with governing empirical formulas and computational-fluid-dynamics (CFD) analysis to identify the following challenges.
- The existing scrubber vessel had limitations in managing maximum gas loading (k-value) before the bulk of the liquid was dragged by the gas to the demister section of the scrubber. Maximum vessel k-value was predicted to be 0.347 m/s, and values were considered to be much higher in this application.
- The inlet device was designed inadequately, leading to incorrect flow paths and a poor distribution of fluids.
- Because of the inertia of the gas, the entry flow into the vessel pushed the gas upward in the vane pack, resulting in poor distribution in the vane-pack region such that only part of the vane pack was used.
- High-velocity zones in the vessel reduced the performance of the vane pack.
- The overall separation performance for the current separator was estimated to be 51%, resulting in a gross liquid carryover of 8,500 B/D.
CFD Analysis of the Existing Scrubber
Fig. 1 shows the predicted gas flow in the vessel, illustrated by path lines. The simulation shows that a relatively large fraction of the gas is directed downward against the liquid pad. This could result in re-entrainment from the already-separated liquid, leading to high liquid carryover.
The poor flow distribution in the inlet vane is caused by a very large and sudden expansion of the flow area in the vane. This undesirable distribution creates zones of high gas velocity, causing droplet breakup and low separation performance in the vessel.
Axial-Flow Cyclones in Separation
Axial-flow cyclones have been applied successfully as demisters in scrubber and separator vessels for more than 20 years. The high efficiency and the tolerance for high liquid loads make axial cyclones well-suited for use as an inlet device. The efficiency of a scrubber is dependent on two sections, the inlet gravity section and the demister section. Consequently, a new type of inlet was suggested that conducted bulk separation using axial-flow cyclones.
Aspects of Axial-Flow-Cyclone Internals in a Separator
- Axial-flow cyclones have a static swirl element that sets the gas into rotation in a pipe. The heavier liquid droplets will hit the wall of the cyclone.
- The liquid is drained from the inside of the pipes through slots in the cyclone wall. The liquid is collected in a separate liquid chamber before being drained back to the liquid compartment in the scrubber vessel.
- The rotational flow will make the heavier liquid move toward the wall of the vessel while the lighter fluid passes through the cyclone. The fluid that hits the wall is extracted to a separate liquid compartment with slits in the wall.
- The gas and liquid collected will enter the scrubber on the lower side of the inlet. Because the inlet has higher pressure than the vessel, the liquid and gas will exit though a downcomer that is not extended into the liquid phase but rather blows the gas and liquid into the vessel.
- The amount of gas that follows the liquid separated in the cyclones will typically be less than 10%. The cyclone is the separation force, and more than 95% of the liquid is separated in the cyclones and introduced underneath the inlet section.
- Reducing the gas load by a factor of 10 to the lower side of the scrubber allows gravity, in combination with a mesh pad, to clean the gas before it is commingled with the main gas flow, achieving a separation performance greater than 98%.
Retrofit Design and Engineering Details
To reduce the liquid loss at the high k-values, a combination of the axial cyclonic inlet and axial-flow cyclones is recommended. The retrofitted scrubber contains
- 116 cyclones in the inlet section for the bulk removal of gas
- 270 axial-flow cyclones with
- 2-in. diameters for the demisting section
- A 150-mm-mesh pad
Inlet Section—Axial Cyclone Inlet. The axial-cyclone-inlet device consists of an inlet compartment, demisting cyclones, mesh pad, and a liquid-drainage/purge-gas system. Gas and liquid enter the inlet compartment through a transition zone between the inlet nozzle and the inlet compartment where guide vanes distribute the flow. The droplet-laden gas flow is distributed over a number of demisting cyclones. Any free liquid in the inlet pipe is transported to the end of the inlet compartment where it is drained through drainage pipes.
Demisting Cyclones. The demisting cyclones are characterized as having a straight-through flow direction. The demisting cyclones exhibit great performance with a relatively low pressure drop at high operating pressures. The demisting cyclones also handle high liquid loadings.
The demisting cyclones work on the principle of a centrifugal field for separation. The gas enters the cyclone tube and is set into rotation by vanes mounted on a central body. The heavier liquid droplets are thrown to the wall by the centrifugal action. The liquid is then transported through slits at the cyclone wall into a liquid-collecting chamber and drained back to the vessel through downcomers.
Final 150-mm-Mesh Pad. The scrubber is supplied with a 150-mm-mesh pad that is installed below the axial-cyclone-inlet device to separate liquid droplets from the purge gas. The clean purge gas passes through the mesh pad before leaving the scrubber. The inlet compartment is equipped with a splitting plate to protect the liquid surface below the plate and improve the gas distribution into the cyclones.
CFD Verification of Retrofit Design
Retrofit design of the scrubber was calculated and verified using CFD analysis. The analysis was conducted to perform the following functions:
- Ensure that the gas is evenly distributed out of the inlet section through individual cyclones
- Assure that the gas that follows the liquid out of the axial cyclone inlet is distributed evenly across the mesh and ensure that the velocity across the liquid surface does not exceed the re-entrainment velocity
- Ensure uniform gas distribution into the gas outlet
The flow in the vessel with the axial cyclone inlet is illustrated in Fig. 2 above with path lines that are colored to indicate the velocity in the scrubber. The gas from the axial cyclone inlet feeds the demister deck, and the flow is distributed uniformly.
The CFD simulation shows that the gas is distributed uniformly out of the gas outlet and the mesh pad.
The last thing to verify with the CFD is flow distribution through the demister section. The flow needs to be uniform to ensure that the demister section is working as intended.
Results show that the flow varies between 97.5% and 107% of the average flow. This confirms that the design layout is well within the ranges of industry standards.
Results of the analytical design and the CFD analysis show that the installation of the axial cyclone inlet and the axial-flow cyclones will increase the separator’s efficiency from 51% to 99.94%.
Installation of the Retrofit Design
One of the greatest advantages of the design is that it does not require welding or hot works for installation of the internals. The entire design of the retrofit scrubber has been created with bolting techniques, and no hot work was necessary for the installation of the axial cyclone inlets.
Because of the modular design of the cyclone deck plates and the demisting cyclones, the entire bolting on the cyclone boxes is performed on the underside of the demisting-cyclone deck plates. Gaskets are installed to ensure proper sealing of the flanges. The internals can be installed step by step with several transition spools that effectively decrease the overall installation labor. All axial cyclone inlets are installed in cyclone boxes that can be lifted in and out through the man-way openings.
An independent third-party verification company was contracted to conduct testing at the facility for the retrofit scrubber design. On the basis of the field data collected and analyzed, the retrofit scrubber design resolved the liquid-carryover issues and provided optimal separation efficiency greater than 99.9%.
Natural-Gas-Liquids Recovery—Retrofit Breathes New Life Into Old Scrubber
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Welcome to the Offshore Facilities feature in this month’s JPT. The three papers featured in this section were selected out of 141 submitted to SPE covering various elements of offshore facilities design, construction, installation, operation, and inspection.
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16 April 2018