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Inlet device in an oil/gas separator is used to direct the flow and absorb the flow momentum of coming stream. However, the inlet has received less attention and “science” than the gas outlets. Actually inlet device play an important role in overall performance of a separation vessel. The followings are a few traditional or typical inlet device
as depicted in Fig. 1, are:
- Flat impact plates
- Dished-head plates
- Half-open pipes
- Open pipes directed at vessel heads
These inlets, although inexpensive, may have the shortcoming of negatively affecting separation performance. The inlets previously mentioned are more appropriate for handling low-momentum fluids (momentum is density times velocity). However, for higher-momentum fluids, these inlets can cause problems. The flat or dished-head plates can result in small drops and foam. The open-pipe designs can lead to fluid short-circuiting or channeling.
Although inlet momentum is a good starting guideline for selection, the process conditions, as well as the demister choice, should also be considered. For example, if the liquid loading is low enough that a demister can handle all the liquid, then inlet devices can be applied beyond their typical momentum ranges.
In recent years, more advaned inlet devices have been developed. The typical examples are inlet vane and inlet cyclone.
For applications of inlet momentum typically less than 9 kPa, a vane inlet can be used. Fig. 2 shows a typical vane inlet.
The fluids are “sliced” off to either side while flowing through the inlet device. The spacing between the blades typically has been designed using computational fluid dynamics (CFD) to achieve uniform flow. Because the area of the vane inlet is several times larger than the inlet nozzle, the fluid velocities are much smaller, allowing for good gas/liquid separation as well as smooth entry into the vessel.
For application of foaming streams, the inlet cyclone are now becoming more widely used. See Figs. 2.5a , 2.5b , and 2.6 for typical examples. The advantages of a cyclonic or vortex inlet are:
- High allowable inlet momentums
- Defoaming characteristics
- Liquid/liquid coalescing benefits
- Gas demisting benefits
- High liquid levels
The inlet cyclone can be used for inlet momentums factor of 10 higher than pipe inlets. Typically, pipe inlets are used for momentums (ρV2, where ρ is bulk density, kg/m3, and V is bulk velocity, m/s) less than 1,000 Pa. Inlet cyclones have been used successfully to 65,000 Pa. Although ρV2 is energy, it is referred to as (transport of) momentum.
Because of the centrifugal flow, large foam bubbles are broken, liquid drops are removed from the gas phase, and liquid/liquid coalescence occurs within the cyclone. A problem with some cyclones is that a poorly designed liquid outlet can shear the liquids, offsetting the benefits of the coalescence and possibly making the situation worse. For cases in which a high liquid level is required in the separator, the inlet cyclone can be submerged up to the gas outlet level. Moreover, the cylone inlet allows the use of a shorter retention time due to the rapid degass process, in which large bubble foam is minimized or eliminated. Without the cyclonic inlet, foam can occupy a considerable volume in the separator. The use of a cyclonic inlet often allows foam to be ignored as a sizing consideration. Thus, for high-capacity crude-oil separators (which are the ones most limited by foam), the cyclone inlet can significantly reduce the required vessel liquid and foam holdup volume, size, weight, and cost.
Fig. 3 shows two different types of inlet cyclones. One cyclone has a simple tangential inlet with or without a gas vortex finder. In cyclones without a vortex finder, gas can escape quickly out the top. This has two effects: the loss of gas yields a lower centrifugal acceleration within the cyclone, and as the gas leaves the top, it carries liquid with it as well as shearing the liquid. More mist is generated, which may impact the downstream demister. With a tangential inlet, the fluids may also circle back on the inlet, disrupting the incoming flow. The bottom of the cyclone is submerged below a liquid level to prevent gas from blowing out the bottom (blow-by). A simple flat plate baffle beneath the liquid outlet spreads the flow out radially.
As the thoughput increase, a cyclone cluster shown in Fig. 4 may be considered to increase vessel capacity. In Fig. 4a, fluids enter the cyclones from a top shell side inlet nozzle and are split to the different cyclones. In Fig. 4b, incoming process fluids are accelerated in the manifold to a desired velocity. Each tube peels off a portion of that stream, which enters the tube tangentially, generating rotational flow. In both cases, within each tube, the swirling fluids create a high force for separation of gas/liquid. The gas accumulates in the center, forming a gas core, exits through an orifice in the top of the tube, and flows into the separator gas phase. Liquids are slung to the tube wall, where they migrate downward as a continuous sheet. They exit the tube through a peripheral gap in the tube wall at the bottom and flow out into the separator liquid bath, in which the bottoms of the tubes are submerged.
The other cyclone has stationary turbine blades used to provide spin. This has the advantage of lowering the shear on the fluids. Lower shear results in less mist generation and droplet shearing. In the liquid section, the liquid outlet with proprietary internals is designed to prevent gas from escaping. The bottom of the cyclone also provides some backpressure through low shear channel flow or a perforated cylinder. This additional backpressure allows the cyclone to operate at higher gas capacities than the first cyclone and, hence, with higher centrifugal accelerations. A bottom flat plate, in conjunction with the perforated cylinder, spreads the liquid flow out more uniformly.
Selection of Inlet Device
The cyclonic-type inlet device is good to diffuse the momentum of the incoming feed stream and allows for the removal of any bulk liquids and solids that may be present. The cyclonic inlet device is designed such that it can operate at both high and low gas/oil ratios without the possibility of gas blow-by and excessive liquid re-entrainment into the gas phase.
The main characteristics to look for in an inlet cyclone are listed next.
- High liquid drainage capacity: This is necessary to prevent internal “choking” of the cyclone. In this case, the liquid carryover into the gas phase will be excessive, which could cause too high a liquid load on the downstream devices. Also, there would be considerable disruption to the internal flow field, which means the cyclone will not operate correctly.
- Low re-entrainment of liquid into the gas phase: This means that the downstream mist eliminating devices will not be overloaded. Also, the mist eliminating section will be working optimally; therefore, the best separation performance from the separator can be realized.
- Liquid re-entrainment: The main cause of liquid re-entrainment within the gas phase is “creep” that is caused by internal pressure differences causing liquid to move along internal surfaces. Within the cyclone, the effect of creep can be minimized by use of rings located around the gas vortex finder.
- Low shear forces on the re-entrained liquids in the gas: Low shear forces are beneficial, given that the droplet distribution of the re-entrained liquids leaving the cyclone is not too fine. The finer the droplets, the more difficult it becomes for the downstream mist eliminating section to remove them—hence, the overall liquid carryover from the separator increases.
- Low shear forces on the dispersed phase within the liquid outlet: Low shear forces are beneficial in that the droplet distribution of the dispersed phase liquid leaving the cyclone is not too small and does not form an emulsion. As the droplets get finer, it becomes more difficult for the downstream gravity settling devices to coalesce and remove them; hence, more is carried over in the exiting streams. This can be accomplished by ensuring that the liquid flow channels within the cyclone are relatively large and that any perforated holes are of the correct size to minimize shear.
- Minimum gas blow-by to the liquid outlet: Gas blow-by is seen when gas exits with the liquid phase from the bottom of the cyclone. If the amount of gas is considerable, a bubbling mass of liquid/gas is formed, which has a negative effect because of foam generation and mixing.
Care must be taken when designing inlet cyclones for separators. Cyclones are designed on the basis of a pressure balance between the pressure drop needed to force the gas up and out the top of the cyclone and that required to push the liquid out the bottom of the cyclone. If the gas pressure drop is higher than the liquid pressure drop, the liquid level inside the cyclone will be lower than that of the level of the surrounding liquid in the separator and vice versa. Flow rate turndown and changes in producing gas-to-oil ratio (GOR) then play important roles in the determination of the operating range of the cyclones. For example, cyclones may be properly designed for, say, 50,000 B/D of liquids at 1,000 GOR. However, if the GOR is actually 1,500, gas may blow out the bottom of the cyclones and create foam throughout the separator.
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Noteworthy papers in OnePetro
Song, J.H., Jeong, B.E., Kim, H.J. and Gil, S.S. Three-Phases Separator Sizing Using Drop Size Distribution. OTC-20558-MS presented at the Offshore Technology Conference, Houston, Texas, USA, 3-6 May. http://dx.doi.org/10.4043/20558-MS.
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