Conventional gas-liquid separators can be equipped with various separator internals in attempt to increase the separation efficiency. Separator internals help promote better separation by more evenly distributing the stream flow, increasing coalescence of both droplets and bubbles, as well as mitigating conditions that are detrimental to good separation. Conditions that involve foaming, liquid waves within the separator, or solid deposition should be avoided if good separation efficiencies are to be maintained.
2.3.1 Inlet Conditioning Devices
The purpose of inlet conditioning devices is to reduce the momentum of the inlet stream flowing into the separator (Bahadori,2014). This decrease in the stream momentum right at the beginning of the separator usually performs the initial bulk gas-liquid separation as well as preconditions the distribution of the gas flow. Ideally, droplet shearing is also minimized within this region to avoid droplet breakup into smaller droplets. There are many different types of inlet conditioning devices, as seen in Figure 2.6.
The diverter plate and dished head function by rapidly changing the direction of the inlet flow. By imposing a sudden change in the direction of the flow, the denser liquid phase with a higher momentum compared to the gas phase strikes the plate, accumulates, and falls to the bottom of the vessel. The less-dense gas phase tends to flow around the plate and disengage from the liquid. The benefit of the diverter plate-type inlet conditioning devices is that they are relatively simple to design and install.
The reversed pipe or half-pipe configuration also consist of a relatively simple design. The reversed pipe is simply a piece of pipe that redirects the flow towards
Figure 2.6: Example of different inlet conditioning devices, adapted from Bothamley (2013a)
the separator. The disadvantage of pipe-type inlet conditioning devices is that they have the potential to further entrain gas into the liquid.
More complex inlet conditioning devices include inlet vanes and inlet cyclones, both relying on centrifugal forces to enhance separation. Inlet vanes consist of a series of curved fins that divert the flow outward into the separator. The curvature of the vanes promotes inertial impact of liquid droplets while the gas flows around them.
Cyclonic inlet conditioning devices generate the centrifugal forces by swirling the in- let stream tangentially, similar to the behavior standalone cyclone separators seen in Section 2.2. The same disadvantages apply to centrifugal inlet conditioning devices as previously mentioned for standalone cyclone separators, namely: the separation performance is sensitive to the inlet flow and the resulting separation produces a rela- tively high pressure drop. The advantage of using centrifugal-type separators as inlet conditioning devices as opposed to standalone units is that if the inlet conditioning device fails to properly separate the gas and liquid phases, the phase separation can
Figure 2.7: Wave breakers inside horizontal separator, adapted from Stewart and Arnold (2008)
still occur within the vessel volume, mitigating to some extent the sensitivity to the flow rate. New separator designs typically employ either vanes or centrifugal inlet conditioning devices (Bothamley, 2013a).
2.3.2 Wave Breakers
Large horizontal separators often require wave breakers to prevent sloshing within the vessel. These anti-wave elements usually consist of perforated baffles po- sitioned along the length of the separator. The baffles help minimize disturbances to the liquid flow and are particularly useful in three-phase separation. An example of these wave breakers can be seen in Figure 2.7.
2.3.3 Foam Breakers
Foaming has the potential to greatly reduce the capacity of gas-liquid separa- tors due to the increased residence times required to dissipate the foam. If foam is still present at the end of a separator’s residence time, the foam can be pulled into the separator outlet, potentially jeopardizing equipment not meant to handle foam further downstream. Foam breakers assist in foam mitigation by forcing the foam
Figure 2.8: Example of foam breaker inside a horizontal separator, adapted from Arnold and Stewart (1998)
through a series of parallel plates. These closely spaced plates, as shown in Figure 2.8, expose the foam to additional surface area, breaking up the foam as it is dragged across the surface of the plate.
2.3.4 Mist Extractors
As the gas flows through the separator, droplets too small to be separated within the bulk of the separator will still be present in the exiting gas stream. To ensure as much of this fine mist is captured as possible, mist extractors are often employed at the gas outlet. Like the inlet conditioning device, there are a variety of different mist extractors available. Common mist extractor designs include wire mesh, vane packs, as well as cyclones.
Wire mesh mist extractors, as the name implies, is a mesh of knitted wires through which the exiting gas stream is passed. Wire meshes are also the most common mist extractors found in production operations (Stewart and Arnold,2008).
The knitting of the wires allows the meshes to have a large surface area and void fraction. The effectiveness of the wire mesh is dependent largly on the gas velocity being in the proper range. If the gas velocity is too low, the fine droplets will simply drift through the mesh without colliding with any of the wire elements. If the gas
Figure 2.9: Wire mesh mist extractor, reproduced with permission from Elsevier, B.V. Stewart and Arnold (2008)
velocity is too high, liquid droplets that collided with the wires will be re-entrained into the gas phase. An example of a wire mesh can be seen in Figure 2.9.
Vane mist extractors function in a similar manner to wire meshes, though parallel plates are instead used as a source of surface area available for droplet colli- sion. Within a vane pack, these parallel plate contain directional changes, impinging droplets onto the surface where they coalesce and fall to the liquid collection area.
An example of a vane mist extractor can be seen in Figure2.10.
Mist extractors are susceptible to plugging if the liquid stream is prone to solid deposition. Bypass lines or removing the mist extractor entirely may be needed to rectify the issue if the plugging is acute (Lyons, 2009).