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Mist capture can occur by three mechanisms, as shown in Fig. 9. It should be kept in mind that there are no sharply defined limits between mechanisms. As the momentum of a droplet varies directly with liquid density and the cube of the diameter, heavier or larger particles tend to resist following the streamline of a flowing gas and will strike objects placed in their line of travel. This is inertial impaction, the mechanism responsible for removing most particles of diameter > 10 μm. Smaller particles that follow the streamlines may collide with the solid objects, if their distance of approach is less than their radius. This is direct impaction. It is often the governing mechanism for droplets in the 1- to 10-μm range. With submicron mists, Brownian capture becomes the dominant collection mechanism. This depends on Brownian motion—the continuous random motion of droplets in elastic collision with gas molecules. As the particles become smaller and the velocity gets lower, the Brownian capture becomes more efficient.
Types of demisters
Almost all mist elimination equipment falls into four categories:
The demisters can be sealed in the liquid or within a gas box with a liquid drain sealed in the liquid. In the later case, enough space must be provided between the bottom of the gas box and the liquid level to prevent siphoning of liquid up the drain tube.
As a vapor stream carrying entrained liquid droplets passes through a knitted mesh, the vapor moves freely through the mesh. However, the inertia of the liquid droplets causes them to contact the wire surfaces, coalesce, and ultimately drain as large droplets.
The knitted mesh can be made in various materials and densities. See Fig. 2. Each manufacturer has its own method of knitting the mesh, which accounts for the differences in separation efficiency. Some meshes have different materials interwoven together to account for different fluids such as glycol and condensate. Other types use layers of different styles of meshes.
For general design, the mesh area can be determined with Eq. 1.
- Vm = design velocity, m/s
- ρg = gas-phase density, kg/m3
- ρl = liquid-phase density, kg/m3
- K = mesh capacity factor, m/s.
The recommended value of K varies and depends upon several factors such as:
- Liquid viscosity
- Surface tension
- Liquid loading
- Operating pressure
Each manufacturer has its own recommended values. For general sizing, a K value of 0.1 m/s can be used as a guideline. Pressure drops are generally a few inches of water (one inch of water equals 25 Pa.)
Vane-type mist extractors
Vane-type mist extractors are widely used in oil/gas separators. They can be of many designs. Fig. 2 shows typical single- and double-pocket vanes.
As the mist-laden gas stream passes through the parallel vane plates, it is forced to change direction several times. The mist droplets are separated by the subsequent centrifugal forces and are collected on the vane blades. The coalesced liquid film is then drained through hooks (single pockets) or slits (double pockets) on the blades.
In the double-pocket design, the liquid is more protected from being re-entrained by the gas. The double-pocket design also can be used in vertical flow, as shown in Fig. 11. This coalesced liquid film is then drained through slits (“double pockets”) in the hollow blades, thereby reducing the gas disturbance. This leads to a higher throughput and greater efficiency in comparison with simpler vane-type separators.
The separating efficiency of a vane mist extractor depends on:
- The number of vanes in the element
- Distance between the vanes
- Angle of the vanes
- Size of liquid particles
It is claimed that this mist extractor will remove all entrained liquid droplets that are 8–10 μm and larger. However, this is generally true only for low pressures, on the order of a few hundred psi. If smaller liquid particles are present in the gas, an agglomerator should be installed upstream.
Eq. 1 can also be used as a guideline for sizing vane-pack areas. Typical K values are in the 0.15–0.25-m/s range.
Typical demisters in production vessels have generally been mesh pads and vane packs. However, axial-flow cyclones are becoming more frequently used because of their advantages: high efficiency at high pressures; high gas/liquid capacities; foam breaking; and nonfouling.
For instance, vane packs cannot remove droplets that are 10–20 μm at pressures greater than 500 to 600 psi. Additionally, cyclones have approximately 10 times the capacity of mesh pads and 4 times that of vane packs. Because of these features, the cyclones are suitable for upgrading existing vessels and for designing smaller, more compact new vessels. Because of their high centrifugal accelerations, the cyclones can be placed horizontally or vertically. Deposition is usually not a concern because of the high velocities. When cyclones are used in conjunction with mesh pads (as coalescers), high turndowns can be achieved.
Fig. 4 shows three types of demisting cyclones:
- Reverse flow
- Nonrecycling axial flow
- Recycling axial flow
In the reverse flow cyclone, flow enters tangentially around the gas outlet tube. The flow travels down, with liquid being spun to the outer wall and draining out the bottom. The gas reverses direction and flows out the inner tube.
In axial flow cyclones, a stationary turbine in the tube spins the flow. Downstream of the turbine, the liquid film is removed through slits, along with some “secondary” purge gas. The liquid drops to the bottom of a box chamber enclosing the cyclone and flows out a drain tube. The main portion of the gas flows straight out of the cyclone. For nonrecycling axial flow cyclones, the secondary gas usually must be cleaned up by a mesh pad.
In the recycling cyclone, the purge gas is educed back into the center of the cyclone through the stationary turbine. (See Fig. 12.) A low-pressure region exists because of the spinning flow, similar to that in a tornado. In this way, the secondary purging gas is cleaned again, and there is no need for a mesh pad. The cyclone demisters have proprietary sizing rules. Typical drop size removal is approximately 5 to 10 μm.
In applying centrifugal force to separation, the separator size is determined by the flow capacity, among other factors. However, the amount of separating force that can be generated at a given rotational velocity decreases as the separator diameter increases. The result is that larger, higher-capacity centrifugal monotube units are less efficient than smaller ones for removing small-entrained mist droplets.
Fig. 5 shows a multitubular cyclone separator, in which the flowstream is processed through a bank of parallel cyclone tubes, each tube taking a fraction of the flow. Each tube keeps a small diameter to maintain high separation efficiency. The unit shown is a recycling separator; that is, a slipstream of the gas is extracted with the liquid from the tubes and recycled to the tube inlet.
Centrifugal force may be used in conjunction with other separation mechanisms for removing oil mist from gas. As previously discussed, Fig. 5 shows two types of cyclone/vortex tube clusters installed in an otherwise conventional separator. Each vertical vortex tube handles a portion of the flowing stream, performing primary separation of entrained mist by means of centrifugal force. The vortex tube device is followed by a disengagement space, where any droplets of oil that have been caught and coalesced, but carried through, will rapidly settle. A mist extractor may be installed in the disengagement space, if needed.
Turndown should be considered when selecting the demister. However, it is difficult to compare turndown of cyclones, vane packs, and mesh because drop size removal is affected differently at varying gas rates.
Coalescers and fiber beds
For scrubbing purposes, filter coalescers merge, or coalesce, small droplets of liquid into larger drops (Fig. 6). Gas is forced to flow through several layers of filter media, each layer having a progressively larger mean pore opening. As droplets compete for the open pores, they coalesce, and the process continues until the larger drops continually collect and drain into a collecting sump. In addition, some coalescers have a patented oleophobic/hydrophobic treatment. Stated removal size is approximately 0.3 μm.
Fiber-bed mist eliminators use small-diameter fibers (0.02 mm) to capture the small droplets. These fiber beds use Browning diffusion or an impaction mechanism to remove drops as small as 0.1 μm. The fiber beds are typically packaged in a cylindrical shape, as shown in Fig. 7.
|Vm||=||design velocity, m/s|
|ρg||=||gas-phase density, kg/m3|
|ρl||=||liquid-phase density, kg/m3|
|K||=||mesh capacity factor, m/s.|
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