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Mesh and Vane Mist Eliminators

1. Types of Mist Eliminators

2. Fundamental Considerations

3. Sizing for Gas Velocity

4. Mesh vs. Vanes - or both

5. Applying Mist Eliminators

6. Curves

APPLYING MIST ELIMINATORS


THE FOLLOWING are some additional considerations that may come into play when applying mesh and vane mist eliminators in specific situations. Like other information in this publication, these guidelines can be useful for preliminary design purposes. However, final decisions should not be made without consulting Amistco’s separation specialists.


Vessel Configurations


The simplified diagrams in Figure 22 show several typical configurations of mist eliminators in vessels. The mist eliminators may be mesh pads, vane units, or combinations. The vessels depicted are cylindrical vapor-liquid separators, often called knockout drums. However, some of the same concepts may also apply to mist eliminators in process vessels, such as vapor-liquid contactor columns, evaporators, chillers, etc.

Considerations affecting selection of a mist eliminator configuration may include the following:
• Mist eliminator cross-sectional area to achieve design velocity with required vapor throughput
• Space available inside existing vessel
• Plant space available for the vessel
• Inlet and outlet locations to fit established piping
• Liquid holding capacity and drainage method
• Worker access for cleaning, replacement, etc.
• Support beams for large horizontal mist eliminators
• Internal flow constraints for efficient operation

Figure 22: Typical mist eliminator configurations in cylindrical knockout drums. Similar configurations can be used in other vessels. The mist eliminators may be mesh, vane or combinations.

— click image to enlarge


Internal Flow Guidelines


The last consideration in the foregoing list—internal flow constraints—is often overlooked but may be of primary importance. There are two main principles:

1. Maintain an even velocity profile across the mist eliminator element—whether mesh, vane, or combination. The object is to avoid situations such as shown in Figure 23. Here, the mist eliminator is mounted too close to the outlet nozzle. Excessive velocity in a region near the center of the mist eliminator results in substantial reentrainment there. Furthermore, deficient velocity in a region around the perimeter causes low droplet removal efficiency in that area. The main key to an even velocity profile is to allow sufficient spacing between the mist eliminator and gas inlets and outlets. Items A through E in Figure 24 show some generally accepted guidelines in this regard for cylindrical vessels with axial flow through the mist eliminator. Flow distribution devices of various sorts can reduce the necessary spacing, but at the risk of violating the following principle.

2. Avoid strong turbulence and fluid shear in the wet part of the vessel. The main objective is to prevent entrainment of the collected liquid. This can be achieved by maintaining adequate separation between the inlet nozzle and the liquid surface as shown in Item F of Figure 24. Another objective is to prevent shearing of droplets into smaller particles that might pass through the mist eliminator.

Figure 23: Example of mist eliminator performance degradation due to uneven velocity profile

Figure 24: Generally accepted spacing guidelines to maintain even velocity profile and avoid entrainment in mist eliminators in cylindrical vessels with axial flow

— click image to enlarge


Application Procedure


Based on all of the principles presented before, the procedure generally followed in designing a mist eliminator application involving mesh, vanes, or both is as follows:

1. Estimate the droplet size distribution (See Table 1).
2. Specify the required separation efficiency.
3. Tentatively choose a mist eliminator (mesh, vane, or combination; mesh or vane style; materials) considering droplet size, efficiency, corrosion, and wettability.
4. Tentatively select a mist eliminator orientation and placement in the vessel (Figure 22, etc.).
5. Calculate the necessary cross-sectional area and mist eliminator dimensions (Figure 19, Table 2, etc.).
6. Estimate separation efficiency and pressure drop within the required turndown range.
7. If the estimated results are not acceptable, repeat steps 3 through 6 with a different mist eliminator or vessel configuration.
8. Check for conformance with internal flow guidelines (Figures 23 and 24, etc.) and revise as necessary. For easy separations that are familiar to the designer, sizing (Step 5) may be the only critical step. In even the simplest applications, however, the possibility of improvements in performance and cost-effectiveness should not be overlooked. In any case, achieving an optimum design requires a great deal of experience and judgement.

Designers and purchasers should always consult with Amistco's separation specialists before making a final decision.


Mesh & Vane Mist Eliminator Brochure (PDF-2.5mb)

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