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 Designing for droplet size distribution
There are many different types of mist eliminator elements, and the variety has greatly increased through the years. Not understanding the liquid source in the upstream process can cause you to select the wrong type of mist eliminator, or to keep a given type when process changes make it inappropriate.
Understanding the process allows you to design for the most efficient mist collection. Most important, selection should not be made until the droplet size distribution is defined, in terms of the proportion of droplets of each size. Assuming an incorrect droplet size distribution can mean that you have designed for a less efficient mist eliminator, and liquid carryover may occur.
See Tables 1 and 2 for some points of reference and rough guidelines in this respect. Be aware that the capture efficiency of a given mist eliminator element does not depend only on droplet size. It is also influenced by gas velocity through the element and mist load in terms of liquid flow rate per unit of cross-sectional area. Then there are variables such as density and viscosity that depend on temperature, pressure, and liquid and gas composition. All else being equal, efficiency generally goes up with higher velocity, finer mesh strands, closer packing of mesh (greater density), closer spacing of vanes, and greater thickness of the mist eliminator element.
 Mesh pad fouling
In some cases, liquid carryover to the compressor is caused by fouling of a mesh-type mist eliminator, on account of the resulting restriction of gas flow and extra holdup of liquid in the pad. Vane-type mist eliminators are a better choice for fouling applications. Due to the relatively wide open spaces between blades, vanes are much less likely to plug. If the fouling deposit can be readily dissolved by a suitable solvent, as might be the case with viscous oils or waxes or certain caked solids, consider installing a spray system as shown in Figure 3 to clean the vanes on-line whenever necessary. Adding a high-efficiency mesh mist eliminator downstream of the vane unit (also shown in Figure 3) can help make up for the inherently lower droplet capture efficiency of the vanes.
Liquid slugs and high liquid loading
In some applications, liquid slugs occasionally come in with the gas feed. These surges can temporarily overwhelm the slug-catching capability of the inlet knockout drum and flood a mesh-type mist eliminator, causing liquid carryover. (See Figure 4.) When liquid slugs or generally high liquid loading are expected, it is recommended to use a vane-type mist eliminator upstream of the mesh pad as shown before in Figure 3. Vanes can generally handle up to 10 times more liquid load than mesh pads.
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