Mist eliminators – Functional Principle

In vane-type mist eliminators with a vertical gas flow the baffle vanes are arranged horizontally or at a slight angle to the horizontal. Since the gas flows upwards in the opposite direction, it is necessary for the baffle vanes to be designed so that certain regions are bypassed by the gas flow.

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Thus zones are provided in which the droplets can be collected without interaction with the gas. These zones also provide a secure diversion of the collected liquid from the surface of the vanes. This requirement can be met in various ways and in function of the profile and shape of the vanes.

As the liquid has to flow in the opposite direction to the gas, the film of liquid that forms on the vane has to be converted back to droplets. To prevent overstressing of the system by droplets being picked up and recirculated by the gas flow, these droplets must be substantially larger than those carried into the separator by the gas.

Date: -07-07 | Author: Christine McAniff

In a world of online information, you will find many options when searching for mist elimination solutions. While many companies can sell you a mist eliminator, it is a wise choice to find a company that will design the right mist eliminator for your process. Below we will talk about the basics of mist eliminators, also known as demister pads.

In the manufacturing of fertilizers, chemicals, and other processes, liquid will encounter flowing gas, as a result, the entrainment of liquid droplets will generate a mist. Mist is corrosive and can result in process inefficiencies and damage your equipment.

A properly designed and installed mist eliminator will collect and remove droplets from the gas stream. Removal happens when droplets within the flowing gas strike the mist eliminator and are then captured. The material coalesces on the surface of the demister pad, causing larger droplets to form. The droplets will join and flow by gravity into the drainage area below where they are recovered or removed.

One major concern in mist elimination is plugging. This occurs when a substance builds upon the surface of the mist eliminator causing a blockage and then pressure drop. It is common for process and gas stream problems to result from plugging of the demister pad. Plugging can result in product loss, excessive energy use, corrosion, reduction of equipment life, plant run times being affected, and not meeting output requirements. Structured media mist eliminators are less prone to plugging than conventional knitted mesh.

When choosing a mist eliminator, it is important to choose a company that will understand your chemical process and design a mist eliminator for your system and process needs. Being a retrofit or a new design, many considerations will come into play for the design of the demister pad. Particle size will determine the type of mist elimination for the application, such as Fiber Bed Filters vs. Structured Mesh Pads. Definition of the droplets size in the current system will be necessary to determine the proper design. Droplets definition are based on diameter. For example Sprays are larger than 10 microns. Mist are 10 microns to 1 micron. Aerosols are smaller than 1 micron.

Droplets are collected on the mist eliminator media by three different methods. Inertial Impaction, Interception, and Brownian diffusion.

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For larger droplets, inertial impaction happens when droplets &#; 20-30 travel in a straight line, away from the gas flow, and then they strike the surface of the media and stick together.

Interception happens when droplets down to 1 to 3 microns in diameter travel along the path but are caught or intercepted because they cannot pass through the fibers of the media.

Brownian diffusion happens when particles smaller than 1 micron collide with gas molecules, then the molecules gain momentum and random motion, therefore likelihood increases that the molecules will contact the fibers.

Mist Eliminators Are Available in Several Different Styles:

Vane packs, also known as chevron or plate-type mist eliminators are composed of corrugated plates that are spaced close together. This creates a tortuous path for the gas to follow. Vanes are more effective for larger droplets and can be quite costly.

Fiber bed Filters are made of fine fibers, typically glass or plastic, and packed or wound around a cylinder. Fiber bed filters can be repacked and replaced. They should not be used where solids exist.

Wire mesh also known as knitted mesh is composed of multiple layers of mesh that is knitted of metals or plastic monofilaments. These pads can be cleaned and reused.

Structured mesh mist eliminators are made of interlocking plastic monofilaments. These pads can also be cleaned and reused. When designed properly, larger diameter polypropylene mesh pads can easily handle particulates that would usually cause pressure drop.

Poorly designed mist eliminators can lead to corrosion of downstream equipment, product waste, and shutdown times that are longer than desired.

Properly designed mist eliminators can reduce plugging and mist carryover, have lower pressure, reduced energy consumption, less cleaning and maintenance, less damage to downstream equipment, reduced corrosion, and longer equipment life and reduced atmospheric emissions. All which will lead increased profitability.

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