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Mist eliminators can protect equipment, recover product and control emissions. Apply these techniques to select the best mist eliminator for your service
Mist eliminators have been used for over 40 years in hundreds of chemical-plant applications to remove liquid aerosols and soluble salt particles from gas streams (Figure 1, above). It is important for engineers to understand how mist eliminators can be utilized to control emissions, protect downstream equipment and recover valuable product. This article covers key design guidelines for proper mist eliminator selection and also provides application insight.
Today, mist eliminators are employed in a multitude of industrial and chemical applications for the removal of liquid aerosols and soluble salt particulates from process streams. Typical applications are those involving sulfuric acid, phosphoric acid, chlorine, ammonium nitrate prilling, vinyl curing and turbine lube-oil venting.
Mist generation
In order to effectively apply mist eliminators to a new application, one needs to understand the mechanisms by which mists or aerosols are generated. Mists are usually formed by one or more of the following mechanisms:
* Mechanical shear: Mists can be formed mechanically by film, droplet, or bubble shattering. In general, the droplets formed by mechanical shear are several microns in diameter - considered large by industry standards - and are collectively called spray [1]. In a packed absorption tower, for instance, mist is mechanically generated at the distributors. The frothing action in the distributor, coupled with bubbling in the downcomers, generates droplets that are swept up into the gas stream. This incident is known as entrainment. If a significant degree of mechanical shear is inherent in the process, a greater number of smaller liquid particles will likely be formed.
* Condensation: The second way to generate mist is by condensing the vapor component in a gas stream. When rapidly cooled to a temperature below its dew point, the vapor condenses into submicron particles. Typical examples of condensation-induced mists include the blue hazes that appear above asphalt storage pots, or hazes that form during food frying and plastic-processing operations. It has also been found that the concentration of foreign nuclei (that is, particulate matter) in the process gas stream affects the resultant mist particle size and loading [2]. Condensation also occurs when there are enough fine, submicron...