A demister is a device often fitted to
vapor–liquid separator vessels to enhance the removal of liquid droplets
entrained in a vapor stream. Demisters may be a mesh-type
coalescer
A coalescer is a device which induces coalescence in a medium. They are primarily used to separate emulsions into their components via various processes, operating in reverse to an emulsifier.
Coalescers are of two main types: mechanical and elec ...
, vane pack or other structure intended to aggregate the mist into droplets that are heavy enough to separate from the vapor stream.
Demisters can reduce the residence time required to separate a given liquid droplet size by reducing the volume and associated cost of separator equipment. Demisters are often used where
vapor quality
In thermodynamics, vapor quality is the mass fraction in a saturated mixture that is vapor; in other words, saturated vapor has a "quality" of 100%, and saturated liquid has a "quality" of 0%. Vapor quality is an intensive property which ca ...
is important in regard to entrained liquids, particularly where separator equipment costs are high (e.g., high-pressure systems) or where space or weight savings are advantageous.
For example, in the process of
brine
Brine (or briny water) is a high-concentration solution of salt (typically sodium chloride or calcium chloride) in water. In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawat ...
desalination
Desalination is a process that removes mineral components from saline water. More generally, desalination is the removal of salts and minerals from a substance. One example is Soil salinity control, soil desalination. This is important for agric ...
on marine vessels,
brine
Brine (or briny water) is a high-concentration solution of salt (typically sodium chloride or calcium chloride) in water. In diverse contexts, ''brine'' may refer to the salt solutions ranging from about 3.5% (a typical concentration of seawat ...
is
flash-heated into vapor. In flashing, vapor carries over droplets of brine, which have to be separated before condensing, otherwise the
distillate
Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
vapor would be contaminated with
salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
. This is the role of the demister. Demisted vapor condenses on tubes in the desalination plant, and product water is collected in the
distillate
Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...
tray.
See also
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Vapor–liquid separator
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Souders–Brown equation
In chemical engineering, the Souders–Brown equation (named after Mott Souders and George Granger Brown) has been a tool for obtaining the maximum allowable vapor velocity in vapor–liquid separation vessels (variously called ''flash drums'', ...
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Vane type separatorbr>
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Vessel Internalsbr>
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Gas Processors Suppliers Association (GPSA
References
{{DEFAULTSORT:Demister (Vapor)
Chemical equipment
Gas technologies