A vapor–liquid separator is a device used in several industrial applications to separate a vapor–liquid mixture.

For the common variety, gravity is utilized in a vertical vessel to cause the liquid to settle to the bottom of the vessel, where it is withdrawn.

In low gravity environments such as a space station

Space station

A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...

, a common liquid separator will not function because gravity is not usable as a separation mechanism. In this case, centrifugal force

Centrifugal force

Centrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...

 needs to be utilized in a spinning centrifugal separator

Centrifuge

A centrifuge is a piece of equipment, generally driven by an electric motor , that puts an object in rotation around a fixed axis, applying a force perpendicular to the axis...

 to drive liquid towards the outer edge of the chamber for removal. Gaseous components migrate towards the center. The gas outlet may itself be surrounding by a spinning mesh screen or grating, so that any liquid that does approach the outlet strikes the grating, is accelerated, and thrown away from the outlet.

Method of operation

The vapor travels gas outlet at a design velocity which minimizes the entrainment

Entrainment (engineering)

Entrainment as commonly used in various branches of engineering may be defined as the entrapment of one substance by another substance. For example:* The entrapment of liquid droplets or solid particulates in a flowing gas, as with smoke....

 of any liquid droplets in the vapor as it exits the vessel.

The feed to a vapor–liquid separator may also be a liquid that is being partially or totally flashed

Flash evaporation

Flash evaporation is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations...

 into a vapor and liquid as it enters the separator.

A vapor–liquid separator may also be referred to as a flash drum, knock-out drum, knock-out pot, compressor suction drum or compressor inlet drum.

When used to remove suspended water droplets from streams of air, a vapor-liquid separator is often called a demister

Demister (Vapor)

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, vane pack or other structure intended to aggregate the mist into droplets that are heavy enough to separate from the...

.

Liquid level monitoring

The separator is only effective as long as there is an air space inside the chamber. The separator can fail if either the mixed inlet is overwhelmed with supply material, or the liquid drain is unable to handle the volume of liquid being collected. The separator may therefore be combined with some other liquid level sensing mechanism such as a sight glass

Sight glass

A sight glass or water gauge is a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within.-Liquid in tanks:...

 or float sensor

Float (liquid level)

Liquid level floats, also known as float balls, are spherical, cylindrical, oblong or similarly-shaped objects, made from either rigid or flexible material, that are buoyant in water and other liquids. They are non-electrical hardware frequently used as visual sight-indicators for surface...

. In this manner, both the supply and drain flow can be regulated to prevent the separator from becoming overloaded.

Where vapor–liquid separators are used

Vapor–liquid separators are very widely used in a great many industries and applications, such as:

• Oil refineries
  Oil refinery
  An oil refinery or petroleum refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, asphalt base, heating oil, kerosene, and liquefied petroleum gas...
  
  
• Natural-gas processing plants
• Petrochemical
  Petrochemical
  Petrochemicals are chemical products derived from petroleum. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as corn or sugar cane....
  
   and chemical plant
  Chemical plant
  A chemical plant is an industrial process plant that manufactures chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transformation and or separation of materials. Chemical plants use special equipment,...
  
  s
• Refrigeration systems
  Refrigeration
  Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
  
  
• Air conditioning
  Air conditioning
  An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
  
  
• Compressor systems for air or other gases
  Gas compressor
  A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
  
  
• Gas pipelines
  Pipeline transport
  Pipeline transport is the transportation of goods through a pipe. Most commonly, liquids and gases are sent, but pneumatic tubes that transport solid capsules using compressed air are also used....
  
  
• Steam condensate flash drums
• Geothermal power plants
• Combined cycle power plants
• Flare Stacks

Preventing pump damage

In refrigeration systems, it is common for the system to contain a mixture of liquid and gas, but for the mechanical gas compressor to be intolerant of liquid.

Some compressor

Compressor

Compressor may refer to:* Gas compressor, a mechanical device that compresses a gas * A device used to apply video compression to a video signal* A device used to apply audio data compression to an audio signal...

 types such as the scroll compressor

Scroll compressor

A scroll compressor is a device for compressing air or refrigerant...

 use a continuously shrinking compression volume. Once liquid completely fills this volume the pump may either stall and overload, or the the pump chamber may be warped or otherwise damaged by the fluid that can not fit into a smaller space.

See also

• Flash evaporation
  Flash evaporation
  Flash evaporation is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations...
  
  
• Vapor-compression refrigeration
  Vapor-compression refrigeration
  Vapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, offices, private residences, hotels, hospitals, theaters, restaurants and automobiles...
  
  
• Souders–Brown equation (for sizing vapor–liquid separators)
• Steam drum
  Steam drum
  A steam drum is a standard feature of a water-tube boiler. It is a reservoir of water/steam at the top end of the water tubes. The drum stores the steam generated in the water tubes and acts as a phase-separator for the steam/water mixture...
  
  

External links

• Gas-Liquid Separation Technology Scroll to page 10 of the brochure.
• Characterization of High-Pressure Natural Gas Scrubbers by Trond Austrheim (preprints of papers based on PhD Thesis at the University of Bergen
  University of Bergen
  The University of Bergen is located in Bergen, Norway. Although founded as late as 1946, academic activity had taken place at Bergen Museum as far back as 1825. The university today serves more than 14,500 students...
  
  , Norway
  Norway
  Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
  
  , 2006)
• Flash Steam Tutorial The benefits of recovering flash steam, how it is done and typical applications.
• Quick Calculator for Horizontal Knock Out Drum sizing Based on minimum time required for liquid droplets of a given minimum size to be separated.
• Design Criteria for Vapor/Liquid Separators
• Detailed explanation of high performance vapor-liquid separators (scrubbers)