chemical engineering Chemical engineering is an engineering field which deals with the study of the operation and design of chemical plants as well as methods of improving production. Chemical engineers develop economical commercial processes to convert raw materials ...

, a vapor–liquid separator is a device used to separate a vapor–liquid mixture into its constituent

phases Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform *Phase space, a mathematica ...

. It can be a vertical or horizontal vessel, and can act as a 2-phase or 3-phase separator. A vapor–liquid separator may also be referred to as a ''flash drum'', ''breakpot'', ''knock-out drum'' or ''knock-out pot'', ''compressor suction drum, suction scrubber'' or ''compressor inlet drum,'' or ''vent scrubber.'' When used to remove suspended water droplets from streams of air, it is often called a demister.

Method of operation

In vapor-liquid separators

gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

is utilized to cause the denser fluid (liquid) to settle to the bottom of the vessel where it is withdrawn, less dense fluid (vapor) is withdrawn from the top of the vessel. In

low gravity Weightlessness is the complete or near-complete absence of the sensation of weight, i.e., zero apparent weight. It is also termed zero g-force, or zero-g (named after the g-force) or, incorrectly, zero gravity. Weight is a measurement of the fo ...

environments such as a

space station A space station (or orbital station) is a spacecraft which remains orbital spaceflight, in orbit and human spaceflight, hosts humans for extended periods of time. It therefore is an artificial satellite featuring space habitat (facility), habitat ...

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

centrifugal force Centrifugal force is a fictitious force in Newtonian mechanics (also called an "inertial" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It appears to be directed radially away from the axi ...

needs to be utilised in a spinning centrifugal separator to drive liquid towards the outer edge of the chamber for removal. Gaseous components migrate towards the center. An inlet diffuser reduces the velocity and spreads the incoming mixture across the full cross-section of the vessel. A mesh pad in the upper part of the vessel aids separation and prevents liquid from being carried over with the vapor. The pad or mist mat traps entrained liquid droplets and allows them to coalesce until they are large enough to fall through the up-flowing vapor to the bottom of the vessel. Vane packs and cyclonic separators are also used to remove liquid from the outlet vapor. The gas outlet may itself be surrounded 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. The vapor travels through the gas outlet at a design velocity which minimises the

entrainment Entrainment may refer to: * Air entrainment, the intentional creation of tiny air bubbles in concrete * Brainwave entrainment, the practice of entraining one's brainwaves to a desired frequency * Entrainment (biomusicology), the synchronization o ...

of any liquid droplets in the vapor as it exits the vessel. A vortex breaker on the liquid outlet prevents the formation of vortices and of vapor being drawn into the liquid outlet.

Liquid level monitoring

The separator is only effective as long as there is a vapor 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 A sight glass or water gauge is a type of level sensor, a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within. Liquid in tanks Simple sight glasses may be just a plastic or glass tube ...

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

Applications

Vertical separators are generally used when the gas-liquid ratio is high or gas volumes are high. Horizontal separators are used where large volumes of liquid are involved. A vapor-liquid separator may operate as a 3-phase separator, with two immiscible liquid phases of different densities. For example natural gas (vapor), water and oil/condensate. The two liquids settle at the bottom of the vessel with oil floating on the water. Separate liquid outlets are provided. The feed to a vapor–liquid separator may also be a liquid that is being partially or totally ''flashed'' into a vapor and liquid as it enters the separator. A slug catcher is a type of vapor–liquid separator that is able to receive a large inflow of liquid at random times. It is usually found at the end of gas pipelines where condensate may be present as slugs of liquid. It is usually a horizontal vessel or array of large diameter pipes. The liquid capacity of a separator is usually defined by the residence time of the liquid in the vessel. Some typical residence times are as shown.

Where vapor–liquid separators are used

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

Oil refineries An oil refinery or petroleum refinery is an industrial process plant where petroleum (crude oil) is transformed and refined into products such as gasoline (petrol), diesel fuel, asphalt base, fuel oils, heating oil, kerosene, liquefied pet ...

Offshore platforms Offshore may refer to: Science and technology * Offshore (hydrocarbons) * Offshore construction, construction out at sea * Offshore drilling, discovery and development of oil and gas resources which lie underwater through drilling a well * Off ...

* Natural-gas processing plants (NGL) *

Petrochemical Petrochemicals (sometimes abbreviated as petchems) are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable s ...

and

chemical plant A chemical plant is an industrial process plant that manufactures (or otherwise processes) chemicals, usually on a large scale. The general objective of a chemical plant is to create new material wealth via the chemical or biological transfor ...

s * Refrigeration systems *

Air conditioning Air conditioning, often abbreviated as A/C (US) or air con (UK), is the process of removing heat from an enclosed space to achieve a more comfortable interior temperature, and in some cases, also controlling the humidity of internal air. Air c ...

* Compressor systems * Gas pipelines *Steam condensate flash drums *Geothermal power plants *Combined cycle power plants *

Flare stack A gas flare, alternatively known as a flare stack, flare boom, ground flare, or flare pit, is a gas combustion device used in places such as petroleum refineries, chemical plants and natural gas processing plants, oil or gas extraction sites ha ...

s * Soil vapor extraction *

Paper mill A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags, and other ingredients. Prior to the invention and adoption of the Fourdrinier machine and other types of paper machine that use an endless belt ...

s *

Liquid ring pump A liquid-ring pump is a rotating positive-displacement gas pump, with liquid under centrifugal force acting as a Seal (mechanical), seal. Description of operation Liquid-ring pumps are typically used as vacuum pumps, but can also be used as gas ...

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 A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor. Many compressors can be staged, that is, the gas is compressed several times in steps o ...

types such as the

scroll compressor A scroll compressor (also called ''spiral compressor'', scroll pump and scroll vacuum pump) is a device for Compressor, compressing air or refrigerant. It is used in air conditioning equipment, as an automobile supercharger (where it is known ...

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

References

External links

Characterization of High-Pressure Natural Gas Scrubbers'' by Trond Austrheim
(preprints of papers based on PhD Thesis at the

University of Bergen The University of Bergen () is a public university, public research university in Bergen, Norway. As of 2021, the university had over 4,000 employees and 19,000 students. It was established by an act of parliament in 1946 consolidating several sci ...

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. The remote Arctic island of Jan Mayen and the archipelago of Svalbard also form part of the Kingdom of ...

, 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)''Vapor Liquid Separator designs and manufacturing process
{{DEFAULTSORT:Vapor-liquid separator Chemical equipment Oil refineries Natural gas technology Heating, ventilation, and air conditioning Industrial equipment Gas-liquid separation Gas technologies