Separation process
Encyclopedia
In chemistry
and chemical engineering
, a separation process, or simply a separation, is any mass transfer
process used to convert a mixture
of substances into two or more distinct product mixtures, at least one of which is enriched in one or more of the mixture's constituents. In some cases, a separation may fully divide the mixture into its pure constituents. Separations are carried out based on differences in chemical properties such as size, shape, mass, or chemical affinity between the constituents of a mixture, and are often classified according to the particular differences they use to achieve separation. In the case that no single difference can be used to accomplish a desired separation, multiple processes will often be performed in combination to achieve the desired end.
Barring a few exceptions, almost every element
or compound
is naturally found in an impure state. Often these impure raw materials must be separated into their purified components before they can be put to productive use, making separation processes essential for the modern industrial economy. In some cases these separations require total purification, as in the electrolysis refining of bauxite
ore for aluminum metal, but a good example of an incomplete separation process is oil
refining. Crude oil occurs naturally as a mixture of various hydrocarbons
and impurities. The refining process splits this mixture into other, more valuable mixtures such as natural gas
, gasoline
and chemical feedstocks, none of which are pure substances, but each of which must be separated from the raw crude. In both these cases a series of separations is necessary to obtain the desired end products. In the case of aluminum refining, bauxite ore is first converted to alumina, a compound of aluminum and oxygen, and then further refined into pure aluminum metal. In the case of oil refining, crude is subjected to a long series of individual distillation
steps, each of which produces a different product or intermediate.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
and chemical engineering
Chemical engineering
Chemical engineering is the branch of engineering that deals with physical science , and life sciences with mathematics and economics, to the process of converting raw materials or chemicals into more useful or valuable forms...
, a separation process, or simply a separation, is any mass transfer
Mass transfer
Mass transfer is the net movement of mass from one location, usually meaning a stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. Mass transfer is used...
process used to convert a mixture
Mixture
In chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...
of substances into two or more distinct product mixtures, at least one of which is enriched in one or more of the mixture's constituents. In some cases, a separation may fully divide the mixture into its pure constituents. Separations are carried out based on differences in chemical properties such as size, shape, mass, or chemical affinity between the constituents of a mixture, and are often classified according to the particular differences they use to achieve separation. In the case that no single difference can be used to accomplish a desired separation, multiple processes will often be performed in combination to achieve the desired end.
Barring a few exceptions, almost every element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
or compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
is naturally found in an impure state. Often these impure raw materials must be separated into their purified components before they can be put to productive use, making separation processes essential for the modern industrial economy. In some cases these separations require total purification, as in the electrolysis refining of bauxite
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
ore for aluminum metal, but a good example of an incomplete separation process is oil
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...
refining. Crude oil occurs naturally as a mixture of various hydrocarbons
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
and impurities. The refining process splits this mixture into other, more valuable mixtures such as natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
, gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
and chemical feedstocks, none of which are pure substances, but each of which must be separated from the raw crude. In both these cases a series of separations is necessary to obtain the desired end products. In the case of aluminum refining, bauxite ore is first converted to alumina, a compound of aluminum and oxygen, and then further refined into pure aluminum metal. In the case of oil refining, crude is subjected to a long series of individual distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
steps, each of which produces a different product or intermediate.
Various types of separation processes
- AdsorptionAdsorptionAdsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
, adhesion of atoms, ions or molecules of gas, liquid, or dissolved solids to a surface. - CentrifugationCentrifugationCentrifugation is a process that involves the use of the centrifugal force for the sedimentation of mixtures with a centrifuge, used in industry and in laboratory settings. More-dense components of the mixture migrate away from the axis of the centrifuge, while less-dense components of the mixture...
and cyclonic separationCyclonic separationCyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. Rotational effects and gravity are used to separate mixtures of solids and fluids...
, separates based on density differences. - ChromatographyChromatographyChromatography is the collective term for a set of laboratory techniques for the separation of mixtures....
separates dissolved substances by different interaction with (i.e., travel through) a material. - CrystallizationCrystallizationCrystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
. - DecantationDecantationDecantation is a process for the separation of mixtures. This is achieved by carefully pouring a solution from a container in order to leave the precipitate in the bottom of the original container...
. - Demister (vapor)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...
, removes liquid droplets from gas streams. - DistillationDistillationDistillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
, used for mixtures of liquids with different boiling points. - DryingDryingDrying is a mass transfer process consisting of the removal of water or another solvent by evaporation from a solid, semi-solid or liquid. This process is often used as a final production step before selling or packaging products. To be considered "dried", the final product must be solid, in the...
, removes liquid from a solid by vaporisation. - ElectrophoresisElectrophoresisElectrophoresis, also called cataphoresis, is the motion of dispersed particles relative to a fluid under the influence of a spatially uniform electric field. This electrokinetic phenomenon was observed for the first time in 1807 by Reuss , who noticed that the application of a constant electric...
, separates organic molecules based on their different interaction with a gelGelA gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
under an electric potential (i.e., different travel). - ElutriationElutriationElutriation, also known as air classification, is a process for separating lighter particles from heavier ones using a vertically-directed stream of gas or liquid . This method is predominately used for particles with size...
. - EvaporationEvaporationEvaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
. - ExtractionExtraction (chemistry)Extraction in chemistry is a separation process consisting in the separation of a substance from a matrix. It may refer to Liquid-liquid extraction, and Solid phase extraction....
.- Leaching.
- Liquid-liquid extractionLiquid-liquid extractionLiquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
. - Solid phase extractionSolid phase extractionSolid-phase extraction is a separation process by which compounds that are dissolved or suspended in a liquid mixture are separated from other compounds in the mixture according to their physical and chemical properties. Analytical laboratories use solid phase extraction to concentrate and purify...
.
- FlotationFlotationFlotation involves phenomena related to the relative buoyancy of objects. The term may refer to:* Flotation, any material added to the hull of a watercraft to keep the hull afloat...
.- Dissolved air flotation, removes suspended solids non-selectively from slurry by bubbles that are generated by air coming out of solution.
- Froth flotationFroth flotationFroth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in several processing industries...
, recovers valuable, hydrophobic solids by attachment to air bubbles generated by mechanical agitation of an air-slurry mixture, which float, and are recovered. - DeinkingDeinkingDeinking is the industrial process of removing printing ink from paperfibers of recycled paper to make deinked pulp.The key in the deinking process is the ability to detach ink from the fibers. This is achieved by a combination of mechanical action and chemical means. In Europe the most common...
, separating hydrophobic ink particles from hydrophilic paper pulp in paper recyclingPaper recyclingPaper recycling is the process of recovering waste paper and remaking it into new paper products. There are three categories of paper that can be used as feedstocks for making recycled paper: mill broke, pre-consumer waste, and post-consumer waste. Mill broke is paper trimmings and other paper...
.
- FlocculationFlocculationFlocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
, separates a solid from a liquid in a colloid, by use of a flocculant, which promotes the solid clumping into flocs. - FiltrationFiltrationFiltration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
, MeshMeshMesh consists of semi-permeable barrier made of connected strands of metal, fiber, or other flexible/ductile material. Mesh is similar to web or net in that it has many attached or woven strands.-Types of mesh:...
, bag and paper filters are used to remove large particulates suspended in fluids (e.g., fly ashFly ashFly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
) while membrane processesMembrane technologyThe membrane technology covers all process engineering measures for the transport of substances between two fractions with the help of permeable membranes...
including microfiltrationMicrofiltrationMicrofiltration is a membrane technical filtration process which removes contaminants from a fluid by passage through a microporous membrane. A typical microfiltration membrane pore size range is 0.1 to 10 micrometres...
, ultrafiltrationUltrafiltrationUltrafiltration is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane...
, nanofiltrationNanofiltrationNanofiltration is a relatively recent membrane filtration process used most often with low total dissolved solids water such as surface water and fresh groundwater, with the purpose of softening and removal of disinfection by-product precursors such as natural organic matter and synthetic organic...
, reverse osmosisReverse osmosisReverse osmosis is a membrane technical filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and...
, dialysis (biochemistry)Dialysis (biochemistry)In biochemistry, dialysis is the process of separating molecules in solution by the difference in their rates of diffusion through a semipermeable membrane, such as dialysis tubing....
utilising synthetic membranes, separates micrometreMicrometreA micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
-sized or smaller species. - Fractional distillationFractional distillationFractional distillation is the separation of a mixture into its component parts, or fractions, such as in separating chemical compounds by their boiling point by heating them to a temperature at which several fractions of the compound will evaporate. It is a special type of distillation...
- Fractional freezingFractional freezingFractional freezing is a process used in process engineering and chemistry to separate two liquids with different melting points. It can be done by partial melting of a solid, for example in zone refining of silicon or metals, or by partial crystallization of a liquid, for example "freeze...
- Oil-water separationAPI oil-water separatorAn API oil-water separator is a device designed to separate gross amounts of oil and suspended solids from the wastewater effluents of oil refineries, petrochemical plants, chemical plants, natural gas processing plants and other industrial sources...
, gravimetrically separates suspended oil droplets from waste water in oil refineriesOil refineryAn 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...
, petrochemicalPetrochemicalPetrochemicals 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 plantChemical plantA 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, natural gas processingNatural gas processingNatural-gas processing is a complex industrial process designed to clean raw natural gas by separating impurities and various non-methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas.-Background:...
plants and similar industries. - Magnetic separationMagnetic separationMagnetic separation is a process in which magnetically susceptible material is extracted from a mixture using a magnetic force. This separation technique can be useful in mining iron as it is attracted to a magnet....
. - PrecipitationPrecipitation (chemistry)Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
. - RecrystallizationRecrystallization (chemistry)-Chemistry:In chemistry, recrystallization is a procedure for purifying compounds. The most typical situation is that a desired "compound A" is contaminated by a small amount of "impurity B". There are various methods of purification that may be attempted , which includes recrystallization...
. - SedimentationSedimentationSedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...
, separates using density differences.- Gravity separationGravity separationGravity separation is an industrial method of separating two components, either a suspension, or dry granular mixture where separating the components with gravity is sufficiently practical: ie the components of the mixture have different specific weight. All of the gravitational methods are common...
.
- Gravity separation
- Sieving.
- StrippingStripping (chemistry)Stripping is a physical separation process where one or more components are removed from a liquid stream by a vapor stream. In industrial applications the liquid and vapor streams can have co-current or countercurrent flows. Stripping is usually carried out in either a packed or trayed column.-...
. - SublimationSublimation (physics)Sublimation is the process of transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase...
. - Vapor-liquid separationSouders-Brown equationThe Souders-Brown equation has been the tool for obtaining the maximum allowable vapor velocity in vapor–liquid separation vessels...
, separates by gravity, based on the Souders-Brown equation. - Winnowing.
- Zone refining.
See also
- Analytical chemistryAnalytical chemistryAnalytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
- Chemical engineeringChemical engineeringChemical engineering is the branch of engineering that deals with physical science , and life sciences with mathematics and economics, to the process of converting raw materials or chemicals into more useful or valuable forms...
- Unit operations