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Distillation, or classical distillation, is the process of separating the components or substances from a liquid
mixture In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically combined. A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in th ...

mixture
by using selective
boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Ther ...
and
condensation Condensation is the change of the state of matter In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. ...

condensation
.
Dry distillation Dry distillation is the heating of solid materials to produce gaseous products (which may condense into liquids or solids). The method may involve pyrolysis or thermolysis, or it may not (for instance, a simple mixture of ice and glass could be ...
is the heating of solid materials to produce gaseous products (which may condense into liquids or solids). Dry distillation may involve chemical changes such as
destructive distillationDestructive distillation is a chemical process in which decomposition of unprocessed material is achieved by heating it to a high temperature; the term generally applies to processing of organic material in the absence of air or in the presence o ...
or cracking and is not discussed under this article. Distillation may result in essentially complete separation (nearly pure components), or it may be a partial separation that increases the concentration of selected components in the mixture. In either case, the process exploits differences in the
relative volatilityRelative volatility is a measure comparing the vapor pressures of the components in a liquid mixture of chemicals. This quantity is widely used in designing large industrial distillation processes. In effect, it indicates the ease or difficulty of us ...
of the mixture's components. In industrial applications, distillation is a
unit operationImage:LOC MI0086 QuincyMine TIF 00027aS.png, 400px, An ore extraction process broken into its constituent unit operations (Quincy Mine, Hancock, MI ca. 1900) In chemical engineering and related fields, a unit operation is a basic step in a process (e ...
of practically universal importance, but it is a physical separation process, not a
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
. Distillation has many applications. For example: * The distillation of
fermented Fermentation is a metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life Life is a characteristic that distinguishes physical entities that have biological processes, such as signaling ...
products produces
distilled beverage Liquor or spirit (also hard liquor, or distilled alcohol) is an alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that ...
s with a high
alcohol In chemistry, alcohol is an organic compound that carries at least one hydroxyl functional group (−OH) bound to a Saturated and unsaturated compounds, saturated carbon atom. The term alcohol originally referred to the primary alcohol ethan ...

alcohol
content, or separates other fermentation products of commercial value. * Distillation is an effective and traditional method of
desalination Desalination is a process that takes away mineral components from saline water Saline water (more commonly known as salt water) is water that contains a high concentration of solvation, dissolved salts (mainly sodium chloride). The salt conce ...
. * In the
petroleum Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...

petroleum
industry, oil stabilization is a form of partial distillation that reduces the vapor pressure of crude oil, thereby making it safe for storage and transport as well as reducing the atmospheric emissions of volatile hydrocarbons. In midstream operations at
oil refineries An oil refinery or petroleum refinery is an industrial process Industrial processes are procedures involving chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some referenc ...
,
fractional distillation Fractional distillation is the separation of a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matt ...
is a major class of operation for transforming
crude oil Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mech ...
into
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical con ...

fuel
s and chemical feed stocks. *
Cryogenic A medium-sized dewar is being filled with liquid nitrogen by a larger cryogenic storage tank In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motio ...
distillation leads to the separation of air into its components – notably
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

oxygen
,
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
, and
argon Argon is a with the  Ar and  18. It is in group 18 of the and is a . Argon is the third-most abundant in the , at 0.934% (9340 ). It is more than twice as abundant as (which averages about 4000 ppmv, but varies greatly), 23 time ...

argon
– for industrial use. * In the
chemical industry The chemical industry comprises the companies A company, abbreviated as co., is a Legal personality, legal entity representing an association of people, whether Natural person, natural, Legal personality, legal or a mixture of both, with a spe ...
, large amounts of crude liquid products of
chemical synthesis As a topic of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they unde ...
are distilled to separate them, either from other products, from impurities, or from unreacted starting materials. An installation used for distillation, especially of distilled beverages, is a distillery. The distillation equipment itself is a
still A still is an apparatus used to distill liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...

still
.


History

Early evidence of distillation was found on
AkkadianAkkadian or Accadian may refer to: * The Akkadian language Akkadian ( ''akkadû'', ''ak-ka-du-u2''; logogram: ''URIKI'')John Huehnergard & Christopher Woods, "Akkadian and Eblaite", ''The Cambridge Encyclopedia of the World's Ancient Languages' ...

Akkadian
tablets dated c. 1200 BCE describing perfumery operations. The tablets provided textual evidence that an early primitive form of distillation was known to the
Babylonia Babylonia () was an and based in central-southern which was part of Ancient Persia (present-day and ). A small -ruled state emerged in 1894 BCE, which contained the minor administrative town of . It was merely a small provincial town dur ...
ns of ancient
Mesopotamia Mesopotamia ( grc, Μεσοποταμία ''Mesopotamíā''; ar, بِلَاد ٱلرَّافِدَيْن ; syc, ܐܪܡ ܢܗܪ̈ܝܢ, or , ) is a historical region of Western Asia situated within the Tigris–Euphrates river system, in th ...

Mesopotamia
. Early evidence of distillation was also found related to
alchemists File:Aurora consurgens zurich 044 f-21v-44 dragon-pot.jpg, Depiction of Ouroboros from the alchemical treatise ''Aurora consurgens'' (15th century), Zentralbibliothek Zürich, Switzerland Alchemy (from Arabic: ''al-kīmiyā''; from Ancient Gree ...
working in
Alexandria Alexandria ( or ; ar, الإسكندرية ; arz, اسكندرية ; Coptic language, Coptic: Rakodī; el, Αλεξάνδρεια ''Alexandria'') is the List of cities and towns in Egypt, third-largest city in Egypt after Cairo and Giza, ...
in
Roman Egypt , conventional_long_name = Roman Egypt , common_name = Egypt , subdivision = Roman province, Province , nation = the Roman Empire , era = Late antiquity , capital = Alexandria , title_leader = Praefectus Augustalis , image_ ...

Roman Egypt
in the 1st century CE. Distillation was practiced in the ancient Indian subcontinent, which is evident from baked clay
retort In a chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo du ...

retort
s and receivers found at
Taxila Taxila (from Pāli Pali () is a Middle Indo-AryanIndo-Aryan refers to: * Indo-Aryan languages ** Indo-Aryan superstrate in Mitanni or Mitanni-Aryan * Indo-Aryan peoples, the various peoples speaking these languages See also *Aryan inva ...
, Shaikhan Dheri, and
Charsadda Chārsadda ( ps, چارسده , ur, ) is a town and headquarters of Charsadda District, in the Khyber Pakhtunkhwa Khyber Pakhtunkhwa ( ps, خیبر پښتونخوا; ), often abbreviated as KP or KPK and formerly known as the North-We ...
in modern
Pakistan Pakistan, . Pronounced variably in English as , , , and . officially the Islamic Republic of Pakistan, is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a popul ...

Pakistan
, dating to the early centuries of the
Common Era Common Era (CE) is one of the year notations used for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Before the Common Era (BCE) is the era before CE. BCE and CE are alternatives ...
. These "
Gandhara Gandhāra was an ancient region in the Kabul Kabul (; ps, , translit=Kābəl, ; prs, , translit=Kābol, ) is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between ...

Gandhara
stills" were only capable of producing very weak
liquor Liquor or spirit (also hard liquor, or distilled alcohol) is an alcoholic drink An alcoholic drink is a drink A drink (or beverage) is a liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance tha ...

liquor
, as there was no efficient means of collecting the vapors at low heat. (2011)
''Economic History of Medieval India, 1200–1500''
Pearson Education Pearson Education is a British British may refer to: Peoples, culture, and language * British people, nationals or natives of the United Kingdom, British Overseas Territories, and Crown Dependencies. ** Britishness, the British identity a ...
. p. 55.
Distilled water Distilled water is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosph ...
has been in use since at least c. 200 CE, when
Alexander of Aphrodisias Alexander of Aphrodisias ( grc-gre, Ἀλέξανδρος ὁ Ἀφροδισιεύς, translit=Alexandros ho Aphrodisieus; AD) was a Peripatetic philosopher and the most celebrated of the Ancient Greek Ancient Greek includes the forms o ...
described the process. Work on distilling other liquids continued in early
Byzantine Egypt , conventional_long_name = Roman Egypt , common_name = Egypt , subdivision = Roman province, Province , nation = the Roman Empire , era = Late antiquity , capital = Alexandria , title_leader = Praefectus Augustalis , image_ma ...
under
Zosimus of Panopolis Zosimos of Panopolis ( el, Ζώσιμος ὁ Πανοπολίτης; also known by the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken i ...
in the 3rd century. Distillation in China may have begun during the
Eastern Han#REDIRECT Han dynasty The Han dynasty () was the second Dynasties in Chinese history, imperial dynasty of China (202 BC – 220 AD), established by the rebel leader Liu Bang and ruled by the House of Liu. Preceded by the short-lived Qin dynas ...
dynasty (1st–2nd centuries CE), but the distillation of beverages began in the Jin (12th–13th centuries) and
Southern Song The Song dynasty (; ; 960–1279) was an imperial dynasty of China that began in 960 and lasted until 1279. The dynasty was founded by Emperor Taizu of Song Emperor Taizu of Song (21 March 927 – 14 November 976), personal name Zhao Kua ...
(10th–13th centuries) dynasties, according to archaeological evidence. Medieval Muslim chemists such as (Latin: Geber, ninth century) and Abū Bakr al-Rāzī (Latin: Rhazes, ) experimented extensively with the distillation of various substances. The distillation of wine is attested in Arabic works attributed to al-Kindī (c. 801–873 CE) and to (c. 872–950), and in the 28th book of al-Zahrāwī's (Latin: Abulcasis, 936–1013) ''Kitāb al-Taṣrīf'' (later translated into Latin as ''Liber servatoris''). In the twelfth century, recipes for the production of ''aqua ardens'' ("burning water", i.e., ethanol) by distilling wine with salt started to appear in a number of Latin works, and by the end of the thirteenth century it had become a widely known substance among Western European chemists. The works of
Taddeo Alderotti Taddeo Alderotti (Latin: Thaddaeus Alderottus, French language, French : Thaddée de Florence), born in Florence (Italy), Florence between 1206 and 1215, died in 1295, was an Italian doctor and professor of medicine at the University of Bologna, ...
(1223–1296) describe a method for concentrating alcohol involving repeated distillation through a water-cooled still, by which an alcohol purity of 90% could be obtained. The invention of
steam distillationImage:Steam dist.svg, alt=Steam Distillation Apparatus, Steam distillation apparatus in a lab. Steam distillation is a separation process that consists in distillation, distilling water together with other Volatility (chemistry), volatile and non-v ...

steam distillation
is attributed to the Persian physician
Avicenna Ibn Sina ( fa, ابن سینا), also known as Abu Ali Sina (), Pur Sina (), and often known in the West as Avicenna (;  – June 1037), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, t ...

Avicenna
. The
fractional distillation Fractional distillation is the separation of a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matt ...
of organic substances plays an important role in the works attributed to Jābir ibn Ḥayyān, such as in the ('The Book of Seventy'), translated into Latin by
Gerard of Cremona European depiction of the Persian physician Rhazes, in Gerard of Cremona's ''Recueil des traités de médecine'' 1250–1260. Gerard de Cremona translated numerous works by Arab scholars. Gerard of Cremona (Latin Latin (, or , ) is a classical ...
(c. 1114–1187) under the title . The Jabirian experiments with fractional distillation of animal and vegetable substances, and to a lesser degree also of mineral substances, is the main topic of the , an originally Arabic work falsely attributed to
Avicenna Ibn Sina ( fa, ابن سینا), also known as Abu Ali Sina (), Pur Sina (), and often known in the West as Avicenna (;  – June 1037), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, t ...

Avicenna
that was translated into Latin and would go on to form the most important alchemical source for
Roger Bacon Roger Bacon (; la, Rogerus or ', also '' Rogerus''; ), also known by the scholastic accolade It was customary in the European Middle Ages, more precisely in the period of scholasticism which extended into early modern times, to designate th ...
(). A still was found in an archaeological site in Qinglong,
Hebei Hebei (; alternately Hopeh) is a coastal province A province is almost always an administrative division Administrative division, administrative unitArticle 3(1). , country subdivision, administrative region, subnational entity, f ...
province, in China, dating back to the 12th century. Distilled beverages were common during the
Yuan dynasty The Yuan dynasty (), officially the Great Yuan (; xng, , , literally "Great Yuan State"), was a successor state Successor is someone who, or something which succeeds or comes after (see success and succession) Film and TV * ''The Succ ...
(13th–14th centuries). In 1500, German alchemist
Hieronymus Braunschweig Hieronymus Brunschwig or Hieronymus Brunschwygk (c. 1450c. 1512) was a German surgeon ("wund artzot"), alchemy, alchemist and botany, botanist. He was notable for his methods of treatment of gunshot wounds and for his early work on distillation, dis ...
published ''Liber de arte destillandi'' (''The Book of the Art of Distillation''), the first book solely dedicated to the subject of distillation, followed in 1512 by a much expanded version. In 1651, John French published ''The Art of Distillation'', the first major English compendium on the practice, but it has been claimed that much of it derives from Braunschweig's work. This includes diagrams with people in them showing the industrial rather than bench scale of the operation. ] As
alchemy Alchemy (from Arabic Arabic (, ' or , ' or ) is a Semitic language The Semitic languages are a branch of the Afroasiatic language family originating in the Middle East The Middle East is a list of transcontinental countri ...
evolved into the science of
chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. T ...

chemistry
, vessels called
retort In a chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo du ...

retort
s became used for distillations. Both
alembic An alembic (from ar, الإنبيق, al-inbīq, originating from grc, ἄμβιξ, ambix, 'cup, beaker') is an alchemical File:Aurora consurgens zurich 044 f-21v-44 dragon-pot.jpg, Depiction of Ouroboros from the alchemical treatise ''Auro ...

alembic
s and retorts are forms of
glassware The list of glassware includes drinking vessels (drinkware) and tableware used to set a table for eating a meal, general glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread ...
with long necks pointing to the side at a downward angle to act as air-cooled condensers to
condense Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

condense
the distillate and let it drip downward for collection. Later, copper alembics were invented. Riveted joints were often kept tight by using various mixtures, for instance a dough made of rye flour. These alembics often featured a cooling system around the beak, using cold water, for instance, which made the condensation of alcohol more efficient. These were called
pot still 275px, Pot stills at the Lagavulin Distillery A pot still is a type of distillation apparatus or still A still is an apparatus used to distill liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that ...

pot still
s. Today, the retorts and pot stills have been largely supplanted by more efficient distillation methods in most industrial processes. However, the pot still is still widely used for the elaboration of some fine alcohols, such as
cognac Cognac ( , also , ) is a variety of brandy Brandy is a liquor Liquor or spirit (also hard liquor, or distilled alcohol) is an alcoholic drink produced by distillation of grains, fruits, or vegetables that have already gone through al ...
,
Scotch whisky Scotch whisky (; sco, Scots whisky/whiskie, whusk(e)y; often simply called whisky or Scotch) is malt whiskyMalt whisky is whisky made from a fermented mashing, mash consisting primarily of malted barley. If the product is made exclusively at a ...
,
Irish whiskey Irish whiskey ( ga, Fuisce or ''uisce beatha'') is whiskey made on the island of Ireland. The word 'whiskey' (or whisky) comes from the Irish , meaning ''water of life''. Irish whiskey was once the most popular spirit in the world, though a lo ...
,
tequila Tequila (; ) is a liquor, distilled beverage made from the Agave tequilana, blue agave plant, primarily in the area surrounding the city of Tequila, Jalisco, Tequila northwest of Guadalajara, Jalisco, Guadalajara, and in the Jaliscan Highlands ...

tequila
,
rum Rum is a liquor Liquor or spirit (also hard liquor, or distilled alcohol) is an alcoholic drink produced by distillation of grains, fruits, or vegetables that have already gone through alcoholic fermentation. The distillation process conce ...
, and some
vodka Vodka ( pl, wódka , russian: водка , sv, vodka ) is a clear distilled alcoholic beverage. Different varieties originated in Poland Poland, officially the Republic of Poland, is a country located in Central Europe. It is div ...

vodka
s. Pot stills made of various materials (wood, clay, stainless steel) are also used by bootleggers in various countries. Small pot stills are also sold for use in the domestic production of flower water or
essential oils An essential oil is a concentrated hydrophobic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, s ...
. Early forms of distillation involved batch processes using one vaporization and one condensation. Purity was improved by further distillation of the condensate. Greater volumes were processed by simply repeating the distillation. Chemists reportedly carried out as many as 500 to 600 distillations in order to obtain a pure compound.Othmer, D. F. (1982) "Distillation – Some Steps in its Development", in W. F. Furter (ed) ''A Century of Chemical Engineering''. In the early 19th century, the basics of modern techniques, including pre-heating and
reflux Reflux is a technique involving the condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (phy ...

reflux
, were developed. In 1822, Anthony Perrier developed one of the first continuous stills, and then, in 1826, Robert Stein improved that design to make his
patent still A column still, also called a continuous still, patent still or Coffey still, is a variety of still consisting of two columns. Column stills can produce rectified spirit (95% alcohol by volume, ABV). Description The first column (called the anal ...

patent still
. In 1830,
Aeneas Coffey Aeneas Coffey (1780–1839) was an Irish inventor and distiller Distillation, or classical distillation, is the process of separation process, separating the components or substances from a liquid mixture by using selective boiling and conde ...

Aeneas Coffey
got a patent for improving the design even further. Coffey's continuous still may be regarded as the
archetype The concept of an archetype (; from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its popu ...
of modern petrochemical units. The French engineer Armand Savalle developed his steam regulator around 1846. In 1877,
Ernest Solvay Ernest Gaston Joseph Solvay (; 16 April 1838 – 26 May 1922) was a Belgian Belgian may refer to: * Something of, or related to, Belgium Belgium, ; french: Belgique ; german: Belgien officially the Kingdom of Belgium, is a country in ...

Ernest Solvay
was granted a U.S. Patent for a tray column for
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

ammonia
distillation, and the same and subsequent years saw developments in this theme for oils and spirits. With the emergence of
chemical engineering Chemical engineering is a certain type of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline ...
as a discipline at the end of the 19th century, scientific rather than empirical methods could be applied. The developing
petroleum Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isoc ...

petroleum
industry in the early 20th century provided the impetus for the development of accurate design methods, such as the
McCabe–Thiele methodThe McCabe–Thiele method is considered to be the simplest and perhaps most instructive method for the analysis of binary distillation Distillation, or classical distillation, is the process of separating the components or substances from a liq ...
by
Ernest Thiele Ernest W. Thiele (1895–1993) was an influential chemical engineering researcher at Standard Oil (then Amoco, now BP) and Professor of Chemical Engineering at the University of Notre Dame. He is known for his highly impactful work in chemical reac ...
and the
Fenske equation frame, Fractionation at total reflux The Fenske equation in continuous fractional distillation Fractional distillation is the separation of a mixture In chemistry Chemistry is the scientific discipline involved with Chemical element, ele ...
. The first industrial plant in the United States to use distillation as a means of ocean desalination opened in
Freeport, Texas Freeport is a city in Brazoria County, Texas, Brazoria County, Texas, United States, located on the Gulf of Mexico. According to the 2010 United States Census, 2010 census, the city population was 12,136, up from 12,049 in 2010. History Freepo ...
in 1961 with the hope of bringing
water security Water security has been defined as "the reliable availability of an acceptable quantity and quality of water for health, livelihoods and production, coupled with an acceptable level of water-related risks". It is realised to the degree that water s ...
to the region. The availability of powerful computers has allowed direct
computer simulation Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be determ ...
s of distillation columns.


Applications

The application of distillation can roughly be divided into four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinals ( herbal distillate), and
food processing Food processing is the transformation of agricultural products into food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of chemical processes with ...
. The latter two are distinctively different from the former two in that distillation is not used as a true purification method but more to transfer all
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily Volatility (chemistry), vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as Refractory (planetary scien ...
from the source materials to the distillate in the processing of beverages and herbs. The main difference between laboratory scale distillation and industrial distillation are that laboratory scale distillation is often performed on a batch basis, whereas industrial distillation often occurs continuously. In
batch distillationBatch distillation refers to the use of distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation. Dry distillation is th ...
, the composition of the source material, the vapors of the distilling compounds, and the distillate change during the distillation. In batch distillation, a still is charged (supplied) with a batch of feed mixture, which is then separated into its component fractions, which are collected sequentially from most volatile to less volatile, with the bottoms – remaining least or non-volatile fraction – removed at the end. The still can then be recharged and the process repeated. In
continuous distillation Continuous distillation, a form of distillation Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture In chemistry Chemistry is the scientific discipline involved wit ...
, the source materials, vapors, and distillate are kept at a constant composition by carefully replenishing the source material and removing fractions from both vapor and liquid in the system. This results in a more detailed control of the separation process.


Idealized model

The
boiling point The boiling point of a substance is the temperature at which the vapor pressure 280px, The ''pistol test tube'' experiment. The tube contains alcohol and is closed with a piece of cork. By heating the alcohol, the vapors fill in the space, inc ...
of a liquid is the temperature at which the
vapor pressure Vapor pressure (or vapour pressure in British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial cod ...

vapor pressure
of the liquid equals the pressure around the liquid, enabling bubbles to form without being crushed. A special case is the
normal boiling point The boiling point of a substance is the temperature at which the vapor pressure Vapor pressure (or vapour pressure in British English British English (BrE) is the standard dialect A standard language (also standard variety, standar ...
, where the vapor pressure of the liquid equals the ambient
atmospheric pressure Atmospheric pressure, also known as barometric pressure (after the barometer A barometer is a scientific instrument that is used to measure air pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the ...
. It is a misconception that in a liquid mixture at a given pressure, each component boils at the boiling point corresponding to the given pressure, allowing the vapors of each component to collect separately and purely. However, this does not occur, even in an idealized system. Idealized models of distillation are essentially governed by
Raoult's law Raoult's law ( law) is a relation of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifyi ...
and
Dalton's law Dalton's law (also called Dalton's law of partial pressures) states that in a mixture of non-reacting gases, the total pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the ...
and assume that vapor–liquid equilibria are attained. Raoult's law states that the vapor pressure of a solution is dependent on 1) the vapor pressure of each chemical component in the solution and 2) the fraction of solution each component makes up, a.k.a. the
mole fraction In chemistry, the mole fraction or molar fraction (''xi'' or ) is defined as unit of the amount of substance, amount of a constituent (expressed in mole (unit), moles), ''ni'', divided by the total amount of all constituents in a mixture (also expr ...
. This law applies to
ideal solution In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...
s, or solutions that have different components but whose molecular interactions are the same as or very similar to pure solutions. Dalton's law states that the total pressure is the sum of the partial pressures of each individual component in the mixture. When a multi-component liquid is heated, the vapor pressure of each component will rise, thus causing the total vapor pressure to rise. When the total vapor pressure reaches the pressure surrounding the liquid,
boiling Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapour pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding atmosphere. Ther ...
occurs and liquid turns to gas throughout the bulk of the liquid. A mixture with a given composition has one boiling point at a given pressure when the components are mutually soluble. A mixture of constant composition does not have multiple boiling points. An implication of one boiling point is that lighter components never cleanly "boil first". At boiling point, all volatile components boil, but for a component, its percentage in the vapor is the same as its percentage of the total vapor pressure. Lighter components have a higher partial pressure and, thus, are concentrated in the vapor, but heavier volatile components also have a (smaller) partial pressure and necessarily vaporize also, albeit at a lower concentration in the vapor. Indeed, batch distillation and fractionation succeed by varying the composition of the mixture. In batch distillation, the batch vaporizes, which changes its composition; in fractionation, liquid higher in the fractionation column contains more lights and boils at lower temperatures. Therefore, starting from a given mixture, it appears to have a boiling range instead of a boiling point, although this is because its composition changes: each intermediate mixture has its own, singular boiling point. The idealized model is accurate in the case of chemically similar liquids, such as
benzene Benzene is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living organ ...

benzene
and
toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon Aromatic compounds are those chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular enti ...

toluene
. In other cases, severe deviations from Raoult's law and Dalton's law are observed, most famously in the mixture of ethanol and water. These compounds, when heated together, form an
azeotrope An azeotrope () or a constant boiling point mixture is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that m ...

azeotrope
, which is when the vapor phase and liquid phase contain the same composition. Although there are computational methods that can be used to estimate the behavior of a mixture of arbitrary components, the only way to obtain accurate
vapor–liquid equilibrium In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quanti ...
data is by measurement. It is not possible to completely purify a mixture of components by distillation, as this would require each component in the mixture to have a zero
partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mix ...
. If ultra-pure products are the goal, then further chemical separation must be applied. When a binary mixture is vaporized and the other component, e.g., a salt, has zero partial pressure for practical purposes, the process is simpler.


Batch or differential distillation

Heating an ideal mixture of two volatile substances, A and B, with A having the higher volatility, or lower boiling point, in a batch distillation setup (such as in an apparatus depicted in the opening figure) until the mixture is boiling results in a vapor above the liquid that contains a mixture of A and B. The ratio between A and B in the vapor will be different from the ratio in the liquid. The ratio in the liquid will be determined by how the original mixture was prepared, while the ratio in the vapor will be enriched in the more volatile compound, A (due to Raoult's Law, see above). The vapor goes through the condenser and is removed from the system. This, in turn, means that the ratio of compounds in the remaining liquid is now different from the initial ratio (i.e., more enriched in B than in the starting liquid). The result is that the ratio in the liquid mixture is changing, becoming richer in component B. This causes the boiling point of the mixture to rise, which results in a rise in the temperature in the vapor, which results in a changing ratio of A : B in the gas phase (as distillation continues, there is an increasing proportion of B in the gas phase). This results in a slowly changing ratio of A : B in the distillate. If the difference in vapour pressure between the two components A and B is large – generally expressed as the difference in boiling points – the mixture in the beginning of the distillation is highly enriched in component A, and when component A has distilled off, the boiling liquid is enriched in component B.


Continuous distillation

Continuous distillation is an ongoing distillation in which a liquid mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams occur over time during the operation. Continuous distillation produces a minimum of two output fractions, including at least one Volatility (chemistry), volatile distillate fraction, which has boiled and been separately captured as a vapor and then condensed to a liquid. There is always a bottoms (or residue) fraction, which is the least volatile residue that has not been separately captured as a condensed vapor. Continuous distillation differs from batch distillation in the respect that concentrations should not change over time. Continuous distillation can be run at a steady state for an arbitrary amount of time. For any source material of specific composition, the main variables that affect the purity of products in continuous distillation are the reflux ratio and the number of theoretical equilibrium stages, in practice determined by the number of trays or the height of packing. Reflux is a flow from the condenser back to the column, which generates a recycle that allows a better separation with a given number of trays. Equilibrium stages are ideal steps where compositions achieve vapor–liquid equilibrium, repeating the separation process and allowing better separation given a reflux ratio. A column with a high reflux ratio may have fewer stages, but it refluxes a large amount of liquid, giving a wide column with a large holdup. Conversely, a column with a low reflux ratio must have a large number of stages, thus requiring a taller column.


General improvements

Both batch and continuous distillations can be improved by making use of a fractionating column on top of the distillation flask. The column improves separation by providing a larger surface area for the vapor and condensate to come into contact. This helps it remain at equilibrium for as long as possible. The column can even consist of small subsystems ('trays' or 'dishes') which all contain an enriched, boiling liquid mixture, all with their own vapor–liquid equilibrium. There are differences between laboratory-scale and industrial-scale fractionating columns, but the principles are the same. Examples of laboratory-scale fractionating columns (in increasing efficiency) include * Condenser (laboratory)#Air condenser, Air condenser * Vigreux column (usually laboratory scale only) * Packed bed, Packed column (packed with glass beads, metal pieces, or other chemically inert material) * Spinning band distillation system.


Laboratory procedures

Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as a ''
still A still is an apparatus used to distill liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...

still
'', consists at a minimum of a reboiler or ''pot'' in which the source material is heated, a condenser in which the heated gas, vapor is cooled back to the liquid phase (matter), state, and a receiver in which the concentrated or purified liquid, called the distillate, is collected. Several laboratory scale techniques for distillation exist (see also :Distillation, distillation types). A completely sealed distillation apparatus could experience extreme and rapidly varying internal pressure, which could cause it to burst open at the joints. Therefore, some path is usually left open (for instance, at the receiving flask) to allow the internal pressure to equalize with atmospheric pressure. Alternatively, a vacuum pump may be used to keep the apparatus at a lower than atmospheric pressure. If the substances involved are air- or moisture-sensitive, the connection to the atmosphere can be made through one or more drying tubes packed with materials that scavenge the undesired air components, or through bubblers that provide a movable liquid barrier. Finally, the entry of undesired air components can be prevented by pumping a low but steady flow of suitable inert gas, like
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
, into the apparatus.


Simple distillation

In simple distillation, the vapor is immediately channeled into a condenser. Consequently, the distillate is not pure but rather its composition is identical to the composition of the vapors at the given temperature and pressure. That concentration follows
Raoult's law Raoult's law ( law) is a relation of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifyi ...
. As a result, simple distillation is effective only when the liquid boiling points differ greatly (rule of thumb is 25 °C) or when separating liquids from non-volatile solids or oils. For these cases, the vapor pressures of the components are usually different enough that the distillate may be sufficiently pure for its intended purpose. A cutaway schematic of a simple distillation operation is shown at right. The starting liquid 15 in the boiling flask 2 is heated by a combined hotplate and magnetic stirrer 13 via a silicone oil bath (orange, 14). The vapor flows through a short Vigreux column 3, then through a Liebig condenser 5, is cooled by water (blue) that circulates through ports 6 and 7. The condensed liquid drips into the receiving flask 8, sitting in a cooling bath (blue, 16). The adapter 10 has a connection 9 that may be fitted to a vacuum pump. The components are connected by ground glass joints (gray).


Fractional distillation

For many cases, the boiling points of the components in the mixture will be sufficiently close that
Raoult's law Raoult's law ( law) is a relation of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifyi ...
must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components by repeated vaporization-condensation cycles within a packed fractionating column. This separation, by successive distillations, is also referred to as rectification. As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a ''theoretical plate'') will yield a purer solution of the more volatile component. In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; ''theoretical plate'' is thus a concept rather than an accurate description. More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Polytetrafluoroethylene, Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.


Steam distillation

Like vacuum distillation, steam distillation is a method for distilling compounds which are heat-sensitive. The temperature of the steam is easier to control than the surface of a heating element, and allows a high rate of heat transfer without heating at a very high temperature. This process involves bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water. Steam distillation of various aroma, aromatic herbs and flowers can result in two products; an essential oil as well as a watery herbal distillate. The
essential oils An essential oil is a concentrated hydrophobic In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, s ...
are often used in perfumery and aromatherapy while the watery distillates have many applications in aromatherapy, food processing and skin care.


Vacuum distillation

Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator. This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure.


Short path and molecular distillation

Molecular distillation is vacuum distillation below the pressure of 0.01 torr. 0.01 torr is one order of magnitude above Hard vacuum, high vacuum, where fluids are in the free molecular flow regime, i.e. the mean free path of molecules is comparable to the size of the equipment. The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure. That is, because the continuum assumptions of fluid dynamics no longer apply, mass transport is governed by molecular dynamics rather than fluid dynamics. Thus, a short path between the hot surface and the cold surface is necessary, typically by suspending a hot plate covered with a film of feed next to a cold plate with a line of sight in between. Molecular distillation is used industrially for purification of oils. Short path distillation is a distillation technique that involves the distillate travelling a short distance, often only a few centimeters, and is normally done at reduced pressure. A classic example would be a distillation involving the distillate travelling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures or to purify small amounts of compound. The advantage is that the heating temperature can be considerably lower (at reduced pressure) than the boiling point of the liquid at standard pressure, and the distillate only has to travel a short distance before condensing. A short path ensures that little compound is lost on the sides of the apparatus. The Kugelrohr apparatus is a kind of short path distillation method which often contains multiple chambers to collect distillate fractions.


Air-sensitive vacuum distillation

Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a "cow" or "pig" adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used. The Perkin triangle, has means via a series of glass or Polytetrafluoroethylene, Teflon taps to allows fractions to be isolated from the rest of the
still A still is an apparatus used to distill liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external force. ...

still
, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of
reflux Reflux is a technique involving the condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (phy ...

reflux
. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such as
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

nitrogen
or
argon Argon is a with the  Ar and  18. It is in group 18 of the and is a . Argon is the third-most abundant in the , at 0.934% (9340 ). It is more than twice as abundant as (which averages about 4000 ppmv, but varies greatly), 23 time ...

argon
) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected.


Zone distillation

Zone distillation is a distillation process in a long container with partial melting of refined matter in moving liquid zone and condensation of vapor in the solid phase at condensate pulling in cold area. The process is worked in theory. When zone heater is moving from the top to the bottom of the container then solid condensate with irregular impurity distribution is forming. Then most pure part of the condensate may be extracted as product. The process may be iterated many times by moving (without turnover) the received condensate to the bottom part of the container on the place of refined matter. The irregular impurity distribution in the condensate (that is efficiency of purification) increases with the number of iterations. Zone distillation is the distillation analog of zone recrystallization. Impurity distribution in the condensate is described by known equations of zone recrystallization – with the replacement of the distribution co-efficient k of crystallization - for the separation factor α of distillation.


Other types

* The process of reactive distillation involves using the reaction vessel as the still. In this process, the product is usually significantly lower-boiling than its reactants. As the product is formed from the reactants, it is vaporized and removed from the reaction mixture. This technique is an example of a continuous vs. a batch process; advantages include less downtime to charge the reaction vessel with starting material, and less workup. Distillation "over a reactant" could be classified as a reactive distillation. It is typically used to remove volatile impurity from the distallation feed. For example, a little lime (material), lime may be added to remove carbon dioxide from water followed by a second distillation with a little sulfuric acid added to remove traces of ammonia. * Catalytic distillation is the process by which the reactants are catalyzed while being distilled to continuously separate the products from the reactants. This method is used to assist equilibrium reactions in reaching completion. * Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous Artificial membrane, membrane. * Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no
azeotrope An azeotrope () or a constant boiling point mixture is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that m ...

azeotrope
with the other components in the mixture. * Flash evaporation (or partial evaporation) is the partial vaporization 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, being equivalent to a distillation with only one equilibrium stage. * Codistillation is distillation which is performed on mixtures in which the two compounds are not miscible. In the laboratory, the Dean-Stark apparatus is used for this purpose to remove water from synthesis products. The Bleidner apparatus is another example with two refluxing solvents. * Membrane distillation is a type of distillation in which vapors of a mixture to be separated are passed through a membrane, which selectively permeates one component of mixture. Vapor pressure difference is the driving force. It has potential applications in seawater desalination and in removal of organic and inorganic components. The unit process of evaporation may also be called "distillation": * In rotary evaporation a vacuum distillation apparatus is used to remove bulk solvents from a sample. Typically the vacuum is generated by a aspirator (pump), water aspirator or a membrane pump. * In a Kugelrohr apparatus a short path distillation apparatus is typically used (generally in combination with a (high) vacuum) to distill high boiling (> 300 °C) compounds. The apparatus consists of an oven in which the compound to be distilled is placed, a receiving portion which is outside of the oven, and a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump. Other uses: *
Dry distillation Dry distillation is the heating of solid materials to produce gaseous products (which may condense into liquids or solids). The method may involve pyrolysis or thermolysis, or it may not (for instance, a simple mixture of ice and glass could be ...
or
destructive distillationDestructive distillation is a chemical process in which decomposition of unprocessed material is achieved by heating it to a high temperature; the term generally applies to processing of organic material in the absence of air or in the presence o ...
, despite the name, is not truly distillation, but rather a
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and t ...

chemical reaction
known as pyrolysis in which solid substances are heated in an inert or redox, reducing atmosphere and any volatile fractions, containing high-boiling liquids and products of pyrolysis, are collected. The destructive distillation of wood to give methanol is the root of its common name – ''wood alcohol''. * Freeze distillation is an analogous method of purification using freezing instead of evaporation. It is not truly distillation, but a Recrystallization (chemistry), recrystallization where the product is the mother liquor, and does not produce products equivalent to distillation. This process is used in the production of American-style lager#Ice beer, ice beer and ice wine to increase ethanol and sugar content, respectively. It is also used to produce Applejack (beverage), applejack. Unlike distillation, freeze distillation concentrates poisonous congeners rather than removing them; As a result, many countries prohibit such applejack as a health measure. Also, distillation by evaporation can separate these since they have different boiling points. * [ Distillation by filtration] In early alchemy and chemistry, otherwise known as natural philosophy, a form of "distillation" by capillary filtration was known as a form of distillation at the time. In this, a series of cups or bowls were set upon a stepped support with a "wick" of cotton or felt-like material, which had been wetted with water or a clear liquid with each step dripping down through the wetted cloth through capillary action in succeeding steps, creating a "purification" of the liquid, leaving solid materials behind in the upper bowls and purifying the succeeding product through capillary action through the moistened cloth. This was called "distillatio" by filtration by those using the method.


Azeotropic process

Interactions between the components of the solution create properties unique to the solution, as most processes entail non-ideal mixtures, where
Raoult's law Raoult's law ( law) is a relation of physical chemistry Physical chemistry is the study of macroscopic The macroscopic scale is the length scale on which objects or phenomena are large enough to be visible with the naked eye, without magnifyi ...
does not hold. Such interactions can result in a constant-boiling
azeotrope An azeotrope () or a constant boiling point mixture is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that m ...

azeotrope
which behaves as if it were a pure compound (i.e., boils at a single temperature instead of a range). At an azeotrope, the solution contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, Ethanol, ethyl alcohol and Water (molecule), water form an azeotrope of 95.6% at 78.1 °C. If the azeotrope is not considered sufficiently pure for use, there exist some techniques to break the azeotrope to give a pure distillate. This set of techniques are known as azeotropic distillation. Some techniques achieve this by "jumping" over the azeotropic composition (by adding another component to create a new azeotrope, or by varying the pressure). Others work by chemically or physically removing or sequestering the impurity. For example, to purify ethanol beyond 95%, a drying agent (or desiccant, such as potassium carbonate) can be added to convert the soluble water into insoluble water of crystallization. Molecular sieves are often used for this purpose as well. Immiscible liquids, such as water and
toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon Aromatic compounds are those chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular enti ...

toluene
, easily form azeotropes. Commonly, these azeotropes are referred to as a low boiling azeotrope because the boiling point of the azeotrope is lower than the boiling point of either pure component. The temperature and composition of the azeotrope is easily predicted from the vapor pressure of the pure components, without use of Raoult's law. The azeotrope is easily broken in a distillation set-up by using a liquid–liquid separator (a decanter) to separate the two liquid layers that are condensed overhead. Only one of the two liquid layers is refluxed to the distillation set-up. High boiling azeotropes, such as a 20 percent by weight mixture of hydrochloric acid in water, also exist. As implied by the name, the boiling point of the azeotrope is greater than the boiling point of either pure component. To break azeotropic distillations and cross distillation boundaries, such as in the DeRosier Problem, it is necessary to increase the composition of the light key in the distillate.


Breaking an azeotrope with unidirectional pressure manipulation

The boiling points of components in an azeotrope overlap to form a band. By exposing an azeotrope to a vacuum or positive pressure, it's possible to bias the boiling point of one component away from the other by exploiting the differing vapor pressure curves of each; the curves may overlap at the azeotropic point, but are unlikely to remain identical further along the pressure axis to either side of the azeotropic point. When the bias is great enough, the two boiling points no longer overlap and so the azeotropic band disappears. This method can remove the need to add other chemicals to a distillation, but it has two potential drawbacks. Under negative pressure, power for a vacuum source is needed and the reduced boiling points of the distillates requires that the condenser be run cooler to prevent distillate vapors being lost to the vacuum source. Increased cooling demands will often require additional energy and possibly new equipment or a change of coolant. Alternatively, if positive pressures are required, standard glassware can not be used, energy must be used for pressurization and there is a higher chance of side reactions occurring in the distillation, such as decomposition, due to the higher temperatures required to effect boiling. A unidirectional distillation will rely on a pressure change in one direction, either positive or negative.


Pressure-swing distillation

Pressure-swing distillation is essentially the same as the unidirectional distillation used to break azeotropic mixtures, but here both Azeotrope#Pressure swing distillation, positive and negative pressures may be employed. This improves the selectivity of the distillation and allows a chemist to optimize distillation by avoiding extremes of pressure and temperature that waste energy. This is particularly important in commercial applications. One example of the application of pressure-swing distillation is during the industrial purification of ethyl acetate after its catalytic synthesis from ethanol.


Industrial process

Large scale industrial distillation applications include both batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely used industrial applications of continuous, steady-state fractional distillation are in oil refinery, petroleum refineries, petrochemical and chemical plants and natural gas processing plants. To control and optimize such industrial distillation, a standardized laboratory method, ASTM D86, is established. This test method extends to the atmospheric distillation of petroleum products using a laboratory batch distillation unit to quantitatively determine the boiling range characteristics of petroleum products. Industrial distillation is typically performed in large, vertical cylindrical columns known as distillation towers or distillation columns with diameters ranging from about and heights ranging from about or more. When the process feed has a diverse composition, as in distilling
crude oil Petroleum, also known as crude oil and oil, is a naturally occurring, yellowish-black liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the mechanics Mech ...
, liquid outlets at intervals up the column allow for the withdrawal of different ''fractions'' or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column and are often called the bottoms. Industrial towers use
reflux Reflux is a technique involving the condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (phy ...

reflux
to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large-scale industrial distillation tower. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficiency of the distillation tower. The more reflux that is provided for a given number of theoretical plates, the better the tower's separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux that is provided for a given desired separation, the fewer the number of theoretical plates required. Chemical engineers must choose what combination of reflux rate and number of plates is both economically and physically feasible for the products purified in the distillation column. Such industrial fractionating towers are also used in cryogenic air separation, producing liquid oxygen, liquid nitrogen, and high purity
argon Argon is a with the  Ar and  18. It is in group 18 of the and is a . Argon is the third-most abundant in the , at 0.934% (9340 ). It is more than twice as abundant as (which averages about 4000 ppmv, but varies greatly), 23 time ...

argon
. Distillation of chlorosilanes also enables the production of high-purity silicon for use as a semiconductor. Design and operation of a distillation tower depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the
McCabe–Thiele methodThe McCabe–Thiele method is considered to be the simplest and perhaps most instructive method for the analysis of binary distillation Distillation, or classical distillation, is the process of separating the components or substances from a liq ...
or the
Fenske equation frame, Fractionation at total reflux The Fenske equation in continuous fractional distillation Fractional distillation is the separation of a mixture In chemistry Chemistry is the scientific discipline involved with Chemical element, ele ...
can be used. For a multi-component feed, simulation models are used both for design and operation. Moreover, the efficiencies of the vapor–liquid contact devices (referred to as "plates" or "trays") used in distillation towers are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation tower needs more trays than the number of theoretical vapor–liquid equilibrium stages. A variety of models have been postulated to estimate tray efficiencies. In modern industrial uses, a packing material is used in the column instead of trays when low pressure drops across the column are required. Other factors that favor packing are: vacuum systems, smaller diameter columns, corrosive systems, systems prone to foaming, systems requiring low liquid holdup, and batch distillation. Conversely, factors that favor plate columns are: presence of solids in feed, high liquid rates, large column diameters, complex columns, columns with wide feed composition variation, columns with a chemical reaction, absorption columns, columns limited by foundation weight tolerance, low liquid rate, large turn-down ratio and those processes subject to process surges. This packing material can either be random dumped packing ( wide) such as Raschig rings or structured packing, structured sheet metal. Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Unlike conventional tray distillation in which every tray represents a separate point of vapor–liquid equilibrium, the vapor–liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns, it is useful to compute a number of "theoretical stages" to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance. Another factor in addition to the packing shape and surface area that affects the performance of random or structured packing is the liquid and vapor distribution entering the packed bed. The number of Theoretical plate, theoretical stages required to make a given separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio will not be correct in the packed bed and the required separation will not be achieved. The packing will appear to not be working properly. The Theoretical plate, height equivalent to a theoretical plate (HETP) will be greater than expected. The problem is not the packing itself but the mal-distribution of the fluids entering the packed bed. Liquid mal-distribution is more frequently the problem than vapor. The design of the liquid distributors used to introduce the feed and reflux to a packed bed is critical to making the packing perform to it maximum efficiency. Methods of evaluating the effectiveness of a liquid distributor to evenly distribute the liquid entering a packed bed can be found in references.Moore, F., Rukovena, F. (August 1987) ''Random Packing, Vapor and Liquid Distribution: Liquid and gas distribution in commercial packed towers'', Chemical Plants & Processing, Edition Europe, pp. 11–15 Considerable work has been done on this topic by Fractionation Research, Inc. (commonly known as FRI).


Multi-effect distillation

The goal of multi-effect distillation is to increase the energy efficiency of the process, for use in desalination, or in some cases one stage in the production of ultrapure water. The number of effects is inversely proportional to the kW·h/m3 of water recovered figure, and refers to the volume of water recovered per unit of energy compared with single-effect distillation. One effect is roughly 636 kW·h/m3. * Multi-stage flash distillation can achieve more than 20 effects with thermal energy input, as mentioned in the article. * Vapor compression evaporation – Commercial large-scale units can achieve around 72 effects with electrical energy input, according to manufacturers. There are many other types of multi-effect distillation processes, including one referred to as simply multi-effect distillation (MED), in which multiple chambers, with intervening heat exchangers, are employed.


In food processing


Beverages

Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution of ethanol in the process. Spirits such as whiskey and
rum Rum is a liquor Liquor or spirit (also hard liquor, or distilled alcohol) is an alcoholic drink produced by distillation of grains, fruits, or vegetables that have already gone through alcoholic fermentation. The distillation process conce ...
are prepared by distilling these dilute solutions of ethanol. Components other than ethanol, including water, esters, and other alcohols, are collected in the condensate, which account for the flavor of the beverage. Some of these beverages are then stored in barrels or other containers to acquire more flavor compounds and characteristic flavors.


Gallery


See also

* Atmospheric distillation of crude oil * Clyssus * Fragrance extraction * Low-temperature distillation * Microdistillery * Sublimation (chemistry), Sublimation * Dixon rings * Random column packing


References


Further reading

* * * Needham, Joseph (1980)
''Science and Civilisation in China''
Cambridge University Press. .


External links





* {{Authority control Distillation, Unit operations Alchemical processes Separation processes Laboratory techniques Phase transitions Gas technologies Ancient inventions