Editing Distillation (section)

==Laboratory procedures==
Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as a ''[[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 [[:Category: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 tube]]s packed with materials that scavenge the undesired air components, or through [[bubbler]]s 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]], into the apparatus.

===Simple distillation===
[[File:Simple distillation apparatus.svg|300 px|thumb|right|Schematic of a simple distillation setup.]]
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]].

As a result, simple distillation is effective only when the liquid boiling points differ greatly (rule of thumb is 25&nbsp;°C)<ref>[https://web.archive.org/web/20080908000656/http://www.iupac.org/didac/Didac%20Eng/Didac05/Content/ST07.htm ST07 Separation of liquid–liquid mixtures (solutions)], DIDAC by [[IUPAC]]</ref> 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 joint]]s.

===Fractional distillation===
{{Main|Fractional distillation}}
For many cases, the boiling points of the components in the mixture will be sufficiently close that [[Raoult's law]] must be taken into consideration. Therefore, fractional distillation must be used 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.<ref name=Perry/>

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 is 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.<ref>[https://web.archive.org/web/20070903202418/http://wulfenite.fandm.edu/labtech/fractdistill.htm Fractional Distillation]. fandm.edu</ref> 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|PTFE]] or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.<ref>[http://www.brinstrument.com/fractional-distillation/spinning_band_distillation.html Spinning Band Distillation] {{webarchive|url=https://web.archive.org/web/20060825161209/http://www.brinstrument.com/fractional-distillation/spinning_band_distillation.html |date=25 August 2006 }}. B/R Instrument Corporation (accessed 8 September 2006)</ref>

===Steam distillation===
{{Main|Steam distillation}}
Like [[vacuum distillation]], steam distillation is a method for distilling compounds which are heat-sensitive.<ref name="Harwood_Moody" />{{rp|pages=151–153}} 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]]tic herbs and flowers can result in two products: an [[essential oil]] as well as a watery [[herbal distillate]]. The [[essential oils]] are often used in perfumery and [[aromatherapy]] while the watery distillates have many applications in [[aromatherapy]], [[food processing]] and [[skin care]].

[[File:Vacuum distillation of DMSO at 70C.jpg|left|thumb|[[Dimethyl sulfoxide]] usually boils at 189{{nbsp}}°C. Under a vacuum, it distills off into the receiver at only 70{{nbsp}}°C.]]
[[File:perkin triangle distillation apparatus.svg|thumb|Perkin triangle distillation setup
{{#invoke:list|horizontal_ordered
 | Stirrer bar/anti-bumping granules
 | Still pot
 | Fractionating column
 | Thermometer/Boiling point temperature
 | Teflon tap 1
 | Cold finger
 | Cooling water out
 | Cooling water in
 | Teflon tap 2
 | Vacuum/gas inlet
 | Teflon tap 3
 | Still receiver
}}]]

===Vacuum distillation===
{{Main|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.

===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 ===
{{Main|Short-path distillation}}
[[File:short path distillation apparatus.svg|thumb|right|Short path vacuum distillation apparatus with vertical condenser (cold finger), to minimize the distillation path;
{{#invoke:list|horizontal_ordered
 | style=display:inline;
 | list_style=display:inline;
 | Still pot with stirrer bar/anti-bumping granules
 | Cold finger – bent to direct condensate
 | Cooling water out
 | cooling water in
 | Vacuum/gas inlet
 | Distillate flask/distillate.
}}]]
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.<ref name="Harwood_Moody" />{{rp|page=150}} 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]], 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]]. 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]] or [[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.<ref>{{cite journal |last=Kravchenko |first=A. I. |title=Зонная дистилляция: новый метод рафинирования|trans-title=Zone distillation: a new method of refining|url=http://dspace.nbuv.gov.ua/handle/123456789/111613 |journal=Problems of Atomic Science and Technology |year=2011 |volume=6 |issue=19 |pages=24–26 |language=ru}}</ref><ref>{{cite journal |last=Kravchenko |first=A. I. |title=Zone distillation: justification |url=http://dspace.nbuv.gov.ua/handle/123456789/79912|journal=Problems of Atomic Science and Technology |year=2014 |volume=1 |issue=20 |pages=64–65 }}</ref><ref>{{cite journal |last=Kravchenko |first=A. I. |title=Разработка перспективных схем зонной дистилляции|trans-title=Design of advanced processes of zone distillation |journal=Perspectivnye Materialy |year=2014 |number=7 |pages=68–72 |language=ru|url=http://j-pm.imet-db.ru/?archive&a=1573}}</ref>

===Closed-system vacuum distillation (cryovap)===
Non-condensable gas can be expelled from the apparatus by the vapor of relatively volatile co-solvent, which spontaneously evaporates during initial pumping, and this can be achieved with regular oil or diaphragm pump.<ref>{{cite journal |last1=Kolesnichenko |first1=I.V. |last2=Goloverda |first2=G.Z. |last3=Kolesnichenko |first3=V.L. |title=A Versatile Method of Ambient-Temperature Solvent Removal |journal=Organic Process Research & Development |date=2020 |volume=24 |issue=1 |pages=25–31 |doi=10.1021/acs.oprd.9b00368|pmid=34092997 |pmc=8174813 }}</ref><ref>{{cite journal |last1=Kolesnichenko |first1=V.L. |last2=Goloverda |first2=G.Z. |last3=Kolesnichenko |first3=I.V. |last4=Wang |first4=G. |title=Methods of Solvent Removal at Ambient Temperatures – Cryovap |journal=US Patent |volume=2021/0178287 Al |url=https://www.patentguru.com/US2021178287A1}}</ref>

===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 distillation 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]] 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 [[solvent]]s 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&nbsp;°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]] or [[destructive distillation]], despite the name, is not truly distillation, but rather a [[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.