Distillation is a process of separating component substances from liquid mixtures through vaporisation and condensation, based on different volatility (vaporization point) of components in the mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction.
Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Air is distilled to separate its components—notably oxygen, nitrogen, and argon—for industrial use. Liquid chemicals for diverse uses are distilled after synthesis to remove impurities and unreacted starting materials. Distillation of fermented solutions can produce distilled beverages
with a higher alcohol content. The premises where distillation is
carried out, especially distillation of alcohol, are known as a
distillery. A
still is the equipment used for distillation.
History
See also: Distilled beverage
Distillation apparatus of Zosimos of Panopolis, from Marcelin Berthelot,
Collection des anciens alchimistes grecs (3 vol., Paris, 1887–1888).
The first evidence of distillation comes from Greek alchemists working in Alexandria in the 1st century AD.
[2] Distilled water has been known since at least c. 200, when Alexander of Aphrodisias described the process.
[3] Distillation in China could have begun during the Eastern Han Dynasty (1st–2nd centuries), but archaeological evidence indicates that actual distillation of beverages began in the Jin and Southern Song dynasties.
[4] A still was found in an archaeological site in Qinglong, Hebei province dating to the 12th century. Distilled beverages were more common during the Yuan dynasty.
[4] Arabs learned the process from the Alexandrians and used it extensively in their chemical experiments.
[citation needed]
Clear evidence of the distillation of alcohol comes from the School of Salerno in the 12th century.
[2][5] Fractional distillation was developed by Tadeo Alderotti in the 13th century.
[6]
In 1500, German alchemist Hieronymus Braunschweig published
Liber de arte destillandi (The Book of the Art of Distillation)
[7] 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 of practice, though it has been claimed
[8]
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.
Hieronymus Brunschwig's
Liber de arte Distillandi de Compositis (Strassburg, 1512) Chemical Heritage Foundation
Old Ukrainian vodka still
Simple liqueur distillation in East Timor
As alchemy evolved into the science of chemistry, vessels called retorts became used for distillations. Both alembics and retorts are forms of glassware with long necks pointing to the side at a downward angle which acted as air-cooled condensers to 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.
[9]
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 stills.
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, Scotch whisky, tequila and some vodkas. 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 the domestic production
[10] of flower water or essential oils.
Early forms of distillation were 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 were reported to carry out as many
as 500 to 600 distillations in order to obtain a pure compound.
[11]
In the early 19th century the basics of modern techniques including pre-heating and reflux were developed, particularly by the French,
[11] then in 1830 a British Patent was issued to Aeneas Coffey for a whiskey distillation column,
[12] which worked continuously and may be regarded as the archetype of modern petrochemical units. In 1877, Ernest Solvay was granted a U.S. Patent for a tray column for ammonia distillation
[13] and the same and subsequent years saw developments of this theme for oil and spirits.
With the emergence of chemical engineering as a discipline at the end of the 19th century, scientific rather than empirical methods could be applied. The developing petroleum industry in the early 20th century provided the impetus for the development of accurate design methods such as the McCabe–Thiele method and the Fenske equation. The availability of powerful computers has also allowed direct computer simulation of distillation columns.
Applications of distillation
The application of distillation can roughly be divided in four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinals (herbal distillate), and food processing.
The latter two are distinctively different from the former two in that
in the processing of beverages, the distillation is not used as a true
purification method but more to transfer all volatiles from the source materials to the distillate.
The main difference between laboratory scale distillation and
industrial distillation is that laboratory scale distillation is often
performed batch-wise, whereas industrial distillation often occurs
continuously. In batch distillation,
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,
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
better control of the separation process.
Idealized distillation model
The boiling point of a liquid is the temperature at which the 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, where the vapor pressure of the liquid equals the ambient atmospheric pressure.
It is a common misconception that in a liquid mixture at a given
pressure, each component boils at the boiling point corresponding to the
given pressure and the vapors of each component will collect separately
and purely. This, however, does not occur even in an idealized system.
Idealized models of distillation are essentially governed by Raoult's law and Dalton's law, 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 aka the
mole fraction. This law applies to ideal solutions,
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 vapor pressure is the sum of the
vapor 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
occurs and liquid turns to gas throughout the bulk of the liquid. Note
that a mixture with a given composition has one boiling point at a given
pressure, when the components are mutually soluble.
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 evaporate also, albeit being less concentrated in the vapor.
Indeed, batch distillation and fractionation succeed by varying the
composition of the mixture. In batch distillation, the batch evaporates,
which changes its composition; in fractionation, liquid higher in the
fractionation column contains more lights and boils at lower
temperatures.
The idealized model is accurate in the case of chemically similar liquids, such as benzene and 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,
which is a composition with a boiling point higher or lower than the
boiling point of each separate liquid. Virtually all liquids, when mixed
and heated, will display azeotropic behaviour. 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 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. If ultra-pure products are the goal, then further chemical separation
must be applied. When a binary mixture is evaporated and the other
component, e.g. a salt, has zero partial pressure for practical
purposes, the process is simpler and is called evaporation in engineering.
Batch distillation
Main article: Batch distillation
A batch still showing the separation of A and B.
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
which 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 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 in turn 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 A : B in the
distillate.
If the difference in vapor 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
Main article: 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 as time
passes during the operation. Continuous distillation produces at least
two output fractions, including at least one volatile
distillate fraction, which has boiled and been separately captured as a
vapor 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 (practically, 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
- Air condenser
- Vigreux column (usually laboratory scale only)
- Packed column (packed with glass beads, metal pieces, or other chemically inert material)
- Spinning band distillation system.
Laboratory scale distillation
Typical laboratory distillation unit
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 vapour is cooled back to the liquid 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 types).
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.
As a result, simple distillation is effective only when the liquid boiling points differ greatly (rule of thumb is 25 °C)
[14]
or when separating liquids from non-volatile solids or oils. For these
cases, the vapor pressures of the components are usually sufficiently
different that the distillate may be sufficiently pure for its intended
purpose.
Fractional distillation
Main article: 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 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.
[15]
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.
[16] 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 Teflon
or metal to force the rising vapors into close contact with the
descending condensate, increasing the number of theoretical plates.
[17]
Steam distillation
Main article: Steam distillation
Like vacuum distillation,
steam distillation is a method for distilling compounds which are heat-sensitive.
[18]
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 aromatic 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.
Dimethyl sulfoxide usually boils at 189 °C. Under a vacuum, it distills off into the receiver at only 70 °C.
Perkin triangle distillation setup1:
Stirrer bar/anti-bumping granules
2: Still pot
3: Fractionating column
4: Thermometer/Boiling point temperature
5: Teflon tap 1
6: Cold finger
7: Cooling water out
8: Cooling water in
9: Teflon tap 2
10: Vacuum/gas inlet
11: Teflon tap 3
12: Still receiver
Vacuum distillation
Main article: 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 is vacuum distillation below the pressure of 0.01 torr.
[19] 0.01 torr is one order of magnitude above 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.
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 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.
Short path distillation
Short path vacuum distillation apparatus with vertical condenser (cold finger), to minimize the distillation path;
1: Still pot with stirrer bar/anti-bumping granules
2: Cold finger – bent to direct condensate
3: Cooling water out
4: cooling water in
5: Vacuum/gas inlet
6: 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.
[20]
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 is a kind of a short path distillation apparatus which often contain multiple chambers to collect distillate fractions.
Zone distillation
Zone distillation is a distillation process in 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 number of repetitions of the process. Zone distillation
is a distillation analog of zone recrystallization. Impurity
distribution in the condensate is described by known equations of zone
recrystallization with various numbers of iteration of process – with
replacement distribution efficient k of crystallization on separation
factor α of distillation.
[21][22]
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 may be added to remove carbon dioxide from water followed by a second distillation with a little sulphuric 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 reach completion.
- Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous 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.
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 water aspirator or a membrane pump.
- In a kugelrohr
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 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 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 where the product is the mother liquor, and does not produce products equivalent to distillation. This process is used in the production of ice beer and ice wine to increase ethanol and sugar content, respectively. It is also used to produce applejack.
Unlike distillation, freeze distillation concentrates poisonous
congeners rather than removing them; As a result, many countries
prohibit such applejack as a health measure. However, reducing methanol
with the absorption of 4A molecular sieve is a practical method for production.[23] Also, distillation by evaporation can separate these since they have different boiling points.
Azeotropic distillation
Main article: Azeotropic distillation
Interactions between the components of the solution create properties
unique to the solution, as most processes entail nonideal mixtures,
where Raoult's law does not hold. Such interactions can result in a constant-boiling
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, ethyl alcohol and 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 an additional 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 a (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,
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 weight percent 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 vapour pressure curves of each; the curves
may overlap at the azeotropic point, but are unlikely to be remain
identical further along the pressure axis 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 vapours 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
Further information: Pressure-Swing Distillation (section on the main Azeotrope page)
Pressure-swing distillation is essentially the same as the
unidirectional distillation used to break azeotropic mixtures, but here
both 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 distillation
Typical industrial distillation towers
Main article: Continuous distillation
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 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.
Automatic Distillation Unit for the determination of the boiling range of petroleum products at atmospheric pressure
Industrial distillation
[15][24] is typically performed in large, vertical cylindrical columns known as
distillation towers or
distillation columns
with diameters ranging from about 65 centimeters to 16 meters and
heights ranging from about 6 meters to 90 meters or more. When the
process feed has a diverse composition, as in distilling crude oil, 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.
Diagram of a typical industrial distillation tower
Industrial towers use 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. Distillation of chlorosilanes also enables the production of high-purity silicon for use as a semiconductor.
Section of an industrial distillation tower showing detail of trays with bubble caps
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 method
[15][25] or the Fenske equation
[15] 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.
Large-scale, industrial vacuum distillation column
[26]
This packing material can either be random dumped packing (1–3" wide) such as Raschig rings or 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 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 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.
[27][28] Considerable work as been done on this topic by Fractionation Research, Inc. (commonly known as FRI).
[29]
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/m
3 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/m
3.
- 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.
Distillation in food processing
Distilled beverages
Main article: Distilled beverage
Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum
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
 |
Chemistry in its beginnings used retorts as laboratory equipment exclusively for distillation processes. |
 |
A simple set-up to distill dry and oxygen-free toluene. |
 |
Diagram of an industrial-scale vacuum distillation column as commonly used in oil refineries |
 |
A rotary evaporator is able to distill solvents more quickly at lower temperatures through the use of a vacuum. |
 |
Distillation using semi-microscale apparatus. The jointless design
eliminates the need to fit pieces together. The pear-shaped flask allows
the last drop of residue to be removed, compared with a similarly-sized
round-bottom flask
The small holdup volume prevents losses. A pig is used to channel the
various distillates into three receiving flasks. If necessary the
distillation can be carried out under vacuum using the vacuum adapter at
the pig. |
