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Fractional distillation of binary solvent mixtureBy - 05/25/2006
in
Objective
To separate the binary mixture by simple distillation using fractionating column
Prinicple/Background of the Experiment
Distillation is apurification technique in which compounds with different boiling points
can be separated by controlled heating. Vapors from a sufficiently heated sample can
be recondensed and collected, purer than the initial mixture.The liquid which has not
vaporized is called the residue, and the liquid which is collected in the receiver is called
the distillate.
Since not all chemicals distill the same way, there are several distillation techniques can
be preferred depending on the nature of constituents to be purified or to be separated.
These include simple distillation, fractional distillation, steam distillation and
vacuum distillation .
A simple distillation (figure 2) is for purifying liquids of one component (separating
nonvolatile liquid impurity or to purify a liquid from solid contaminants), multiple liquids
where the differences in boiling points is very large (a low boiling liquid from a high
boiling liquid)(b.p difference around 50 -70C). Simple distillations are not effective in
removing multiple solvents from one another with a high degre e of success.
In fractional distillation (figure 3), a fractionating column is inserted between the
distillation flask and the distillation head. The fractionating column provides a large
surface area in which the mixture can be continuously vaporized and condensed.
The principle of a fractionating column is that, as the vapours ascend the column from
the boiling mixture below, the high boiling components are condensed and returned to
the flask, the ascending column of vapour being thus steadily scrubbed by the
descending column of liquid condensate. The ascending column of the vapour becomes
therefore steadily richer in the lowest boiling component, and the descending column of
condensate steadily richer in the highest boiling component.
Figure 1 represents the typical curve for simple and fractional distillation. In an ideal
fractional distillation, two distinct fractions are obtained. The first corresponds to the
component with the lower boiling point and the second to the high -boiling point
component. What characterizes a good fractional distillation is the sudden increase intemperature between both fractions, or in other words, a very small volume distilled at
temperatures other than the boiling points of the pure liquids. In simple distillation, a
much more gradual increase in temperature is observed, reflecting the impure nature of
the distillate
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Figure 1. Simple and fractional distillation curves
Steam distillation is used for separating mixtures of chemicals such as oils, resins,
hydrocarbons, etc. which are insoluble in water and may decompose at their B.P.
Vacuum distillation is used for separating liquids boiling above 200C
Fractional Distillation Apparatus
The set-up for fractional distillation is shown in Figure 3. It consists of a round -bottomed
distillation flask where the liquid is placed, a fractionalting column, a distillation head that
connects the distillation flask to the condenser; and a distillation adaptor that connects
the condenser to the receiving flask. The condenser is a tube surrounded by a water
jacket to cool and condense vapors. The distillation head holds a thermometer to allow
the temperature of the vapors to be monitored during the distillation.
Figure 2 Simple distillation set -up
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Procedure:
Assemble the apparatus as shown in Figure 3, use graduated cylinder to collect the
distillate. Place the liquid (25 mL of cyclohexane (b.p.80.7C) and 25 mL of
toluene(b.p.111C) or 25 mL of benzene (b.p.80C)or 25 mL of of toluene) to be distilled
into the distillation flaskPlace a boiling chip in the distillation flask to ensure smooth
bubbling and prevent bumping of the liquid up into the distillation head. Heat the liquids
and distil slowly, so that the total distillation occupies about 1 hour and 30 minutes. As
the liquid boils, vapors rise into fractionating column and to the distillation head and
condensed liquid will be seen dripping from the thermometer bulb. Eventually the vapors
enter the side arm of the distillation head and continue into the condenser. Once in the
condenser the vapors are cooled due to the water circulating in the outer jacket; the
vapors condense back to a liquid that runs down the condenser and is collected in the
receiving flask. Increase the heating rate near the midpoint of the distillation otherw ise
the head temperature will drop.
Take temperature readings at the first drop and at each 2 ml increment. Continue until
you have distilled 48 ml(collect the fractions having boiling points (a) 80 85, (b) 85-
107 (c) 107-111, these fractions should have the volumes about 23, 2, 23 mL
respectively). Construct a table in your notebook as given below, to record the
temperature at the distillation "head" as a function of volume distilled.
Volume distilled (mL) 2 4 6 8 10 12 14 16 1820
Temperature without column
Temperature with column
Make a plot of Temperature (C) Vs. volume of distillate Co llected (ml) and determine
the b.p of each of these solvents.
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The separation or purification of liquids by vaporization and condensation is a
very important step in one of our oldest professions, The word "still" lives onas a tribute to the importance of organic chemistry. There are two important
points here.
1.Vaporization . Turning a liquid to a vapor.
2.Condensation. Turning a vapor to a liquid.
Remember these. They show up on quizzes. But when do I use distillation?That is a very good question. Use the guidelines below to pick your special
situation, and turn to that section. But you should read all the sections
anyway.
1.Class1: Simpledistillation. Separating liquids boiling below 150C atone atmosphere (1 atm) from
a. Nonvolatile impurities.b. Another liquid boiling at least 25C higher than the first. The liquids
should dissolve in each other.
2.Class 2:Vacuumdistillation. Separating liquids boiling above 150C at1 atm from
a. Nonvolatile impurities.b. Another liquid boiling a t least 25C higher than the first. They
should dissolve in one another.
3.Class 3: Fractional distillation. Separating liquid mixtures, soluble ineach other, that boil at less than 25C from each other at 1 atm.
4.Class 4: Steam distillation. Isolating tars, oils, and other liquidcompounds that are insoluble, or slightly soluble, in water at alltemperatures. Usually, natural products are steam distilled. Theydo nothave to be liquids at room temperatures. (For example, caffeine, a
solid, can be isolated from green tea.)
Remember, these are guides. If your compound boils at 150.0001C, don't
scream that you must do a vacuum distillation or both you and your productwill die. I expect you to have some judgment and to pay attention to your
instructor's specific directions.
Chapter XXXIII. Distillation Under Reduced Pressure
Vacuum distillation in the petroleum industry has not up to the present received the
attention it deserves. Plants for the distillation of lubricating oils are often operated under
vacuum, but this is seldom sufficiently high to give the best results, in many cases,indeed, being so low as to have little marked effect on the distillation. The advantages ofdistilling under vacuum are: -The fractions are distilled off at relatively lower
temperatures, cracking or decomposition being thus largely avoided. The distillatesobtained are of better colour and of higher flash -point; the residues are also of better
quality, as they have not been cracked and so contain little or no free carbon. From waxbase oils, forexample, a better yield, not .only of the heavier lubricating oils but also of
the higher grade waxes, may be obtained. ood distillates suitable for concentrating tocylinder oils may thus be obtained. The difference in character between wax and asphalt
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oils, as far as yield of lubricating oils is concerned, largely disappears under highvacuum distillation.1 Vacuum distillation plants are naturally more costly as regards
capital outlay and operating wages. This is, however, counterbalanced by the fact thatthe products obtained are of better quality and require less subsequent chemical
treatment. The system finds application, in the petroleum industry, chiefly for themanufacture of lubricating oils ; but there is no reason why it should not be applied to
other operations as well, the treatment of which lies outside the scope of this work.
Vacuum distillation may be applied either to periodic or continuous distillation. Thesimpler forms of plant consist of a still of the ordinary type, structurally strengthened to
withstand the external pressure, with dephlegmators, coolers, and receiving boxes, allunder vacuum. The system is connected to the top of a barometric condenserandexhausted to the required vacuum by an air pump. Many forms of plant, differing in detail
but similar in principle, have been devised, e.g. that of Henderson, designed as far backas 1883 (Eng. Pat. 5401 of 1883 and 17332 of 1889), those of Lennard (Eng. Pat. 944 of
1892) (applied in the coal -tar industry), of Zaloziecki, Palmer, Wanklyn and Cooper (Eng.Pat. 4097 of 1893) and others, all of which operate at relatively low vacuums. Fig. 150
illustrates the main features of a simple vacuum pla nt.
1 L. Singer, Petroleum, Berlin, 10, 605. 352
The still is of the ordinary type, internally strengthened to withstand t he externalpressure. It is fitted with perforated pipes in the usual way, as the distillation is invariably
conducted with the aid of steam.
The vapour pipe, which is always of large diameter, is connected to several domes onthe still. It leads to dephlegmators, or atmospheric condensers, where the heavier
fractions are condensed. The first air -cooled condenser often takes the form of a numberof large diameter horizontal pipes. The condensate from these air coolers flows through
a cooler into a receiving tank connected to the air pump. The receiving tanks are induplicate, so that that which receives the distillate can always be kept under vacuum,
while the other is being pumped out. Several air -cooled condensers may be employed,and also water-cooled condensers, the condensates from which run off through coolers
to receiving tanks under vacuum. The vapours containing the lightest fractions and thesteam are finally condensed in a spray condenser, elevated and connected with a water
pipe terminating in a water seal below, of such a height that it functions as a barometer
tube, the vacuum in this barometric condenser being maintained by an air pump.An apparatus designed for working at high vacuum is that of Steinschneider ( .S. Pat.981953 of 1911). One of the chief objections to the previously described schemes is thatthe receivers for the distillates are under vacuum, an inconvenient arrangement, as they
are not under complete observation and control. Further, if evacuation takes place via
the distillate receiver, the lighter vapours are retained in contact with the distillates,whereas they should be removed as quickly as possible. This could be avoided byplacing each receiver tank at the bottom of an oil barometer, but this would necessitate
either building the plant very high, or else much excavating, both of which areexpensive. Steinschneider avoids these difficulties in the following way (Fig. 151). The
distillate vapours pass from the still s through dephlegmators or air condensers d to thecooler c and on to the elevated barometric condenser b. The distillates condensed in the
dephlegmators d flow away through the coolers e to the receivers r, which may be fitted
with floats for regulating the level of the liquid contained in them.
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Laboratory di
lay of ditillation: 1: A heating devi e 2: Still pot3: Still head 4:Thermometer/Boiling point
temperature 5:Condenser6: Cooling waterin 7: Cooling water out8: Distillate/receiving flask9:Vacuum/gas
inlet10: Still receiver11: Heat control12: Stirrer speed control13:Stirrer/heat plate 14: Heating (Oil/sand)
bath15: Stirring means e.g.(shown), anti-bumping granules or mechanical stirrer16: Cooling bath.[1]
Distillationis a method ofseparatingmixtures based on differences in theirvolatilitiesin a boiling
liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical
reaction.
Commercially, distillation has a number of applications. Itis used to separate crude oilinto more
fractions for specific uses such as transport,power generation and heating. Wateris distilled to
remove impurities, such as salt from seawater. Airis distilled to separate its components
notably oxygen,nitrogen, and argonforindustrialuse. Distillation offermentedsolutions has been
used since ancienttimes to produce distilled beverages with a higher alcohol content. The premises
where distillation is carried out, especially distillation of alcohol, are known as a distillery.
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Contents
[hide]
1 History
2 Applications of distillation
3 Idealized distillation model
o 3.1 Batch distillation
o 3.2 Continuous distillation
o 3.3 General improvements
4 Laboratory scale distillation
o 4.1 Simple distillation
o 4.2 Fractional d istillation
o 4.3 Steam distillation
o 4.4 Vacuum distillation
o 4.5 Air-sensitive vacuum distillation
o 4.6 Short path distillation
o 4.7 Other types
5 Azeotropic distillation
o 5.1 Breaking an azeotrope with unidirectional pressure manipulation
o 5.2 Pressure-swing distillation
6 Industrial distillation
7 Distillation in food processing
o 7.1 Distilled beverages
8 Gallery
9 Notes
10 Further reading
11 External links
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Distillation
Old Ukrainian vodka still
As alchemy evolved into the science ofchemistry, vessels called retorts became sed for distillations.
Both alembics and retorts are forms ofglassware 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 sing vario s
mi t res, for instance a do gh made of rye flo r.[8]
These alembics often feat red a cooling system
aro
nd the beak,
sing cold water for instance, which made the condensation of alcohol more
efficient. These were calledpot stills. Today, the retorts and pot stills have been largely s pplanted by
more efficient distillation methods in most ind strial processes. However, the pot still is still widely
sed for the elaboration of some fine alcohols s ch as cognac, Scotch whisky,te
ila and
some vodkas. Pot stills made of vario s materials (wood, clay, stainless steel) are also sed
bybootleggers in vario s co ntries. Small pot stills are also sold for the domestic prod ction[9]
of
flower water oressential oils.
Early forms of distillation were batch processes sing one vaporization and one condensation. P rity
was improved by f rther distillation of the condensate. Greater vol mes were processed by simply
repeating the distillation. Chemists were reported to carry o t as many as 500 to 600 distillations in
order to obtain a p re compo nd[10]
.
In the early 19th cent ry the basics of modern techni es incl ding pre heating and refl were
developed, partic larly by the French[10]
, then in 1830 a British Patent was iss ed to Aeneas
Coffey for a whiskey distillation col mn[11], which worked contin o sly and may be regarded as
thearchetype of modern petrochemical nits. In 1877, Ernest Solvay was granted a U.S. Patent for a
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tray column forammonia distillation[12] and the same and subsequent years saw developments ofthis
theme for oil and spirits.
With the emergence ofchemical engineering as a discipline atthe end ofthe 19th century, scientific
ratherthan empirical methods could be applied. The developingpetroleumindustry in the early 20th
century provided the impetus forthe development of accurate design methods such as theMcCabe-
Thiele method and the Fenske equation. The availability of powerful computers has also allowed
directcomputer 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 lattertwo are distinctively different from the formertwo in thatin the processing of beverages,
the distillation is notused as a true purification method but more to transfer allvolatiles from the
source materials to the distillate.
The main difference between laboratory scale distillation and industrial distillation is thatlaboratory
scale distillation is often performed batch-wise, whereas industrial distillation often occurs
continuously. Inbatch distillation, the composition ofthe source material, the vapors ofthe distilling
compounds and the distillate change during the distillation. In batch distillation, a stillis 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 atthe 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 ofthe separation process.
Ideali ed distillation model
Theboiling point of a liquid is the temperature at which the vapor pressure ofthe liquid equals the
pressure in the liquid, enabling bubbles to form without being crushed. A special case is the normal
boiling point, where the vapor pressure ofthe liquid equals the ambientatmospheric pressure.
Itis a common misconception thatin a liquid mixture at a given pressure, each component boils atthe
boiling point corresponding to the given pressure and the vapors of each component will collectseparately and purely. This, however, does not occur even in an ideali ed system. Ideali ed models of
distillation are essentially governed by Raoult s law and Dalton's law, and assume thatvapor-liquid
equilibria are attained.
Raoult's law assumes that a component contributes to the totalvapor pressure ofthe mixture in
proportion to its percentage ofthe mixture and its vapor pressure when pure, or succinctly: partial
pressure equals mole fraction multiplied by vapor pressure when pure. If one component changes
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another component's vapor press re, or if the volatility of a component is dependent on its percentage
in the mi t re, the law will fail.
Dalton's law states that the total vapor press re is the s m of the vapor press res of each individ al
component in the mi t re. When a m lti component li id is heated, the vapor press re of each
component will rise, th
s ca
sing the total vapor press
re to rise. When the total vapor press
re
reaches the press re s rro nding the li
id,boiling occ rs and li
id t rns to gas thro gho t the b lk
of the li id. Note that a mi t re with a given composition has one boiling point at a given press re,
when the components are m t ally sol ble.
An implication of one boiling point is that lighter components never cleanly "boil first". At boiling
point, all volatile components boil, b t for a component, its percentage in the vapor is the same as its
percentage of the total vapor press re. Lighter components have a higher partial press re and th s are
concentrated in the vapor, b t heavier volatile components also have a (smaller) partial press re and
necessarily evaporate also, albeit being less concentrated in the vapor. Indeed, batch distillation and
fractionation s cceed by varying the composition of the mi t re. In batch distillation, the batch
evaporates, which changes its composition; in fractionation, li id higher in the fractionation col mn
contains more lights and boils at lower temperat res.
The idealized model is acc rate in the case of chemically similar li ids, s ch asbenzene and tol ene.
In other cases, severe deviations from Rao lt's law and Dalton's law are observed, most famo sly in
the mi t re of ethanol and water. These compo nds, when heated together, form an azeotrope, which
is a composition with a boiling point higher or lower than the boiling point of each separate li id.
Virt ally all li
ids, when mi ed and heated, will display azeotropic behavio r. Altho gh there
are comp tational methods that can be sed to estimate the behavior of a mi t re of arbitrary
components, the only way to obtain acc rate vapor li id e ilibri m data is by meas rement.
It is not possible to completely p rify a mi t re of components by distillation, as this wo ld re
ire
each component in the mi t re to have a zeropartial press re. If ltra p re prod cts are the goal, then
f rtherchemical separation m st be applied. When a binary mi t re is evaporated and the other
component, e.g. a salt, has zero partial press re for practical p rposes, the process is simpler and is
called evaporation in engineering.
Batch di tillati
Main article:Batch distillation
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A batch still showing the separation of A and B.
Heating an ideal mi t re of two volatile s bstances A and B (with A having the higher volatility, or
lower boiling point) in a batch distillation set p (s ch as in an apparat s depicted in the opening
fig re) ntil the mi t re is boiling res lts in a vapor above the li id which contains a mi t re of A
and B. The ratio between A and B in the vapor will be different from the ratio in the li id: the ratio
in the li id will be determined by how the original mi t re was prepared, while the ratio in the vapor
will be enriched in the more volatile compo nd, A (d e to Rao lt's Law, see above). The vapor goes
thro gh the condenser and is removed from the system. This in t rn means that the ratio of
compo nds in the remaining li id is now different from the initial ratio (i.e. more enriched in B than
the starting li
id).
The res lt is that the ratio in the li
id mi t re is changing, becoming richer in component B. This
ca
ses the boiling point of the mi
t
re to rise, which in t
rn res
lts in a rise in the temperat
re in the
vapor, which res lts in a changing ratio of A :B in the gas phase (as distillation contin es, there is an
increasing proportion ofB in the gas phase). This res lts in a slowly changing ratio A :B in the
distillate.
If the difference in vapor press re between the two components A and B is large (generally e pressed
as the difference in boiling points), the mi t re in the beginning of the distillation is highly enriched
in component A, and when component A has distilled off, the boiling li id is enriched in component
B.
Continuous distillation
Contin o s distillation is an ongoing distillation in which a li
id mi t re is contin o sly (witho t
interr ption) fed into the process and separated fractions are removed contin o sly as o tp t streams
as time passes d ring the operation. Contin o s distillation prod ces at least two o tp t fractions,
incl ding at least one volatile distillate fraction, which has boiled and been separately capt red as a
vapor condensed to a li id. There is always a bottoms (or resid e) fraction, which is the least volatile
resid e that has not been separately capt red as a condensed vapor.
Contin o s distillation differs from batch distillation in the respect that concentrations sho ld not
change over time. Contin o s distillation can be r n at a steady state for an arbitrary amo nt of time.
For any so
rce material of specific composition, the main variables that affect the p
rity of prod
cts
in contin o s distillation are the refl ratio and the n mber of theoretical e ilibri m stages
(practically, the n mber of trays or the height of packing). Refl is a flow from the condenser back
to the col mn, which generates a recycle that allows a better separation with a given n mber of trays.
E ilibri m stages are ideal steps where compositions achieve vapor li id e ilibri m, repeating the
separation process and allowing better separation given a refl ratio. A col mn with a high refl
ratio may have fewer stages, b t it refl es a large amo nt of li
id, giving a wide col mn with a
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to separate the components well by repeated vaporization condensation cycles within a packed
fractionating col mn. This separation, by s ccessive distillations, is also referred to
as recti ication[14].
As the sol tion to be p rified is heated, its vapors rise to the fractionating col mn. As it rises, it cools,
condensing on the condenser walls and the s
rfaces of the packing material. Here, the condensate
contin es to be heated by the rising hot vapors; it vaporizes once more. However, the composition of
the fresh vapors are determined once again by Rao lt's law. Each vaporization condensation cycle
(called a theoretical plate) will yield a p j rer solj tion of the more volatile component.[15] In reality,
each cycle at a given temperat j re does not occj r at ek actly the same position in the fractionating
col j mn; theoretical plate is thj s a concept rather than an accj rate description.
More theoretical plates lead to better separations. A spinning band distillation system j ses a spinning
band ofTeflon or metal to force the rising vapors into close contact with the descending condensate,
increasing the nj mber of theoretical plates.[16]
[edit]Steam distillation
Main article: Steam distillation
Like vac l l m distillation, steam distillation is a method for distilling compo l nds which are heatm
sensitive.[17]
This process involves l sing bl bbling steam thro l gh a heated mi n t l re of the raw material.
By Raol lt's law, some of the target compo l nd will vaporize (in accordance with its partial pressl re).
The vapor mi n tl re is cooled and condensed, l s l ally yielding a layer of oil and a layer of water.
Steam distillation of vario l s aromatic herbs and flowers can res l lt in two prodl cts; an essential oil as
well as a watery herbal distillate. The essential oils are often
l
sed in perf
l
meryand aromatherapywhile the watery distillates have many applications in aromatherapy, food
processing and skin care.
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Dimethyl so lfo ideo so ally boils at 189 C. Under a vaco o m, it distills off into the receiver at only 70C.
erkin triangle distillationsetup
1: Stirrer bar/antibo mping grano les 2: Still pot3: Fractionating colo mn 4:Thermometer/Boiling point
temperato re 5:Teflon tap 1 6:Cold finger7:Cooling water oo t 8:Cooling water in 9: Teflon tap 2 10:Vaco o m/gas
inlet 11: Teflon tap 3 12: Still receiver
[edit]Vacuum distillation
Main article: Vacuum distillation
Some compo nds have very high boiling points. To boil s ch compo nds, it is often better to lower
the press re at which s ch compo nds are boiled instead of increasing the temperat re. Once the
press re is lowered to the vapor press re of the compo nd (at the given temperat re), boiling and the
rest of the distillation process can commence. This techni e is referred to as vacuum distillation and
it is commonly fo nd in the laboratory in the form of therotary evaporator.
This techni e is also very sef l for compo nds which boil beyond theirdecomposition
temperat re at atmospheric press re and which wo ld therefore be decomposed by any attempt to boil
them nder atmospheric press re.
Molecular distillation is vac m distillation below the press re of 0.01 torr.[18]
0.01 torr is one order
of magnit de above high vac m, where fl ids are in the free molec lar flow regime, i.e. the mean
free path of molec les is comparable to the size of the e
ipment. The gaseo s phase no longer e erts
significant press re on the s bstance to be evaporated, and conse
ently, rate of evaporation no
longer depends on press re. That is, beca se the contin m ass mptions of fl id dynamics no longer
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apply, mass transport is governed by molec lar dynamics rather than fl id dynamics. Th s, a short
path between the hot s rface and the cold s rface is necessary, typically by s spending a hot plate
covered with a film of feed ne t to a cold plate with a clear line of sight in between. Molec lar
distillation is sed ind strially for p rification of oils.
[edit]Air-sensitivevacuum distillationSome compo nds have high boiling points as well as being air sensitive. A simple vac m distillation
system as e emplified above can be sed, whereby the vac m is replaced with an inert gas after the
distillation is complete. However, this is a less satisfactory system if one desires to collect fractions
nder a red ced press re. To do this a "pig" adaptor can be added to the end of the condenser, or for
better res lts or for very air sensitive compo nds a Perkin triangle apparat s can be sed.
The Perkin triangle, has means via a series of glass orTeflon taps to allows fractions to be isolated
from the rest of the still, witho t the main body of the distillation being removed from either the
vac m or heat so rce, and th s can remain in a state ofrefl . To do this, the sample is first isolated
from the vac m by means of the taps, the vac m over the sample is then replaced with an inert gas
(s ch as nitrogen orargon) and can then be stoppered and removed. A fresh collection vessel can then
be added to the system, evac ated and linked back into the distillation system via the taps to collect a
second fraction, and so on, ntil all fractions have been collected.
[edit]Short path distillation
Short path vacz z m distillation apparatz s with vertical condenser (cold finger), to minimize the distillation
path; 1: Still pot with stirrer bar/anti{
bz
mping granz
les 2:Cold finger{
bent to direct condensate 3:Cooling wateroz t 4: cooling water in 5: Vacz z m/gas inlet 6: Distillate flask/distillate.
Short path distillation is a distillation techni| e that involves the distillate travelling a short distance,
often only a few centimeters, and is normally done at red ced press re.[19]
A classic e ample wo ld
be a distillation involving the distillate travelling from one glass b lb to another, witho t the need for
a condenser separating the two chambers. This techni| e is often sed for compo nds which are
nstable at high temperat res or to p rify small amo nts of compo nd. The advantage is that the
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heating temperature can be considerably lower (at reduced pressure) than the boiling point ofthe
liquid at standard pressure, and the distillate only has to travel a short distance before condensing. A
short path ensures thatlittle compound is lost on the sides ofthe apparatus. The Kugelrohris a kind of
a short path distillation apparatus which often contain multiple chambers to collect distillate fractions.
[edit]Other types
The process ofreactive distillationinvolves using the reaction vessel as the still. In this process,
the productis usually significantly lower-boiling than its reactants. As the productis formed from
the reactants, itis vapori} ed 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.
Pervaporationis a method forthe separation of mixtures ofliquids by partial vapori~ ation through
a non-porous membrane.
Extractive distillationis 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 throttlingvalve or other
throttling device. This process is one ofthe simplestunit operations, being equivalentto 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 ofevaporation may also be called "distillation":
In rotary evaporation a vacuum distillation apparatus is used to remove bulksolvents from a
sample. Typically the vacuum is generated by a wateraspiratoror a membrane pump.
In a kugelrohra 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 ofthe oven, and
a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump.
Otheruses:
Dry distillation ordestructive distillation, despite the name, is nottruly distillation, but rather
a chemical reaction known aspyrolysisin which solid substances are heated in an inert
orreducingatmosphere and any volatile fractions, containing high-boiling liquids and products of
pyrolysis, are collected. The destructive distillation ofwoodto give methanolis the root ofits
common name -wood al ohol.
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Freeze distillationis an analogous method of purification using freezinginstead of evaporation. It
is nottruly distillation, but a recrystallization where the productis the motherliquor, and does not
produce products equivalentto distillation. This process is used in the production ofice
beerand ice wineto increase ethanol and sugarcontent, respectively. Itis also used to
produce applejack. Unlike distillation, freeze distillation concentrates poisonous congeners ratherthan removing them.
[edit]Azeotropic distillation
Mai arti l Azeotropi distillation
Interactions between the components ofthe solution create properties unique to the solution, as most
processes entail nonideal mixtures, where Raoult's law does not hold. Such interactions can resultin a
constant-boiling azeotrope which behaves as ifit were a pure compound (i.e., boils at a single
temperature instead of a range). At an azeotrope, the solution contains the given componentin the
same proportion as the vapor, so that evaporation does not change the purity, and distillation does noteffect separation. For example, ethyl alcohol and waterform an azeotrope of 95.6% at 78.1 C.
Ifthe azeotrope is not considered sufficiently pure foruse, there exist some techniques to breakthe
azeotrope to give a pure distillate. This set oftechniques are known as azeotropic distillation. Some
techniques achieve this by "jumping" overthe azeotropic composition (by adding an additional
componentto 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 aspotassium carbonate) can be added to convertthe soluble water
into insoluble water of crystallization. Molecular sieves are often used forthis 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 ofthe azeotrope is lowerthan the
boiling point of either pure component. The temperature and composition ofthe azeotrope is easily
predicted from the vapor pressure ofthe pure components, withoutuse ofRaoult'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 ofthe 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 ofthe azeotrope is greaterthan the boiling point of eitherpure component.
To break azeotropic distillations and cross distillation boundaries, such as in the DeRosierProblem, it
is necessary to increase the composition ofthe light key in the distillate.
[edit]Breaking an azeotrope with unidirectional pressure manipulation
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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 atthe
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 sothe azeotropic band disappears.
This method can remove the need to add other chemicals to a distillation, butit has two potential
drawbacks.
Under negative pressure, power for a vacuum source is needed and the reduced boiling points ofthe
distillates requires thatthe condenser be run coolerto prevent distillate vapours being lostto 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 highertemperatures required to effect boiling.
A unidirectional distillation will rely on a pressure change in one direction, either positive or negative.
ressure-swing distillation
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.[clarification needed]
This has an importantimpact on the selectivity ofthe distillation and allows a chemist[citation needed]
to
optimize a process such that fewer extremes of pressure and temperature are required and less energyis consumed. This is particularly importantin commercial applications.
Pressure-swing distillation is employed during the industrial purification ofethyl acetate afterits
catalytic synthesis from ethanol.
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[edit]Ind strial distillation
Typical ind strial distillation towers
Main article: Continuous distillation
Large scale industrial distillation applications incl de both batch and contin o s fractional, vac m,
azeotropic, e tractive, and steam distillation. The most widely sed ind strial applications of
contin
o
s, steady
state fractional distillation are inpetrole
m refineries,petrochemical and chemical
plantsand nat ral gas processing plants.
Ind strial distillation[14][20]
is typically performed in large, vertical cylindrical col mns known
as distillation towers ordistillation columns with diameters ranging from abo t 65 centimeters to 16
meters and heights ranging from abo t 6 meters to 90 meters or more. When the process feed has a
diverse composition, as in distilling cr de oil, li id o tlets at intervals p the col mn allow for the
withdrawal of differentfractions or prod cts having differentboiling points or boiling ranges. The
"lightest" prod cts (those with the lowest boiling point) e it from the top of the col mns and the
"heaviest" prod cts (those with the highest boiling point) e it from the bottom of the col mn and are
often called the bottoms.
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Diagram of a typicalindustrial distillation tower
Industrialtowers use refluxto achieve a more complete separation of products. Reflux refers to the
portion ofthe condensed overhead liquid product from a distillation or fractionation towerthatis
returned to the upper part ofthe tower as shown in the schematic diagram of a typical, large-scale
industrial distillation tower. Inside the tower, the downflowing refluxliquid provides cooling and
condensation ofthe upflowing vapors thereby increasing the efficiency ofthe distillation tower. The
more refluxthatis provided for a given number oftheoretical plates, the betterthe tower's separation
oflower boiling materials from higher boiling materials. Alternatively, the more refluxthatis
provided for a given desired separation, the fewerthe number oftheoretical plates required.
Such industrial fractionating towers are also used in air separation, producing liquid oxygen, liquid
nitrogen, and high purity argon. Distillation ofchlorosilanes also enables the production of high-
purity silicon foruse as asemiconductor.
Section of an industrial distillation tower showing detail oftrays with bubble c aps
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Design and operation of a distillation tower depends on the feed and desired prod cts. Given a simple,
binary component feed, analytical methods s ch as the McCabe Thiele method[14][21]
or theFenske
e ation[14] can be sed. For a m lti component feed, sim lation models are sed both for design and
operation. Moreover, the efficiencies of the vapor li id contact devices (referred to as "plates" or
"trays")
sed in distillation towers are typically lower than that of a theoretical 100%efficient e ilibri m stage. Hence, a distillation tower needs more trays than the n mber of theoretical
vapor li
id e
ilibri m stages.
In modern ind strial ses, generally a packing material is sed in the col mn instead of trays,
especially when low press re drops across the col mn are re
ired, as when operating nder vac m.
Large scale, ind
strial vac
m distillation col
mn[22]
This packing material can either be random d mped packing (1 3" wide) s ch as Raschig
rings orstr ct red sheet metal. Li
ids tend to wet the s rface of the packing and the vapors pass
across this wetted s rface, where mass transfertakes place. Unlike conventional tray distillation in
which every tray represents a separate point of vapor li id e ilibri m, the vapor li id e ilibri m
c rve in a packed col mn is contin o s. However, when modeling packed col mns, it is sef l to
comp te a n mber of "theoretical stages" to denote the separation efficiency of the packed col mn
with respect to more traditional trays. Differently shaped packings have different s rface areas and
void space between packings. Both of these factors affect packing performance.
Another factor in addition to the packing shape and s rface area that affects the performance of
random or str ct red packing is the li id and vapor distrib tion entering the packed bed. The n mber
oftheoretical stages re
ired to make a given separation is calc lated sing a specific vapor to li
id
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Distillation for Compound Identification: Boiling Point Determination
The organic teaching labs employ distillation routinely, both forthe
identification and the purification of organic compounds. The boiling point of acompound determined by distillation is well-defined and thus is one ofthephysical properties of a compound by which it can be identified. Distillation is
used to purify a compound by separating it from a non -volatile orless-volatile
material. Because different compounds often have different boiling points, thecomponents often separate from a mixture when the mixture is distilled.
The boiling pointis the temperature at which the vapor pressure ofthe liquidphase of a compound equals the external pressure acting on the surface ofthe
liquid. The external pressure is usually the atmospheric pressure. Forinstance,consider a liquid heated in an open flask. The vapor pressure ofthe liquid will
increase as the temperature ofthe liquid increases, and when the vapor pressure
equals the atmospheric pressure, the liquid will boil. Different compounds boilat differenttemperatures because each has a different, characteristic vapor
pressure: compounds with higher vapor pressures will boil atlowertemperatures.
Boiling points are usually measured by recording the boiling point (or range) ona thermometer while performing a distillation. This method is used whenever
there is enough ofthe compound to perform a distillation. The distillationmethod of boiling point determination measures the temperature ofthe vapors
above the liquid. Since these vapors are in equilibrium with the boiling liquid,they are the same temperature as the boiling liquid. The vaportemperatureratherthan the pottemperature is measured because if you put a thermometer
actually in the boiling liquid mixture, the temperature reading would likely behigherthan that ofthe vapors. This is because the liquid can be superheated or
contaminated with other substances, and therefore its temperature is not anaccurate measurement ofthe boiling temperature.
If you are using the boiling pointto identify a solid compound which you haveisolated in the lab, you will need to compare its boiling point with that ofthe
true compound. Boiling po ints are listed in various sources of scientific data, asreferenced on the Chem Info page on this orgchem site:
y Hazard and PhysicalProperty Data for Compounds
If youlookup the boiling point of a compound in more than one source, you
may find thatthe values reported differ slightly. The literature boiling pointdepends on the method and ability ofthe technician taking the boiling point, and
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also on the purity ofthe compound. While theore tically all boiling points should
be constant from source to source, in reality the reported boiling pointssometimes vary. Therefore:
y Always reference the source ofthe physicaldata which you write in your
lab report
Distillation for Compound Purification
Simple distillations are used frequently in the organic chemistry teaching labs.They are usefulin the following circumstances:
y the liquid is relatively pure to begin with (e.g., no more than 10% liquid
contaminants)y the liquid has a non-volatile component, for example, a solid contaminant
y the liquid is contaminated by a liquid with a boiling pointthat differs by
atleast 70C
Simple distillation may be a miss -leading term to the beginning organicchemistry student, since ittakes a lot of pract ice in simple distillation to becomeproficientin this technique. Itis especially importantto do a perfect simple
distillation when determining a boiling point foridentification purposes.
How to do a Simple Distillation
Very detailed photos and steps in a distillation set-up:
y details of distillation set-up
Quicklinkto photo of a simple distillation set -up:
y simple
Fractional and Vacuum Distillations
Mixtures ofliquids whose boiling points are similar (separated by less than70C) cannot be separated by a single simple distillation. In these situations, a
fractional distillation is used.
Vacuum distillation is distillation at a reduced pressure. Since the boiling point
of a compound is lower at a lower external pressure, the compound will nothave to be heated to as high a temperature in order foritto boil. Vacuum
distillation is used to distill compounds that have a high boiling point or anycompound which mightundergo decomposition on heating at atmospheric
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pressure. The vacuum is provided either by a water aspirator or by a mechanical
pump.
Always check forstar cracksin the flasks before beginning a vacuum
distillation.
Quicklinks to photos of fractional and vacuum distillation set -ups:
y fractional
y vacuum