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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