distillation principles and processes

TO "FRACTIONAL DISTILLATION" Duringthe past eighteen years Ihave been engagedin investigations which necessitated the preparation of chemicalmaterialsin the purestpossible state, and as

CORNELL UNIVERSITY LIBRARY

ENGINEERING

TP 156Y76 "

Distillation principlesandprocesses

1924 003 958 216

Cornell University

Library

the United States on the use of the text.

http://www.archive.org/details/cu31924003958216

DISTILLATION PRINCIPLES

MACMILLAN AND

LONDON•BOMBAY CALCUTTA•MADRAS

MELBOURNE

NEWYORK BOSTON CHICAGODALLAS•SAN FRANCISCO

PROFESSOR OFCHEMISTRYIN DUBLIN UNIVERSITY

WITH THE COLLABORATION OF

Lieut.-Col E BRIGGS, D.S.O., B.Sc

THOS H DURRANS, M.Sc, F.I.C

The Hon. F R HENLEY, M.A., F.I.C

JOSEPH REILLY, M.A., D.Sc, F.R.C.Sc.L, F.I.C

WITH TWO HUNDRED AND TEN ILLUSTRATIONS

103

MACMILLAN AND CO., LIMITED

1922

The volume on " Fractional Distillation" was written in the hope

thatitwouldbeofassistancetochemistsinovercomingthedifficulties

sofrequentlymetwithinthe laboratory,not onlyinthe actualcarrying

outof the fractional distillationof a complexmixture but also in theinterpretationoftheresultsobtained

Thelastcopy was soldshortlyafterthe declarationofwar,and thequestion then arose whether a revised second edition of the book in

the Publishersthat itwouldbewiser to extendthescope of thework

so as to include distillation on the large scale as carried out in the

manufactureofimportantproducts

Thatthis changein the character of thebook was reallyadvisable

became more and more evident asthe war increasedin intensity andmagnitude, because of the immense importance of such materials asacetone, toluene, petrol, glycerine,and so on, theproduction ofwhichinvolved processes of distillation on an enormous scale, andfor some

of which newsources or methods of formation had to be sought and

investigated

It was obvious that the larger book could only be of realvalue

if the sections on manufacturing processes were written by chemiststhoroughly conversant with the various subjects dealt with, and it

was eventuallydecidedthatIshouldeditthebookifIcouldsecuretheco-operationofexpertsinthedifferentbranchesofmanufacture

In these preliminary negotiations Ireceivedmostvaluable

Professor W.E Adeney,D.Sc, and my sincere thanks are especially

dueto Professor Francis for the great interest he has taken in the

productionof thebook

Unfortunately theheavy pressure of workentailed on all chemists

bythewarcausedseriousdelayincommencingthebook, andprogress

hasnecessarilybeenslow

vi DISTILLATION PRINCIPLES AND PROCESSES

practically a revision of " Fractional Distillation." In this sectionfull reference is made to the valuable researches of Wade, Merriman, and Pinnemore, of Rosanofi and his co-workers, and of Lecat; there

isalsoan additionalchapteronSublimation

The remaining sections, dealing with manufacturing processes, are

asfollows:

2. DistillationofAcetone and»-Butyl AlcoholontheManufacturing

Scale, by Joseph Reilly, M.A., D.Sc, F.R.C.Sc.L, F.I.C., chargeattheRoyal NavalFactory,HoltonHeath,Dorset,andtheHon.

Chemist-in-F.R Henley, M.A.,F.I.C

3. Distillation ofAlcohol on the Manufacturing Scale, by theHon.

F.R Henley andDr.Reilly

4. Fractional Distillation asapplied in the PetroleumIndustry, by James Kewley, M.A., F.I.C, Member of Council of the Institution of

PetroleumTechnologists

5. Fractional Distillation in the Coal Tar Industry, by T Howard

Butler, Ph.D., M.Sc, F.I.C, Managing Director of William Butler &

6. The Distillation of Glycerine, by Lieut.-Col E Briggs, D.S.O.,B.Sc, TechnicalDirector, BroadPlain Soap Works, Bristol

7. The Distillation of Essential Oils, by Thos H. Durrans, M.Sc.(London), F.I.C, ofMessrs A Boake Roberts& Co., Ltd., London.

TO "FRACTIONAL DISTILLATION"

Duringthe past eighteen years Ihave been engagedin investigations

which necessitated the preparation of chemicalmaterialsin the purestpossible state, and as the great majority of these substances were

resorted tofor their purification

ThedifficultiesImetwithinsomeofthe separations ledmetomake

acarefulinvestigationofthewholesubject, and Iwasthus enabledtodevise some new methods and forms of apparatus,which have been

describedfromtimetotimeinvariousscientificjournals

It is inthe hopethat the solution of thedifficultieswhichso oftenoccur in carrying out afractional distillation may be rendered easier,

andthat thevalueand economyofhighlyefficientstill-headsintory work may come to be more widely recognisedthan is generallythe case at present, thatthis bookhas beenwritten

labora-Mysincere thanks aredue to Professor J Campbell Brownfor theloan ofvaluable ancientworks byLibavius and Ulstadius,from which

MS andforthe perusaloftheproofs

Inthe descriptionandillustrationofthestillsemployedincommerce

I have derived muchassistancefrom articlesin Thorpe's "Dictionary

ofApplied Chemistry"and Payen's " Precisde chimieindustrielle."

I have made much use of the experimental data of Brown,

not beenpracticable

Several fractional distillations and numerous experiments havebeen carriedoutwhile thebook wasbeing writtenand theresultshave

inmanycasesnotbeenpublished elsewhere

S. Y

Bristol,August 1903

CHAPTER IXRelation between the Boiling Points of Residue and Distillate 123

DISTILLATION PRINCIPLES AND PROCESSES

"Constant Temperature" Still-heads . 148

IndirectMethod of separating the' Components of a Mixture of

CHAPTER XIX

General Rf.marks.—Purposes for which Fractional Distillationis

required InterpretationofExperimental Results Choice

CHAPTER XX

PAGR

CHAPTER XXIV

CHAPTER XXV

SCALE CHAPTER XXVI

Distillation of Mixtures of Ethyl Alcohol and Water

DISTILLATION PRINCIPLES AND PROCESSES

Various Distillation Methods . 356

CHAPTER XXXVIII

PAGE

Fractional Separation of the Naphthas and Light Oils . 392

CHAPTER XXXIX

DISTILLATION PEINCIPLBS AND PROCESSES

CHAPTEE I

INTEODUCTION ObjectofDistillation.— Theobjectof distillationisthe separation

the separationoftwo ormoreliquids of different volatility

Ifonly onecomponentofa mixtureisvolatile,thereisnodifficulty

in obtaining it ina pure state by distillation, and in many cases theconstituents of a mixture of two or more volatile liquids may beseparated—though frequently at much cost of time and material—

by means of the simple apparatusdescribed in this chapter Forthefractional distillation in the laboratory of such complex mixtures as

petroleum or fusel oil, the improved still-heads described in Chapters

X to XII must be employed Still-heads employed on the large

petroleum, etc.

Quantitative Analysis byDistillation.— Thedetermination, by

ordinary analyticalmethods, of the relativequantities oftwo ormore

organic compounds in a mixture is often amatter of great difficulty,

approximately and, not seldom, with considerable accuracy from the

This methodhasprovedofconsiderable value

Difficulties Encountered — Thesubject of fractional distillation

not onlyintheexperimentalwork, but alsoininterpreting theresultsobtained

substances, such difficulties areof common occurrenceand aredueto

one or other of three causes:

The separation of two liquidswhich boilat temperatures even20°

or 30°apart, suchasethyl alcohol andwater, orbenzene and isobutyl

minimumor, lessfrequently,ofmaximumboilingpoint Itis,indeed,

onlyin the case of substances whichare chemicallyclosely related to

DISTILLATION PRINCIPLES AND PROCESSES

each other that the statement canbedefinitelymadethat thedifficulty

of separating thecomponents ofa mixturediminishesasthedifference

betweentheirboilingpoints increases

In any other case, we must consider the relation between the

boiling points, or the vapour pressures, ofmixtures of the substancesand their composition, and unless somethingis known of the form of

the curve representing one orother ofthese relations, it is impossible

topredictwhetherthe separationwillbe aneasyoneor,indeed,whether

The form ofthese curvesdependslargely onthe chemicalship ofthe components, andit is nowpossible, ina moderate number

substances, ofthe extent towhich the curves would deviate from thenormal form, and therefore to predict the behaviour of a mixture on

distillation

Fractionaldistillationisfrequently avery tediousprocessandthere

is necessarily considerable loss of material by evaporation and by

repeatedtransferencefromthereceivers to thestill,butagreatamount

ofboth timeand materialmay be savedbythe use of avery efficient

composition of a mixture, very much greater accuracy is thereby

attained

Apparatus Ancient Apparatus — The process of distillation is evidently avery ancient one, for Aristotlex mentions that pure water may beobtained from sea-waterbyevaporation, but he does not explainhow

Fig 1.—Alexandrianstillwith

head, or alembic.

Fig 2.—Ancientstillwithwater

condenser.

the condensation ofthe vapour can be effected Aprimitive method

of condensation is described by Dioscorides and by Pliny, who state

thatan oil may be obtainedbyheating rosin in avessel, in theupperpart ofwhichisplacedsomewool Theoil condensesin thewool andcanbesqueezed outofit.

TheAlexandrian chemistsaddedasecondvessel,theheador cover,

formofapparatus usedby themisshownin Fig 1

Later on, the side tube was cooledby passing it through avesselcontaining water The diagram, Fig 2, is taken from Libavius,

Syntagma Alchymiae Arcanorum, 1611

time issimilarin principle,but, in addition, athermometeris used to

In Fig 3 the ordinary

form of apparatus is

shown, and we may

dis-tinguish the following

parts:— The still, a; the

condenser, c, in which

thevapourisdeprivedof

heat byacurrentof cold

water; the receiver, d;

the thermometer, b In

the laboratory thestill is

usuallyheated by means

ofaBunsenburner

_,, _ , ,, Fig 3.—Ordinary still, with Liebig's condenser.

de-livery tube from the still-headpasses through asecond corkinthecondensingtube Forliquids

whichboilata hightemperature,

orwhichactchemicallyoncork,

it is more convenient to have

the still and still-head in one

piece and to elongate the

ifnecessary,throughthe Liebig's

condenser (Fig.4).

The Still.—If a glass flask

is used it should be globular,

because a flat-bottomed flask is

thanisnecessaryfortheamountofliquidtobedistilled.

The Still-head.— The still-head should not be very narrow, orthethermometer may be cooledslightlybelowthetemperature of the

vapour It is a good plan to seal a shortlength of wider tubing to

narrower tubing belowto pass throughthecork inthestill, as shown

in Fig.3.

FIG 4.—Modifiedformol still with condenser.

AND PROCESSES

position that not only the mercury in the bulb but also that in thestem is heated by the vapour of the boiling liquid ; otherwise atroublesome and somewhat uncertain correction must be applied

air may be carriedback fromtime to timeas far as thethermometerbulb and the temperature registered by the thermometer will then

fluctuateandwill, onthe whole,betoolow(p. 25)

The longer and wider the still-head and the higher the boiling

point of the liquid distilled, the greater will be the amount of densed liquid flowing back to the still. The lower end of the still-

con-head should be wide enough to ensure that no priming takes place

Withthe bottom ground obliquely asin Fig 6 a much narrower tube

may beusedthan whenthe endiscutoffhorizontally(Pig.3).

The Condenser —Ifthe boilingpoint ofthe liquid tobe distilled

ishigherthan about170°, thecondensing tube should notbe cooledby

running water for fear of fracture A long tube should be used and*the coolingeffectofthesurroundingair willthenbesufficient

Whena Liebig'scondenser isusedthere isno advantagein having

either the inner orthe outer tube verywide; aninternaldiameter of

If the outer tube is muchwider it is unwieldy, and, when filled with

water, it isinconveniently heavy Amistake that isratherfrequently

made may bereferred to here Itisusualtoseala short wide tubeto

thelong, narrowcondensing tubefortheinsertion ofthedeliverytube

proceeding alittlepoolofliquid

5, a), and, in the fractional

(b) correct construction •*

fault is easily remedied by

heating thewide tube close tothe junctionwith the narrowoneuntil

The Source of Heat — For laboratory purposes an ordinary

Bunsen burner is usuallyemployed Wire gauze, asbestos cardboard,sandbaths,or wateroroilbathsare not, as arule,to berecommended

because the supplyofheatcanbe much moreeasilyregulated withoutthem, anda round-bottomed flask,ifproperlyblown, issothinwalledthat there is no danger of fracture when the naked flame is applied

I INTRODUCTION 7

disulphide, whichboil quite regularly underthe ordinaryatmospheric

reduced, unless special precautions are taken Under a pressure of

361 mm. carbon disulphide boils at 25°, and if a quantity of it be

thebottom oftheflask, it may happen thatthe whole ofthe carbon

disulphide willpassover withoutanyebullitionwhatevertakingplace

superheated, and if a bubble does form there will be a sudden andperhapsviolent rushoftheextremely inflammable vapour If,however,

the top of theburner be placed onlyabout 2mm. below thebottom

oftheflask, sothat the minuteflame touches theglass, ebullition willtakeplace quietlyandregularly

Thereare liquidswhich cannot be preventedfrom bumpingin this

way,and the bestplanisthento addafewsmallfragmentsof porous

porcelain [aclay pipebrokenin small piecesanswersthepurpose very

slow current of air through the liquid, but a

smallerror inthe boilingpointisthereby

intro-duced Theexplanationof thisisgivenonp 24

Asuitable flask,described by Wade and

Merri-man,1

is shown in Mg. 6. A water or oil

bath need only be used when asolidsubstance

is present in theflask, as,forinstance,when a

pent-oxide,orwhentheliquidis liabletodecompose

whenheated with thenakedflame

Itiscustomaryto employ a water-bath for

whether this is necessary or even advisable

exceptin the case of the ethereal solution of

When an accident occurs it is almost invariably because, owing

to "bumping," or to the distillation being carried on too rapidly,

some ofthe vapour escapes condensationand comes in contact with

a flame in the neighbourhood, generally that below the water-bath

Ifanakedflamewere usedthedistillationcould bemuch more easilyregulated, and there would probably be really less danger than if a

water-bathwere employed

Protection of Flame from Draughts.—In order that satisfactory

proceed withgreatregularity, andthe heat supplymust therefore not

be subject to fluctuations The most important point is to guard

A "Apparatusfor Fractional Distillation at Pressures otherthantheAtmosphericPressure,'

DISTILLATION PRINCIPLES AND PROCESSES

Fia 7.

Simple flameprotector.

against draughts, and, to do this, the ordinary conicalflame protector

may be used, or a simple and efficient guard may be made from a

large beaker by cutting off the bottom andtaking a piece outof theside (Fig 7).

Electrical Heating.— For many purposes,notablyfordistillationunderreducedpressure,

it is convenient to employ an electrically

heated coil of platinum wire as the source ofheat T W. Eichards and J H Matthews*

strongly recommend this method of heating,

and consider that electrical heating gives

super-heatingthanordinaryflameheating

Inorder that the fine platinum wire may

stillbecompletelyimmersed whenthequantity

of liquid in the flask has becomevery small, Richardsand Matthews

recommenda vessel ofthe form shown in Fig 8. The coil actually

employedconsistedof about 40cm.ofplatinumwireand had a

resist-ance of0-7 ohm A current of ten to fifteen amperes was passedthrough the coil, the ends of which were

sealed into two glass tubes in which were

stout copper wires Connection between

the platinum and copperwires waseffected

by meansofalittlemercury

Beckmann.2

H. S.Bailey3makesuseofaflaskwhich

issonarrowatthebottomthat 90 per cent

of the liquid can be distilled with the coil

stillcompletely immersed Bailey

recom-mends acoil ofGerman silver or nichrome

wire instead ofplatinum

Rosanofi and Easley* windthe heating

wire on a glass rod bentin the form of a

conical spiral, the apex of which extends

almosttothebottomofthevessel

W R. G Atkinsfinds it convenient for

many purposes to employ an electrically

heated metal plate, placed a little distance

below the flask The amount of heat reaching the flaskis regulated

by moving a piece of asbestos pasteboard over the heated plate

Superheatingof thevapour, when the amount of liquid has become

1Richards and Matthews, "Electrical Heating in Fractional Distillation," J. Amer.Ohem Soc,1908, 30,1282; 1909, 31,1200; Zeilschr, physik.Chem.,1908, 64, 120.

2 Beckmann," Erfahrungeniiber elektrisehesHeizenbei ebullioskopischenBestimmungenundbei der fraktionierten Destination," Zeitschr physik.Chem.,1908, 64, 506.

Fig 9.—Still withsteamjacket.

asbestos pasteboard with a circular hole cut in the centre, so that

theheat onlyreaches asmall area at thebottomoftheflask.

Allen and Jacobs1

encase the distillation flaskinthetwo halvesof

a pear-shaped mould onthe innerside ofwhichis wounda resistancewirewhichissuitablyheated electrically

Steam as Source of Heat.— On the

steam under ordinary or increased

through the pipe a, and the condensed

water runoffatb (see also pp 331,398)

The Thermometer —In carrying out

not onlytoread a constant or nearly

con-stant temperature with great accuracy,

but also totake readings of rapidly rising

temperatures These requirementsare best

fulfilled by the ordinary mercurial

ther-mometer, which is therefore, notwithstanding its many drawbacks,used in preference to the airorthe platinumresistancethermometer

If accurate results are to be obtained the*following points must beattendedto.

itwouldbea greatadvantageif allthermometerswerecomparedwith

an air thermometer, for two mercurial thermometers, constructed of

(a) In the first place, it is impossible to obtain an absolutely

Various methods have been devised for calibrating the stem,2 3 butevenwhenthisisdonethereremainother sourcesof error

(6) The position of the mercury in the stem at any temperaturedependson theexpansion bothof themercury and theglass, and, for

bothsubstances,the rateofexpansionincreaseswithriseoftemperature

(c) Different kinds of glass have different rates of expansion, sothattwothermometersmadeof differentmaterials—evenifthe capillary

tubes were perfectly cylindrical— would give different readings at the

sametemperature It is therefore necessary to compare the readings

of a mercurial thermometer with those of an air thermometer, or of

anothermercurialthermometerwhich haspreviouslybeenstandardised1

AllenandJacobs, " ElectricallyHeatedStill for Fractional Distillation," Dept of Inter., Bur of Mines, U.S.A., Bull 19, 1 ; J Soc.Ohem.Intl., 1912, 31, 18.

2 "Methods employedin Calibration of MercurialThermometers," British Association Report for 1SS2, 145.

DISTILLATION PRINCIPLES AND PROCESSES

by means of an air thermometer Or, instead of this, a number offixed points maybe determined byheating thethermometerwith thevapoursofa seriesof pureliquids boilingunder knownpressures

Table 1 contains a list of suitable substances with their boilingpoints, and the variation of temperature for a difference of 10 mm.

fromthe normalatmosphericpressure

I INTRODUCTION 11

graduated

(6) Ifa thermometer —even after itszero point has beenrendered

as constant as possible—be heated and then cooled very rapidly, a

greater part of this fall will be recovered, and the remainder after along period

cheaper thermometers there is a vacuum above the mercury and,

when the mercuryin thestemis strongly heated, volatilisation takes

when, therefore, the temperature is really constant it appears to be

gradually falling The betterthermometers, which are graduated up

to hightemperatures, contain nitrogenoverthemercury, abulb beingblown near the top of the capillary tube to prevent too great a rise

of pressure by the compression ofthe gas; but thermometers which

are only required for moderate temperatures, say, not higher than

such thermometers are used for the distillation of liquids boiling at

so lowatemperatureas 100°, oreven80°, a quite perceptibleamount

of mercury may volatilise and, after prolonged heating, errors

amountingto 0-2° or 0-3° may occur Itwouldbe muchbetter if allthermometers required to register temperatures higher than 60° werefilledwithnitrogen

4. Correction for Unheated Column ofMercury.— As already

men-tioned, the thermometer should, if possible, be so placed in the

apparatus thatnot onlythe mercuryin thebulb but alsothatin the

stem is heated by the vapour of the boiling liquid; otherwise thefollowingcorrection, which, at the best, is somewhat uncertain, must

beapplied:

—

To the temperature read, add -0001 43(T -t)N, where T is the

observed boiling point, t the temperature of the stem above the

vapour, and N the length of the mercury column not heated by the

vapour, expressedin scale divisions

expan-sionofmercury andthatof glass—isveryfrequentlyemployed, butit

is foundin practice to betoo high; and Thorpe has shown that the

value 0-000143givesbetterresults

Table 2, on page12, givenby Thorpe] maybe founduseful

1 Thorpe, "Onthe Relationbetweenthe MolecularWeightsof Substancesandtheir Specific Gravitieswhenin the Liquid State," Trans.Chem Soc,1880, 37, 159.

AND PROCESSES

i INTRODUCTION 13

5. Superheating of Vapour.— When the amount of liquid in the

still isverysmall,thevapourisliableto be superheatedbytheflame,

and unlessthebulb ofthe thermometeris thoroughlymoistened withcondensed liquid, too high a temperature will be registered If a

verylittlecotton wool,or, fortemperaturesabove230°, alittlefibrousasbestos, be wrappedroundthe bulb ofthe thermometer, itremains,

nakedflame,thethermometerregistersaperfectlyconstanttemperature

With a water or oil bath the danger of superheating is greater,

and the cotton wool may become dry at the end of the distillation

In that case'the temperature registered may betoo high, though, as

protected

If it is of special importance to determine the boiling point of a

liquid with great accuracy during the course of a distillation, one ofthe two forms of still devised by Eichards and Barryx may beemployedwith advantage

6. Correction of Boiling Point for Pressure.— The barometer mustalways be read and corrected to 0° (p. 229) and, in alongdistillation

orin unsettledweather, itmay benecessary to readitfrequently, forthe boiling point ofa liquid varies greatlywiththe pressure

It is impossible to give any accurate and generally applicable

formulaforcorrectingtheobservedboilingpoint tothatundernormal

pressure (760mm.), butthe followingmay betaken asapproximately

correct:

—

=0-00012(760 -p)(273+1),

where is the correction in centigrade degrees to be added to the

observedboiling point,t, and pisthebarometricpressure.2

This correction isapplicable, withoutmuch error, to the majority

formula

=0-00010(760-2>)(273+1).

Crafts3

has collected together the dataforanumberofsubstances,

from which the values of cinthe formula 0=c (760-p) (273+t) are

absolute scale, T, the values of cand those of dpjdtfor some of the

substancesreferred to by Crafts andalso for aconsiderablenumber ofadditionalones.5

1 "AnAdvantageous Formof Still for theExactMeasurementof Boiling Point during Fractional Distillation," J.Amer.Cliem Soc., 1914, 36, 1787.

2Ramsayand Young,"SomeThermodynamicalRelations," Phil.Mag.,1885, [V.], 20, 515.

3 Crafts, "On the Correction of the Boiling Point for Barometric Variations," Berl Berichte, 1887, 20, 709.

4 There are afewmisprints in the table givenbyCrafts, and since itwas publishedmanyadditional accurate determinations of boiling point and vapourpressure have beenmade

5 Young," Correction of the Boiling Points of Liquidsfrom ObservedtoNormalPressure,"

AND PROCESSES The values marked with an asterisk have been determined in-directly,andarenottoberegardedasso well established astheothers.

When a boiling point is to be corrected, the constant c for thesubstance may usually be found byreference to Table 3 Either theconstantforthat substanceinthetable mostclosely related totheone under examination is to be used, or the constant may be altered in

conformity with oneorotherofthe followinggeneralisations

Table 3

Substance.

Examples:— Benzene andamono-derivative; naphthalene and naphthalene.

bromo-4. Byreplacing one halogenby another no change isusually duced Examples:— Thefourhalogenderivativesofbenzene

pro-5. Allcompounds containing a hydroxyl group—alcohols, phenols,water, acids—have very lowvalues Butthe influenceofthehydroxylgroup in lowering the constant diminishes as the complexity ofthe rest of the molecule increases Thus, with methyl, ethyl, andpropylalcoholstheconstantsmustbeasmuchas0-000035 lowerthan

those of the correspondinghydrocarbons, but with amyl alcoholit isonly 0-000029, and with phenol only 0-000015 lower On the other

hand, the constant tends in general to be lowered as the molecularcomplexityincreases,andthesetwofactors,actinginoppositedirections,neutraliseeachother moreorless completely; thus,inthe caseofthealcohols atanyrate,thereisapparentlynorelationbetweenthe values

oftheconstantandthemolecularweight

molecularweight

Modifications ofthe Still.—For ordinarylaboratorypurposes a

round-bottomedglass flaskis themost convenientformof still, butif

alargequantityof liquid hastobedistilled,especiallywhenit isveryinflammable, it is safer to employ a metal vessel Metal vessels aregenerallymadeuseofonthe largescale

Anyalterationintheshape ofthestillasaruleis merelyamatter

ofconvenienceanddoesnotcallforspecialmention

Modifications ofthe still-head areof great importance and willbe

consideredlaterin Chapters X.to XIII

Modifications ofthe Condenser —Forliquids boilingabovethe

ordinary temperature, but below about

usuallyemployed, but various more

temperature than 170°, there would be

danger of fracture if the glass delivery

tube werecooled by water The cooling

must be cooled by ice or by a

freezing-mixture (poundedice andsalt, or iceand

concentrated hydrochloric acid are

con-venientformoderatelylowtemperatures)

shouldbeused(Fig 10) Condensation of moistureinthe receiveris,

Fig 10.

Condenser for volatile liquids.

16 DISTILLATION PRINCIPLES AND PROCESSES

Modifications of the Receiver —Ifa liquidboilsat a very hightemperature, or if it suffers decomposition at its ordinary boilingpoint, it may be necessary to distil it under reduced pressure For

cases of simple distillation the apparatus shown in Fig 11 may be employed, but if the distillate is to becollected inseparate portions,

the removal of the receiver would necessitate admission of air into

the apparatus and a fresh exhaustion after each change In order

to introduce successive fractions into the still without disturbing thevacuum, Noyes and Skinner1 fuse a separatory funnel and the still-

headtotheneckofa Claissenbulb [Thelarge globeinFig 11 serves

Fig 11.—Simple apparatusfor distillationunder reducedpressure.

to keepthe pressure steady and toprevent oscillation ofthe mercury

in the gauge.] Various methods have been devised to allow of the

receivers being changed without altering the pressure, of which the

followingmay be mentioned

1. Thome's Apparatus.— A series of stopcocks may be arranged in

such a mannerthat airmay be admitted intothereceiver and afresh

oneputinits placewhilethe distillationbulb remains exhausted (Fig.12) Thestopcock bis closed,andthe three-way stopcock c isturned

so as to admitairinto thereceiver, whichisthen disconnected and a

fresh one is put in its place The stopcock a is then closedto shut

off the still from the pump, and c is turned so as to connect the

pump with thenewreceiver,whichisthenexhausteduntilthepressure

This method, though comparatively simple, isattended by several

disadvantages; thereissome riskofleakagewhensomany stopcocks

areused— even whena three-way stopcockisemployed (asin Fig 12)

in place of two simple ones— and this is especially the case becauseordinarylubricants cannotas arule be used forb, through which thecondensed liquid flows Moreover, the changing of the receiver, themanipulationofthe stopcocks, andtheexhaustionofthefresh receiver

1 Noyes and Skinner, "An Efficient Apparatus for Fractional Distillation under

take up sometime,duringwhichthe progressofthedistillationcannot

Fig 12/-Thome'sapparatus for distillation

under reducedpressure.

FIG 13.—Bredt's apparatus lor distillation

under reducedpressure.

2. Bredt's Apparatus.— To the end of the delivery tube from the

which are sealed three narrow tubes a, b, and c, approximately atright anglestoit(Fig 13),anda fourthtubed,whichserves toadmit

cylindrical vessel by means of a perforated cork.* The long-necked

that the firstfraction collects in the flask; when a change is to bemade, theneck of theflaskis rotated until

the dropsof distillate fall into one of the

cylindrical receivers and each of these in

turncan be broughtverticallybelowtheend

ofthe delivery tube

3. Bruhl'sApparatus.— A numberof

test-tubes are placed in a circular stand which

may berotated within an exhausted vessel

de-livery tube This arrangementisconvenient,

asthechangeofreceivercan beeffectedwith

the greatest easeandrapidity

4. Wade and Merriman's Apparatus for

Pressure.— To keep the pressure constant a

pressure regulator or "manostat" was

devisedon a principle similar to that of a

thermostat The air inlet passage is

auto-matically left uncovered by the mercuryof a manometer when thepressurefalls below the limit to which the manostat is set. Fortheconstructionoftheinstrumentandthemethodofusingit,theoriginal

Fig 14.—Bruhl's apparatus for distillation under reduced pressure.

18 DISTILLATION PRINCIPLES AND PROCESSES

paper1 should be consulted In employing the instrument fortionaldistillation anair reservoir of about10litres capacityis placedbetween the manostat and the vacuum pump or aircompressor and

frac-a second air reservoir of abouttwicethe capacitybetweenthe

mano-stat and the manometer anddistillationapparatus A suitable

mer-curialaircompressorisdescribed inthe paper

In order tocollectthefractions without disturbing the distillation

the modification of F D Brown's apparatus shown in Fig 15 was

employed The separator consists essentially of a tap-funnel B, into

thewide neckofwhichissealedasleevec, of sufficientcalibre toallowthe passage of the condenser tube; the latter,which should project

wellbelow thesleeve, is madetightby a shortlength ofstoutrubber

Fia 16<7, Fig. 166.

tubing Thetap Pis preferablyof large bore, 3 or4 mm The stem

of the tap-funnelissealed to theupper end ofa second sleeve L, thelower endof whichcarriestherubber stopper ofthereceivingtube orflask E The two sleeves are respectivelyfurnishedwith T tubes tt',

the ends of which are sealed to oppositeways of a three-way tap h,

thethirdwayofwhichisopentotheair. Connectionwith thevacuum

pump is made through a branch t" of the upper sidetube t'. The

tap Pcannot,asarule,belubricated,butas anyleakageisinwards no

stoppers (Fig 16a)whichcan be actuatedfromoutside This stopper

ismounted ona stoutglass rod, whichpassesthrougha closely-fitting

1WadeandMerriman, "Apparatusfor theMaintenanceofConstantPressuresaboveandbelowtheAtmosphericPressure. Applicationto Fractional Distillation," Trans.Chem.Soc.,

i INTRODUCTION 19

pressure tubing This tubing, which is wired to the rod and sleeve,acts as a spring which normally keeps the stopper away from its

embouchure inthe upper, bentpart ofthe funnel stem In changing

rubber tube until it closes the embouchure, and on relieving thepressure by means of the three-way tap, the stopper is held firmly

in positionbythe pressure of thecompressed airinthe funnel Any

leakage is downwards and internal and the receiver can be changed

so quickly that there is no danger of loss. In a distillation under

increased pressure all the rubber joints and stoppers must be wired

tothe respective glassvessels andtubes A convenient fasteningforthe receiveris affordedby a stoutmetal ring m (Fig 16b) which fits

hooks mm', one of which isradial, the otherparallel to the ring; a

hooks, and the two ends, after traversingthe stopper on either side

keepingitsecurelyin position

ConvenientseparatorshavebeendescribedbyEosanoffandEasleyJ

(seealsoRosanofi, Bacon andWhite),2

and by Hahn.3

Prevention of Leakage — When a liquid is distilled underreducedpressure,it isnecessarythatalljointsintheapparatus should

stillbecause this substance isattacked or dissolved by many organic

the apparatus and then covering the cork with the ordinary liquid

gum, soldinbottles (not gumarabic,whichisapttocrackwhendry)

the applicationmaybe repeatedseveraltimes

Apparatusforfractionaldistillationunder reducedpressurehasalso

beendescribedby

—

Lothar Meyer,Berichte, 1887, 20, 1834

H. Gautier, Bull Soc Chim., 1889, 2, 675

Rosanoff, Baconand White,"ARapid LaboratoryMethod ofMeasuring the Partial

VapourPressures of LiquidMixtures"(Apparatus), J.Amer Chem, Soc,1914, 36, 1806.

3 Hahn,"Fractionating Device," Ber., 1910, 43, 1725 ; J Soc.Chem.Ind., 1910, 29, 842.

CHAPTER II

THE BOILING POINT OP A PURE LIQUID

The Statical Method

under a given pressure may be determined, the statical and thedynamical Bythefirst method thepressures exerted bythevapour

oftheliquidataseriesof

tempera-tures are ascertained and plottedagainst the temperatures, a curve,

the vapour pressure curve, beingthen drawn through the points

(Fig 17)

This curve has a twofold

mean-ing; it represents not only thevapour pressures of the liquid atdifferent temperatures,butalsothe

boiling points of the liquid under

vapour pressure of water at 50°is91-98 mm., andwater boils at 50°under apressure of91-98mm.

no 17.

Pressure

-Vapourpressure curve.

Evaporation in Absence of Air.— The vapour pressures of a

liquid attemperatures lower thanits boilingpoint underatmosphericpressure may be determined by placing some of the liquid over themercury in a barometer tube and heating the tube to different tem-

peratures The difference between the height of the barometer and

that of the column of mercury in the tube, after correcting for theexpansion of the mercury, and, if necessary, forits vapour pressure,givesthevapourpressureoftheliquid

It is necessary to take great care that the liquid introduced is

quite free from dissolved air, otherwise, under the reduced pressureand atthe higher temperature,some of thisairwould beexpelled and

themeasuredpressurewould bethesumofthepressuresofthevapour and of the air. If the liquidis pure and free from air, the pressure

variesonly with the temperature and does not depend on therelative

volumes ofliquid and vapour solong as both are present This