elements of fractional distillation fourth edition

Presentation ofVapor-Liquid Equilibrium Data 16 3.. Usually this is obtained from informationconcerning the composition ofthe vapor which is in equilibrium with dis-theliquid.. The vapor

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Osmania University Librarq

Elements of

BY CLARK SHOVE ROBINSON

AND

t>y

ProfessorofChemical Engineering

Afassachusetts Institute of Technology

FOURTH EDITION SKCOND

NKW YORK TORONTO LONDON

1950

ELEMENTS OF FRACTIONAL DISTILLATION

Copyright, 1922, 1930, 1939, 1950, by the McGraw-Hill Book Company, Inc Printed in theUnitedStates ofAmerica All rights reserved. This book, orparti

THE MAPLEPRESS

PREFACE TO THE FOURTH EDITION

The

firs^^ditipn.of-thisbook andthe early revisionsweretheresult

of the efforts of Professor Robinson, and he took an active part inguiding the revision of the previous edition. His death

tmade it

necessary to prepare this edition without his helpful guidance and

counsel

The present revision differs extensively from the previous edition

Thematerial hasbeen modifiedtobring it more closely intolinewith

the graduate instruction in distillation at Massachusetts Institute ofTechnology Muchgreateremphasishasbeenplacedonthemeasure-

believed thatthisisoneofthemostserious limitations indesignlations. Greater emphasis has also been placed upon the use ofenthalpy balances, and the treatment of batch distillation has been

calcu-considerably expanded Unfortunately, the design calculations forthistypeofoperationarestillinanunsatisfactory status. Azeotropic

and extractive distillation are considered as an extension of

conven-tional multicomponent problems The sections on column design

and columnperformance have been completelyrewrittenandincreased

in scope Inall cases quantitativeexamples have beengiven because

descriptive material

over the conventional Sorelmethod, and it isbelievedthat the law ofdiminishing returns hasbeenapplyinginthis field forsometime It is

investi-gators to transfer their efforts to more critical problems, such asvapor-liquid equilibria, batch distillation, transient conditionswithinthe distillationsystem, andcolumnperformance.

EDWIN RICHARD GILLILAND

CAMBRIDGE, MASS

July, 1960

PREFACE TO THE FIRST EDITION

The subject of fractional distillation has received but scant tion from,writers inthe English languagesinceSidneyYoungpublishedhis book "Fractional Distillation" in 1903 (London). French and

atten-Germanauthors have, onthe other hand, produced anumberofbooks

on thesubject, amongthemore importantofwhichare the following:

"La Rectification et les colonnes rectificatriccs en distillerie,"

E Barbet, Paris, 1890; 2d ed., 1895

"Der Wirkungsweise der Rcctificir und Destillir Apparate,"

fraktionierten Destination," J. P Kuenen, Leipzig, 1906

Destination," C von Rechenberg, Leipzig, 1910.

"La Distillation fractione*e et la rectification," Charles Manlier,Paris, 1917

has to do almost entirely with the aspects of the subject as viewedfrom the chemicallaboratory, andthere has beenliterally no work inEnglishavailable forthe engineerandplant operator dealing with the

applications ofthe laboratory processes tothe plant

The use of the modern types of distilling equipment is growing at

a veryrapidrate. Manufacturers ofchemicalsare learningthat they

must refine their products in orderto market them successfully, and

it is often true that fractional distillation offers the most available

if not the only way of accomplishing this. There has consequentlyarisen awidedemand amongengineersandoperatorsforabookwhich

willexplain the principlesinvolvedinsuch awaythat these principles

It has therefore been the purpose of the writer of this book to

physical chemistry and chemical engineering, the principles of tional distillation, illustrating these principles with a few carefully

frac-selected illustrations. This book is to be regardedneither as a

com-plete treatise nor as an encyclopedia on the subject but, as the title

anintroductiontoits

Viii PREFACE TO THE FIRST EDITION

Ingeneral, ithasbeen dividedintofive parts The first partdeals

with fractional distillation from the qualitative standpoint of thephase rule. The second part discusses some of the quantitativeaspects from the standpoint of the chemical engineer. Part threediscusses the factors involved in the design of distilling equipment

Part four gives a few examples of modern apparatus, while the lastportionincludes a number ofuseful reference tableswhich have been

compiled from sources mostly out of print andunavailable except inlargelibraries.

Thewriterhasdrawnatwillontheseveral books mentionedabove,

someofthetables beingtaken nearly bodily from them,and has alsoderivedmuchhelpfrom Findlay's "Phase Rule" (London, 1920) and

from "The General Principles of Chemistry" by Noyes and Sherrill(Boston, 1917) He wishes especially to express his gratitude forthe inspiration and helpful suggestions from Dr W. K Lewis of the

associates at the Instituteand ofthe E B Badger &Sons Company.

Finally, he wishes to express his appreciation of the assistance of

CLARK SHOVE ROBINSON

CAMBRIDGE, MASS

June, 1920,

1. DeterminationofVapor-Liquid Equilibria . .3

2. Presentation ofVapor-Liquid Equilibrium Data 16

3. Calculation ofVapor-Liquid Equilibria . .26

4. Calculation ofVapor-Liquid Equilibria (Continued) . .79

5. General Methods ofFractionation . 101

6. Simple Distillation andCondensation 107

7. RectificationofBinary Mixtures . 118

8. Special BinaryMixtures . 192

/9/Rectification ofMulticomponent Mixtures . , 214

10. Extractiveand Azeotropic Distillation . 285

i^l. Rectification ofComplexMixtures . 325

12. Alternate Design MethodsforMulticomptonent Mixtures , 336

13. Simultaneous Rectificationand Chemical Reaction . 361

15. Vacuum Distillation 393

16. Fractionating Column Design 403

17. FractionatingColumn Performance 445

18. FractionatingColumn Auxiliaries 471

Definition of Fractional Distillation. By the expressionfractionaldistillation was originally meantthe process ofseparating so far as it

may be feasible a mixtureof two ormore volatile substances into its

of heat, condensing thevaporsinsuch awaythatfractions ofvaryingboilingpointsare obtained,revaporizingthese fractionsandseparating

theirvaporsinto similarfractions, combining fractions of similar ing points,andrepeatinguntilthedesireddegreeofseparationis finallyobtained

the chemical laboratory, but it is a laborious and time-consuming

operationwhichhasitschiefvalueasaproblemforthe student, forthe

ofvolatile substances It ispossible to carry ona fractional tion by meansofcertainmechanicaldeviceswhicheliminatealmostall

distilla-of thislaborandtimeandwhichpermit separations not only equal tothose obtained by this more tedious process but far surpassing it inquality andpurityofproduct Thepurposeof thisbookisto indicate

how such devicesmay beprofitablyusedin thesolutionof distillationproblems

Origin of Fractional Distillation. Like all the older industries,fractional distillation is an art that originated in past ages and thatdeveloped,asdidallthearts,bythe gradual accumulationofempiricalknowledge It isprobable thatitsgrowthtookplace along with that

of the distilled alcoholic beverages, and to the average person today

the word "still" is synonymous with apparatus for making rum,

brandy, and other distilled liquors. To France, which has been thegreatproducerofbrandy, belongs thecredit fortheinitialdevelopment

ofthemodernfractionating still.

Physical Chemistry and Fractional Distillation. Fractional tillation has labored under the same sort of burden that the otherindustrial arts have borne Empirical knowledge will carry an

dis-industry toa certain point,andthenfurther advancesarefewandfarbetween Ithas beenthe functionofthesciencestocometothe rescue

2 FRACTIONAL DISTILLATION

oftheartsatsuchtimesandthuspermit advancementtogreater fulness. The science that has raised fractional distillation from an

use-empirical to a theoretical basisis physical chemistry Byits aid the

chemistry

CHAPTER 1

DETERMINATION OF VAPOR-LIQUID EQUILIBRIA

Theseparationofamixtureof volatileliquidsby meansof fractionaldistillation ispossiblewhen the composition ofthevapor coming from

theliquidmixtureisdifferentfrom thatoftheliquid. Theseparation

is theeasierthe greaterthe difference betweenthe composition ofthe

when the difference is small The relation between the vapor and

liquidcompositionsmustbeknownin ordertocomputefractional tillation relationships. Usually this is obtained from informationconcerning the composition ofthe vapor which is in equilibrium with

dis-theliquid. On this account a knowledge ofvapor-liquid equilibriumcompositions is usually essential for the quantitative design of frac-tional distillation apparatus In mostcasesthe studyismade onthebasis of the composition of the vapor in equilibrium with theliquid.

for separation However, most of the equipment employed depends

on the use of a vaporization type of operation, and the equilibrium

can be considered in two main classifications: (1) the experimentaldetermination of equilibrium compositions and (2) the theoreticalrelationships

EXPERIMENTAL DETERMINATIONS OF VAPOR-LIQUID EQUILIBRIA

simple Ahighlydeveloped laboratory techniqueis thereforeneeded

to obtainreliabledatainanyoftheseveralmethodsdescribedhere.CirculationMethod A common methodforobtaining vapor-liquidequilibrium (Refs 11, 13, 16, 23, 27, 35) is by circulating the vapor

liquid until no further change in the composition of either takesplace A schematic diagram of such a system is shown in Fig 1-1.

FRACTIONAL DISTILLATION

num-ber of complications:

quan-tityofmaterialwill continuallyvary andthe equilibrium compositions

2. Thequantitiesofliquidandofvaporwhenequilibriumisobtained

themconstantit is necessaryforthe systemtoremain isothermaland

for the total volume to remain constant The chief variation in the

touse areciprocatingpump Theerrordueto thisvariationisusually

The pumps are generally of a mercury-piston type; t.e., a mercury

as the piston ofthe pump. This makes itpossible tohave antiallyleakproofpump andallowsthe pumpingoperation to becarried

This type of has been used most under

con-DETERMINATION OF VAPOR-LIQUID EQUILIBRIA 5

ditionswhere the vapor does not condense at room temperature. If

itwerenecessarytooperate thepumpingsystemata high temperature

to avoid condensation of the vapor, difficulties might be encountered

due to the vapor pressure of the mercury, in which case other lower

and of liquid to vary is the rate of flow. The rate of recirculationvaries the pressure drop through the apparatus and thereby changes

the quantity,ofvaporpresent Inmost casestherate of recirculation

is such that the pressure differential for recirculation is not great.

due to recirculation can be madeless detrimental by makingthe

vol-ume of theliquid in vessel A large.

5 It is necessary to ensure that there is no entrainment of liquid

by the use of lowvelocity and by efficient entrainment separators in

con-densatewillbeof different compositionandtheresultswillbein error.

It is particularly suitable for very low temperature studies such asthose involved in the equilibria associated with liquid air. In thiscase vessel A is maintained in a low-temperature cryostat, and therecycle vapor stream is heat-exchanged with the exit vapor; the rest

the difficultieswith the operation isthe fact that the vaporsample is

obtained as a vapor and, unless the pressure ishigh, the quantity of

analysis

bubbledthrough theliquid untilequilibriumisobtained Theoreticallyexact equilibrium is not obtained because of the fact that there arepressure differentials in the system Thusthe vapor entering at the

bottom of A mustbe underapressurehigherthanthevaporleavingA,

at leastby an amountequaltothe hydrostatichead oftheliquid in A

Since the vapor-liquidequilibriadepend on pressure,it is obvious thatthere cannot be exact equilibrium However, the change in thecompositionofthe equilibriumvapor dueto thissmall changeof pres-

most It could be serious inthe critical region

FRACTIONAL DISTILLATION

the best forobtainingtrue equilibrium

liquidsampleisplacedinaclosedevacuatedvessel. Itisthenagitated

byrocking, orby othermeans, at constanttemperatureuntil

equilib-rium is obtained between the vapor and the liquid. Samples of the

The method appears simple, but it involves certain difficulties.

material, and these pressure changes can be large in magnitude In

ordertoavoid them,it iscustomarytoaddsomefluid,suchas mercury,

Rockingmechanism

Sampling line

-fjf

- Constanttemperature

FIG 1-2. Bombapparatus.

any vaporization or condensation. Another difficulty with the

method is the fact thatin most cases it is necessary touse sampling

lines ofsmall cross sections Thesemayfillupwithliquidduring theinitialpartoftheoperation,andthisliquidmaynevercometothe trueequilibrium It is necessary to purge the sampling lines to remove

such liquid. This liquid holdup is particularly serious inthe case of

sample A schematic diagram of the bomb-type apparatusis shown

in Fig 1-2.

used (Refs. 10, 21, 25, 37) for the determination ofvapor-liquidlibria isoneinwhichavaporispassedthroughaseriesof vesselscon-taining liquids of a suitable composition The vapor entering the

equilibrium vapor, but asit passes through the system it tends to

liquid composition, thevapor will more nearly approach equilibrium

asitpassesthroughtheunit, The numberof vesselsemployedshould

equilib-rium

DETERMINATION OF VAPOR-LIQUID EQUILIBRIA 7

it ispossible todispensewiththeanalysis oftheliquidsample,i.e.,theliquids can be made ofa knowncomposition, and sincethechange inthe lastvesselis small, it ispossible toassumethatthe compositionoftheliquid in this caseisequaltothatoriginallycharged. Aschematic

It isobvious thatitcannot be anexact equilibriumsystembecause

ofthefactthatapressuredropisinvolvedinpassing thevapor through

the system; i.e., there are pressure variations which will affect librium There is also the dangerofentrainment, although this can

Ina great many cases, the gas introduced into the firstvessel has.been carriergas oflow solubilityand not a component ofthe system

FIG 1-3. Dynamicflow method.

Thus, in the determination of the vapor-liquid equilibria forsystems

such as ammonia and water, ammoniacal solutions areplaced in thevessels, anda gassuchasnitrogenisbubbledintothefirst oftheseand

the resultingnitrogen-ammonia-water vapormixtureispassedthrough

the succeeding vessels obtaining a closer approach to equilibrium.Equilibrium obtained in such a manner is not the true vapor-liquidequilibria forthewatervapor-ammoniasystem Itcloselyapproaches

true equilibriumfor the binarysystem under atotalpressureequal tothepartialpressureoftheammoniaandthewater vaporinthe gaseousmixture Eventhisisnotexact Thecarriergas hassomesolubility

intheliquidphase,andthepartialpressureof theseaddedconstituentsmodifies theenergyrelations oftheliquidandvaporphases. Usually

forlow-pressure operationthese errors arenotlarge in magnitude, but

as the pressure becomeshigher theerrors are serious andthe method

can give erroneous resultsifthe true vapor-liquid equilibria for tureswithoutthe carrier gas are desired.

mix-Dew andBoiling-point Method In essencethistechniqueconsists

in introducing a mixture of known composition into an evacuated

equilibrium containerofvariablevolume (Refs 6, 7, 9, 15, 17, 18, 20,

8 FRACTIONAL DISTILLATION

pressure vs. temperaturefor a number of theseprepared samples areobtained and, by cross-plotting, conditions of phase equilibrium may

pressure

The pressures are determined in two ways. One involves the

point being indicated bythediscontinuities inthe curveat the ning and the end of condensation The other employs a glass orquartz equilibrium cell, and the conditions are determinedvisually.

condi-tions to be determined, gives data on specific volumes of mixturesathigh pressures, and requires no analysis of the phases since the totalcomposition of the mixture is accurately determined gravimetrically

upon charging

simplerunitsusingmercuryasavariablevolumeconfiningfluidcannot

be used below the freezingpoint ofmercury In addition, the

mate-rials usedmust beverypure andfreein particularfromtraces of fixedgases, for in thecriticalregionthe saturation pressureisquite sensitive

to small amounts of fixed gases Further, a large amount of

onlyon binary systems. Asthephaseruledictatesthatmorecomplex

equilibrium

repeated contact of the vapor with the liquid and thus afforded the

time necessary for theattainment of equilibrium The dynamic tillationmethod (Refs. 2, 5, 11, 19, 24, 26, 34, 39) involvesadifferentprocedure(seeFig 1-4). Inthissystemadistillingvesselisconnected

dis-toa condenserand areceiver.

In the simplest case, a small sample of distillate is taken, and thecompositionsof thissampleandtheliquid in the still aredetermined

During such a distillation the composition of the distillate and the

DETERMINATION OF VAPOR-LIQUID EQUILIBRIA 9

Toreducethiscomposition variationthe quantityof liquidinthe still

ismade large incomparisontothe quantityof distillate. Frequentlysuccessive samples ofcondensateare obtained, andthese are analyzed

and the composition plotted vs. the quantity of liquidthat has been

distilled. An extrapolation of this curve back to zero quantity ofliquidremoved istakenasthe compositionofthevaporinequilibrium

with the original liquid.

Topheater

Charge

FIG 1-4. Dynamicdistillation apparatus.

obtainedby boiling aliquid isin equilibrium with theliquid. There

con-siderationswould tend to indicate that equilibrium should not beobtained The few experimental data thatare availablewould indi-catethat thedifference inthe compositionbetweenthevaporobtained

in thismanner andthetrueequilibriumisnotgreat inmostcases, but

After thevaporleavestheliquid, anycondensationintheupperpart

10 FRACTIONAL DISTILLATION

therefore introduce errors. Such condensation is usually reduced oreliminated by havingtheupperpart of the systemjacketed and ata

higher temperature than the condensation temperature ofthe vapor

in such aboilingsystemthereisacertainamountofsprayand

splash-ing. The spray that landson the heated walls will tend to vaporizetotallyandgive avaporofthe composition oftheliquidratherthan ofthe equilibrium composition

The pressure involvedin sucha systemisof courseessentially thatprevailing inthe receiver, andthismethodcan be used either for nor-

mal pressures, high pressures, orvacuum Theexact temperature ofthe operation is usually not known because the liquid is generallysuperheated The vapor and theliquid therefore are not in thermalequilibrium, and it is doubtful whether they are in true compositionequilibrium The apparatus hasbeen extensivelyused because of its

simplicity, andthe resultsare ofsufficientaccuracytobeofrealvalue

in distillation calculations

In order to obtain a closer approach to equilibrium, various

com-plicatingarrangements have beenused;forexample, Rosanoff modified

involve distillingaliquid, condensing thevaporsample, andrecycling

the condensate back to the still. A schematic drawing of such an

equilibrium still is givenin Fig 1-5. Thissystem was developed by Yamaguchi (Ref. 38) and Sameshima (Ref. 29) andhas been modified

advan-tage that it is simple, and the unit can be placed in operation and

allowed to cometoasteady statewithoutanygreatamount oftion. The same precautions relative to entrainment, condensation

atten-andtotalvaporization ofsplashed liquid mustbeobserved inthestill

aswasindicatedforthedynamicdistillationmethod Thecondensatecollectsuntil thelevel ishighenoughto flowover the trapandbacktothestill. Attheendofthedistillation, thiscondensateisremoved and

analyzed to determine the composition ofthe vapor, and a sample is

removed fromthestill todeterminethestill composition

This method suffers from the same difficulties as the dynamic tillationmethodin thatit isopentothe questionofwhetherthevapor

difficult to obtain the true liquid temperature because of the

super-effects. The ismaintainedbythe pressureintheexit

DETERMINATION OF VAPOR-LIQUID EQUILIBRIA 11

tube, andinnormalpressuredeterminationsthis isusuallyopentotheatmosphere Thistheoreticallyoffers thepossibility oferrors in that

it allows Oxygen and nitrogen to dissolve in the condensate sample,

solu-bilityofsuchgasesisusually smalland theerroris slight, butinpressure operations the useofthisgassystemcanlead to seriouserrors.

high-FIG 1-5 Continuous distillation equilibrium still.

The gas pressuring system, however, isextremely desirable in that it

regulates the condenser cooling capacity so that it exactly balancesheat input tothestill. Athighpressurestheerrors becomeso seriousthat this benefit must beforegone. Figure 1-6 indicates a type of

the pressure remains constant without the necessity of a sealing gas.

con-densate returned to the still is of a different composition from theliquid in the still and in general is of lower boiling point If thisvaporizes beforeit iscompletelymixedwith alloftheliquidinthestill,this will not beanequilibrium vapor.

12 FRACTIONAL DISTILLATION

a closer approach to equilibrium, this is not the case If the vapor

evolvedfromtheliquidisnotanequilibrium vapor,thistypeofrecycle

Vent*.

Condenser, Top heater

FIG 1-6 Continuous distillation still for high-pressure operation.

equi-librium bysuccessive contacts Therecyclingdoesgiveasteady-statecondition, but the approach to equilibrium is only that obtained by

boilingtheliquid.

In orderto eliminatesomeofthesources of errorin continuous tillation modifications^have been made The most

dis-DETERMINATION OF VAPOR-LIQUID EQUILIBRIA 13

theheatisaddedtorevaporize the condensatestreaminstead ofing a new vaporinthe still. Such anapparatusisshown inFig. 1-7,

form-In this case, the result is equivalent to recycling the vapor, and theoperation tends to be equivalent to the usual recycle system. It is

more difficult tooperate than the conventional continuousdistillationsystem The condensate must be completely vaporized. If any

Thermometer

well

FIG 1-7. Continuous distillation still with re vaporized condensate.

liquid is allowed to return to the still, the purpose of the system is

defeatedandtherateof distillationdecreases;i.e., lessvaporleavesthe

still If thevapor returnedto the still is greatly superheated, it willcause additional evaporation in the still and the operation will speed

up By proper adjustment a satisfactory balance can be obtained

It is believed that this apparatus is a definite improvement over the

regular continuous distillation system, and comparative data on the

sys-tem showdefinite differences ofthe type thatwouldbeanticipated.

Both the continuous distillation system and the modifications of it

suffer from the difficultythat the vapormust be totally condensable

14 FRACTIONAL DISTILLATION

It is also necessary that the condensate be a homogeneous mixture

,Thus,ifthe condensate separatesintotwolayers, the operationisnotsatisfactory Theother vapor-liquid equilibriummethodsare suitableformultilayer systemseither inthestillor in thevaporsample

ACCURACY OF VAPOR-LIQUID EQUILIBRIUM DATA

Asatisfactory investigation of the accuracy of the various

con-cerningalargeamountofthe published experimental vapor-liquidlibrium data The circulation and the bomb methods have thepotentialityofgiving high accuracy,andthe valueoftheresultsdepends

equi-on the careemployed bythe experimentalistin eliminatingsources oferror. The dynamic distillation and the continuous distillation

liquid is of a composition that isin equilibrium with the remainingliquid. Theadequacy of thisassumption has notbeen proved experi-mentally, but there are experimental results which cast doubt on its

validity for all cases. Analysis of the published data obtained by

maygivevaluesforthedifferencesofthevapor andliquidcompositions

at a given liquid composition thatarewithin 10 to 15per cent ofthetrue values Whenresults forthesame system are compared on this basis, it is not uncommonto find deviations of 10 per cent between

different investigators employingessentially the same techniques. A

critical study ofthe methods ofdeterminingvapor-liquid equilibria is

needed

References

1. ADEY, S. M thesis M.I.T., 1941.

2. BALY,Phil.Mag., (V), 49, 517 (1900).

3. BEBGANTZ, Sc.D thesis, M.I.T., 1941.

5. BROWN, Trans.Chem.Soc., 35,547 (1879).

6. CALINGAERT andHITCHCOCK,J.Am Chem.Soc., 49,750 (1927).

7. CAUDET, Compt.rend., 130,828 (1900).

8. COLBURN,JONES,and SCHOENBORN,Ind.Eng Chem., 35,666 (1943).

9. CUMMINGS, Ind. Eng Chem., 23,900(1931).

10. DOBSON,J. Chem.Soc.,127,2866 (1925).

12. FEDORITBNKO andRUHEMANN, Tech Phys., U.S.S.R., 4, 1 (1937).

13. FERGUSON and FUNNELL,/ Phys Chem., 33, 1 (1929).

15. HOLST and HAMBURG, Z.physik Chem.,91,513 (1919).

640

DETERMINATION OF VAPOR-LIQUID EQUILIBRIA 15

17. KAY, Ind.Eng.Chem., 30, 459(1938),

18. KTJBNEN, VERSCHOYLE, and VAN UBK, Communs. Phys.Lab., Univ Leiden,

19. LEHFELDT,Phil. Mag (V), 46, 42 (1898).

22. OTHMER, Ind. Eng Chem.,20,743 (1928).

23. QUINN, Sc.D.thesis,-M.I.T., 1940.

24. RAYLEIGH,Phil. Mag (VI), 4, 521 (1902).

25. RIGNAULT, Ann chim et phys., (3) 16, 129(1845).

26. ROSANOFF, BACON, and WHITE,J. Am Chem.Soc.,36, 1993(1914).

27. ROSANOFF, LAKE, andBREITHUT, /.Am Chem.Soc., 48,2055 (1909).

28. SAGE andLACEY, Ind.Eng.Chem., 26, 103 (1934).

29. SAMESHIMA,J.Am Chem.Soc., 40, 1482, 1503 (1918).

30. SCATCHARD,RAYMOND,and GILMANN,J. Am.Chem.Soc., 60, 1275 (1938).

31. SCHEELINE andGILLILAND, Ind.Eng.Chem., 31, 1050 (1939).

32 SIMS, Sc.D.thesis, M.I.T., 1933.

34. TAYLOR,/ Phys Chem., 4, 290(1900).

35. TOROCHESNIKOV, Tech Phys., U.S.S.R., 4,337 (1937).

36. VERSCHOYLE, Trans Roy Soc (London), A230, 189 (1931).

37. WILLand BREDIG, Ber., 22, 1084 (1889).

38. YAMAGUCHI,/. Tokyo Chem Soc., 34, 691 (1913).

39. ZAWIDSKI,Z physik. Chem.,36, 129 (1900).

CHAPTER 2

PRESENTATION OF VAPOR-LIQUID EQUILIBRIUM DATA

It is usually desirable to present the experimental vapor-liquidequilibrium data graphically A number of methods of presentation

temperature-composition and the vapor-liquid composition diagrams

numberof independent variablescan be obtainedfrom thephase rule

F is equal tothe numberof components Cplus 2.

In the usualvapor-liquid equilibriatwophasesareinvolved:liquidand

vapor ?Iowever,insomesystemsmorethan oneliquidphasemaybeencountered Forthetwo-phase systemthe phaserule statesthat thedegreesoffreedomorvarianceareequaltothenumberofcomponents

Thus a binary system has two degrees of freedom and can be sented by two variables*

repre-on rectangular coordinates

presented on triangular coordinates. Multicomponent systems with

more than three components are difficult to present, and special

methods are employedfor such systems

Any point onthe curve ABCgivesthe composition x, ofa mixtureof

use of the "mol fraction" greatly facilitates calculations of pressure phenomena It is the ratio of the number of molecularweightsofonecomponentina mixture dividedbythesumofthenum-

vapor-berofthe molecular weightsofall components Molper centisequal

to 100 times the mol fraction The line ADC represents the

com-position of the vapor that is in equilibrium with the liquid at any

PRESENTATION OF VAPOR-LIQUID EQUILIBRIUM DATA 17

given temperature Thus a liquid with the composition xi will have

equilibrium withit will have the composition yi = #2.

Starting with a mixture of the composition xi, at a constant totalpressure equalto760mm., andatatemperaturebelow 2 , therewillbe

vertical line at Xi intersects the curve ABC, when a vapor phase of

the composition x2 willappear Sincethere arenowtwophasesandthepressureis fixed, there can be but onevariable, temperature, and thecomposition of the phases will depend upon it. Let the temperature

being no longer independent variables, must change accordingly,

compositiony* = rc 4 It should be remembered that the quantityof

there-fore,thechangeinthe compositionsoftheliquidandthevaporincludes

suchacorresponding changein the relative proportionsof each phasethat the totalcomposition ofthe system remains the same, x\. Fur-thermore, the relative proportions of the liquid phase and the vapor

18 FRACTIONAL DISTILLATION

be seen that, as the temperature is raised farther, the proportion ofliquid phase decreases until, when the temperature reaches a pointcorrespondingto the intersection ofthe vertical linex\ and the curve

com-position as the original liquid, and the liquid phase disappears. At

higher temperatures, there is but one phase, and the system again

Fia 2-2 Equilibrium y,x data for CC14-CS2 mixtures.

region of superheated vapor

Ifthe foregoingprocessisreversed, the steps canbe followed inthe

sameway. Startingwithsuperheatedvaporofa composition3/5 = xi

separateout Furthercoolingwillchange boththe compositionoftheliquidandthevaporalong thelinesABC and ADC, respectively, untiltheliquidhasreached the compositionx\ whenall thevaporwillhave

disappeared

eitherconstant pressure or constant temperature Thedatapresented

were forconstant pressure so

PRESENTATION OF VAPOR-LIQUID EQUILIBRIUM DATA 19

Fig 2-2 for the same conditions In this curve the composition

yi = x2isplottedasordinate with compositionx\ asabscissa, y* x\

as ordinatewithz3 as abscissa,andsoforth. Thisparticular relation

is very useful in distillation calculations. Itdoes not give so much

information as Fig. 2-1, owing to the elimination of temperature

separation between the components and the temperaturesare allowed

x,Mol fraction in liquid

Fio 2-3 Temperature-composition diagram.

1.0

toadjust themselvesaccordingly On Pig. 2-2 the 45 line represents

important, this variable can be plotted vs. the liquid composition onthe same figure.

The curves given in Figs. 2-1 and 2-2 are termedthe normal type.

2-3 temperature-compositiondiagrams forconstant total pressure aregiven forfourdifferent typesof binary mixtures, and in Fig. 2-4 thecorresponding vapor-liquid diagrams are given for the four same

mixtures

Type I isnormal, i.e., the composition of the equilibrium vaporis

liquid, thus by repeated operations it ispossible to obtain complete

20 FRACTIONAL DISTILLATION

In type II the temperature-composition diagram passes through a

diagonal Thustherearemixtures thathavelowerboilingpointsthan

either ofthe pure componentsat thesamepressure. In other words,the mixtureistheminimumboiling-point type. Whensuch tempera-

ture-composition diagrams are encountered, the vapor-liquid

com-positioncurvewill alwayscrossthe 45 line. In the regionbelowthisintersection with the diagonal, the equilibrium vapor isricher in one

componentthantheliquid;abovethisintersection,the vaporispoorer

in thiscomponent than the corresponding liquid from which it came

Thus the volatilities have reversed Where the vapor-liquid curvecrosses the 45 line, thevapor has thesame composition as theliquid

and operations based on producing an equilibrium vapor from thisliquid would not be able to separate mixtures of this composition.This particular composition is called a constant boiling mixture, orazeotropicmixture, sincejt^will yaporize^without anychange inCom-

positionand, theipfore, withoutanychangeintemperatureduring theevaporation

*~

TyplFHTTs the reverse oftype II. In thiscase there are mixturesthathaveboiling pointshigherthaneither ofthepurecomponentsatthesame It will be notedin 2-4that the curve

PRESENTATION OF VAPOR-LIQUID EQUILIBRIUM DATA 21

III also crosses the 45 line but curve II cuts the 45 line with theslope less than 1 while curve III crosses the 45 line with the slopegreaterthan 1. CurveIIIis ofthemaximumboiling-point type, and

the particular composition at which the curve crosses the 45 line is

called a maximum constant boiling mixture or a maximum boiling

mix-to the value given in the two-phase region The y,x data for this

diagonal in the two-phase region; thus at this intersection the

com-positionofthevaporisthesameasthatofthecombinedliquid phases.

Such a mixture can be evaporated to dryness at constant pressure

particular composition istermed a pseudo-azeotrope This

usageofthetermazeotropeby Wadeand Merriman (Ref. 50) impliedthat the liquid could be evaporated to dryness without change incomposition Onlythetwo-phase mixture correspondingtotheinter-section ofthe y,xcurvewith the y = xlinecanbe evaporatedwithout

In other casesthe y,xcurve maynot cross the diagonal inthe

they may form, andusuallydo, trueazeotropesin one of their

LiteratureData Thevapor-liquidequilibria foralarge numberofmixtures have been experimentally determined, and Table 2-1 lists

some of the more reliable anduseful determinations

The data given in the table represent a large amount of

determina-tionsa number of investigatorshave tried to develop theoretical and

empirical methods ofpredicting such vapor-liquid equilibria fromthephysical properties of the pure components While certain correla-tionshave beendevelopedbythismethod,reliableexperimental deter-minationsare to be suchcalculated values

PRESENTATION OF VAPOR-LIQUID EQUILIBRIUM DATA 23

24 FRACTIONAL DISTILLATION

References

1. BAERE, MeVICAR, and FERGUSON,/ Phys Chem., 34,1310 (1930).

2. BEEBE,COULTER,LINDSAY,and BAKER,2nd.Eng Chem.t 34, 1501 (1942).

3. BOSNJAKOVIC, "Technische Termodynamik II," diagrams, T. Steinkopf,Dresden,1937.

5. BREDIG and BAYER,Z physik Chem., 130, 15 (1927).

6. BROMILEY andQUIGGLE,Ind. Eng Chem., 25,1136 (1933).

7. BRUNJES and FURNAS, Ind.Eng Chem., 27,396 (1935).

8. CAREY,Sc.D thesis inchemicalengineering, M.I.T., 1929.

9. and VOLANTE,Ind.

PRESENTATION OF VAPOR-LIQUID EQUILIBRIUM DATA 25

12. FERGUSON, FREED, and MORRIS, Landolt-Bernstein Tabellen, egIIIc, 2470,and /.Phys Chem., 7,87 (1933).

15. GRISWOLD andDINWIDDIE, Ind. Eng.Chem., 34, 1188 (1942).

16. HARRISON and BERG,Ind. Eng Chem., 38, 117 (1946).

17. HIRSHBERG,Bull soc.Mm. Beiges, 41,163-195 (1932).

19. "

International Critical Tables," McGraw-Hill Book Company, Inc., New

York, 1926-1930.

22. LEE,/ Phys Chem., 36, 3569 (1931).

23. LEWIS and CAREY, Ind.Eng Chem., 24,882 (1932).

26. MCDONALDand McMiLLAN, Ind.Eng Chem., 36, 1175 (1944).

27. OTHMER,Ind. Eng.Chem., Anal Ed., 4, 232 (1932).

28. OTHMER, Ind.Eng Chem., 20,743 (1928).

34. QUINN,Sc.D.thesis inchemicalengineering, M.I.T., 1940.

36. RHODES, WELLS, and MURRAY,Ind. Eng Chem., 17, 1199 (1925).

37. ROSANOFF, BACON, and WHITE,J.Am.Chem.Soc., 36, 1803 (1914).

39. SAMESHIMA,/.Am Chem.Soc., 40, 1482 (1918).

40. SCATCHARD, WOOD, and MOCHEL,/. Am Chem.Soc., 62,712(1940).

41. SCHEELINE andGILLILAND, Ind.Eng Chem., 31, 1050 (1939).

43 SIMS, Sc.D thesis inchemicalengineering, M.I.T., 1933.

45. STOCKHARDT and HULL,Ind. Eng.Chem., 23, 1438 (1931).

47. TREYBAL, WEBER, and DALEY,Ind. Eng.Chem., 38,815 (1946).

48. TYRER,J. Chem.Soc., 101, 81, 1104 (1912).

50. WADEand MERRIMAN, /.Chem.Soc., 99,997 (1911).

51. WASHBURNand HANDORF,J.Am Chem.Soc., 67, 441 (1935).

52. WEISSENBERGER, HENKE,and KAWENSKI,/ prakt Chem., 113, 171 (1926).

53. WILLIAMS,ROSENBERG, and ROTHENBERG,Ind.Eng.Chem., 40,1273(1948).

55. YORKand HOLMES,Ind. Eng.Chem., 34, 345(1942).

56. ZAWIDSKI,Z physik. Chem., 36, 129 (1900).

22

CHAPTER 3

CALCULATION OF VAPOR-LIQUID EQUILIBRIA

The calculation of vapor-liquid equilibria is important because ofthe difficulty of obtainingexperimental values andbecause itgives apicture of the general behavior of liquid-vapor mixtures The basic

thermodynamic relationships for such equilibria are complex, and in

most cases they involve unknown factors or quantities which limittheir usefulness until simplifying assumptions are made It is thelimitations ofthe simplifyingassumptionsthat restrictthe applicabil-ity of the thermodynamic relations. However, even for the cases in

rela-tionships,theydoserve asvaluable criteriaforestimating the behavior

of a distillation process and they help to clarify and explain thedivergenciesthat arefrequently noted

VAPOR PRESSURES OF COMPLETELY MISCIBLE LIQUID MIXTURES

AT CONSTANT TEMPERATURE

Raoult'sLaw Whe^<m&liquid isdissolved in another, thepartialpressure of each is decreased Assume two liquids, the molecules of

of molecular association, chemical combination, and the like. In an

equimolecularmixtureoftwosuch liquids,eachunitof surfaceareaoftheliquid mixture will have in its surface half as many molecules ofeachcomponentasexistintheliquid surface ofthatcomponentinthepure state. Hence the escaping tendency or partial pressure of each

componentin themixture willbe halfthat ofthe same componentinthepure state. Similarly, in a mixture containing 25molper cent of

exert a partial pressure 25 per cent of that of this component in thepure state. In more general terms, for any such mixturethe partialpressure ofany component will equal thevaporpressure ofthatcom-

Thisgeneralizationisknownas Raoult'slaw (Ref. 19) It isexpressed

in the relationship, pa = Paxa , where pa is thepartial pressure ofthe

component A in thesolution, xa is its molfraction inthesolutionand

CALCULATION OF VAPOR-LIQUID EQUILIBRIA 27

relationship is shown graphically in Fig.

3-1,

By in the liquid portion. The ordinates are pressures, C being the

BCrepresent thepartialpressures ofthecomponentsoveranymixture,while theline CD isthe totalpressure ofthe mixture

Deviations from Raoult'sLaw In view of the above assumptions

as to equal molecular size, absence of association, etc., it is not prising to find Raoult's law honored more in the breach than in theobservance Nonetheless mixtures of some organic liquids, such asbenzene-toluene, deviate from it

mix-tures of hydrocarbons of thesame

seriescanusuallybeneglectedfora

great deal ofengineeringwork,and

even for mixtures of a number of

series this isoftentrue. For

mix-Mol entA

tures ofaromaticandaliphatic

pounds, however, the deviations Raoult's law.

are oftenlarge,thoughneverofthe

order of magnitude of such mixtures as hydrochloric acid and water,andthe like. Organic stereoisomers obeyit very closely aswould beexpected from the considerations upon which it is based However,

thedataforthe great majorityofotherliquids,whenplotted asshown

in Fig 3-1, deviate largely from the lines BC and AD When very

near pointsC andD, the deviationforany componentis slight ifthat

expressed bysaying that indilutesolution Raoult'slawapplies tothesolvent Since the deviationfromRaoult'slawmaybeeither positive

or negative, great or small, this graphical generalization serves as a

convenientstandard ofcomparison

applies to the vapor pressure of the solute in dilute solutions, justasRaoult'slawapplies tothatofthesolvent. Henry's lawstatesthatthepartialpressureofthesoluteisproportional toitsconcentrationinthesolution In analogy with Raoult's law it may be expressed by theequation

= kx