ethanol-water system (1)

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ETHANOL-WATER

SYSTEM

Vapor-Liquid Properties

at High Pressures

ERTAIK operations used in the manufacture of an-

hydrou s alcohol, ethylene from alcohol, sil ica aerogel,

C and o ther products depend on th e high-pressure vapor-

liquid behavior of ethanol-water. Da ta of satisfactory accu-

racy and pressure range have not heretofore been available.

VAPOR-LIQUID EQUILIBRIA

Vapor-liquid equilibria up to 15 atmospheres were re-

po r te d by G rum bt 3). However, a stu dy of his dat a shows

serious scattering of the points and self-inconsistencies which

he attribu ted to refluxing in the vapor line of his app aratus .

Th e vapor-liquid equilibria of the system were determined

a t several constant tem peratures with a n all-steel recirculation

type appara tus. The deve lopment of th is appara tus and the

high-pressure vapor-liquid equilibrium

of

the benzene-toluene

The photograph shows a cont rol panel for continuous rectification

of

190 proof alcohol from wheat mash , at the plant of Joseph E. eagram

Sons, Inc . : section

of

column may be seen in the background.

70

are extended to high pressures to make

possible more intelligent control over

certain industrial processes.

Y - X - P - T

curves are evaluated up to a tempera-

ture

of

275 C. at saturation pressures.

Critical temperatures and pressures

of

the system are also obtained.

JOHN

The

GRISWOLD J. D. HANEYl

Jniversity of Texas, Austin,

Texas

AND

v. A. KLEIN2

system determined with it are reported in another article

(1A). The pressure gage had a to ta l range of 1500 pounds

and was graduated in 10-pound divisions. Experim ental

pressures were read to

1

pound, and the gage was checked

against

a

dead-weight tester. Tem peratures were determined

by calibrated iron-cons tantan thermocouples and

a

low-range

potentiom eter. Th e accuracy of the temperatu re observations

was approximately

0.5

e. A

combination check on thermo-

couples and gage was obtained by observing the vapor pres-

sure-temperature curve for water in the same appar atus, up

to 1500 pounds pressure . The e thanol used throughout the

work was

U.

S.P. grade material. Distilled water was taken

from the laboratory supply. Analysis of samples was ob-

tained from densities at 20 C., determined by the balance-

plummet- thermosta t method

of

Osborne, McKelvy, and

Bearce (8).

1

Present address, Joseph

E.

Seagiam Sons, Inc., Lawrenceburg, Ind.

Present address,

Dow

Chemical Company, Freeport, Texas.



702 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 35, No. 6

-

VAPOR iaum EQUIL IERIUM

OF

E THANO1

-

WA

TER

I CONSTANT

TEMPERA

T U K S

4. APOR LIQUID EQUILIBRIUM

OF

FTHANOL WA TER

CONSTANT

PRESSURES

b CAREY

8

LEWIS 1

A NOYES

8

WARFEL

ALCULATED

F

EXPERIMENTAL

THANOL IN

iiauio

v IO

2 3

4,O

5

6

7

8 9

The exper imenta l vapor- l iquid equi l ibr ium da ta a re sum-

marized in Table

I

and p lo t ted as i so thermal Y -X curves on

Figure 1A.

T o

develop the constant pressure

Y-X

plot of

Figure lB, pressure isotherms were plotted and then curves

of

vapor composition against pressure for several constant

liquid compositions were constructed. Inte rpo latio n

of

t h e

latter curves gave the isopiestic Y-X graph (Figure 1B).

CRITICAL

TEMPERATURES

Cri t ica l tempera tures were de te rmined by observing the

behavior of mixtures

of

known composition when sealed into

glass

tubes and hea ted . The sea ling technique and the hea te r

were described in an earlier article

2 ) .

The sa mp le t ube s

were of Pyrex, 4 mm. 0.d. with a 1-m m. wall thickness and

I

CR I T I C A L TFhPER.4 TURES *-f-f3

PER CCNT

ETHANOL

ro 2 30

4

so

60

70 80

30

10

2

30

40

5

6

7

8

9

-7

Figure 1

an over-all length

of

a b o u t

60

mm . Th e obse rve d t e m-

pera tures (a f te r emergent s tem correc t ions) were accura te

to approximate ly

1 C.

Th e cons tant volume behavior of th is system as th e c r i t ica l

s t a t e

is

approached was found to d i f fe r somewhat f rom tha t

characteristic of pure compounds and hydrocarbon mixtures.

The tubes were charged with alcohol solution to approxi-

mate ly one th i rd the i r volume a t room tempera tu re and were

the n sealed . I n the de te rmina tion id i ich fo llowed, the l iquid

volume or meniscus level rose wit h temp eratu re. TT'ithin

the last

1

C.

below th e crit ical, the meniscus rose from abo ut

two th i rds of the tube he ight to comple te ly f i l l the tube .

The vapor phase app arent ly became zero , and d isappearance

of the meniscus could no t be observed. On

slow

cooling from

1' C.

above th is tem pera ture , a white cloud 1%-ould uddenly

appear , quickly condense , and revea l a l iquid meniscus.

These rising an d falling tem pera ture s differed by less than

1'

C.

for

all cases in which the tu bes were charged t o between

20 and

40

per cent

of

the i r volume a t room condi tions. This

tempe ra ture i s therefore taken

as

the t rue c r i tica l. Fur t her

support of this hypothesis is obtained from a study of rela-

tions

for

re la t ive vapor and l iquid volumes a t constant to ta l



June, 1943 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 703

volume. Exper imental critical tempera tures are summarized

point. Th e ethanol-water azeotrope, which occurs a t 95.6

in Tab le

I1

and plot ted on Figure 1C.

Th e figure also shows weight per cent alcohol at atmospheric pressure, lies at 95.3

the recent data of W hite (IO), which lie

2

t o 5 C. ower weight per cent alcohol a t a tota l pressure of 1450 m m . (9)

than the new values .

This indicates only

a

slow change in the azeotropic compo-

Th e critical tempera ture of this system ap-

proaches linearity with composition (on thc

weight basis) much more closely than doe?

t h a t of binary hydrocarbon mixtures

7).

CRITICAL PRESSURES

Critical pressures were determined by the

same procedure and with an appa ratus s imi-

lar to th at used in an earl ier art icle 9). T h e

equipment (F igure 2) consisted

of

a steel

bomb connected to

a

pressure gage by a

loop of l/*-inch 0.d. annealed steel tubing.

The bom b was suspended in a fused salt

(IETS) bath by a sliding linkage to a motor-

driven cam. This gave the bomb vert ical

reciprocating agitation. Th e pressure gage

described under Vapor-Liquid Equilibria

was utilized when the pressures were be-

low 1500 pounds. At higher pressures a

3000-pound gage graduated in 20-pound

divisions and checked against a dead-

weight tester was used.

T h e b o m b was charged with

100

cc. of

alcohol solution and heated to the boiling

point to e l iminate air . Th e gage tubing

(used to vent air) was then connected to

the gage, and temperature-pressure data

were taken through the cri t ical temperature

of the mixture ,

as

read from Figure 1C.

The usual range was from 20 below to 20 C. above the

critical temperature, in which six to eight observations were

made. Th e ba th temperature and observed pressure were

constant for at least

10

minutes prior to readings.

Figure 2.

Apparatus for Determina tion of Critical Pressures

TABLE

.

VAPOR-LIQUIDQUILIBRIUM

F

ETHANOLWATER

Tempe ratur e, Mole %,Eth anol Mole 7 Ethanol Pressure

c. in Liquid in Vapor Lb./Sq. In. kbs.

150

200

250

7 . 3

1 3 . 8

2 6 . 5

5 1 . 4

6 3 . 9

3 3 .4

4 1 . 4

4 8 .7

6 1 . 6

6 9 . 5

5 . 8 2 4 . 7

1 1 .4 3 3 .8

2 3 .7 4 3 .3

4 9 .7 5 8 .5

6 3 . 3 6 8 . 1

107

119

130

145

147

300

331

370

415

424

275

1 2 . 6

2 6 . 0

40 .0

2 4 . 5

3 4 . 4

4 2 . 5

1176

1341

1492

The data were plot ted and the pressures at the critical

temperatures read from the plots .

The results are included

in Table I1 and plot ted on Figure 1D. Th e critical pressure

is seen to be sub stantially linear with weight per c ent ethanol

at concentrations below 70 per cent.

BEHAVIOR

OF

AZEOTROPE

Separation of a mixture b y fractional dis ti l la t ion may b e

limited by the existence of either an azeotrope or

a

critical

sition with tempe rature a nd pressure. Azeotropic composi-

t ions and behavior a t higher temperatures a nd pressures have

apparent ly not been reported. Th e cri tical temp erature

of

anhydrous ethanol is 243 C. F rom F igure 1C the cri t ical

composition at

250

C. is approximately

80

mole per cent

alcohol, and a t 275 it is approximately 45 mole per cent.

A minimum-boiling azeotrope mu st exhibit

a)

no difference

in composition between liquid and equilibrium vapor, and

b ) an isotherm al maximum p ressure at some definite compo-

sition. To show the aeeotropic behavior more clearly at

250

O

C. an d above, the pressure-composition diagram of

Figure 3 was constructed.

It is evident that at 275

C.

no

azeotrope exists. A t 250 an equilibrium determ ination

gave 68.8 mole per cent alcohol in the liquid and 69.6 in the

vapor at

a

pressure of

1010

pounds. Since th e critical

composition is 80 mole per cent alcohol and the critical

pressure is slightly abov e

1000

pounds a t 250 C. , the presence

TABLE1. SUMMARIZEDATA OR CRITICALTEMPERATURES

AND

PRESSURES

Critical Critical

Weight

Mole

Temp Weight Mole Pressure

Ethanoy Ethan07 C. Ethanol Etheno?

Lb./Sq.

Ih.

100

9 4 . 0

8 8 . 7

8 4 . 0

7 9 . 0

7 4 . 1

6 9 . 2

6 4 . 4

6 1 . 0

5 5 . 1

4 9 . 6

4 5 . 1

4 0 .0

3 5 . 8

2 6 . 9

2 4 .0

1 8 . 7

100

8 6 . 1

7 5 . 5

6 7 .3

5 9 . 4

5 2 .8

4 6 .8

4 1 .5

3 8 . 0

3 2 .4

2 7 .8

2 4 . 4

2 0 .6

1 7 .9

1 2 .6

1 1 . 0

8 . 3

2 4 3 .0

248.0

2 5 3 .1

2 5 9 .2

2 6 4 . 2

2 7 0 .3

2 7 7 .3

2 8 4 .4

288.4

296.5

307.5

3 1 1 .6

3 1 7 .6

325.7

3 3 4 .8

339.8

344.9

100

8 6 . 5

80 0

6 3 . 9

4 6 . 0

3 0 . 9

1 6 . 3

100

7 1 . 5

6 1 . 0

4 0 . 9

2 5 . 0

1 4 . 9

7 . 1

925

1100

1220

1618

2060

2440

2830



704

I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 35, No. 6

MOLE PER ChN T E T H NOL

Figure

3

is an exceptional rather than a general situation 4 ) . T h e

system th us does no t fall into either of tw o classes which

exhibi t re trograde condensat ion as enunciated by Katz and

K u r a t a (6). A necessary condition for retrograde con-

densation is the existence of separate points of m axim um

pressure and of niaxiniuin tempe rature during th e coexistence

of liquid and rapor, by a mixture of some definite composi-

t ion. This requires tha t the peak of a border curve be round

rather tha n sharp. Although few of the present

l--X

d a t a

lie close to the critical locus, vapor and liquid isotherms

extrapolate so near the same point

on

the locus curl-e th at

zones of retrog rade con densation must b e either extremely

small

or

nonexistent.

ACKNOWLEDGMEhT

S .S

Sutherla nd assisted in th e construction of th e diagrams.

or absence of a n azeotrope between 70 and

80

mole per cent

alcohol a t this temperature canno t be definitely ascertained.

PRESSURE-TEMPERATURE RELATIONS

The most widely used method of obtalning high-pressure

vapor-liquid equ ilibrium da ta on binary systems heretofore

has been to observe dew and

bubble points on samples of

definite composition in a vari-

able-volume calibrated glass pres-

sure tube (6).

The resul t ing P-V-T da ta a re

customarily p lotted directly as

envelope curves. Th e envelope

for each composition is tangent

to the critical locus curve.

Vapor-liquid equilibria may be

calculated from P-X-Y plots

of dew and bubble point curres.

On the o the r hand , the p resen t

metho d yields direct Y-X da ta .

Hon-ever, a

P-T

diagram was

constructed for ethanol-water

(Figure

4)

since it is of interest

for comparison with other sys-

tems s imilarly plot ted. To de-

velop this diagram, the data

were interpolated from a P-X

chart w ith pressure

o n

a logaritli-

mic scale, The spacing

of

t h e

isotherms was nearly l inear with

temperature , and P-T values

for

compositions of 25, 50, and i t?

niole per cent alcohol were read

from the plot . The resul ts a long

with vapor pressures of water,

ethanol, and the critical locus

are shown on Figure 4.

The critical locus contains

no poin t of high er pressure or

higher temperature th an the

critical values for aater. This

LITEHATUHE CITED

( l j Carey and Lewis,

IND.

B G .CHEM., 4, 882 (1932).

(1A) Griswold, Andres, an d Klei n, Trans. Am.

Inst

Chem.

Engis.

39,

223 (1943) ;

Petroleum

Ref i nerg ,

22,

No. 6

(1943).

(2) Griswold and Kasch,

1x11.

BG. CHEM.,

4,

804 (1942).

(3) Grumbt ,

J. A.

Tech Mech .

Thermodgnam.

1,

309, 349 1930).

4)

Hougen and Watso n, Industrial Chemical Calculations ,

2nd

ed., pp. 406, 407, New York, John

TTiley

Sons, 1936.

(5) Katz and Kura ta ,

IND.

NG.

CHEM. 2, 817

(194CI)

(6) Kay, W. B., Ib id . , 30, 459 (1938).

(7)

Mayfield, F. D.,

I b i d . , 34,

844 (1942).

(74.) Noyes and Warfel, J .

Am.

Chem. Soc , 23, 463 (1901).

(8) Osborne, McKelvy, and Bearce, Bur. Standards,

Bd l .

9, 371

(9) Wade and Merriman, J . Chem. Sac., 99, 997 (1911).

(1913).

(10) White, J . F., T ra n s . Am. I n s t . Chem. Engrs. ,

38,

435 (1942).

Figure 4

ethanol-water system (1)

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