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.
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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
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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
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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
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