till
Vapor-Recirculating Equilibriu
DATA-ON HEPTANE-TOLUENE AND METHANOL-CARBON TETRACHLORIDE HOWARD HIPKIN AND H. S. %\ITERS C . F. Rraiin & Co., A l h a m b r a , Calif.
MAXY
schemes have been used to determine vapor-liquid equilibria. Gilliland ( 6 ) has revieyed the various kinds of equilibrium etills and classified them into six main types, with numerous modifications. The t'wo most common t,ypes can be classified as liquid-recirculating and vapor-recirculating, This paper describes an improved vapor-recirculating still that eliniinates the opcrat'ing difficulties associated wit,h previous designp.
The sound theoret,ical principles of the vapor-recirculating still outbveigh its operational disadvantages. An improved design with a good cont,rol should minimize operational troubles, vihilc> no amount of design could entirely remedy the theoretical :h > ortcomings of the liquid-recirculating still. APPARATUS AND PROCEDURE
Figure 1 shows the still that was constructed. X separatp vapor jacket replaces the usual heater winding t o maintain adiabatic operation. All parts, except the Teflon sample valvps, are made from glass. Arrows on Figure 1 show the vapor flow through the still. Vapor rises from the vaporizer through tt series of annular spaces, bubbles from eight s m d l holes into tlip liquid in the contactor, and then passes into t'he condenser, where it is totally condensed. The condensate returns t o the vaporizer and is again vaporized, thereby completing the cycle. A t equilibrium, samples are taken from the contactor and fro:n t,he condensate-return line to the vaporizer. The center tube of the contactor is open a t both the top arid the bottom. It has t v o purposes: it promotes civculation within the contac%orby the Cottrell principle; and it provides n calming
LIQUID-RECIRCULATING STILLS
A literature survey showed that liquid-recirculating still3 are the type most commonly used ( 1 , 5, 1 1 ) . I n thia type of still, the liquid is boiled, condensed, and then recycled to the reboiler. Liquid samples are taken from the reboiler, and vapor samples are removed as liquid from t,he condensate recycle. Operation is simple and requires very little attention, and the apparatus is easy to build. Theoretically, holyever, the liquid-recirculat,ing still has sevrral drawbacks. Its operation is based on the assumption that vapor arising from the boiling liquid is in equilibrium with the liquid. Gilliland (6) points out that there has been no adequate proof of this assumption. Unless this assumpt,ion is valid, continued recirculation does not bring t'he system nearer equilibrium. The liquid-recirculating still requires pcrfect mixing in the rcboiler. If any portion of the recycling condensate vaporizes before it mixes intimatel. n-ith the liquid in the still, equilibrium will not be reached.
dk.
,CONDENSER
TO JACKET-MANOMETER
VAPOR-RECIRCULATIX'C, STILL
From a theoretical standpoint, the vapor-recirculating still is a better design. Here t'he condensate stream is revaporized before it is returned to the still; in effect, vapor is bubbled through liquid and recycled until equilibrium is reached. The aseumptions required by the liquid-recirculating still are unneces.ary, but in general, vapor-recirculttt,in~stills are difficult to control. T h e contactor section must be kept adiabatic t o maintain steadystate operation. Several vapor-recirculating stills have been described ( 2 , 4,8, 12). I n the still described by Jones, Schoenborn, and Colburn (8)) the condensate recycle is totally vaporized in a flash boiler. An esternal winding is used t o keep the contactor adiabatic. If this winding is too hot, the boiler floods. If the uinding is too cold, the boiler goes dry. Heat leaks from the contact,or along the therniondl or sample line must, be balanced by an incrense in mdl temperature under the nindinge, arid so the wv:rll may h e hotter than the boiling liquid. Amicli, JTeiss, and Kirshenbauni (2) used an air space betxeen the winding and the contactor in an at,t,cnipt t o overcome t,his difficulty. The still described by Scatchard, Raymond, and Gilrnann ( I @ ) uses a submerged heater to vaporize the condensate. The vapor rising from the reboiler jackets the contactor. Because of hezt loss from the walls of the jacket, a vertical temperature gradient probably exists in the vapor jacket and along the ~ v a l of l the contactor. The upper section of the contactor is not jacketed, and some heat leakage is unavoidable a t this point. These tlieoreticnl ehortcomings niay have negligible effect on the results from these stills, but it seemed advisable to try to eliminate such possible sources of error. 2524
-r
THERMOWELLS-'
EIGHT HO-ES I/$ 0 45'APART
CAD?-LARY TbBING 2 w m BC9E
LIQUIO SAUPLE PdIhT
EIGHT TRIANGULAR SLOTS 2 m n DEEP 45' APART
-
VCPOR JCCKET
2 5 0 WATT HEATER SEALED INTO PYREX d-VCPOii
./i
Figure 2 .
SAMPLE-POlhT
Vapor-Recirculaeinga~~~~n~ Still
Two 25Q-watr heaters are sealed into vapor jarliet. is shown
O n l y one
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1954
zone, relatively free from vapor bubbles, where the liquid samples are withdrawn. The top of the contactor is necked into the shape of a Venturi. This causes the boiling liquid to impinge on the tip of the thermocouple and increases the vapor velocity past the couple.
SAN'PLE
Figure 2.
Sample Valves for Equilibrium Still A. B.
Two-way valve Three-way valve
To ensure satisfactory operation, the still must be kept adiabatic. An external heater winding has several disadvantages. Installation is difficult-particularly over the ends of a vessel, and over any irregular shapes or projections. Hot spots are unavoidable, and precise control is difficult if not impossible. Most of the operational difficulties of previous vapor-recirculating stills can be traced directly to the adiabatic winding. VAPORJACKET. A vapor jacket eliminates the problems of an external winding. Figure 1 shows how it is constructed, All parts of the still are enclosed by the jacket, except the condenser and the condensate-return line. Two immersion heaters are seakd into the bottom of the jacket. One is used as a fixed heater controlled by a variable transformer, the other operates intermittently from a differmtial temperature controller. A small condenser attached to the top of the vapor jacket provides additional heat loss from the jacket and thus smooths out the operation. Such a condenser is not absolutely necessary, but for low-boiling materials, where the heat loss is normally low, it is helpful. Almost any reasonably pure liquid can be used in the vapor jacket. Normally the jacket liquid need not be of as high a Duritv - . , as the comDonents used for eauilibrium measurements, but the boiling range should be not more than about 0.5' C. The boiling point should be about the same as the temperature in the still, or a little higher. Thir keeps the jacket a t atmospheric pressure or under a slight vacuum. Temperature traverses along the jacket thermowell have shown no appreciable differences in temperature over the length of the still. Operation of the vapor jacket is simple and automatic. At the start of each run, the jacket is evacuated until the liquid boils and is then sealed off. Two thermocouples, one in the jacket and one in the contactor, are attached to a differentialtemperature controller. If the jacket couple is a t a lower temperature than the contactor couple, the controller turns on the jacket heater. This raises the temperature in the jacket. When the jacket gets as hot as the contactor, the heat goes off. Maximum temperature fluctuations with this type of control are about 10.l0c.
2525
HEATERS.The three heaters are of a bayonet type, commercially available. The heating elements are fused into a borosilicate glass tube, and are completely enclosed with glass. The heaters are ring-sealed directly into the still. This arrangement eliminates any possibility of leakage or contamination from the heaters. SAMPLEVALVES. Materials such as toluene and methyl ethyl ketone are difficult to hold with stopcocks, even though special greases are used, and there is often the problem of contamination from the grease. Figure 2 shows details of the two sample valves used on the still instead of stopcocks. One is a two-way valve, the other a three-way. All internal parts are made from Teflon. These valves make a positive, leakproof seal. They are easily attached to glass tubing, and they do not cause contamination. MEASUREMENTS. Temperatures in the contactor are measured with a copper-constantan thermocouple and a Leeds and Northrup potentiometer. The thermocouple was calibrated a t the ice point, the steam point, and the boiling points of the pure components. The pressure in the still is held a t 760 mm. by bleeding nitrogen manually into the system. A 5-gallon surge bottle provides enough capacity so t h a t nitrogen additions are infrequent. SAMPLING. Experiment showed that it is necessary t o flush about 5 or 6 ml. of liquid from the contactor sample line to ensure a good sample. This is roughly ten times the holdup of the line. The three-way valve on the condensate line requires only about a %mi. flush, since the main body of the valve is cor)tinuously purged by the condensate flow. The three-way valve must be operated so that the sample is drawn from the condensate leg rather than from the vaporizer, because the compositions of the condensate and the vaporizer liquid are not the same. T o eliminate any possibility of back-leakage, the condensate ample is taken slowly enough to keep the level in the condensate leg always higher than the level in the vaporizer. INITIAL TESTS
LINE-OUTTIME. The time required for the still to reach equilibrium was checked by two different methods and for two binary mixtures. In the first test, a heptane-toluene mixture was charged to the still, the same mixture to both the contactor and the vaporizer. Sufficient charge was added to the contactor t o cover the boiling tube, and enough to the vaporizer to cover the heater. During the run these levels remained essentially con-
STEINHAUSER 8
0 MOLE % HEPTANE
2526
INDUSTRIAL AND ENGINEERING CHEMISTRY
'
were carried out. Both refractive index and infrared were used for analysis. Again equilibrium was established in 30 to 45 minutes. ENTRAIXVENT. Solutions of silver nitrate and sodium chloride were used to check entrainment,-silver nitrate in the vaporizer and sodium chloride in the contactor. Any entrainment: either into the vaporizer or into the cont,actor, caused precipitation, but even a t the highest rates, about 18 gram-molce per hour, there were no signs of cloudiness. As this is 3 sensitive test, it, \?-as concluded that entrainment is negligible.
I
*
Vol. 46, No. 12
BRAUN EQUILIBRIUM-STILL BRQMILEY AND QUIGGLE, IND. ENG. CHEM., 25, I136 (1933) STEINHAUSER AND WHITE, IND. ENG. CHEM ,41, 2912 (1944) I I 1 1 1
--I ' l,9EA
EQUILIBRIUM DATA
1.1
0
I
'
I
10
20
30
7 '
I
I I
I
40
50
60
70
"
1
80
0
90
MOLE PERCENT HEPTANE IN LIQUID
Figure 4.
Two binary systems, heptane-toluene and methanol-carbon tetrachloride, were used t,o check the accuracy of the still. The heptane was a reference-fuel grade, redistilled through a BO-plate Oldershaw column. The toluene was Baker and Adamson reagent grade, redistilled through a column 17 mm. in inside diameter., packed to a height of 180 em. Kith l/le-inch stainless steel helices. This column had 100 theoretical plates \Then tested with heptane-methylcyclohexane at total reflux. The Baker and ,.idamson reagent-grade methanol was dried over calcium and redistilled in the 60-plate Oldershax column. The carbon tetrachloride \?-as Baker reagent grade, redistilled twice in the Oldershai\- column. Refractive indices of the purified materials are compared with published values in the following table.
Relative Volatility for System HeptaneToluene
Refractive Index a t 20' C . , n Experimental Literature 1.3877 I 38704 1.4989 1.49693 1,8286 1.3288
":
Compound n-Heptane Toluene XI et llano1 Carbon tetrachloride
1,4602
1.4007
lteferencc KRS-API NBS-API Lanse ( 8 ) hierck Index (IO)
Refractive index was used to analyze vapor and liquid samples from the still. Calibration data are shown in Table I. Because of the wide spread b e h e e n the refractive indices of the pure components in each binary mixture, the accuracy of this method of analysis is reasonably good-about =i=0.002mole fraction.
T?&LEI. REFRSCTIVG ISOEXClLIBRATIONS FOR htIXTURES OF HEPTANE-TOLUEXE A K D ~IETHIXOL-CARBOS TETRACHLORIDE hlole '7i Heptane, HeptaneToluene 0 10 20 80 40 50 GO 70 80 90 100 I
I
0
IO
20
30
40
I
I
50
60
1.4989
1.4818 1,4674 1,4545 1.4429 1.4322 1.4219
1.4119 1.4027 1.3946 1.3877
XIole Methanol, XIethanol-
cc1.
Refractive Index at 200 c
0 10 20 30 40 50 80 70 80 90 100
I
70
80
90
100
MOLE % HEPTANE IN LIQUID
Figure 5.
Reflactire Index a t 20' C.
Acti>ity Coefficients for System HeptaneTo1u en e
stant. After boiling had begun, the vapor and the liquid iyere sampled a t 15-minute intervals until the refractive indices of successive samples m-ere the same. Several such determinations showed that the still comes to equilibrium in about 30 to 45 minutes. The second test was made with a mixture of toluene and methylethylketone. This time the still was shut down, drained, and recharged with an identical mixture after each sampling. Runs of 30 minutes, 45 minutes, 1 hour, 2 hours, and 4 hours
The data for this system are listed in HEPTANE-TOLUESE. Table I1 and plotted in Figures 3, 4, and 5 . The temperaturecomposition diagram, Figure 3 , also shows the experimental points of Steinhauser and White ( 1 6 ) and the liquid-composition points of Bromiley and Quiggle (3). From a thermodynamic analysis of the agstem, St,einhauser and White concluded that their measurcd temperatures Jvere a little low, while the temperatures of Bromiley and Quiggle were as much as 1" C. high. Figure 3 shows that the temperatures meaeured in this invrstigation fall consistently above Steinhauser and White's, and are as much as 0.7" C . below the temperatures of Bromiley and Quiggle. Figure 4 is a plot of relative volatilit'y as a function of composition for t,he heptane-toluene system. The average scatter of t,he experimental points from the beat line is less than that of the previous investigators.
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1954 I
l
l
' FIGURE 6
78
D I A G R A M FOR
TEMPERATURE-COMPOSITION
77 76
THE S Y S T E
75 74 73 72 71
*
70 69 68
u ' -
67
5
66 65 64
9
E 5 I-
63
62 61 60
Xeither the Van Laar nor the Margules equations fit the expcrimental data precisely. Differences between the experimental points and the theoretical curves are within the analytical accuracy, but the measured values for heptane are consistently belo\? and the measured values for toluene are consistently above the predicted curves. hfETHANOL-CARBON TETRACHLORIDE. The data for this system are tabulated in Table 111, and plotted in Figures 6, 7, and 8. This system was chosen for investigation because it is highly nonideal. T h e temperature-composition d i a gram (Figure 6 ) shows the azeotrope a t 55.1 mole % methanol and 55.7" C. Horsley (7) lists the azeotrope a t 55.41 mole % methanol and 55.70' C. Soday 80 90 100 and Bennett ( 1 4 )report values from 56.1 to 58.9 mole % methanol a t temperatures from 54.2" to 54.5' C. Figure 7 is the relative volatility curve for the system. This plot is divided, the left side being the volatility of methanol relative to carbon tetrachloride, and the right side being the volatility of carbon tetrachloride relative to methanol. Again, the scatter of the points is small. Figure 8 shows the activity coefficients for the two components as a function of composition. The dashed lines are the activity coefficients from the Margules and Van Laar equations. The constants were again evaluated by the Gilliland technique, and are: b = 0.815; c = 0.154; A = 1.22; B = 327.
-
59 58
57 56 55 54
53 10
0
20
50
40
30
MOLE
*/o
60
70
METHANOL
Figure 5 is a plot of activity coefficients as a function of composition, and includes the data of Steinhauser and White. The dashed lines are the activity coefficients predicted by the Van Laar and the Margules equations. The form of the equations used is that given by Gilliland (6). Margules
Log
*,I
= b(1
Log
y 2
=
- z ) ~+ c ( l - x
) ~
bx2 $. 3/2cz2 - ex3
4c
30
The constants for these equations were determined from the experimental points by the graphjcal procedure outlined by Gilliland (6). They were: b = 0.022; c = 0.133; A = 1.37; B = 54.3.
TABLE11. VAPOR-LIQUIDEQUILIBRIUM DATAFOR SYSTEM HEPTAA-E-TOLUENE
KO. 1 2 3 4 5
% -!?! Vapor 0.0 5.3 12.4 19.1 19.4
(Pressure = 760 mm. Hg absolute) ~ ~ i Activity ~ t Coefficient, i ~ ~ Heptane Temp., Volatility, Y Liquid C. Alpha Heptane Toluene 0.0 110.6 ... ... 1.00 3.0 109.7 1.81 1.29 1.002 7.4 108.5 1.77 1.26 1.005 12.2 107.3 1.70 1.22 1.013 12.3 107.3 1.72 1.23 1.010
8 9 10
27.1 28.2 32.3 33.7 39.5
18.4 19.3 22.8 24.0 29.4
106.0 105.8 105.1 105.0 104.0
1.65 1.64 1.62 1.G1 1.57
1.18 1.18 1.17 1.16 1.14
1.02 1.02 1.03 1.02 1.04
11 12 13 14 15
43.0 44.5 49.2 50.5 55.5
32.9 34.5 39.9 41.1 47.0
103.8 103.3 102.7 102.5 102.0
1.54 1.52 1.46 1.46 1.41
1.12 1.12 1.09 1.09 1.06
1.05 1.06 1.06 1 .ox
16 17 18 19 20
60.2 65.0 70.3 77.4 83.3
52 7 58.8 65.5 74.2 81.3
101.2 100.8 100.2 99.6 99.2
1.36 1.30 1.25 1.19 1.15
LO5 1.03 1.02 1,008 1,000
1.11 1.13 1,17 1.21 1.25
21 22 23 24
88.2 91.7 95.8 100.0
86.8 90.6 95.2 100.0
99.0 98.9 98.8 98.4
1.14 1.15 1.15
0.998 1,000 0.995 1.000
1.26 1.25 1.24
G
c
...
I !
! I !
A B S O L U T E PRESSURE
7 6 0 m m Hg
I
I
I
20
a
Run
2527
I
V O L A T I L I T Y OF METHANOL RELATIVE
!
r
n
1
-
a
: 9
IO
?
e
+
7
J
f
'
r
U
0
w
-
t
4
2
-t
W
a
?
1.04
...
2
M O L E % METHANOL IN LlQUlO
Figure 7.
Relative Volatility for System MethanolCarbon Tetrachloride
INDUSTRIAL AND ENGINEERING CHEMISTRY
2528
22
_.__.. .... MARGULES EQUATIONS, b = 0 8 1 5 , c = 0 1 5 4 ~
---
VAN L A A R EQUATIONS, A = 1.22
~
B = 327
Vol. 46, No. 12
The isot’hermal data of Scatchard, Wood, and Rlochel ( 1 3 ) a t 55’ C. are also plotted on Figure 8. The agreement betreen their isothermal data and the isobaric data of this study is excellent. This is probably due to the fact that for over 707, of the isobaric range, t’he temperature is within 2” C. of the t,emperature used by Scatchard. CONCLUSIONS
The vapor-recirculating still described here is easy to operate, it reaches equilibrium rapidly, and it has negligible entrainment and low pressure drop. Operation of the st’illwith several binary systems has yielded consistent data t’hatagree well with published information. N OMES C LATURE
A: E , b, c = constank = temperature, degrees IC. = vapor pressure = mole fraction of more volatile component in liquid = mole fraction of more volatile component in vapor 71zj = activity coefficient, defined by y = -
Px
=
total pressure LITERATURE CITED
Altsheler, W.B., Unger, E. D., and Rolochov, P., IND.ENG. CHEM.,43, 2559 (1951). hmick, E. H., Weias, AI. A , and Kirshenbautn, &I. S., Ibid., 43, 969 (1951). Bromiley, E. P., and Quiggle, D., Ibid., 25, 1136 (1933). Dodge, B. F., and Dunbar, A. K., J . B n z . Chem. Soc., 49, 501 (1927). Gillespie, 73. T. C . , I s u . Esc. C H E X . , ASAL. ED., 18, 575
MOLE PERCENT METHANOL IN LIQUID
Figure 8.
Coefficients for System MethanolCarbon Tetrachloride
Activity
(1946).
Gilliland, E. Jt., ”Elements of Fractional Distillation,” 4th ed., McGraw-Hill Book Co., S e w York, 1950. IIorsley, L. H., Anal. C h e m . , 19, 508 (1947). Jones, C. A , Schoenborn, E. XI., and Colburn, A. P., IXD. ENG. CHEY.,35, 666 (1943). Lange, “Handbook of Chemistry,” 8th ed., Handbook Puhlishers, h e . , Sandusky, Ohio, 1952. Merck and Co., Inc., Rahway, N. J., “Merck Index,” 6th ed., 1952. Othmer, D. F., IND. EXG.CHEM.,20, 743 (1928). Scatchard, G., Raymond, C . L., and Gilmann, H. H . , Z. Am. Chem. Soc., 60, 127.5 (1938). Scatchard, G., Wood, S. E., and Rlochel, J. AI., Ibid..58, 19GO (1946). Soday, F. J.. a n d Bennett, G. W., J . Chem. Educ., 7, 1336 (1930). Steinhauser, H. H., and White, R. R., ISD. ENGI.CHEX.,41, 2912 (1949).
TABLEI11
Run NO. 1 2 3 4 5 0 7 8 9
10 11 12 13 14 15 16 17 18 19 20
TUOR-LIQII D EQUILIBRIUM D4-r.4 FOR Sr.;mai METH~NOL-CARBOY TETRXCRLORIUE (Pressure = 760 iiim Hg absoluto) Activity Relative Mole Methanol Coefficient. y Temp., Volatility, _._____ Liquid Vapor c. CClr Alphaa Methanol 0.0 0.2 0.2 0.4 1.3 1.7 3.0 5.05 10.7 12.4
24.8 40.1 50.5 55,0 60.3 72.7 100.0 99.9 99.7 99.3
0.0 2.0 2.7 12.0 24.15 26.4 38.3 44.5 49.0 50.0
76.7 76.1 75,85 72 35 67.6 66,85 62.0 59.4 57.2 56.95
52.2 53.65 54.85 55.2 56.1 59.5 100.0 99.5 98.8 9 6 , 65
56.25 55,s 52.7 56.65 55.7 56.0 64.7 64.6 64.5 64.1
3.31 1.74 1,19 -1.01 -1.19 -1.81
, . .
10.2 13.9 34.0 24 2 20.7 20.0 l5,l 8.0 7.17
...
6.62 9.03 22.7 16.7 14 3 14.2 11.0 6.27 6.33
1.00 0 . s!m
n ,u w
1 ni 1 .03 1 .0P I 0-1 I 0.5 1 I1 1 12
-5.0 -4.04 -4.92
2.99 1.95 1.58 1.46 1.35 1.17 1. 0 0 0.997 0.995 0.999
7.42 .j.95 7 25
, . .
1.27 1.57 1.86 2 07 2.25 2.99
98.6 97.9 96.2 94.8 93.8 Yl.75 88.3 89.7 86.8 83.8
93.9 91.0 86.4 82.3 80.3 7L3 69.55 71.6 67.7 64.9
63.5 62.8 61.8 60. 85 60.4 59.5 58.2 58.6 57.7 37.1
-4.58 -4.61 -4.00 -3.92 -3.71 -3.65 -3.30 -3.45 -3.14 -2.80
0.997 1 .oo 1.00 1.02 1.02 1.02 1.03 1.03 1.05 1.06
6.71 6.79 5.90 5.78 ,a. 49 5.35 4.87 5.08 4.65 4.21
31 32
81.3 76.4 72.5 67.6 62.5 59.65 56.55 49.8 45.25
63.0 60.5 59.1 57.6 56.3 55.8 55.2 54.5 54.1
56.75 56.35
-2.55 -2.11 -1.83
1.08 1.12 1.17 1.24 1.31 1.36 1.42 1.59 1.74
3.90 3.34 3.00 2.67 2.38 2.24 2.10 1.85 1.71
E.!?.. OO.iD 55.75 55.7 56.7 55.75 55.75
--1,54 -1.29 -1.17 -1.06 1.21 1.42
April 16. 1 Y 5 i .
ACCEPTED July 2 1 , 1954.
...
21 22 23 24 25 26 27 28 29 30
33 34 35 36 37 38 39
RECEIVED for r e v i e w
a Positive values are for volatility of methanol relative to CCh. Vegative values for volatility of CCla relative t o methanol.
Polymeric Plasticizers. Preparation and Characterization of a Series of Terrninated Polyesters-Correction I n the article on “Polymeric Plasticizers. Preparation and Characterizations of a Series of Terminated Polyesters” [Iioroly, J. E., and Beavers, E. AI., IND.EIVG.CHEX, 4 5 , 1060 ( l W 3 ) ] , the legend below Figure 2 should read as follows: THEORETICAL n
Figure 2. @
0
Ring Formation
Values of molecular weight derived f r o m saponification e q u i v a l e n t Ebulliometric values (acetone)
E. 11. BEAVERS
