I
BILLY G. HARPER and JOHN C. MOORE Texas Division, The Dow Chemical Co., Freeport, Tex.
Vapor-Liquid Equilibrium New Still and Method for Determining Vapor-Liquid Equilibrium Equilibrium data on binary mixtures may be measured by this novel method without analyzing the vapor and liquid samples
THE
evolution of modern distillation theory and practice, brought about by its industrial importance, has made accurate vapor-liquid equilibrium data practically a necessity. To acquire such data is no easy task and much effort has been put forth in the past 50 years to minifnize the effects of or eliminate known sources of error. Errors d u e to superheating of the liquid phase, fractionation on the walls of the vessel above the liquid, entrainment of liquid in vapor, and improper mixing of returning cold condensate with the main liquid seem to be most important. Good reviews of the sources of error in previous vapor-liquid equilibrium stills were presented by Fowler ( 3 ) in 1948 and briefly by Ellis (2) in 1952. A method proposed by Sameshima (8) in 1918 seems to be the forerunner of the conventional apparatus used today, in that it was the first to provide a vapor trap whereby the vapors are condensed and returned to the liquid continuously. Othmer (6) designed a similar apparatus in 1928, modifications of which are widely accepted today. T h e methods most used today depend upon analyzing liquid and vapor samples taken simultaneously.
Apparatus This work presents a modification of the recirculation-type still, which further minimizes known sources of error and provides more dependable data. I t also presents a new and simple method for arriving a t the vapor and liquid composition. In this work temperature was used to analyze the liquid phase. A 'platinum resistance thermometer and Mueller bridge (Leeds & Northrup 8067) provided for its deterrhination to O.O0lo C. T h e constancy and reproducibility of the boiling temperature of a given solution are evidently important and define many of the features employed in the
apparatus used. T h e apparatus is shown in Figure 1. Condensed vapor enters the boiling chamber, B, through line K, while four inlet holes, E, allow the separate liquid phase to enter. This arrangement reduces error due to improper mixing of cold returning condensate. A boiling chamber similar to this was successfully used by Ellis ( 2 ) in 1952. T h e magnetic stirrer, C, and open Nichrome wire heater, D, provide for small bubble formation, which is necessary for the maintenance of constant boiling temperature. A three-outlet Cottrell-type pump, F, leads the vapor-liquid mixture over the resistance thermometer, I, and minimizes the effect of superheating. T h e threeoutlet pump was found to give steadier boiling temperature than other types. A major portion of the pump is immersed in the liquid of the still. Insulation, H , reduces condensation and fractionation on the walls of the vapor chamber, G. I t was evident early that when small vapor outlets were used the temperature increased as the rate of distillation increased. This was found to be due to pressure changes through the small outlet and was satisfactorily eliminated by using a large outlet. A large outlet, J , and joint, N , make the apparatus easy to dismantle for cleaning, modification, and repair. T h e vapor condenses on the walls of LI and passes into the condensed-vapor chamber, M . Stopcock SI allows for recycle while chamber M is either empty or full, and along y i t h S,allows for taking samples of liquid and vapor simultaneously for analyses. I t was necessary to place the manostat in a constant temperature bath to eliminate changes due to changes i n room temperature. T h e pressure is believed to be constant to rt0.l mm. of mercury. This apparatus gives boiling points of pure compounds constant to rt0.002° C. over a fairly wide distillation-rate range. \
T h e boiling points of many mixtures were not as constant as the boiling points of pure compounds. T h e variation of boiling temperatures of the systems reported here was always less than rtO.01 O C. and usually less than 0.005° C. Experimental Materials Acetone, redistilled over Linde molecular sieve. A center cut was used, which distilled with no apparent change in temperature. Allyl alcohol. Eastman white label material was redistilled and a center cut was used, which distilled with no apparent change in temperature. Analysis by gas-liquid partition chromatography proved it to be better than 99.9% pure. Methanol. Fisher grade methanol was used. This material had a purity of 99.9% and contained less than 50 p.p.m. of water. Water, distilled.
Method of Operation T h e method used to determine the compositions of vapor and liquid is a modification of the conventional method. A curve of composition us. temperature is first constructed and later used for analysis of the liquid phase. I n this work equilibrium between liquid and vapor is assumed to exist after the mixture has boiled a t constant temperature 10.01O C. for 30 minutes. T h e 30-minute limit was determined by plotting the boiling temperature of several compounds and mixtures against time. T h e boiling in all cases became constant in from 5 to 20 minutes and remained constant for several hours. As the temperature drifted or became erratic after becoming constant, only in case of equipment failure, the 30-minute time limit is thought to be a reasonable one. T h e curve of temperature us. composition is determined by placing a known
-
VOL. 49, NO. 3
MARCH 1957
41 1
A 6
l i q u i d phase chamber Separate boiling chamber C. Magnetic stirrer D. Nichrome wire heater E Inlet holes F Three-outlet Cottrell type pump G Vapor chamber H. Insulation 1. Resistance thermometer J V a p o r outlet K. Vapor inlet line 11, Lz, L a . Condensers M Condensed-vapor chamber N Joint 0. Outlet t o manometer, surge tank, and manostat SI,S7 Stopcocks
D Figure 1.
quantity of solution of known coinposition i n the liquid phase chamber, A . Boiling is started and stopcock S,is adjusted so that the condensed vapors are returned to A instead of filling -\I. When the temperature has been constant for 30-minutes, it is noted. T h e compo-
4 12
*
Diagram of apparatus
sition is then changed by adding a kno\vn quantity of either component. lvirh or without the withdrawal of a kno\vn amount of mixture, and noted and the procedure is repeated. These data are then plotted on graph paper where one division on thr abscissa equals O.lc.; and
INDUSTRIAL AND ENGINEERING CHEMISTRY
one division o n I I I C ordi1lCitr c-cluals 0.ci53 CI. Slnooth, fine curves i t t . ~ obtained in this manner. u.hicI1 ~jass through 7.5';; of all data points. 'The curve is thrn checked by Inaking new solutions in three areas of coi1cc:niration and detrrlnininq their boiling tclrlpera-
V
= weight
x1
= weight per cent of component 1
yl
of vapor sample
in liquid phase upon sampling = weight per cent of component 1 in vapor phase upon sampling
But
wl =
M’”
-
- wh - v
(2)
Substituting and solving for y l Y1
=
woxo - [ W 0-
wg
w,- U’h
+ +
- V]W,
wh
(3)
As the same equipment is used to determine the equilibrium data as to draw the curve of temperature us. composition, W , and W , will cancel in Equation 3, so that Yl
ture. With this curve the temperature and quantity of material in the liquid .chamber before and after a sample of vapor has been taken are all that is necessary to determine the vapor composition. After a known weight of solution is placed in A, boiling is started and stopcock S1 is adjusted so that condensed vapors are returned to A without filling M . When equilibrium is attained, the temperature is noted. Stopcock SI is then adjusted so that chamber M is filled to overflowing. Its overflow is allowed to continue until equilibrium is reached and the temperature is again noted. The condensed vapor is drawn off through SI and weighed. It is necessary
E
wlxl
+
wgyl
+
why1
+ vyl
(1)
where
W0 = weight of initial charge of binary mixture xo
woxo
- W,(WO - V ) V
(4)
These terms cannot be immediately neglected in determining the curve of temperature us. composition, as doing so results in a displacement of the curve. The volume of the liquid phase, A , at its normal level of operation is 400 ml., while the volume of the vapor chamber, M , when filled to overflowing is 54.5 ml. The volume of K is 0.7 ml. The volume of the vapor holdup is approximately 800 ml. If the quantity of material that would occupy 800 ml. as a vapor is added to the line holdup, the total holdup is approximately 0.35% of the liquid chamber and 2.5y0 of the vapor chamber. A first approximation of the effect of this holdup on the curve of temperature us. composition. made at several points on the curve, showed a displacement of close boiling compounds cannot be studied? unless some other analysis of the liquid phase is provided.
Table
I.
Acetone-Methanol niiii I-lrt __ Wt C; A l i r t n i i c >
(Piemure = i s 2 -
T , * C.
LIole D/o .icetoile Liquid t-apor
62.39 61.93 60.52 59.87 59.35 58.64 57.12 56.78 55.61 55.45 55.07 55.37 55.39
5.8 7.8 13.6 16.7 20.1 22.9 36.3 39.8 58.4 61.1 74.6 91.7 92.1
Liqud
11.8 15.3 25.2 29.5 35.3 38.1 50.1 52.6 65.3 66.6 75.9 90.7 91.3
10.1 12.9 22.2 26.7 31.3 35.0 50.8 54.5 71.1 74.0 84.2 95.2 95.9
\
_\ I _t l T
I t \ [ i)ciflr~lc.llt ~
\Lrtillle
. ~ ~ C J I
19.5 24.7 37.9 43.1 50.1 52.7 64.5 66.8 77.4 78.4 85.1 94.7 95.0
AI~tli,lll~Jl
1.642 1.573 1,499 1.547 1.664 1.314 1.332 1.292 1.138 1.117 1.061 1.020 1.022
1.009 1.018 1.029 1,030 1.016 1.015 1.068 1.086 1.215 1.260 1.420 1.640 1.840
Experimental Results
T h e acetone-methanol system !cas considered suitable for testing the apparatus and method because the ana1J.tical difficulties presenced bv this system make its use logical, the relatively close boiling range and minimum boiling azeotrope make a fairly severe test for the apparatus and method. and the disagreement in available data makes i t a desirable system to study. The allyl alcohol-\rater system \vas briefly studied for comparison \vith the literature. The experimental data and activity coefficients for the acetone-methanol system are given in Table I and for the allyl alcohol-{rater system in Table 11. The acetone-methanol results are compared with the data of Othmer (6’) in Figure 2 and n i t h those of Grkvold and Buford ( 4 ) in Figure 3. Figure 4 compares the allyl alcohol-\rater data with literature values (Y), The solid line in each of these graphs represents vaporliquid equilihium curves calculated from a Van Laar equation. The still ivhile in operation required very little attention. : I reliable cuive of temperature 18s. composition and as
many as en points on the x-1 equilibrium curve may be determined in one day. The control of the still require so little attention that the operator may carrlout necessary Meighing and calculations irhile the next equilibrium point is beinq reached. The reliability of the acetone-methan01 data was tested. becausr i t did not agree \\.it11 data of Othmer (6).Bergstrom ( T ) , or Petit ( 7 ) . ;in integrated form of the GibbsDuhem equation. such as the \’an Laar or Margulrs equation. is capable of fitting most of the reliable determinations of activity coefficienrs. The data on a numbcr of‘ systems indicatc that thc majority of the determinations can be fitted brlter by a Van Laar equation ( i ) . A Van Laar equation \vas found to fit the data obtained in this rrork veri \vel1 (Van Laar constants A = 0.25S and B = 0.311). Figure 5 compares activity coefficients with a Van Laar equation derived from these data and a Van Laar equation derived from the azeotropic compositions reporied by Grisivold and Buford. The equations w r e
1
3
-
Calcd. Van Laor Eqn. m Othmer Data
2
Table II.
Allyl Alcohol-Water
(Pre-.iiie 7 5 2 imn. I l g )
92.0 94.1 96.3 91.6 88.6
16.0 9.5 4.5 94.7 81.3
56.9 48.6 37.4 85.2 74.3
similar and ririthrr \louId fit thy data given by Othmer. The agreement between these data and those of Griswold and Buford is considcred good. Thr results for the allyl alcohol watcr system agree very \vrll \vith thosc presented in the literature ( 0 ) . .I Van Laar equation derived from the azeotropic composition fits both sets of data a t one end of the curve but falls slightly tx.low the data o n the other end (Van 1,aar constants :l = 0.948 and R = 0.454). Acknowledgment
Thanks are due to V. .I. Klein for many helpful suggestions and advice, to S. 11. Ziinmernian for the \\zork in gasliquid partition chromatography. and to G. \,Ir.Marrs and J. H. Shannon for the glass construction \rork. literature Cited
Carlson, H. C.. Coburn. A P., IND.
L
.-.-uuc
ENG. CHEM.
34, 581-9 (~1942j.
Ellis, S. R . hf., l’runs. Inst. (;‘hem. h n t r s . (London) 30, 58-64 (1952).
wc 9)
~
0
0
* e
.-> .-
41 9 8 7
6
20
40 MOLE
Figure
41 4
5. Data
O h
60 ACETONE
on acetone-methanol
INDUSTRIAL AND ENGINEERING CHEMISTRY
80
100
Fowler, H . T., I d . Chemis! 1948, 717-
21, 823-37. ( 4 ) Griwold, J., Buford. C!. B., ISD. I,NG. CHEM. 4 1 , 2347-51 (1949). ( 5 ) Hausbrand, E., “Principles and Practices of Industrial Distillation,” pp. 223-32, Wiley, New York, 1926. ( 6 ) Othmer, I). F., I s u . EKG. CHEM.20, 743 (1928 1. ( 7 ) Pettit, J. H.: .I. P h w Chem. 3, 340 (1 899 ). ( 8 ) Sameshima, ,J,, .I. Am. Chem. sibc. 40, 1482 (1918). ( 9 ) Shell Chemical Co., Tech. Pub. SC:46-32,51 (1 950 ). RECEWEU for review March 31, 1956 ACCF,PTEDSeptembrr 4. 1956
