BILLY G. HARPER and JOHN C. MOORE Texas Division, The Dow Chemical Co., Freeport, Tex. I

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 ac- curate 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 due to super- heating of the liquid phase, fractionation on the walls of the vessel above the liq- uid, entrainment of liquid in vapor, and improper mixing of returning cold con- densate 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. The methods most used today depend upon analyzing liquid and vapor samples taken simul- taneously.

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 com- position.

In this work temperature was used to analyze the liquid phase. A 'platinum resistance thermometer and Mueller bridge (Leeds & Northrup 8067) pro- vided for its deterrhination to O.O0lo C. The constancy and reproducibility of the boiling temperature of a given solu- tion are evidently important and define many of the features employed in the

apparatus used. The 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 re- duces error due to improper mixing of cold returning condensate. A boiling chamber similar to this was successfully used by Ellis ( 2 ) in 1952.

The magnetic stirrer, C, and open Nichrome wire heater, D, provide for small bubble formation, which is neces- sary for the maintenance of constant boil- ing temperature.

A three-outlet Cottrell-type pump, F, leads the vapor-liquid mixture over the resistance thermometer, I, and minimizes the effect of superheating. The three- outlet 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 va- por chamber, G.

I t was evident early that when small vapor outlets were used the temperature increased as the rate of distillation in- creased. 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, modi- fication, and repair. \

The 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 th S, allows for taking samples of liquid and vapor simul- taneously for analyses.

I t was necessary to place the manostat in a constant temperature bath to elimi- nate changes due to changes in room temperature. The 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.

The boiling points of many mixtures were not as constant as the boiling points of pure compounds. The variation of boiling temperatures of the systems re- ported here was always less than rtO.01 O C. and usually less than 0.005° C.

Experimental Materials

Acetone, redistilled over Linde molec- ular 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 ap- parent change in temperature. Analy- sis by gas-liquid partition chromatog- raphy 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

The 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. The 30-minute limit was determined by plotting the boiling temperature of several compounds and mixtures against time. The 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.

The curve of temperature us. composi- tion is determined by placing a known

- VOL. 49, NO. 3 MARCH 1957 41 1

A l iquid phase chamber 6 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 Vapor outlet K. Vapor inlet line 11, Lz, L a . Condensers M Condensed-vapor chamber N Joint 0.

SI, S7 Stopcocks

Outlet to manometer, surge tank, and manostat

D Figure 1. Diagram of apparatus

quantity of solution of known coinposi- * sition is then changed by adding a kno\vn tion in the liquid phase chamber, A. quantity of either component. lvirh or Boiling is started and stopcock S, is ad- without the withdrawal of a kno\vn justed so that the condensed vapors are amount of mixture, and noted and the returned to A instead of filling -\I. procedure is repeated. These data are When the temperature has been constant then plotted on graph paper where one for 30-minutes, it is noted. The compo- division on thr abscissa equals O.lc.; and

4 12

one division o n I I I C ordi1lCitr c-cluals 0.ci53 CI. Slnooth, fine curves i t t . ~ ob- tained 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-

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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 com- position.

After a known weight of solution is placed in A, boiling is started and stop- cock 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. I t is necessary

to stop the boiling about 1 minute before the sample is drawn off, so that vapor of a different composition will not come over.

From this the vapor composition can be determined. WflO E wlxl + wgyl + why1 + vyl (1) where

W0 = weight of initial charge of bi-

xo = weight per cent of component 1

Wl = weight of liquid phase upon

nary mixture

in initial charge of mixture

sampling

sampling Wg = weight of gas holdup upon

W, = weight of line holdup upon sampling

V x1

= weight of vapor sample = weight per cent of component 1

in liquid phase upon sam- pling

= weight per cent of component 1 in vapor phase upon sam- pling

yl

But wl = M’” - - wh - v ( 2 )

Substituting and solving for y l

woxo - [W0 - w, - U’h - V]W, wg + wh + Y1 =

( 3 )

As the same equipment is used to de- termine the equilibrium data as to draw the curve of temperature us. composition, W, and W , will cancel in Equation 3, so that

(4) V

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 cham- ber. 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 <0.08 weight yo. I t is believed that this does not produce a se- rious error in the final vapor-liquid equi- librium data.

woxo - W,(WO - V ) Yl =

Mole% Allyl Alcohol in Liquid

Figure 4. Data on allyl alcohol-water Figure 3. Data on acetone-methanol

. VOL. 49, NO. 3 MARCH 1957 4 13

The experimental procedure may be repeated immediately. if the quantity of liquid remaining in ..i \rill alloiv, FVhen necessary, a quantity of either component may be added and the procedure re- peated.

This method is advantageous Lrith s!-s- tems that cannot be analyzed by refrac- tive index. density. or other ordinary methods. In many cases it provides data more rapid]!- than other methods. One obvious disadvantage is that ver>- close boiling compounds cannot be studied? unless some other analysis of the liquid phase is provided.

Experimental Results

The acetone-methanol system !cas considered suitable for testing the ap- paratus and method because the ana1J.t- ical 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 disagree- ment in available data makes i t a de- sirable 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 com- pared with the data of Othmer (6’) in Figure 2 and n i th those of Grkvold and Buford ( 4 ) in Figure 3. Figure 4 com- pares the allyl alcohol-\rater data with literature values (Y), The solid line in each of these graphs represents vapor- liquid 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

- T , * C.

6 2 . 3 9 6 1 . 9 3 60 .52 5 9 . 8 7 5 9 . 3 5 5 8 . 6 4 5 7 . 1 2 5 6 . 7 8 5 5 . 6 1 5 5 . 4 5 5 5 . 0 7 5 5 . 3 7 5 5 . 3 9

Table I . Acetone-Methanol (Piemure = i s 2 niiii I-lrt

__ Wt C; Al i r tn i i c> LIole D/o .icetoile Liquid t-apor L i q u d \ . ~ ~ C J I

5 . 8 1 1 . 8 1 0 . 1 1 9 . 5 7 . 8 1 5 . 3 1 2 . 9 2 4 . 7

1 3 . 6 2 5 . 2 2 2 . 2 3 7 . 9 1 6 . 7 2 9 . 5 2 6 . 7 4 3 . 1 2 0 . 1 3 5 . 3 3 1 . 3 5 0 . 1 2 2 . 9 3 8 . 1 3 5 . 0 5 2 . 7 3 6 . 3 50.1 50 .8 6 4 . 5 3 9 . 8 5 2 . 6 5 4 . 5 6 6 . 8 5 8 . 4 6 5 . 3 7 1 . 1 7 7 . 4 6 1 . 1 6 6 . 6 7 4 . 0 7 8 . 4 7 4 . 6 7 5 . 9 8 4 . 2 8 5 . 1 9 1 . 7 9 0 . 7 9 5 . 2 9 4 . 7 9 2 . 1 9 1 . 3 9 5 . 9 9 5 . 0

\ I t l T I t \ _ _ ~

\Lrtillle 1.642 1 . 5 7 3 1 , 4 9 9 1 . 5 4 7 1 . 6 6 4 1 . 3 1 4 1 . 3 3 2 1 . 2 9 2 1 .138 1 . 1 1 7 1 . 0 6 1 1 . 0 2 0 1 .022

[ i)ciflr~lc.llt

A I ~ t l i , l l l ~ J l

1 . 0 0 9 1 .018 1 . 0 2 9 1 , 0 3 0 1 .016 1 . 0 1 5 1 . 0 6 8 1 . 0 8 6 1 .215 1 . 2 6 0 1 . 4 2 0 1 . 6 4 0 1 . 8 4 0

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 carrl- out necessary Meighing and calculations irhile the next equilibrium point is beinq reached.

The reliability of the acetone-meth- an01 data was tested. becausr i t did not agree \\.it11 data of Othmer (6). Berg- strom ( T ) , or Petit ( 7 ) .

;in integrated form of the Gibbs- Duhem equation. such as the \’an Laar or Margulrs equation. is capable of f i t - ting 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 ac- tivity coefficients with a Van Laar equa- tion derived from these data and a Van Laar equation derived from the azeo- tropic compositions reporied by Gris- ivold and Buford. The equations w r e

3 1 - Calcd. Van Laor Eqn.

m Othmer Data 2

L c u u

9) 0 0 * >

.- .- c w-

e .- .- 4 1 9 8

7

6 20 40 60 80 100

M O L E O h ACETONE

Figure 5. Data on acetone-methanol

Table II. Allyl Alcohol-Water (Pre-.iiie 752 imn. I lg)

9 2 . 0 1 6 . 0 5 6 . 9 9 4 . 1 9 . 5 4 8 . 6 9 6 . 3 4 . 5 3 7 . 4 9 1 . 6 9 4 . 7 8 5 . 2 8 8 . 6 8 1 . 3 7 4 . 3

similar and ririthrr \louId fit thy data given by Othmer. The agreement be- tween these data and those of Griswold and Buford is considcred good.

Thr results for the allyl alcohol watcr system agree very \vrll \vith thosc pre- sented 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 gas- liquid partition chromatography. and to G. \,Ir. Marrs and J. H. Shannon for the glass construction \rork.

l i terature Cited

Carlson, H. C.. Coburn. A P., IND.

Ellis, S. R . hf., l’runs. Inst. (;‘hem. h n t r s .

~ Fowler, H. T., I d . Chemis! 1948, 717-

(4 ) Griwold, J., Buford. C!. B., ISD. I,NG.

( 5 ) Hausbrand, E., “Principles and Prac- tices of Industrial Distillation,” pp. 223-32, Wiley, New York, 1926.

(6 ) Othmer, I). F., I su . 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.

E N G . C H E M . 34, 581-9 (~1942j.

(London) 30, 58-64 (1952).

21, 823-37.

CHEM. 41 , 2347-51 (1949).

SC:46-32, 51 (1 950 ). RECEWEU for review March 31, 1956

ACCF,PTED Septembrr 4. 1956

4 1 4 INDUSTRIAL AND ENGINEERING CHEMISTRY