lle sistemas binarios

7/24/2019 LLE Sistemas Binarios

1/10

Fluid Phase Equilibria, 40 1988) 279-288

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

279

LIQUID LIQUID COEXISTENCE CURVES FOR BINARY SYSTEMS

DONACIANO BERNABE, ASCENCION ROMERO-MARTINEZ and ARTURO TRl JO*

Xnstituto Mexican0 de1 PetrHeo, Subdirecci6n de LB. P., Eje Central Lizaro Grdenas 152,

07730 MPxico, D. F MPxico)

(Received May 11, 1987; accepted in final form September 29, 1987)

ABSTRACT

Bernabe, D., Romero-Martinez, A. and Trejo, A., 1988. Liquid-liquid coexistence curves for

binary systems. Fluid Phase Equilibria, 40: 279-288.

The liquid-liquid coexistence curves of polar+non-polar binary systems have been

determined experimentally. The polar compounds studied were ethanenitrile, methanol and

N-methylpyrrolidone, whereas the non-polar compounds were chosen from the n-alkane

series. The upper critical solution temperature for each set of mixtures increases with

increasing n-alkane chain length, and the critical composition of the polar component also

increases in this fashion.

INTRODUCTION

The experimental investigation of phase equilibria in fluid mixtures has

revealed a wide variety of phase behaviour. The importance of such investi-

gations extends into the fields of both industrial applications and science; this

is demonstrated by the continuous flow of publications in this field. In

particular, a knowledge of liquid-liquid equilibria (LLE) is of the utmost

importance for the design, operation and optimization of different kinds of

separation equipment. Equally important is the information on the interac-

tions between unlike molecules, which may be derived from LLE data of

carefully selected mixtures through the use of theories of the liquid state or

solution models.

There exist several possible types of liquid-liquid coexistence curves,

which have been studied and discussed in the literature. It is well known that

many binary liquid mixtures that form a single, homogeneous phase will

separate into two liquid phases when cooled below a characteristic temper-

* Author to whom correspondence should be addressed.

037%3812/88/$03.50

0 1988 Elsevier Science Publishers B.V.


2/10

280

ature, i.e. the mutual miscibility of the components decreases with de-

creasing temperature. The characteristic temperature is the maximum of the

coexistence curve and is called the upper critical solution temperature

(UCST). In other cases, the mutual miscibility of the components decreases

with increasing temperature; thus, the coexistence curve shows a minimum

corresponding to the lower critical solution temperature (LCST). Some other

systems present closed coexistence curves, hence possessing both upper and

lower critical points.

Binary systems that present liquid-liquid coexistence curves with UCS

phenomena are more abundant than any of the other behaviours mentioned

above. This is verified from the wealth of experimental data compiled by

Francis (1961) and by Sorensen and Arlt (1979/1980). Upper critical

solution phenomena characterize binary mixtures formed by a polar com-

pound and a non-polar compound, where specific interactions between

unlike pairs of molecules are absent.

In this report of our work on the LLE of binary mixtures, we present

experimental results of the coexistence curve for systems formed by

ethanenitrile, methanol or N-methylpyrrolidone as the polar compound, and

an n-alkane containing between 5 and 13 carbon atoms as the non-polar

compound. Previous work on the liquid-liquid phase behaviour of this type

of system included measurements of the upper coexistence temperature for

26 mixtures composed of n-alkanenitriles and n-alkanes (McLure et al.,

1982); the binary mixtures studied contained ethanenitrile, propanenitrile,

or n-butanenitrile, and an n-alkane containing between 5 and 18 carbon

atoms.

TABLE 1

Sources, grades and estimated purity of materials used in the liquid-liquid determinations

Substance Source and grade Estimated purity (mol W)

Ethanenitrile

Methanol

N-Methylpyrrolidone

N-Pentane

n-Hexane

n-Heptane

n-Nonane

n-Undecane

n-Tridecane

= h.p.1.c.

b h.p.1.c.

L.R.

d L.R.

e pure

a h.p.1.c.

e pure

e pure

e pure

99.9

99.8

98.0

99.0

99.0

99.6

99.0

99.0

99.0

a Baker. b Merck. Matheson Colleman & Bell. d Sigma. e Phillips Petroleum.


3/10

281

EXPERIMENTAL

aterials

The source, grade and estimated purity of each substance used are listed

in Table 1. All compounds were further purified by placing them in contact

with a drying agent. The polar compounds were dried with a molecular

sieve, whereas the n-alkanes were dried with sodium.

4

375

x

:

35

325

3

I I

1

I

I

I

I

1

I

Fig. 1. Experimental coexistence temperature-composition curves for ethanenitrile (l)+ n-

alkane (2) systems.


4/10

282

Coexistence temperatures

The liquid-liquid coexistence curves of the three sets of binary systems

were determined using the sealed-tube technique (Trejo, 1979; McLure et

al., 1982). The samples were prepared by weight in Pyrex glass tubes. Two

stainless-steel ball-bearings were placed inside the sample tube for stirring of

the mixture during measurements. Each sample was thoroughly degassed by

several freeze-pump-thaw cycles before being flame sealed under vacuum.

375

350

5

h-

3

300

275

I 1

I

I I

I I

0.5 I

Xl

Fig. 2. Experimental coexistence temperature+zomposition curves for N-methylpyrrolidone

(l)+ n-alkane (2) systems.


5/10

283

The measurement of temperature was carried out with a Hewlett-Packard

quartz thermometer, model 2804A. The samples were brought into thermal

equilibrium in a water bath for temperatures up to 340 K and a ethylene

TABLE 2

Experimental coexistence temperature-composition data for ethanenitrile 1) + n-alkane 2)

systems

T 6)

Xl T 6) 4

n-Pentane

329.11

336.19

339.12

339.36

340.26

340.49

340.48

340.20

339.57

338.11

336.11

328.44

321.34

312.27

n-Heptane

328.37

338.26

348.58

349.32

351.68

353.74

356.94

356.84

357.30

356.89

356.43

356.05

353.43

348.93

341.76

336.56

333.33

332.87

321.94

0.2146

0.2972

0.3819

0.3894

0.4644

0.4862

0.5457

0.6093

0.6526

0.6975

0.7316

0.8013

0.8407

0.9008

0.1591

0.2181

0.3153

0.3231

0.3669

0.4071

0.5522

0.5705

0.6250

0.6738

0.7202

0.7391

0.7977

0.8382

0.8494

0.8921

0.9060

0.9076

0.9303

n-Hexane

319.80

332.51

341.75

341.96

342.07

344.56

345.98

348.29

348.34

348.51

348.65

348.51

348.31

347.22

342.32

333.74

313.56

n-Tridecane

340.82

358.28

363.45

372.10

372.60

386.68

398.98

394.67

396.27

398.57

399.16

399.46

399.25

397.14

384.76

0.1465

0.2050

0.2955

0.2979

0.2996

0.3460

0.3808

0.5013

0.5096

0.5764

0.6000

0.6111

0.6564

0.7098

0.7996

0.8531

0.9123

0.1873

0.2791

0.2886

0.3401

0.3471

0.4825

0.5096

0.5702

0.6108

0.6648

0.7527

0.8050

0.8505

0.8944

0.9465


6/10

284

glycol bath for measurements above 340 K. The coexistence temperatures

were determined by observing the onset of mixing, marked by the presence

of opalescence, after the samples had been subjected to changes in temper-

ature with heating rates of about 0.03 K mm-. Repeated measurements of

the temperature at which the liquid-liquid meniscus of the same sample

disappeared gave the same value within fO.O1 K. Thus, the coexistence

temperatures reported here are a mean of several determinations for each

one of the compositions studied.

RESULTS AND DISCUSSION

The experimental coexistence temperature-composition data are given in

Tables 2-4 for ethanenitrile, methanol and N-methylpyrrolidone with n-al-

TABLE 3

Experimental coexistence temperature-composition data for methanol (1) + n-alkane (2)

systems

T (K)

XI

T K)

XI

n-Pentane

276.89

283.71

287.49

288.29

288.68

289.28

289.20

289.19

288.67

288.35

283.92

279.93

269.99

n-Nonane

329.61

340.88

349.83

354.54

355.78

355.94

355.75

355.22

354.36

341.44

0.100

0.220

0.313

0.374

0.413

0.438

0.480

0.491

0.565

0.600

0.702

0.760

0.803

0.192

0.281

0.395

0.511

0.604

0.659

0.700

0.751

0.803

0.902

n-Heptane

322.18

326.79

327.69

328.57

329.03

329.17

328.83

328.53

326.71

325.10

319.97

314.69

n-Undecane

348.11

356.56

364.34

370.32

374.44

376.13

376.16

375.78

374.94

370.59

363.15

0.277

0.376

0.394

0.449

0.508

0.555

0.587

0.661

0.701

0.745

0.826

0.852

0.200

0.310

0.395

0.499

0.596

0.698

0.750

0.803

0.851

0.900

0.920


7/10

285

TABLE 4

Experimental coexistence temperature-composition data for N-methylpyrrolidone (1) + n-

alkane (2) systems

T W

x,

n-Pentane

309.12

319.45

323.76

324.14

324.18

324.20

323.14

324.36

323.54

319.32

309.51

290.72

n-Nonane

318.66

327.16

332.36

332.98

333.05

332.95

332.85

332.66

332.07

325.87

306.62

0.1030

0.1999

0.3000

0.3502

0.3791

0.3990

0.4323

0.4501

0.4960

0.5980

0.7010

0.8012

0.2011

0.2996

0.4003

0.5015

0.5350

0.5692

0.6044

0.6492

0.6891

0.7970

0.8938

n-Undecane

318.94

331.61

337.48

340.28

341.17

341.42

341.36

341.10

338.18

322.22

n-Heptane:

312.69

317.62

321.05

324.35

325.13

325.41

325.74

325.73

324.34

320.82

l

0.1622

0.2006

0.2518

0.3061

0.3502

0.3978

0.4442

0.4958

0.6063

0.7037

0.1938

0.3016

0.4030

0.4925

0.5580

0.5954

0.6493

0.6981

0.7960

0.9004

kanes, respectively. Figures 1-3 show the solubility curves for the three sets

of systems. From these results it is evident that the solubility of the

n-alkanes decreases as their chain length increases, hence the upper critical

solution temperatures (UCST) for a given set of systems increases as the

chain length of the n-alkane increases; the critical composition (X,C) of the

polar component also increases in this fashion. This behaviour may also be

observed if, instead of referring the changes to the variation of the n-alkane

chain length, any other convenient property is used (e.g. molar volume or

boiling temperature). Table 5 contains UCST and Xc values for all the

systems studied.

Our UCST values for ethanenitrile with C,, C, and C, agree reasonably

well with the upper coexistence temperatures of McLure et al. (1982) and

with the UCST of Zieborak and Olszewski (1956a). No reported values of

the UCST for ethanenitrile with C,, were found in the literature. The UCST


8/10

286

273-

0

Fig. 3. Experimental coexistence temperature-composition curves for methanol (l)+n-alkane

(2) systems.

value

determined here for methanol with C, agrees well with the value

quoted by Francis (1961) and it is slightly higher than the values reported by

Kiser et al. (1961) and by Zieborak et al. (1956b). Our value of the UCST


9/10

287

TABLE 5

Comparison of upper critical solution temperatures (UCST) and critical compositions (X,C)

for the systems studied in this work

System This work

UCST (K)

Ethanenitrile

+n-C,

340.21

n-C,

348.80

n-C,

357.35

n-G

399.83

Methanol

+ n-C,

288.32

n-C,

329.27

n-C9

355.88

n-C,,

376.04

N-methylpyrrolidone

+ n-C,

324.35

n-C,

325.76

n-C,

333.19

n-C,,

341.38

Literature

X,c UCST (K) X,E

0.5094

341.2 a 0.687

=

0.5557

350.2 a 0.714

d

0.5972

358.0 a, 351.8 0.736

, 0.63 b

0.7907

_

_

0.461

287.40 , 287.90 , 0.502

c

287

0.551

324.4 , 324.2 d,e 0.601

, 0.54 e

0.660

_

0.737

376 d.e _

0.3478

_ -

0.4659

_ _

0.5438

- _

0.6148

- _

a McLure et al. (1982). b Zieborak and Olszewski (1956a). Kiser et al. (1961). d Francis

(1961). e Zieborak et al. (1956b).

for methanol with C, is 5 K higher than the values given by Kiser et al.

(1961), Francis (1961) and Zieborak et al. (1956b). No comparison is carried

out for the UCST with C, since no data were available from the literature.

Our UCST with C,, agrees very well with the value reported by Zieborak et

al. (1956b). It was not possible to compare our UCST values for N-methyl-

pyrrolidone + n-alkanes due to lack of reported data in the literature for this

set of systems.

The compositions given in Table 5 for ethanenitrile + n-alkane systems by

McLure et al. (1982) correspond to equivolume mixtures, thus a direct

comparison with X,C values of this work is not possible. Our X,C values for

methanol with C, and C, are lower than those reported by Kiser et al.

(1961), although our value with C, agrees with that reported by Zieborak et

al. (1956b). As for the UCST values, no reported d t of critical composi-

tions are available in the literature for N-:methylpyrrolidone + n-alkane

systems.

LIST OF SYMBOLS

LLE liquid-liquid equilibria

UCST upper critical solution temperature


10/10

288

LCST lower critical solution temperature

T temperature (K)

X,

mole fraction of component

x

critical composition of component i

REFERENCES

Francis, A.W., 1961. Critical solution temperature. Am. Chem. Sot., No. 31, Adv. Chem. Ser.,

Washington, DC.

Kiser, RW., Johnson, G.D. and Shetlar, M.D., 1961. Solubilities of various hydrocarbons in

methanol. J. Chem. Eng. Data, 6: 338-341.

McLure, LA., Trejo, A., Ingham, P.A. and Steele, J.F., 1982. Phase equilibria for binary

n-alkanenitrile-n-alkane mixtures. I. Upper liquid-liquid coexistence temperatures for

ethanenitrile, propanenitrile and n-butanenitrile with some

C5 Cl8

n-alkanes. Fluid Phase

Equilibria, 8: 271-284.

Sorensen, J.M. and Arlt, W., 1979/1980. Liquid-liquid equilibrium data collection, DE-

CHEMA Chemistry Data Series, Vol V, 3 parts. Frankfurt.

Trejo, A., 1979. A thermodynamic study of polar+non-polar fluid mixtures. Ph.D. Thesis,

University of Sheffield, Sheffield.

Zieborak, K. and Olszewski, K., 1956a. Heteropolyazeotropic systems. II. Acetonitrile-n-

paraffinic hydrocarbons. Bull. Acad. Pol. Sci. Cl. III, 4: 823-827.

Zieborak, K., Maczynska, 2. and Maczynski, A., 1956b. Heteropolyazeotropic systems. I.

System methanol-n-paraffinic hydrocarbons. Bull. Acad. Pol. Sci. Cl. III, 4: 153-157.

lle sistemas binarios

Documents