Fluid Phase Equilibria, 43 (1988) 317-327 317 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

QUATERNARY LIQUID-LIQUID EQUILIBRIUM: WATER-ETHANOL-2-ETHYLHEXANOL-DECALIN AT 25 ° C. U N U S U A L SHAPE OF THE SOLUBILITY SURFACE

FRANCISCO RUIZ, VICENTE GOMIS and ROGELIO F. BOTELLA

Divisibn de Ingenierla Qulmica, Universidad de Alicante, Aptdo. 99 Alicante (Spain)

(Received December 12, 1987; accepted in final form March 9, 1988)

ABSTRACT

Ruiz, F., Gomis, V. and Botella, R.F., 1988. Quaternary liquid-liquid equilibrium: water-ethanol-2-ethylhexanol-decalin at 25 o C. Unusual shape of the solubility surface. Fluid Phase Equilibria, 43: 317-327.

Mutual solubility and tie line data at 25°C are presented for the quaternary system water-ethanol-2-ethylhexanol-decalin. The experimental results show unusual features such as a large hump in the solubility surface. This unusual behaviour can be associated with quaternary systems containing two pairs of partly-miscible compounds with very different solubilities.

INTRODUCTION

Liquid-liquid equilibrium (LLE) data for the quaternary system water (W)-ethanol (E)-2-ethylhexanol (EH)-decahydronaphthalene (decalin) (D) at 25 °C and atmoshperic pressure are presented in this work. This system contains two pairs of partly-miscible compounds (W-EH and W-D), which could be of interest to industry in the recovery by liquid-liquid extraction of ethanol manufactured in a fermentation process. Ethylhexanol has a large distribution coefficient for ethanol but a small separation factor with respect to the water. Decalin has a large separation factor but a small distribution coefficient. The use of both solvents could improve the distribution coeffi- cient/separation factor ratio.

EXPERIMENTAL

All chemicals (Merck) were used as supplied. The contents of volatile impurities were determined by chromatographic analysis. With the exception of decalin all the compounds contained < 0.2 wt.% impurities. Decalin was a mixture of cis and trans isomers. Therefore, considering this system as a

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318

quaternary system is not strictly correct since five components are present. Fortunately, it was found that the change in composition of the decalin (c i s / t rans ratio) was negligible in equilibrium separations, justifying as- sumption of a quaternary mixture.

Data for the binodal curves of the component ternary and quaternary systems were determined by using the cloud-point method. The experimental device was that used by Ruiz and Prats (1983).

Equilibrium data were obtained by preparing mixtures of known overall composition by weighing the components, stirring intensely and settling for 2 h at constant temperature (25 + 0.1°C). At the end of each experiment samples were taken from both phases and analysed by means of gas chromatography. Good separation of the three components was obtained on a 2 m × 3 mm column packed with Chi'omosorb 101 100/120. The column temperature was 190 ° C and detection was carried out by thermal conductiv- ity for the organic phases and by flame ionization for the aqueous phases. The helium flow rate was 40 ml min -1. To obtain quantitative results, we applied the internal standard method, 1-propanol being the standard com- pound used for this purpose. Furthermore, the addition of 1-propanol prevents phase separation effects. The relative accuracy of the weight fraction measurements was 2%.

The methodology applied in selecting the points to be determined experi- mentally was as reported in a previous paper (Ruiz et al., 1984): six equidistant W - E - ( E H - D mixture) planes were selected, each one char- acterized by a value of M defined as M = X D / ( X D + XEH), X D and XEH being the weight percentages of decalin and 2-ethylhexanol, respectively. M = 0.0 denotes the ternary system W - E - E H , and M = 1.0 the ternary system W - E - D . Initial mixtures were selected such that X w = X D + XEH for each M value ( M = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0); the ethanol levels L were increased stepwise until the homogeneous region was reached.

RESULTS AND DISCUSSION

Table 1 shows mutual solubility data for the ternary system W - E - D at 25 ° C. Quaternary solubility points concerning the planes M = 0.2, 0.4, 0.6 and 0.8 are shown in Table 2.

The tie lines for the ternary systems W - E - D and W - E H - D appear in Tables 3 and 4. Mutual solubility and tie-line data at 25 °C for the ternary system W - E - E H were reported in a previous paper (Ruiz et al., 1987). The compositions for the quaternary tie lines are shown in Table 5.

The tetrahedral representation of the solubility surface of the quaternary system is shown in Fig. 1 and presents an unusual shape with a large hump in the side of the ternary plane W - E - E H .

T A B L E 1

Mutua l solubility da ta (wt.%) for water ( W ) - e t h a n o l (E ) -deca l in (D) at 25 ° C

319

W E D

0.9 49.1 50.0 1.5 60.0 38.5 2.6 66.7 30.7 4.0 72.2 23.8 7.9 77.3 14.8

11.3 78.3 10.4 21.1 75.0 3.9 25.8 71.8 2.4

T A B L E 2

Qua te rna ry solubili ty data (wt.%) for water ( W ) - e t h a n o l (D) at 25 o C

(E) -2-e thy lhexanol ( E H ) - d e c a l i n

Xw XE XE- H XD Xw XE XE- H XD

M = 0.2 M = 0.4 4.2 16.4 63.5 15.9 7.3

10.4 30.8 47.0 11.8 12.9 15.8 37.1 37.7 9.4 17.8 20.6 40.4 31.2 7.8 21.5 23.2 41.5 28.3 7.0 22.8 28.3 43.1 22.9 5.7 23.1 33.2 44.7 17.7 4.4 25.2 38.2 45.4 13.1 3.3 29.1 42.0 47.4 8.5 2.1 34.8 43.1 51.9 4.0 1.0 46.5 51.6 1.5 0.4

M = 0.6 M = 0.8 3.3 18.0 31.5 47.2 3.2 7.2 28.9 25.6 38.3 5.1 9.9 51.1 15.6 23.4 6.7

10.3 58.5 12.4 18.8 8.5 11.1 62.8 10.4 15.7 10.7 13.3 66.8 8.0 11.9 12.9 15.1 69.6 6.1 9.2 15.2 17.6 70.5 4.8 7.1 18.0 19.9 71.4 3.6 5.1 21.3 23.3 70.7 2.4 3.6 25.3 28.2 68.6 1.3 1.9 29.5 33.8 64.4 0.7 1.1 35.0 63.2 0.7 1.1

26.8 39.5 26.4 35.6 30.9 20.6 40.6 25.0 16.6 46.1 19.4 13.0 54.8 13.4 9.0 61.6 9.2 6.1 64.0 6.5 4.3 63.6 4.4 2.9 61.3 2.3 1.6

31.5 13.1 52.2 55.2 7.9 31.8 67.3 5.2 20.8 71.8 3.9 15.8 74.0 3.0 12.3 74.6 2.5 10.0 74.7 2.0 8.1 74.1 1.6 6.3 73.2 1.1 4.4 71.4 0.7 2.6 68.8 0.3 1.4

320

T A B L E 3

Tie-line data (wt.%) for water (W)-2-e thy lhexanol ( E - H ) - d e c a l i n (D) at 25 ° C

Aqueous phase Organic phase

M (initial) X w XE- H XD Xw XE- H XD

0.0 99.9 0.085 0.0 2.42 97.6 0.0 0.2 99.9 0.07 N D 1.67 78.6 19.8 0.4 99.9 0.059 N D 1.10 59.3 39.6 0.6 100.0 0 . 0 4 2 N D 0.61 39.7 59.7 0.8 100.0 0.04 N D 0.26 19.9 79.8 1.0 100.0 0.00 N D N D 0.0 100.0

ND, no detectable composit ion.

Figure 2 shows the pseudoternary representation of the solubility curve for each W - E - ( E H - D mixture). A representation using equilateral triangles has been found preferable to that using isosceles triangles of different size, which are the real sections of the tetrahedron, as can be seen in Fig. 1. The equilateral plot produces a slight deformation but it simplifies both the graphical representation and the comparison of the plots since the size of all of them is the same.

T A B L E 4

Tie l ine data (wt.%) for water ( W ) - e t h a n o l (E) -deca l in (D) at 25 ° C

Ini t ial Organic phase Aqueous phase mixture

X w X E X D X W X E X D L a

5.0 89.8 10.18 N D N D 0.3 99.9 10.0 81.4 18.6 0.008 N D 0.17 99.8 15.0 73.8 26.2 0.022 N D 0.24 99.8 20.0 66.8 33.2 0.028 N D 0.35 99.7 25.0 60.3 39.6 0.073 N D 0.41 99.6 30.0 54.2 45.7 0.15 N D 0.46 99.5 35.5 48.0 51.7 0.31 N D 0.58 99.4 40.0 43.2 56.3 0.55 N D 0.61 99.4 45.0 37.8 61.3 0.89 N D 0.63 99.4 50.0 32.7 65.9 1.44 N D 0.66 99.3 55.0 28.5 69.4 2.09 N D 0.77 99.2 60.0 24.4 72.5 3.10 N D 0.89 99.1 65.0 19.7 75.8 4.49 N D 1.10 98.9 70.0 16.6 77.3 6.09 N D 1.25 98.8

a E thanol level (defined as L = X E in the overall init ial mixture). ND, no detectable composit ion.

321

T A B L E 5

Tie-l ine da ta (wt.%) for water ( W ) - e t h a n o l (E) -2-e thy lhexanol ( E H ) - d e c a l i n (D) at 25 ° C

Aqueous phase Organic phase

L (initial) X w X E XEH X D X w XE XEH XD

M(ini t ia l ) = 0.2 5.0 93.2 6.7 0.08 N D 2.16 3.51 75.4 19.0

10.0 86.9 13.0 0.10 N D 2.71 7.32 71.8 18.1 15.0 81.4 18.5 0.14 N D 3.57 11.9 67.5 17.0 20.0 76.0 23.8 0.23 N D 4.67 16.9 62.7 15.7 25.0 71.1 28.5 0.44 N D 6.44 22.3 56.8 14.4 30.0 66.0 33.0 0.98 0.005 8.73 28.0 50.6 12.7 35.0 59.5 37.9 2.52 0.04 12.3 33.4 43.1 11.2 40.0 50.8 42.5 6.45 0.29 18.6 39.3 33.1 9.0

M(ini t ia l ) = 0.4 5.0 92.5 7.44 0.072 N D 1.48 3.01 57.3 38.2

10.0 86.2 13.7 0.11 N D 2.01 7.20 54.5 36.3 15.0 81.6 18.3 0.15 N D 2.62 11.2 51.7 34.5 20.0 74.4 25.2 0.35 N D 3.45 15.3 48.6 32.7 25.0 67.8 31.5 0.66 N D 4.16 19.6 45.5 30.8 30.0 62.2 36.5 1.28 0.022 5.50 23.8 41.9 28.8 35.0 56.1 40.8 2.93 0.24 "7.74 29.2 37.1 26.0 40.0 46.4 46.2 6.16 1.27 10.12 32.6 32.3 25.0

M(ini t ia l ) = 0.6 5.0 91.9 8.0 0.060 N D 0.75 1.73 39.1 58.5

10.0 84.7 15.2 0.085 N D I).89 4.15 38.1 56.8 15.0 78.1 21.8 0.13 N D 1.35 7.37 36.4 54.8 20.0 72.0 27.7 0.27 N D 2.00 10.8 34.5 52.7 25.0 65.9 33.4 0.70 0.007 2.95 14.7 32.4 50.0 30.0 59.0 39.2 1.73 0.065 :3.2 17.4 30.3 49.1 35.0 50.8 45.0 3.95 0.30 13.3 19.2 27.4 50.1 40.0 42.4 49.2 6.71 1.66 13.2 19.0 23.6 54.2 45.0 34.3 52.5 9.6 3.6 11.9 14.1 16.8 67.2 50.0 28.4 56.2 10.3 5.1 0.78 7.9 7.6 83.8 55.0 24.4 60.0 8.7 5.9 0.34 5.4 1.84 92.4

M(ini t ia l ) = 0.8 5.0 91.5 8.5 0.05 N D 0.28 1.06 19.7 79.0

10.0 83.9 16.0 0.09 N D 0.32 2.6 19.4 77.7 15.0 75.8 24.0 0.17 N D 0.50 4.81 18.8 75.9 20.0 68.7 31.0 0.34 0.0024 0.76 6.24 18.2 74.8 25.0 61.4 37.7 0.90 0.033 0.84 7.62 17.3 74.2 30.0 54.7 43.1 1.90 0.30 1.0 8.52 15.6 74.9 35.0 48.3 47.6 3.35 0.72 1.05 8.52 13.0 77.4 40.0 41.0 52.7 4.90 1.41 0.52 6.10 8.9 84.5 45.0 35.2 57.4 5.50 1.96 0.31 4.31 5.55 89.9 50.0 30.8 61.5 5.30 2.45 0.24 3.00 3.82 93.0 55.0 27.4 64.6 4.80 3.20 0.15 2.65 3.15 94.0 60.0 23.1 68.5 4.20 4.20 0.045 2.15 3.00 94.8

ND, no detectable composi t ion.

DI

~w

322

EH

Fig. 1. Tetrahedral representation of the solubility surface of the quaternary system water (W)-ethanol (E)-2-ethylhexanol (EH)-decalin (D).

In Fig. 2, the projection of the experimental tie lines having a value of M in the global initial mixture belonging to that plane is represented. Actually, a tie line does not lie on a plane W - E - ( E H - D mixture) because the values of M are different for the tie line extremes and for any global heterogeneous mixture splitting in those extremes. The EH and the D in the overall intitial mixture are not tied together. Since the solubility of EH in W is greater than that of D, the aqueous phase extracts more EH than D, giving a value of M higher than that of the overall initial mixture. On the other hand, the ratio of EH to D in the organic phase is smaller than in the overall mixture, giving a smaller value of M for this phase. Therefore, the tie lines do not terminate on the curve of solubility points of a determined triangle W - E - ( E H - D mixture) and some of them cross each other, as can be seen in Fig. 2, since the only point of a tie line belonging to that plane is that corresponding to the global initial mixture; the other points are the projections on that plane.

Another feature which is also impossible for a genuine ternary system (Francis, 1954) is the large "hump" in the solubility curves of Fig. 2a,b with a predictable plait point (intersection of the solubility curve and the line joining the extremes of the equilibrium mixtures) placed in a concave position. The large difference in the solubilities of the systems W - E H and W - D is again the cause of this unusual behaviour: the system W - E - E H presents a much smaller heterogeneous region than that of W - E - D and produces the aforementioned unusual hump in the quaternary solubility

(a)

323

40

20

M=0.2

8 0

6 0

6 0 4 0

8 0

i i J - - - i

, - - - - 2 0 4 0 6 o 8 0

( b )

20

rl=O. 4

6 0

4 0

BO

i i ~ - , i

EH-~ 20 40 60 80 14

Fig. 2. Pseudoternary representation of the solubility curve and projected tie lines for each water (W)-ethanol (E)-(2-ethylhexanol (EH)-decal in (D)) plane characterized by the value of M: (a) M = 0.2, (b) M = 0.4, (c) M = 0.6 and (d) M = 0.8.

324

EH-D

(d)

20 40 60 8 0

~o/ ,:0.6 \~o

6 0

4 0

EH-D

Fig. 2 (continued).

8 0

J

.._.~..-.z~

20

20 40 60 80 W

325

,...1

<

.= II

o r g

r,.) I

I <

i

<

r,.) 0

t 'q r . ~

,<

.=,

• 'a ~,

c5 ~ ~ e-i ~ d

¢-,q

326

20 Z \ 80

40 ,4/ / / / \ \ & 60

r1:o.o

f l i i E H- D 20 40 60 80 W

Fig. 3. Simultaneous pseudoternary representation of the solubility curves for each M value.

curves. When the experimental solubility curves are all represented together, as in Fig. 3, it can be seen that the heights of the aqueous branches increase suddenly when M changes from 0 to 0.2 to 0.4 while the heights of the organic branches remain almost constant. The result is a hump in the solubility curve and a predictable plait point in a concave position for this range of M.

In order to verify these explanations about the solubilities as causes of the unusual behaviour of the system, a review of the papers in the literature with experimental data of quaternary systems A - B - C - D was completed. Several papers on systems with humps in the solubility curves were found. The references of the papers and the solubilities of the compounds therein are shown in Table 6. It can be seen that all the referred systems with this behaviour contain two pairs of partly-miscible compounds A - C and A - D with very different solubilities of C and D in A.

LIST OF SYMBOLS

D decalin E ethanol E H 2-ethylhexanol

327

L

M

W

X

e t h a n o l l eve l ( d e f i n e d as L = X E in t h e i n i t i a l m i x t u r e )

M = X D / ( X D ~- XEH ) i n t h e i n i t i a l m i x t u r e w a t e r

w e i g h t p e r c e n t a g e

REFERENCES

Francis, A.W., 1954. Liquid equilibria of water-methanol-anil ine-benzene system. Ind. Eng. Chem., 45: 205-207.

Fritzsche, R.H. and Stockton, D.L., 1946. Systems containing isobutanol and tetrachloro- ethane. Ind. Eng. Chem., 38: 737-740.

Prutton, C.F., Walsh, T.J. and Desai, A.M., 1950. Solvent extraction of tar acids from coal tar hydrocarbons. Ind. Eng. Chem., 42: 1210-1217.

Rudakovskaya, T.S., Timofeev, V.S. and Serafimov, L.A., 1972. Study of l iquid-liquid phase equilibrium and mutual solubility of the components in the four component mixture methanol-vinyl acetate-methyl acetate-water. Zh. PilE. Khim., 45:2122-2124 (English translation).

Ruiz, F. and Prats, D., 1983. Quaternary liquid-liquid equilibria: experimental determination and correlation of equilibrium data. Part I: system water-acetone-acetic acid-chloro- form. Fluid Phase Equilibria, 10: 77-93.

Ruiz, F., Prats, D., Gomis, V. and Var6, P., 1984. Quaternary liquid-liquid equilibrium: water-acetic ac id- l -butanol -n-butyl acetate at 25 o C. Fluid Phase Equilibria, 18: 171-183.

Ruiz, F., Gomis, V. and Botella, R.F., 1987. Extraction of ethanol from aqueous solution: solvent less volatile than ethanol: 2-ethylhexanol. Ind. Eng. Chem. Res., 26: 696-699.