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Liquid-liquid equilibria for the systems triethylene glycol - toluene - heptane, propylene carbonate - toluene - heptane and propylene carbonate -o-xylene- heptane
Fluid Phase Equilibria, 86 (1993) 351-361 Elsevier Science Publishers B.V.. Amsterdam
351
Liquid-liquid equilibria for the systems triethylene glycol- toluene- heptane, propylene carbonate- toluene- heptane and pi-opylene carbonate-o -xylene- heptane Abu Bakr S.H. Salem Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261 (Saudi Arabia) (Received
November
22, 1991; accepted
in final form October
11, 1992)
ABSTRACT Salem, A.B.S.H., 1993. Liquid-liquid equilibria heptane, propylene carbonate-toluene-heptane tane. Fluid Phase Equilibria 86: 351-361 Liquid-liquid equilibrium data propylene carbonate-toluene-heptane determined at 25°C. Solvent capacity The propylene carbonate capacity higher than that of triethylene glycol aromatics than paraffins at the same
for the systems triethylene glycol-tolueneand propylene carbonate-o-xylene-hep-
for
the systems triethylene glycol-toluene-heptane, and propylene carbonate-o-xylene-heptane were and selectivity at 25°C were calculated. towards aromatics was found to be two to three times but the latter is about two times more selective towards temperature.
INTRODUCTION
Many solvents are commonly used for extracting aromatics from reformate such as diethylene glycol (DEG), sulpholane (tetramethylene sulmorpholine (N-formyl morpholine) and phone), dimethyl sulphoxide, methyl carbamate (Yorulmaz and Karpuzcu, 1985). Work is still in progress for exploring other solvents with high selectivity and solvent capacity (Awwad et al., 1988). Badertscher et al. (1954) have used ethylene carbonate as solvent to extract benzene from heptane and the process has been patented. A literature survey revealed that propylene carbonate has not yet been evaluated as a solvent for extraction of aromatics from paraffins. The object of this work is to evaluate propylene carbonate as a solvent for the recovery of aromatic hydrocarbons from reformate. Evaluation of this solvent will be based on the determination of the equilibrium data of 0378-3812/93/$06.00
G 1993 - Elsevier
Science Publishers
B.V. All rights reserved
352
A.B.S.H.
Salem / Fluid Phase Equilibria
86 (1993) 351-361
some ternary systems containing this solvent and two types of aromatics and a paraffin. The results obtained are to be compared with a commonly used solvent. Triethylene glycol solvent has been selected for such comparison. Power and selectivity
of solvents
If a liquid solvent is added to a solution of some solute, A, in a second solvent, either immiscible or only partially miscible with it, then the solute will be distributed between the two liquid phases until equilibrium is established. At equilibrium, the ratio of the concentrations of solute in the extract phase, Y, and raffinate phase, X, is called the distribution coefficient, K. This coefficient is a measure of the affinity of the solute for the two phases. K.4 = Y*I&
(1)
The solvent power (capacity) can be defined as the distribution coefficient, KA, (Bailes et al., 1976) or as the mass fraction extracted, Y,, (Voetter and Kosters, 1963). If only one solute is involved, such as in the recovery of an impurity from an effluent stream, it is desirable that the distribution coefficient for this solute should be as large as possible. In other cases, however, the aim is to achieve a separation between two solutes. While a large distribution coefficient for the solute being extracted is still desirable, consideration also has to be given to the selectivity of the solvent towards one solute, A, compared with that towards the other solute, B. This is measured by the separation factor, S, which is the analog of relative volatility in distillation: S,,
= K%l&
(2)
The amount of solute that can be recovered from a particular feed solution by equilibration with a solvent depends on both the distribution coefficient and the volumetric ratio of extract to raffinate phase. The amount of separation achieved in a single equilibration can easily be calculated from the distribution coefficient of the solute and an overall mass balance for this component. The selection of a solvent for an extraction duty depends upon the solvent power (capacity) measured by the solute distribution coefficient (eqn. (1)) and also by its selectivity estimated by eqn. (2). In the case of the recovery of aromatics from reformates, for example, trials are made to screen a number of solvents in order to select a solvent with the largest possible capacity and highest selectivity towards aromatics under some specified conditions.
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Salem / Fluid Phase Equilibria 86 (1993) 351-361
353
EXPERIMENTAL
Some data for the triethylene glycol-benzene-heptane system are available in the literature (Graham, 1962). It was decided to start with a similar system containing this solvent for the sake of comparison. However, toluene was used in the present work since it is usually present in reformate in larger proportions than benzene. The equilibrium data for this system were determined by refractometry, since the purity of the available triethylene glycol was less than 95%. Chromatography was initially tried for the analysis of the phases but input and output masses were not balanced. The other systems containing propylene carbonate solvent were analyzed by chromatography without difficulties. Materials The materials used in this study were toluene, n-heptane, o-xylene, propylene carbonate, and triethylene glycol. Toluene, n-heptane, and oxylene were obtained from BDH Chemicals Ltd., U.K. Propylene carbonate and triethylene glycol were obtained from Surechem Products Ltd., U.K. Acetone was obtained from the Fischer Company. The physical properties of these chemicals are given in Table 1. Equilibrium
data
The equilibrium data for the systems used were determined at 25°C using the Smith-Bonner method (Smith and Bonner, 1950). Equal amounts of the paraffin and the solvent were stirred with a certain weight of an aromatic component for two hours. The mixture was then allowed to settle in a separating funnel for 24 hours. The phases were separated and weighed, and the solute content in each phase was determined by gas chromatography. Refractometry was used for analyzing the system containing triethylene glycol at 25°C. TABLE
1
Physical
properties
Acetone Heptane o-Xylene Propylene carbonate Toluene Triethylene glycol
of liquids used Density
Boiling point (“C)
Refractive index
0.791 0.684 0.982 1.189 0.867 1.125
56 98 142 240 111 285
1.359 1.387 1.502 1.421 1.496 1.455
A.B.S.H.
354 TABLE
2
Solubility
data for the system triethylene
(a) Triethylene Weight
Salem / Fluid Phase Equdibrra 86 (1993) 351-361
glycol-rich
glycol-toluene-heptane
at 25°C
phase
(g)
Percentage
R;
by weight
TEG
Toluene
Heptane
TEG
Toluene
Heptane
56.25 56.25 56.25 56.25 56.25
0.000 4.330 6.062 7.794 12.124
0.08538 0.11953 0.14514 0.18783 0.20490
99.850 92.654 90.062 87.573 82.020
0.000 7.132 9.706 12.134 17.680
0.15000 0.19688 0.23237 0.29241 0.29877
(b) Heptane-rich Weight
phase Percentage
(g)
by weight
RP,
Heptane
Toluene
TEG
Heptane
Toluene
TEG
34.150 34.150 34.150 34.150 34.150 34.150
00.000 08.630 17.260 25.890 34.520 43.150
0.39375 0.42188 0.49219 0.56250 0.59063 0.60469
98.8500 79.0500 65.8000 56.3500 49.3065 43.8356
00.00 19.98 33.26 42.72 49.84 55.40
1.140 0.9700 0.9450 0.9300 0.8525 0.7744
d Refractive
1.4558 1.4598 1.4610 1.4622 1.4652
1.3892 1.4084 1.4216 1.4278 1.4348 1.4420
index.
In order to calibrate the refractometer, the solubility data of this system were determined. The heptane phase data were determined by titrating different mixtures of known compositions of heptane and the solute with triethylene glycol until the appearance of the first permanent turbidity. The triethylene glycol phase data were determined by a similar procedure in which different mixtures of known compositions of triethylene glycol and the toluene were titrated with heptane until the appearance of the first permanent turbidity. The refractive indices of these phases were measured by an Abbe refractometer at 25°C. The solubility data for this system are given in Table 2. The equilibrium data for the triethylene glycol system were then obtained by the procedure outlined above. The raffinate and extract aromatic concentrations were determined from the corresponding calibration graphs. Gas chromatography was used for analyzing the propylene carbonate system. Liquid samples from the extract and raffinate phases were analyzed in a gas chromatograph (Perkin Elmer 8500 with flame ionization detector). The separation column consisted of a glass tube (l/4” diameter and 2 m long);
A.B.S.H.
Salem 1 Fluid Phase Equilibria
86 (1993) 351-361
355
it was packed with Apiezon L on Chromosorb W-NAW (So-100 mesh). Acetone was used as solvent for the liquid mixtures used. The separation of all components started at 60°C and finished at 150°C for the propylene carbonate peak, with a temperature rate of increase of 20°C min-‘. The injector and detector temperatures were 200°C and 3OO”C, respectively. The carrier gas used was helium, flowing at 20 ml min-‘. Sample volumes for analysis were 0.5 ml. Calibration of the gas chromatograph was done by injecting standard solutions containing known amounts of the components of interest in order to obtain the peak areas for these components. The response factor of a component i in a standard sample was calculated from the corresponding peak area as follows: J;. = Cl-%,
(3)
where5 is the response factor of component i in the standard sample, Ci is the concentration of component i in the standard sample (mass fraction) and A,., is the peak area of component i in the standard sample. Each standard sample was injected three times and an average response factor was calculated for each component. The computed response factors were then used to calculate the concentration of the components in the equation was unknown phase. For the raffinate phase, the following used: (4) where X, is the concentration of component i in the raffinate phase sample (mass fraction) and A, is the peak area of component i in the raffinate phase sample. The concentration of a component i in the extract phase, Y,, was determined by a similar procedure.
RESULTS
Triethylene
glycol-
toluene - heptane system
Equilibrium data obtained for this system at 25°C using refractometry are given in Table 3, Parts a and b. Part a contains the phase analysis data. Part b shows the estimated equilibrium constants of toluene and heptane and the solvent selectivity towards toluene with respect to heptane. The error in the solute balance was less than 2%. The selectivity increased from 37.24 to 85.56, corresponding to a solvent capacity increase of 0.06 to 0.1677. The table indicates that this solvent, although very highly selective, has a very small capacity for extraction of aromatics from paraffins.
356 TABLE
A.B.S.H.
Salem 1 Fluid Phase Equilibria
86 (1993) 351-361
3
Equilibrium (a) Tie-line 3-component TEG (%)
48.544 47.619 46.729 45.872 45.045
data for the system triethylene data
W)
2.913 4.762 6.542 8.257 9.910
(b) Equilibrium to heptane
YT
Percentage
system Toluene
Heptane (%I)
Heptane
48.544 47.619 46.729 45.872 45.045 constants
XT
glycol-toluene-heptane
and selectivity
KyS
of toluene phase
fraction
0.003 0.007 0.014 0.021 0.026
0.050 0.080 0.110 0.140 0.155
0.0600 0.0875 0.1273 0.1500 0.1677
Propylene
carbonate
systems
in TEG phase
Xr (%)
R,
Y, (%)
R,
5 8 11 14 15.5
1.3932 1.3960 1.3988 1.4010 1.4030
0.3 0.7 1.4 2.1 2.6
1.4560
of triethylene
YH
glycol towards
X”
1.4562 1.4566 1.4569
1.4572 toluene
with respect
S+
KH+
H
H
T
fraction
at 25°C
fraction
fraction
0.00152 0.00155 0.00159 0.00163 0.00166
0.94553 0.91840 0.88955 0.86685 0.84620
0.001611 0.001684 0.001792 0.001879 0.001960
37.240 51.960 7 1.040 79.830 85.560
Propylene carbonate- toluene -heptane system Data obtained for this system at 25°C are given in Table 4. In part a of this table, data for six samples of mixtures of different compositions (amounts of extract and raffinate phases and percentage error data) are given. The error in these measurements was less than 1% as shown in the table. The toluene and heptane concentrations in the extract and raffinate phases, Y,, XT, Y, and X,, determined are given in part b of this table. Also included are the estimated values of the distribution coefficients of toluene and heptane, KT and KH, and the selectivity of the solvent towards toluene with respect to heptane, S. This table shows that the solvent capacity towards toluene ranges from 0.354 to 0.441 in the range of concentration studies. Correspondingly the selectivity falls from 14 to 5.97.
A.B.S.H. TABLE
Salem 1 Fluid Phase Equilibria 86 (1993) 351-361 4
Equilibrium data for the system 25°C (a) Total mass balance Weight
(g)
PC
T
H
50
5 10 20 30 40 50
50 50 50 50 50 50
50 50 50 50 50
d
o/
0
357
propylene
(PC) -toluene
(T) -heptane
Mixture,
Extract,
Raffinate,
E+R
M (g)
E (g)
R (g)
(9)
105 110 120 130 140 150
50.8987 52.9717 55.3892 57.4600 60.0257 61.8442
53.16790 56.01700 64.15900 72.00900 79.25920 87.09000
104.068 108.898 119.548 129.469 139.285 148.934
Erroi_/-(E+R) M
(b) Equilibrium constants respect to heptane
% Error
(H) at
d
0.8870 0.9190 0.3756 0.4080 0.5100 0.7100
x 100. and
selectivity
K-,=2
YT
carbonate
of propylene
carbonate
towards
toluene
with
Y,
T
fraction
fraction
0.0282 0.0527 0.0918 0.1296 0.1637 0.1918
0.0795 0.1409 0.2437 0.3211 0.3863 0.4346
0.35400 0.37400 0.37700 0.40360 0.42381 0.44130
fraction
fraction
0.0232 0.027 1 0.0293 0.0329 0.0344 0.0400
0.9181 0.8568 0.7506 0.6663 0.5968 0.5411
0.02527 0.03166 0.03908 0.04942 0.05773 0.07396
14.000 11.800 9.650 8.167 7.342 5.970
Comparing Table 4 with Table 3, it can be seen that propylene carbonate has a greater capacity for extracting toluene from heptane than triethylene glycol at the same temperature. Table 4 also shows that propylene carbonate is not as selective as triethylene glycol, and also that selectivity decreases as the percentage aromatic in the feed increases. This is the opposite of the behavior of the triethylene glycol solvent. Propylene
carbonate -o-xylene
- heptane system
Data obtained for this system at 25°C are given in Table 5 parts a and b. The percentage error in the total mass balance of this system was less than 1.5%. The solvent capacity towards o-xylene increased from 0.258 to 0.3373 in the range of concentration studied. Correspondingly the selectivity decreased from 13.4 to 7.22. It can be seen that the behavior of this system is similar to that of toluene.
A.B.S.H.
358 TABLE
5
Equilibrium data for the system (a) Total mass balance Weight
(g)
PC
o-x
Hep.
50 50 50 50 50 50
5 10 20 30 40 50
50 50 50 50 50 50
a o/
0
Salem 1 Fluid Phase Equilibria 86 (1993) 351-361
Error
=
(b) Equilibrium constants respect to heptane
.,
Yo-x
A,-X
fraction
fraction
0.019892 0.038485 0.067701 0.102553 0.131847 0.152246
0.07713 0.14275 0.25414 0.33656 0.40042 0.45133
carbonate-o-xylene-heptane
at 25°C
Mixture,
Extract,
Raffinate,
E+R
M (g)
E (g)
R (g)
(g)
105 110 120 130 140 150
51.1100 52.0500 54.4800 55.7300 56.5300 57.8700
52.55000 56.31000 64.3 1000 72.98000 8 1.82000 90.90000
103.660 108.360 118.790 128.710 138.350 148.770
M - (E+ R) x A4
propylene
% Error
d
1.2700 1.4900 1.0083 0.9920 1.1800 0.8200
100. and selectivity
Yo-x K 0-X =p x O-X
0.25800 0.26960 0.27000 0.30500 0.32930 0.33730
of propylene
carbonate
Y”
X”
fraction
fraction
0.01776 0.01903 0.01923 0.02425 0.02446 0.02527
0.92230 0.85546 0.74287 0.65956 0.59508 0.54098
towards
0.01926 0.02225 0.02589 0.03676 0.04110 0.04671
o-xylene
with
13.3975 12.1350 10.4300 8.2970 8.0122 7.2200
DISCUSSION
A comparison of the equilibrium and selectivity data obtained in this work with those of other published work using other solvents for the extraction of aromatics from paraffins is given in Table 6. The table shows that some investigators have tried to add percentages of water to the solvent used in order to increase its capacity or selectivity towards the aromatic component. However, different temperatures were used. Graham (1962) used triethylene glycol with 7.7% water at 121°C for the extraction of benzene from heptane. He obtained a reasonable capacity (0.2-0.4) compared with sulfolane, which is considered to be one of the most powerful solvents in the field of aromatics extraction. However, the selectivity obtained was low and decreases from 17 to 2 at higher concentrations. In contrast,
Toluene
Toluene
Sulfolane
Sulfolane + 4% water Diethylene glycol Methyl ether Sulfolane
Triethylene glycol Propylene carbonate Propylene carbonate
6
8 9 10
7
Toluene Toluene o-Xylene
Aromatic
Toluene
Heptane Heptane Heptane
Non-aromatic
Isooctane
Heptane
Heptane
Heptane
Toluene
4
3
Heptane
Heptane
Paraffin
Benzene
Benzene
Triethylene glycol -t 7.1% water Diethylene glycol + 7.7% water Sulfolane
1
2
Aromatic
System
25 25 25
25
25
30
30
30
121
121
Temp. (“C)
0.1-0.2 0.3-0.4 0.2-0.3
0.3-0.6
0.7-0.8
0.3-0.6
0.3-0.5
0.3-0.8
0.1-0.2
0.2-0.4
Solvent capacity
37-85 14-6 13-7
40-15
6-3
88-2.7
23- 19
38-1.7
21-12
17-Z
Selectivity
Hassan & Fahim (1988) This work This work This work
Yorulmaz & Karpuzcu (1985) Hassan & Fahim (1988) Yorulmaz & Karpuzcu (1985) Bottinl (1986)
Graham (1962)
Graham (1962)
Reference
of propylene carbonate solvent performance with other solvents for aromatic extraction from paraffins
Solvent
NO.
Comparison
TABLE 6 -
A.B.S.H.
360
Salem / Fluid Phase Equilibria
86 (1993) 351-361
in the present work, low temperature extraction decreased the capacity to 0.1-0.2 at 25°C but increased the selectivity to an attractive figure of 37-85. However, the increase in selectivity of this solvent at higher solute concentration at such a low temperature may be exceptional. Table 6 also reveals that propylene carbonate has a reasonable capacity (0.3-0.4) which can be compared to that of sulfolane (0.3-0.6). However, the selectivity of sulfolane is almost twice that of propylene carbonate. Work has to be done to find possible additives or conditions to increase the selectivity of this solvent.
CONCLUSIONS
Propylene carbonate is a promising solvent which can be used for extracting aromatics from reformate. It has a reasonable capacity similar to that of solvents commonly used for this purpose. However, its selectivity is not very high. Nevertheless, owing to its relatively low price it may become an alternative to such solvents.
ACKNOWLEDGMENT
During his final year project at the Chemical Engineering Department, Qatar University, Mohamad N. Al-Muthafari was involved in the experimental work of this study. This is acknowledged.
LIST OF SYMBOLS
4 4, Cl
r” rk M PC R R, s TEG X K
peak area of component i in the raffinate phase sample peak area of component i in the standard sample concentration of component i in the standard sample (mass fraction) extract response factor of component i in the standard sample distribution coefficient mixture propylene carbonate raffinate refractive index selectivity triethylene glycol concentration of component i in the raffinate phase (mass fraction) concentration of component i in the extract phase (mass fraction)
A.B.S.H.
Salem
1 Fluid Phase
Equilibria
86 (1993)
351-361
361
Subscripts A, B H o-x T
components heptane 0 -xylene Toluene
REFERENCES Awwad, A.M., Salman, M.A. and Hassan, F.A., 1988. Liquid-liquid equilibria for the ternary system y-butyrolactone-n-heptane-benzene/toluene/~-xylene. J. Chem. Eng. Data, 33: 263-265. Badertscher, D.E., Francis, A.W. and Johnson, G.C., 19.54. U.S. Patent 2, 688, 645, Sept. 7. Bailes, P.J., Hanson, C. and Hughes, M.A., 1976. Liquid-liquid extraction: nonmetallic materials. Chem. Eng., 84: 115-120. Bottinl, S.B., 1986. Liquid-liquid equilibria for the system toluene-iso-octane-diethylene glycol methyl ether. J. Chem. Eng. Data, 31: 84-86. Graham, H.L., 1962. Extraction of benzene with di- and tri-ethylene glycols. J. Chem. Eng. Data, 7(2): 214-217. Hassan, M.S. and Fahim, M.A., 1988. Correlation of phase equilibria of naphtha reformate with sulfolane. J. Chem. Eng. Data, 33: 1622165. Smith, T.E. and Bonner, R.F., 1950. n-Propyl alcohol-n-propyl acetate-water solubility data at 20°C and 35°C. Ind. Eng. Chem., 42(5): 896-898. Voetter, H. and Kosters, W., 1963. The sulfolane extraction process. Paper presented at the International Conference of Solvent Extraction, The Netherlands Section III, Paper II, June 26, 131-145. Yorulmaz, Y. and Karpuzcu, F., 1985. Sulfolane versus diethylene glycol in recovery of aromatics. Chem. Eng. Res. Des., 63: 184-190.
351
Liquid-liquid equilibria for the systems triethylene glycol- toluene- heptane, propylene carbonate- toluene- heptane and pi-opylene carbonate-o -xylene- heptane Abu Bakr S.H. Salem Chemical Engineering Department, King Fahd University of Petroleum & Minerals, Dhahran 31261 (Saudi Arabia) (Received
November
22, 1991; accepted
in final form October
11, 1992)
ABSTRACT Salem, A.B.S.H., 1993. Liquid-liquid equilibria heptane, propylene carbonate-toluene-heptane tane. Fluid Phase Equilibria 86: 351-361 Liquid-liquid equilibrium data propylene carbonate-toluene-heptane determined at 25°C. Solvent capacity The propylene carbonate capacity higher than that of triethylene glycol aromatics than paraffins at the same
for the systems triethylene glycol-tolueneand propylene carbonate-o-xylene-hep-
for
the systems triethylene glycol-toluene-heptane, and propylene carbonate-o-xylene-heptane were and selectivity at 25°C were calculated. towards aromatics was found to be two to three times but the latter is about two times more selective towards temperature.
INTRODUCTION
Many solvents are commonly used for extracting aromatics from reformate such as diethylene glycol (DEG), sulpholane (tetramethylene sulmorpholine (N-formyl morpholine) and phone), dimethyl sulphoxide, methyl carbamate (Yorulmaz and Karpuzcu, 1985). Work is still in progress for exploring other solvents with high selectivity and solvent capacity (Awwad et al., 1988). Badertscher et al. (1954) have used ethylene carbonate as solvent to extract benzene from heptane and the process has been patented. A literature survey revealed that propylene carbonate has not yet been evaluated as a solvent for extraction of aromatics from paraffins. The object of this work is to evaluate propylene carbonate as a solvent for the recovery of aromatic hydrocarbons from reformate. Evaluation of this solvent will be based on the determination of the equilibrium data of 0378-3812/93/$06.00
G 1993 - Elsevier
Science Publishers
B.V. All rights reserved
352
A.B.S.H.
Salem / Fluid Phase Equilibria
86 (1993) 351-361
some ternary systems containing this solvent and two types of aromatics and a paraffin. The results obtained are to be compared with a commonly used solvent. Triethylene glycol solvent has been selected for such comparison. Power and selectivity
of solvents
If a liquid solvent is added to a solution of some solute, A, in a second solvent, either immiscible or only partially miscible with it, then the solute will be distributed between the two liquid phases until equilibrium is established. At equilibrium, the ratio of the concentrations of solute in the extract phase, Y, and raffinate phase, X, is called the distribution coefficient, K. This coefficient is a measure of the affinity of the solute for the two phases. K.4 = Y*I&
(1)
The solvent power (capacity) can be defined as the distribution coefficient, KA, (Bailes et al., 1976) or as the mass fraction extracted, Y,, (Voetter and Kosters, 1963). If only one solute is involved, such as in the recovery of an impurity from an effluent stream, it is desirable that the distribution coefficient for this solute should be as large as possible. In other cases, however, the aim is to achieve a separation between two solutes. While a large distribution coefficient for the solute being extracted is still desirable, consideration also has to be given to the selectivity of the solvent towards one solute, A, compared with that towards the other solute, B. This is measured by the separation factor, S, which is the analog of relative volatility in distillation: S,,
= K%l&
(2)
The amount of solute that can be recovered from a particular feed solution by equilibration with a solvent depends on both the distribution coefficient and the volumetric ratio of extract to raffinate phase. The amount of separation achieved in a single equilibration can easily be calculated from the distribution coefficient of the solute and an overall mass balance for this component. The selection of a solvent for an extraction duty depends upon the solvent power (capacity) measured by the solute distribution coefficient (eqn. (1)) and also by its selectivity estimated by eqn. (2). In the case of the recovery of aromatics from reformates, for example, trials are made to screen a number of solvents in order to select a solvent with the largest possible capacity and highest selectivity towards aromatics under some specified conditions.
A.B.S.H.
Salem / Fluid Phase Equilibria 86 (1993) 351-361
353
EXPERIMENTAL
Some data for the triethylene glycol-benzene-heptane system are available in the literature (Graham, 1962). It was decided to start with a similar system containing this solvent for the sake of comparison. However, toluene was used in the present work since it is usually present in reformate in larger proportions than benzene. The equilibrium data for this system were determined by refractometry, since the purity of the available triethylene glycol was less than 95%. Chromatography was initially tried for the analysis of the phases but input and output masses were not balanced. The other systems containing propylene carbonate solvent were analyzed by chromatography without difficulties. Materials The materials used in this study were toluene, n-heptane, o-xylene, propylene carbonate, and triethylene glycol. Toluene, n-heptane, and oxylene were obtained from BDH Chemicals Ltd., U.K. Propylene carbonate and triethylene glycol were obtained from Surechem Products Ltd., U.K. Acetone was obtained from the Fischer Company. The physical properties of these chemicals are given in Table 1. Equilibrium
data
The equilibrium data for the systems used were determined at 25°C using the Smith-Bonner method (Smith and Bonner, 1950). Equal amounts of the paraffin and the solvent were stirred with a certain weight of an aromatic component for two hours. The mixture was then allowed to settle in a separating funnel for 24 hours. The phases were separated and weighed, and the solute content in each phase was determined by gas chromatography. Refractometry was used for analyzing the system containing triethylene glycol at 25°C. TABLE
1
Physical
properties
Acetone Heptane o-Xylene Propylene carbonate Toluene Triethylene glycol
of liquids used Density
Boiling point (“C)
Refractive index
0.791 0.684 0.982 1.189 0.867 1.125
56 98 142 240 111 285
1.359 1.387 1.502 1.421 1.496 1.455
A.B.S.H.
354 TABLE
2
Solubility
data for the system triethylene
(a) Triethylene Weight
Salem / Fluid Phase Equdibrra 86 (1993) 351-361
glycol-rich
glycol-toluene-heptane
at 25°C
phase
(g)
Percentage
R;
by weight
TEG
Toluene
Heptane
TEG
Toluene
Heptane
56.25 56.25 56.25 56.25 56.25
0.000 4.330 6.062 7.794 12.124
0.08538 0.11953 0.14514 0.18783 0.20490
99.850 92.654 90.062 87.573 82.020
0.000 7.132 9.706 12.134 17.680
0.15000 0.19688 0.23237 0.29241 0.29877
(b) Heptane-rich Weight
phase Percentage
(g)
by weight
RP,
Heptane
Toluene
TEG
Heptane
Toluene
TEG
34.150 34.150 34.150 34.150 34.150 34.150
00.000 08.630 17.260 25.890 34.520 43.150
0.39375 0.42188 0.49219 0.56250 0.59063 0.60469
98.8500 79.0500 65.8000 56.3500 49.3065 43.8356
00.00 19.98 33.26 42.72 49.84 55.40
1.140 0.9700 0.9450 0.9300 0.8525 0.7744
d Refractive
1.4558 1.4598 1.4610 1.4622 1.4652
1.3892 1.4084 1.4216 1.4278 1.4348 1.4420
index.
In order to calibrate the refractometer, the solubility data of this system were determined. The heptane phase data were determined by titrating different mixtures of known compositions of heptane and the solute with triethylene glycol until the appearance of the first permanent turbidity. The triethylene glycol phase data were determined by a similar procedure in which different mixtures of known compositions of triethylene glycol and the toluene were titrated with heptane until the appearance of the first permanent turbidity. The refractive indices of these phases were measured by an Abbe refractometer at 25°C. The solubility data for this system are given in Table 2. The equilibrium data for the triethylene glycol system were then obtained by the procedure outlined above. The raffinate and extract aromatic concentrations were determined from the corresponding calibration graphs. Gas chromatography was used for analyzing the propylene carbonate system. Liquid samples from the extract and raffinate phases were analyzed in a gas chromatograph (Perkin Elmer 8500 with flame ionization detector). The separation column consisted of a glass tube (l/4” diameter and 2 m long);
A.B.S.H.
Salem 1 Fluid Phase Equilibria
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355
it was packed with Apiezon L on Chromosorb W-NAW (So-100 mesh). Acetone was used as solvent for the liquid mixtures used. The separation of all components started at 60°C and finished at 150°C for the propylene carbonate peak, with a temperature rate of increase of 20°C min-‘. The injector and detector temperatures were 200°C and 3OO”C, respectively. The carrier gas used was helium, flowing at 20 ml min-‘. Sample volumes for analysis were 0.5 ml. Calibration of the gas chromatograph was done by injecting standard solutions containing known amounts of the components of interest in order to obtain the peak areas for these components. The response factor of a component i in a standard sample was calculated from the corresponding peak area as follows: J;. = Cl-%,
(3)
where5 is the response factor of component i in the standard sample, Ci is the concentration of component i in the standard sample (mass fraction) and A,., is the peak area of component i in the standard sample. Each standard sample was injected three times and an average response factor was calculated for each component. The computed response factors were then used to calculate the concentration of the components in the equation was unknown phase. For the raffinate phase, the following used: (4) where X, is the concentration of component i in the raffinate phase sample (mass fraction) and A, is the peak area of component i in the raffinate phase sample. The concentration of a component i in the extract phase, Y,, was determined by a similar procedure.
RESULTS
Triethylene
glycol-
toluene - heptane system
Equilibrium data obtained for this system at 25°C using refractometry are given in Table 3, Parts a and b. Part a contains the phase analysis data. Part b shows the estimated equilibrium constants of toluene and heptane and the solvent selectivity towards toluene with respect to heptane. The error in the solute balance was less than 2%. The selectivity increased from 37.24 to 85.56, corresponding to a solvent capacity increase of 0.06 to 0.1677. The table indicates that this solvent, although very highly selective, has a very small capacity for extraction of aromatics from paraffins.
356 TABLE
A.B.S.H.
Salem 1 Fluid Phase Equilibria
86 (1993) 351-361
3
Equilibrium (a) Tie-line 3-component TEG (%)
48.544 47.619 46.729 45.872 45.045
data for the system triethylene data
W)
2.913 4.762 6.542 8.257 9.910
(b) Equilibrium to heptane
YT
Percentage
system Toluene
Heptane (%I)
Heptane
48.544 47.619 46.729 45.872 45.045 constants
XT
glycol-toluene-heptane
and selectivity
KyS
of toluene phase
fraction
0.003 0.007 0.014 0.021 0.026
0.050 0.080 0.110 0.140 0.155
0.0600 0.0875 0.1273 0.1500 0.1677
Propylene
carbonate
systems
in TEG phase
Xr (%)
R,
Y, (%)
R,
5 8 11 14 15.5
1.3932 1.3960 1.3988 1.4010 1.4030
0.3 0.7 1.4 2.1 2.6
1.4560
of triethylene
YH
glycol towards
X”
1.4562 1.4566 1.4569
1.4572 toluene
with respect
S+
KH+
H
H
T
fraction
at 25°C
fraction
fraction
0.00152 0.00155 0.00159 0.00163 0.00166
0.94553 0.91840 0.88955 0.86685 0.84620
0.001611 0.001684 0.001792 0.001879 0.001960
37.240 51.960 7 1.040 79.830 85.560
Propylene carbonate- toluene -heptane system Data obtained for this system at 25°C are given in Table 4. In part a of this table, data for six samples of mixtures of different compositions (amounts of extract and raffinate phases and percentage error data) are given. The error in these measurements was less than 1% as shown in the table. The toluene and heptane concentrations in the extract and raffinate phases, Y,, XT, Y, and X,, determined are given in part b of this table. Also included are the estimated values of the distribution coefficients of toluene and heptane, KT and KH, and the selectivity of the solvent towards toluene with respect to heptane, S. This table shows that the solvent capacity towards toluene ranges from 0.354 to 0.441 in the range of concentration studies. Correspondingly the selectivity falls from 14 to 5.97.
A.B.S.H. TABLE
Salem 1 Fluid Phase Equilibria 86 (1993) 351-361 4
Equilibrium data for the system 25°C (a) Total mass balance Weight
(g)
PC
T
H
50
5 10 20 30 40 50
50 50 50 50 50 50
50 50 50 50 50
d
o/
0
357
propylene
(PC) -toluene
(T) -heptane
Mixture,
Extract,
Raffinate,
E+R
M (g)
E (g)
R (g)
(9)
105 110 120 130 140 150
50.8987 52.9717 55.3892 57.4600 60.0257 61.8442
53.16790 56.01700 64.15900 72.00900 79.25920 87.09000
104.068 108.898 119.548 129.469 139.285 148.934
Erroi_/-(E+R) M
(b) Equilibrium constants respect to heptane
% Error
(H) at
d
0.8870 0.9190 0.3756 0.4080 0.5100 0.7100
x 100. and
selectivity
K-,=2
YT
carbonate
of propylene
carbonate
towards
toluene
with
Y,
T
fraction
fraction
0.0282 0.0527 0.0918 0.1296 0.1637 0.1918
0.0795 0.1409 0.2437 0.3211 0.3863 0.4346
0.35400 0.37400 0.37700 0.40360 0.42381 0.44130
fraction
fraction
0.0232 0.027 1 0.0293 0.0329 0.0344 0.0400
0.9181 0.8568 0.7506 0.6663 0.5968 0.5411
0.02527 0.03166 0.03908 0.04942 0.05773 0.07396
14.000 11.800 9.650 8.167 7.342 5.970
Comparing Table 4 with Table 3, it can be seen that propylene carbonate has a greater capacity for extracting toluene from heptane than triethylene glycol at the same temperature. Table 4 also shows that propylene carbonate is not as selective as triethylene glycol, and also that selectivity decreases as the percentage aromatic in the feed increases. This is the opposite of the behavior of the triethylene glycol solvent. Propylene
carbonate -o-xylene
- heptane system
Data obtained for this system at 25°C are given in Table 5 parts a and b. The percentage error in the total mass balance of this system was less than 1.5%. The solvent capacity towards o-xylene increased from 0.258 to 0.3373 in the range of concentration studied. Correspondingly the selectivity decreased from 13.4 to 7.22. It can be seen that the behavior of this system is similar to that of toluene.
A.B.S.H.
358 TABLE
5
Equilibrium data for the system (a) Total mass balance Weight
(g)
PC
o-x
Hep.
50 50 50 50 50 50
5 10 20 30 40 50
50 50 50 50 50 50
a o/
0
Salem 1 Fluid Phase Equilibria 86 (1993) 351-361
Error
=
(b) Equilibrium constants respect to heptane
.,
Yo-x
A,-X
fraction
fraction
0.019892 0.038485 0.067701 0.102553 0.131847 0.152246
0.07713 0.14275 0.25414 0.33656 0.40042 0.45133
carbonate-o-xylene-heptane
at 25°C
Mixture,
Extract,
Raffinate,
E+R
M (g)
E (g)
R (g)
(g)
105 110 120 130 140 150
51.1100 52.0500 54.4800 55.7300 56.5300 57.8700
52.55000 56.31000 64.3 1000 72.98000 8 1.82000 90.90000
103.660 108.360 118.790 128.710 138.350 148.770
M - (E+ R) x A4
propylene
% Error
d
1.2700 1.4900 1.0083 0.9920 1.1800 0.8200
100. and selectivity
Yo-x K 0-X =p x O-X
0.25800 0.26960 0.27000 0.30500 0.32930 0.33730
of propylene
carbonate
Y”
X”
fraction
fraction
0.01776 0.01903 0.01923 0.02425 0.02446 0.02527
0.92230 0.85546 0.74287 0.65956 0.59508 0.54098
towards
0.01926 0.02225 0.02589 0.03676 0.04110 0.04671
o-xylene
with
13.3975 12.1350 10.4300 8.2970 8.0122 7.2200
DISCUSSION
A comparison of the equilibrium and selectivity data obtained in this work with those of other published work using other solvents for the extraction of aromatics from paraffins is given in Table 6. The table shows that some investigators have tried to add percentages of water to the solvent used in order to increase its capacity or selectivity towards the aromatic component. However, different temperatures were used. Graham (1962) used triethylene glycol with 7.7% water at 121°C for the extraction of benzene from heptane. He obtained a reasonable capacity (0.2-0.4) compared with sulfolane, which is considered to be one of the most powerful solvents in the field of aromatics extraction. However, the selectivity obtained was low and decreases from 17 to 2 at higher concentrations. In contrast,
Toluene
Toluene
Sulfolane
Sulfolane + 4% water Diethylene glycol Methyl ether Sulfolane
Triethylene glycol Propylene carbonate Propylene carbonate
6
8 9 10
7
Toluene Toluene o-Xylene
Aromatic
Toluene
Heptane Heptane Heptane
Non-aromatic
Isooctane
Heptane
Heptane
Heptane
Toluene
4
3
Heptane
Heptane
Paraffin
Benzene
Benzene
Triethylene glycol -t 7.1% water Diethylene glycol + 7.7% water Sulfolane
1
2
Aromatic
System
25 25 25
25
25
30
30
30
121
121
Temp. (“C)
0.1-0.2 0.3-0.4 0.2-0.3
0.3-0.6
0.7-0.8
0.3-0.6
0.3-0.5
0.3-0.8
0.1-0.2
0.2-0.4
Solvent capacity
37-85 14-6 13-7
40-15
6-3
88-2.7
23- 19
38-1.7
21-12
17-Z
Selectivity
Hassan & Fahim (1988) This work This work This work
Yorulmaz & Karpuzcu (1985) Hassan & Fahim (1988) Yorulmaz & Karpuzcu (1985) Bottinl (1986)
Graham (1962)
Graham (1962)
Reference
of propylene carbonate solvent performance with other solvents for aromatic extraction from paraffins
Solvent
NO.
Comparison
TABLE 6 -
A.B.S.H.
360
Salem / Fluid Phase Equilibria
86 (1993) 351-361
in the present work, low temperature extraction decreased the capacity to 0.1-0.2 at 25°C but increased the selectivity to an attractive figure of 37-85. However, the increase in selectivity of this solvent at higher solute concentration at such a low temperature may be exceptional. Table 6 also reveals that propylene carbonate has a reasonable capacity (0.3-0.4) which can be compared to that of sulfolane (0.3-0.6). However, the selectivity of sulfolane is almost twice that of propylene carbonate. Work has to be done to find possible additives or conditions to increase the selectivity of this solvent.
CONCLUSIONS
Propylene carbonate is a promising solvent which can be used for extracting aromatics from reformate. It has a reasonable capacity similar to that of solvents commonly used for this purpose. However, its selectivity is not very high. Nevertheless, owing to its relatively low price it may become an alternative to such solvents.
ACKNOWLEDGMENT
During his final year project at the Chemical Engineering Department, Qatar University, Mohamad N. Al-Muthafari was involved in the experimental work of this study. This is acknowledged.
LIST OF SYMBOLS
4 4, Cl
r” rk M PC R R, s TEG X K
peak area of component i in the raffinate phase sample peak area of component i in the standard sample concentration of component i in the standard sample (mass fraction) extract response factor of component i in the standard sample distribution coefficient mixture propylene carbonate raffinate refractive index selectivity triethylene glycol concentration of component i in the raffinate phase (mass fraction) concentration of component i in the extract phase (mass fraction)
A.B.S.H.
Salem
1 Fluid Phase
Equilibria
86 (1993)
351-361
361
Subscripts A, B H o-x T
components heptane 0 -xylene Toluene
REFERENCES Awwad, A.M., Salman, M.A. and Hassan, F.A., 1988. Liquid-liquid equilibria for the ternary system y-butyrolactone-n-heptane-benzene/toluene/~-xylene. J. Chem. Eng. Data, 33: 263-265. Badertscher, D.E., Francis, A.W. and Johnson, G.C., 19.54. U.S. Patent 2, 688, 645, Sept. 7. Bailes, P.J., Hanson, C. and Hughes, M.A., 1976. Liquid-liquid extraction: nonmetallic materials. Chem. Eng., 84: 115-120. Bottinl, S.B., 1986. Liquid-liquid equilibria for the system toluene-iso-octane-diethylene glycol methyl ether. J. Chem. Eng. Data, 31: 84-86. Graham, H.L., 1962. Extraction of benzene with di- and tri-ethylene glycols. J. Chem. Eng. Data, 7(2): 214-217. Hassan, M.S. and Fahim, M.A., 1988. Correlation of phase equilibria of naphtha reformate with sulfolane. J. Chem. Eng. Data, 33: 1622165. Smith, T.E. and Bonner, R.F., 1950. n-Propyl alcohol-n-propyl acetate-water solubility data at 20°C and 35°C. Ind. Eng. Chem., 42(5): 896-898. Voetter, H. and Kosters, W., 1963. The sulfolane extraction process. Paper presented at the International Conference of Solvent Extraction, The Netherlands Section III, Paper II, June 26, 131-145. Yorulmaz, Y. and Karpuzcu, F., 1985. Sulfolane versus diethylene glycol in recovery of aromatics. Chem. Eng. Res. Des., 63: 184-190.