1369
I n d . Eng. Chem. Res. 1989,28, 1369-1374 Hirao, A.; Hamazaki, T.; Terano, T.; Nishikawa, T.; Tamura, A.; Kumagai, A.; Sajiki, J. Lacent 1980, 11, 1132. Ikushima, Y.; Goto, T.; Arai, M. Bull. Chem. SOC. Jpn. 1987, 60, 4145. Ikushima, Y.; Hatakeda, K.; Ito, S.; Saito, N.; Asano, T.; Goto, T. Ind. Eng. Chem. Res. 1988,27, 818. Karger, B. L.; Eon, C.; Synder, L. R. J . Chromatogr. 1978,125, 71. Lawson, D. D. Proceedings of the DOE Chemical/Hydrogen Energy Contractor Review Systems, National Technical Information Service, Springfield, 1978.
Lossonczy, T. 0. Am. J . Clin. Nutr. 1978, 31, 1340. Sako, T.; Yokochi, T.; Sugeta, T.; Nakazawa, N.; Hakuta, T.; Suzuki, 0.; Sato, S.; Yoshitome, H. J . Jpn. Oil Chem. SOC. 1986,35, 463. Sanders, T. A. B.; Younger, K. M. Br. J . Nutr. 1981,45, 613. Schoermakers, P. J.; Billiet, H. A. H.; Tijssen, R.; de Galan, L. J . Chromatogr. 1978, 149, 519. Yamaguchi, K.; Murakami, M. J. Jpn. Oil Chem. SOC.1986,35,260.
Received for review December 28, 1988 Accepted J u n e 12, 1989
Liquid-Liquid Equilibrium of Sulfolane-Benzene-Pentane and Sulfolane-Toluene-Pentane George W. Cassell,+ Nilufer Dural, and Anthony L. Hines* Chemical Engineering Department, University of Missouri, Columbia, Missouri 6521I
Ternary liquid-liquid equilibrium data for the systems sulfolane-toluene-pentane and sulfolanebenzene-pentane were obtained a t 17, 25, and 50 "C. Experimental tie line data were measured by gas chromtographic analyses and correlated with the nonrandom two liquid (NRTL) and universal quasi-chemical (UNIQUAC) equations. Ternary liquid-liquid equilibrium data are essential for the design of liquid-liquid extraction processes and for the selection of solvents. Sulfolane (tetramethylene sulfone) is widely used as a solvent in the recovery of high-purity aromatics, such as benzene, toluene, and xylenes, from refinery process streams (Deal et al., 1959; Voetter and Kosters, 1966; Broughton and Asselin, 1967). Although relatively little quantitative phase equilibrium data on sulfolane have been published (Hartwig et al., 1955; Hanson et al., 1969; Tripathi et al., 1975; De Fre and Verhoeye, 1976; Rawat and Gulati, 1976; Ashcroft et al., 1982; Mukhopadyay and Dongaonkar, 1983; Hassan et al., 1988),some ternary systems containing sulfolane have been reported in the data collection of Sorensen and Arlt (1980), including tables of experimental tie lines, diagrams of these tie lines with corresponding predictions of phase envelopes, and distribution coefficients. In the present work, two new systems involving sulfolane have been investigated: sulfolane-toluene-pentane and sulfolane-benzene-pentane. The experimental equilibrium data were collected at 17, 25, and 50 "C.
Experimental Section Materials. Benzene and toluene were obtained from the J. T. Baker Chemical Co., and pentane and sulfolane were obtained from Burdick & Jackson Laboratory and from Alfa Products, respectively. Different purification methods were utilized to purify the chemicals used. Recrystallization was used exclusively for sulfolane since its melting point is 27 "C and because heating for distillation caused a discoloration. Recrystallization was carried out three times by heating the sulfolane to a temperature of 50 "C and then allowing it to cool slowly until approximately 90% had solidified; the remaining 10% was discarded. The other chemicals were purified with a Bughi Rotavapor-R rotary evaporator operated a t atmospheric pressure. The water bath used to heat the rotary evaporator was controlled to produce a slow evaporation rate, generating 400 mL/h of final product. A heart cut was
* To whom correspondence should be directed. 'Current address: Conoco Oil Co., Ponca City, OK 74602.
0888-5885/89/2628-1369$01.50/0
collected by discarding approximately the first 10% of the distillate and the last 10% residual. The chemicals were injected into the packed column gas chromatograph before and after purification to determine their purities. All of the chemicals used in this study had purities greater than 99.9% after the purification process. Data Collection. The ternary LLE data for the systems investigated were obtained by using a gas chromatograph, and the size of the binodal curves was predicted by a cloud point determination (Tripathi et al., 1975). In order to analyze the sample mixtures with the gas chromatograph, it was necessary to dilute the mixtures with an appropriate solvent. Among the several solvents examined (methanol, butanol, methylene chloride, acetone, carbon tetrachloride, carbon disulfide, freon, and pyridine), carbon disulfide was selected for several reasons. First, the flame ionization detector sensitivity to carbon disulfide is very low, and if the concentration ranges were carefully controlled, carbon disulfide does not cause peak interference with most of the chemicals studied. Second, carbon disulfide is inexpensive and easy to handle compared with other solvents. It is available in a relatively pure form and can be further purified quite easily by distillation. Since detector sensitivity to some components was nonlinear, standards were prepared over the same concentration ranges as the samples. The first standard concentration was estimated by using an area analysis produced from the gas chromatographic test analysis. Once a few points on the binodal curve were determined, a new standard concentration was calculated from the trends present. Sample mixtures were prepared within the two-phase region using Supelco sample bottles with Teflon-lined septum caps. The samples were then placed in a shaker bath and brought to equilibrium a t the specified temperature (f0.2 "C, as indicated by a digital thermometer). Batch studies carried out in this work and by others (Tripathi et al., 1975) indicated that the equilibrium was reached within 30-45 min. However, the mixtures were left in the shaker bath about 24 h before sampling in order to ensure that equilibrium was obtained. After attainment of equilibrium, the agitation of the samples were stopped to allow proper phase separation prior to sampling. Both 0 1989 American Chemical Society
1370 Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 Table I. Tie Line Data for the Sulfolane-Toluene-Pentane System at 17, 25, and 50 "C phase rich in sulfolane, wt %
temp, "C 17
pentane 1.11 1.36 1.62 1.72 2.31 2.85 3.70 4.16 6.57 1.30 1.30 1.67 1.84 2.31 2.58 2.32 4.57 5.80 3.61 3.55 3.30 3.46 3.01 2.88 3.25 7.64
25
50
70
toluene 0 3.09 6.39 10.30 14.55 19.99 28.46 34.24 42.57
sulfolane 98.89 95.55 91.99 87.98 83.14 77.16 67.84 61.60 50.86 98.70 95.32 91.36 87.34 79.61 77.88 81.48 66.30 58.78 96.40 92.79 90.43 85.87 78.98 77.69 80.72 60.80
0
3.38 6.98 10.82 18.07 19.53 16.20 29.12 35.42 0 3.66 6.27 10.67 18.01 19.43 16.03 31.56
1
0 0
A
phase rich in pentane, w t % pentane 99.94 88.53 79.29 67.27 57.04 48.74 31.35 27.15 19.95 99.72 86.26 75.14 62.21 47.11 46.27 52.52 32.10 24.90 99.56 87.18 80.09 68.53 49.92 46.57 55.15 30.51
toluene 0 11.08 20.02 31.28 40.98 48.15 61.43 61.82 58.94 0
13.31 23.94 36.27 49.73 50.26 45.06 59.39 61.42 0 12.23 18.84 28.99 45.03 46.31 41.69 58.02
sulfolane 0.06 0.39 0.69 1.45 1.98 3.12 7.23 11.03 21.10 0.28 0.43 0.92 1.52 3.16 3.47 2.43 8.51 13.69 0.44 0.59 1.07 2.49 5.06 7.11 3.16 11.47
-r=170C T=25"C T=50°C
70
selectivity
distribution coeff pentane 89.95 65.34 48.86 39.06 24.65 17.10 8.47 6.53 4.05 76.70 66.41 45.27 33.74 20.36 17.91 22.67 7.02 4.29 27.62 24.56 24.29 20.95 16.61 16.18 16.96 4.00
toluene
toluene
3.58 3.13 3.04 2.82 2.41 2.16 1.81 1.44
18.25 15.61 12.85 8.74 7.10 3.92 3.61 2.81
3.94 3.43 3.35 2.75 2.57 2.78 2.04 1.73
16.86 13.20 10.07 7.40 6.97 8.15 3.44 2.48
3.34 3.00 2.34 2.50 2.38 2.60 1.84
7.35 8.10 8.95 6.64 6.80 6.52 2.17
c
I
0
T=17'C
A
T=25%2 T=50"C
60 al
50 C
-m
0 40 C
al
2
E
30
.u
5 0,
z.-
20
10
$
, 20
I
40
I
I
I
I
1
I
6C
Weight Percent of Sulfolane Figure 1. Ternary LLE phase diagrams for sulfolane-toluenepentane a t 17, 25, and 50 "C.
Weight Percent of Sulfolane Figure 2. Ternary LLE phase diagrams for sulfolane-benzenepentane a t 17, 25, and 50 "C.
top and bottom phases were sampled with a 0-50-pL syringe. To prepare the samples for gas chromatographic analysis, a sample of 25 p L was injected into a '/2-dram Supelco sample bottle which contained 1.5 mL of carbon disulfide solvent. Gas chromatography was used to determine tie line data for this study, employing two Hewlett-Packard gas chromatographs. The first, a Model 5793 packed column gas chromatograph, was used with a 1.5-m-long, 3-mm-diameter stainless steel column with 2% OV-101 methylsilicone fluid on a Chromasorb W-HP 100/120 Hewlett-Packard packing. The second gas chromatograph employed was a Model 5792 capillary column chromatograph. The column used for this chromatograph contained cross-linked methylsilicone and was constructed of fused silica, 12 m
long, with an inside diameter of 6 mm (Hewlett-Packard column). Both instruments employed a single-channel flame ionization detector and a Hewlett-Packard Model 3390 integrator. The chromatographs were operated in a temperature-programmed mode to reduce the run time and obtain well-defined peaks. For each sample, three analyses were performed to obtain the mean value. The average absolute deviation from the mean value was estimated to be 0.05 wt%. The chromatographic method was used because impurities could be readily detected. Results and Discussion The ternary liquid-liquid equilibrium data for the sulfolane-toluene-pentane and sulfolane-benzene-pentane systems were measured at 17, 25, and 50 "C and are
Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 1371 Table 11. Tie Line Data for the Sulfolane-Benzene-Pentane System at 17,25, and 50 OC temp, "C 17
25
50
phase rich in sulfolane, wt % pentane benzene sulfolane 0.88 0 99.12 0.95 4.75 94.29 1.09 9.32 89.60 1.30 14.75 83.95 21.34 77.16 1.50 1.50 23.19 75.31 1.67 28.21 70.13 2.66 31.35 66.00 4.33 39.29 56.39 10.37 47.16 42.48 0.88 0 99.12 1.10 94.11 4.79 1.12 9.72 89.15 1.78 82.39 15.83 2.06 76.46 21.48 2.13 74.83 23.04 2.50 68.49 29.01 2.91 64.70 32.40 4.18 56.32 39.50 1.12 0 98.89 1.28 5.05 93.67 1.22 9.45 89.33 82.41 1.55 16.04 77.64 2.26 20.10 74.08 2.65 23.26 66.99 2.65 30.37 3.21 34.04 62.75 56.23 4.08 39.70
phase rich in pentane, wt % pentane benzene sulfolane 99.34 0 0.66 0.77 89.70 9.53 19.01 1.18 79.81 28.07 1.79 70.14 36.40 2.80 60.80 38.42 3.74 57.83 45.71 4.75 49.54 50.37 6.28 43.35 58.31 8.46 33.24 58.79 14.87 26.34 0 99.29 0.71 89.78 0.93 9.29 1.14 80.50 18.36 67.92 2.10 29.98 2.47 36.50 61.03 57.90 3.50 38.60 47.22 5.28 47.50 40.90 6.41 52.69 31.67 57.12 11.21 0.71 99.29 0 0.86 10.50 88.64 1.44 19.32 79.24 2.46 28.87 67.67 60.74 3.79 35.46 55.79 5.54 38.67 46.04 6.04 47.92 8.44 38.82 52.74 29.88 14.10 56.02
presented in Tables I and 11, respectively. To demonstrate the consistency of these data, the binodal curves for different temperatures are presented on a single graph for each system, and the results are presented in Figures 1and 2. To predict ternary liquid-liquid equilibrium, it is necessary to have a method of predicting activity coefficients. Regular solution theory (Hildebrand et al., 1970) offers a model to determine the activity coefficients without experimental data. However, this theory does not apply to polar substances and therefore is not applicable to liquid-liquid equilibrium. Pierotti et al. (1959) correlated the molecular structure of components with infinite dilution activity coefficients for binary systems. However, for multicomponent liquid-liquid systems, infinite dilution activity coefficients cannot be used to make predictions without a serious loss of accuracy. The best methods for predicting liquid-liquid equilibrium without the direct use of experimental data are the group contribution models, such as the analytical solution of groups (ASOG) of Derr and Deal (1969) and the UNIQUAC functional-group activity coefficients (UNIFAC) of Fredenslund et al. (1975). Three of the more successful methods for predicting excess Gibbs energy are the Wilson, NRTL, and UNIQUAC models proposed by Wilson (19641, Renon and Prausnitz (1968), and Abrams and Prausnitz (1975), respectively. These three models utilize the local composition concept introduced by Wilson (1964). On a microscopic scale, the local composition concept suggests that the composition changes from point to point, which indicates that a structural pattern exists in the molecular configuration rather than random mixing. The NRTL and UNIQUAC models were used to correlate and predict the LLE data in the present work. The NRTL equation proposed by Renon and Prausnitz (1968) is an empirical model that is used to predict the excess Gibbs free energy. This model is an improvement over the Wilson (1964) equation for the simultaneous representation of vapor-liquid equilibria, liquid-liquid equilibria, and
distribution coeff pentane benzene 112.63 94.02 2.01 73.35 2.04 1.90 54.08 1.71 40.45 1.66 38.48 1.62 29.68 1.61 16.27 1.48 7.68 1.25 3.03 112.83 81.99 1.94 71.69 1.89 38.26 1.89 29.57 1.70 27.16 1.68 18.93 1.64 14.07 1.63 1.45 7.58 89.05 2.08 69.25 2.05 64.74 1.86 43.55 1.76 26.90 1.66 21.04 1.58 17.39 1.55 12.10 7.35 1.41
selectivity pentane/ benzene 46.78 35.96 28.46 23.65 23.18 18.32 10.11 5.19 2.66 42.26 37.93 20.24 17.39 16.17 11.54 8.63 5.23 33.29 31.58 23.41 15.28 12.67 11.01 7.81 5.21
T:25'C .Experimental aUNlQUAC Plait Point
&? E
3
0
20 40 60 80 Weight Percent of Sulfolane
Figure 3. Comparison of sulfolane-toluene-pentane data with predicted phase diagrams at 25 "C.
I
100
experimental
limiting activity coefficients in binary multicomponent mixtures. It is, however, a three-parameter equation that makes use of the local composition concept developed by Wilson (1964) and Scott's two-liquid theory (Scott, 1956). The local mole fraction concept is modified by introducing the nonrandomness parameter, a. The nonrandomness parameter is related inversely to the coordination number of the liquid by Guggenheim's quasi-chemical theory (1952). The multicomponent form of the NRTL equation is given by M
k=l
where
and
Gji = e x p ( - c ~ ~ ~ ~ ~ J
1372 Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 Table 111. Specific Interaction and Structural Parameters for the Sulfolane-Toluene-Pentane System componentn UNIQUAC NRTL (CY = 0.2) temp, O C i i k A,, A Rb Qb A,, A,, -301.42 425.95 3.83 3.32 136.58 -112.55 17 1 2 1 792.26 1879.50 3.92 2.97 547.66 98.23 1 3 2 -74.46 4.04 3.20 546.63 137.08 -8.27 2 3 3 -351.24 388.77 3.83 3.32 128.42 -134.88 25 1 2 1 786.34 3.92 2.97 1890.30 577.41 86.01 1 3 2 4.04 3.20 -93.18 567.22 147.99 -20.61 2 3 3 -297.94 3.83 3.32 386.22 104.71 -97.65 50 1 2 1 975.22 1790.80 3.92 2.97 521.08 54.38 1 3 2 -69.10 505.26 4.04 3.20 177.26 -24.63 2 3 3
SI,
SN
0.84
0.80
0.43
0.36
0.73
1.36
,I
(1
1, pentane; 2, toluene, 3, sulfolane.
Table IV. Specific Interaction and Structural Parameters for the Sulfolane-Benzene-Pentane System componento UNIQUAC NRTL (a = 0.2) temp, "C i j k AI, 4 1 Rk Qk All All 17 1 2 1 282.73 -190.83 3.83 3.32 172.62 -102.70 1 3 2 538.68 185.95 3.19 2.40 1511.90 832.47 2 3 3 141.47 -50.98 4.04 3.20 674.51 -268.03 25 1 2 1 295.38 -171.63 3.83 3.32 140.29 53.03 1 3 2 532.04 187.84 3.19 2.40 1338.20 955.24 -188.83 3.20 618.10 -36.08 4.04 3 151.02 2 3 50 1 2 1 179.86 -95.70 3.83 3.32 65.42 147.67 1254.40 10007.80 3.19 2.40 2 375.93 247.77 1 3 -162.77 3.20 593.73 -6.28 4.04 3 131.51 2 3
SU
SN
0.82
0.74
0.61
0.42
0.53
0.52
1, pentane; 2, benzene; 3, sulfolane.
In the development of the UNIQUAC equation proposed by Abrams and Prausnitz (1975), the local composition concept introduced by Wilson (1964) is applied to a modified form of Guggenheim's quasi-chemical theory (1952). For multicomponent systems, the UNIQUAC equation is given as follows:
c
801
i
3
T:25'C
.Experimental
AUNIQUAC Plait Point
60 c
,:--NRTL Model
1
C
40
2 E .$ 20 3
ob
0
where (5)
and
(7)
Although only two adjustable parameters are required for each binary, the UNIQUAC equation is applicable to a wide range of nonideal mixtures. Unlike some other methods available, it is based on structural parameters and can be used with polymer solutions. These models are significantly different, but both may be applied to the same types of systems. Interaction parameters for specific systems are obtained by fitting the equilibrium data to the model. Compositions are predicted from the NRTL and UNIQUAC models and are compared with the experimental data for the systems studied in Figures 3 and 4. The corresponding standard deviations and the specific model parameters for both models are presented in Tables
I
20
'
'
40
'
'
60
'
'
80
'
-. I
1
1 0 Weight Percent of Sulfolane Figure 4. Comparison of sulfolane-benzene-pentane experimental data with predicted phase diagrams a t 25 "C.
111 and IV. It is important to note, however, that the parameters obtained by correlating the data are not unique. Thus, care must be taken in interpreting the significance of the curve fit parameters. The third NRTL parameter ( a )was set equal to 0.2 for the systems studied. This is a reasonable assumption considering the strong relationship of a to the inverse of the coordination number. Also in the original presentation of this theory by Renon and Prausnitz (1968), it is stated that a is a positive constant of the order 0.1-0.3. They found a good fit, however, to the data by using values for a that ranged from 0.2 to 0.5. A modified version of the computer program written by Sorensen and Arlt (1980) was utilized to find the six interaction parameters that best fit the data. The best fit was obtained by adjusting the model parameters to minimize an objective function. For this purpose, an activity and concentration objective function, which was essentially a combination of the two traditional methods, was employed. This type of application enables one to overcome the disadvantage of using only an activity objective function or a concentration objective function. The method involves minimizing the concentration objective function in such a way that the predicted concentrations
Ind. Eng. Chem. Res., Vol. 28, No. 9, 1989 1373 are consistent with the isoactivity criteria. For a specific model, the activities of each component in both phases are calculated based on experimental concentrations. Since the intent is to determine an activity that is consistent with the concentration objective function by changing the appropriate concentrations by the smallest amount possible, the derivatives of the activity with respect to concentration are examined. The procedure is simplified by choosing an appropriate activity at its original calculated value and adjusting the calculated concentrations. In order to do this under the constraints of the concentration objective function, the derivative of the activity with respect to concentration is examined. The activity whose derivative has the minimum magnitude is chosen as the appropriate activity for each tie line. Once the calculated tie lines have been determined from experimental data, the concentration objective function is then used in the traditional manner. The program calculates a set of tie lines based on either the NRTL or the UNIQUAC equations for a given set of interaction parameters. The modification in the program consists of incorporating experimental data and comparing these data to predicted values in order to obtain the appropriate model parameters. The first parameter estimation step involves comparing experimental data to predicted values by linear interpolating between tie lines. Once the parameters were obtained in the neighborhood of the desired best fit, the experimental data were used directly as an initial guess to calculate a tie line. The concentration of each component was adjusted based on the relative size of the derivative of the activity with respect to concentration. The final parameter adjustment was based on this step and was obtained by minimizing the objective function given by N
2
M
C C C Wilm(Cjlm i=ll=lm=l
except for the sulfolanetoluene-pentane system at 50 "C (Tables I11 and IV). This suggests that the NRTL equation yields better results for the systems investigated, but either model adequately predicts the LLE behavior. Nomenclature A,, Aji = specific interaction parameters, K C = concentration gE = molar excess Gibbs energy G j j = empirical constant M = number of components N = number of molecules q = pure component area parameter Q k = UNIQUAC structural parameter, K r = pure component volume parameter R = gas constant Rk = UNIQUAC structural parameter, K SN= standard deviation corresponding to NRTL correlation Su = standard deviation corresponding to UNIQUAC correlation T = temperature Uij= energy characteristicof interactions between lattice sites W = weighting factor xi = mole fraction of component i Greek Letters aij = parameter in NRTL equation aj = segment fraction Oi = area fraction 7
= empirical constant
o = coordination number Superscripts
E = excess = calculated
i = 1,2,3,...,N (components), 1 = 1,2 (phases), m = 1,2,3,...,M (tie lines)
Subscripts i, j , k = components 1 = phases m = tie lines
where C is the experimental weight or mole percent, 6 is the calculated weight or mole percent, and W is 1 for all tie lines except the last one. For this tie line, W is 0.5. The weighting function W was set at a different value for the last tie line because of instability near the plait point. The degree of this instability was estimated based on the degree of difficulty required to obtain these tie lines. In its correlated form, the data obtained in the present work represent both the particular measured tie lines and the entire binodal curve. The consistency of the experimental data obtained can be best observed in Figures 1 and 2 where the binodal curves of the systems studied at different temperatures are presented on a single graph to illustrate the temperature effect on the LLE data for these systems. For both systems, the influence of temperature decreased as the temperature increased. The temperature dependency of the specific parameters can be observed in Tables I11 and IV. The random behavior of these parameters suggests that they are weak functions of temperature. Although the models employed in the present work do not provide an adequate test for the true temperature behavior, the existing data are all theoretically consistent. In the vicinity of the plait point where the system is highly unstable, however, the differences between the experimental data and predicted values are greater than the deviations observed in other regions, which are approximately 1.0% by weight. The standard deviations corresponding to the UNIQUAC equation have been found to be greater than the ones corresponding to the NRTL equation for all cases,
Abrams, D. S.; Prausnitz, J. M. Statistical Thermodynamics of Liquid Mixtures: A New Expression for the Excess Gibbs Energy of Partly or Completely Miscible Systems. AZChE J . 1975, 21, 116-128. Ashcroft, S. J.; Clayton, A. D.; Shearn, R. B. Liquid-Liquid Equilibria for Three Ternary and Six Quaternary Systems Containing Sulfolane, n-Heptane, Toluene, 2-Propanol, and Water a t 303.15K. J . Chem. Eng. Data 1982, 27, 148-151. Broughton, D. B.; Asselin, G. F. Production of High Purity Aromatics by the Sulfolane Process. World Pet. Congr., Proc., 7th 1967, 4, 65-73. Deal, C. H., Jr.; Evans, H. D.; Oliver, E. D.; Papadoupoulos, M. N. A Better Way to Extract Aromatics. Pet. Refin. Congr. 1959,38, 185-192. De Fre, R. M.; Verhoeye, L. A. Phase Equilibria in Systems Composed of an Aliphatic and an Aromatic Hydrocarbon and Sulfolane. J . Appl. Chem. Biotechnol. 1976, 26, 469-487. Derr, E. L.; Deal, C. H. Analytical Solutions of Groups: Correlation of Activity Coefficients through Structural Group Parameters. Inst. Chem. Eng., Symp. Ser. 1969, 32, 3-40. Fredenslund, A.; Jones, R. L.; Prausnitz, J. M. Group-Contribution Estimation of Activity Coefficients in Nonideal Liquid Mixtures. AIChE J . 1975, 1086-1099. Guggenheim, E. A. Mixtures; Clarendon Press: London, 1952. Hanson, C.; Patel, A. N.; Chang-Kakuti, D. K. Separation of Thiophen from Benzene by Solvent Extraction. J. Appl. Chem. 1969, 19, 320-323. Hartwig, G. M.; Hood, G. C.; Maycock, R. L. Quaternary Liquid Systems with Three Liquid Phases. J . Phys. Chem. 1955, 59, 52-54. Hassan, M. S.; Fahim, M. A.; Mumford, C. J. Correlation of Phase Equilibria of Naphtha Reformate with Sulfolane. J. Chem. Eng. Data 1988, 33, 162-165.
- @ilm12
(8)
Literature Cited
1374
I n d . Eng. Chem. Res. 1989, 28, 1374-1379
Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L. Regular and Related Solutions; Van Nostrand Reinhold: New York, 1970. Mukhopadyay, M.; Dongaonkar, K. R. Prediction of Liquid-Liquid Equilibria in Multicomponent Aromatics Extraction Systems by Use of the UNIFAC Group Contribution Model. Ind. Eng. Chem. Process Des. Dev. 1983, 22, 521-532. Pierotti, G. J.; Deal, C. H.; Derr, E. L. Activity Coefficients and Molecular Structure. Ind. Eng. Chem. 1959,51, 95. Rawat, B. S.; Gulati, I. B. Liquid-Liquid Equilibrium Studies for Separation of Aromatics. J . Appl. Chem. Biotechnol. 1976, 26, 425-435. Renon, H.; Prausnitz, J. M. Local Compositions in Thermodynamic Excess Functions for Liquid Mixtures. AIChE J . 1968, 14, 135-144. Scott, R. L. Corresponding States Treatment of Nonelectrolyte Solutions. J . Chem. Phys. 1956,25, 193-205.
Sorensen, J. M.; Arlt, W. Liquid-liquid Equilibrium Data Collection; DECHEMA Chemistry Data Series; DECHEMA: Frankfurt, 1980; Vol. V, Parts 1-3. Tripathi, R. P.; Raja Ram, A.; Bhimeshwara, Rao. P. Liquid-Liquid Equilibria in Ternary System Toluene-n-Heptane-Sulfolane. J. Chem. Eng. Data 1975,20, 261-264. Voetter, H.; Kosters, W. C. G. New Applications of the Sulfolane Extraction Process and Industrial Experiences with This Process. Erdoel Kohle 1966, 19, 267-271. Wilson, G. M. Vapor-Liquid Equilibrium. XI-A New Expression for the Excess Free Energy of Mixing. J . Am. Chem. SOC.1964,86, 127.
Received for review November 30, 1988 Revised manuscript received April 4, 1989 Accepted June 19, 1989
Temperature-Heat Diagrams for Complex Columns. 1. Intercooled/Interheated Distillation Columns Brenda E. Terranova and Arthur W. Westerberg* Department of Chemical Engineering and Engineering Design Research Center,’ Carnegie-Mellon University, Pittsburgh, Pennsylvania 15213
We show how to establish heating and cooling curves for distillation columns featuring interheating and intercooling. The approach is to construct a diagram that plots column pinch temperatures versus reboiler and condenser duties. No assumptions are needed on ideal behavior. Introduction A distillation column is conveniently thought of as a device to degrade heat to produce separation work. Heat enters into the reboiler of a typical column, the hottest point in the column. It is removed from the condenser, the coldest point in the column. Often in industry, one will see columns in which heat has been removed from or added to a column on an intermediate tray. Figure l a illustrates this. Such heat addition and heat removal are termed interheating and intercooling, respectively. The advantage obtained for using interheating or intercooling is that the heat is added or removed at a temperature that is between the temperatures of the reboiler and condenser for the column. Thus, heat can be added a t a lower temperature than the reboiler temperature or removed at a temperature higher than the condenser temperature, each of which is advantageous from a second-law point of view. Andrecovich and Westerberg (1985) presented a diagram to illustrate the flow of heat in a column on a temperature versus heat (T-Q)diagram. The axes for this diagram are the same as used in heat-exchanger network synthesis calculations (Hohmann, 1971; Linnhoff et al., 19821, allowing this very useful “cascade“ representation to be used on the same diagram as one would use to design heat-exchanger networks. Figure l b illustrates such a diagram for an interheated/intercooled column. The top of the area representing the column corresponds to heat entering the column at two temperatures: the hotter is the reboiler and the other is the interheater. Similarly, heat is shown being removed at two temperatures: from the colder condenser temperature and from the intercooler. The main question to be answered in this paper is how does one construct this cascade diagram for a column in
* Author
t o whom correspondence should be addressed.
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which there is interheating and intercooling taking place; i.e., how does one construct Figure I b given the column in Figure la? Temperature Profiles for Heating and Cooling. The temperatures for the heat entering ( Q R and Qk) and leaving (Qc and Q’c) are shown here to be constant. Since the material being vaporized or condensed will generally be a mixture, the temperature can in fact vary from the dew point to bubble point for condensing and the reverse for vaporizing. Often condensing or reboiling is carried out by having the liquid mixture in a pool on the shell side covering the tubes of the heat exchanger. In this case, the temperature is constant and is the bubble point for the liquid mixture in the pool. For the top and bottom of a column having a total condenser (not uncommon) and a partial reboiler (a little thought will show this to be the only reasonable type of reboiler), the temperature is the bubble point of the respective products. In both cases, the temperature is the bubble point, and our sketch is appropriate. For interheating or intercooling, one can imagine coils being placed in the trays of the column to effect the heat addition or removal; the heating and cooling would be at the bubble point of the liquid on the tray, again a constant temperature. For external heat removal from a tray, the temperature profile would be from the dew point to the bubble point. The bubble point is a safe “lowest” temperature at which to assume the heat is removed. For external heat addition to a tray, the liquid would likely be circulated and be on the shell side in a pool covering the tubes. Only a small fraction of the liquid would vaporize with each pass of the liquid and would again vaporize at essentially the bubble point temperature of the liquid on the tray. Finally, the diagram can easily be modified if desired to show a temperature range for any heat transfer. Approximately Equal Heats for Base Case. Andrecovich and Westerberg (1985) argue that the amount (C 1989 American
Chemical Society
