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Ind. Eng. Chem. Res. 2001, 40, 4596-4602
Separation of Closely Boiling Compounds of Catechol and 4-Methoxyphenol with the Aid of 1,4-Butanediol Ming-Jer Lee, Fu-Li Wu, and Ho-mu Lin* Department of Chemical Engineering, National Taiwan University of Science and Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan
Solid-liquid equilibrium data were measured for two binary systems of 1,4-butanediol with closely boiling compounds: catechol and 4-methoxyphenol. The experimental results revealed that a 1:1 adducted compound formed in 1,4-butanediol + catechol, whereas 1,4-butanediol + 4-methoxyphenol remained a simple eutectic system. The eutectic loci were further investigated for the mixture of 1,4-butanediol + catechol + 4-methoxyphenol, from which a ternary solidliquid equilibrium phase diagram was obtained. A feasible separation sequence, on the basis of the phase diagram, was then proposed to separate the eutectic mixture of 4-methoxyphenol + catechol by means of extractive crystallization with 1,4-butanediol as an auxiliary agent. Both the ideal-chemical model and the regular solution-chemical model were applied to correlate the experimental data of the binary systems. The regular solution-chemical model with the determined parameters predicts the equilibrium temperatures of the ternary system to a grand average absolute deviation of 1.24%. Introduction Separation of catechol (Tb ) 518.15 K) and 4-methoxyphenol (Tb ) 516.15 K) is needed in the process of manufacturing methoxyphenols. Conventional distillation appears to be economically infeasible to separate such closely boiling compounds. Extractive or adductive crystallization, among others, can be advantageous. In the use of this alternative technique, the separation sequence is governed mainly by the phase equilibrium behavior of the related mixtures. Measurements of phase equilibrium properties, therefore, play an important role in the development and design of the processes. Lee et al.1 reported the phase diagram of solid-liquid equilibrium (SLE) for 4-methoxyphenol + catechol as shown in Figure 1. In the present study, the SLE behavior was investigated for catechol and 4-methoxyphenol in the presence of 1,4-butanediol, which is a potential auxiliary agent to separate the eutectic mixture of the closely boiling compounds catechol and 4-methoxyphenol by means of extractive crystallization. A ternary phase diagram was also prepared from the results of SLE measurements, and subsequently a feasible separation sequence was proposed on the ground of the phase diagram. Lee et al.2 found that a 2:1 adducted compound formed in tert-butyl alcohol + catechol and tert-butyl alcohol + 4-methoxyphenol. Also, a 1:2 complex species exists in the mixtures of ethylenediamine + 4-methoxyphenol and piperazine + 4-methoxyphenol.1 The present work is undertaken to study the phase behaviors of 1,4-butanediol + 4-methoxyphenol, 1,4-butanediol + catechol, and 1,4-butanediol + 4-methoxyphenol + catechol. The SLE data of these systems are not available in the literature at comparable conditions. A physical-chemical treatment is essential to represent the SLE behavior for an adducted system; that is, * Corresponding author. Tel.: +886-2-2737-6643. Fax: +8862-2737-6644. E-mail:
[email protected].
Figure 1. Solid-liquid phase boundary for 4-methoxyphenol (1) + catechol (2).
not only an activity coefficient model but also a complex formation mechanism should be specified. Prior to assuming a reaction mechanism, the molar ratio of dissimilar constituents in the complex species needs to be known. It can be found from the congruent composition of the corresponding binary system. Because an adducted compound forms in the mixtures of 1,4butanediol + catechol + 4-methoxyphenol, both the ideal-chemical model (ICM) and the regular solutionchemical model (RSCM)3,4 were employed to correlate the binary SLE data. These two models with the determined parameters were then applied to predict the SLE boundaries for the ternary system.
10.1021/ie001020c CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001
Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 4597 Table 1. Properties of Pure Compounds Tm (K) substance
this work
literature
1,4-butanediol 292.9 293.055 4-methoxyphenol 328.2 catechol 377.7 377.809
VL at ∆fusH 298.15 K δ [(kJ (kJ mol-1) (cm3 mol-1) cm-3)1/2] 18.706 18.301 22.541
88.9747 84.47a,b 53.95a,b
0.7848 0.901c 1.149c
a Liquid molar volumes were estimated from the modified Rackett equation10 V ) (RTc/Pc)ZRA{1 + [1 - (T/Tc)2/7]} with ZRA ) 0.29056 - 0.08775ω, where Tc ) 756.04 K, Pc ) 51.46 bar, and ω ) 0.5874 for 4-methoxyphenol and Tc ) 764.46 K, Pc ) 75.61 bar, and ω ) 0.6965 for catechol. The critical properties were estimated from the Joback model,11 and the acentric factor ω was computed from the Lee-Kesler equation.11 b At subcooled liquid state. c The heat of vaporization (∆vapH) was calculated from a tworeference-fluid method,12 and the internal energy change of vaporization (∆vapU) was estimated from ∆vapH - RT.
Experimental Method 4-Methoxyphenol (99+%) and catechol (99+%) were purchased from Merck (Germany), and 1,4-butanediol (99+%) was supplied by Aldrich (Milwaukee, WI). Analysis by gas chromatography confirmed the claimed purity of the chemicals. All of these substances were used without further purification. Table 1 lists the properties of the chemicals. The SLE data were measured in the present study by a solid-disappearance method.4,13 Each mixture (about 3 g) was prepared by weighing pure compounds to (0.1 mg. The accuracy of the sample composition was about (0.0002 mole fraction. A small amount of homogenized sample (about 0.5 g) was transferred and sealed in a 1-mL glass syringe. The advantages of using the syringe over a glass vial are better heat transfer and smaller free space for sample vaporization. The syringe was immersed in a cold bath to crystallize the liquid sample. The solidified sample was then shaken vigorously in a visual thermostated bath (Neslab, TV-4000, stability ) (0.03 K, operable from room temperature up to 503 K) for observation of solid disappearance at a fixed temperature. The bath temperature was elevated by a tiny increment each time, if the solid still existed after about a 30-min observation. The increment was as small as 0.1 K, when the temperature is near that of solid disappearance. To operate the bath at temperatures lower than room temperature, an external refrigeration circulator (Neslab, RTE-110, stability ) (0.01 K) was connected. The bath temperature was measured by a Hart Scientific Microtherm (model 1506) with a thermistor probe to (0.015 K. The uncertainty of the reported solid-disappearance temperatures was about (0.2 K under normal experimental conditions and (0.5 K around the vicinity of the eutectic point. It was estimated by repeatedly measuring the disapperance temperatures of the same sample. Experimental Results and Discussion Binary Systems. The experimental results of 1,4butanediol + 4-methoxyphenol and 1,4-butanediol + catechol are given in Table 2. Figure 2 presents the SLE phase diagram of 1,4-butanediol + 4-methoxyphenol, indicating that this binary is a simple eutectic system. Two eutectic points and one congruent point are exhibited in Figure 3 for 1,4-butanediol + catechol. The congruent point is very close to the first eutectic point; the existence of the congruently melting compound can
Figure 2. Solid-liquid phase boundary for 1,4-butanediol (1) + 4-methoxyphenol (2). Table 2. SLE for 1,4-Butanediol Binary Systems x1
T (K)
x1
T (K)
0.1597 0.2529 0.3246 0.3642 0.4585 0.4858 0.5032
1,4-Butanediol (1) + 4-Methoxyphenol (2) 317.8 0.5490 311.1 0.5975 304.9 0.6183 300.8 0.6408 290.5 0.6732 286.8 0.8008 285.0 0.8949
279.3 271.3 270.2 272.7 274.5 282.7 288.6
0.3022 0.3453 0.4521 0.4814 0.4917 0.4936 0.5011 0.5236 0.5477
1,4-Butanediol (1) + Catechol (2) 345.2 0.5660 338.2 0.5930 318.0 0.7466 310.0 0.7692 306.1 0.7836 300.9 0.7880 301.8 0.8501 301.1 0.9001 300.7 0.9479
300.3 298.3 285.6 281.2 280.0 280.6 283.5 286.2 290.4
further be proved by the experimental results of the ternary system. The locations of the eutectic and the congruent points for these two systems are given in Table 3 which were determined by interpolation from the experimental SLE data. The congruent composition of 1,4-butanediol + catechol is about 0.5 mole fraction of catechol, implying that a 1:1 adducted compound, 1,4butanediol-catechol, forms in this binary system. The adducted compound formation may change dramatically the phase behavior of the ternary system, leading to the separation that becomes much more feasible. Ternary Systems. In addition to the binary systems of 1,4-butanediol + 4-methoxyphenol and 1,4-butanediol + catechol, seven pseudobinary (pseudoternary) systems were also investigated in this study. These pseudobinary samples were prepared with constant molar ratios of 4-methoxyphenol:catechol ) 20:80, 50:50, and 70:30, 1,4butanediol:4-methoxyphenol ) 60:40, 50:50, and 30:70, and 1,4-butanediol:catechol ) 65:35. The experimental results are compiled in Table 4. Figure 4 shows the phase boundary of the pseudobinary system of 1,4butanediol + (4-methoxyphenol:catechol ) 50:50). A congruent point is obviously exhibited on the liquidus
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Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 Table 4. SLE for a Ternary System of 1,4-Butanediol, 4-Methoxyphenol, and Catechol x1
T (K)
x1
T (K)
x1
T (K)
1,4-Butanediol (1) + (4-Methoxyphenol:Catechol ) 20:80) (2) 0.2498 337.6 0.4750 296.1 0.7181 281.0 0.3484 321.0 0.5010 296.2 0.7364 278.3 0.3835 314.4 0.5208 294.7 0.7498 277.2 0.4120 303.5 0.5489 293.9 0.7598 277.9 0.4328 300.1 0.5899 292.0 0.7990 280.0 0.4471 295.9 0.6995 282.8 1,4-Butanediol (1) + (4-Methoxyphenol:Catechol ) 50:50) (2) 0.1005 326.1 0.3836 276.8 0.6491 275.9 0.2154 307.9 0.4082 279.5 0.6788 273.8 0.2901 291.8 0.4514 281.5 0.6972 274.7 0.3333 284.0 0.5052 282.2 0.7196 276.1 0.3478 280.5 0.5993 279.0 0.8007 282.7 1,4-Butanediol (1) + (4-Methoxyphenol:Catechol ) 70:30) (2) 0.1065 305.6 0.5065 273.8 0.6274 267.5 0.2978 295.1 0.5515 267.0 0.6997 272.9 0.4005 285.6 0.5771 264.9 0.4512 280.3 0.6023 266.0
Figure 3. Solid-liquid phase boundary for 1,4-butanediol (1) + catechol (2). Table 3. Locations of Invariant Points
Catechol (1) + (1,4-Butanediol:4-Methoxyphenol ) 60:40) (2) 0.0964 272.5 0.2438 276.6 0.3959 290.9 0.1475 272.9 0.3006 279.4 0.6091 336.9 0.1635 273.0 0.3472 280.7 0.6615 348.0 0.1758 273.2 0.3735 281.2 0.2013 274.7 0.3846 285.5
1,4-Butanediol (1) + 4-Methoxyphenol (2) eutectic 0.6144 270.1
Catechol (1) + (1,4-Butanedio:4-Methoxyphenol ) 50:50) (2) 0.1102 284.8 0.3015 279.7 0.4038 303.2 0.2208 280.9 0.3188 279.4 0.5049 326.1 0.2527 280.3 0.3446 285.0
1,4-Butanediol (1) + Catechol (2) eutectic 1a 0.4953 congruentb 0.5000 eutectic 2c 0.7827
Catechol (1) + (1,4-Butanedio:4-Methoxyphenol ) 30:70) (2) 0.1202 304.3 0.2994 295.8 0.3971 312.3 0.2047 301.4 0.3136 293.9 0.4986 324.7 0.2721 298.0 0.3299 296.5 0.5692 340.1
type of invariant point
x1
T (K)
301.2 301.3 280.0
1,4-Butanediol + 1,4-butanediol-catechol. b 1,4-Butanediolcatechol. c 1,4-Butanediol-catechol + catechol. a
line that is experimental evidence of the formation of an adducted compound. The eutectic composition of each pseudobinary system was estimated by interpolation, and the determined eutectic loci are presented in Figure 5. The eutectic loci divide the phase diagram into four regions, marked respectively as A-D. Each region corresponds to a solid that will preferentially crystallize from the mother liquor, i.e., region A for 1,4-butanediol, region B for catechol, region C for 4-methoxyphenol, and region D for the adducted compound 1,4-butanediol-catechol. Among these four areas, region C is of particular interest in the development of the separation processes. A separation sequence of extractive crystallization is proposed according to the SLE phase diagram. As shown in Figure 5, the conceptual separation procedure is as follows: (1) Add auxiliary agent 1,4-butanediol into the eutectic mixture of 4-methoxyphenol and catechol up to 0.28 mole fraction. (2) Produce 4-methoxyphenol solid from the ternary solution in the first crystallizer at about 275 K, while the composition of the corresponding mother liquor is located at the intersection of regions B-D. (3) Recover 1,4-butanediol from the mother liquor of the first crystallizer by distillation, while the bottom residue is about 1,4-butanediol-free. (4) Transfer the residues into the second crystallizer to produce catechol solid at about 310 K. The composition of the mother liquor is about the eutectic mixture of 4-methoxyphenol + catechol, which is recycled to the feed of the first crystallizer.
4-Methoxyphenol (1) + (1,4-Butanediol:Catechol ) 65:35) (2) 0.3034 276.6 0.3301 273.6 0.3799 282.3 0.3237 274.4 0.3519 278.2 0.3989 286.2
Figure 4. Solid-liquid phase boundary for the pseudobinary system of 1,4-butanediol (1) + (4-methoxyphenol:catechol ) 50: 50) (2).
The proposed procedure requires two crystallizers and one distillation column. In comparison with the methods using tert-butyl alcohol as the auxiliary agent (Lee et al.2), this new treatment reduces the amounts of auxiliary compound by about 20-53% on a mole basis.
Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 4599
the objective function π over the entire composition range: n
π)
∑ [|Tcalc,k - Texpt,k|/Texpt,k]/n
(5)
k)1
Figure 5. Phase diagram of 1,4-butanediol (A) + catechol (B) + 4-methoxyphenol (C) at SLE.
Correlation of SLE Data
where Tcalc is solved from eq 1. The parameters as determined are given in Table 5. The dashed lines in Figures 1 and 2 illustrate the calculated solid-liquid phase boundaries by using the RSM for 4-methoxyphenol + catechol and 1,4-butanediol + 4-methoxyphenol, respectively. The agreement between the calculated and the experimental values is generally satisfactory. Adducted Binary System. As mentioned earlier, a 1:1 adducted species forms in the mixtures of 1,4butanediol + catechol. The RSCM was utilized to correlate these SLE data. For a complex formation system, the SLE criterion, eq 1, becomes3
Simple Eutectic Binary Systems. The criterion of SLE for a system without complex formation is approximately expressed by3
ln ai ) ln(xiγi) )
(
∆fusHi 1 1 R Tm,i T
)
(1)
where ai, γi, Tm,i, and ∆fusHi are the activity, activity coefficient, melting temperature, and molar enthalpy of fusion for compound i, respectively. R is the gas constant, and xi is the solid solubility of compound i at temperature T. In accordance with the RSCM that was adopted in the next section to correlate the SLE data of the adducted system (1,4-butanediol + catechol), the regular solution model (RSM)14 was applied in this work to represent the nonideality of the liquid mixtures of 1,4-butanediol + 4-methoxyphenol and catechol + 4-methoxyphenol. Although the original RSM is predictive (without need of any adjustable parameters), it always obtains positive deviations from an ideal solution. This model fails to describe the phase behavior of the liquid mixtures containing highly polar components such as 1,4-butanediol, 4-methoxyphenol, and catechol. Adjustable parameters were thus introduced into the RSM to overcome the problem; that is,
(
m m
RT ln γi ) Vi
∑ ∑ ΦjΦk Dji j)1k)1
)
Djk 2
(2)
ln ai ) ln(ziRi) )
(
∆fusHi 1 1 R Tm,i T
)
where zi and Ri are the “true” mole fraction and the “true” activity coefficient of species i in the liquid phase, respectively. The species i may be 1,4-butanediol (denoted as A), catechol (B), or the adducted compound (AB) that is dependent on the “apparent” composition of the liquid mixtures, xA or xB. The solid-disappearance temperature (T) at a given “apparent” composition can be calculated from eq 6, in which the “true” mole fractions (zA, zB, and zAB) are solved from the chemical equilibrium and material balance equations simultaneously. Provided that the mechanism of the complex AB formation is simply assumed as
A + B T AB
KAB )
zABRAB (zARA)(zBRB)
and m
Φj ) xjVj/(
∑ xkVk) k)1
(4)
where m is the number of species and λjk is a binary interaction constant. The variables Vi, δi, and Φi in eqs 2-4 are the liquid molar volume, solubility parameter, and volume fraction for species i, respectively. The molar volumes and solubility parameters are listed in Table 1 for each constituent compound. For a simple eutectic binary system, λjk is the only adjustable parameter. Its optimal value is determined by minimizing
(8)
where KAB is the chemical equilibrium constant. The temperature dependence of KAB can be expressed by the van’t Hoff relationship with the assumption of that the enthalpy change of complex formation is independent of temperature, i.e.,
ln KAB ) -∆cpxHAB/RT + ∆cpxSAB/R (3)
(7)
the chemical equilibrium is given by
with
Djk ) (δj - δk)2 + 2λjkδjδk
(6)
(9)
where ∆cpxHAB and ∆cpxSAB stand for the molar enthalpy and the molar entropy changes of the complex formation, respectively. These two variables were treated in this work as model parameters. In addition to eq 8, two independent material balance equations are needed to calculate zA, zB, and zAB. These two equations are
xA )
zA + zAB zA + zB + 2zAB
(10)
and
1 ) zA + zB + zAB
(11)
While the “true” mole fractions are solved simultaneously from eqs 8, 10, and 11, the “true” activity
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Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001
Table 5. Correlated Results for Binary Systems with RSCM or RSM mixture A + B
model
4-methoxyphenol + catechol 1,4-butandiol + 4-methoxyphenol 1,4-butandiol + catechol 1,4-butandiol + catechol
RSM RSM ICM RSCM
a
complex compound
AB AB
-∆cpxHAB/R (K)
Tm,AB (K)
316.2 319.3
-∆cpxSAB/R
2563.6 2512.5
5.311 5.385
λij
AAD (%)
-0.041 -0.033
0.32 0.15 0.76 0.43
-0.024
n AAD (%) ) (100/n)∑k)1 [|Tcalc,k - Texpt,k|/Texpt,k], where n is the number of data points.
coefficients (Ri) were calculated from the RSM as defined in eqs 2-4 with zi in place of xi. For this adducted binary system, the number of species (m) is 3, including A, B, and AB. For the purpose of simplification, the values of the three binary interaction parameters of the system are assumed to be identical (i.e., λA-B ) λA-AB ) λB-AB ) λ12) to reduce the number of adjustable parameters. If the “apparent” compositions locate between two eutectic compositions, the properties of the adducted compound AB, including its solubility parameter, liquid molar volume, “true” melting temperature, and molar enthalpy of fusion, are input variables in the SLE calculations. The solubility parameter and the liquid molar volume for the adducted compound AB were estimated from the method of Harris and Prausnitz:15
Table 6. Predicted Results for a Ternary System AAD (%) mixture
ICM
4-methoxyphenol:catechol 20:80 50:50 70:30 1,4-butandiol:4-methoxyphenol 60:40 50:50 30:70 1,4-butandiol:catechol 65:35 grand AAD (%) a
RSCM with RSCM with λC-AB ) 0 λC-AB ) -0.024
1.17 1.50 4.35
0.78 1.43 1.13
0.93 1.51 0.81
3.71 4.13 2.96
2.13 2.61 1.45
2.06 0.75 0.95
4.90 2.86
2.47 1.57
1.46 1.24
n AAD (%) ) (100/n)∑k)1 [|Tcalc,k - Texpt,k|/Texpt,k].
δAB ) x(δA2VA + δB2VB)/VAB
(12)
to an AAD of 0.76%. Figure 3 presents a graphical comparison of these calculated results with the experimental values.
VAB ) VA + VB - 6.0
(13)
SLE Calculation for Ternary System
where
The solubility parameter and liquid molar volume for 1,4-butanediol (A) and catechol (B) are given in Table 1. The “true” melting point of complex AB (Tm,AB) is defined as the equilibrium temperature of solid AB and liquid AB. The liquid mixture at the congruent point, however, does not compose AB only. Actually, it contains species A, B, and AB at the chemical equilibrium concentrations. The experimental congruent temperature (T′m,AB), thus, is not the “true” melting temperature of solid AB. Its “true” melting temperature can be calculated from eq 6 by replacing T with T′m,AB:
There are four “true” species: 1,4-butanediol (A), catechol (B), 4-methoxyphenol (C), and adducted compound (1,4-butanediol-catechol; AB), in the ternary system of 1,4-butanediol + catechol + 4-methoxyphenol. The “true” mole fractions of these compounds (zA, zB, zC, and zAB) in the liquid mixtures can be solved simultaneously from the chemical equilibrium relation, eq 8 with eq 9, and the following material balance equations:
1 ) zA + zB + zC + zAB
Tm,AB ) T′m,AB{1 - [R ln(zABRAB)/∆fusSAB]} (14) where ∆fusSAB is the molar entropy of fusion for the adducted solid AB ()∆fusHAB/Tm,AB) and was estimated from the values of the monomers by the method of Beardmore et al.:16
1 1 1 ∆fusSAB ) ∆fusSA + ∆fusSB - R ln 2 2 2
()
(15)
As mentioned above, there are three adjustable parameters including ∆cpxHAB, ∆cpxSAB, and λ12 in the RSCM for 1,4-butanediol + catechol. If the assumption of Ri ) 1 is made, the RSCM becomes the so-called ICM. The ICM contains two adjustable parameters (∆cpxHAB and ∆cpxSAB) for 1,4-butanediol + catechol. The optimized values were determined from SLE by minimizing the objective function as defined in eq 5. The calculation procedure has been detailed in Lee and Lien.4 Table 5 presents the correlated results from the ICM and the RSCM. The RSCM reproduces the equilibrium temperatures to an average absolute deviation (AAD) of 0.43%, whereas the ICM correlated the same experimental data
(16)
xB )
zB + zAB zA + zB + zC + 2zAB
(17)
xC )
zC zA + zB + zC + 2zAB
(18)
and
The solid component i may be A, B, C, or AB depending on the apparent composition of the feed as mentioned earlier. The procedure for solving the solid-disappearance temperature is similar to that of Lee and Lien.4 In the calculation of “true” activity coefficients for this ternary system, the interaction parameter λC-AB is needed but has not yet been determined. Because the interactions between 4-methoxyphenol (C) and the adducted compound AB exist only in the ternary system, parameter λC-AB cannot be obtained from binary data. To minimize the number of adjustable parameters, we assumed that the values of all of the interaction parameters related to the complex AB are identical; i.e., λC-AB ) λA-AB ) λB-AB ) λA-B ) -0.024. The calculated results from the ICM and the RSCM are listed in Table 6, indicating that the ICM estimates the equilibrium
Ind. Eng. Chem. Res., Vol. 40, No. 21, 2001 4601
Figure 6. Comparison of predicted equilibrium temperatures with experimental values for the ternary system of 1,4-butanediol + catechol + 4-methoxyphenol.
∆H ) enthalpy of change (kJ mol-1) ICM ) ideal-chemical model K ) chemical equilibrium constant m ) number of species n ) number of data points P ) pressure (bar) R ) gas constant (kJ mol-1 K-1) RSCM ) regular solution-chemical model ∆S ) entropy of change (kJ mol-1 K-1) T ) temperature (K) Tm,AB ) “true” melting temperature of complex AB (K) T′m,AB ) congruent temperature (K) ∆U ) change of internal energy (kJ mol-1) V ) liquid molar volume (cm3 mol-1) x ) “apparent” mole fraction z ) “true” mole fraction ZRA ) parameter in the modified Rackett equation R ) “true” activity coefficient γ ) activity coefficient δ ) solubility parameter [(kJ cm-3)1/2] λ ) binary interaction parameter π ) objective function Φ ) volume fraction ω ) acentric factor Subscripts
temperatures to a grand AAD of 2.86%. The RSCM with λC-AB ) -0.024 predicts the equilibrium temperatures to a grand AAD of 1.24%, whereas the grand AAD is as high as 1.57% by assuming λC-AB ) 0. The predicted results are compared with experimental values in Figure 6. While the RSCM with λC-AB ) -0.024 well reproduces the phase boundary, the ICM obviously overestimates the equilibrium temperatures over a majority of composition ranges as shown in Figure 6. Conclusions SLE data were experimentally determined for binary systems of 1,4-butanediol + catechol and 1,4-butanediol + 4-methoxyphenol. The experimental results showed the formation of a 1:1 adducted species in 1,4-butanediol + catechol. A ternary SLE phase diagram of 1,4butanediol + catechol + 4-methoxyphenol was also prepared, from which a procedure was proposed to separate the closely boiling compounds of 4-methoxyphenol and catechol via an extractive crystallization with 1,4-butanediol as an adducted agent. The SLE data were correlated with the RSCM that was found applicable to represent the equilibrium behavior for the adducted system. This model with the adjusted parameters from binary data was successfully applied to predict the solid-liquid phase boundaries for the multicomponent system of 1,4-butanediol + catechol + 4-methoxyphenol. Acknowledgment Financial support from the National Science Council of ROC (NSC89-2214-E011-013) is gratefully acknowledged. Nomenclature a ) activity AAD ) average absolute deviation D ) variable in the RSM
A, B ) for species A and B, respectively AB ) for complex species AB b ) boiling c ) critical property calc ) calculated value cpx ) complex formation expt ) experimental value fus ) fusion m ) melting i ) component i ij ) for i-j pair interaction vap ) vaporization
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Received for review November 30, 2000 Accepted July 3, 2001 IE001020C
