Phase Equilibrium and Phase Diagram for the System of Benzene

Jul 29, 2014 - The mutual solubility for ternary systems of terephthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, phthalic acid + isophthalic...
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Phase Equilibrium and Phase Diagram for the System of Benzene Dicarboxylic Acid + N‑Methyl-2-pyrrolidone Gan-Bing Yao,* Ya-Ping Sun, Ling Wang, Jian-Kang Yi, Long Meng, and Xu Zhang College of Chemistry & Chemical Engineering, YangZhou University, YangZhou, Jiangsu 225002, People’s Republic of China ABSTRACT: The mutual solubility for ternary systems of terephthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, and phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone were measured at 303.15 K and 313.15 K under normal pressure. Six phase diagrams were plotted on the basis of the measured solubility data. Two pure solids were formed for each system, and they were recognized by the method of Schreinemakers’ wet residue. Two crystalline adducts were formed, which were the adduct of isophthalic acid with N-methyl-2-pyrrolidone (named as N, isophthalic acid/N-methyl-2pyrrolidone = 1:2 in mole ratio) and adduct of terephthalic acid with N-methyl-2-pyrrolidone (named as M, terephthalic acid/N-methyl-2-pyrrolidone = 1:2 in mole ratio). Each phase diagram exhibited only one eutectic point. The crystallization fields of phthalic acid, adduct N and adduct M were increased as the temperature increased. At the same temperature, the crystallization field of adduct M is the largest. In addition, the densities of the equilibrium liquid phase were obtained.

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INTRODUCTION As a crystallization separation method, adductive crystallization is a kind of operation that generates a crystalline solid phase, which is the adduct of the feed, and an extraneous agent added to the feed. Then the crystalline adduct can be separated from the mother liquid by filtering.1−7 It is very easy for the adduct crystal to decompose. As a result, the highly purified solute can be acquired, and the solvent can be recovered and recycled easily. Benzene dicarboxylic acid has three isomers, which are terephthalic acid, phthalic acid, and isophthalic acid. They are all important raw materials in the production of polyester and plasticizers.8−10 To date, separation of the benzene dicarboxylic acid isomers is still challenging. Several methods have been proposed to isolate the benzene dicarboxylic acid isomers.8−13 Of these methods adductive crystallization is an effective method to separate the isomers.4−8 The adductive crystallization method has been used to separate and purify terephthalic acid in previous works.1,12,13 The formation process for the adducts crystallization of terephthalic acid with N,N-dimethylacetamide (DMAC) and N-methyl-2-pyrrolidone (NMP) is spontaneous and rapid. It was also found that, in the adduct crystallization, the intermolecular hydrogen bonding between the amide solvent molecules and terephthalic acid dominated. The precipitation of terephthalic acid adduct © 2014 American Chemical Society

crystals and the dissolution of terephthalic acid solid were carried out simultaneously during the adduct crystallization process. The high-purity terephthalic acid adduct crystals precipitated, and impurities were dissolved in the liquid phase. The adduct crystallization process of terephthalic acid with DMAC and NMP solvent can effectively remove the impurities in the terephthalic acid residue and recover pure terephthalic acid products from the terephthalic acid residue or crude terephthalic acid products. Cheng and co-workers studied the thermodynamics of adduct crystallization of terephthalic acid in amide solvents, such as N,N-dimethylacetamide (DMAC) and N-methyl-2-pyrrolidone (NMP).14 The choice of the adductive crystallization process depends on the solid−liquid equilibrium and phase diagram of the system. The solid−liquid equilibrium is essential to the design of an adductive crystallization process just as the vapor−liquid phase equilibrium is to the design of a distillation process. During the separation process of benzene dicarboxylic acid from its isomeric mixtures via adductive crystallization, the mutual solubility of the benzene dicarboxylic acid isomers in a solvent should be known in advance. It is very important to Received: June 14, 2014 Accepted: July 21, 2014 Published: July 29, 2014 2686

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performance liquid-phase chromatograph (HPLC), which was equipped with a 250 mm × 4.6 mm chromatographic column (model unimicro Kromasil C18). The column temperature was set to 303.15 K, and the wavelength of the UV single spectrophotometric detector was 242 nm. The mobile phase consisted of methanol and water in volume ratio 75:35, and the flow rate was 1.0 mL·min−1. The concentration ranges of solutes for construction of the calibration curves were from (0 to 1) mg·mL−1. Experimental data points correspond to an average of at least three repetitive measurements. The uncertainty of the measured solubility values is approximately 2.0 %.

determine the mutual solubility of the benzene dicarboxylic acid + solvent system. Although the solubility of terephthalic acid, isophthalic acid, and phthalic acid in various solvents was reported,15−22 research on the mutual solubility for two isomers of terephthalic acid, isophthalic acid, and phthalic acid in a solvent was not found in previous publications. The aim of this investigation is to study the ternary phase equilibrium of the systems of terephthalic acid + isophthalic acid + N-methyl-2pyrrolidone, phthalic acid + isophthalic acid + N-methyl-2pyrrolidone, and phthalic acid + terephthalic acid + N-methyl2-pyrrolidone at different temperatures by Schreinemakers’ method of wet residues.23−25 In addition the ternary phase diagrams of these systems were constructed, and the temperature dependence of the ternary phase diagrams were analyzed.

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RESULTS AND DISCUSSION The mutual solubility and the density of equilibrium liquid phase for the ternary systems of isophthalic acid + terephthalic acid + N-methyl-2-pyrrolidone, phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone, and phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone at 303.15 K and 313.15 K are shown in Tables 1 to 3. The corresponding phase diagrams for the three ternary systems at 303.15 K and 313.15 K are plotted in Figures 1 to 6 on the basis of measured solubility in Tables 1 to 3. The solubilities of terephthalic acid and isophthalic acid in Nmethyl-2-pyrrolidone at 303.15 K and 313.15 K were taken

EXPERIMENTAL SECTION Materials and Apparatus. Phthalic acid (0.996 in mass fraction), terephthalic acid (0.994 in mass fraction), isophthalic acid (0.993 in mass fraction), and N-methyl-2-pyrrolidone (0.997 in mass fraction) were purchased from Sinopharm Chemical Reagent Co., Ltd., China, and were used as received without any further purification. Deionized water for the preparation of the mobile phase was produced in-laboratory with a conductivity of < 1·10−5 S·m−1. The smart thermostatic bath (model DZKW-4) was provided by Ningbo Scientz Biotechnology Co., Ltd. and had an uncertainty of ± 0.01 K. The analytical balance (model BSA224S) with an uncertainty of ± 0.0001 g used in this experiment was provided by Sartorius Scientific Instruments (Beijing) Co., Ltd. The densities (ρ) of the equilibrium liquid phase were determined by using a DMA 4500 M digital densimeter with a precision of 1.0·10−5 g·cm−3 (Anton Paar, Austria). Procedure. In this work, the method of isothermal dissolution was employed as described in detail in our previous publications.24,25 Briefly a smart thermostatic bath was employed in the experiment. An excess terephthalic acid, isophthalic acid, and/or phthalic acid were added to N-methyl2-pyrrolidone to obtain the saturated solutions. The system was placed in a water jacketed vessel with motor stirring in a constant temperature water bath for 2 days to ensure equilibrium. To prevent N-methyl-2-pyrrolidone from evaporating, a condenser is attached to the flask. The upper liquid phase was extracted at 1 h intervals using plastic syringes coupled with polypropylene filters (0.45 μm) which were heated previously with the purpose of avoiding any solid precipitation, and subsequently, the liquid phase composition was determined by a high-performance liquid-phase chromatograph. When the composition of liquid phase became constant, the system was assumed to be equilibrium. The equilibrium state was confirmed by measuring the content of solute repetitively after 2 additional days. Result showed that the equilibrium time of the system was about 12 h. After the equilibrium was reached, stirring was stopped for 1 h to allow any solid to settle out of the liquid. The upper equilibrium liquid phase and the solid phase adhering some saturated liquid phase were taken out and analyzed using high performance liquid chromatography (HPLC), respectively. The ratio of the solute and solvent was changed to get different compositions of the solid and liquid phase. Analysis. The saturated solutions of benzene dicarboxylic acid and the wet residues were added to a volumetric flask. The compositions of terephthalic acid, isophthalic acid, and phthalic acid were determined by the use of a Shimadzu-6A high-

Table 1. Mass Fraction Solubility of Ternary Isophthalic Acid (1) + Terephthalic Acid (2) + N-Methyl-2-pyrrolidone (3) System at 303.15 K and 313.15 K under Pressure of 0.1 MPaa liquid phase 100w1

moist solid phase

100w2

T = 303.15 K 0 5.896 1.012 5.543 2.904 4.549 6.481 4.092 9.745 3.968 12.04 4.091 13.58 3.952 14.75 3.164 15.82 2.305 16.04 1.339 16.24 0 T = 313.15 K 0 8.754 3.038 8.334 6.021 7.879 8.452 7.181 11.81 6.583 13.64 6.014 15.73 5.808 17.02 4.647 18.26 3.903 19.42 1.882 19.57 0

density

100w1

100w2

g·mL−1

solid phase

0 0.3605 1.011 3.622 5.048 6.553 15.32 28.23 29.74 30.21 36.43

38.56 33.37 31.76 22.38 24.45 22.49 14.60 1.771 1.154 0.7193 0

1.0295 1.0310 1.0349 1.0415 1.0471 1.0557 1.0608 1.0553 1.0507 1.0469 1.0361

M M M M M M M+N N N N N

0 1.631 2.932 4.416 7.878 6.504 15.22 30.77 36.42 35.35 35.87

37.64 25.60 26.45 25.31 19.63 26.91 23.29 2.532 1.934 1.117 0

1.0315 1.0371 1.0438 1.0489 1.0569 1.0629 1.0741 1.0631 1.0585 1.0479 1.0385

M M M M M M N+M N N N N

a Notation: w, mass fraction; M, adduct of terephthalic acid with Nmethyl-2-pyrrolidone; N, adduct of isophthalic acid with N-methyl-2pyrrolidone. The relative standard uncertainty of the mass fraction solubility is ur(w) = 0.023 and for the measured temperature, u(T) = 0.02 K.

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Table 2. Mass Fraction Solubility of Ternary Phthalic Acid (1) + Terephthalic Acid (2) + N-Methyl-2-pyrrolidone (3) System at 303.15 and 313.15 K under Pressure of 0.1 MPaa liquid phase 100w1 T = 303.15 K 0 2.438 5.834 7.951 10.03 12.04 12.93 14.28 14.93 15.80 T = 313.15 K 0 1.138 3.403 4.492 6.929 9.261 11.89 14.23 15.36 15.82 16.76 18.70

100w2

moist solid phase 100w1

100w2

Table 3. Mass Fraction Solubility of Ternary Phthalic Acid (1) + Isophthalic Acid (2) + N-Methyl-2-pyrrolidone (3) System at 303.15 and 313.15 K under Pressure of 0.1 MPaa

density −1

g·mL

liquid phase 100w1

solid phase

5.896 5.397 4.902 4.573 4.009 3.681 3.223 2.536 1.872 0

0 1.251 3.894 5.042 5.629 20.82 32.56 30.25 33.98 45.61

38.56 25.75 18.94 19.61 22.35 11.82 2.611 1.907 1.293 0

1.0295 1.0317 1.0433 1.0571 1.0635 1.0722 1.0646 1.0469 1.0407 1.0378

M M M M M P+M P P P P

8.754 8.421 8.028 7.982 7.224 6.673 5.857 5.478 4.573 3.292 2.064 0

0 0.4805 1.263 2.571 3.187 5.844 5.672 25.85 32.51 30.60 35.31 44.89

37.64 29.80 32.86 24.61 28.06 21.88 27.35 12.11 3.421 2.435 1.514 0

1.0385 1.0392 1.0408 1.0428 1.0569 1.0638 1.0746 1.0872 1.060 1.0542 1.0488 1.0428

M M M M M M M P+M P P P P

100w2

T = 303.15 K 0 16.24 1.963 16.11 5.801 14.40 8.705 12.81 10.32 12.22 10.71 10.49 11.51 7.552 11.91 5.538 13.20 3.417 13.67 1.853 15.80 0 T = 313.15 K 0 19.57 3.084 18.92 5.381 18.17 7.305 16.82 10.70 15.23 12.91 14.85 13.68 11.00 14.40 8.503 15.01 6.141 17.33 2.548 18.70 0

a

moist solid phase

density

100w1

100w2

g·mL−1

solid phase

0 0.7503 3.182 5.714 30.66 32.81 32.15 34.08 33.58 36.34 45.61

36.43 34.73 28.66 24.23 28.79 7.851 5.804 4.085 2.409 1.161 0

1.0361 1.0401 1.0510 1.0580 1.0694 1.0584 1.0517 1.0482 1.0427 1.0402 1.038

N N N N N+P P P P P P P

0 1.261 2.487 3.276 5.224 10.19 35.30 33.19 33.24 38.87 44.89

35.87 35.02 32.60 33.08 31.07 38.99 8.222 6.646 4.623 1.781 0

1.0385 1.0467 1.0542 1.0596 1.0775 1.0876 1.0743 1.0659 1.0553 1.0488 1.0428

N N N N N N+P P P P P P

a

Notation: w, mass fraction; P, phthalic acid; M: adduct of terephthalic acid with N-methyl-2-pyrrolidone;. The relative standard uncertainty of the mass fraction solubility is ur(w) = 0.020 and for the measured temperature, u(T) = 0.02 K.

Notation: w, mass fraction; p, phthalic acid; N, adduct of isophthalic acid with N-methyl-2-pyrrolidone. The relative standard uncertainty of the mass fraction solubility is ur(w) = 0.022 and for the measured temperature, u(T) = 0.02 K.

from the literature. They were collected and given in Table 4. It can be seen from Table 4 that the reported solubilities of terephthalic acid in N-methyl-2-pyrrolidone are different from each other. As a result we remeasured the solubility of terephthalic acid and isophthalic acid in the solvent N-methyl2-pyrrolidone by using the method of isothermal dissolution. The results are also listed in Table 4. The solubility values of terephthalic acid and isophthalic acid are approximately the same with those reported in ref 22), but different from the values given in ref 14 and 19. No work has hitherto been made for the solubility of phthalic acid in N-methyl-2-pyrrolidone. The System of Isophthalic Acid + Terephthalic Acid + N-Methyl-2-pyrrolidone. Figures 1 and 2 are the phases diagram of ternary isophthalic acid + terephthalic acid + Nmethyl-2-pyrrolidone systems at 303.15 K and 313.15 K, respectively. In the phase diagrams shown in Figures 1 and 2, the points M and N represent the pure solids of adduct M (terephthalic acid: N-methyl-2-pyrrolidone = 1:2 in mole ratio) and adduct N (isophthalic acid: N-methyl-2-pyrrolidone = 1:2 in mole ratio), respectively. Points S1 and S2 represent the solubility of adduct N in N-methyl-2-pyrrolidone at 303.15 K and 313.15 K, and T1 and T2 stand for the solubility of adduct M in N-methyl-2-pyrrolidone at 303.15 K and 313.15 K. T1C11 and T2C12 are boundary curves along which the saturated solution that are in equilibrium with pure adduct M at 303.15 K and 313.15 K, while solubility curves C11S1 and C12S2 indicate along which the saturated solutions are in equilibrium with the pure adduct N. Points C11 and C12 are invariant points for the

Figure 1. Phase diagram of ternary isophthalic acid + terephthalic acid + N-methyl-2-pyrrolidone system at 303.15 K. ■, composition of equilibrium liquid phase; ●, composition of wet solid phase; M, adduct of terephthalic acid with N-methyl-2-pyrrolidone, in which the mole ratio of the two compositions is 1:2; N, adduct of isophthalic acid with N-methyl-2-pyrrolidone, in which the mole ratio of the two compositions is 1:2; NMP, N-methyl-2-pyrrolidone; T1, solubility of adduct M in NMP at 303.15 K; S1, solubility of adduct N in NMP at 303.15K; C11, cosaturation point of adduct M and adduct N at 303.15 K.

ternary isophthalic acid + terephthalic acid + N-methyl-2pyrrolidone system, at which the equilibrium solid phase contains both M and N. The compositions of the invariant 2688

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cosaturation point is constant, but the composition representing the solid phase may vary, which depends on the original mass ratio of the two solutes. The compositions of crystalline adducts M and N determined by the method of Schreinemakers’ wet residue are in accordance with that reported in the literature.12,14,26 The System of Phthalic Acid + Terephthalic Acid + NMethyl-2-pyrrolidone. The phase diagrams for the ternary system of phthalic acid + terephthalic acid + N-methyl-2pyrrolidone at 303.15 K and 313.15 K are plotted in Figures 3

Table 4. Mass Fraction Solubility of Terephthalic Acid and Isophthalic Acid in N-Methyl-2-pyrrolidone at 303.15 and 313.15 K under Pressure of 0.1 MPa: Comparison of the Data Determined in This Work with Those Reported in the Literatures terephthalic acid

isophthalic acid

303.15 K

313.15 K

ref

0.02579 0.02216 0.06406 0.05896 0.1612 0.1624

0.03062 0.02574 0.08832 0.08754 0.1940 0.1957

14 19 22 this work 22 this work

Figure 3. Phase diagram of ternary phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone system at 303.15 K.; P1, solubility of phthalic acid in NMP at 303.15 K; C21, cosaturation point of adduct M and phthalic acid at 303.15 K. M, adduct of terephthalic acid with Nmethyl-2-pyrrolidone, in which the mole ratio of the two compositions is 1:2; NMP, N-methyl-2-pyrrolidone; T1, solubility of adduct M in NMP at 303.15 K.

Figure 2. Phase diagram of ternary isophthalic acid + terephthalic acid + N-methyl-2-pyrrolidone system at 313.15 K. ■, composition of equilibrium liquid phase; ●, composition of wet solid phase; T2, solubility of adduct M in NMP at 313.15 K; S2, solubility of adduct N in NMP at 313.15 K; C12, cosaturation point of adduct M and adduct N at 313.15 K. M, adduct of terephthalic acid with N-methyl-2pyrrolidone, in which the mole ratio of the two compositions is 1:2; N, adduct of isophthalic acid with N-methyl-2-pyrrolidone, in which the mole ratio of the two compositions is 1:2; NMP, N-methyl-2pyrrolidone.

and 4. It is noted that phthalic acid does not form an adduct with solvent N-methyl-2-pyrrolidone. As shown in Figures 3 and 4, the points M, T1, and T2 have the same meaning as described in Figures 1 and 2. Points P1 and P2 stand for the equilibrium solubility of phthalic acid in N-methyl-2-pyrrolidone at 303.15 K and 313.15 K, respectively. Solubility curves

points are shown in Table 1. The phase diagrams are divided into four fields by two solubility curves: I, unsaturated region, II, crystalline region of pure solid M, IV, the crystalline region of pure solid N, and III, the crystalline region of mixture solids of adducts M and N. In the crystallization region of adduct N (region IV), the points representing the equilibrium liquid phase, wet solid phase, and the pure adduct N must lie on a single straight line along the solubility curves C11S1 and C12S2. So when the composition points of the saturated liquid phase and corresponding wet solid phase are connected and extended, the joint point of these tie-lines is approximately the pure solidphase component for adduct of isophthalic acid with N-methyl2-pyrrolidone, in which the mole ratio of the two compositions is 1:2, named as adduct N in the present work. In the crystallization region of adduct M (region II), along solubility curves T1C11 and T2C12, when the composition points of the saturated liquid phase and corresponding wet solid phase are connected and extended, the joint point of these tie-lines approximates the solid-phase component for the adduct of terephthalic acid with N-methyl-2-pyrrolidone, in which the mole ratio of the two compositions is 1:2, named as adduct M. The region III is the crystallization region of the mixture solid of adducts M and N. In this region, the composition of the

Figure 4. Phase diagram of ternary phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone system at 313.15 K.; P2, solubility of phthalic acid in NMP at 313.15 K; C22, cosaturation point of adduct M and phthalic acid at 313.15 K. T2, solubility of adduct M in NMP at 313.15 K; M, adduct of terephthalic acid with N-methyl-2-pyrrolidone, in which the mole ratio of the two compositions is 1:2; NMP, N-methyl2-pyrrolidone. 2689

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C21P1 and C22P2 indicate the compositions of equilibrium saturated solutions which are in equilibrium with the pure solid phthalic acid at 303.15 K and 313.15 K, and the solubility curves T1C21 and T2C22 indicate the compositions of equilibrium saturated solutions which are in equilibrium with the pure solid M. Points C21 and C22 are invariant points for the ternary system of phthalic acid + terephthalic acid + Nmethyl-2-pyrrolidone, at which the two equilibrium solids are adduct M and phthalic acid. The phase diagram has three crystallization fields: adduct M crystallization field (II), phthalic acid crystallization field (IV), and mixture solids of adduct M and phthalic acid crystallization field (III). The phase diagram includes two invariant solubility curves and one cosaturation point at each temperature as shown in Figures 3 and 4. In the crystallization field of phthalic acid (IV), along the solubility curves C21P1 and C22P2, connecting the composition points representing the saturated liquid phase and wet solid phase and extrapolating the curve gives the intersection point of these lines which approximates the pure solid-phase composition for phthalic acid. Region III is the crystallization region of the solid mixtures of compound M and phthalic acid. In this field, the composition of the cosaturation point is fixed, but the composition representing the solid phase may vary, which also depends on the original mass ratio of the two solutes. The System of Phthalic Acid + Isophthalic Acid + NMethyl-2-pyrrolidone. Figures 5 and 6 show the phase

Figure 6. Phase diagram of ternary phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone system at 313.15 K.; C32, cosaturation point of adduct N and phthalic acid at 313.15 K. S2, solubility of adduct N in NMP at 313.15 K; N, adduct of isophthalic acid with N-methyl-2pyrrolidone, in which the mole ratio of the two compositions is 1:2; NMP, N-methyl-2-pyrrolidone; P2, solubility of phthalic acid in NMP at 313.15 K.

The phase diagrams include three crystallization fields: compound N crystallization field (II), phthalic acid crystallization field (IV), and mixture solids of adduct N and phthalic acid crystallization field (III). The phase diagram includes two invariant solubility curves and one cosaturation point at two temperatures as shown in Figures 5 and 6. Figures 1 to 6 further illustrate the phase diagram dependence on the temperature for the ternary system of terephthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone and phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone. The invariant points of the three ternary systems move upward when the temperature increases. The crystalline regions of three pure solids (phthalic acid, adducts M and N) increase as the temperature increases. The crystallization region of compound M is larger than those of adduct N and phthalic acid at the same temperature. The densities of equilibrium liquid phase were obtained and listed in Tables 1, 2, and 3 for the studied system. The relationship between the density values of the equilibrium solution and the composition was drawn as shown in Figures 7 to 9 for the ternary system of terephthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, and phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone, respectively. Figure 7 was employed to illustrate the density value-composition relationship diagram. The density values of the equilibrium solution are increased first then decreased with the increasing isophthalic acid concentration. The maximum points were in accordance with the invariant points in Figures 1 and 2. At the same composition of isophthalic acid, the density at 313.15 K is greater than that at 303.15 K. It is obvious that Figures 8 and 9 were similar to Figure 7, so they were not further illustrated in this work.

Figure 5. Phase diagram of ternary phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone system at 303.15 K.; C31, cosaturation point of adduct N and phthalic acid at 303.15 K. P2, solubility of phthalic acid in NMP at 313.15 K; N, adduct of isophthalic acid with N-methyl2-pyrrolidone, in which the mole ratio of the two compositions is 1:2; P1, solubility of phthalic acid in NMP at 303.15 K; NMP, N-methyl-2pyrrolidone; S1, solubility of adduct N in NMP at 303.15 K.

diagram of the ternary phthalic acid + isophthalic acid + Nmethyl-2-pyrrolidone system at 303.15 K and 313.15 K. It is obvious that the phase diagrams are similar to those in Figures 3 and 4. The points N, S1, S2, P1, and P2 have the same meaning as described in Figures 1 to 4. Solubility curves S1C31 and S2C32 show the compositions of saturated solution that are in equilibrium with adduct N at 303.15 K and 313.15 K, and solubility curves C31P1 and C32P2 indicate the compositions of saturated solutions are in equilibrium with the pure solid phthalic acid at 303.15 K and 313.15 K. Points C31 and C32 are cosaturation points at which a solution is saturated with the two solids adduct N and phthalic acid.

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CONCLUSION The solubility of three ternary systems of terephthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone, and phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone at 303.1 K and 2690

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pure solids formed for each system that correspond to adduct M and adduct N for the terephthalic acid + isophthalic acid + N-methyl-2-pyrrolidone system, adduct M and phthalic acid for the phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone system, and adduct N and phthalic acid for the phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone system. Each phase diagram includes three crystallization regions, two invariant curves, and one cosaturation point. The solubility of terephthalic acid, isophthalic acid, and phthalic acid increases with an increase in temperature. At the same temperature, the crystallization region of adduct M is larger than that of adduct N and phthalic acid. The phase diagrams, the mutual solubility data for these ternary systems, and the densities of the equilibrium liquid phase can provide the basis data for a separation process of the binary benzene carboxylic acid isomers.

Figure 7. Density value−composition relationship diagram for the ternary isophthalic acid + terephthalic acid + N-methyl-2-pyrrolidone system at 303.15 K and 313.15 K: ■, experimental data point at 303.15 K; ●, experimental data point at 313.15 K, , experimental relationship diagram.

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AUTHOR INFORMATION

Corresponding Author

*Tel: + 86 514 87975568. Fax: + 86 514 87975244. E-mail: [email protected]. Funding

We thank the Yangzhou City Science and Technology Bureau, China, for their support (Project No.: 2012038-3 and YZ2011139). Notes

The authors declare no competing financial interest.

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REFERENCES

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Figure 8. Density value−composition relationship diagram for the ternary phthalic acid + terephthalic acid + N-methyl-2-pyrrolidone system at 303.15 K and 313.15 K: ■, experimental data point at 303.15 K; ●, experimental data point at 313.15 K, , experimental relationship diagram.

Figure 9. Density value−composition relationship diagram for the ternary phthalic acid + isophthalic acid + N-methyl-2-pyrrolidone system at 303.15 K and 313.15 K: ■, experimental data point at 303.15 K; ●, experimental data point at 313.15 K, , experimental relationship diagram.

313.15 K were measured experimentally under normal pressure, and the corresponding phase diagrams were plotted on the basis of the measured mutual solubility data. There are two 2691

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