Article pubs.acs.org/jced
Investigation on Thermodynamics in Separation for Ethylene Glycol + Neopentyl Glycol System by Azeotropic Distillation Hao Yu, Qing Ye,* Hong Xu, Xin Dai, Xiaomeng Suo, and Rui Li Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China ABSTRACT: The isobaric vapor−liquid equilibrium (VLE) data for two binary systems of ethylene glycol (EG) + neopentyl glycol (NPG) and NPG + para-xylene (PX) have been measured under pressure of 101.3 kPa, respectively. The thermodynamic consistency of the experimental isobaric VLE data was tested by means of Herington area test. Meanwhile, the experimental isobaric VLE data have been satisfactorily correlated with Wilson, NRTL, and UNIQUAC models. A conventional azeotropic distillation process has been designed to achieve high-purity products based on preceding results. The product purities of EG and NPG can both achieved at 99.9 mol % with minimal total annual cost (TAC) through sequential iterative optimization procedure.
1. INTRODUCTION Ethylene glycol (EG) and neopentyl glycol (NPG) are important chemical raw materials and widely used in resin synthesis. These two compounds exhibit close-boiling behavior, which requires many stages of a regular column and also large energy consumption to meet stringent product purity specifications. In recent years, several studies on the isobaric vapor−liquid equilibrium (VLE) data of EG with glycol system have been presented.1,2 Azeotropic distillation as the commonly used operation has been adopted to separate EG + NPG mixture and para-xylene (PX) will be selected as the entrainer for the heterogeneous azeotropic system in this article. The thermodynamics of VLE data of EG−NPG system, which are indispensable for distillation design, have not been available in open literature so far. The present work has focused on isobaric VLE of the EG + NPG and NPG + PX binary systems as EG + PX forms a heterogeneous azeotrope at atmospheric pressure. The Wilson,3 NRTL,4 and UNIQUAC5 models are adopted to fit the isobaric VLE data and are confirmed the thermodynamic consistency by Herington area test.6 A feasible conventional azeotropic distillation process has been designed based on preceding results. With sequential iterative optimization procedure, minimal total annual cost (TAC) of this azeotropic distillation process can be obtained.
ionization detector (FID). The sample description are showed in Table 1. Table 1. Sample Description
EGa PXc NPGd a
source
initial mass fraction purity
purification method
Chinasun Lingfeng BASF
0.9995 0.9900 0.9997
none none none
final mass fraction purity
analysis method GCb GC GC
EG = ethylene glycol. bGC = gas chromatography. cPX = para-xylene. NPG = neopentyl glycol.
d
2.2. Apparatus and Methods. In this work, the modified Ellis recirculation equilibrium still was used to take measurements.7 The temperatures were measured with precision mercury thermometers. The standard uncertainty in the temperature measurements was 0.1 K in this work. The measuring pressure was maintained under pressure of 101.3 kPa with the aid of a pressure control system. The vapor (condensate) and liquid were analyzed by a gas chromatograph (GC-950), which produced by Shanghai Haixin Chromatograph Instrument Co., Ltd. The GC column was an SE-30 capillary column (30 m × 0.25 mm × 0.25 μm). Injector and column temperatures were set at 200 and 170 °C, respectively. Nitrogen was taken as carrier gas at a constant flow rate of 30 mL/min. The FID temperature was set at 200 °C. Each sample was detected three times to ensure accuracy.
2. EXPERIMENTAL SECTION 2.1. Materials. EG (>99.9 wt %) was provided by Chinasun Specialty Products Co., Ltd. PX (>99.9 wt %) was provided by Shanghai Lingfeng Chemical Reagent Co., Ltd. NPG (>99.9 wt %) was provided by BASF Company. The purities of above materials were detected by a gas chromatography with a flame © XXXX American Chemical Society
chemical name
Received: December 9, 2015 Accepted: May 25, 2016
A
DOI: 10.1021/acs.jced.5b01044 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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3. THERMODYNAMIC MODELING 3.1. Experimental Data. The experimental isobaric VLE data and calculated activity coefficients for EG + NPG and NPG + PX systems were listed in Table 2 and Table 3, respectively. The activity coefficient γi was calculated with the following equation ⎡ V L(p − ps ) ⎤ i ⎥ s s ⎢ i exp pyi φ̑ V = xiγφ p i i i ⎢⎣ ⎥⎦ RT
The constants of Antoine equation for vapor pressures of the pure compounds were listed in Table 4. Table 4. Antoine Constants of EG, NPG, and PX
T/K
x1
y1
1.0000 0.9242 0.8524 0.7727 0.6841 0.6115 0.4966 0.4165 0.2439 0.1333 0.0587 0.0000
1.0000 0.9382 0.8763 0.8047 0.7216 0.6518 0.5394 0.4601 0.2870 0.1696 0.0813 0.0000
γ1 0.9994 0.9981 0.9960 0.9936 0.9921 0.9925 0.9969 1.0293 1.0815 1.1430
NPG
PX
A B C D E
72.577 −10411 −8.198 1.65 × 10−18 6
75.011 −11276 −8.336 4.45 × 10−18 6
77.207 −7741.2 −9.869 6.08 × 10−6 2
γ2 1.0025 1.0138 1.0236 1.0312 1.0347 1.0351 1.0318 1.0177 1.0071 1.0016
3.2. Thermodynamic Consistency Test. The thermodynamic consistency tests for the two binary systems were based on the Gibbs−Duhem equation9,10 and verified with the Herington area test.11 The Gibbs−Duhem equation was
ln(γ1/γ2)
HE VE d T − dp (4) RT RT 2 Under constant pressure circumstance, the Gibbs−Duhem equation can be simplified as x1dln γ1 + x 2dln γ2 =
−0.0031 −0.0156 −0.0273 −0.0371 −0.0419 −0.0420 −0.0344 0.0113 0.0713 0.1320
∫x
x1 = 1
ln
1=0
γ1 γ2
dx1 = −
∫x
x1 = 1
ln
1=0
HE dT RT 2
(5)
As Herington proposed, the experimental isobaric VLE data can be regarded thermodynamically consistent when |D − J| < 10. D and J were calculated in the equations 1
a
Standard uncertainties are u(P) = 1 kPa, u(T) = 0.1 K, u(x1) = u(y1) = 0.001.
T/K
x1
y1
γ1
γ2
ln(γ1/γ2)
411.21 414.70 419.62 427.69 432.33 442.49 449.35 456.64 466.21 472.57 475.19 478.95
0.0000 0.0751 0.2237 0.3512 0.4310 0.5764 0.6626 0.7508 0.8711 0.9468 0.9752 1.0000
0.0000 0.0097 0.0218 0.0497 0.0776 0.1631 0.2465 0.3634 0.5824 0.7754 0.8775 1.0000
0.7152 0.8455 0.9122 0.9460 0.9936 1.0127 1.0233 1.0183 1.0057 1.0004
0.9992 0.9898 0.9762 0.9638 0.9288 0.8996 0.8698 0.8785 1.0005 1.0307
−0.3343 −0.1576 −0.0678 −0.0186 0.0674 0.1184 0.1625 0.1477 0.0052 −0.0262
|∫ ln(γ1/γ2)dx1| 0 1
D = 100 ×
∫0 |ln(γ1/γ2)|dx1
Table 3. Experimental VLE data for Temperature T, LiquidPhase Mole Fraction x, and Gas-Phase Mole Fraction y, for the System Neopentyl Glycol (1) + para-Xylene (2)a at 101.3 kPa
J = 150 ×
(6)
Tmax − Tmin Tmin
(7)
Tmax and Tmin are the highest and lowest temperature, respectively. The results of thermodynamics consistency test are shown in Table 5, the experimental isobaric VLE data of the two binary systems both passed the Herington area test. Table 5. Thermodynamics Consistency Test for Two Binary System at 101.3 kPa system
D
J
|D − J|
EG + NPG NPG + PX
5.068 23.972
1.857 20.932
3.211 3.040
3.3. Data Regression. The interaction parameters of the two binary systems were regressed by means of Wilson, NRTL, and UNIQUAC models and the values were listed in Table 6. The interaction parameter α in NRTL was fixed at 0.3.4 The root-mean-square deviations (σT and σy1) between the evaluated and experimental data were defined as12
a
Standard uncertainties are u(P) = 1 kPa, u(T) = 0.1 K, u(x1) = u(y1) = 0.001.
Under mesolow circumstances, the values of φ̑ V and φsi can both thought to equal to 1. Equation 1 can be simplified as pyi = xiγipis
EG
(1)
Table 2. Experimental VLE Data for Temperature T, LiquidPhase Mole Fraction x, and Gas-Phase Mole Fraction y, for the System Ethylene Glycol (1) + Neopentyl Glycol (2)a at at 101.3 kPa 470.22 470.80 471.28 471.77 472.29 472.71 473.31 473.76 474.79 475.76 476.63 478.95
Antoine equation
N
σT =
∑i = 1 (Tical − Tiexp)2 N
(8)
(2) N
The saturation vapor pressure was calculated by Antoine equation8 B ln pis = A + + C ln T + DT E (3) T
σy1 =
∑i = 1 (y1 cal − y1 exp )2 i
i
N
(9)
The calculated root-mean-squared deviations were also given in Table 6. The comparison results indicated that NRTL model B
DOI: 10.1021/acs.jced.5b01044 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 6. Binary Interaction Parameters of Wilson, NRTL, and UNIQUAC Models for Two Binary System model parameters models
A12/(J·mol−1)a
Wilson NRTL UNIQUAC
−590.2820 −449.4958 235.3980
Wilson NRTL UNIQUAC
488.4102 896.7799 −502.2751
root-mean-square deviation
A21/(J·mol−1)a EG (1) + NPG (2) System 334.3378 686.3341 −365.8704 NPG (1) + PX (2) System −958.3601 −602.2680 353.6040
α
σT/Kb
σy1b
0.3000
0.5025 0.5000 0.4978
0.0015 0.0005 0.0019
0.3000
0.8548 0.9119 0.9198
0.0086 0.0091 0.0077
a Wilson: A12 = (λ12 − λ11)/R, A21 = (λ21 − λ22)/R. NRTL: A12 = (g12 − g22)/R, A21 = (g21 − g11)/R. UNIQUAC: A12 = (u12 − u22)/R, A21 = (u21 − exp 2 N cal exp 2 1/2 1/2 u11)/R. bσT = [∑i N= 1(Tcal i − Ti ) /N] ; σy1 = [∑i = 1(y1i − y1i ) /N]
was more suitable to regress the interaction parameters in this study than others did, which fit the experimental isobaric VLE data in a similar way. The experimental isobaric VLE data of the two binary systems were plotted in Figure 1 and Figure 2, in which the NRTL model was showed in solid lines.
Figure 3. RCM for the EG + NPG + PX heterogeneous azeotropic system at 101.3 kPa simulated by Aspen Plus using NRTL model.
Figure 1. T−x−y diagram for EG (1) + NPG (2) at 101.3 kPa: (red ● and black ■), experimental data; solid lines, NRTL equation.
Figure 4. Simulation flowsheet of the conventional azeotropic distillation process.
Figure 2. T−x−y diagram for NPG (1) + PX (2) at 101.3 kPa: (red ● and black ■), experimental data; solid lines, NRTL equation.
computed by Aspen Plus with NRTL model based on the regressed interaction parameters. Only a binary heterogeneous azeotrope with composition of 89.14 mol % PX/10.86 mol % EG and an azeotropic temperature of 135.42 °C existed in EG + NPG + PX heterogeneous azeotropic system.13 The
4. DISTILLATION PROCESS DESIGN Figure 3 illustrated the residue curve map (RCM) for EG + NPG + PX heterogeneous azeotropic system at 101.3 kPa C
DOI: 10.1021/acs.jced.5b01044 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 7. Effect of NT2 on the TAC of the conventional azeotropic distillation process.
column C1, which served as a heterogeneous azeotropic distillation column. The organic phase 2 (containing lesser entrainer) was returned to the first tray of column C2, which served as a product column. Several parameters should be optimized for the design with optimized operation conditions of this azeotropic distillation process. Optimum parameters can be achieved through elaborate comparison of the minimal TAC values. TAC was defined as the sum of the annual operation cost and capital investment divided by a three-year payback period. 15 Parameters of the columns were obtained based on the TAC results by the methods proposed by Douglas.16 The capital investment was considered as the investment of the major piece of equipment such as vessels and HXs. Other minor pieces such as pipes, valves, and pumps can usually be neglected as the investment were much less than that of major ones. Only utility consumption was taken into consideration when calculating operating cost in this study. The price of high-pressure steams and the relation between equipment sizes with investment were followed the recommendation by Luyben.15 Sequential iterative optimization procedure for the conventional azeotropic distillation process (Figure 5) is follows: (1) Fix total number of trays (NT) of column C2. (2) Fix NT of column C1. (3) Give the feed tray (NF) of the fresh feed. (4) Change the reboiler duty (QR) of the two columns until two product meet their specifications. (5) Back to the third step, if the previous step can not proceed, until the minimized TAC obtained. (6) Back to the second step, if the previous step can not proceed, until the minimized TAC obtained. (7) Back to the first step, if the previous step can not proceed, until the minimized TAC obtained. The optimum NT of the azeotropic distillation column is 37 with feed location at fifth tray. The optimum NT of the product column is 4. The effect of NT1 and NT2 on the TAC of the conventional heterogeneous azeotropic distillation process were showed in Figures 6 and 7, respectively. The results for all of the streams in each column were shown in Table 7.
Figure 5. Sequential iterative optimization procedure for the conventional azeotropic distillation process.
Figure 6. Effect of NT1 on the TAC of the conventional azeotropic distillation process.
conventional two-column/decanter flowsheet14 was designed for separating the heterogeneous azeotropic system (Figure 4). Both of the two columns were operated at 1.0 atm and the bottom purities were set at 99.9 mol %. The compositions of the two vapor streams fed to a single heater exchanger (HX) were almost the same. The vapor condensate separated into aqueous and organic phases in a decanter. The organic phase 1 (containing mainly entrainer) was fed to the first tray of
5. CONCLUSION The thermodynamics was studied in detail in order to make an appropriate azeotropic distillation process design in this article. The experimental isobaric VLE data for the two binary systems of EG + NPG and NPG + PX were measured under pressure of D
DOI: 10.1021/acs.jced.5b01044 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 7. Specifications for Flows in Column C1 and Column C2 column C1 mole flow/kmol·h−1 temperature/K EG/mol fraction NPG/mol fraction PX/mol fraction
column C2
B1
V1
B2
V2
50.00 483.88 7.69 × 10−2 99.92 4.81 × 10−31
531.78 408.58 11.11 7.23 × 10−33 88.88
50.00 471.58 99.92 7.69 × 10−2 4.86 × 10−10
0.52 408.56 10.86 2.01 × 10−3 89.14
(6) Herington, E. F. G. Tests for the Consistency of Experimental Isobaric Vapor-Liquid Equilibrium Data. J. Inst. Petrol. 1951, 37, 457− 470. (7) Walas, S. M. Phase Equilibria in Chemical Engineering; Butterworth-Heineman: Boston, 1985. (8) Daubert, T. E.; Danner, R. P. Physical and Thermodynamic Properties of Pure Chemicals; Hemisphere: New York, 1989. (9) Abbott, M. M.; Smith, J. M.; Van Ness, H. C. Introduction to Chemical Engineering Thermodynamics; McGraw-Hill: New York, 2001. (10) Jackson, P. L.; Wilsak, R. A. Thermodynamic Consistency Tests Based on the Gibbs-Duhem Equation Applied to Isothermal, Binary Vapor-Liquid Equilibrium Data: Data Evaluation and Model Testing. Fluid Phase Equilib. 1995, 103, 155−197. (11) Chen, S.; Bao, Z.; Lü, Z.; Yang, Y.; Xu, W.; Chen, Z.; Ren, Q.; Su, B.; Xing, H. Vapor-Liquid Equilibrium for the 1,1,1-Trifluorotrichloroethane + Sulfuryl Chloride System at 101.3 kPa. J. Chem. Eng. Data 2014, 59, 16−21. (12) Giner, B.; Martín, S.; Haro, M.; Artigas, H.; Lafuente, C. Experimental and predicted vapor-liquid equilibrium for cyclic ethers with 1-chloropentane. Ind. Eng. Chem. Res. 2005, 44, 6981−6988. (13) Horsley, L. H. Table of azeotropes and nonazeotropes. Anal. Chem. 1949, 21, 831−873. (14) Doherty, M. F.; Malone, M. F. Conceptual Design of Distillation Systems; McGraw-Hill: New York, 2001. (15) Luyben, W. L. Principles and Case Studies of Simultaneous Design; John Wiley & Sons: New York, 2012. (16) Douglas, J. M. Conceptual Design of Chemical Processes; McGraw Hill: New York, 1988.
101.3 kPa. Reasonable thermodynamic consistency were confirmed by means of Herington area tests. The experimental isobaric VLE data were regressed by Wilson, NRTL, and UNIQUAC models. A conventional azeotropic distillation process was designed and optimized with minimal TAC to achieve 99.9 mol % of EG and NPG based on preceding results.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 519 86330355. Fax: +86 519 86330355. E-mail:
[email protected]. Funding
There is no grant of financial support in the study. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful for assistance from the staff at the School of Petrochemical Technology (Changzhou University).
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NOMENCLATURE VLE = vapor−liquid equilibrium EG = ethylene glycol NPG = neopentyl glycol PX = para-xylene NRTL = nonrandom two-liquid UNIQUAC = universal quasichemical activity coefficient TAC = total annual cost FID = flame ionization detector ADD = average absolute deviation NT = total number of trays QR = reboiler duty HX = heater exchanger NF = feed tray
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REFERENCES
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DOI: 10.1021/acs.jced.5b01044 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
