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Measurements and modeling of high-pressure phase behavior of binary CO2–amides systems
Fluid Phase Equilibria 208 (2003) 53–68
Measurements and modeling of high-pressure phase behavior of binary CO2 –amides systems Hun-Soo Byun a,∗ , Nak-Hyun Kim b , Chul Kwak b a
b
Department of Chemical Engineering, Yosu National University, San 96-1, Dundeok, Yosu, Chonnam 550-749, South Korea School of Chemical Engineering, Kyungnam University, Masan, Kyungnam 631-701, South Korea Received 14 June 2002; accepted 3 December 2002
Abstract High-pressure vapor–liquid equilibrium (VLE) data were measured for binary CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems at various temperatures (318.2–398.2 K) and pressure up to 25 MPa. These CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems exhibits type-I phase behavior, which is characterized by an uninterrupted critical mixture curve which has a maximum in pressure. The solubility of N,N-dimethylformamide, N,N-diethylformamide and N,N-dibutylformamide for the CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems increases as the temperatures increases at constant pressure. The CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems have continuous critical mixture curves that exhibit maximums in pressure at temperatures between the critical temperatures of CO2 and N,N-dimethylformamide, N,N-diethylformamide or N,N-dibutylformamide. Also, three phases were not observed of any of the systems studied. In each systems, the mixture critical point increases as the temperatures increases, and also the mixture critical pressure does as molecular weight increases. The experimental results for CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems is modeled using the Peng–Robinson equation of state. A good fit of the calculated data is obtained with the Peng–Robinson equation of state. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Bubble- and dew-point behavior; Carbon dioxide; N,N-Dimethylformamide; N,N-Diethylformamide; N,N-Dibutylformamide; Peng–Robinson equation
1. Introduction Thermodynamic knowledge of high-pressure phase behavior experimental data of solutes and supercritical carbon dioxide mixtures plays an essential role in the basic design of various separation processes ∗
Corresponding author. Tel.: +82-61-659-3296; fax: +82-61-653-3659. E-mail address: [email protected] (H.-S. Byun). 0378-3812/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-3812(02)00324-2
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and fine chemical industries. As a result, the attention has been placed on the thermodynamic understanding of supercritical fluid systems [1,2]. The information on the high-pressure behavior of fluids at supercritical states has been valuable in the design of new separation processes in various fields such as food, pharmaceutical and related industries [3]. The high-pressure phase equilibrium information of mixtures composed of CO2 and amides is of importance and has been actively studied for various utilities. The phase behavior experimental data relations for the CO2 –N,N-dimethylformamide system were reported by Duran-Valencia et al. [4] at 293.95–338.05 K and pressure up to 12 MPa, and by Chang et al. [5] at 290.8–310.1 K and pressure up to 12.34 MPa. Chang et al. [5] used a static phase equilibrium apparatus and obtained experimental data for liquid phase mole fraction. Duran-Valencia et al. [4] used a static–analytic experimental apparatus to determine the vapor–liquid equilibria data but did not observe three-phase separation. Also, Byun and Jeon [6] studied the phase behavior of CO2 –N,N-dimethylacetamide and CO2 –N,N-diethylacetamide systems. Recently, we have performed phase behavior experiments for containing carbon dioxide [7–10]. In this paper, a static-type experimental apparatus was designed for the present study and measurement of the phase behavior experimental data for CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems. The isothermal phase behavior experimental data of the binary mixture CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems have been measured in the temperature range from 313.2 to 398.2 K and pressure up to 25 MPa. Therefore, the purpose of this study was to determine the bubble, critical and dew points experimentally for binary mixtures of CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems. Also, the pressure– composition isotherms experimental data were modeled using the Peng–Robinson equation of state with two adjustable binary interaction parameters. 2. Experiment 2.1. Chemicals Carbon dioxide was provided by Deasung Oxygen Co. (Korea) with a certified purity of 99.9%. The N,N-dimethylformamide (99.9% purity), N,N-diethylformamide (99.0% purity) and N,N-dibutylformamide (99.0% purity) used in this work are obtained from Aldrich. The chemicals were used without further purification. 2.2. Apparatus and procedure Bubble-, dew- and critical-point curves are obtained using a high-pressure variable-volume that has a 1.59 cm i.d., an o.d. of 7.0 cm and a working volume of 28 cm3 , equipped with a window for visual observation and a movable piston. Fig. 1 shows a diagram of the experimental set-up. The experimental apparatus and procedure have been described in detail elsewhere [9,10]. The temperature was measured using a thermocouple placed inside wells drilled directly into the body of the equilibrium cell. The temperature of the cell is measured using a platinum resistance thermometer (Thermometrics Corp., class A) and a digital multimeter (Yokogawa, model 7563, accurate to within ±0.005%). It contains an efficient magnetic stirrer to ensure fast equilibrium. The pressure was measured by means of a pressure transducer (Dresser Ind., model CM-35790, 690 bar) for which the accuracy of pressure readings was found to be
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Fig. 1. Schematic diagram of the experimental apparatus used in this study.
better than 0.01 MPa. The mixture inside the cell is viewed on a video monitor using a camera coupled to a borescope (Olympus Corp., model F100-038-000-50) placed against the outside of the sapphire window. The empty cell is purged several times with nitrogen followed by carbon dioxide to ensure that all of the air is removed. The liquid solute is loaded into the cell to within ±0.002 g using a syringe and carbon dioxide is transferred into the cell gravimetrically to within ±0.004 g using a high-pressure bomb. In this work, the uncertainty was reported for the analysis of the composition in mole fraction for both vapor and liquid phases was estimated to be within <1.0%. The solution in the cell is compressed to the desired operating pressure by displacing a movable piston using water pressurized by a high-pressure generator (HIP Inc., model 37-5.75-60). The equilibrium cell is maintained at the desired working temperature, controlled to within 0.1 K. The solution in the cell is stirred by a magnetic stir bar, which is activated by an external magnet beneath the cell. To reach thermal equilibrium, the cell is maintained at the temperature for at least 30–40 min. When equilibrium was reached, the mixture in the cell is compressed to a single phase and the pressure is then slowly decreased until a second phase appeared at a fixed temperature. A bubble point is obtained if a small bubble appears in the cell, and a dew point is obtained if a fine mist appears in the cell. The transition occurs in the mixture critical point if critical opalescence is observed during the transition process and if two phases of equal volume are present when the mixture phase separates. After this pressure has been determined at a given temperature, the procedure is repeated at a new temperature, until a pressure–temperature isopleth for the solution has been obtained. The estimated accuracy of the pressure measurements is 0.07 MPa. 3. Results and discussion 3.1. Experimental results The accuracy and reproducibility of experimental apparatus have been already certified of phase equilibria data consistency for binary carbon dioxide–acetic acid mixture by Byun et al. [11].
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Table 1 Pressure–composition isotherms data for the CO2 –N,N-dimethylformamide system Mole fraction of N,N-DMFA
Pressure (MPa)
Transition
T = 318.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.171 0.151 0.121 0.100 0.078 0.059 0.038 0.028 0.014 0.006
3.86 4.66 6.21 7.59 8.17 8.45 8.48 8.79 9.00 9.14 9.26 9.35 9.35 9.52 9.41 9.15 9.07
BP BP BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP
T = 338.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.155 0.098 0.080 0.055 0.048 0.041 0.034 0.031 0.028 0.023
4.52 5.38 8.07 9.98 10.72 11.15 11.36 12.31 12.43 12.44 12.43 12.45 12.35 12.30 12.31 12.19 11.90
BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP DP DP
T = 358.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.172 0.118
5.10 6.66 10.17 12.50 13.90 14.45 14.41 16.04 16.03
BP BP BP BP BP BP BP BP BP
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Table 1 (Continued ) Mole fraction of N,N-DMFA
Pressure (MPa)
Transition
15.76 15.77 15.77 15.62 15.75 15.64 15.62 15.35
BP BP BP BP BP BP CP DP
T = 378.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.171 0.137 0.103 0.083 0.075
5.69 7.76 12.24 15.10 16.52 17.38 17.48 18.48 18.38 18.26 18.10 17.97
BP BP BP BP BP BP BP BP BP BP CP DP
T = 398.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.169 0.135 0.113 0.102 0.087 0.079
6.28 8.79 14.14 17.35 18.97 19.79 20.00 20.67 20.66 20.46 20.49 20.24 20.03
BP BP BP BP BP BP BP BP BP BP CP DP DP
0.122 0.107 0.090 0.074 0.066 0.059 0.056 0.052
BP, bubble point; DP, dew point; CP, critical point.
Bubble-, dew- and critical-point data for CO2 –amides systems are reproduced to within ±0.03 MPa at least twice for a given loading of the cell. Tables 1–3 present the data of the CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems obtained in this work. The mole fractions are accurate to within ±0.002. The mole fractions for the solubility isotherms from 318.2 to 398.2 K are arranged according to the value at least two independent data points which have an accumulated error of less than ±1.0%. Table 1 and Fig. 2 shows the experimental pressure–composition (P–x) isotherms at 318.2, 338.2, 358.2, 378.2 and 398.2 K, and the range of pressures of 3.86–20.67 MPa for the CO2 –N,N-dimethylformamide
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Table 2 Pressure–composition isotherms data for the CO2 –N,N-diethylformamide system Mole fraction of N,N-DEFA
Pressure (MPa)
Transition
T = 318.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.057 0.048 0.034 0.026 0.014 0.010 0.009 0.006
2.90 4.42 5.70 6.79 7.65 8.46 8.86 9.14 9.26 9.35 9.40 9.41 9.35 9.29 9.28 9.07
BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP DP
T = 338.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.057 0.048 0.034 0.031 0.026 0.014 0.010
3.46 5.32 7.18 8.86 10.31 11.44 12.17 12.45 12.45 12.45 12.45 12.40 12.19 11.87 11.55
BP BP BP BP BP BP BP BP BP BP CP DP DP DP DP
T = 358.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.057 0.048 0.034 0.026 0.014
3.90 6.35 8.85 10.93 13.21 14.66 15.77 15.90 15.90 15.76 15.23 14.92 13.76
BP BP BP BP BP BP BP BP CP DP DP DP DP
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Table 2 (Continued ) Mole fraction of N,N-DEFA
Pressure (MPa)
Transition
T = 378.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.071 0.057 0.048 0.034 0.026
4.44 7.35 10.49 13.15 15.97 17.69 18.52 18.52 18.50 18.38 17.98 17.21 16.46
BP BP BP BP BP BP BP BP CP DP DP DP
T = 398.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.088 0.085 0.080 0.077 0.048 0.034
4.86 8.44 12.05 15.35 18.26 20.16 20.72 20.59 20.59 20.45 20.38 19.90 18.86
BP BP BP BP BP BP BP BP CP DP DP DP DP
BP, bubble point; DP, dew point; CP, critical point.
system. Three phases were not observed at any of the five temperatures. The P–x isotherms shown in Fig. 2 are consistent with those expected for a type-I system [12,13] where a maximum occurs in the critical mixture curve. Table 2 and Fig. 3 shows the experimental P–x isotherms at 318.2, 338.2, 358.2, 378.2 and 398.2 K, and the range of pressure of 2.90–20.72 MPa for the CO2 –N,N-diethylformamide system. Also, the type-I phase behavior is observed for CO2 –N,N-diethylformamide system. Table 3 and Fig. 4 presents the phase behavior experimental data at 318.2, 338.2, 358.2, 378.2 and 398.2 K and pressure up to 22.66 MPa for the CO2 –N,N-dibutylformamide system. As shown in Fig. 4, the mixture critical pressures shows 9.42 MPa (at 318.2 K), 13.42 MPa (at 338.2 K), 17.30 MPa (at 358.2 K), 20.41 MPa (at 378.2 K) and 22.66 MPa (at 398.2 K), respectively. Three phases are not observed for the CO2 –N,N-dibutylformamide system. The pressure of each mixture critical point continually increases as the temperature increases. The solubility of carbon dioxide decreases as temperatures shift higher at the constant pressure.
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Table 3 Pressure–composition isotherms data for the CO2 –N,N-dibutylformamide system Mole fraction of N,N-DBFA
Pressure (MPa)
Transition
T = 318.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.048 0.038 0.021 0.011 0.009 0.005
2.45 3.76 4.86 6.34 7.17 8.45 8.88 9.14 9.19 9.24 9.31 9.33 9.36 9.38 9.40 9.42
BP BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP
T = 338.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.048 0.038 0.021 0.011 0.009
3.00 4.41 5.93 8.31 9.59 11.69 12.69 13.07 13.12 13.17 13.28 13.41 13.42 12.48 12.38
BP BP BP BP BP BP BP BP BP BP BP BP CP DP DP
T = 358.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.048 0.038 0.021 0.011
3.59 5.28 9.31 10.17 12.07 15.03 16.66 17.07 17.12 17.17 17.28 17.30 16.14 15.07
BP BP BP BP BP BP BP BP BP BP BP CP DP DP
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Table 3 (Continued ) Mole fraction of N,N-DBFA
Pressure (MPa)
Transition
T = 378.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.056 0.048 0.038 0.021
4.16 6.28 8.62 12.17 14.72 18.10 20.14 20.28 20.23 20.25 20.38 20.41 20.24 18.31
BP BP BP BP BP BP BP BP BP BP BP CP DP DP
T = 398.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.056 0.048 0.038 0.021
4.79 7.21 9.93 14.03 17.00 20.93 22.59 22.67 22.59 22.66 22.66 22.52 22.24 19.52
BP BP BP BP BP BP BP BP BP BP CP DP DP DP
BP, bubble point; DP, dew point; CP, critical point.
3.2. Thermodynamic modeling The isothermal phase equilibria experimental data in this work is modeled using the Peng–Robinson equation of state. The equation of state is briefly described here. For the correlation with the experimental data, we used the Peng–Robinson equation of state [14] with the following mixing rules: amix = xi xj aij (1) i
j
aij = (aii ajj )1/2 (1 − kij ) xi xj bij bmix = i
j
(2) (3)
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Fig. 2. Experimental isotherms for the CO2 –N,N-dimethylformamide system obtained in this study at 318.2, 338.2, 358.2, 378.2 and 398.2 K.
bij = 0.5[(bii + bjj )](1 − ηij )
(4)
where kij and ηij are the interaction binary parameters that are determined by fitting pressure–composition data and aii and bii the pure component parameters as defined by Peng and Robinson. The expression for the fugacity coefficient using these mixing rules is given by Peng and Robinson [14] and is not reproduced here. The critical parameters (Tc and Pc ) and acentric factors (ω) used in this work are reported in Table 4 [15–18]. The properties of N,N-diethylformamide and N,N-dibutylformamide were calculated by group-contribution method [15]. The boiling points of N,N-diethylformamide and N,N-dibutylformamide were obtained by Aldrich.
Fig. 3. Experimental isotherms for the CO2 –N,N-diethylformamide system obtained in this study at 318.2, 338.2, 358.2, 378.2 and 398.2 K.
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Fig. 4. Experimental isotherms for the CO2 –N,N-dibutylformamide system obtained in this study at 318.2, 338.2, 358.2, 378.2 and 398.2 K.
Fig. 5 shows a comparison of CO2 –N,N-diethylformamide experimental results with calculations obtained using Peng–Robinson equation at a temperature of 358.2 K. The binary interaction parameters of the Peng–Robinson equation of state are fitted by the experimental data at 358.2 K. The values of the adjusted parameters for the Peng–Robinson equation of state of the CO2 –N,N-diethylformamide system are kij = 0.012 and ηij = −0.052. A reasonable fit of the data is obtained over most of the composition range even if no mixture parameters are used. But if two mixture parameters, are used the fit of the experimental results is significantly better. These sets of parameters are used to predict the vapor–liquid equilibria at two other temperatures, namely, 318.2, 338.2, 378.2 and 398.2 K. Fig. 6 shows a comparison of experimental with calculated data at the temperatures of 318.2, 338.2, 358.2, 378.2 and 398.2 K for the CO2 –N,N-diethylformamide mixture. These isotherms are calculated using the optimized values of kij = 0.012 and ηij = −0.052 determined at 358.2 K. A good fit of the data is obtained with Peng–Robinson equation of state using two adjustable mixture parameters for the CO2 –N,N-diethylformamide system. Fig. 7 shows predicted P–x isotherms for the CO2 –N,N-dimethylformamide mixture at 318.2, 338.2, 358.2, 378.2 and 398.2 K using the Peng–Robinson equation of state with kij = 0.048 and ηij = −0.050. These optimized values of the mixture parameters are obtained by fitting the 358.2 K isotherm between the experimental and calculated results. Table 4 Pure component parameters [15–18] used with the Peng–Robinson equation of state Component
Tc (K)
Pc (MPa)
Acentric factor
CO2 N,N-DMFA N,N-DEFA N,N-DBFA
304.2 649.7 655.5 691.4
7.39 4.37 3.73 2.48
0.225 0.312 0.485 0.691
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Fig. 5. Comparison of the best fit of Peng–Robinson equation of state to the CO2 –N,N-diethylformamide system at 358.2 K.
Fig. 8 shows predicted P–x isotherms for the CO2 –N,N-dibutylformamide mixture at 318.2, 338.2, 358.2, 378.2 and 398.2 K using the Peng–Robinson equation of state with kij = 0.015 and ηij = −0.025. A good fit of the data is obtained with Peng–Robinson equation of state using two adjustable mixture parameters for the CO2 –N,N-dibutylformamide system. Fig. 9 shows the mixture critical curve for the CO2 –N,N-dimethylformamide system predicted by the Peng–Robinson equation of state. The calculated mixture critical curve is type-I, in agreement with experimental observations. As shown Fig. 9, the solid line represent the vapor pressure for pure CO2 [15,16] and N,N-dimethylformamide [15,17,18]. The solid circles represent the critical point for pure CO2 and N,N-dimethylformamide. The upper part of the dashed line is single phase (fluid), the lower part is two phases (vapor–liquid). The open squares are the mixture critical points determined from
Fig. 6. Comparison of the experimental data (symbols) for the CO2 –N,N-diethylformamide system with calculated values (solid lines) obtained with Peng–Robinson equation of state with kij = 0.012 and ηij = −0.052.
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Fig. 7. Comparison of the experimental data (symbols) for the CO2 –N,N-dimethylformamide system with calculated values (solid lines) obtained with Peng–Robinson equation of state with kij = 0.048 and ηij = −0.050.
isotherms measured in this experiment. The dashed line represent the calculated value obtained using the Peng–Robinson equation of state. The binary mixture parameters are then obtained from the Peng–Robinson equation, with kij = 0.048 and ηij = −0.050. Fig. 10 shows the mixture critical curve for the CO2 –N,N-diethylformamide system predicted by the Peng–Robinson equation of state. The calculated mixture critical curve is type-I, in agreement with experimental observations. As shown Fig. 10, the solid line represent the vapor pressure for pure CO2 [15,16] and N,N-diethylformamide [15]. The vapor pressure of N,N-diethylformamide obtained by Lee–Kesler method [15]. The dashed line represent the calculated value obtained using the Peng–Robinson equation of state. The binary mixture parameters are then obtained from the case Peng–Robinson equation, with
Fig. 8. Comparison of the experimental data (symbols) for the CO2 –N,N-dibutylformamide system with calculated values (solid lines) obtained with Peng–Robinson equation of state with kij = 0.015 and ηij = −0.025.
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Fig. 9. Pressure–temperature diagram for the CO2 –N,N-dimethylformamide system. The solid line and the solid circles represent the vapor–liquid line and the critical point for pure CO2 [15,16] and N,N-dimethylformamide [15,17,18]. The open squares are critical points determined from isotherms measured in this study. The dashed line represent calculated values obtained using Peng–Robinson equation of state with kij = 0.048 and ηij = −0.050.
kij = 0.012 and ηij = −0.052. The obtain process of optimum binary interaction parameters are identified with the method in the CO2 –N,N-dimethylformamide system. Fig. 11 shows the mixture critical curve for the CO2 –N,N-dibutylformamide system predicted by the Peng–Robinson equation of state. The mixture critical curves calculated by the two mixture parameters are type-I, and three phases (liquid–liquid–vapor) are not observed. As shown Fig. 11, the solid line represent the vapor pressure for pure CO2 [15,16] and N,N-dibutylformamide [15]. The vapor pressure
Fig. 10. Pressure–temperature diagram for the CO2 –N,N-diethylformamide system. The solid line and the solid circles represent the vapor–liquid line and the critical point for pure CO2 [15,16] and N,N-diethylformamide [15]. The open squares are critical points determined from isotherms measured in this study. The dashed line represent calculated values obtained using Peng–Robinson equation of state with kij = 0.012 and ηij = −0.052.
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Fig. 11. Pressure–temperature diagram for the CO2 –N,N-dibutylformamide system. The solid line and the solid circles represent the vapor–liquid line and the critical point for pure CO2 [15,16] and N,N-dibutylformamide [15]. The open squares are critical points determined from isotherms measured in this study. The dashed line represent calculated values obtained using Peng–Robinson equation of state with kij = 0.015 and ηij = −0.025.
of N,N-dibutylformamide obtained by Lee–Kesler method [15]. The dashed line represent calculation obtained using the Peng–Robinson equation of state, with kij = 0.015 and ηij = −0.025. The agreement between calculated and experimental mixture critical curves is reasonably good using two adjustable parameters with Peng–Robinson equation of state.
4. Conclusion We shows vapor–liquid equilibria data for the CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems. The experimental phase behavior data obtained for each mixture covers several compositions and different temperatures (318.2, 338.2, 358.2, 378.2 and 398.2 K). The CO2 –N,N-dimethyformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems exhibit type-I phase behavior, which is characterized by an uninterrupted critical mixture curve. Also, three phases were not observed at any temperatures of all the system studied. The Peng–Robinson equation is capable of accurately predicting the phase behavior for the CO2 –N,N-dimethyformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems studied using two independent temperature mixture parameters. The results shows good agreement between experimental data and calculated values by the Peng–Robinson equation of state.
Acknowledgements This work was supported by Grant No. R05-2001-000-01177-0 from the Basic Research Program of the Korea Science & Engineering Foundation. Also, this work was supported by the Kyungnam University Research Fund (2002).
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R.E. Fornari, P. Alessi, I. Kikic, Fluid Phase Equilib. 57 (1990) 1–33. C. Borch-Jensen, A. Staby, J. Mollerup, J. Supercrit. Fluids 7 (1994) 231–244. M.A. McHugh, V.J. Krukonis, Supercritical Fluid Extraction, Butterworths, Stoneham, MA, 1994. C. Duran-Valencia, A. Valtz, L.A. Galicia-Luna, D. Richon, J. Chem. Eng. Data 46 (2001) 1589–1592. C.J. Chang, C.Y. Chen, H.C. Lin, J. Chem. Eng. Data 40 (1995) 850–855. H.S. Byun, N.S. Jeon, Fluid Phase Equilib. 167 (2000) 113–120. H.S. Byun, M.A. McHugh, Ind. Eng. Chem. Res. 39 (2000) 4658–4662. H.S. Byun, Fluid Phase Equilib. 198 (2002) 299–312. H.S. Byun, J. Chem. Eng. Data 47 (2000) 359–362. H.S. Byun, C. Kwak, Hwahak Konghak 38 (2000) 366–372. H.S. Byun, K. Kim, M.A. McHugh, Ind. Eng. Chem. Res. 39 (2000) 4580–4587. R.L. Scott, P.B. Van Konynenburg, Discuss. Faraday Soc. 49 (1970) 87–97. M.A. McHugh, V.J. Krukonis, Supercritical Fluid Extraction: Principles and Practice, 2nd ed., Butterworths, Stoneham, MA, 1993. D.Y. Peng, D.B. Robinson, Ind. Eng. Chem. Fundam. 15 (1976) 59–63. R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Liquids and Gases, 4th ed., McGraw-Hill, New York, 1987. N.B. Vargaftick, Handbook of Physical Properties of Liquid and Gases, Springer-Verlag, Berlin, 1983. R.H. Perry, D.W. Green, The Properties of Gases & Liquids, 4th ed., McGraw-Hill, New York, 1987. T.F. Daubert, R.P. Danner, Data Compilation of Properties of Pure Compounds, Parts 1–4, DIPPR Project, AIChE, New York, NY, 1985–1992.
Measurements and modeling of high-pressure phase behavior of binary CO2 –amides systems Hun-Soo Byun a,∗ , Nak-Hyun Kim b , Chul Kwak b a
b
Department of Chemical Engineering, Yosu National University, San 96-1, Dundeok, Yosu, Chonnam 550-749, South Korea School of Chemical Engineering, Kyungnam University, Masan, Kyungnam 631-701, South Korea Received 14 June 2002; accepted 3 December 2002
Abstract High-pressure vapor–liquid equilibrium (VLE) data were measured for binary CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems at various temperatures (318.2–398.2 K) and pressure up to 25 MPa. These CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems exhibits type-I phase behavior, which is characterized by an uninterrupted critical mixture curve which has a maximum in pressure. The solubility of N,N-dimethylformamide, N,N-diethylformamide and N,N-dibutylformamide for the CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems increases as the temperatures increases at constant pressure. The CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems have continuous critical mixture curves that exhibit maximums in pressure at temperatures between the critical temperatures of CO2 and N,N-dimethylformamide, N,N-diethylformamide or N,N-dibutylformamide. Also, three phases were not observed of any of the systems studied. In each systems, the mixture critical point increases as the temperatures increases, and also the mixture critical pressure does as molecular weight increases. The experimental results for CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems is modeled using the Peng–Robinson equation of state. A good fit of the calculated data is obtained with the Peng–Robinson equation of state. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Bubble- and dew-point behavior; Carbon dioxide; N,N-Dimethylformamide; N,N-Diethylformamide; N,N-Dibutylformamide; Peng–Robinson equation
1. Introduction Thermodynamic knowledge of high-pressure phase behavior experimental data of solutes and supercritical carbon dioxide mixtures plays an essential role in the basic design of various separation processes ∗
Corresponding author. Tel.: +82-61-659-3296; fax: +82-61-653-3659. E-mail address: [email protected] (H.-S. Byun). 0378-3812/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0378-3812(02)00324-2
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and fine chemical industries. As a result, the attention has been placed on the thermodynamic understanding of supercritical fluid systems [1,2]. The information on the high-pressure behavior of fluids at supercritical states has been valuable in the design of new separation processes in various fields such as food, pharmaceutical and related industries [3]. The high-pressure phase equilibrium information of mixtures composed of CO2 and amides is of importance and has been actively studied for various utilities. The phase behavior experimental data relations for the CO2 –N,N-dimethylformamide system were reported by Duran-Valencia et al. [4] at 293.95–338.05 K and pressure up to 12 MPa, and by Chang et al. [5] at 290.8–310.1 K and pressure up to 12.34 MPa. Chang et al. [5] used a static phase equilibrium apparatus and obtained experimental data for liquid phase mole fraction. Duran-Valencia et al. [4] used a static–analytic experimental apparatus to determine the vapor–liquid equilibria data but did not observe three-phase separation. Also, Byun and Jeon [6] studied the phase behavior of CO2 –N,N-dimethylacetamide and CO2 –N,N-diethylacetamide systems. Recently, we have performed phase behavior experiments for containing carbon dioxide [7–10]. In this paper, a static-type experimental apparatus was designed for the present study and measurement of the phase behavior experimental data for CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems. The isothermal phase behavior experimental data of the binary mixture CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems have been measured in the temperature range from 313.2 to 398.2 K and pressure up to 25 MPa. Therefore, the purpose of this study was to determine the bubble, critical and dew points experimentally for binary mixtures of CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems. Also, the pressure– composition isotherms experimental data were modeled using the Peng–Robinson equation of state with two adjustable binary interaction parameters. 2. Experiment 2.1. Chemicals Carbon dioxide was provided by Deasung Oxygen Co. (Korea) with a certified purity of 99.9%. The N,N-dimethylformamide (99.9% purity), N,N-diethylformamide (99.0% purity) and N,N-dibutylformamide (99.0% purity) used in this work are obtained from Aldrich. The chemicals were used without further purification. 2.2. Apparatus and procedure Bubble-, dew- and critical-point curves are obtained using a high-pressure variable-volume that has a 1.59 cm i.d., an o.d. of 7.0 cm and a working volume of 28 cm3 , equipped with a window for visual observation and a movable piston. Fig. 1 shows a diagram of the experimental set-up. The experimental apparatus and procedure have been described in detail elsewhere [9,10]. The temperature was measured using a thermocouple placed inside wells drilled directly into the body of the equilibrium cell. The temperature of the cell is measured using a platinum resistance thermometer (Thermometrics Corp., class A) and a digital multimeter (Yokogawa, model 7563, accurate to within ±0.005%). It contains an efficient magnetic stirrer to ensure fast equilibrium. The pressure was measured by means of a pressure transducer (Dresser Ind., model CM-35790, 690 bar) for which the accuracy of pressure readings was found to be
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Fig. 1. Schematic diagram of the experimental apparatus used in this study.
better than 0.01 MPa. The mixture inside the cell is viewed on a video monitor using a camera coupled to a borescope (Olympus Corp., model F100-038-000-50) placed against the outside of the sapphire window. The empty cell is purged several times with nitrogen followed by carbon dioxide to ensure that all of the air is removed. The liquid solute is loaded into the cell to within ±0.002 g using a syringe and carbon dioxide is transferred into the cell gravimetrically to within ±0.004 g using a high-pressure bomb. In this work, the uncertainty was reported for the analysis of the composition in mole fraction for both vapor and liquid phases was estimated to be within <1.0%. The solution in the cell is compressed to the desired operating pressure by displacing a movable piston using water pressurized by a high-pressure generator (HIP Inc., model 37-5.75-60). The equilibrium cell is maintained at the desired working temperature, controlled to within 0.1 K. The solution in the cell is stirred by a magnetic stir bar, which is activated by an external magnet beneath the cell. To reach thermal equilibrium, the cell is maintained at the temperature for at least 30–40 min. When equilibrium was reached, the mixture in the cell is compressed to a single phase and the pressure is then slowly decreased until a second phase appeared at a fixed temperature. A bubble point is obtained if a small bubble appears in the cell, and a dew point is obtained if a fine mist appears in the cell. The transition occurs in the mixture critical point if critical opalescence is observed during the transition process and if two phases of equal volume are present when the mixture phase separates. After this pressure has been determined at a given temperature, the procedure is repeated at a new temperature, until a pressure–temperature isopleth for the solution has been obtained. The estimated accuracy of the pressure measurements is 0.07 MPa. 3. Results and discussion 3.1. Experimental results The accuracy and reproducibility of experimental apparatus have been already certified of phase equilibria data consistency for binary carbon dioxide–acetic acid mixture by Byun et al. [11].
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Table 1 Pressure–composition isotherms data for the CO2 –N,N-dimethylformamide system Mole fraction of N,N-DMFA
Pressure (MPa)
Transition
T = 318.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.171 0.151 0.121 0.100 0.078 0.059 0.038 0.028 0.014 0.006
3.86 4.66 6.21 7.59 8.17 8.45 8.48 8.79 9.00 9.14 9.26 9.35 9.35 9.52 9.41 9.15 9.07
BP BP BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP
T = 338.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.155 0.098 0.080 0.055 0.048 0.041 0.034 0.031 0.028 0.023
4.52 5.38 8.07 9.98 10.72 11.15 11.36 12.31 12.43 12.44 12.43 12.45 12.35 12.30 12.31 12.19 11.90
BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP DP DP
T = 358.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.172 0.118
5.10 6.66 10.17 12.50 13.90 14.45 14.41 16.04 16.03
BP BP BP BP BP BP BP BP BP
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Table 1 (Continued ) Mole fraction of N,N-DMFA
Pressure (MPa)
Transition
15.76 15.77 15.77 15.62 15.75 15.64 15.62 15.35
BP BP BP BP BP BP CP DP
T = 378.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.171 0.137 0.103 0.083 0.075
5.69 7.76 12.24 15.10 16.52 17.38 17.48 18.48 18.38 18.26 18.10 17.97
BP BP BP BP BP BP BP BP BP BP CP DP
T = 398.2 K 0.789 0.671 0.457 0.378 0.300 0.250 0.224 0.169 0.135 0.113 0.102 0.087 0.079
6.28 8.79 14.14 17.35 18.97 19.79 20.00 20.67 20.66 20.46 20.49 20.24 20.03
BP BP BP BP BP BP BP BP BP BP CP DP DP
0.122 0.107 0.090 0.074 0.066 0.059 0.056 0.052
BP, bubble point; DP, dew point; CP, critical point.
Bubble-, dew- and critical-point data for CO2 –amides systems are reproduced to within ±0.03 MPa at least twice for a given loading of the cell. Tables 1–3 present the data of the CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems obtained in this work. The mole fractions are accurate to within ±0.002. The mole fractions for the solubility isotherms from 318.2 to 398.2 K are arranged according to the value at least two independent data points which have an accumulated error of less than ±1.0%. Table 1 and Fig. 2 shows the experimental pressure–composition (P–x) isotherms at 318.2, 338.2, 358.2, 378.2 and 398.2 K, and the range of pressures of 3.86–20.67 MPa for the CO2 –N,N-dimethylformamide
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Table 2 Pressure–composition isotherms data for the CO2 –N,N-diethylformamide system Mole fraction of N,N-DEFA
Pressure (MPa)
Transition
T = 318.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.057 0.048 0.034 0.026 0.014 0.010 0.009 0.006
2.90 4.42 5.70 6.79 7.65 8.46 8.86 9.14 9.26 9.35 9.40 9.41 9.35 9.29 9.28 9.07
BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP DP
T = 338.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.057 0.048 0.034 0.031 0.026 0.014 0.010
3.46 5.32 7.18 8.86 10.31 11.44 12.17 12.45 12.45 12.45 12.45 12.40 12.19 11.87 11.55
BP BP BP BP BP BP BP BP BP BP CP DP DP DP DP
T = 358.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.057 0.048 0.034 0.026 0.014
3.90 6.35 8.85 10.93 13.21 14.66 15.77 15.90 15.90 15.76 15.23 14.92 13.76
BP BP BP BP BP BP BP BP CP DP DP DP DP
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Table 2 (Continued ) Mole fraction of N,N-DEFA
Pressure (MPa)
Transition
T = 378.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.077 0.071 0.057 0.048 0.034 0.026
4.44 7.35 10.49 13.15 15.97 17.69 18.52 18.52 18.50 18.38 17.98 17.21 16.46
BP BP BP BP BP BP BP BP CP DP DP DP
T = 398.2 K 0.829 0.674 0.518 0.386 0.284 0.197 0.136 0.088 0.085 0.080 0.077 0.048 0.034
4.86 8.44 12.05 15.35 18.26 20.16 20.72 20.59 20.59 20.45 20.38 19.90 18.86
BP BP BP BP BP BP BP BP CP DP DP DP DP
BP, bubble point; DP, dew point; CP, critical point.
system. Three phases were not observed at any of the five temperatures. The P–x isotherms shown in Fig. 2 are consistent with those expected for a type-I system [12,13] where a maximum occurs in the critical mixture curve. Table 2 and Fig. 3 shows the experimental P–x isotherms at 318.2, 338.2, 358.2, 378.2 and 398.2 K, and the range of pressure of 2.90–20.72 MPa for the CO2 –N,N-diethylformamide system. Also, the type-I phase behavior is observed for CO2 –N,N-diethylformamide system. Table 3 and Fig. 4 presents the phase behavior experimental data at 318.2, 338.2, 358.2, 378.2 and 398.2 K and pressure up to 22.66 MPa for the CO2 –N,N-dibutylformamide system. As shown in Fig. 4, the mixture critical pressures shows 9.42 MPa (at 318.2 K), 13.42 MPa (at 338.2 K), 17.30 MPa (at 358.2 K), 20.41 MPa (at 378.2 K) and 22.66 MPa (at 398.2 K), respectively. Three phases are not observed for the CO2 –N,N-dibutylformamide system. The pressure of each mixture critical point continually increases as the temperature increases. The solubility of carbon dioxide decreases as temperatures shift higher at the constant pressure.
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Table 3 Pressure–composition isotherms data for the CO2 –N,N-dibutylformamide system Mole fraction of N,N-DBFA
Pressure (MPa)
Transition
T = 318.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.048 0.038 0.021 0.011 0.009 0.005
2.45 3.76 4.86 6.34 7.17 8.45 8.88 9.14 9.19 9.24 9.31 9.33 9.36 9.38 9.40 9.42
BP BP BP BP BP BP BP BP BP BP BP BP BP BP BP CP
T = 338.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.048 0.038 0.021 0.011 0.009
3.00 4.41 5.93 8.31 9.59 11.69 12.69 13.07 13.12 13.17 13.28 13.41 13.42 12.48 12.38
BP BP BP BP BP BP BP BP BP BP BP BP CP DP DP
T = 358.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.048 0.038 0.021 0.011
3.59 5.28 9.31 10.17 12.07 15.03 16.66 17.07 17.12 17.17 17.28 17.30 16.14 15.07
BP BP BP BP BP BP BP BP BP BP BP CP DP DP
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Table 3 (Continued ) Mole fraction of N,N-DBFA
Pressure (MPa)
Transition
T = 378.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.056 0.048 0.038 0.021
4.16 6.28 8.62 12.17 14.72 18.10 20.14 20.28 20.23 20.25 20.38 20.41 20.24 18.31
BP BP BP BP BP BP BP BP BP BP BP CP DP DP
T = 398.2 K 0.747 0.639 0.521 0.391 0.297 0.193 0.112 0.083 0.075 0.066 0.056 0.048 0.038 0.021
4.79 7.21 9.93 14.03 17.00 20.93 22.59 22.67 22.59 22.66 22.66 22.52 22.24 19.52
BP BP BP BP BP BP BP BP BP BP CP DP DP DP
BP, bubble point; DP, dew point; CP, critical point.
3.2. Thermodynamic modeling The isothermal phase equilibria experimental data in this work is modeled using the Peng–Robinson equation of state. The equation of state is briefly described here. For the correlation with the experimental data, we used the Peng–Robinson equation of state [14] with the following mixing rules: amix = xi xj aij (1) i
j
aij = (aii ajj )1/2 (1 − kij ) xi xj bij bmix = i
j
(2) (3)
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Fig. 2. Experimental isotherms for the CO2 –N,N-dimethylformamide system obtained in this study at 318.2, 338.2, 358.2, 378.2 and 398.2 K.
bij = 0.5[(bii + bjj )](1 − ηij )
(4)
where kij and ηij are the interaction binary parameters that are determined by fitting pressure–composition data and aii and bii the pure component parameters as defined by Peng and Robinson. The expression for the fugacity coefficient using these mixing rules is given by Peng and Robinson [14] and is not reproduced here. The critical parameters (Tc and Pc ) and acentric factors (ω) used in this work are reported in Table 4 [15–18]. The properties of N,N-diethylformamide and N,N-dibutylformamide were calculated by group-contribution method [15]. The boiling points of N,N-diethylformamide and N,N-dibutylformamide were obtained by Aldrich.
Fig. 3. Experimental isotherms for the CO2 –N,N-diethylformamide system obtained in this study at 318.2, 338.2, 358.2, 378.2 and 398.2 K.
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Fig. 4. Experimental isotherms for the CO2 –N,N-dibutylformamide system obtained in this study at 318.2, 338.2, 358.2, 378.2 and 398.2 K.
Fig. 5 shows a comparison of CO2 –N,N-diethylformamide experimental results with calculations obtained using Peng–Robinson equation at a temperature of 358.2 K. The binary interaction parameters of the Peng–Robinson equation of state are fitted by the experimental data at 358.2 K. The values of the adjusted parameters for the Peng–Robinson equation of state of the CO2 –N,N-diethylformamide system are kij = 0.012 and ηij = −0.052. A reasonable fit of the data is obtained over most of the composition range even if no mixture parameters are used. But if two mixture parameters, are used the fit of the experimental results is significantly better. These sets of parameters are used to predict the vapor–liquid equilibria at two other temperatures, namely, 318.2, 338.2, 378.2 and 398.2 K. Fig. 6 shows a comparison of experimental with calculated data at the temperatures of 318.2, 338.2, 358.2, 378.2 and 398.2 K for the CO2 –N,N-diethylformamide mixture. These isotherms are calculated using the optimized values of kij = 0.012 and ηij = −0.052 determined at 358.2 K. A good fit of the data is obtained with Peng–Robinson equation of state using two adjustable mixture parameters for the CO2 –N,N-diethylformamide system. Fig. 7 shows predicted P–x isotherms for the CO2 –N,N-dimethylformamide mixture at 318.2, 338.2, 358.2, 378.2 and 398.2 K using the Peng–Robinson equation of state with kij = 0.048 and ηij = −0.050. These optimized values of the mixture parameters are obtained by fitting the 358.2 K isotherm between the experimental and calculated results. Table 4 Pure component parameters [15–18] used with the Peng–Robinson equation of state Component
Tc (K)
Pc (MPa)
Acentric factor
CO2 N,N-DMFA N,N-DEFA N,N-DBFA
304.2 649.7 655.5 691.4
7.39 4.37 3.73 2.48
0.225 0.312 0.485 0.691
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Fig. 5. Comparison of the best fit of Peng–Robinson equation of state to the CO2 –N,N-diethylformamide system at 358.2 K.
Fig. 8 shows predicted P–x isotherms for the CO2 –N,N-dibutylformamide mixture at 318.2, 338.2, 358.2, 378.2 and 398.2 K using the Peng–Robinson equation of state with kij = 0.015 and ηij = −0.025. A good fit of the data is obtained with Peng–Robinson equation of state using two adjustable mixture parameters for the CO2 –N,N-dibutylformamide system. Fig. 9 shows the mixture critical curve for the CO2 –N,N-dimethylformamide system predicted by the Peng–Robinson equation of state. The calculated mixture critical curve is type-I, in agreement with experimental observations. As shown Fig. 9, the solid line represent the vapor pressure for pure CO2 [15,16] and N,N-dimethylformamide [15,17,18]. The solid circles represent the critical point for pure CO2 and N,N-dimethylformamide. The upper part of the dashed line is single phase (fluid), the lower part is two phases (vapor–liquid). The open squares are the mixture critical points determined from
Fig. 6. Comparison of the experimental data (symbols) for the CO2 –N,N-diethylformamide system with calculated values (solid lines) obtained with Peng–Robinson equation of state with kij = 0.012 and ηij = −0.052.
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Fig. 7. Comparison of the experimental data (symbols) for the CO2 –N,N-dimethylformamide system with calculated values (solid lines) obtained with Peng–Robinson equation of state with kij = 0.048 and ηij = −0.050.
isotherms measured in this experiment. The dashed line represent the calculated value obtained using the Peng–Robinson equation of state. The binary mixture parameters are then obtained from the Peng–Robinson equation, with kij = 0.048 and ηij = −0.050. Fig. 10 shows the mixture critical curve for the CO2 –N,N-diethylformamide system predicted by the Peng–Robinson equation of state. The calculated mixture critical curve is type-I, in agreement with experimental observations. As shown Fig. 10, the solid line represent the vapor pressure for pure CO2 [15,16] and N,N-diethylformamide [15]. The vapor pressure of N,N-diethylformamide obtained by Lee–Kesler method [15]. The dashed line represent the calculated value obtained using the Peng–Robinson equation of state. The binary mixture parameters are then obtained from the case Peng–Robinson equation, with
Fig. 8. Comparison of the experimental data (symbols) for the CO2 –N,N-dibutylformamide system with calculated values (solid lines) obtained with Peng–Robinson equation of state with kij = 0.015 and ηij = −0.025.
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Fig. 9. Pressure–temperature diagram for the CO2 –N,N-dimethylformamide system. The solid line and the solid circles represent the vapor–liquid line and the critical point for pure CO2 [15,16] and N,N-dimethylformamide [15,17,18]. The open squares are critical points determined from isotherms measured in this study. The dashed line represent calculated values obtained using Peng–Robinson equation of state with kij = 0.048 and ηij = −0.050.
kij = 0.012 and ηij = −0.052. The obtain process of optimum binary interaction parameters are identified with the method in the CO2 –N,N-dimethylformamide system. Fig. 11 shows the mixture critical curve for the CO2 –N,N-dibutylformamide system predicted by the Peng–Robinson equation of state. The mixture critical curves calculated by the two mixture parameters are type-I, and three phases (liquid–liquid–vapor) are not observed. As shown Fig. 11, the solid line represent the vapor pressure for pure CO2 [15,16] and N,N-dibutylformamide [15]. The vapor pressure
Fig. 10. Pressure–temperature diagram for the CO2 –N,N-diethylformamide system. The solid line and the solid circles represent the vapor–liquid line and the critical point for pure CO2 [15,16] and N,N-diethylformamide [15]. The open squares are critical points determined from isotherms measured in this study. The dashed line represent calculated values obtained using Peng–Robinson equation of state with kij = 0.012 and ηij = −0.052.
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Fig. 11. Pressure–temperature diagram for the CO2 –N,N-dibutylformamide system. The solid line and the solid circles represent the vapor–liquid line and the critical point for pure CO2 [15,16] and N,N-dibutylformamide [15]. The open squares are critical points determined from isotherms measured in this study. The dashed line represent calculated values obtained using Peng–Robinson equation of state with kij = 0.015 and ηij = −0.025.
of N,N-dibutylformamide obtained by Lee–Kesler method [15]. The dashed line represent calculation obtained using the Peng–Robinson equation of state, with kij = 0.015 and ηij = −0.025. The agreement between calculated and experimental mixture critical curves is reasonably good using two adjustable parameters with Peng–Robinson equation of state.
4. Conclusion We shows vapor–liquid equilibria data for the CO2 –N,N-dimethylformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems. The experimental phase behavior data obtained for each mixture covers several compositions and different temperatures (318.2, 338.2, 358.2, 378.2 and 398.2 K). The CO2 –N,N-dimethyformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems exhibit type-I phase behavior, which is characterized by an uninterrupted critical mixture curve. Also, three phases were not observed at any temperatures of all the system studied. The Peng–Robinson equation is capable of accurately predicting the phase behavior for the CO2 –N,N-dimethyformamide, CO2 –N,N-diethylformamide and CO2 –N,N-dibutylformamide systems studied using two independent temperature mixture parameters. The results shows good agreement between experimental data and calculated values by the Peng–Robinson equation of state.
Acknowledgements This work was supported by Grant No. R05-2001-000-01177-0 from the Basic Research Program of the Korea Science & Engineering Foundation. Also, this work was supported by the Kyungnam University Research Fund (2002).
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