Vapor–liquid phase equilibrium behavior of mixtures containing supercritical carbon dioxide near critical region

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J. of Supercritical Fluids 44 (2008) 273–278

Vapor–liquid phase equilibrium behavior of mixtures containing supercritical carbon dioxide near critical region Hung-Yu Chiu, Rong-Feng Jung, Ming-Jer Lee ∗ , Ho-Mu Lin Department of Chemical Engineering, National Taiwan University of Science & Technology, 43 Keelung Road, Section 4, Taipei 106-07, Taiwan Received 27 February 2007; received in revised form 28 August 2007; accepted 11 September 2007

Abstract A new visual and volume-variable high-pressure phase equilibrium analyzer (PEA) was installed for determining the vapor–liquid phase boundaries over a wide pressure range, including near critical region, by a synthetic method. This apparatus was tested with the measurement of CO2 + 1-octanol. The PEA was also utilized to measure the isothermal vapor–liquid phase boundaries of CO2 + dimethyl sulfoxide, CO2 + propylene glycol monomethyl ether acetate, and CO2 + quinoline at temperatures from 308.15 to 358.15 K and pressures up to 28 MPa or near critical pressures. These phase boundary data were correlated with the Soave, the Peng–Robinson and the Patel–Teja equations of state, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: Supercritical carbon dioxide; Vapor–liquid equilibrium; Synthetic method; Cubic equation of state

1. Introduction The phase behavior of the mixtures containing supercritical fluid (SCF) at elevated pressures has received particular attention in the last few decades. These phase equilibrium properties are closely related to the development of supercritical fluid techniques applying to SCF extraction [1], reaction [2], fractionation [3], and nanometric particles formation [4–7]. Carbon dioxide is widely used in SCF applications because it has mild critical conditions (Tc = 304.25 K, Pc = 7.38 MPa), is inexpensive, nontoxic, nonflammable, and readily available. Regarding the ultra-fine particle formation with the supercritical anti-solvent (SAS) method, the vapor–liquid equilibrium (VLE) behavior of solvent/anti-solvent mixtures plays an important role to manipulate the particle size distribution of the resultant products [8]. In the present study, a visual and volume-variable phase equilibrium analyzer (PEA) was installed to investigate the phase transition phenomena of mixtures changing from single-phase region into vapor–liquid coexistence region. Since the operation of the PEA is based on a synthetic method, it is especially applicable to determine the phase boundaries near critical region, in which dynamic apparatus often fail. This apparatus was used ∗

Corresponding author. Tel.: +886 2 2737 6626; fax: +886 2 2737 6644. E-mail address: [email protected] (M.-J. Lee).

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to measure the isothermal vapor–liquid phase boundaries for the binary systems composed of CO2 with 1-octanol, dimethyl sulfoxide (DMSO), propylene glycol monomethyl ether acetate (PGMEA) or quinoline at temperatures from 308.15 K to 358.15 K and pressures up to 28 MPa or critical pressures over a wide composition range. These new binary VLE data were correlated with the Soave (SRK) [9], the Peng–Robinson (PR) [10] and the Patel–Teja (PT) [11] equations of state (EOS). A comparison of the correlated results from different models was made. 2. Experimental 2.1. Materials Carbon dioxide (purity of 99.8 mass%) was purchased from Liu-Hsiang Co. (Taiwan), 1-octanol (99+ mass%) from Arcos (USA), and DMSO (99.9 mass%), PGMEA (99.9 mass%) and quinoline (99.9 mass%) from Aldrich (USA). All the chemicals were used without further purification. The physical properties of these compounds are listed in Table 1. 2.2. Apparatus and procedure Fig. 1 is the schematic diagram of the visual and volumevariable PEA. This apparatus consists of a high-pressure

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Table 1 Physical properties of pure compounds Compound Carbon dioxide 1-Octanol DMSO PGMEA Quinoline a b c d

MW

Tc (K)

Pc (MPa)

ω

44.01 130.23 78.13 132.16 129.16

304.1a

7.38a

652.5b 478.96c 597.9c 782b

2.86b 5.705c 3.01c 5.7b

0.225a 0.587b 0.280d 0.472d 0.396b

Reid et al. [12]. Database of PE (phase equilibrium) software (Pfohl et al. [13]). Determined from the Joback group contribution model (Reid et al. [12]). Determined from the Lee–Kesler model (Reid et al. [12]).

generator (Model 62-6-10, High Pressure Equipment Co., USA) equipped with a sapphire window (Part No. 742.0106, Bridgman closure, SITEC, Switzerland), a rupture disk and a circulation jacket. The internal volume of the cell can be adjusted manually from 35 cm3 to 60 cm3 with a piston screw pump and the cell is operable up to 50 MPa and 473.15 K. The cell’s temperature was controlled by circulated thermostatic water and was measured by an inserted thermocouple, whose reading has been calibrated to within an uncertainty of ±0.1 K. A pressure transducer (PDCR 407-01, Druck, UK) with a digital display (DPI 280, Druck, UK) measured the cell’s pressure to within an uncertainty of ±0.04%. The operation procedure is described as follows. The empty cell was purged several times with carbon dioxide to remove the entrapped air. After evacuating the cell, two precision syringe pumps (Model: 260D, Isco Inc., USA) were employed to charge carbon dioxide and organic compounds, respectively. The loaded mixture in the cell was compressed to the desired pressure by displacing movable piston fitted within the cell. A magnetic stir bar was placed inside the cell to promote the mixing of the mixture. The weight of each loaded material was calculated from the known charged volume (accurate to ±0.01 cm3 ) and its density at pre-specified pressure and temperature. While the densities of

carbon dioxide were taken from the NIST Chemistry WebBook [14], the densities of the organic liquids were determined experimentally by a high-pressure densitometer (DMA 512P, Anton Paar, Austria) with an oscillation period indicator (DMA 48, Anton Paar, Austria) to an uncertainty of ±0.0001 g/cm3 . The uncertainty of the composition of the prepared mixtures was estimated to be ±0.003 in mole fraction. The phase behavior of the loaded mixture in the cell was observed with an aid of a digital camera (Model: DSC-F88/S, Sony, Japan), LED-white light, and a television (Model: PV-C2062, Panasonic, Japan). The loaded sample was compressed to form a single phase at a fixed temperature, and the mixture was maintained homogeneously at least 20 min. The pressure was then slowly decreased until the second phase appeared by manipulating manually the position of piston screw pump. The uncertainty of the observed phase transition pressures was estimated to be ±0.02 MPa. In case of a small bubble appeared in the cell, a bubble point was obtained. A dew point was determined upon a fine mist droplet existing in the cell. Moreover, critical pressure was found as critical opalescence was observed during the phase transition process. 3. Results and discussions 3.1. Phase transition near critical region Three kinds of phase transition behavior were observed in the different phase boundary regions, including bubble, dew, and near critical points. It is worthy to note the phenomenon of phase transition passing through the vapor–liquid phase boundary near the critical region. Fig. 2 presents an example of CO2 (1) + 1-octanol (2) mixture at 348.15 K with 0.9076 in the mole fraction of CO2 . At 18.47 MPa, the mixture existed as a single phase. The mixture changed from colorless to golden transparent as pressure was decreased to 18.42 MPa, and then it became darker and darker as pressure kept decreasing. When the pres-

Fig. 1. Schematic diagram of the experimental apparatus. (1) CO2 cylinder, (2) syringe pump, (3) sapphire window, (4) vacuum pump, (5) pressure transducer, (6) thermocouple, (7) thermostatic bath, (8) LED light, (9) magnetic stirrer, (10) digital camera and (11) TV monitor.

H.-Y. Chiu et al. / J. of Supercritical Fluids 44 (2008) 273–278

275

Fig. 2. Photographs of phase transition near critical region for CO2 + 1-octanol at 348.15 K and xCO2 = 0.9076. (a) P = 18.47 MPa, (b) P = 18.42 MPa, (c) P = 18.23 MPa and (d) P = 18.21 MPa.

sure was decreased to 18.23 MPa, the cell became totally dark, and that pressure was recorded as phase transition pressure. Vapor–liquid phase coexistence appeared while pressure was reduced to 18.21 MPa.

aries of CO2 + DMSO [16,17]. The literature data [16] are also shown in the graph for comparison. The agreement between our results and those of Gonzalez et al. [16] is consistently well. Fig. 5 depicts the vapor–liquid equilibrium phase diagram for

3.2. Verification of new apparatus

Table 2 Experimental phase boundaries of carbon dioxide (1) + 1-octanol (2)

To verify the reliability of our new PEA apparatus, the VLE phase boundaries were measured for the binary mixtures of CO2 (1) + 1-octanol (2) at 313.15 K, 328.15 K and 348.15 K. Table 2 lists the experimental results and Fig. 3 compares the experimental results with literature data [15]. The graph shows good agreement in the bubble point and dew point regions; however, the literature values are obviously higher than the results of this work near the critical regions. The discrepancy may be attributed to the experimental uncertainty of Weng and Lee [15] that becomes large in these regions due to the limitation of the semi-flow apparatus. 3.3. Vapor–liquid phase boundaries The PEA apparatus was further employed to measure the isothermal VLE phase boundaries for three binary systems containing carbon dioxide and organic solvents in a temperature range from 308.15 K to 358.15 K and in a pressure range from 1.76 MPa to 27.71 MPa. Tables 3–5 report the determined phase boundary data for CO2 + DMSO, CO2 + PGMEA, and CO2 + quinoline, respectively. Fig. 4 presents the phase bound-

x1

P (MPa) 313.15 K

0.216 0.229 0.325 0.354 0.468 0.539 0.637 0.651 0.744 0.816 0.841 0.856 0.870 0.887 0.907 0.921 0.943 0.958 0.984 a b

Bubble point. Dew point.

3.25a – 4.73a – 6.82a 7.74a 8.58a 9.33a 12.86a 15.54a 15.74a 15.95a 16.12a 16.03b 15.83b 15.22b 14.45b 12.88b 9.56b

328.15 K

348.15 K

– 3.92a – 5.62a 7.62a 9.22a 11.27a – 14.42a 15.62a 15.99a 16.07a 16.25a 16.23b 16.14b 15.83b 15.28b 14.45b 12.21b

– 4.86a – 7.03a 9.45a 11.15a 13.52a – 16.58a 17.57a 17.95a 18.05a 18.21a 18.29a 18.23b 17.95b 17.55b 16.78b 14.79b

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H.-Y. Chiu et al. / J. of Supercritical Fluids 44 (2008) 273–278 Table 4 Experimental phase boundaries of carbon dioxide (1) + PGMEA (2) x1

P (MPa) 318.15 K

0.301 0.549 0.697 0.805 0.831 0.859 0.890 0.910 0.935 0.946 0.965 0.985 0.992 a b

Fig. 3. Comparison of the calculated results from the PR EOS with the experimental values for CO2 (1) + 1-octanol (2).

CO2 + PGMEA, indicating that the critical points were located in the carbon dioxide-rich regions, where the mole fractions of carbon dioxide are greater than 0.9. Fig. 6 illustrates the isothermal phase boundaries varying with the composition of carbon dioxide for CO2 + quinoline. It is clearly shows that the bubble pressures increase dramatically as the mole fractions of carbon dioxide change from 0.65 to 0.70, especially at temperatures lower than 318.15 K. Since the experimental conditions cover the entire critical region, the critical point at a given temperature can be estimated from the maximum pressure of the isothermal vapor–liquid phase boundary by interpolation. The estimated results are reported in Table 6. For CO2 + 1-octanol and CO2 + quinoline, their critical compositions are insensitive to temperature over the experimental conditions.

328.15 K

338.15 K

348.15 K

1.76a

2.04a

2.77a

3.08a

4.38a 5.76a 6.97a 7.29a 7.60a 7.91a 8.17a 8.53a 8.61a 8.89a 8.92b 8.73b

4.77a 6.59a 8.14a 8.42a 9.01a 9.33a 9.58a 9.97a 10.06a 10.13b 9.86b 9.13b

5.36a 7.55a 9.33a 9.74a 10.37a 10.71a 11.01a 11.26a 11.34a 11.18b – 9.14b

6.07a 8.68a 10.58a 10.98a 11.58a 12.08a 12.29a 12.37a 12.38b 11.99b – –

3.4. Correlation of vapor–liquid equilibrium data The new VLE data were correlated with the Soave (SRK) [9], the Peng–Robinson (PR) [10], and the Patel–Teja (PT) [11] equations of state with the van der Waals one-fluid oneparameter mixing rule (MR-Q1) and the van der Waals one-fluid two-parameter mixing rule (MR-Q2), respectively. The vdW one-fluid mixing rules for equation constants am , bm , and cm (cm for the PT EOS only) were defined as am =

nc nc  

xi xj aij

(1)

xi xj bij

(2)

i=1 j=1

bm =

nc nc   i=1 j=1

Table 5 Experimental phase boundaries of carbon dioxide (1) + quinoline (2)

x1

x1

0.289 0.435 0.607 0.702 0.759 0.800 0.805 0.830 0.852 0.879 0.879 0.894 0.913 0.939 0.964 0.990 a b

313.15 K

328.95 K

338.15 K

348.15 K

3.95a 5.86a 8.06a 8.79a 9.08a 9.26a – 9.44a 9.53a – 9.58a 9.59a 9.56b 9.49b 9.37b 9.21b

4.61a 7.00a 9.66a 10.96a 11.80a 12.31a – 12.50a 12.60a – 12.64a 12.65a 12.53b 12.12b 11.66b 10.62b

5.45a 8.46a 11.75a 13.23a 14.23a 14.85a – 15.01a 15.11a – 15.04b 14.98b 14.80b 14.18b 13.41b 11.87b

6.51a 9.97a 13.83a 15.72a 16.82a – 17.30a 17.51a 17.63a 17.48b – 17.34b 17.06b 16.17b 15.08b 13.91b

Bubble point. Dew point.

3.52a 6.78a 9.59a 11.75a 12.17a 12.85a 13.31a 13.36b 13.28b 13.21b – – –

Bubble point. Dew point.

Table 3 Experimental phase boundaries of carbon dioxide (1) + DMSO (2) P (MPa)

358.15 K

P (MPa)

0.246 0.443 0.560 0.601 0.648 0.710 0.758 0.794 0.817 0.845 0.865 0.885 0.905 0.935 0.959 0.990 a b

308.15 K

313.15 K

318.15 K

328.15 K

333.15 K

3.48a 6.40a 8.18a 9.05a 11.28a 19.55a 21.85a 22.87a 23.53a 23.73b 23.02b 22.49b 21.35b 17.17b 12.72b 7.92b

3.62a 6.63a 9.17a 10.35a 12.66a 20.40a 22.58a 23.61a 24.35a 24.49b 23.84b 23.38b 22.25b 18.21b 14.04b 8.76b

3.74a 7.58a 10.48a 11.86a 13.92a 21.99a 23.75a 24.51a 25.14a 25.32b 24.63b 24.21b 23.12b 19.30b 15.38b 9.82b

4.03a 8.61a 13.03a 14.63a 18.15a 24.28a 25.58a 26.18a 26.69a 26.89b 26.32b 25.91b 24.87b 21.54b 17.59b 11.89b

4.41a 9.10a 14.21a 16.7a 20.15a 25.44a 26.53a 27.18a 27.65a 27.71b 27.15b 26.55b 25.75b 22.53b 18.96b 12.67b

Bubble point. Dew point.

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Fig. 4. Comparison of the calculated results from the PR EOS with the experimental values for CO2 (1) + DMSO (2).

cm =

nc nc  

xi xj cij

(4) (5)

π=

with

(1 − lij )(bi + bj ) 2 ci + cj cij = 2 bij =

Fig. 6. Comparison of the calculated results from the PR EOS with the experimental values for CO2 (1) + quinoline (2).

The variables of kij and lij are binary interaction parameters. In the mixing rule MR-Q1, the value of lij was set to zero. The optimal values of the binary interaction parameters at a given temperature were obtained from isothermal flash calculation by minimization of the following objective function π:

(3)

i=1 j=1

√ aij = (1 − kij ) ai aj

277

n  

expt 2

expt 2

calc calc (x1,k − x1,k ) + (y2,k − y2,k )

 (7)

k=1

(6)

where nc is the number of components and the subscripts of m, i, j and ij represent the parameters for mixture, component i, component j, and i–j pair interactions, respectively.

where n is the number of experimental data points and the superscripts of calc and expt refer to the calculated and the experimental values, respectively. The calculation results for CO2 + 1-octanol, + DMSO, + PGMEA, and + quinoline (2) are Table 6 Estimated critical points for the binary systems of carbon dioxide (1) + 1-octanol (2), + DMSO (2), + PGMEA (2), and + quinoline (2)

Fig. 5. Comparison of the calculated results from the PR EOS with the experimental values for CO2 (1) + PGMEA (2).

Comp. (2)

T (K)

Pc (MPa)

x1c

1-Octanol

313.15 328.15 348.15

16.13 16.26 18.31

0.872 0.876 0.892

DMSO

318.15 328.95 338.15 348.15

9.59 12.67 15.11 17.63

0.890 0.879 0.860 0.854

PGMEA

318.15 328.15 338.15 348.15 358.15

8.98 10.14 11.34 12.41 13.36

0.976 0.960 0.946 0.933 0.908

Quinoline

308.15 313.15 318.15 328.15 333.15

23.79 24.56 25.38 26.92 27.78

0.837 0.835 0.837 0.838 0.834

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compiled in Tables S1–S4 as suuplementary data, where the entries of x1 and y1 were defined as follows: n

x1 =

1  calc expt |x1,k − x1,k | n

(8)

k=1 n

y1 =

1  calc expt |y1,k − y1,k | n

(9)

k=1

These tabulated values show that the mixing rules are more influential than the EOS on the quality of phase equilibrium calculations. Only the calculated results from the Peng–Robinson EOS are thus presented in the following graphical comparisons. Figs. 3 and 4 illustrate that the MR-Q1 mixing rule often fails to accurately correlate the VLE data near critical region, and the MR-Q2 may improve the presentation of the phase boundaries. Fig. 5 shows that reasonable correlations were obtained for the mixtures at 318.15 K and 338.15 K, whereas overestimations are obvious for the phase boundary at 358.15 K. Fig. 6 indicates that significant deviations were exhibited in the system for the mixtures of CO2 + quinoline, especially around the critical regions and in the vapor phase. More complicated mixing rules may be necessary to improve the presentation of the phase behavior. 4. Conclusions The vapor–liquid phase transition phenomena have been observed for CO2 + 1-octanol system in a temperature range of 313.15–348.15 K by using a new visual and volume-variable high-pressure phase equilibrium analyzer. Different transition phenomena were observed when the experiments were conducted in bubble point, dew point, and near critical point regions. The PEA apparatus was also successfully employed to measure the vapor–liquid phase boundaries for CO2 + 1-octanol, CO2 + DMSO, CO2 + PGMEA, and CO2 + quinoline at temperatures from 313.15 K to 358.15 K and up to critical pressures. The critical points have been determined by interpolation of the measured phase boundary data. The new vapor–liquid equilibrium data were correlated with the Soave, the Peng–Robinson, and the Patel–Teja equations of state by using the van der Waals one-fluid mixing rules with one adjustable parameter and with two adjustable parameters. The capabilities of these three cubic equations of state are quite similar when the same mixing rule was utilized. In general, the use of two-parameter mixing rule significantly improved the representation for the phase boundaries, especially near critical region. However, more sophisticated models may be necessary to quantitatively correlate the phase envelopes over wide temperature and pressure ranges.

Acknowledgement The authors gratefully acknowledged the financial support from the National Science Council, Taiwan, through grant No. NSC95-2221-E011-154-MY3. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.supflu.2007.09.026. References [1] M.A. McHugh, V.J. Krukonis, Supercritical Fluid Extraction: Principles and Practice, 2nd ed., Butterworth Heinemann, Stoneham, MA, 1994. [2] K. Nishi, Y. Morikawa, R. Misumi, M. Kaminoyama, Radical polymerization in supercritical carbon dioxide—use of supercritical carbon dioxide as a mixing assistant, Chem. Eng. Sci. 60 (2005) 2419–2426. [3] C.A. Eckert, M.P. Ekart, B.L. Knutson, K.P. Payne, D.L. Tomasko, C.L. Liotta, N.R. Foster, Supercritical fluid fractionation of a nonionic surfactant, Ind. Eng. Chem. Res. 31 (1992) 1105–1110. [4] E. Reverchon, Supercritical antisolvent precipitation of micro- and nanoparticles, J. Supercrit. Fluids 15 (1999) 1–21. [5] J. Jung, M. Perrut, Particle design using supercritical fluids: literature and patent survey, J. Supercrit. Fluids 20 (2001) 179–219. [6] H.T. Wu, M.J. Lee, H.M. Lin, Nano-particles formation for pigment red 177 via a continuous supercritical anti-solvent process, J. Supercrit. Fluids 33 (2005) 173–182. [7] H.T. Wu, M.J. Lee, H.M. Lin, Precipitation kinetics of pigment blue 15:6 sub-micro particles with a supercritical anti-solvent process, J. Supercrit. Fluids 37 (2006) 220–228. [8] E. Reverchon, G. Caputo, I. De Marco, Role of phase behavior and atomization in the supercritical antisolvent precipitation, Ind. Eng. Chem. Res. 42 (2003) 6406–6414. [9] G. Soave, Equilibrium constants from a modified Redlich–Kwong equation of state, Chem. Eng. Sci. 27 (1972) 1197–1203. [10] D.Y. Peng, D.B. Robinson, A new two-constant equation of state, Ind. Eng. Chem. Fundam. 15 (1976) 59–64. [11] N.C. Patel, A.S. Teja, A new cubic equation of state for fluids and fluid mixtures, Chem. Eng. Sci. 37 (1982) 463–473. [12] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, 4th ed., McGraw-Hill, New York, 1984. [13] O. Pfohl, S. Petkov, G. Brunner, Phase Equilibria, Herbet Utz Verlog, Wissenschaft Muenchen, 1998. [14] Isothermal properties for carbon dioxide, NIST Chemistry WebBook, NIST Standard Reference Database No. 69-March 2003 Release, National Institute of Standard and Technology, USA, 2003 (http://webbook.nist.gov/chemistry/). [15] W.L. Weng, M.J. Lee, Phase equilibrium measurements for the binary mixtures of 1-octanol plus CO2 , C2 H6 , and C2 H4 , Fluid Phase Equilibr. 73 (1992) 117–127. [16] A.V. Gonzalez, R. Tufeu, P. Subra, High-pressure vapor–liquid equilibrium for the binary systems carbon dioxide + dimethyl sulfoxide and carbon dioxide + dichloromethane, J. Chem. Eng. Data 47 (2002) 492–495. [17] A.E. Andreatta, L.J. Florusse, S.B. Bottini, C.J. Peters, Phase equilibria of dimethyl sulfoxide (DMSO) + carbon dioxide and DMSO + carbon dioxide + water mixtures, J. Supercrit. Fluids 42 (2007) 60–68.