ELSEVIER Fluid Phase Equilibria 101 (1994) 237-245

High-Pressure Phase Equilibria for Carbon Dioxide-2- Methyl-2-propanol and Carbon Dioxide-2-Methyl-2- propanol-Water: Measurement and Prediction

Jin-Soo Kim, Ji-Ho Yoon, and Huen Lee *

Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-l Kusung-dong, Yusung-gu, Taejon, 305-701, South Korea

Received 2 April 1994; accepted in final form 20 July 1994

Keywords: experiment, equation of state, mixing rule, high-pressure VLE, three-phase region

ABSTRACT

High-pressure vapor-liquid equilibria for the binary system carbon dioxide-2-methyl-2- propanol were measured at 299.0, 323.2, and 343.2 K. For the ternary system carbon dioxide-2-methyl-2-propanol-water equilibrium compositions were measured at 323.2 K and pressures of 60, 80, 100, and 120 bar. An isothermal three-phase region at 323.2 K and phase equilibria at 60 and 80 bar are also presented. The experimental data were correlated with the Soave-Redlich-Kwong equation of state with the modified Huron-Vidal second- order mixing rule. The model was able to provide good predictions of the phase behavior of all systems considered in this study.

INTRODUCTION

Among many alternatives to energy intensive industrial separation processes,

the use of a supercritical fluid solvent has been received much attention.

Particularly due to the ease of forming two liquid phases, supercritical carbon dioxide-alcohol-water mixture can be utilized successfully to the extraction process. The measurement and prediction of high-pressure equilibrium for aqueous solutions containing several alcohols thus have become important.

Several studies of the phase behavior for such systems have been reported in the literature. Efremova and Shvartz (1969, 1970) measured the liquid-liquid

and gas-liquid critical end points for carbon dioxide-ethanol (and methanol)- water systems. They also investigated the three-phase equilibria of those

systems and a higher-order critical end point at which the three-phase

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238 J.S. Kim et al./ Fluid Phase Equilibria IO1 (1994) 237-245

equilibrium behavior terminated. Takishima et al. (1986) measured the tie lines

in the two-phase region and three-phase equilibrium compositions at

temperatures near the critical point of carbon dioxide for the carbon dioxide-

ethanol-water system. Panagiotopoulos and Reid (1986b) reported that the

carbon dioxide-n-butanol-water system exhibited an extensive three-phase region in the vicinity of a four-phase point. Di Andreth and Paulaitis (1987)

and Di Andreth et al. (1987) suggested an experimental technique for the

accurate measurement of both the composition and the molar volume of

individual phases in multiphase equilibria and measured the three- and four-

phase equilibrium compositions and molar volumes for the carbon dioxide-2- propanol-water system. In previous work, Yoon et al. (1993b) measured the high-pressure equilibria of the carbon dioxide-methanol-water system, and

reported that the original and modified versions of the Huron-Vidal (1979)

mixing model showed better predictions of the high-pressure equilibria than all

of the local composition models considered. In this work, we present new data

for two systems carbon dioxide-2-methyl-2-propanol and carbon dioxide-2-

methyl-2-propanol-water, and test the applicability of the modified Huron-

Vidal second-order (MHV2) mixing rule to the prediction of the two- and

particularly the three-phase behavior for this system.

EXPERIMENTAL

The phase equilibrium apparatus used in this work is of the circulation type

in which the coexisting phases are recirculated, on-line sampled, and analyzed. The experiment began by charging the equilibrium cell with a mixture of

liquid, and then the cell was pressurized with carbon dioxide by a metering

pump. When the cell reached the equilibrium condition at the desired temperature and pressure, a small amount of sample for each phase was

injected into a gas chromatograph where the composition of each sample was

analyzed. Each sample was analyzed at least three times for each equilibrium

point. A more detailed description on the experimental procedure covering the

calibration technique is given elsewhere (Yoon et al., 1993a,b). The reproducibilities of the vapor and liquid mole fractions for a binary system are

within +O.OOl and f0.002, respectively. For the ternary system the vapor- and liquid-phase mole fractions are reproducible to within kO.002 and f0.003,

respectively. The carbon dioxide (purity of 99.9%) used in this work was supplied by World Gas Co. in South Korea, the 2-methyl-2-propanol (99.5%) was supplied by Aldrich, and HPLC grade distilled water supplied by Aldrich was used. These chemicals were used without further purification.

J.S. Kim et al./ Fluid Phase Equilibria 101 (1994) 237-245 239

THERMODYNAMIC MODEL

The Soave-Redlich-Kwong equation of state (SRK-EOS) was used,

p,RT_ a v-b v(v+b)

(1)

where the mixture parameter, b, is derived from the conventional mixing rule.

b=Ax,b, (2) r=l

For the mixture parameter, a, the MIIV2 mixing rule was applied to the SRK-

EOS, because of its capability to correlating the high-temperature and high-

pressure phase equilibria. In particular, the MHV2 mixing rule was found to be

superior to the local-composition models such as Panagiotopoulos and Reid (1986a) mixing rule to the prediction of the phase

dioxide-methanol-water system (Yoon et al., 1993b).

mixing rule is given by

41Ca-Txiaj)+q2( i=l

02-~$ai2)=$+$xiln t i=l I 1 0 I

behavior of the carbon The explicit form of this

(3)

where a = a/bRT, ai = ai/bi RT, and gE * is the excess Gibbs energy at zero

pressure. The recommended values of q1 and q2 are -0.478 and -0.0047,

respectively (Dahl and Michelsen, 1990), and the pure component parameters,

ai and bi, are given in the literature (Soave, 1972). Any appropriate excess

Gibbs energy model can be used for the MHV2 mixing rule. In the present work, the UNIQUAC excess Gibbs energy model was used with the mixing

rule and the structural parameters R and Q of the UNIQUAC model are given

elsewhere (Gmehling et al., 198 1; Sander et al., 1983).

RESULTS AND DISCUSSION

The equilibrium compositions and critical points for the carbon dioxide-2-

methyl-2-propanol system were measured at 299.0, 323.2, and 343K and are listed in Table 1. Using these experimental data, the UNIQUAC molecular

interaction parameters were determined by a fitting procedure to minimize the

average absolute deviarion (AAD). The interaction parameters of 2-methyl-2- propanol-water and carbon dioxide-water binaries required for the prediction

of the ternary system were determined from equilibrium data in the literature (Gmehling et al., 1981; Wiebe and Gaddy, 1940). The interaction parameters

240 J.S. Kim et al./Fluid Phase Equilibria 101 (1994) 237-245

TABLE 1 Equilibrium Compositions and Critical Points for Carbon Dioxide( l)-2-Methyl-2- Propanol(2) System.

T, K P, bar Yl P, bar yl

299.0 7.3 0.975 0.055 10.8 0.983 0.084 19.9 0.989 0.168 30.7 0.990 0.298 40.0 0.991 0.427

323.2 6.4 0.957 0.040 17.7 0.984 0.121 26.4 0.987 0.183 36.4 0.988 0.267 50.3 0.989 0.391 60.7 0.988 0.500

343.2 9.7 0.940 0.055 21.3 0.964 0.127 33.3 0.973 0.199 45.6 0.976 0.287 53.4 0.976 0.341

50.6 0.991 0.633 57.0 0.991 0.795 61.0 0.993 0.876 63.3 0.995 0.937 65.4” 1.0 1.0 70.2 0.986 0.606 79.5 0.985 0.737 87.8 0.982 0.859 91.4 0.978 0.915 92.3c 0.945 0.945

67.9 0.974 0.458 79.4 0.973 0.543 94.6 0.970 0.669

108.5 0.954 0.823 111.6c 0.880 0.880

v Vapor pressure of pure carbon dioxide. c Measured critical point.

TABLE 2

UNIQUAC Molecular Interaction Energy Parameters and Average Absolute Deviation.

system T, K P, bar Ul2, K azl, K AAD Xla AAD y,”

carbon dioxide(l)- 299.0 7.3 - 65.4 -57.8 193.1 5.9 0.71 TBA(2) 323.2 6.3 - 92.3 -76.3 174.6 2.6 0.25

343.2 9.7-l 11.6 -89.6 166.3 1.2 0.36

TBA( I)-water(2) b 323.2 0.12 - 0.26 346.2 -33.0 27.9 2.1

carbon dioxide(l)- 323.2 25.3-709.3 904.0 124.0 6.3 0.12 water(2) C

a ADDx,= (100 / NP) f I(x~‘“. - xy”) / xleyp.l ; NP = number of data points. i=l

b Gmehling et al., 1981, c Wiebe and Gaddy, 1940.

and AAD values for these three binary systems are given in Table 2. The

experimental and calculated results for the binary system carbon dioxide-2-

methyl-2-propanol are shown in Fig.1. As shown in this figure there is quite

good agreement between the experimental and calculated results over the wide

range of temperature and pressure considered.

J.S. Kim et al./ Fluid Phase Equilibria I01 (1994) 237-245 241

n 299.0 K

0 323.2 K

0 342.2 K

~ calculated

0 0.2 0.4 0.6 0.8 1

Mole Fraction of Carbon Dioxide

Fig. 1. Vapor-liquid equilibria and critical points for carbon dioxide-2-methyl-2-propanol.

TABLE 3

Equilibrium Compositions for Carbon Dioxide( l)-2-methyl-2-propano1(2)-Water(3) System at 323.2 K.

vapor liquid, liquid,

YZ Y3 XI x2 x3 Xl x2 x3

60 0.989 0.003 0.008 0.019 0.937

0.988 0.003 0.009

0.988 0.003 0.009

80 0.021 0.942

0.983 0.003 0.014 0.021 0.949

0.982 0.004 0.014

100 0.022 0.941 0.037 0.288 0.359 0.353

0.021 0.947 0.032 0.442 0.224 0.334

0.019 0.958 0.023 0.756 0.072 0.172

0.022 0.957 0.021 0.884 0.028 0.088

0.022 0.963 0.015 0.966 0.006 0.028

120 0.022 0.939 0.039 0.292 0.352 0.356

0.022 0.946 0.032 0.435 0.230 0.335

0.021 0.954 0.025 0.762 0.066 0.172

0.023 0.958 0.019 0.899 0.016 0.085

0.044

0.037

0.030

0.215 0.437 0.348

0.226 0.389 0.385

0.334 0.216 0.450

0.297 0.339 0.364

0.512 0.161 0.327

0.587 0.102 0.311

242 J.S. Kim ef al./ Fluid Phase Equilibria 101 (1994) 237-245

A few two-phase equilibrium tie-lines are presented in the literature (Kander,

1987) for the ternary carbon dioxide-2-methyl-2-propanol-water system. This

study covers two- and three-phase equilibrium compositions at 323K and

pressures of 60, 80, 100, and 120 bar. The experimental data are listed in Table

3, where liquid, is water-rich liquid phase and liquid, is the middle phase of intermediate density. In Fig.2 the experimentally observed phase behavior for

this ternary system are presented together with the calculated results. Similar to

the phase behavior of the carbon dioxide-n-butanol-water system, this system exhibits extensive three-phase regions at 60 and 80 bar. However three-phase

equilibria were not observed at 100 and 120 bar, and the phase behavior at

these two pressures were very similar each other. It can be seen that the

predictions of using the MHV2 model are quite accurate as shown in Fig.2.

co2 Ha’J co2

three-phase O---O two-phase -~ calculated

Fig. 2. Phase equilibria for carbon dioxide-2-methyl-2-propanol-water system at 323.2 K.

J.S. Kim et al./Fluid Phase Equilibria 101 (1994) 237-245 243

CONCLUSIONS

Equilibrium compositions and critical points for the binary carbon dioxide- 2-methyl-2-propanol system were measured at 299.0, 323.2, and 343.2K. For the ternary system carbon dioxide-2-methyl-2-propanol-water, two- and three-phase equilibria were measured at 323K and pressures of 60, 80, 100, and 120 bar. In order to correlate the experimental data, the SRK-EOS and the MHV2 mixing rule was used; the UNIQUAC binary interaction parameters were determined from the three constituent binary systems. The agreement between the experimental and the predicted results was quite good for the systems investigated in this work.

Moreover, based on the results of this and the previous work, the prediction of high-pressure multiphase equilibria of systems consisting of carbon dioxide-alcohols-water, the SRK-MHV2 model can be expected to reproduce many features of the measured behavior, although the model needs to be tested with other systems.

ACKNOWLEDGMENT

This work was supported by the Korea Science and Engineering Foundation and University Awards Program of the Korea Institute of Science and Technology.

LIST OF SYMBOLS

a = parameter in the equation of state au = interaction energy parameter of UNIQUAC model, K b = parameter in the equation of state g = Gibbs free energy P = pressure, bar Q = structural parameter in UNIQUAC model ql, q2 = mixing rule constants in eq (3) R = gas constant R = structural parameter in UNIQUAC model T = temperature v = molar volume x = mole fraction

Greek Letters

244 J.S. Kim et al./ Fluid Phase Equilibria IO1 (1994) 237-245

a = equation of state parameter, a = a / bRT

Superscripts

E * = excess property at zero pressure

Subscripts

i, j = molecular species

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