Enhanced Solubility of Hydrogen in CO2-Expanded Liquids

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Ind. Eng. Chem. Res. 2008, 47, 570-576

Enhanced Solubility of Hydrogen in CO2-Expanded Liquids Zulema K. Lopez-Castillo, Sudhir N. V. K. Aki,† Mark A. Stadtherr, and Joan F. Brennecke* Department of Chemical and Biomolecular Engineering, UniVersity of Notre Dame, Notre Dame, Indiana 46556

Vapor-liquid equilibrium (VLE) data for the systems CO2/H2/acetonitrile, CO2/H2/acetone, and CO2/H2/ methanol were determined at 40 °C and 25, 60, and 90 bar. A variable volume view cell was used coupled with an online gas chromatograph. The solubility of H2 in expanded solvents was improved by the presence of CO2 but not significantly; the solubility is less than one can achieve with pure H2 at the same total pressure. Nevertheless, CO2 does improve H2 solubility in all three solvents at the conditions studied, even though the H2 solubility enhancement by CO2 is very modest. The Peng-Robinson equation of state predicted the ternary VLE remarkably well using only binary interaction parameters. 1. Introduction Synthesis of chemical products in systems involving CO2expanded liquids is of current interest. This is due to the search for more environmentally friendly solvent systems and limitations of traditional organic solvents, such as poor solubility of reactant gases. CO2-expanded liquids have attracted research interest especially for homogeneously catalyzed reactions. Advantages of CO2-expanded liquids include the potential of enhancement of reaction rates, increased solubility of reactant gases, good catalyst solubility, reduction of hazardous waste, and minimization of safety concerns. These advantages can be tailored by the amount of volume replacement of the organic solvent by CO2. In particular, homogeneously and heterogeneously catalyzed hydrogenation reactions have been performed in these novel solvent systems.1-6 Hydrogenation reactions are usually limited by the low solubility of hydrogen in the solvent where catalysts and reactants are dissolved. The hydrogenation reaction for the synthesis of naproxen in CO2-expanded methanol has been studied by Combes et al.1,2 They found improved reaction rates in CO2-expanded methanol compared to neat methanol, possibly due to an increase in H2 solubility in CO2-expanded methanol. Solinas et al.5 performed the catalytic hydrogenation of imines in CO2-expanded ionic liquids. Conversions were favored by the presence of CO2 in the reaction mixture, and they presented NMR data that suggested this was due to the enhancement of hydrogen solubility. Hence, it is clear from these examples that the phase behavior of CO2-expanded solvents is of paramount importance in optimizing these novel solvent systems for the synthesis of fine chemicals. The solubility of H2 in CO2-expanded solvents is of particular interest since H2 is a key reactant in both hydrogenation and hydroformylation reactions. We recently reported on the solubility of oxygen and carbon monoxide in CO2-expanded liquids.7 Here we present an extension of our previous work and report on the solubility of hydrogen in CO2-expanded liquids. There is limited information in the literature about phase equilibrium of ternary systems involving hydrogen in CO2-expanded solvents. Yin and Tan8 reported the solubility of hydrogen in CO2-expanded toluene at temperatures of 305, 323, and 343 K and pressures from 1.3 * To whom correspondence should be addressed. Tel.: (574) 6315847. Fax: (574) 631-8366. E-mail: [email protected]. † Current address: Intermediates R&D, Invista S.a.r.l., Sabine River Laboratory B 568, P. O. Box 1003, Orange, TX 77631-1003.

to 10.5 MPa (from 13 to 105 bar); however, construction of ternary diagrams at constant pressure is not possible with the experimental data gathered. They used the Peng-Robinson equation of state (PR EoS) with van der Waals mixing rules to correlate the experimental data, and a satisfactory agreement was found. Also, enhancement factor (EF) values were reported. Additionally, Freitag and Robinson9 reported the phase behavior of the CO2/H2/n-pentane system at 0 and 50 °C and pressures between 6900 and 27 600 kPa (69 and 276 bar). Recently, Bezanehtak et al.10 reported the vapor-liquid equilibrium data for the ternary system CO2/H2/methanol at 278, 288, and 298 K and pressures between 20 and 200 bar. They presented data at temperatures equal to and below room temperature, which has limited applicability for some homogeneous hydrogenation reactions. Also, Shenderei et al.11 measured the solubility of H2 in methanol in the presence of CO2 at 228 and 247 K. They found that the solubility of H2 increases with an increase in CO2 content. The objective of this study is to understand the role of CO2 in improving the solubility of hydrogen in typical liquid solvents. In this paper, we report the solubility of H2 in the presence of CO2 in three different organic solvents, viz., acetonitrile, acetone, and methanol. We report the effect of pressure on the vaporliquid equilibrium (VLE) behavior of the CO2/H2/organic systems at 40 °C. Furthermore, the experimental data for the ternary systems are modeled using the Peng-Robinson equation of state (PR EoS) using only binary interaction parameters that were fit to binary VLE measurements. 2. Experimental Section The experimental apparatus is shown in Figure 1. The details of this experimental setup have been described previously.7 Briefly, the apparatus consists of a variable volume view cell, stir plate, fan, light bulb, multiport sampling valves for liquid and vapor phases, temperature controller, temperature and pressure indicators, CO2 cylinder, hydrogen cylinder, and syringe pump. The variable volume view cell consists of a tubular reactor, a quartz window, a piston, a stir bar, and sampling valves with 9.6 and 6.03 µL sample loops for the liquid and vapor phases, respectively. The window allows for visual monitoring of the phases. The piston moves freely in the main body of the reactor and separates the system side from the pressurization side. N2 was used as the pressurization fluid. The sample lines were heated with heating tapes to avoid condensation of the liquids. Analyses of the phases were determined

10.1021/ie070105b CCC: $40.75 © 2008 American Chemical Society Published on Web 06/30/2007

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Figure 1. Variable volume view cell apparatus: (1) CO2 cylinder; (2) syringe pump; (3) N2 cylinder; (4) equilibrium cell; (5) high-pressure side; (6) piston; (7) system side; (8) window; (9) multiport sampling valves; (10) magnetic stirrer; (11) stir plate; (12) air bath; (13) fan; (14) light bulb.

analytically by using a gas chromatograph (Varian Star 3400 CX) with a thermal conductivity detector (TCD) and two 6 ft. × 1/8 in. HayeSep Q 80/100 packed columns (Alltech 2801PC), one for the detector and one for the reference line, using argon as the carrier gas. The carrier gas flow rate was set at 26 mL/ min. Calibration curves for each of the pure components were obtained as described previously. For the organic compounds, a 5 µL calibrated syringe (Hamilton CAL 88011 7105KH PT2) was used. For the gases, the high-pressure system and a sampling valve were used, with the moles of the compound estimated from an accurate equation of state. For hydrogen, densities were obtained from a truncated virial expression and second virial coefficients from the DIPPR 801 database.12 For CO2, densities were obtained from Span and Wagner.13 The experiments were carried out as described previously. For a specified temperature and pressure, several different tie lines were obtained. All organic materials used were of HPLC grade. Methanol was 99.93% from Sigma-Aldrich, acetonitrile was 99.9% from Fisher, and acetone was 99.9+% from Sigma-Aldrich. All organics were used as received except for drying over molecular sieves. Carbon dioxide (99.99%), hydrogen (ultrahigh purity, 99.999%), and argon (ultrahigh purity, 99.999%) were purchased from Mittler Supply, Inc. 3. Results and Discussion This work represents the continuation of our recently reported investigation on the solubility of oxygen and carbon monoxide in CO2-expanded liquids. Here, we measured the solubility of H2 in the same three organicssacetonitrile, acetone, and methanolsin the presence of CO2 at 40 °C and three different pressures (25, 60, and 90 bar). As mentioned previously, H2 is an important reactant gas in the hydrogenation and hydroformylation reactions. Vapor-liquid equilibrium data of the ternary systems consisting of CO2/H2/organic were collected using a variable volume view cell, and for every system studied several different tie lines were obtained at a given pressure. We modeled the experimental results for the ternary systems using the PengRobinson equation of state (PR EoS) with van der Waals mixing rules, using temperature-dependent binary interaction parameters estimated from binary VLE data. 3.1. CO2/H2/Acetonitrile. Table 1 shows the experimental results for the CO2/H2/acetonitrile system at 40 °C and total pressures of 25, 60, and 90 bar. The data in the table are at approximately the specified temperature and pressure, and every entry in the table represents a tie line. Mole fractions for all components were determined analytically by gas chromatog-

raphy. For clarity, only the CO2 and H2 mole fractions are shown in the table, with the balance being acetonitrile. In general, the solubility of H2 and CO2 increased with an increase in pressure. As described in our previous work, to understand the effect of the presence of CO2 on the solubility of H2 in CO2-expanded acetonitrile, we compare the H2 solubility in pure acetonitrile (i.e., no CO2) at the same hydrogen fugacity as in the ternary system and the results are shown in Table 1. The PR EoS was used to calculate the fugacities of each component in the ternary mixtures. Since no experimental data were found in the literature for the binary system H2/acetonitrile at 40 °C, an interpolated binary interaction parameter kij value at 40 °C was used to generate values for the solubility of H2 in pure acetonitrile at 40 °C and at the same hydrogen fugacity as in the ternary system. These values are discussed in the Modeling section. The values of H2 solubility in pure acetonitrile at the same H2 fugacity as the ternary systems are reported in Table 1. They were calculated using the PR EoS with the isothermal LNGFLASH program; the details of this procedure are described elsewhere7 and later in the paper. We notice that the solubility of hydrogen in acetonitrile at these conditions is less than 1.2 mol %. Brunner14 measured the solubility of H2 in different organics, including acetonitrile, at three different temperatures (298.15, 323.15, and 373.15 K) and pressures up to 10 MPa (100 bar). He found low H2 solubility in acetonitrile, consistent with our modeled results. As we reported in our previous work, to understand the effect of CO2 in the system, we use the enhancement factor (EF) to compare the solubility of H2 in the ternary and in the binary systems at the same conditions of temperature and gas fugacity. The EF is defined as follows:

EF )

xmixture gas xsolvent gas

and expresses the ratio of the solubility of H2 in the liquid phase of the ternary system CO2/H2/acetonitrile to the solubility of pure H2 in acetonitrile at the same conditions of temperature and H2 fugacity. From the EF definition, EF values greater than 1 are desirable, meaning that the presence of CO2 increases the solubility of H2. In Table 1 we see that EF values greater than 1 are obtained for all conditions studied. At any particular pressure the EF increases with increasing CO2 composition in the vapor and liquid phases; i.e., the highest EFs occur when the gas mixture is mostly CO2. In general, the EFs appear to be a bit higher at higher pressures. In most cases the uncertainty in the EF values is well below (0.1. However, it is important to note that the experimental solubility of H2 in the liquid phase of the ternary systems is quite small, especially at the lowest partial pressures of hydrogen, so the EF values are the ratio of two small numbers. At the lowest H2 partial pressures this results in significant uncertainty in the EF values, as high as (0.3 in a couple cases. As previously reported, EF > 1 are probably due to the high solubility of CO2 in the liquid phase and the ability of CO2 to increase the free volume, which favors H2 solubility. If we compare the solubility of H2 in the ternary system to the solubility of H2 in pure acetonitrile at the same total pressure instead of the same H2 fugacity, then there is no enhancement. The dilution effect is greater than the solubility enhancement at all conditions. This means that the enhancement of the solubility of H2 is at the cost of higher total pressure. 3.2. CO2/H2/Acetone. Experimental VLE data for the CO2/ H2/acetone ternary system at approximately 40 °C and 25, 60,

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Table 1. Vapor-Liquid Equilibrium for CO2 (1)/H2 (2)/Acetonitrile (3) liquid-phase composition

vapor-phase composition

T (°C)

Ptotal (bar)

x1

x2

y1

y2

fH2 (bar)

xH2, pure gasa mole fraction

EF

40.2 40.1 40.0 40.3 40.3 40.0 40.3 40.2 40.2 40.2 40.1 40.2 40.3 40.3 40.0 40.2 40.2 40.3 40.2 40.3

25.4 25.7 25.0 24.9 25.1 25.3 25.1 25.0 59.9 59.8 59.7 59.7 59.8 59.6 90.1 90.0 89.9 89.7 90.0 90.1

0.03 0.09 0.12 0.18 0.20 0.24 0.25 0.29 0.16 0.22 0.28 0.36 0.40 0.49 0.26 0.40 0.44 0.44 0.52 0.64

0.0053 0.0044 0.0039 0.0026 0.0019 0.0013 0.0012 0.0004 0.0103 0.0091 0.0093 0.0082 0.0076 0.0024 0.0204 0.0150 0.0153 0.0146 0.0142 0.0141

0.10 0.29 0.40 0.64 0.72 0.80 0.84 0.95 0.28 0.40 0.45 0.57 0.71 0.90 0.35 0.52 0.56 0.57 0.66 0.79

0.89 0.70 0.59 0.35 0.26 0.19 0.14 0.03 0.71 0.59 0.54 0.43 0.28 0.09 0.65 0.47 0.43 0.42 0.33 0.20

21.0 18.2 15.1 9.1 7.1 5.2 3.9 0.9 44.2 37.9 35.0 28.5 20.2 7.4 62.8 49.5 46.2 44.9 38.2 26.9

0.0042 0.0037 0.0030 0.0018 0.0014 0.0010 0.0008 0.0002 0.0087 0.0075 0.0070 0.0057 0.0041 0.0015 0.0122 0.0097 0.0091 0.0089 0.0076 0.0054

1.27 1.21 1.29 1.42 1.38 1.32 1.53 2.68 1.18 1.22 1.33 1.44 1.87 1.59 1.67 1.54 1.68 1.65 1.88 2.61

a

Calculated from PR EoS and kij in Table 4 using LNGFLASH.

Table 2. Vapor-Liquid Equilibrium for CO2 (1)/H2 (2)/Acetone (3) liquid phase composition

vapor phase composition

T (°C)

Ptotal (bar)

x1

x2

y1

y2

fH2 (bar)


EF

40.1 40.0 40.0 39.9 39.9 40.2 40.1 40.0 40.0 40.1 39.9 40.0

25.1 25.0 25.0 24.9 60.0 60.0 60.1 60.2 90.2 90.0 89.9 90.1

0.03 0.16 0.23 0.28 0.05 0.18 0.32 0.46 0.22 0.38 0.48 0.60

0.0072 0.0047 0.0030 0.0020 0.0232 0.0198 0.0148 0.0108 0.0441 0.0247 0.0239 0.0210

0.09 0.42 0.66 0.76 0.07 0.22 0.43 0.69 0.25 0.41 0.50 0.71

0.88 0.54 0.30 0.20 0.91 0.76 0.55 0.29 0.74 0.57 0.48 0.28

21.0 14.0 8.2 5.4 52.5 47.5 35.6 20.9 70.6 57.7 50.2 33.9

0.0067 0.0044 0.0025 0.0016 0.0166 0.0150 0.0114 0.0067 0.0220 0.0182 0.0159 0.0108

1.08 1.07 1.21 1.21 1.40 1.31 1.30 1.61 2.00 1.36 1.50 1.94

a


and 90 bar are reported in the same way as the CO2/H2/ acetonitrile data in Table 2. The liquid and vapor mole fractions of CO2 and H2 are presented, with the balance being acetone. We also give the H2 fugacity, the mole fraction of H2 in pure acetone at the same H2 fugacity as in the ternary system, and the EF. Every entry in the table represents a tie line at approximately the specified temperature and pressure. As with the system CO2/H2/acetonitrile, CO2 and H2 solubilities in the liquid phase increase with an increase in pressure. For the CO2/ H2/acetone system higher solubilities of hydrogen in the liquid phase were obtained, with values up to 4.4 mol % observed at 90 bar. The solubility of hydrogen in pure acetone was calculated in the same way as in the CO2/H2/acetonitrile system. No VLE data for the binary system H2/acetone were available in the literature at exactly 40 °C; Brunner14 reported the solubility of hydrogen in acetone at 298.15, 323.15, and 373.15 K. Therefore, we fit kij values in the PR EoS to the Brunner data and interpolated to obtain a kij value at 313.15 K. This value was used with the isothermal flash program LNGFLASH with the PR EoS to estimate H2 solubility in acetone at the hydrogen fugacity of the ternary system. The resulting EF values, the ratio of H2 solubility in the ternary system to the H2 solubility in pure acetone at 40 °C and at the same H2 fugacity, for the CO2/H2/acetone system are reported in the last column of Table 2. Trends similar to those found for CO2/H2/acetonitrile were observed. In all conditions studied EF values were >1. At a given pressure, EF values increased as the concentration

of CO2 increased in the liquid (and vapor) phase. EF values were somewhat higher at 90 bar pressure, where the EF was as high as 2 even at low CO2 concentration. In general, the EF values for the CO2/H2/acetone system are slightly lower than those for the CO2/H2/acetonitrile system. Perhaps this is due to the fact that H2 solubility in pure acetonitrile is significantly less than in pure acetone, providing a greater opportunity for enhancement. 3.3. CO2/H2/Methanol. Vapor-liquid equilibrium experimental results for the CO2/H2/methanol system at 40 °C and 25, 60, and 90 bar are reported in Table 3, in the same way as in the two previous ternary systems. The solubility of H2 in pure methanol was calculated from the PR EoS with a twophase isothermal flash program (LNGFLASH) using a binary interaction parameter interpolated from fits of the PR EoS to the binary VLE data reported by Brunner et al.15 As with the other two systems, H2 and CO2 solubility in the liquid phase increased with pressure and the EF values were >1 at all conditions. EF values increase with total pressure and with CO2 mole fraction in the liquid (and vapor) phase. Higher EF values are obtained for the highest pressure investigated (90 bar). The solubility of H2 is greater in acetone than in either acetonitrile or methanol. In addition, the solubility of CO2 is less in methanol than it is in either acetonitrile or acetone, and the greatest EF values were obtained for the CO2/H2/methanol system. This was also observed for the CO2/O2/organic systems reported in our previous work, where EF values for O2 were

Ind. Eng. Chem. Res., Vol. 47, No. 3, 2008 573 Table 3. Vapor-Liquid Equilibrium for CO2 (1)/H2 (2)/Methanol (3) liquid-phase composition

vapor-phase composition

T (°C)

Ptotal (bar)

x1

x2

y1

y2

fH2 (bar)


EF

40.2 40.2 40.0 40.2 40.2 40.3 40.3 40.3 40.1 40.2 40.2 40.1 40.3 40.0 40.1

25.9 25.4 25.2 58.9 59.6 59.0 59.9 90.1 90.2 90.1 90.2 90.3 90.2 90.2 90.2

0.06 0.07 0.10 0.08 0.21 0.23 0.31 0.14 0.22 0.26 0.30 0.44 0.50 0.54 0.59

0.0039 0.0034 0.0019 0.0092 0.0067 0.0052 0.0022 0.0120 0.0127 0.0118 0.0120 0.0084 0.0064 0.0058 0.0054

0.39 0.47 0.71 0.30 0.66 0.75 0.88 0.41 0.53 0.68 0.71 0.85 0.88 0.90 0.91

0.58 0.51 0.27 0.68 0.33 0.24 0.10 0.58 0.46 0.31 0.28 0.14 0.10 0.07 0.06

15.6 13.4 7.2 42.1 23.0 17.3 8.5 57.9 48.5 36.0 33.5 20.5 16.6 14.4 13.5

0.0027 0.0024 0.0012 0.0074 0.0041 0.0031 0.0015 0.0102 0.0086 0.0064 0.0059 0.0036 0.0029 0.0025 0.0024

1.43 1.43 1.50 1.24 1.64 1.70 1.46 1.17 1.48 1.84 2.02 2.30 2.20 2.27 2.26

a


Table 4. Binary Interaction Parameters in the PR EoS for the Systems of Interest at Different Temperaturesa compounds

T (°C)

kij PR EoS

CO2 CO2 CO2 CO2

ACN acetone MeOH H2

40.0 25-60 40.0 40.0

0.06 0.01 0.06 0.32

H2

ACN

40.0

0.51

H2

acetone

40.0

0.39

H2

MeOH

40.0

-0.24

Table 5. %AARD for the Peng-Robinson Equation of State Modeling of Vapor-Liquid Equilibrium %AARD

comments use as reported7 use as reported7 use as reported7 extrapolated from kij values fit to literature data20 interpolated from kij values fit to literature data14 interpolated from kij values fit to literature data14 interpolated from kij values fit to literature data15

a

All parameter fitting was done using LNGFLASH, and kij as a function of temperature was regressed using either a linear or polynomial fit.

greater in methanol than in acetonitrile and acetone. The same argument as in our previous work could be applied for the system CO2/H2/methanol. Since the solubility of H2 is so low in methanol, the presence of any CO2 is effective in enhancing the solubility of the sparingly soluble H2 gas. In summary, in all three organics, the presence of CO2 in the liquid phase can increase the solubility of H2 in the liquid. The amount of H2 present in the liquid phase is strongly dependent on the amount of CO2 present. 3.4. Modeling. For modeling the ternary systems investigated we use the same strategy as previously reported.7 The PengRobinson EoS was used for modeling the ternary systems with the van der Waals mixing rule using only binary interaction parameters kij that have been fit to binary phase equilibrium data. The flash algorithm LNGFLASH was used with the PengRobinson EoS to model the ternary systems. As in our previously reported work, calculated values were checked for phase stability with a completely reliable routine based on interval mathematics. Binary interaction parameters (kij) were fit to experimental VLE data. When data were not available at the temperature of interest, kij values were interpolated or extrapolated from the temperatures available. In Table 4 binary interaction parameters used for this study are reported as well as their source. With the optimized binary interaction parameters and the LNGFLASH program, we calculated VLE data for the systems investigated, using the experimental vapor-phase composition as the feed composition in the flash calculation. When no results were obtained in the two-phase region, then slightly richer organic compositions were used lying on approximately the same experimental tie line. We compared experimental and

ternary system

T (°C)

x1

x2

x3

CO2 (1)/H2 (2)/acetonitrile (3) CO2 (1)/H2 (2)/acetone (3) CO2 (1)/H2 (2)/methanol (3)

40 40 40

7.90 7.94 16.74

15.10 11.27 38.22

3.05 4.72 13.44

calculated values using the percent absolute average deviation (%AARD) defined as follows:

%AARD )

100 ND

ND

∑1

|xexp - xcal| xexp

and the results are reported in Table 5 for the liquid phase. In general, the results are very satisfactory considering the fact that only binary interaction parameters were used. For the acetonitrile and acetone systems, results are remarkably good. The highest %AARD values were observed for the methanol systems. An explanation for this is given below. In Figures 2-4 are shown the experimental and calculated values for the ternary CO2/H2/acetonitrile system at 40 °C and 25, 60, and 90 bar. Calculated values were obtained using k12 ) 0.32, k23 ) 0.51, k13 ) 0.06, and the LNGFLASH program. Again, excellent agreement was observed between the experimental data and values predicted by the PR EoS. In general good agreement is observed for all three pressures studied. In addition, the PR EoS predicts the ternary CO2/H2/acetone system at 25 and 60 bar extremely well. As seen in Figures 5 and 6, many of the experimental and calculated tie lines overlap. However, the predictions at 90 bar, shown in Figure 7, are not as good, with the model overestimating the CO2 solubility in the liquid phase and somewhat underestimating the H2 solubility. Figures 8-10 show the comparison between the experimental and PR EoS model predictions for the CO2/H2/methanol system at 40 °C and 25, 60, and 90 bar. The phase compositions and the slopes of the modeled and experimental tie lines are in excellent agreement at the two lower pressures of 25 and 60 bar, where the CO2 concentration in the liquid phase is relatively low (less than 30 mol %). The poorest predictions of the multicomponent phase behavior with the PR EoS were obtained at 90 bar, where concentrations of CO2 in the liquid phase are high. As noted in our previous work, we recognize that the PR EoS provides a particularly poor fit for the CO2/methanol binary system at high CO2 compositions in the liquid phase. Therefore, we do not expect the PR EoS to predict liquid-phase composi-

574


Figure 2. Vapor-liquid equilibrium of the CO2 (1)/H2 (2)/acetonitrile (3) system at 40 °C and 25 bar. Calculated values are from the PR EoS with k12 ) 0.32, k23 ) 0.51, and k13 ) 0.06. (O) Experimental (solid line); (0) calculated (dashed line).


tions at high CO2 concentrations for this system very well, as shown in Figure 10 at 90 bar. Adrian et al.16 compared experimental and calculated values for the CO2/methanol binary system using the PR EoS with different mixing rules in the temperature range 273-323 K and at pressures of 0.2-10.6 MPa (2-106 bar). They found that in most cases the agreement with experimental results is poor, so changing the mixing rule does not improve the calculated values. Chang et al.17 compared fits of the CO2/methanol system at temperatures between 291 and 313 K and pressures up to 8 MPa (80 bar) using the PatelTeja equation and the PR EoS with the van der Waals mixing rule. The Patel-Teja equation did perform better than the PR EoS in the region of interest, i.e., high CO2 mole percent in the liquid phase. Yoon et al.18 also investigated this binary system at 313.15 K with the Patel-Teja EoS with the mixing rule of Wilson. They noted that the calculated values were in good agreement with experimental data except in the near-critical region. The statistical associating fluid theory (SAFT) EoS has also been used by Li et al.19 in the correlation of CO2/methanol VLE data. Again, the model predicts poorly in the region of high CO2 concentration, near the critical point of the binary mixture. While some improvement in the predictions may be


Figure 5. Vapor-liquid equilibrium of the CO2 (1)/H2 (2)/acetone (3) system at 40 °C and 25 bar. Calculated values are from the PR EoS with k12 ) 0.32, k23 ) 0.39, and k13 ) 0.01. (O) Experimental (solid line); (0) calculated (dashed line).

possible using a different equation of state, we focus here on the PR EoS for consistency. However, due to the limitations of the PR EoS, it is not surprising that the quality of calculated values for ternary systems involving CO2/methanol at high CO2 concentrations in the liquid-phase is not very good. Recently, Bezanehtak et al.10 reported experimental VLE data for one of the systems investigated here: the ternary CO2/H2/ methanol system at 278, 288, and 298 K and pressures between 20 and 200 bar. While direct comparison of the data is not possible since they are at different temperatures, we did perform model calculations using the LNGFLASH program with the PR EoS at some of the conditions, viz., 298 K and 70, 110, 170, and 200 bar, used in their experiments. In the model calculations we used binary interaction parameters, k12 ) 0.22, k23 ) -0.36, and k13 ) 0.05, determined from binary experimental data.20,21 Not surprising, the calculated values for the liquid phase deviate from the experimental data at high CO2 concentrations (>20 mol %). However, there is also extremely poor agreement between the predicted and experimental values for the vapor phase. The reported experimental vapor-phase methanol concentrations are extremely high. For example, for a particular tie line Bezanehtak et al. report an experimental methanol vapor-

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Figure 8. Vapor-liquid equilibrium of the CO2 (1)/H2 (2)/methanol (3) system at 40 °C and 25 bar. Calculated values are from the PR EoS with k12 ) 0.32, k23 ) -0.24, and k13 ) 0.06. (O) Experimental (solid line); (0) calculated (dashed line).

phase composition at 298 K and 110 bar of 9.41 mol %, while the calculated value using the PR EoS for the same tie line is just 0.43 mol %. Although the disagreement is less pronounced



at higher CO2 concentrations, it is clear that the vapor-phase concentrations of methanol reported by Bezanehtak et al. are unrealistically high and should be regarded with some skepticism. From the modeling of the ternary H2/CO2/organic systems, it is clear that the PR EoS can be used to predict the solubility of H2 in CO2-expanded solvents using values for the binary interaction parameters obtained from binary experimental data. Even though the concentration of CO2 in the liquid phase is not properly predicted by the PR EoS in some cases, the H2 concentration in the liquid phase, which is of primary importance here, is captured reasonably well. The fact that the solubility of H2 in CO2-expanded liquids is modeled adequately by the Peng-Robinson equation of state, using only binary interaction parameters, is probably due to the relatively small solubility of H2 in the liquid phase. With this in mind, only binary VLE data and a small number of experimental ternary data are required to predict the phase behavior of CO2-expanded systems involving H2. 4. Conclusions We presented the phase behavior of H2 in three CO2-expanded solvents (acetonitrile, acetone, and methanol) at 40 °C and

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pressures of 25, 60, and 90 bar. The solubility of H2 in the CO2expanded liquids was greater than that in the absence of CO2 at the same H2 fugacity for all three systems at all conditions studied. Enhancement factors as high as 2.68 were observed. The EFs generally increased with increasing CO2 solubility in the liquid phase. However, higher CO2 concentration in the liquid phase means the vapor phase is richer in CO2 and, subsequently, the partial pressure of H2 is not high. Therefore, H2 solubility in the liquid phase is not high, even in the cases where EFs are high. The net effect is that the solubility of H2 can be increased by the addition of CO2, but only at the cost of higher total pressures. As in our previous report with O2 and CO solubility, H2 solubility did not substantially exceed what could be obtained at the same total pressure with pure H2. Thus, CO2 does slightly enhance the solubility of H2 in acetonitrile, acetone, and methanol but the dilution effect of the CO2 outweighs any enhancement. The Peng-Robinson equation of state predicted the ternary systems very well at the conditions studied with only binary interaction parameters obtained from readily available binary VLE data. Hence, the solubility of gases in CO2-expanded solvents can be estimated using the PengRobinson equation of state with reliable binary vapor liquid equilibrium data and only a limited number of experiments for the ternary system. Acknowledgment This research was supported by the National Science Foundation, Engineering Research Centers Program, Grant EEC0310689. Literature Cited (1) Combes, G.; Coen, E.; Dehghani, F.; Foster, N. Dense CO2 expanded methanol solvent system for synthesis of naproxen via enantioselective hydrogenation. J. Supercrit. Fluids 2005, 36, 127-136. (2) Combes, G. B.; Dehghani, F.; Lucien, F. P.; Dillow, A. K.; Foster, N. R. Asymmetric Catalytic Hydrogenation in CO2 Expanded Methanols an Application of Gas Anti-Solvent Reaction (GASR). In Reaction Engineering for Pollution PreVention; Abraham, M. A., Hesketh, R. P., Eds.; Elsevier: Amsterdam, 2000; pp 173-181. (3) Fujita, S.; Akihara, S.; Zhao, F. Y.; Liu, R. X.; Hasegawa, M.; Arai, M. Selective hydrogenation of cinnamaldehyde using ruthenium-phosphine complex catalysts with multiphase reaction systems in and under pressurized carbon dioxide: Significance of pressurization and interfaces for the control of selectivity. J. Catal. 2005, 236, 101-111. (4) Jessop, P. G.; Stanley, R. R.; Brown, R. A.; Eckert, C. A.; Liotta, C. L.; Ngo, T. T.; Pollet, P. Neoteric solvents for asymmetric hydrogenation: supercritical fluids, ionic liquids, and expanded ionic liquids. Green Chem. 2003, 5, 123-128. (5) Solinas, M.; Pfaltz, A.; Cozzi, P. G.; Leitner, W. Enantioselective

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ReceiVed for reView January 16, 2007 ReVised manuscript receiVed May 4, 2007 Accepted May 24, 2007 IE070105B

Enhanced Solubility of Hydrogen in CO2-Expanded Liquids

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