Solubility of carbon dioxide in dimethylsulfoxide and N-methyl-2-pyrrolidone at elevated pressure

J. of Supercritical Fluids 31 (2004) 227–234

Solubility of carbon dioxide in dimethylsulfoxide and N-methyl-2-pyrrolidone at elevated pressure Rueben Rajasingam, Liwan Lioe, Q. Tuan Pham, Frank P. Lucien∗ School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, UNSW Sydney, NSW 2052, Australia Accepted 12 December 2003

Abstract Experimental data on the solubility of CO2 in dimethylsulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP) are reported at temperatures of 298, 308 and 318 K and pressures approaching the mixture critical point for each binary system. The corresponding volumetric expansions of DMSO and NMP with CO2 are also presented. The solubility data are correlated with the Peng–Robinson equation of state (PR-EOS). At a given temperature and pressure, higher solubilities of CO2 are obtained in NMP compared with DMSO and this trend is consistent with the volumetric expansions of the solvents. A plot of the volumetric expansion data as a function of the solubility of CO2 in the liquid phase suggests that it is unlikely, as a general rule, that the expansion isotherms for the various systems collapse onto a single expansion curve. The existence of a single expansion curve appears to hold only for a given binary system over a limited range of temperature. The use of two adjustable binary interaction parameters with the PR-EOS provides a superior correlation of the liquid phase composition in comparison with the standard PR-EOS, particularly in the vicinity of the mixture critical point. © 2003 Elsevier B.V. All rights reserved. Keywords: Vapour–liquid equilibria; Volumetric expansion; Carbon dioxide; Dimethylsulfoxide; N-Methyl-2-pyrrolidone

1. Introduction The miscibility of a dense gas with a liquid solvent is a fundamental requirement of a range of precipitation techniques which utilise a gaseous or supercritical antisolvent. This requirement is commonly assessed by measuring the volumetric expansion of the solvent when it is contacted with the dense gas. At a given temperature and pressure, however, different solvents exhibit varying degrees of volumetric expansion with the same dense gas. This is a consequence of the solubility of the dense gas in the solvent. A more complete understanding of the expansion behaviour of a solvent therefore requires data on the solubility of the dense gas in the solvent. Such information can be used, for example, to tune equations of state which can then be used to predict the volumetric expansion over a wide range of operating conditions [1]. The optimum solvent for a given precipitation process is influenced by the type of solute being precipitated in addi∗ Corresponding author. Tel.: +61-2-9385-4302; fax: +61-2-9385-5966. E-mail address: [email protected] (F.P. Lucien).

0896-8446/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2003.12.003

tion to the expansion behaviour of the solvent. In the most recent applications of supercritical antisolvent technology, there has been a clear trend towards the precipitation of macromolecular compounds such as pharmaceuticals [2–5] and polymers [6–9]. In nearly all cases, these processes are restricted to the use of very polar organic solvents in order to attain a sufficient loading of the solute in the solution to be expanded. Such solvents include acetone, dimethylsulfoxide (DMSO), N,N-dimethylformamide, dichloromethane and N-methyl-2-pyrrolidone (NMP). In this paper, we present experimental data on the solubility of CO2 in DMSO and NMP at elevated pressure. These solvents are of particular interest in an associated study on the recovery of precious metals from dipolar aprotic solvents using the gas antisolvent process [10]. A substantial amount of vapour–liquid equilibria (VLE) and volumetric expansion data has been published for the CO2 -DMSO binary system, as shown in Table 1. In the case of NMP, however, there is a lack of such data in the literature. New experimental data are therefore presented on the solubility of CO2 in NMP at temperatures of 298, 308 and 318 K and pressures approaching the mixture critical point. For comparative purposes, solubility data for DMSO are reported at the same values of

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R. Rajasingam et al. / J. of Supercritical Fluids 31 (2004) 227–234

Table 1 Published experimental vapour–liquid equilibria and volumetric expansion data for the CO2 -DMSO and CO2 -NMP binary systems System

P (MPa)a

Reference

0.2–6.6 4.1–12.6 1.0–9.0

[11] [12] This work

1.0–8.4

This work

303, 307 303 308 308, 318

1.0–6.2 0.2–6.6 1.2–7.6 0.5–9.1

[13] [11] [14] This work

298, 308, 318

0.6–8.9

This work

T (K)

Vapour–liquid equilibria CO2 -DMSO 298, 303 309, 314, 321, 329 298, 308, 318 CO2 -NMP

298, 308, 318

Volumetric expansion CO2 -DMSO 298, 298, 295, 298, CO2 -NMP a

and volumetric expansion data were measured using the same apparatus which is shown schematically in Fig. 1. The equilibrium cell consisted of a high-pressure sight gauge with an internal volume of approximately 70 ml. The temperature of the water bath was maintained constant to ±0.2 K. Equilibrium between the vapour and liquid phases was achieved by recirculation of the liquid phase from the bottom of the sight gauge to the top using a metering pump. The temperature, pressure and liquid level in the system typically attained stable values within 30 min using this technique. Volumetric expansion curves were determined for each solvent prior to the measurement of the solubility of CO2 in the liquid phase. This information provided a means of determining an allowable range of pressure for solubility measurements at a given temperature and for a given solvent. Furthermore, the measurement of volumetric expansion was based upon a fixed amount of liquid initially in the sight gauge, as indicated in Eq. (1), which precluded any sampling during this procedure. The volumetric expansion of each solvent with CO2 was ascertained from a scale which was fitted along the visible length of the sight gauge. The volume of liquid in the sight gauge, as a function of the scale length, was determined by calibration with ethanol. Ethanol was selected for this purpose because it is available in high purity and its density is accurately known. A certain mass of ethanol was added to the sight gauge and recirculated for a period of 30 min at atmospheric pressure and 298 K. Care was taken to remove all of the air trapped in the connecting line between the bottom and top of the sight gauge. From a knowledge of the density of ethanol it was possible to calculate the total volume of ethanol corresponding to the liquid level in the sight gauge. The volume of liquid at a given level also included that portion of the liquid in the connecting line between the top and the bottom of the gauge. This procedure was repeated for several different masses of ethanol, covering the range of the scale, and a calibration curve was constructed.

Minimum and maximum pressure used.

temperature. The corresponding volumetric expansions of DMSO and NMP with CO2 are also presented. The solubility data are correlated with the Peng–Robinson equation of state (PR-EOS).

2. Experimental 2.1. Materials DMSO (99%) and NMP (99.5%) were obtained from Sigma–Aldrich. Due to the hygroscopic nature of these solvents, they were dried prior to use with a type 4A molecular sieve (Labchem). Ethanol (99.7%) was purchased from CSR Distilleries and acetone (99.5%) was purchased from Asia Pacific Specialty Chemicals. Liquid CO2 (99.5%) was obtained from Linde Gas. 2.2. Apparatus and procedure A detailed description of the apparatus and procedure has been reported previously [15]. Both the solubility data H

T

PT

V2

V3

V1

TO VENT B

V4

PC

EC

CO 2

MP

ST

V5 SP

WB

E

W

Fig. 1. Schematic diagram of the experimental apparatus. B, burette; CO2 , carbon dioxide cylinder; EC, equilibrium cell; E, ethanol; H, heater; MP, metering pump; PC, preheating coil; PT, pressure transducer; SP, syringe pump; ST, solvent trap; T, thermocouple; V, valve; W, water; WB, water bath.

R. Rajasingam et al. / J. of Supercritical Fluids 31 (2004) 227–234

The volumetric expansion (E) of the liquid phase at a given temperature (T) and pressure (P) was calculated using the following equation: E (%) =

VL (T, P) − VL∗ (T, P ∗ ) × 100 VL∗ (T, P ∗ )

(1)

where VL is the liquid volume and VL∗ the liquid volume at the reference condition and P∗ represents the reference pressure. The reference condition represents the liquid phase saturated with CO2 at a atmospheric pressure. The volume of liquid contained in the sight gauge when the level was at the uppermost part of the scale represented around 80% of the total internal volume of the sight gauge, thus ensuring that a vapour phase was present at all experimental conditions. The maximum volumetric expansion obtainable between the lower and upper limits of the scale was around 300%. The reproducibility of the technique for measuring the volumetric expansion data was assessed in preliminary work in which data were obtained for different initial volumes of solvent in the equilibrium cell. In a binary system, where the components are distributed between vapour and liquid phases, the volumetric expansion of the liquid phase is independent of the initial volume of liquid once T and P are specified [15]. This fact was verified in the preliminary work which revealed that the deviations between runs, obtained with different initial volumes of liquid, typically varied between 5 and 10%. The uncertainty in the volumetric expansion data reported here is therefore estimated to be less than 10%. In the determination of the solubility of CO2 in the liquid phase, equilibrium between the vapour and liquid phases was established in the manner described above except that the actual attainment of equilibrium was confirmed by monitoring the composition of the liquid phase over time. Samples of the liquid phase were removed from the equilibrium cell via a three-way valve located at the inlet of the metering pump (V5). Separation between the solvent and the CO2 was carried out in a solvent trap filled with ethanol. The volume of CO2 associated with the sample was measured with a burette connected directly to the solvent trap as shown in Fig. 1. The amount of solvent collected in the ethanol was determined by gas chromatography. In a typical experiment, the sampling procedure resulted in a drop in pressure of less than 0.1 MPa in the equilibrium cell. After each sample was taken, the desired pressure was restored in the equilibrium cell followed by recirculation of the liquid phase for a period of at least 30 min. The composition of the liquid phase was determined from the average of at least three measurements, with a relative standard deviation (R.S.D.) less than 5%. The R.S.D. was calculated with respect to the ratio of the organic solvent to CO2 in the sample. The composition of the liquid phase is reported as the mole fraction of CO2 in the liquid phase.

229

3. Data correlation Experimental data on the solubility of CO2 in DMSO and NMP were correlated using both a 1-parameter and a 2-parameter version of the PR-EOS. In the 1-parameter version, conventional mixing rules were used for calculating the attractive (amix ) and repulsive (bmix ) terms for the fluid mixture as follows: amix =

N N



xi xj (1 − kij )(ai aj )1/2

(2)

i=1 j=1

bmix =

N


xi bi

(3)

i=1

where xi refers to the mole fraction of component i in a given phase, kij is the binary interaction parameter, ai the attractive parameter of component i, bi the repulsive parameter of component i and N the number of components in the mixture. The attractive and repulsive parameters of the pure components (ai and bi ) were calculated with the conventional equations. The critical properties and acentric factors for the components of interest in this study are listed in Table 2. For mixtures in which one of the components is polar, a second interaction parameter can be introduced to achieve greater accuracy in the correlation of the data. This technique is also often required in hydrocarbon systems in order to accurately model VLE data obtained at pressures approaching the mixture critical point. In the 2-parameter version of the PR-EOS, Eq. (2) was used for calculating amix and a second binary interaction parameter (lij ) was incorporated into the mixing rule for bmix as follows: bmix =

N N



xi xj (1 − lij )

i=1 j=1

bi + b j 2

(4)

Note, if the second interaction parameter is set to zero, Eq. (4) reduces to Eq. (3). Values of the binary interaction parameters were regressed from the solubility data using a bubble pressure calculation procedure. The procedure involves the calculation of the vapour-phase composition and the system pressure from the liquid phase composition and the system temperature. The reader is referred to elsewhere for a detailed description of the calculation procedure and for Table 2 Values of the critical temperatures (Tc ), critical pressures (Pc ) and acentric factors (ω) used in the PR-EOS Component

Molar mass

Tc (K)

Pc (MPa)

ω

Carbon dioxidea DMSOb NMPb

44.01 78.14 99.13

304.12 726 724

7.374 5.65 4.78

0.225 0.2094 0.3577

a b

[16]. [17].

230

R. Rajasingam et al. / J. of Supercritical Fluids 31 (2004) 227–234

the relevant equations applicable to the 2-parameter PR-EOS employed here [18,19]. The binary interaction parameters were allowed to vary with temperature. At a given temperature and for a given CO2 /solvent combination, the optimum values of kij and lij were obtained by minimising the sum of squared relative deviations (SSRD) with respect to pressure:  M 
Pcalc − Pexp 2 (5) SSRD = Pexp i i=1

where M is the number of data points and Pcalc and Pexp the calculated and experimental values of pressure, respectively. In the discussion which follows, the average absolute relative deviation (AARD) is defined as:  M  100
 Pcalc − Pexp  AARD (%) = (6)   Pexp M i i=1

4. Results and discussion

Table 4 Volumetric expansion of NMP with CO2 at various temperatures T = 298.2 K

T = 308.2 K

T = 318.2 K

P (MPa)

E (%)

P (MPa)

E (%)

P (MPa)

E (%)

0.57 1.21 1.75 2.25 2.82 3.30 3.76 4.25 4.53 4.81 5.11 5.20 5.30 5.39 5.49 5.54 5.59 5.66 5.71

5.1 12.8 20.5 25.6 35.9 46.2 59.0 74.4 87.2 103 132 143 160 185 206 223 244 262 280

1.61 2.71 3.66 4.42 5.04 5.45 5.85 6.23 6.52 6.73 6.89 7.00

16.7 26.9 42.3 57.7 75.7 91.1 110 139 170 203 241 279

1.33 2.78 4.29 5.40 6.28 6.98 7.55 7.99 8.31 8.55 8.76 8.94

9.0 18.0 33.3 53.9 69.3 88.5 114 140 168 200 239 282

4.1. Volumetric expansion

Table 3 Volumetric expansion of DMSO with CO2 at various temperatures T = 298.2 K

T = 308.2 K

T = 318.2 K

P (MPa)

E (%)

P (MPa)

E (%)

P (MPa)

E (%)

1.09 1.67 2.31 2.89 3.37 3.91 4.27 4.57 4.82 5.00 5.19 5.35 5.49 5.58 5.71 5.76 5.78 5.80 5.83

5.0 10.0 17.5 25.0 35.0 47.5 60.0 75.0 87.5 97.5 113 130 146 163 193 208 228 248 260

1.35 2.55 3.76 4.53 5.15 5.65 6.08 6.40 6.66 6.85 7.01 7.12 7.22 7.27

11.3 22.5 37.5 52.5 67.5 86.3 105 125 147 172 198 225 255 283

1.05 2.11 3.56 4.62 5.47 6.19 6.78 7.29 7.83 8.07 8.38 8.62 8.81 9.00 9.13

10.0 15.0 27.5 40.0 53.8 68.8 85.0 103 125 139 165 192 220 259 300

Ref.[13] - 298 K

Ref.[11] - 298 K

Ref.[14] - 308 K

300 250

298 K

200 E (%)

Volumetric expansion data for DMSO and NMP with CO2 at 298, 308 and 318 K are presented in Tables 3 and 4. Elvassore et al. [13] and Kordikowski et al. [11] have also measured the expansion of DMSO with CO2 at 298 K and these data are shown in Fig. 2. It can be seen that some significant discrepancies exist between the two sets of published data and this is possibly attributed to the differences in the experimental techniques used to obtain the data. Kordikowski et al. [11] determined the volumetric expansion by measuring the density of the expanded liquid phase

using a densitometer. With the known density of the liquid phase, as well as its composition, the volumetric expansion was calculated with an equation equivalent to Eq. (1). Elvassore et al. [13] employed UV-Vis spectroscopy to measure the volumetric expansion. In this technique, an internal standard consisting of potassium hexacyanoferrate(III) was added to the solvent at a very low concentration (<0.1 wt.%). The absorbance of the internal standard was then measured as a function of pressure. Based on the Lambert–Beer law, which relates absorbance to concentration, they showed that the relative variation of the absorbance signal can be directly related to the volumetric expansion of the liquid phase. Despite the inconsistencies in the published data for DMSO at 298 K, it can be seen that the data measured at 298 K in this study are intermediate to the published sets of

150 100 308 K

50 0 3

4

5

6

7

8

P (MPa) Fig. 2. Comparison of volumetric expansion data for DMSO. The data measured in this work are shown as points connected by lines.

R. Rajasingam et al. / J. of Supercritical Fluids 31 (2004) 227–234 acetone

DMSO

Table 5 Solubility (x) of CO2 (1) in DMSO (2) at various temperatures

NMP

300 250 E (%)

200 150 100 50 0

0

2

4 P (MPa)

6

231

8

Fig. 3. Comparison of volumetric expansion data for acetone, DMSO and NMP at 308 K.

data. A comparison between the data for DMSO at 308 K and the data reported by Yeo et al. [14] is also shown in Fig. 2. Yeo et al. determined the volumetric expansion by direct observation using a technique similar to the one reported here, except that the liquid phase was not recirculated. With the exception of a couple of data points at the highest values of pressure, it can be seen that their data agree closely with the present data. It was noted that in the present study, longer recirculation times were required to achieve a stable liquid level at higher values of pressure, mainly due to the longer time required for saturation of the solvent with CO2 . The time required for equilibration was around 30 min. In contrast, Yeo et al. reported that a duration of 10 min was used for equilibration in their work, using a sight gauge of comparable volume to the one described in this work. Some underestimation of the volumetric expansion at higher values of pressure is therefore conceivable using their technique, in view of the absence of recirculation and the shorter equilibration time. The data in Tables 3 and 4 demonstrate that at a given pressure the volumetric expansion decreases with increasing temperature. This is due to the decrease in the density of pure CO2 and the corresponding reduction in the solubility of CO2 in the liquid phase. A comparison of the volumetric expansion data for both solvents at 308 K is shown in Fig. 3. Data for acetone at 308 K, measured using the same technique described in Section 2.2, are also shown in this figure. The relative expansion of the three solvents can be explained qualitatively in terms of solvent polarity. The dipole moments for acetone, DMSO and NMP are around 2.8, 3.8 and 4.0 Debye, respectively [20]. Carbon dioxide behaves essentially as a nonpolar solvent and is most soluble in the least polar solvent which in this case is acetone. Thus, at a given pressure, the level of volumetric expansion for acetone is significantly greater than that for the other two solvents. The similarity between the expansion curves for NMP and DMSO is also consistent with their dipole moments.

P (MPa)

x1 (mole fraction)

E (%)

T = 298.2 K 1.03 2.00 3.01 4.02 5.01 5.51

0.0666 0.1358 0.2311 0.3282 0.4775 0.5906

4.5 13.9 27.5 51.3 98.3 150

T = 308.2 K 1.04 3.00 5.01 6.01 6.51 7.04

0.0524 0.1633 0.3128 0.4134 0.4919 0.5571

8.5 28.1 64.1 102 134 205

T = 318.2 K 1.01 5.00 6.01 7.04 8.52 9.04

0.0439 0.2507 0.3196 0.3937 0.5411 0.6004

9.6 46.2 65.0 94.0 181 272

4.2. Solubility of CO2 The experimental data for the solubility of CO2 in DMSO and NMP are presented in Tables 5 and 6. The corresponding values of the volumetric expansion of the liquid phase are also provided in these tables. The values of E shown were obtained by linear interpolation of the experimental data presented in Tables 3 and 4. The solubility data for CO2 Table 6 Solubility (x) of CO2 (1) in NMP (2) at various temperatures P (MPa)

x1 (mole fraction)

E (%)

T = 298.2 K 1.00 2.02 2.89 4.07 4.48 5.48

0.0887 0.1935 0.2782 0.4332 0.4771 0.6657

10.3 23.3 37.4 68.7 84.9 203

T = 308.2 K 0.96 4.01 5.00 5.49 6.02 6.67

0.0693 0.3286 0.4342 0.4894 0.5571 0.6775

9.5 49.4 74.5 93.0 123 193

T = 318.2 K 1.00 4.06 5.97 6.89 8.07 8.43

0.0633 0.2840 0.4496 0.5199 0.6519 0.7118

6.6 31.0 63.8 86.0 147 184

232

R. Rajasingam et al. / J. of Supercritical Fluids 31 (2004) 227–234 298 K

308 K

318 K

Ref.[11] - 298 K

10

D (318K)

N (308K)

N (318K)

250

6

200 E (%)

P (MPa)

D (308K)

N (298K)

300

8

4

150 100

2 0 0.0

D (298K)

50 0.2 0.4 0.6 x 1 (mole fraction)

0 0.0

0.8

Fig. 4. Comparison of P − x1 data for the CO2 -DMSO binary system.

in DMSO are shown graphically in Fig. 4. Kordikowski et al. [11] have also measured the solubility of CO2 in DMSO at 298 K and these data are included in Fig. 4. Overall, the published solubility data are around 20% higher than the present data, which appears to be consistent with the comparison of the corresponding sets of volumetric expansion data for this solvent in Fig. 2. In any event, the internal consistency of the solubility data for DMSO is evident. At a given pressure, the solubility of CO2 decreases with increasing temperature in accordance with the decrease in the density of pure CO2 . At relatively low values of pressure (<5 MPa), the solubility of CO2 displays a near-linear dependence on pressure, which is consistent with Henry’s law on the solubility of gases in liquids. As the mixture critical point is approached, the slope of the P − x1 curve decreases and the solubility of CO2 becomes much more sensitive to pressure, as indicated also by the rapid increase in the volumetric expansion of the solvent. Similar characteristics are observed for the solubility data for NMP (not shown). These trends are commonly observed in binary systems consisting of CO2 and an organic solvent [11,15,21,22]. At a given pressure, the solubility of CO2 increases in the order: DMSO < NMP < acetone. The relationship between volumetric expansion and the solubility of the antisolvent in the liquid phase has been investigated in some detail by Kordikowski et al. [11]. They measured volumetric expansion data and solubility data for six different polar organic solvents with CO2 . The use of ethane and ethene was also considered for a few of the solvents. From a qualitative viewpoint, their results demonstrate that for a given antisolvent, the expansion data for various solvents appear to coincide when plotted as a function of the solubility of the antisolvent in the liquid phase. However, a closer examination of their data reveals that at a given solubility of the antisolvent, the variation in the volumetric expansion of the different solvents encompasses an interval of around 50%. For example, when the mole fraction of CO2 in the liquid phase is around 0.6, the expansion data vary from 100 to 150%.

0.2 0.4 0.6 x 1 (mole fraction)

0.8

Fig. 5. Relationship between volumetric expansion and the solubility of CO2 in DMSO (D) and NMP (N).

The relationship between volumetric expansion and the solubility of CO2 in the solvents considered in this work is shown in Fig. 5. The data presented are identical to the data given in Tables 5 and 6. Overall, it is evident that the expansion curves for the various systems do not coincide, even if some allowance is made for experimental error. For a given solvent, however, there is some measure of coincidence of the different isotherms. For example, the data for DMSO at 308 and 318 K coincide. The data for NMP at 298 and 308 K also coincide. The significance of this result is that it provides a means of estimating the solubility of CO2 in the liquid phase simply from a knowledge of the volumetric expansion. It can also provide an indication of the consistency of the solubility data. 4.3. Data correlation Optimised values of kij and lij from the correlation of the solubility data are presented in Table 7. The AARD with respect to pressure is in the range of 14–18 and 8–13% for Table 7 Optimised values of kij and lij from the PR-EOS System

T (K)

kij

lij

SSRD

0.0807 0.0828 0.0747

– – –

0.1557 0.2001 0.2578

298.2 308.2 318.2

0.0619 0.0540 0.0408

– – –

0.0468 0.0929 0.1394

2-Parameter PR-EOS CO2 -DMSO 298.2 308.2 318.2

0.0479 0.0514 0.0496

−0.0402 −0.0460 −0.0484

0.0007 0.0005 0.0004

0.0340 0.0258 0.0198

−0.0250 −0.0330 −0.0352

0.0010 0.0011 0.0014

1-Parameter PR-EOS 298.2 CO2 -DMSO 308.2 318.2 CO2 -NMP

CO2 -NMP

298.2 308.2 318.2

R. Rajasingam et al. / J. of Supercritical Fluids 31 (2004) 227–234 DMSO

suggests that it is unlikely, as a general rule, that the expansion isotherms for the various systems collapse onto a single expansion curve. The existence of a single expansion curve appears to hold only for a given binary system over a limited range of temperature. The use of two adjustable binary interaction parameters with the PR-EOS provides a superior correlation of the liquid phase composition in comparison with the standard PR-EOS, particularly in the vicinity of the mixture critical point.

NMP

8

P (MPa)

6 4 2 0 0.0

0.2

0.4

233

0.6

0.8

x 1 (mole fraction) Fig. 6. Correlation results from the PR-EOS for the CO2 -DMSO and CO2 -NMP binary systems at 308 K. The dashed lines represent the 1-parameter model and the solid lines represent the 2-parameter model.

the CO2 -DMSO and CO2 -NMP systems, respectively, using the 1-parameter model. The values of the AARD obtained with the 2-parameter model are less than 1 and 2% for the CO2 -DMSO and CO2 -NMP systems, respectively. A comparison of the correlation results obtained from the 1-parameter and 2-parameter models is shown in Fig. 6 for the CO2 -DMSO and CO2 -NMP systems at 308 K. The data shown in Fig. 6 indicate that the 1-parameter model typically underestimates the bubble pressure in the lower range of solubility while the opposite situation occurs in the higher range of solubility. The same trends are observed for the solubility data measured at 298 and 318 K, although this is not shown here. With the 2-parameter model, it can be seen that the correlation results follow the contour of the experimental data very well, even as the mixture critical pressure is approached. As a final point of interest, the calculated composition of the vapour phase from the 2-parameter model is generally greater than 99 mol%, with respect to CO2 , in both the CO2 -DMSO and CO2 -NMP systems for the three temperatures investigated. This highlights another useful characteristic of these solvents in that they behave as nonvolatile solvents when contacted with CO2 at elevated pressure. This characteristic is highly desirable in the context of the GAS process because it minimises the loss of solvent when the system is depressurised.

5. Conclusion Solubilities of CO2 as high as 60 mol% can be achieved in DMSO and NMP at pressures well below 10 MPa over the range of temperature investigated. At a given temperature and pressure, higher solubilities of CO2 are obtained in NMP compared with DMSO and this trend is consistent with the measured volumetric expansions and dipole moments of the solvents. A plot of the volumetric expansion data as a function of the solubility of CO2 in the liquid phase

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