Supercritical Fluid Engineering Science - American Chemical Society

Chapter 2

High-Pressure Vapor—Liquid Equilibria in Carbon Dioxide and 1-Alkanol Mixtures 1

Downloaded by NORTH CAROLINA STATE UNIV on January 15, 2013 | http://pubs.acs.org Publication Date: December 17, 1992 | doi: 10.1021/bk-1992-0514.ch002

David W. Jennings, Michael T. Gude, and Amyn S. Teja

School of Chemical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0100

CO + 1-alkanol systems are of interest as solvent/co-solvent pairs in the supercritical extraction of biomaterials, in alkanol dehydration processes, in the processing of syngas, and in supercritical drying. In spite of these interests, however, only the CO + methanol and CO + ethanol systems have received much attention in the litera ture. Moreover, there appears to be some disagreement between the different sets of published data even for these systems. In order to compile a reliable set of data for modelling processes which involve CO + 1-alkanol mixtures, we have measured vapor - liquid equi libria in CO + ethanol, CO + 1-butanol, and CO + 1-pentanol mixtures at temperatures ranging from 314 to 337 Κ and pressures from 4.63 to 11.98 MPa. In this paper, we compare our data with data reported in the literature, and note trends as the 1-alkanol in creases in size. The correlation of the data with a simple equation of state and an equation of state based on Statistical Associating Fluid Theory is also discussed. 2

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High pressure vapor - liquid equilibria in C 0 + 1-alkanol systems are of interest in a number of applications. These applications include the extraction of bioma terials with supercritical C 0 + 1-alkanol mixtures, the separation of 1-alkanols from aqueous solutions using supercritical carbon dioxide [1-8] , the production of alkanols from syngas [9-11] , and the supercritical drying of aerogels [12]. Ther modynamic modelling of these processes requires a knowledge of binary vapor liquid equilibria (VLE). In spite of this need, however, only the C 0 + methanol and C 0 + ethanol systems have received much attention in the literature. This is evident from the summary of vapor - liquid equilibrium measurements given in Tables I and II. Critical properties of C 0 + methanol mixtures have been measured by Brunner [13]; those of C 0 + ethanol mixtures have been measured 2

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Corresponding author 0097-6156/93/0514-0010$07.00/0) © 1993 American Chemical Society

In Supercritical Fluid Engineering Science; Kiran, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


2. JENNINGS ET AL.

High-Pressure Equilibria in C0 and 1-Alkanol Mixtures 1 2

Table I: Vapor - Liquid Equilibrium Measurements in C Q 4- 1-Alkanol Systems 2

System

Temperature Range (K)

Pressure Range (MPa)

Reference

C0 /Methanol

288.15-333.15 298.15 298.15-313.15 213.15-273.15 323.15-398.15 263.15-273.15 298.15-473.15 230.0-330.0 313.4 298.15-348.15 304.2-308.2 307.85-337.85 304.6-323.1 304.2-323.4 313.46-333.27 313.4-333.4 314.2-337.2 305.6-313.1 313.4-333.4 298.15 313.15-383.15 314.6-337.4 313.15 314.3-337.5 323.4-401.8 343.3-402.0

1.6-4.3 0.22-6.13 0.58-8.06 0.01-1.69 0.5-18.50 1.72-1.78 0.41-15.06 0.69-10.65 0.68-7.71

323.4-402.0

10.0-53.20

Hemmaplardh and King, Jr. [16] Katayama et al. [17] Ohgaki and Katayama [18] Schneider [19] Semenova et al. [20] Lazalde-Crabtree et al. [21] Brunner et al. [22] Hong and Kobayashi [23] Suzuki et al. 19] Gupta et al. [24] Takishima et al. [4] Panagiotopoulos [25] Yao et al. [261 Feng et al. [5] Nagahama et al. [6] Suzuki et al. Î9] Jennings et al. [271 Yao et al. [26 Suzuki et ai 91 Massoudi and King, Jr. [28] King et al. [29] Jennings et al. [27] Winkler and Stephan [30l Jennings et al. |31| Nickel and Schneider [32 4 Nickel and Schneider |32 4 Katzenski-Ohling [33Γ Hôlscher [34Γ Friedrich and Schneider [35]

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C0 /Ethanol 2

C0 /l-Propanol 2

C0 /l-Butanol 2

C0 /l-Pentanol C0 /l-Hexanol C0 /l-Decanol C0 /1-Dodecanol C 0 /1 - Hexadecanol C 0 /1- Octadecanol 2

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1

2

A

3.75-7.67 1.25-10.95 2.22-9.17 3.31-7.96 0.53-10.63 0.51-10.65 5.55-10.85 2.10-8.04 0.52-10.82

1

2.02-16.45 4.63-11.78 «0.2-8.2 5.18-11.98 9.10-19.50 13.0-24.80

2

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2

1

3

1= Vapor Phase Only 2 = Bubble/Dew Point Measurements 3 = Graphical Results Only 4 = Phase Compositions Reported in Concentrations. No Phase Densities Given


4


SUPERCRITICAL FLUID ENGINEERING SCIENCE

Table Π: Measurements of Carbon Dioxide Solubility in 1-Alkanols System

Temperature Range (K)

Pressure Range (MPa)

Reference

C0 /Methanol

291.15-307.15 298.15-348.15 273.15-348.15 223.15-348.15 194.45-273.15 213.15-247.15 228.15-247.15 223.65-328.15 243.15-273.15 283.15-313.15 258.0 298.15 247.15 248.15-273.15 233.15-298.15 243.2-298.1 293.15· •373.15 291.15-•307.15 212.65-•247.95 293.15- 373.15 283.15- 313.15 212.65- 247.95 212.65 247.95 282.66 313.64 284.01-313.49

«0.1 5.1-8.6 0.69-6.97 0.1-3.04 0.01-0.10 0.1-1.62

Kunerth [36] Krichevsky and Koroleva [37] Krichevsky and Lebedewa [38] Bezedel and Teodorovich [39] Shenderei et al. [40' Shenderei et al. 411 Shenderei et al. 42| Otsuka and Takada [43] Yorizane et al. .[44] Tokunaga et al. [45] Ferrell and Rousseau [46] Won et al. [47] Takeuchi et al. [48] Zeck and Knapp [49] Weber et al. [50] Chang and Rousseau (511 Sander [52J Kunerth [36] Shenderei et al. [40] Sander [521 Tokunaga [53] Shenderei et al. [401 Shenderei et al. 40 Wilcock et al. 54 Wilcock et al. 54

2

C0 /Ethanol 2

C02/1-Propanol

CJU /l-ButanôT CQ /l-Octanor 2

2

— ' — ' — I — ' — • — • — Γ

0.4

0.6

MOLE FRACTION C O

0.8 o

2 Figure 5. C0 + ethanol vapor-liquid equilibria at 325.2 K. 2

1 2 -τ11 -

• Δ

THIS WORK, 337.2 Κ PANAGIOTOPOULOS, 337.8 Κ Δ

1ο 03 Q.

Έ LU CC

9 -

8 -

(/) ω LU

7 -

ce α.

6 5 4 0.0

"Γ 0.4

0.2

0.6

-ι

1

1

0.8

M O L E FRACTION C O Figure 6. C0 + ethanol vapor-liquid equilibria at 337.2 K. 2


1.0

18


dioxide compositions than the data of Suzuki et al.. It will be shown below that the liquid phase data of Suzuki et al. are higher in carbon dioxide composition than is justified by measurements of CO2 -f ethanol, CO2 + 1-butanol, and CO2 + 1-pentanol mixtures. The critical point of Gurdial et al. appears to be in agreement with the measurements presented in Figure 7. C0 + 1-Butanol System. Vapor - liquid equilibrium data in C 0 + 1butanol mixtures were measured in our apparatus at 314.8 Κ, 325.3 Κ, and 337.2 Κ [27]. The measurements at 314.8 Κ are compared with those of King et al. at 313.15 Κ [29] and with the liquid phase data of Winkler and Stephan at 313.15 [30] in Figure 8. The critical point of Gurdial et al. [15] is also included in Fig ure 8. The data of Winkler and Stephan were obtained by digitizing the graphical results presented in their paper. However, due to the small 1-butanol composi tions in the vapor phase and the scale of the graph of Winkler and Stephan, the vapor phase compositions could not be determined by digitization. As is obvi ous, the results obtained by us are in considerable disagreement with the data of King et al. even after allowance is made for the temperature differences in the two sets of measurements. However, there is good agreement with the liquid phase measurements of Winkler and Stephan. We have noted elsewhere [27] that measurements by King et al. [58] for the system CO2 + n-hexadecane at 333.15 Κ also showed higher carbon dioxide compositions in the liquid phase than mea surements of Charoensombut-amon et al. [59] and D'Souza et al. [60] at the same temperature. Additionally, the measurements of King et al. were made in a blind cell and, because of the odd shape of the isotherm in the critical region, King et al. hypothesized that the data points at the highest two pressures were indicative of two liquid phases. No liquid - liquid equilibria were observed in this work or in the work of Winkler and Stephan. This is further supported by the recent work of Lam et al. [61], who report that the liquid - liquid - vapor locus in the C 0 + 1-butanol system terminates at an upper critical end point of 22.99 bar and 259.25 K . Liquid - liquid phase separation is therefore unlikely at the conditions studied by King et al. and their measurements must be considered erroneous. As with the C 0 + ethanol systems, the critical point of Gurdial et al. again appears to be too high in pressure.


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C 0 + 1-Pentanol System. Vapor - liquid equilibrium data were measured in our apparatus for C 0 + 1-pentanol mixtures at 314.6, 325.9, and 337.4 Κ [31]. No other literature data are available for this system. The data at 314.6 Κ are shown in Figure 9 along with the critical point measured by Gurdial et al. [15]. The phase behavior is similar to that exhibited by other CO2 + 1-alkanol systems discussed above and the critical point of Gurdial et al. appears to be slightly higher than is justified by the vapor - liquid equilibrium measurements. 2

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C0 4- Higher 1-Alkanol Systems. Vapor - liquid equilibria for C 0 + higher 1-alkanol systems has been measured by Schneider and coworkers for the mixtures C 0 + 1-hexanol [32], C 0 + 1-decanol [32], C 0 + 1-dodecanol [33], 2

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2. JENNINGS ET AL.


9

8

°fi

•

χ

χ CO

7

-

6

-


CL LU CC D (0

•

5

χ

en UJ

ce o.

;i 0.0

•

SUZUKI ET A L , 313.4 K

Χ YAO ET A L , 313.1 K A GURDIAL ET A L (CRIT. POINT 313.4 K) — ι — Τ 1.0 0.8 0.6 0.4

—I— 0.2

M O L E FRACTION C Figure 7. C0 + 1-propanol vapor-liquid equilibria at **313 Κ. 2

1 0

ο ο

9 H 8 H CD

CL

Ε

7

LU CC

6 -

en en LU

ο

5 -

ce CL

4 • ο Ο A Τ

3 2

— ι —

THIS WORK, 314.8 Κ KING ET A L , 313.5 K WINKLER AND STEPHAN, 313.5 Κ GURDIAL ET AL. (CRIT. POINT 314.8 K) -i 1 r T

0.4

0.2

0.6

0.8

M O L E FRACTION C O Figure 8. C0 + 1-butanol vapor-liquid equilibria at 314.8 Κ. 2


1

20


C 0 + 1-hexadecanol [34], and C 0 + 1-octadecanol [35] using a near-infrared spectroscopic technique. However, the phase compositions are reported in units of (g/cm ) and no phase densities are reported. Solubilities of carbon dioxide in 1-octanol and 1-decanol at atmospheric pressure have also been reported by Wilcock et al. [54]. 2

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Discussion of C 0 + 1-Alkanol Systems. In order to check the consistency of the data sets, the C 0 + 1-alkanol measurements were plotted together. The data of some of the other investigators were also included for comparison. The liquid phase compositions at about 314 Κ are presented in Figure 10. The data obtained in this study show regular trends with the size of the 1-alkanol (also seen in the estimated critical pressures and phase compositions). The critical pressures of the mixtures show a consistent increase as the size of the 1-alkanol increases from ethanol to 1-pentanol. The liquid phase compositions show a consistent crossover behavior. At the lower pressures, the systems with the larger 1-alkanol also contain higher amounts of carbon dioxide. However, as the systems approach their respective critical regions, a crossover occurs. When the data of the other investigators are examined, it can be seen that there are inconsistencies in some of their measurements. It was previously shown that good agreement was obtained with the C 0 + ethanol measurements of Suzuki et al. This is seen again in Figure 10, which also shows that the C 0 + 1-propanol measurements of Suzuki et al. are in disagreement with the trends exhibited by the C 0 + 1-alkanol systems in this study. The inconsistency in the trends exhibited by the C 0 + 1-propanol data of Suzuki et al. is surprising, since good agreement was obtained with their C 0 + ethanol measurements. The liquid composition measurements of C 0 + 1-alkanol systems near 337 Κ are shown in Figure 11. The data of Suzuki et al. at 333.4 Κ are also included. Again, the data of this study show the same regular trends as noted previously at 314 K . Due to the temperature differences in the various data sets, it is not possible to directly compare the results of this study with the data of Suzuki et al. However, the C 0 -f ethanol and C 0 + 1-propanol data of Suzuki et al. at 333.4 Κ appear to exhibit slightly different behavior from that at 313.4 Κ (different crossover behavior and less difference in phase compositions at lower pressures). This would indicate that the C 0 + 1-propanol data may be suspect. As shown earlier, fairly good agreement is obtained between the C 0 + ethanol measurements of Panagiotopoulos and those of this study. 2


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The vapor phase compositions for the measurements of C 0 + 1-alkanol sys tems near 314 Κ are shown in Figure 12. With the exception of the data of King et α/., the measurements in general appear to exhibit expected trends. The same is true for the vapor phase compositions of C 0 + 1-alkanol systems near 337 Κ (Figure 13), although on this expanded scale slight differences can be seen between the C 0 + ethanol measurements of Panagiotopoulos and those of this study. 2

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2. JENNINGS ET AL.

High-Pressure Equilibria in CO, and 1-Alkanol Mixtures 21

8H CO


a. LU

DC 7 CO

cn UJ

CC CL • •

0.3

—ι— 0.4

THIS WORK, 314.6 Κ GURDIAL ET AL. (CRIT. POINT 314.6 K)

— I —

— I —

— I —

0.5

0.6

0.7

—ι— 0.8

—ι— 0.9

M O L E FRACTION C O Figure 9. C0 + 1-pentanol vapor-liquid equilibria at 314.6 K. 2

9.0 8.0ce 7 . 0 o. ω ce (/) (/>

6.05.0-

111

oc Q.

4.0-

-•-ETHANOL

314.5 K, THIS STUDY

- • - E T H A N O L 313.4 K, SUZUKI ET A L -H -1-PROPANOL 313.4 Κ, SUZUKI ET AL.

3.0-

—A—1-BUTANOL 314.8 K, THIS STUDY —•-1-PENTANOL

2 0

— I

0.1

314.6 K, THIS STUDY

Τ

1

1

1

1

1

1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1—

0.9

M O L E FRACTION C O Figure 10. C0 + 1-alkanol liquid phase compositions at **314 K. 2


22


1 3

12H

H

11


S.

1 C/> c/> HI DC CL

8 7

-

•—ETHANOL - Β -ETHANOL

6

-

••te-

337.2 Κ, THIS STUDY 333.4 Κ, SUZUKI ET AL.

ETHANOL

337.8 K,

PANAGIOTOPOULOS

- * -1-PROPANOL 333.4 K, SUZUKI ET AL. 5

-

—A-1-BUTANOL

337.2 K, THIS STUDY

—•—1-PENTANOL 337.4 K, THIS STUDY "Τ Τ T τ 0.2

0.1

0.4

0.3

0.5

0.6

Ο.7

Ο.8

0.9

M O L E FRACTION C O Figure 11. CO, + 1-alkanol liquid phase compositions at «337 Κ.

9.0

Η

8.0

ω* CL

LU ÛC => w

CO

7.0

Η

6.0

5

D

'

a.

¥ ι ι

0 u

LU ÛC

4.0

H

-ETHANOL 314.5

K, THIS STUDY

- • - E T H A N O L 313.4 K, SUZUKI ET A L — X -1-PROPANOL 313.4

3.0

H

Φ

1-BUTANOL 313.15 K, KING ET A L -1-BUTANOL 314.8 -1-PENTAN0L

2.0

0.985

*

K, THIS STUDY

314.6

• · — I —

0. 9 8 0

I I I

K, SUZUKI ET A L |

K, THIS STUDY r

-τ

J—Γ

0.990

• · — I —

r

0.995

1

M O L E F R A C T I O N CC> Figure 12. C0 + 1-alkanol vapor phase compositions at «314 K. 2


.00


2. JENNINGS ET AL.


Β

S

- • - E T H A N O L 337.2 K, THIS STUDY • Β - E T H A N O L 333.4 K, SUZUKI ET AL. 'tr- ETHANOL 337.8 K, PANAGIOTOPOULOS •H-1-PROPANOL -*-1-BUTANOL -•—1-PENTANOL 1

0.90

— I — «

0.92

333.4 K, SUZUKI ET AL. 337.2 K, THIS STUDY 337.4 K, THIS STUDY " Γ

0.94

ΊΓ 0.96

0.98

1 .0

MOLE FRACTION C O Figure 13. C0 + 1-alkanol vapor phase compositions at «337 K. 2



24


C r i t i c a l Behavior A knowledge of the critical locus facilitates the extrapolation of vapor - liquid equilibrium data in the critical region and allows a closure of the bubble and dew point surfaces. However, measurement techniques for vapor - liquid equilibria are least accurate in the critical region, since even small pressure disturbances result in large density fluctuations due to the large compressibility of a near critical fluid. As shown previously (Figures 4-9), the critical pressures reported by Gurdial et al. [15] in general appear to be high in relation to the phase envelopes measured by us or by other workers. A possible cause could be that the experimental technique used by Gurdial et ai did not allow them to ascertain that the total density in the view cell corresponded to the critical density of the mixture. This is necessary for the measurement of the true critical point, since opalescence can be observed not only at the critical point, but also in its immediate vicinity. Measured "critical " (or phase transition) temperatures at densities higher or lower than the critical density would be lower than the true critical temperatures. This is only true for pure and dilute mixtures, where the phase boundary in Τ — ρ coordinates in the critical region is symmetric with the critical point at the top. Critical pressures corresponding to given critical temperatures would therefore be too high, as is the case with the data of Gurdial et al. Further, the data reported by Gurdial do not show the expected trend as the carbon number of the solute increases in the 1-alkanol series. The critical pressures of CO2 + 1pentanol mixtures appear to be almost identical with those of 1-butanol, whereas the solute influence on the critical properties is strongly noticeable as the solute is changed from ethanol to 1-propanol or from 1-pentanol to 1-hexanol. We have also compared the slopes (^f) and (^f) of the critical lines at infinite dilution (where χ is the mole fraction of carbon dioxide) obtained from the data of Gurdial et al with those estimated from the vapor - liquid equilibrium data of this work. The slopes of the Gurdial et al. data do not exhibit regular trends with increasing carbon number as exhibited by the slopes obtained from the vapor - liquid equilibrium data. It should, however, be noted that there is greater uncertainty in the slopes estimated from the vapor - liquid equilibrium data. The slopes of the critical lines were also correlated using the Patel - Teja equation of state (PT EOS) [62]. The slopes of the critical lines are given by:

\~dT)v.T RT

_

(

d?P \ \dVdx)

T

(*£-) \dTdV)

x

The equation of state could be used to correlate either set of slopes provided a binary interaction parameter kij is used in the calculations. The experimental


2. JENNINGS ET AL.

High-Pressure Equilibria in CO- and 1-Alkanol Mixtures 25

Table III: Slopes of critical lines at infinite dilution. E x p e r i m e n t and prediction. O p t i m a l k{j values i n parentheses

( * ) Γ

Supercritical Fluid Engineering Science - American Chemical Society

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