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Solubility measurements of naphthol isomers in supercritical CO2 by a recycle technique
37
Fluid Phase Equilibria, 34 (1987) 37-47
Elsevier Science Publishers B.V., Amsterdam
-
Printed in The Netherlands
SOLUBILITY MEASUREMENTS OF NAPHTHOL ISOMERS IN SUPERCRITICAL CO, BY A RECYCLE TECHNIQUE CHUNG-SUNG
TAN * and JIN-YIH WENG
Department of Chemical Engineering (Republic of China)
National Tsing Hua University, Hsincku, Taiwan 30043
(Received September 23, 1986; accepted in final form December 1, 1986)
ABSTRACT Tan, C.-S. and Weng, J.-Y., 1987. Solubility measurements of naphthol isomers in supercritical CO, by a recycle technique. Fluid Phase Equilibria, 34: 37-47. A recycle technique has been developed for measurement of equilibrium solubility of a solid in a supercritical fluid. The solubilities of naphthalene in supercritical carbon dioxide obtained by this technique agreed well with published data. Solubilities in supercritical carbon dioxide in a temperature range of 308-328 K and a pressure range of 90-170 atm were also measured for naphthol isomers. Solubilities of mixtures of naphthol isomers at different compositions at 308 K and 130 atm are also reported in this study.
INTRODUCTION
Extraction using supercritical fluid has received widespread attention over the past few years. The advantages of this separation technology, described by Gangoli and Thodos (1977), Williams (1981) and Paulaitis et al. (1983), may be summarized as ease of separation of the extracted materials by varying temperature and pressure, high selectivity, possibly less energy consumed and lower mass transfer resistances compared with conventional methods. The existing and potential applications of this technology in the food, drug, chemical and petrochemical industries can be found in an excellent review by Paulaitis et al. (1983). To understand supercritical fluid extraction, we need to study some fundamental aspects. One of these is the determination of the equilibrium solubility of a solid in a supercritical fluid. Tsekhanskaya et al. (1964) used a static gravimetric method with a magnetic stirrer to measure equilibrium solubilities of naphthalene in compressed carbon dioxide and ethylene. Prausnitz and Benson (1959) and Kaul and Prausnitz (1978) used a flow
* Author to whom all correspondence 0378-3812/87/$03.50
should be addressed.
0 1987 Elsevier Science Publishers B.V.
38
system to obtain solubility data. In their experiments, solid was collected in a cold trap after expansion of compressed gas through a needle valve. A similar experimental apparatus was also used by Johnston and Eckert (1981) Kurnik et al. (1981), Johnston et al. (1982) and Schmitt and Reid (1986) to obtain solubility information. McHugh and Paulaitis (1980) used a sampling valve located after the extractor in a flow system to measure the solubilities of solids in supercritical fluids. To achieve solid-fluid equilibrium in a continuous operation, as used by the above authors, the supercritical fluid flow rate and the length of the packed extractor usually need to be adjusted. For less volatile compounds, caution is especially needed to avoid underestimating the equilibrium solubility. The objective of this study is to develop an experimental method for measurement of equilibrium solubility of a solid in a supercritical fluid. The fluid is recycled in a batch system to increase the contact time with the solid. Carbon dioxide and naphthol isomers were used as solvent and solutes, respectively. EXPERIMENTAL
A schematic diagram of the apparatus is shown in Fig. 1. The whole system was constructed with o.d. 0.63 cm S.S. 316 tubing, except the extractor and the separator. At the beginning all the valves in Fig. 1 were closed. Carbon dioxide with 99.0% purity -at ambient temperature first passed through a silica gel bed to remove the possible content of water
Regulator
Compressor
I-Q Air
u
Pump
Cylinder
Heating
‘Constant
Temperature
Fig. 1. Schematic diagram of apparatus.
Bath
lci
Bath
Tape
39
moisture and was then compressed to a surge tank by a diaphragm compressor (Superpressure Inc.) with the minimum charge pressure of 47.6 atm. The range of the operating pressure in this study varied from 90 to 170 atm. In each experiment the pressure was maintained within + 1.0% deviation from the desired value. The extractor with o.d. 1.89 cm and 20 cm in length was packed with about 20 g of 0.6 cm reagent grade naphthol particles (Merck Co.). Glass wool was plugged at both ends of the extractor to prevent entrainment of fine solid particles during the experiment. The extractor and separator were immersed in a constant temperature bath. Three temperatures, 308, 318 and 328 K, were used and controlled within kO.5 K. The volume of the separator was 9.58 cm3, determined by measuring the amount of distilled water needed to fill it. The compressed carbon dioxide was allowed to fill the whole system by opening the metering valves 2-4 and the three-way valves 8 and 9. A metering plunger pump (Milton Roy Co., Model A) was equipped in the system. When it stopped, the system became a closed batch system. When this pump turned on, the supercritical fluid in the system could be circulated, which was classified as an internal recycle operation. This circulation can increase the contact time between fluid and solid and the diffusion rates. Though the variations of temperature and pressure caused by circulation were found to be small, the pump turned on every 30 min for a period of 15 min. After a certain period of operation the three-way valves were switched to isolate the separator from the system. The solubility could thus be measured by analysing the amount of solid in the separator. This analysis could be done by first expanding the compressed gas in the separator across valve 7. The precipitated solid was then collected by two U-tubes which were immersed in an ice bath. The gas coming out of the U-tube finally passed into an isopropanol flask which trapped residual solid in the gas stream. A known amount of isopropanol was used to wash these two U-tubes to dissolve the collected solid. Since some solid might be precipitated in the separator and in valve 7, a known amount of isopropanol was also used to wash them. The samples of isopropanol solutions from separator, U-tubes and flask were sent to gas chromatography (G.C.) (Varian 3700) for analysis. Before the separator was put back into operation, it was flushed with air first, subsequently with compressed carbon dioxide, and then it was filled with carbon dioxide at the same pressure as in the extractor. The same operation described above would then be repeated until the solubilities obtained were unchanged, i.e. equilibrium was reached. RESULTS AND DISCUSSION
It can be seen from Fig. 2 that there is good agreement for equilibrium solubilities of naphthalene in compressed carbon dioxide between the pre-
40
-
Tsekhanskaya et al. (1964 1
4.0
l
McHugh
A
Present
and Pauloitis(l980)
l
328 )c
.
study
/
0 PRESSURE,
atm
Fig. 2. Comparison of solubilities of naphthalene
in supercritical CO,.
sent and previous studies. In the present study, at least three samples were taken in each experiment to ascertain the equilibrium being reached. The average deviation among samples which were believed to have reached equilibrium was about 2.0%. The reproducibility tests indicated a maximum deviation of about 3.5%. The density of carbon dioxide used to calculate the amount of carbon dioxide in the separator was obtained through the information provided by Tsekhanskaya et al. (1964) and Angus et al. (1976). If the three-way valve 9 in Fig. 1 was not operated during the experiment (i.e. the recycle part was not used), the present apparatus would be similar to those used by some previous researchers, notably Kaul and Prausnitz (1978) and Kurnik et al. (1981), and the equilibrium data would be obtained by the gravimetric flow method. Under this condition it was found that the solubility of naphthalene in compressed carbon dioxide always reached equilibrium in the superficial velocity range of 5.0 x 10-3-2.0 x lo-* cm s-l, as 6.0 g of naphthalene particles, 0.488 cm in diameter, was packed in the extractor. But for the benzoic acid and carbon dioxide system, the equilibrium was not reached in the same range of superficial velocity, which is indicated in Fig. 3. It can be seen from Fig. 3 that the solubilities were quite close below velocities of 10e2 cm s-‘. However, this observation may not justify the assumption that equilibrium was reached. Using the developed recycle method, the solubilities of (Y-and /3-naphthol in supercritical carbon dioxide were also measured, which are shown in Table 1 and Figs. 4 and 5. From these two figures it can be seen that the solubilities of naphthol isomers in supercritical carbon dioxide are about one order of magnitude less than that of naphthalene in carbon dioxide. This is probably due to the existence of a hydroxyl group in naphthols. This
41 .._ 0.9
I
71328
K
P= 158 atm
0.8 u” \ u
0
00
0.7-
0.3 0
1 5
I 10
SUPERFICIAL
I 15 VELOCITY
I 20
I 25 (x103),cm/s
Fig. 3. Extraction rates of 0.488 cm benzoic acid particles in a packed-bed extractor at T = 328 K and P = 158 atm (equilibrium mole fraction is 2.27 X 10m3 given by Kumik et al., 1981).
observation supports the postulate by Stahl et al. (1978) and Dandge et al. (1985) that the presence of hydroxyl group in hydrocarbons might lower the extraction ability of compressed carbon dioxide. Figures 4 and 5 also show that the trends of temperature and pressure effects on equilibrium solubilities of (Y-and P-naphthol in supercritical carbon dioxide are the same as those for naphthalene. These trends may be described by the equation (Prausnitz, 1969), which interprets well for the naphthalene and carbon dioxide system, y
=p”$
(1)
where y is the mole fraction of solid in a fluid phase, E is the enhancement factor and ps is the vapour pressure of solid. The solubility of cY-naphthol in supercritical carbon dioxide is higher than that of j?-naphthol, which can be seen in Figs. 4 and 5. These two isomers have similar structures and close boiling points (7 K in difference), but have a larger difference in their melting points (ar-naphthol is 26 K higher than /3-naphthol). Therefore, the melting point may be a significant factor affecting equilibrium solubilities of naphthol isomers. For an ideal solution, the solubility may be expressed by (Prausnitz, 1969) ln($)=($)(l--$-)
(2)
42 TABLE 1 Solubilities of OL-and /Snaphthols in supercritical carbon dioxide Pressure (atm)
Temperature (K)
Mole fraction
a-Naphthol 91 105 120 130 140 160
308 308 308 308 308 308
7.35 x 1o-4 9.40x 1o-4 1.o4x1o-3 1.15 x10-s 1.21 x 10-3 1.31 x10-s
98 108 118 130 140 150
318 318 318 318 318 318
4.17x1o-4 7.20~10-~ 9.82 x 1O-4 1.15x10-3 1.35x10-3 1.47x10-3
105 115 124 141 150 170
328 328 328 328 328 328
3.30 x 1o-4 5.30 x 10-4 8.30~10-~ 1.32~10-~ 1.47x10-3 1.78~10-~
/3-Naphthol 91 105 115 130 130 150 160
308 308 308 308 308 308 308
2.38 x 1O-4 3.39x1o-4 3.76 x 1O-4 4.53 x 1o-4 4.58 x 1O-4 5.10x 1o-4 5.40 x 10-4
98 108 118 130 140 150
318 318 318 318 318 318
1.58 x 1O-4 2.92x 1O-4 3.95 x 10-4 4.80x 1O-4 5.58~10-~ 6.47~10-~
101 110 118 130 150 170
328 328 328 328 328 328
8.64x 10F5 1.61 x 1O-4 2.82x 1O-4 4.91 x 10-4 6.86 x 1o-4 8.98~10-~
43
16 -
6-
2080
’
1 100
1 120
1
1
1 140
PRESSURE,
Fig. 4. Experimental
1 160
1
I 180
I
2 0
atm
solubilities of cy-naphthol in supercritical CO,.
PRESSURE,
Fig. 5. Experimental
1
atm
solubilities of /3-naphthol in supercritical co2 .
44
where T, is the melting point and Ah is the heat of fusion of the solute. When y is small, eqn. (2) may be rewritten as C*=p*
(3)
exp
where C* and p* are the dimensionless solubility and density of the fluid normalized by critical density, respectively. Equation (3) is similar to an equation proposed by Chrastil (1982), who did not consider supercritical fluid as an ideal solution, which is expressed by
c* =p*k
exp($
(4
+b*)
With one more adjustable parameter, eqn. (4) is probably better than eqn. (3) at describing supercritical fluid behavior. If eqn. (4) was used to correlate the solubility data of naphthol isomers, two equations could be obtained: For a-naphthol c * =
p*4.5668
-
exp
3527.4 T
+ 3.9574
15.0.
8.0
13.0 -
11.0 -
9.0 -
7.0 / 5.0 -
1’
-3P
O/ 3.0-
,’
-
Pure 1-Naphthol
- -
Pure 2-Naphthol
A
I-Naphthol
.Mixture
o
INophthol,
Mixture
1.0 -
0.070
_ 2.0
w P
-1.0
’
90 1
’
110 1
1 130 1 1 150 1 1 170 1 1
PRESSURE,
190a0
atm
Fig. 6. Experimental solubilities of cr-naphthol and fi-naphthol mixtures at a composition of SO:50 at T= 308 K.
45 TABLE
2
Mole fractions 130 atm
of (Y- and P-naphthol
mixtures
in supercritical
carbon
dioxide
at 308 K and
Mole fraction
Composition ol-naphthol
fl-naphthol
cr-naphthol
/3-naphthol
100 80 50 20 0
0 20 50 80 100
1.15 x10-3 1.2ox1o-3 1.13x1o-3 1.12x lo-’ 0
0 3.15 5.29 4.62 4.53
x x x x
10-4 1O-4 1O-4 1o-4
For P-naphthol c
* =
,,* 4.8236
exp
- 4948.2 T
+ 7.4543
The average deviations of eqns. (5) and (6) were 2.1 and 4.296, respectively. Figure 6 indicates the solubilities of cu-naphthol and /3-naphthol mixtures at a composition of 50 : 50 at 308 K. The mixtures were prepared by mixing equal amount of isomer pellets. Note that the same data were also obtained from mixtures prepared by mixing isomer powders first and then pressing to form pellets. It can be observed that the presence of j%naphthol lowers the solubility of a-naphthol in supercritical carbon dioxide, but the presence of a-naphthol increases the solubility of /3-naphthol. But at other compositions this may not be the case, and the change of solubility for P-naphthol is more pronounced than that for cY-naphthol, which are shown in Table 2. CONCLUSIONS
A recycle technique which can increase the contact time between supercritical fluid and solid was developed. A good agreement for solubilities of naphthalene in supercritical carbon dioxide obtained by the present and previous methods indicates that the developed technique is a reliable method for measurement of equilibrium solubility of solids in a supercritical fluid. With the recycle method, solubilities of naphthol isomers in supercritical carbon dioxide were measured at 308, 318 and 328 K over a pressure range of 90-170 atm. It was found that solubilities of (Y-and /3-naphthol are about one order of magnitude less than those of naphthalene in supercritical carbon dioxide. This indicates that the presence of a hydroxyl group in hydrocarbons may lower the extraction power of compressed carbon dioxide. Solubilities in supercritical carbon dioxide for (Y- and /?-naphthol mixtures at different compositions at 308 K and 130 atm were also measured in this study.
46 ACKNOWLEDGEMENTS
The support acknowledged.
of the National
Science Council of ROC is gratefully
LIST OF SYMBOLS
constants in eqn. (3) a*, b* constants in eqn. (4) C solubility of solid in supercritical fluid (g cmequilibrium solubility (g cm- 3, CS defined as C/p, C* E enhancement factor heat of fusion of solute Ah P pressure (atm) P” vapour pressure of solid (atm) R universal gas constant mole fraction of solid in supercritical fluid Y temperature (K) T melting point of solid (K) T, density of supercritical fluid (g cm-3) P critical density of fluid (g cmP3) PS defined as p/p, P* a, b
REFERENCES Angus, S., Armstrong, B. and de Reuck, K.M., 1976. Carbon Dioxide: International Thermodynamics Tables of the Fluid State-3, Pergamon Press, New York. Chrastil, J., 1982. Solubility of solids and liquids in supercritical gases. J. Phys. Chem., 86: 3016-3021. Dandge, D.K., Heller, J.P. and Wilson, K.V., 1985. Structure solubility correlations: organic compounds and dense carbon dioxide binary systems. Ind. Eng. Chem. Prod. Res. Dev., 24: 162-166. Gangoli, N. and Thodos, G., 1977. Liquid fuels and chemical feedstocks from coal by supercritical gas extraction. Ind. Eng. Chem. Prod. Res. Dev., 16: 208-216. Johnston, K.P. and Eckert, C.A., 1981. An analytical Carnahan-Starling-van der Waals model for solubility of hydrocarbon solids in supercritical fluids. AIChE J., 27: 773-778. Johnston, K.P., Zlger, D.H. and Eckert, C.A., 1982. Solubilities of hydrocarbon solids in supercritical fluids. Ind. Eng. Chem. Fundam., 21: 191-197. Kaul, B.K. and Prausnitz, J.M., 1978. Solubilities of heavy hydrocarbons in compressed methane, ethane and ethylene: dew-point temperature for gas mixtures containing small and large molecules. AIChE J., 24: 223-231. Kumik, R.T., Holla, S.J. and Reid, R.C., 1981. Solubility of solids in supercritical carbon dioxide and ethylene. J. Chem. Eng. Data, 26: 47-51. McHugh, M. and Paulaitis, M.E., 1980. Solid solubilities of naphthalene and biphenyl in supercritical carbon dioxide. J. Chem. Eng. Data, 25: 326-329.
47
Paula&is, M.E., Krukonis, V.J., Kurnik, R.T. and Reid, R.C., 1983. Supercritical fluid extraction. Rev. Chem. Eng., 1: 179-250. Prausnitz, J.M., 1969. Molecular Thermodynamics of Fluid Phase Equilibria. Prentice-Hall, Englewood Cliffs, NJ. Prausnitz, J.M. and Benson, P.R., 1959. Solubility of liquids in compressed hydrogen, nitrogen and carbon dioxide. AIChE J., 5: 161-164. Schmitt, W.J. and Reid, R.C., 1986. Solubility of monofunctional organic solids in chemically diverse supercritical fluids, J. Chem. Eng. Data, 31: 204-212. Stahl, E., Schilz, W., Schi.itz, E. and Willing, E., 1978. A quick method for microanalytical evaluation of the dissolving power of supercritical gases, Angew. Chem. Int. Ed. Engl., 17: 731-738. Tsekhanskaya, Y.V., Iomtev, M.B. and Mushkina, E.V., 1964. Solubility of naphthalene in ethylene and carbon dioxide. Russ. J. Phys. Chem., 38: 1173-1176. Williams, D.F., 1981. Extraction with supercritical gases. Chem. Eng. Sci., 36: 1769-1788.
Fluid Phase Equilibria, 34 (1987) 37-47
Elsevier Science Publishers B.V., Amsterdam
-
Printed in The Netherlands
SOLUBILITY MEASUREMENTS OF NAPHTHOL ISOMERS IN SUPERCRITICAL CO, BY A RECYCLE TECHNIQUE CHUNG-SUNG
TAN * and JIN-YIH WENG
Department of Chemical Engineering (Republic of China)
National Tsing Hua University, Hsincku, Taiwan 30043
(Received September 23, 1986; accepted in final form December 1, 1986)
ABSTRACT Tan, C.-S. and Weng, J.-Y., 1987. Solubility measurements of naphthol isomers in supercritical CO, by a recycle technique. Fluid Phase Equilibria, 34: 37-47. A recycle technique has been developed for measurement of equilibrium solubility of a solid in a supercritical fluid. The solubilities of naphthalene in supercritical carbon dioxide obtained by this technique agreed well with published data. Solubilities in supercritical carbon dioxide in a temperature range of 308-328 K and a pressure range of 90-170 atm were also measured for naphthol isomers. Solubilities of mixtures of naphthol isomers at different compositions at 308 K and 130 atm are also reported in this study.
INTRODUCTION
Extraction using supercritical fluid has received widespread attention over the past few years. The advantages of this separation technology, described by Gangoli and Thodos (1977), Williams (1981) and Paulaitis et al. (1983), may be summarized as ease of separation of the extracted materials by varying temperature and pressure, high selectivity, possibly less energy consumed and lower mass transfer resistances compared with conventional methods. The existing and potential applications of this technology in the food, drug, chemical and petrochemical industries can be found in an excellent review by Paulaitis et al. (1983). To understand supercritical fluid extraction, we need to study some fundamental aspects. One of these is the determination of the equilibrium solubility of a solid in a supercritical fluid. Tsekhanskaya et al. (1964) used a static gravimetric method with a magnetic stirrer to measure equilibrium solubilities of naphthalene in compressed carbon dioxide and ethylene. Prausnitz and Benson (1959) and Kaul and Prausnitz (1978) used a flow
* Author to whom all correspondence 0378-3812/87/$03.50
should be addressed.
0 1987 Elsevier Science Publishers B.V.
38
system to obtain solubility data. In their experiments, solid was collected in a cold trap after expansion of compressed gas through a needle valve. A similar experimental apparatus was also used by Johnston and Eckert (1981) Kurnik et al. (1981), Johnston et al. (1982) and Schmitt and Reid (1986) to obtain solubility information. McHugh and Paulaitis (1980) used a sampling valve located after the extractor in a flow system to measure the solubilities of solids in supercritical fluids. To achieve solid-fluid equilibrium in a continuous operation, as used by the above authors, the supercritical fluid flow rate and the length of the packed extractor usually need to be adjusted. For less volatile compounds, caution is especially needed to avoid underestimating the equilibrium solubility. The objective of this study is to develop an experimental method for measurement of equilibrium solubility of a solid in a supercritical fluid. The fluid is recycled in a batch system to increase the contact time with the solid. Carbon dioxide and naphthol isomers were used as solvent and solutes, respectively. EXPERIMENTAL
A schematic diagram of the apparatus is shown in Fig. 1. The whole system was constructed with o.d. 0.63 cm S.S. 316 tubing, except the extractor and the separator. At the beginning all the valves in Fig. 1 were closed. Carbon dioxide with 99.0% purity -at ambient temperature first passed through a silica gel bed to remove the possible content of water
Regulator
Compressor
I-Q Air
u
Pump
Cylinder
Heating
‘Constant
Temperature
Fig. 1. Schematic diagram of apparatus.
Bath
lci
Bath
Tape
39
moisture and was then compressed to a surge tank by a diaphragm compressor (Superpressure Inc.) with the minimum charge pressure of 47.6 atm. The range of the operating pressure in this study varied from 90 to 170 atm. In each experiment the pressure was maintained within + 1.0% deviation from the desired value. The extractor with o.d. 1.89 cm and 20 cm in length was packed with about 20 g of 0.6 cm reagent grade naphthol particles (Merck Co.). Glass wool was plugged at both ends of the extractor to prevent entrainment of fine solid particles during the experiment. The extractor and separator were immersed in a constant temperature bath. Three temperatures, 308, 318 and 328 K, were used and controlled within kO.5 K. The volume of the separator was 9.58 cm3, determined by measuring the amount of distilled water needed to fill it. The compressed carbon dioxide was allowed to fill the whole system by opening the metering valves 2-4 and the three-way valves 8 and 9. A metering plunger pump (Milton Roy Co., Model A) was equipped in the system. When it stopped, the system became a closed batch system. When this pump turned on, the supercritical fluid in the system could be circulated, which was classified as an internal recycle operation. This circulation can increase the contact time between fluid and solid and the diffusion rates. Though the variations of temperature and pressure caused by circulation were found to be small, the pump turned on every 30 min for a period of 15 min. After a certain period of operation the three-way valves were switched to isolate the separator from the system. The solubility could thus be measured by analysing the amount of solid in the separator. This analysis could be done by first expanding the compressed gas in the separator across valve 7. The precipitated solid was then collected by two U-tubes which were immersed in an ice bath. The gas coming out of the U-tube finally passed into an isopropanol flask which trapped residual solid in the gas stream. A known amount of isopropanol was used to wash these two U-tubes to dissolve the collected solid. Since some solid might be precipitated in the separator and in valve 7, a known amount of isopropanol was also used to wash them. The samples of isopropanol solutions from separator, U-tubes and flask were sent to gas chromatography (G.C.) (Varian 3700) for analysis. Before the separator was put back into operation, it was flushed with air first, subsequently with compressed carbon dioxide, and then it was filled with carbon dioxide at the same pressure as in the extractor. The same operation described above would then be repeated until the solubilities obtained were unchanged, i.e. equilibrium was reached. RESULTS AND DISCUSSION
It can be seen from Fig. 2 that there is good agreement for equilibrium solubilities of naphthalene in compressed carbon dioxide between the pre-
40
-
Tsekhanskaya et al. (1964 1
4.0
l
McHugh
A
Present
and Pauloitis(l980)
l
328 )c
.
study
/
0 PRESSURE,
atm
Fig. 2. Comparison of solubilities of naphthalene
in supercritical CO,.
sent and previous studies. In the present study, at least three samples were taken in each experiment to ascertain the equilibrium being reached. The average deviation among samples which were believed to have reached equilibrium was about 2.0%. The reproducibility tests indicated a maximum deviation of about 3.5%. The density of carbon dioxide used to calculate the amount of carbon dioxide in the separator was obtained through the information provided by Tsekhanskaya et al. (1964) and Angus et al. (1976). If the three-way valve 9 in Fig. 1 was not operated during the experiment (i.e. the recycle part was not used), the present apparatus would be similar to those used by some previous researchers, notably Kaul and Prausnitz (1978) and Kurnik et al. (1981), and the equilibrium data would be obtained by the gravimetric flow method. Under this condition it was found that the solubility of naphthalene in compressed carbon dioxide always reached equilibrium in the superficial velocity range of 5.0 x 10-3-2.0 x lo-* cm s-l, as 6.0 g of naphthalene particles, 0.488 cm in diameter, was packed in the extractor. But for the benzoic acid and carbon dioxide system, the equilibrium was not reached in the same range of superficial velocity, which is indicated in Fig. 3. It can be seen from Fig. 3 that the solubilities were quite close below velocities of 10e2 cm s-‘. However, this observation may not justify the assumption that equilibrium was reached. Using the developed recycle method, the solubilities of (Y-and /3-naphthol in supercritical carbon dioxide were also measured, which are shown in Table 1 and Figs. 4 and 5. From these two figures it can be seen that the solubilities of naphthol isomers in supercritical carbon dioxide are about one order of magnitude less than that of naphthalene in carbon dioxide. This is probably due to the existence of a hydroxyl group in naphthols. This
41 .._ 0.9
I
71328
K
P= 158 atm
0.8 u” \ u
0
00
0.7-
0.3 0
1 5
I 10
SUPERFICIAL
I 15 VELOCITY
I 20
I 25 (x103),cm/s
Fig. 3. Extraction rates of 0.488 cm benzoic acid particles in a packed-bed extractor at T = 328 K and P = 158 atm (equilibrium mole fraction is 2.27 X 10m3 given by Kumik et al., 1981).
observation supports the postulate by Stahl et al. (1978) and Dandge et al. (1985) that the presence of hydroxyl group in hydrocarbons might lower the extraction ability of compressed carbon dioxide. Figures 4 and 5 also show that the trends of temperature and pressure effects on equilibrium solubilities of (Y-and P-naphthol in supercritical carbon dioxide are the same as those for naphthalene. These trends may be described by the equation (Prausnitz, 1969), which interprets well for the naphthalene and carbon dioxide system, y
=p”$
(1)
where y is the mole fraction of solid in a fluid phase, E is the enhancement factor and ps is the vapour pressure of solid. The solubility of cY-naphthol in supercritical carbon dioxide is higher than that of j?-naphthol, which can be seen in Figs. 4 and 5. These two isomers have similar structures and close boiling points (7 K in difference), but have a larger difference in their melting points (ar-naphthol is 26 K higher than /3-naphthol). Therefore, the melting point may be a significant factor affecting equilibrium solubilities of naphthol isomers. For an ideal solution, the solubility may be expressed by (Prausnitz, 1969) ln($)=($)(l--$-)
(2)
42 TABLE 1 Solubilities of OL-and /Snaphthols in supercritical carbon dioxide Pressure (atm)
Temperature (K)
Mole fraction
a-Naphthol 91 105 120 130 140 160
308 308 308 308 308 308
7.35 x 1o-4 9.40x 1o-4 1.o4x1o-3 1.15 x10-s 1.21 x 10-3 1.31 x10-s
98 108 118 130 140 150
318 318 318 318 318 318
4.17x1o-4 7.20~10-~ 9.82 x 1O-4 1.15x10-3 1.35x10-3 1.47x10-3
105 115 124 141 150 170
328 328 328 328 328 328
3.30 x 1o-4 5.30 x 10-4 8.30~10-~ 1.32~10-~ 1.47x10-3 1.78~10-~
/3-Naphthol 91 105 115 130 130 150 160
308 308 308 308 308 308 308
2.38 x 1O-4 3.39x1o-4 3.76 x 1O-4 4.53 x 1o-4 4.58 x 1O-4 5.10x 1o-4 5.40 x 10-4
98 108 118 130 140 150
318 318 318 318 318 318
1.58 x 1O-4 2.92x 1O-4 3.95 x 10-4 4.80x 1O-4 5.58~10-~ 6.47~10-~
101 110 118 130 150 170
328 328 328 328 328 328
8.64x 10F5 1.61 x 1O-4 2.82x 1O-4 4.91 x 10-4 6.86 x 1o-4 8.98~10-~
43
16 -
6-
2080
’
1 100
1 120
1
1
1 140
PRESSURE,
Fig. 4. Experimental
1 160
1
I 180
I
2 0
atm
solubilities of cy-naphthol in supercritical CO,.
PRESSURE,
Fig. 5. Experimental
1
atm
solubilities of /3-naphthol in supercritical co2 .
44
where T, is the melting point and Ah is the heat of fusion of the solute. When y is small, eqn. (2) may be rewritten as C*=p*
(3)
exp
where C* and p* are the dimensionless solubility and density of the fluid normalized by critical density, respectively. Equation (3) is similar to an equation proposed by Chrastil (1982), who did not consider supercritical fluid as an ideal solution, which is expressed by
c* =p*k
exp($
(4
+b*)
With one more adjustable parameter, eqn. (4) is probably better than eqn. (3) at describing supercritical fluid behavior. If eqn. (4) was used to correlate the solubility data of naphthol isomers, two equations could be obtained: For a-naphthol c * =
p*4.5668
-
exp
3527.4 T
+ 3.9574
15.0.
8.0
13.0 -
11.0 -
9.0 -
7.0 / 5.0 -
1’
-3P
O/ 3.0-
,’
-
Pure 1-Naphthol
- -
Pure 2-Naphthol
A
I-Naphthol
.Mixture
o
INophthol,
Mixture
1.0 -
0.070
_ 2.0
w P
-1.0
’
90 1
’
110 1
1 130 1 1 150 1 1 170 1 1
PRESSURE,
190a0
atm
Fig. 6. Experimental solubilities of cr-naphthol and fi-naphthol mixtures at a composition of SO:50 at T= 308 K.
45 TABLE
2
Mole fractions 130 atm
of (Y- and P-naphthol
mixtures
in supercritical
carbon
dioxide
at 308 K and
Mole fraction
Composition ol-naphthol
fl-naphthol
cr-naphthol
/3-naphthol
100 80 50 20 0
0 20 50 80 100
1.15 x10-3 1.2ox1o-3 1.13x1o-3 1.12x lo-’ 0
0 3.15 5.29 4.62 4.53
x x x x
10-4 1O-4 1O-4 1o-4
For P-naphthol c
* =
,,* 4.8236
exp
- 4948.2 T
+ 7.4543
The average deviations of eqns. (5) and (6) were 2.1 and 4.296, respectively. Figure 6 indicates the solubilities of cu-naphthol and /3-naphthol mixtures at a composition of 50 : 50 at 308 K. The mixtures were prepared by mixing equal amount of isomer pellets. Note that the same data were also obtained from mixtures prepared by mixing isomer powders first and then pressing to form pellets. It can be observed that the presence of j%naphthol lowers the solubility of a-naphthol in supercritical carbon dioxide, but the presence of a-naphthol increases the solubility of /3-naphthol. But at other compositions this may not be the case, and the change of solubility for P-naphthol is more pronounced than that for cY-naphthol, which are shown in Table 2. CONCLUSIONS
A recycle technique which can increase the contact time between supercritical fluid and solid was developed. A good agreement for solubilities of naphthalene in supercritical carbon dioxide obtained by the present and previous methods indicates that the developed technique is a reliable method for measurement of equilibrium solubility of solids in a supercritical fluid. With the recycle method, solubilities of naphthol isomers in supercritical carbon dioxide were measured at 308, 318 and 328 K over a pressure range of 90-170 atm. It was found that solubilities of (Y-and /3-naphthol are about one order of magnitude less than those of naphthalene in supercritical carbon dioxide. This indicates that the presence of a hydroxyl group in hydrocarbons may lower the extraction power of compressed carbon dioxide. Solubilities in supercritical carbon dioxide for (Y- and /?-naphthol mixtures at different compositions at 308 K and 130 atm were also measured in this study.
46 ACKNOWLEDGEMENTS
The support acknowledged.
of the National
Science Council of ROC is gratefully
LIST OF SYMBOLS
constants in eqn. (3) a*, b* constants in eqn. (4) C solubility of solid in supercritical fluid (g cmequilibrium solubility (g cm- 3, CS defined as C/p, C* E enhancement factor heat of fusion of solute Ah P pressure (atm) P” vapour pressure of solid (atm) R universal gas constant mole fraction of solid in supercritical fluid Y temperature (K) T melting point of solid (K) T, density of supercritical fluid (g cm-3) P critical density of fluid (g cmP3) PS defined as p/p, P* a, b
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47
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