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Phase equilibria and critical phenomena in fluid mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane up to 423 K and 100 MPa
J. Chem. Thermodynamics 1975,7,805-814
Phase equilibria and critical phenomena in fluid mixtures of carbon dioxide + 2,6,10,15,19,23=hexamethyltetracosane up to 423 K and 100 MPa K. G. LIPHARD
and G. M. SCHNEIDER
Institute of Physical Chemistry, University of Bochum, Germany (Received10 December1974)’ In an optical high-pressure cell with sapphire windows and magnetic stirring, phase equilibria in fluid binary mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane) have been measured up to 423 K and 100 MPa. The critical curve of the mixture starts at the critical point of the pure alkane (Tc = 933 K, pc = 0.51 MPa (calculated values)). With decreasing temperatures the critical curve runs first through a pressure maximum at about 39 MPa and 473 K, then through a pressure minimum at p = (36.3 j, 0.2) MPa and T = (373 & 2) K, and at last through a temperature minimum at T = (328.2 f 0.5) K and p = (70.5 -i: 2.0) MPa. Finally the critical curve rises steeply to higher pressures with increasing temperatures. The results are discussed from a phasetheoretical point of view and are compared with phase equilibrium data of other carbon dioxide + alkane mixtures. It follows from this discussion that there are continuous transitions between liquid + gas, liquid -I- liquid, and gas + gas equilibria in these systems.
1. Introduction In recent years the phase equilibria of binary mixtures of carbon dioxide with n-alkanes up to hexadecane have been measured. (l-*) These mixtures are of considerable interest in phase theory since the physical properties of the alkanes and thus of the binary mixtures can be easily and gradually varied over a wide range by adding more and more carbon atoms to the alkane chain. These mixtures show very different types of fluid phase equilibria (see figure 6). Up to hexane only liquid + gas equilibria occur; from heptane on, however, additionally liquid + liquid immiscibility is observed. For mixtures of larger PZalkanes (e.g. hexadecane) + carbon dioxide liquid + gas and liquid + liquid equilibria are superimposed. It can be extrapolated from these results that in binary mixtures of carbon dioxide with alkanes of much higher molar mass the phase equilibria and critical phenomena will become more and more similar to typical gas + gas equilibria of the second type thus verifying the hypothesis that in these mixtures continuous transitions between liquid + gas, liquid + liquid, and gas -t- gas equilibria o~ur.(9* lOv18~19)
For the present investigation 2,6,10,15,19,23-hexamethyltetracosane (squalane) was chosen; its molar mass is approximately twice as high as that of the largest 54
806
K. G. LIPHARD
AND G. M. SCHNEIDER
alkane investigated before (hexadecane); it has also the advantage of being liquid at room temperature and as a stationary phase in gas chromatography it is readily available in good purity. The present paper deals with measurements of the phase equilibria and the critical phenomena in fluid binary mixtures of carbon dioxide and this alkane up to 423 K and 100 MPa.
2. Experimental The measurements were carried out in an optical high-pressure cell (see figure la). It is made of a non-magnetic stainless steel and is sealed by a copper ring (CR). The maximum cell volume is 7.8 cm3. The liquid mixture in the cell can be stirred efficiently a
b
FIGURE 1. Apparatus (for details see text). a, Optical high-pressure autoclave; b, Experimental set-up (schematic).
by a magnetic stirrer (MS); in order to improve mixing a coiled wire is attached to this magnet. It is driven from outside the cell by another rotating magnet (RM). All phase transitions in the cell can be observed through two sapphire windows (WI, W2). The whole apparatus is shown schematically in figure lb. The pressure is created with liquid decane in a rotating pump (RP) and transmitted to the mixture under test by a closely fitted piston (P) which is additionally tightened with viton O-rings (0). By calibration runs the pressure difference across the piston was found to be within the accuracy of the pressure readings (see below). The pressure is measured with a
PHASE
EQUILIBRIA
M
CO2 + SQUALANE
MIXTURES
807
Heise gauge (M). The cell can be cooled with a water jacket (not shown in figure 1) and heated with four electric heating elements (HE) mounted from the bottom side of the cell, The heating current is controlled with an electronic regulator (R). The temperature is measured directly in the mixture under test with a calibrated steelsheathed thermocouple (T& introduced into the cell through a capillary tube. The temperature of the measuring and of the regulating thermocouple (T, and TR respectively) are registered with a recorder (RE). For a run the alkane was filled into the cell with a syringe, its mass being determined by weighing the syringe before and after filling; then liquid carbon dioxide was condensed into the cooled cell from a small high-pressure cylinder, the mass of carbon dioxide being also determined by weighing. For the measurements the mixture was brought to conditions of temperature and pressure in the homogeneous region. Then the temperature or the pressure was lowered slowly until a second phase appeared. The temperature could be measured within +O.l K; it was maintained constant to better than +O.l K. The pressure was accurate to 50.2 MPa. The error in mass fraction is estimated to f0.2 per cent. The mole fraction purity of the carbon dioxide used (Messer-Griesheim) was greater than 0.99995 (mole fractions of impurities: 0,, < 5 x 10m6, Nz, ~45 x 10e6). The 2,6,10,15,19,23-hexamethyltetracosane (W. Guenther Analysen, Dusseldorf) had a mole fraction purity of 0.984 determined by gas chromatography. Both substances were used for the measurements without further purification. A small sample of the alkane was additionally purified by preparative gas chromatography; when used for the phase-equilibrium measurements the deviations were within the limits of experimental error given above. 3. Results In order to help the understanding of the rather complicated phase relationships in carbon dioxide + alkane mixtures a schematic three-dimensional representation of the two-phase surface in pressure, temperature, composition space (X denotes mole fraction) is given in figure 2a. It has been found in earlier investigations that this type of phase behaviour is typical for binary mixtures of carbon dioxide with alkanes greater than tridecane ; it has been discussed in detail elsewhere.(g* lo7 18, lg) In figure 2b five characteristic p(T) sections for constant concentration (so-called isopleths) through the three-dimensional phase diagram of figure 2a are schematically shown; they correspond (from left to right) to p(T) sections at the mole fractions I, II, III, IV, and V marked in figure 2a. Some experimental results on carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane are given in table 1 and plotted in figures 3, 4, and 5.(12-r4) The experimental curves in figure 3 are parts of p(T) sections for constant mass fraction w of alkane in the temperature range 273 to 423 K and in the medium mass-fraction range. All isopleths represented in figure 3 have a characteristic form: within the experimental temperature and pressure range they exhibit a pressure and a temperature minimum and a steep ascent to increasing temperatures and pressures in the high-
K. G. LIPHARD
AND
G. M. SCHNEIDER
0
--+
T
FIGURE 2. Phase behaviour of binary mixtures of carbon dioxide with alkanes greater than tridecane (schematically, not to scale; x denotes mole fraction, HC = alkane; C = critical endpoint; see text). a, Three-dimensional pressure, temperature, composition diagram (I to V refer to the isopleths in figure 2b). b, Selected p(T) sections for constant mole fraction (so-called isopleths) (schematically; positions of the sections I to V are marked in figure 2a by arrows). TABLE 1. Fluid-phase equilibria in binary mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): p(T) curves for constant mass fraction w of alkane (see figure 3) p/MPa
T/K-273.15
w = 0.069 100.0 41.2 80.0 39.5 38.7 40:o ZEi 33.8 30.4 30.2 30.4 31.4 32.8 33.6 34.7
40.1 44.9 53.7 64.1 69.3 78.0 92.0 105.7 124.1 143.0
p/MPa
T/K-273.15
w = 0.119 100.0 50.8 80.0 50.0 70.0 49.6 60.0 49.8 50.0 51.9 40.7 58.3 35.0 69.7 33.8 81.6 33.6 83.6 33.8 87.5 34.8 106.0 35.8 125.9 36.7 145.0
pIMPa
T/K-273.15
w = 0.161 100.0 55.1 80.0 53.6 62.5 53.2 50.0 56.1 44.0 59.7 38.0 71.7 35.5 86.5 35.3 90.0 35.7 95.4 36.3 104.8 36.8 114.7 37.5 124.5 38.0 134.9
p/MPa
T/K-273.15
w = 0.239 100.0 58.8 80.0 57.4 60.0 58.2 50.0 61.9 45.0 65.8 40.0 74.3 37.2 86.4 36.9 90.2 96.7 36.7 36.8 103.2 37.0 112.1 37.6 124.4 38.3 138.2
PHASE
p/MPa
T/K-273.15
EQUILIBRIA
p/MPa
w = 0.287 100.0 80.0 60.0 50.0 45.0 39.8 38.3 36.6 36.5 36.5 36.8 37.7 38.7
IN
co,
+ SQUALANE
TABLE
l-continued
T/K-273.15
p/MPa
w = 0.401
57.4 55.9 56.8 60.2 66.2 74.3 79.5 91.4 95.5 99.4 104.5 120.5 144.5
100.0 80.0 60.0 50.0 40.0 36.7 35.7 35.4 35.6 35.9 36.7 37.4 38.1
p/MPa
T/K-273.15
49.0 47.6 46.9 48.1 53.0 62.6 70.1 77.6 83.8 86.1 99.0 123.2 148.0
w= 100.0 70.0 40.0 10.0 6.5 5.9 8.1 10.1 13.0 15.0 17.6 21.3
loo.0 80.0 60.0 50.0 40.0 37.2 34.3 33.1 33.0 33.1 33.8 34.8 36.4
pIMPa
T/K-273.15
52.5 50.8 51.1 53.5 59.4 64.3 74.0 84.6 88.5 94.6 107.1 122.9 146.9
p/MPa
w = 0.604 100.0 70.0 50.0 40.0 30.0 25.0 22.4 22.1 22.3 23.1 24.5 27.4 29.5
41.3 35.5 34.4 34.6 39.0 46.9 54.6 62.0 69.8 79.7 99.0 123.4 147.7
w=
0.728 13.2 9.6 7.2 12.6 18.7 22.6 37.9 50.3 69.3 84.7 105.2 147.9
T/K-273.15
p/MPa
T/K-273.15
100.0 80.0 60.0 50.0 40.0 37.5 33.5 32.3 32.0 32.2 32.8 34.1 35.7
w = 0.480 52.4 49.7 49.8 52.0 58.9 61.2 72.1 79.2 86.0 92.0 100.4 119.7 144.6
w = 0.467
56.6 54.1 55.0 58.1 68.1 78.9 86.4 92.1 97.4 104.8 120.8 129.7 144.9
w = 0.516 100.0 80.0 60.0 50.0 40.0 32.8 31.2 30.4 30.3 30.4 31.1 32.9 34.6
MIXTURES
4.0 5.1 5.9 7.2 7.8 8.8 9.5 10.5 11.6 12.2
0.837 22.5 34.7 47.5 66.3 75.0 87.1 97.3 109.4 127.6 143.3
T/K-273.15 w = 0.654
100.0 70.0 50.0 30.0 22.5 18.2 16.0 15.9 16.1 17.4 20.5 23.4 26.2
29.1 24.0 22.9 25.1 28.4 32.6 41.6 44.6 50.2 64.6 89.6 115.4 147.0 w = 0.930
1.6 1.9 2.3 2.7 3.3 3.5 3.7 4.0 4.2 4.4 4.7 4.8
14.3 27.2 39.7 56.4 80.0 86.7 93.6 109.9 117.2 125.9 140.6 148.7
810
K. G. LIPHARD
AND G. M. SCHNEIDER
pressure region. The experimental curves plotted in figure 3 correspond to the branches of the type IV isopleths in figure 2b (IV) within the hatched frame. Experiments below 273 K and above 423 K, at low pressures (e.g. on the low pressure branches of the isopleths such as those shown in figure 2b) and at very high and very low mass fractions of carbon dioxide (e.g. for isopleths of type I, II, and V in figure 2b) could not be established with the apparatus used (see section 2); they were, however, less important within the scope of the present investigation which was mainly concerned with high-pressure critical phenomena.
50
100 T/K - 273.15
150
FIGURE 3. Phase separation in fluid mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): p(T) sections for constant mass fraction w of alkane. The numbers attached to the curves are values of 10%~.
From these results isothermal pressure-composition sections and isobaric temperature-composition sections were obtained by interpolation. Some results are plotted in figures 4 and 5; here the lower abscissa represents the mass fraction w of alkane and the upper abscissa the mole fraction x of the alkane. Figures 4 and 5 thus demonstrate that in this system the regions of phase separation are displaced to very low mole fractions of the alkane. Additionally an interesting retrograde solubility behaviour is shown: for the isotherm at 326.2 K in figure 4 the mutual miscibility first increases and then decreases again with increasing pressure; for temperatures above 328 K even two completely separated two-phase regions exist, a first at low and a second at high pressures. A similar retrograde behaviour can also be deduced from the temperature-composition isobars in figure 5.
PHASE EQUILIBRIA
IN COz + SQUALANE
811
MIXTURES
Do-
loo-
80 -
t
60.
40-
20 .
OO
20
40
60
80
90
: 0
lO?v(alkane) FIGURE 4. Phase separation in fluid mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): isotherms of pressure against mass fraction w of alkane.
The envelope of all p(T) isopleths in figure 3 is the p(T) projection of the critical curve; in the pressure and temperature range of the measurements of the present paper it coincides more or less with a p(T) isopleth for approximately 29 mass per cent of alkane. The critical curve will start from the critical point of the pure alkane (T, = 933 K,p, = 0.51 MPa (calculated values cl 5’), and with decreasing temperatures it will first pass through a pressure maximum at about 39 MPa and 473 K (estimated values). Then it runs through a pressure minimum at p = (36.3 & 0.2) MPa and T = (373 &‘2) K, and at last through a temperature minimum at T = (328.2 + 0.5) K and p = (70.5 rt 2.0) MPa; finally the critical curve rises steeply to higher pressures with increasing temperatures. From the discussion of the critical p(T) curve above, a value of approximately 29 mass per cent of alkane was deduced for the critical composition. Additional information about the changes of the critical composition along the critical curve can be obtained from figures 4 and 5 where extreme values correspond to points on the critical curve. The experiments show that the critical composition is approxi-
812
K. G. LIPHARD
mately constant for the isotherms however, the critical composition alkane as has already been found such as octane,(‘) decane,(5) and
AND G. M. SCHNEIDER
and isobars plotted. With increasing temperatures, will be more and more displaced towards the pure for mixtures of carbon dioxide with smaller alkanes hexadecane.“) x(alkane)
20
.40
60
lO%(alkane) FIGURE 5. Phase separation in fluid mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): isobars of temperature against mass fraction w of alkane.
4. Discussion In figure 6 the phase behaviour of the mixture is compared to that of three binary carbon dioxide mixtures with n-alkanes of very different chain length, namely octane, tridecane, and hexadecane according to data taken from the literature.“) For carbon dioxide + octane the liquid + gas critical curve (LG) runs through the usual pressure maximum. At temperatures far below the critical temperature
PHASE EQUILIBRIA
IN CO2 + SQUALANE
MIXTURES
813
of pure carbon dioxide (approximately 304 K) additional separation into two liquid phases takes place. The branch LL of the critical curve corresponds to upper critical solution temperatures (UCST) that are slightly raised with increasing pressures. For mixtures of carbon dioxide with n-alkanes smaller than heptane the branch LL is situated below the crystallization surface. The more the mutual miscibility of the two components decreases the more the branch LL of the critical curve is displaced to
T/K-273.15
FIGURE 6. p(T) projectionsof the phasediagrams of binary carbondioxide+ alkanemixtures. Full lines,critical curves; hatchedlines,vapour pressurecurvesof carbon dioxide and octane respectively;Ce, octane; CX3,tridecane;C16,hexadecane;CaO,2,6,10,15,19,23-hexamethyltetracosane(squalane); the three-phase linesLLG areomitted.
higher temperatures. It can finally penetrate the ranges of temperature and pressure for liquid + gas critical phenomena and may pass continuously into the critical curve LG whereas the branch of the critical curve starting from the critical point of the pure carbon dioxide ends at a critical end point on the three-phase line LLG (see figure 2a). Carbon dioxide + hexadecane belongs to this type, whereas carbon dioxide + tridecane corresponds to a transition type. Figure 6 demonstrates that binary mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane show phase behaviour similar to that of carbon dioxide + hexadecane,“) the pressure minimum and maximum, however, being much less pronounced. For a detailed phase theoretical discussion see references 7, 9, IS, and 19,
814
K. G. LIPHARD
AND G. M. SCHNEIDER
For the mixtures in figure 6 the steeply ascending branch of the critical curve can be attributed more or less definitely to liquid + liquid phase-separation phenomena. It can be extrapolated, however, from these results that in mixtures of carbon dioxide with still larger alkanes the pressure minimum and maximum in the critical curve will vanish completely and the fluid phase equilibria in these systems will correspond to typical gas + gas equilibria of the second kind. It is, however, interesting to note that in carbon dioxide + n-alkane mixtures with n-alkanes larger than about pentacosane the steeply ascending branch of the critical curve and the corresponding two-phase region will probably disappear below the crystallization surface as has been already shown for mixtures of carbon dioxide + n-alkanes smaller than heptane (see above). Since because of branching the melting temperature of 2,6,10,15,19,23-hexamethyltetracosane is much lower than that of the corresponding n-alkane (triacontane) liquid -I- liquid equilibria can still be detected in binary mixtures of carbon dioxide + this branched alkane. These findings are not only of interest from a phase-theoretical and thermodynamic point of view but also for separation methods such as extraction with supercritical carbon dioxide and for the so-called supercritical fluid chromatography (SFC); a detailed discussion of the relationships between phase behaviour and chromatographic data is presented elsewhere.(ll’ 14* 16’ “I The authors thank Dr D. Bartmann and Mr R. Masselink for their help in constructing the measuring autoclave. Financial support of the Deutsche Forschungsgemeinschaft (DFG) and of the Verband der Chemischen Industrie e.V. (Fonds der Chemischen Industrie) is gratefully acknowledged. REFERENCES 1. 2. 3. 4. ii: 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Donnelly, H. G.; Katz, D. L. Znd. Eng. Chem. 1954, 46, 511. Kuenen, J. P. 2’. Phys. Chem. 1897,24,667. Poettmann, H. F.; Katz, D. L. Znd. Eng. Chem. 1945, 37, 847. Reamer, H. H.; Sage, B. H.; Lacey, W. N. Znd Eng. Chem. 1951,43,2515. Olds, R. H.; Reamer, H. H.; Sage, B. H.; Lacey, W. N. Znd. Eng. Chem. 1949,41,475. Reamer, H. H.; Sage, B. H. J. Chem. Eng. Data 1963, 8,508; J. Chem. Eng. Data 1965, 10, 49. Schneider, G. M.; Alwani, Z.; Heim, W.; Horvath, E.; Franck, E. U. Chem. Zng. Tech. 1967, 39, 649. Ku&arm, A. A.; Zorah, B. J.; Luks, K. D.; Kohn, J. P. J. Chem. Eng. Dutu 1974, 19, 92. Schneider, G. M. Ber. Bunsenges. Phys. Chem. 1966, 70, 10. Schneider, G. M. Ber. Bunsenges. Phys. Chem. 1966, 70, 497. Bartmann, D.; Schneider, G. M. J. Chromatogr. 1973, 83, 135. Liphard, K. G. Diplomthesis, University of Bochum, Germany, 1974. Jockers, R.; Liphard, K. G.; Schneider, G. M. paper presented at the 12th Annual Meeting of the European High Pressure Research Group, March 1974, Marburg, Germany. _ . -. Schneider, G. M.; Liphard, K. G.; van Wasen, U.; Bartmann, D. Paper presented at the Joint Meeting GVC-AIChE, September 1974, Munich, Germany. Luck. A. P. In Ullmanns Encyklopiidie der Technischen Chemie, Vol. I. Verlag Chemie: Weinheim/ Bergstr, 1972, p. 55. Bartmann, D. Thesis, University of Bochum, Germany, 1972. Schneider, G. M.; v. Wasen, U. Chromatographia, in the press. Schneider, G. M. Adv. Chem. Phys. 1970,17, 1. Schneider, G. M. In Chem. Sot. Spec. Rep.: Chemical Thermodynamics (Vol. 2) (in the press).
Phase equilibria and critical phenomena in fluid mixtures of carbon dioxide + 2,6,10,15,19,23=hexamethyltetracosane up to 423 K and 100 MPa K. G. LIPHARD
and G. M. SCHNEIDER
Institute of Physical Chemistry, University of Bochum, Germany (Received10 December1974)’ In an optical high-pressure cell with sapphire windows and magnetic stirring, phase equilibria in fluid binary mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane) have been measured up to 423 K and 100 MPa. The critical curve of the mixture starts at the critical point of the pure alkane (Tc = 933 K, pc = 0.51 MPa (calculated values)). With decreasing temperatures the critical curve runs first through a pressure maximum at about 39 MPa and 473 K, then through a pressure minimum at p = (36.3 j, 0.2) MPa and T = (373 & 2) K, and at last through a temperature minimum at T = (328.2 f 0.5) K and p = (70.5 -i: 2.0) MPa. Finally the critical curve rises steeply to higher pressures with increasing temperatures. The results are discussed from a phasetheoretical point of view and are compared with phase equilibrium data of other carbon dioxide + alkane mixtures. It follows from this discussion that there are continuous transitions between liquid + gas, liquid -I- liquid, and gas + gas equilibria in these systems.
1. Introduction In recent years the phase equilibria of binary mixtures of carbon dioxide with n-alkanes up to hexadecane have been measured. (l-*) These mixtures are of considerable interest in phase theory since the physical properties of the alkanes and thus of the binary mixtures can be easily and gradually varied over a wide range by adding more and more carbon atoms to the alkane chain. These mixtures show very different types of fluid phase equilibria (see figure 6). Up to hexane only liquid + gas equilibria occur; from heptane on, however, additionally liquid + liquid immiscibility is observed. For mixtures of larger PZalkanes (e.g. hexadecane) + carbon dioxide liquid + gas and liquid + liquid equilibria are superimposed. It can be extrapolated from these results that in binary mixtures of carbon dioxide with alkanes of much higher molar mass the phase equilibria and critical phenomena will become more and more similar to typical gas + gas equilibria of the second type thus verifying the hypothesis that in these mixtures continuous transitions between liquid + gas, liquid + liquid, and gas -t- gas equilibria o~ur.(9* lOv18~19)
For the present investigation 2,6,10,15,19,23-hexamethyltetracosane (squalane) was chosen; its molar mass is approximately twice as high as that of the largest 54
806
K. G. LIPHARD
AND G. M. SCHNEIDER
alkane investigated before (hexadecane); it has also the advantage of being liquid at room temperature and as a stationary phase in gas chromatography it is readily available in good purity. The present paper deals with measurements of the phase equilibria and the critical phenomena in fluid binary mixtures of carbon dioxide and this alkane up to 423 K and 100 MPa.
2. Experimental The measurements were carried out in an optical high-pressure cell (see figure la). It is made of a non-magnetic stainless steel and is sealed by a copper ring (CR). The maximum cell volume is 7.8 cm3. The liquid mixture in the cell can be stirred efficiently a
b
FIGURE 1. Apparatus (for details see text). a, Optical high-pressure autoclave; b, Experimental set-up (schematic).
by a magnetic stirrer (MS); in order to improve mixing a coiled wire is attached to this magnet. It is driven from outside the cell by another rotating magnet (RM). All phase transitions in the cell can be observed through two sapphire windows (WI, W2). The whole apparatus is shown schematically in figure lb. The pressure is created with liquid decane in a rotating pump (RP) and transmitted to the mixture under test by a closely fitted piston (P) which is additionally tightened with viton O-rings (0). By calibration runs the pressure difference across the piston was found to be within the accuracy of the pressure readings (see below). The pressure is measured with a
PHASE
EQUILIBRIA
M
CO2 + SQUALANE
MIXTURES
807
Heise gauge (M). The cell can be cooled with a water jacket (not shown in figure 1) and heated with four electric heating elements (HE) mounted from the bottom side of the cell, The heating current is controlled with an electronic regulator (R). The temperature is measured directly in the mixture under test with a calibrated steelsheathed thermocouple (T& introduced into the cell through a capillary tube. The temperature of the measuring and of the regulating thermocouple (T, and TR respectively) are registered with a recorder (RE). For a run the alkane was filled into the cell with a syringe, its mass being determined by weighing the syringe before and after filling; then liquid carbon dioxide was condensed into the cooled cell from a small high-pressure cylinder, the mass of carbon dioxide being also determined by weighing. For the measurements the mixture was brought to conditions of temperature and pressure in the homogeneous region. Then the temperature or the pressure was lowered slowly until a second phase appeared. The temperature could be measured within +O.l K; it was maintained constant to better than +O.l K. The pressure was accurate to 50.2 MPa. The error in mass fraction is estimated to f0.2 per cent. The mole fraction purity of the carbon dioxide used (Messer-Griesheim) was greater than 0.99995 (mole fractions of impurities: 0,, < 5 x 10m6, Nz, ~45 x 10e6). The 2,6,10,15,19,23-hexamethyltetracosane (W. Guenther Analysen, Dusseldorf) had a mole fraction purity of 0.984 determined by gas chromatography. Both substances were used for the measurements without further purification. A small sample of the alkane was additionally purified by preparative gas chromatography; when used for the phase-equilibrium measurements the deviations were within the limits of experimental error given above. 3. Results In order to help the understanding of the rather complicated phase relationships in carbon dioxide + alkane mixtures a schematic three-dimensional representation of the two-phase surface in pressure, temperature, composition space (X denotes mole fraction) is given in figure 2a. It has been found in earlier investigations that this type of phase behaviour is typical for binary mixtures of carbon dioxide with alkanes greater than tridecane ; it has been discussed in detail elsewhere.(g* lo7 18, lg) In figure 2b five characteristic p(T) sections for constant concentration (so-called isopleths) through the three-dimensional phase diagram of figure 2a are schematically shown; they correspond (from left to right) to p(T) sections at the mole fractions I, II, III, IV, and V marked in figure 2a. Some experimental results on carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane are given in table 1 and plotted in figures 3, 4, and 5.(12-r4) The experimental curves in figure 3 are parts of p(T) sections for constant mass fraction w of alkane in the temperature range 273 to 423 K and in the medium mass-fraction range. All isopleths represented in figure 3 have a characteristic form: within the experimental temperature and pressure range they exhibit a pressure and a temperature minimum and a steep ascent to increasing temperatures and pressures in the high-
K. G. LIPHARD
AND
G. M. SCHNEIDER
0
--+
T
FIGURE 2. Phase behaviour of binary mixtures of carbon dioxide with alkanes greater than tridecane (schematically, not to scale; x denotes mole fraction, HC = alkane; C = critical endpoint; see text). a, Three-dimensional pressure, temperature, composition diagram (I to V refer to the isopleths in figure 2b). b, Selected p(T) sections for constant mole fraction (so-called isopleths) (schematically; positions of the sections I to V are marked in figure 2a by arrows). TABLE 1. Fluid-phase equilibria in binary mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): p(T) curves for constant mass fraction w of alkane (see figure 3) p/MPa
T/K-273.15
w = 0.069 100.0 41.2 80.0 39.5 38.7 40:o ZEi 33.8 30.4 30.2 30.4 31.4 32.8 33.6 34.7
40.1 44.9 53.7 64.1 69.3 78.0 92.0 105.7 124.1 143.0
p/MPa
T/K-273.15
w = 0.119 100.0 50.8 80.0 50.0 70.0 49.6 60.0 49.8 50.0 51.9 40.7 58.3 35.0 69.7 33.8 81.6 33.6 83.6 33.8 87.5 34.8 106.0 35.8 125.9 36.7 145.0
pIMPa
T/K-273.15
w = 0.161 100.0 55.1 80.0 53.6 62.5 53.2 50.0 56.1 44.0 59.7 38.0 71.7 35.5 86.5 35.3 90.0 35.7 95.4 36.3 104.8 36.8 114.7 37.5 124.5 38.0 134.9
p/MPa
T/K-273.15
w = 0.239 100.0 58.8 80.0 57.4 60.0 58.2 50.0 61.9 45.0 65.8 40.0 74.3 37.2 86.4 36.9 90.2 96.7 36.7 36.8 103.2 37.0 112.1 37.6 124.4 38.3 138.2
PHASE
p/MPa
T/K-273.15
EQUILIBRIA
p/MPa
w = 0.287 100.0 80.0 60.0 50.0 45.0 39.8 38.3 36.6 36.5 36.5 36.8 37.7 38.7
IN
co,
+ SQUALANE
TABLE
l-continued
T/K-273.15
p/MPa
w = 0.401
57.4 55.9 56.8 60.2 66.2 74.3 79.5 91.4 95.5 99.4 104.5 120.5 144.5
100.0 80.0 60.0 50.0 40.0 36.7 35.7 35.4 35.6 35.9 36.7 37.4 38.1
p/MPa
T/K-273.15
49.0 47.6 46.9 48.1 53.0 62.6 70.1 77.6 83.8 86.1 99.0 123.2 148.0
w= 100.0 70.0 40.0 10.0 6.5 5.9 8.1 10.1 13.0 15.0 17.6 21.3
loo.0 80.0 60.0 50.0 40.0 37.2 34.3 33.1 33.0 33.1 33.8 34.8 36.4
pIMPa
T/K-273.15
52.5 50.8 51.1 53.5 59.4 64.3 74.0 84.6 88.5 94.6 107.1 122.9 146.9
p/MPa
w = 0.604 100.0 70.0 50.0 40.0 30.0 25.0 22.4 22.1 22.3 23.1 24.5 27.4 29.5
41.3 35.5 34.4 34.6 39.0 46.9 54.6 62.0 69.8 79.7 99.0 123.4 147.7
w=
0.728 13.2 9.6 7.2 12.6 18.7 22.6 37.9 50.3 69.3 84.7 105.2 147.9
T/K-273.15
p/MPa
T/K-273.15
100.0 80.0 60.0 50.0 40.0 37.5 33.5 32.3 32.0 32.2 32.8 34.1 35.7
w = 0.480 52.4 49.7 49.8 52.0 58.9 61.2 72.1 79.2 86.0 92.0 100.4 119.7 144.6
w = 0.467
56.6 54.1 55.0 58.1 68.1 78.9 86.4 92.1 97.4 104.8 120.8 129.7 144.9
w = 0.516 100.0 80.0 60.0 50.0 40.0 32.8 31.2 30.4 30.3 30.4 31.1 32.9 34.6
MIXTURES
4.0 5.1 5.9 7.2 7.8 8.8 9.5 10.5 11.6 12.2
0.837 22.5 34.7 47.5 66.3 75.0 87.1 97.3 109.4 127.6 143.3
T/K-273.15 w = 0.654
100.0 70.0 50.0 30.0 22.5 18.2 16.0 15.9 16.1 17.4 20.5 23.4 26.2
29.1 24.0 22.9 25.1 28.4 32.6 41.6 44.6 50.2 64.6 89.6 115.4 147.0 w = 0.930
1.6 1.9 2.3 2.7 3.3 3.5 3.7 4.0 4.2 4.4 4.7 4.8
14.3 27.2 39.7 56.4 80.0 86.7 93.6 109.9 117.2 125.9 140.6 148.7
810
K. G. LIPHARD
AND G. M. SCHNEIDER
pressure region. The experimental curves plotted in figure 3 correspond to the branches of the type IV isopleths in figure 2b (IV) within the hatched frame. Experiments below 273 K and above 423 K, at low pressures (e.g. on the low pressure branches of the isopleths such as those shown in figure 2b) and at very high and very low mass fractions of carbon dioxide (e.g. for isopleths of type I, II, and V in figure 2b) could not be established with the apparatus used (see section 2); they were, however, less important within the scope of the present investigation which was mainly concerned with high-pressure critical phenomena.
50
100 T/K - 273.15
150
FIGURE 3. Phase separation in fluid mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): p(T) sections for constant mass fraction w of alkane. The numbers attached to the curves are values of 10%~.
From these results isothermal pressure-composition sections and isobaric temperature-composition sections were obtained by interpolation. Some results are plotted in figures 4 and 5; here the lower abscissa represents the mass fraction w of alkane and the upper abscissa the mole fraction x of the alkane. Figures 4 and 5 thus demonstrate that in this system the regions of phase separation are displaced to very low mole fractions of the alkane. Additionally an interesting retrograde solubility behaviour is shown: for the isotherm at 326.2 K in figure 4 the mutual miscibility first increases and then decreases again with increasing pressure; for temperatures above 328 K even two completely separated two-phase regions exist, a first at low and a second at high pressures. A similar retrograde behaviour can also be deduced from the temperature-composition isobars in figure 5.
PHASE EQUILIBRIA
IN COz + SQUALANE
811
MIXTURES
Do-
loo-
80 -
t
60.
40-
20 .
OO
20
40
60
80
90
: 0
lO?v(alkane) FIGURE 4. Phase separation in fluid mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): isotherms of pressure against mass fraction w of alkane.
The envelope of all p(T) isopleths in figure 3 is the p(T) projection of the critical curve; in the pressure and temperature range of the measurements of the present paper it coincides more or less with a p(T) isopleth for approximately 29 mass per cent of alkane. The critical curve will start from the critical point of the pure alkane (T, = 933 K,p, = 0.51 MPa (calculated values cl 5’), and with decreasing temperatures it will first pass through a pressure maximum at about 39 MPa and 473 K (estimated values). Then it runs through a pressure minimum at p = (36.3 & 0.2) MPa and T = (373 &‘2) K, and at last through a temperature minimum at T = (328.2 + 0.5) K and p = (70.5 rt 2.0) MPa; finally the critical curve rises steeply to higher pressures with increasing temperatures. From the discussion of the critical p(T) curve above, a value of approximately 29 mass per cent of alkane was deduced for the critical composition. Additional information about the changes of the critical composition along the critical curve can be obtained from figures 4 and 5 where extreme values correspond to points on the critical curve. The experiments show that the critical composition is approxi-
812
K. G. LIPHARD
mately constant for the isotherms however, the critical composition alkane as has already been found such as octane,(‘) decane,(5) and
AND G. M. SCHNEIDER
and isobars plotted. With increasing temperatures, will be more and more displaced towards the pure for mixtures of carbon dioxide with smaller alkanes hexadecane.“) x(alkane)
20
.40
60
lO%(alkane) FIGURE 5. Phase separation in fluid mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane (squalane): isobars of temperature against mass fraction w of alkane.
4. Discussion In figure 6 the phase behaviour of the mixture is compared to that of three binary carbon dioxide mixtures with n-alkanes of very different chain length, namely octane, tridecane, and hexadecane according to data taken from the literature.“) For carbon dioxide + octane the liquid + gas critical curve (LG) runs through the usual pressure maximum. At temperatures far below the critical temperature
PHASE EQUILIBRIA
IN CO2 + SQUALANE
MIXTURES
813
of pure carbon dioxide (approximately 304 K) additional separation into two liquid phases takes place. The branch LL of the critical curve corresponds to upper critical solution temperatures (UCST) that are slightly raised with increasing pressures. For mixtures of carbon dioxide with n-alkanes smaller than heptane the branch LL is situated below the crystallization surface. The more the mutual miscibility of the two components decreases the more the branch LL of the critical curve is displaced to
T/K-273.15
FIGURE 6. p(T) projectionsof the phasediagrams of binary carbondioxide+ alkanemixtures. Full lines,critical curves; hatchedlines,vapour pressurecurvesof carbon dioxide and octane respectively;Ce, octane; CX3,tridecane;C16,hexadecane;CaO,2,6,10,15,19,23-hexamethyltetracosane(squalane); the three-phase linesLLG areomitted.
higher temperatures. It can finally penetrate the ranges of temperature and pressure for liquid + gas critical phenomena and may pass continuously into the critical curve LG whereas the branch of the critical curve starting from the critical point of the pure carbon dioxide ends at a critical end point on the three-phase line LLG (see figure 2a). Carbon dioxide + hexadecane belongs to this type, whereas carbon dioxide + tridecane corresponds to a transition type. Figure 6 demonstrates that binary mixtures of carbon dioxide + 2,6,10,15,19,23-hexamethyltetracosane show phase behaviour similar to that of carbon dioxide + hexadecane,“) the pressure minimum and maximum, however, being much less pronounced. For a detailed phase theoretical discussion see references 7, 9, IS, and 19,
814
K. G. LIPHARD
AND G. M. SCHNEIDER
For the mixtures in figure 6 the steeply ascending branch of the critical curve can be attributed more or less definitely to liquid + liquid phase-separation phenomena. It can be extrapolated, however, from these results that in mixtures of carbon dioxide with still larger alkanes the pressure minimum and maximum in the critical curve will vanish completely and the fluid phase equilibria in these systems will correspond to typical gas + gas equilibria of the second kind. It is, however, interesting to note that in carbon dioxide + n-alkane mixtures with n-alkanes larger than about pentacosane the steeply ascending branch of the critical curve and the corresponding two-phase region will probably disappear below the crystallization surface as has been already shown for mixtures of carbon dioxide + n-alkanes smaller than heptane (see above). Since because of branching the melting temperature of 2,6,10,15,19,23-hexamethyltetracosane is much lower than that of the corresponding n-alkane (triacontane) liquid -I- liquid equilibria can still be detected in binary mixtures of carbon dioxide + this branched alkane. These findings are not only of interest from a phase-theoretical and thermodynamic point of view but also for separation methods such as extraction with supercritical carbon dioxide and for the so-called supercritical fluid chromatography (SFC); a detailed discussion of the relationships between phase behaviour and chromatographic data is presented elsewhere.(ll’ 14* 16’ “I The authors thank Dr D. Bartmann and Mr R. Masselink for their help in constructing the measuring autoclave. Financial support of the Deutsche Forschungsgemeinschaft (DFG) and of the Verband der Chemischen Industrie e.V. (Fonds der Chemischen Industrie) is gratefully acknowledged. REFERENCES 1. 2. 3. 4. ii: 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Donnelly, H. G.; Katz, D. L. Znd. Eng. Chem. 1954, 46, 511. Kuenen, J. P. 2’. Phys. Chem. 1897,24,667. Poettmann, H. F.; Katz, D. L. Znd. Eng. Chem. 1945, 37, 847. Reamer, H. H.; Sage, B. H.; Lacey, W. N. Znd Eng. Chem. 1951,43,2515. Olds, R. H.; Reamer, H. H.; Sage, B. H.; Lacey, W. N. Znd. Eng. Chem. 1949,41,475. Reamer, H. H.; Sage, B. H. J. Chem. Eng. Data 1963, 8,508; J. Chem. Eng. Data 1965, 10, 49. Schneider, G. M.; Alwani, Z.; Heim, W.; Horvath, E.; Franck, E. U. Chem. Zng. Tech. 1967, 39, 649. Ku&arm, A. A.; Zorah, B. J.; Luks, K. D.; Kohn, J. P. J. Chem. Eng. Dutu 1974, 19, 92. Schneider, G. M. Ber. Bunsenges. Phys. Chem. 1966, 70, 10. Schneider, G. M. Ber. Bunsenges. Phys. Chem. 1966, 70, 497. Bartmann, D.; Schneider, G. M. J. Chromatogr. 1973, 83, 135. Liphard, K. G. Diplomthesis, University of Bochum, Germany, 1974. Jockers, R.; Liphard, K. G.; Schneider, G. M. paper presented at the 12th Annual Meeting of the European High Pressure Research Group, March 1974, Marburg, Germany. _ . -. Schneider, G. M.; Liphard, K. G.; van Wasen, U.; Bartmann, D. Paper presented at the Joint Meeting GVC-AIChE, September 1974, Munich, Germany. Luck. A. P. In Ullmanns Encyklopiidie der Technischen Chemie, Vol. I. Verlag Chemie: Weinheim/ Bergstr, 1972, p. 55. Bartmann, D. Thesis, University of Bochum, Germany, 1972. Schneider, G. M.; v. Wasen, U. Chromatographia, in the press. Schneider, G. M. Adv. Chem. Phys. 1970,17, 1. Schneider, G. M. In Chem. Sot. Spec. Rep.: Chemical Thermodynamics (Vol. 2) (in the press).