Chapter 4
Three-Phase Behavior in Binary Mixtures of Near-Critical Propane and Triglycerides E. J. M . Straver, J . L. de Roo, C. J. Peters, and J . de Swaan Arons Department of Chemical Engineering and Materials Science, Laboratory of Applied Thermodynamics and Phase Equilibria, Delft University of Technology, Julianalaan 136, 2628 BL Delft, Netherlands
This contribution reports on the three-phase behavior liquid-liquid-vapor(1 1 g)in binary mixtures of near-critical propane and triglycerides. The triglycerides, investigated in this work, range in carbon number from 51 (tripalmitin) up to 57 (tristearin). Using a Cailletet apparatus the three-phase equilibria, including the lower and upper critical endpoints (LCEP and UCEP), were measured. A comparison of the experimental results with earlier measurements of three-phase equilibria 1 1 g in near-critical propane and heavy normal paraffins lead to some interesting conclusions. It was established that the length of the three-phase region ΔΤ = T(UCEP) - T(LCEP) in binaries of triglycerides and normal paraffins with the same carbon number is identical. This suggests that the three-phase behavior in the binaries of triglycerides is no longer influenced by the presence of functional groups and branching of the triglyceride molecules. 1
2
1
2
Carbon dioxide and propane both have been studied as near-critical solvents for the recovery, purification and fractionation of vegetable oils, animal fats and fish oils; see for example McHugh and Krukonis (i), Lee et al. (2), Eggers et al. (3) and del Valle and Aguilera (4). The interest stems of course from the special features of near-critical solvents: both product and residue can be obtained in a highly pure state due to the very high volatility of the solvent at reduced pressures. A significant advantage of the use of highly volatile solvents, in comparison with solvents like liquid hexane, is the strongly reduced presence of minute amounts of the solvent in the product. Although carbon dioxide is attractive for its relative inertness, propane is of special interest as a solvent for various reasons:
0097-6156/93/0514-0046$06.00/0 © 1993 American Chemical Society
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- its critical point is at conditions were most oils are liquids with an appreciable volatility; - oils mainly have a paraffinic nature and consequently are more related to propane than to carbon dioxide. In addition to the latter point it is known that (Peters et al. (5)) binary mixtures of near-critical propane and normal paraffins with a carbon number higher than 29, show liquid-liquid immiscibility, whereas binary mixtures of carbon dioxide and normal paraffins already show this phenomenon at a carbon number higher than 6. For details on the behavior of propane as a near-critical solvent for paraffinic and polyaromatic hydrocarbons one is referred to Peters et al. (5,6). Similarly as mineral oils, edible oils are natural products also with a complex composition. From techniques used in the mineral oil industry it is known that broad ranges of components can be characterized in their behavior by selecting a limited number of key components. For example Cotterman et al. (7,8) discussed how from a selection of a limited number of key components predictions for multicomponent mixtures can be obtained. It is to be expected that for the prediction of the behavior of edible oils a similar approach will be applicable. Therefore, the current research also contributes to the knowledge of the selection of suitable key components for edible oils. Some details on this aspect of the current study were already discussed by Straver et al. (9). Previously Coorens et al. (10) reported on the phase behavior of the binary propane + tripalmitin (PPP). Recently Peters et al. (11) and Straver et al. (12) determined experimentally the phase behavior of the binary propane + triolein (OOO) and propane + tristearin (SSS) respectively. In this study we will focus on the three-phase behavior liquid-liquid-vapor Ol^ê) °f binaries of near-critical propane and pure triglycerides. In particular it will be investigated how this three-phase behavior compares with similar phase behavior, reported earlier by Peters et al. (J), in binaries of near-critical propane and normal paraffins with a comparable number of carbon atoms. For that purpose this three-phase behavior was determined experimentally in the propane binaries of tripalmitin (PPP), α-stearodipalmitin (SPP), α-palmitodistearin (PSS) and tristearin (SSS). Phase Behavior The fluid phase behavior occurring in binaries of propane and triglycerides is of type V in the classification of Van Konynenburg and Scott (13). This type of phase behavior was first recognized by Kuenen (14). More recently, detailed dicussions on this type of phase behavior were given by Diepen (15), Luks (16), Peters (17) and Peters et al. (18). The characteristics of this type of phase behavior will be recalled here only briefly. Figure 1 shows a schematical ρ,Τ,χ-projection for propane binaries of triglycerides, including the presence of the solid phase of the triglyceride. From Figure 1 it can be seen that in this type of phase behavior partial miscibility in the liquid phase occurs, i.e. in the near-critical region of propane a three-phase equilibrium l ^ g exists. This three-phase equilibrium starts in the lower critical endpoint (LCEP) and
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terminates in the upper critical endpoint (UCEP). Figure 1 also shows that the critical locus is separated into two branches. One branch originates in the LCEP of the ^^-equilibrium and, via a maximum in pressure, terminates in the critical point of the pure triglyceride. The other branch originates in the critical point of propane and ends in the UCEP of the ^^-equilibrium. For a detailed discussion on this type of phase behavior one is also referred to Peters et al. WO).
Experimental The experimental work was carried out in a high pressure Cailletet equipment. In this equipment pressures up to approximately 15 MPa can be achieved and temperatures from 250 Κ up to 450 Κ can be attained. This equipment was discussed in the literature by Van der Kooi (21) and De Loos et al. (22). The experimental procedures used were quite similar as discussed previously by Peters et al. (23,24) in their studies on the binaries ethane + pentacosane and ethane + tetracosane, respectively. In addition detailed information on the experimental procedures was also given by Van der Kooi (21), Peters (17) and Coorens et al. (10). The temperature in the experimental work could be measured with an accuracy of 0.02 Κ and the pressure could be determined within 0.002 MPa. The purity of the propane used was 99.95 mole % and was checked by measuring its saturated vapor pressure curve. Excellent agreement with data of Goodwin and Haynes (25) was obtained. All other chemicals had a minimum purity of at least 99.0 mole % and were used without further purification. Experimental Results In Table I the experimental data of the three-phase equilibria l ^ g of the four binaries, investigated in this work, are summarized. Table II summarizes the data of the critical endpoints separately. In general it appeared to be possible to determine the complete three-phase region on samples with an overall composition of approximately 2 mole %. Because it was not possible to sample the coexisting phases, the composition of these phases could not be determined. Discussion and Conclusions As a result of earlier work, Peters et al. (5), the conjectures of Rowlinson and Freeman (26) and Rowlinson and Swinton (27), that mixtures of propane and n-alkanes will not demix for carbon numbers less than 37 or 39, were found to be incorrect. Experimentally a three-phase equilibrium was already established in the propane binary of dotriacontane, i.e. a normal paraffin with a carbon number as low as 32. Thisfindingwas also in disagreement with that of Leder
STRAVER ET AL.
Mixtures of Near-Critical Propane and Triglycerides
Figure 1. Schematical ρ,Τ,χ-projection of the phase behavior of binary mixtures of propane and triglycerides. Table I. Experimental results of the three-phase behavior l l g in binary systems of near-critical propane and triglycerides x
T/K 348.99 349.48 350.11 352.07 357.04 359.39 360.78 361.01 362.33 364.46 368.77 369.23 369.75 369.91 370.03 370.31
SSS
PSS
SPP
PPP
2
p/MPa
T/K
p/MPa
T/K
p/MPa
T/K
p/MPa
2.876 2.900 2.940 3.036 3.336 3.481 3.590 3.610 3.676 3.831 4.147 4.182 4.217 4.249 4.251 4.258
346.72 347.25 350.22 354.21 358.21 362.21 366.22 370.20
2.770 2.797 2.959 3.191 3.443 3.701 3.988 4.277
344.93 345.00 345.95 348.80 350.81 353.69 355.67 358.58 360.52 363.45 365.37 368.32 369.76 370.14
2.660 2.665 2.709 2.870 2.977 3.150 3.266 3.460 3.574 3.780 3.908 4.131 4.240 4.264
343.10 343.20 343.52 345.48 346.03 348.01 350.44 352.91 355.42 357.88 360.41 362.85 365.20 367.84 369.82 370.11
2.574 2.579 2.594 2.693 2.733 2.843 2.965 3.126 3.259 3.422 3.574 3.750 3.902 4.105 4.253 4.268
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Table Π. LCEP'S and UCEP's of the three-phase equilibria \\ g in binary systems of near-critical propane and triglycerides 2
LCEP TRIGLYCERIDE T / K PPP SPP PSS SSS
348.99 346.72 344.93 343.10
UCEP p/MPa T / K
p/MPa
2.876 2.770 2.660 2.574
4.258 4.277 4.264 4.268
370.31 370.20 370.14 370.11
et al. (28), who found no liquid-liquid immiscibility in the propane binary of a normal paraffin with a carbon number of 36. For the propane + normal paraffin systems Figure 2 shows the temperatures of the UCEP's and LCEP's as a function of the carbon number. The experimental data of the three-phase equilibria, including the critical endpoints, can be found elsewhere (Peters et al. (5)). It is known from the classical theory of tricritical points, as discussed by Griffiths (29) and Scott (30), that the 2/3 power of the temperature region ΔΤ = T(UCEP) - T(LCEP) of the three-phase region is proportional to the carbon number. The group of Scott, Knobler and collaborators found no departures from linearity in the ethane systems, but their range of carbon numbers was restricted. However, Figure 3 shows that in the propane systems for carbon numbers less than 40 also linearity is observed, but nonlinear behavior is observed for carbon numbers beyond 40. Figure 4 is similar to Figure 2, but now the LCEP's and U C E F s (open triangles) of the three-phase equilibria l l g of the propane binaries of the triglycrides have been included. The open circles represent the critical endpoints of the propane systems with the normal paraffins, whereas the open triangles are the critical endpoints of the propane systems with the triglycerides. The black triangles are LCEP's measured by Hixson and Bockelmann (31). Figure 5 is similar to Figure 3, but now the ΔΤ-data of the propane systems of the triglycerides (open triangles) have been included. From both Figure 4 and 5 it can be seen that the open triangles (data of the triglyceride systems) are a continuous extension of the loci connecting the open circles (data of the normal paraffin systems). This suggests that the three-phase behavior l ^ g of the propane systems of the triglycerides is identical to that of the propane systems of the normal paraffins with the same carbon number. Due to the presence of long aliphatic chains in the triglyceride molecules, the influence of the functional groups of these molecules no longer plays a role in the occurrence of the three-phase equilibrium l ^ g . Also it should be recognized that a triglyceride molecule can be considered as a branched paraffin and that this fact apparently does not cause deviation from the three-phase behavior in comparison to that of the normal paraffins. Hence, in conclusion, it can be established that triglyceride molecules with carbon numbers beyond 50 have similar three-phase behavior l l g in near-critical propane as normal paraffins with the same carbon number. x
2
1
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/
/
/
yΡ
/
2
3
Figure 3. Plot of ΔΤ / versus carbon number η of the three-phase region of propane binaries of normal paraffins.
^
^~-o
UCEP
ο—ο——ΟΔ--Δ--Δ--Δ1
\x
25
THREE PHASES
LCEP^
3^0
ΔΤ
1
1
30
35
1
1
1
1
*>5 • η
50
55
Figure 4. Experimental data of the LCEP's and UCEP's of propane binaries with normal paraffins (open circles) and triglycerides (open triangles). Black triangles are LCEP's take from Hixson and Bockelmann (31). Full curves are best fits to the experimental data.
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Figure 5. Plot of ΔΤ / versus carbon number η of the three-phase region of propane binaries of normal paraffins (open circles) and triglycerides (open triangles). Acknowledgement The authors gratefully acknowledge the supply of the pure triglycerides by Unilever Research, Vlaardingen (The Netherlands). Literature Cited 1. McHugh, M.; Krukonis, V.; Supercritical fluid extraction. Butterworth, London, England, 1986. 2. Lee, A.K.K.; Bulley, N.R.; Fattori, M.; Meisen, Α.;J.Am. Chem. Soc. 1986, 63, 921. 3. Eggers, E.; Sievers, U.; Stein, W.; J. Am. Chem. Soc. 1985, 62, 1222. 4. Valle, J.M. del; Aguilera, J.M.; Ind. Eng. Chem. Res. 1988, 27, 1551. 5. Peters, C.J.; Kooi, H.J. van der; Roo, J.L. de; Swaan Arons, J. de; Gallagher, J.S.; Levelt Sengers, J.M.H.; Fluid Phase Equilibria 1989, 51, 339. 6. Peters, C.J.; Rijkers, M.P.W.M.; Roo, J.L. de; Swaan Arons, J. de; Fluid Phase Equilibria 1989, 52, 373. 7. Cotterman, R.L.; Dimitrelis, D.; Prausnitz, J.M.; Ber. Bunsen Physik. Chem. 1984, 87, no 9. 8. Cotterman, R.L.; Bender, R.; Prausnitz, J.M.; Ind. & Eng. Chem. Fund. 1985, 24, 194. 9. Straver, E.J.M.; Roo J.L. de; Peters, C.J.; Swaan Arons, J. de; Proceedings 2nd International Symposium on Supercritical Fluids, Boston, Massachusetts, U.S.A., 1991, 482. 10. Coorens, H.G.A.; Peters, C.J.; Swaan Arons, J. de; Fluid Phase Equilibria 1988, 40, 135. 11. Peters, C.J.; Roo, J.L. de; Swaan Arons, J. de; Fluid Phase Equilibria 1993, in preparation. 12. Straver, E.J.M.; Peters, C.J.; Swaan Arons, J. de; Fluid Phase Equilibria, 1993, in preparation.
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13. Konynenburg, P.H. van; Scott, R.L; Phil. Trans. of the Roy. Soc. (London) 1980, 298, 495. 14. Kuehnen, J.P.; Theorie der Verdampfung und Verflüssigung von Gemischen und der fraktionierten Destillation. Verlag von Johann Ambrosius Barth, Leipzig, Germany, 1906. 15. Diepen, G.A.M.; De oplosbaarheid van vaste stoffen in superkritische gassen. Ph. D. Thesis: Delft University of Technology, The Netherlands, 1947. 16. Luks, K.D.; Proc. 2nd Int. Conf. on Phase Equilibria and Fluid Properties in the Chemical Industry, West-Berlin, 1980. 17. Peters, C.J.; Phase behavior of binary mixtures of ethane + n-eicosane and statistical mechanical treatment of fluid phases. Ph. D. Thesis: Delft University of Technology, The Netherlands, 1986. 18. Peters, C.J.; Lichtenthaler, R.N.; Swaan Arons, J. de; Fluid Phase Equilibria 1986, 29, 495. 19. Peters, C.J.; Roo, J.L. de; Lichtenthaler, R.N.; Fluid Phase Equilibria 1987, 34, 287. 20. Peters, C.J.; Spiegelaar, J.; Swaan Arons, J. de; Fluid Phase Equilibria 1988, 41, 245. 21. Kooi, H.J. van der; Metingen en berekeningen aan het systeem methaan n-eicosaan. Ph. D. Thesis: Delft University of Technology, The Netherlands, 1981.
22. Loos, Th. W. de; Kooi, H.J. van der; Poot, W.; Ott, P.L.; Delft Progr. Rep. 1983, 8, 200. 23. Peters, C.J.; Roo, J.L. de; Swaan Arons, J. de; J. Chem. Therm. 1987, 19, 265. 24. Peters, C.J.; Kooi, H.J. van der; Swaan Arons, J. de;J.Chem. Therm. 1987, 19, 395. 25. Goodwin, R.D.; Haynes, W.M.; Thermodynamic properties of propane from 85 to 700 Κ at pressures to 70 MPa: NBS Monograph 170, US Department of Commerce, NBS, 1982. 26. Rowlinson, J.S.; Freeman, P.I.; Pure Appl. Chem. 1961, 2, 329. 27. Rowlinson, J.S.; Swinton, F.L.; Liquids and liquid mixtures. 1982, London, England. 28. Leder, F.; Irani, C.A.; McHenry, J.Α.; AIChE J. 1976, 22, 199. 29. Griffiths, R.B.; J. Chem. Phys. 1974, 60, 195. 30. Scott, R.L.; J.Chem. Phys. 1987, 86, 4106. 31. Hixson, A.W.; Bockelman, J.B.; Trans. Am. Chem. Eng. 1942, 38, 891. RECEIVED May 14, 1992
