Supercritical Fluid Engineering Science - American Chemical Society

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Chapter 5

Multiphase Equilibrium Behavior of a Mixture of Carbon Dioxide, 1-Decanol, and n-Tetradecane C. L. Patton, S. H. Kisler, and Κ. D. Luks Department of Chemical Engineering, The University of Tulsa, Tulsa, OK 74104-3189

The liquid-liquid-vapor (llg) phase equilibrium behavior of the mixture CO + 1-decanol + n-tetradecane was experimentally studied using a visual cell (stoichiometric) technique with pressure discrimination of ± 0.01 bar, achieved by coupling a dead weight gauge to the cell pressure transducer. The portion of the three-phase surface of the ternary system examined is bounded by the llg loci of the binary mixtures CO + 1-decanol and CO + n­ -tetradecane and from above by an upper critical end point (l-l=g) locus. Phase compositions and molar volumes are reported for the two liquid phases along the 298.15 Κ isotherm. This three-phase llg surface is unusual in that it has a two-phase (lg) "hole" in it which is completely bounded by l=l-g critical end points. A rationale for the occurrence of this hole in the llg surface is offered. 2

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During the past several years, our group has been engaged i n the study of l i q u i d - l i q u i d - v a p o r ( l l g ) phase e q u i l i b r i a i n well-defined systems such as C0 + hydrocarbon, ethane + hydrocarbon and nitrous oxide + hydrocarbon binary mixtures, where the hydrocarbon has frequently been one of the members of the homologous series of η-paraffins or nalkylbenzenes. The goals of these studies were to map out the patterns of the multiphase e q u i l i b r i a of these prototype mixtures i n thermodynamic phase space and to generate phase equilibrium data that would support the prediction of phase e q u i l i b r i a within and near regions of l l g i m m i s c i b i l i t y . With the studies of these e s s e n t i a l l y nonpolar systems as a point of reference, we have undertaken a series of studies of the multiphase e q u i l i b r i a of binary mixtures composed of both polar and nonpolar species. Studies were performed on the l l g i m m i s c i b i l i t y behavior of c e r t a i n members of the homologous series of ethane + 1-alkanol, C0 + 1-alkanol, and nitrous oxide + 1-alkanol binary mixtures (1-3). More 2

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recently, boundaries and l i q u i d phase properties of the l l g surface f o r the ternary mixture ethane + methanol + 1-decanol were reported (4). This system has a " f o l d " i n i t s l l g surface when projected onto pressure-temperature coordinates. The presence of a f o l d i n such ternary systems where the solutes are r e l a t i v e l y n o n v o l a t i l e i n the gas solvent has implications with respect to the s e p a r a b i l i t y function of the solutes between the two l i q u i d phases, often marking a s h i f t i n that function across the value of unity at the f o l d (5,6). In t h i s present study, the goals are to map out the three-phase l l g region for a system composed of the gas solvent C0 and the solutes l-decanol and n-tetradecane and examine the s e p a r a b i l i t y of the solutes between the two l i q u i d phases. The solutes are r e s p e c t i v e l y polar and nonpolar i n nature, although the s i z e of the p a r a f f i n i c part of 1decanol diminishes to some degree the influence of i t s dipole moment [This decrease i n influence of the hydroxyl group with chain length i n 1-alkanols has been discussed by Teja et a l . ( 7 ) ] . The solutes are s i m i l a r l y l l g - i m m i s c i b l e with CO,, with t h e i r respective l l g l o c i located just below the C0 vapor pressure curve i n pressure-temperature space and extending from a s l l g point upwards to an UCEP (1-1-g) point. For C0 + 1-decanol, the s l l g point i s at 32.14 bar, 270.54 Κ (2), and the UCEP i s at 77.48 bar, 307.15 K. For C0 + n-tetradecane, the s l l g point i s at 30.82 bar, 269.10 Κ (8), and the UCEP i s at 82.54 bar, 311.15 K. The binary UCEP's c i t e d here were measured i n t h i s present study and are close to the values reported by Lam et a l . (2) and Hottovy et a l (8). Studies of the binary l l g system C0 + n-tetradecane have also been performed by van der Steen et a l . (9) and Laugier et a l . (10). The n-tetradecane appears to e x h i b i t higher v o l a t i l i t y than 1-decanol i n the configuration of a C0 -rich l l g binary system. At a given temperature, the l l g pressure f o r C0 + n-tetradecane i s less than that of C0 + 1-decanol, as much as 1.5 bar less at about 307 K. The ternary l l g surface has a f o l d when projected onto pressure-temperature coordinates. Reported i n t h i s study are the P-T l o c i of the ternary UCEP's and the c r i t i c a l end points l = l - g that bound the two-phase l g hole i n the l l g surface. Also reported i s the barotropic inversion locus where the l and 1 mass d e n s i t i e s are i d e n t i c a l . At temperatures below t h i s locus, the C0 -rich 1 phase i s denser than the s o l u t e - r i c h l phase. Compositions and molar volumes for the two l i q u i d phases are reported at Τ = 298.15 Κ. 2

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Experimental A d e t a i l e d d e s c r i p t i o n of the apparatus i s given i n F a l l and Luks (11) and i s updated i n Jangkamolkulchai and Luks (12). The experimental procedure employs a v o l u m e t r i c a l l y c a l i b r a t e d v i s u a l glass equilibrium c e l l with a t y p i c a l t o t a l volume of 8 to 9 cm . Known amounts of 1-decanol and n-tetradecane are loaded i n the c e l l , at which time the vapor space i s thoroughly flushed with C0 . Measured amounts of C0 are added to the c e l l from a high-pressure bomb. The c e l l contents are then brought to equilibrium by a magnetically actuated s t e e l b a l l s t i r r e r . Mass balances f o r conjugate measurements at a given temperature and pressure are used to c a l c u l a t e the compositions and molar volumes. The conjugate measurements herein are respectively dominant (volumetrically) i n each of the l i q u i d phases l and 1 . (Since ntetradecane and 1-decanol are r e l a t i v e l y non-volatile, the vapor phase 1

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1-Decanol, and n-Tetradecane

i s assumed to be pure C0 ; thus, only l - and lj-dominant measurements are employed.) The c e l l temperature was measured with a Pt- resistance thermometer to an estimated accuracy of + 0.02 K. Pressures were measured with a precision of +0.01 bar using pressure transducers c a l i b r a t e d with a dead-weight gauge after each experiment. Phase volumes i n the v i s u a l c e l l were determined using a cathetometer to an accuracy of + 0.005 cm'. The 1-decanol and n-tetradecane were obtained from A l d r i c h Chemical Co. with a stated p u r i t y of 99+ mol %. Chromatographic analyses of these solutes were consistent with the stated p u r i t y . No further p u r i f i c a t i o n was performed. The C0 was purchased from A i r Products and Chemicals, Inc. as "Coleman Grade" with a stated p u r i t y of 99.99 mol %. The C0 was f i r s t transferred to an evacuated storage bomb immersed i n an i c e bath. The vapor phase was vented to remove any l i g h t gas i m p u r i t i e s . The p u r i t y was v e r i f i e d by l i q u e f y i n g the C0 at 298.15 K. The vapor pressure at t h i s temperature was within 0.03 bar of that reported by Vargaf t i k (13). In addition, the c r i t i c a l temperature and pressure were within 0.06 Κ and 0.03 bar of those reported by Vargaftik (13). Single-phase C0 data were obtained from Angus et al.(14) f o r performing necessary stoichiometric computations. 2

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Results Figure 1 i s a projection of the ternary l l g surface onto pressuretemperature space. The pressure coordinate i s represented by the difference between the actual pressure and the pure C0 vapor pressure (extrapolated above the c r i t i c a l temperature, i f necessary). Experimental data corresponding to (1) the l-l=g boundary, (2) a l g hole bounded by l=l-g points, and (3) a locus of barotropic inversion points (i.p.) i n Figure 1 are presented i n Table I . The l l g surface i s bounded compositionally by the two binary l l g l o c i for C0 n-tetradecane and C0 + 1-decanol, and from above (temperature-wise) by the ternary l-l=g locus. No data were taken on the s l l g - t y p e boundaries that would terminate the l l g surface from below. The temperatures and pressures of the c r i t i c a l end points l-l=g and l=l-g are estimated (by our experience) to be precise to + 0.05 Κ and + 0.03 bar respectively. Figure 2 i s a plot of the l g hole i n terms of temperature and the mole f r a c t i o n X of n-tetradecane on a C0 -free basis. Since the two solutes are e s s e n t i a l l y nonvolatile at the temperatures of the l g hole and l = l - g i s the nature of the c r i t i c a l end points bounding the l g hole, the values of X w i l l be v i r t u a l l y i d e n t i c a l with the loadings of the solutes, which are performed outside the experimental bath using a scale s e n s i t i v e to + 0.00001 g and r e l i a b l e to at least + 0.0001 g. Thus the values of X i n Table I and Figure 2 should be precise to at least + 0.001 f o r the unstable l i q u i d phase at the boundary of the l g hole. The barotropic inversion points i n Table I and Figure 1 for the ternary system extend from about 283 to 293 K. Temperatures and pressures for these points are d i f f i c u l t to determine accurately since the equal densities of the two l i q u i d phases n a t u r a l l y prevents these phases from s e t t l i n g out separately. Thus, the data i n Table I for t h i s locus should be considered as rough estimates. The locus i s i n two parts, being interupted by the l g hole presented above. A barotropic inversion of the l i q u i d phases had been reported e a r l i e r at about 283 Κ f o r the binary l l g system C0 + 1-decanol (3) and at 293 Κ f o r the 2

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