Supercritical fluid-liquid-phase equilibrium measurement

In this paper, we describe the apparatus we use to make phase equilibrium measurements on mixtures of conqponents with greatly differing volatilities, putting particular emphasis on recent inqprove-ments over the previous version (6-7). We also describe quantitative measurements of the solubility of methyl oleate in supercritical fluids which can provide a basis for choosing a solvent to separate fatty acids in edible oils. In the following paper (JB.) we explore the utility of cubic equations of state to describe the results of supercritical fluid - liquid phase equilibrium measurements. Some additional experimental results on the mutual solubility of methyl linoleate and carbon dioxide are presented there also. 

Experimental Measurement of Supercritical Fluid—Liquid Phase Equilibrium... 

Solubilities of meso-tetraphenylporphyrin (normal melting temperature 444°C) in pentane and in toluene have been measured at elevated temperatures and pressures. Three-phase, solid-liquid-gas equilibrium temperatures and pressures were also measured for these two binary mixtures at conditions near the critical point of the supercritical-fluid solvent. The solubility of the porphyrin in supercritical toluene is three orders of magnitude greater than that in supercritical pentane or in conventional liquid solvents at ambient temperatures and pressures. An analysis of the phase diagram for toluene-porphyrin mixtures shows that supercritical toluene is the preferred solvent for this porphyrin because (1) high solubilities are obtained at moderate pressures, and (2) the porphyrin can be easily recovered from solution by small reductions in pressure. 

Statistical mechanics, the science that should yield parameters like A/x , is hampered by the multibody complexity of molecular interactions in condensed phases and by the failure of quantum mechanics to provide accurate interaction potentials between molecules. Because pure theory is impractical, progress in understanding and describing molecular equilibrium between phases requires a combination of careful experimental measurements and correlations by means of empirical equations and approximate theories. The most comprehensive approximate theory available for describing the distribution of solute between phases—including liquids, gases, supercritical fluids, surfaces, and bonded surface phases—is based on a lattice model developed by Martire and co-workers [12, 13]. 

Because of the close relationship between the MNM transition and the vapor-liquid transition, it is to be expected that immiscibility in the mercury-helium system reaches up to the critical point, or even into the supercritical region. This expectation is confirmed by measurements of the phase diagram at very low helium concentrations and at pressures close to the critical pressure of pure mercury. The experiments extend up to 1610 °C and to pressures up to 3325 bar (Marceca et al., 1996). The p — T — X phase equilibrium surface obtained is qualitatively like the one shown schematically in Fig. 6.4 for a binary fiuid-fluid system of the first kind. The critical line starts at the critical point of pure mercury (Tc(l) = 1478 °C, Pc(l) = 1673 bar) and runs to higher temperatures and pressures as the helium composition X2 increases. 

Ratzsch MT, Browarzik D, Kehlen H (1989) Refined continuous thermodynamic treatment for the liquid-liquid equilibrium of copolymer solutions. J Macromol Sci Chem A 26 903-920 Ratzsch MT, Kehlen H, Browarzik D, Schirutschke M (1986) Cloud-point curves for the system copoly(ethylene-vinyl acetate) plus methylacetate. Measurement and prediction by continuous thermodynamics. J Macromol Sci Chem A 23 1349-1361 Ratzsch MT, Browarzik D, Kehlen H (1990) Liquid-liquid equilibrium of copolymer solutions with broad and asymmetric chemical distribution. J Macromol Sci Chem A 27 809-830 Browarzik C, Browarzik D, Kehlen H (2001) Phase-equilibrium calculation for solutions of poly(ethylene-co-vinyl acetate) copolymers in supercritical ethylene using a cubic equation of state. J Supercrit Fluids 20 73-88... 

Isobaric-isothermal methods are often also called dynamic methods. One or more fluid streams are pumped continuoirsly into a thermostated equilibriirm cell. The pressure is kept constant during the experiment by controlling an effluent stream, irsually of the vapor phase. One can distinguish between continuorrs-flow methods and semi-flow methods. In continuous-flow methods, both phases flow throrrgh the eqrrihbrirrm cell. They can be used only for systems where the time needed to attain phase equilibrium is sufficiently short. Therefore, such equipment is usually not applied to polymer solutions. In semi-flow methods, only one phase is flowing while the other stays in the equilibrium phase. They are sometimes called gas-saturation methods or pure-gas circulation methods and can be used to measure gas solubilities in liquids and melts or solubilities of liquid or solid substances in supercritical fluids. 

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