Supercritical fluids phase equilibria

Chen, S.J., et al.. Mass-spectrometer composition probe for batch cell studies of supercritical fluid phase-equilibria. Journal of Chemical and Engineering Data, 1993. 38(2) p. 211-216. 

Keywords Carbon dioxide Polyethylene glycol Phase behavior Biphasic solvent system Supercritical fluids Phase equilibrium... 

Chapters 17 and 18 use thermodynamics to describe solutions, with nonelectrolyte solutions described in Chapter 17 and electrolyte solutions described in Chapter 18. Chapter 17 focuses on the excess thermodynamic properties, with the properties of the ideal and regular solution compared with the real solution. Deviations from ideal solution behavior are correlated with the type of interactions in the liquid mixture, and extensions are made to systems with (liquid + liquid) phase equilibrium, and (fluid -I- fluid) phase equilibrium when the mixture involves supercritical fluids. 

Fundamental studies on the adsorption of supercritical fluids at the gas-solid interface are rarely cited in the supercritical fluid extraction literature. This is most unfortunate since equilibrium shifts induced by gas phase non-ideality in multiphase systems can rarely be totally attributed to solute solubility in the supercritical fluid phase. The partitioning of an adsorbed specie between the interface and gaseous phase can be governed by a complex array of molecular interactions which depend on the relative intensity of the adsorbate-adsorbent interactions, adsorbate-adsorbate association, the sorption of the supercritical fluid at the solid interface, and the solubility of the sorbate in the critical fluid. As we shall demonstrate, competitive adsorption between the sorbate and the supercritical fluid at the gas-solid interface is a significant mechanism which should be considered in the proper design of adsorption/desorption methods which incorporate dense gases as one of the active phases. 

They showed that the Peng-Robinson equation of state using mixing rules based on conformal solution theory can predict the fluid phase equilibrium of high molecular weight liquids in supercritical fluids more accurately than others (18.19). 

The miscibility of lactic acid and lactide with supercritical CO2 was studied by Gregorowicz [24]. The solubility of the monomer in a supercritical solvent depends on temperature, pressure, and overall composition of the reacting mixture equilibrium between different phases. It is possible that the concentration of the monomer in the supercritical fluid phase may vary over a wide range. In general, the solute solubility in compressed carbon dioxide increases as pressure increases from 50 to 200 bar (Figure 2.1). At pressures below 130 bar, the solubility decreases as temperature increases. Thus, it is... 

There is a similar expression for polymer activity. However, if the fluid being sorbed by the polymer is a supercritical gas, it is most useful to use chemical potential for phase equilibrium calculations rather than activity. For example, at equilibrium between the fluid phase (gas) and polymer phase, the chemical potential of the gas in the fluid phase is equal to that in the liquid phase. An expression for the equality of chemical potentials is given by Cheng (12). 

Supercritical fluids represent a different type of alternative solvent to the others discussed in this book since they are not in the liquid state. A SCF is defined as a substance above its critical temperature (Tc) and pressure (Pc)1, but below the pressure required for condensation to a solid, see Figure 6.1 [1], The last requirement is often omitted since the pressure needed for condensation to occur is usually unpractically high. The critical point represents the highest temperature and pressure at which the substance can exist as a vapour and liquid in equilibrium. Hence, in a closed system, as the boiling point curve is ascended, increasing both temperature and pressure, the liquid becomes less dense due to thermal expansion and the gas becomes denser as the pressure rises. The densities of both phases thus converge until they become identical at the critical point. At this point, the two phases become indistinguishable and a SCF is obtained. 

Optimizing solvents and solvent mixtures can be done empirically or through modeling. An example of the latter involves a single Sanchez-Lacombe lattice fluid equation of state, used to model both phases for a polymer-supercritical fluid-cosolvent system. This method works well over a wide pressure range both volumetric and phase equilibrium properties for a cross-linked poly(dimethyl siloxane) phase in contact with CO2 modified by a number of cosolvents (West et al., 1998). 

A commercial Pt-Sn on y-Al203 catalyst showed 2-3 times higher activity in the catalytic dehydrogenation of a mixture of Ci0—CJ2 alkanes to linear monoalkenes when applied in a supercritical phase.332 The strong shift of the equilibrium under supercritical conditions is believed to be due to the high solubility of the product in supercritical fluids or the rapid desorption of alkenes from the catalyst surface. 

The method is applicable only to binary two-phase systems. Since only the supercritical-fluid rich effluent phase is sampled, the composition of the equilibrium heavy phase cannot be determined. However, there are some pitfalls in this technique, which the experimentalist should be aware of ... 

The basic for developing a high pressure liquid extraction unit is the phase equilibrium for the (at least) ternary system, made up of compound A and compound B, which have to be separated by the supercritical fluid C. Changing pressure and temperature influences on one hand the area of the two phase region, where extraction takes place, and on the other hand the connodes, representing the equilibrium between extract and raffinate phase. 

We refer to Fig. 6.7-1. Reaching once equilibrium between the supercritical fluid SCF1 and the feed in the extractor El is enough for separation. By changing pressure and temperature the produced extract EX1 and raffinate R1 concentrations can be varied following the ternary phase equilibrium. The supercritical solvent-to-feed flow rate ratio affects the amounts of products obtained from a given feed. The apparatus required to apply this method are a normal stirred reactor, where contact of the two phases takes place, followed by a separator eliminating the extract from the extraction gas, which is recycled back to the extractor. 

Figure 14.13 The three-phase (liquid + liquid + gas) region for a type II system. The shaded area represents the surface where three phases are in equilibrium. Reproduced with permission from W. B. Streett, Chapter 1 in Chemical Engineering at Supercritical Fluid Conditions, M. E. Paulaitis, J. M. L. Penninger, R. D. Gray Jr., and P. Davidson, editors, Ann Arbor Science Press, 1983.

For any pure chemical species, there exists a critical temperature (Tc) and pressure (Pc) immediately below which an equilibrium exists between the liquid and vapor phases (1). Above these critical points a two-phase system coalesces into a single phase referred to as a supercritical fluid. Supercritical fluids have received a great deal of attention in a number of important scientific fields. Interest is primarily a result of the ease with which the chemical potential of a supercritical fluid can be varied simply by adjustment of the system pressure. That is, one can cover an enormous range of, for example, diffusivities, viscosities, and dielectric constants while maintaining simultaneously the inherent chemical structure of the solvent (1-6). As a consequence of their unique solvating character, supercritical fluids have been used extensively for extractions, chromatographic separations, chemical reaction processes, and enhanced oil recovery (2-6). 

Stradi, B. A. Stadtherr, M. A. Brennecke, J. F. Multicomponent Phase Equilibrium Measurements and Modeling for the Allylic Epoxidation of traw.y-2-hexen-l-ol to (2R,3R)-( + )-3-Propyloxiranemethanol in High-Pressure Carbon Dioxide. J. Supercrit. Fluid 2001b, 20 (1), 1-13. 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

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