Supercritical regions binary systems

Chapter 14 describes the phase behavior of binary mixtures. It begins with a discussion of (vapor -l- liquid) phase equilibria, followed by a description of (liquid + liquid) phase equilibria. (Fluid + fluid) phase equilibria extends this description into the supercritical region, where the five fundamental types of (fluid + fluid) phase diagrams are described. Examples of (solid + liquid) phase diagrams are presented that demonstrate the wide variety of systems that are observed. Of interest is the combination of (liquid + liquid) and (solid 4- liquid) equilibria into a single phase diagram, where a quadruple point is described. 

Experimental results are presented for high pressure phase equilibria in the binary systems carbon dioxide - acetone and carbon dioxide - ethanol and the ternary system carbon dioxide - acetone - water at 313 and 333 K and pressures between 20 and 150 bar. A high pressure optical cell with external recirculation and sampling of all phases was used for the experimental measurements. The ternary system exhibits an extensive three-phase equilibrium region with an upper and lower critical solution pressure at both temperatures. A modified cubic equation of a state with a non-quadratic mixing rule was successfully used to model the experimental data. The phase equilibrium behavior of the system is favorable for extraction of acetone from dilute aqueous solutions using supercritical carbon dioxide. 

Type III behavior indicates the most extreme asymmetry between the components of a binary mixture. Nearly all H2 systems supply striking examples of type III behavior. CO2 mixtures with 2,5-hexanediol and 1-dodecanol are also classified as type III. The system CO2 -I- n-tridecane is peculiar because it was classified by van Konynenburg and Scott as type III, whereas Enick et al. have classified it as type IV, owing to experimental identification of a three-phase region. The system CO2 -I- n-tetradecane is a variation on type III, where the solute-rich locus terminates in a solid(wax)-liquid-liquid boundary. Several important systems fall into a similar category. For example, CO2 + naphthalene is commonly used as a model system for supercritical extraction. The naphthalene system differs from the n-tetradecane system in that the solute-rich locus terminates at a higher temperature... 

The solvent power of the supercritical phase, and the selectivity of the column can be enhanced by using a cosolvent, usually called an entrainer or modifier in this application. A two-phase region may be inadvertently entered at P-T conditions for which the pure supercritical fluid is in one phase. Knowledge of the phase behavior of the binary system is therefore necessary. 

Figure 14. p(T) diagrams (a, b) and selected isothermal p(x) sections (c, d) for binary systems with the occurrence of a solid phase in the supercritical region fluid-fluid in simple cases (for symbols see legend of Fig. 4, Tr = triple point, Q = quadruple point according to [4,12]).

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. 

Values of the thermodynamic functions based on experiments for the two binary systems H20-NaCl and H2O-CO2 in the fluid one-phase region at high temperatures and pressures have not yet been sufficiently determined. New work in this field is being done at present. As one example. Fig, 7 shows partial mola volumes of NaCl in H2O which were calculated recently (17) from a critical compilation of existing data. Fig. 8 gives excess Gibbs free energy values of H2O-CO2 mixtures for two supercritical temperatures. ... 

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