Retrograde condensation, phase equilibria

Other phenomena can be simply explained by the fact that the critical pressure and temperature for a given mixture is not, as it happens for a pure fluid, the maximum temperature and pressure that allows the coexistence of a vapour and liquid phase in equilibrium. Retrograde condensation phenomena can be easily explained in this way. 

Retrograde Condensation For near-ideal mixtures, the intersection of the (p, X2 or y ) isotherm with the critical locus occurs at the maximum in the (p, y2) equilibrium line. For example, an enlargement of the two-phase (p, xi or yi) section for (xi oryi)Ar + (x2 or j2)Kr at T— 177.38 K is shown in Figure 14.12. The point of intersection with the critical locus at point (c) gives rise to an... 

Figure 14.12 The top of the (vapor + liquid) isotherm for ( iAr + Kr) at T = 177.38 K. Point (c) is the intersection with the critical locus. The curve marked g gives the composition of the vapor phase in equilibrium with the liquid curve marked 1. The tubes shown schematically to the right demonstrate the changes in phase when the fluid is compressed at a mole fraction given by (a), or at a mole fraction corresponding to (b) where retrograde condensation occurs. Reprinted with permission from M. L. McGlashan, Chemical Thermodynamics, Academic Press, London, 1979, p. 276.

There are several areas of interest in chemical processing that should be reviewed in this chapter. These areas include retrograde condensation, polymers, and electrolytes. In each case, the material is more extensive than can be covered in any detail in this chapter. Reference should be made to Prausnitz, Lichtenthaler, and Gomes de Acevedo. Furthermore, the principles covered in this chapter form the basis for viewing and calculating the phase equilibrium that applies. The subjects of polymers and electrolytes are so complex that they require a chapter by themselves. However, a brief statement follows describing retrograde condensation. 

The phase equilibrium curve is calculated for the methane -i- n-butane mixture at 330 K (Fig. 2). The force field model used reproduces experimental data [3] on methane solubility in liquid butane rather well up to 80 atm. It reproduces the existence of the retrograde condensation region for the mixture under consideration at this temperature. The existence of the region follows from the fact that the phase equilibrium curve does not reach 100 % methane molar fraction. 

The lower limit of the region of retrograde condensation CC is often called critical condensation point. At CC the highest concentration of the low boiler in the vapor phase is obtained in equilibrium with the liquid phase. At this point the dew-point curve runs vertically and thus the slope for a given temperature is... 

The right points on the plots of Figs. 8 and 9 correspond to the limits for capillary condensation. In the retrograde region, these limits are determined by the fact that the equilibrium liquid phase approaches the spinodal boundary. In the region of normal condensa-... 

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