Binary Mixtures of Fluids at Low Pressures

The intersection of those two one-phase lines satisfies (9.2.2) and therefore identifies the vapor-liquid saturation point. The lines for one-phase liquids terminate at the spinodal—they become unstable— and the unstable portion of the van der Waals loop is represented by the broken line on the fP plot. 

We now describe the phase behavior exhibited by binary mixtures at modest pressures. The kinds of behavior observed in Nature include vapor-liquid equilibria (VLE, 9.3.1-9.3.3), azeotropes ( 9.3.4), critical points ( 9.3.5), liquid-liquid equilibria (LLE, 9.3.6), and vapor-liquid-liquid equilibria (VLLE, 9.3.7). When solid-fluid equilibria occur ( 9.4), many (but not all) of the resulting phase diagrams are analogous to their counterparts in fluid-fluid equilibria for example, many liquid-solid diagrams are analogous to vapor-liquid diagrams. 

Consider a mixture of components 1 and 2 in vapor-liquid equilibrium in a closed vessel at temperature T and pressure P. Let the composition of the liquid phase be represented by the mole fraction Xi and that of the vapor by mole fraction i/j. The properties of first importance are the four measurables T, P, X, and yx- absence 

Consider the one-phase liquid state A, which is at 330 K, 25 bar, and overall composition Zi = 0.8. From point A a reversible isothermal expansion will trace the vertical path through points B, C, D, and E. During the expansion along AB, the mixture remains one-phase liquid, but the pressure decreases as the volume expands. At point 

B the pressure has reached 16.8 bar and the mixture is a saturated liquid any further expansion will cause a bubble of vapor to form. Hence the line through B is called the bubble-P curve) it relates pressures to the compositions of saturated liquid mixtures. 

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BINARY MIXTURES OF FLUIDS AT LOW PRESSURES 379 Introducing the partial molar volume from Table 6.2 and rearranging, we have... 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

Sections 9.3-9.S present the common phase behavior of binary mixtures 9.3 describes vapor-liquid, liquid-liquid, and vapor-liquid-liquid equilibria at low pressures 9.4 considers solid-fluid equilibria and 9.5 discusses common high-pressure fluid-phase equilibria. Then 9.6 briefly describes the basic vapor-liquid and liquid-liquid equilibria that can occur in ternary mixtures. This chapter describes many apparently different phase behaviors, and so we try to show when those differences are more apparent than real. The organization is intended to bring out underlying similarities, thereby reducing the number of different things to be learned. 

Whereas gas-gas equilibria had been a curiosity of phase theory as lately as 10 years ago they have now proved to be as important as the classical types of gas-liquid and liquid-liquid equilibria. They are not at all restricted to some special cases but represent the normal type of two-phase equilibrium in systems of components that differ considerably in size, shape, volatility, and polarity, and consequently show a low mutual solubility even up to rather high temperatures. Thus, fluid systems where the phase-separation effects have to be attributed to the solubility of gas in a liquid (or of a liquid in a gas) at normal conditions of temperature and pressure will frequently exhibit gas-gas critical phenomena at higher temperatures some examples for binary mixtures of He, N2, CH4, CO2, etc. with organic liquids and liquid water are given in Sections 2 and 3.f... 

Dahmen, N. and Schneider, G.M. (1993) Phase separation in binary mixtures of trifluo-romethane with propane, butane and xenon at low temperatures between 200 and 280 K, and at pressures up to 200 MPa, Fluid Phase Equilibria 87, 295-308. 

The first chiral separation using pSFC was published by Caude and co-workers in 1985 [3]. pSFC resembles HPLC. Selectivity in a chromatographic system stems from different interactions of the components of a mixture with the mobile phase and the stationary phase. Characteristics and choice of the stationary phase are described in the method development section. In pSFC, the composition of the mobile phase, especially for chiral separations, is almost always more important than its density for controlling retention and selectivity. Chiral separations are often carried out at T < T-using liquid-modified carbon dioxide. However, a high linear velocity and a low pressure drop typically associated with supercritical fluids are retained with near-critical liquids. Adjusting pressure and temperature can control the density of the subcritical/supercritical mobile phase. Binary or ternary mobile phases are commonly used. Modifiers, such as alcohols, and additives, such as adds and bases, extend the polarity range available to the practitioner. 

In figure 3.29c the pressure has now been increased to a value greater than the critical pressure for the SCF-B mixture. The SCF is now miscible in all proportions with B, and the binodal curve no longer intersects the SCF-B binary axis of the ternary diagram. Even at this elevated pressure, the SCF still remains virtually insoluble in A, as would be the case if the supercritical fluid were a low molecular weight hydrocarbon and component A were water (Culberson and McKetta, 1951). As shown in figure 3.29c, the binodal curve intersects the SCF-A binary axis in two locations. The tie lines for the ternary system now indicate that a liquid phase, mostly a mixture of A and B, is in equilibrium with a fluid phase, mainly the SCF with component B. 

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