Carbon dioxide critical locus

Two early studies of the phase equilibrium in the system hydrogen sulfide + carbon dioxide were Bierlein and Kay (1953) and Sobocinski and Kurata (1959). Bierlein and Kay (1953) measured vapor-liquid equilibrium (VLE) in the range of temperature from 0° to 100°C and pressures to 9 MPa, and they established the critical locus for the binary mixture. For this binary system, the critical locus is continuous between the two pure component critical points. Sobocinski and Kurata (1959) confirmed much of the work of Bierlein and Kay (1953) and extended it to temperatures as low as -95°C, the temperature at which solids are formed. Furthermore, liquid phase immiscibility was not observed in this system. Liquid H2S and C02 are completely miscible. 

For mixtures of carbon dioxide and hydrogen sulfide, a binary critical locus extends from the critical point of COz and terminates at the critical point of H2S. This is the case for H2S and COz, but not for all binary mixtures. 

For binary mixtures of hydrogen sulfide and carbon dioxide, the critical locus extends uninterrupted from the critical point of C02 to that of H2S. The critical point of a binary mixture can be estimated from the next two figures. Figure 3.4 shows the critical temperature as a function of the composition, and figure 3.5 gives the critical pressure. 

Huron, M.-J., G.-N. Dufour, and J. Vidal. 1978. "Vapor-Liquid Equilibrium and Critical Locus Curve Calculations with the Soave Equation for Hydrocarbon Systems with Carbon Dioxide and Hydrogen Sulphide" Fluid Phase Equil., 1 247-265. 

Figure 7.4. Phase diagrams for type I (top) and type II (bottom) binary mixtures with carbon dioxide as one component (L = liquid and v = vapor). The UCST line indicates the temperature at which the two immiscible liquids merge to form a single liquid phase. The critical mixture curve is the locus of critical mixture points spanning the entire composition range. (From ref. [44] American Chemical Society).

Type I mixtures have continuous gas-liquid critical line and exhibit eomplete miseibil-ity of the liquids at all temperatures. Mixtures of substances with eomparable eritieal properties or substances belonging to a homologous series form Type I unless the size difference between components is large. The critical locus could be convex upward with a maximum or concave down with a minimum. Examples of Type I mixtures are Water -l-1-propanol, methane -i- n-butane, benzene -I- toluene, and carbon dioxide -I- n-butane. 

Type II have systems have liquid-liquid immiscibility at lower temperatures while locus of liquid-liquid critical point (UCST) is distinct from gas liquid critical line. Examples include water -l- phenol, water -l- tetralin, water -l- decalin, carbon dioxide -l- n-oetane, and carbon dioxide -I- n-decane. 

When the mutual immiscibility of two components becomes large, the locus of liquid-liquid critical solution moves to higher temperatures and it eventually interacts with gas-liquid critical curve disrupting the gas-liquid locus. This particular class is type in and some examples include water -l- n-hexane, water -l- benzene, carbon dioxide -t- n-tridecane, and carbon dioxide + water. 

Figure 1.3 Phase behaviour of carbon dioxide/water system at temperatures between the critical hydrate temperature and the upper critical solution temperature, (a) Typical pressure/composi-tion diagram for carbon dioxide/water (a Class B2 system) at temperatures below the critical temperature of carbon dioxide but above the critical hydrate formation temperature. Data for arms B and C are shown in (b) and (c) respectively, (b) Solubility of liquid CO2 in water as a function of temperature and pressure (arm C in (a)), (c) Solubility of water in liquid CO2 as a function of temperature and pressure (arm B in (a)), (d) The three phase pressure curve compared with the vapour pressure curve of carbon dioxide showing the critical locus CsU (i.e. locus of points such as C on (e) where vapour properties merge with those of solvent-rich liquid). (Data reference [75].) (e) Detail of the isothermal pressure/composition diagram at 25°C (on left) and at temperature between Tc and Tu (on right). Subscripts 1 and 2 denote water-rich and C02-rich phase. Critical point C is shown as blocked-in circle. (Data reference for (b) and (c) is [81].)...

Figure 1.6 Pressure/composition diagrams for carbon dioxide/water system at temperatures above 250°C. Critical points are shown as blocked-in circles. Position of minimum M in critical locus estimated from data assembled in ref. [76] (see also ref. [79]).

Figure 1,9 Typical critical locus curves for Class B systems, (a) General forms for the locus curves for Class B systems. Branch I is very limited in extent in some systems. Branch II behaviour of types a, b, c, d, e (in order of increasing dissimilarity of the components) is known. Cs and Ch are the vapour/liquid critical points for the volatile component and less volatile component. (In the present context there are the solvent and solute.) In many cases of interest Ch will not be accessible due to thermal decomposition, (b) Critical loci for system carbon dioxide/water. (Cw is the critical point of water.) (c) Critical loci for system carbon dioxide//i-hexadecane.

The critical locus curve for the carbon dioxide/water system is approximately of the form shown by curve (d) in Figure 1.9 (a) that of the carbon dioxide/squalane system is more nearly described by curve (c), though the critical point of squalane is unattainable due to thermal decomposition. Critical loci for these and other systems in which carbon dioxide, ethane, ethylene and propane are the volatile component are given by Schneider [17]. 

---