Supercritical fluids triple point

The liquid state exists only below the critical point pressure and above the triple point pressure. When a vapor below the triple point pressure is cooled down, we encounter a discontinuous and abrupt phase change to solid but, above the critical point pressure, a cooled vapor turns into the supercritical state where the properties of the fluid... 

Figure 1. Solid-liquid-gas-supercritical fluid phase diagram. TP = triple point, CP - critical point, Pc = critical pressure, Tc = critical temperature.

Figure 3.32 Phase diagram illustrating the domains of the solid (S), gaseous (G) and liquid (L) phases as a function of pressure (p) and temperature (T). tp is the triple point, at which three phases co-exist, cp is the critical point, which forms the end of the vapour pressure curve (between tp and cp). The area in the top right corner indicated by SF represents the domain of supercritical fluids.

On a phase diagram, the triple point is where three phases are in equilibrium. Above the critical point, a substance is a supercritical fluid. 

Figure 1. Phase diagram for CO 2 "with coordinates of the triple point (T(f, PtrJ and the liquid-gas critical point T, P. The loci of the supercritical fluid (SCF) domain is also shown.

FIGURE 9.1 Idealized phase diagram for a pure compound describing the formation of a supercritical fluid. Abbreviations T, triple point CP, critical point Pc, critical point pressure Tc, critical point temperature. 

Nonvariant regions are points in the phase diagram (triple points r, i = 3) indicating equilibrium of three phases (g, 1, s or s, s, 1 or s, s, g, etc.). The vapor pressure curve (equilibrium 1 g) terminates in a critical point C. Above the critical temperature, liquid and gaseous states are indistinguishable (supercritical fluid state). 

In Fig. 11, we draw schematically the case of fluid-solid phase behavior for the Type-I fluid mixture water-NaCl. For critical temperatures this far apart, the three-phase line Sb-L-V from the low-temperature quadruple point (where four three-phase lines meet) to the solutes triple point develops a high maximum that reaches above water s critical pressure and temperature. If a salt solution is heated at a pressure above the critical pressure of water, the vapor-liquid critical line is crossed first, and a two-phase L-V region entered. At high enough temperature the three-phase line Sb-L-V may be crossed, and solid salt will form. Thus supercritical water, fully miscible with air constituents and hydrocarbons, becomes a poor solvent for salts. 

When the triple point temperature of the solute increases, the solid phase beings to intrude into the fluid part of the phase diagram. We refer to Peters [ 16] for a full treatment. In Fig. 12, we show the special case where the solid phase intrudes into the supercritical... 

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]).

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