Liquid—vapor critical points

Fig. 6. Qualitative pressure—temperature diagrams depicting critical curves for the six types of phase behaviors for binary systems, where Ca or C corresponds to pure component critical point G, vapor L-, liquid U, upper critical end point and U, lower critical end point. Dashed curves are critical lines or phase boundaries (5). (a) Class I, the Ar—Kr system (b) Class II, the C02—C8H18 system (c) Class III, where the dashed lines A, B, C, and D correspond to the H2-CO, CH4-H2S, He-H2, and He-CH4 system, respectively (d) Class IV, the CH4 C6H16 system (e) Class V, the C2H6 C2H5OH...

At the critical point, the vapor, liquid, and critical volumes are the same, (V- I = 0, and expansion gives... 

F liquid + vapor G liquid + vapor (critical point) H vapor the first dashed line (at the lower temperature) is the normal melting point, and the second dashed line is the normal boiling point. The solid phase is denser because of the positive slope of the solid/liquid equilibrium line. 

C is correct. Above the critical point, liquid and vapor water have the same density. The critical temperature will be the highest temperature on the graph where the two lines meet. 

The critical point refers to the certain combination of temperature and pressure at which the liquid density is equal to the vapor density. At its critical point, liquid will become vapor and is easily removed. We cannot directly remove water using its critical point because the temperature and pressure of the critical point (374°C and 22 MPa) are too high and may damage the specimen. Alternatively, we can replace water with a transitional fluid that has a critical point with lower temperature and pressure. Liquid CO2 or Freon is often used as the transitional fluid. The critical point for liquid CO2 is 31.1 °C and 7.4 MPa. The common procedure is described as follows. First, water content in a specimen is removed by dehydration with an ethanol series (30, 50, 75 and 100%). Then, the dehydrated specimen is transferred into an ethanol-filled and cooled chamber in the critical-point drying apparatus. The transitional fluid is introduced until it completely displaces ethanol in the chamber. The chamber is gradually heated and pressurized to reach the critical point of the transitional fluid. After reaching the critical point, the transitional fluid vaporizes, and this vapor is slowly released from the chamber until atmospheric pressure is reached. Then, we can retrieve the intact, dry specimen from the chamber. 

Water is an ecologically safe substance widespread throughout nature. Below the critical point, the vapor pressure curve separates the liquid and gaseous phase. 

The numerical value of the exponent ft proves to be quite close to that of the critical exponent 0.35 over the entire range of pressures. Generally the exponent A takes on different values on the vapor and liquid branches of the saturation boundary. The numerical range of X easily can be estimated, using the critical point, the low-pressure vapor, and the low-pressure liquid as reference points. Near the critical point, the vapor-pressure equation is... 

In making a preliminary selection of feasible separator types, we find our experience to indicate that those operations marked by an a in Table 1.1 should be given initial priority unless other separation operations are known to be more attractive. To compare the preferred operations, one will find certain physical properties tabulated in handbooks and other references useful. " " These properties include those of the ptire species—normal boiling point, critical point, liquid density, melting point, and vapor pressure—as well as those involving the species and a solvent or other MSA—liquid diffusivity, gas solubility, and liquid solubility. In addition, data on thermal stability are important if elevated temperatures are anticipated. 

These conditions identify both vapor-liquid and liquid-liquid critical points. For vapor-liquid equilibria, they are satisfied when the spinodal coincides with the vapor-liquid saturation curve. However, that point need not occur either at the maximum in the saturation envelope or at the maximum in the spinodal see Figure 8.12. Along a spinodal the one-phase metastable system is balanced on the brink of an instability at a critical point that balance coincides with a two-phase situation and the resulting fluctuations cause critical opalescence, just as they do at pure-fluid critical points. 

Supercritical fluids are defined as a fluid at a pressure above the critical pressure and a temperature above the critical temperature. Below the critical point, the vapor-the pressure curve separates the Hquid and gaseous phase. The vapor pressure ends up at the critical point. Beyond the critical point, the density of the fluids can be varied continuously from liquid-hke to gas-like values without phase transition. This variability of density corresponds to diversity of properties. Supercritical fluids are tunable solvents [26] for which the properties can be adjusted as a function of temperature and pressure. This chapter focuses on the utilization of supercritical CO2 and water. The properties of these two supercritical fluids will now be introduced. 

At room temperature and atmospheric pressure, ammonia is a pungent, colorless gas. It may be compressed and cooled to a colorless liquid. Between the melting and critical points, liquid ammonia exerts a vapor pressure which increases with rising temperature. When anhydrous ammonia in a closed container is in equilibrium with anhydrous ammonia vapor, the pressure within... 

The solid/liquid border extends upward indefinitely. However, the vapor/liquid border terminates at a condition called the critical point. Liquid and vapor are indistinguishable above the critical point. Vapor transforms into liquid without condensation. The vapor gets denser and denser until it is a liquid. Why That s thermodynamics -we warned you in the first chapter that we would occasionally have to postpone why until later in the curriculum. The critical point is at very high temperature and pressure for substances that are liquid at 25°C and 1 atm. For H2O, the critical temperature is 374°C and the critical pressure is 218 atm. For n-hexane, the critical point is 234°C and 29 atm. 

Expression 6.1g, known as the Clausius-Clapeyron equation, provides an excellent representation of the solid vapor pressure but is valid only over a limited temperature range of the liquid vapor pressure. It breaks down, in particular, near the critical point where vapor and liquid densities approach each other in magnitude. Various semiempirical modifications have been proposed as a result, among which the Antoine equation (1888) has proved to be particularly successful. If fakes the form... 

Type of data critical points of vapor-liquid equilibrium ... 

Va.por Pressure. Vapor pressure is one of the most fundamental properties of steam. Eigure 1 shows the vapor pressure as a function of temperature for temperatures between the melting point of water and the critical point. This line is called the saturation line. Liquid at the saturation line is called saturated Hquid Hquid below the saturation line is called subcooled. Similarly, steam at the saturation line is saturated steam steam at higher temperature is superheated. Properties of the Hquid and vapor converge at the critical point, such that at temperatures above the critical point, there is only one fluid. Along the saturation line, the fraction of the fluid that is vapor is defined by its quaHty, which ranges from 0 to 100% steam. 

The Kellogg and DePriester charts and their subsequent extensions and generahzations use the molar average boiling points of the liquid and vapor phases to represent the composition effect. An alternative measure of composition is the convergence pressure of the system, which is defined as that pressure at which the Kvalues for aU the components in an isothermal mixture converge to unity. It is analogous to the critical point for a pure component in the sense that the two... 

Physical and Chemical Properties - Physical State at 15 X and 1 atm. Liquid Molecular Weight S5.W -, Boiling Point at 1 atm. Decomposes-, Freezing PoirU -5.8, -21,252 Critical Temperature Not pertinent Critical Pressure Not pertinent Specific Gravity 0.925 at 25 °C (liquid) Vapor (Gas) Density Not pertinent Ratio of Specific Heats of Vapor (Gas) 1.074 Latent Heat of Vaporization No... 

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