Phase Equilibria Including a Vapor Phase
As already presented in Sect. 1.2, some of the atoms or molecules tied to a solid or liquid, can, at a fixed temperature, leave the condensed phase. In a closed system a certain vapor pressure p is established, which depends, according to Eq. (1.1b), on t
- PDF / 2,261,292 Bytes
- 19 Pages / 439.37 x 666.142 pts Page_size
- 34 Downloads / 264 Views
Phase Equilibria Including a Vapor Phase
S.l Vapor-Liquid Equilibrium in a One-Component System As already presented in Sect. 1.2, some of the atoms or molecules tied to a solid or liquid, can, at a fixed temperature, leave the condensed phase. In a closed system a certain vapor pressure p is established, which depends, according to Eq. (1.1 b), on the temperature. Equation (1.1 b) is only valid when the enthalpy of vaporization dH v is independent of the temperature. This is not always true. In such cases, to represent the vapor pressure as a function of temperature, the following expression has been found applicable:
10g(~)= ~+ bolOg(~)+coT+d Po T To
(5.1a)
The constant a has the dimension of [K], c the dimension [11K]; band dare dimensionless. Further we have Po = 1 Pa, To = 1 K. In many tabular works, the dimensionally not correct - simplified equation a logp= -+ bologT+coT+d T
(5.1b)
is used. As an example, the values of the constants for solid NaCl, valid between room temperature and the melting point are (p in Pa): a= -1.658 .106 ; b = -120; c = -61.10- 3; d= 1907 At high pressure and low temperatures intermolecular interactions occur in gases. The ideal gas law p·V=R·T
(5.2)
is no longer valid; V = molar volume. With increasing pressure the volume of real gases decreases faster than predicted by Eq. (5.2). Figure 5.1 represents the volume of a liquid and of the coexisting vapor as a function of the temperature close to the critical point. With increasing temperB. Predel et al., Phase Diagrams and Heterogeneous Equilibria © Springer-Verlag Berlin Heidelberg 2004
156
5 Phase Equilibria Including a Vapor Phase
Fig.S.l Specific volume of vapor and liquid, which are in equilibrium near the critical point K
Specific Volume [cm 3/g]
ature the volume of the vapor decreases along the line D - K, the volume of the liquid increases along the line F - K. If the critical temperature TK is reached, liquid and vapor become identical (point K). At a temperature T < TK and fixed total volume the ratio of the specific volume of liquid and vapor is given by the lever rule. When the critical point is approached, the specific volumes of the liquid and vapor in equilibrium approach each other. Simultaneously the enthalpy of vaporization of the liquid decreases, becoming zero when the TK is reached. As an example, the critical point of water is at TK =638 K and PK =20.3 . 106 Pa.
5.2 Phase Equilibria Between Liquid and Vapor in Binary Systems, without a Miscibility Gap In a two-component system A - B the total pressure P is formed additively from the partial pressure of the components p A, PB: (5.3)
In an ideal binary system, where at a given temperature the vapor pressures of the components are equal, the composition of the vapor phase must equal that of the vaporizing liquid. In real systems this is not the case. The different vapor pressures of the components at a fixed temperature cause the composition of the vapor phase to differ from that of the liquid phase. Figure 5.2 represents a simple p-x diagram of a b
Data Loading...
