Liquid Equilibrium and Enthalpy for a Pure Substance

VAPOR-LIQUID EQUILIBRIUM AND ENTHALPY FOR A PURE SUBSTANCE 

As indicated above, the substance undergoing interphase transfer in these operations is the material constituting the liquid phase, which diffuses in the form of a vapor. The equilibrium vapor-pressure characteristics of the liquid are therefore of importance. 

Whenever a process involves bringing a sample of fluid across the vapor-pressure curve, such as the isobaric process ADE or the isothermal process ABF, there will be a change of phase. This will be accompanied by the evolution (for condensation) or absorption (for vaporization) of the latent heat of vaporization at constant temperature, for example, at points 5 or Z) in the processes mentioned above. Heat added or given up with changing temperatures is called sensible heat. 

Interpolation beriveen data For such common liquids as water, many refrigerants, and others, the vapor-pressure-temperature curve has been established at many points. For most liquids, however, only relatively few data are available, so that it is necessary frequently to interpolate between, or extrapolate beyond, the measurements. The curve on arithmetic coordinates (Fig. 7.1) is very inconvenient for this because of the curvature, and some method of linearizing the curve is needed. Most of the common methods stem from the Clausius-Clapeyron equation, which relates the slope of the vapor-pressure curve to the latent heat of vaporization 

Equation (7.3) suggests that a plot of logp against l/T will be straight for short temperature ranges. It also suggests a method of interpolating between points listed in a table of data. 

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Vapor-Liquid Equilibrium and Enthalpy for a Pure Substance Vapor-Pressure Curve.5... 

According to the vapor pressure curve P = P T), there is a unique pressure for each temperature where a vapor-liquid equilibrium for a pure substance is possible, as long as both temperature and pressure are below their critical values. Similarly, there is a functional relationship between the enthalpy of vaporization and the boiling temperature... 

The Van der Waals interactions of the force fields for CO2 and C2H6 were described by two LJ 12-6 sites and one point quadrupole (16). Both force fields were empirically parameterized to experimental critical temperatures, saturated liquid densities, and vapor pressures by means of a nonlinear optimization algorithm. For both pure substances, the vapor-liquid equilibrium properties from simulation deviate by less than 1% from experimental saturated liquid density data and less than 3% from experimental vapor pressure and enthalpy of vaporization data. 

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