PREDICTED EFFECT OF PRESSURE ON BOILING POINT

TABLE 3A. PREDICTED EFFECT OF PRESSURE ON BOILING POINT  

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In cases where only the normal azeotropic boiling point is known, it is possible to predict the effect of pressure on the system by drawing the azeotrope curve through the normal boiling point with a slope equal to the average slopes of the component vapor pressure curves. This procedure will permit a fairly accurate prediction of whether the azeotrope will cease to exist below the critical pressure. 

This procedure is the basis for the series of relations shown in Figures 1-6 which can be used to predict azeotropic data for related systems. The procedure also indicates the effect of pressure on azeotropic composition if one determines the difference in boiling point of the system at any desired pressure and applies the data to the appropriate curve. This is illustrated in Figure 6. 

Many procedures have been developed to predict vapor pressure, and the predictive error of eleven different methods have been evaluated using a series of PCBs. A number of approaches use a set of known vapor pressures to develop a correlation with molecular properties that can be used for predictions of unknowns. For example, the free energy of vaporization has been correlated with molecular surface area to predict vapor pressures of PCB congeners. More direct approaches are based on the Clausius-Clapeyron equation, and vapor pressures can be predicted quite effectively for some series of compounds using only boiling points along with melting points for compounds that are sohds at ambient temperatures. 

Consideration of these characteristics makes it clear that only very special liquid pairs could conceivably form ideal solutions. It would be necessary that the molecules of the constituents be very similar in every respect, for example in structure, size, and chemical nature. Thus, solutions of optical isomers, adjacent members of an homologous series, and similar mixtures would be expected to be nearly ideal, but actually all solutions can at best only approach ideality as a limit. Solutions which form immiscible liquid phases are of necessity extremely nonideal, and extraction operations depend upon this. The extent to which solutions depart from ideality is manifested by deviations of the properties of the solutions from the characteristics listed above, and a study of these deviations will permit us to some extent to predict their behavior in extraction operations. The most useful characteristics of the ideal solution for these purposes is that of vapor pressure, since considerable information has now been accumulated for many mixtures on this and related properties such as boiling points of solutions, azeotropism, and vapor-liquid equilibria. Classifications of compounds according to the effect of intermolecular forces on properties of mixtures also provide much useful material, but the second and third characteristics in the list above are of limited value owing to lack of experimental data to which we can refer. 

Raoult s law predicts that when we increase the mole fraction of nonvolatile solute particles in a solution, the vapor pressure over the solution will be reduced. In fact, the reduction in vapor pressure depends on the total concentration of solute particles, regardless of whether they are molecules or ions. Remember that vapor-pressure lowering is a colligative property, so it depends on the concentration of solute particles and not on their kind. In our applications of Raoult s law, however, we will limit ourselves to solutes that are not only nonvolatile but nonelectrolytes as well. We consider the effects of volatile substances on vapor pressure in the "Closer Look" box in this section, and we will consider the effects of electrolytes in our discussions of freezing points and boiling points. 

AIChE (1994) describes a procedure developed by Baker et al. (1975) and Tang et al. (1996) for determining both the peak overpressure and impulse due to vessels bursting from pressurized gas. This procedure is too detailed to be described in detail here. The method results in an estimate of the overpressure and impulse due to blast waves from the rupture of spherical or cylindrical vessels located at groimd level. The method depends on the phase of the vessel s contents, its boiling point at ambient pressure, its critical temperature, and its actual temperature. An approach is also presented to determine blast pressures in the near-field, based on the results of numerical simulations. These methods are only for the prediction of pressure effects. 

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