Dissimilar binary system

Figure 20.1.6 Binary solid - supercritical system. Phase behavior of dissimilar binary systems...

Figure 3.10 shows the vapor pressure/composition curve at a given temperature for an ideal solution. The three dotted straight lines represent the partial pressures of each constituent volatile liquid and the total vapor pressure. This linear relationship is derived from the mixture of two similar liquids (e.g., propane and isobutane). However, a dissimilar binary mixture will deviate from ideal behavior because the vaporization of the molecules A from the mixture is highly dependent on the interaction between the molecules A with the molecules B. If the attraction between the molecules A and B is much less than the attraction among the molecules A with each other, the A molecules will readily escape from the mixture of A and B. This results in a higher partial vapor pressure of A than expected from Raoult s law, and such a system is known to exhibit positive deviation from ideal behavior, as shown in Figure 3.10. When one constituent (i.e., A) of a binary mixture shows positive deviation from the ideal law, the other constituent must exhibit the same behavior and the whole system exhibits positive deviation from Raoult s law. If the two components of a binary mixture are extremely different [i.e., A is a polar compound (ethanol) and B is a nonpolar compound (n-hexane)], the positive deviations from ideal behavior are great. On the other hand, if the two liquids are both nonpolar (carbon tetrachloride/n-hexane), a smaller positive deviation is expected.

The excess Gibbs energy for binary systems consisting of liquids not too dissimilar in cherai nature is represented to a reasonable approximation by the equation... 

Figure 5.1(a) A completely separated binary system (b) a randomized binary system (c) an ideal or ordered binary system (d) a segregated binary system (e) a cohesive structure containing self-loving particles (jf) a cohesive structure with dissimilar particle preference... 

The exact behaviour of branch II of the critical locus depends on the extent to which the components are dissimilar and progresses in the sequence (a), (b), (c), (d) as the dissimilarity increases. This figure which is based on Rowlinson [16] is for binary systems but similar principles would apply if a mixture of solute components were present. 

For a binary system whose components are not too dissimilar chemically but have different molar volumes, we can assume that interaction coefficients involving more than two molecules can be neglected. Then, the Wohl expression becomes ... 

Composition Dependence of Alloy Viscosity. Attempts have been made to calcnlate the viscosity of a dilnte liquid alloy from a theoretical standpoint, but with little success. This is primarily due to the fact that little is known about the interaction of dissimilar atoms in the liquid state. Empirical relationships for the viscosity of dilute liquid alloys have been developed, but these are generally limited to specific alloy systems—for example, mercury alloys with less than 1% impurities. The viscosities of binary liquid alloys have been empirically described using a quantity called the excess viscosity, (not to be confused with the excess chemical potential), which is defined as the difference between the viscosity of the binary mixture (alloy), pa, and the weighted contributions of each component, xipi and X2P2-... 

A few results of quantum moment calculations for dissimilar rare gas pairs are shown in Tables 5.1, 5.2 and 3.1, and will be discussed below. The classical fourth binary moment was also reported [79], Table 5.1 compares quantum, semi-classical and classical calculations based on sum formulae (Eqs. 5.37, 5.38, 5.39) with moments obtained by integration of computed line shapes for the He-Ar system at 295 K. 

The prediction of efficiencies for multicomponent systems is also discussed by Chen and Fair (1984b). For mixtures of dissimilar compounds the efficiency can be very different from that predicted for each binary pair, and laboratory or pilot-plant studies should be made to confirm any predictions. 

The equation, which is generalized to multi-component mixtures, requires pure component data for the van der Waals area and volume parameters, and r. Additionally, the binary interaction parameters (Uj -Uj and (Wy - are also required and are generally determined from binary phase equilibrium data. Temperature dependency is incorporated in the equation, similar to the Wilson and NRTL equations. The UNIQUAC equation is applicable to many classes of components, including mixtures containing considerably dissimilar molecules, and is also applicable to liquid-liquid equilibrium systems. It can represent temperature dependency over moderate ranges but is not necessarily more accurate than simpler equations in spite of its theoretical foundation. 

In a ternary system where there are two similar and one dissimilar species, the dissimilar one will have an efficiency close to the binary value, and the efficiencies of the similar species will differ from each other and from the binary value. 

In a binary electrolyte solution such as this one, terms containing A, 0, or tf/ are zero, since these involve interactions with two dissimilar anions or cations. In most such cases, the parameter is unnecessary, because it is invoked to account for exceptionally strong ion-ion interactions. In fact, Pitzer shows that should approach -K/2 in the limit of infinite dilution, where K is the association constant for the ion-pair. The work of Harvie and Weare (1980), Eugster, Harvie and Weare (1980), and Harvie, Eugster and Weare (1982), who modeled solubility equilibria in the multicomponent oceanic salt system is considered a milestone in the application of the Pitzer equations, and the set of parameters in Harvie, Mller and Weare (1984) is considered a sort of standard for modeling of seawater evaporitic systems. 

The binary azeotropes considered in Chapters 5 and 6 were homogeneous, that is, a single liquid phase. The interaction between molecules that have similar structures and elements is usually small, and the liquid-liquid behavior is ideal. The molecular interaction increases as the molecules become more dissimilar. In many systems the molecules repel each other and result in nonideal behavior with activity coefficients greater than unity. For example, methanol and water are somewhat dissimilar, so their activity coefficients are modestly greater than unity. The methanol-water system does not have an azeotrope because the nonideality is modest. Ethanol and water are more dissimilar, so their activity coefficients are fairly large. The result is a homogeneous azeotrope. 

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