Limited azeotropy

The conditions for an azeotrope may, for some systems, be satisfied for all values of T, while for others only in a certain temperature range. Following Lecat, we may call the first case absolute azeotropy, and the second limited azeotropy." ... 

In our discussion of (vapor + liquid) phase equilibria to date, we have limited our description to near-ideal mixtures. As we saw in Chapter 6, positive and negative deviations from ideal solution behavior are common. Extreme deviations result in azeotropy, and sometimes to (liquid -I- liquid) phase equilibrium. A variety of critical loci can occur involving a combination of (vapor + liquid) and (liquid -I- liquid) phase equilibria, but we will limit further discussion in this chapter to an introduction to (liquid + liquid) phase equilibria and reserve more detailed discussion of what we designate as (fluid + fluid) equilibria to advanced texts. 

Many azeotropic combinations exist among hydrocarbons themselves. Aromatic hydrocarbons, for example, are almost always found in petroleum fractions distilling below the true boiling point of the aromatics. Marschner and Cropper (41) accurately delineated the limits of azeotropy for benzene and toluene with saturated hydrocarbons, and Denyer et al. (11) did the same for the thiols. Consideration of such data is desirable in the design and operation of equipment for the distillation of gasoline fractions to produce specialized products. 

Compared to Fig. 5.9 the line of reactive azeotropy is now shifted to higher concentrations for the cases in Fig. 5.11(b) and (c). Consequently, for a sufficiently low feed concentration, total conversion with pure products B and C is possible in all three cases. Theoretical limitations only occur for high feed concentration. In many practical applications the theoretical limitation illustrated in Fig. 5.11b, c is beyond the solubility limit of the reactants, and has then no practical significance. 

In conclusion let us make some remarks concerning the existence of the lines both of the limited and partial azeotropes proposed earlier in the literature and discussed in Sect. 4.5. The realization of the conditions of the limited or partial azeotropy in the course of the terpolymerization process means that the representing point x(p) of the system doesn t leave these azeotropic lines under the change... 

Solid-fluid phase diagrams of binary hard sphere mixtures have been studied quite extensively using MC simulations. Kranendonk and Frenkel [202-205] and Kofke [206] have studied the solid-fluid equilibrium for binary hard sphere mixtures for the case of substitutionally disordered solid solutions. Several interesting features emerge from these studies. Azeotropy and solid-solid immiscibility appear very quickly in the phase diagram as the size ratio is changed from unity. This is primarily a consequence of the nonideality in the solid phase. Another aspect of these results concerns the empirical Hume-Rothery rule, developed in the context of metal alloy phase equilibrium, that mixtures of spherical molecules with diameter ratios below about 0.85 should exhibit only limited solubility in the solid phase [207]. The simulation results for hard sphere tend to be consistent with this rule. However, it should be noted that the Hume-Rothery rule was formulated in terms of the ratio of nearest neighbor distances in the pure metals rather than hard sphere diameters. Thus, this observation should be interpreted as an indication that molecular size effects are important in metal alloy equilibria rather than as a quantitative confirmation of the Hume-Rothery rule. 

Critical Azeotropy. Systems with critical azeotropy form azeotropes up to the critical region. The type shown in Figure 3a corresponds to a positive azeotrope and has been found e.g. for binary mixtures of H2O (component II) with ethanol, 1-propanol, or acetone (component I).t Severalp(x) isotherms are schematically represented for this type in Figure 3b, the cusp-like isotherm for T = const. = T% being especially interesting since here the limits of material and mechanical stability coincide. ... 

Azeotropies do not necessarily occur the foregoing are merely limitations on their occurrence. On the other hand, they can always be made to occur by a suitable choice of fields. With each point the two-phase coexistence surface we may associate a new coordinate system in which any number up to c of the coordinate axes is diosen parallel to... 

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