Positive azeotropy

AZEOTROPIC SYSTEM. A system of two or more components that has a constant boiling point at a particular composition. If the constant boiling point is a minimum, the system is said to exhibit negative azeotropy, if it is a maximum, positive azeotropy. 

Azeotropic Systems. An azeotropic system is one wherein two or more components have a constanl boiling point at a particular composition. Such mixtures cannot be separated by conventional distillation methods. If rhe constant boiling point is a minimum, the system is said lo exhibit negomv azeotropy if it is a maximum, positive azeotropy. Consider a mixture of water and alcohol in the presence of the vapor. This system of two phases and two components is divarianl. Now choose some fixed pressure and study the composition of the system at equilibrium us a function of temperature. The experimental results arc shown schematically in Fig. 5. 

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. 

We have already seen that (c/. 18.68 and 18.69) if this expression is positive the azeotrope corresponds to a minimum in T, or a maximum in p. In this case we call the azeotropy positive, and the derivative 8X2I8T has the same sign as 2... 

In other words, if the azeotropy is positive, an increase in temperature increases the mole fraction in the azeotrope of the component which has the greater heat of evaporation from the solution. 

Let us suppose that the azeotropy is positive (a>0) and choose the indices so that T [Pg.466]

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. ... 

Jenner and Kellou recently studied the pressure effect on azeotropy in free-radical terpolymerization of MA with acrylonitrile, dielthyl fumarate, methyl acrylate, methyl methacrylate, methyl vinyl ketone, vinylidene chloride, norbornene, a-methylstyrene, indene, and vinyl acetate, with styrene as the second comonomer common in all cases. It was found that ternary azeotropes were only possible for those systems where the first comonomers had positive e values, i.e., diethyl fumarate, acrylonitrile, methyl acrylate, methyl methacrylate, methyl vinyl ketone, and vinylidene chloride. Surprisingly, the coordinates of the ternary azeotropes were very little affected by variations of the pressure from 1-3,000 bars. Since reactivity ratios in multi-component polymerizations are sensitive to pressure, causing terpolymer composition to also be pressure dependent, a shift of the ternary azeotropic point would be expected. Why this occurs awaits further clarification. 

---