Positive azeotrope

For a solution or mixture of two or more distinct liquid components, an azeotrope is that composition (typically measured in mole fractions or percent weight and referred to as the azeotropic solution) with which there is either a maximum point (a negative azeotrope) or a minimum point (a positive azeotrope) in a boiling point versus composition diagram at constant pressure. 

Minimum boiling point azeotrope (positive azeotrope)... 

Handling the separation problem involves essentially the ternary mixture phenol/ cyclohexanone/cyclohexanol. As Table 5.10 indicates, phenol is the highest boiler (181.9°C), followed by cyclohexanol (160.8°C) and cyclohexanone (155.4°C). Phenol forms positive azeotropes with cyclohexanone and cyclohexanol, of similar composition (roughly 75% mol phenol) and very similar boiling points. 

The vapour pressure curves behave in a different way these are either straight lines or convex curves, with or without a maximum. As we know, such a maximum results in positive azeotropism, i.e. 

If we consider the case where a>0, then the system will have a positive azeotrope (c/. 28.22), so that from (28.43) we deduce that if two components, whose vapour pressures are approximately equal, form a positive azeotrope, then a rise in temperature increases the mole fraction in the azeotrope of that component whose saturation vapour pressure when pure increases the more rapidly with temperature," ... 

We define A as the absolute difference between the boiling points of the two components, and S as the absolute difference between the boiling point of the azeotrope and that of the more volatile component, for positive azeotropes, and the less volatile for negative azeotropes. The quantity S is called the azeotropic deviation, (figs. 28.5 (a) and (6)). 

Liquid mixtures having an extremum (maximum or minimum) vapor pressure at constant temperature, as a function of composition, are called azeotropic mixtures, or simply azeotropes. Mixtures that do not show a maximum or minimum are called zeotropic. Azeotropes in which the pressure is a maximum are often called positive azeotropes, while pressure-minimum azeotropes are called negative azeotropes. The coordinates of an azeotropic point are the azeotropic temperature t, pressure, and liquid-phase composition, usually expressed as mole fractions. At the azeotropic... 

Lowly compatible behavior by the components in the liquid state with weaker particle interactions leads to a positive deviation from Raoult s law. Correspondingly, one can observe a vapor pressure maximum (Fig. 14.22a) as well as a boiling point minimum (Fig. 14.22b). Acetone/carbon disulfide and ethanol/water are examples of positive azeotropes (minimum-boiling or maximum-pressure azeotropes). 

Mixtures that have a maximum in the Px curve exhibit positive deviations from ideal-solution behavior that is, the activity coefficients are greater than unity. Such mixtures are called positive deviants and their azeotropes are called positive azeotropes. Since such mixtures have minima in their Tx curves, those same azeotropes are also called minimum boiling-point azeotropes. Positive deviants usually occur when attractive intermolecular forces between molecules of the same species are stronger than those between molecules of different species. 

Figure 9.11 Effect of temp>erature on positive azeotropes formed in mixtures of methyl ace-tate(l) and methanol(2). Calculated using the Wilson model for activity coefficients, with parameters from Appendix E.

Figure 9.12 Tests of the Clausius-Clapeyron equation (9.3.22) for correlating azeotropic pressures and temperatures. Circles are positive azeotropes formed in mixtures of methyl acetate and methanol (see Figure 9.11). Triangles are negative azeotropes that occur in mixtures of acetone and chloroform (see Figure 9.10). Lines are least-squares fits.

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. ... 

Bobbo, S. Fedele, L. Camporese, R. Stryjek, R- VLE measurements and modeling for the strongly positive azeotropic R32 +... 

Binary homogeneous (single-phase) liquid mixtures having an extremum (maximum or minimum) vapor pressure P at constant temperature T, as a function of composition, are called azeotropic mixtures, or simply azeotropes. The composition is usually expressed as mole fractions, where for component 1 in the liquid phase and i for component 1 in the vapor phase are identical. Mixtures that do not show a maximum or minimum are called zeotropic. A maximum (minimum) of the P xi) or P(yi) curves corresponds to a minimum (maximum) of the boiling temperature T at constant P, plotted as a function of. Vj or yi [see T xj) and T(yi) curves. Types 1 and 111, in Fig.l]. Azeotropes in which the pressure is a maximum (temperature is a minimum) are often called positive azeotropes, while pressure-minimum (temperature-maximum) azeotropes are called negative azeotropes. The coordinates of an azeotropic point are the azeotropic temperature Taz> pressure arid the vapor-phase composition, which is the same as the liquid-phase composition... 

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