Azeotropic mixtures maximum-boiling

Mixtures forming an azeotrope with maximum, boiling point distillate the comj)o-nent in excess, pure bottom product azeotropic mixture of the two components. 

Examples of azeotropic mixtures of maximum boiling point are tabulated below these are not as numerous as those of minimum boiling point. 

All extractive distillations correspond to one of three possible residue curve maps one for mixtures containing minimum boiling azeotropes, one for mixtures containing maximum boiling azeotropes, and one for nonazeotropic mixtures. Thus extractive distillations can be divided into these three categories. 

FIG. 13-12 Liq iiid boiling points and vapor condensation temperatures for maximum-boiling azeotrope mixtures of chloroform and acetone at 101.3 kPa (1 atm) total pressure. 

FIG. 13-57 (Continued) Schematic isoharic-phase diagrams for binary azeotropic mixtures, (b) Homogeneous maximum-boiling azeotrope. 

An important system in distillation is an azeotropic mixture. An azeotrope is a liquid mixture which when vaporized, produces the same composition as the liquid. The VLE plots illustrated in Figure 11 show two different azeotropic systems one with a minimum boiling point and one with a maximum boiling point. In both plots, the equilibrium curves cross the diagonal lines. 

FIGURE 8.42 The temperature-composition diagram showing a maximum-boiling azeotrope (such as acetone and chloroform). When this mixture is fractionally distilled, the (less volatile) azeotropic mixture is left in the flask. 

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. 

At atmospheric pressure, sulfuric acid has a maximum boiling azeotrope at approximately 98.48% (78,79). At 25°C, the minimum vapor pressure occurs at 99.4% (78). Data and a discussion on the azeotropic composition of sulfuric acid as a function of pressure can also be found in these two references. The vapor pressure exerted by sulfuric acid solutions below the azeotrope is primarily from water vapor above the azeotropic concentration S03 is the primary component of the vapor phase. The vapor of sulfuric acid solutions between 85% H2S04 and 35% free S03 is a mixture of sulfuric acid, water, and sulfur trioxide vapors. At the boiling point, sulfuric acid solutions containing <85% H2S04 evaporate water exclusively those containing >35% free S03 (oleum) evaporate exclusively sulfur trioxide. 

If azeotropes are not present, a fractional distillation can eventually separate the mixture into the pure components, with the component with the higher vapor pressure ending up as the distillate and the less volatile component (known as the residue) left in the distillation pot. For a minimum boiling azeotrope, a fractional distillation can produce a distillate with the azeotropic composition and a residue that is one of the pure components, depending on the composition of the starting mixture. For a maximum boiling azeotrope a fractional distillation can produce one of the pure components as the distillate, and a residue with the azeotropic composition. 

The mixture with the maximum boiling point is called maximum bailing azeotrope and behaves as if it is a pure chemical compound of two components, because it boils at a constant temperature and the composition of the liquid and vapour is the same. But the azeotrope is not a chemical compound, because its composition is not constant under conditions and rarely corresponds to stoichiometric proportions. 

Azeotrope mixtures reach a point at which liquid and vapor compositions become the same at a certain temperature and pressure. Some azeotropes show a maximum boiling temperature, while others show a minimum boiling temperature. Table 1.8 shows some examples of binary and ternary azeotropes. Azeotrope mixtures cannot be separated into their pure species by a single distillation column. 

Schematic DRDs are particularly useful in determining the implications of possibly unknown ternary saddle azeotropes by postulating position 7 at interior positions in the temperature profile. Also note that some combinations of binary azeotropes require the existence of a ternary saddle azeotrope. As an example, consider the system acetone (56.4°C), chloroform (61.2°C), and methanol (64.7°C) at 1-atm pressure. Methanol forms minimum-boiling azeotropes with both acetone (54.6°C) and chloroform (53.5°C), and acetone-chloroform forms a maximum-boiling azeotrope (64.5°C). Experimentally there are no data for maximum- or minimum-boiling ternary azeotropes for this mixture. Assuming no ternary azeotrope, the temperature profile for this system is 461325, which from Table 13-18 is consistent with DRD 040 and DRD 042. However, Table 13-18 also indicates that the pure-component and binary azeotrope data are consistent with three temperature profiles involving a ternary saddle azeotrope, namely, 4671325, 4617325, and 4613725. All three of these temperature profiles correspond to DRD 107. Calculated residue curve trajectories for the acetone-chloroform-methanol system at 1-atm pressure, as...

U.S. Patent 4,093,633 (1978)], and maximum-boiling azeotropes of hydrogen chloride-water and formic acid-water (Horsley, Azeotropic Data-III, American Chemical Society, Washington, 1983). Since distillation boundaries move with pressure-sensitive azeotropes, the pressureswing principle can also be used for overcoming distillation boundaries in multicomponent azeotropic mixtures. 

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