Maximum boiling mixture

Type III The behaviour of solution of type HI is similar to that of type II discussed above. The only difference being that the residue tends towards the maximum boiling mixture of composition D while the distillate tends towards pure constituents either A or B. 

Mixtures which have the above properties are known as azeotropes. The azeotropic composition is itself a function of temperature or pressure for example, the proportions of alcohol and benzene which give rise to a maximum boiling mixture vary somewhat as the pressure of ebullition is changed. 

Minimum- and maximum-boiling azeotropic mixtures of the type shown in Figs. 9.7 and 9.10 can be treated by the methods already described, except that it will be impossible to obtain two products of compositions which fall on opposite sides of the azeotropic composition. In the rectification of a minimum-boiling azeotrope (Fig. 9.7), for example, the distillate product may be as close to the azeotropic composition as desired. But the residue product will be either rich in A or rich in B depending upon whether the feed is richer or leaner in A than the azeotropic mixture. With maximum-boiling mixtures (Fig. 9.10) the residue product will always approach the azeotropic composition. These mixtures can sometimes be separated completely by addition of a third substance, as described later. 

Figure A2.5.5. Phase diagrams for two-eomponent systems with deviations from ideal behaviour (temperature T versus mole fraetion v at eonstant pressure). Liquid-gas phase diagrams with maximum (a) and minimum (b) boiling mixtures (azeotropes), (e) Liquid-liquid phase separation, with a eoexistenee eurve and a eritieal point.

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. 

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. 

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