Minimum-boiling mixtures

In the system of water-ethanol, the point M corresponds to a minimum boiling temperature of 78.13° and a composition of95.57% ethanol by weight. If any solution of composition of pure water and 95.57% ethanol is distilled, then we get a residue of pure water and a constant minimum boiling mixture of 95.57% alcohol in the distillate. No pure ethanol can be recovered. On the contrary, if a solution of composition between pure alcohol and 95.57% ethyl alcohol is distilled, then we get a mixture containing 95.57% ethanol and pure alcohol. No pure water will be recovered. 

Binary azeotropic mixtures cannot be separated by fractional distillation. A mixture of two components which form maximumboiling or minimum-boiling mixtures may be separated into portions of different compositions, but cannot be separated into pure... 

The formation of minimum-boiling mixtures makes azeotropic distillation a useful tool in those cases where separation by fractional distillation is not feasible. In the example cited for separation operation (10) in Table 1.1, n-butyl acetate, which forms a heterogeneous minimum-boiling azeotrope with water, is used to facilitate the separation of acetic acid from water. The azeotrope is taken overhead, the acetate and water layers are decanted, and the MSA is recirculated. 

C), forms a ternary azeotrope (boiling point 64.9 "C), which is a minimum-boiling mixture. Benzene and Ethanol form a binary azeotrope (boiling point... 

Minimum-boiling mixtures—azeotropes When the positive deviations from ideality are sufficiently large and the vapor pressures of the two components are not too far apart, the total-pressure curves at constant temperature may rise through a maximum at some concentration, as in Fig. 9.1a. Such a mixture is said to form an azeotrope, or constant-boiling mixture. The significance of this is more readily seen by study of the constant-pressure section (Fig. 9.1b or c). TTie liquid- and vapor-composition curves are tangent at point L, the point of... 

Many liquid mixtures exhibit a minimum boiling point (e.g. methanol and chloroform -propanol and water) whilst others show a... 

At z in the curve, however (the minimum of vapour pressure), the solution and vapour are in equilibrium and the liquid at this point will distil without any change in composition. The mixture at z is said to be azeotropic or a constant boiling mixture. The composition of the azeotropic mixture does vary with pressure. 

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 minimum boiling point are collected in Table I, 4, A. 

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. 

Minimum Boiling Azeotropes. AH extractive distillations of binary minimum boiling azeotropic mixtures are represented by the residue curve map and column sequence shown in Figure 6b. Typical tray-by-tray composition profiles are shown in Figure 7. 

Phase Diagrams. For binary mixtures, it is weU known that when a Hquid—Hquid envelope merges with a minimum boiling vapor—Hquid-phase envelope the resulting azeotropic phase diagram has the form shown in Figure 13. When the Hquid composition, as in Figure 13a, then the vapor... 

FIG. 13-11 liquid boiling points and vapor oondensation temperatures for minimum-boiling azeotrope mixtures of ethyl aeetate and ethanol at 101.3 kPa (1 atm) total pressure. 

The simplest case of combining T E and LLE is the separation of a binaiy heterogeneous azeotropic mixture. One example is the dehydration of 1-butanol, a self-entraining system, in which butanol (117.7°C) and water form a minimum-boiling heterogeneous azeotrope (93.0°C). As shown in Fig. 13-69, the fresh feed may be added... 

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 8.41 The temperature composition diagram of a minimum-boiling azeotrope (such as ethanol and benzene). When this mixture is fractionally distilled, the (more volatile) azeotropic mixture is obtained as the initial distillate. 

---