Minimum azeotrope

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). 

Fig. 14.22 (a) Vapor pressure diagram with azeotropic maximum of a binary system of two lowly compatible substances, (b) Corresponding boiling point diagram with azeotropic minimum. 

Fig. 14.24 Fractional distillation using the example of a boiling point diagram with azeotropic minimum.

The phase diagram of Auj Cu is shown in Figure 2.3. At higher temperatures a complete statistical miscibility of Au and Cu in the solid phase is observed. This can be described as solid solution. The phase separating line shows an azeotropic minimum. The layered structure for x = 0.5 (AuCu) and the structure for x = 0.75 (AuCuj) with copper surrounding the Au atoms (Figure 2.2) form at lower temperatures. The formation of AB and ABj structures is typical for these systems. 

Mixtures with miscibihty gaps in the liquid phase veiy often - but not always -form azeotropes within the miscibility gap. Since miscibility gaps decrease the boihng point of mixtures (see Chap. 5), such heteroazeotropes are always low boiling azeotropes (minimum azeotropes). 

Figure 9.16 Different types of liquid-vapor phase diagrams for a binary liquid mixture of component A and B as functions of the mole fraction of the component with the higher boiling temperature, (a) The phase diagram for a system with a low-boiling azeotrope (minimum boiling point) and (b) the phase diagram for a system with a high-boiling azeotrope (maximum boiling point). The arrows show how the paths for various distillation processes depend upon the position of the initial composition relative to the azeotrope.

Azeotrope Minimum Maximum Minimum Maximum Minimum... 

The results of the studies on separating minimum and maximum boiling azeotropes with light, intermediate, or heavy entrainers are compared according to their operation steps and feasibility domains. The decisive property for designing an effective BED process, separating azeotropes, is the relative position of the entrainer to the azeotrope in the bubble point series. But the type of the azeotrope (minimum or maximum) can modify the existence of some limiting parameters (FA a , N,., >.,). 

Figure 3.8a shows the temperature-composition diagram for a minimum-boiling azeotrope that is sensitive to changes in pressure. This azeotrope can be separated using two columns operating at different pressures, as shown in Fig. 3.86. Feed with mole fraction of A Ufa)) of, say, 0.3 is fed to the high-pressure column. The bottom product from this high-pressure column is relatively pure B, whereas the overhead is an azeotrope with jcda = 0-8, jcdb = 0.2. This azeotrope is fed to the low-pressure column, which produces relatively pure A in the bottom and in the overhead an azeotrope with jcda = 0.6, jcdb = 0.4. This azeotrope is added to the feed of the high-pressure column.

Figure 3.8 Separation of a minimum boiling azeotrope by pressure change. (From Holland, Gallun, and Lockett, Chemical Engineering, March 23, 1981, 88 185-200 reproduced by permission.)...

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. 

More curved the boundary, less recycle requited (generally less favorable for minimum boiling azeotropes because of large overhead recycles). 

Particularly good for components that form minimum boiling azeotropes with water and are immiscible with water tend to have extremely high volatihties. 

The physical and thermodynamic properties of benzene are shown in Table 1 (2). Azeotrope data for benzene with selected compounds are shown in Table 2 (3). Benzene forms minimum-boiling azeotropes with many alcohols and hydrocarbons. Benzene also forms ternary azeotropes. 

Fig. 4. Boiling poiat (a) and phase diagram (b) for a minimum boiling biaary azeotropic system at constant pressure. B and C, D are representative...

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

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. 

Because there is no azeotrope, these mixtures could be separated without adding a solvent. This, however, would be a difficult and expensive separation. Thus there is no minimum feed ratio (minimum solvent flow) and the only way to determine the optimal solvent-to-process feed ratio is by determining the sequence cost over a range of feed ratios. The best reflux ratios are again 1.2—1.5 times the minimum. 

Fig. 12. Pressure-swing distillation of a minimum boiling binary azeotrope, (a) Temperature—composition phase diagram showing the effect of pressure on...

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... 

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