Minimum-boiling

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.)...

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

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. 

Bina System. The first task is to examine the characteristics of the 2-propanol-water-phase equiUbria (VLE, LLE, SEE) to determine the compositions of interest and any critical features. 2-Propanol forms a minimum boiling a2eotrope with water (80.4°C at 101.3 kPa (760 tort), 68 mol % 2-propanol). The a2eotrope is between the feed and the IPA product and is a distillation boundary, thus it is impossible to obtain both desired products from any single-feed... 

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...

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. 

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... 

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. 

Schematic DRD shown in Fig. 13-59 are particularly useful in determining the imphcations of possibly unknown ternary saddle azeotropes by postulating position 7 at interior positions in the temperature profile. It should also be noted that some combinations of binary azeotropes require the existence of a ternaiy saddle azeotrope. As an example, consider the system acetone (56.4°C), chloroform (61.2°C), and methanol (64.7°C). 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 ternaiy azeotropes. The temperature profile for this system is 461325, which from Table 13-16 is consistent with DRD 040 and DRD 042. However, Table 13-16 also indicates that the pure component and binary azeotrope data are consistent with three temperature profiles involving a ternaiy saddle azeotrope, namely 4671325, 4617325, and 4613725. All three of these temperature profiles correspond to DRD 107. Experimental residue cui ve trajectories for the acetone-...

THE-water Minimum boiling azeotrope None Alternative to extractive distillation... 

Methyl acetate-methanol Minimum boiling azeotrope None Element of recovery system for alternative to production of methyl acetate by reactive distillation alternative to azeotropic, extractive... 

Alcohol-ketone systems Minimum boiling azeotropes None ... 

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