Minimum-boiling azeotropism

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.)...

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. 

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. 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 ... 

Biitanol-water Minimum boiling azeotrope Self-entraining ... 

Exploitation of boundaiy curvature for breaking azeotropes is veiy similar to exploiting pressure sensitivity from a mass-balance point of view, and suffers from the same disadvantages. Separation schemes have large recycle flows, and in the case of minimum-boiling azeotropes, the recycle streams are distillates. However, in the case of maximum-boihng azeotropes, these recycles are underflows and... 

For a minimum boiling azeotrope the partial pressures of the components will be greater than predicted by Raoult s Law, and the activity coefficients will be greater than 1.0. 

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. 

Solutions in which intermolecular forces are stronger in the solution than in the pure components have negative deviations from Raoulfs law some form maximum-boiling azeotropes. Solutions in which intermolecular forces are weaker in the solution than in the pure components have positive deviations from Raoulfs law some form minimum-boiling azeotropes. 

Figure 12.1 Separation of minimum-boiling azeotrope by pressure change.

An equimolar mixture of ethanol and ethyl acetate is to be separated by distillation into relatively pure products. The mixture forms a minimum-boiling azeotrope, as detailed in Table 12.1. However, the composition of the azeotrope is... 

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