Separating Heterogeneous Minimum-Boiling Azeotropes

A fresh feed with a composition in region LI (single liquid that is predominantly component 1) may be distilled in a conventional column to produce essentially pure component 1 in the bottoms and an overhead at or close to the azeotropic composition X. If the feed composition is in region L2 (single liquid that is predominantly component 2), it may be separated by conventional distillation into a bottoms product that is relatively pure component 2 and an overhead that is around the azeotropic composition The overhead from either column may be cooled 

FIGURE 10.2 Separating heterogeneous minimum-boiling azeotropes. 

Examples of the separation of heterogeneous azeotropes using such a process include n-butanol-water, ethyl acetate-water, and ethyl ether-water (Othmer, 1963). 

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Figure 6-9 Use of a two-column system to separate a heterogeneous minimum-boiling azeotrope. (M. Van Winkle, Distillation, 1967, by courtesy McGraw-Hill Book Company.)...

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. 

Azeotrope formation in general, and heterogeneous azeotropes in particular, can be usefully employed to achieve difficult separations. As discussed in Chapter 1, in azeotropic distillation an entrainer is added (frequently near the bottom of the column) for the purpose of removing a component that will combine with the ent to form a minimum boiling azeotrope, which is then recovered as the distillate. 

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 example of heterogeneous-azeotrope formation is shown in Fig. 13-13 for the water-normal butanol system at 101.3 kPa. At hquid compositions between 0 and 3 mole percent butanol and between 40 and 100 mole percent butanol, the liquid phase is homogeneous. Phase splitting into two separate liquid phases (one with 3 mole percent butanol and the other with 40 mole percent butanol) occurs for any overall liquid composition between 3 and 40 mole percent butanol. A minimum-boiling heterogeneous azeotrope occurs at 92°C (198°F) when the vapor composition and the overall composition of the two liquid phases are 75 mole percent butanol. 

The best ideal entrainer for the separation using heterogeneous azeotropic distillation is one that only forms one additional minimum-boiling heterogeneous azeotrope with water. 

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