Fractionation of Mixtures with Heteroazeotropes

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). 

Processes for completely fractionating binary mixtures with heteroazeotropes consist of two distillation columns and one decanter (Fig. 11.3-1). As the azeotrope lies within the miscibility gap of the liqnid the azeotrope can be broken by decantation. The two fractions from the decanter are at different sides of the azeotropic point. Purification of these two rather impnre fractions is performed by distillation. The pure products are recovered as bottoms from the distillation columns C-1 and C-2. 

The process shown in Fig. 11.3-1 utilizes the specific advantages of distillation and decantatioa The advantage of distillation is the ability to produce pure fractions. However, distillation cannot break azeotropes. On the other hand, decantation can break heteroazeotropes but cannot produce pure products. Thus, the combination of distillation and decantation is a very effective process for fractionating mixtures with heteroazeotropes. Such processes are extensively used in industiy. Table 11.3-1 lists some important binary mixtures fiactioiated by the process shown in Fig. 11.3-1. 

Toluene/water Ethyl propyl ether/water 

The combination of distillation and decantation can also be applied to multicomponent mixrnres. A process for the separation of the temaiy mixture acetone/water/1-butanol is depicted in Fig. 11.3-2. One organic compound (acetone) is miscible with water, the other one (1-butanol) is inmiscible. The mixture water/l-butanol exhibits a heteroazeotrope. A boundary distillation line runs from the heteroazeotrope to the low boiler acetone. 

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