Azeotropic distillation is accomplished by adding to the liquid phase, a volatile third component, which changes the volatility of one of the two components more than the other so that the components are separated by distillation. The two components to be separated often are close boiling point components which do or do not azeotrope in the binary mixture, but sometimes they are components which do azeotrope although they aren't close boiling point components [1-37]. It is likely that in some cases, one system can be separated either by azeotropic distillation or by extractive distillation, for instance, alcohol/water, acid/water, etc.
The added third component, sometimes called the entraîner, may form a ternary azeotrope with the two components being separated. However, it must be sufficiently volatile from the solution so that it is taken overhead with one of the two components in the azeotropic distillation column. If the entraîner and the component taken overhead separate into two liquid phases when the vapor overhead is condensed, the entraîner phase is refluxed back to the column. The other phase can be fractionated to remover the dissolved entraîner and the residual amount of the other component before it is discarded in the solvent (entraîner) recovery column). Alternatively, this second liquid phase is recycled to some appropriate place in the main process scheme.
Azeotropic distillation is usually divided into two types: homogeneous azeotropic distillation and heterogeneous azeotropic distillation. They are illustrated in Figs. 1 and 2, respectively. In homogeneous azeotropic distillation, phase split doesn't appear in the liquid along the whole column. Whereas, in heterogeneous azeotropic distillation, two liquid phases exist in some regions of the composition space. Heterogeneous azeotropic distillation is more widely used for separating the close boiling point components or azeotropes than homogeneous azeotropic distillation. In particular, the case of heterogeneous mixtures without decanter at the top of azeotropic distillation column can be looked upon as the one of homogeneous mixtures, and at the same time the liquid composition on a tray or a section of packing is replaced by the overall liquid composition. The operation, controlling and optimization of the azeotropic distillation column have been studied in [56-59],
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