Distillation of non ideal mixtures

The aim of this dossier is to present the theoretical and practical elements required to design distillation units for the separation of non-ideal mixtures.

Broadly speaking, there are three main classes of problems in separating an A-B mixture by distillation:

• 1. distillation of ideal mixtures (see box);

• 2. azeotropic distillation, which involves adding a third body E, called the entrainer, to the charge to be distilled;

• 3. extractive distillation, which also involves the addition of an E-carrier, but which is fed continuously during certain stages of the process.

More often than not, a distillation mixture of two substances A-B behaves in a non-ideal way, as it may form an azeotrope or have a relative volatility close to unity. However, it is impossible to separate an azeotropic mixture in a conventional distillation column, as the azeotrope behaves like a pure body and is obtained at the top or bottom of the column, depending on its boiling temperature, instead of the pure bodies of the mixture, A or B. To remedy this, we can add a third body (called a solvent or entrainer, noted E) which has a more pronounced affinity for one of the constituents of the azeotropic mixture than for the other, so as to "entrain/extract" the first with it.

As indicated in [J 1 072] Material transfer. Ideal compartmental distillation, the main question in azeotropic or extractive distillation is the choice of the third body. This file [J 2 611] and the following one [J 2 612] provide the theoretical and practical elements to answer this question. Theoretical tools for the design of azeotropic and extractive distillation units mainly concern the analysis of the thermodynamic properties of mixtures, particularly residue, univolatility and unidistribution curves. In ternary diagrams, in particular A-B-E, it can also be performed graphically.