The most general definition of homogeneous azeotropic distillation is the separation of any single liquid-phase mixture containing one or more azeotropes into the desired pure component or azeotropic products by continuous distillation. Thus, in addition to azeotropic mixtures which require the addition of a miscible separating agent in order to be separated, homogeneous azeotropic distillation also includes self-entrained mixtures that can be separated without the addition of a separating agent.
The first step in the synthesis of a homogeneous azeotropic distillation sequence is to determine the separation objective. Sometimes it is desirable to recover all of the constituents in the mixture as pure components other times it is sufficient to recover only some of the pure components as product. In our example, we would like to recover the cyclohexane product at 90% purity and recycle the separating agent back to the initial separating column for further use. The second step is to sketch the residue curve map for the mixture to be separated. The residue curve map allows one to determine whether the goal can be reached and if so how to reach it, or the goal needs to be redefined.
Distillation boundaries for continuous distillation are approximated by simple distillation boundaries. It is a good approximation for mixtures with nearly simple distillation boundaries. For a separation to be feasible by distillation, the simple distillation boundary should not be crossed, i.e. the distillate and bottom composition should lie in the same distillation region. A more detail calculation method involving the composition will be discuss in the later section.
In the most common situation, a separating agent is added to separate a minimum boiling binary azeotrope into its two constituent pure components by homogeneous azeotropic distillation. Michael F. D. and Jeffrey P. K. presented seven of the most favorable residue curve maps for this task. Of the seven, the map representing extractive distillation is by far the most common and the most important.