Distillation, staged graphical analysis

The concept will be introduced in the context of the McCabe-Thiele construction for the graphical analysis of binary distillation problems. This method uses y-x diagrams of the form shown in figure 6.2. Such diagrams are a graphical representation of equilibrium data, and so if the composition of the resultant vapour from an equilibrium stage is known, then the composition of the resultant liquid (in equilibrium with that vapour) can be obtained from the curve on the appropriate y-x diagram. 

However, while ideal equilibrium stages may be a poor approximation, they are usually the language in which distillation problems are discussed. Thus anyone who studies distillation should analyze any target separation in terms of a McCabe-Thiele analysis even though he knows that this analysis may not be that close to what actually happens. The graphical picture of what is happening is just too good to abandon. 

---