Boiling-Point Diagrams Theory of Distillation

We turn now to the T-xB boiling-point diagram (7.43) at fixed pressure (e.g., P= 1 atm). Under these conditions, the T-xeq curve displays the quantitative variation of the normal boiling point of solution as a function of liquid concentration. The boiling-point diagram therefore provides a basis for exact theoretical analysis of the ancient art of distillation, a staple of chemical laboratory practice. 

To describe the formal theory of fractional distillation, let us consider the boiling-point diagram of a near-ideal A/B binary solution, as shown in Fig. 7.10. The solution is initially at high concentration x of the high-boiling component B. Consider the following four steps, as illustrated in the figure  

The solution at composition x is heated to its boiling point at Tx, producing vapor of composition x2 (enriched in low-boiling component A). 

The vapor of composition x2 is collected and condensed to liquid at temperature T2. 

The condensate from Step 2 is boiled at T2, producing vapor of composition x3. 

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