Separation by Forming Two Binary Azeotropes

For certain close boilers or difficult-to-separate binaries, an entrainer may be selected 

FIG U RE 10.4 Separation by forming two binary azeotropes, (a) amount of entrainer by graphical material balances, (b) liquid composition isotherms, and (c) typical separation flowsheet. 

The required amount of entrainer is calculated by material balance. From overall 

The mole fraction subscripts represent components A, B, and E in the feed and in the two azeotropes. It is assumed in these approximate calculations that components A and B and the entrainer are totally involved in forming the azeotropes and that the separation between the two azeotropes is perfect. Combining the above equations gives the required ratio of entrainer to feed  

A typical flowsheet for the separation is shown in Eigure 10.4c. The fresh feed is combined with the entrainer and then fed to the azeotropic column, where the two azeotropes are separated. Each azeotrope product is sent to a liquid-liquid extraction column to dissolve the entrainer in some suitable solvent. The combined entrainer-solvent solution from both extractors is sent to a distillation column to separate the entrainer and solvent. These are recycled to the azeotropic column and extractors with makeup added to each, as needed. 

---