Recovery by Multieffect Azeotropic Distillation

If the acid concentration is too low for conventional recovery by extraction, and if recovery of water is of interest as well, an elegant industrially proven solution of the problem is a multieffect azeotropic distillation. This process was developed by the BASF in Ludwigs-hafen/Germany [58]. 

It was realized that in the water/acetic acid system, in the range of low acid concentrations, the vapor/liquid equilibrium line very nearly coincides with the diagonal, so that obtaining pure water by simple distillation would require a monstrous number of stages (in the order of 150) as well as a huge reflux ratio (in the order of 9). In view of this situation, various considerations have led the BASF to choose azeotropic distillation using butyl acetate as entrainer. 

Point A represents the water/butyl acetate azeotrope at 28.719 % by weight of water, with an atmospheric boiling point of 91.0 C. 

Condensation of the head vapors leads to two liquid phases An organic phase consisting essentially of butyl acetate with very little water, and an aqueous phase consisting essentially of water with very little acetate. The organic phases are used as refluxes, while the combined aqueous phases are fed through a cooler into the head of a small fourth column, in which the water is stripped of its acetate by means of a reboiler energized with low-pressure steam. The fourth column is operated at the same pressure as the third column, the head vapors of the third and fourth columns being liquefied in a condenser maintained at 0.5 ATM by means of a vacuum pump. 

---