Acetic Anhydride Production from Acetic Acid

The process under analysis comprises four major sections: (1) Acetic Acid Pyrolysis; (2) Ketene Absorption; (3) Purification; and (4) Acetic Acid Recovery.

Acetic Acid Pyrolysis. Initially, part of the fresh glacial acetic acid is mixed with recycled acetic acid, vaporized and mixed with triethylphosphate catalyst. The mixture is fed into a multitubular furnace where the acetic acid is cracked to ketene and water at temperatures of about 750 °C and pressures of about 0.25 bar. The reaction effluent receives an injection of ammonia in order to neutralize the catalyst, preventing the ketene from reacting with the water or with the unconverted acetic acid.

Ketene Absorption. The outlet gas from the pyrolysis is cooled down to about 0 °C in such a way that an aqueous solution of acetic acid is condensed and separated from the ketene gas. The ketene gas is then absorbed by the countercurrent contact with glacial acetic acid in a series of two absorbers. The Crude Acetic Anhydride obtained is sent to the purification section. The off-gas from the second absorber is sent to two scrubbers operated in series, in order to recover the acetic acid entrained by these gases. The recovered aqueous acetic acid solution is sent to the acetic acid recovery section.

Purification. The purification area is composed of three distillation columns. The light-ends column removes the volatile liquids and a small part of acetic acid, which are recycled to the absorption section. In the acetic acid separation column, glacial acetic acid is obtained and recycled to the pyrolysis. Finally, heavy-ends are separated in the acetic anhydride purification column and Acetic Anhydride with a purity of 99 wt.% is obtained and directed to storage facilities located outside battery limits.

Acetic Acid Recovery. In this section, acetic acid is dehydrated and purified for its reuse along the process. Water is removed by azeotropic distillation using n-butyl acetate as the entrainer. Subsequently, glacial acetic acid is obtained after removal of heavy impurities by means of distillation and recycled to the pyrolysis section.