How acetic anhydride is made

Water slowly turns acetic anhydride back into acetic acid, so plants keep the whole process bone dry.

stations: 5
things upstream: 4
raw sources: 2
steps from the ground: 2

The short version

Acetic anhydride is a colorless liquid with a sharp vinegar smell. The largest commercial plants make it from methyl acetate and carbon monoxide, using a rhodium catalyst and an iodide promoter in a loop kept free of water. A flash step returns the catalyst, and distillation gives the product. Its main customer is the cellulose acetate industry.

Make methyl acetate

Plants begin with methyl acetate, an ester made by reacting methanol with acetic acid. An integrated site gets much of that acid from its own later steps.

Goes in
Methanol
Run the carbonylation loop

Carbon monoxide with a little hydrogen, which keeps the catalyst active, meets the ester in pressure reactors in liquid phase. The chemistry goes through methyl iodide and acetyl iodide. Water stays out, because it would break the product back down, and keeping the catalyst stable without it is the central problem.

Goes in
Carbon monoxide
Flash off the light liquids

Leaving the reactor, the liquid enters a flash tank or evaporator. Lighter components boil off toward purification, while the rhodium and other heavy material go back to the reactor.
Distill the crude liquid

Distillation splits the finished liquid from acetic acid and other light components, which return to the process.
Recycle the co-product

At a cellulose acetate site, the product reacts with cellulose and frees acetic acid in the process. That acid goes back to meet methanol and form more methyl acetate, closing the circle.

End of the lineAcetic anhydride Next it goes into aspirin, acetaminophen, ibuprofen and 1 more.

Worth repeating

An integrated plant can turn out cellulose acetate from only methanol, carbon monoxide and cellulose, with no outside acetic acid.
The older ketene route to acetic anhydride dates to 1922, when Wacker Chemie was chasing demand for cellulose acetate.