Salicylic acid (ortho-hydroxybenzoic acid) has been around forever. Its
main end use is in the preparation of ortho-acetylsalicylic acid (Aspirin).
Competition from acetaminophen (Tylenol), ibuprofen (Advil), and naproxen
(Aleve) have led to slow to no market growth for acetylsalicylic acid and
hence for salicylic acid, too. Worldwide production of salicylic acid is roughly
60,000 metric tons/yr.
Salicylic acid is produced commercially via the Kolbe-Schmitt process. Here
phenol and
sodium hydroxide are reacted to make sodium phenoxide. The phenoxide is contacted
with CO2
to form sodium salicylate. The salicylate is acidified to give salicylic acid.
The acid is usually crystallized from aqueous solution to give a technical
grade 99.5% salicylic acid product. For a pharmaceutical grade product a further
purification step is required. Typically this is achieved by sublimation.
The Kolbe-Schmitt synthesis is very old and a number of manufacturing methods
have been used. The old Wacker process was carried out in phenol solution.
Later processes have used ball mill autoclaves, counter-bladed kneader/mixers,
and other batch reaction means. In this review, we present a semi-continuous
process that uses a fluidized bed reactor for the CO2 addition step. Our design
capacity is 10,000 metric tons/yr.
