Butanediol Production from Propylene Oxide

The process under analysis comprises three major sections: (1) Isomerization; (2) Butanediol synthesis; and (3) Purification.

Isomerization. Fresh and recycled propylene oxide streams are mixed, vaporized and superheated before being fed to the isomerization reactor. The isomerization reaction is conducted in the gas phase in a fixed-bed reactor. The reaction is promoted by a basic lithium phosphate catalyst, supported on high-purity silica. Allyl alcohol and several by-products are formed during the isomerization reaction. The isomerization effluent is then sent to two distillation columns, where unconverted propylene oxide is recovered and recycled, and the allyl alcohol is separated from the other contaminants.

Butanediol synthesis. The allyl alcohol is reacted with synthesis gas in a hydroformylation reaction, promoted by a catalyst system composed of a rhodium complex and a diphosphine. The reaction is conducted in two steps, first in a stirred tank reactor, then in a bubble column. The 4-hydroxybutyraldehyde product is recovered from the reaction medium by water extraction and it is directed to the next reaction step. The organic-phase with the catalyst system is recycled to the hydroformylation reaction. The aqueous effluent, containing the 4-hydroxybutyraldehyde is directed to a hydrogenation reactor, where the 4-hydroxybutyraldehyde reacts with hydrogen, yielding 1,4-Butanediol. This reaction is carried out in a vertical shell and tube reactor filled with a Raney nickel catalyst.

Purification. The output stream from the hydrogenation is fed to a stirred tank, where it is mixed with a sodium hydroxide solution to precipitate the dissolved nickel catalyst, which is removed by filtration. Then, propanol and water are separated in a low-boiling distillation column followed by a triple effect evaporator. n-propanol is further recovered to be sold as by-product. Finally, the concentrated stream from evaporators is directed to three distillation columns in series, where high-purity 1,4-Butanediol product is obtained as final product and 2-methyl-1,3-propanediol is obtained as by-product.