Hexamethylenediamine Production from Butadiene

The process under analysis comprises four major sections: (1) Hydrocyanation; (2) Adiponitrile Separation; (3) Hydrogenation; and (4) HDMA Purification.

Hydrocyanation. In the first reaction stage, butadiene is fed to the reactor, along with hydrogen cyanide. This reaction produces different isomers of pentenenitriles and branched butenenitriles. Catalyst and unconverted reactants are separated from reaction mixture and recycled to the reactor. As the hydrocyanation of branched butene nitriles does not yield adiponitrile, they must also be separated from linear nitriles prior to second hydrocyanation step. Separation is accomplished in a distillation column and separated branched nitriles are further treated for recovery of 2-methyl-3-butenenitrile (2M3BN). 2M3BN is fed into an isomerization reactor, where it is converted to useful 3-pentenenitrile (3PN), which is mixed with the linear pentenenitriles previously obtained and fed to the second hydrocyanation reactor. The reactor is also fed with fresh HCN, as well as fresh and recycled catalyst solution. Adiponitrile is the main product formed in the reaction, but some yield losses may occur due to the formation of by-products.

Adiponitrile Separation. First, catalyst is separated from reaction mixture and recycled to the reactor. The product stream is passed through three distillation steps to recover mononitriles, dinitriles and dimers as well as other high boiling compounds. Purified adiponitrile is routed to the hydrogenation section.

Hydrogenation. Adiponitrile, fresh and recycle hydrogen are fed to a “gas-lift” reactor, where they quickly react on the surface of RaneyTM nickel catalysts yielding Hexamethylenediamine (HMDA). High catalyst concentrations are used to keep the concentration of unreacted nitriles very low. Water and caustic soda are fed to the reactor so as to make up for losses leaving in the crude HMDA.

HDMA Purification. Crude HMDA is passed through a distillation train, so as to remove water, 1,2-diaminocyclohexane (DCH) and high boilers. DCH is still sent to a column, in which residual HDMA is recovered, before it is purged.