Patent ID: 7501047

Claim:
A process for purifying crude caprolactam comprising 1) converting a mixture (I) comprising 6-aminocapronitrile and water to a mixture (II) comprising caprolactam, ammonia, water, high boilers and low boilers in the presence of a catalyst, 2) removing ammonia from the mixture (II) to obtain a mixture (III) comprising caprolactam, water, high boilers and low boilers, 3) completely or partly removing water from the mixture (III) to obtain crude caprolactam (IV) comprising caprolactam, high boilers and low boilers, and 4) distilling the crude caprolactam (IV) comprising a) feeding the crude caprolactam (IV) and an inorganic acid which has a boiling point above the boiling point of caprolactam to a first distillation apparatus C 1 , b) distilling the crude caprolactam IV and the inorganic acid in the first distillation apparatus C 1 , and removing a first substream in the bottom region and a second substream in the top region of the distillation apparatus C 1 , c) feeding the second substream from step b) to a second distillation apparatus C 2 , d) distilling the second substream from step b) in the second distillation apparatus C 2 , and removing a first substream in the bottom region and a second substream in the top region of the distillation apparatus C 2 , e) feeding the first substream from step d) to a third distillation apparatus C 3 , f) distilling the first substream from d) in the third distillation apparatus C 3 , and removing a first substream in the bottom region of distillation apparatus C 3 and removing purified caprolactam in the top region of distillation apparatus C 3 , and g) feeding the first substream from step f) to the first distillation apparatus C 1 .