GB811646A - Process for recovering -e-caprolactam from waste lyes of the -e-caprolactam production - Google Patents

Abstract

e -Caprolactam is recovered from waste lyes of the e -caprolactam production by reducing the water content by at least 50 per cent and extracting the concentrated lyes by means of a solvent which is immiscible with water, distilling off approximately 90 per cent of the solvent from the extract, lixiviating the residue to remove the lactam and evaporating the aqueous solution thus obtained. The crude lactam so produced can be further treated in a known manner to produce pure lactam. The solvent recovered in distilling off the extract and washing the evaporation residue is returned direct to the process which is carried out in a continuous manner. Trichlorethylene is a suitable solvent for the continuous process. In a method of operation shown diagrammatically, waste lye is forced continuously by a centrifugal pump into a circulation evaporator heated with steam at 16 atm. abs. and thereby concentrated to 50 to 60 per cent, and the thickened lye is passed to a sediment container wherein ammonium sulphate is precipitated. A reciprocating pump conveys the lye via a Grafe meter to a mixing pump where the emulsification and extraction are carried out with trichloroethylene in a proportion of 1 : 4. The mixture passes through a cooler to a separator where the residual lye is separated from the trichloroethylene-lactam extract. The extract flows via a syphon into a settling tower to separate residual lye and then enters a cascade evaporator wherein 90 per cent of the trichloroethylene is evaporated. The condensed solvent is returned by a centrifugal pump to the mixing pump. The residue from the evaporator is passed to the top of a washing tower and the lactam contained therein dissolves in the condensate water flowing up the tower and passes out at the head of the tower as a 35 per cent aqueous solution. The remaining 10 per cent of trichloroethylene is recovered from the foot of the washing tower and pumped back to the settling tower. The aqueous lactam solution is passed into a device where it is heated with steam at 3 atm. abs. to expel the water and yield a crude lactam. The latter is worked up into pure lactam by distillation in vacuum with the addition of acid or alkaline refining agents, e.g. sodium hydroxide. A detailed example is described.