GB778751A - Process of purifying caprolactam - Google Patents

Abstract

Caprolactam is purified by passing hydrogen through an aqueous solution of the caprolactam at a temperature below the boiling point thereof in the presence of a hydrogenation catalyst, and recovering the caprolactam from the reaction liquid. It is preferred to employ lactam concentrations of 15-30 per cent by weight and to subject the solution to a preliminary purification with anion or cation exchangers as described in Specification 762,879. Higher concentrations of 40, 60 or 90 per cent by weight may also be used. The preferred hydrogenation temperature is 85-100 DEG C., but temperatures of 15-25 DEG C. or 30-50 DEG C. may be employed. The hydrogen pressure may be maintained at 1-2 atmospheres gauge. Suitable catalysts are iron, nickel, palladium, platinum and copper, or metal oxides, e.g. molybdenum oxide, copper oxide, vanadium oxide, or mixtures of metals and/or metal oxides. The catalysts may be deposited on carriers such as kieselguhr, diatomaceous earth, silica gel or alumina. Foraminate catalysts prepared by removal of aluminium or silicon from their alloys with nickel, cobalt, iron or copper are also suitable. The catalyst may be suspended in the lactam solution or may be employed in a fixed bed through which the lactam solution and hydrogen are passed. The purified caprolactam may be recovered by distillation, or the reaction liquid may be passed over anion or cation exchangers, or both anion and cation exchangers simultaneously or in succession, to remove the hydrogenated impurities whereafter the lactam solution may be evaporated to remove the water. In an example, an impure aqueous caprolactam solution is passed successively through columns of adsorbent carbon, cation exchanger and anion exchanger, and then is passed downwardly through three vertical reaction vessels filled with palladium-on-alumina catalyst, in countercurrent relation to the flow of hydrogen, and then is passed successively through a column of cation exchanger and a column of anion exchanger and subsequently evaporated to dryness.