Kombinationsverfahren zur Herstellung von Harnstoff und Melamin

摘要

1,216,100. Production of urea and melamine. MITSUI TOATSU CHEMICALS Inc. 10 July, 1968 [19 July, 1967], No. 33001/68. Heading C2C. Urea and melamine are obtained in a process comprising reacting ammonia with CO 2 at a urea-forming temperature and pressure in a urea-synthesis zone, expanding and passing the resulting urea-synthesis effluent through a highpressure distillation zone kept at a head temperature of 90-130‹ C. and a still temperature of 130-170‹ C. under a gauge pressure of 10- 30 kg/cm.2, and then through a low-pressure distillation zone kept at a pressure lower than that of the said high-pressure distillation zone to separate in each stage unreacted ammonium carbamate as a gaseous mixture of ammonia, carbon dioxide and water vapour, thereby obtaining urea, at least a part of the urea being heated in the presence of ammonia and a catalyst, the resulting gaseous reaction product comprising melamine, ammonia and CO 2 being cooled with, as coolant, water or an aqueous solution of NH 3 , ammonium carbamate, or ammonium carbamate and melamine to separate melamine and, concurrently, to form an aqueous solution containing by-produced ammonia and CO 2 as ammonium carbamate, the aqueous solution of ammonium carbamate being subjected to distillation at a gauge pressure of 3-10 kg/cm.2 to separate a gaseous mixture of ammonia, CO 2 and water vapour, the gaseous mixture separated from the said aqueous solution of ammonium carbamate and the gaseous mixtures separated by the said lowpressure distillation and the said high-pressure distillation from the said urea-synthesis effluent being recovered and recirculated to the said urea-synthesis zone, wherein there is absorbed in a medium pressure absorption column at a gauge pressure of 3 to 10 kg/cm.2 the said gaseous mixture, by-produced in producing melamine, into the ammonium carbamate solution obtained by absorbing into an absorbent the said gaseous mixture separated in the low-pressure distillation zone to form a first absorbate, the pressure of the said first absorbate is increased, a part of the said first absorbate having the pressure increased is introduced into the said high-pressure distillation zone thereby the ammonium carbamate contained therein is decomposed together with the ammonium carbamate in the said urea-synthesis effluent to form a single gaseous mixture of NH 3 , CO 2 and water vapour and the thus separated gaseous mixture is absorbed into the remaining part of the said first absorbate in a high-pressur absorption column to form a second absorbate for recirculation to the said urea-synthesis zone. The absorbent employed may be water, an aqueous ammonia solution or an aqueous urea solution and the amount of the first absorbate which is introduced into the high-pressure distillation column may be varied depending on the composition of said first absorbate.