Verfahren zur Herstellung von Harnstoff

摘要

Urea is made by a process which comprises introducing ammonia and carbon dioxide in a molar ratio of at least 2:1 into a urea synthesis zone maintained at elevated conditions of temperature and pressure whereby a corrosive ammonium carbamate melt is formed as an intermediate and whereby a product stream containing urea, water vapor, ammonia and carbon dioxide is produced, the reaction being effected in apparatus constructed in such a manner that the surfaces thereof exposed to the reaction mixture are zironium, separating from the product stream a first gaseous mixture having a pressure within the range from about 70 p.s.i.a. to about 280 p.s.i.a. and consisting essentially of ammonia, carbon dioxide and water vapor, the molar ratio of ammonia to carbon dioxide being within the range of from about 20:1 to about 2:1 and the amount of water vapor being up to the limit of saturation; separating from the product stream a second gaseous mixture having a pressure within the range from 15 p.s.i.a. to 50 p.s.i.a. and consisting essentially of ammonia, carbon dioxide and water vapor, the molar ratio of ammonia to carbon dioxide being within the range from about 10:1 to about 1:1 and the amount of water vapor being up to the limit of saturation; contacting the first gaseous mixture with a liquid alkylolamine of at least 50% by weight alkylolamine concentration in a first contact stage maintained at a pressure within the range of 70 p.s.i.a. to 280 p.s.i.a. wherein all the carbon dioxide and water are absorbed, introducing sufficient liquid ammonia into the first contact stage to maintain the bottoms alkylolamine temperature at the maximum level precluding excessive alkylolamine degradation which would otherwise occur, removing unabsorbed ammonia gas from the first contact stage, removing the alkylolamine enriched with carbon dioxide, water and ammonia from the first contact stage, contacting the second gaseous mixture with both the enriched alkylolamine from the first contact stage and liquid alkylolamine of at least 50% by weight alkylolamine concentration in a second contact stage maintained at a pressure within the range of 15 p.s.i.a. to 50 p.s.i.a. wherein all the carbon dioxide and water are absorbed from the gaseous mixture and ammonia is desorbed from the enriched alkylolamine, removing ammonia gas from the second contact stage, removing the alkylolamine enriched with carbon dioxide and water from the second contact stage and passing it to a third stage and therein heating it whereby absorbed carbon dioxide and water vapor are liberated, withdrawing carbon dioxide gas and water vapor from the third stage, withdrawing liquid alkylolamine substantially free of water and carbon dioxide from the third stage, liquefying the ammonia removed from the first contact stage and the second contact stage and returning the liquid ammonia to the urea synthesis zone, preferably wherein carbon dioxide gas and water vapor withdrawn from the third stage is passed to a carbon dioxide separation stage wherein the carbon dioxide gas is separated from the water vapor, and the carbon dioxide is removed from the separation stage and returned to the urea synthesis zone. The invention is descibed with reference to a flow sheet (not shown).