GB885691A - Urea synthesis using excess ammonia - Google Patents

Abstract

A process for the production of urea by the reaction of 4-5 moles of ammonia with 1 mole of carbon dioxide at 180 DEG C. and 280 atmospheres pressure includes the steps of subjecting the urea melt from the synthesis zone to distillation at 90 DEG C.-120 DEG C. and 10-15 atmospheres pressure to separate a mixture containing a major proportion of unreacted ammonia and a minor proportion of unreacted carbon dioxide; scrubbing the mixture with a limited quantity of water to remove substantially all the CO2 contained therein, thus producing pure ammonia which is liquified for delivery under pressure to the synthesis zone, and an aqueous ammonium carbonate solution; subjecting the treated urea melt to a further distillation at 1-3 atmospheres pressure to remove substantially all the remaining NH3 and CO2 together with a substantial proportion of the water produced in the synthesis: dehumidifying the latter mixture to produce a gaseous product containing 7%-10% water; absorbing said gaseous product in said aqueous ammonium carbonate solution at 80 DEG C.-140 DEG C. and 20-50 atmospheres pressure; and recycling to the synthesis zone the NH3- and CO2-containing product so obtained, having a water content of 10%-15%. An intermediate distillation of the urea melt may be carried out after the first distillation at 10-15 atmospheres, said second distillation being at 20-50 atmospheres to separate a mixture of unreacted NH3 and CO2 which is absorbed directly in the aqueous ammonium carbonate solution. A process for the dehumidification step by a rectification procedure is described.