Process for the preparation of urea

Claim 1

In a process for preparing urea by reacting ammonia and carbon dioxide to form an ammonium carbamate intermediate product in a first reaction zone, and then converting ammonium carbamate into urea in an additional reaction zone at a higher pressure, partially expanding the resulting urea synthesis solution and subjecting the urea synthesis solution to a stripping treatment, and separating gases released in the stripping treatment and from the urea synthesis solution and recycling the separated gases to the first reaction zone, the improvement comprising conducting the formation of ammonium carbamate from ammonia and carbon dioxide in said first reaction zone at a pressure of 90 - 140 atmospheres and a temperature of about 150° to 173° C, thereafter partly converting the ammonium carbamate into urea in a second reaction zone at substantially the same pressure as said first reaction zone and a temperature of about 165 to 183° C until at least 40% of the equilibrium amount of urea has been formed, and thereafter converting ammonium carbamate present in the synthesis solution from the second reaction zone into additional urea in a third reaction zone at a pressure of at least 160 atmospheres and a temperature of about 190° to 220° C until at least 85% of the equilibrium amount of urea that can be obtained in this zone has been formed.