A two-stage evaporative concentration under vacuum, with a limited residence time in the evaporator to limit biuret formation, produces a urea stream containing about 0.5% water which can be sprayed into a prilling tower.

I want the Process Flow Diagram of this system.

a The reaction that produces urea from ammonia and carbon dioxide takes place in two stages; in the first, ammonium carbamate is formed: 2NH3 + CO2 = NH, COONH4 In the second, the carbamate is dehydrated to give urea: NH2COONH4 = CO(NH2)2 + H2O Both reactions are reversible, the first being exothermic and going almost to completion, while the second is endothermic and goes to 40 to 70% of completion. Ammonia and carbon dioxide are fed to the reactor, a stainless steel vessel with a series of trays to assist mixing. The reactor pressure is 125 bar and the temperature is 1858C. The reactor residence time is about 45 seconds, a 95% approach to equilibrium being achieved in this time. The ammonia is fed directly to the reactor, but the carbon dioxide is fed to the reactor upwardly through a stripper, down which flows the product stream from the reactor. The carbon dioxide decomposes some of the carbamate in the product stream, and takes ammonia and water to a high-pressure condenser. The stripper is steam heated and operates at 1808C, while the high-pressure condenser is at 1708C and the heat released in it by recombination of ammonia and carbon dioxide to carbamate is used to raise steam. Additional recycled carbamate solution is added to the stream in the high-pressure condenser, and the combined flow goes to the reactor. The product stream leaving the stripper goes through an expansion valve to the low pressure section, the operating pressure there being 5 bar. In a steam-heated rectifier, further ammonia and carbon dioxide are removed and, with some water vapor, are condensed to give a weak carbamate solution. This is pumped back to the high-pressure condenser