2NH3 + CO2 NH2COONH4 -37.4 Kcal/mol
NH2COONH4 NH2CONH2 + H2O + 6.3 Kcal/mol
While going main reactions, the undesirable side reaction taking place is
2NH2CONH2 NH2CONHCONH2 (Biuret) + NH3
This process is further complicated by the formation of a substance called biuret, NH2CONHCONH2, which should be kept low because it adversely affects the growth of some plants. Figure. Schematic representation of urea synthesis
We can consider that the totally process which can be divided to four sub processes.
Ammonia and CO2 are compressed separately and fed at high pressure (180 atm) to the …show more content…
This liquid effluent is let down to 27 atm and fed to a special flash-evaporator containing a gas-liquid separator and condenser. Unreacted NH3, CO2 and H2O are thus removed and recycled. An aqueous solution of carbamate-urea is passed to the atmospheric flash drum where further decomposition of carbamate takes place.
NH2COONH4 2NH3 + CO2
The pressure is then reduced and a solution of urea dissolved in water and free of other impurities remains. At each stage the unconsumed reactants are absorbed into a water solution which is recycled to the secondary reactor. The excess ammonia is purified and used as feedstock to the primary reactor.
75% of the urea solution is heated under vacuum, which evaporates off some of the water, increasing the urea concentration from 68% w/w to 80% w/w. At this stage some urea crystals also form. The solution is then heated from 80 to 110oC to re-dissolve these crystals prior to evaporation. In the evaporation stage molten urea (99% w/w) is produced at 140oC. 4- …show more content…
The main reaction is carried out with excess ammonia at similar conditions as the once through process. The reactor effluent containing 80% urea passes through a high-pressure separator where excess ammonia is stripped using steam. The ammonia is then recovered in the absorption tower and pumped back to the reactor. It is necessary to recover ammonia before passing it directly to the carbamate stripper because excess ammonia will hinder the decomposition of ammonium carbamate. In addition, recovering ammonia covers some of the production costs. The stream containing the carbamate, urea and water goes to the high- then to the low-pressure carbamate decomposers where the carbamate decomposes to NH3 and CO2. Also, the urea solution gets separated and goes to the finishing for further