Urea is manufactured by the following method: first, ammonia (NH3) and carbon dioxide (CO2) are subjected to reaction to produce ammonium carbamate (NH2COONH4) as represented by Formula (1), and then, ammonium carbamate is subjected to dehydration reaction to produce urea (NH2CONH2) and water (H2O) as represented by Formula (2).2NH3+CO2→NH2COONH4  (1)NH2COONH4→NH2CONH2+H2O  (2)Both reactions are the equilibrium reaction but the reaction of Formula (1) is the exothermic reaction while the reaction of Formula (2) is the endothermic reaction. For this reason, various schemes have been studied to increase the conversion ratio from the raw materials of ammonia and carbon dioxide to urea.
Patent Literature 1 has described the improved urea synthesis method with the characteristics below. In this method, ammonia and carbon dioxide react with each other under the urea synthesis temperature and pressure in the urea synthesis zone. The resulting urea synthesis solution containing urea, unreacted ammonia, unreacted carbon dioxide, and water is brought into contact with at least a portion of the raw material carbon dioxide under heating and under the pressure substantially equal to the urea synthesis pressure in the stripping zone. This causes the unreacted ammonia and the unreacted carbon dioxide to be separated as the mixed gas of ammonia, carbon dioxide, and water. The urea synthesis solution containing the unreacted ammonia and the unreacted carbon dioxide which are not separated is processed further; thus, the urea is obtained. Meanwhile, the mixed gas separated in the stripping zone is introduced to the bottom of the vertical condensation zone and is brought into contact with the absorbing medium while being cooled. This causes the mixed gas to be condensed. The resulting condensate circulates in the urea synthesis zone.
In the third example of Patent Literature 1, ammonia as the raw material is heated up to 175° C. in the heat exchanger and then introduced into the ejector. The ejector plays the role of sending the solution from the condenser to the reactor under the boosted pressure. Carbon dioxide (CO2) is introduced into the reactor and the stripper. The temperature of the solution in the condenser is adjusted to 185° C. The solution goes through the ejector to be introduced into the reactor. In the reactor, urea is synthesized through the dehydration reaction of the ammonium carbamate. This reaction is endothermic reaction. By increasing the temperature of the raw material ammonia up to 175° C., the temperature of the reactor is maintained so as not to decrease below 185° C.
According to Patent Literature 2, the temperature of the reaction zone where the urea synthesis is carried out is increased by introducing at least a portion of a gas mixture discharged from the stripping zone into the reaction zone and condensing the introduced gas mixture.