Urea manufacturing method and urea manufacturing apparatus

A urea manufacturing method comprising:

• a synthesis step of reacting carbon dioxide and ammonia in a reactor under a condition of excessive ammonia, thereby providing a synthesis mixture containing urea, ammonium carbamate, water, unreacted ammonia, and unreacted carbon dioxide;
• a decomposition step of decomposing the ammonium carbamate by heating the synthesis mixture and stripping using at least a portion of raw material carbon dioxide as an auxiliary agent, thereby providing a decomposed gas containing ammonia and carbon dioxide, and a urea synthesis solution containing ammonia, carbon dioxide, water, and urea;
• a pressure reduction step of placing the urea synthesis solution under reduced pressure, thereby providing a gas-liquid mixture;
• a heating step of heating the gas-liquid mixture with the decomposed gas;
• a purification step of separating water and unreacted substances including ammonia and carbon dioxide from the heated gas-liquid mixture, thereby providing a purified urea and water and recovering the separated water and unreacted substances;
• a condensation step of condensing the decomposed gas after heating the gas-liquid mixture in a condenser together with at least a portion of the water and unreacted substances recovered in the purification step that is introduced through a first recovered unreacted substance line directly connected to the condenser, thereby providing a condensate;
• and
• a condensate introduction step of introducing the condensate to the reactor;
• wherein in the heating step, the decomposed gas is introduced into a tube side of a shell-and-tube heat exchanger while the gas-liquid mixture is introduced into a shell side of the shell-and-tube heat exchanger;
• and
• wherein a portion of the water and the unreacted substances recovered in the purification system is introduced into the tube side of the shell-and-tube heat exchanger through a second recovered unreacted substance line, that is directly connected to the shell-and-tube heat exchanger, together with the decomposed gas.