Urea synthesis method

A method for improving an existing urea synthesis apparatus to reduce a urea synthesis pressure in a urea synthesis tower, wherein

• the existing urea synthesis apparatus is a urea synthesis apparatus for reacting ammonia with carbon dioxide containing corrosion prevention oxygen fed from a carbonic acid gas compressor at a urea synthesis temperature and a urea synthesis pressure in a urea synthesis tower, bringing a urea synthesis liquid containing at least produced urea, unreacted ammonia, unreacted carbon dioxide and water into contact with at least part of the raw material carbon dioxide under heating at a pressure substantially equal to the urea synthesis pressure in a stripper to separate the unreacted ammonia and the unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide and water, further treating the urea synthesis liquid containing unseparated unreacted ammonia and unreacted carbon dioxide to obtain urea, while introducing the mixed gas separated in the stripper into a bottom of a condenser to bring the mixed gas into contact with an absorbing medium under cooling to thereby condense the mixed gas, and circulating the thus obtained condensate to the urea synthesis tower, and
• a material of piping connecting the stripper and the condenser in the existing urea synthesis apparatus is neither of the materials (a) and (b) below,
• the method including, replacing the piping connecting the stripper and the condenser in the existing urea synthesis apparatus with piping including the material (a) or (b) below and reducing an oxygen feed concentration with respect to the raw material carbon dioxide in the carbonic acid gas compressor to enable reduction of the urea synthesis pressure in the urea synthesis tower to 125 to 145 bar, a urea synthesis temperature of 175 to 190° C. in the urea synthesis tower, and a pressure of 130 to 140 bar in operating conditions of the stripper,
• (a) a general-purpose austenitic-ferritic duplex stainless steel with a Cr content of 21 to 26 masse, a Ni content of 4.5 to 7.5 mass %, a Mo content of 2.5 to 3.5 mass %, a N content of 0.08 to 0.30 mass %, a C content of 0.03 mass % or less, a Si content of 1.0 mass % or less, a Mn content of 2.0 mass % or less, a P content of 0.04 mass % or less, and a S content of 0.03 mass % or less, and
• (b) an austenitic-ferritic duplex stainless steel with a Cr content of 26 to 35 mass %, a Ni content of 3 to 10 mass %, a Mo content of 1.0 to 4.0 mass %, and a N content of 0.2 to 0.6 mass %.

A method for improving an existing urea synthesis apparatus to reduce a urea synthesis pressure in a urea synthesis zone, wherein

• the existing urea synthesis apparatus is a urea synthesis apparatus for reacting ammonia with carbon dioxide containing corrosion prevention oxygen fed from a carbonic acid gas compressor at a urea synthesis temperature and a urea synthesis pressure in a urea synthesis zone, bringing a urea synthesis liquid containing at least produced urea, unreacted ammonia, unreacted carbon dioxide and water into contact with at least part of the raw material carbon dioxide under heating at a pressure substantially equal to the urea synthesis pressure in a stripper to separate the unreacted ammonia and the unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide and water, further treating the urea synthesis liquid containing unseparated unreacted ammonia and unreacted carbon dioxide to obtain urea, while introducing the mixed gas separated in the stripper into a bottom of a condensation zone to bring the mixed gas into contact with an absorbing medium under cooling to thereby condense the mixed gas, and circulating the thus obtained condensate to the urea synthesis zone,
• the urea synthesis zone and the condensation zone are present in the same container or in separate containers,
• the condensation zone is present in a horizontal container, and
• a material of piping connecting the stripper and the condensation zone in the existing urea synthesis apparatus is neither of the materials (a) and (b) below,
• the method including, replacing the piping connecting the stripper and the condensation zone in the existing urea synthesis apparatus with piping including the material (a) or (b) below and reducing an oxygen feed concentration with respect to the raw material carbon dioxide in the carbonic acid gas compressor to enable reduction of the urea synthesis pressure in the urea synthesis zone to 125 to 145 bar, a urea synthesis temperature of 175 to 190° C. in the urea synthesis zone, and a pressure of 130 to 140 bar in operating conditions of the stripper,
• (a) a general-purpose austenitic-ferritic duplex stainless steel with a Cr content of 21 to 26 mass %, a Ni content of 4.5 to 7.5 mass %, a Mo content of 2.5 to 3.5 mass %, a N content of 0.08 to 0.30 mass %, a C content of 0.03 mass % or less, a Si content of 1.0 mass % or less, a Mn content of 2.0 mass % or less, a P content of 0.04 mass % or less, and a S content of 0.03 mass % or less, and
• (b) an austenitic-ferritic duplex stainless steel with a Cr content of 26 to 35 mass %, a Ni content of 3 to 10 mass %, a Mo content of 1.0 to 4.0 mass %, and a N content of 0.2 to 0.6 mass %.