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Modeling Urea Processes: A New
Thermodynamic Model and Software Integration Paradigm
Stamicarbon Process (Carbon Dioxide Stripping)
"NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of approximately 140 bar and a temperature of 180-185° C. The molar NH3/CO2 ratio applied in the reactor is 2.95. This results in a CO2 conversion of about 60% and an NH3 conversion of 41%. The reactor effluent, containing unconverted NH3 and CO2 is subjected to a stripping operation at essentially reactor pressure, using CO2 as stripping agent. The stripped-off NH3 and CO2 are then partially condensed and recycled to the reactor. The heat evolving from this condensation is utilized to produce 4.5 bar steam, some of which can be used for heating purposes in the downstream sections of the plant. Surplus 4.5 bar steam is sent to the turbine of the CO2 compressor.
The NH3 and CO2 in the stripper effluent are vaporized in a 4 bar decomposition stage and subsequently condensed to form a carbamate solution, which is recycled to the 140 bar synthesis section. Further concentration of the urea solution leaving the 4 bar decomposition stage takes place in the evaporation section, where a 99.7% urea melt is produced." (6)
Figure 1: Total Recycle CO2 Stripping Urea Process (6)
Snamprogetti Process (Ammonia Stripping)
"NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of 150 bar and a temperature of 180° C. A molar ratio of 3.5 is used in the reactor giving a CO2 conversion of 65%. The reactor effluent enters the stripper where a large part of the unconverted carbamate is decomposed by the stripping action of the excess NH3. Residual carbamate and CO2 are recovered downstream of the stripper in two successive stages operating at 17 and 3.5 bar respectively. NH3 and CO2 vapors from the stripper top are mixed with the recovered carbamate solution from the High Pressure (HP)/Low Pressure (LP) sections, condensed in the HP carbamate condenser and fed to the reactor. The heat of condensation is used to produce LP steam. The urea solution leaving the LP decomposition stage is concentrated in the evaporation section to a urea melt." (6)
Figure 2: Total Recycle NH3 Stripping Urea Process (6)
Urea processes are challenging to model from a thermodynamic point of view. From one side, accurate low pressure equilibrium thermodynamic equilibrium is necessary to model aqueous urea solutions, while accurate high pressure modeling is necessary to properly model the high pressure synthesis reactor. The thermodynamic package also has to properly take into account the formation of new chemical species, some which are ionic. The effect of minute amounts of inerts in the saturation bubble pressure also has to be taken into account. In addition, the model has to provide reasonable enthalpy and entropy values for flowsheeting calculations. Last but not the least, some operations in the urea process require special behavior from the property package calculation engine and proper communication between the unit operations and the property package system has to be implemented.
The thermodynamic modeling is conveniently divided into high pressure and medium / low pressure areas. In the high-pressure section we have a non-aqueous ionic system while in the medium / low pressure areas we have an aqueous ionic system.