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The steady-state simulation of an ammonium carbamate decomposer (stripper) is presented. This falling-film reactor is usually located in the high-pressure section of industrial urea plants in order to concentrate the urea solution leaving the synthesis reactor.
The stripper consists in a bundle of small diameter tubes installed in a heating shell. The aqueous urea solution (also containing ammonium carbamate, NH3, CO2 and inerts) is fed to the top of the stripper and flows by gravity effects as a thin film on the internal wall of the tubes. The ammonium carbamate is decomposed through the endothermic reaction: NH2COO- + NH4+ == 2NH3 + CO2 , at around 200 ºC and 150 atm. The energy for the reaction and the evaporation of the volatile components is supplied by means of saturated steam (shell side). The stripped gas (containing NH3, CO2, H2O and N2) is collected in the central section of the tubes and flows upwards countercurrently with the urea solution. The gas leaves the stripper by the top to be recycled to the urea synthesis section. The concentrated urea solution is sent to the medium pressure section of the plant for further purification.
The countercurrent gas-liquid flow is represented as a series of stages with perfect mixing for both phases.
The ø-ø method is used as thermodynamic model to describe the phase equilibrium. The mass transfer limitations (assumed to be concentrated in the gas film) are represented by the Maxwell-Stefan equations.
The mass and energy balances for both phases, the equilibrium equations and the flows through the gas-liquid interface for the multicomponent system are considered for all the stages.
The influence of the main operating variables on the stripper performance is studied. The model predicts stripped gas flowrates, outlet compositions and temperatures values in agreement with those found in a large scale stripper.