Steady State Analysis of the High-Pressure Section of an Industrial Urea Plant

D. K. CÉSARI, N.S. SCHBIB, D.O. BORIO

Zacatecas (México)

Congreso; Chemical Reaction Engineering X (CRE X); 2005

COMPLETAR

Modern urea plants with total recycle of non-converted reactants are divided in three main sections: High Pressure (HP), Medium Pressure (MP) and Low Pressure (LP). Urea (NH2CONH2) is produced from liquid ammonia and gaseous carbon dioxide in the synthesis reactor located in the HP section (around 160 bar), according to the following reactions: 2 NH3 + CO2 Û NH2COONH4                                                     (1) NH2COONH4 Û NH2CONH2 + H2O                                            (2) The first reaction (exothermic) reaches the equilibrium quickly; the second reaction (endothermic) occurs slowly and determines the reactor volume. The CO2 conversion in the reactor is between 60-65%. The effluent solution of the synthesis reactor (containing urea, ammonium carbamate, NH3, CO2 and inerts) enters the stripper (a falling-film reactor operating at the same pressure level as the urea reactor), where the main fraction of the unconverted carbamate is decomposed using saturated steam (reverse of reaction 1). Vapors of NH3, CO2 and H2O coming from the top of the stripper, after mixing with a carbonate recycle solution from the MP and LP sections, are condensed in the carbamate condenser (HP section), where the exothermic reaction (1) takes place. After separating the inert gases, the carbamate solution leaving the condenser is mixed with a stream containing fresh NH3 and recycled to the reactor bottom. The steady-sate simulation of the high-pressure section of a large-scale urea plant is presented. A Modular-Sequential procedure is selected. The urea synthesis reactor, the stripper and the carbamate condenser are represented using detailed heterogeneous models. The convergence of the recycle stream (carbamate solution from the condenser) is reached by means of an iterative process using a Quasi-Newton Method.