(78b) Modeling and Simulation of Ammonia and Urea Processes: Challenges and Best Practices

The ammonia and urea synthesis reactors are the most important units in the urea synthesis process. Rigorous simulation of the urea reactor is necessary and significant. However, the complicated thermodynamic system and chemical reactions result in difficulties of modelling and simulation.

Nitrogen based fertilizers are the most widely produced types of fertilizers, accounting for 82.79 million tones produced worldwide between 1998 and 1999 (1). Amongst all nitrogen based fertilizers urea is the most widely produced, with 37.57 million tones produced between 1997 and 1998 (1). It is significant to notice that urea consumption is increasing significantly, jumping from 8.3 million tones in 1973-1974 to 37.57 million tones in 1997-1998 corresponding to about 46% of the total world consumption of nitrogen. The importance of urea production and the availability of modern flowsheeting tools motivated us to apply basic thermodynamic principles and software engineering for the creation of a tool that can be used for modeling the most significant aspects of the urea production processes currently used. Albeit several technologies are available for the production of urea (2, 3, 4, 5, 6), the Stamicarbon and Snamprogetti processes correspond to approximately 76% of the world market (7) and therefore our modeling efforts concentrated on these two production technologies. Basic Principles The commercial production of urea is based on the reaction of ammonia and carbon dioxide at high pressure and temperature to form ammonium carbamate, which in turn is dehydrated into urea and water. A mechanism model is presented based on a rigorous thermodynamic model, reaction kinetics and fluid dynamics. A stage-by-stage method is adopted to solve the urea reactor model that consists of a series of non-linear equations. The numerical solution of the equations is substantially performed with a conventional iterative method and the algorithm is simple and robust.