(672g) An Effective Novel Approach for Modeling Thermodynamic Properties of Urea Process

Urea (NH2CONH2) is the most widely produced and used nitrogen based fertilizer. Its production is a physicochemical process involving complex ionic interaction and reactions among the compounds. Because of its complexity, a theoretical model for calculating the thermodynamic properties of Urea systems with reasonable fidelity involves calculations that are in the order of 50-100 times more computationally expensive than for typical hydrocarbon systems. Consequently, such a model has been a non starter for dynamic simulation. On the other hand, an empirical or regression model cannot provide the desired fidelity outside of narrow operating conditions. These challenges have stymied others in their attempts to build a high fidelity thermodynamic model for Urea systems that is applicable over a sufficiently broad range of operating conditions for dynamic simulation, especially for Operator Training System (OTS) applications which range from start up, shut down, and other abnormal operating conditions.

Presented in this paper is a complete thermo property package developed within the UniSim Design platform as a high fidelity model for Urea production process. Process engineers can build their Urea process simulation models by using this package in UniSim Design, and evaluate chemical and vapor-liquid equilibrium, as well as physical properties of components in the whole system. It has been tested through real customer projects for a wide operation range, and proven to be very robust both on thermodynamic model and software user interface.

There are three major innovations in this model development: 1. Incorporate an appearance carbamate formation reaction, the most important intermediate production reaction, into flash and property calculations, such as fugacity and enthalpy calculations so that the reaction effects on VLE and Energy balance will be automatically accounted; 2. Create a virtual molecular model instead of full ionic model to reduce the complexity of the application. This perceptive and creative simplification to the theoretical model has reduced the computational complexity dramatically and made it possible for dynamic process simulation and OTS applications; 3. Use multiple sets of parameters associated with different operating pressures ranges through a newly developed user-friendly interface ? the first successful attempt to overcome the limitation of the activity coefficient models to low pressure conditions in industrial applications.

This package has been well accepted by the process simulation engineers for modeling operator training systems of the urea production process.