Study On Thermodynamic System Of Urea Process And Development Of Flowsheeting

Urea is one of the most important nitrogen-based fertilizers in the world. Since 1930s, more researches on basic theory and industry application related urea synthesis system have been carried out and now have been an important academic research region in chemical engineering. With the energy crisis and the severe environmental pollution, researches on the urea process are more and more attached importance so as to lower raw materials and energy costs, and reducing waste generation. In this paper, we start our work with the basic thermodynamics principles, then develop the unit module models and flowsheeting system of urea synthesis high-pressure loop. The main contents of this paper are as follows:1. A rigorous thermodynamic model is critical in chemical process simulation. Based on the Edwards model, a vapor-liquid equilibrium model of NH3-CO2-H20 system is proposed. We focused on the program to resolve this mathematics model and developed a robust algorithm that combined the equation-based approach and the iterative approach considering the real reaction process to ensure the program to be convergent steadily. This model is applied fo" calculating the models of the condensers in urea process. The calculated results are compared with the design values.2. With the extended UNIQUAC equation, we calculate the vapor-liquid equilibrium (VLE) of NH3-CO2-H2O-Urea thermodynamic system involved in the urea process system. Considering reactor synthesis conditions, the coefficients of calculating the ammonia standard fugacity have been optimized and the results of thermodynamic model agree well with experimental data. It is for the first time to introduce the concept of "groups of species" into the extended UNIQUAC equation so that the number of interaction parameters can be reduced radically. This new and simple approach has particular significance for the multicomponent volatile electrolyte system. The VLE of NH3-CO,-H2O-Urea system in the range of middle and low pressure is calculated with this new approach and the result is compared with the experimental data.3. The equilibrium stage model and non-equilibrium stage model (non-CSTR model, considering mass transfer flux between the phases)of urea reactor are proposed based on the VLE of NH3-CO2-H2O-urea system and the numerical solutions of the equations are performed with conventional iterative methods. The calculated results are compared with design and industrial data. Because we use the rigorous thermodynamic model, the profiles of parameters along the urea reactor are agreed well with the practical process.4. Based on rigorous thermodynamic model, an equilibrium stage model is developed to simulate CO, stripper and NH3 stripper. The results are compared with the conclusions obtained with the analysis of the high-pressure phase graph. The applicability of the thermodynamic model is also proved.5. Considering that the Urea synthesis high-pressure loop is a complex process with a number of recycle streams, we attempt to use simultaneous-modular approach to simulate the urea high-pressure loop. Resolving the linear equations of the flowsheet level and calling the rigorous unit models alternately until the coefficient matrices are convergent, the simulation implement can be finished. This approach can avoid resolving very large nonlinear equations and partitioning and tearing the recycle loops. The simulation results are closed to the design data.6. Many real-world design problems involve multiple objectives that have to be optimized simultaneously. Sometimes these objectives are usually conflicting and non-commensurable, and must be satisfied simultaneously. We select for the first time the equilibrium conversion of CO2 into urea and unreacted NH3 as two objective functions and the design variables are temperature, NH3/CO2 and H2O/CO2. In order to reduce the load of calculation, the multi-objective micro-genetic algorithm b...