尿素生产系统热力学研究及流程模拟开发
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摘要
尿素作为一种重要的化学肥料，在世界范围内其生产和使用居首位，此外它也是一种用途广泛的工业原料。自二十世纪三、四十年代以来，与尿素生产相关的理论研究和工业应用研究就一直为人们所关注，成为化学工业一个重要的学术研究领域。当今日益严重的能源危机和日益恶化的环境状况促使人们对这一工业过程进行更为深入的研究，以达到节能降耗、清沾生产的目的。本文从尿素生产系统的热力学气液平衡机理模型的研究入手，对尿素工艺高压流程模拟系统进行开发研究，以期为尿素工业生产提供一定的理论指导。论文的主要内容有：
1．热力学模型是进行化工过程模拟的基础，是决定模拟结果是否准确的最关键因素之一。论文首先基于Edwards模型建立了NH_3-CO_2-H_20体系气液平衡热力学机理模型，着重对模型的求解算法进行了详细的研究，将联立求解方程组的方法与模拟实际反应过程、以反应进度稳定为收敛准则的迭代法相结合，设计出稳定收敛的算法，以满足尿素流程低压设备模拟计算的需要。将热力学模型用于尿素生产流程中冷凝工段各冷凝器的模拟计算，所得计算结果与实际相符，可将其作为全流程模拟的单元模块，同时也验证了热力学模型的可靠性，
2．本文采用扩展的UNIQUAC方程对NH_3-CO_2-H_2O-urea体系气液平衡进行模拟计算，以满足尿素生产系统中高压系统的流程模拟计算的需要，并重新回归厂模型氨组分的标准态逸度，使模型计算结果与实际数据更为相符。首次将“物种群”概念引入求液相物种活度系数的扩展UNIQUAC方程中，用“物种群”间的交互作用代替实际物种间的交互作用，大大减少了模型参数个数，简化了计算及参数回归，将该模型用于NH_3-CO_2-H_2O-urea体系中低压部分的气液平衡模拟计算，与原模型计算值相比，平衡压力平均相对误差降低了3.6％。所提出的模型同样适用于伴随有化学反应的多组分溶液体系的气液平衡热力学计算。
3．以NH_3-CO_2-H_2O-urea体系气液平衡热力学机理模型为基础，采用平衡级模型及非平衡级模引(基于非全混流模型，考虑传质通量)对尿素生产系统的关键设备——尿素合成塔进行了模拟计算，将计算结果同实际设计及实际操作数据作了比较。我们首次将该热力学机理模型用于非平衡级模型的计算，所得到的合成塔内的参数分布与实际的分析结果更为相符。
4．采用平衡级模型对高压汽提塔进行了模拟计算，计算结果同高压相图的分析结果进行了比较，模型适用于CO_2汽提塔及NH-3汽提塔。也以此验证了NH_3-
大江搜工大学N门，学位论义
CO。－H。O－。lrea体系热力学机理拱型及单元设备模型的正确性。问时对其他高压
设爷进行了建模分析与计算。
5｛f对尿素高压部分各单元设备建立严格模型并求解的基础上，考虑到该系统媚
环流股多、流程复杂的特点，我们采川联立模块法开发尿素高压流程模拟系统，
交替求解模块水平上各单元的机理模型和流程水平上的简化模型，从而完成对
高压圈的模型计算。本方法既保证了各单元模块机理模型的使用，又避免了由
丁流程复杂而导致采用序贯模块法时模拟过程不易收敛、计算时问过长等问
题，同样也无需求解采用联立方程法时所形成的大型仆线性方程组。模拟计算
结果同设计数据吻合。
6 在实际工程应用中，许多忧化问题常常需要同时满足多个目林的需求，而这些
口标函数又常常是相互冲突的。本文首伙以二氧化碳平衡转化率和未反应的氮
量作为两个目标函数，同时优化求解尿素合成过程的主要控制参数——操作硫
度、氨碳比和水碳比。采用多口标微观遗传算法对该多目标优化问题求解，汕
策者可从求解得到的优化解集Pareto fronts中根据自己的意向和各日标所赋权
哲的大小寻求满意解。本二－作对实际厂业中尿素合成过程操作参数的优选有巫
要的指导意义。
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
1．热力学模型是进行化工过程模拟的基础，是决定模拟结果是否准确的最关键因素之一。论文首先基于Edwards模型建立了NH_3-CO_2-H_20体系气液平衡热力学机理模型，着重对模型的求解算法进行了详细的研究，将联立求解方程组的方法与模拟实际反应过程、以反应进度稳定为收敛准则的迭代法相结合，设计出稳定收敛的算法，以满足尿素流程低压设备模拟计算的需要。将热力学模型用于尿素生产流程中冷凝工段各冷凝器的模拟计算，所得计算结果与实际相符，可将其作为全流程模拟的单元模块，同时也验证了热力学模型的可靠性，
2．本文采用扩展的UNIQUAC方程对NH_3-CO_2-H_2O-urea体系气液平衡进行模拟计算，以满足尿素生产系统中高压系统的流程模拟计算的需要，并重新回归厂模型氨组分的标准态逸度，使模型计算结果与实际数据更为相符。首次将“物种群”概念引入求液相物种活度系数的扩展UNIQUAC方程中，用“物种群”间的交互作用代替实际物种间的交互作用，大大减少了模型参数个数，简化了计算及参数回归，将该模型用于NH_3-CO_2-H_2O-urea体系中低压部分的气液平衡模拟计算，与原模型计算值相比，平衡压力平均相对误差降低了3.6％。所提出的模型同样适用于伴随有化学反应的多组分溶液体系的气液平衡热力学计算。
3．以NH_3-CO_2-H_2O-urea体系气液平衡热力学机理模型为基础，采用平衡级模型及非平衡级模引(基于非全混流模型，考虑传质通量)对尿素生产系统的关键设备——尿素合成塔进行了模拟计算，将计算结果同实际设计及实际操作数据作了比较。我们首次将该热力学机理模型用于非平衡级模型的计算，所得到的合成塔内的参数分布与实际的分析结果更为相符。
4．采用平衡级模型对高压汽提塔进行了模拟计算，计算结果同高压相图的分析结果进行了比较，模型适用于CO_2汽提塔及NH-3汽提塔。也以此验证了NH_3-
大江搜工大学N门，学位论义
CO。－H。O－。lrea体系热力学机理拱型及单元设备模型的正确性。问时对其他高压
设爷进行了建模分析与计算。
5｛f对尿素高压部分各单元设备建立严格模型并求解的基础上，考虑到该系统媚
环流股多、流程复杂的特点，我们采川联立模块法开发尿素高压流程模拟系统，
交替求解模块水平上各单元的机理模型和流程水平上的简化模型，从而完成对
高压圈的模型计算。本方法既保证了各单元模块机理模型的使用，又避免了由
丁流程复杂而导致采用序贯模块法时模拟过程不易收敛、计算时问过长等问
题，同样也无需求解采用联立方程法时所形成的大型仆线性方程组。模拟计算
结果同设计数据吻合。
6 在实际工程应用中，许多忧化问题常常需要同时满足多个目林的需求，而这些
口标函数又常常是相互冲突的。本文首伙以二氧化碳平衡转化率和未反应的氮
量作为两个目标函数，同时优化求解尿素合成过程的主要控制参数——操作硫
度、氨碳比和水碳比。采用多口标微观遗传算法对该多目标优化问题求解，汕
策者可从求解得到的优化解集Pareto fronts中根据自己的意向和各日标所赋权
哲的大小寻求满意解。本二－作对实际厂业中尿素合成过程操作参数的优选有巫
要的指导意义。
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
引文
[1] 万勇．尿素合成塔高效塔板的应用，大氮肥，1998，21(3)：154-157
[2] 李芳玲，路春荣，陆为民．尿素工艺进展，石油化工动态，1998，6(2)：64-70
[3] 王威，刘鸿雁．斯塔米卡邦尿素2000-TM技术，中氮肥，1999，3：4-6
[4] 周家驹，许志宏．挥发性弱电解质水溶液三元系的汽液平衡 NH_3—CO_2—H_2O，NH_3—H_2S—H_2O，NH_3—SO_2—H_2O 体系，化工学报，1983，3：234-244
[5] 左有祥，郭天民．用状态方程描述弱电解质 NH_3—CO_2—H_2O 体系的气液平衡行为，化工学报，1991，42(2)：162-169
[6] 李玉刚，周传光等．水溶液全循环法尿素生产流程模拟软件的开发，化学工程，1999，26(1)：51-53
[7] Bernadis M,Carvoli G.,Delogu P.,NH3-CO2-H2O VLE Calculations Using an Extended UNIQUAC Equation,AIChE J.,1989a ,35(2) : 314-317
[8] Bernadis M.,Carvoli G.,Santini M..urea-NH,-CO2-H2O VLE Calculations Using an Extended UNIQUAC Equation,Fluid Phase Equilibria,1989b,53: 207-218
[9] Edwards T.J.,Newman J.,Prausnitz J.M..Thermodynamics of aqueous solutions containing volatile weak electrolytes,AIChE J.,1975,21: 246-259
[10] Edwards T.J.,Newman J.,Prausnitz J.M.,Vapor-Liquid Equilibrium in Multicomponent Aqueous Solutions of Volatile Electrolyte,AIChE J.,1978,24(6) : 966-976
[11] Dente M.,Pierucci S.,Sogaro A..,Simulation Program for Urea Plant,Comput.Chem.Engng., 12(5) ,1988: 389-400．
[12] Dente M.,Rovaglio M.,Bozzano G.,Sogaro A..Gas-liquid reactor in the synthesis of urea,Chem.Eng. Sci.,1992,47(9-11) : 2475-2480
[13] Goppert U.,Maure G.,Vapor-Liquid Equilibrium in Aqueous Solutions of Ammonia and Carbon Dioxide at Temperature Between 333 and 393 K,and Pressures Up to 7MPa,Fluid Phase Equilibria,1988,41(1-2) : 153
[14] Kotula,E.A.,A VLE Model of the NH3-CO2-H2O-Urea System at Elevated Pressure, J.Chem.Tech.Biotechml.1981,31: 103-110．
[15] Kurz F.,Rumf B.,Maurer G.,Vapor-Liquid-Solid equilibrium in the system NH3-CO2-H2O,from around 310 to 470K: New experimental data and modeling, Fluid Phase Equilibria,1995,104:261-275
[16] Muller G.,Bender E.,Maurer G..Das Dampf-Flussigkeitsgleichgewicht des ternaren System Ammoniak-Koheledioxid-Wasser bei hohen Wassergehalten im Bereich zwischen 373 und 473 Kelvin.,Ber.Bunsenges.Phys.Chem.,1988,92:148-160
[17] Satyro M.A.,Li Y.K.,Agarwal R.K.,Santollani 0． J..Modeling Urea Process: A New Thermodynamic Model and Software Integration Paradigm,from http//www.cheresources.com
[18] Satyro M.A.,Li Y.K.,Agarwal R.K.,Santollani R.K..Modeling Urea Processes: A New Thermodynamic Model and Software Integration Paradigm,the 2000 Clearwater Convention on Phosphate Fertilizer & Sulfuric Acid Technology,Sheraton Sand Key Resort,Clearwater Beach,Florida,June 16-18,2000
[2] 李芳玲，路春荣，陆为民．尿素工艺进展，石油化工动态，1998，6(2)：64-70
[3] 王威，刘鸿雁．斯塔米卡邦尿素2000-TM技术，中氮肥，1999，3：4-6
[4] 周家驹，许志宏．挥发性弱电解质水溶液三元系的汽液平衡 NH_3—CO_2—H_2O，NH_3—H_2S—H_2O，NH_3—SO_2—H_2O 体系，化工学报，1983，3：234-244
[5] 左有祥，郭天民．用状态方程描述弱电解质 NH_3—CO_2—H_2O 体系的气液平衡行为，化工学报，1991，42(2)：162-169
[6] 李玉刚，周传光等．水溶液全循环法尿素生产流程模拟软件的开发，化学工程，1999，26(1)：51-53
[7] Bernadis M,Carvoli G.,Delogu P.,NH3-CO2-H2O VLE Calculations Using an Extended UNIQUAC Equation,AIChE J.,1989a ,35(2) : 314-317
[8] Bernadis M.,Carvoli G.,Santini M..urea-NH,-CO2-H2O VLE Calculations Using an Extended UNIQUAC Equation,Fluid Phase Equilibria,1989b,53: 207-218
[9] Edwards T.J.,Newman J.,Prausnitz J.M..Thermodynamics of aqueous solutions containing volatile weak electrolytes,AIChE J.,1975,21: 246-259
[10] Edwards T.J.,Newman J.,Prausnitz J.M.,Vapor-Liquid Equilibrium in Multicomponent Aqueous Solutions of Volatile Electrolyte,AIChE J.,1978,24(6) : 966-976
[11] Dente M.,Pierucci S.,Sogaro A..,Simulation Program for Urea Plant,Comput.Chem.Engng., 12(5) ,1988: 389-400．
[12] Dente M.,Rovaglio M.,Bozzano G.,Sogaro A..Gas-liquid reactor in the synthesis of urea,Chem.Eng. Sci.,1992,47(9-11) : 2475-2480
[13] Goppert U.,Maure G.,Vapor-Liquid Equilibrium in Aqueous Solutions of Ammonia and Carbon Dioxide at Temperature Between 333 and 393 K,and Pressures Up to 7MPa,Fluid Phase Equilibria,1988,41(1-2) : 153
[14] Kotula,E.A.,A VLE Model of the NH3-CO2-H2O-Urea System at Elevated Pressure, J.Chem.Tech.Biotechml.1981,31: 103-110．
[15] Kurz F.,Rumf B.,Maurer G.,Vapor-Liquid-Solid equilibrium in the system NH3-CO2-H2O,from around 310 to 470K: New experimental data and modeling, Fluid Phase Equilibria,1995,104:261-275
[16] Muller G.,Bender E.,Maurer G..Das Dampf-Flussigkeitsgleichgewicht des ternaren System Ammoniak-Koheledioxid-Wasser bei hohen Wassergehalten im Bereich zwischen 373 und 473 Kelvin.,Ber.Bunsenges.Phys.Chem.,1988,92:148-160
[17] Satyro M.A.,Li Y.K.,Agarwal R.K.,Santollani 0． J..Modeling Urea Process: A New Thermodynamic Model and Software Integration Paradigm,from http//www.cheresources.com
[18] Satyro M.A.,Li Y.K.,Agarwal R.K.,Santollani R.K..Modeling Urea Processes: A New Thermodynamic Model and Software Integration Paradigm,the 2000 Clearwater Convention on Phosphate Fertilizer & Sulfuric Acid Technology,Sheraton Sand Key Resort,Clearwater Beach,Florida,June 16-18,2000