Microsoft Word - CET--006.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 59, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Zhuo Yang, Junjie Ba, Jing Pan Copyright © 2017, AIDIC Servizi S.r.l. ISBN 978-88-95608- 49-5; ISSN 2283-9216 Application of Computer Process Simulation in Chemical Synthesis of Urea Xiaohui Wang, Shengpu Li* Pingdingshan University, College of Computer Science and Technology, Pingdingshan Henan, China lsp1519@163.com The computer process simulation technology can simulate the chemical process of urea, effectively reflect the technological parameters of the chemical production process, and provide practical guidance for chemical production of urea. This paper conducts computer process simulation of the high-pressure urea synthesis tower, the center piece of the chemical synthesis of urea. The chemical parameters in the production process of the synthesis tower are simulated and analyzed with the aid of the equilibrium stage model and computer technology. The calculation results are consistent with the design values of the synthesis tower, indicating that the equilibrium stage model satisfies the technological design requirements. Suffice it to say that the research provides meaningful guidance for the chemical production of urea. 1. Introduction Urea is not only the most-produced and most-used chemical fertilizer in the world, but also a multi-purpose industrial raw material (Naz and Sulaiman, 2016; Puspita et al., 2017). Since the 1930s and 1940s, urea production has been a hot subject amongst theorists and industrial practitioners. Over the years, it has developed into an important research field in the chemical industry (Edris et al., 2016). Facing increasingly severe energy crisis and deteriorating environment, researchers are digging deeper into the industrial process, hoping to reduce energy consumption and realize cleaner production. In chemical production, process simulation is the most fundamental computer technology and an essential tool for computer-aided design, condition analysis, and optimization operations (Yao et al., 2016). High-pressure urea synthesis tower is the key to the synthesis of urea. The synthetic ratio and quality of urea hinge on the parameters of the tower (e.g. temperature, pressure). Figure 1 illustrates the urea synthesis process in the tower. Figure 1: Sketch map of high-pressure urea synthesis tower DOI: 10.3303/CET1759103 Please cite this article as: Xiaohui Wang, Shengpu Li, 2017, Application of computer process simulation in chemical synthesis of urea, Chemical Engineering Transactions, 59, 613-618 DOI:10.3303/CET1759103 613 The high-pressure urea synthesis tower is separated into several reaction chambers by a number of tower trays that prevent materials from back-mixing. For the equilibrium stage model selected in this paper, it is assumed that each reaction chamber is a continuous mixing reactor containing well-blended gas-liquid mixture (Sahu et al., 2009), that is, an ideal mixing model, that gas-liquid equilibrium is achieved at the outlet of each tray, and that the reaction process fulfils the adiabatic requirements (Victor-Ortega et al., 2016; Gimenes et al., 2016; Freitas and Guirardello, 2016; Villarroel et al., 2016; De Andrade et al., 2016; Lee and Zaini, 2017). Based on the basic principle of chemical reaction equilibrium and chemical reaction kinetics, and on the assumptions of the equilibrium stage model, this paper conducts the computer process simulation of the urea synthesis procedure in the high-pressure urea synthesis tower. The experimental results effectively reflect the parameter variation in the synthesis process, and provide practical guidance for application of chemical technologies. 2. Calculation and analysis of computer process simulation of urea synthesis tower 2.1 Introduction to chemical equations The main constituents of urea include NH3, CO2 and H2O. The three substances are turned into a gas-liquid equilibrium mixture of NH3-CO2-H2O-urea in the synthesis tower. When the temperature falls in the range of 160~190°C and the pressure remains between 10 and 20Mpa, the chemical reaction in the synthesis tower can be divided into the following two steps (Boggs et al., 2009): CO (I) + 2NH (I) → H NCOONH − 100.4kj (1) H NCOONH (I) → H NCOONH (I) + H O(I) + 27.6KJ (2) The two equations are combined into Equation (3): CO (I) + 2NH (I) → H NCOONH (urea) (3) Equation (1) describes the rapid carbothermic reaction of the intermediate product: ammonium carbamate; Equation (2) illustrates the slow endothermic process of urea production through the hydrolysis of ammonium carbamate. The two reactions automatically reach a thermal equilibrium in the synthesis tower (Rafiee, 2015). 2.2 Calculation and analysis of urea synthesis tower based on equilibrium stage model 2.2.1 Establishment of mathematical models for the urea synthesis tower Considering the material balance, energy balance, phase equilibrium equation, chemical kinetics equation and normalization equation on each tray, the author establishes the following equilibrium stage models (MESHR equations) for each tray in the tower (Kuwabata et al., 1998). The mathematical models are established as follows (MESHR equations). For the components on the j-th stage of the tower: V ∙ y . + L ∙ x , = V ∙ y , + L ∙ x , + V ∙ u (4) y , = k , ∙ x , ,∑ x , = 1 (5) ∆p = ∆p + ∆p + ∆p (6) Urea generation equation (kinetic equation) (Popoola et al., 2016): u = R ∙ ∆V,V ∙ H + L ∙ H = V ∙ H + L ∙ H + Q (7) In the above equations: x , , y , , x , and y , stand for the mole fraction of the liquid-gas two-phase component i on the j-th tray at the inlet and outlet of the 8-layer synthesis tower, respectively (Hazmi et al., 2016); k , refers to the phase equilibrium constant of component i on the j-th tray; V , L , V and L represent the gas-liquid two phase flow on the j-th tray, kmol/h; H ,H , H and H denote the gas-liquid two-phase molar enthalpy on the j-th tray at the inlet and outlet, kj/kmol; u means the urea production on the j- th tray, kmol/h; ∆V is the equivalent volume of reactants on the j-th tray; Q represents heat loss; i=1, 2, 3 and 4 refer to H2O, NH3, CO2 and urea, respectively (Watts et al., 1995). 2.2.2 Solution of mathematical models for the urea synthesis tower The established models are calculated in the computer. The model for each tray is solved in the following steps: 614 (1) The equipment parameters and the structural parameters of the tray are taken as given values; the temperature, composition, pressure, flow, etc. of the gas-liquid two-phase components at the inlet are regarded as the known inlet parameters. (2) Assuming that the outlet temperature is Tj the j-th tray at the outlet and that the liquid-phase component on the tray at the outlet is x , , calculate the gas phase composition for thermodynamic gas-liquid equilibrium; then, obtain the urea production on the tray by the kinetic equation (Nicoletti et al., 2016). (3) Solve the gas-liquid flow by overall energy balance (Pang et al., 1997). (4) Calculate the single-tray pressure drop, and get the outlet pressure of the tray p ; (5) Treat the outlet parameters of the j-th tray as the inlet parameters of the next plate, and repeat steps (1) - (4) in the subsequent simulation. The material composition of the last tray is the outlet material composition of the entire synthesis tower (Hamidipour et al., 2005). The above steps are simulated on the computer to obtain the distribution of flow, composition and temperature across the tower. The procedure is an important part of computer simulation of urea chemical production (Zhang et al., 2005). 2.3 Calculation results and analysis of process simulation 2.3.1 Calculation results Table 1 lists the results of the calculation results of the equilibrium stage model, namely gas-liquid components, temperature, pressure. Table 2 compares the calculation results of the model and the design values. Table 1: Simulation results of urea synthesis tower Tray Temperature(t/℃) P/Mpa x(Mole fraction) y(Mole fraction) NH3 CO2 urea NH3 CO2 Inlet 167 14.10 0.6568 0.2309 0.0002 0.6963 0.2321 1 169 14.07 0.6272 0.2076 0.0238 0.6923 0.2541 2 170.63 14.04 0.6000 0.1882 0.0451 0.674 0.2641 3 172.54 14.02 0.5736 0.1713 0.0653 0.6614 0.2666 4 174.67 13.99 0.5482 0.1558 0.0847 0.6506 0.2673 5 176.75 13.97 0.5248 0.1417 0.1028 0.6422 0.265 6 178.81 13.94 0.5028 0.1285 0.1201 0.6357 0.2601 7 180.76 13.91 0.4831 0.1165 0.136 0.6313 0.2526 8 182.66 13.89 0.4645 0.1051 0.1512 0.628 0.2429 Table 2: The comparison of the results of this model with the literature design data The overhead outlet Temperature(t/℃) P/Mpa x(Mole fraction) y(Mole fraction) NH3 CO2 urea NH3 CO2 Calculated value 167 14.10 0.6568 0.2309 0.0002 0.6963 0.2321 Design value 169 14.07 0.6272 0.2076 0.0238 0.6923 0.2541 Relative deviation (%) 170.63 14.04 0.6000 0.1882 0.0451 0.674 0.2641 The overhead outlet Gas and liquid flow(kmol/hr) CO2 Conversion rate Gas Liquid Calculated value Design value Relative deviation (%) 945.39 1016.58 7.09 8529.68 8522.58 0.083 58.99 56.2 4.96 As shown in Table 2, the relative error between the calculation values and the design values is 7.09% for gas- phase components and 0.083% for liquid-phase components; the relative error of CO2 conversion rate stands at 4.96%. All the errors are within the tolerance range for the chemical technologies. Then, the author analyzes how the temperature of each tray, the flow of each component and the production of urea varies during the urea synthesis process within the tower. The variations are calculated by the equilibrium stage model. Figure 2 shows the temperature trend along the tray level, and Figure 3 presents the urea production trend along the tray level. 615 Figure 2: The temperature trend along the tray level Figure 3: The Urea production along the tray level According to Figures 2 and 3, the temperature rises with the increase of the tray level and peaks at 182.66°C; the urea production also increases with the tray level. The results are in good agreement with the actual chemical production results. The above-mentioned chemical equations in the synthesis tower are, in essence, the gas-liquid phase conversion of NH3, CO2 and H2O under different pressure and temperature conditions. The variation in the flow of each liquid-phase component along the tray level is shown in Figure 4. As shown in the figure, as the material enters the synthesis tower from the bottom tray, the liquid-phase molar flows of NH3 and CO2 decrease gradually, but flow of urea rises along the tray level. The result also echoes the actual situation in chemical production. Influenced by opening rate, tray, etc., the CO2 conversion rate is a key determinant of urea production. Through computer flow simulation, the rate is calculated as 58.99%, while the design conversion rate is 56.2%. The relative error is merely 4.96%, signifying high accuracy. The CO2 conversion rate is mainly reflected by the distribution of calculated pressure in the equilibrium stage model. This means the model offers some instructions on the design of structure parameters of the trays in the tower. 616 Figure 4: Liquid-phase composition flow along the tray level 3. Conclusion In order to realize the process simulation of the technologies in urea synthesis tower, this paper calculates the variation in temperature and each component in the tower based on the theory of equilibrium stage model and with the aid of computer flow simulation. The conclusions are drawn as below: (1) The calculated temperature increases with tray level, and the pressure goes in the opposite direction. The trend is consistent with the results of the actual chemical process. (2) The simulation results are proved accurate by the minimal relative errors between the calculated parameters and the design parameters of the actual chemical process. (3) The application of computer process simulation technology in urea production demonstrates the guiding effect of theoretical simulation on the actual production. References Boggs B.K., King R.L., Botte G.G., 2009, Urea electrolysis: direct hydrogen production from urine. Chemical Communications, 32, 4859. DOI: 10.1039/b905974a. 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