The results show that the operating parameters of the designed urea hydrolysis reaction device are consistent with the calculated results, and the model establishment and calculation method are feasible. When the operating conditions are controlled at 150℃ and 0.6MPa, the higher the concentration of urea solution, the greater the ammonia production capacity of the reactor and the lower the H2O content of the outlet gas, which is in line with theoretical calculation.
According to the statistics of the monthly report of China Electric Power Enterprise Association, by the end of March 2065438+2006, the installed capacity of thermal power of 6000 kilowatts and above in China was1kloc-0/0 million kilowatts. It is estimated that by 2020, the installed capacity of thermal power in China will probably exceed 654.38+0.2 billion kilowatts. Among them, reducing the environmental pollution caused by nitrogen oxide emissions from coal-fired power plants will be paid more and more attention. With the stricter institutional agreement of environmental protection industry, the potential danger of liquid ammonia is stipulated, and the denitration technology of urea hydrolysis to ammonia in coal-fired power plants has been widely concerned as a preparation method of denitration reducing agent.
Due to the lack of domestic technology, power plants basically buy foreign U2A urea ammonia hydrolysis reactors directly. In recent years, some domestic institutions have developed urea hydrolysis reactors with independent property rights. However, due to the confidentiality of the technology and the limitation of the use of hydrolysis reactors, there is almost no public information.
In order to fill this technical gap, since 20 12, the research team led by overseas experts of "Thousand Talents Program" has carried out research on urea hydrolysis technology, preliminarily designed hydrolysis reaction flow and hydrolysis reactor parameters through theoretical calculation, built a pilot plant for urea hydrolysis to produce ammonia for denitration in coal-fired power plants, analyzed the effects of operating temperature, operating pressure and mass flow on urea hydrolysis rate and ammonia production, and verified the applicability of the design scheme and theoretical calculation method, thus developed.
In this paper, firstly, based on the establishment of the equilibrium constant of urea hydrolysis reaction (assuming it follows the reaction equilibrium constant of urea synthesis process) and the calculation method of phase equilibrium of NH3-CO2-H2O-CO(NH2)2 quaternary system, the PR equation of state is combined with the modified UNIQUAC model, and the simulation calculation is carried out by using ASPEN software, which not only verifies the feasibility of the method, but also simulates the denitrification process of urea water on the built pilot-scale experimental platform, optimizes the reaction control factors and conditions, and obtains the following results.
Theoretical calculation method of 1
The basic principle of urea hydrolysis to produce ammonia is as follows:
Different from the deep hydrolysis section of condensate in urea synthesis process, the concentration of urea aqueous solution in denitration unit is high, generally 40% and 50% by weight, which belongs to high concentration urea hydrolysis process. However, the concentration of urea hydrolysis in urea synthesis plant is only about 0.003 ~ 0.006 mol/kg, and the contents of NH3, CO2 and urea are 3.5 ~ 5.5%, 2 ~ 3% and 0.4 ~ 2% respectively, which belongs to low concentration urea hydrolysis and is a reactive distillation process. Its chemical reaction and phase equilibrium calculation model of NH3-CO2-H2O-Co (NH2).
Obviously, the calculation method of phase equilibrium in deep hydrolysis and low concentration distillation process is not suitable for high concentration urea hydrolysis system, and the calculation of hydrolysis equilibrium of high concentration urea is rarely reported.
Thermodynamic calculation of hydrolysis of 1 urea
As the reverse process of urea synthesis reaction, urea hydrolysis can learn from the relatively mature research theory of urea synthesis system.
The reaction equilibrium constant is 1. 1
The reaction equilibrium constant k is a key parameter of simulation calculation, which has nothing to do with pressure and composition, but is a function of temperature. When the heat capacity does not change obviously before and after the process, as shown in Formula (2):
Therefore, it should be possible to learn from the equilibrium constant of low concentration urea hydrolysis process. However, the hydrolysis process of low concentration urea is accompanied by the ionization equilibrium of weak electrolyte of ammonia and CO2 and the chemical reaction between ammonia and CO2. The equilibrium system of low concentration urea aqueous solution belongs to the phase equilibrium state of weak electrolyte solution, and the electrostatic force term plays an important role in the calculation of low concentration activity coefficient.
Therefore, the thermodynamic calculation of urea hydrolysis equilibrium system in the condensate of urea synthesis process is not suitable for urea hydrolysis equilibrium system for denitrification, but the urea concentration in the synthesis section is high, and the temperature and pressure of different urea production processes are between 180-2 10℃ and 13-24MPa, which belongs to the phase equilibrium state of non-electrolyte solution, and can be referred to as the equilibrium constant or.
At the same time, the activity coefficient is used to correct the non-ideality of liquid molecules. When calculating the activity coefficient, the electrostatic force term and the binary interaction between urea neutral molecules and other particles are ignored. For example, the reaction equilibrium can also be expressed as Formula (4):
Where: mi is the mass concentration of each component, γ is the activity coefficient of each component, and αw is the activity of water.
Phase equilibrium of 1.2 quaternary system
The calculation of phase equilibrium of quaternary system is very complicated, and the ionization equilibrium of each component should be considered. At present, there is no more accurate method to obtain it. The ionization reaction formula of low concentration urea hydrolysis process is as follows:
The system includes many components. The gas phase contains water, ammonia and CO2, and the equilibrium liquid phase contains 10: urea hydrolysis, ionization equilibrium of ammonia and CO2, and equilibrium reaction of ammonium formate ion generation.
In this paper, Edwards model is used to obtain the activity coefficient of ternary system, and a urea hydrolysis constraint equation is added to the liquid phase to obtain the phase equilibrium calculation of quaternary system.
1.3 urea hydrolysis rate
The hydrolysis of urea is a reversible process. When the temperature is lower than 60℃, the hydrolysis reaction hardly occurs. With the increase of temperature, the hydrolysis speed is accelerated. When the temperature reaches 80℃, the hydrolysis amount of urea is only 0.5% at 1h, which can be increased to 3% at 1 10℃, and when the heating solution temperature is higher than1h.
The expression of urea hydrolysis rate is as follows:
Where Ue and U0 are the initial urea concentration before the reaction and the final urea concentration after the reaction, mg/L respectively; τ is the residence time of urea solution in the reactor, and minn is the number of hydrolysis reactors; K is the rate constant of urea hydrolysis reaction; T is the hydrolysis reaction temperature.
2 Model building and simulation
On the basis of thermodynamic calculation of urea hydrolysis, combined with urea hydrolysis reaction model and reaction kinetics model, ASPEN is used for process simulation calculation, and the physical parameters of each operating point calculated by HYSYS process are introduced into HTRI for calculation and selection of reactors and heat exchangers, as shown in figure 1.
As shown in figure 1, 50w% urea aqueous solution as a stream (1) exchanges heat with water vapor (5) at 180℃ and 1.0MPa in heat exchanger B 1. After the temperature of urea aqueous solution is raised to 60℃, it becomes the feed stream (2) of hydrolysis reactor B2.
Fig. 2 shows the comparison results of simulation calculation of the molar concentration of each component in the hydrolysate and the thermal power of the reactor under different feed concentrations. It can be seen that with the increase of urea solution concentration, the concentration of NH3 in the hydrolysate increases, the concentration of H2O decreases, and the energy consumption per unit of ammonia production decreases. When the concentration of urea solution is increased from 50w% to 60w%, the mole fraction of NH3 in the product gas is increased from 0.37 to 0.47, and the mole fraction of H2O is decreased from 0.43 to 0.28.
With the decrease of H2O concentration in product gas, not only the latent heat of vaporization absorbed by the evaporation of excess water in the reaction liquid can be reduced, but also the consumption of heating steam in the reactor can be reduced, thus effectively improving the economy of urea hydrolysis reactor.
3 Pilot test
3. 1 pilot reactor system
The pilot plant is running, the preparation of urea aqueous solution is completed by the preparation system, and the concentration of urea in the aqueous solution is controlled. The raw materials used in the pilot process are bagged urea, with total nitrogen content ≥46.3%, biuret content ≤0.9% and water (H2O) content ≤0.5%, which meets the requirements of national standard GB2440-200 1.
The process flow of the urea hydrolysis pilot plant shown in Figure 3 is as follows: one path of softened water in the discharge tank is pumped to the urea dissolving tank through feed to be mixed with urea particles to make urea solution, and the other path is preheated by a heat exchanger and sent to an electric boiler to generate high-temperature steam. The urea solution is sent to the hydrolysis reactor by the feed pump, and the hydrolysis reaction produces ammonia gas. The heat required for the reaction is provided by the steam flowing in the sub-cylinder, and the steam releases heat to become saturated water, which is cooled by the heat exchanger and returned to the drain tank. The gas phase product is discharged from the top of the reactor. The reaction residue is sent to the waste water tank for post-treatment.
The device operates at constant pressure, continuously feeds, and the flow rates of heating steam and product gas are recorded in real time by the mass flowmeter installed on the pipeline. When the reaction system reaches equilibrium, the heating steam flow rate and product gas flow rate remain stable, while the gas phase temperature of the hydrolysis reactor gradually decreases until it is stable.
3.2 product gas analysis
Analysis of export product gas by on-line chromatograph. As shown in Figure 4, with the increase of the mass concentration of urea solution, the component concentrations of NH3 and CO2 in urea hydrolysate increase, while the component concentration of H2O decreases, which is consistent with the conclusion of simulation study, and the detection results meet the reaction design requirements of the device.
3.3 Material balance and heat balance of the system
In order to further revise the process design and calculation method, the design parameters such as reactor heat exchange area, reactor size, feed rate and steam generator power were checked, and the material balance and heat balance of the urea hydrolysis device were analyzed taking the experimental data during operation as an example.
Among them, the material balance is to check the material balance of the system through mass flowmeters installed at the feed inlet and product gas outlet of urea hydrolysis reactor, and at the same time keep the liquid level in the reactor constant. The heat balance of urea hydrolysis reactor mainly includes the balance of heat absorption of working medium, heat release of steam and heat conduction of heating coil.
The parameters of heating steam are 1.0MPa and 180℃, which flows in the coil and releases the latent heat of vaporization, and conducts the heat to the urea solution in the reactor through the tube wall. The endothermic process of urea solution can be simplified as boiling heat transfer treatment. The heat released by steam in the heating coil mainly means that the latent heat of vaporization released by saturated steam becomes the heat of saturated water. The heat absorption capacity of urea solution outside the heating coil consists of three parts: the heat required by urea solution from feed temperature to reaction temperature; Chemical reaction heat absorbed by hydrolysis reaction of heated urea solution; The latent heat of vaporization absorbed by the remaining water in the reactor after the hydrolysis reaction is vaporized into steam.
The heat transfer coefficient corresponding to the total heat exchange capacity consists of three parts: convection heat transfer coefficient in the coil, thermal conductivity of the coil and boiling heat transfer coefficient outside the coil. Check the heat balance of the reaction system according to the detected heating steam flow, as shown in Figure 5.
As can be seen from the figure, when the reaction system reaches an equilibrium state, the mass of substances entering and leaving the system is equal. When the reaction system reaches an equilibrium state, the total heat released by heating steam is equal to the total heat absorbed by the reactor and the total heat exchange calculated by the heat exchange coefficient.
3.4 Others
The liquid product of hydrolysis reaction is not the main target of pilot evaluation. After cooling and depressurization, it can be detected by the sampling device at the bottom of the reactor and then compared with the results of phase equilibrium calculation. The experiment shows that the concentration of urea and its derivatives in the reaction solution decreases with the increase of operating pressure under different feed concentrations.
4 conclusion
The reaction system of urea hydrolysis to produce ammonia in thermal power plant belongs to high concentration urea aqueous solution system. In this paper, the urea hydrolysis process is simulated by ASPEN software, and the ammonia production capacity of the reactor is obtained under the assumption that the reaction equilibrium constant of urea synthesis process follows, and the feasibility of the assumption is verified by pilot test.
The results show that the revised assumptions are feasible and consistent with the actual operation results of the device. In the multi-batch test, the maximum ammonia output of the device is 9.9kg/h and the minimum ammonia output is13.65 kg/h, which is consistent with the design value of ammonia output10 kg/h. The device can meet the requirements of ammonia load change and denitrification system adjustment.
With the increase of feed urea solution concentration, the ammonia concentration in the hydrolysate increases, the water vapor concentration decreases, and the energy consumption per unit of ammonia production decreases. When the mass concentration of urea solution is increased from 50w% to 60w%, the volume concentration of ammonia component in product gas is increased from 37.5% to 48%, and water vapor is decreased from 43% to 28%. Reducing the energy loss caused by the latent heat of vaporization caused by excessive water consumption can not only improve the feed concentration and reduce the energy consumption caused by excessive water consumption, but also help reduce the operating cost of hydrolysis process.
From the dynamic point of view, ammonia productivity is another important factor affecting the operating cost of hydrolysis ammonia production process. With the increase of feed concentration, the urea concentration in the reaction solution increases in equilibrium, and the operating temperature required for the same ammonia production rate decreases, thus reducing the energy consumption of the system, improving the response ability of the device to variable load, helping to improve the economy of the hydrolysis device, and providing basic parameters for the next process design of urea hydrolysis for ammonia production from flue gas denitrification and the development of hydrolysis reactor equipment.
At present, urea hydrolysis ammonia production technology has been successfully applied to Huaneng Yantai Power Generation Co., Ltd., Guodian Longhua Yanji Thermal Power Co., Ltd. and Huaneng Zuo Quan Power Plant. The equipment has been put into operation stably, and its main parameters have reached the advanced level in the industry.
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