流化床聚合反应器颗粒生长与流动模式的研究
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摘要
气相流化床乙烯聚合生产工艺中,床层流化的均一性、床层的传递特性、流态化的特征参数、反应效果、聚合物后处理以及聚合物产品均受到流化床内颗粒粒径分布的影响。因此,在乙烯气相聚合反应中,聚合物颗粒粒径及其分布的检测、预测和调控,对于优化操作生产,实现聚合物产品的设计,具有非常重要的意义。另一方面,与颗粒粒径分布密切相关的流动模式是流化床聚合反应器设计所必需的基本条件,直接影响到聚合反应特性和传热传质性能,揭示其本质特点及相互间的转变规律,不仅有助于从理论上加深对流化床聚合反应器的认识,而且其研究结果对反应器的设计和改造等工业过程开发具有直接的应用价值。但是所有研究问题的瓶颈都在于目前尚未有效的检测方法进行工业流化床聚合反应器的测量。因此,采用新型测量技术,研究流化床聚合反应器颗粒长大的规律、流体力学特性以及它们之间的相互作用关系,成为本论文的主要任务。
受古人“听音辨琴”的启示,并发现“气固流化床运行过程中发出的声波主频变化反映为声波各尺度(频段)能量分布的改变,正是由于不同粒径的颗粒所产生的声波能量叠加,导致声波在各尺度的能量分布不同”。因此,本论文通过对流化床运行过程中发出的包含有丰富信息的声信号检测,经非线性的多尺度解析,发明了新型的气固流化床参数在线测量技术,实现了气固体系颗粒流动模式的实验检测和结块与料位高度的在线监控。在此基础上,建立了工业气固流化床稳态操作下聚乙烯颗粒粒径分布的预测模型和定制模型,模型可以较好地预测聚合物的颗粒粒径分布,并在定制模型的指导下,通过调节流化床聚合反应器各操作变量,可以实现定制的聚合物颗粒的粒径分布。通过研究流化床壁面颗粒的运动活跃程度,获得了流化床存在大小循环流的流动模式特征,以及流动模式随颗粒粒径和表观气速变化的实验证据。本论文主要创新性的工作包括:
(1) 在考虑聚乙烯颗粒在气相流化床反应器中生长、磨损和扬析的基础上,建立了乙烯气相聚合工艺中稳态操作条件下颗粒生长与粒径分布的预测模型。论文首次对工业气相流化床反应器的聚乙烯颗粒生长和粒径分布进行了模拟研究,计算得到的粒径及其分布与生产数据基本一致,说明该模型能够较为真实地反映气相流化床反应器内聚乙烯颗粒的反应历程和生长规律,在预测聚乙烯粒径分布方面具有良好的应用前景。
(2) 首次提出了聚乙烯颗粒粒径分布定制模型。由定制的最优粒径分布数据,
In gas phase polymerization fluidized-bed reactor, polymerization rate, process stability, the characteristic parameters of fluidization, transport characteristic, reaction effect, cost of polymer process stage and polymer properties may be affected by polymer particle size distribution (PSD). Therefore, it is vital to predict and control the PSD of polyethylene particles for optimization of production and design of polymer product. Gas phase polymerization in fluidized bed reactor is used widely to produce polyolefin all over the world during decade years with significant commercial success. But the inherent disadvantages of gas phase process exist obviously due to weak ability of heat transfer in gas fluidized bed reactor. Generally there is a relationship between heat transfer and flow pattern within the reactor. So investigation of flow pattern or structure of fluidization has been a hot theme for a long time. However, there is not an effective apparatus to measure the flow pattern to this day. Agglomeration is an incipient fault for the gas-solid fluidized bed. And the bed level is an important variable which effects on the product quality and the yield. Both the bed level and agglomeration are monitored by fatal y-ray. So it is more necessary and urgent to develop a novel technique to detect the bed level and agglomeration replacing the γ-ray.The changes of acoustic signal main-frequency in gas phase polymerization fluidized-bed reactor reflect the changes of acoustic energy in different segment. The nestification of different size particles' acoustic energy results in different distribution of acoustic energy. And the PSD could be got by acoustic signals analyses and mal-functional operation online in fluidized bed reactor could be monitored. Based on this PSD measurement, a novel technique was used by means of an acoustic emission (AE) sensor to investigate flow pattern, agglomeration and the bed level in the fluidized bed in this work. Solid fluidization generally releases various AE signals comprised of particle-particle or particle-chamber collisions impact sound, particle-particle or particle-chamber friction sound, and air turbulence in fluidized bed. Therefore, measurement of AE energy and analysis could reflect the particle activity and size (agglomeration). And this energy further reveals the flow pattern and mixing of particles and bed level in fluidized bed. The innovative results can be summarized as following:
(1) Polymerization rate, devolatilization rate, the cost of polymer processing and polymer properties might be affected by polymer particle size distribution in gas phase polymerization fluidized-bed reactor. For modeling polyethylene particle size distribution (PSD) in FBR and investigating the effects of operation parameters and kinetic parameters on PSD, a steady-state particle size distribution model was developed based on mass balance. The particle growth, attrition and elutriation based on solid population balance were synthetically taken into account in the model. The result showed that for uniform size catalyst feed, the PSD in bed became boarder and shifted into larger size by increasing either initial catalyst size or superficial gas velocity. And the effect of bed temperature and pressure on PSD could be neglected. It was also shown, that the multi-size catalyst particles which consist of the different size catalyst particles ratio strongly affected PSD in bed. Bimodal PSD could be obtained by using a catalyst feed consisting of multiple distinct but uniform catalyst size. The polyethylene PSD in the industrial fluidized bed reactor could be predicted by this model with fairly good accuracy.(2)< On the basis of steady-state particle size distribution model in continuous polymerization process presented in the previous papers, a particle size distribution tailored model (PSD-tailored model) was presented. After considering the restriction of technology, such as operation parameter ranges, the novel model was solved by an algorithm named as Particle Swarm Optimization. And the polyethylene PSD could attain the objective value by the optimization of manipulated variable values, such as PSD of catalyst particles, superficial gas velocity, polymerization temperature, the concentration of ethylene and butylene. Then the optimal fluidization quality could be attained and the stability of production could be improved greatly. The particle swarm optimization method could reduce optimized parameters and much more time of calculation, and attain global optimization during the calculation. The PSD tailored operation strategy of three types polyethylene PSD in the gas phase ethylene polymerization technology were illuminated as examples, which could clarify the optimal strategy of polyethylene PSD tailored production in detail. It was shown that the commercial process could be enormously beneficial from the optimal strategy.(3) The flow pattern of a gas-solid fluidized bed was experimentally investigated by a novel Acoustic Emission technique. The fluidization of polyethylene particles with the
average size 460 urn in 250 fluidized bed under the superficial gas velocity ranging from 0.3 to 0.7 m/s has a multi-circulation flow pattern, in which there are main-circulation zone, circulation cell zone and stagnant zone on the wall of column. And the multi-circulation flow pattern will transit to the single circulation flow pattern if the superficial gas velocity and properties of particles were changed. The results of flow pattern in fluidized bed can be used to improve the process operation and reactor design.By the criterion of the energy percentage at different frequency increasing and decreasing sharply, a novel approach can be developed to judge whether the fluidization runs stably based on the wavelet analysis, and it can replace the y-ray in future.According to the regular change of the acoustic emission energy, the criterion of the mean square deviation ratio of AE energy attaining the maximum is presented.(4) For the first time, the chaotic characteristic parameters including biggest Lyapunov exponent, correlation dimension and Kolmogorov entropy were devoted to analyzing the acoustic emission signals in fluidized bed. The results showed that regime transitions among static bed, bubbling fluidization and turbulence fluidization could be characterized sensitively by chaotic characteristic parameters. Chaotic characteristic parameters changed regularly with the operating conditions and they would provide a novel thought for Regime distinguishes.The relation of chaotic characteristic and the gas velocity exposes convex curve function during the process of particles fluidization. It was demonstrated that the chaotic characteristic was existed in the regime from bubbling fluidization to turbulence fluidization and the appearance of remarkable bubbles was the most significant factor for particle motion with chaos characteristic.Compared with correlation dimension and Kolmogorov entropy, biggest Lyapunov exponent can distinctly reflect regime transition from static bed to minimum fluidization for particles motion, and correlation dimension and Kolmogorov entropy could be used to distinguish transitional region from bubbling fluidization to turbulence fluidization distinctly.
(5) A new method to measure the agglomeration in gas-solid fluidized beds was presented based on AE measurement and wavelet analysis. It showed that some agglomeration appeared when the energy percentage of d5 increased suddenly, and the bigger agglomeration come forth when the energy percentage of d6-d7 increased obviously. And this method could be used to replace the fatal y-ray measurement and to detect the fluidization quality in-line as a new indicator.A criterion was determined to measure the agglomeration size quantificationally in gas-solid fluidized bed based on the chaotic characteristic parameters analysis, including chaotic malfunctioned coefficients (Clei, Cd2 and Cia)- These chaotic malfunctioned coefficients were sensitive to measure and forecast the agglomeration in gas-solid fluidized bed effectively. The experimental results proved the validity of the criterion in the diameter of 100mm and the pilot reactor with the diameter of 420mm.(6) The software package of surveillance system in gas phase polymerization fluidized-bed reactor was established and applied to industrial equipment successfully. The item was favorably appraised by China Petroleum & Chemical Corporation.In the end, it is much significant to invent the technique to monitor and control the gas-solid fluidized bed instead of the fatal y-ray.
受古人“听音辨琴”的启示,并发现“气固流化床运行过程中发出的声波主频变化反映为声波各尺度(频段)能量分布的改变,正是由于不同粒径的颗粒所产生的声波能量叠加,导致声波在各尺度的能量分布不同”。因此,本论文通过对流化床运行过程中发出的包含有丰富信息的声信号检测,经非线性的多尺度解析,发明了新型的气固流化床参数在线测量技术,实现了气固体系颗粒流动模式的实验检测和结块与料位高度的在线监控。在此基础上,建立了工业气固流化床稳态操作下聚乙烯颗粒粒径分布的预测模型和定制模型,模型可以较好地预测聚合物的颗粒粒径分布,并在定制模型的指导下,通过调节流化床聚合反应器各操作变量,可以实现定制的聚合物颗粒的粒径分布。通过研究流化床壁面颗粒的运动活跃程度,获得了流化床存在大小循环流的流动模式特征,以及流动模式随颗粒粒径和表观气速变化的实验证据。本论文主要创新性的工作包括:
(1) 在考虑聚乙烯颗粒在气相流化床反应器中生长、磨损和扬析的基础上,建立了乙烯气相聚合工艺中稳态操作条件下颗粒生长与粒径分布的预测模型。论文首次对工业气相流化床反应器的聚乙烯颗粒生长和粒径分布进行了模拟研究,计算得到的粒径及其分布与生产数据基本一致,说明该模型能够较为真实地反映气相流化床反应器内聚乙烯颗粒的反应历程和生长规律,在预测聚乙烯粒径分布方面具有良好的应用前景。
(2) 首次提出了聚乙烯颗粒粒径分布定制模型。由定制的最优粒径分布数据,
In gas phase polymerization fluidized-bed reactor, polymerization rate, process stability, the characteristic parameters of fluidization, transport characteristic, reaction effect, cost of polymer process stage and polymer properties may be affected by polymer particle size distribution (PSD). Therefore, it is vital to predict and control the PSD of polyethylene particles for optimization of production and design of polymer product. Gas phase polymerization in fluidized bed reactor is used widely to produce polyolefin all over the world during decade years with significant commercial success. But the inherent disadvantages of gas phase process exist obviously due to weak ability of heat transfer in gas fluidized bed reactor. Generally there is a relationship between heat transfer and flow pattern within the reactor. So investigation of flow pattern or structure of fluidization has been a hot theme for a long time. However, there is not an effective apparatus to measure the flow pattern to this day. Agglomeration is an incipient fault for the gas-solid fluidized bed. And the bed level is an important variable which effects on the product quality and the yield. Both the bed level and agglomeration are monitored by fatal y-ray. So it is more necessary and urgent to develop a novel technique to detect the bed level and agglomeration replacing the γ-ray.The changes of acoustic signal main-frequency in gas phase polymerization fluidized-bed reactor reflect the changes of acoustic energy in different segment. The nestification of different size particles' acoustic energy results in different distribution of acoustic energy. And the PSD could be got by acoustic signals analyses and mal-functional operation online in fluidized bed reactor could be monitored. Based on this PSD measurement, a novel technique was used by means of an acoustic emission (AE) sensor to investigate flow pattern, agglomeration and the bed level in the fluidized bed in this work. Solid fluidization generally releases various AE signals comprised of particle-particle or particle-chamber collisions impact sound, particle-particle or particle-chamber friction sound, and air turbulence in fluidized bed. Therefore, measurement of AE energy and analysis could reflect the particle activity and size (agglomeration). And this energy further reveals the flow pattern and mixing of particles and bed level in fluidized bed. The innovative results can be summarized as following:
(1) Polymerization rate, devolatilization rate, the cost of polymer processing and polymer properties might be affected by polymer particle size distribution in gas phase polymerization fluidized-bed reactor. For modeling polyethylene particle size distribution (PSD) in FBR and investigating the effects of operation parameters and kinetic parameters on PSD, a steady-state particle size distribution model was developed based on mass balance. The particle growth, attrition and elutriation based on solid population balance were synthetically taken into account in the model. The result showed that for uniform size catalyst feed, the PSD in bed became boarder and shifted into larger size by increasing either initial catalyst size or superficial gas velocity. And the effect of bed temperature and pressure on PSD could be neglected. It was also shown, that the multi-size catalyst particles which consist of the different size catalyst particles ratio strongly affected PSD in bed. Bimodal PSD could be obtained by using a catalyst feed consisting of multiple distinct but uniform catalyst size. The polyethylene PSD in the industrial fluidized bed reactor could be predicted by this model with fairly good accuracy.(2)< On the basis of steady-state particle size distribution model in continuous polymerization process presented in the previous papers, a particle size distribution tailored model (PSD-tailored model) was presented. After considering the restriction of technology, such as operation parameter ranges, the novel model was solved by an algorithm named as Particle Swarm Optimization. And the polyethylene PSD could attain the objective value by the optimization of manipulated variable values, such as PSD of catalyst particles, superficial gas velocity, polymerization temperature, the concentration of ethylene and butylene. Then the optimal fluidization quality could be attained and the stability of production could be improved greatly. The particle swarm optimization method could reduce optimized parameters and much more time of calculation, and attain global optimization during the calculation. The PSD tailored operation strategy of three types polyethylene PSD in the gas phase ethylene polymerization technology were illuminated as examples, which could clarify the optimal strategy of polyethylene PSD tailored production in detail. It was shown that the commercial process could be enormously beneficial from the optimal strategy.(3) The flow pattern of a gas-solid fluidized bed was experimentally investigated by a novel Acoustic Emission technique. The fluidization of polyethylene particles with the
average size 460 urn in
(5) A new method to measure the agglomeration in gas-solid fluidized beds was presented based on AE measurement and wavelet analysis. It showed that some agglomeration appeared when the energy percentage of d5 increased suddenly, and the bigger agglomeration come forth when the energy percentage of d6-d7 increased obviously. And this method could be used to replace the fatal y-ray measurement and to detect the fluidization quality in-line as a new indicator.A criterion was determined to measure the agglomeration size quantificationally in gas-solid fluidized bed based on the chaotic characteristic parameters analysis, including chaotic malfunctioned coefficients (Clei, Cd2 and Cia)- These chaotic malfunctioned coefficients were sensitive to measure and forecast the agglomeration in gas-solid fluidized bed effectively. The experimental results proved the validity of the criterion in the diameter of 100mm and the pilot reactor with the diameter of 420mm.(6) The software package of surveillance system in gas phase polymerization fluidized-bed reactor was established and applied to industrial equipment successfully. The item was favorably appraised by China Petroleum & Chemical Corporation.In the end, it is much significant to invent the technique to monitor and control the gas-solid fluidized bed instead of the fatal y-ray.
引文
[1] Mitsui.Toatsu, US patent 3, 530107, 1970
[2] Hercules, US patent 4, 186107. 1980
[2] Hercules, US patent 4, 186107. 1980