Study On Polyethylene Particle Evolution And Flow Pattern In Gas Phase Ethylene Polymerization Fluidized Bed Reactor Based On Acoustic Emission Measurement

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 theaverage 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.