Study On Acoustic Emission Measurement And Multi-scale Structure Of Fluidized Bed Polymerizer

Hydrodynamics of fluidization, behaviour of reaction and properties of resin produced in gas phase polymerization fluidized bed reactor are strongly affected by the movement of the particles in fluidized bed. Therefore, it is the frontal and full of challenge in chemical engineering research. The thesis focuses on the growth of particles and its evolvement of particle size distribution (PSD), including occurrence of agglomeration, definition of minimum fluidization velocity, flow pattern, bed height and so on in view of the multi-scale approach based on the acoustic emission (AE) measurement. Not only the novel measurement and analytical method were established, but also discover many new phenomena and then theory established. All the results have the important significant academically and commercially.The signals emitted from the fluidized particles were the synthesis of the multi-scale information, and they were so immingle that could not easily to be understood. Traditional measurements had been devised to measure the signals, but they may not be used reliably on an industrial scale under hostile conditions of atmosphere, temperature and pressure. A novel and non-invasive AE measurement was used in this study. Further, the AE signals would be decomposed in multi-scale based on wavelet analysis and R/S analysis. The creative works can be summarized as following:(1) Compared AE signals of the multi-scale structure character with pressure signals. The criterion of dividing the information to three characteristic scales was established by Hurst analysis of the wavelet decomposed signals: micro-scale signals with all Hurst exponents less than 0.5 (lst-5th wavelet decomposing scales); macro-scale signals with all Hurst exponents more than 0.5 (6th-7th wavelet decomposing scales); meso-scale with two characteristic Hurst exponents (8th-10th wavelet decomposing scales), and the three scales reflected the action of particles, average dynamics and bubbles and agglomeration respectively. Energy profiles of the three scales components confirmed that the AE signals and pressure signals mainly reflect apart the micro-scale and meso-scale.(2) The character of the polymerization fluidized bed reactor in micro-scale was studied. The AE signals came from not only the experiment apparatus with the diameter of 100mm and 150mm, the pilot reactor with the diameter of 420mm, butalso the industrial units with diameter of 3500 mm (LLDPE), 3500 mm (HDPE) and 5000 mm (Bimodal PE). The signal main-frequency measurement Model, Hou-Yang Model and the new criterion of Umf were based.(a) A model of signal main-frequency measurementi. 1 2 v5J =- = ■r 5.7was presented for the gas-solid fluidized bed. It was found the signals main-frequency decreased with the increasing of the particles' size, temperature and density and with the decreasing of gas velocity. The experiment in the experimental fluidized bed showed that a satisfactory agreement was obtained between calculation and experimental data with respect of the main-frequency, and the average error all less than 8.3%. When this model was applied to the industrial fluidized bed of LLDPE, HDPE and bimodal polyethylene, the error between calculation and experimental data were less than 8.8% showed the model is effective.(b) Hou-Yang Model of particle size distribution (PSD)7=1 7 = 1was devoted to combine Hou-Yang Model with AE signals measurement and the wavelet analysis to determine the PSD in the fluidized bed with three different polyethylene powders of LLDPE, HDPE and Bi-mode PE respectively. It was found the corresponding deviations between Hou-Yang method and sieves method was less than 8.8%. When using between Hou-Yang Model to predict PSD in the industrial fluidized bed with the diameter of 3500mm of LLDPE, 3500mm of HDPE and 4000mm of Bimodal HDPE, the average deviations Hou-Yang method and sieves method was less than 15.8%. Farther study found Hou-Yang Model could be applied to some measurement that there were many wavelet scales in multi-scale, and gas velocity beyond 3Umf.(c) A new method to define minimum fluidization velocity in fluidized beds was put forward with AE measurement and wavelet packet analysis. When using this method to define the Umr with the particles of ordinary PE (LLDPE) and especial PE (Bimodal PE) under normal temperature conditions and LLDPE particles on the temperature increasing process, it showed that a satisfactory agreement was obtained between this new method and traditional ways. The traditional ways could not estimate Umf accurately with the Bimodal PE particles on the temperature increasingprocess, but the new method was effective. It was found Umf of LLDPE particles could not changed with temperature and Umf of Bimodal PE particles increased with the temperature increasing. Further, Using the new method give the result that Umr arranged from low to high in the same average particle size was FCC-type, bipeak-type, flat PSD, binary PSD, Gaussian-type and narrow cut PSD.(3) The character of the polymerization fluidized bed reactor in meso-scale was studied. It was found in the experiment apparatus with the diameter of 100mm and 150mm and the pilot reactor with the diameter of 420mm that the change of PSD affected the energy percentage of d'-d5 in multi-scale, and the agglomeration size increasing brought on the energy percentage of d6-d7 increased in turn. A formula to measure the agglomeration size was:S = a(Edb-E°d6) + b(Ed1 -E°d1) + c(Ed&-￡°8).and 3=580, b= 1920, c=6800. the error between calculation and experimental data were less than 11.2%. On the other hand, the result showed there are reversible and non-reversible phases in the agglomerating process.(4) Investigate the AE signals with the different height in fluidized beds by using energy analysis, mean square deviation and complexity analysis. The flow pattern in the experimental apparatus and industrial unit (HDPE) was worked over. It was found that there are two solid circulations cells with smaller size and solid circulation with larger size (main circulation zone), and a stagnant zone located between the two circulations cells. On the other hand, the bed height could be measured in experimental apparatus and industrial unit (HDPE), and the error between the prediction by AE measurement and actual height was less than 2%. In a word, the character of macro-scale in fluidized beds could be measured availably by AE sensor array.