| Gas phase ethylene polymerization is widely applied in industry as its simpleness, strong production capacity, good flexibility, low material and energy consumption and security. Space time yield of the reactor can be greatly improved through evaporation of the condensed-liquid to move out amount of reaction heat. However, adding liquid will affect the flow conditions and mixing efficiency of the fluidized bed reactor. It will cause agglomeration of the particles or unnormal fluctuations of material level, and even cause bed collapse, which will force to stop production. Meanwhile, adding liquid to the fluidized bed reactor can affect the heat balance, causing a large bed axial temperature gradient, thereby producing products with specific properties. Therefore, it is necessary to further understand the effect of liquid on fluidization characteristics and motion characteristics of particles and bed temperature distribution.In this thesis, gas phase polyethylene condensed mode technology was studied respectively with acoustic emission detection technology, combined with statistical analysis and Hilbert-Huang transform (HHT) analysis, through single bubble and bed collapsing experiments. We studied the effects of liquid on fluidization characteristics and motion characteristics of particles; A multi-layers model was builded for the fluidized bed reactor in the condensed process, and the effect of liquid contents on the bed temperature profile was studied. The results had important guiding significance for safe and stable production, optimizing operating conditions in the gas phase polyethylene condensed mode technology. This thesis mainly focused on the following three aspects:1. We revealed the relationship between the flow types and the liquid contents by cold molding experiments.(1) In the3-dimensional gas solid fluidized bed, the air velocity was adjusted to increase from0to high and then decline gradually. According to observation and the bed pressure drop varing with the gas velocity, the minimum fluidization velocity (umf) and minimum bubbling velocity (umb) of polyethylene (PE) particles with differnt liquid contents were detected. The results showed that, after adding the liquid, umf of PE particles increased, and umb was significantly higher than umf. When the superficial gas velocity was greater than umf and less than umb, the bed expanded obviously as Geldart’s group A particles typically. When the liquid content is large (12～16wt%), there even were a small amount of agglomerate and channel at bottom bed as Geldart’s Group C particles typically.(2) In the3-dimensional gas solid fluidized bed, acoustic emission (AE) technique was used to analyze the collapse processes of different Geldart’s particles. A criterion to distinguish the Geldart’s group classification was presented. The variation curve of the acoustic energy at the bottom bed with time for Group A particles shows a linear segment but there is no linear part for Group B/D particles.. On this basis, by usingo AE technique to analyze the collapse processes of particles with different liquid contents, we studied the relationship between the flow types and the liquid contents. The results showed that, adding a small amount of liquid (4-8wt%) could enhance the aeration of wet PE particles and improve fluidized quality and the fluidization characteristics turn to Group A. We also found that adding a lot of liquid would lead to agglomeration and the fluidization characteristics turn to Group C.(3) Based Molerus’s interparticle force theory, combined with simulation and calculation of liquid bridge force, we investigated the effect of liquid bridge force on the ratio of flag force and interparticle force. The results show that the liquid bridge force enlarged the interparticle force, so the magnitude of the nterparticle force can no longer be neglected compared to the fluid drag force, and the fluidization characteristics of wet PE particles turn to Group A. Meanwhile, when the liquid content is high (eg.16wt%), fluidization characteristics of some small particles may turn to Group C2. We revealed the relationship between the particles’motion characteristics and the liquid contents by cold molding experiments.(1) Based on AE detection technology, we simulated the friction and collision between particles and fluidized bed wall on a plexiglass panel. The AE signals were analyzed by Hilbert-Huang transform (HHT) method to extract the characteristic signals of collision and friction between the particles and the wall. A relationship between AE signal frequency and the motion of particles in the fluidized bed was founded. The results showed that, the AE energy generated by friction between particles and the wall was concentrated in the low frequency region as20-50kHz, the AE energy generated by collision between particles and the wall was concentrated in the high frequency region as50-200kHz.(2) In the2-dimensional gas-solid fluidized bed, AE technique was used to detect AE signals of different Geldart’s particles at the top, side and wake of a bubble. The signals were analyzed by HHT method. We found that, Group A particles collided strong with the wall at the top and wake of the bubble, Group B particles only collided weakly with the wall in the bubble wake, while Group D particles almost didn’t collide with the wall. On this basis, we used AE techniques to detect the AE signal in the bubble wake for particles with different liquid contents, combined with HHT method for analysis to investigate the effects of liquid on particles’motion characteristics. The results showed that, after adding the liquid, PE particles almost didn’t collide with the wall in the bubble wake. As the liquid content increaseing, PE particles’motion characteristic trun to Group D from Group B.(3) In the3-dimensional gas-solid fluidized bed, AE technique was used to detect AE signals of different Geldart’s particles during collapse, and the AE signal were analyzed by HHT method. We found that particles were easier to collide with the wall in aggregative fluidization, and the particles at the bottom and surface of bed wer easier to collide with the wall, In a fluidized bed, Group A particles collided stronger with the wall, Group B particles collided weaker, while Group D particles almost didn’t collide with the wall. On this basis, we used AE techniques to detect the AE signal of PE particles with different liquid contents during collapse, combined with HHT method for analysis to investigate the effects of liquid on particles’motion characteristics. The results showed that, after adding the liquid, PE classes particles in the bed almose didn’t collide with the wall during collapse. As the liquid content increaseing, PE particles’motion characteristic trun to Group D from Group B.3. Based on the two-phase fluidized bed model, combining some assumptions of bubble growth model and reactors-in-series model, we established the multi-layers model(MLM) for fluidized bed reactor in condensed technology. The emulsion phase and bubble phase are divided into multiple layers in the axial direction, and each layer is considered fully mixed. The length of each layer is equal to the bubble size at that height. There is solid-exchange between emulsion phases of two adjacent layers through the bubble wakes. Based on the heat balance equation of each layer, we obtained emulsion phase temperature, bubble phase temperature and superficial bed temperature varying with height through Matlab. The simulation results of superficial bed temperature matched the industrial data well. On this basis, we studied the effects of consended-liquid contents and entrance location of consended-liquid on the superficial bed temperature distribution. The results showed that, as the consended-liquid content increasing or elevating the entrance location, the region of lower temperature spreaded, which is beneficial for coexisting of multiple temperature zones in the reactor, and for producing of polymer products with specific properties. |
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