| The gas-solid fluidized beds are extremely widely applied in the industrial fields due to their high efficiency of heat and mass transfer and large handling capacity. From the development of catalyst to the description of reactor hydrodynamic characteristics and establishment of industrial plant, gas-solid fluidized beds are always the research hotspot at home and abroad. As a typical multi-phase flow reactor, there exists bubble agitation, particle motion and phase interaction in a gas-solid fluidized bed. It is a non-linear transient system, and the most significant feature of fluidized bed is the fluctuation of various flow parameters generated by the above complex phenomenon. Hence, compared to the time-averaged flow parameters, the fluctuating parameters contain much more abundant information about flow structures and phase transfer. The flow structures in a fluidized bed include bubbles, vortex and particle clusters, whose behaviors and evolution are the bottleneck of revealing multi-phase flow features and achieving process intensification and optimal design. Therefore, it is of great significance to study the local and overall fluctuating signals generated by bubbles and particles in the fluidized bed, based on which the different flow structures are described and analyzed. By this way, the multi-phase flow field characteristics can be comprehended deeply and the reactor can be designed accurately.In order to overcome the shortages of current fluctuating signal measuring and flow structure characterizing methods, our work employed the computational fluid dynamics (CFD) software Fluent to simulate the fluidized bed and obtain the fluctuating parameters. Based on the similarity of single-phase turbulent fluctuation and fluidized bed fluctuation, we established the characterizing method of multi-phase flow filed and flow structures. The hydrodynamic characteristics of fluidized bed were studied by this method. Besides, we developed the multiple-temperature-zone ethylene polymerization process with condensing mode and implemented the industrial test, achieving remarkable effects. The main research achievements of this paper are as follows.1. Based on the similarity of single-phase turbulent fluctuation and fluidized bed fluctuation, we established the analytical methods of fluidized bed fluctuating signals and the characterizing methods of multi-phase flow filed and flow structures, including the analysis of particle fluctuating energy spectrum, flow field intermittency, multi-scale evolution of particle vortex, granular temperature, and the characterization and extraction of coherent structures. The aim was to extract regularities in random signals, and to discover order in disorder.2. By comparing the simulated results with experimental or literature data, the CFD models of mono-dispersed and bi-dispersed fluidized bed were validated. The particle fluctuating velocity signals were monitored by CFD simulation, and analyzed by power spectrum and wavelet decomposition. According to that, the particle fluctuating energy spectrum and flow field intermittency in mono-dispersed and bi-dispersed fluidized bed was studied. The particle fluctuating energy spectrum can be divided into energy-containing region, inertial region and dissipation region. The Levy-Kolmogorov scaling law was obeyed in the inertial region. The flow field intermittency distribution with frequency was investigated by the wavelet flatness factor. The intermittency was weak in low frequency and large scale fluctuation, while it was strong in high frequency and small scale fluctuation. For bi-dispersed system, the wavelet flatness factors of the two kinds of particles showed similar distribution trend, however the differences appeared in frequency higher than10Hz. The bubble dynamics was also investigated by the voidage fluctuation spectrum. The dominant frequency agreed well with the empirical formula calculated value. That meant CFD simulation can provide accurate information of bubble motion.3. The relationship between probability density function (PDF) of wavelet coefficients and coherent structures was established. The coherent structure signals were extracted and the Extended Self Similarity (ESS) scaling law was applied before and after the extraction. The effects of coherent structures on the flow field property were investigated and the extracting method was validated. The time scale and multi-scale evolution of particle vortex was studied by autocorrelation analysis. The universal relation of particle vortex time scales and wavelet scales was established. The vortex at some scales moved regularly as time passed. Particle vortexes at adjacent scales merged with or separated from others. The variation of particle vortex evolution was related to the fluidization transition.4. The relation of CFD simulated granular temperature and experimental acoustic emission (AE) energy generated by particle impacting wall was established. The granular temperature was proved to be able to indicate the flow pattern. The radial and axial distribution of granular temperature in bi-dispersed fluidized bed was studied. Above the distributor, when turbulent granular temperature dropped suddenly, particles moved into the "stagnant" zone and lost their vitality, corresponding to the boundary of flotsam and jetsam layer. As the superficial gas velocity increased, the height of "stagnant" zone decreased. The distribution of particle time-averaged velocity and Reynolds stress was also applied to indicate the flow pattern and phase interaction.5. In order to study the gas phase ethylene polymerization process with condensing mode, the calculating model of bubbling fluidized bed and central jetting fluidized bed were established. The CFD simulation was used to investigate the effects of gas ejection from wall on hydrodynamic characteristics. The gas ejection promoted horizontal motion of particle phase and strengthened the particle fluctuating activity near wall, which can reduce particle agglomeration on the wall. Besides, the gas ejection can stabilize the particle flow pattern and modulate the evolution of particle vortex. The work provided a theoretical guide of condensing liquid evaporation destroying the particle vortex hotspots.6. Based on the theoretical analysis, the scheme of multiple-temperature-zone ethylene polymerization process was proposed and was verified preliminarily in the industrial plants. The feasibility of this technique was discussed firstly. The gas-liquid separator and liquid ejecting equipment was developed. The characteristics of this technique and their effects on resin performance were analyzed, including the differences of polymerizing temperature, monomer concentration and temperature sensitivity of catalyst between the two reaction zones. The polyethylene of low density, high molecular weight and more branched chains can be produced in the lower reaction zone, while the polyethylene of high density, low molecular weight and less branched chains can be produced in the above reaction zone. The new process was tested in the120000ton/year ethylene polymerization plants of Sinopec Tianjin Company. The output was increased by8%and the resin with excellent performance and distinct characteristics was produced successfully. This process was named as "Packaged technology of gas-liquid fluidized bed ethylene polymerization" and selected as "Sinopec Ten Dragons" project in2012. |
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