(448w) Realization, Control and Stability Analysis of Multiple Temperature Zones in Liquid-Containing Gas-Solid Fluidized Bed Reactor

Multiple temperature zones fluidized bed reactors (MTZFBRs) can overcome the limitations of uniform temperature in traditional gas-solid fluidized beds, and thus are widely used in numerous chemical engineering processes, such as fluid catalytic cracking, coal gasification, drying and granulation to mention a few. Investigations of the common scientific problems from the perspective of multiple temperature zones and multiple fluidization patterns coexistence, will greatly enhance the understanding of the fluidization characteristics and control mechanism of MTZFBR. Various mature technologies have shown that spraying liquid into a fluidized bed is one of the most effective means to realize MTZFBR. Therefore, this thesis proposes a novel MTZFBR by means of liquid-spraying within a fluidized bed reactor based on gas-phase fluidized bed polyethylene condensed mode operation process. Due to its optimization and improvement in the fluidized bed reactor process, high performance polyethylene products can be expected, which provides a new research direction and realization for product optimization and reactor process development. However, liquid-containing operation significantly affects hydrodynamic behavior and fluidization stability of fluidized beds. Realization and control of multiple temperature zones in liquid-containing gas-solid fluidized beds on the premise of stable fluidization states, is an extremely challenging research task which possesses great theoretical significance and industrial value. This thesis focuses on realization, control and stability of the MTZFBR and thus the research work has been carried out from the following five aspects:

1. A new research methodology, with combination of time scale analysis and force balance analysis, has been proposed in this thesis and thus the stability analysis model for liquid-bridge induced particle agglomerations is acquired. Firstly, the judgment of the effective particle agglomerations among liquid-coating particles is made based on time scale analysis of liquid-related key processes (such as droplet evaporation, droplet-particle collision and liquid-coating particles collision). To be specific, the particle agglomerations are induced by the liquid bridge when the droplet evaporation time scale is greater than the collision time scale among liquid-coating particles. Furthermore, the accurate judgment of fluidization instability caused by particle agglomerations is made according to the proposed force balance analysis and important criterion. Finally, based on results of stability analysis model, the spectrums of gas-liquid-solid (G-L-S) three phases fluidization patterns under different liquid-solid contact states are obtained as well as the particle agglomeration behaviors and the dominant mechanisms during the liquid-containing fluidization processes.
2. Multi-scale characterization method has been established with multiple measurement techniques including acoustic emission, pressure fluctuation and camera, which has been used for the first time to reveal the variation law of particle, bubble behavior and overall fluidization states simultaneously in the liquid-containing gas-solid fluidized bed reactor. Results demonstrate that when particle size reflected by acoustic signal varies significantly, the bubble scale (from pressure fluctuation) and overall fluidization state scale (from camera) behaviors show regular variation trends, such as formation of bubble shrinkage and gas channeling. Moreover, through Hurst and V-statistics analysis of the acoustic signal, the cyclic behavior characteristics induced by increased liquid in the fluidized bed are distinguished for the first time. The frequency of the characteristic cycle component is 400 Hz and thus the cycle time is 2.5 ms, which indicates the newly formed characteristic behavior is correlated to motion behavior of the meso-scale particle agglomeration. There are verification and complementarity among three kinds of measurements results, and all the three techniques can reflect the unstable fluidization states such as agglomerations and gas channeling during the liquid addition process.
3. With a self-designed hot mode fluidized bed apparatus, systematic experiments have been performed to realize MTZFBR. Results demonstrate that compared with the bottom liquid-spraying scheme, the upper liquid-spraying scheme shows larger temperature differences between upper zone and bottom zone, as well as higher fluidization stability, and thus the upper liquid-spraying scheme is preferable in the realization of MTZFBR. Based on upper liquid-spraying scheme, liquid flow-rate, bed height and gas inlet temperature are found to be the most significant operating parameters to affect multiple temperature zones. Moreover, studies have shown that liquid evaporation and liquid bridge are the two competitive factors during liquid-containing fluidization and thus the relative intensity of these two factors will significantly affect the temperature differences and fluidization stability. Furthermore, based on the stability analysis method proposed in the Chapter 4, two different three phases fluidization patterns are found to coexist in the stable MTZFBR. One is G-L-S three phases fluidization pattern with dynamic particle agglomerations mechanism dominated by a balanced action between liquid evaporation and liquid bridge, and the other is G-L-S three phases fluidization pattern with agglomeration breakup mechanism dominated by liquid evaporation action. 
4. In the MTZFBR, the action law of particle, bubble and particle circulation pattern induced by increased liquid flow-rate have been revealed. The study shows that both increases in liquid flow-rate and decreases in gas velocity cause lower gas-liquid relative action intensity, and thus the liquid-containing fluidization process dominant mechanism shifts from gas flow (or liquid evaporation) controlling mechanism to liquid bridge controlling mechanism. As a result, the particle circulation pattern changes from one mode (particles move upward in the center and particles move downward in the wall) to another mode (downward motion of agglomerations has been enhanced significantly), meanwhile the particle and bubble motion behaviors also change with regular trends. Besides, the minimum fluidization velocity of particles is found to increase with liquid content increase. Based on temperature profile measurements, one feasible method is proposed to reflect particle circulation modes in the fluidized beds, which helps to provide a new means to study particle circulation modes.
5. For industrial multiple temperature zones fluidized bed polymerization reactor, based on the concept of G-L-S and G-S fluidization patterns coexistence, an improved emulsion-bubble two phase fluidized bed model has been proposed with help of the liquid evaporation model. The model can simulate accurately the temperature profile of industrial reactor, which proved that the industrial MTZFBR can be studied by means of two fluidization patterns coexistence. Finally, the process simulation results show that high performance products can be realized by using the reactor with composite fluidization patterns.