【作者】 周业丰；

【导师】 陈建峰； 阳永荣； 王靖岱；

【作者基本信息】 浙江大学 ， 化学工程， 2014， 博士

【摘要】 多温区流化床反应器,由于其突破了传统流化床中温度或浓度分布均匀的限制,已应用于流化催化裂化、煤气化、干燥及造粒等化工过程。从多温区／多流型共存的角度出发,研究此类反应器的共性问题,能帮助揭示多温区流化床反应器的流化特性及其调控机制。众多成熟工艺技术已证明,向单个流化床中喷液是构建多温区流化床反应器的有效手段。故本文针对气相法流化床聚乙烯工艺,向流化床反应器中喷入大量冷凝液,提出了一种新型的多温区流化床聚合反应器,可以生产高性能的聚乙烯产品,为反应器的设计开发和产品结构的优化提供了新思路。然而,持液操作将显著影响流化床中流体力学行为及流化稳定性。在保证流化状况良好的前提下,如何在持液气固流化床中构建稳定共存的多温区并对其进行有效调控,是一个极富挑战性的研究课题,具有重大的理论意义和工业价值。本文围绕多温区流化床反应器的构建、调控及稳定性开展如下五方面的研究工作。1.提出了液滴蒸发、液滴-颗粒碰撞和覆液膜颗粒碰撞的时间尺度分析与力平衡分析相结合的研究方法,建立了液桥诱导聚团的稳定性分析模型。稳定性分析模型由两步骤组成：根据液滴相关过程的时间尺度分析,对覆液膜颗粒能否发生有效团聚进行判断,认为当液滴蒸发时间尺度大于碰撞时间尺度时,可能发生液桥诱导团聚；进一步,采用力平衡分析,对流体曳力与液桥力、颗粒聚团重力进行比较,进而对颗粒聚团是否导致流化失稳进行准确判断。基于稳定性分析模型,获得了不同的液固接触状态下气液固三相流型谱图、各流型下颗粒聚团的表现形式和主导作用机制。2.建立了声波、压力脉动及摄像多种测量手段相结合的多层次表征方法,首次对持液气固流化床中的颗粒、气泡及整体流化状态进行多层次分析,揭示了颗粒、气泡以及流化状态随液体含量的变化规律。研究表明,当声波测量所反映的颗粒尺寸呈现显著变化时,比颗粒尺度更大的气泡尺度(压力脉动测量反映)及整体流化状态尺度(摄像法所反映)行为也会相应发生显著变化,比如气泡变小并最终出现气体沟流。进一步对声信号进行Hurst和V统计分析,首次分辨出了液体增加过程中出现的周期行为的信号特征,此周期成分的循环时间为2.5ms,对应的频率为400Hz,此频率处于颗粒和气泡之间的特征频段,故推断其为微观的颗粒尺度和宏观的反应器尺度之间的介尺度作用行为。三种测量方法相互间存在验证和互补,且均能有效地反映液体增加所导致的颗粒团聚、气体沟流等不稳定流化状态。3.在自行设计的热态流化床装置中,研究发现,相比于下部喷液方法,上部喷液方法构建的多温区具有更大的温区间温差和更高的流化稳定性,为优选的多温区构建方式,且液体流率、静床高和进气温度等操作参数对多温区的温区间温差和流化稳定性影响最为显著。研究同时表明,多温区流化床中液体蒸发和液体架桥是相互竞争的两种液体调控作用,两者的强弱将显著影响多温区的区间温差及流化稳定性。通过分析多温区流化床内的液体作用机制与颗粒聚团特性,发现稳定共存的多温区流化床反应器可看作是由液体架桥和液体蒸发作用相制约平衡的气液固三相流型(动态聚团机制)和由液体蒸发作用主导的气液固三相流型(聚团破碎机制)共存的模式。4.在多温区流化床反应器中,研究并揭示了液体对颗粒、气泡及颗粒循环模式的作用规律。研究发现,液体流率增加和气速降低均导致液体作用强度增加,气流／液体蒸发作用主导的流化机制会向液体架桥作用主导的流化机制转变；与此对应的是,气流／液体蒸发作用主导时的中心向上、壁面向下的颗粒循环模式逐渐转变为液桥作用主导时的颗粒聚团向下流动的颗粒循环模式,最终导致颗粒聚团无法循环,出现流化失稳等不稳定现象,此时相应的颗粒及气泡行为也发生显著变化。研究提出的通过床层温度分布测量来反映颗粒循环模式的方法,为揭示流化床内颗粒循环提供了新手段。5.针对工业多温区流化床聚合反应器,基于气液固和气固多流型共存的思想,结合液体蒸发模型,建立了改进的乳化相-气泡相模型,较为准确模拟了工业聚合反应器冷凝态操作下的温度分布,具有明显的多温区特征,证实了工业多温区流化床聚合反应器中气液固和气固双流型／双温区复合假设的合理性,最后采用流程模拟的方法,发现复合流型的反应器能够实现产品的高性能化。更多还原

【Abstract】 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 is400Hz and thus the cycle time is2.5ms, 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 Chapter4, 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.更多还原