Abstract:

The external heat extractor is one of the key equipment to maintain the thermal balance of the catalytic cracking reaction system and keep the device running smoothly. It can process feed oils into products with higher value, such as gasoline, diesel, and olefins. The catalytic cracking reaction belongs to a parallel series reaction, and its desired products are the reaction intermediate. Meanwhile, a lot of heat is absorbed during reaction. The heat is mainly provided by coke-burning regeneration of the catalysts in regenerator. When processing heavy oils, the carbon amount on the catalysts will be increased, which can generate the extra heat during the catalysts coke-burning regeneration. A fluidized bed with external catalyst cooler can take the extra heat away to control the reaction temperature effectively and keep the heat balance between reaction and regeneration. The downflow external catalyst coolers have been widely used due to their high heat load and flexible adjustment. Both optimal design and effective control of downflow external coolers in industrial application require a deep understanding of gas-solid flow characteristics, heat transfer characteristics and their correlation. Therefore, a pilot scale cold mode experimental setup was built to investigate the effect of superficial gas velocity and solid mass flux on heat transfer, solid holdup and bubble frequency. The results showed that the instantaneous heat transfer coefficient presented the characteristics of low-frequency and high-amplitude as well as high-frequency and low-amplitude, and the former played a dominate role. The fluctuation periods of instantaneous heat transfer coefficient and instantaneous local solid holdup were both 25 s, which indicated that the heat transfer process between bed and heat transfer tube was directly related to the local solid holdup. Increasing superficial gas velocity can reduce local solid holdup, increase local bubble frequency, and enhance the heat transfer between bed and heat transfer surface. Moreover, local solid holdup and local bubble frequency were increased with solid mass flux. Heat transfer coefficient was decreased with solid mass flux at u_g=0.1 m/s, while increased with solid mass flux at u_g≥0.4 m/s. Influence of gas-solid flow characteristics on heat transfer characteristics was various under different flow patterns of fluidized beds. In bubbling bed flow pattern, local bubble frequency near the heat transfer tubes had a great influence on the heat transfer process, due to high local solid holdup impeded the renewal of particles on the heat exchange surface. In turbulent bed flow pattern, heat transfer coefficient was increased with local solid holdup and bubble frequency. Empirical correlations for predicting heat transfer coefficient at different flow patterns were built based on operation conditions and gas-solid flow characteristics. Their mean relative deviations between predicted value and experimental value were 6.9% and 1.3% respectively.

Key words: fluidized bed, heat transfer, hydrodynamics, empirical correlations

摘要：

外取热器是维持催化裂化反应-再生系统热平衡和保持装置平稳运行的关键设备之一。外取热器的优化设计和合理调控，要求深入理解外取热器内的流动特性、换热特性及两者之间关系。在一套大型冷模热态实验装置上，分别考察了表观气速、颗粒质量流率对换热管附近的局部固含率和气泡频率、床层与换热管间传热系数的影响。结果表明：增加表观气速可以降低局部固含率、增加局部气泡频率、强化床层与换热管间换热；随着颗粒质量流率增加，局部固含率和局部气泡频率均增加；在较低表观气速下，增加颗粒质量流率不利于换热，而在较高表观气速下，传热系数随颗粒质量流率逐渐增加。不同流型下，气固流动特性对换热特性的影响不同。在鼓泡床流型下，过高的局部固含率不利于颗粒在换热表面的更新，增加换热管附近的局部气泡频率可以明显强化换热；而在湍流床流型下，换热管附近的局部固含率和气泡频率的增加，均使传热系数逐渐增大。建立了针对不同流型的换热经验关联式，预测值与实验值的平均相对偏差分别为6.9%和1.3%。

关键词: 流化床, 传热, 流体动力学, 经验关联