关键词: 流化床, 传热, 流体动力学, 经验关联

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