催化裂化外取热器传热与流动特性的大型冷模实验

Large Cold-Model Experiment on Heat Transfer and Hydrodynamics of an FCC External Catalyst Cooler

为了深入认识催化裂化外取热器内的传热和流动特性,在1套ф500mm×3.0m、内设9根竖直翅片取热管的大型催化裂化外取热器冷态模型中,采用和工业装置类似的传热机制,测量了不同操作条件下外取热器内取热管传热系数的变化规律,并结合床层密度轴、径向变化规律进行了分析。结果表明,取热管传热系数随表观气速(u)增大呈现先增大后减小的趋势,峰值点出现在0.4m/s时,这一趋势和床层从鼓泡到湍动的流域转变密切相关,工业设计中可以选用床层的起始湍动速度作为最佳操作气速;由于中心区域气泡和颗粒更为强烈的运动,因此中心处取热管的传热系数显著高于边壁区域取热管的传热系数,和床层中心稀、边壁浓的床层密度径向分布相对应;随着床层高度的不断降低,取热管传热系数呈现单调下降的趋势,因此改变床层高度可以作为调节外取热器取热负荷的一个有效手段。

In order to obtain deeper insights on the heat transfer properties and hydrodynamics of FCC external catalyst cooler, a large cold fluidized-bed model （4500 mm× 3.0 m） with 9 vertical finned heat exchange tubes was established. With a similar heat transfer mechanism adopted in commercial units, the heat transfer coefficient changes of heat exchange tube was measured and analyzed based on the bed density changes along axial and radial direction. The experimental results indicated that, as superficial gas velocity increased, the heat transfer coefficient first increased and then decreased at 0.4 m/s, which is related to the flow regime transition from bubbling to turbulent regime. In industrial design, the predicted onset turbulent velocity can be adopted as the optimum operating gas velocity. Due to stronger bubble and particle movement, the heat transfer coefficient of heat exchange tube in the central bed was higher than that in the side bed, which agreed with the measured radial bed density profiles. As heat transfer coefficient decreased monotonously with the decrease of bed height, changing bed height can be an effective measure to change the heat removal load of an industrial external catalyst cooler.