用临界功率表征多相搅拌槽中搅拌效果

Critical power criterion for characterizing mixing in multiphase stirred vessels

多相搅拌过程中常常根据临界搅拌转速表征搅拌效果。由于各种搅拌桨的功率准数不同,低临界转速的搅拌桨对应的搅拌功耗可能大于高临界转速的搅拌桨,因此,本文提出用临界功率作为表征手段。选取表面曝气和固体悬浮2个典型的多相搅拌过程进行了验证,结果表明:(1)在表面曝气过程中,达到临界曝气时六斜叶下推圆盘涡轮(PBRTD)桨的搅拌功耗比标准Rushton(RT)桨低,相同功耗下PBRTD桨的kLa也更高,与临界曝气转速的判定结果不同,临界功率法认为PBRTD桨更适合作表面曝气桨,这与表面曝气机理的认识相符;(2)下推折叶透平(PBTD)桨和三叶后掠(TBHA)桨的固体颗粒悬浮实验结果也表明TBHA桨的临界离底悬浮转速高但功耗更低,TBHA桨的整体循环能力更强,有利于固体悬浮。

In multiphase processes, the mixing stirred by different impellers in a multiphase stirred tank are generally compared and characterized based on their critical speed. As the power numbers of different impellers differ greatly, the power consumption of the impellers with lower critical speeds may be greater than those with higher critical speeds under the critical conditions. So a new criterion based on critical power is proposed to characterize the mixing in this work. Surface aeration and solid suspension experiments are taken to validate this method and the following results are obtained： （1） In surface aeration, the power consumed by a pitched-bladed disc turbine downflow （PBRTD） impeller is lower than that by a Rushton turbine （RT） impeller under the critical aeration condition, and the volumetric gas-liquid mass transfer coefficient of a PBRTD impeller is higher than that of a RT impeller under the same power consumption. The PBRTD impeller is more suitable for surface aeration than the RT impeller based on the critical power criterion, which is different from the conclusion by the critical speed method, and the flow produced by the PBRTD impeller is more suitable for the surface aeration based on this mechanism. （2） The results of solid suspension stirred by a pitched-bladed turbine downflow （PBTD） impeller and a Three-bladed helical agitator （TBHA） impeller show that the different conelnsions are drawn by using the critical speed method and the critical power method. The TBHA impeller with higher critical impeller speed has lower power consumption, possibly due to that the TBHA impeller brings better flow circulation beneficial for solid suspension. So the solid-suspension ability of the TBHA impeller is assessed to be better than that of the PBTD impeller.