Circulation in bubble columns

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Abstract
Liquid circulation in bubble columns influences many system properties such as mass transfer and catalyst dispersion. Conditions favorable for liquid circulation often exist only for highly turbulent slurries. Experiments under conditions of lowered surface tension proved conclusively that uniform circulation can exist even in the bubbly‐flow regime. Moreover, the measured circulating velocity profile is more plug‐shaped in the center core than traditional models, which take a parabolic shape. These observations led to the application of a circulation theory based on an energy‐dissipation model for turbulent eddy viscosity, which can be easily computed as a simple function of superficial‐gas velocity. Two length scales were used in model calculations depending on flow regime: bubble diameter for bubbly flow and column diameter for churn turbulence. The liquid‐velocity profile contained a tuned dimensionless proportionality constant, which resulted in two different average values, k = 2.51 for bubbly flow and 0.0242 for churn turbulence, when tuned to experimental data. The integral average eddy viscosity is numerically equivalent to the eddy dispersion coefficient for bubbly flow. Predicted liquid velocity compared very favorably with new measured values under conditions of low surface tension and with literature values.