Hydrodynamics and gas holdup in a cocurrent air-water-fiber bubble column
Gas-liquid-fiber flows are widely found in various unit operations in the pulp and paper industry and similar flow conditions may be found in other industrials, such as wastewater treatment, food processing, biological organism production, and pharmaceutical manufacturing;Flocculated fiber suspensions are considered as mixtures of fiber and suspending liquid with network structures comprising flocs and inter-floc regions. The fundamental mechanisms of the fiber influences on bubble motions and gas holdup in gas-liquid-fiber bubble columns are connected to the unique structure and properties of fiber suspensions;An experimental study is completed to investigate the hydrodynamics and gas holdup in a cocurrent air-water-fiber bubble column. Generally, gas holdup increases with increasing superficial gas velocity without a local maximum, decreases with increasing superficial liquid velocity, and changes nonlinearly with increasing fiber mass fraction. When flocculation is significant in the fiber suspension, gas holdup decreases with increasing fiber mass fraction. These trends are similar for all the studied fiber types. Gas distribution method significantly affects the gas holdup trends with increasing superficial liquid velocity or fiber mass fraction. Fiber type has a significant effect on gas holdup in the cocurrent air-water-fiber bubble column. Gas flow regimes in the air-liquid-fiber bubble column are identified based on the drift-flux model. Three gas flow regimes (i.e., dispersed bubble, vortical-spiral, and turbulent flow) are identified. When fiber mass fraction is higher than a certain value (which is a function of fiber type), the dispersed bubble flow regime disappears because bubble coalescence is enhanced at low superficial gas velocities by flocculating fibers. Superficial liquid velocity does not affect gas flow regime transition;A parameter (Ic) is identified to characterize the fiber effects on gas holdup in the airwater-fiber bubble column that satisfies the following condition: when this parameter is constant, the gas holdup in different fiber suspensions is generally similar at most operating conditions;A gas holdup model is developed for cocurrent air-water-fiber bubble flows based on the drift-flux model. The model coefficients are estimated with a nonlinear least square error curve fitting method using all data collected in the air-water-fiber system investigated in this study. The gas holdup model correlates gas holdup with superficial gas and liquid velocity and fiber type and mass fraction. The characterization parameter Ic is used to represent the effect of fiber type and mass fraction. The model reproduces most air-water-fiber system data within +/-10% error. It also predicts the gas holdup data in air-water systems, which is not used in estimating the model coefficients, within +/-10% error.