The effect of bubbling regime on gas and liquid phase mixing in bubble column reactors

An experimental study has been aimed at linking the extent of axial mixing in gas and liquid phases in tall bubble column reactors with the respective bubbling regimes (homogeneous, transition, heterogeneous) encountered in such reactors and with corresponding macro-scale flow patterns of both phases. The experimental programme, carried out in a bubble column reactor of 0.14 m in diameter and 4.1 m in height, included determination of gas and liquid phase residence time distribution (RTD) characteristics for different modes of primary gas dispersion and flow visualisation experiments aimed at identification of macro-scale flow patterns in the reactor. The experimental results proved an essential effect of gas dispersion mode (bubbling regime) on the extent of gas and liquid phase mixing in the reactor. The respective dependences of PeG and PeL on the superficial gas velocity, obtained with the distributing plate generating sequentially homogeneous, transition and heterogeneous bubbling regimes, indicated very sensitively the transition from the homogeneous to the heterogeneous bubbling mode (the onset of the transition bubbling regime) occurring at u0G = 0.04 m s−1 with the appearance of local macroeddies in the bed. The comparison of our mixing data with corresponding dependences of gas holdup and kLaL on the superficial gas velocity revealed the existence of direct links between the variations of RTD and mass transfer characteristics induced by the changes of gas dispersion (bubbling) mode. The results of the analysis of variances of gas phase RTD curves justified (in agreement with corresponding PeG data) the plug flow approximation for the gas phase in the homogeneous bubbling regime and indicated the convective mechanism of gas mixing within the whole region of heterogeneous bubbling conditions. Analogically, the data for the liquid phase yielded the convective mechanism of liquid mixing within the whole range of experimental conditions. The mixing data obtained for both phases in the heterogeneous bubbling regime were adequately described by the model of consecutive circulation cells with the backflow between adjacent cells, proposed on the basis of the flow visualisation experiments as a realistic simplified representation of the macro-scale flow structures in heterogeneous bubble beds.