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 mi xing in gas and liquid phases in tall bubble column readers with the r espective bubbling regimes (homogeneous, transition, heterogeneous) en countered in such reactors and with corresponding macro-scale flow pat tens of both phases. The experimental programme, carried out in a bubb le column reader of 0.14 m in diameter and 4.1 m in height, included d etermination of gas and liquid phase residence time distribution (RTD) characteristics for different modes of primacy gas dispersion and now visualisation experiments aimed at identification of macro-scale flow patterns in the reader. The experimental results proved an essential effect of gas dispersion mode (bubbling regime) on the extent of gas a nd liquid phase mixing in the reader. The respective dependences of Pc (o) and Po-2 on the superficial gas velocity, obtained with the distri buting plate generating sequentially homogeneous, transition and heter ogeneous bubbling regimes, indicated very sensitively the transition f rom the homogeneous to the heterogenous bubbling mode (the msd of the transition bubbling regime) occurring at k(L)a(L) = 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 k(L)a(L) on the superficial gas velocity revealed the existence of direct links between the variations of RTD and mass transfer characteristics induc ed 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 Pc(o) data) the plug now approximation for the gas phase in the homogeneous bubbling regime and indicated the convective mechanism of gas mixing within the whole region of heterogeneous bubb ling conditions. Analogically, the data for the liquid phase yielded t he convective mechanism of liquid mixing within the whole range of exp erimental conditions. The mixing data obtained far both phases in the heterogeneous bubbling regime were adequately described by the model o f consecutive circulation cells with the backflow between adjacent cel ls, proposed on the basis of ihc now visualisation experiments as a re alistic simplified representation of the macro-scale now structures in heterogeneous bubble beds.