Duality of the gas-liquid flow regimes in bubble column reactors

The purpose of the present work was to examine the effect of design and working parameters of bubble column reactors on the formation and stability of the homogeneous and the heterogeneous bubbling regimes and to demonstrate the effect of bubbling regime transition on the hydrodynamic and mass transfer characteristics of gas-liquid beds in bubble column reactors. The factors determining the mode of gas-liquid contacting in bubble column reactors included the type and geometry of distributing plates and column geometry, as well as liquid-phase properties. Experimental evidence demonstrated extreme sensitivity of gas and liquid-phase mixing data to the transition from the homogeneous to the heterogeneous bubbling regime and, at the same time, indicated existence of direct links between the variations of RTD characteristics of both phases and values of bubble-bed voidage and k(L)a(L) induced by the changes of bubbling mode. The effect of design parameters on bubble-bed behaviour has been illustrated for air-water system. In contrast with the heterogeneous bubble beds, the range of the homogeneous regime stability and corresponding values of gas holdup have been found to be significantly influenced by the column diameter and aspect ratio, with obvious implications for column design and scale-up. Experimental data obtained in aqueous solutions of electrolytes and aliphatic alcohols demonstrated significant synergistic effect of solute concentration and gas dispersion mode on the character of a bubble bed and on the values of its voidage. The influence of solutes on gas holdup has been considerably more pronounced in the homogeneous and transition bubbling regimes than in the heterogeneous bubbling regime while, at the same time, our data demonstrated significantly enhanced stability of the homogeneous bubbling regime in the presence of surface active solutes. Increased viscosity of the liquid phase suppresed formation of homogeneous bubble beds already at mu(L) greater than or equal to 8 mPa s. Gas holdup data corresponding to the homogeneous bubbling regime were adequately described on the basis of the slip velocity concept while the Zuber and Findlay drift-flux model fitted well the data obtained under the heterogeneous bubbling conditions both in a bubble column and in an external-loop reactor. (C) 1997 Elsevier Science Ltd.