DUALITY OF THE GAS-LIQUID FLOW REGIMES IN BUBBLE-COLUMN REACTORS

The purpose of the present work was to examine the effect of design an d working parameters of bubble column reactors on the formation and st ability of the homogeneous and the heterogeneous bubbling regimes and to demonstrate the effect of bubbling regime transition on the hydrody namic and mass transfer characteristics of gas-liquid beds in bubble c olumn reactors. The factors determining the mode of gas-liquid contact ing in bubble column reactors included the type and geometry of distri buting plates and column geometry, as well as liquid-phase properties. Experimental evidence demonstrated extreme sensitivity of gas and liq uid-phase mixing data to the transition from the homogeneous to the he terogeneous 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 c hanges 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 st ability and corresponding values of gas holdup have been found to be s ignificantly 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 an d gas dispersion mode on the character of a bubble bed and on the valu es of its voidage. The influence of solutes on gas holdup has been con siderably more pronounced in the homogeneous and transition bubbling r egimes than in the heterogeneous bubbling regime while, at the same ti me, our data demonstrated significantly enhanced stability of the homo geneous bubbling regime in the presence of surface active solutes. Inc reased viscosity of the liquid phase suppresed formation of homogeneou s bubble beds already at mu(L) greater than or equal to 8 mPa s. Gas h oldup data corresponding to the homogeneous bubbling regime were adequ ately described on the basis of the slip velocity concept while the Zu ber and Findlay drift-flux model fitted well the data obtained under t he heterogeneous bubbling conditions both in a bubble column and in an external-loop reactor. (C) 1997 Elsevier Science Ltd.