PREDICTION OF MINIMUM GAS VELOCITY IN SUSPENDED BUBBLE-COLUMNS AND AIRLIFT REACTORS

For design and operation of bubble columns and airlift reactors with suspended solid particles it is necessary to know the minimum gas velocity for maintaining the state of complete solid suspension. To predict this critical gas velocity information on flow bebaviour of solid particles in gas-liquid dispersions is required. For this purpose a simplified model is proposed which is based on an energy balance containing mean values of liquid velocity in the upstream region. Whereas within the riser region of airlift reactors the solid content was found to be constant, it shows a gradient as a function of height in the upstream region of bubble columns, which is described by the Sedimentation-Dispersion Model. In this way for each of the two reactor types a relation is obtained in which the minimum gas velocity is given as a function of sedimentation velocity of solid particles and liquid velocity. These functions are confirmed by results from measurements in both a bubble column and an airlift reactor with dispersions of glass particles of different grain size in the system air/water.