Bubble recirculation regimes in an internal-loop airlift reactor

Bubble recirculation regimes have been studied in an internal-loop airlift reactor with two phases (water and air) and three phases (water, air, and polystyrene particles). The airlift was operated both in batch mode and with throughflow. In batch mode with increasing superficial gas velocity three regimes may be defined, depending on the state of gas phase in the downcomer: (1) no air bubbles, (2) bubbles remain stationary and (3) bubbles flow downwards and into the riser. In this paper the transition of regime 2 to regime 3 is studied. This transition is reflected by the increase of (i) the difference between gas holdups in the riser and downcomer and (ii) the liquid velocity in the downcomer. In regime 2 these two parameters are constant, while in regime 3 they increase with increasing superficial gas velocity. The ratio between the gas holdups in downcomer and riser is usually reported to be constant. However, in this work it is shown that the ratio is not constant and depends on the regime with parameters based on the regime transition. In regime 2 the gas holdups in riser and downcomer increase at the same rate, and in regime 3 the gas holdup in the riser increases faster than in the downcomer. In throughflow mode the regimes were less discernible. The presence of solids resulted in a decrease of the gas holdups, and lower values of the ratio between gas holdups in downcomer and riser. (C) 1999 Elsevier Science Ltd. All rights reserved.