Hydrodynamics and Bubble Size Distribution in a Stirred Reactor

A study of the hydrodynamics of two-phase stirred tanks is presented. The hydrodynamics in large-scale reactors is shown to be mainly a function of the superficial gas velocity and the stirring in the system. Six configurations for the stirrer have been tested: two single-stage and four two-stage configurations. The three hydrodynamic regimes (short circuit, load and flood) were observed for these configurations. The results show that the gas holdup, measured using the difference in level between aerated and non-aerated states, achieves a maximum value for the mixed two-stage combination. For this configuration, a study of the residence time distribution was carried out by employing the tracer (pulse injection) method, thus allowing the determination of the dead volume and the modeling of the flow in the reactor, corresponding to a perfectly mixed reactor. In stirred tank reactors, the study of the bubble size distribution has a great importance on the flow dynamics, the dimensions of bubbles are measured photographically; this investigation shows the presence of fine bubbles (d < 10 mm) with the experimental bubble size distribution curves exhibiting classical log-normal function traits within +/- 3%. The characterization of the hydrodynamics and the flow regimes in the stirred reactor permits to optimize the operating parameters (stirrer type and configuration, stirring speed, gas velocity) within the reactor in order to treat the water contaminated by persistent pollutants.