Coherent structures and axial dispersion in bubble column reactors

In this paper resulls of measurements of the local and time-dependent behaviour of the two-phase flow in a bubble column are presented. Measurements with Laser Doppler Anemometry (LDA) and with glass fibre probes were performed in two homogeneously aerated air/water bubble columns, of 15 and of 23 cm dia. These measurements show that considering the flow field as stationary considerably underestimates the velocities present. Although the time averaged liquid velocity profiles resemble textbook data, these averaged values are a result of the passage of coherent structures. LDA measurements showed that these swarms have typical velocities and that at different radial positions, different typical velocities are dominant. The measurements performed with sets of glass fibre probes show that these swarms are typically of the order of the column diameter, indicating that dispersive transport in the axial direction is limited to a distance of approximately the column diameter. Axial dispersion in a bubble column is thus regarded as transport with a typical velocity over a typical distance. A simple model is proposed, defining the axial dispersion coefficient as the product of the typical velocities with the column diameter. Agreement of results obtained with this model with existing literature data is good, especially at lower superficial gas velocity conditions.