Time-dependent hydrodynamics of bubble columns

Bubble column reactors, where gas is injected at the bottom of an initially stagnant liquid, are widely used in chemical engineering. They often operate in the heterogeneous regime combining thus high gas concentrations (15 to 40%) and strong velocity fluctuations (25-30% times the mean). Furthermore, processes are usually dynamic as the injected gas flowrate may be unsteady, they can suffer sudden change of operating parameters, etc… Owing to such flow complexity, no reliable scaling rules are available for reactor designers, and scale-up from the lab units to industrial units still relies on empirical formulæ.

We present experimental results for bubble columns with different diameters in both the homogenous and heterogeneous regime. We cover two different time-dependent situations. First, the startup of the bubble column is considered. This unsteady regime consists on injecting a fixed gas flowrate starting from zero. The time that bubbles require to reach the free surface and the duration of the unsteady regime are then analyzed. Later, we focus on the fluctuations of several quantities (void fraction, bubble velocity…) within stable operating conditions. Our results show that even within steady gas injection conditions, all studied parameters present significant fluctuations.