Influence of the bubble size distribution on the stability of bubble columns

Bubble columns are a fundamental operation unit in chemical engineering; nevertheless, their dimensioning is still based on empirical models. Here, one of the most important parameter is the point of change from the homogenous to the heterogeneous regime. Despite intensive research in the past decades, no deeper understanding of the underlying, local processes was gained. From theoretical deliberations, the lift force was identified as the possibly crucial parameter for the stability in bubble columns in the past (Lucas et al. 2005). The pre-factor of the lift force, the lift force depends on the shear rate, changes its sign when the bubbles reach a certain size (Tomiyama et al. 2002). Consequently, large bubbles tend to velocity peaks in a bubble column, which amplify the heterogeneous character; small bubbles, on the other hand, are driven away from velocity peaks, which homogenizes the flow.
Based on experiments, the turnover from the homogeneous regime to the heterogeneous by solely changing the bubble size distribution (BSD) is shown. For this purpose, the volume flow in a bubble column with evenly distributed needle spargers is kept constant; the BSD is varied by using different needles. By means of the BSD and the liquid velocity at different heights, the theoretically obtained stability criterion (Lucas et al. 2005) is compared to the experiments. The findings presented help to better understand the local process in a bubble column that lead to the turnover to the heterogeneous regime. In the end, with general, local models for these processes, bubble columns of arbitrary shape and other facilities can be specially designed to reach the desired homogenous regime at higher gas hold ups.