Uniform Flow in Bubble Columns

• Published in: Industrial & engineering chemistry research 2009-01, Vol.48 (1), p.148-158
• Main Authors: Mudde, R. F, Harteveld, W. K, van den Akker, H. E. A
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical engineering; Exact sciences and technology; General Research
• ISSN: 0888-5885; 1520-5045
• DOI: 10.1021/ie8000748

Uniform bubbly flow in a 15 cm bubble column is investigated. We use a special needle sparger consisting of 559 separate needles, uniformly distributed over the bottom. With this sparger, we can ensure that all bubbles generated are of the same size and that the bubble injection is very uniform over the entire bottom of the column. Detailed experiments are reported, using optical glass fibers to measure the local gas fraction and bubble size and velocity and using laser Doppler anemometry to measure the liquid axial velocity field. We find that the homogeneous flow regime extends up to a gas fraction of 55% well beyond the predictions of theory. The superficial gas velocity at which the homogeneous regime looses its stability depends on the water quality: fresh water looses its stability much earlier than old water. However, the gas fraction as a function of the superficial gas velocity is in the homogeneous regime independent of the water quality. The overall gas fraction can be described by a Richardson and Zaki type of relation or by the proposal by Garnier et al. We have indications that, at the point of instability, the bubble size has increased to a critical value at which the lift force reverses sign. This causes the radial gas fraction to change from flat with a small wall peaking to core peaking provoking the instability as suggested by Lucas et al. [ Chem. Eng. Technol. 2005, 60, 3609 ]. Alternatively, at higher gas fractions, the swarm gets denser and the bubble wakes get suppressed. According to Fox and co-workers [ Chem. Eng. Sci. 2007, 62, 3159 ], this causes the flow to lose its stability.