Prediction of coalescence properties of gas bubbles in a gas-liquid reactor using persistence time measurements

The continuous coalescence and redispersion process of gas bubbles is an important phenomenon that has great significance on bubble size, gas holdup and gas-liquid mass transfer in gas-liquid reactors. The physical laws that determine the bubble coalescence rate are not yet well understood. Therefore, there is a great need for more detailed information about the coalescence process. The coalescence models can be utilized, for example, in multiphase CFD simulations. A combination of CFD and population balances with coalescence and break-up models is a promising topic, which enables a more detailed calculation of local hydrodynamics, gas holdup and gas-liquid mass transfer. The measurement of persistence time, defined as a time that a bubble remains attached at free gas-liquid surface, was used in this work to predict the coalescence properties in a bubble column. The persistence time was evaluated for different liquids and different bubble sizes using highspeed video camera. The applied method for measuring the persistence time is very simple and fast. The measured persistence times range from 10 ms for de-ionized water up to 15 s for water isopropanol solution. In addition, bubble size distributions were photometrically determined in a bubble column at several height positions using different liquids and operational conditions. The measured persistence times were applied in a simplified bubble column simulation model together with population balances and the measured and predicted bubble sizes were compared. A good agreement with calculated and measured size distributions was observed.