In this paper bubble size distributions and flow fields in bubble columns are calculated numerically. The population balance is simplified and reduced to a balance equation for the average bubble volume. Models are developed in order to predict the rate of bubble break-up and coalescence based on physical principals. The flow fields are numerically calculated for bubble columns with cylindrical cross section using the Euler-Euler method. The newly derived balance equations for the average bubble volumes are implemented into a commercial CFD-code. The solutions of the balance equation result for high superficial gas velocities mainly in two fractions, one for the fraction with small and one for the fraction with large bubble diameters. Both are considered as pseudo continuous phases in addition to the liquid phase. The calculated flow fields are characterized by several large scale vortices. The local volume fractions of gas and liquid are locally inhomogeneous and highly time dependent. The time averaged flow field is axisymmetric and stationary. The calculated volume fractions, velocities and bubble size distributions agree well with experimental results for bubble columns up to 0.3m in diameter.

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