Prediction of bubble size and interfacial area in bubble column CFD simulations through solution of population balance equations
Multi-scale and/or multi-disciplinary approach to process-product innovation
CFD & Multiscale Modelling in Chemical Engineering (T3-4P)
Keywords: bubble column, CFD, population balances, bubble coalescence, antifoaming agent
In bubble column reactors the bubble size and the interfacial area available for gas-liquid mass transfer is determined by continuous coalescence and breakage of bubbles. The changes in bubble size, or more generally size distribution, can be taken into account by the solution of population balances. In the solution of population balance equations it is necessary to ensure that at least two moments of the size distributions are conserved. In practice, the moments are chosen to conserve the total number and volume (or mass) of the distribution. This ensures that the numerical solution does not cause any error in the total number or mass of the bubbles. Some special techniques or correction factors has to be implemented to ensure the moment conservation.
In this work the solution of population balances together with moment conservation was implemented in ANSYS CFX 10 as an external user defined routine. In this routine some phenomenological models for bubble coalescence and breakage are also implemented. Bubble coalescence model include some solution dependent experimental parameters, which has to be estimated based on experimental data. The package contains a method for discretization of population balances of multi phase system (G/L/S) based on the Multi Phase Multi Size Group model by Sha et. al (2006). The obtained CFD model was used to calculate flow hydrodynamics and bubble size distributions in a cylindrical column with a diameter of 0.19 m and height of 1.7 m. To minimize the computational load a 2D model was used in the calculation.
Experimental data was collected from similar column using industrial wastewater with different concentrations of antifoaming agent. The experimental data included bubble size distribution, average gas holdup and flow hydrodynamics for liquid and gas phase. Bubble size distribution was measured using photometrical method. Flow fields of gas and liquid were measured by using Particle Image Velocimetry (PIV). The experimental results were compared to the results from CFD calculations to verify the developed CFD model.
1. Sha, Z., Laari, A., Turunen, I., Multi-Phase-Multi-Size-Group Model for the Solution of Population Balances in CFD Simulation of Gas-Liquid Bubbly Flows, Chemical Engineering & Technology, 29 (2006), 550-559.
Presented Tuesday 18, 13:30 to 15:00, in session CFD & Mutliscale Modelling in Chemical Engineering (T3-4P).
