CFD analyses of the mixing characteristics in bubble columns and airlift reactors

The mixing characteristics in bubble columns and airlift reactors are analyzed using computational fluid dynamics. In the simulations, an Eulerian-Eulerian approach was used to model air as the dispersed phase within a continuous phase of water using the commercial software FLUENT. The Schiller-Naumann drag model was employed along with virtual mass and the standard κ - ∈ turbulence model. An effective bubble diameter was specified for each case studied and depended on the inlet gas velocity specified. The predicted flow field in the airlift geometry showed a regular oscillation of the gas flow due to flow recirculating from the downcomer and connectors, whereas the bubble column oscillations were random and resulted in flow moving through the center of the column. The profiles of gas holdup, gas velocity, and liquid velocity versus column width showed that the airlift reactor flow is asymmetric and the profile shape varied along the height of the column. The bubble column flow became independent of height after 20 cm above the inlet because there was less mixing than the airlift reactor. It was shown that the airlift reactor increased the mixing of the gas-liquid flow due to the addition of the downcomer. The airlift reactor showed less gas holdup in the riser than the bubble column but its velocity and gas holdup never became independent of column diameter like in the bubble column. The gas and liquid flow field showed increased mixing with increasing inlet velocity.