A mathematical model predicting the hydrodynamic behaviour of three-phase airlift reactors, working with low-density solids and with high solids loading, was developed. The model allows for the prediction of local gas holdup and liquid velocity in airlift bioreactors. Model was validated for an external-loop airlift reactor and an internal-loop airlift reactor with an enlarged degassing zone, being a good agreement obtained between calculated and experimental data.