Predictions of hydrodynamic performance of an air-lift reactor (ALR) have traditionally considered bulk gas and liquid phase characteristics. This study aims to present an alternative approach of modelling an ALR by estimating the energy input to the reactor and gas hold-up in the riser based on dynamics of individual bubbles. The bubbles have been assumed to rise in discrete layers in the riser having a diameter in equilibrium with the surrounding pressure. The model predictions of gas hold-up and liquid velocity in the downcomer for ALRs have been compared with the reported experimental data.