An 1D hydrodynamic model has been developed for gas hold-up and liquid circulation velocity prediction in air-lift reactors. The model is based on momentum balance equations and has been adjusted to experimental data collected on a pilot plant reactor equipped with two types of gas distributors and using water and water/butanol as the liquid phase. Agreement between the hydrodynamic model and pilot experimental paints is shown to be fairly good. Different techniques of signal analysis have also been applied to pressure fluctuations in order to extract information about flow regimes and regime transitions. A good knowledge of the flow pattern is essential to establish adequate hydrodynamic correlations. This model has also been combined with mass transfer and the kinetics of a chemical reaction to yield a complete model of the performance of a reactor.