A mathematical model for predicting the hydrodynamic behavior of a multi-stage external loop airlift reactor operating in three stages has been developed. The present model is based on macroscopic balances of the gas-liquid separator, external downcomer and spatially averaged, 1 D mass and momentum balances in the riser. Using only the physical properties of the gas and liquid phases, the reactor dimensions and the superficial gas velocity, the model predicts gas holdup profiles, gas and liquid velocity profiles, and pressure profiles in the riser for a two-phase bubbly flow. It has been observed that the values of gas holdup obtained are in the range of 44-52%. Empirical correlations are used to represent frictional an drag effects, but there are no adjustable parameters in the model. Results indicate that a pressure of 9.3 x 10(-4) N/m(2) were achieved at a dimensionless riser position of 1.0. The results predicted by the model are in excellent agreement with the experimental data reported in literature for a multi-stage external loop airlift reactor. (C) 2008 Elsevier B.V. All rights reserved.