A mathematical model of an airlift reactor (ALR) is presented, which is based on the careful application of the first law of thermodynamics to each of the regions of the reactor. The model requires as input data the pressures and the holdup in each region, and allows the calculation of the energy dissipation in them. Using formulations of the drift-flux model, the range of possible gas recirculation can be obtained from energetic considerations. A "global shear rate" can be calculated for each of the regions of an ALR. An example is presented, which compares the energy dissipation as a function of the gas flow rate in two ALRs which differ only in the bottom clearance.