A mathematical model for riser liquid superficial velocity in a concentric-tube airlift reactor is proposed. The model is based on an energy balance incorporating acceleration coefficients to quantify deviations from ideal flow. The acceleration coefficients at the draft-tube and downcomer entrance an determined experimentally. based on static pressure profile measurements. The model could predict liquid velocities over a broad range, including an almost 50-fold variation of liquid circulation velocity and a four-fold change in reactor height. The model predictions agreed with the measurements to within +/- 28%.