The role of the gas±liquid separator on hydrodynamic characteristics in an internal-loop airlift reactor (ALR) was investigated. Both gas holdup and liquid velocity were measured in a 30 dm 3 airlift reactor with two different head con®gurations: with and without an enlarged separator. A magnetic tracer method using a neutrally buoyant magnetic particle as¯owfollower was used to measure the liquid velocity in all sections of the internal-loop airlift reactor. Average liquid circulation velocities in the main parts of the ALR were compared for both reactor con®gurations. At low air¯ow rates the separator had no in¯uence on gas holdup, circulation velocity and intensity of turbulence in the downcomer and separator. At higher super®cial air velocities, however, the separator design had a decisive effect on the hydrodynamic parameters in the downcomer and the separator. On the other hand, the gas holdup in the riser was only slightly in¯uenced by the separator con®guration in the whole range of air¯ow. Circulation¯ow regimes, characterising the behaviour of bubbles in the downcomer, were identi®ed and the effect of the separator on these regimes was assessed. # 2001 Society of Chemical Industry Keywords: airlift reactor; separator; circulation regimes; circulation velocity;¯owfollower

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A low-cost and simple magnetic particle tracer method was adapted to characterize the hydrodynamic behavior of an internal-and an external-loop airlift reactor (ALR). The residence time distribution of three magnetic particles differing in diameter (5.5, 11.0 and 21.2 mm) and with a density very close to that of water was measured in individual reactor sections. The measured data were analyzed and used to determine the velocity of the liquid phase. Validation of the experimental results for liquid velocity was done by means of the data obtained by an independent reference method. Furthermore, analysis of the differences found in the settling velocity of the particle in single-liquid and gas-liquid phases was carried out, using a simplified 3D momentum transfer model. The model considering particle-bubble interaction forces resulting from changes in the liquid velocity field due to bubble motion was able to predict satisfactorily the increase in the particle settling velocity in the homogeneous bubbly regime. The effective drag coefficient in two-phase flow was found to be directly dependent on particle Reynolds number to the power of −2 but independent of gas flow-rate for all particle diameters studied. Based on the experimental and theoretical investigations, the valid exact formulation of the effective buoyancy force necessary for the calculation of the correct particle settling velocity in two-phase flow was done. In addition, recommendations concerning the use of flow-following particles in internal-loop ALRs for liquid velocity measurements are presented.  2006 Society of Chemical IndustryKeywords: multiphase flow; airlift reactors; hydrodynamics; momentum transfer; solid mechanics, flow followerRe p Reynolds number of particle