Application of a Magnetic Tracer Method for the Characterization of Hydrodynamics in Internal-Loop Airlift Bioreactors

Nowadays there is still a lack of measuring techniques, which would give reliable information about the hydrodynamics in internal-loop airlift reactors (ALR) not only with model media but also during real fermentation processes. Hydrodynamic parameters (liquid residence time distribution, linear circulation velocity, intensity of turbulence) are of particular importance for the verification of the validity of hydrodynamic models or the scale-up procedure. Thus, a magnetic tracer method was developed allowing the measurement of the liquid circulation velocity in individual sections of internal-loop airlift bioreactors during fermentation processes. We attained a signal with a very low noise to signal ratio, which gave reproducible information on the residence time of a magnetic particle in the appropriate section of the ALR. Moreover, the linear liquid circulation velocity, VL, could be calculated if the settling velocity of the tracer particle was known. The results attained were compared with the pulse response method using hot water. Differences of VL values between both measuring methods were within ±20 %. A proper formulation of the effective buoyancy in a gas-liquid dispersion is discussed in this paper. Our results demonstrate that the effective buoyancy is based on the liquid density, so the Archimedes buoyancy force is the proper expression for the formulation of the effective buoyancy. It seems that probably a critical diameter of the classifying particle with respect to the diameter of surrounding particles or bubbles exists, which determines the formulation of the effective buoyancy.