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 re liable information about the hydrodynamics in internal-loop airlift reactor s (ALR) not only with model media but also during real fermentation process es. Hydrodynamic parameters (liquid residence time distribution, linear cir culation velocity, intensity of turbulence) are of particular importance fo r the verification of the validity of hydrodynamic models or the scale-up p rocedure. Thus, a magnetic tracer method was developed allowing the measure ment of the liquid circulation velocity in individual sections of internal- loop airlift bioreactors during fermentation processes. We attained a signa l with a very low noise to signal ratio, which gave reproducible informatio n on the residence time of a magnetic particle in the appropriate section o f the ALR. Moreover, the linear liquid circulation velocity, V-L, could be calculated if the settling velocity of the tracer particle was known. The r esults attained were compared with the pulse response method using hot wate r. Differences of V-L values between both measuring methods were within +/- 20%. A proper formulation of the effective buoyancy in a gas-liquid disper sion 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 re spect to the diameter of surrounding particles or bubbles exists, which det ermines the formulation of the effective buoyancy.