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Abstract: Aerated stirred tank reactors are widely used in chemical industry and bioprocess engineering. One major parameter to characterize the heat- and mass transfer performance of such aerated stirred tank reactors is the mixing time necessary to homogenize the reactor volume. Despite its importance, the prediction of the mixing time is still challenging due to the complex hydrodynamic inhomogeneities induced by the gaseous phase, which becomes strongly apparent in large scale systems. A precise measurement of the two phase flow on the other hand requires a volumetric insight and is thus difficult to realize. To overcome this problem a transparent stirred tank reactor on industrial scale has been erected at the Hamburg University of Technology in cooperation with Boehringer Ingelheim Pharma GmbH & Co.KG. With the decolouration method the temporal and spatial development of mixing can be taken into account. The results indicate the importance of the local inhomogeneities on large scale. A first characterization can be done by taking into account buoyancy driven flows superimposing the flow imposed by the impeller. A correlation is presented to estimate the transition between a loading and flooding regime on large scale. This correlation enables the calculation of mixing times for a wide range of stirrer frequencies and superficial gas velocities. Furthermore, this publication emphasizes the challenges of scale-up on the basis of laboratory experiments in small scale.