Aerated stirred tank reactors have virtually become a standard piece of equipment in bioprocess engineering. No other bioprocess engineering bioreactor is as versatile, meaning that significant time- and cost savings are made. Sophisticated methods of numerical flow simulation such as those used at the Institute for Biochemical Engineering (IBVT) at the University of Stuttgart make these reactors even more effective.
These simulations are increasingly complementing the classical experimental examinations and interpretations relating to the optimal adaptation of the stirred reactors to the actual operation situation. CFD enables the realistic modelling of the different reactor constituents independently of scale. CFD is particularly suited for assessing the effect of different mixing elements on the mixing in the reactor. Apart from the design and size, it is also possible to simulate different angles of inclinations and assemblies. This led to the discovery that certain stirrer types are insufficient for axial mixing, but on the other hand enable the excellent supply of oxygen to the reaction medium – and with other stirrer types it is exactly the opposite. That is why nowadays the practice is to combine several different stirrer types. “The simulations provide us with details about which stirrers to use, where to position them and how heavily the broth has to be stirred,” said Reuss summarising the advantages of the new method that is already used in industry. But the method also has its limits. Two- and multiphase systems such as for example gas-fluid dispersion cannot be effectively simulated with CFD. “In addition, many phenomena are not yet physically understood and can therefore not be described mathematically – for example the occurrence of turbulence,” said Reuss.This will not hamper the success of numerical flow simulations in the field of bioprocess engineering because the method already enables new and fascinating insights into the inner life of stirred tank reactors, something that would otherwise not be possible.