Biotech: Erlenmeyer-flask

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Mixing processes are fundamental unit operations for many sectors within the process-engineering industry. Due to their frequent utilization and also their long duration in e.g. pharmacy, analytics of substances and chemistry, a fluidmechanical optimization of mixing processes offers a considerable potential for enhanced efficiency and cost reductions.

The Erlenmeyer-flask is a widely used containment for e.g. investigations in laboratories as well as production processes in pharmacy. With the Erlenmeyer-flask agitation is achieved either by a circular shaking motion on shaker tables, or by the use of stirrers inserted from the top of the flask. Recently, magnetic stirring has emerged as an innovative method. Agitation is performed by a magnetic stirrer placed at the bottom of the flask in combination with a rotating magnetic field within the surface of the flask platform. This agitation process is characterized by a significantly increased efficiency compared to the shaking motion. In addition, the flask can be sealed with simple means, e.g. plugs. Complicated shaft seals and alignment procedures as needed for conventional stirrers are avoided.

The fluid flow in an Erlenmeyer-flask with magnetic stirrer has been investigated at a filling volume of 2 liters. The stirrer of triangular-prismatic shape rotates at 1000 rpm at the bottom of the vessel. In Figure 1 the resulting cone-shaped free surface of the fluid (colored blue) can be identified in the upper part of the Erlenmeyer-flask. In addition, characteristic vortex structures in the center and at the bottom of the flask are shown. These vortices predominantly effect an efficient mixing of the liquid. In a vertical cross section, shown in Fig. 2, the circulation of liquid due to the stirrer is visible: At the bottom, liquid is transported in radial direction towards the vessel walls, the liquid then rises along these walls, and is finally accelerated on a helical path at the central vortex downwards to the stirrer again.

A careful selection of stirrer geometry, rotational speed and filling volume allows for highly efficient mixing processes. Also additional constrains, e.g. a maximum permissible shear rate, can be taken into account during the optimization.