| Many biochemical processes developed by pharmaceutical and biotechnological firms rely on the culture of shear-sensitive mammalian, insect, or plant cells. Because of the specific nature of the fluid to mix, these processes are often achieved in the laminar regime which induces new problems for these industries like flow compartmentalization and the presence of segregated zones. Coaxial mixers seem good candidates to handle these phenomena. In the process industry, coaxial mixers are more and more used in a wide range of applications, such as dispersion, emulsification or viscous blending. However, the knowledge of these devices is still poor and their design and operation remain often empirical in industry. They are based on a combination of two impellers mounted on two independently actuated and centered coaxial shafts: a high speed open turbine promotes efficient pumping, and a low speed close-clearance impeller cleans out the vessel wall. On our experimental bench, a Rushton turbine was used in combination with an anchor in both co- and counter-rotating modes to mix viscous Newtonian and non-Newtonian fluids. The hydrodynamics performances of this coaxial mixing system were studied experimentally by flow visualization. Image analysis of a discoloration reaction was used to quantify the mixing evolution in the tank. The mixing curves generated by this method were used to reveal the presence of flow compartmentalization and dead zones. It was shown that the co-rotating mode is better to avoid the flow compartmentalization and the counter-rotating mode appears to be more efficient to break the segregated regions. Finally, new ways of operating the agitators, like switching on the anchor after the turbine or changing the direction of rotation, were successfully tested. |
