MIXING AND RHEOLOGY PART TWO

We can say that in the design of fermentors, the choice of impellers to execute the mixing regime has often been too over simplified. It is strange that despite the complex rheology of various fermentation broths, nost models of impellers are based on two main classes of mixings that is euther radial or axial.
Radial mixings are generated by impellers which are often based on simple flat blade design or variations of the Rushton turbine design. In radial flows all the mixing particles are brought out to the walls of the fermentor subject to tangentianal and centrifugal forces leading to formation of the common close loop pattern too commonly encountered in most standard text books in fermentation technology
On the other hand, the axial mixing are often brought about by the propeller design inpeller leading to the axial flow commonly visualised in ceiling fans
There are variations of mixings which integrate both the radial and axial flows io create a more effective hybrid flows
The important point here is that in the design of these main types of impellers, too much emphasis is given on the following:
1The fermentation broth subjected to mixing is Newtonian by nature
2 The properties of the fermentation broth do not change or remain constant
These kinds of conditions are not commonly found in the industrial fermentation industries. The fermentation broth is often more complex than expected to adhere to the Newtonian characteristics. Second, the volume of fermentation broth or fermentor is huge. This will make it very difficult for radial or even axial mixing to be efficient.
If we learn from blendings carried out in various chemical or even food industries, the type of mixers are often simpler. However in their cases their main requirements are just homogenization and that aseptic requirements are often not carried out during the mixing process itself
It is therefore very important that we need to come up with a very specific design for the mixing of specific fermentation broth and not have the “inbox mentality” of using the same standard impellers


Type rest of the post here.