CHOOSING THE RIGHT STIRRER

One of the most fundamental requirements in any type of fermentor is the provision of a device or mechanism to mix the contents of the fermentor. Good mixing will result in homogenous conditions of the fermentation broth which will help improve good mass transfer leading to an efficient fermentation process.
It is therefore not surprising to see in fermentors the provision of a stirrer or some sort of mixing device to execute this important function. Good mixing is not only restricted in liquid fermentation but also in solid substrate fermentation such as cocoa fermentation and even composting.
Many of us are so ingrained by the ‘brainwashing’ of fermentation literature that a standard fermentor is and always must be accompanied by a standard mixing device such as the motor, shaft and impeller combination. To this influence we are also brainwashed that Rushton turbine is the ‘best’ impeller system. Due to this also we dare not think ‘outside the box’ and are happily satisfied with the standard Rushton turbine despite the fact that the kind of fermentation we are using is not the same as those reported for the standard stirrer. Part of the blame for this ignorance is to be blamed on the users for failing to understand themselves the kind of fermentation they are carrying out, their limitations and failing to appreciate the rheology of their own broth.
The only exceptions to this poor thinking which I observed are those involved in disposable bag reactors and those dealing with the cultivation of plant and mammalian cells. In plant cell cultivation the mixing is brought about by the circulation of the fine air bubbles. In the disposable bag fermentation they use gentle waves which help in the mixing of the fermentation broth.
In the market there are various types of stirrers, shafts and impellers. You must be able to choose the right combination for your purpose.
In this aspects it is important that we need to know :
1 What sort of fermentation are we carrying out?
2 What is the size and geometry of the fermentor
3 The rheology of the fermentation broth
4 The kind of flow we expect for the fermentation
It is only after we have considered the above we custom fit the stirring configurations.
The type of flow generated by the stirrer is important. It is easy to get laminar flow or even non laminar flow for Newtonian rheology. Things do becomes complex if the fermentation broth which we are dealing are of the Non Newtonian type. In certain cases, the types of Non Newtonian transformations become even more complex.

The choice of the proper stirrer will have to be seen from:
1 The shaft component
2 The blades or the impellers
If we are dealing with viscous fermentation broth it is advisable we use shafts with bigger diameter. The length of the shaft too is critical to the efficiency of the shaft
The choice of the type of blades will determine if the flow produced will be radial or axial. Propeller stirrer shaft will produce axial flows. Which will result the flow moving away from the shaft? The inclination or direction in change of rotation will have impact on the direction of the flow
In cases where the blades are arranged on the disc will have flow attributes which will show strong shearing properties. The flow will be radial and directed outwards. Such flow will generate strong axial suction ia top and bottom axis
Impeller stirrer shaft will provide strong radial flows .
There are the so called anchor design stirrers with the shaft fitting in the centre of a “U” shape structure. This effective in generating tangential flows but poor axial forces or movement of the fermentation broth
There are also hybrid stirrers where both type of propeller blades and impeller blades share the same shaft. Usually the propeller blade is located at the top to induce downward flow to the blades with radial flows
It is important therefore in decoding the choice of stirrers the user must understand the rheology and viscosity of their fermentation broth before rushing like a fool to carry out just any fermentation using the ‘standard stirrer’ The best part of these fermentation studies not only they will not be able to determine the optimal fermentation conditions for their fermentation study but they have faith in “ stupid data’ their studies generated.
I just love the way the fools rushed in where angels fear to tread..