FLOW TRACING IN FERMENTOR

In most fermentation especially involving liquid substrate fermentation, water is the main medium of the fermentation reaction. In fermentor, the vessel supports the fermentation broth which is a water based suspension supporting all the different components of the fermentation process such as the substrate, microorganisms and the fermentation products. As the fermentation process in the fermentor represents different phases such as solids, liquids and gases, it is most important in a successful fermentation that a homogenous condition or composition is attained throughout the fermentation broth. There must not be dead zones or physical and chemical gradients formed anywhere in the fermentor.

In reality however, this homogenous condition throughout the fermentor is difficult to achieve except at small scale fermentors where homogenous mixing is easily obtained by efficient stirring by the stirrer. In very large bioreactors, it is quite difficult to achieve homogenous conditions throughout the fermentor. This is one of the biggest problem facing any scale up exercise involving large fermentors.

Efficient mixing of a fermentor are affected by many parameters especially:
1 Size and geometry of fermentor
2 Power
3 Fermentation broth rheology
4 Stirrer design and configuration

Mixing of the fermentation broth is a very complex phenomenon and it changes temporaly and spatially. Sometimes in most fermentation technology books or practicals the phenomenon of mixing is so often oversimplified and explained in terms of primary, secondary and tertiary mixings. It assumes so much on the laminar concept of flows when in reality it is far more complex

Thus it is important to carry out water or flow tracing studies to observe visually how the mixing pattern of the fermentation broth appears at different parametric conditions in order to achieve the optimal mixing conditions and to detect formation of dead zones and formation of gradients so that steps can be taken to solve the problem.



Many courses in fermentation technology often try to demonstrate the phenomenon of mixing by using erroneous models. One poor example is by using plastic rings or balls to show the mixing pattern generated by mixing. It does not truly reflect the true mixing pattern as that occuring in the fermentor. The choice of the models and mixing regimes must be as close as possible to real conditions in order to show what really happened in the mixing of the broth.

There are many types of water tracers or flow tracers which can be exploited to show the true phenomenon of mixing. We have physical, chemical and even microbiological tracers to choose and many kinds of detectors and sensors to follow the mixing pattern evolving in the fermentor.

What is important in the choice of the tracers, it must reflect the true situation and it does not influence the mixing pattern! Every type of tracer used has its limitations so the correct tracer must be properly chosen with the objective in mind

One of the weakest point in trying to show mixing using tracers is that the time for mixing to show occur rapidly and within a short time all established observations will be lost. This can be avoided by using rapid Polaroid photography and even infra red techniques where situation is required