WEISENBERG EFFECT- PROBLEMS OF STIRRING BIOPOLYMERS IN FERMENTORS

It is the purpose in any engineering design of a fermenter to achieve uniform and homogenous mixing. This objective is to ensire that the content of the fermenter is homogenous and good mass transfers occur between the microorganisms and the surrounding environment.
Frequently though this objective is always kept in mind, the weakest link in the process is the failure to understand the rheological properties of each fermentation broth and the provision of unsuitable stirrers or mixers to achieve this end.
Complications arise due to the fact that most if not all fermentation broth are non newtonian and show very complex properties spatially or even temporally during the fermentation run. Just providing any standard stirrer is not the proper solution to this problem. Each fermentation process have its own unique problems and conditions and almost require its own specific stirrer and mixing regime.
One good example often faced in fermentation industries is the problems of mixing viscous broth. This is often encountered in food fermentation such as yogurt fermentation or fermentation of biopolymers or those involving use of sticky sugar substrates.
In theory we expect any mixing of fermentation broth would result in homogenous conditions, improved mass transfers. In viscous broth we expect the effect of stirring would be to stretch and thin out the broth to improve mass transfers
The problem is during the fermentation mixing or stirring there is the tendency for the broth to creep up the stirrer shaft, instead of being dispersed throughout the fermenter. This effect is called the Weisenberg Effect
The Weisenberg effect refers to a common phenomenon when a spinning rod is placed in a solution containg liquid polymers. This will result in the entanglement of liquid polymers to the spinning rod or shaft leaving the free ends of the polymers in the solution. Tensional forces acting on both ends of the biopolymers will try to reduce the distance between the two ends resulting in the polymers to move along the shaft.
Given time a mass of biopolymers will be cumulated at the end of the spinning shaft of the stirrer.
IMPACT OF WEISSENBERG
1 Extra load at tip of moving shaft will generate extra torque and if the shaft of stirrer is long will throw out the shaft out of its normal oscilation
1 Inefficient mixing as the impellers will be covered by the cumulated biopolymers
3 Entrapment of biomass that will not contribute to fermentation product
4 more downstream problems and loss of time in cleaning the stirrer system

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