IMPORTANCE OF FERMENTOR DESIGN IN MINIMIZING ENERGY CONSUMPTION AND INCREASING YIELD

The main function of the fermentor tank is to hold the volume fermentation broth. In fact any vessels or container can be used to carry out the fermentation process. Fermentors differed from any container by allowing optimal fermentation process to occur through design and control

It is almost unbelievable that the concept of CSTR still dominates the day, years after its inception during penicillin fermentation. Only lately have attempts been made to come up with fermentors with novel or new designs.

The change towards new design fermentors is more to:
1 New fermentation products
2 Shift from high volume low value products to low volume and high value products

Any changes in the fermentor design must be with the purpose of improving the efficiency of the fermentation process in terms of yield and productivity or increasing its mass transfers and lowering energy consumption

In the old days it seems that almost any product fermentation is just forced into the CSTR. This was the mental block that hover over most engineers and fermentation technologists that one fermentor can be used for all types of fermentation!. It just doesn’t make sense!

Nowadays the design of the fermentor is more to accommodate the demands of the type of fermentation process. It seems in the past the engineers building or designing the fermentors are not able to think outside the box. The rigidity of their thinking is as rigid as the fermentor design

The fermentation process is considered complex with physical chemical and microbiological continually changing. With this in mind the design of fermentors must always be responsive. The fermentor design must not only be looked upon as fixed unit processes undergo by the fermentor such as standard stirrer specifications, aeration and even the sterilizing specifications. It is doubtful one standard design fermentor can bring out the best for a variety of fermentation carried out in the lifetime of the fermentor.

So, it is back to the drawing board in the design of the fermentors and a compulsory re education in fermentation technology for those involved in the design of fermentors. Reengineering and novel engineering is a must for the future of fermentors and fermentation.

The trouble with the design of fermentors in the past is that there is too much emphasis on engineering and math with poor understanding of the fermentation process. Sheets and sheets of the blueprint is the testimony to this

It is recommended that:

1 Understand the fermentation process first
2 Determine the rate limiting steps of the fermentation process
3 Understand the characteristics of the fermentation broth
4 Carry out feasibility lab studies before the design is accepted
5 Determine the flow and mixing pattern of the fermentor before determining the final optimal geometry
6 And other relevant studies

The key performance index in all the designs of the fermentors is cost. One of the major liabilities is energy. A fermentor is not a dead or innate structure metaphorically speaking. It consumes and even generates energy depending on the situation. Energy costs may not be a problem if you are dealing with small fermentors or working in government fermentation laboratories. But in industries, the energy costs could be very prohibitive and will add cost to the final fermentation products

If we analyse the energy consumption in a typical fermentor, most of the energy requirements are for:

1 Aeration
2 Stirring
3 Heating and cooling activities

Aeration is necessary in any aerobic fermentation process. The amount of aeration should reflect the oxygen demand of the fermentation process at that temporal and spatial status. What is important is that the aeration process should always be able to provide enough oxygen in the narrow window during the fermentation process. Providing more not only add up to the energy costs but might even complicate the fermentation process. Insufficient aeration might result in certain zones of the fermentor being deprived of oxygen and affect the fermentation process

In deciding the aeration requirements we have to consider many factors such as:
1 Type of aerators or spargers used
2 Location of spargers within the fermentors
3 Demand of oxygen as function of time of fermentation progress
4 Presence of mixers or stirrers in fermentors
5 Feed back loops to stirrer and aerator volumetric rate

Easier said than done. There is really no such thing of using the optimal aeration conditions of one fermentation process to be used on another. No two fermentations are the same. The optimal aeration conditions for fermentation have to be individually determined. This itself is the problem of having customized designed fermentor as it might only be suitable for one type of fermentation and not another

It is a known fact that experience in fermentation beats all the formal knowledge or information from books.

The choice of aerator is important and will decide whether the aeration costs will be higher or lower. It has been shown that aeration by air compression is only about 40% efficient in terms of electrical energy while agitation by turbine is about 90% efficient. Surprisingly in my experience as consultant on a trouble shooting job I have seen many clients are convinced by very qualified engineers who proposed air compression. Poor clients!

Use of sensors n feed back lops important. We need a reactive fermentor to detect changes and operate within narrow window of process optimization

Whole objective of stirring to homogenize or keep broth in suspension. How powerful we stir are often determined by the nature of the rheology of the fermentation broth. Mixing and aeration should be interplayed where a compromise is made between keeping the components of the broth in suspension yet still keeping the minimal amount of dissolved oxygen concentration enough to support the needs of the microorganisms. Excessive stirring may not only be demanding on the energy but also damaging to the microorganisms


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