A fermenter is a vessel in which high scale fermentation is carried out. There are 3 main steps involved in the fermentation process. They are upstream processing, the fermentation process itself and downstream processing.
Fermenters have several parts. Each part has its own use and importance. The most important part though is the agitator which brings about mixing of the contents contained in the fermenter vessel.
Based on the type of mixer present in the fermenter vessel we have mechanically agitated fermenters and airlift fermenters. When we compared the 2 it was found that each type of fermenter has its own advantages and disadvantages and a slight change in the desing of the fermenter can lead to better mixing and decreased mortality rates of the microorganisms that are used in the fermentation process.
A fermenter is a vessel in which large scale fermentation is carried out. It provides the right atmosphere with all the appropriate conditions for the entire fermentation process to take place. The fermentation process is made up of 3 steps. They are as follows:
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Upstream processing: this involves sterilisation of the fermenter vessel, strain selection, isolation, preservation, media preparation, innoculum preparation, introduction of the media into the fermenter (which occupies about 70-80% of the volume of the fermenter vessel) and addition of the innoculum to the fermenter.
Fermentation itself: where the selected strain utilises the substrate and nutrients to produce the crude form of the desired product
Downstream processing: this includes steps such as separation of the biomass from the supernatant fluid by centrifugation, filtration (by vaccum), cell disruption, protein purification, lyophilisation and product recovery.
There are several parts in a fermenter. These in general include: the fermenter vessel itself (which can either be made from pyrex glass or stainless steel depending on its capacity), the water inlet and outlet, the outer cooling jacket(which is filled with either tap water of refrigerated water), internal cooling coils( these are needed for large fermenters in order to ensure efficient heat transfer), a motor (to drive the impeller), speed controller, mechanical seal, a stirring shaft, baffle plates (to produce axial liquid flow and prevent vortexing), sparger, sparger ring, air filter, additional inlet, air exhaust outlet, a foam breaker, a pH controller, steam inlet and a harvest outlet. They are shown in the diagram below:
These are used for the mass production of commercially important substances such as antibiotics, monoclonal antibodies, etc.. There are several considerations which go into the choice of fermenter that is to be used. For example, size of the reactor required for optimal production, processing conditions prevalent inside the fermenter, mode of operation(i.e. batch, fed-batch or continuous), mass energy balance, heat transfer, mass transfer, mixing, aeration, cost effectiveness etc.. The type of fermemter used depends upon the product that is being produced.
Most importance is given to the appropriate provision of adequate mixing and aeration in the design of a fermenter since most fermentation processes are oxygen-requiring i.e. aerobic.
What is mixing ?
This is a physical operation that is carried out in all fermenters. It reduces the non-uniformity in a fluid. This is done by the elimination of gradients of concentration, temperature and other such properties. This process is achieved by interchanging substances between different parts of the fermenter. This brings about uniform mingling of the components that are present in the various parts of the fermenter. If the system is homogenous then the various physical as well as chemical properties of that system will be more or less homogenous.
The following steps are involved in mixing:
Mingling of soluble components e.g. sugars
Dispersion of air in the form of gas bubbles throughout the liquid
Maintenance of suspension of solid particles such as cells
Dispersion of immiscible liquids in the form of a suspension of fine droplets of as emulsions
Promotion of heat transfer.
Mixing is of prime importance in a bioprocess operation. In order to produce an optimal environment for fermentation to occur, the cells which are present in the fermenter must have access to all the necessary substrates and nutrients in order to produce the desired end product. Such an environment can only be produced when the contents that have been filled into the fermenter are uniformly mixed, if the contents of the fermenter are not mixed properly then zones of nutrient depletion will occur in parts of the fermenter. It is also important to maintain a uniform biomass suspension since substrate concentration can reach zero in parts where the cells stay out of suspension.
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Another important aspect of mixing is that it produces heat transfer. This is important since it is necessary to maintain a particular temperature during fermentation for manufacture of the desired product. Cooling coils containing water are present around the fermenter in order to take up any excess heat that is given of during fermentation. The rate at which heat gets transferred from the medium in the fermenter to the cooling water is directly dependant on the mixing condition prevalent in the fermenter vessel. And, the effectiveness of the mixing in turn depends upon the rheological properties of the culture fluid.
Mixing can be attained in several ways. Depending upon the type of mixing equipment present in the fermenter they are divided into 2 types. They are mechanically agitated and airlift fermenters.
Mechanically agitated fermenters :
A good example of this type of fermenter is the stirred tank fermenter. They can be of 2 types depending on where they are being used. When used in the laboratory they are small and are made of pyrex glass. When used on an industrial scale they are large and made of stainless steel. These are the most commonly used variety of fermenters for industrial purposes. They provide low capital and operating costs. Their height to diameter ratio can be varied depending upon the heat removal specifications.
The basic operating principle of the stirred tank bioreactor is fairly simple to understand. In this fermenter the medium along with the innoculum are introduced into the vessel from the top. The air supply required for the fermentation process to take place is provided from the bottom of the fermenter. The air bubbles that are produced by the air supply are disrupted with the help of the agitator that is present in the fermenter. The agitators also produce turbulence. The turbulence so produced aids in the oxygen control and also appropriate mass transfer. There are several types of agitators that can be used in the stirred tank fermenter. A few examples are four blade disc turbine, pitched blade disc turbine, anchor, helical ribbon type agitator, curved blade disc turbine, pitched blade paddle and so on.
Pitched blade disc turbine curved blade disc turbine helical ribbon
Anchor Four blade high efficiency impeller
Pitched blade paddle impeller
Among these the most commonly used agitator in the stirred tank bioreactor is the four blade disc turbine, although now there are several new designs with 12-18 blades and also concave bladed agitators. These have the ability to improve hydrodynamics.
This fermenter apart from having agitators is also provided with baffles that are generally attached to the walls of the fermenter. The presence of these baffles aids in the prevention of whirlpool formation which could cause a hindrance in the mixing process. The number of baffles present ranges from four to eight in number.
The waste gases that are produced during the fermentation process are removed from the top the the fermenter and the product that is formed flows in the downward direction and is collect through a drain tube that is present at the bottom of the fermenter.
These fermenters are also known as tower reactors. An airlift fermenter can be described as being a bubble column with a draught tube. There are several types of airlift fermenters currently in use.
In a typical airlift fermenter the air is fed in through a sparger ring which is located at the bottom of the central draught tube. This central draught tube controls the medium and also the circulation of air. This air then flows in the upward direction and the waste gases that are produced are removed from the top of the fermenter. The liquid that is present in the fermenter contains the desired product. It flows downwards and is drained at the bottom of the vessel through a tube.
The heat exchanger for this type of a fermenter can either be present internally of can be present as an external loop. The airlift external loop fermenters are generally used for batch operations. These types of reactors use induced in order to direct the air and liquid throughout the vessel. This type of a fermenter consists of a riser and a external down-comer which are attached to the main vessel at the top and bottom respectively. the waste gases that are produced during the course of the fermentation process are removed from the top of the vessel and the liquid descends to the bottom through the down-comer and is collected from the bottom of the vessel.
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Figure 5 shows a cut-away view, that is, the vessel
and downcomer are actually taller than shown for the
particular diameter drawn.
Comparison between relative merits of airlift and mechanically agitated fermenters :
Airlift fermenters have several advantages over the more commercially used stirrted tank fermenters which make use of mechanical agitators. These fermenters have a simple designs with no moving parts or agitators. They have a lower maintenance cost as compared to mechanically agitated fermenters. They also have a lesser risk of defects in design and are much simpler to sterilize. The shear rate that is produced by airlift is much lesser when compared to that which is produced by mechanical agitators. These fermenters(airlift) can be used for both plant and animal cells, hence are more flexible in their use in terms of the cells being used. The efficiency of airlift in gas-phase disengagement is higher than in the mechanically agitated fermenters. These fermenters also have a well controlled flow and have good mixing efficiency rates. They also provide well defined residence times for all phases. There is an increased mass-transfer achieved in airlift fermenters due to the enhanced oxygen solubility that is achieved in these large tanks owing to the greater pressure in the vessel.
When compared to mechanically agitated fermenters, very large volume tanks of airlift fermenters can be maintained with ease and it has also been shown that these large tanks have a greater product output. The efficiency of heat transfer is also greater in airlift fermenters.
There are also certain disadvantages of airlift fermenters. The main disadvantage of these fermenters is that a high initial capital investment has to be made due to the large scale of processes. Also, these fermenters need greater air throughput and much higher operating pressures when compared to the conventional mechanically agitated fomenters. Another point to be noted that can work as a disadvantage is the fact that airlift fermenters require low friction with an optimal hydraulic diameter for the riser and downcomer to function appropriately but, there are no such issues involved with the mechanically agitated fermenters. These fermenters also have a lower efficiency of gas compression when compared to the mechanically agitated fermenters. It is also difficult to maintain consistent levels of substrate, nutrient and oxygen in such fermenters since the organisms are conctantly circulating through the fermenter and there are constantly changing conditions within the fermenter vessel. When foaming occurs these fermenters are inefficient in maintaining gas/liquid separation where as mechanically agitated fermenters have a good ability to tackle foam formation and can thus maintain gas/liquid separation. The above mentioned disadvantages can however be overcome be a few changes in design such as increase in the number of feed points. This would lead to the elimination of the risk of the organism undergoing continuous cycles of high growth followed by starvation which leads to the production of undesired by-products, low yield of the desired product and high mortality rates of the organism being used in the fermentation process. Apart from that modification in design, multiple entry points must also be provided for oxygen. Most of this oxygen must be supplied from the bottom of the vessel since it would aid in the circulation of the fluid throughout the fermenter vessel. This modification also reduces mortality rates of the organism being used in the fermentation process.
Mechanically agitated fermenters also have their own advantages. They have low initial capital investment and also low operating costs. The use of mechanical agitators reduces the risk of contamination of the contents of the fermenter vessel and also prevent cell mutation from occurring. This is mainly due to the fact that the cells have a brief period of growth in these fermenters. Mechanically agitated fermenters also provide high conversion rates of the growth material present in the vessel. Flexibility of choice in mechanical agitatior provides options of working with various biological systems. The use of a mechanical agitator also allows the manipulation of cell growth rate, this is to say that they aid in increasing cell growth rate. But, the drawback of this manipulation is that it also increases maintenance costs by a certain amount.
In such systems varying levels of dilution can be used. This aids in the production of secondary metabolites. As opposed to the airlift fermenters, consisten levels of oxygen, nutrient and substrate can be maintained in mechanically agitated fermenters. Also, levels of major constituents such as nutrients can be maintained in these fermenters. Further, these fermenters already have multiple feed points for nutrients as well as air flow included in their design as compared to the airlift fermenters which have only one feed point for nutrient and only one feed point for the air in the commonly used designs of airlift fermentrs. They thus have an advantage over airlift fermenters in terms of mortality rates of the organism being used in the fermentation process.
We see above that both airlift fermenters and mechanically agitated fermenters have their own advantages and disadvantages. A few modification in the design of both can make them much more efficient in their working. We notice that very few changes have to be made in the design of either type of fermenter and we can achieve greater efficiency in their mixing abilities. This increased efficiency in mixing would lead to a more efficient fermentation process and thus better quality of the product being manufactured.