Different Types of Fermentors / Bioreactors

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The heart of the fermentation or bioprocess technology is the Fermentor or Bioreactor. A bioreactor is basically a device in which the organisms are cultivated to form the desired products. it is a containment system designed to give right environment for optimal growth and metabolic activity of the organism.
A fermentor usually refers to the containment system for the cultivation of prokaryotic cells, while a bioreactor grows the eukaryotic cells (mammalian, insect cells, etc).

                                               
Types of Bioreactor

1. Continuous Stirred Tank Bioreactor
2. Airlift  Bioreactor
3. Fluidized Bed  Bioreactor
4. Packed Bed  Bioreactor
5. Photobioreactor
6. Membrane Bioreactor

Continous Stirred Tank Bioreactor 

In Continous Stirred Tank Bioreactor, the contents of the vessel no longer vary with time, this applies to the hold up of micro-organisms and the concentration of the components of the medium in the fermentor.Steady state conditions can be achieved by either Chemostatic or Turbidostatic principles.The former involves the adjustment of the flow rate of the fermentor to an appropriate and constant value and allowing the micro-organisms, substrates and biochemical product concentration to attain their natural levels. The turbidostat requires an experimental determination of the turbidity (ie, indirect measurement of microbial concentration). This thus used to control the flow rate. Both these methods have been employed in practice, though the former is obviously the simpler from every view point.

The most successful continuous systems to date have been those employing yeasts and bacteria, in which the desired products are the cells or primary metabolites, compounds that form the chemical 'inventory' of a microbe, (e.g. enzymes and amino acids), or some product clearly associated with growth or energy producing mechanisms (e.g. the production of alcohol).

The most widely used continous process based on CSTF (Continous Stirred Tank Fermentor) is the activated sludge process used in waste water treatment industry.

In continuous processing the autocatalytic (a reaction in which one of the products of the reaction increases the overall rate of a reaction) nature of microbiological reactions takes on a further significance. This is because the presence of one of the products, additional micro-organisms, enhances the overall rate of reaction. In the absence of micro-organisms no reaction can take place. Therefore, it is essential to retain at least a portion within the fermentor. It follows that if the flow rate is raised to a high value, then all the micro-organisms will be swept from the fermentor, and the conversion will cease. This phenomenon is commonly known as 'Wash-out'. if micro-organisms are fed to the fermentor simultaneously with the substrate feed, the problems associated with wash-out are abated, and the reaction proceeds normally.

Advantages of Stirred Tank Bioreactor

1. Continuous operation
2. Good temperature control
3. Easily adapts to two phase runs
4. Good control
5. Simplicity of construction
6. Low operating (labor) cost
7. Easy to clean

Airlift Bioreactor

This kind of fermenter works on the principle of an air lift pump. It is of two kinds: 

1. Internal loop type
2. External loop type.

The reactor's volume is determined by its capacity, kinetic data, and specific growth rate of the organism used. The rate of airflow of the reactor depends on the volumetric mass transfer coefficient in the reactor system. It is a uniform cylindrical cross type and has an internal loop or external loop riser configuration, 

diverging converging. The external loop riser configuration is adjustable and the change in the configuration improves the O2 transfer rate vis-a-vis mass transfer coefficient for a particular rate of airflow. This helps provide required particular dissolved O2 concentration for specific microbial system. This reactor reduces the operating cost for pumping air through the bioreactor.

Advantages:

• Simple design with no moving parts or agitator for less maintenance, less risk of defects.
• Easier sterilization (no agitator shaft parts)
• Low Energy requirement vs stirred tank : Obviously doesn’t need the energy for the moving parts (agitator shaft).
• Greater heat-removal vs stirred tank: At the Airlift bioreactor it doesn’t need the heat plate to control the temperature, because the Draught-Tube which is inside the bioreactor can be designed to serve as internal heat exchanger. It is difference to the Stirred tank bioreactor that needs the heat coat or plate surrounding the tank to make warm bioreactor. It is clear enough that the Airlift bioreactor has greater heat-removal compare to Stirred tank.
• Very low cost

Fluidized Bed  Bioreactor

This is a characteristic of beds of regular particles suspended in an up flowing liquid stream. 

If an additional gas phase is involved, there is a tendency for the particles in the bed to become less evenly distributed. 

There are two important features of the beds of mixed particle sizes: 

(i) The increase in porosity from the bottom to the top of the bed, and 

(ii) The decreased particle movement when compared with beds containing particles of constant size.

Since porosity or voidage is a measure of the local free space within a bed, it also represents a measure of the microbial hold-up when expressed as wet volume per unit bed volume. Thus, a variation in microbial hold-up is to be expected within a 'fluidised bed' fermentor  on fluidisation, the smaller particles rise relative to the larger particles, and produce a situation where the smaller particles are at the top and the larger 

particles are at the bottom of the bed.

As the smaller particles have the lowest settling velocity, the bed arranges itself, so that the smaller particles may be in the region of the highest porosity and the lowest linear velocity. The tower fermentor (developed for the continuous production of beer) is based upon these general principles (Ault et al, 1969). In this process yeast flocs are maintained in suspension by the upward movement of the nutrient medium. Moreover, any entrained particles are returned by means of a sedimentation device at the top of the tower. 

Essentially, the fermentor consists of a vertical cylinder with an aspect ratio (length to diameter) of approximately 10:1 At the top of the tower a separator is provided to induce the gas bubbles produced by 

th,e reaction, to coalesce and escape from the liquid phase.

Within the separator there is a quiescent lone, free of the rising gas, so that the yeast may settle and return to the main body of the tower, and clear beer can be removed. A flocculent yeast (i.e. a yeast capable of achieving  relatively large floc sizes) is essential for an alcoholic fermentation in a PBP at acceptable flow rates, otherwise a large proportion of the yeast would be washed out. As a result of this, an insufficient yeast concentration is maintained. A mean yeast concentration of 25 % by weight (expressed as centrifuged wet weight) is typical with values as high as 30-35% by weight at the bottom of the tower, and as low as 5-10% by weight at the top.

 A significant feature of the tower is the progressive and continuous fall in the specific gravity of the nutrient medium between the bottom and the top of the tower. There is an initial rapid fall at the bottom of the tower. It is followed by a slower fall over the middle and the top of the tower. This gradual fall in the specific gravity is due to the fermentation of the sugars.

Advantages of Fluidized Bed Reactor:

• Uniform Particle Mixing: Due to the intrinsic fluid-like behavior of the solid material, fluidized beds do not experience poor mixing as in packed beds. This complete mixing allows for a uniform product that can often be hard to achieve in other reactor designs. The elimination of radial and axial concentration gradients also allows for better fluid-solid contact, which is essential for reaction efficiency and quality.

• Uniform Temperature Gradients: Many chemical reactions require the addition or removal of heat. Local hot or cold spots within the reaction bed, often a problem in packed beds, are avoided in a fluidized situation such as an FBR. In other reactor types, these local temperature differences, especially hotspots, can result in product degradation. Thus FBRs are well suited to exothermic reactions. Researchers have also learned that the bed-to-surface heat transfer coefficients for FBRs are high.

• Ability to Operate Reactor in Continuous State: The fluidized bed nature of these reactors allows for the ability to continuously withdraw product and introduce new reactants into the reaction vessel. Operating at a continuos process state allows manufacturers to produce their various products more efficiently due to the removal of startup conditions in batch process.

Photobioreactor

Advantages of Photobioreactor

• Cultivation of algae is in controlled circumstances, hence potential for much higher productivity
• Large surface-to-volume ratio. PBRs offer maximum efficiency in using light and therefore greatly improve productivity. Typically the culture density of algae produced is 10 to 20 times greater than bag culture in which algaeculture is done in bags – and can be even greater.
• Better control of gas transfer.
• Reduction in evaporation of growth medium.
• More uniform temperature.
• Better protection from outside contamination.
• Space saving – Can be mounted vertically, horizontally or at an angle, indoors or outdoors.
• Reduced Fouling – Recently available tube self cleaning mechanisms can dramatically reduce fouling.

Membrane Bioreactor

Membrane bioreactors successfully applied to various microbial bioconversions such as alcoholic fermentation, solvents, organic acid production, waste water treatment, etc.

In membrane bioreactor the soluble enzyme and substrate are introduced on one side of ultrafilter membrane by means of a pump. product is forced out through the membrane. membrane holds back the enzyme. good mixing in the reactor can be achieved by using a stirrer.

The most widely used membrane materials includes polysulfonte, polyamide and cellulose acetate.

Advantages of Membrane Bioreactor

1. The loss of enzyme is reduced.
2. Enzyme lost by denaturation can be make up by periodic addition of enzyme.
3. Substrate and enzyme can be easily replaced.

References

Food Biotechnology Course material,by K V Anand Raj