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Fermentation is essential part of modern life. Industries such as food and beverage survive on it. In order to make any of this possible the apparatus in need is the fermenter. Although designed for the same purpose there are many different types of fermenters which have been designed specifically so as to be efficient in different situations. This essay discusses the two main type agitation systems in fermenters, namely mechanical agitation and air lift. It goes on to further state the construction and working of these types and how they compare with each other in different areas of performance. Some of the areas that have been taken into consideration are operating characteristics, cost and practicality, O2 transfer and mixing.
Since the work of Louis Pasteur in the late 19th century fermentation has become an important and indelible part of our human existence. From what we eat, to what we drink and even how our muscles work can be credited to the process. Usually referred to as the method by which complex substances like sugars are broken down into simpler ones, it can be called nature's way of deriving energy by the oxidation of organic compounds in an anaerobic environment. It however is not the most effective way to generate energy as the products formed due to fermentation cannot be further metabolized due to the lack of oxygen and thus must be termed as waste. The two main types of fermentation are ethanol fermentation and lactic acid fermentation. While the former is used in the manufacture of bread, wine and in brewing the latter is used by muscles to generate energy faster than the rate at which blood can supply oxygen to it.
Fermentation has existed long before the science behind it was discovered. It was more chance than science that led to the manufacture of products like cheese, bread, wine and beer. However it was at the frustration of the French wine makers that they were producing vinegar more than they were producing wine that led Pasteur to look into the science behind this then unknown process. What was discovered was a microscopic plant called yeast that was responsible for the biological activity. When unwanted microbes infected the wine and consumed the alcohol they left behind waste that destroyed the wines flavour. This led Pasteur to lay down the foundation for modern day fermenters, because once the process had been identified it could now be controlled.
A fermenter is the setup required to carry out the process of fermentation. They may vary in size from those used in laboratories to those of industrial capacity. A fermenter can be differentiated from a bio-reactor as it is used for the mass culture of microorganisms in place of plant or animal cells. The main types of fermenters include
Stirred Tank Fermenter
Bubble Column Fermenter
Figure 1: Fermenter
Factors that are to be considered before constructing a typical fermenter:
The challenge in designing a reactor lies in supplying the required amount of mixing and air supply for the fermenters that require oxygen. It is comparatively easy to construct anaerobic fermenter as they do not require air supply.
Fermenters require a sterile atmosphere to work efficiently in. Contaminations from the outside can jeopardise the process. This requires that the apparatus be sterilized along with everything that enters into it. Points of entry for contaminants are through the valves that control flow of liquids as well as the stirrer shafts. Diaphragm valves are used to maintain sterility of valves and mechanical seals or magnetic drives are used for stirrers.
Inoculation and Sampling
Inocula used in large scale fermentations are transferred from their own smaller reactor. In order to prevent contamination both these vessels are maintained under high pressure. Transfer from the smaller container to the larger one is carried out by connecting the two of them using valves and then blowing the medium out of the smaller one by filling it with sterile air. There are also sampling ports fitted on fermenters to allow the removal of the medium for testing and analysis. A series of valves is used to make sure that the main fermenter does not get contaminated while the sample is being drawn.
The main points to keep in mind when constructing a fermenter is that the material it is made out of can withstand the multiple sterilization process and at the same time must not react or absorb the culture medium inside it. Glass and stainless steel are the two most common materials used to construct fermenters as they satisfy these conditions with ease.
Design of Spargers
In stirred fermenters a sparger, impeller and baffles control the rate at which mixing and oxygen transfer occurs. The most commonly used types are porous, orifice and nozzle. Different types of spargers are used for different mediums but whatever be the design, it should be possible to open and clean them.
Control of Evaporation
If the air entering the chamber is dry and devoid of moisture, it will most likely take with it some of the moisture content as vapour when leaving the chamber. This leads to a significant loss of water over time and can be a problem. This problem is solved by pre humidifying the air that enters into the fermenter by bubbling it through water.
Stirred Tank Fermenter (Mechanically Agitated fermenters)
A conventional fermenter is the Stirred Tank reactor, one that has internal mechanical agitation. The main asset of such a reactor is that it is flexible and can administer a high volumetric mass-transfer coefficient (kLa). A typical stirred tank reactor consists of a mechanical agitator which is responsible for mixing and bubble dispersion. A high amount of energy is required for this process. Baffles are employed to reduce vortexing which might occur due to swift agitation by the mechanical agitators. Spargers are used for aeration. Impellers are used for producing flow patterns in the reactor vessel which help in gas dispersions in the tank. It is essential for increasing residence time and breaking down large bubbles into small ones. Only 70-80% of the volume of the the reactor is filled with liquid so that sufficient amount of head space is available to hold foam if formed. Addition of antifoam agents is the most common and effective way to deal with foam formation.
Figure 2: Stirred Tank reactor
Different types of agitators can be employed for mixing depending on the type of the liquid which is present in the reactor vessel. Agitators can have shafts with more than one impeller which can enter the vessel through the top, side or the bottom, though the most commonly used one is the one that enters through the top. The most commonly used impellers include disc turbine type,pitch-blade and high efficiency turbine types of impellers amongst others. Clustering of small bubbles into largers ones is unacceptable as it decreases the total interfacial area and gas hold-up.This property depends upon the type of the liquid which is present, if it is a coalescing liquid then a larger bubble formation will occur and otherwise if it is a non-coalescing liquid. Impellor flooding should be checked because an impeller, if makes contact with gas does not make proper contact with liquid which leads in ppor mixing and gas dispersion. The rate of mixing in a reactor is associated with its flow pattern. Aspect Ratio of stirred vessels, that is, the ratio of the height to the diameter varies widely. The most inexpensive shape to build is the one with aspect ratio one as this shape has the least surface area and hence needs the most minimum material to construct for a given volume. It is only increased when aeration is required as it provides longer contact times. Internals cooling coils are used for heat transfer and temperature control.
Figure 3: Types of Turbine Agitators
(Gates et al.,1995)
Stirred tank reactors are generally adopted for the culture of free and immobilized enzyme reactions. They are also used for the culture of suspended and immobilized cells. To prevent any sort of damage due to the high impeller speed adequate amount of care has to be taken which culturing sensitive cells.
Air Lift Fermenters
An air lift fermenter is a based on the physical principle of an air lift pump, which can be used to circulate, aerate and agitate the medium in a closed space. They are generally used for the culture of plant and animal cells as their shear levels are less when compared to the stirred tank reactors. It is a simple and inexpensive apparatus which is made using an inverted, wide mouth, pyrex solution bottle and other accessories. A simple metal clamp is used to secure a rubber stopper to the mouth of the bottle. The rubber stopper contains four holes which can be used to allow in four pyrex tubes. Tube 3 has a two inch internal bend with a funnel attached to it in order to deflect the spray of the culture medium. This reduces the amount of medium sucked out with the air exhaust. The level of the medium is maintained at a minimum of two inches below the funnel.
Figure 4: Functional Diagram of Air-lift Fermenter
(Russell et al., 1955)
As the air enters the column through the Y tube inlet the sterile and humidified air which is under pressure forces the culture medium up tube 1. This forceful expulsion of the medium at the bottom of the bottle can be classified as both mechanical agitation as well as aeration. This is coupled with another compressed and sterile air inlet through tube 4. This treatment makes sure that the culture medium is neither contaminated nor evaporates. It is imperative however that the fermenter and its accessories such as the air filters, humidifiers and others are sterilized beforehand. The inoculum -which is usually added before the process is started- is supplied to the fermenter from an Erlenmyer flask. Air bubbles introduced at the bottom of the vessel help in aeration and agitation. The aerated medium being lighter than the non aerated one creates an imbalance which makes the medium rise and hence results in mixing along with aeration. The bubbles then move out through the outlet tube and are then sent for recirculation. Oxygen setup in this type of reactor vessel is excellent but has some problems in scaling up. Air lift reactors show better mixing when compared to bubble column reactors. Mixing time is the factor that illustrates the mixing property and the rate of heterogeneity in the vessel. It is calculated by recording the difference of time between the start of an impulse to the achievement of steady state. This is recorded by measuring the shift of pH from the acidic to the basic range. Draft tube height in an airlift fermenter relatively affects the mixing time where, the short draft tubes show better mixing time when compared to the large ones. Thus the importance of working within a prescribed range of liquid volume is directly correlated to the mixing characteristics of the system.
Figure 5: Correlation between mixing times and air flow rates for different draft tube positions.
(Suskan et al. ,1987)
The air lift fermenters are employed for continuous, semi-continuous and batch fermentations. Substrates can be continuously supplied by manipulating inlet and outlet hose lines at regular intervals. Some of the advantages of the air-lift fermenter are mentioned below.
Successful in the production of antibiotics, vitamins and enzymes
Capable in obtaining adequate measures of microbial cells for chemical analysis
Can be used for both continuous and semi-continuous fermentations
Temperature in the fermenter can be easily controlled
Effects of different gas mixtures on microbial growth can be studied
Low cost oxygen transfer
Energy consumption is less when compared to stirred tank reactors
Although the efficiency of an air lift fermenter is inferior in comparison to a mechanical -type fermenter, its simple design and low cost of construction makes it ideal as an aid to teaching and for research.
Comparison between mechanically agitated and air lift fermenters
Based on Operating Characteristics:
For treating a large volume of low viscosity culture air lift fermenters are better when compared to stirred tank reactors as they are cheaper and easier to operate. Using mechanical agitators is illogical as they require a high amount of power generation to bring about mixing. On the other hand, if the culture to be treated has high viscosity then using stirred tank reactors is a better option. Greater power for mixing and mass transfer can be induced using the mechanical agitators. Stirred tank reactors generate more heat when compared to sparging which in certain cases poses as a problem where the removal of frictional stirrer heat is difficult.
Based on O2 transfer:
Oxygen transfer is a major aspect which is to be considered while checking the performance of a fermenter. Air lift fermenters give better production rates when compared to stirred tank reactors in terms of oxygen transfer. They require less power input, for example production of Itaconic acid by Aspergillus terreus (Matsushima et al. , 1972). Power input can be further decreased by using O2 enriched air in contrast to normal air, making the air lift fermenters more suitable for usage against Stirred tank reactors.
Based on Mixing:
Mixing is one of the most important factors in a fermenter. Stirred tank reactors are generally used in submerged processes. Mechanical agitators cause shear which is undesirable as it damages the cells subjected for treatment, but this in the case of air lift fermenters is of disregard as mixing occurs through spargers. Maintaining a homogenous system is another factor that has to be taken care of to render even mixing throughout the medium. Homogenization of the medium relies upon the supply of oxygen a system gets which makes airlift fermenters slightly more restricted in contrast to with the stirred vessel, as oxygen is sufficiently supplied by the presence of agitators and baffles.
Based on cost and practicality
For any industry or laboratory to setup any machine cost is an important factor that needs to be taken into consideration. It is also important to take into account the type of work that will be expected out of the apparatus. A small company might prefer a cheaper and easily replaceable alternative compared to a larger more focussed company that might want require perfection in their fermentation process. The cost of a fermenter varies with the complexity of the reaction vessel. As the air-lift fermenter has a simpler structure it would be cheaper to purchase in comparison to a mechanically agitated fermenter which is of a more complex design.
Having compared the two different types of fermenters it can be observed that despite being designated for the same purpose they are most efficient in fermenting their own respective type of culture. Depending on the nature of the culture medium appropriate reactor vessels have to be used. The mechanically agitated fermenters can be used for the treatment of high viscosity fluid like non Newtonian fluids whereas air-lift fermenters can be employed for low viscosity broths, the Newtonian fluids. Both have their own merits and demerits in their own rights. Based on that it is important to analyse which situation demands what kind of a fermenter to employ to derive maximum efficiency.