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The use of airlift and mechanical agitators in fermenters in the field of biotechnology and bioprocessing are important as they can be applied in different industries. Airlift agitation is suitable for use in processes that require low shear stress and need aseptic conditions for maximum productivity, while mechanical agitators are presently well understood and easily controlled, thus their widespread use in most fermentation processes. The agitators have merits that enable their use in different industries.
Fermentation is a process that has been around for a very long time, it dates back to the prehistoric period (6000BC), when cheese was first made from goat and cow's milk in Iraq. The Egyptians followed 2000 years later with the discovery of the use of yeast in wine and leavened bread production; the Sumerians were the first to produce beer by barley fermentation. The table below shows a summary of important events in the history of the fermentation process (Shetty, 2006).
TABLE 1: FERMENTATION HISTORY
APPLICATION OF FERMENTATION
Wine preparation promoted by the Roman Emperor Marcus Aurelius Probus
Production of spirits (ethanol)
Vinegar manufacturing industry
Yeast fermentation is discovered by Erxleben
Description of lactic acid fermentation by Pasteur
Discovery of fermentation enzymes in yeast by Buchner
Discovery of penicillin by Fleming
Discovery of many other antibiotics
TABLE 1: FERMENTATION EVENTS IN HISTORY; significant events that have occurred in the developmental process of fermentation.
The development of the fermentation process in the early twentieth century saw the industrial production of acetic acid, lactic acid, alcohol, butanol, citric acid, glycerol and acetone. Initially, anaerobic fermentations were carried out in submerged cultures, while aerobic fermentations were carried out in surface culture. The increase in demand for improved productivity of aerobic industrial fermentation for bakers yeast and organic acid production, led to the utilization of deep tank vessels that were sparged with large quantities of sterile air. The development of the penicillin production process during the Second World War reasserted the development of industrial fermentation processes. Towards the latter part of the twentieth century, with improved technological knowledge and advances, efficient fermentation systems for biochemical production were developed.
There are different types of growth systems used in fermentation, the most commonly used one is the suspended growth system, the other is the attached or supported growth system; some systems contain both the suspended and supported systems.
Currently, the process of fermentation has gained increased credence due to the power of microbe design biotechnology, perceived hazards to people and the environment of chemical synthesis, and better economies from use of renewable raw materials.
A fermenter is an equipment or container which is used in carrying out biological processes like fermentation; it is also called a bioreactor. Organisms, nutrients and substrates are usually brought together under favourable environmental conditions. The fermenter could be a closed or open vessel, but more often it is a closed vessel with conditions for introduction of desired organisms, access for sterile liquid, gaseous nutrients and control fluids all under aseptic conditions. There is also the provision for sterile outlets for exhaust gases, liquid streams and control of environmental parameters that may include temperature, pH, dissolved oxygen concentration and homogeneity (Winkler, 2001).
Bioreactors design must attain certain requirements to be considered fit for use in industry, and they include but are not limited to the following:
Good mixing capacity
There are different types of Fermenters, and the major distinguishing factor between them is usually the method of agitation used in maintaining homogeneity and gas dissolution promotion i.e. either by mechanical agitation or by gas injection also called sparging. The fermenters that are usually mechanically agitated are the stirred tank fermenters while those without mechanical agitation are the Gas sparged fermenters that include the major types i.e. bubble columns and airlift fermenters
Agitation is a very important unit of operation that is used in the chemical industry. Agitation as a unit of operation can be defined as the process by which particles of the component of a mass of materials are put in such space relation to one another so that the desired result may be obtained in the minimum time possible and with the least energy expenditure. A device used in agitation is an essential part of virtually all unit machinery process (Hixson, 1944).
MASS TRANSFER HEAT TRANSFER MOMENTUM TRANSFER
Figure1: Design Considerations for Bioreactor: There are certain things to consider before the design of a bioreactor, and they are shown in the figure above.
TYPES OF FERMENTERS
There are different types of fermenters, but in this paper my attention will be on the bioreactors with mechanical and airlift agitation systems (stirred tank fermenters and airlift fermenters).
These are fermenters or bioreactors that do not have any form of mechanical agitation, they can be used in fermentation processes that have low shear and require low energy. In air-lift reactors, the medium of reaction is usually pneumatically mixed or agitated by a stream of compressed air, gas or mixture of gases (Merchuk, 1990).
The main pattern of flows in this type of fermenters is usually dependent on the design of the bioreactors/fermenter; a channel for liquid or gas up-flow and a different channel for down-flow are available. The two channels are linked at the top and bottom, thus creating a closed loop. The reactor is also separated into an unaerated and aerated region that generates a vertically circulating flow (Merchuk, 1990).
There are different types of air-lift fermenters, and they are classified based on their structure; Baffled vessels, where appropriate baffles are added to create the required channels for circulation. The external loop vessels; circulation in this vessel occurs through distinct conduits (Merchuk, 1990).
There are varying industrial applications of these types of fermenters; they are used in water treatment, chemical industries, and as biochemical reactors e.t.c (Dhaouadi, Poncin, Midoux, & Wild, 2001).
This bioreactor offers an environment that has a low - shear and this has enabled the cultivation of cells that are filamentous and sensitive to shear (Garcia-Ochoa & Gomez, 2009).
Figure 2: Diagram showing (a) air-lift fermenter and (b) bubble column fermenter, both are gas sparged bioreactors source: (Winkler, 2001).
These are fermenters or bioreactors that achieve agitation in their vessels mechanically. It is also the most conventionally used fermenter design in most fermentation processes in different industries, as it is very well understood (Winkler, 2001). The most commonly used fermenters with this kind of agitation are called the stirred tank fermenters.
The agitation is as a result of the rotation of an agitator in the vessel or fermenter .The agitator usually comprises of a rotating shaft on which are mounted one or more impellers. Usually, but not always, the shaft is concentric with the axis of the cylindrical vessel. The agitator could be bottom driven or suspended from the above the vessel, depending on the level of operation. Impeller choice is based on the characteristic (physical and biological) of the growth medium. A kind of ring sparger is used to supply sterile air to the broth in aerobic fermentation. For vortex formation to be prevented baffles are provided; they also assist in mixing improvement. This entry point is a major contamination risk and is the principal reason for non-mechanical agitation being preferred for some applications (Winkler, 2001).
The performance of stirred tank fermenters is dependent on the viscosity and coalescence of the liquid in the fermenter and this change often during the fermentation process.
Figure 3: Diagram showing a mechanically stirred bioreactor (Stirred-tank fermenter) Source: (Winkler, 2001)
This is a physical process that aids the reduction of non uniformities in mixtures (liquid and gases) by the elimination of concentration gradients, temperature gradients and other properties associated with mixtures (Doran, 2009). The characteristic of this process include exchanging of materials between different locations in the mixture to produce a mingling of components, thereby improving contacts between the different phases in the mixture.
Mixing is a very important process in varying industries, and it has been estimated that the cost of inefficient mixing industrially or lack of the process of mixing cost the USA approximately ten billion dollars annually. In order to obtain products with high value, the mixing process must satisfy not only the need of heat and mass transfer, but also the required homogeneity in the bioreactor in the shortest time possible.
MIXING: AIRLIFT AGITATORS
Bioreactors with this type of agitation, usually achieve mixing by the action of the compressed air or gas that is utilized in the process. In the laboratory, reactors show a higher degree of mixing than the ones used on an industrial scale.
Therefore mixing is usually accomplished in the absence of mechanical agitation, and the levels of shear are lower than in stirred vessels.
MIXING: MECHANICAL AGITATORS
Mixing is accomplished here by the action of the impeller that is usually mounted inside the vessel/tank/fermenter. The impeller or impellers (depending on the type of fermenter and process) is located over a stirrer shaft that is centrally located. In some instance, the vessel is designed in such a way that the stirrer shaft enters at the bottom of the vessel. The stirrer motor drives the stirrer shaft rapidly; the impeller in turn rotates, thus it pumps the liquid and creates a flow pattern, hence mixing. Impeller speed, type of impellers and gas flow rates are some factors that affect mixing in this fermenter (stirred-tank fermenter) (Garcia-Ochoa & Gomez, 2009).
There are different types of impellers that are designed for different types of mixing processes (Doran, 2009). The impellers are usually required to have a shearing action, to enable breakage of air stream into small bubbles, and a pumping action that creates bulk movement of the liquid in the vessel.
Figure 4: Examples of multi- impellers that are used for experiments on a small scale:
A modified double disk impeller (b) A hollow PBTD450 impeller (c) The six pipe impeller (d) A modified four pipe impeller (e) Hydrofoil impeller
(f) PBTD450 impeller Source: (Kalkarni, Anand, & Jyeshtharaj, 2008).
This is a process of oxygen transfer in a bioreactor, and it is usually between the same or different phases (i.e. gas and liquid); there could be a transfer of oxygen from a gas bubble that is rising into a liquid phase, and finally to the site where oxidative phosphorylation takes place within the cell (Garcia-Ochoa & Gomez, 2009). Aeration is very important in biotechnological applications that involve microorganisms, plant cells and animal cells that depend on oxygen supply (Chisti & Moo-Young, 1993).
In the aerobic fermentation process, the rate of oxygen consumption by the cells in the bioreactor is the main determinant of the rate of oxygen transfer from gas to liquid. Therefore, an accurate estimation of the oxygen transfer rate (OTR) in different bioreactors and at different scales is very important in the selection of different types of fermenters for the fermentation process (Garcia-Ochoa & Gomez, 2009).
The molecules of oxygen also have to overcome a number of transport resistance before they can be utilized by cells. The resistance include; movement across the gas-liquid interface, transportation through bulk liquid, transport from the bubble interior to the gas - liquid interface e.t.c (Doran, 2009).
Figure 5: Summary of resistance to oxygen transport from a bubble of gas to the cell where it is required. Source: (Garcia-Ochoa & Gomez, 2009).
AERATION: MECHANICAL AGITATORS
Aeration is provided in a stir tank fermenter by the injection of sterile air through a sparger that is at the base of the vessel, into the growth medium. The mechanical agitators in this vessel have a major duty to dissolve the oxygen into the broth and then mix the oxygen rich culture. In an aeration - agitation system such as the STF (Stirred- tank Fermenter), the oxygen transfer rate (OTR) is affected strongly by the characteristic of the culture medium , it's composition, and most importantly the viscosity and mechanical design of the fermenter (Winkler, 2001). The OTR does not increase in proportion with increasing power input of the agitator or increasing rate of air flow.
This type agitation offers a high oxygen transfer rate which is a merit in some fermentation processes.
AERATION: AIRLIFT AGITATION
In the airlift bioreactor, aeration is achieved by the introduction of air unevenly into the vessel that carries the culture medium. This in turn induces circulation and aeration, as the liquid rises in the section that is aerated. Oxygen transfer rate (OTR) in this bioreactor does not increase in proportion with the flow rate of air.
In aerobic fermentation, energy increases the cost of the process. During agitation energy is consumed, it is also consumed during compression of air and refrigeration (Alves & Vasconcelos, 1996).
A good choice of agitator is a way to reduce energy cost in bioreactor application, as the particular agitator will be well suited for the fermentation process.
In airlift bioreactors the main source of agitation is air/gas, and on an industrial scale the amount of energy required for fluid circulation and dispersion, will be reasonably higher than the energy needed in the stirred-tank fermenters.
COMPARISON OF AIRLIFT AND MECHANICAL AGITATORS IN FERMENTERS
The shaft entry point in the stir tank fermenter is a major drawback to its use in some fermentation process as it is a contamination risk, and this is why airlift fermenters are preferred in some processes that require aseptic conditions.
Mixing is not perfect in airlift fermenters (low intensity) in comparison to stirred tank fermenters which have high mixing intensity (Znad, Bales, Markos, & Kavase, 2004),
The airlift reactors are not as complicated to construct and cost less in comparison to the stirred tank reactors (Poulsen & Iversen, 1999), and they also offer less shear stress on cells thus their preferred application in some biotechnological industries (Znad, Bales, Markos, & Kavase, 2004).
In fermentation involving highly viscous liquids, the stirred tank fermenters are more advantageous as they offer a higher volumetric mass transfers in comparison to the airlift fermenters that have a low volumetric mass transfer coefficient.
MERITS OF AIRLIFT AND MECHANICAL AGITATION IN FERMENTERS
Maintenance cost of airlift fermenters is low as there is an absence of mechanical stirring; reasonable gas - liquid mass transfer rates are obtained with low power input.
There is a high surface area to volume ratio in airlift fermenters, thus encouraging growth due to photosynthesis, it also encourages oxygen utilization. When fitted with a draft tube, it aids better circulation of liquid.
Mechanical agitators help achieve excellent mixing in the fermentation process (Garcia-Ochoa & Gomez, 2009).
The use of airlift agitation enables the bioreactor to handle large amount of gas while also maintaining a flow that is homogenous; it also enables the prevention of large concentration gradients with its short mixing time and circulation rates that are high.
The mechanical agitation in stirred-tank fermenters enables high oxygen transfer rates which promotes biomass productivity. These agitators have low operating costs and require low investments; these factors are important in an aerobic fermentation process.
For the industrial application of airlift fermenters, they offer inadequate sterilization, high capital investments and aeration requirements.
The major function of a bioreactor is to provide a favourable environment that will enable microorganisms carry out the essential biological activity that is required in the fermentation process. To ensure maximum and high productivity, certain conditions have to be met and bioreactors are designed in such a way that they encourage the maximum productivity during fermentation.
One of the important factors in bioreactor design is agitator type, as it contributes to productivity in fermentation. The agitators that were highlighted in this paper have both advantages and disadvantage during their utilization in fermentation. Their advantages enable them to be used in certain fermentation processes; these include enabling high productivity by supporting the good environmental conditions required by microorganism, less shearing for shear sensitive cells, low energy use and low cost. All of these are important factors in fermentation.
More studies need to be carried out on the airlift agitators in fermenters, as they have not been very well understood and thus cannot be controlled adequately.
Therefore, the airlift agitator and mechanical agitator should be used in fermentation processes for which they are best suited, as this will save cost and there will be high productivity.