Fermentation is a process involve microbial cells to breakdown or catabolise the organic compounds into smaller molecules. Fermentation is performed under aerobic or anaerobic conditions. The end products of fermentation are our daily products such as alcohol, vitamins, enzymes rennets, antibiotics such as penicillin and lactic acid. In the fermentation process, it is very crucial to maintain optimum mixing and aeration in the reaction. Optimum mixing is to ensure best amount of products conversion as well as to avoid wastage of biomass and substrate. Whereas, aeration is the oxygen transfer rate, also the rate-limiting step in the aerobic bioprocess which is very crucial in design, operation and scale-up of bioreactors. Mixing and aeration in the fermenter is carried out by air-lift or mechanical agitators. Air-lift fermenter uses air injected from the bottom of its draft-tube, while mechanical type uses motor driven shaft agitator. Both of the air-lift and mechanical agitator are differ in mixing, aeration, biological efficiency, energy consumption, operation and construction, application and cost. Different types of fermentation biomass and substrate's characteristic is the factor to determine types of air-lift or mechanical agitators to be used.
Advances of biotechnology today has been making use of microbiology in different perceptions of the natural processes to produce our daily products such as cheese, antibiotics, alcohols, biofuel, hormones, microbial enzymes such as rennets and vitamins. The process employed to produce these products is known as fermentation, in which catabolism of organic compound take place. The breakdown of the organic compounds is performed by microorganisms under aerobic and anaerobic conditions to yield end-products that are our daily products as mentioned (Fermentation, Chapter 1 2009). In the process of fermentation, the microbial cells obtain energy through glycolysis to break down the complex organic compounds to simpler molecules. Usually, the by product of the process is excreted by the cell in the form of acetone, lactic acid and alcohol.
In the history of fermentation, in year 1680 Antony van Leeuwenhoek was the first biologist discover the process through fermenting beer under observation with microscope (Fermentation, Chapter 1 2009). Later, in the 19th century, Louise Pasteur discovered and understood the process of fermenting alcohol by using yeast to convert sugar to alcohol and carbon dioxides(Fermentation, Chapter 1 2009). Besides, in 1929 Alexander Fleming discovered the production of antibiotics Penicillin to fight against bacteria infection (Fermentation, Chapter 1 2009). Since then, various antibiotics are produced by fermentation using various bacteria and fungi.
There are several factors to be taken into accounts as designing the fermenters such as scale-up issues, fermenter types, impeller types, immobilised systems, peripheral equipment and fermenter measurements. Impeller plays a vital role in mixing during fermentation because it maintains optimum substrate and biomass concentration in the fermenter throughout the whole process. It also keeps the solids suspended, disperse oxygen to keep maximum total bubble surface area and entrap air bubbles to avoid the air escape before all the oxygen is dissolved (Freitas C. et al. 2000). Fundamentally, there are 2 apparatus used for mixing which are the mechanical agitators and the air-lift fermenters. Air-lift fermenters are sometimes known as the bubble column, can be divided into free rise, draft tube, propeller-assisted and pumped liquid jet-assisted.
Air-lift fermenter is an efficient contactor for the reactions involved gases, liquids and solids. There are two types of air-lift fermenters which are the internal loop and the external loop. The internal loop has a draft tube in its inner tube, in which the up-flowing gasses liquid and the down-flowing liquid is separated by the draft tube. Draft tubes are used in some processes to promote better mass transfer, mixing and inducing circulatory motion to reduce bubble coalescence. The external loop has two streams flow in two separate pipes connected at top and bottom. In this way, the air-lift fermenters improve the circulation and oxygen transfer and equalize shear forces in the reactor. Both of the internal and external loop air-lift fermenters have been investigated to the hydrodynamic behaviour and other design factors (Fermentation, Chapter 1 2009). The figure below shows the structure of an air-lift bioreactor with an internal loop.
Unlike the mechanical agitation system, air-lift fermenters do not have motor, shaft and impeller blades. As such, the mixture inside the fermenters is agitated by injecting air from the bottom of the tube. Sterile atmospheric air is injected into the fermenters if the microorganism is aerobic, while for the anaerobic microorganism is fed with inert gas (Fermentation, Chapter 1 2009). Mixing in air-lift fermenters is very gentle hence it is suitable for batch culture of shear sensitive cells and tissues such as the mammalian and plant cells (European patent application 2009). Likewise, high shearing stress causes damage to cells could be avoided. Batch culture of plant and animal cells can be cost intensive. However, in cases where the demand for the plant or animals culture products is low and batch cycles are long, the high capital cost can exclude the economical production (European patent application 2009). One of the application of air-lift fermenters is the large scale production of monoclonal antibodies (Fermentation, Chapter 1 2009).
Stirred tank fermenters are most commonly used in fermentation. It is a cylindrical vessel with an agitator driven by motor to stir the mixture contents in the tank. There two types of agitator used which are the top-entry stirrer and the bottom entry stirrer. The top entry stirrer is more commonly used because the operation is easier to manage, more reliable and robust, whereas the bottom entry model is rarely to be used. The bench scale fermenters are usually made of borosilicate glass with a stainless steel lid. In the laboratory, top-entry stirrer is used, which consist of a motor attached to the shaft and together with impellers (Bloch H.P. and Soares C. 2007). The criteria's of manufacturing the fermenters are the high grade of stainless steel, a well-polished internal surface to prevent adhesion of contents to the fermenter's walls, and contain smooth joints and free of pin holes to avoid de-mixing. The typical volume of the fermenter used in the laboratory is range from 1 to 100 litres (Bloch H.P. and Soares C. 2007). The figure below is the basic structure of a stirred tank fermenter.
There are several categories of mechanical mixing equipment, the types of mechanical drivers require to accommodate speed, shaft and power; the kinds of impellers used for specific process, the needs to seal the tanks to against high pressure caused by the processes and stabilization of the impeller's devices and the steady bearings in the tank (Bloch H.P. and Soares C. 2007).
The main purpose of the mechanical agitator is to provide homogenous mixing, provide aeration and removal of waste gaseous such as carbon dioxide produced during the fermentation process. Usually, the agitator is consisted of motor driven shaft with impellers of 4 to 6 blades. Several types of impellers are more commonly used which are Rushton blade or disc turbine, open turbine impellers and marine impellers. Among the three, Rushton turbine is most commonly used because its design is more simple, easy operation methods and its robustness (Fermentation, Chapter 1 2009). Figure 3 and 4 illustrate the structure of Ruston turbine and marine impeller.
However, mixing in the fermenter is poor and energy intensive as scaling up the process. The problem caused will affect the concentration, reaction rate and heat removal. Therefore, baffles will be used in the fermentation process to prevent energy waste in bulk circulation and to introduce liquid slurry turbulence in order to ensure better mixing, better suspension of solids and better retention of air bubbles. The baffles are fixed on the wall of the fermenters. Both of the action of agitators and baffle produce axial and radial flow patterns in the mixing contents and prevent formation of vortex (Fermentation, Chapter 1 2009).
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Compare and contrast the relative merits of air-lift and mechanical agitator
Air-lift and mechanical agitator are widely employed in chemical and bioprocessing reaction as the efficient machine for mass and heat transfer. Both of the fermenters are applied in different types of mixture nature. For instance, air-lift fermenter has gentle agitation and low cost oxygen transfer, hence it is only suitable for less viscous liquid. Whereas, mechanical agitator fermenter perform poorly in highly viscous non-Newtonian media, not aerated at high rate and has relatively poor mixing pattern as compare to the air-lift fermenter. However, the mechanical fermenter has broader range of application as compared to the air-lift fermenter (Abashar M. E. E. 2002).
In terms of biological efficiency, the mechanical type of fermenter is more efficient as compare to the air-lift fermenter. However, due to the simplicity operation of air-lift fermenter, it is useful in the laboratory teaching and research. In a laboratory experiment, air-lift fermenter has successfully produce antibiotics, enzymes, microbial cells and vitamins for microbial analysis (AIR-LIFT LABORATORY FERMENTOR 2009).
Operation and construction
The operation of the air-lift fermenter is much simple and easier than the mechanical agitator fermenter. This is illustrated by the control of temperature in the air-lift fermenter, in which the temperature is readily controlled in an incubator or by attaching a heating tape externally to adjust to higher temperature. The construction of air-lift fermenter also more simpler as compared to the mechanical agitator fermenter, as such air-lift fermenter is applicable to continuous and semicontinuous fermentation. The effect of various gas mixture on microbial growth can be readily studied by the air-lift fermenter because the propagation of obligate thermophilic bacteria at 55 C has been successful (AIR-LIFT LABORATORY FERMENTOR 2009).
Mixing and aeration
In comparison of mixing in highly viscous non-Newtonian media, the conventional mechanical agitator performs relatively poorer than the air-lift fermenter. Also, conventional agitator has lower aeration rate due to its impeller flooding. Chisti Y. & Jauregui-Haza U.J. (2002) had find out a better solution to overcome the limitation by creating mechanically stirred hybrid airlift bioreactor. This hybrid airlift bioreactor had one or more downward pumping axial flow impeller located at the bottom of the reactor. It able to overcome the limitation of both the mechanical and airlift reactor by providing confined annular zone with better aeration system. It also manages to improve the directional and fluid circulation for the suspending solids and minimize concentration gradients between nutrients and oxygen to the substrate (Chisti Y. & Jauregui-Haza U.J. 2002).
In the other case, mechanical agitator, the marine propeller is installed at the bottom of the draft-tube, to carry out a fermentation of highly viscous non-Newtonian media with Saccharopolyspora erythraea broth. The results show that the yield of antibiotic erythromycin was increased by 45% compared to the air-lift configuration or conventional mechanical agitator fermenter alone. In another study, highly viscous broth of mircrofungus N. sitophila was fermented in the airlift fermenter with low-shear axial flow impeller. The end result obtained from hybrid reactor shown that it is more effective as compared to the conventional Rushton turbine-stirred fermenter (Chisti Y. & Jauregui-Haza U.J. 2002).
In short, installation of mechanical agitator such as marine propeller or impeller in the draft-tube of air-lift fermenter can significantly improve the fluid circulation and hence increase viscous aerobic fermentation.
Energy consumption of an operation system is the major constitute of the capital cost in one production. Generally, the energy consumption of air-lift fermenter is relatively lower than the mechanical agitator fermenter. In a production of a single cell protein, a low pressure air-lift fermenter was designed to reduce the energy usage. The fermenter broth was kept below 120cm in depth and air is injected to supply the oxygen, also to cool and agitate the broth. In this way, the low pressure air-lift fermenter reduces the energy consumption by producing 1 kWh/kg of protein, which could save 70% of energy as compared to mechanical agitator. Hence, it eliminates the investment of mechanical agitators, air compressors and heat exchangers (Chen N. Y., Kondis E. F., & Srinivasan S. 1986).
In contrast to mechanical agitator fermenter, air-lift fermenter has higher efficiency in mass transfer at the same power input. This is because air-lift fermenter is not using mechanical agitation, which in turn makes mass transfer perform faster (Chen N. Y., Kondis E. F., & Srinivasan S. 1986). The application prove mechanical agitator has lower mass transfer is the performance of Rushton turbine impeller which involve high cells densities to support oxygen transfer rate and also the highly viscous broth cause turbulence and result in the poor mass transfer. However, air-lift fermenter with the draft tube manages to give better mixing and fluid circulation result in better mass transfer efficiency (Boodhoo K. 2006).
An example of mechanical agitator, submerged agitating system is used in gas-liquid mixture. The mechanically rotational agitator is used to intensify the turbulent mixture between gas and liquid. In this system, pressurized gas is injected through the spargers to the agitating area, in the form of bigger bubbles which then broken into smaller bubbles by the mechanical agitator's mixing power. However, the efficiency of the power consumption is generally greater in the mechanical agitator. Nonetheless, mechanical agitator also increase the residence time of the bubble and caused efficient gas-liquid mass transfer. However, the relative complication of the mechanical agitator has counterbalance the energy efficiency. Mechanical system complication's includes driving motors, gear reducers, submerged agitators, long shaft and the expensive gas compression system (Chen N. Y., Kondis E. F., & Srinivasan S. 1986).
Capital cost is the main factor to be considered in the industrial fermentation process. Fermenter cooling, mass transport between nutrients and microorganism and oxygen transfer are the main factors in affecting the design of the fermenter. The high demand of substrate to the oxygen amount increase the energy cost and caused significant increase in the production cost. Due to the simplicity operation in air-lift fermenter, it is generally lower in cost than the mechanical agitator fermenter (AIR-LIFT LABORATORY FERMENTOR 2009). This is because mechanical system involves extensive pipe lines, diffusers, distribution manifolds, expensive compressor and filtration system. As a result, it caused high power cost and capital costs. Moreover, the frequent plugging of the diffusers make the maintenance for the mechanical system is very troublesome particularly in the reaction involve solids such as aerobic wastewater treatment and industrial fermentation (AIR-LIFT LABORATORY FERMENTOR 2009).
Both of the air-lift fermenter and mechanical agitator is used in the fermenter tube to aid in mixing and aeration. Mixing is very important to ensure optimum homogenous mixture of substrate and biomass throughout the entire process. Air-lift fermenter is suitable to plant and mammalian cells fermentation due to its low shear rate, protect cells from damage. The application of air-lift fermentation is the production of monoclonal antibodies. Whereas, mechanical agitator is more vigorous compare to air-lift fermenter, hence it is not suitable for fermentation involve plant and animal cells. The examples of the mechanical agitators are Rushton blade or disc turbine, open turbine impellers and marine impellers. In the above, comparisons was made between both of the air-lift and mechanical agitator in terms of biological efficiency, operation and construction, mixing and aeration, energy consumption and cost. Mechanical agitator has better biological efficiency than the air-lift fermenter, but air-lift fermenter has better mass transfer rate at the same power input. The operation and construction of air-lift fermenter is less complex than mechanical agitator. Likewise, energy consumption is relatively lower in air-lift fermenter. Nonetheless, both of the fermenter's agitator types have its pros and cons. Choice of fermenter types is depend on the fermenter substrates and biomass biological nature.
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