Agitators are used for mixing of fluids, suspend solid particles in fluids, disperse gases, emulsify liquids and even enhance heat transfer between a fluid and a solid surface or increase mass transfer between phases. There are differnet types of agitators. They are either of stirred tank type in which mechanical motor driven stirrers are provided or of air lift type in which no mechanical stirrers are used and the agitation is achieved by the air bubbles generated by the air supply. The mixing of liquids or solids or any mixture within a container is achieved by agitator, which consists of a motor that have a fixed sparge ring for circulating liquid. The agitator consists of a motor driven shaft of to which solid disc is attached to one side and agitator blades on the other side of disc. The circular cover plate that is attached to the bottom end of agitator blades allows the induction of fluid within the inner area of the agitator blades and some area from the sparge ring. This helps the liquid to be expelled from all the outer ends of agitator blades which results in better distribution of a liquid around the sparge ring and also improves mixing of gas with a liquid without drawing the gas into the input area of the agitator.
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The differences in temperatures and concentrations of various constituents in fermenters are equilibrated using the agitators. The heat exchange between fermentation broth and thermostatting elements are intensified using agitators. Furthermore, the sedimentation of cells in fermentation are also being prevented by the use of agitators and enhancing the distribution. It also helps in dispersion of gas phase in fermentation broth. In procees industries, various agitator types are used. Disc agitator is considered to be the most widely used agitator. For example Rushton turbine, a standard disc agitator have six perpendicularly arranged blades that generates a radial flow to the agitator axis. The flow vertices forms a high dispersion effect above and beneath the agitator. On the other hand, in inclined-blade agitator the angle of agitator blades is changeable , but is usually placed at 45 degress. It helps in attaining high effective mixing with the help of axial transport direction coupled with radial component. The major disadvantage of these two agitators is that such agitators are easily flooded and are no longer useful to disperse the gas, In highly flooded gas situations.
The air lift agitators are the ones in which the liquid that is to be fermented is mixed equally with the help of air. These Agitators require no mechanical agitation and eliminates the need for a stirrer system. It is very efficient for large unit plants and for fermentations that require a high and constant aeration degree. The airlift fermenters are mostly applied in tissue culture and also used for aerobic bioprocessing technology. For tissues the normal mixing is impossible as they are sensitive to shear and with airllift the mixing is achieved as the shear levels are low. They are fitted in a long, thin vessel with an aspect ratio of around 10:1 (height to base diameter). The vessel is divided by baffle or draft tube into two interconnected zones. The zone that receives air is called 'riser' and the one that doesn't receive air is called 'downcomer'. The liquid flows up the riser zone and is flown downwards in the downcomer zone. A conical section is often used on the top of vessel to give widest possible area for gas exchange. In the steel base section the sensors are mounted. A stream of air enters at the base of the vessel and is passed through the fluid present in the vessel for both mixing and aeration. The air is being moved upwards with the help of a draft tube fitted in the center of the vessel. The double walled draft tube allows heating and cooling by thermo circulator system. When the agitator rotates the aerated fluid goes to the top of the draft tube and spills down through the spacing between the outer wall of the draft tube and inner wall of the vessel. There is an increase in the density of the fluid going down to the bottom by the gas transfer from liquid to gas phase at the headspace of the draft tube. This descending liquid, which goes down to the bottom, is again aerated and rises to the top. There are two types of air lift fermentors based on their circulation types: Natural and Forced. In natural circulation the liquid is recirculated by the gradient density generated by air whereas, in the forced circulation type fermentors an external mechanical power is applied for lifting the liquid by air which results in recirculation of the liquid in fermentor. In both these types of fermentors the liquid that is heated up while fermentation is cooled by the external refrigirator that is attatched to the fermentor itself. An aeration rate of 1.5 to 3.0 liters of air per minute per liter of medium has been found effective.
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The main types of airlift fermenters are Internal-loop fermenters, that consists of a single container with a draft tube in the centre of the vessel which creates circulation channels in the interior for keeping the volume and circulation at a constant rate for fermentation. External loop fermenters are the ones that contain external loops to circulate the liquid through separate independent channels. Depending on the type of fermentation being used the fermenters can be modified accordingly. Two different fermenters are used for the temperature dependent product formation, which are known as two stage airlift fermenters. As the temperature is difficult to increase from 320 to 400 in the same vessel, the growing cells are transferred from one fermenter (maintained at temperature 300C into another fermenter (at temperature 420C). The cells that grow in the first fermenter are transferred into second fermenter with the help of fitted valves, a transfer tube and pump. Tower fermenters are those with a high hydrostatic pressure, which is generated at the bottom of the reactor that helps in increasing the solubility of O2 in the medium. CO2 is expelled as the pressure is reduced with expanding the top. The cycle completes with the medium flowing back into the downcomer. The advantage with Tower fermenter is that it has high aeration capacities without having moving parts. Gas holdup, mixing, liquid circulation and gas-liquid oxygen transfer can be characterized in a large (approx 1.5 m~3) draft-tube airlift bioreactor agitated with impellers placed in the draft-tube.
The advantages of airlift fermentor are in low shear there is low mixing which means the fermenter can be used for growing plant and animal cells, since there is no agitation sterility is easily maintained, in a large vessel the height of the liquid can be as high as 60m and the pressure at the bottom of the vessel will increase the oxygen solubility, extremely large vessels can be constructed in which the microorganisms will undergo a biochemical reaction and release large heat which is not achieved by conventional stirred tank design. The main disadvantages of airlift fermentors are high capital costs with large scale vessels, high energy costs although an agitator is not required for most of the fermenters, a greater air throughput is necessary and the air has to be at high pressure, particulary on a large scale. Also the efficiency of the gas compression is low, as the microorganisms circulate through the bioreactor, the conditions change and it is impossible to maintain consistent levels of carbon source, nutrients and oxygen throughout the vessel, the seperation of gas from liquid is not very efficient when foam is present. In the design of an airlift fermenter, these disadvantages have to be minimised. If the feed comes in at only one location, the organism reaches starvation stage after all nutrients being used up in continuous growth cycles. This would result in the production of undesirable by-products, low yields and high death rates. Therfore, particularly on a large scale, multiple feed points should be used. Similarly, air should be admitted at various points up the column. However, for the circulation of fluid through the reactor the air must enter from the bottom.
Mechanical agitators are used in many industrial plants for mixing. Mechanical agitation is necessary when you must add materials a portion at a time so that they have immediate intimate contact with the bulk of solution. Efficient mechanical agitation reduces the time for completion of a reaction and can be used to control rate of reaction as well as improves the yields of products. High viscous fluids are mixed with the help of helical screw agitator, but agitators with draught tube found to be effective than others. Mixing chamber shapes, impellers and presence of draught tubes are the main factors that affect the perfomance of an agitator. The components of mechanical agitator are a drive motor, a geared reducer (also called a gear box), a gearbox output shaft and impellers. The drive motor can be air-operated or electric. The stirrers can be either of continuous or intermittent duty. The Characterstics and efficiency of mixing operation in a specific liquid solution in mechanically agitated fermenters is determined by the physical shape, size and speed of rotation of agitator. mixing efficiency was investigated with various geometrical configurations by tracking the fractions of particle distributions.The choice of agitator in a particular fermenter depends on specific gravity and viscosity of solution, speed of motor, size, shape and volume of container, shaft length and type of material used for it, charecterstics of reactants and products, and operation being used (mixing, homogenizing, etc.). The main objective of mechanical agitation system is uniform suspension of all solids, appropriate application of shear, homogeneous fluid proporties throughout the system, and economical application of applied power. Most mechanical agitators are driven by eletric motors. These motors must be rated for explosion-proof duty (to ensure that motor, starters, and wiring meet specifications for local codes and operating criteria) and may be mounted horizontally or vertically . Motors may be coupled to or direct-face mounted to a gear reducer that in turn drives the impeller shaft. Impellers are mounted on the shaft at a specific distance off the tank bottom to achieve desired results. Some mechanically agitated fermenters involve a substrate bed that sits on a perforated plate, such that air is blown through the whole cross-section of the bed. A mechanical agitator embeded in the bed mixes the bed. In the case of the 50-L fermenter the bed is mixed with a planetary mixer, that is, the mixer blade rotates around the central axis of the fermenter. These mechanically agitated fermenters can be readily used in either the continuously-mixed or intermittently-mixed mode because they give good aerationof the bed when it is static. Other mechanically agitated fermenters are built in a way that air only enters at specific points, and not over a wide cross section of the bed. In this case the efficiency of aeration of the bed depends on degree ofmixing achieved by the agitation system, because it is the mixing action that brings the substrate particles into the aeration zone. The design and the operation of the agitator are crucial for fermenters with mechanical agitators, since they determine the effectivness of mixing. Neverthless, it is not easy to setup the general principles, since optimal design and operation of agitators will be affected by the proporties of substrate bed, which can vary between different substrates. The main advantages of mechanical agitators are it suits most need due to the variety of gearbox and impeller combinations, it is effective for large and deep tanks, it can also be designed based on more or less shear needed to induce, it helps in cooling the mixture by exposing the mixture to atmosphere. The various disadvantages associated with mechanical agitators are that they cannot blend fluids if the system has different compartments, Higher initial cost, Heavier than other agitators and requires more space, Electricity required to run the motors and it may also require installation of baffles.
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As an example, Streptomyces fradiae was cultivated in both an air-lift fermenter and a mechanically agitated jar-fermenter with various agitation rates from 200 to 800 rpm to investigate differences in neomycin production between the two reactors. Final neomycin concentrations in the jar-fermenter operated at 600 rpm and the air-lift fermenter were 3.19 and 1.39 g/l, respectively. On the other hand, levels of soybean oil consumption in the two reactors were 25.9 and 9.4 g/l, respectively. Shear stress due to mechanical agitation caused changes in the morphology of mycelia and influenced neomycin production. The morphological changes of the mycelia in the jar-fermentor caused the viscosity of the culture broth to decrease by half, and soybean oil consumption and fatty acid uptake rate to increase 3- and 1.8-fold, respectively, in comparison with those of the air-lift bioreactor. The product yield coefficient determined from the level of soybean oil consumption in the air-lift bioreactor was similar to that of the jar-fermentor at 600 rpm, but the neomycin yield was less than one-half. In the case of the jar-fermentor, the yield increased with increasing agitation rate and was maximum at 600 rpm. To maximize neomycin production in S. fradiae cultures using soybean oil as sole carbon source, it was necessary to provide a degree of shear stress to the mycelia and to optimize liquid mixing. In an air-lift bioreactor, the soybean oil consumption may be suppressed due to a low degree of liquid mixing.
The aeration rates are given by volume of air at standard conditions per volume of liquid per minute or standard cubic feet of air per hour per gallon. Airlift agitators are used in large-scale fermenters as they save lot of energy and are also cost effective when compared to mechanically agitated fermenters. For airlift fermenters it is seen that with an increasing agitator speed the gas hold up also increases. On the other hand there is only little mass transfer that can be achieved even with increasing mechanical energy input. The upper impellers mainly circulate the fluid and contribute very little to bubble dispersion and oxygen transfer. The main difference between airlift and mechanical agitators are there are no shaft seals or moving parts, so the design of airlift is very simple. The defects are minimal and are easier to sterilize when compared to mechanical agitators. In fermenters with airlift agitators there is a low energy input with large interfacial contact area. The flow in airlift is well controlled which results in efficient mixing. The oxygen solubility achieved by higher pressures in large tanks enhances the mass transfer in airlift fermenters. It is easy to build large airlift fermenters that help in increasing the yield.
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