Agitation systems are integral to all fermenters. The process of Fermentation in biochemical industry requires an additional equipment called Agitator which consists of shaft, impellers with 4 to 6 blades and motor to drive. Shafts should have double seals to prevent leakage of the contents. The main function of the agitator is mixing of the contents, aeration, and removal of carbon dioxide produced during fermentation process by mixing action. Most fermenters employ free organisms, which are suspended in the fermentation broth either as individual cells or as flocs.
On smaller units, mixing is generally accomplished by direct-drive mechanical stirring through a seal in the head plate. Some models offer either magnetically coupled agitators, or air-lift systems to eliminate mechanical seals, which are usually limited to the smaller volume fermenters where low torque produces effective agitation. The transfer of energy, nutrients, substrate, and metabolite within the bioreactor must be brought about by a suitable mixing device.
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The efficiency of any one nutrient may be crucial to the efficiency of the whole fermentation. For the three phases, mixing fermenter contents causes the following:
1) Air dispersion in the medium
2) Homogenisation, which equalises temperature and nutrient concentration through out the fermenter
3) Suspension of culture and nutrients
4) Dispersion of immiscible liquids.
The purpose of agitation is generally to ensure homogeneity of the vessel contents, to promote mass and heat transfer (especially oxygen transfer in aerobic fermentation), and to suspend particles in a fluid. Increased agitation will improve the performance of the system in these aspects. Unfortunately, increasing agitation increases shear and other forces to which the vessel contents will be exposed. The problems of shear in stirred tanks arise from the need to compromise between conflicting requirements such as promotion of oxygen transfer and reduction in shear damage.
When discussing agitation needs, shear rate depends on many variables, including impeller design, tip speed, distance to compartment walls, baffling, particulate concentration, particle size distribution, fluid density, plastic velocity, gel strength, and yield point, among others.
Airlift fermenter is the fermenter in which the circulation and aeration of the culture medium is achieved by injection of air into some lower part of the fermenter and is related to gas lift systems where an inert gas is used to achieve circulation in anaerobic conditions. The vessel has an inner draft tube (the concentric tube air-lift is common) through which the air bubbles and the aerated medium rise since aerated medium is lighter than non aerated one; this results in mixing of the culture as well as aeration. The air bubbles lift to the top of the medium and the air passes out through an outlet. The degassed liquid then flows down the annular space outside the draft to the bottom of the bioreactor. Cooling can be provided by either making the draft tube an internal heat exchanger or with a heat exchanger in an external recirculation loop.
Oxygen supply is quite efficient but scaling up presents certain problems. Thus both mixing and aeration can be achieved without mechanical stirring Airlift
is ideal for the production of monoclonal antibodies on a large scale.
Amongst other advantages of this air-lift design is low shear, but oxygen transfer is not as good as in mechanically agitated stirred tanks. Airlifts are increasingly being used for tissue culture, where the oxygen requirements of the culture are low, but the cells are fragile (the tissues are shear sensitive and normal mixing is not possible). The hydrodynamics of airlifts are complex and not yet fully understood.
Mechanical agitation (with a paddle or propeller) is the most commonly used mixing technique in the chemical process industry Mechanical agitation includes any mixing technology whereby an impeller, paddle, propeller, turbine, or ribbon is rotated within a process vessel to cause mixing or create dispersions. Impeller-based systems are generally classified as either radial-flow or axial-flow, depending on the flow patterns generated in the process vessel..It is less expensive than other mixing techniques for most industrial process vessels. Moreover, mechanical agitation appears to be considered excellent for many mixing applications (e.g., gas dispersion) based on the tremendous amount of research and process knowledge. Most tank mixing companies only sell mechanical agitation equipment. The objective of a properly designed mechanical agitation system is uniform suspension of all solids, appropriate application of shear, homogeneous fluid properties through out the system, and economical application of applied power. The major advantage of stirred fermenter is that the scale-up is not a problem.
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Stirred tanks (mechanically agitated fermenters) are the most commonly used fermenters. They are cylindrical vessels with a motor driven agitator to stir the contents in the tank. The Top-entry stirrer (agitator) model is most commonly used because it has many advantages like ease of operation, reliability, and robustness. The Bottom-entry stirrer (agitator) model is rarely used.
A major advantage of the stirred vessel over other designs and one, which may largely account for its popularity, is the degree of operational flexibility, which it provides even when installed. This arises largely because mixing and mass transfer are influenced both by the action of the impeller(s) and by the rate of aeration, which can, within fairly wide limits, be varied independently, albeit at the expense of changing the impeller speed or geometry. By contrast, in the airlift, mixing and mass transfer are both dependent, in a given piece of equipment, on the rate of aeration, and cannot readily be varied independently.
The need for mechanical agitation beyond oxygen mass transfer is not clearly understood in fermentation broths which have a viscosity before inoculation of a Newtonian fluid, but change to pseudoplastic (non-Newtonian) after growth starts. Air-agitated fermenters exist in industry today for a wide range of products. It is a viable alternative to mechanically agitated systems. The advantages are the following:
1) Improved sterility because of no top- or bottom-entering agitator shaft.
2) The design of the fermenters is not limited to the parameters such as Shaft length, size of the motor thereby large fermenters can be possibly constructed.
3) Refrigeration requirements are reduced 20 to 35% because of no mechanical agitation.
4) Usage of structural steel is less, as there is no need for agitator, gear box or crane rail and so cheaper fermenter design results.
5) No maintenance of motors, gear boxes bearings or seals.
6) The air-agitated fermenter is a variable mixing power unit, like a variable speed drive with no motor and drive noise.
7) Air compressors can be steam driven to reduce power cost and continue to operate during power outages in large plants that have minimal power generation for controls to achieve the high energy input required for aeration and mass transfer in large fermenters without the problems associated with massive agitators.
Airlift fermenters offer unique advantages over mechanically agitated fermenters in particulate solid substrate fermentation systems such as microbial desulfurization of coal or bacterial ore leaching (in general, for three phase (solid-liquid-gas) fermentation systems). Airlift reactors eliminate the potential particle grinding problems encountered in agitated reactors. Also, due to lower shear force in airlift fermenters, the extent of cell desorption from particle surfaces would be less as compared to agitated systems. Therefore, airlift fermenters are probably more suitable for kinetic studies on microbial utilization of particulate solid substrates. Among other advantages of airlift fermenters are:
They are easy to operate and more energy efficient compared to agitated fermenters.
The airlift fermenters require only compressed air for aeration and agitation and eliminate the need for mechanical agitation.
The oxygen transfer efficiency in some airlift fermenters (kg o2 transferred/kWh) is higher than mechanical agitated fermenters.
The many attractive features of airlift reactors have led to increasing usage of these devices in environmental remediation technology, the chemical process industry and the biotechnology-based manufacture. Airlift reactors have an established niche in high-strength activated sludge type treatment of wastewater where the high oxygen transfer capability, low power requirements and non-mechanical agitation are particular advantages of these systems. Bioremediation of soil fines in airlift devices is being investigated as a promising new pollution abatement application of this technology. Similarly, applications in treatment of gaseous effluents are expected.
Airlift reactors are often chosen for culture of plant and animal cells and immobilised catalyst because shear level are low.
COMPARISON OF NEOMYCIN PRODUCTION FROM STREPTOMYCES-FRADIAE CULTIVATION USING SOYBEAN OIL AS THE SOLE CARBON SOURCE IN AN AIRLIFT BIOREACTOR AND A STIRRED-TANK REACTOR
Streptomyces fradiae was cultivated in both an air-lift bioreactor and a 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 bioreactor 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-fermenter 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-fermenter at 600 rpm, but the neomycin yield was less than one-half. In the case of the jar-fermenter, the yield increased with increasing agitation rate and was maximum at 600 rpm. To maximise 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.
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Oxygen transfer and mixing in mechanically agitated airlift bioreactors:
Gas hold-up, mixing, liquid circulation and gas-liquid oxygen transfer were characterised in a large (1.5 m3) draft-tube airlift bioreactor agitated with ProchemÂ® hydrofoil impellers placed in the draft-tube. Measurements were made in water and in cellulose fibre slurries that resembled broths of mycelial microfungi. Use of mechanical agitation generally enhanced mixing performance and the oxygen transfer capability relative to when mechanical agitation was not used; however, the oxygen transfer efficiency was reduced by mechanical agitation. The overall volumetric gas-liquid mass transfer coefficient declined with the increasing concentration of the cellulose fibre solids; however, the mixing time in these strongly shear thinning slurries was independent of the solids contents (0-4% w/v). Surface aeration never contributed more than 12% to the total mass transfer in air-water
Application of an airlift bioreactor to the nystatin biosynthesis
Pilot plant studies were performed using a concentric-tube airlift bioreactor of 2.5 m3 fermentation volume. The results have proven the relative merits of such a system in the biosynthesis of nystatin, produced by Streptomyces noursei, in submerged aerobic cultivation and batch operation mode. The results were compared to those obtained in a pilot-scale stirred tank bioreactor of 3.5 m3 fermentation volume.
The fermentation processes in the two fermentation devices were similar with respect to substrate utilization, biomass production and nystatin biosynthesis
In the riser section, the dissolved oxygen concentration was higher than that in the downcomer. The volumetric oxygen mass transfer coefficient was dependent on the rheological behaviour of the biosynthesis liquids, which was not constant during the fermentation process. The total energy consumption for nystatin production in the airlift bioreactor was 56% of that in the stirred tank, while the operating costs represented 78% of those in the stirred tank bioreactor.
Mass cultivation of catharanthus roseus cells using a nonmechanically agitated bioreactor
Batch suspension cultures of Catharanthus roseus G. Don were grown in a 5 L LKB Ultraferm fermenter, converted to operate as an airlift bioreactor, to test the suitability of such a system for the mass culture of plant cells. Results show that the airlift system has considerable merits as a culture vessel for such a purpose, including: conversion rates of carbohydrate substrate to cell mass equivalent to > 50% under optimum conditions. (Operating under these conditions, growth rates of approximately 0.4 d-1 are typical). In the absence of the mechanical shear normally associated with mechanically driven bioreactors, the gently agitated environment of the airlift vessel proves to be an ideal system for the growth of fragile plant cells. Use of a nozzle sparger reduces the possibility of a high mass transfer coefficient, except at very high gassing rates, thereby eliminating any interference with the growth rate caused by high rates of gaseous exchange.
Comparing the production of lipase by Geotrichum candidum in mechanically agitated stirred fermenter and air-lift fermenter.
The studies revealed that there were similar lipase yields in both fermentations but in airlift type, its cost effective as energy consumed is less which could be achieved in less time, i.e.: 30 hrs of fermentation and the productivity was more than 60% compared to the mechanically agitated stirred fermenter wherein it took 54hrs of fermentation to achieve the same yield.
Air-Lift Bioreactors for Algal Growth on Flue Gas:
Air-lift reactors have great potential for industrial bioprocesses, because of the low level and homogeneous distribution of hydrodynamic shear. One growing field of application is the flue-gas treatment using algae for the absorption of CO2. The measured removal efficiency of CO2 was significant (82.3 Â± 12.5% on sunny days and 50.1 Â± 6.5% on cloudy days) and consistent with the increase in the algal biomass.
Cultivation of a filamentous mold in a glass pilot-scale airlift fermenter:
Results of pilot plant studies using a glass airlift fermentation device (55 litre fermentation volume) have proven the relative merits of such a system in the fermentation of a filamentous mold, Monascus purpureus, on 4% (w/w) starch media. The resultant overall yield of cell mass (Yx/s) of 0.38 was an appreciable increase over the 0.32 obtained with a pilot scale stirred tank fermentor previously studied. Power requirements of the airlift fermentor were approximately 50% of those for the mechanically agitated system. The lack of mechanical shear in the airlift system provides a more gentle environment or the cultivation of organisms than does the high degree of shear prevalent in the mechanically agitated vessels. Mass transfer of oxygen to the aqueous phase of the fermentation volume is improved significantly through use of the airlift device. Mass transfer coefficients in the range of 200 reciprocal hr were obtained to approximately 80 reciprocal hr in the stirred tank fermentor.
Investigation of the bacitracin biosynthesis in an airlift bioreactor:
Results of pilot plant studies using an external-loop airlift bioreactor have proven the relative merits of such a system in the bacitracin biosynthesis produced by the Bacillus licheniformis submerged aerobic cultivation.
The results were compared to those obtained in a pilot-scale stirred-tank bioreactor with the same values of kLa. Excepting the aeration rate of 0.2 vvm, the fermentation process performed at 0.5 vvm and 1/0 vvm, respectively, unfolded similarly in the two fermentation devices with respect to the cell mass production, substrate utilization and bacitracin production during the fermentation process.
In the riser section of the airlift bioreactor, the dissolved oxygen levels were higher, while in the downcomer section they were lower than those realized in the stirred tank bioreactor.
Power requirements of the airlift fermenter were by 17-64% lower than those for a mechanically agitated system, depending on the aeration rates, which led to an important energy saving
Moreover, the lack of mechanical devices in the airlift system provides safety and a more gentle environment for the cultivation of microorganisms.
Growth and development of fern gametophytes in an airlift fermenter:
Spores of the fernsPteridium aquilinum andAnemia phyllitidis were grown in an airlift fermenter and subsequent growth and development of gametophytes was monitored. Both species produced greater biomass than that generated in any other solid- or liquid-based culture system tested.Pteridium generated more tissue thanAnemia in every system. The morphology of airlift-grown gametophytes was similar to that of soil-grown plants; fewer gametophytes with perturbed development were observed in airlift cultures than in the other liquid-based systems. No attempt was made to optimise airlift conditions for the species and tissue employed, so it is concluded that airlift cultivation is a promising system for the bulk production of fern gametophytic tissue.
Mixing by Air Agitation in Horizontal Milk Tanks:
The results of laboratory experiments were applied to larger tanks and comparisons made of air and mechanical mixing. Cheese colour was blended with water to study the efficiency of air and mechanical agitation in horizontal tanks. An air supply pipe with uniformly spaced holes required longer mixing than one with a single hole at the centre. Mixing times decreased with increased air rate or liquid depth. For mixing a cream layer into milk, air and mechanical agitation were equally effective, but for standardising, mechanical agitation gave uniform composition more rapidly than air agitation. Mixing cheese colour into water provided useful information for blending fluids with similar physical properties, including cream and milk mixtures.
Scientists at Marlow Foods are currently growing F.graminearum in an air-lift fermenter.. Air-lift fermenters have no moving parts and use the difference in specific gravity of aerated culture in the riser and the air depleted culture in the downcomer to obtain continuous circulation of the culture around the fermenter loop.In the production of mycoprotein,a complete flow cycle is achieved in about 2 minutes.The use of this continuous culture system ensures that F.graminearum for mycoprotein production is cultivated under environmental conditions which remain constant.
Biological activity depends basically on the microenvironment surrounding each cell.In practice,control of the microenvironment must be exercised rather indirectly through control of the overall conditions in the apparatus in which the process is conducted.It is clearly necessary to ensure that the overall supply of nutrients is adequate and that there should be provision for the removal of excess heat and of volatile products of metabolism,but this is insufficient to ensure optimal activity unless local variations in conditions can be kept within acceptable limits.Mechanical agitation is frequently employed to meet this latter need,but its effects are imperfectly understood and inadequately characterised.
Large-scale fermenters, for energy savings in production equipment, use air-agitated fermenters. The cost savings are not apparent when comparing the cost of operating a fermenter agitator to the cost of the increased air pressure required. However, when the total capital and operating costs of fermentation plants (utilities included) for the two methods of fermentation are compared, the non-mechanically agitated fermenter design is cheaper. The questions are, How much mixing horsepower is available from aeration, versus how much turbine horsepower is effective for aeration and mixing? Scale-up of an agitated fermenter "both kLa and gas hold-up" increase with an increasing gas rate and agitator speed. Most of the agitator's power is spent in mixing the fluid.
Consequently, there is a continuing need for improved fermenters of even larger capacity than those built to date, and capable of conducting aerobic fermentation processes at high cell densities and high productivities. At the same time, a fermenter which does not require moving parts for effective foam control, which is simple in construction, economical to manufacture and to maintain, and which gives good results in terms of achievable oxygen transfer rate and power consumption would provide obvious advantages.
The airlift fermenter is likely to find increased applications for production "of "bulk products, since it has several advantages for large scale, aerated fermentations. The construction is suitable for large volume fermenters and the oxygen transfer efficiency is high. In addition, because the power for agitation and aeration is supplied to the air compressor, it is possible to use alternative sources of power (other than direct electric drive) depending on local availability and costs.