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A fermenter is an apparatus that provides suitable conditions for growth of microbes and prevents entry or growth of contaminating microbes. Fermenters can be defined as a medium that induces fermentation. Basically fermenters are used in large scale and commercial production of food, antibiotics and hormones. Fermenters in fermentation processes exhibit the basic fundamental characteristic of establishing an appropriate balance of bacteria for the desired product. The process of fermentation can be divided into three stages.
- Upstream Processing: The upstream processing is the initializing step where the liquid for the medium is prepared. Chemicals involved in the process of fermentation for particulate selection, sterilization and purification are selected.
- Fermentation: One of the critical stages of complete fermentation process. This stage mainly relies on fermenters. The desired product of the process in dependent on this stage of fermentation. Fundamental step carried out in this stage is ‘conversion'. The substrates are converted into desired product with the help of microorganisms.
- Downstream Processing: The downstream process is the final determining stage in fermentation. Downstream mainly deals with purification and adjusting appropriate parameters like concentration, PH of the desired product.
Major components of fermenters are
- Drive motor, heater pumps and gas controller.
- Microbial cells.
- Vessel and its corresponding accessories.
- Microbial enzymes.
- Microbial metabolites
- Primary metabolites: Ethanol, citric acid, lysine and vitamins.
- Secondary metabolites: All antibiotic fermentation.
There are several fermentation processes available to produce desired products. Out of which the batch fed and continuous fermentation processes are commercially viable.
A tank of fermenter is filled with raw materials. The temperature and microbial pH is adjusted appropriately with respect to desired product. The start to end process is controlled by a ‘batch fed fermenter'. With recent advances a batch fed fermenter is robust non linear fed controller apparatus that analyzes and controls the desired product by tracking (Chidambaram, 2001). At regular intervals the nutritive supplements are added in batches. The later stages constitute the process of sterilization and purification.
Continuous fermentation process
Generally the continuous fermentation process is carried out in towers in sterile conditions. The upper end of the tower is normally sterilized with steam. The fermentable and raw materials are added to the lower end of the tower. Unlike the batch fed fermentation the continuous fermentation implies the continuous intake of nutrients rather than in batches.
The growth of the microorganisms in batch fed is shows a growth curve which then correspond to lag phase which is followed by a logarithmic phase.
Production of commercial products, large scale industries use a typical ‘industrial fermenter'. Industrial fermenters have a capacity 2,00,000 litres of cultures. Specific nutrients are induced to cells in a controlled environment for the desired growth. As these fermenters are of commercial use, industrial fermenters are made up of stainless steel mainly to produce to acid. The internal component of industrial fermenter is integrated with sterilisble probe which record and analyze parameters like the pressure, stirrer speed pH and oxygen.
An agitator is an apparatus that initiates shaking, stirring or mixing in a vessel by motion. The most common example of an agitator is the working mechanism of washing machine. The production of desired product in a fermentation process primarily depends on the agitator used. The basic design of an agitator constitutes 2 agitator blades which are appropriately arranged radially to the axis of rotation. These blades are inclined at an angle of attack in the direction of the rotation relative to the axis of rotation. The angle of attack in the vessel varies continuously or step wise
- Top end: changes to 25 to 35 degrees.
- Lower end: Changes to 55 to 65 degrees.
An agitators is mainly used in a process for two reasons
- “Creating necessary hydrodynamic conditions in the vessel.
- Simplicity of construction” (V belynskii,1976).
While selecting a agitator for commercial use the basic parameters like the recirculation factor, uniform distribution of velocity gradients and turbulent pulsations are considered. The operating efficiency of an agitator can be derived or calculated directly by the power consumed.
Air lift agitators utilize compressed air to continuously recycle the slurry. One of the components' of an air lift agitator is ‘pipe', which requires simple piping in vessel. Unlike other agitatating apparatus or devices' air lit agitators have no moving parts and have negligible maintenance. One of the significant characteristics of an air lift agitator is that it provides with homogenous slurry.
Air lift agitators are mainly used in air lift bioreactors or fermenters. Basically airlift bioreactors are tower shaped reactors; most of the reactions carried out in these reactors are aerobic and run under optimal conditions. Air lift agitators are used in airlift bioreactors when the microorganisms must be cultivated in large plants, generally around > 500 m3. “Airlift bioreactors generate a homogenous mixture or apparatus of the growth medium and aeration by injecting air by an air lift agitator into the lower part of the bioreactor vessel”. As shown in figure 6
The movement of air is initiated from the bottom of the bioreactor vessel which produces a re-circulating flow. The air provided to the bioreactor vessel through the airlift agitator lowers the density of the broth and causes it to rise near to the draught tube. The gases which are subdued within the broth are separated and taken out from the top of the bioreactor vessel. As these gases are moved out the broth density increases, the remaining broth re-circulates and resides down to the bottom end of bioreactor vessel. The bioreactor is then subjected with oxygen, as the waste gases are eliminated; the bioreactor is controlled by the diffused gases (bubbles). For maximum efficacy and efficiency airlift bioreactors follow a height to diameter aspect ratio. Generally 8 : 1 ratio is considered which give these bioreactors a tall and slender vessel for enhanced mixing.
“A mechanical agitator can be defined as an agitating device for mixing a sample within a sample container”.
Usage: It is usually applied to infusion, oral liquid, blood preparation, injection biology, fine chemical and fermentation industries.
Technical parameters: Material adopts 316L or 304, inside surface roughness is Ra0.4µm, and outside surface roughness is Ra0.8µm.
Agitation mode: top mechanical agitator.
Impeller type: Propeller type, Spiral type, anchor type or according to client's specific requirement.
Jacket form: dimple jacket, full jacket or coil jacket.
Insulation material adopts rock wool, polyurethane or pearl cotton.
Components: vent filter, sight glass, pressure gauge, manhole, cleaning ball, wheel, thermometer, liquid level gauge, and control system or according to clients' specific requirement.
Some major components of a mechanical agitator are
- Base: Provides with solidity and rigidity.
- Container holder: Basically used for holding a sample container. The sample container moves corresponding to the movement of the base.
- Agitation drive unit: The agitation drive unit is placed on the base specifically to provide the agitation drive force to the corresponding container holder.
- Agitation vessel: Increases the energy transmission efficiency in an agitation apparatus. The retained liquid and the raw materials are agitated by sound waves.
Mechanism of action of a mechanical agitator: Stirred tank bioreactors
Stirred tank bioreactors use mechanical agitating devices like impellers and baffles. The working mechanism of mechanically agitated stirred tank bioreactor completely reciprocates to the working mechanism of an airlift bioreactor. The basic function is performed by a mechanical agitator is turbulence. Turbulence mainly allows the finer control of oxygen and appropriate mass transfer processes. Like airlift bioreactors, stirred tank bioreactors also follow dimension constraints. Generally, height to diameter ratio is 3: 1 stirred tank bioreactor considers.
As many mechanical agitating devices (impellers or baffles) can be used, the internal configuration of the vessel directly corresponds to the mixing and final product of the respective process. Commonly in stirred tank bioreactors a rotating impeller and a fixed baffle is used for agitation, approximately around 4 - 6 baffles and a series of impellers are used. There variety of agitators available which agitate mechanically, the specific type of agitator to be used directly depends on the microorganism. There are two basic types of impellers in stirred tank bioreactors
- Axial dispersion: The fluid in the vessel is moved to the top of the vessel. This movement is similar to the movement of the ship propeller.
- Radial flow: The fluid in the tank is moved laterally.
These two basic types are used individually or in combination. The agitating power to the mechanical agitator system is provided by an external motor, the external motor is either placed at top or at the bottom of the bioreactor vessel (Williams,2002).
Relative merits of airlift agitators
When bioreactors use airlift agitator for agitation these systems have some advantages over conventional systems which use mechanical agitators. Air lift systems have no mechanical pumps or agitators, so there is no need external electrical supply. Air lift agitators have no moving parts; additional attachment of shaft seal is not required. The phenomenon of ‘no moving parts' provides with extended advantage of less maintenances. The lesser are the number of parts moving the lesser are the chances of defects occurring in components of airlift systems. An airlift agitator system provides with easier means of sterilization. Airlift systems have a lower shear rate. Shear rate is defined as a gradient of velocity in a flowing material. As the airlift agitator have low shear rate this provides with flexibility, this allows with options of growing cells of both plants and animals in a single system.
The phenomenon of ‘gas phase disengagement' is successfully exhibited by an airlift agitator. As explained in the working mechanism of air lift bioreactor waste gases are eliminated from top of the vessel, this characteristic of gas phase disengagement is accomplished by an airlift agitator. The air in the airlift bioreactor is introduced into the vessel by an air sparger from the bottom end. This specifically provides with large interfacial contact area. The interfacial area can be defined as area where two materials are brought to contact. The large interfacial area generated by an airlift agitator is generally used in heat transfer calculations. When two materials are in contact it's with lower energy levels. The flow air to the medium can be controlled manually or automatically using an optimization device. Control of the flow of air is helpful in mixing.
Air lift agitator postulates quality residence time for each of the phases in a bioreactor. As an air lift agitator assert enhanced oxygen solubility this gives impetus to mass transfer in large tanks with greater pressure. An airlift agitator has the capacity to perform the basic function of agitation in large tanks, thereby increasing the final output (salser, 1973).
Airlift agitators in bioreactors have some undue disadvantages. One of them is them that, air lift agitating systems have longer growth period. Due to this there is a chance of cell contamination or cell mutation. As the growth period is longer the bioreactor system needs to be checked and maintained at regular intervals. Airlift agitators do not support the growth of filamentous organisms, as these organisms require a homogenous mixture. Most of airlift agitators generate a viscous and a heterogeneous mixture.
Relatives merits of mechanical agitator
Mechanical agitator when used in bio reaction systems have some relative benefits. Mechanical agitators like impeller, propeller or turbines when used reduce the risk of cell contamination or cell mutation. The reduced risk of contamination is mainly due to the brief growth period the cells have in such bioreactor systems. They also provide with higher conversion of growth materials in the system. Bioreactor systems which are installed with mechanical agitators have lower capital investment when compared with air lift systems. Mechanical agitators have flexibility of choices, which provides with options of on working in varying biological systems.
A mechanical agitator permits manipulations to the growth rate of cells. Such manipulations increase the growth rate and maintenance of the entire system. The dilution levels of the whole system can be varied, this provides with new opportunity to control the whole biomass concentration and production of secondary metabolite production. The basic functions integrated with mechanical agitators are of system investigation and analysis, this is because of the variables used are the same and certain checkpoints are used in the process.
Mechanical agitators relatively have high friction which directly corresponds to the optimal hydraulic diameter of the whole vessel. Friction helps in the purpose of rising and then coming down to the bottom of the vessel. A mechanical agitator maintains consistent level of oxygen, nutrients and substrate. The levels of major constituents are consistently maintained even when they are subjected to changes in the conditions. Bioreactors that use mechanical agitators have multiple feed points for nutrients in the vessel.
A mechanical agitator in food processing is generally used for mixing and blending. Mechanical agitators successfully agitate the liquid mixture with induced solid suspensions in it. These agitators mix in such a manner, so that there is no damage to solid suspension (pardo, 1988).