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How to Design a Fermenter: Provisions, Features and Vessel!
A process in which a chemical product of human utility is produced involving microorganisms is called as fermentation. The vessel in which it is carried out is known as fermenter. An ideal fermenter should provide congenial environmental conditions, which promote the optimum growth of an organism and produce maximum product. Fermenter plays a critical role in the product yield. Hence design of a fermenter is important in the process of fermentation.
The fermenter may be simple for fermentation products, which does not require aseptic, conditions, while fermenter requiring an aseptic conditions have to be designed to prevent interference by the contaminating organisms.
Fermenter supports best possible growth, biosynthetic conditions and ease of manipulation for all operations associated with the use of it.
For the better production of a desired product an ideal fermenter should have following provisions:
1. Fermenter body
2. Agitator
3. Coil
4. Gas outlet
5. Inoculation port
6. Thermometer
7. pH electrode
Characteristic Features of a Good Fermenter:
1. It must be strong enough to withstand pressure of large volume of aqueous medium.
2. It must not corrode and contribute toxic ions to the growing microorganism.
3. Make provision for control or prevention of growth of contaminating microorganisms.
4. Provision for rapid incorporation of sterile air into the medium.
5. Carbon dioxide released during fermentation must be flushed out.
6. Stirrer must be available to mix the medium and microorganisms to facilitate the availability of nutrients and oxygen.
7. Intermittent addition of antifoaming agent.
8. Provision for controlling temperature.
9. Aseptic withdrawal of culture sample during fermentation and introduction of inoculum at the initiation of fermentation.
10. Determination of the pH and its adjust mentill, if required.
11. Some means of sterilization of medium and addition of antifoaming agent.
12. Air filter.
13. Drain in the bottom.
14. Access to the inside of the fermenter to clean it.
Fermenters used in microbial fermentation represent a wide range of devices from a simple type of tube aeration system fermenter operating on the air lift system or the deep jet principle or devices with rotary stirrers in which the air is sucked in or distributed under pressure into the stirrer space.
There are wide variety of designs of fermenter (bioreactor) available. Selection of a fermenter design for a particular process depends on a variety of factors such as mass transfer considerations, mixing, sheer sensitivity, broth viscosity, oxygen demand, reliability of operations, sterilization considerations, the cost of construction and operation.
Stirred tank reactors use sparged air and submerged impellers to aerate and mix the broth. They are versatile and are specially adapted to highly aerobic cultures and highly viscous fermentations. Even in this, there are many variations in design such as the style, number and placement of impellers, the height to diameter ratio, the number and placement of coils or baffles that affect the mixing characteristics of the vessels. The main drawbacks are high-energy input and the use of rotating scale on the agitator shaft which may cause contamination risk.
Airlift fermenters (Fig. 2.5) mix broth with air from the sparger.
Some designs have an internal shaft tube to direct the flow of fluid. Most airlift designs have a much greater height to diameter ratio than stirred tank vessels to improve oxygen transfer. The mixing is not as good as in a stirred tank but the energy input and shear forces are much lower, thus, useful for shear sensitive cultures or in processes where the energy cost of agitation is a significant factor.
Ability to clean the vessel and maintenance cost are important factors for the selection of a bioreactor. Some reactor designs have excellent characteristics but in the pilot plant are not good choice for larger scale operation due to mechanical complexity that causes sterility and maintenance problem on scale up. Most large-scale airlift fermenters are used for plant effluent treatment production or for baker’s yeast or for fungal fermentations where the size of the mycelial pellets is controlled by shear forces.
Fermenters with mechanical stirrers are used to mix the reactant mixture and they are called stirred tank fermenters (Fig. 2.6a).
Fermenter with a draft tube is a hollow perforated tube that improves circulation and oxygen transfer. The air is introduced from the bottom of the fermenter that lifts the draft tube and it is known as Airlift Fermenter (Fig. 2.6b). The fermenters can also fluidize its bed where the microbial cells are immobilized on small particles. These particles move along with the fluid and as a result, nutrient easily stick to it that enable high rate of oxygen and nutrient transfer to cells.
On the other hand, flocculated or packed bed reactor (fermenter) contains larger particles which immobilize cells and cannot move along with the liquids (Fig. 2.6c). The reactor can be operated in either up-flow or down-flow mode that is the liquid containing the substrate can be introduced either at the top or the bottom of the reactor. These type of fermenters are employed in sewage treatment where cells are immobilized by flocculation.
They may also be used for bioconversion of small molecule. Bubble column fermenter (reactor) is another type of reactor in which the agitation and aeration are provided by a bottom sparger. To ensure even agitation, the sparger nozzles must be distributed uniformly over the cross-section of the bottom. Either a ring with regularly spaced holes, a small number of parallel pipes or star like arrangement of pipes is used (Fig. 2.6d).
According to the size they may be classified as laboratory fermenter (Fig. 2.7), 500 ml to 50 liters in volume, pilot plant with 50 to 500 liters in volume and production fermenter with 500 liters and above one lakh liters (Fig. 2.8).
The fermenter shape may vary from cylindrical to spherical, to tubular usually with a D-shaped bottom. It is closed at the top and bottom.
The material with which a fermenter is constructed vary according to the type of fermentation process. For example, fermentation of alcohol and lactic acid is carried out in a wooden fermenter, where sterilization is not necessary or where there is no chance of corrosion of inner lining of the fermenter.
However, present day fermenters are constructed with inner surface lined with stainless steel or copper or iron or glass, which are chemically inert. Normally fermenters upto 1000 litres capacity have an external jacket and larger vessels have internal coils. Both, provide a mechanism for vessel sterilization and temperature control during the fermentation.
1. Provision for Control of Microbial Growth:
Since most industrial fermentations utilize pure cultures, fermenters should be designed in such a way that promotes luxuriant growth of microbe but prevent the growth of contaminating microorganisms.
2. Provision for Incorporation of Sterile Air (Oxygen):
Most aerobic fermentation processes requires oxygen supply, which is called as aeration. Aeration is done by passing sterile air under pressure into the fermenter. The required air is sterilized by passing it through a sterile filter consisting of glass wool or some other finely powdered material that help in trapping microorganisms present in the air. The sterile air bubbled into the liquid medium through a sparger in order to make oxygen distributes uniformly in the medium.
3. A Device for Removal of CO2:
During fermentation process carbon dioxide and hydrogen gases are liberated which collect in the head space of the fermenter. The fermenter should be provided with a device to release these gases outside aseptically.
4. An Impeller:
An impeller, a rotating device, is generally provided to most of the fermenters, which accomplishes vigorous stirring and agitation of the medium. The rotation is carried out either by indirect or direct methods. In indirect method, the impeller is mounted on a shaft, which is driven by an electric motor fitted at the top of the fermenter.
In direct method, impeller action is varied by using different impeller blades and is driven by a magnetic coupling fitted to a motor which is mounted beneath the fermenter. The impeller blades are arranged at different heights to achieve vigorous stirring and agitation of the medium.
5. A Device for Addition of Antifoam Agent:
Aeration and agitation of the liquid medium causes the production of foam. Media with high levels of proteins or peptides cause more foam than with pure sugars and inorganic salts. Proteolytic bacteria that degrade proteins into peptides and amino acids also produce more foam. Appearance of foam leads to problems like contamination of the medium and impediment of aeration.
The formation of foam is undesirable, can be prevented by the addition of antifoam agents. An antifoam agent lowers surface tension of the foam and thereby it collapses, which leads to the disappearance of foam. Antifoam agents may be added to the medium either manually or mechanically. Manual addition requires the presence of some device in the fermenter to add antifoam agent aseptically, whenever needed.
In mechanical addition, which is done automatically, an electrical sensing mechanism is provided at the top of the fermenter. It consists of two electrodes projecting into the headspace of the fermenter. They are connected to a pump of antifoam reservoir. When foam builds up in the headspace and touches the electrodes, current flows between the electrodes and activates the pump for addition of antifoam. When foam collapses the electrodes get disconnected and addition of antifoam ceases.
6. A Device for Temperature Control:
Microorganisms widely differ in their temperature dependence for growth. However, they grow well at optimum fermentation temperature, which may be below or above ambient temperature. During fermentation lot of temperature is generated due to metabolic activities of microorganism, which leads to a rise in the temperature in the fermenter. For maintaining optimum temperature in the fermenter, one of the following devices is provided (Fig. 2.9).
i. Sparging cold water on the fermenter,
ii. By circulating cold water through the jacketed walls of fermenter, or
iii. Through coils arranged along the inside walls of the fermenter.
7. A Device for Addition or Withdrawal of Inoculum:
The fermenter should also be provided with a device to introduce inoculum at the beginning of the fermentation, and its withdrawal aseptically during fermentation.
8. A Provision for pH Adjustment:
pH, which plays an important role in the growth and metabolism of microbes, influences fermentation process. A mechanism for determining pH of the fermentation broth intermittently and adjusting the values is often required. This is usually accomplished by withdrawing a sample from the fermenter for pH measurement, followed by addition of alkali or acid to the fermentation medium to adjust pH.
9. Seed Tanks:
Inoculum of 1-10% is required to inoculate production tanks to reduce incubation period. They are also called as inoculum tanks. They are generally small sized fermenters in which inoculum is produced under controlled conditions.
10. Medium Preparation Vessel:
Fermentation requires additional vessels for the preparation of medium. Required nutrients for the medium is transferred to the fermenter from these vessels.
11. Sterilization of the Medium:
Most of the fermentations require pure culture and needs sterilized medium. For this purpose, the medium is passed through retention tubes and heat exchangers before passing into the large, empty and sterilized fermentation tank. The retention tubes contain steam waters jet that inject high pressure steam into the medium to sterilize it as it passes through the pipes and the rate of passage is adjusted in such a way that there will be complete sterilization.
The heat exchanger, which consists of a pipe containing the medium within a second pipe containing cool water moving in the opposite direction, cool the medium before it is passed into the fermenter. After entry into the fermenter the medium is diluted with sterile water.
12. Device for Withdrawal of Used Medium:
There must be a device at the bottom of the fermenter or some mechanism may be provided for removing the completed fermentation broth from the tank. The fermenter should be accessible for cleaning after fermentation is completed.
Fermentation system must be efficiently controlled in order to optimize productivity, product yield and ensure reproducibility. The key physical and chemical parameters involved largely depend on the bioreactor, its mode of operation and the microorganism being used. They are primarily aeration, mixing, temperature, pH and foam control. The control and maintenance at optimum levels inside the reactor is mediated by sensors (electrodes) along with compatible control systems and data logging.