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Design of a Fermenter
Fermenter or bioreactor refers to a device that provides all the basic necessities important for biological product extraction. A fermenter contains different devices that help to maintain the environmental factor inside it which in turn leads to the production of biological products. Therefore the main objective of a fermenter is to maintain a controlled environment that supports the growth of the bacteria or any other organism. There are several important factors that need to be accurate to design a fermenter. Those factors are following
- The vessel should be well equipped to maintain aseptic conditions inside it for a number of days.
- Aeration and agitation are important for the production of biological metabolites. However, controlled agitation is required to prevent any damage to the cells.
- It should be less expensive in terms of power consumption.
- Temperature is an important environmental factor required for microbial growth. Therefore, a temperature control system is required.
- Optimum pH is important for the growth of the organism; therefore, the fermenter must be equipped with a pH controller.
- The fermentation of a huge culture is a time-consuming process. It needs to be contamination-free until the process is complete. Apart from that, it is also important to monitor the growth rate of the organism. Therefore, an aseptic sampling system is needed to design a fermenter.
- The fermenter vessel should be designed properly to minimize the labor involved in cleaning, harvesting, etc.
- It should be designed in such a way that it reduces evaporation.
- The vessel needs to be equipped with a smooth internal surface to support adequate mixing.
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As fermentation required adequate aseptic conditions, for better yield of biomass or product, it is important to select a material for the body of the fermenter, which restricts the chances of contamination. Moreover, it needs to be non-toxic and corrosion free. Glass is a material that provides a smooth surface inside the vessel and also non-toxic in nature. Apart from that, it is corrosion-proof and due to the transparency, it is easy to examine the inside of the vessel. There are mainly two types of glass fermenters
- A glass vessel with a flat bottom and a top plate with a diameter of 60 cm (approximately). Sterilization of this type of vessel is performed by general autoclaving. Borosilicate battery jars were used as large glass vessels.
- The second glass vessel contains stainless steel plates at the top and bottom of the glass vessel. In situ sterilization is possible for this type of glass vessel. However, it is more expensive than the glass top vessels due to the use of stainless steel plates.
The main disadvantage of glass vessels is that it is difficult to top to design a pilot-scale fermenter with glass. It is difficult to handle glass as a pilot-scale fermenter. Therefore, another non-toxic, corrosion-proof material, stainless steel, was used for pilot scale fermenter. According to Americal Iron and Steel Institute, steel contains more than 4% chromium is standardized as stainless steel. However, the minimum amount of chromium required to protect the steel from corrosion depends on the corroding agent present in a specific environment. In a pilot-scale fermenter normally the steel contains around 10-13% of chromium. In many cases nickel is also mixed in high concentration with the chromium to make the steel more corrosion resistant and it also provides engineering advantages. In this modern-day, stainless steel fermenters are mostly used for industrial production. However, small scale production requires glass vessels.
During the fermentation process heat can be produced mainly in two ways, firstly microbial biochemical reactions and secondly mechanical agitation. In case of fermentation, a temperature control helps to control the temperature at the optimum level by removing or providing heat. In small scale production vessel the amount of produced heat is negligible. Therefore, extra heat is provided by hot bath or internal heat coil or heating jacket with a water circulation system or silicon heating jacket. The silicon heating jacket consists of silicon rubber mats with heating wires and it is wrapped around the fermenter. In the case of pilot-scale fermenters, it is not possible to use silicon jackets due to large size. In such cases, an internal heating coil is used for providing extra heat while cold water circulation helps to remove excess heat.
The objectives of the impeller used in fermenters are bulk fluid and gas mixing, air dispersion, heat transfer, oxygen transfer, suspension of solid particles, maintain the uniform environment inside the vessel, etc. Air bubbles often cause problems inside the fermenter. Impellers involved in breaking the air bubbles produced in a liquid medium. There are mainly three types of agitators used in industrial-scale bioreactors
- Disc Turbine: It consists of a disc with a series of rectangular vanes connected in a vertical plane around the disc.
- Vaned disc: In this case, the rectangular vanes are attached vertically to the underside of a disc.
- Variable Pitch open turbine: This type of agitator lacks disc and the vanes are directly connected to a center shaft.
Stirrer glands and bearings
The most important factor of designing a fermenter is to maintain aseptic conditions inside the vessel. It is highly challenging in the case of pilot-scale fermenters. Therefore stirrer shafts are required. These stirrer shafts play an important role to seal the openings of a bioreactor. As a result, it restricts the entry of air from outside. There are several types of seals used for this purpose, which are following
- The Stuffing Box: The shafted is sealed by several layers of packing rings of asbestos or cotton yarn which is pressed against the shaft by gland follower. At high stirrer speeds, the packing wears quickly and excessive pressure may need to ensure the tightness of fit. The packing may be difficult to sterilize properly because of unsatisfactory heat penetration and it is necessary to check and replace the packing rings regularly.
- The Mechanical Seal: It is used in both small scale and large scale fermenters. The seal is divided into two parts, first is the stationary bearing housing and the second rotates on the shaft. These two parts are pressed together by springs. Steam condensate is used to lubricate and cool the seals during operation and provides protection against the contamination.
- Magnetic Drives: This type of seals helps to counter the problem originated by the impeller shaft which is going through the top or bottom of the fermenter plate. The magnetic drive is made up of two magnets one is driving and one driven. The driven magnet held in bearings in housing on the outside of the head plate and connected to a drive shaft. The internal driven magnet is placed on one end of the impeller shaft and held in bearings in a suitable housing on the inner surface of the head plate. When multiple ceramic magnets have been used it has been possible to transmit power across a gap of 16mm. Using this drive water can be stirred in baffled vessels up to 300 dm3 capacity at speeds of 300 to 2000 rpm.
There are four baffles that are present inside of an agitated vessel to prevent a vortex and improve aeration efficiency. Baffles are made up of metal strips roughly one-tenth of the vessel diameter and attached to the wall. The agitation effect is slightly increased with wider baffles but drops sharply with narrower baffles. After installation of the baffle there a gap between them and the vessel wall which facilitates scouring action around the baffles and minimizes microbial growth on the baffles and the fermenter wall. Baffles are often attached to cooling coils to increase the cooling capacity of the fermenter.
The aeration system (sparger)
A sparger is a device that introduces air into the liquid medium in a fermenter. There are three main types of fermenter used in industrial-scale bioreactors such as
- Porous Sparger: It is made up of sintered glass, ceramics or metals’ and are mostly used in laboratory-scale bioreactors. As it introduces air inside a liquid medium, bubbles are formed. These bubbles are always 10 to 100 times larger than the pore size of the aerator. The air pressure is generally low in these devices and a major disadvantage of using porous sparger is that microbial growth may occur on the pores which hamper the airflow.
- Orifice Sparger: These are used in small stirred fermenters where perforated pipes are used and attached below the impeller in the form of a ring. The air holes are mostly drilled under the surface of the tubes. Orifice spargers were used to a limited extent in yeast manufacture, effluent treatment and production of single-cell proteins.
- Nozzle Sparger: This is used in industrial-scale fermenters. The main characteristic of this kind of sparger is that it contains a single open or partially closed pipe as an air outlet. The pipe needs to be positioned below the impeller. The design helps to overcome troubles related to sparger blockage.
pH control sensors
All types of fermenters are attached with a pH control sensor which consists of a pH sensor and a port to maintain the pH inside of the fermenter. pH alteration can lead to death of the organism which leads to product loss. Therefore, it is a crucial instrument for a fermenter and needs to be checked regularly.
Principles of fermentation technology (2nd Edition) by P.F. Stanbury. A. Whitaker and S.J. Hall.
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