Design of a Fermentor
- A bioreactor is a specially designed vessel which is built to support the growth of high concentration of microorganisms.
- It must be so designed that it is able to provide the optimum environments or conditions that will allow supporting the growth of the microorganisms.
- Bioreactors are commonly cylindrical vessels with hemispherical top and/or bottom, ranging in size from some liter to cube meters, and are often made of stainless steel and glass.
- The difference between a bioreactor and a typical composting system is that more parameters of the composting process can be measured and controlled in bioreactors.
- The sizes of the bioreactor can vary over several orders of magnitudes. The microbial cell (few mm3), shake flask (100-1000 ml), laboratory fermentor (1-50 L), pilot scale (0.3-10m3 ) to plant scale (2 — 500 m3) are all examples of bioreactors.
- The vessel is a single-walled cylinder of borosilicate glass with a flat bottom or as a glass- jacked system having a typically round bottom.
- The top plate is made from stainless steel and is compressed onto the vessel flange by an easily released clamping system.
- The vessel glass and the top plate is separated by a seal. Port fittings of various sizes are provided for insertion of probes, inlet pipes, exit gas cooler, cold fingers, sample pipes, etc.
- These work by compressing the sides of the probe/pipe against the seal. Culture can be withdrawn into a sampling device or a reservoir bottle via a sample pipe situated in the bulk of the fermentor fluid.
- A gas sparger is also fixed into the top plate and this terminates in a special assembly, which ensures that incoming air is dispersed efficiently within the culture by the flat-bladed ‘Rushton-type’ impellors fixed to the drive shaft. A drive motor is attached for the purpose of stirring.
- A narrow platinum resistance temperature sensor is also attached with the vessel, which control the temperature either by direct heating with a heater pad or by circulating warm water around the vessel jacket. A cold finger is also used to cool the vessel contents, if direct heating is used.
2) Peripheral parts and accessories
i) Reagent pumps
Pumps are used for the essential parts of the instrumentation system for measuring pH and antifoam and control. Peristaltic pumps are basically used and the flow rate is normally fixed with a timed system of control. For extremely high accuracy of addition, the reservoir bottles are placed on analytical balances, which can be used to determine how much reagent has been pumped in a given time.
ii) Medium feed pumps and reservoir bottles –
Medium feed pumps are often used with variable speeds so as to give the maximum possible range of feed rates. An effluent pump isoften used to remove culture fluid form the fermentor vessel into a storage reservoir bottle.
iii) Rotameter/gas supply –
A laboratory air supply is provided from a separate pump, which must be oil free. A variable area flow meter or rotameter is used to control the airflow rate into the fermentor vessel. A sterile filter (usually 0.22m) is used as a bridge between the tubing form the rotameter and that connected to the air sparger of the fermentor.
iv) Sampling device –
This allows culture fluid to be removed aseptically during the fermentation at intervals decided by the user. The frequencies of sampling and the size of each sample are determined empirically according to the needs of the experiment.
3) Alternative vessel designs
There are various types of alternative vessel designs commonly found for the fermentation processes. Usually alternative vessel designs are tried when the standard vessel configuration does not allow adequate growth of the organism.
i) Air Lift –
By the use of airlift the need for a stirrer can be minimized or can be neglected.
ii) Fluidized bed –
In fermentation process the microbes/cells are trapped in a physical medium and then put into the vessel by a mesh. The medium is re-circulated with a pump, which is easily adapted to give a continuous/semi-continuous flow to allow the trapped cells to effect chemical changes on constituents of the medium without being washed out along with spent medium.
iii) Hollow fiber –
In this cells are embedded in fibers contained in a cartridge, which are bathed in circulating culture medium. This is often used for mammalian cell culture, where anchorage-dependent cell lines can be perfused with oxygenated medium.
4) Additional accessories and peripherals
i) Feed pumps –
For fed-batch or continuous applications a separate feed pump is used.
ii) Peristaltic pump —
The pumps used for medium feed are typically variable speed units, which can accept signals from an external source such as a computer or process controller to allow adjustment of feed rates based on for example, respiratory quotient (RQ) or optical density(OD).
iii) Syringe pump –
This type of pump is typically used in medical applications for the precise delivery of drugs. Most now allow for computer control.
iv) Exit gas analysis –
The exit gas may need to be conditioned (e.g. moisture removed) before going into the analyzer, depending on the type of instrument used.
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