Major types of bioreactors or Fermenters
A
bioreactor or fermenter is basically a device in which the organisms are
cultivated to form the desired products. It is a containment system designed to
give right environment for optimal growth and metabolic activity of the
organism so that maximum production of the desired product can be achieved.
Bioreactors
can be classified based on different criteria.
Based on the mode of operation there are Batch fermenter, Continuous fermenter
and Fed-batch fermenter and based on the on the energy input and aeration
methods adopted, there are Liquid phase, Gas phase or Combined phase fermenters.
The
major structural variations of or types of bioreactors used in industry are
discussed below:
1.
Stirred tank
reactors or Continuous Stirred Tank Bioreactors
A continuous
stirred tank bioreactor consists of a cylindrical vessel with motor driven
central shaft that supports one or more agitators (impellers). It is provided
with a baffle and a rotating stirrer is attached either at the top or at the
bottom of the bioreactor. Baffles are
flat vertical plates whose width is about one-tenth of the vessel diameter.
Normally, 4-6 baffle plates are fitted to the inside vessel walls to aid mixing
and mass transfer by increasing turbulence, preventing vortex formation and
eliminating ‘dead spaces’. The air is
added to the culture medium under pressure through a device called sparger. The
sparger may be a ring with many holes or a tube with a single orifice. The
sparger along with impellers (agitators) enables better gas distribution system
throughout the vessel.
Continuous operation
Good mixing of the
contents
Efficient gas transfer
Good temperature control
Low operating (labor) cost
Easy to clean
2.
Bubble column
reactors
Air or gas is
introduced at the base of the column through perforated pipes or plates, or
metal micro porous spargers. This is a
cylindrical column with aspect ratio of 4-6. Sparging is done at the bottom
with perforated pipes/plates, sintered glass or metal micro-porous spargers.
The vessel is
divided into two interconnected zones by means of a baffle or draft tube. In
one of the two zones referred to a riser, the air/gas is pumped. The other zone
that receives no gas is the down comer. The dispersion flows up the riser zone
while down flow occurs in the down comer.
These reactors are
similar to bubble column reactors, but differ by the fact that they contain a
draft tube. The draft tube is typically an inner tube which improves circulation
and oxygen transfer and equalizes shear forces in the reactor.
There are two
types of airlift bioreactors.
Internal-loop
airlift bioreactor has a single container with a central draft tube.
External loop
airlift bioreactor possesses an external loop so that the liquid circulates
through separate independent channels.
Advantages
Simple design with
no moving parts or agitator, so less maintenance, less risk of defects.
Easier
sterilization (no agitator shaft parts)
Low Energy
requirement
Greater
heat-removal - Draught-Tube can be designed to serve as internal heat
exchanger.
Very low cost
Tower reactor
Air
lift reactors with large dimensions constitute a tower bioreactor. A high
hydrostatic pressure generated at the bottom of the reactor increases the
solubility of O2 in the medium. At the top of the riser, (with
expanded top) reduces pressure and facilitates expulsion of CO2. The
medium flows back in the down comer and completes the cycle. Tower bioreactor
has high aeration capacities.
Tower
fermenter can be defined as an elongated non-mechanically stirred fermenter
with an aspect ratio of at least 6:1 for the tubular section or 10:1 overall,
through which there is a unidirectional flow of gases. Several different types
of tower fermenter exist. The simplest
types of fermenter are those that consist of a tube which is air sparged at the
base (bubble columns).
4.
Fluidized bed
reactors (FBR)
Here, a fluid (gas
or liquid) is passed through a solid granular material (usually immobilized
enzyme or microbial cells) at enough velocities to suspend the solid in
solution and cause it to behave as a fluid. This process is known as
fluidization. This allows Uniform
Particle Mixing and temperature control.
Advantages of Fluidized Bed Reactor:
Uniform Particle Mixing
Uniform Temperature Gradients
Ability to Operate in Continuous State
5.
Packed bed
reactors
In packed bed
reactors, cells are immobilized on large particles. These particles do not move
with the liquid. Packed bed reactors are simple to construct and operate but
can suffer from blockages and from poor oxygen transfer.
A nutrient media
flows continuously over the immobilised cells or enzymes. The products are
released into the fluid. The flow of the
fluid can be upward or downward.
The trickle-bed reactor is a variation of
the packed bed reactors.
6.
Flocculated cell
reactors
Flocculated cell reactors suspend cells by
allowing them to flocculate. These reactors are used mainly in wastewater
treatment
7.
Photo bioreactors
(PBR)
These are the
bioreactors specialised for fermentation that can be carried out either by
exposing to sunlight or artificial illumination. They are made up of glass or transparent plastic.
The culture can be circulated through array of tubes or flat panels (the solar
receivers) by using centrifugal pumps or airlift pumps. The cells should be in
continuous circulation without forming sediments. Adequate penetration of
sunlight should be maintained. The tubes should also be cooled to prevent rise
in temperature. Microalgae and cyanobacteria are normally used.
Advantages of
Photobioreactor
Large
surface-to-volume ratio.
Better control of
gas transfer.
Reduction in
evaporation of growth medium.
More uniform
temperature.
Space saving – Can
be mounted vertically, horizontally or at an angle, indoors or outdoors.
8.
Acetator and
Cavitator
These
are designed especially for vinegar production. They differ in mechanical operation.
Acetators operates in semibatch mode and cavitator in continuous mode. Aeration
in acetator is by fast rotating ceramic disc over an air nozzle to provide
finely dispersed air bubbles. Whereas in cavitator, nutrient liquid and air are
sucked down a hollow tube extending from the liquid surface so that agitation
and cavitation can cause air bubbles.
Frings generator or
acetator fermenter used for the manufacture of vinegar. The fermenter, which
can be used semicontinuously or continuously, employs vortex stirring
9.
Membrane
Bioreactor
Membrane
bioreactors successfully applied to various microbial bioconversions such as
alcoholic fermentation, solvents, organic acid production, waste water
treatment, etc. In membrane bioreactor the soluble enzyme and substrate are
introduced on one side of ultrafilter membrane by means of a pump. Product is forced
out through the membrane and membrane holds back the enzyme. The most widely
used membrane materials includes polysulfonte, polyamide and cellulose acetate.
Advantages
of Membrane Bioreactor
The
loss of enzyme is reduced.
Enzyme
lost by denaturation can be make up by periodic addition of enzyme.
Substrate
and enzyme can be easily replaced.
10.
Cylindro-conical vessels
The vessel consists of a stainless-steel
vertical tube with a hemispherical top and a conical base with an included
angle of approximately 70°. Aspect ratios are usually 3:1 and fermenter heights
are 10 to 20 m. Vessels are not normally agitated unless a particularly
flocculant yeast is used. In the vessel,
the wort is inoculated with yeast and the fermentation proceeds for 40 to 48
hours. Mixing is achieved by the generation of carbon dioxide bubbles that rise
rapidly in the vessel. This is generally employed for brewing.
Adantages of this vessel in brewing are:
1. Reduced process times
2. Primary fermentation and conditioning may be
carried out in the same vessel.
3. The sedimented yeast may be easily removed
4. The maturing time may be reduced
11.
The Waldhof-type
fermenter
The
investigations on yeast growth in sulphite waste liquor in Germany, Japan and
the United States of America led to the development of the Waldhof-type
fermenter. The fermenter was of carbon
steel, clad in stainless steel, 7.9 m in diameter and 4.3-m high with a centre
draught tube 1.2 m in diameter. Non-sterile air was introduced into the
fermenter through a rotating pin-wheel type of aerator, composed of open-ended
tubes rotating at 300 rpm. The broth passed down the draught tube from the
outer compartment and reduced the foaming.
Top view of Waldhof
aerator
12.
Deep jet fermenter
There will be a pump to circulate the liquid
medium from the fermenter through a gas entrainer and back to the fermenter. Two basic types are the injector and the
ejector. In injector a jet of medium
will be surrounded by a jet of compressed air.
In ejector the liquid jet enters into a larger nozzle and entrains the
gas around it.
13.
The cyclone column
This is useful for filamentous cultures. The culture liquid will be pumped from the
bottom to the top of the cyclone column through a closed loop. The descending liquid ran down the walls of
the column in a relatively thin film. Nutrients
and air will be fed in near the base of the column and exhaust gases will leave
at the top of the column.
Advantages
1.
Good gas exchange
2.
Lack of foaming
3.
Limited wall growth
14.
Rotating disc fermentor (Rotating disc contactors)
Rotating disc contactors are used in effluent
treatment. The microorganisms are grown
in thin microbial films on slow rotating discs and they oxidise the
effluent. The discs (2-3 m diameter with
1-2 cm spacing between the discs) are fitted on a central drive shaft and
rotate slowly (0.5 to 15 rpm) through the effluent so that 40-50% of the disc
surface are submerged. The discs are
made from synthetic material (Polystyrene, PVC). `
15.
Hollow fibre reactor
Hollow
fiber reactors are used to provide a high growth surface- volume ratio. Cells
are immobilized on the external surfaces of hollow fibers, and nutrients pass
through the tubes. Anchorage dependent
cells can be cultured using hollow fibers held together in cartridge
chambers. The cells are grown in extra
capillary spaces within the cartridge.
16.
Microcarriers
Microcarriers of
DEAE or dextran are used for anchorage dependent cells. Cells grow on the
surface of the microcarriers, usually in the form of monolayers and sometimes
as multilayers. Microporous microcarriers are also used in which cells grow
inside them.
17.
Perfusion cultures
This is a
technique where modified fermenter is gently stirred and broth is withdrawn
continuously from the vessel and passed through a steel or ceramic filter. This is also known as spin culture since the
filter will be spinning to prevent cell blocking.
References
•
Principles of fermentation technology, PF
Stanbury, A Whittakker, SJ Hall
•
Industrial Biotechnology, Lesson 6: Types Of
Bioreactor, Rai University
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