Fermentor Design

8/12/2019 Fermentor Design

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Design and Operation of

Conventional Bioreactor


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Basic functions of a fermentor for microbial or animal cell

cultures:1. The vessel should capable of aseptic operation for number of days

2. Adequate aeration/agitationshould be provided to the microbes

3. Mixingshould not cause any damage to the organism

4. Power consumption should be low

5. System of temp., pHcontrol and sampling facilities should be

provided

6. Vessel should be designed in such that minimal use of labour in

operation, harvesting, cleaning and maintenance

7. Similar geometryto both smaller and larger vessels


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Baffle


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Operation of a Conventional bioreactor

Basically involves 5 steps

1. Sterilization

2. Inoculation and Sampling

3. Aeration

4. Control systems

5. Cleaning

1. Sterilizationi) Batch sterilization ( direct or indirect method)

Bioreactor + medium is sterilized

Injecting steam directly in to the Vessel containing the medium Or

Injecting the pressurized steam in to the coil surrounding the vessel

Whole system is heated to 121oC/20 mins


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DisadvantageTakes longer time (2-3hrs) to reach sterilization temp(121 C)

Once temp reached it should be maintained for another20-60mins,followed by cooling for 1 hour

Alter nutrient solution

Change in color and pH occurs and Destroy vitamins

ii) Continuous sterilization

Bioreactor

Nutrient solution (rapidly heated to 140 C for 30-120sec)

by injecting the steam directly or medium can be sterilized through

Heat exchangers

(Sterilized by Injecting the steam directly)


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2. Inoculation and Sampling

Stock culturelarge volume of inoculum is prepared until the

inoculum has a volume of 5 -10% of the working volume of the

production fermenter .

After inoculationFermenter is regularly sampled to examine culture

contamination and to measure product concentration

Aseptic inoculation and sampling is performed using steam locked

port (T-shaped arrangement of piping with a valve in each arm)


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Fermentor

Feed Tank

Cooling

Tank

2ndHeat exchanger

90-120 C

for 20-30 sec

140 C for 30 -120 Sec

1stHeat exchanger

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Continuous Sterilization by Heat Exchangers


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Usually small fermenters with Steam locked port is used for

inoculum preparation and linked to the production vessel via piping.

These T arm pipes are always maintained in sterile condition by

passing pressurized steam. During inoculation and sampling, valve

to the steam supply is shut down.


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Fermenter attached with steam locked port

Fermenter


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3.Aeration

Aeration of a fermentation medium has two functions

1. To supply oxygen to the production organism

2. To remove CO2 from the bioreactor

Thus the aeration system is designed to promote exchange of gases

between air and medium.

Oxygen- supplied from air, is compressed by a compressor and stored

in tanks

Then it is passed in to the fermenter through flow meter (regulate flow

rate), then a sterilizing filter


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Usually air supplied at the bottom of the fermenter via sparger -

(hollow plate perforated with small holes)- the air supplied into the

medium as small bubbles

Smaller bubbles have greater surface volume ratio and more efficient

in gas exchange. This gas exchange occurs at the interface betweenthe two bubbles surface

After exchange , air bubbles rise to the surface of the medium,

breaks up and releases the gas they contain ( for this purpose,

fermenters are not filled completely, 20% capacity of vessel left free)


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i) Airlift System of aeration

- Simple aeration system

-These fermenters do not have mechanical agitation systems (motor,

shaft, impeller blades)

- Involves sparging of air in at the base of the bioreactor

- Bubbles rise up producing upward flow of the medium

- Thus the system is called Airlift system.

A tube is placed at the centre of an airlift fermenter to channel

the liquid flow around the system. This tube is called draft tube

- Most suitable for fermentation of animal and plant cell cultures

since the mechanical agitation produces high shearing stress that

may damage the cells.


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-Most widely used for large-scale production of monoclonal

antibodies.

Advantage:It has no moving parts , easy maintenance and

operation

Disadvantage: Aeration capacity is low

Aeration capacityis a measure of the maximum rate at which

oxygen can be dissolved into the medium

For aerobic organism, aeration capacity should be > O2demand( rate at which the growing cultures requires O2)

The aeration capacity of an airlift system is directly

proportional to the air flow rate and internal pressure


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ii) Stirred system of aeration

-Aeration capacity of the medium is enhanced by stirring

-Stirring is done by impellers

-Aeration capacityproportional to the stirrer speed, rate of air flow

and internal pressure.

Small fermenter -one impeller at the base of the shaft

Larger fermenter- several impellers throughout the shaft


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Types of impellers

1. Disc impeller (most common type)

48 radial blades are projected out atthe edge of the disc

2. Turbine impeller

Blades are in curved shape

Turbine stirrer requires 50% less air when compared with

disc stirrer, for same yield


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3. MIG Impeller and INTERMIG Impeller

-Both consist of stirring arms with blades attached at an angle

-They requrie 25% and 40% of less air for the same yeild when

compared with disc-turbine stirrer.


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Uses of stirrers/impellers

-Used to mix bluk fluid and gas phase

-Air dispersion

-Oxygen transfer and heat transfer

-Maintaining uniform environment throughout the vessel

Advantage of stirred system

- produce higher aeration capacity than air lift system

Disadvantage of stirred system

- requires energy for stirring

- more maintenance is required


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Baffles

- are narrow rectangular metal plates, fixed to the internal walls-Usually 4 baffles are fixed in STR

-Increase turbulence and there by increase aeration capacity

-Increase residence time of the bubble

-Increase the energy required for stirring

-Wider baffles increase agitation effect where as narrow baffles

drops sharply


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-In large fermenters (>100 m3), two baffles are used as heat

exchangers

-These baffles are installed in such that a gap should be present

between baffles and fermenter wall

-This allows scouring action around and behind the baffles

- There by minimize the growth on baffles and fermenter wall


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4. Control systems

During fermentations, parameters( temp, pH, DO conc. and nutrient

conc. will vary

To maintain optimal conditions for growth and product formation,

such variables are monitored and controlled

Biosensors - used for online measurement and control of important

parameters in a more accurate manner.


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II) pH

During fermentation, organism grows and they change the pH of themedium

pH of the medium is measured using pH electrode which must be in

direct contact with the medium and fixed in the vessel wall using air

tight seal.

This electrode is linked to a control unit which activates pumps to

pump in acid or alkali as required.


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III)Oxygen tension

Conc. Of dissolved oxygen in the medium is measured using an oxygen

electrode, which is in direct contact with the medium and linked to a

control unitalter the air flow rate or stirrer speed to increase, there by

increase in DO takes place and vice versa

IV) Nutrient Concentration

Nutrient conc. in the medium is limited to prevent wastage or toprevent inhibitory effects of high conc. Of nutrients

Using ion-selective and enzyme selective electrodes, it is possible to

control nutrients like ammonia and glucose

V) F


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V) Foam

Media are rich in proteins which form stable foam during agitation

and aeration

Accumulation of foam takes place in head space.

Foam probe fixed in head space will sense the foam and send the

signal to control unit ,which in turn pump in anti-foam as required

These antifoams lowers the surface tension of the medium and cause

bubbles to collapse

Instead of antifoam , some fermenters are equipped with mechanical

foam braker which is placed at the top of the fermenter

VI Cleaning

Fermentor Design

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