lect bio react. dsgn)

8/8/2019 Lect Bio React. Dsgn)

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BIOREACTORDESIGNProf Madya Dr. Umi Kalsom Md Shah

Dept. Bioprocess Technology

BTK5301 (Bioreactor System)

22/07/10


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Types of Bioreactor

Major Function:

To provide controlled environment for

growth of microorganism (or mixture)

to obtain desired product.


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Points to be considered in designing and

constructing bioreactor:

1)M

icrobiological and biochemical characteristicsof cell systems.

2) Hydrodynamic characteristics of the bioreactor.

3) Mass and heat characteristics of the bioreactor.

4) Kinetic of cell growth and product formation.

5) Genetic stability characteristics of the cell system.

6) Aseptic equipment design.

7) Control of bioreactor environment (macro and

microenvironments).

8) Implication of bioreactor design on downstream

product separation.

9) Capital and operating costs of the bioreactor.

10) Potential for bioreactorscale up.


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Vary in size and complexity,

test tube (10 ml) tocomputer controlled

fermenters (>100 m3).


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Little or no power used for aeration.Aeration - dependent on transfer of oxygen

through the still surface of the culture.

1. STANDING CULTURES

PoorRate of oxygen transfer- due to small

surface area.

Used in small-scale (oxygen supply is not

critical).

Ex: biochemical tests for identification of

bacteria (test-tubes containing 5-10 mL of

media).


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a. T- Flasks

Small scale culture of animal cells. Incubated

horizontally to increase the surface area for

oxygen transfer.

Surface aeration rate can be increased

using large volume flasks.


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b. Fernback Flasks

Fig. 1 shows a 3 L "Fernback" flaskcontaining 1 L of medium and a 250 mL

Erlenmeyer flask containing 100 mL of

medium. Note how the former has a large

surface area.

Figure 1

Fernback

flask


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Large Pyrex flasks are used for small-

scale production of fermented products.

Ex: Kombucha tea, (a tea brewed bymixture of yeasts and acetic acid

bacteria).


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c. Surface Cultures

Standing culture aeration is not

restricted to the laboratory. In some

countries, where availability of

electricity is unreliable, citric acid is

produced using surface culture

techniques.


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Aspergillus nigermycelia are grown on

surface of liquid media in large shallowtrays. The medium is neither gassed

nor agitated.


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Ex: Aerobic solid substrate

fermentations (biomass is grown

on solid biodegradable substrates

such as water softened bran, rice orbarley, solids continuously or

periodically turned over to improve

aeration and to regulate culture

temperature).


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Ex: 1. Production ofkoji by

Aspergillus oryzae on soya beans,

(part ofsoya sauce process).

2. Mushroom cultivation.


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For small scale cell cultivation

(continuous shaking of culture fluid),

higher oxygen transfer rates.

Shaking breaks liquid surface and

provides greatersurface area for oxygen

transfer.

Increased rates of oxygen transfer are

also achieved by entrainment of oxygen

bubbles at the surface of the liquid.

2. SHAKE FLASKS


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a. Factors Affecting KLa (volumetric

oxygen transfer rate)

Rate of oxygen transferis dependent on:

1. shaking speed,

2. liquid volume,

3. shake flask design

KLa increases with

liquid surface area KLa is higher when

baffles are present

KLa decreases with

liquid volume


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y KLa increase with shaking speed.

y At high shaking speeds, bubbles

become entrained into medium tofurther increase oxygen transfer rate.


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3. MECHANICALLY STIRRED

BIOREACTORS

Most important bioreactorfor industrial

application, (pharmaceutical industry).

y Low capital cost, low operating costs,best understood and flexible.

y Non-sparged mechanically agitated

bioreactors can supply sufficient aerationfor microbial fermentations with liquid

volumes up to 3 L.


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y However, stirring speeds of up to

600 rpm may be required before the

culture is not oxygen limited.

In non-sparged reactors, oxygen istransferred from head-space above

fermenter liquid.

Agitation continually breaks the liquid

surface and increases surface area foroxygen transfer.


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Air-sparged fermenters can have liquid

volumes of greater than 500,000 L


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Airlift fermenters are more expensive

to construct than bubble column

reactors.

An airlift fermenter differs from bubblecolumn bioreactors by the presence of a

draft tube provides better mass and heat

transfer efficiencies and more uniform

shear conditions.


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2. Airlift Bioreactors - draft tube

y The main functions ofdraft tube:

1. Increase mixing through the reactor

2. The draft tube enhances axial

mixing throughout the whole reactor

3. Reduce bubble coalescence.


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Due to circulatory effect, draft tube induces in

the reactor. Circulation occurs in one

direction and hence the bubbles also travel inone direction.


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5. AIRLIFT BIOREACTOR

a. Air-riser and Down-comerThree regions: air-riser, down-comerand

disengagement zone.


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y Air-riser: region into which

bubbles are sparged (inside or outside ofdraft-tube).

y The rising bubbles in the air-riser

cause the liquid to flow in a vertical

direction. To counteract these upwardforces, liquid will flow in a downward

direction in the down-comer.

y

This leads to liquid circulation and thusimproved mixing efficiencies as

compared to bubble columns.


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y Enhanced liquid circulation causes

bubbles to move in a uniform direction at arelatively uniform velocity.

y This bubble flow pattern reduces

bubble coalescence and thus results in higherKLa values as compared to bubble column

reactors.

y Disengagement zone: add volume tothe reactor, reduce foaming minimize

recirculation of bubbles through the down

comer.


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6. FIXED BED REACTORS

y Cells are immobilized by

absorption on or entrapment in solid,non-moving solid surfaces(e.g: plastic

blocks, concrete blocks, wood shavings or

fibrous material such as plastic or glass

wool)..


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y The liquid feed is either pumpedthrough or allowed to trickle over the

surface of the solids where the immobilized

cells convert the substrates into products.

y

Once steady state has been reached there

will be a continuous cell loss from the solid

surfaces.

y


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Used in waste treatment and asbiological filters in small aquarium

water recycling systems.

y In other types of fixed-bed

fermenters, the cells are immobilizedin solidified gels such as agaror

carragenan.


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y Cells are trapped inside pores of gels

(better cell retention and a large effective

surface area for cell entrapment).

y

Increase surface area for cell

immobilization: hollow fibres and pleated

membranes as immobilization surfaces.

y


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Industrial applications: waste watertreatment, production of enzymes and

amino acids, steroid transformations

y Advantages: non-growing

cells can be used. Cells enzymatically

act on substrates in the feed. The

cells can be eitherinactivated ornot

fed nutrients required for growth.


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7. PACKED BED BIOREACTOR

y Rate of mass transfer between cells

and medium depends on flow rate and on

thickness of biomass film on or near the

surface of the solid particles.

y Problems: poor mass transfer rates

and clogging.

y Used commercially with

enzymatically catalysts and with slowly or

non-growing cells.


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8. TRICKLE FLOW BIOREACTORS

y

Class of packed bed reactors, inwhich medium flows (or trickles) over the

solid particles. Particles are not immersed in

the liquid.

y Used in aerobic treatment of

sewage. Oxygen transfer is enhanced by

ensuring cells are covered by only a very thin

layer of liquid, thus reducing the distanceover which the dissolved oxygen must diffuse

to reach the cells.


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y

Because stirring is not used,considerable capital and energy

costs are saved. However, oxygen

transfer rates per unit volume are

low compared with sparged stirredtank systems.


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9. FLUIDIZED BED BIOREACTORS

y

Maintaining high biomassconcentrations, good mass transfer rates

in continuous cultures.

y Mixing is assisted by the action of a

pump.y

cells or enzymes are immobilized in and/or

on the surface of light particles.


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y A pump located at base of tank

causes the immobilized catalysts to move

with the fluid. The pump pushes fluid and

particles in a vertical direction.

y Upward force of the pump is

balanced by the downward movement ofthe particles due to gravity.

y This results in good circulation. For

aerobic microbial systems, sparging is used

to improve oxygen transfer rates.y


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y Fluidized-bed microcarrier cultures

can be operated both in batch andcontinuous mode.

Use of small particles increases the

surface area for cell immobilization andmass transfer. Because the particles are

small and light, they can be easily made

to flow with the liquid (ie. fluidized).


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y Fluidization of particles leads to the

surface of particles being continuously

turned over (increases mass transfer rate).y

y Used in wastewater treatment.

Aquarium scale fluidized bioreactors for

biological nitrification are readily available.

2 Category: 2 phase system (not

aerated) and 3 phase system (aerated

by sparging).


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Used for animal cell culture. Animal

cells are trapped in gels or on the

surface of special particles known as

"microcarriers". Ex: perfusion culture

technology used for animal cell

culture.

6.10 INDUSTRIAL BIOREACTORS

y Worlds largest fermenter (fromChemical and Engineering News), 200 ft high

and 25 ft diameter.


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