1. Industrial MicrobiologyIndustrial Microbiology
BASIS OF BIOREACTOR FORBASIS OF BIOREACTOR FOR
BIOPHARMACEUTICALSBIOPHARMACEUTICALS
Angel L. Salamán, PhDAngel L. Salamán, PhD
angelsalaman@yahoo.comangelsalaman@yahoo.com

2. Tutorial on BioreactorsTutorial on Bioreactors
1. Introduction1. Introduction
2. O2 uptake and Stoichiometry2. O2 uptake and Stoichiometry
3. Surface aeration3. Surface aeration
4. Methods of aeration4. Methods of aeration
5. Mechanically stirred bioreactors5. Mechanically stirred bioreactors
6. Bubble driven bioreactors6. Bubble driven bioreactors
7. Airlift bioreactors7. Airlift bioreactors
8. Packed bed and trickle flow bioreactors8. Packed bed and trickle flow bioreactors
9. Fluidized bed bioreactors9. Fluidized bed bioreactors

3. Bioreactors- IntroductionBioreactors- Introduction
 Previous lectures have stress the importance of consideringPrevious lectures have stress the importance of considering
process engineering factors when culturing cells.process engineering factors when culturing cells.
 Biological factors include the characteristics of the cells, theirBiological factors include the characteristics of the cells, their
maximum specific growth rate, yield coefficient, pH range andmaximum specific growth rate, yield coefficient, pH range and
temperature range.temperature range.
 We have seen however that the productivity of a fermentation isWe have seen however that the productivity of a fermentation is
determined by the mode of operation of the fermentationdetermined by the mode of operation of the fermentation
process; eg. the advantages of fed-batch and continuousprocess; eg. the advantages of fed-batch and continuous
fermentations over batch fermentations.fermentations over batch fermentations.

4.  The oxygen demand of an industrial process isThe oxygen demand of an industrial process is
generally satisfied by aeration and agitationgenerally satisfied by aeration and agitation
 Productivity is limited by oxygen availability andProductivity is limited by oxygen availability and
therefore it is important to the factors that affect atherefore it is important to the factors that affect a
fermenters efficiency in supplying Ofermenters efficiency in supplying O22
 We are going to discuss OWe are going to discuss O22 requirement,requirement,
quantification of Oquantification of O22 transfer and factors influencingtransfer and factors influencing
the transfer of Othe transfer of O22 into solutioninto solution
Bioreactors- IntroductionBioreactors- Introduction

5. Bioreactors- IntroductionBioreactors- Introduction
 Likewise mass transfer, in particular, oxygenLikewise mass transfer, in particular, oxygen
transfer was highlighted as an important factortransfer was highlighted as an important factor
which determined how a reactor must bewhich determined how a reactor must be
designed and operated.designed and operated.
 Cost was also described as an importantCost was also described as an important
consideration. The larger the reactor or theconsideration. The larger the reactor or the
faster the stirrer speed, the greater the costsfaster the stirrer speed, the greater the costs
involved.involved.
 How bioreactors are designed to meet cost,How bioreactors are designed to meet cost,
biological and engineering needs…biological and engineering needs…

6. MASS TRANSFER and PHASESMASS TRANSFER and PHASES
Different phases present -IntroductionDifferent phases present -Introduction
Fundamental concept in fermentation technology is transfer of materialsFundamental concept in fermentation technology is transfer of materials
(e.g nutrients, products, gases etc.) through different phases (e.g gas into(e.g nutrients, products, gases etc.) through different phases (e.g gas into
a liquid).a liquid).
Major problem associated with provision of oxygen to the cell - is a rateMajor problem associated with provision of oxygen to the cell - is a rate
limiting step and thus serves as a model system to understand masslimiting step and thus serves as a model system to understand mass
transfer.transfer.
The rate of oxygen transfer = driving force / resistanceThe rate of oxygen transfer = driving force / resistance.. E.gE.g
resistance to mass transfer from medium to mo`s are complex and mayresistance to mass transfer from medium to mo`s are complex and may
arise from;arise from;
•• Diffusion from bulk gas to gas/liquid interfaceDiffusion from bulk gas to gas/liquid interface
•• Solution of gas in liquid interfaceSolution of gas in liquid interface
•• Diffusion of dissolved gas to bulk of liquidDiffusion of dissolved gas to bulk of liquid
•• Transport of dissolved gas to regions of cellTransport of dissolved gas to regions of cell
•• Diffusion through stagnant region of liquid surrounding the cellDiffusion through stagnant region of liquid surrounding the cell
•• Diffusion into cellDiffusion into cell
•• Consumption by organism (depends on growth/respiration kinetics)Consumption by organism (depends on growth/respiration kinetics)

7. The following diagram serves to illustrate the differentThe following diagram serves to illustrate the different
phases and material that are relevant in general transportphases and material that are relevant in general transport
processes associated with fermentation technology;processes associated with fermentation technology;
Dispersed gases Dissolved
nutrients
Solid and
Immiscible
liquid
nutrients
Floc
Cells
Products in
water
MASS TRANSFER

8. Phases present in bioreaction /Phases present in bioreaction /
bioreactorbioreactor
Non aqueous phase Aqueous phase Solid phase
(Reactants / products) Dissolved reactants /
products
Reaction
Gas (O2, CO2, CH4 etc) Cells
Liquids (e.g oils) Sugars Organelles
Solid (e.g particles of
substrate)
Minerals
Enzymes
Enzymes
......... 1 →
← 2 ..........
1 = reactant supply and utilisation
2 = product removal and formation

9. • One of the most critical factors in the operation of a fermenter is
the provision of adequate gas exchange.
•The majority of fermentation processes are aerobic
• Oxygen is the most important gaseous substrate for microbial
metabolism, and carbon dioxide is the most important gaseous
metabolic product.
• For oxygen to be transferred from a air bubble to an individual
microbe, several independent partial resistance’s must be overcome
Mass Transfer

10. 1) The bulk gas phase in the bubble
2) The gas-liquid interphase
3) The liquid film around the bubble
4) The bulk liquid culture medium
5) The liquid film around the microbial cells
6) The cell-liquid interphase
7) The intracellular oxygen transfer resistance
1
2
3
4
5
6
7
Gas bubble
Liquid film
Microbial cell
Oxygen Mass Transfer Steps

11. Stoichiometry of respirationStoichiometry of respiration
To consider the Stoichiometry of respiration the oxidation
of glucose may be represented as;
C6H12O6 + 6O2 = 6H2O + 6CO2
Atomic weight of Carbon
Hydrogen
Oxygen
12
1
16
Molecular weight of glucose is 180
How many grams of oxygen are required to oxidise 180g of glucose?
Answer 192g

12. Solubility of OxygenSolubility of Oxygen
 Both components oxygen and glucose must be inBoth components oxygen and glucose must be in
solution before they become available tosolution before they become available to
microorganismsmicroorganisms
 Oxygen is 6000 times less soluble in water thanOxygen is 6000 times less soluble in water than
glucoseglucose
 A saturated oxygen solution contains only10mgA saturated oxygen solution contains only10mg
dmdm-3-3
of oxygenof oxygen
 Impossible to add enough oxygen to a microbialImpossible to add enough oxygen to a microbial
culture to satisfy needs for complete respirationculture to satisfy needs for complete respiration
 Oxygen must be added during growth at aOxygen must be added during growth at a
sufficient rate to satisfy requirementssufficient rate to satisfy requirements

13. Comparison of conc. driving forces and uptakeComparison of conc. driving forces and uptake
rates for glucose and oxygen by yeastrates for glucose and oxygen by yeast
Problems encountered in oxygen transport can be illustrated byProblems encountered in oxygen transport can be illustrated by
comparing transport of glucose vs oxygen;comparing transport of glucose vs oxygen;
1% Sugar (glucose)1% Sugar (glucose) Broth OBroth O22 satsat
@ 25@ 25oo
CC
Conc. in bulk brothConc. in bulk broth 10,000 ppm10,000 ppm approx. 7 ppmapprox. 7 ppm
Critical conc .Critical conc . 100 ppm100 ppm 0.8 ppm0.8 ppm
(growth stops)(growth stops)
Rate of demandRate of demand 2.8 mmoles/ g cells /h2.8 mmoles/ g cells /h 7.7 mmoles/7.7 mmoles/
g cells /hg cells /h

14. MASS TRANSFER and RESPIRATIONMASS TRANSFER and RESPIRATION
(a) Mass balance(a) Mass balance
StoichiometryStoichiometry of respiration e.g glucose;of respiration e.g glucose;
CC66HH1212OO66 + 6O+ 6O22 ⇒⇒ 6H6H22O + 6 COO + 6 CO22
Oxidation ofOxidation of 180 gms Glucose180 gms Glucose requiresrequires 192 gms O192 gms O22
Compare with a hydrocarbon (i.eCompare with a hydrocarbon (i.e 6 CH6 CH22))

15. The Oxygen requirements ofThe Oxygen requirements of
industrial fermentationsindustrial fermentations
 Oxygen demand dependant on convertion of Carbon (C)Oxygen demand dependant on convertion of Carbon (C)
to biomassto biomass
 Stoichiometry of conversion of oxygen, carbon andStoichiometry of conversion of oxygen, carbon and
nitrogen into biomass has been elucidatednitrogen into biomass has been elucidated
 Use these relationships to predict the oxygen demandUse these relationships to predict the oxygen demand
for a fermentationfor a fermentation
 Darlington (1964) expressed composition of 100g of dryDarlington (1964) expressed composition of 100g of dry
yeastyeast CC 3.923.92 HH 6.56.5 OO 1.941.94

16. OO22 RequirementsRequirements
6.67CH6.67CH22O + 2.1OO + 2.1O22 = C= C 3.923.92 HH 6.56.5 OO 1.941.94 + 2.75CO+ 2.75CO22 + 3.42H+ 3.42H22OO
7.14CH7.14CH22 + 6.135O+ 6.135O22 = C= C 3.923.92 HH 6.56.5 OO 1.941.94 + 3.22CO+ 3.22CO22 + 3.89H+ 3.89H22OO
where CHwhere CH22 = hydrocarbon= hydrocarbon
CHCH22O = carbohydrateO = carbohydrate
From the above equations to produce 100g of yeast fromFrom the above equations to produce 100g of yeast from
hydrocarbon requires three times the amount of oxygenhydrocarbon requires three times the amount of oxygen
than from carbohydratethan from carbohydrate

17. Compare solubility of Oxygen vs Glucose ( e.g. oxygen = 9.0Compare solubility of Oxygen vs Glucose ( e.g. oxygen = 9.0
mg/l @ 20mg/l @ 20oo
C, 11.3 mg/l @ 10C, 11.3 mg/l @ 10oo
C)C)
Thus must consider;Thus must consider;
••Requirement for oxygen important in biotechnologicalRequirement for oxygen important in biotechnological
processesprocesses
••Quantification of oxygen transfer (to avoid rate limiting step)Quantification of oxygen transfer (to avoid rate limiting step)
importantimportant
•• Factors influencing rate of transfer (e.g. viscosity) importantFactors influencing rate of transfer (e.g. viscosity) important

18. Case Study:Case Study:
Give the chemical properties of oxygen, why is itGive the chemical properties of oxygen, why is it
so important to life?so important to life?
Give examples of biochemical pathways (of commercialGive examples of biochemical pathways (of commercial
significance) influenced by oxygen (i.e aerobic vs anaerobic).significance) influenced by oxygen (i.e aerobic vs anaerobic).
What type of bioreactor is used in the production of the productsWhat type of bioreactor is used in the production of the products
chosen?chosen?

19. Dissolved Oxygen Concentration
QO2
Ccritical
Effect of dissolved O2 concentration
on the QO2 of a microorganism
Specific O2 uptake increases with increase in dissolved
O2 levels to a certain point Ccrit

20. Critical dissolved oxygen levels for aCritical dissolved oxygen levels for a
range of microorganismsrange of microorganisms
Organism Temperature Critical dissolved
o
C Oxygen concentration
(mmoles dm -3
)
Azotobacter sp. 30 0.018
E. coli 37 0.008
Saccharomyces sp. 30 0.004
P. chrysogenum 24 0.022
Azotobacter vinelandii is a large, obligate aerobic soil bacterium which
has one of the highest respiratory rates known among living
organisms

21. Critical dissolved oxygen levelsCritical dissolved oxygen levels
 To maximize biomass production you must satisfy the organismsTo maximize biomass production you must satisfy the organisms
specific oxygen demand by maintaining the dissolved Ospecific oxygen demand by maintaining the dissolved O22 levelslevels
above Cabove Ccritcrit
 Cells become metabolically disturbed if the level drops below CCells become metabolically disturbed if the level drops below Ccritcrit
 In some cases metabolic disturbance may be advantageousIn some cases metabolic disturbance may be advantageous
 Or high dissolved OOr high dissolved O22 levels may promote product formationlevels may promote product formation
 Amino acid biosynthesis by Brevibacterium flavumAmino acid biosynthesis by Brevibacterium flavum
 Cephalosporium synthesis by Cephalosporium sp.Cephalosporium synthesis by Cephalosporium sp.

22. FACTORS AFFECTING OXYGEN DEMANDFACTORS AFFECTING OXYGEN DEMAND
•• Rate of cell respirationRate of cell respiration
•• Type of respiration (aerobic vs anaerobic)Type of respiration (aerobic vs anaerobic)
•• Type of substrate (glucose vs methane)Type of substrate (glucose vs methane)
•• Type of environment (e.g pH, temp etc.)Type of environment (e.g pH, temp etc.)
•• Surface area/ volume ratioSurface area/ volume ratio
large vs small cells (bacteria v mammalian cells)large vs small cells (bacteria v mammalian cells)
hyphae, clumps, flocks, pellets etc.hyphae, clumps, flocks, pellets etc.
•• Nature of surface area (type of capsule etc)Nature of surface area (type of capsule etc)

23. Diffusivity of gas
BULK LIQUID
?
?
?
?
?
?
CELLS
O2

24. Size of sparger
gas bubble
Gas composition, volume &
velocity
Design of Impeller
size, no. of blades
rotational speed Baffles
width, number
FACTORS INFLUENCING OXYGEN SUPPLY
Foam/antifoam
Temperature
Type of liquid
Height/width ratio
‘’Hold up’’
Process factors

25. Methods of AerationMethods of Aeration
 A bioreactor is a reactor system used for the culture ofA bioreactor is a reactor system used for the culture of
microorganisms. They vary in size and complexity from a 10 mlmicroorganisms. They vary in size and complexity from a 10 ml
volume in a test tube to computer controlled fermenters withvolume in a test tube to computer controlled fermenters with
liquid volumes greater than 100 mliquid volumes greater than 100 m33
. They similarly vary in cost. They similarly vary in cost
from dollars to a few million dollars.from dollars to a few million dollars.
 In the following sections we will compare the following reactorsIn the following sections we will compare the following reactors
• Standing culturesStanding cultures
• Shake flasksShake flasks
• Stirred tank reactorsStirred tank reactors
• Bubble column and airlift reactorsBubble column and airlift reactors
• Fluidized bed reactorsFluidized bed reactors

26. Standing culturesStanding cultures
 In standing cultures, little or no power is used forIn standing cultures, little or no power is used for
aeration. Aeration is dependent on the transfer ofaeration. Aeration is dependent on the transfer of
oxygen through the still surface of the culture.oxygen through the still surface of the culture.

27. Standing culturesStanding cultures
 The rate of oxygen transfer will be poor due to theThe rate of oxygen transfer will be poor due to the
small surface area for transfer. Standing culturessmall surface area for transfer. Standing cultures
are commonly used in small scale laboratoryare commonly used in small scale laboratory
systems in which oxygen supply is not critical. Forsystems in which oxygen supply is not critical. For
example, biochemical tests used for theexample, biochemical tests used for the
identification of bacteria are often performed inidentification of bacteria are often performed in
test-tubes containing between 5-10 ml of media.test-tubes containing between 5-10 ml of media.
 T-flasks used in the small scale culture of animalT-flasks used in the small scale culture of animal
cells are another example of a standing culture. T-cells are another example of a standing culture. T-
flasks are normally incubated horizontally toflasks are normally incubated horizontally to
increase the surface area for oxygen transfer.increase the surface area for oxygen transfer.

28.  The surface aeration rate in standing cultures can be increasedThe surface aeration rate in standing cultures can be increased
by using large volume flasks.by using large volume flasks.
 The following photograph shows a 250 ml Erlenmeyer flaskThe following photograph shows a 250 ml Erlenmeyer flask
containing 100 ml of medium and a 3 litre "Fernback" flaskcontaining 100 ml of medium and a 3 litre "Fernback" flask
containing 1 litre of medium.containing 1 litre of medium.
Note how the latter has a large surface area.

29. Standing culturesStanding cultures
• Large Pyrex flasks are used for the small scale production
of fermented products.
• Standing culture aeration is not restricted to the laboratory.
• In some countries, where the availability of electricity is
unreliable, citric acid is produced using surface culture
techniques.
• In these cultures, the Aspergillus niger mycelia are grown
on the surface of liquid media in large shallow trays.
• The medium is neither gassed nor agitated.

30. Aspergillus nigerAspergillus niger myceliamycelia

31. Standing culturesStanding cultures
 Aerobic solid substrate fermentations are anotherAerobic solid substrate fermentations are another
example of standing cultures. In these fermentations,example of standing cultures. In these fermentations,
the biomass is grown on solid biodegradablethe biomass is grown on solid biodegradable
substrates.substrates.
 The solids may be continuously or periodically turnedThe solids may be continuously or periodically turned
over to improve aeration and to regulate the cultureover to improve aeration and to regulate the culture
temperature. One example of a commercial scale,temperature. One example of a commercial scale,
solid substrate fermentation is the production of kojisolid substrate fermentation is the production of koji
byby Aspergillus oryzaeAspergillus oryzae on soya beans which is part ofon soya beans which is part of
the soya sauce process.the soya sauce process.
 Another is mushroom cultivation. ConsiderableAnother is mushroom cultivation. Considerable
research is currently being invested into theresearch is currently being invested into the
feasibility of producing biochemicals by solidfeasibility of producing biochemicals by solid
substrate fermentations.substrate fermentations.

32. Shake flasksShake flasks

33. Shake flasksShake flasks
 Shake flasks are commonly used for small scaleShake flasks are commonly used for small scale
cell cultivation.cell cultivation.
 Through continuous shaking of the culture fluid,Through continuous shaking of the culture fluid,
higher oxygen transfer rates can be achieved ashigher oxygen transfer rates can be achieved as
compared to standing cultures.compared to standing cultures.
 Shaking continually breaks the liquid surface andShaking continually breaks the liquid surface and
thus provides a greater surface area for oxygenthus provides a greater surface area for oxygen
transfer.transfer.
 Increased rates of oxygen transfer are alsoIncreased rates of oxygen transfer are also
achieved by entrainment of oxygen bubbles atachieved by entrainment of oxygen bubbles at
the surface of the liquid.the surface of the liquid.

34. Shake flasksShake flasks
 Although higher oxygen transfer rates can beAlthough higher oxygen transfer rates can be
achieved with shake flasks than with standingachieved with shake flasks than with standing
cultures, oxygen transfer limitations will still becultures, oxygen transfer limitations will still be
unavoidable particularly when trying to achieveunavoidable particularly when trying to achieve
high cell densities.high cell densities.
 The rate of oxygen transfer in shake flasks isThe rate of oxygen transfer in shake flasks is
dependent on thedependent on the
• shaking speedshaking speed
• the liquid volumethe liquid volume
• shake flask designshake flask design

35. Shake flasks OShake flasks O22 TransferTransfer
kLa decreases with liquid volume kLa is higher when baffles are present
kLa increases with liquid surface area
kLa
kLa
k
L
a
kLa

36. Shake flasks OShake flasks O22 TransferTransfer
 The kThe kLLa will increase with the shaking speed.a will increase with the shaking speed.
 At high shaking speeds, bubbles becomeAt high shaking speeds, bubbles become
entrained into the medium to further increasesentrained into the medium to further increases
the oxygen transfer rate.the oxygen transfer rate.
 The presence of baffles in the flasks will furtherThe presence of baffles in the flasks will further
increase the oxygen transfer efficiency,increase the oxygen transfer efficiency,
particularly for orbital shakers.particularly for orbital shakers.
 The following photographs show how bafflesThe following photographs show how baffles
increase the level of gas entrainment in a shakeincrease the level of gas entrainment in a shake
flask being shaken in an orbital shaker at 150flask being shaken in an orbital shaker at 150
rpmrpm

37. Unbaffled flask Baffled flask

38. Shake flasks OShake flasks O22 TransferTransfer
 Note the high level of foam formation in the baffled flaskNote the high level of foam formation in the baffled flask
due to the higher level of gas entrainment.due to the higher level of gas entrainment.
 The same improvement in oxygen transfer is not asThe same improvement in oxygen transfer is not as
evident with horizontal reciprocating shakers.evident with horizontal reciprocating shakers.
 The appropriate liquid volume is determined by the flaskThe appropriate liquid volume is determined by the flask
volume. For example, for a standard 250ml flask, thevolume. For example, for a standard 250ml flask, the
liquid volume should not exceed 70 ml while for a 1 litreliquid volume should not exceed 70 ml while for a 1 litre
flask, the liquid volume should be less than 200 ml.flask, the liquid volume should be less than 200 ml.

Larger liquid volumes can be used with wide based flasksLarger liquid volumes can be used with wide based flasks

39. Mechanically stirredMechanically stirred
bioreactorsbioreactors

40.  For aeration of liquid volumes greater than 200 ml,For aeration of liquid volumes greater than 200 ml,
various options are available.various options are available.
 Non-sparged mechanically agitated bioreactors canNon-sparged mechanically agitated bioreactors can
supply sufficient aeration for microbial fermentationssupply sufficient aeration for microbial fermentations
with liquid volumes up to 3 litres.with liquid volumes up to 3 litres.
 However, stirring speeds of up to 600 rpm may beHowever, stirring speeds of up to 600 rpm may be
required before the culture is not oxygen limited.required before the culture is not oxygen limited.
 In non-sparged reactors, oxygen is transferred fromIn non-sparged reactors, oxygen is transferred from
the head-space above the fermenter liquid. Agitationthe head-space above the fermenter liquid. Agitation
continually breaks the liquid surface and increasescontinually breaks the liquid surface and increases
the surface area for oxygen transfer.the surface area for oxygen transfer.
Mechanically stirredMechanically stirred
bioreactorsbioreactors

41. Mechanically stirred reactors -Mechanically stirred reactors -
Sparged stirred tank bioreactorsSparged stirred tank bioreactors
 For liquid volumes greater than 3 litres, air sparging isFor liquid volumes greater than 3 litres, air sparging is
required for effective oxygen transfer.required for effective oxygen transfer.
 The introduction of bubbles into the culture fluid byThe introduction of bubbles into the culture fluid by
sparging, leads to a dramatic increase in the oxygensparging, leads to a dramatic increase in the oxygen
transfer area.transfer area.
 Agitation is used to break up bubbles and thus furtherAgitation is used to break up bubbles and thus further
increase kincrease kLLa.a.
 Sparged fermenters required significantly lower agitationSparged fermenters required significantly lower agitation
speeds for aeration efficiencies comparable to thosespeeds for aeration efficiencies comparable to those
achieved in non-sparged fermenters.achieved in non-sparged fermenters.
 Air-sparged fermenters can have liquid volumes greaterAir-sparged fermenters can have liquid volumes greater
than 500,000 litres.than 500,000 litres.

43. Bubble driven bioreactorsBubble driven bioreactors
 Sparging without mechanical agitation can also be used forSparging without mechanical agitation can also be used for
aeration and agitation. Two classes of bubble drivenaeration and agitation. Two classes of bubble driven
bioreactors arebioreactors are bubble column fermentersbubble column fermenters andand airliftairlift
fermentersfermenters..
 Bubble driven bioreactors are commonly used in the cultureBubble driven bioreactors are commonly used in the culture
of shear sensitive organisms such as moulds and plantof shear sensitive organisms such as moulds and plant
cells. An airlift fermenter differs from bubble columncells. An airlift fermenter differs from bubble column
bioreactors by the presence of a draft tube which providesbioreactors by the presence of a draft tube which provides
better mass and heat transfer efficiencies.better mass and heat transfer efficiencies.
 Airlift fermenters are however considerably more expensiveAirlift fermenters are however considerably more expensive
to construct than bubble column reactors. There areto construct than bubble column reactors. There are
several designs for air-lift fermenters although the mostseveral designs for air-lift fermenters although the most
commonly used design is one with a central draft tube.commonly used design is one with a central draft tube.

44. Bubble driven bioreactorsBubble driven bioreactors

45. Bubble driven bioreactorsBubble driven bioreactors
 An airlift fermenter differs from bubble column bioreactors byAn airlift fermenter differs from bubble column bioreactors by
the presence of a draft tube which providesthe presence of a draft tube which provides
• better mass and heat transfer efficienciesbetter mass and heat transfer efficiencies
• more uniform shear conditions.more uniform shear conditions.
 Bubble driven fermenters are generally tall with liquid height toBubble driven fermenters are generally tall with liquid height to
base ratios of between 8:1 and 20:1.base ratios of between 8:1 and 20:1.
 The tall design of these fermenters leads to high gas hold-ups,The tall design of these fermenters leads to high gas hold-ups,
long bubble residence times and a region of high hydrostaticlong bubble residence times and a region of high hydrostatic
pressure near the sparger at the base of the fermenter.pressure near the sparger at the base of the fermenter.
 These factors lead to high values of kThese factors lead to high values of kLLa and Ca and Coo
**
thus enhancedthus enhanced
oxygen transfer ratesoxygen transfer rates

46. Airlift bioreactorsAirlift bioreactors
 An airlift fermenter differs from bubble columnAn airlift fermenter differs from bubble column
bioreactors by the presence of a draft tube.bioreactors by the presence of a draft tube.
 The main functions of the draft tube are to:The main functions of the draft tube are to:
• Increase mixing through the reactorIncrease mixing through the reactor
The presence of the draft tube enhancesThe presence of the draft tube enhances
axial mixing throughout the whole reactoraxial mixing throughout the whole reactor
• Reduce bubble coalescence.Reduce bubble coalescence.
This presumably occurs due to circulatoryThis presumably occurs due to circulatory
effect that the draft tube induces in theeffect that the draft tube induces in the
reactor. The circulation occurs in onereactor. The circulation occurs in one
direction and hence the bubbles also traveldirection and hence the bubbles also travel
in one direction.in one direction.

47. Airlift bioreactorsAirlift bioreactors
Small bubbles lead to an increased surface area for oxygen transfer.

48. Airlift bioreactorsAirlift bioreactors
 Equalize shear forces throughout the reactor.Equalize shear forces throughout the reactor.
Major reason why the productivity of cells grown in airliftMajor reason why the productivity of cells grown in airlift
bioreactors havebioreactors have higher productivitieshigher productivities than those grown inthan those grown in
stirred tank reactors.stirred tank reactors.

49. Airlift bioreactorsAirlift bioreactors
 The major disadvantages of air-lift fermenters areThe major disadvantages of air-lift fermenters are
 high energy requirementshigh energy requirements
 excessive foamingexcessive foaming
 cell damage due to bubble bursting; particularlycell damage due to bubble bursting; particularly
with animal cell culturewith animal cell culture

50. Airlift bioreactorAirlift bioreactor
Air-riser and down-comerAir-riser and down-comer
 An air-lift reactor is divided into threeAn air-lift reactor is divided into three
regions:regions:
- the air-riser- the air-riser
- down-comer- down-comer
- disengagement zone.- disengagement zone.

51. Airlift bioreactorAirlift bioreactor

52. Airlift bioreactorAirlift bioreactor
 The region into which bubbles are sparged is called theThe region into which bubbles are sparged is called the air-riserair-riser..
The air-riser may be on the inside or the outside of the draft-tube.The air-riser may be on the inside or the outside of the draft-tube.
The latter design is preferred for large scale fermenters as itThe latter design is preferred for large scale fermenters as it
provides better heat transfer efficiencies.provides better heat transfer efficiencies.
 The rising bubbles in the air-riser cause the liquid to flow in aThe rising bubbles in the air-riser cause the liquid to flow in a
vertical direction. To counteract these upward forces, liquid willvertical direction. To counteract these upward forces, liquid will
flow in a downward direction in theflow in a downward direction in the down-comerdown-comer. This leads to. This leads to
liquid circulation and thus improved mixing efficiencies asliquid circulation and thus improved mixing efficiencies as
compared to bubble columns.compared to bubble columns.
 The enhanced liquid circulation also causes bubbles to move in aThe enhanced liquid circulation also causes bubbles to move in a
uniform direction at a relatively uniform velocity. This bubble flowuniform direction at a relatively uniform velocity. This bubble flow
pattern reduces bubble coalescence and thus results in higher kpattern reduces bubble coalescence and thus results in higher kLLaa
values as compared to bubble column reactors.values as compared to bubble column reactors.

53. Airlift bioreactors -Airlift bioreactors -
Disengagement zoneDisengagement zone

54. Airlift bioreactors -Airlift bioreactors -
Disengagement zoneDisengagement zone
 The roles of the disengagement zone are toThe roles of the disengagement zone are to
• add volume to the reactor,add volume to the reactor,
• reduce foaming andreduce foaming and
• minimise recirculation of bubbles throughminimise recirculation of bubbles through
the down comer.the down comer.

55. Airlift bioreactors -Airlift bioreactors -
Disengagement zoneDisengagement zone
 The sudden widening at the top of the reactor slows the bubbleThe sudden widening at the top of the reactor slows the bubble
velocity and thus disengages the bubbles from the liquid flow.velocity and thus disengages the bubbles from the liquid flow.
 Carbon-dioxide rich bubbles are thus prevented from enteringCarbon-dioxide rich bubbles are thus prevented from entering
the downcomer.the downcomer.
 The reduced bubble velocity in the disengagement zone alsoThe reduced bubble velocity in the disengagement zone also
leads to a reduction in the loss of medium due aerosol formation.leads to a reduction in the loss of medium due aerosol formation.
 The increase in area will also helps to stretch bubbles in foams,The increase in area will also helps to stretch bubbles in foams,
causing the bubbles to burst. The axial flow circulation caused bycausing the bubbles to burst. The axial flow circulation caused by
the draft tube also helps to reduce foamingthe draft tube also helps to reduce foaming

56. Packed bed and trickle flowPacked bed and trickle flow
bioreactorsbioreactors
 The topic of packed bed bioreactors was discussed inThe topic of packed bed bioreactors was discussed in
another lecture on immobilisation.another lecture on immobilisation.

57. Packed bed bioreactorsPacked bed bioreactors
 The rate of mass transfer between the cells and the mediumThe rate of mass transfer between the cells and the medium
depends on the flow rate and on the thickness of the biomassdepends on the flow rate and on the thickness of the biomass
film on or near the surface of the solid particles.film on or near the surface of the solid particles.
 Packed bed reactors often suffer from problems caused by poorPacked bed reactors often suffer from problems caused by poor
mass transfer rates and clogging. Despite this they are usedmass transfer rates and clogging. Despite this they are used
commercially with enzymatically catalysts and with slowly orcommercially with enzymatically catalysts and with slowly or
non-growing cells.non-growing cells.
 They are also used in the anaerobic treatment of high strengthThey are also used in the anaerobic treatment of high strength
wastewaters (eg. food processing wastes). Large plastic blockswastewaters (eg. food processing wastes). Large plastic blocks
are used as solid supports for the cells. These blocks have aare used as solid supports for the cells. These blocks have a
large surface area for cell immobilization and when packed inlarge surface area for cell immobilization and when packed in
the reactor are difficult to clog.the reactor are difficult to clog.

59. Trickle flow bioreactorsTrickle flow bioreactors
 Trickle bed reactors are a class of packed bedTrickle bed reactors are a class of packed bed
reactors in which the medium flows (or trickles)reactors in which the medium flows (or trickles)
over the solid particles. In these reactors, theover the solid particles. In these reactors, the
particles are not immersed in the liquid.particles are not immersed in the liquid.
The liquid medium trickles over the surface of the solids
on which the cells are immobilized
They are used widely in aerobic treatment of sewage.

60. Trickle flow bioreactorsTrickle flow bioreactors
 Oxygen transfer is enhanced by ensuring that the cells are coveredOxygen transfer is enhanced by ensuring that the cells are covered
by only a very thin layer of liquid, thus reducing the distance overby only a very thin layer of liquid, thus reducing the distance over
which the dissolved oxygen must diffuse to reach the cells.which the dissolved oxygen must diffuse to reach the cells.

61. Trickle flow bioreactorsTrickle flow bioreactors
 Because stirring is not used, considerable capitalBecause stirring is not used, considerable capital
costs are saved.costs are saved.
 However, oxygen transfer rates per unit volume areHowever, oxygen transfer rates per unit volume are
low compared with spared stirred tank systems.low compared with spared stirred tank systems.
 Trickle flow systems are used widely for the aerobicTrickle flow systems are used widely for the aerobic
treatment of sewage.treatment of sewage.
 They are used to polish effluent from the activatedThey are used to polish effluent from the activated
sludge or anaerobic digestion process and for thesludge or anaerobic digestion process and for the
nitrification of ammonia.nitrification of ammonia.

62. Fluidized bed reactorsFluidized bed reactors

63. Fluidized bed reactorsFluidized bed reactors
 Fluidised bed bioreactors are one method of maintaining highFluidised bed bioreactors are one method of maintaining high
biomass concentrations and at the same time good massbiomass concentrations and at the same time good mass
transfer rates in continuous cultures.transfer rates in continuous cultures.
 Fluidised bed bioreactors are an example of reactors in whichFluidised bed bioreactors are an example of reactors in which
mixing is assisted by the action of a pump. In a fluidised bedmixing is assisted by the action of a pump. In a fluidised bed
reactor, cells or enzymes are immobilised in and/or on thereactor, cells or enzymes are immobilised in and/or on the
surface of light particles.surface of light particles.
 A pump located at the base of the tank causes the immobilisedA pump located at the base of the tank causes the immobilised
catalysts to move with the fluid. The pump pushes the fluid andcatalysts to move with the fluid. The pump pushes the fluid and
the particles in a vertical direction. The upward force of thethe particles in a vertical direction. The upward force of the
pump is balanced by the downward movement of the particlespump is balanced by the downward movement of the particles
due to gravity. This results in good circulation.due to gravity. This results in good circulation.

64. Fluidised bed reactorsFluidised bed reactors
 For aerobic microbial systems, sparging is used toFor aerobic microbial systems, sparging is used to
improve oxygen transfer rates.improve oxygen transfer rates.
 A draft tube may be used to improve circulationA draft tube may be used to improve circulation
and oxygen transfer. Both aerobic and anaerobicand oxygen transfer. Both aerobic and anaerobic
fluidised bed bioreactors have been developed forfluidised bed bioreactors have been developed for
use in waste treatment.use in waste treatment.
 Fluidised beds can also be used with microcarrierFluidised beds can also be used with microcarrier
beads used in attached animal cell culture.beads used in attached animal cell culture.
 Fluidised-bed microcarrier cultures can beFluidised-bed microcarrier cultures can be
operated both in batch and continuous mode. Inoperated both in batch and continuous mode. In
the former the fermentation fluid is recycled in athe former the fermentation fluid is recycled in a
pump-aroundpump-around loop.loop.

65. Fluidized bed reactorsFluidized bed reactors

66. SummarySummary
 Looked at methods of aeration in differentLooked at methods of aeration in different
bioreactorsbioreactors
 Aeration in standing culturesAeration in standing cultures
 Oxygen transfer in shake flasksOxygen transfer in shake flasks
 Advantages and applications of mechanicallyAdvantages and applications of mechanically
stirred bioreactorsstirred bioreactors
 Bubble driven bioreactorsBubble driven bioreactors
 Airlift bioreactorsAirlift bioreactors
 Packed bed and trickle flow bioreactorsPacked bed and trickle flow bioreactors
 Fluidised bed bioreactorsFluidised bed bioreactors