bioreactor basis

Industrial MicrobiologyIndustrial MicrobiologyBASIS OF BIOREACTOR FOR BASIS OF BIOREACTOR FOR

BIOPHARMACEUTICALSBIOPHARMACEUTICALSAngel L. Salamán, PhDAngel L. Salamán, PhD

[email protected]@yahoo.com

Tutorial on BioreactorsTutorial on Bioreactors

1. Introduction1. Introduction 2. O2 uptake and Stoichiometry 2. 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

Bioreactors- IntroductionBioreactors- Introduction Previous lectures have stress the importance of considering Previous 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, their Biological factors include the characteristics of the cells, their

maximum specific growth rate, yield coefficient, pH range and maximum specific growth rate, yield coefficient, pH range and temperature range. temperature range.

We have seen however that the productivity of a fermentation is We have seen however that the productivity of a fermentation is determined by the mode of operation of the fermentation determined by the mode of operation of the fermentation process; eg. the advantages of fed-batch and continuous process; eg. the advantages of fed-batch and continuous fermentations over batch fermentations.fermentations over batch fermentations.

The oxygen demand of an industrial process is The oxygen demand of an industrial process is generally satisfied by aeration and agitationgenerally satisfied by aeration and agitation

Productivity is limited by oxygen availability and Productivity is limited by oxygen availability and therefore it is important to the factors that affect a therefore 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 O2 2 transfer and factors influencing transfer and factors influencing the transfer of Othe transfer of O22 into solution into solution

Bioreactors- IntroductionBioreactors- Introduction

Bioreactors- IntroductionBioreactors- Introduction

Likewise mass transfer, in particular, oxygen Likewise mass transfer, in particular, oxygen transfer was highlighted as an important factor transfer was highlighted as an important factor which determined how a reactor must be which determined how a reactor must be designed and operated. designed and operated.

Cost was also described as an important Cost was also described as an important consideration. The larger the reactor or the faster consideration. The larger the reactor or the faster the stirrer speed, the greater the costs involved. the stirrer speed, the greater the costs involved.

How bioreactors are designed to meet cost, How bioreactors are designed to meet cost, biological and engineering needs…biological and engineering needs…

MASS TRANSFER and PHASESMASS TRANSFER and PHASESDifferent phases present -IntroductionDifferent phases present -Introduction

Fundamental concept in fermentation technology is transfer of materials (e.g Fundamental concept in fermentation technology is transfer of materials (e.g nutrients, products, gases etc.) through different phases (e.g gas into a liquid).nutrients, products, gases etc.) through different phases (e.g gas into a liquid).

Major problem associated with provision of oxygen to the cell - is a rate limiting Major problem associated with provision of oxygen to the cell - is a rate limiting step and thus serves as a model system to understand mass transfer.step and thus serves as a model system to understand mass transfer.

The rate of oxygen transfer = driving force / resistanceThe rate of oxygen transfer = driving force / resistance.. E.g resistance E.g resistance to mass transfer from medium to mo`s are complex and may arise from;to mass transfer from medium to mo`s are complex and may 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)

The following diagram serves to illustrate the different The following diagram serves to illustrate the different phases and material that are relevant in general transport phases and material that are relevant in general transport processes associated with fermentation technology; processes associated with fermentation technology;

Dispersed gases Dissolvednutrients

Solid and

Immiscible

liquid

nutrients

Floc

Cells

Products in

water

MASS TRANSFER

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 OrganellesSolid (e.g particles of

substrate)MineralsEnzymes

Enzymes

......... 1 2 ..........

1 = reactant supply and utilisation2 = product removal and formation

• 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

1) The bulk gas phase in the bubble2) The gas-liquid interphase3) The liquid film around the bubble4) The bulk liquid culture medium5) The liquid film around the microbial cells 6) The cell-liquid interphase7) The intracellular oxygen transfer resistance

1

2

3

4

5

67

Gas bubble

Liquid film

Microbial cell

Oxygen Mass Transfer Steps

Stoichiometry of respirationStoichiometry of respirationTo consider the Stoichiometry of respiration the oxidation of glucose may be represented as;

C6H12O6 + 6O2 = 6H2O + 6CO2

Atomic weight of CarbonHydrogenOxygen

12116

Molecular weight of glucose is 180

How many grams of oxygen are required to oxidise 180g of glucose?

Answer 192g

Solubility of OxygenSolubility of Oxygen Both components oxygen and glucose must be in Both components oxygen and glucose must be in

solution before they become available to solution before they become available to microorganismsmicroorganisms

Oxygen is 6000 times less soluble in water than Oxygen is 6000 times less soluble in water than glucoseglucose

A saturated oxygen solution contains only10mg dmA saturated oxygen solution contains only10mg dm--

33 of oxygen of oxygen Impossible to add enough oxygen to a microbial Impossible 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 a Oxygen must be added during growth at a

sufficient rate to satisfy requirements sufficient rate to satisfy requirements

Comparison of conc. driving forces and uptake Comparison 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 by Problems 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 sat @ 25sat @ 25ooCC

Conc. in bulk brothConc. in bulk broth 10,000 ppm10,000 ppm approx. 7 ppm approx. 7 ppm

Critical conc .Critical conc . 100 ppm100 ppm 0.8 ppm 0.8 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

MASS TRANSFER and RESPIRATIONMASS TRANSFER and RESPIRATION

(a) Mass balance(a) Mass balance

StoichiometryStoichiometry of respiration e.g glucose;of respiration e.g glucose;

CC66HH1212OO6 6 + 6O + 6O22 6H 6H22O + 6 COO + 6 CO22

Oxidation of Oxidation of 180 gms Glucose180 gms Glucose requires requires 192 gms O192 gms O22

Compare with a hydrocarbon (i.e Compare with a hydrocarbon (i.e 6 CH6 CH22))

The Oxygen requirements of The 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 and Stoichiometry of conversion of oxygen, carbon and nitrogen into biomass has been elucidatednitrogen into biomass has been elucidated

Use these relationships to predict the oxygen demand Use these relationships to predict the oxygen demand for a fermentationfor a fermentation

Darlington (1964) expressed composition of 100g of dry Darlington (1964) expressed composition of 100g of dry yeastyeast C C 3.923.92 H H 6.56.5 O O 1.941.94

OO22 Requirements Requirements

6.67CH6.67CH22O + 2.1OO + 2.1O22 = C = C 3.923.92 H H 6.56.5 O O 1.94 1.94 + 2.75CO+ 2.75CO22 + 3.42H + 3.42H22OO

7.14CH7.14CH22 + 6.135O + 6.135O22 = C = C 3.923.92 H H 6.56.5 O O 1.94 1.94 + 3.22CO+ 3.22CO22 + 3.89H + 3.89H22OO

where CHwhere CH22 = hydrocarbon = hydrocarbonCHCH22O = carbohydrateO = carbohydrate

From the above equations to produce 100g of yeast from From the above equations to produce 100g of yeast from hydrocarbon requires three times the amount of oxygen hydrocarbon requires three times the amount of oxygen than from carbohydratethan from carbohydrate

Compare solubility of Oxygen vs Glucose ( e.g. oxygen = 9.0 Compare solubility of Oxygen vs Glucose ( e.g. oxygen = 9.0 mg/l @ 20mg/l @ 20ooC, 11.3 mg/l @ 10C, 11.3 mg/l @ 10ooC)C)

Thus must consider;Thus must consider;

Requirement for oxygen important in biotechnological Requirement 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

Case Study:Case Study:

Give the chemical properties of oxygen, why is it Give the chemical properties of oxygen, why is it so important to life?so important to life?

Give examples of biochemical pathways (of commercial Give 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 products What type of bioreactor is used in the production of the products

chosen?chosen?

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

Critical dissolved oxygen levels for a Critical dissolved oxygen levels for a range of microorganismsrange of microorganisms

Organism Temperature Critical dissolvedoC 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

Critical dissolved oxygen levelsCritical dissolved oxygen levels To maximize biomass production you must satisfy the organisms To maximize biomass production you must satisfy the organisms

specific oxygen demand by maintaining the dissolved Ospecific oxygen demand by maintaining the dissolved O22 levels levels 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 formation levels 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.

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 large vs small cells (bacteria v mammalian cells)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)

Diffusivity of gas

BULK LIQUID

??

? ??

?

CELLS

O2

Size of sparger gas bubble

Gas composition, volume & velocity

Design of Impellersize, no. of bladesrotational speed Baffles

width, number

FACTORS INFLUENCING OXYGEN SUPPLY

Foam/antifoam

Temperature

Type of liquid

Height/width ratio

‘’Hold up’’

Process factors

Methods of AerationMethods of Aeration A bioreactor is a reactor system used for the culture of A bioreactor is a reactor system used for the culture of

microorganisms. They vary in size and complexity from a 10 ml microorganisms. They vary in size and complexity from a 10 ml volume in a test tube to computer controlled fermenters with volume 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 reactors In the following sections we will compare the following reactors

Standing cultures Standing cultures

Shake flasks Shake flasks

Stirred tank reactors Stirred tank reactors

Bubble column and airlift reactors Bubble column and airlift reactors

Fluidized bed reactorsFluidized bed reactors

Standing culturesStanding cultures

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


The rate of oxygen transfer will be poor due to the The rate of oxygen transfer will be poor due to the small surface area for transfer. Standing cultures small surface area for transfer. Standing cultures are commonly used in small scale laboratory are commonly used in small scale laboratory systems in which oxygen supply is not critical. For systems in which oxygen supply is not critical. For example, biochemical tests used for the example, biochemical tests used for the identification of bacteria are often performed in identification 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 animal T-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 to flasks are normally incubated horizontally to increase the surface area for oxygen transfer.increase the surface area for oxygen transfer.

The surface aeration rate in standing cultures can be increased The 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 flask The following photograph shows a 250 ml Erlenmeyer flask containing 100 ml of medium and a 3 litre "Fernback" flask containing 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.


• 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.

Aspergillus nigerAspergillus niger mycelia mycelia


Aerobic solid substrate fermentations are another Aerobic 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 biodegradable the biomass is grown on solid biodegradable substrates. substrates.

The solids may be continuously or periodically turned The solids may be continuously or periodically turned over to improve aeration and to regulate the culture over 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 koji solid substrate fermentation is the production of koji by by Aspergillus oryzaeAspergillus oryzae on soya beans which is part of on soya beans which is part of the soya sauce process. the soya sauce process.

Another is mushroom cultivation. Considerable Another is mushroom cultivation. Considerable research is currently being invested into the research is currently being invested into the feasibility of producing biochemicals by solid feasibility of producing biochemicals by solid substrate fermentations.substrate fermentations.

Shake flasksShake flasks


Shake flasks are commonly used for small scale Shake 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 as higher oxygen transfer rates can be achieved as compared to standing cultures. compared to standing cultures.

Shaking continually breaks the liquid surface and Shaking continually breaks the liquid surface and thus provides a greater surface area for oxygen thus provides a greater surface area for oxygen transfer. transfer.

Increased rates of oxygen transfer are also Increased rates of oxygen transfer are also achieved by entrainment of oxygen bubbles at achieved by entrainment of oxygen bubbles at the surface of the liquid. the surface of the liquid.


Although higher oxygen transfer rates can be Although higher oxygen transfer rates can be achieved with shake flasks than with standing achieved with shake flasks than with standing cultures, oxygen transfer limitations will still be cultures, oxygen transfer limitations will still be unavoidable particularly when trying to achieve unavoidable particularly when trying to achieve high cell densities. high cell densities.

The rate of oxygen transfer in shake flasks is The rate of oxygen transfer in shake flasks is dependent on the dependent on the

shaking speed shaking speed the liquid volume the liquid volume shake flask designshake flask design

Shake flasks OShake flasks O22 Transfer Transfer

kLa decreases with liquid volume kLa is higher when baffles are present

kLa increases with liquid surface area

kLa

kLa

k

L

a

kLa


The kThe kLLa will increase with the shaking speed. a will increase with the shaking speed.

At high shaking speeds, bubbles become entrained At high shaking speeds, bubbles become entrained into the medium to further increases the oxygen into the medium to further increases the oxygen transfer rate. transfer rate.

The presence of baffles in the flasks will further The 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 baffles The following photographs show how baffles increase the level of gas entrainment in a shake increase the level of gas entrainment in a shake

flask being shaken in an orbital shaker at 150 rpmflask being shaken in an orbital shaker at 150 rpm

Unbaffled flask Baffled flask


Note the high level of foam formation in the baffled flask Note 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 as The 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 flask The appropriate liquid volume is determined by the flask volume. For example, for a standard 250ml flask, the volume. For example, for a standard 250ml flask, the liquid volume should not exceed 70 ml while for a 1 litre liquid 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

Mechanically stirred Mechanically stirred bioreactorsbioreactors

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 can Non-sparged mechanically agitated bioreactors can supply sufficient aeration for microbial fermentations supply 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 be However, 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 from In non-sparged reactors, oxygen is transferred from the head-space above the fermenter liquid. Agitation the head-space above the fermenter liquid. Agitation continually breaks the liquid surface and increases continually breaks the liquid surface and increases the surface area for oxygen transfer.the surface area for oxygen transfer.

Mechanically stirred Mechanically stirred bioreactorsbioreactors

Mechanically stirred reactors - Mechanically stirred reactors -

Sparged stirred tank bioreactorsSparged stirred tank bioreactors For liquid volumes greater than 3 litres, air sparging is required For liquid volumes greater than 3 litres, air sparging is required

for effective oxygen transfer. for effective oxygen transfer.

The introduction of bubbles into the culture fluid by sparging, The introduction of bubbles into the culture fluid by sparging,

leads to a dramatic increase in the oxygen transfer area.leads to a dramatic increase in the oxygen transfer area. Agitation is used to break up bubbles and thus further increase Agitation is used to break up bubbles and thus further increase

kkLLa. a.

Sparged fermenters required significantly lower agitation speeds Sparged fermenters required significantly lower agitation speeds for aeration efficiencies comparable to those achieved in non-for aeration efficiencies comparable to those achieved in non-sparged fermenters. sparged fermenters.

Air-sparged fermenters can have liquid volumes greater than Air-sparged fermenters can have liquid volumes greater than

500,000 litres.500,000 litres.

Bubble driven bioreactorsBubble driven bioreactors Sparging without mechanical agitation can also be used for Sparging without mechanical agitation can also be used for

aeration and agitation. Two classes of bubble driven aeration and agitation. Two classes of bubble driven bioreactors are bioreactors are bubble column fermentersbubble column fermenters and and airlift airlift fermentersfermenters..

Bubble driven bioreactors are commonly used in the culture Bubble driven bioreactors are commonly used in the culture of shear sensitive organisms such as moulds and plant cells. of shear sensitive organisms such as moulds and plant cells. An airlift fermenter differs from bubble column bioreactors by An airlift fermenter differs from bubble column bioreactors by the presence of a draft tube which provides better mass and the presence of a draft tube which provides better mass and heat transfer efficiencies. heat transfer efficiencies.

Airlift fermenters are however considerably more expensive Airlift fermenters are however considerably more expensive to construct than bubble column reactors. There are several to construct than bubble column reactors. There are several designs for air-lift fermenters although the most commonly designs for air-lift fermenters although the most commonly

used design is one with a central draft tube.used design is one with a central draft tube.

Bubble driven bioreactorsBubble driven bioreactors

Bubble driven bioreactorsBubble driven bioreactors An airlift fermenter differs from bubble column bioreactors by the An airlift fermenter differs from bubble column bioreactors by the

presence of a draft tube which provides presence of a draft tube which provides

better mass and heat transfer efficiencies better mass and heat transfer efficiencies

more uniform shear conditions. more uniform shear conditions.

Bubble driven fermenters are generally tall with liquid height to Bubble 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 hydrostatic long 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 enhanced thus enhanced

oxygen transfer ratesoxygen transfer rates

Airlift bioreactorsAirlift bioreactors An airlift fermenter differs from bubble column An 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 enhances axial The presence of the draft tube enhances axial mixing throughout the whole reactor mixing throughout the whole reactor

Reduce bubble coalescence.Reduce bubble coalescence. This presumably occurs due to circulatory effect This presumably occurs due to circulatory effect that the draft tube induces in the reactor. The that the draft tube induces in the reactor. The circulation occurs in one direction and hence circulation occurs in one direction and hence the bubbles also travel in one direction.the bubbles also travel in one direction.

Airlift bioreactorsAirlift bioreactorsSmall bubbles lead to an increased surface area for oxygen transfer.

Airlift bioreactorsAirlift bioreactors Equalize shear forces throughout the reactor. Equalize shear forces throughout the reactor.

Major reason why the productivity of cells grown in airlift Major reason why the productivity of cells grown in airlift bioreactors have bioreactors have higher productivitieshigher productivities than those grown in than those grown in stirred tank reactors.stirred tank reactors.

Airlift bioreactorsAirlift bioreactors

The major disadvantages of air-lift fermenters areThe major disadvantages of air-lift fermenters are high energy requirements high energy requirements excessive foaming excessive foaming cell damage due to bubble bursting; particularly cell damage due to bubble bursting; particularly

with animal cell culture with animal cell culture

Airlift bioreactor Airlift bioreactor Air-riser and down-comerAir-riser and down-comer

An air-lift reactor is divided into three An air-lift reactor is divided into three regions:regions:

- the air-riser - the air-riser

- down-comer - down-comer

- disengagement zone. - disengagement zone.

Airlift bioreactorAirlift bioreactor

Airlift bioreactorAirlift bioreactor The region into which bubbles are sparged is called the The region into which bubbles are sparged is called the air-riserair-riser. The . 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 it provides latter design is preferred for large scale fermenters as it provides better heat transfer efficiencies. better heat transfer efficiencies.

The rising bubbles in the air-riser cause the liquid to flow in a vertical The rising bubbles in the air-riser cause the liquid to flow in a vertical direction. To counteract these upward forces, liquid will flow in a direction. To counteract these upward forces, liquid will flow in a downward direction in the downward direction in the down-comerdown-comer. This leads to liquid circulation . This leads to liquid circulation and thus improved mixing efficiencies as compared to bubble columns. and thus improved mixing efficiencies as compared to bubble columns.

The enhanced liquid circulation also causes bubbles to move in a The enhanced liquid circulation also causes bubbles to move in a uniform direction at a relatively uniform velocity. This bubble flow uniform 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 kLLa a

values as compared to bubble column reactors.values as compared to bubble column reactors.

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 and reduce foaming and minimise recirculation of bubbles through minimise recirculation of bubbles through

the down comer. the down comer.


The sudden widening at the top of the reactor slows the bubble The 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 entering Carbon-dioxide rich bubbles are thus prevented from entering the downcomer. the downcomer.

The reduced bubble velocity in the disengagement zone also The 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 by causing 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

Packed bed and trickle flow Packed bed and trickle flow bioreactorsbioreactors

The topic of packed bed bioreactors was discussed in The topic of packed bed bioreactors was discussed in

another lecture on immobilisation.another lecture on immobilisation.

Packed bed bioreactorsPacked bed bioreactors

The rate of mass transfer between the cells and the medium The rate of mass transfer between the cells and the medium depends on the flow rate and on the thickness of the biomass depends 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 poor Packed bed reactors often suffer from problems caused by poor mass transfer rates and clogging. Despite this they are used mass transfer rates and clogging. Despite this they are used commercially with enzymatically catalysts and with slowly or commercially with enzymatically catalysts and with slowly or

non-growing cells.non-growing cells. They are also used in the anaerobic treatment of high strength They are also used in the anaerobic treatment of high strength

wastewaters (eg. food processing wastes). Large plastic blocks wastewaters (eg. food processing wastes). Large plastic blocks are used as solid supports for the cells. These blocks have a are used as solid supports for the cells. These blocks have a large surface area for cell immobilization and when packed in large surface area for cell immobilization and when packed in the reactor are difficult to clog.the reactor are difficult to clog.

Trickle flow bioreactorsTrickle flow bioreactors Trickle bed reactors are a class of packed bed Trickle 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, the over 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.

Trickle flow bioreactorsTrickle flow bioreactors

Oxygen transfer is enhanced by ensuring that the cells are Oxygen transfer is enhanced by ensuring that the cells are covered by only a very thin layer of liquid, thus reducing the covered by only a very thin layer of liquid, thus reducing the distance over which the dissolved oxygen must diffuse to reach distance over which the dissolved oxygen must diffuse to reach the cells.the cells.

Trickle flow bioreactorsTrickle flow bioreactors Because stirring is not used, considerable capital Because stirring is not used, considerable capital

costs are saved. costs are saved. However, oxygen transfer rates per unit volume are However, 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 aerobic Trickle flow systems are used widely for the aerobic

treatment of sewage. treatment of sewage. They are used to polish effluent from the activated They are used to polish effluent from the activated

sludge or anaerobic digestion process and for the sludge or anaerobic digestion process and for the

nitrification of ammonia.nitrification of ammonia.

Fluidized bed reactorsFluidized bed reactors Fluidised bed bioreactors are one method of maintaining high Fluidised bed bioreactors are one method of maintaining high

biomass concentrations and at the same time good mass biomass 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 which Fluidised bed bioreactors are an example of reactors in which mixing is assisted by the action of a pump. In a fluidised bed mixing is assisted by the action of a pump. In a fluidised bed reactor, cells or enzymes are immobilised in and/or on the reactor, 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 immobilised A pump located at the base of the tank causes the immobilised catalysts to move with the fluid. The pump pushes the fluid and catalysts to move with the fluid. The pump pushes the fluid and the particles in a vertical direction. The upward force of the the particles in a vertical direction. The upward force of the pump is balanced by the downward movement of the particles pump 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.

Fluidised bed reactorsFluidised bed reactors

For aerobic microbial systems, sparging is used to For aerobic microbial systems, sparging is used to improve oxygen transfer rates. improve oxygen transfer rates.

A draft tube may be used to improve circulation and A draft tube may be used to improve circulation and oxygen transfer. Both aerobic and anaerobic fluidised oxygen transfer. Both aerobic and anaerobic fluidised bed bioreactors have been developed for use in waste bed bioreactors have been developed for use in waste treatment. treatment.

Fluidised beds can also be used with microcarrier Fluidised 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 be operated Fluidised-bed microcarrier cultures can be operated both in batch and continuous mode. In the former the both in batch and continuous mode. In the former the

fermentation fluid is recycled in a fermentation fluid is recycled in a pump-aroundpump-around loop. loop.

SummarySummary Looked at methods of aeration in different Looked 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 Advantages and applications of

mechanically stirred bioreactors mechanically stirred 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

bioreactor basis

Science

o22 transfer

phasesmass transfer

oxygenlikewise mass

bulk gas

process engineering

provision of oxygen

g gas intoe

fermentation process