Industrial Microbiology

INDM 4005

Lecture 14

23/03/04

Tutorial on Bioreactors

1. Introduction 2. Methods of aeration 3. Surface aeration 4. Shake flasks 5. Mechanically stirred bioreactors

(5.1) Sparged stirred tank bioreactors 6. Bubble driven bioreactors 7. Airlift bioreactors

(7.1) Air-riser and downcomer (7.2) Disengagement zone

8. Packed bed and trickle flow bioreactors

9. Fluidised bed bioreactors

Bioreactors- Introduction

• Previous lectures have stress the importance of considering process engineering factors when culturing cells.

• Biological factors include the characteristics of the cells, their maximum specific growth rate, Monod constant, yield coefficient, pH range and temperature range.

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

Bioreactors- Introduction• Likewise mass transfer, in particular, oxygen transfer was

highlighted as an important factor which determined how a reactor must be designed and operated.

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

• In this lecture, we shall look into how bioreactors are designed to meet cost, biological and engineering needs

2. Methods of Aeration• A bioreactor is a reactor system used for the culture of microorganisms.

They vary in size and complexity from a 10 ml volume in a test tube to computer controlled fermenters with liquid volumes greater than 100 m3. They similarly vary in cost from a few cents to a few million dollars.

• In the following sections we will compare the following reactors

Standing cultures

Shake flasks

Stirred tank reactors

Bubble column and airlift reactors

Fluidized bed reactors

3. Standing cultures

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

Standing cultures

• The rate of oxygen transfer will be poor due to the small surface area for transfer. Standing cultures are commonly used in small scale laboratory systems in which oxygen supply is not critical. For example, biochemical tests used for the identification of bacteria are often performed in test-tubes containing between 5-10 ml of media.

• T-flasks used in the small scale culture of animal cells are another example of a standing culture. T-flasks are normally incubated horizontally to increase the surface area

for oxygen transfer.

• The surface aeration rate in standing cultures can be increased by using large volume flasks.

• The following photograph shows a 250 ml Erlenmeyer flask containing 100 ml of medium and a 3 litre "Fernback" flask containing 1 litre of medium.

Note how the latter has a large surface area.

Standing cultures

• Large Pyrex flasks are used for the small scale production of fermented products. One example is Kombucha tea which is a tea brewed by mixture of yeasts and acetic acid bacteria.

• 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 niger mycelia

Standing cultures• Aerobic solid substrate fermentations are another example of

standing cultures. In these fermentations, the biomass is grown on solid biodegradable substrates such as water softened bran, rice or barley.

• The solids may be continuously or periodically turned over to improve aeration and to regulate the culture temperature. One example of a commercial scale, solid substrate fermentation is the production of koji by Aspergillus oryzae on soya beans which is part of the soya sauce process.

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

4. Shake flasks

Shake flasks

• Shake flasks are commonly used for small scale cell cultivation.

• Through continuous shaking of the culture fluid, higher oxygen transfer rates can be achieved as compared to standing cultures.

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

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

Shake flasks

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

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

shaking speed

the liquid volume

shake flask design

Shake flasks O2 TransferkLa decreases with liquid volume kLa is higher when baffles are present

kLa increases with liquid surface area

kLa

kLa

k

L

a

kLa

Shake flasks O2 Transfer

• The kLa will increase with the shaking speed.

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

• The presence of baffles in the flasks will further increase the oxygen transfer efficiency, particularly for orbital shakers.

• The following photographs show how baffles increase the level of gas entrainment in a shake flask being shaken in an orbital

shaker at 150 rpm

Unbaffled flask Baffled flask

Shake flasks O2 Transfer

• Note the high level of foam formation in the baffled flask due to the higher level of gas entrainment.

• The same improvement in oxygen transfer is not as evident with horizontal reciprocating shakers.

• The appropriate liquid volume is determined by the flask volume. For example, for a standard 250ml flask, the liquid volume should not exceed 70 ml while for a 1 litre flask, the liquid volume should be less than 200 ml.

• Larger liquid volumes can be used with wide based

flasks

5. Mechanically stirred bioreactors

Mechanically stirred bioreactors• For aeration of liquid volumes greater than 200 ml, various

options are available.

• Non-sparged mechanically agitated bioreactors can supply sufficient aeration for microbial fermentations with liquid volumes up to 3 litres.

• However, stirring speeds of up to 600 rpm may be required before the culture is not oxygen limited.

• In non-sparged reactors, oxygen is transferred from the head-space above the fermenter liquid. Agitation continually breaks the liquid surface and increases the surface area for

oxygen transfer.

(5.1) Mechanically stirred reactors

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

effective oxygen transfer.

• The introduction of bubbles into the culture fluid by sparging, leads to a

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

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

• Air-sparged fermenters can have liquid volumes greater than 500,000

litres.

6. Bubble driven bioreactors • Sparging without mechanical agitation can also be used for aeration

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

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

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

with a central draft tube.

Bubble driven bioreactors

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

presence of a draft tube which provides

better mass and heat transfer efficiencies

more uniform shear conditions.

• Bubble driven fermenters are generally tall with liquid height to base ratios of between 8:1 and 20:1.

• The tall design of these fermenters leads to high gas hold-ups, long bubble residence times and a region of high hydrostatic pressure near the sparger at the base of the fermenter.

• These factors lead to high values of kLa and Co* thus enhanced oxygen

transfer rates

7. Airlift bioreactors • An airlift fermenter differs from bubble column bioreactors by

the presence of a draft tube.

• The main functions of the draft tube are to:

Increase mixing through the reactor The presence of the draft tube enhances axial mixing throughout the whole reactor

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

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

Airlift bioreactors• Equalise shear forces throughout the reactor.

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

Airlift bioreactors

• The major disadvantages of air-lift fermenters are

- high energy requirements

- excessive foaming

- cell damage due to bubble bursting; particularly with animal cell culture

(7.1) Airlift bioreactor Air-riser and down-comer

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

- the air-riser

- down-comer

- disengagement zone.

Airlift bioreactor

Airlift bioreactor• The region into which bubbles are sparged is called the air-riser. 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 better heat transfer efficiencies.

• 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 downward direction in the down-comer. This leads to liquid circulation and thus improved mixing efficiencies as compared to bubble columns.

• The enhanced liquid circulation also causes bubbles to move in a uniform direction at a relatively uniform velocity. This bubble flow pattern reduces bubble coalescence and thus results in higher kLa values as compared to

bubble column reactors.

(7.2) Airlift bioreactors - Disengagement zone

Airlift bioreactors - Disengagement zone

• The roles of the disengagement zone are to

add volume to the reactor,

reduce foaming and

minimise recirculation of bubbles through the down comer.

Airlift bioreactors - Disengagement zone

• The sudden widening at the top of the reactor slows the bubble velocity and thus disengages the bubbles from the liquid flow.

• Carbon-dioxide rich bubbles are thus prevented from entering the downcomer.

• The reduced bubble velocity in the disengagement zone also leads to a reduction in the loss of medium due aerosol formation.

• The increase in area will also helps to stretch bubbles in foams, causing the bubbles to burst. The axial flow circulation caused by

the draft tube also helps to reduce foaming

8. Packed bed and trickle flow bioreactors

• The topic of packed bed bioreactors was discussed in another

lecture on immobilisation.

Packed bed bioreactors• The rate of mass transfer between the cells and the medium depends on the

flow rate and on the thickness of the biomass film on or near the surface of the solid particles.

• Packed bed reactors often suffer from problems caused by poor mass transfer rates and clogging. Despite this they are used commercially with enzymatically

catalysts and with slowly or non-growing cells.

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

Trickle flow bioreactors• Trickle bed reactors are a class of packed bed reactors in

which the medium flows (or trickles) over the solid particles. In these reactors, the 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 bioreactors• Oxygen transfer is enhanced by ensuring that the cells are covered by only

a very thin layer of liquid, thus reducing the distance over which the dissolved oxygen must diffuse to reach the cells.

Trickle flow bioreactors• Because stirring is not used, considerable capital costs are saved.

• However, oxygen transfer rates per unit volume are low compared with sparged stirred tank systems.

• Trickle flow systems are used widely for the aerobic treatment of sewage.

• They are used to polish effluent from the activated sludge or

anaerobic digestion process and for the nitrification of ammonia.

9. Fluidised bed reactors

Fluidised bed reactors• Fluidised bed bioreactors are one method of maintaining high

biomass concentrations and at the same time good mass transfer rates in continuous cultures.

• Fluidised bed bioreactors are an example of reactors in which mixing is assisted by the action of a pump. In a fluidised bed reactor, cells or enzymes are immobilised in and/or on the surface of light particles.

• A pump located at the base of the tank causes the immobilised catalysts to move with the fluid. The pump pushes the fluid and the particles in a vertical direction. The upward force of the pump is balanced by the downward movement of the particles due to gravity.

This results in good circulation.

Fluidised bed reactors

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

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

• Fluidised beds can also be used with microcarrier beads used in attached animal cell culture.

• Fluidised-bed microcarrier cultures can be operated both in batch and continuous mode. In the former the fermentation

fluid is recycled in a pump-around loop.

Fluidised bed reactors

Summary

• Looked at methods of aeration in different bioreactors

• Aeration in standing cultures• Oxygen transfer in shake flasks• Advantages and applications of mechanically

stirred bioreactors • Bubble driven bioreactors• Airlift bioreactors• Packed bed and trickle flow bioreactors• Fluidised bed bioreactors