Types of Types of BioreactorsBioreactors

Unit - 2Unit - 2

Continuous Stirred Tank Continuous Stirred Tank ReactorsReactors

A Bioreactor is a device that is used A Bioreactor is a device that is used to carry out one or more biochemical to carry out one or more biochemical reactions to convert substrate into reactions to convert substrate into productproduct

The product of conversion may be The product of conversion may be biomass, metabolites,etc.biomass, metabolites,etc.

Many different kinds of bioreactors Many different kinds of bioreactors are available for different types of are available for different types of products .products .

A bioreactor is used in all kinds of A bioreactor is used in all kinds of bioprocesses like bioprocesses like food processing,food processing, treating of waste water,treating of waste water, for making vaccines, antibiotics etc..for making vaccines, antibiotics etc..

Bioreactors are designed both for Bioreactors are designed both for submerged culturing and solid state submerged culturing and solid state fermentationsfermentations

Only 70-80% of the volume of stirred Only 70-80% of the volume of stirred reactors is filled with liquid.reactors is filled with liquid.

This allows adequate head space for This allows adequate head space for disengagement of droplets from the disengagement of droplets from the exhaust gas and to accommodate the exhaust gas and to accommodate the foam which may develop.foam which may develop.

If foaming is a problem, an impeller If foaming is a problem, an impeller called a foam breaker may be installed.called a foam breaker may be installed.

Alternatively, chemical antifoam Alternatively, chemical antifoam agents are added to the broth, but they agents are added to the broth, but they reduce the rate of O2 transfer ,so reduce the rate of O2 transfer ,so mechanical foam dispersal is generally mechanical foam dispersal is generally preferred.preferred.

The aspect ratio of stirred vessels,i.e the The aspect ratio of stirred vessels,i.e the ratio of height to diameter ,can be varied ratio of height to diameter ,can be varied over a wider range but the least expensive over a wider range but the least expensive shape to build has an aspect ratio of 1.shape to build has an aspect ratio of 1.

When aeration is required the aspect ratio When aeration is required the aspect ratio is usually increased. this provides longer is usually increased. this provides longer contact times between the rising bubbles contact times between the rising bubbles and liquid.and liquid.

Stirred fermenters are used for free and Stirred fermenters are used for free and immobilized enzyme reactions and for immobilized enzyme reactions and for culture of suspended and immobilized culture of suspended and immobilized cells.cells.

Care is required for sensitive cells, Care is required for sensitive cells, particularly in plant and animal cell particularly in plant and animal cell culture especially when impellers are run culture especially when impellers are run at high speeds.at high speeds.

Stirred tank bioreactors consist of a Stirred tank bioreactors consist of a cylindrical vessel with a motor driven cylindrical vessel with a motor driven central power shaft that supports one or central power shaft that supports one or more agitators.more agitators.

Microbial cultures are generally Microbial cultures are generally provided with four baffles placed provided with four baffles placed equidistant around peripheral of the equidistant around peripheral of the tank.tank.

Baffles run the entire height of the tank Baffles run the entire height of the tank to prevent vortexingto prevent vortexing

Baffle width is 1/10 or 1/12 of the Baffle width is 1/10 or 1/12 of the tank diameter.tank diameter.

The aspect ratio of vessel is between The aspect ratio of vessel is between 3 and 5.3 and 5.

The number of impellers depend on The number of impellers depend on aspect ratio of the vessel.aspect ratio of the vessel.

The lower most impeller is locted The lower most impeller is locted about one third of the tank diameter about one third of the tank diameter above the bottom of the tank.above the bottom of the tank.

Rushton turbines are used for Rushton turbines are used for microbial cell cultures.microbial cell cultures.

Marine propellers are used for Marine propellers are used for animal cell cultures.animal cell cultures.

Aspect ratio is less than 2 for animal Aspect ratio is less than 2 for animal cell culture vessels.cell culture vessels.

Animal cell culture vessels are Animal cell culture vessels are unbaffled.unbaffled.

In microbial fermenters ,gas is In microbial fermenters ,gas is sparged below the lowermost impeller sparged below the lowermost impeller using a perforated ring sparger with using a perforated ring sparger with ring diameter slightly smaller than ring diameter slightly smaller than impeller diameterimpeller diameter

Aeration velocity is generally kept less Aeration velocity is generally kept less than 0.05ms-1.than 0.05ms-1.

For animal cell culture vessels For animal cell culture vessels aeration velocity is less than 0.01ms-1.aeration velocity is less than 0.01ms-1.

Stirred tank are commonly used for Stirred tank are commonly used for Aerobic or Anaerobic (air-sparged if Aerobic or Anaerobic (air-sparged if

aerobic)aerobic) bacterial cellsbacterial cells

Airlift bioreactorsAirlift bioreactors

In ALB the fluid In ALB the fluid volume of the vessel volume of the vessel is divided in to two is divided in to two zones by means of a zones by means of a baffle or a draft tubebaffle or a draft tube

The sparged zone is The sparged zone is known asknown as riser riser,the ,the zone that receives no zone that receives no gas is the gas is the downcomerdowncomer

The bulk density of gas liquid The bulk density of gas liquid dispersion in the riser tend to dispersion in the riser tend to be less than that in the be less than that in the downcomer.downcomer.

Consequently dispersion flows Consequently dispersion flows up in riser and downwards in up in riser and downwards in downcomer.downcomer.

Sometimes riser and Sometimes riser and downcomer are two separate downcomer are two separate vertical pipes that are vertical pipes that are interconnected at top and interconnected at top and bottom to form an external bottom to form an external circulation loop circulation loop

External loop configurations are External loop configurations are used less when compared with used less when compared with internal loop.internal loop.

AFB are generally used in AFB are generally used in Bacterial and yeast culturesBacterial and yeast cultures Fermentation of mycelial fungiFermentation of mycelial fungi Animal and plant cell cultureAnimal and plant cell culture Wastewater treatmentWastewater treatment

AFB are highly energy efficient AFB are highly energy efficient relative to stirred fermenters.relative to stirred fermenters.

Heat and mass transfer Heat and mass transfer characteristics are good and it is characteristics are good and it is efficient in suspending the solids efficient in suspending the solids than bubble columns.than bubble columns.

For optimal mass transfer ,the riser For optimal mass transfer ,the riser to downcomer cross section area to downcomer cross section area ratio must be in between 1.8 and 4.3 ratio must be in between 1.8 and 4.3

All performance characteristics of AFB All performance characteristics of AFB are related with gas injection rate and are related with gas injection rate and resulting rate of liquid circulation.resulting rate of liquid circulation.

The liquid circulation rate depends on The liquid circulation rate depends on the difference between gas holdup (vol the difference between gas holdup (vol fraction of gas in gas-liq dispersion) fraction of gas in gas-liq dispersion) between riser and downcomer.between riser and downcomer.

Liquid velocity is also affected by Liquid velocity is also affected by geometry of the vessel and viscosity of geometry of the vessel and viscosity of fluid.fluid.

In general ,liquid circulation velocity In general ,liquid circulation velocity increases with square root of height increases with square root of height of the airlift device.of the airlift device.

So aspect ratios are generally high So aspect ratios are generally high between 6 and 8 or even in between 6 and 8 or even in hundreds.hundreds.

Circulation is enhanced if there is Circulation is enhanced if there is little or no gas in down comer.little or no gas in down comer.

ALBs can be used for both free and ALBs can be used for both free and immobilized cells.immobilized cells.

The advantages of Airlift reactors The advantages of Airlift reactors are the elimination of attrition are the elimination of attrition effects generally encountered in effects generally encountered in mechanical agitated reactors. mechanical agitated reactors.

It is ideally suited for aerobic It is ideally suited for aerobic cultures since oxygen mass transfer cultures since oxygen mass transfer coefficient are quite high in coefficient are quite high in comparison to stirred tank reactorscomparison to stirred tank reactors

Which type of reactor is the Which type of reactor is the world's largest industrial world's largest industrial

fermenter?fermenter?

A STR can never be very large because A STR can never be very large because of the heat-exchanger, mixing and of the heat-exchanger, mixing and aeration problems. aeration problems.

The world's largest fermenter is The world's largest fermenter is Air-Air-lift Bioreactor!lift Bioreactor! The fermenter is 200' The fermenter is 200' high and 25 ft diam. The first photo high and 25 ft diam. The first photo shows the fermenter being transported shows the fermenter being transported on vehicles with tank treads. on vehicles with tank treads.

The next photo shows the fermenter being raised into position The next photo shows the fermenter being raised into position

at the ICI, Ltd. factory, Billingham, UK.at the ICI, Ltd. factory, Billingham, UK.

Fluidized bed bioreactorsFluidized bed bioreactors Suited to reactions Suited to reactions

involving a fluid involving a fluid suspended particulate suspended particulate biocatalyst such as biocatalyst such as immobilized enzyme immobilized enzyme or microbial flocsor microbial flocs

An upflowing stream An upflowing stream of liquid is used to of liquid is used to suspend or “fluidize” suspend or “fluidize” the relatively dense the relatively dense solids.solids.

The reactor is expanded in the top to The reactor is expanded in the top to reduce the superficial velocity of the reduce the superficial velocity of the liquid to a value below that needed to liquid to a value below that needed to keep the solids in suspensionkeep the solids in suspension

Consequently solids sediment in the Consequently solids sediment in the expanded zone and drop back into the expanded zone and drop back into the narrower reactor columnnarrower reactor column

Hence the solids are retained in the Hence the solids are retained in the column and liquid flows out. column and liquid flows out.

A liquid fluidized bed may be sparged A liquid fluidized bed may be sparged with air or any gas to produce a gas-with air or any gas to produce a gas-liquid-solid fluidised bed.liquid-solid fluidised bed.

If the solid particles are too light they If the solid particles are too light they may have to be artificially weighted, may have to be artificially weighted, for example by embedding stainless for example by embedding stainless steel balls.steel balls.

A high density of solids improve solid-A high density of solids improve solid-liquid mass transfer and denser solids liquid mass transfer and denser solids are easier to sediment.are easier to sediment.

But the density should not be too high, But the density should not be too high, otherwise fluidization itself may be otherwise fluidization itself may be difficult.difficult.

Even with lighter solid particles, Even with lighter solid particles, superficial velocity needed to suspend superficial velocity needed to suspend the solids may cause the liquid to leave the solids may cause the liquid to leave from the reactor too quickly.from the reactor too quickly.

This leads to insufficient solid liquid This leads to insufficient solid liquid contact time for the reaction and liquid contact time for the reaction and liquid may have to be recycled to obtain may have to be recycled to obtain sufficiently long cumulative time with the sufficiently long cumulative time with the biocatalyst.biocatalyst.

The minimum fluidization velocity –The minimum fluidization velocity –the superficial liquid velocity needed the superficial liquid velocity needed to just suspend the solids from a to just suspend the solids from a settled state depends on settled state depends on Density difference between two phasesDensity difference between two phases Shape and diameter of the particlesShape and diameter of the particles Viscosity of the liquid.Viscosity of the liquid.

A photograph of a fluidized bed bioreactor (inside volume 450 cm3) used to produce butanol from whey permeate using C. acetobutylicum P262.