BIOREACTOR

CONFIGURATIONS

• Usually, a cylindrical tank, either stirred or unstirred.

• Reactor design– Provision of adequate mixing and – aeration for the large proportion of

fermentations requiring oxygen

Stirred Tank Reactors (STRs)

• Mixing and bubble dispersion, achieved by mechanical agitation;– High input of energy per unit volume.

• Baffles for reducing vortexing.

• Impellers for different flow patterns inside fermentor

• Multiple impellers in tall fermentors, to improve mixing

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

• Adequate headspace for– disengagement of droplets from the exhaust

gas– accommodating any foam which may develop

• A supplementary impeller – “foam breaker”

• Or, chemical antifoam agents added to broth– Reduces rate of O2 transfer

• Aspect ratio: Ratio of height to diameter

• Internal cooling coils: For temperature control and heat transfer

• Used for free- and immobilised cells

Bubble Column Reactors

• No mechanical agitation• Aeration and mixing, achieved by gas sparging.

– Requires less energy than mechanical stirring.

• Generally cylindrical vessels with height > twice the diameter

• A sparger for entry of compressed air• Typically, no internal structures.• For industrial production of bakers’ yeast, beer

and vinegar (H:D ratio of ~ 3:1 to 6:1)• For treatment of wastewater.

• Perforated horizontal plates to break up and redistribute coalesced bubbles.

• Advantages– Low capital cost,– Lack of moving parts, and – Satisfctory heat- and mass-transfer

performance

Airlift Reactors (ALRs)

• Mixing without mechanical agitation.

• For culture of plant and animal cells and immobilised catalysts– Because shear levels are much lower than in

STRs

• Patterns of liquid flow are more defined

• Physical separation of up-flowing and down-flowing streams.

• Gas is sparged into part of the vessel cross-section called Riser.

• Gas hold-up and decreased fluid density cause liquid in the riser to move upwards.

• Gas disengages at the top of the vessel leaving heavier bubble-free liquid to recirculate through the downcomer.

• Liquid circulation is a result of density difference between riser & downcomer.

• Internal-loop ALRs– Riser and downcomer are separated by an

internal baffle or draft tube– Air may be sparged into either draft tube or

annulus

• External-loop or outer-loop ALRs– Separate vertical tubes, connected by short

horizontal sections at the top and bottom.– Riser & downcomer are further apart in

external-loop vessels

• So, gas disengagement is more effective

• Density difference between fluids in the riser and downcomer is greater

• So, circulation of liquid is faster.

• Thus, mixing is better in external-loop than internal-loop ALRs

• In production of SCP

• For plant and animal cell culture

• In municipal and industrial waste treatment.

• Height of ALRs is typically ~10 times the diameter

• For deep-shaft systems, H / D ratio, increased up to 100.

Packed Bed Reactors (PBRs)

• Used with immobilised or particulate biocatalysts.

• Consists of a tube, usually vertical, packed with catalyst particles.

• Medium can be fed either at the top or bottom of the column

• Medium forms a continuous liquid phase between the particles.

• Damage due to particle attrition is minimal• Used for production of aspartate and fumarate,

conversion of penicillin to 6-aminopenicillanic acid, and resolution of amino acid isomers.

• Operated with liquid recycle

• Catalyst is prevented from leaving the column by screens at the liquid exit.

• Particles should be relatively incompressible and able to withstand their own weight in the column without deforming and occluding liquid flow.

• Recirculating medium, to be clean & free of debris to avoid clogging the bed.

PBR with medium recycle

Fluidised Bed Reactor (FBRs)

• Packed beds are operated in upflow mode with catalyst beads of appropriate size and density

• Basis: Bed expands at high liquid flow rates due to upward motion of the particles.

• Particles in fluidised beds are in constant motion– Channelling and clogging of the bed are avoided– Air can be introduced directly into the column.

• Used in waste treatment, brewing and for production of vinegar.

Trickle Bed Reactor

• A variation of the packed bed

• Liquid is sprayed onto the top of the packing

• Liquid trickles down through the bed in small rivulets.

• Air may be introduced at the base

• Used widely for aerobic wastewater treatment.