BIOREACTOR CONFIGURATIONS. Usually, a cylindrical tank, either stirred or unstirred. Reactor design...
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Transcript of BIOREACTOR CONFIGURATIONS. Usually, a cylindrical tank, either stirred or unstirred. Reactor design...
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.