BIOREACTOR CONFIGURATIONS

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BIOREACTOR CONFIGURATIONS
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• Usually, a cylindrical tank, either stirred or
  unstirred.
• Reactor design
• Provision of adequate mixing and
• aeration for the large proportion of
  fermentations requiring oxygen

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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

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• 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

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• 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

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Bubble Column Reactors

• No mechanical agitation
• Aeration and mixing, achieved by gas sparging.
• Requires less energy than mechanical stirring.
• Generally cylindrical vessels with height gt twice
  the diameter
• A sparger for entry of compressed air
• Typically, no internal structures.
• For industrial production of bakers yeast, beer
  and vinegar (HD ratio of 31 to 61)
• For treatment of wastewater.

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• 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

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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.

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• 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.

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• 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

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• 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

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• 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.

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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.

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

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• 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.

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PBR with medium recycle
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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.

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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.

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