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.

External-loop or outer-loop ALRs 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.