Industrial Microbiology INDM 4005 Lecture 6 17/02/04

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Industrial MicrobiologyINDM 4005Lecture
617/02/04
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Questions for today

• 1. What is a fermentation system?
• 2. What is the most widely used fermenter?
• 3. What are the other types of fermenter?
• 4. How do you control a fermentation system?
• 5. Why is mass transfer important?

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Lecture Overview

• 1) Basic design criteria and limitations
• 2) Stirred Tank Reactor (STR)
• 3) Modifications and Industrial Examples

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Biotechnological processing
Types of Process Fermentation Design
Fermenter Design
Performance
Optimisation
Construction
Configuration
Control
Stirred Tank Reactor
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What is a Fermenter?

• Vessel or tank in which whole cells or cell-free
  enzymes transform raw materials into biochemical
  products and/or less undesirable by-products
• Also termed a Bioreactor

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Fermenter - Basic Function

• The basic function of a fermenter is to provide a
  suitable environment in which an organism can
  efficiently produce a target product that may be
• - cell biomass,
• - a metabolite,
• - or bioconversion product.

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Fermentation System

• In this lecture we will concentrate on fermenters
  used in traditional microbial, plant and animal
  cell culture
• However with the advent of recombinant DNA
  technology alternate systems for producing
  specific cell products are now available

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Two Types of Fermentation Systems

• closed or open.
• A closed system implies that all the nutrient
  components are added at the beginning of the
  fermentation process and, as a result, the growth
  rate of the contained organisms will eventually
  proceed to zero due either to diminishing
  nutrients or accumulation of toxic waste
  products. A modification of the batch process is
  the fed batch system. Here, volumes of nutrients
  may be added to augment depletion of nutrients.
  Overall, the system, however, remains closed and
  there is no continuous flow.
• In contrast to the above types, in the open
  system, organisms and nutrients can continuously
  enter and leave the fermenter.

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Fermenter General Functions

• What it should be capable of
• Biomass concentration must remain high
• Maintain sterile conditions
• Efficient power consumption
• Effective agitation
• Heat removal
• Correct shear conditions
• Sampling facilities

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• Fermenters range from simple stirred tanks to
  complex integrated systems involving varying
  levels of computer input.
• Fermenter design involves cooperation in
  Microbiology, Biochemistry, Chemical Engineering,
  Mechanical Engineering, Economics
• There are 3 groups of bioreactor currently used
  for industrial production
• - non-stirred, non-aerated
• - non-stirred, aerated
• - stirred, aerated

(Beer and wine)
(Biomass, eg Pruteen)
(Antibiotics)
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Fermenter construction

• All materials must be corrosion resistant to
  prevent trace metal contamination of the process
• Materials must be non-toxic so that slight
  dissolution of the material or components does
  not inhibit culture growth
• Materials of the fermenter must withstand
  repeated sterilization with high pressure steam
• Fermenter stirrer system and entry ports be
  sufficiently robust not to be deformed under
  mechanical stress
• Visual inspection of the medium and culture is
  advantageous, transparent materials should be
  used

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Basic fermenter configuration

• A microbial fermentation can be viewed as a
  three-phase system, involving liquid-solid,
  gas-solid, and gas-liquid reactions.
• The liquid phase contains dissolved nutrients,
  dissolved substrates and dissolved metabolites.
• The solid phase consists of individual cells,
  pellets, insoluble substrates, or precipitated
  metabolic products.
• The gaseous phase provides a reservoir for oxygen
  supply and for CO2 removal.

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Optimisation of the Fermenter System

• Fermenter should be designed to exclude entrance
  of contaminating organisms as well as containing
  the desired organisms
• Culture volume should remain constant,
• Dissolved oxygen level must be maintained above
  critical levels of aeration and culture agitation
  for aerobic organisms
• Parameters such as temperature of pH must be
  controlled, and the culture volume must be well
  mixed.
• Therefore a need for control exists

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Control of Chemical and Physical Conditions

• Intensive properties (cannot be balanced)
• - temperature, concentration, pressure,
  specific heat
• Extrinsive properties (can be balanced)
• - mass, volume, entropy and energy
• Mass and energy levels should balance at the
  start and finish of fermentations.
• Combining this with determination of
  thermodynamic properties and rate equations we
  can build computer and mathematical models to
  control processes.

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Basic Fermenter Design Criteria

• (i). Nature of microbial (or mammalian, plant
  tissue) cell
• (a) Hydrodynamic characteristics
• (b) Mass and Heat Transfer
• (c) Kinetics
• (d) Genotype and Phenotype
• (ii). Environmental Control and Monitoring of the
  process
• (a) pH, temperature, dissolved oxygen etc.
• (b) Asepsis and avoidance of contamination
• (iii). Process factors
• (a) Effect on other unit operations
• (b) Economics
• (c) Potential for scale-up

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Types of Fermenter

• Aerobic fermenters may be classified depending on
  how the gas is distributed
• Stirred Tank Reactor
• Airlift
• Loop Reactor
• Immobilised System

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Stirred Tank Reactors

• Most commonly fermenter used
• Made from stainless steel when over 20 Litres
• Height to Diameter ratio 21 and 61
• Baffles prevent a large central vortex
• Also used to carry coolants in large systems

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Stirred Tank Reactor
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STR - Control systems

• An agitator system
• An oxygen delivery system
• A foam control system
• A temperature control system
• A pH control system
• Sampling ports
• A cleaning and sterilization system.
• A sump and dump line for emptying of the
  reactor.

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Aeration and agitation

• The transfer of energy, nutrients, substrate and
  metabolite within the bioreactor must be brought
  about by a suitable mixing device. The efficiency
  of any one nutrient may be crucial to the
  efficiency of the whole fermentation.
• For the three phases, the stirring of a
  bioreactor brings about the following
• Dispersion of air in the nutrient solution
• Homogenisation to equalise the temperature and
  the concentration of nutrients throughout the
  fermenter
• Suspension of microorganisms and solid nutrients
• Dispersion of immiscible liquids

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Basic features of a stirred tank bioreactor

• Agitation system
• The function of the agitation system is to
• provide good mixing and thus increase mass
  transfer rates through the bulk liquid and bubble
  boundary layers.
• provide the appropriate shear conditions required
  for the breaking up of bubbles.
• The agitation system consists of the agitator and
  the baffles.
• The baffles are used to break the liquid flow to
  increase turbulence and mixing efficiency.

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Agitator design and operation
Radial flow impellers - Rushton turbine The
most commonly used agitator in microbial
fermentations Like all radial flow impellers,
the Rushton turbine is designed to provide the
high shear conditions required for breaking
bubbles and thus increasing the oxygen transfer
rate.
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Mass Transfer

• One of the most critical factors in the
  operation of a fermenter is the provision of
  adequate gas exchange.
• Oxygen is the most important gaseous substrate
  for microbial metabolism, and carbon dioxide is
  the most important gaseous metabolic product.
• For oxygen to be transferred from a air bubble
  to an individual microbe, several independent
  partial resistances must be overcome

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Oxygen Mass Transfer Steps
Gas bubble
Liquid film
Microbial cell
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2
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1) The bulk gas phase in the bubble 2) The
gas-liquid interphase 3) The liquid film around
the bubble 4) The bulk liquid culture medium 5)
The liquid film around the microbial cells 6)
The cell-liquid interphase 7) The intracellular
oxygen transfer resistance
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Air lift reactors

• In such reactors, circulation is caused by the
  motion of injected gas through a central tube
  with fluid re-circulating through the head space
  where excess air and the by-product CO2
  disengage.
• The degassed liquid then flows down the annular
  space outside the draught tube

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Airlift reactors
Draught tube
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Airlift reactors

• Advantages
• Low shear
• Easier to maintain sterility
• Increased oxygen solubility (KLa)
• Can allow large vessels
• Disadvantages
• High capital cost
• High energy costs
• Hard to control conditions
• Foaming hinders gas -liquid separation

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SOME MODIFICATIONS

• (i) Important in tank reactor design
• 1. Continuous flow (activated sludge waste
  treatment)
• Suitable when substrate at low conc.
• Allows greater control on growth rate\ cell
  physiology
• 2. Immobilised cells - may be membrane (e.g.
  hollow fibre reactor), immobilised onto support
  such as ceramic (e.g packed-bed) or in polymers
  (e.g alginate beads)
• Increases rate of reaction
• Microenvironment created protects cells e.g.
  from shear damage
• 3. Low energy aeration\ mixing Air-lift,
  draft-tubes, loop reactors etc.
• Increase height to diameter ratio. Increased
  path length of bubble, improves mass transfer
• Results in decreased shear levels, important in
  floc systems.

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SOME MODIFICATIONS

• (ii) Industrial examples of modified STR /
  bioreactors
• (i) Waste treatment. - Activated sludge system.
• Characterised by Low substrate conc. Therefore
  require (a) recycle of biomass, (b) continuous
  operation, (c) Low cost aeration / mixing.
• (ii) Brewing - Cylindro-conical fermenter
• Note no aeration but gas produced by yeast cells
  contributes to mixing, closed to capture carbon
  dioxide produced, cone helps sedimentation of
  yeast, Low shear environment promotes
  flocculation.
• (iii) Tissue culture - low shear, anchored and
  immobilised systems.
• (iv) Solid-state fermentations e.g. silage,
  mushroom production etc.

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In Summary Major considerations include

• 1. Bioreactor size - to provide required
  production capacity
• 2. Mass transfer - to provide nutrients to cells,
  well dispersed, adequate oxygen etc
• 3. Control systems
• (a) temperature, pH, etc.
• (b) sterilisation/ aseptic operation
• (c) representative sampling
• (d) heat transfer - example sterilisation of
  media
• 4. Requirement for asepsis / containment

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Critical Concepts or Questions

• What are the objectives in fermenter design?
• Draw a diagram of a STR
• How does a STR relate to structure and function?
• How can fermentation systems be controlled?

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Conclusion

• This lecture introduced the various parameters
  involved in design of an industrial fermenter.
• Using a STR it illustrated the optimisation and
  control of a fermentation system.