Measurement of Bioreactor KLa

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Measurement of BioreactorKLa
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Motivations

• Biotech/pharmaceutical industry employing more
  Chemical Engineers
• Process Engineering
• Validation
• Management
• Pilot testing
• Scale-up

• 2. Good example of mass transfer at gas-liquid
  interface
• 3. Experience modeling in both semi-empirical
  and factorial methods

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Types of Products

• Natural Products
• Drugs
• Penicillin is early example
• Taxol
• Mupricin
• Cyclosporin A, etc.
• Foods
• Fermented beverages
• Fermented dairy products

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Types of Products

• Transgenic Products
• Gene for a therapeutic protein inserted in
  foreign expression system
• Factor IX
• a-1-antitrypsin
• EPO
• Antibodies
• antithrombin III
• tissue plasminogen activator (TPA)
• Interferons, etc.

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Expression Systems

• Bacterial Cells
• Fungal Cells
• Plant Cells
• Insect Cells
• Mammalian Cells

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Types of Bioreactors (fermenter)(often depends
on shear sensitivity)

• Stirred tank
• Aerobic or Anaerobic (air-sparged if aerobic)
• Most common for bacterial cells
• Bubble or airlift column
• Good for shear-sensitive cells
• Fixed bed systems
• Trickle beds, hollow membrane fiber (mammalian
  cells), etc.

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Industrial Stirred Fermenter
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Experimental Apparatus
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Transport in Bioprocess Systems
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Why is KLa Important?

• Dissolved oxygen is an important substrate in
  aerobic fermentations. Since oxygen is sparingly
  soluble in water, it may be the growth-limiting
  substrate in these fermentations. For bacteria
  and yeast cultures, the critical oxygen
  concentration is about 10 to 50 of the
  saturated DO (dissolved oxygen concentration).

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Equation for Transport
Oxygen transfer is usually limited by the liquid
film surrounding the gas bubbles
where mO2 is the rate of oxygen transfer per
volume of bioreactor (mass O2/ L3 t), kL is the
oxygen transport coefficient, L/t, a is the
gas-liquid interfacial area per volume of reactor
L2/L3, kLa is the volumetric oxygen transfer
coefficient 1/t, C is saturated DO (dissolved
oxygen) concentration m/L3 (approx. 7 mg/l at
25 deg. C and 1 atm.), CL is the actual DO
concentration in the liquid m/L3
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Terms affecting rate

• KLa
• What we are trying to determine and correlate
  with mixing speed and aeration rate
• Two quantities multiplied together
• Liquid side (essentially overall mass transfer
  coefficient)
• Total area of bubbles in bioreactor
• Cant be separated

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Some Interactions Affecting Oxygen Transport in
Aerobic Systems
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Terms affecting rate

• C (saturation oxygen concentration max
  solubility of the gas in liquid)
• - Constant at a given T and P
• - Available in tables (see on-line lab
  manual)
• CL (C(t)) the oxygen concentration at a given
  time during the run what we measure
• - C- CL driving force

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Terms affecting rate

• KL -mass transfer coefficient
• a- interfacial area for mass transfer

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Probe response rate needed to get real CL(t)
value

1. Gaseous oxygen dissolves in water at bubble
   interface and disperses in the bioreactor
2. Dissolved O2 crosses probe membrane at tip.
3. O2 in probe is sensed and sent to meter

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Time constant 1/kLa
Time constant 1/kp
Fast
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Some Interactions Affecting Oxygen Transport in
Aerobic Systems
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Data Acquisition