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Measurement of Bioreactor K L a

Measurement of Bioreactor K L a

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Measurement of Bioreactor K L a

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  1. Measurement of BioreactorKLa

  2. Motivations 2. Good example of mass transfer at gas-liquid interface 3. Experience modeling in both semi-empirical and factorial methods • Biotech/pharmaceutical industry employing more Chemical Engineers • Process Engineering • Validation • Management • Pilot testing • Scale-up

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

  4. 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.

  5. Expression Systems • Bacterial Cells • Fungal Cells • Plant Cells • Insect Cells • Mammalian Cells

  6. 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.

  7. Industrial Stirred Fermenter

  8. Experimental Apparatus

  9. Transport in Bioprocess Systems

  10. 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).

  11. 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

  12. 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 • Can’t be separated

  13. Some Interactions Affecting Oxygen Transport in Aerobic Systems

  14. 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”

  15. Terms affecting rate • KL -mass transfer coefficient • a- interfacial area for mass transfer

  16. Probe response rate needed to get “real” CL(t) value • Gaseous oxygen dissolves in water at bubble interface and disperses in the bioreactor • Dissolved O2 crosses probe membrane at tip. • O2 in probe is sensed and sent to meter 1 2 3 Time constant =1/kLa Time constant =1/kp Fast

  17. Some Interactions Affecting Oxygen Transport in Aerobic Systems

  18. Data Acquisition