Measurement of Bioreactor K L a

Motivations 2. Good example of mass transfer at gas- liquid interface 3. Experience modeling in both semi- empirical and factorial methods 1.Biotech/pharmaceutical industry employing more Chemical Engineers Process Engineering Validation Management Pilot testing Scale-up

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

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.

Expression Systems Bacterial Cells Fungal Cells Plant Cells Insect Cells Mammalian Cells

Types of Bioreactors (fermentors) (often depends on shear senstivity) 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.

Industrial Stirred Fermenter

Experimental Apparatus

Why is K L a Important? Oxygen is an important substrate in aerobic fermentations Since oxygen is sparingly soluble in water, it may be 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).

Equation for Transport Oxygen transfer is usually limited by the liquid film surrounding the gas bubbles: where m O2 is the rate of oxygen transfer per volume of bioreactor (mass O 2 / L 3 t), k L is the oxygen transport coefficient, [=]L/t, a is the gas-liquid interfacial area per volume of reactor [=] L 2 /L 3, k L a is the volumetric oxygen transfer coefficient [=]1/t, C* is saturated DO (dissolved oxygen) concentration [=] m/L 3 (approx. 7 mg/l at 25 deg. C and 1 atm.), C L is the actual DO concentration in the liquid [=] m/L 3

Terms affecting rate K L a –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

Some Interactions Affecting Oxygen Transport in Aerobic Systems

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) C L (C(t)) the oxygen concentration at a given time during the run; what we measure - {C*- C L } = “driving force”

Experimental Apparatus

Probe response rate needed to get “real” C L (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 1 23 Time constant =1/kLaTime constant =1/kpFast

Some Interactions Affecting Oxygen Transport in Aerobic Systems

Data Acquisition