Measurement of BioreactorKLa

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

Transport in Bioprocess Systems

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).

Equation for TransportOxygen transfer is usually limited by the liquid film surrounding the gas bubbles:

LLO CCakm *2

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

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

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)

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

Experimental Apparatus

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

1 2 3

Time constant =1/kLa Time constant =1/kp Fast

Some Interactions Affecting Oxygen Transport in Aerobic Systems

Assignment / Procedures

1) Measure probe response time, -put probe in beaker of sodium sulfite solution and stir until probe stabilizes at zero O2 concentration. -set meter to zero (offset calibration) -quickly move probe to bioreactor that is being sparged with O2 and immediately start data acquisition -determine from 63.2% response

2) Determine KLa at 4 combinations of flow (Q, Vol/time) and impeller speed (N, RPM)= 16 runs (“full factorial”)

3) Use the results of items 1-2 to determine coefficients/exponents a, b, c in an equation of the form:

cs

bL vaeak (1)

where e is power dissipated in volume of liquid phase (W/m3) and s is the superficial gas velocity in meters per second (note, you will need to measure the appropriate variables to obtain power input).

Data Acquisition

Calibration Screen