The aeration process and stirring mechanisms are essential in pharmaceutical bioprocessing, influencing oxygen transfer, mixing, shear forces, and product quality. B. Pharm students must understand aeration strategies (spargers, diffusers), impeller types (axial, radial), mass transfer coefficient (kLa), oxygen solubility, power input, Reynolds number, and bubble dynamics to design and operate fermenters and sterile mixing systems. Practical knowledge of scale-up, foam control, DO sensors, and the effect of agitation on cells and formulations helps optimize yields and stability. This introduction outlines core concepts, key parameters, and practical implications for drug manufacturing and bioprocessing. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. Which parameter primarily quantifies the overall rate of oxygen transfer from gas to liquid in an aerated bioreactor?
- Oxygen solubility
- kLa
- Bubble terminal velocity
- Foam fraction
Correct Answer: kLa
Q2. Which impeller type produces predominantly radial flow and high shear useful for gas dispersion?
- Axial flow hydrofoil
- Rushton turbine
- Pitch blade turbine
- Marine propeller
Correct Answer: Rushton turbine
Q3. The volumetric mass transfer coefficient kLa has units of:
- mol·L−1·s−1
- s−1
- m·s−1
- m3·s−1
Correct Answer: s−1
Q4. Which factor most directly increases kLa in a stirred bioreactor?
- Increasing antifoam concentration
- Reducing gas flow rate
- Increasing agitation speed
- Raising medium viscosity significantly
Correct Answer: Increasing agitation speed
Q5. Which sparger type produces many small bubbles and high surface area for mass transfer?
- Single nozzle coarse sparger
- Porous diffused sparger
- Downcomer sparger
- External loop sparger
Correct Answer: Porous diffused sparger
Q6. For scale-up of aerobic fermentation where oxygen transfer is limiting, which criterion is commonly preferred?
- Constant impeller diameter
- Constant tip speed
- Constant power per unit volume (P/V)
- Constant Reynolds number
Correct Answer: Constant power per unit volume (P/V)
Q7. Which expression correctly gives the Reynolds number for an impeller-driven stirred tank?
- Re = ρND/μ
- Re = ρN2D3/μ
- Re = ρND2/μ
- Re = μ/ρND2
Correct Answer: Re = ρND2/μ
Q8. Which of the following most reduces oxygen transfer efficiency in a fermenter?
- Higher superficial gas velocity
- Smaller mean bubble diameter
- Excessive foaming at the gas–liquid interface
- Increased impeller submergence
Correct Answer: Excessive foaming at the gas–liquid interface
Q9. Which device measures dissolved oxygen electrochemically and is commonly used in fermentation?
- pH electrode
- Polarographic DO electrode
- Mass spectrometer
- Conductivity probe
Correct Answer: Polarographic DO electrode
Q10. In the two-film theory of gas transfer, the main resistance to oxygen transfer is usually in which phase?
- Gas film around the bubble
- Liquid film surrounding the bubble
- Bulk gas phase
- Solid reactor wall
Correct Answer: Liquid film surrounding the bubble
Q11. A commonly used empirical correlation for kLa includes terms for power input and gas flow. Which two parameters are typically included?
- Power per volume (P/V) and dissolved solids
- Power per volume (P/V) and superficial gas velocity (Vs)
- Impeller material and tank color
- Media osmolarity and pH
Correct Answer: Power per volume (P/V) and superficial gas velocity (Vs)
Q12. Which impeller is best described as an axial-flow impeller often used for bulk mixing with low shear?
- Rushton turbine
- Pitched blade turbine
- Flat-blade turbine
- Centrifugal impeller
Correct Answer: Pitched blade turbine
Q13. Which parameter is most affected by medium viscosity during aeration and stirring?
- Oxygen molecular weight
- Bubble coalescence and kLa
- Sparger material
- Ambient atmospheric pressure
Correct Answer: Bubble coalescence and kLa
Q14. What is the primary purpose of antifoaming agents during aeration?
- Increase bubble surface area
- Stabilize foam to improve oxygen transfer
- Collapse foam to prevent overflow and mass-transfer hindrance
- Increase medium viscosity
Correct Answer: Collapse foam to prevent overflow and mass-transfer hindrance
Q15. Which of the following most directly increases gas hold-up in the liquid?
- Lower gas flow rate
- Smaller bubble size and better dispersion
- Decreasing agitation speed
- Adding a large amount of antifoam
Correct Answer: Smaller bubble size and better dispersion
Q16. The power consumed by an impeller in a Newtonian fluid can be estimated by P = Po·ρ·N3·D5. In this equation, Po stands for:
- Power output
- Power number (dimensionless)
- Operational pressure
- Partial oxygen concentration
Correct Answer: Power number (dimensionless)
Q17. Which effect does increasing sparger submergence depth generally have on aeration?
- Decreases bubble residence time
- Increases bubble residence time and gas–liquid contact
- Eliminates the need for agitation
- Reduces liquid density
Correct Answer: Increases bubble residence time and gas–liquid contact
Q18. For cultures sensitive to shear, which stirring strategy is preferred?
- High-speed Rushton turbine with coarse sparger
- Gentle axial-flow impeller and increased aeration with microbubbles
- No aeration and no mixing
- Use of metal beads for agitation
Correct Answer: Gentle axial-flow impeller and increased aeration with microbubbles
Q19. Which measurement provides a direct indication of oxygen transfer limitation in a fermenter?
- Stable high dissolved oxygen (DO) throughout operation
- Frequent DO dips to near-zero during high respiration phases
- Constant pH
- Low foam only
Correct Answer: Frequent DO dips to near-zero during high respiration phases
Q20. Which factor does NOT significantly influence kLa in an aerobic process?
- Agitator speed
- Temperature
- Gas composition (O2 concentration)
- Color of the vessel walls
Correct Answer: Color of the vessel walls
Q21. Which bubble characteristic increases the interfacial area available for mass transfer?
- Larger average bubble radius
- Smaller average bubble radius
- Higher bubble rise velocity only
- Bubble transparency
Correct Answer: Smaller average bubble radius
Q22. During scale-up from lab to pilot scale, maintaining constant specific power (P/V) typically affects which of the following?
- Maintains identical shear rates and mixing times at all scales automatically
- Aims to preserve similar mixing intensity and oxygen transfer per unit volume
- Ensures identical bubble sizes regardless of geometry
- Makes sparger design irrelevant
Correct Answer: Aims to preserve similar mixing intensity and oxygen transfer per unit volume
Q23. Which mixing parameter is most used to quantify how fast a tracer becomes uniformly distributed?
- Mixing time ™
- kLa
- Foam stability time
- Impeller blade thickness
Correct Answer: Mixing time ™
Q24. What is the typical effect of dissolved salts and proteins on oxygen solubility in aqueous fermentation media?
- They significantly increase oxygen solubility
- They have no effect on oxygen solubility
- They slightly decrease oxygen solubility
- They convert oxygen into nitrogen
Correct Answer: They slightly decrease oxygen solubility
Q25. Which gas–liquid contactor is commonly used in pharmaceutical fermenters to introduce air or oxygen?
- Membrane oxygenator only
- Sparger
- Static mixer outside the vessel only
- Condenser
Correct Answer: Sparger
Q26. Which control strategy is used to maintain a desired dissolved oxygen (DO) setpoint in aerobic cultures?
- Varying impeller geometry during run
- Cascade control: adjust agitation speed and gas flow based on DO feedback
- Change the vessel material
- Varying culture inoculum dynamically
Correct Answer: Cascade control: adjust agitation speed and gas flow based on DO feedback
Q27. Which phenomenon reduces effective interfacial area by causing adjacent bubbles to merge?
- Bubble breakup
- Bubble coalescence
- Bubble nucleation
- Bubble entrainment
Correct Answer: Bubble coalescence
Q28. In a gas–liquid reactor, superficial gas velocity (Vs) is defined as:
- The actual gas velocity within bubbles
- Gas volumetric flow rate divided by tank cross-sectional area
- Liquid flow rate divided by tank height
- Impeller peripheral velocity
Correct Answer: Gas volumetric flow rate divided by tank cross-sectional area
Q29. Which impeller characteristic most strongly influences shear stress experienced by suspended cells?
- Blade color
- Tip speed
- Vessel headspace volume
- pH probe location
Correct Answer: Tip speed
Q30. When designing an aeration system for an oxygen-sensitive recombinant culture, which approach minimizes cell damage while meeting oxygen demand?
- Maximize shear with high-speed Rushton turbine and no antifoam
- Use low-shear axial impellers, fine bubble sparging, and controlled DO via cascade
- Stop aeration and rely on oxygen diffusion through the headspace
- Use very high antifoam concentration to stabilize foam and increase kLa
Correct Answer: Use low-shear axial impellers, fine bubble sparging, and controlled DO via cascade
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
Mail- Sachin@pharmacyfreak.com