Introduction:
This quiz collection on Impeller Design and Agitation Principles is tailored for M.Pharm students studying Bioprocess Engineering and Technology. It focuses on core concepts such as impeller types, flow patterns, power consumption, dimensionless numbers (Reynolds, Power, Flow), scale-up criteria, and effects of viscosity, shear and gas dispersion on mixing performance. Questions address both theoretical fundamentals and practical design considerations encountered in bioreactors and stirred tanks, including baffles, blade geometry, tip speed, and oxygen transfer. These MCQs will help reinforce analytical thinking, prepare for exams, and support design choices when optimizing agitation systems for pharmaceutical bioprocesses.
Q1. What is the primary flow pattern produced by a Rushton turbine in a stirred tank?
- Predominantly axial flow with strong top-to-bottom circulation
- Strong radial flow with intense shear and mixing near the impeller plane
- Laminar swirling flow with minimal vertical circulation
- Pure plug flow without recirculation
Correct Answer: Strong radial flow with intense shear and mixing near the impeller plane
Q2. Which dimensionless number is most commonly used to characterize the hydrodynamic regime (laminar, transitional, turbulent) in stirred tanks?
- Power number (Np)
- Froude number (Fr)
- Reynolds number (Re)
- Flow number (Fl)
Correct Answer: Reynolds number (Re)
Q3. The power drawn by an impeller in turbulent conditions is often correlated using which relation?
- P = ρ N^3 D^5 × constant (with constant = power number, Np)
- P = μ N D^3 × constant (with constant = flow number)
- P = kLa × V × ΔC
- P = ρ g Q H
Correct Answer: P = ρ N^3 D^5 × constant (with constant = power number, Np)
Q4. For scale-up of mixing under geometric similarity, which scale-up criterion preserves the same mixing intensity per unit volume?
- Constant tip speed
- Constant power per unit volume (P/V)
- Constant Reynolds number
- Constant impeller diameter to tank diameter ratio (D/T)
Correct Answer: Constant power per unit volume (P/V)
Q5. Which impeller type is typically preferred for low-viscosity, high gas–liquid mass transfer applications to enhance dispersion?
- Axial-flow hydrofoil impeller
- Radial-flow Rushton turbine
- Pitched-blade turbine at low pitch
- Anchor impeller
Correct Answer: Radial-flow Rushton turbine
Q6. What effect do baffles have on mixing in a stirred tank?
- Reduce power draw and eliminate circulation
- Promote tangential swirling and reduce axial flow
- Suppress bulk rotation, increase turbulent mixing and power draw
- Convert radial flow into laminar flow
Correct Answer: Suppress bulk rotation, increase turbulent mixing and power draw
Q7. The Power number (Np) for an impeller is defined as:
- Np = P / (ρ N^3 D^5)
- Np = P / (μ N D^3)
- Np = ρ N D / μ
- Np = V / (N D^3)
Correct Answer: Np = P / (ρ N^3 D^5)
Q8. In viscous fermentations where Re is low, which impeller design is generally more effective?
- Rushton turbine with many small blades
- Hydrofoil impeller designed for high-speed, low-viscosity fluids
- Pitched-blade or helical ribbon impeller for axial pumping
- Bubble cap impeller
Correct Answer: Pitched-blade or helical ribbon impeller for axial pumping
Q9. Which parameter most directly describes the maximum local shear imposed by an impeller useful for shear-sensitive cell cultures?
- Tip speed (π D N)
- Power number (Np)
- Flow number (Fl)
- Reynolds number (Re)
Correct Answer: Tip speed (π D N)
Q10. For gas–liquid reactors, the volumetric oxygen transfer coefficient kLa is influenced most by which agitation-related factor?
- Impeller material composition
- Gas solubility in liquid only
- Turbulent dissipation and interfacial area generated by impeller
- Height of liquid above the impeller only
Correct Answer: Turbulent dissipation and interfacial area generated by impeller
Q11. Which of the following best describes the Flow number (Fl) of an impeller?
- Fl = Q / (N D^3), a measure of volumetric flow generated by the impeller
- Fl = P / (ρ N^3 D^5), a measure of power consumption
- Fl = μ / (ρ N D^2), a measure of fluid viscosity effect
- Fl = Re / Np, a dimensionless shear index
Correct Answer: Fl = Q / (N D^3), a measure of volumetric flow generated by the impeller
Q12. Increasing impeller diameter (D) while keeping rotational speed (N) constant will generally:
- Decrease power draw because of lower tip speed
- Increase power draw dramatically (P ∝ D^5 for turbulent regime)
- Have no effect on power in turbulent flow
- Only affect mixing time but not power
Correct Answer: Increase power draw dramatically (P ∝ D^5 for turbulent regime)
Q13. Cavitation near impeller blades is primarily caused by:
- High dissolved oxygen concentration
- Local pressure drops below vapor pressure due to high tip speed
- Insufficient baffling in the tank
- Excessive fluid viscosity
Correct Answer: Local pressure drops below vapor pressure due to high tip speed
Q14. Which approach is most appropriate when scaling up a bioreactor for shear-sensitive mammalian cell culture?
- Maintain the same tip speed as the laboratory reactor
- Maintain constant P/V and accept increased shear
- Use scale-up criteria that keep shear rate or power per unit mass within safe limits
- Increase impeller speed proportionally to vessel diameter
Correct Answer: Use scale-up criteria that keep shear rate or power per unit mass within safe limits
Q15. Which impeller is typically chosen to produce strong axial flow to improve bulk circulation and reduce dead zones?
- Flat-blade Rushton turbine
- Pitched-blade turbine or hydrofoil (axial-flow) impeller
- Anchor impeller used for very low-viscosity fluids
- Propeller with radial vanes
Correct Answer: Pitched-blade turbine or hydrofoil (axial-flow) impeller
Q16. Mixing time in a stirred tank is most reduced by which of the following changes?
- Decreasing impeller speed while increasing liquid height
- Increasing impeller speed or using an impeller with higher flow number
- Removing baffles to allow free swirling
- Decreasing impeller diameter while keeping speed constant
Correct Answer: Increasing impeller speed or using an impeller with higher flow number
Q17. Which effect does increasing fluid viscosity have on impeller performance at constant speed?
- Decreases power consumption and increases flow rate
- Raises power consumption in laminar regime and reduces mixing efficiency
- Has negligible effect if Reynolds number is high
- Always improves gas dispersion
Correct Answer: Raises power consumption in laminar regime and reduces mixing efficiency
Q18. In aerated bioreactors, impeller position relative to gas sparger is important because:
- It determines the pH of the culture
- Optimal position improves bubble entrainment and gas dispersion, affecting kLa
- Impeller position has no influence on oxygen transfer
- It only affects cleaning-in-place (CIP) performance
Correct Answer: Optimal position improves bubble entrainment and gas dispersion, affecting kLa
Q19. Which computational tool is frequently used to predict flow patterns and shear fields around impellers during design optimization?
- HPLC simulation
- Computational Fluid Dynamics (CFD)
- Differential scanning calorimetry (DSC)
- Gas chromatography (GC)
Correct Answer: Computational Fluid Dynamics (CFD)
Q20. The presence of gas bubbles typically affects the effective Reynolds number in a stirred tank by:
- Always increasing effective fluid viscosity and reducing Re
- Reducing the effective density and viscosity such that Re effects are negligible
- Altering apparent fluid properties and flow regime; gas holdup and bubble-induced turbulence can change hydrodynamics
- Having no effect on hydrodynamics at any scale
Correct Answer: Altering apparent fluid properties and flow regime; gas holdup and bubble-induced turbulence can change hydrodynamics
