Scale-down theory MCQs With Answer
Introduction: Scale-down theory is an essential topic for M.Pharm students specializing in bioprocess engineering and technology. This blog presents focused multiple-choice questions that probe principles used to design lab-scale systems which faithfully reproduce large-scale bioreactor behaviour. Questions emphasize similarity criteria (geometric, kinematic, dynamic), dimensionless numbers (Reynolds, Froude, power number), oxygen transfer and mixing scaling, hydrodynamic stress, and practical scale-down strategies for shear-sensitive biologics. These MCQs will strengthen conceptual understanding and help apply quantitative scaling rules when developing representative small-scale models for process development, troubleshooting, and regulatory submissions.
Q1. What is the primary objective of scale-down model development in bioprocess engineering?
- To build the smallest possible reactor regardless of performance
- To replicate critical large-scale process performance and stresses in a controlled laboratory system
- To reduce cost by changing process chemistry at small scale
- To eliminate the need for pilot-scale testing entirely
Correct Answer: To replicate critical large-scale process performance and stresses in a controlled laboratory system
Q2. Which similarity criterion is most appropriate when the main concern is matching mixing intensity and shear exposure between scales?
- Geometric similarity
- Constant impeller tip speed
- Constant power per unit volume (P/V)
- Constant residence time distribution
Correct Answer: Constant power per unit volume (P/V)
Q3. Which dimensionless number is most useful to compare inertial and viscous forces and to characterize flow regime in stirred bioreactors?
- Froude number
- Power number
- Reynolds number
- Schmidt number
Correct Answer: Reynolds number
Q4. When oxygen transfer is limiting and must be preserved at small scale, which parameter is commonly conserved during scale-down?
- Impeller diameter
- Volumetric mass transfer coefficient (kLa)
- Pump head
- Vessel wall thickness
Correct Answer: Volumetric mass transfer coefficient (kLa)
Q5. For turbulent stirred tanks, scaling based on constant P/V typically implies what relationship between agitation speed (N) and vessel volume (V)?
- N scales proportional to V^(1/3)
- N scales proportional to V^(-1/3)
- N does not change with V
- N scales proportional to V^(-1)
Correct Answer: N scales proportional to V^(-1/3)
Q6. Which of the following is a limitation of simple geometric similarity when scaling down bioreactors?
- It guarantees identical hydrodynamic stress distribution
- It ensures identical oxygen transfer coefficients across scales
- It often fails because power input and mass transfer do not scale linearly with size
- It automatically preserves mixing times
Correct Answer: It often fails because power input and mass transfer do not scale linearly with size
Q7. Which criterion would you prioritize when scale-down aims to reproduce shear-sensitive cell damage observed at production scale?
- Constant kLa
- Constant volumetric power input (P/V)
- Constant oxygen concentration in off-gas
- Constant sampling frequency
Correct Answer: Constant volumetric power input (P/V)
Q8. The power number (Np) for an impeller is defined as which combination of variables made dimensionless?
- P/(ρ N^3 D^5)
- P/(μ N^2 D^3)
- P/(ρ g H^2)
- P/(N D)
Correct Answer: P/(ρ N^3 D^5)
Q9. When scale-down targets constant impeller tip speed, what is a likely consequence for oxygen transfer (kLa) as scale decreases?
- kLa will necessarily remain constant
- kLa tends to decrease if power per unit volume drops
- kLa will increase proportional to tip speed squared
- kLa is independent of tip speed
Correct Answer: kLa tends to decrease if power per unit volume drops
Q10. Which similarity approach explicitly accounts for gravitational effects important for free-surface flows and gas dispersion?
- Reynolds scaling
- Froude scaling
- Geometric scaling
- Thermal scaling
Correct Answer: Froude scaling
Q11. In scale-down design for oxygen-limited aerobic cultures, why might one simulate sparger performance at small scale?
- Sparger design does not affect gas-liquid mass transfer
- Gas bubble size distribution and dispersion affect kLa and local DO gradients
- Spargers only impact sterility and not mass transfer
- Spargers affect only heat transfer
Correct Answer: Gas bubble size distribution and dispersion affect kLa and local DO gradients
Q12. What is the typical effect of non-Newtonian rheology on scale-down similarity choices?
- Non-Newtonian behavior simplifies scaling because viscosity is constant
- It requires consideration of local shear rates and apparent viscosity, making simple P/V scaling unreliable
- It allows use of Froude number as the sole scaling metric
- Rheology has no effect on kLa or mixing
Correct Answer: It requires consideration of local shear rates and apparent viscosity, making simple P/V scaling unreliable
Q13. Which experimental measurement is often used to validate a scale-down model of mixing performance?
- Protein sequence analysis
- Mixing time measured by tracer decay or pH step
- Gas chromatogram of off-gas components
- Vessel wall thermal conductivity
Correct Answer: Mixing time measured by tracer decay or pH step
Q14. In population balance approaches integrated with scale-down, what phenomenon is typically being modeled?
- Heat transfer across the vessel wall
- Particle/bubble size distribution dynamics including breakage and aggregation
- Genomic variation in cell line
- Electrical conductivity of medium
Correct Answer: Particle/bubble size distribution dynamics including breakage and aggregation
Q15. Computational Fluid Dynamics (CFD) is increasingly used in scale-down design because it can:
- Replace all physical experiments with perfect accuracy
- Provide detailed local fields of velocity, shear and gas hold-up to guide scale-down decisions
- Guarantee identical kLa across scales without validation
- Negate the need to understand dimensionless numbers
Correct Answer: Provide detailed local fields of velocity, shear and gas hold-up to guide scale-down decisions
Q16. Which of the following is a common practical compromise when exact similarity cannot be achieved at small scale?
- Change the biological system to fit the scale
- Prioritize reproducing the most critical performance attributes (e.g., kLa or shear) and accept deviations in less critical metrics
- Ignore scale effects entirely
- Run production-scale fermentations at lab environmental conditions
Correct Answer: Prioritize reproducing the most critical performance attributes (e.g., kLa or shear) and accept deviations in less critical metrics
Q17. For shear-sensitive mammalian cells, which scale-down parameter is often limited to minimize damage while attempting to match large-scale conditions?
- Sterility assurance level
- Maximum local energy dissipation rate
- Culture medium osmolarity
- Sampling interval
Correct Answer: Maximum local energy dissipation rate
Q18. When replicating gas-liquid mass transfer using constant superficial gas velocity at different scales, what must also be considered to ensure similarity?
- Impeller geometry, mixing regime and bubble coalescence dynamics
- Only the vessel color
- Protein molecular weight
- Ambient room temperature alone
Correct Answer: Impeller geometry, mixing regime and bubble coalescence dynamics
Q19. Which monitoring strategy can improve the relevance of a scale-down model by ensuring comparable process dynamics?
- Real-time monitoring of DO, pH and off-gas combined with feedback control to match large-scale profiles
- Only end-point sampling
- Monitoring only external temperature
- Weekly manual cell counts
Correct Answer: Real-time monitoring of DO, pH and off-gas combined with feedback control to match large-scale profiles
Q20. What is a recommended first step when designing a scale-down model for a new production bioprocess?
- Assume geometric similarity is sufficient and proceed with the smallest vessel
- Perform risk assessment to identify critical large-scale attributes (e.g., kLa, P/V, shear) and select similarity criteria accordingly
- Immediately run full-scale batches to get data
- Change the strain to a more robust organism
Correct Answer: Perform risk assessment to identify critical large-scale attributes (e.g., kLa, P/V, shear) and select similarity criteria accordingly
