Equipment design in scale-down studies MCQs With Answer

Introduction: Equipment design in scale-down studies is a critical topic for M.Pharm students specializing in bioprocess engineering. Scale-down models allow researchers to emulate large-scale bioreactors and production equipment in laboratory settings to predict process performance, evaluate critical quality attributes, and troubleshoot design or operational issues. Understanding how geometric, kinematic and dynamic similarities are translated into small-scale systems, along with the role of dimensionless numbers (Reynolds, Froude, Peclet), mass transfer (kLa), mixing times, shear effects, and heat transfer, is essential. This quiz set focuses on practical and theoretical aspects of scale-down equipment design, validation strategies, and common pitfalls encountered during scale reduction.

Q1. In scale-down studies for stirred tank bioreactors, which scaling criterion is most appropriate when preserving both mixing and oxygen transfer characteristics for aerobic cultures?

• Constant geometric similarity (same aspect ratio)
• Constant tip speed
• Constant power per unit volume (P/V)
• Constant Reynold’s number

Correct Answer: Constant power per unit volume (P/V)

Q2. Which dimensionless number primarily describes the relative importance of inertial to viscous forces and is widely used in scaling mixing processes?

• Peclet number (Pe)
• Froude number (Fr)
• Reynolds number (Re)
• Damköhler number (Da)

Correct Answer: Reynolds number (Re)

Q3. When scaling down from pilot to lab scale for shear-sensitive mammalian cell cultures, which of the following is the most critical design consideration?

• Matching vessel diameter exactly
• Minimizing maximum local shear and tip speed
• Maintaining the same impeller material
• Using identical sparger hole size

Correct Answer: Minimizing maximum local shear and tip speed

Q4. Which technique is commonly used to measure volumetric oxygen transfer coefficient (kLa) in a lab-scale bioreactor?

• Sulfite oxidation (sodium sulfite) method
• High-performance liquid chromatography (HPLC)
• Dynamic gassing-out method using dissolved oxygen probe
• Gas chromatography of headspace

Correct Answer: Dynamic gassing-out method using dissolved oxygen probe

Q5. For scale-down models aiming to reproduce bubble size distribution found at large scale, which factor in the small-scale reactor must be carefully matched?

• The pH of the medium
• Sparger geometry and gas superficial velocity
• Impeller shaft material
• Sampling port location

Correct Answer: Sparger geometry and gas superficial velocity

Q6. Which scaling rule is typically preferred when heat transfer is the primary concern in scaling down thermal processes?

• Constant power per volume
• Constant heat transfer coefficient (U) and surface area to volume ratio
• Constant mixing time
• Constant kinematic viscosity

Correct Answer: Constant heat transfer coefficient (U) and surface area to volume ratio

Q7. In scale-down studies, residence time distribution (RTD) is evaluated to detect which of the following large-scale issues?

• Foam formation
• Non-ideal mixing and dead zones leading to broad RTD
• Electrode fouling
• pH probe calibration errors

Correct Answer: Non-ideal mixing and dead zones leading to broad RTD

Q8. Which impeller selection principle is most appropriate for promoting axial flow and bulk mixing in scale-down stirred tanks?

• Use of radial flow Rushton turbines
• Use of pitched-blade or hydrofoil impellers that promote axial flow
• Use of multiple small disc turbines
• Use of static mixers only

Correct Answer: Use of pitched-blade or hydrofoil impellers that promote axial flow

Q9. When employing geometric similarity in scale-down, which ratio must be maintained between large and small vessels?

• Same absolute impeller diameter
• Same aspect ratio (height/diameter) and relative impeller diameter (D/T)
• Same liquid volume in liters
• Same absolute baffle width

Correct Answer: Same aspect ratio (height/diameter) and relative impeller diameter (D/T)

Q10. Which approach is best for capturing large-scale shear gradients in a scale-down reactor for filamentous organisms?

• Using a single well-mixed mini-reactor with identical P/V
• Designing a multi-compartment scale-down model that mimics high and low shear zones
• Matching only the overall kLa
• Using static batch flasks without agitation

Correct Answer: Designing a multi-compartment scale-down model that mimics high and low shear zones

Q11. In CFD-assisted scale-down design, what primary output is most useful to compare hydrodynamics between scales?

• Local temperature maps only
• Velocity fields, shear distribution, and turbulence kinetic energy maps
• Only global power consumption
• pH distribution maps

Correct Answer: Velocity fields, shear distribution, and turbulence kinetic energy maps

Q12. Which dimensionless number is important when gravity-driven effects (e.g., free surface vortexing) must be preserved during scaling?

• Peclet number
• Damköhler number
• Froude number (Fr)
• Prandtl number

Correct Answer: Froude number (Fr)

Q13. During scale-down validation for a fed-batch process, which measurable attribute is most critical to correlate between scales?

• Color of the medium
• Substrate consumption rate, product titer, and key metabolite profiles
• Weight of the vessel
• Exact sparger hole diameter in mm

Correct Answer: Substrate consumption rate, product titer, and key metabolite profiles

Q14. What is a major limitation of the sulfite oxidation method for kLa measurement when applied to scale-down bioreactors with biological cultures?

• It overestimates pH control requirements
• It does not account for biological oxygen uptake and can misrepresent kLa under foaming or reactive gas conditions
• It requires expensive isotopes
• It gives instantaneous bubble size distributions

Correct Answer: It does not account for biological oxygen uptake and can misrepresent kLa under foaming or reactive gas conditions

Q15. Which scaling strategy helps emulate intermittent nutrient and oxygen fluctuations experienced by cells at large scale?

• Continuous single-zone well-mixed model
• Multi-compartment scale-down that recreates cyclic exposures to gradients
• Eliminating sparging entirely
• Maintaining a static cell-free medium

Correct Answer: Multi-compartment scale-down that recreates cyclic exposures to gradients

Q16. For high cell density cultures where oxygen demand is high, which scale-down design feature is most important to ensure representativeness?

• Large headspace with no agitation
• Properly matched oxygen transfer (kLa) and gas flow regimes including bubble residence time
• Use of identical stainless-steel grade
• Exact replication of pilot-scale control software

Correct Answer: Properly matched oxygen transfer (kLa) and gas flow regimes including bubble residence time

Q17. Which parameter is commonly used as a surrogate for mixing performance and often matched in scale-down studies?

• Mixing time (t95) — time required to reach 95% homogeneity
• Number of operator interventions per run
• Absolute baffle thickness in mm
• Cost of the vessel

Correct Answer: Mixing time (t95) — time required to reach 95% homogeneity

Q18. When scale-down modeling focuses on fouling or deposit formation on heat transfer surfaces, which similarity should be prioritized?

• Geometric similarity only
• Thermal boundary layer similarity and local shear at the wall
• Matching impeller color and finish
• Reproducing the operator’s work schedule

Correct Answer: Thermal boundary layer similarity and local shear at the wall

Q19. Which of the following is a common regulatory expectation for scale-down models used in process characterization and validation?

• Scale-down models can be informal and undocumented
• Demonstration of representativeness with experimental comparability data and rationale for scaling choices
• Only theoretical justification without experimental data is sufficient
• Use of any lab glassware is acceptable

Correct Answer: Demonstration of representativeness with experimental comparability data and rationale for scaling choices

Q20. In scale-down studies, which strategy helps reduce discrepancies due to probe placement and sensor response time between large and small reactors?

• Ignoring sensor dynamics since they are negligible
• Characterizing and compensating for sensor time constants and deliberately matching probe immersion depth relative to impeller
• Using only offline assays instead of sensors
• Placing sensors arbitrarily near the vessel wall

Correct Answer: Characterizing and compensating for sensor time constants and deliberately matching probe immersion depth relative to impeller

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