Process Validation – Mixing MCQs With Answer

Process Validation – Mixing MCQs With Answer

Introduction: This quiz set focuses on process validation for mixing operations, a core topic in M.Pharm Scale Up & Technology Transfer. It covers fundamental mixing principles, scale-up criteria, impeller selection, turbulence and laminar behavior, tracer studies, and validation lifecycle considerations. Questions emphasize practical aspects such as power and Reynolds number correlations, shear-sensitive materials, solids suspension, baffles, multiple impellers, and the use of PAT and CFD for predicting performance. These MCQs are designed to deepen understanding of how mixing affects product quality and reproducibility, and to prepare students for designing and validating robust mixing processes during technology transfer and commercial scale manufacturing.

Q1. What is the primary objective of process validation for mixing in pharmaceutical scale-up?

• To minimize energy consumption during mixing
• To demonstrate that mixing consistently produces material meeting quality specifications
• To select the cheapest impeller available
• To validate cleaning procedures only

Correct Answer: To demonstrate that mixing consistently produces material meeting quality specifications

Q2. Which of the following expressions correctly represents the impeller power number (Np) used in mixing scale-up correlations?

• P/(rho N^3 D^5)
• P/(mu N^2 D^3)
• rho N D^2 / mu
• N D / (g H)

Correct Answer: P/(rho N^3 D^5)

Q3. Which scale-up criterion is most appropriate when trying to maintain similar bulk mixing intensity across scales?

• Constant impeller diameter
• Constant tip speed
• Constant power per unit volume (P/V)
• Constant motor torque

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

Q4. The mixing Reynolds number for an agitated tank is given by which formula?

• Re = rho N D^2 / mu
• Re = rho V D / mu
• Re = N D / g
• Re = P/(rho N^3 D^5)

Correct Answer: Re = rho N D^2 / mu

Q5. For suspending coarse solids uniformly in a tall tank with moderate viscosity liquid, which impeller type is generally preferred?

• Radial Rushton turbine
• Axial-flow hydrofoil (or pitched blade) impeller
• High-shear rotor-stator
• High solidity raked blade

Correct Answer: Axial-flow hydrofoil (or pitched blade) impeller

Q6. Which mixing device is most suitable for producing fine emulsions requiring high shear at small scales?

• Pitched blade turbine
• Rushton turbine
• High-shear rotor-stator mixer
• Anchor stirrer

Correct Answer: High-shear rotor-stator mixer

Q7. The Rushton turbine is primarily characterized as which type of impeller and is commonly used for what application?

• Axial flow impeller used for solids suspension
• Radial flow impeller used for gas dispersion and dispersion/intensive mixing
• High-shear stator used for emulsification
• Low-shear anchor used for viscous liquids

Correct Answer: Radial flow impeller used for gas dispersion and dispersion/intensive mixing

Q8. In mixing validation, how is “mixing time” most commonly defined?

• The time for the impeller to reach full speed from rest
• The time required to reach a specified degree of homogeneity (e.g., 95%) after tracer addition
• The time taken to fill the tank
• The time for temperature to reach setpoint during heating

Correct Answer: The time required to reach a specified degree of homogeneity (e.g., 95%) after tracer addition

Q9. What is the main functional effect of installing four equally spaced vertical baffles in a stirred tank?

• Reduce power draw by reducing turbulence
• Prevent vortex formation and increase turbulent mixing
• Eliminate the need for an impeller
• Improve heat transfer only without affecting flow

Correct Answer: Prevent vortex formation and increase turbulent mixing

Q10. Which scale-up rule is most appropriate when the product is shear-sensitive and shear must be limited during scale-up?

• Maintain constant power per volume (P/V)
• Maintain constant tip speed
• Maintain constant Reynolds number
• Maintain constant impeller diameter-to-tank diameter ratio

Correct Answer: Maintain constant tip speed

Q11. In the laminar mixing regime, how does power consumption typically scale with speed and viscosity?

• P ∝ rho N^3 D^5
• P ∝ mu N^2 D^3
• P ∝ N D
• P ∝ N^1.5 D^4

Correct Answer: P ∝ mu N^2 D^3

Q12. Which experimental technique is most widely used to quantify mixing time and validate mixing performance in a liquid system?

• Viscometry with rotational rheometer only
• Tracer studies using conductivity, pH or dye with time-resolved measurement
• Sampling for microbial count
• Measuring motor current alone

Correct Answer: Tracer studies using conductivity, pH or dye with time-resolved measurement

Q13. Why are non-Newtonian fluids particularly challenging during scale-up of mixing processes?

• They have constant viscosity regardless of shear
• Shear-dependent viscosity changes flow regime and invalidates simple geometric scale-up rules
• They always require radial impellers
• They do not respond to baffles

Correct Answer: Shear-dependent viscosity changes flow regime and invalidates simple geometric scale-up rules

Q14. The power number (Np) for many impellers becomes approximately constant in which flow regime?

• Laminar regime
• Transitional regime
• Turbulent regime
• Buoyancy-dominated regime

Correct Answer: Turbulent regime

Q15. During powder addition into a liquid, which factors most strongly influence wetting time and prevention of clumping?

• Impeller material and motor brand
• Liquid surface tension and impeller-induced shear/flow at the powder-liquid interface
• Color of the powder
• Tank color and lighting

Correct Answer: Liquid surface tension and impeller-induced shear/flow at the powder-liquid interface

Q16. How does computational fluid dynamics (CFD) contribute to mixing process validation and scale-up?

• Replaces all experimental validation and regulatory documentation
• Predicts detailed flow patterns, identifies dead zones and helps define scale-up parameters to reduce experimental burden
• Only predicts temperature profiles and is irrelevant to mixing
• Eliminates the need for tracer studies

Correct Answer: Predicts detailed flow patterns, identifies dead zones and helps define scale-up parameters to reduce experimental burden

Q17. When is it advisable to use multiple impellers on a single shaft in a stirred vessel?

• When the tank diameter is extremely small
• For tall tanks with large liquid height where a single impeller cannot provide uniform mixing vertically
• To reduce the Reynolds number
• Only when using laminar flow

Correct Answer: For tall tanks with large liquid height where a single impeller cannot provide uniform mixing vertically

Q18. In a mixing validation study for a liquid blend, which acceptance criterion is commonly used to demonstrate content uniformity across sampling locations?

• All samples must have identical pH to two decimal places
• Batch samples show relative standard deviation (RSD) ≤ 5% across sampling locations
• Each sample must show zero turbidity
• Sampling locations must have identical temperatures

Correct Answer: Batch samples show relative standard deviation (RSD) ≤ 5% across sampling locations

Q19. Which tracer type is most appropriate and commonly used for aqueous mixing time studies because it provides rapid, quantitative, non-reactive signals?

• Radioactive tracer
• Conductivity tracer (e.g., NaCl addition with conductivity probe)
• Suspension of solids
• Colorless oil droplet

Correct Answer: Conductivity tracer (e.g., NaCl addition with conductivity probe)

Q20. Within the Process Validation lifecycle, which stage is primarily intended to demonstrate reproducible mixing performance under routine production conditions?

• Installation Qualification (IQ)
• Operational Qualification (OQ)
• Process Performance Qualification (PQ)
• Factory Acceptance Testing (FAT)

Correct Answer: Process Performance Qualification (PQ)

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