Introduction: Packed bed reactors are widely used in pharmaceutical bioprocessing for immobilized cell cultivations, enzyme-catalyzed conversions, and continuous biotransformations. This set of MCQs focuses on the theory, design, operation, and practical challenges of packed bed bioreactors relevant to M.Pharm students. Questions cover hydrodynamics, mass and heat transfer, pressure drop, residence time distribution, immobilization supports, scale-up considerations, and common failure modes such as channeling and fouling. The aim is to deepen understanding of how packing properties, flow regime, and biological constraints interact to affect productivity and product quality, and to prepare students for designing, troubleshooting, and optimizing packed bed systems in pharmaceutical applications.
Q1. What is the primary distinguishing feature of a packed bed reactor in bioprocessing?
- Free-floating suspended cells in a stirred tank
- Porous or solid packing that supports immobilized biocatalysts through which fluid flows
- High-shear turbulent mixing without immobilization
- Aerated membrane modules with liquid circulation
Correct Answer: Porous or solid packing that supports immobilized biocatalysts through which fluid flows
Q2. Which equation is commonly used to estimate pressure drop through a packed bed containing spherical particles?
- Navier–Stokes equation
- Michaelis–Menten equation
- Ergun equation
- Hagen–Poiseuille equation
Correct Answer: Ergun equation
Q3. In a downflow packed bed with immobilized cells, a major operational risk is:
- Excessive axial mixing improving RTD
- Channeling leading to bypass and reduced contact time
- Complete plug flow behaviour ensuring uniform conversion
- Spontaneous bed densification increasing mass transfer
Correct Answer: Channeling leading to bypass and reduced contact time
Q4. Which parameter most directly decreases when particle size of packing is reduced (all else constant)?
- Specific surface area available for immobilization
- Bed void fraction
- Pressure drop across the bed increases
- Inter-particle porosity increases
Correct Answer: Pressure drop across the bed increases
Q5. The main mass transfer limitation inside an immobilized cell particle is best described as:
- External convective mass transfer in bulk fluid only
- Internal diffusion resistance through the particle matrix
- Instantaneous equilibrium between phases
- Negligible for all immobilization supports
Correct Answer: Internal diffusion resistance through the particle matrix
Q6. Which residence time distribution (RTD) characteristic is most associated with ideal packed bed reactors?
- Exponential RTD of a CSTR
- Delta function (perfect plug flow)
- Broad Gaussian due to strong back-mixing
- Multi-modal RTD from dead zones and channeling
Correct Answer: Multi-modal RTD from dead zones and channeling
Q7. For aerobic biotransformations in a packed bed, which factor is most critical to maintain reaction rate?
- High mechanical agitation inside packing
- Efficient oxygen transfer to immobilized cells
- Low external mass transfer resistance only
- Elimination of all axial gradients
Correct Answer: Efficient oxygen transfer to immobilized cells
Q8. In scale-up of packed bed bioreactors, which correlation is commonly adjusted to maintain similar hydrodynamic behavior?
- Reynolds and Peclet numbers
- Arrhenius and Monod parameters
- Henry’s law constant
- Van’t Hoff factor
Correct Answer: Reynolds and Peclet numbers
Q9. Trickle-bed reactors differ from typical packed beds by:
- Operating with gas and liquid phases flowing co-currently over packing
- Having no solid packing and only a liquid phase
- Using only freely suspended cells in liquid
- Being designed exclusively for high-pressure single-phase reactions
Correct Answer: Operating with gas and liquid phases flowing co-currently over packing
Q10. Which packing attribute most enhances external mass transfer coefficient (kL) for liquid-phase reactions?
- Large smooth spherical particles with low specific surface area
- High specific surface area and surface roughness of packing
- Extremely high bed porosity with minimal contact
- Hydrophobic packing that repels liquid
Correct Answer: High specific surface area and surface roughness of packing
Q11. The effectiveness factor (η) for immobilized enzyme particles indicates:
- The ratio of observed reaction rate to rate if entire particle were at bulk substrate concentration
- The packing density of the bed
- The pressure drop per unit bed length
- The fraction of gas holdup in the bed
Correct Answer: The ratio of observed reaction rate to rate if entire particle were at bulk substrate concentration
Q12. Axial dispersion in a packed bed is best quantified by which dimensionless group?
- Damköhler number
- Schmidt number
- Peclet number
- Biot number
Correct Answer: Peclet number
Q13. A common advantage of packed bed reactors for pharmaceutical biotransformations is:
- Easy agitation of large liquid volumes
- Ability to operate continuously with high catalyst loading and long-term stability
- Negligible pressure drop regardless of scale
- Complete absence of mass transfer limitations
Correct Answer: Ability to operate continuously with high catalyst loading and long-term stability
Q14. Which operational strategy helps mitigate hot spots in an exothermic packed bed bioreactor?
- Increasing particle size to reduce surface area
- Implementing segmented cooling jackets or internal heat-exchange tubes
- Reducing bed porosity to increase residence time
- Eliminating feed pre-heating
Correct Answer: Implementing segmented cooling jackets or internal heat-exchange tubes
Q15. Fouling in a packed bed commonly results in:
- Decreased pressure drop and increased flow
- Increased pressure drop and decreased effective bed porosity
- Improved mass transfer due to smoother surfaces
- Instant regeneration of packing
Correct Answer: Increased pressure drop and decreased effective bed porosity
Q16. When immobilizing cells on porous carriers, which parameter most directly affects internal diffusion resistance?
- Bulk fluid viscosity only
- Pore size and tortuosity of the carrier
- External reactor geometry exclusively
- Feed substrate concentration only
Correct Answer: Pore size and tortuosity of the carrier
Q17. In packed bed design, the term “bed void fraction” (ε) refers to:
- The fraction of solid particle volume relative to reactor volume
- The fraction of void (fluid) volume relative to total bed volume
- The porosity within each particle only
- The fraction of bed occupied by microorganisms
Correct Answer: The fraction of void (fluid) volume relative to total bed volume
Q18. Which dimensionless number compares reaction rate to mass transfer rate and is useful in packed bed analysis?
- Reynolds number
- Damköhler number
- Prandtl number
- Sherwood number
Correct Answer: Damköhler number
Q19. A practical cleaning-in-place (CIP) concern for packed beds with biological fouling is:
- CIP is always unnecessary for immobilized systems
- Achieving uniform cleaning without disrupting the immobilized biomass or packing integrity
- Using only water at ambient temperature is sufficient
- High-velocity turbulent jets cannot remove fouling
Correct Answer: Achieving uniform cleaning without disrupting the immobilized biomass or packing integrity
Q20. Which application is least suited for packed bed bioreactors in pharmaceutical processing?
- Continuous enzymatic synthesis with immobilized enzyme beads
- High-cell-density free suspension fermentation needing frequent sampling and agitation
- Biocatalytic column for stereoselective conversion
- Trickle-bed bioreactor for gas-liquid biotransformations
Correct Answer: High-cell-density free suspension fermentation needing frequent sampling and agitation
