Types of bioreactors: Tower reactor MCQs With Answer
This short quiz set is designed for M.Pharm students studying bioprocess engineering to deepen understanding of tower-type bioreactors (packed/tower, trickle-bed and related vertical configurations). The questions cover core concepts such as hydrodynamics, gas–liquid mass transfer, packing materials, flow regimes, immobilized-cell applications, sterilization, fouling control and scale-up challenges. Each MCQ emphasizes practical design and operational considerations you will encounter in pharmaceutical bioprocessing and continuous production systems. Use these items to test conceptual clarity, troubleshoot design choices, and prepare for exam questions and practical process development tasks involving tower reactors.
Q1. What best defines a tower (packed) bioreactor?
- A vertical column containing packing where gas and liquid contact is enhanced for mass transfer and biochemical reactions
- A horizontally stirred vessel with internal baffles and spargers
- A membrane-separated flat plate reactor designed for shear-sensitive cells
- A batch fermenter with rotating drum agitation
Correct Answer: A vertical column containing packing where gas and liquid contact is enhanced for mass transfer and biochemical reactions
Q2. What is the principal advantage of using a tower-type reactor for aerobic bioprocesses?
- High shear for cell disruption
- Enhanced gas–liquid interfacial area leading to improved oxygen transfer
- Precise temperature gradients along radial direction
- Complete elimination of biofilm formation
Correct Answer: Enhanced gas–liquid interfacial area leading to improved oxygen transfer
Q3. Which materials are commonly used as packing in tower bioreactors for pharmaceutical applications?
- Raschig rings, saddles and structured metal or ceramic packings
- Only cotton wool and paper fibers
- Disposable plastic bags filled with beads
- Activated carbon granules exclusively
Correct Answer: Raschig rings, saddles and structured metal or ceramic packings
Q4. Which flow arrangement in a packed tower typically maximizes driving force for mass transfer between gas and liquid phases?
- Counter-current flow (gas and liquid flow in opposite directions)
- Co-current flow (gas and liquid flow in same direction)
- Crossflow (gas flows across liquid flow laterally)
- Static (no imposed flow)
Correct Answer: Counter-current flow (gas and liquid flow in opposite directions)
Q5. A major scale-up challenge unique to tall packed tower bioreactors is:
- Increased pressure drop and risk of channeling with scale, which reduces effective mass transfer
- Complete elimination of oxygen transfer limitations at large scale
- Ability to use any packing without affecting hydrodynamics
- Lower risk of fouling compared with lab scale
Correct Answer: Increased pressure drop and risk of channeling with scale, which reduces effective mass transfer
Q6. Tower reactors are particularly well suited for which bioprocess configuration?
- Immobilized-cell or immobilized-enzyme continuous reactors (packed-bed/trickle-bed)
- Suspension aerobic cultures requiring intense mechanical stirring
- Lyophilization of bioproducts
- Batch solid-state fermentation on trays only
Correct Answer: Immobilized-cell or immobilized-enzyme continuous reactors (packed-bed/trickle-bed)
Q7. Compared to a bubble column, a packed-tower reactor typically provides:
- Higher specific interfacial area and often better gas–liquid contact per unit volume
- Greater homogenous mixing by turbulent eddies
- Lower overall surface area for transfer
- Better capability to handle very high solids slurries
Correct Answer: Higher specific interfacial area and often better gas–liquid contact per unit volume
Q8. Which operational symptom most consistently indicates channeling in a packed tower?
- Sudden decrease in pressure drop across the bed and reduced conversion or mass transfer
- Linear increase in pressure drop with flow as expected
- Constant or improved conversion with time
- Complete elimination of liquid maldistribution but higher heat transfer
Correct Answer: Sudden decrease in pressure drop across the bed and reduced conversion or mass transfer
Q9. What sterilization approach is generally preferred for industrial tower reactors when feasible?
- Steam-in-place (SIP) combined with validated cycle times and steam distribution
- Complete autoclaving of the assembled tower in a single unit
- Ultraviolet irradiation through the packed bed
- Dry-heat sterilization at ambient pressure
Correct Answer: Steam-in-place (SIP) combined with validated cycle times and steam distribution
Q10. Excessive shear or high superficial gas velocity in a packed tower containing immobilized cells will most likely cause:
- Detachment of biomass from the support and loss of activity
- Improved immobilization and stronger biofilm adhesion
- Complete resistance to channeling
- Formation of larger, more stable immobilized cell pellets
Correct Answer: Detachment of biomass from the support and loss of activity
Q11. Which experimental method is most appropriate to determine the overall volumetric mass transfer coefficient (KLa) in a tower reactor?
- Dynamic gassing-out (oxygen de-saturation and re-aeration) method
- Direct measurement of biomass dry weight only
- Using a calorimeter to measure heat generation
- Measuring electrical conductivity of the liquid phase alone
Correct Answer: Dynamic gassing-out (oxygen de-saturation and re-aeration) method
Q12. One advantage of structured packing over random packing in tower bioreactors is:
- Lower pressure drop per unit mass transfer and improved predictable liquid distribution
- Complete immunity to fouling
- Ability to use without any liquid redistribution trays
- Guaranteed sterilization without SIP
Correct Answer: Lower pressure drop per unit mass transfer and improved predictable liquid distribution
Q13. In pharmaceutical continuous bioprocessing, tower reactors are commonly applied for:
- Continuous biotransformations using immobilized enzymes or cells and gas–liquid reactions (e.g., oxygen-dependent conversions)
- High-shear homogenization of lipid emulsions
- Dissolution testing of tablets only
- Direct sterilization of packaging materials
Correct Answer: Continuous biotransformations using immobilized enzymes or cells and gas–liquid reactions (e.g., oxygen-dependent conversions)
Q14. The idealized residence time distribution (RTD) for a well-wetted packed tower operating without channeling approximates which behavior?
- Plug flow with some axial dispersion (non-ideal but closer to plug flow than CSTR)
- Perfectly mixed reactor behavior (single CSTR)
- Completely segregated batch-wise pockets only
- Infinite mixing in both axial and radial directions
Correct Answer: Plug flow with some axial dispersion (non-ideal but closer to plug flow than CSTR)
Q15. Which design parameter is most critical when sizing a tower reactor for an oxygen-limited aerobic process?
- Overall volumetric oxygen transfer coefficient (KLa) and available gas–liquid interfacial area
- Color of the packing material
- Total reactor height only, irrespective of packing or flow
- Electrical conductivity of the feed
Correct Answer: Overall volumetric oxygen transfer coefficient (KLa) and available gas–liquid interfacial area
Q16. Which operational practice helps control fouling and maintain bed performance in a packed tower?
- Periodic backwashing or pulsed liquid flushes and controlled flow reversal when compatible with biology
- Never stopping flow to prevent disturbance
- Constant use of the highest possible superficial gas velocity
- Applying only dry cleaning methods while reactor is in operation
Correct Answer: Periodic backwashing or pulsed liquid flushes and controlled flow reversal when compatible with biology
Q17. When selecting construction materials for a pharmaceutical tower reactor, which choice is most suitable for corrosion resistance and cleanability?
- 316L stainless steel and glass- or ceramic-lined internals where necessary
- Plain carbon steel without protective coating
- Uncoated aluminum alloy for all wetted parts
- Wooden internal supports to reduce cost
Correct Answer: 316L stainless steel and glass- or ceramic-lined internals where necessary
Q18. A trickle-bed reactor is best described as:
- Gas flowing continuously through a packed bed while liquid trickles over packing, used for gas–liquid–solid catalytic or biocatalytic processes
- A reactor where liquid circulates turbulently without a packing bed
- An agitated flat-flask used for small-scale screening
- A closed membrane module operating in dead-end filtration
Correct Answer: Gas flowing continuously through a packed bed while liquid trickles over packing, used for gas–liquid–solid catalytic or biocatalytic processes
Q19. For aerobic bioprocesses in tubular/tower configurations, improving oxygen transfer without increasing shear is often achieved by:
- Optimizing packing geometry and increasing specific surface area rather than increasing superficial gas velocity excessively
- Removing all packing to create a stagnant column
- Maximizing mechanical agitation inside the packed bed
- Using pure oxygen without changing packing or flows
Correct Answer: Optimizing packing geometry and increasing specific surface area rather than increasing superficial gas velocity excessively
Q20. Which operational change will most directly increase KLa in a packed tower up to practical limits?
- Increasing superficial gas velocity (gas flow rate) and optimizing liquid distribution across packing
- Lowering liquid feed temperature to near freezing
- Decreasing available packing surface area dramatically
- Sealing the column completely to stop gas flow
Correct Answer: Increasing superficial gas velocity (gas flow rate) and optimizing liquid distribution across packing
