Introduction: Types of bioreactors: Airlift reactor MCQs With Answer is aimed at M.Pharm students who need a concise, application-focused review of airlift reactors within bioprocess engineering. This blog covers internal and external airlift designs, draft tube function, hydrodynamics, gas holdup, mass transfer (kLa), scale-up considerations, and suitability for different biological systems. The questions probe conceptual understanding, practical implications for aerobic fermentations and cell cultures, and modelling approaches used in development and scale-up. Use these MCQs to test and strengthen your ability to select and design airlift reactors for pharmaceutical bioprocesses involving shear-sensitive microbes and oxygen-limited reactions.
Q1. What fundamentally distinguishes an airlift reactor from other gas-liquid reactors?
- Mixing driven primarily by an external mechanical impeller
- Circulation established by density difference created by gas injection into a riser and downcomer
- Operation as a packed bed with fixed catalyst particles
- Use of sparged liquid jets without gas-induced circulation
Correct Answer: Circulation established by density difference created by gas injection into a riser and downcomer
Q2. Which pair correctly names the two main airlift reactor configurations?
- Stirred-tank and bubble column
- Internal-loop and external-loop airlift reactors
- Baffled and unbaffled airlift reactors
- Packed-bed and fluidized-bed airlift reactors
Correct Answer: Internal-loop and external-loop airlift reactors
Q3. What is the primary function of a draft tube in an internal-loop airlift reactor?
- To inject nutrients into the culture medium
- To separate the riser and downcomer zones and enhance circulation velocity
- To provide mechanical agitation to break bubbles
- To act as a heat exchanger for temperature control
Correct Answer: To separate the riser and downcomer zones and enhance circulation velocity
Q4. In a draft-tube airlift, how does gas holdup typically compare between the riser and the downcomer?
- Gas holdup is higher in the downcomer than riser
- Gas holdup is equal in riser and downcomer under all conditions
- Gas holdup is higher in the riser than in the downcomer
- Gas holdup is negligible in both riser and downcomer
Correct Answer: Gas holdup is higher in the riser than in the downcomer
Q5. Which characteristic makes airlift reactors especially suitable for shear-sensitive cultures?
- High-intensity mechanical shear from impellers
- Large-scale turbulent eddies created by baffles
- Low shear environment with gentle gas-driven circulation
- Intermittent high-shear mixing from pulsating pumps
Correct Answer: Low shear environment with gentle gas-driven circulation
Q6. Which operational parameter most directly increases the volumetric oxygen transfer coefficient (kLa) in airlift reactors?
- Increasing substrate concentration while keeping gas flow constant
- Increasing superficial gas velocity (gas flow per cross-sectional area)
- Decreasing liquid height only
- Operating at lower temperature to increase gas solubility
Correct Answer: Increasing superficial gas velocity (gas flow per cross-sectional area)
Q7. Compared with an equivalent stirred-tank reactor, the power input requirement of an airlift reactor is typically:
- Much higher for the same oxygen transfer rate
- Lower per unit volume for comparable circulation and aeration
- Identical because mixing is always matched
- Negligible and independent of gas flow
Correct Answer: Lower per unit volume for comparable circulation and aeration
Q8. Which type of bioprocess is generally less suitable for airlift reactors without modification?
- Aerobic fermentations of suspension bacteria
- Mammalian cell cultures that are shear-sensitive
- Highly viscous or filamentous biomass processes prone to clogging
- Algal cultures requiring low shear
Correct Answer: Highly viscous or filamentous biomass processes prone to clogging
Q9. What is the primary driving force for liquid circulation in an airlift reactor?
- Mechanical pumping by external pumps
- Density difference between aerated riser and deaerated downcomer due to gas holdup
- Thermal convection from heat exchange surfaces
- Electro-osmotic flow induced by applied electrical fields
Correct Answer: Density difference between aerated riser and deaerated downcomer due to gas holdup
Q10. Which scale-up criterion is commonly applied for airlift reactors to maintain similar hydrodynamics?
- Constant impeller tip speed
- Constant superficial gas velocity
- Constant number of baffles per tank diameter
- Constant stirrer geometry
Correct Answer: Constant superficial gas velocity
Q11. Which mathematical approach is widely used to model two-phase flow in airlift reactors?
- Michaelis-Menten kinetics
- Two-fluid (Euler–Euler) multiphase model
- Langmuir adsorption isotherm
- Fourier conduction equation only
Correct Answer: Two-fluid (Euler–Euler) multiphase model
Q12. How does increasing hydrostatic pressure (holding gas flow constant) generally affect gas holdup in an airlift reactor?
- Gas holdup increases because bubbles expand under pressure
- Gas holdup decreases because gas compressibility reduces volumetric fraction
- Gas holdup is independent of pressure
- Gas holdup becomes infinite at high pressure
Correct Answer: Gas holdup decreases because gas compressibility reduces volumetric fraction
Q13. What is the main structural difference between a bubble column and a draft-tube airlift reactor?
- Bubble columns always have mechanical agitators, while airlifts do not
- Draft-tube airlifts include an internal partition (draft tube) to create riser and downcomer
- Bubble columns operate under vacuum, airlifts at positive pressure
- Bubble columns use immiscible liquids, airlifts use miscible liquids
Correct Answer: Draft-tube airlifts include an internal partition (draft tube) to create riser and downcomer
Q14. Compared to a stirred-tank reactor at similar power input, how does an airlift reactor typically perform in terms of kLa?
- Generally higher kLa for same power input
- Generally lower kLa for same power input
- Exactly the same regardless of geometry
- kLa does not depend on reactor type
Correct Answer: Generally lower kLa for same power input
Q15. For cultivation of shear-sensitive mammalian cells, what feature of airlift reactors is most beneficial?
- High-intensity turbulent mixing near impellers
- Low shear stress and gentle mixing environment
- Frequent formation of large inertial eddies
- Aggressive bubble breakup by static mixers
Correct Answer: Low shear stress and gentle mixing environment
Q16. The flow pattern inside many airlift reactors is best approximated as which of the following?
- Perfectly mixed continuous stirred-tank behavior throughout
- Plug flow with axial dispersion in the riser/downcomer
- Purely laminar single-phase flow with no dispersion
- Stationary stratified layers with no circulation
Correct Answer: Plug flow with axial dispersion in the riser/downcomer
Q17. Which type of biochemical process most commonly benefits from airlift reactor operation?
- Anaerobic digestion with no gas injection
- Aerobic fermentations requiring oxygen transfer and gentle mixing
- High-shear enzymatic reactions needing impeller-induced shear
- Solid-phase peptide synthesis in packed columns
Correct Answer: Aerobic fermentations requiring oxygen transfer and gentle mixing
Q18. A simple experimental method to estimate gas holdup in an airlift reactor is:
- Measuring dissolved oxygen only
- Measuring the difference in liquid height between aerated and unaerated columns (static height method)
- Counting bubbles visually at the sparger only
- Estimating from temperature measurements alone
Correct Answer: Measuring the difference in liquid height between aerated and unaerated columns (static height method)
Q19. How does introducing a concentric draft tube typically change reactor performance?
- It eliminates axial circulation and reduces gas holdup
- It enhances axial circulation by separating riser and downcomer, increasing circulation velocity
- It reduces oxygen transfer capability to zero
- It transforms the reactor into a packed-bed reactor
Correct Answer: It enhances axial circulation by separating riser and downcomer, increasing circulation velocity
Q20. Which of the following is a common limitation of airlift reactors for pharmaceutical bioprocesses?
- Excessive mechanical shear that always destroys mammalian cells
- Limited applicability for highly viscous broths and processes prone to solids accumulation
- Inability to provide any oxygen transfer for aerobic processes
- Inherent requirement for corrosive construction materials only
Correct Answer: Limited applicability for highly viscous broths and processes prone to solids accumulation
