Effects of agitation on mass transfer MCQs With Answer

Introduction

Effects of agitation on mass transfer MCQs With Answer is a concise question set designed for M.Pharm students studying Bioprocess Engineering and Technology. This blog-style quiz focuses on how agitation influences mass transfer in bioreactors, covering concepts such as volumetric mass transfer coefficient (kLa), bubble dynamics, mixing time, shear effects, impeller selection, power input, and scale-up considerations. Questions probe both theory and practical implications for oxygen transfer, gas hold-up, and liquid–liquid systems, reinforcing critical thinking required in pharmaceutical bioprocess design and operation. Each MCQ includes four options and the correct answer to support efficient exam preparation and classroom revision.

Q1. What is the volumetric mass transfer coefficient (kLa) primarily used to describe in stirred bioreactors?

• Rate constant for chemical reaction in the liquid phase
• Rate of mass transfer per unit volume between gas and liquid phases
• Rate of heat transfer per unit area
• Diffusion coefficient of solute in the liquid

Correct Answer: Rate of mass transfer per unit volume between gas and liquid phases

Q2. How does increasing impeller speed generally affect the volumetric mass transfer coefficient (kLa) in a gas–liquid stirred tank?

• kLa decreases because turbulence reduces gas–liquid contact
• kLa remains constant because impeller speed does not influence mass transfer
• kLa increases due to smaller bubble size and enhanced interfacial renewal
• kLa increases only via temperature rise, not by speed

Correct Answer: kLa increases due to smaller bubble size and enhanced interfacial renewal

Q3. Which effect of agitation most directly improves gas–liquid mass transfer in aerobic fermentations?

• Reduction of microbial metabolic rate
• Decrease in dissolved oxygen concentration
• Increase in gas–liquid interfacial area via bubble break-up
• Increase in liquid viscosity

Correct Answer: Increase in gas–liquid interfacial area via bubble break-up

Q4. In the two-film theory for gas–liquid mass transfer, agitation primarily reduces resistance in which region?

• Gas-phase bulk resistance only
• Liquid-side stagnant film resistance by renewing the interface
• Intrinsic molecular resistance inside cells
• Solid catalyst surface resistance

Correct Answer: Liquid-side stagnant film resistance by renewing the interface

Q5. Which parameter is most directly increased by adding baffles to a stirred tank?

• Axial mixing and reduction of vortex formation
• Decrease in power draw of impeller
• Increase in bubble coalescence
• Reduction of oxygen solubility in liquid

Correct Answer: Axial mixing and reduction of vortex formation

Q6. How does liquid viscosity affect the influence of agitation on mass transfer?

• Higher viscosity amplifies bubble break-up leading to increased kLa
• Higher viscosity reduces turbulence and decreases kLa
• Viscosity has no effect on mass transfer
• Lower viscosity always prevents oxygen transfer

Correct Answer: Higher viscosity reduces turbulence and decreases kLa

Q7. For a given impeller and tank geometry, which relationship best approximates power input per unit volume as impeller speed (N) changes?

• Power per volume ∝ N
• Power per volume ∝ N^2
• Power per volume ∝ N^3
• Power per volume ∝ N^0 (independent of N)

Correct Answer: Power per volume ∝ N^3

Q8. What is the typical effect of increasing aeration (gas flow rate) at constant agitation on kLa?

• kLa decreases because bubbles get larger and coalesce more
• kLa increases primarily due to increased gas holdup and interfacial area
• kLa remains unchanged by aeration rate
• kLa increases only if temperature decreases

Correct Answer: kLa increases primarily due to increased gas holdup and interfacial area

Q9. Which impeller characteristic most strongly favors oxygen transfer in a low-viscosity medium?

• Large-diameter axial-flow impeller with low shear
• High-shear disc turbine producing radial flow and fine dispersion
• Very small impeller with slow rotation
• Smooth stationary baffle without impeller

Correct Answer: High-shear disc turbine producing radial flow and fine dispersion

Q10. What is the effect of increased impeller tip speed on shear-sensitive cell cultures?

• Improved cell viability due to gentle mixing
• Potential cell damage and decreased viability due to high shear
• No effect on cells since shear only affects gas
• Increased oxygen solubility protecting cells

Correct Answer: Potential cell damage and decreased viability due to high shear

Q11. When scaling up agitation from lab to pilot scale, which dimensionless number is most relevant to maintain similar mixing regimes?

• Reynolds number to preserve flow regime
• Prandtl number to match thermal diffusion
• Schmidt number to preserve chemical reaction rates
• Biot number to keep surface heat transfer

Correct Answer: Reynolds number to preserve flow regime

Q12. Gas holdup in a stirred tank generally changes with agitation in which way?

• Gas holdup decreases as agitation increases because bubbles escape faster
• Gas holdup increases with agitation due to dispersion and entrainment of gas
• Gas holdup is independent of agitation and depends only on solubility
• Gas holdup becomes zero at moderate agitation

Correct Answer: Gas holdup increases with agitation due to dispersion and entrainment of gas

Q13. Which statement correctly describes the effect of agitation on mass transfer resistance distribution?

• Agitation eliminates both gas-side and liquid-side resistances completely
• Agitation primarily reduces liquid-side film resistance and interfacial renewal frequency
• Agitation only affects gas-phase diffusion coefficient
• Agitation increases boundary layer thickness and thus increases resistance

Correct Answer: Agitation primarily reduces liquid-side film resistance and interfacial renewal frequency

Q14. Which dimensionless correlation is commonly used to relate kLa to power input and superficial gas velocity?

• kLa ∝ (P/V)^a * Vs^b where P/V is power per volume and Vs is superficial gas velocity
• kLa ∝ Reynolds number only
• kLa ∝ Henry’s constant times viscosity
• kLa ∝ molecular weight of solute

Correct Answer: kLa ∝ (P/V)^a * Vs^b where P/V is power per volume and Vs is superficial gas velocity

Q15. In high-shear agitation, which change in bubble behavior improves mass transfer?

• Bubbles grow larger and rise faster with less surface area
• Bubbles fragment into smaller sizes, increasing surface area per volume
• Bubbles completely dissolve eliminating interfacial resistance
• Bubbles coalesce more frequently reducing gas hold-up

Correct Answer: Bubbles fragment into smaller sizes, increasing surface area per volume

Q16. For viscous fermentation broths, which strategy helps to improve mass transfer without excessive shear?

• Decrease impeller diameter and increase speed drastically
• Use multiple low-shear axial-flow impellers and higher gas flow
• Eliminate aeration and rely on diffusion alone
• Switch to a stationary stirrer to avoid shear

Correct Answer: Use multiple low-shear axial-flow impellers and higher gas flow

Q17. How does temperature indirectly influence the effect of agitation on mass transfer?

• Temperature only affects impeller geometry
• Temperature changes liquid properties (viscosity, diffusion) altering kLa
• Temperature has no impact on gas–liquid mass transfer
• Temperature decreases gas holdup by creating a vacuum

Correct Answer: Temperature changes liquid properties (viscosity, diffusion) altering kLa

Q18. Which of the following best explains why increased agitation may not always increase oxygen uptake rate (OUR) in a bioreactor?

• Cells may be substrate-limited, so extra oxygen transfer has no effect
• Agitation always proportionally increases OUR without limitation
• Agitation reduces metabolic activity by increasing dissolved oxygen excessively
• OUR is solely determined by gas flow rate and unaffected by agitation

Correct Answer: Cells may be substrate-limited, so extra oxygen transfer has no effect

Q19. Which parameter is most useful for predicting scale-up performance of oxygen transfer when changing vessel diameter?

• Maintaining constant impeller tip speed alone
• Maintaining constant power per unit volume (P/V) and similar impeller geometry
• Changing to a completely different impeller geometry regardless of P/V
• Keeping gas composition identical but ignoring hydrodynamics

Correct Answer: Maintaining constant power per unit volume (P/V) and similar impeller geometry

Q20. In liquid–liquid extraction systems, what is one main negative effect of excessive agitation on phase separation?

• Improved coalescence leading to faster separation
• Formation of very fine dispersed droplets increasing stability of emulsion and slowing separation
• Reduction in interfacial area making extraction instantaneous
• Complete miscibility of phases eliminating need for separation

Correct Answer: Formation of very fine dispersed droplets increasing stability of emulsion and slowing separation

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