Factors affecting mass transfer coefficient MCQs With Answer

Introduction: Factors affecting mass transfer coefficient MCQs With Answer is designed for M.Pharm students studying Bioprocess Engineering and Technology. This short guide highlights key physical, chemical and operational variables that control the mass transfer coefficient (k, kL, kG, kLa) in gas–liquid, liquid–liquid and solid–liquid systems used in bioprocesses. Topics include diffusion, hydrodynamics, interfacial area, turbulence, temperature, viscosity, surfactants, particle size, biofilms, non‑Newtonian fluids, and scale‑up implications. Understanding these factors is essential for optimizing oxygen transfer, solvent extraction, fermentation efficiency and downstream unit operations. The following 20 MCQs test core concepts and practical considerations with clear answers for revision and exam preparation.

Q1. What is the common linear expression that relates mass flux to concentration driving force in film theory?

• N = k (C_bulk)
• N = k (C_surface)
• N = k (C1 – C2)
• N = D (dC/dx)

Correct Answer: N = k (C1 – C2)

Q2. Which property most directly affects the molecular diffusion coefficient of a solute in a liquid?

• Agitation speed
• Temperature only
• Molecular size and shape
• System pressure

Correct Answer: Molecular size and shape

Q3. Why does increasing agitation generally increase the mass transfer coefficient in a liquid phase?

• It increases the chemical solubility
• It reduces liquid boundary layer thickness and increases turbulence
• It increases the molecular weight of solute
• It decreases the interfacial area

Correct Answer: It reduces liquid boundary layer thickness and increases turbulence

Q4. In a stirred bioreactor, which pair of operational variables most strongly influences the volumetric mass transfer coefficient kLa?

• Temperature and pressure
• Agitation speed and superficial gas velocity (aeration rate)
• Sterility and pH
• Feed composition and inoculum age

Correct Answer: Agitation speed and superficial gas velocity (aeration rate)

Q5. Which definition correctly represents the Sherwood number (Sh)?

• Sh = kL * L / D
• Sh = Re * Pr
• Sh = Nu / Re
• Sh = kL / (L * D)

Correct Answer: Sh = kL * L / D

Q6. How do non‑Newtonian (shear‑thickening or shear‑thinning) fluids typically affect the mass transfer coefficient compared with Newtonian fluids?

• They always increase k due to higher viscosity
• They reduce turbulence and often increase boundary layer thickness, lowering k
• They have no effect on mass transfer
• They convert mass transfer control from liquid to gas side

Correct Answer: They reduce turbulence and often increase boundary layer thickness, lowering k

Q7. What is the usual effect of adding surfactants to a gas–liquid system on the mass transfer coefficient?

• They always increase k by enhancing diffusion
• They decrease k by forming interfacial resistance and damping surface renewal
• They have no measurable effect
• They convert liquid film resistance to gas film resistance

Correct Answer: They decrease k by forming interfacial resistance and damping surface renewal

Q8. How does increasing temperature generally influence the mass transfer coefficient in liquid systems?

• It decreases k by increasing viscosity
• It increases k by increasing diffusivity and lowering viscosity
• It has no impact on k but increases solubility
• It converts k from kL to kG

Correct Answer: It increases k by increasing diffusivity and lowering viscosity

Q9. What is the primary effect of reducing particle size in solid–liquid extraction on mass transfer?

• Decreases external surface area and lowers k
• Increases external surface area and shortens internal diffusion path, increasing mass transfer rate
• Eliminates need for agitation
• Reduces solute diffusivity dramatically

Correct Answer: Increases external surface area and shortens internal diffusion path, increasing mass transfer rate

Q10. In bubble column reactors, which variable most directly increases the gas–liquid interfacial area (a) and therefore can raise kLa?

• Larger bubble size
• Higher gas holdup and smaller bubble size
• Lower gas flow rate
• Higher liquid viscosity only

Correct Answer: Higher gas holdup and smaller bubble size

Q11. What does kLa represent in bioprocess oxygen transfer?

• The gas film mass transfer coefficient only
• The product of liquid film mass transfer coefficient and interfacial area per unit volume
• Henry’s constant times solubility
• The solute diffusion coefficient times reactor volume

Correct Answer: The product of liquid film mass transfer coefficient and interfacial area per unit volume

Q12. During scale‑up of an aerobic fermenter, which operational parameter is most critical to maintain similar kLa between scales?

• Same impeller diameter only
• Power input per unit volume and gas flow rate (or superficial gas velocity)
• Same absolute agitation speed (rpm)
• Same cell concentration

Correct Answer: Power input per unit volume and gas flow rate (or superficial gas velocity)

Q13. According to film theory, what is the effect of decreasing the thickness of the boundary layer on the local mass transfer coefficient?

• It decreases k because diffusion path increases
• It increases k because resistance across the film is reduced
• It has no effect on k
• It converts diffusive transport to convective transport only

Correct Answer: It increases k because resistance across the film is reduced

Q14. How does membrane fouling affect mass transfer coefficient across a membrane module?

• Fouling typically increases k by forming channels
• Fouling adds resistance and decreases effective mass transfer coefficient
• Fouling only affects selectivity, not k
• Fouling converts convective flux to pure diffusion with no change in k

Correct Answer: Fouling adds resistance and decreases effective mass transfer coefficient

Q15. In high cell density fermentations, how does biomass concentration commonly influence the apparent mass transfer coefficient for oxygen?

• High biomass increases k by creating micro‑mixing
• High biomass reduces k due to increased viscosity, cell layer formation and oxygen uptake near interfaces
• Biomass concentration has no effect on k
• It only affects kG, not kL

Correct Answer: High biomass reduces k due to increased viscosity, cell layer formation and oxygen uptake near interfaces

Q16. The Schmidt number (Sc) is defined as which ratio relevant to mass transfer correlations?

• Sc = Re / Pr
• Sc = ν / D (kinematic viscosity divided by diffusion coefficient)
• Sc = D / ν
• Sc = kL / a

Correct Answer: Sc = ν / D (kinematic viscosity divided by diffusion coefficient)

Q17. Increasing Reynolds number in a stirred tank typically causes which effect on the mass transfer coefficient?

• Decreases k by laminarizing the flow
• Increases k by promoting turbulence and thinning the boundary layer
• Has no effect on k once agitation exceeds 10 rpm
• Only changes kG and not kL

Correct Answer: Increases k by promoting turbulence and thinning the boundary layer

Q18. What is the general effect of lowering interfacial tension on droplet formation and mass transfer in liquid–liquid extraction?

• Fewer, larger droplets form and interfacial area decreases
• Smaller droplets form, interfacial area increases and mass transfer often improves
• No change in droplet size or mass transfer
• It always stops mass transfer completely

Correct Answer: Smaller droplets form, interfacial area increases and mass transfer often improves

Q19. Which experimental technique is most commonly used to determine kLa for oxygen transfer in laboratory bioreactors?

• Dynamic (gassing‑out) method measuring dissolved oxygen recovery
• Static headspace gas analysis without aeration
• Chromatography of off‑gas only
• pH titration method

Correct Answer: Dynamic (gassing‑out) method measuring dissolved oxygen recovery

Q20. According to a linear mass transfer relationship, what happens to mass flux if the concentration driving force doubles while the mass transfer coefficient remains constant?

• Flux decreases by half
• Flux remains unchanged
• Flux doubles
• Flux increases by the square of the change

Correct Answer: Flux doubles

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