Mass transfer theory in fermentation MCQs With Answer

Introduction: Mass transfer theory in fermentation is central to designing and operating bioreactors for M.Pharm students focused on production of pharmaceuticals. This blog presents targeted MCQs that cover gas–liquid mass transfer fundamentals, the two-film model, kLa estimation methods, oxygen transfer and uptake rates (OTR/OUR), effects of agitation and sparging, interfacial area, and scale-up considerations. Questions emphasize quantitative understanding (dimensionless numbers, correlations) and practical measurement techniques (dynamic gassing-out, sulfite method). The set is designed to deepen conceptual clarity and problem-solving skills required for formulation of fermentation processes, optimization of oxygen supply, and critical evaluation of mass transfer limitations in bioprocess development.

Q1. What does the parameter kLa commonly represent in aerobic fermentation?

• The rate of oxygen consumption by microbes per unit biomass
• Volumetric mass transfer coefficient combining liquid film mass transfer coefficient and interfacial area
• The Henry’s law constant for oxygen solubility at reactor conditions
• The stirring power input per unit volume

Correct Answer: Volumetric mass transfer coefficient combining liquid film mass transfer coefficient and interfacial area

Q2. According to the two-film theory, which resistances dominate gas–liquid mass transfer?

• Convective resistance in the bulk liquid and bulk gas
• Resistance in the stagnant films on the gas and liquid sides
• Internal diffusion inside microbial cells only
• Heat transfer resistance across the gas–liquid interface

Correct Answer: Resistance in the stagnant films on the gas and liquid sides

Q3. Which measurement method is widely used to determine kLa experimentally in stirred tanks?

• Static headspace equilibration
• Dynamic gassing-out (or oxygen depletion/re-aeration) method
• Viscosity titration
• Infrared absorption of dissolved oxygen

Correct Answer: Dynamic gassing-out (or oxygen depletion/re-aeration) method

Q4. In the dynamic gassing-out method, what parameter is directly monitored to calculate kLa?

• Biomass concentration over time
• Dissolved oxygen concentration recovering after gas switch
• pH change after aeration
• Temperature increase during sparging

Correct Answer: Dissolved oxygen concentration recovering after gas switch

Q5. The overall oxygen transfer rate (OTR) in a fermenter at a given DO is best expressed as which formula?

• OTR = kLa × (C* – C_L), where C* is saturation concentration and C_L is liquid DO
• OTR = μ × X, where μ is growth rate and X is biomass
• OTR = Henry’s constant × partial pressure of O2
• OTR = Power input × impeller speed

Correct Answer: OTR = kLa × (C* – C_L), where C* is saturation concentration and C_L is liquid DO

Q6. Which factor generally increases kLa in a stirred-tank fermenter?

• Decreasing agitation speed while keeping gas flow constant
• Increasing liquid viscosity at fixed agitation
• Increasing power input (agitation) and gas dispersion (higher gas flow)
• Adding antifoam to enhance bubble coalescence

Correct Answer: Increasing power input (agitation) and gas dispersion (higher gas flow)

Q7. How does increasing broth viscosity (e.g., due to high biomass or polymers) typically affect mass transfer?

• It increases interfacial area and raises kLa
• It decreases diffusivity and usually reduces kLa
• It has no effect on gas–liquid mass transfer
• It lowers liquid density but increases oxygen solubility

Correct Answer: It decreases diffusivity and usually reduces kLa

Q8. Which dimensionless number correlates viscous diffusion to convective transport and influences mass transfer near bubbles?

• Reynolds number (Re)
• Peclet number (Pe)
• Schmidt number (Sc)
• Prandtl number (Pr)

Correct Answer: Schmidt number (Sc)

Q9. For small oxygen bubbles in water, what effect does reducing bubble diameter have on mass transfer?

• Decreases specific interfacial area (a) and lowers OTR
• Increases interfacial area per volume (a) and typically raises kLa
• Changes only gas composition, not mass transfer
• Causes Henry’s law constant to increase

Correct Answer: Increases interfacial area per volume (a) and typically raises kLa

Q10. Which correlation form is commonly used to express kLa dependence on power input and superficial gas velocity?

• kLa = A × exp(B × T)
• kLa = α × (P/V)^β × Vs^γ, where P/V is power per volume and Vs is superficial gas velocity
• kLa = Henry’s constant × solubility
• kLa = μ × X × Y where μ is viscosity

Correct Answer: kLa = α × (P/V)^β × Vs^γ, where P/V is power per volume and Vs is superficial gas velocity

Q11. What is the significance of the term C* (C star) in oxygen transfer calculations?

• The oxygen concentration in the gas phase above the liquid
• The oxygen saturation concentration in the liquid at given temperature and partial pressure
• The critical concentration at which cells start to die
• The concentration of dissolved CO2 in equilibrium

Correct Answer: The oxygen saturation concentration in the liquid at given temperature and partial pressure

Q12. When scaling up a fermentation, maintaining constant kLa is often targeted. Which scale-up rule helps approximate constant kLa?

• Geometric similarity only
• Constant impeller tip speed across scales
• Keeping constant power per unit volume (P/V) and gas flow/volume or using empirical kLa correlations
• Adjusting pH to match small-scale conditions

Correct Answer: Keeping constant power per unit volume (P/V) and gas flow/volume or using empirical kLa correlations

Q13. The sulfite oxidation method for kLa measurement provides what main limitation when applied to fermentation broths?

• It is too slow and cannot measure kLa in minutes
• It uses chemical oxygen demand not representative of biological oxygen uptake and can overestimate kLa in real broths
• It directly measures microbial respiration, so results are identical to biological systems
• It requires specialized dissolved CO2 probes

Correct Answer: It uses chemical oxygen demand not representative of biological oxygen uptake and can overestimate kLa in real broths

Q14. Which phenomenon reduces interfacial area and thus kLa in the presence of excessive foam?

• Increased bubble breakup leading to microbubbles
• Bubble coalescence and stable foam layer preventing gas dispersion into the bulk
• Enhanced liquid mixing under foam layer
• Lowering of gas viscosity inside foam

Correct Answer: Bubble coalescence and stable foam layer preventing gas dispersion into the bulk

Q15. How does temperature generally affect oxygen solubility and kLa in fermentation?

• Higher temperature increases oxygen solubility and decreases kLa
• Higher temperature decreases oxygen solubility but often increases mass transfer coefficient kL and thus may change kLa unpredictably
• Temperature has no effect on either solubility or kLa
• Higher temperature makes Henry’s law invalid

Correct Answer: Higher temperature decreases oxygen solubility but often increases mass transfer coefficient kL and thus may change kLa unpredictably

Q16. In terms of resistance-in-series for gas–liquid transfer, which side usually controls oxygen transfer into aqueous fermentation broths?

• Gas-phase resistance is typically dominant
• Liquid-film resistance or liquid-side resistance is often rate-limiting for oxygen in aqueous systems
• Cell/membrane internal diffusion is always dominant over film resistances
• Solid reactor walls dominate mass transfer resistance

Correct Answer: Liquid-film resistance or liquid-side resistance is often rate-limiting for oxygen in aqueous systems

Q17. Which strategy directly increases the interfacial area (a) in a bioreactor?

• Reducing gas flow to barely sparge the liquid
• Using a sparger and impeller combination that creates fine bubbles
• Operating at extremely low temperatures
• Adding salt to increase liquid density

Correct Answer: Using a sparger and impeller combination that creates fine bubbles

Q18. The oxygen uptake rate (OUR) of a culture is measured as 5 mmol·L^-1·h^-1 and kLa is 0.2 h^-1 with C* = 8 mmol·L^-1. What steady-state DO (C_L) results? (Use OTR = kLa(C* – C_L) = OUR)

• C_L = 3 mmol·L^-1
• C_L = 5 mmol·L^-1
• C_L = 8 mmol·L^-1
• C_L = 0 mmol·L^-1

Correct Answer: C_L = 3 mmol·L^-1

Q19. Which dimensionless group often correlates mass transfer around bubbles in empirical correlations together with Re?

• Nusselt number only used for heat transfer
• Schmidt number (Sc) or Sherwood number (Sh) paired with Re for mass transfer correlations
• Biot number for internal cell diffusion only
• Fourier number for transient thermal diffusion

Correct Answer: Schmidt number (Sc) or Sherwood number (Sh) paired with Re for mass transfer correlations

Q20. During scale-up, if superficial gas velocity is kept constant but power per volume drops, what is the expected trend for kLa and DO availability?

• kLa will increase and DO will always rise
• kLa will likely decrease due to lower turbulence despite constant gas flow, potentially lowering DO availability
• kLa and DO remain unchanged because gas velocity is constant
• kLa becomes independent of agitation and only depends on temperature

Correct Answer: kLa will likely decrease due to lower turbulence despite constant gas flow, potentially lowering DO availability

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