Measurement of mass transfer coefficient MCQs With Answer

Introduction

Measurement of mass transfer coefficient MCQs With Answer is a focused quiz set designed for M.Pharm students studying Bioprocess Engineering and Technology. This collection reinforces core concepts such as local and volumetric mass transfer coefficients (k and kLa), two-film and penetration theories, dimensionless groups (Sherwood, Schmidt, Reynolds), experimental methods (dynamic gassing-out, sodium sulfite), and practical influences like agitation, temperature, and interfacial area. Each question is crafted to deepen understanding of how mass transfer controls rates in gas–liquid and liquid–liquid systems commonly encountered in pharmaceutical bioprocesses, and to prepare students for exam-style reasoning and process-scale decisions. Answers are provided for self-evaluation.

Q1. What is the physical meaning of the liquid-phase mass transfer coefficient (kL)?

• The concentration at the interface
• The rate of mass transfer per unit interfacial area per unit concentration driving force
• The total amount of solute transferred per second
• The diffusion coefficient divided by film thickness squared

Correct Answer: The rate of mass transfer per unit interfacial area per unit concentration driving force

Q2. The volumetric mass transfer coefficient (kLa) is best described as:

• The interfacial area per unit volume divided by the liquid mass transfer coefficient
• The liquid-film mass transfer coefficient multiplied by interfacial area per unit reactor volume
• The gas-film mass transfer coefficient only
• The partition coefficient between phases

Correct Answer: The liquid-film mass transfer coefficient multiplied by interfacial area per unit reactor volume

Q3. Which dimensionless number is the mass-transfer analogue of the Nusselt number and relates convective mass transfer to diffusion?

• Reynolds number
• Schmidt number
• Sherwood number
• Peclet number

Correct Answer: Sherwood number

Q4. The two-film theory for interphase mass transfer assumes which primary resistance?

• Resistance is only in the bulk phases and interfaces are equilibrium instantaneously
• Mass transfer resistance resides in thin stagnant boundary layers (films) on both phases
• There is no concentration gradient anywhere
• Resistance is dominated by chemical reaction in the bulk

Correct Answer: Mass transfer resistance resides in thin stagnant boundary layers (films) on both phases

Q5. Penetration theory of mass transfer differs from steady film theory by assuming:

• A permanently fixed boundary layer at the interface
• Surface renewal of fluid elements contacting the interface for finite contact times
• The interface is chemically reactive and controls rate purely by kinetics
• Mass transfer occurs only by convective transport

Correct Answer: Surface renewal of fluid elements contacting the interface for finite contact times

Q6. Which experimental technique is commonly used to determine kLa in aerated bioreactors using dissolved oxygen measurements?

• Sodium chloride addition method
• Dynamic gassing-out (step change of dissolved oxygen) method
• pH titration method
• Mass spectrometry of vapour phase

Correct Answer: Dynamic gassing-out (step change of dissolved oxygen) method

Q7. The Sherwood correlation for many stirred systems is often written Sh = a Re^b Sc^c. In this form, what does Sc represent?

• The ratio of inertial to viscous forces
• The ratio of momentum diffusivity to mass diffusivity (ν/D)
• The ratio of convective to conductive heat transfer
• The ratio of species concentration to solubility

Correct Answer: The ratio of momentum diffusivity to mass diffusivity (ν/D)

Q8. How does temperature generally affect the liquid-phase mass transfer coefficient (kL) in aqueous systems?

• kL decreases with temperature because viscosity increases
• kL is independent of temperature
• kL increases with temperature due to higher diffusion coefficients and lower viscosity
• kL falls exponentially with temperature due to lower gas solubility only

Correct Answer: kL increases with temperature due to higher diffusion coefficients and lower viscosity

Q9. Interfacial area per unit volume (a) in gas–liquid reactors is most directly increased by:

• Increasing bubble diameter while keeping gas holdup constant
• Reducing gas holdup
• Creating smaller bubbles and/or increasing gas holdup
• Decreasing agitation speed only

Correct Answer: Creating smaller bubbles and/or increasing gas holdup

Q10. If liquid-film resistance dominates mass transfer, the overall mass transfer coefficient will be approximately equal to:

• The gas-film mass transfer coefficient
• The sum of gas and liquid coefficients
• The liquid-film mass transfer coefficient (or the smaller of the two individual coefficients)
• Zero, because equilibrium is instantaneous

Correct Answer: The liquid-film mass transfer coefficient (or the smaller of the two individual coefficients)

Q11. The Schmidt number (Sc) is defined as which ratio?

• Viscosity divided by density
• Kinematic viscosity divided by mass diffusivity (ν/D)
• Mass diffusivity divided by thermal diffusivity
• Reynolds number times Prandtl number

Correct Answer: Kinematic viscosity divided by mass diffusivity (ν/D)

Q12. What are the SI units of the volumetric mass transfer coefficient kLa?

• m/s
• s^-1 (reciprocal seconds)
• m^3/s
• dimensionless

Correct Answer: s^-1 (reciprocal seconds)

Q13. The sodium sulfite method for kLa measurement is based on:

• Chemical consumption of oxygen by sodium sulfite, allowing measurement of oxygen ingress rate
• Direct measurement of bubble size distribution by microscopy
• pH change due to CO2 absorption
• Measuring electrical conductivity changes upon aeration

Correct Answer: Chemical consumption of oxygen by sodium sulfite, allowing measurement of oxygen ingress rate

Q14. Which dimensionless number quantifies the relative importance of inertial forces to viscous forces in fluid flow?

• Schmidt number
• Reynolds number
• Sherwood number
• Biot number

Correct Answer: Reynolds number

Q15. In many gas–liquid systems involving small solute molecules, which film resistance typically controls mass transfer?

• Gas-film resistance always controls
• Liquid-film resistance often controls because diffusion in liquid is much slower than in gas
• Neither; bulk convection controls exclusively
• Interfacial chemical reaction is always rate limiting

Correct Answer: Liquid-film resistance often controls because diffusion in liquid is much slower than in gas

Q16. Which theoretical model predicts that the mass transfer coefficient scales approximately with the square root of the molecular diffusivity (k ∝ D^1/2)?

• Steady-state film theory
• Penetration theory (surface renewal/short contact time models)
• Perfect-mixing model
• Henry’s law equilibrium model

Correct Answer: Penetration theory (surface renewal/short contact time models)

Q17. The interfacial flux J across a liquid film is commonly expressed as J = kL (Cbulk – Cinterface). Which concentration difference drives the flux?

• Cinterface – Cbulk
• Cbulk – Cinterface
• Cbulk only, independent of Cinterface
• Cinterface only, independent of Cbulk

Correct Answer: Cbulk – Cinterface

Q18. A commonly used empirical correlation for mass transfer from a single sphere is Sh = 2 + 0.6 Re^0.5 Sc^0.33. This correlation is known as:

• Stokes-Einstein relation
• Ranz–Marshall correlation (mass/heat transfer analogue)
• Arrhenius equation
• Henry’s law correction

Correct Answer: Ranz–Marshall correlation (mass/heat transfer analogue)

Q19. During scale-up of an aerobic bioreactor, which operational change is typically most effective at increasing kLa?

• Reducing agitation speed while increasing working volume
• Increasing agitation (impeller speed) to thin boundary layers and increase gas dispersion
• Decreasing gas flow rate
• Raising solute concentration uniformly

Correct Answer: Increasing agitation (impeller speed) to thin boundary layers and increase gas dispersion

Q20. The presence of soluble surfactants at the gas–liquid interface most commonly has which effect on gas–liquid mass transfer?

• Always increases kLa by breaking up bubbles
• Has no effect on mass transfer
• Often decreases mass transfer by creating an interfacial film and inhibiting renewal
• Converts mass transfer control from diffusion to convection exclusively

Correct Answer: Often decreases mass transfer by creating an interfacial film and inhibiting renewal

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