Chemical Engineering — MCQ Practice Questions & Answers

Question 1

Which of the following is the correct definition of mass transfer coefficient in a binary diffusion system?

Accepted Answer

Correct answer: A. Rate of mass transfer per unit area per unit concentration difference — Mass transfer coefficient (k) is defined as the rate of mass transfer per unit interfacial area per unit concentration difference, with units of length/time.

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B. Total molar flux divided by pressure gradient

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C. Diffusivity multiplied by viscosity

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D. Molecular weight ratio of components

Question 2

What is the Schmidt number (Sc) in mass transfer? Select the most accurate definition.

Accepted Answer

Correct answer: A. Sc = μ/(ρ × D_AB) — Schmidt number is Sc = kinematic viscosity/diffusivity = μ/(ρ × D_AB). It is a dimensionless number analogous to Prandtl number in heat transfer.

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B. Sc = ρ × D_AB/μ

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C. Sc = μ × D_AB/ρ

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D. Sc = ρ/μ × D_AB

Question 3

In the film theory of mass transfer, what is assumed about the concentration profile across the stagnant film?

Accepted Answer

Correct answer: A. Linear concentration gradient — Film theory assumes a linear concentration gradient across the stagnant boundary layer, making the analysis mathematically simple and suitable for engineering calculations.

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B. Logarithmic concentration gradient

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C. Exponential concentration gradient

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D. Parabolic concentration gradient

Question 4

A textile industry needs to determine the diffusivity of dye in water at 25°C. The Stokes-Einstein equation is: D_AB = kT/(6πμr). What does 'r' represent?

Accepted Answer

Correct answer: A. Molecular radius of solute — In the Stokes-Einstein equation, r is the molecular radius (or hydrodynamic radius) of the solute particle diffusing through the solvent.

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B. Radius of solvent molecules

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C. Average pore radius of medium

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D. Radius of curvature of interface

Question 5

A cooling tower is designed for evaporative cooling where water evaporates into dry air. The mass transfer occurs due to:

Accepted Answer

Correct answer: A. Concentration gradient of water vapor in air — In evaporative cooling towers, mass transfer is driven by the difference in water vapor concentration between the saturated air at the water surface and the bulk air stream (concentration gradient).

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B. Temperature gradient between water and air

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C. Pressure gradient at the interface

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D. Density difference between liquid and vapor

Question 6

The eddy diffusivity (ε_D) in turbulent flow represents contribution of _____ to overall mass transfer.

Accepted Answer

Correct answer: B. Turbulent eddies and mixing — Eddy diffusivity (ε_D) accounts for the enhanced mass transfer due to turbulent mixing and eddy formation. Total diffusivity = D_AB + ε_D in turbulent regions.

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A. Molecular diffusion only

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C. Bulk flow movement

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D. Gravity and pressure gradients

Question 7

In the context of mass transfer, what does the Schmidt number (Sc) represent?

Accepted Answer

Correct answer: A. The ratio of kinematic viscosity to mass diffusivity — Schmidt number (Sc = ν/D_AB) represents the relative importance of momentum diffusion and mass diffusion in a fluid.

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B. The ratio of thermal conductivity to mass diffusivity

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C. The ratio of momentum diffusivity to heat diffusivity

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D. The ratio of heat diffusivity to mass diffusivity

Question 8

In a gas absorption process using a packed tower, if the interfacial area 'a' increases, how does it affect the mass transfer rate?

Accepted Answer

Correct answer: B. Mass transfer rate increases proportionally — Since mass transfer rate = k_G * a * ΔP, an increase in interfacial area directly increases the mass transfer rate (linear relationship).

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A. Mass transfer rate remains unchanged

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C. Mass transfer rate decreases exponentially

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D. Mass transfer rate becomes zero

Chemical Engineering - MCQ Practice Questions