A heat engine operating between 800 K and 300 K has actual efficiency of 40%. Its carnot efficiency is:

The correct answer is A: 62.5%. Carnot efficiency = 1 - T_c/T_h = 1 - 300/800 = 500/800 = 0.625 or 62.5%. Actual efficiency (40%) is always less than Carnot efficiency.

In an orifice meter with diameter ratio β = 0.6, what is the typical discharge coefficient (Cd) used for flow calculations?

The correct answer is B: 0.61. For sharp-edged orifices, discharge coefficient typically ranges from 0.60-0.65, with 0.61 being standard for β = 0.6

Which reactor configuration gives maximum yield for a series reaction A→B→C where k₁ >> k₂?

The correct answer is A: Batch reactor with optimized residence time. For A→B→C series, batch allows control of residence time to maximize intermediate B before it converts to C

In a tubular reactor with axial dispersion, increasing the Peclet number (Pe) results in:

The correct answer is A: RTD approaching that of an ideal PFR. Peclet number Pe = uL/D. High Pe means low diffusion relative to convection, approaching ideal PFR behavior with narrow RTD.

Which of the following dimensionless numbers represents the ratio of inertial forces to viscous forces in fluid flow?

The correct answer is B: Reynolds number. Reynolds number (Re) = inertial forces/viscous forces. It is the fundamental dimensionless number in fluid mechanics.

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Chemical Engineering
Mass Transfer

Process design, thermodynamics, reactions

32 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 1–10 of 32

Topics in Chemical Engineering

All Mass Transfer 100 Heat Transfer 100 Fluid Mechanics 100 Chemical Reaction Engineering 100 Thermodynamics 97

Q.1 Hard Mass Transfer

For membrane distillation with a hydrophobic membrane, the mass transfer resistance in the gas phase across the membrane pores is overcome by:

A Increasing membrane thickness

B Reducing pore size below a critical value

C Maintaining vacuum or pressure difference across the membrane

D Increasing feed temperature only

Correct Answer: C. Maintaining vacuum or pressure difference across the membrane

EXPLANATION

In membrane distillation, vapor pressure difference drives transport through pores. Vacuum or pressure difference across the hydrophobic membrane promotes vapor transport while preventing liquid penetration.

Test

Q.2 Hard Mass Transfer

In industrial scale-up of absorption columns, the L/G ratio (liquid to gas ratio) is adjusted to:

A Keep residence time constant

B Maintain operating line slope and absorption efficiency while avoiding flooding

C Minimize energy consumption only

D Maximize column diameter

Correct Answer: B. Maintain operating line slope and absorption efficiency while avoiding flooding

EXPLANATION

L/G ratio determines the operating line slope in McCabe-Thiele diagram. Proper L/G balances absorption efficiency and prevents flooding/weeping, critical for scale-up design.

Test

Q.3 Hard Mass Transfer

For a gas diffusing into a liquid droplet, the internal circulation (if present) causes:

A Decrease in mass transfer rate compared to rigid sphere

B Increase in mass transfer rate compared to rigid sphere

C No change in mass transfer rate

D Complete stagnation at droplet surface

Correct Answer: B. Increase in mass transfer rate compared to rigid sphere

EXPLANATION

Marangoni convection and internal circulation in liquid droplets enhance mass transfer by reducing the effective liquid-film resistance, increasing overall k_c compared to rigid spheres.

Test

Q.4 Hard Mass Transfer

In evaporative cooling, the heat and mass transfer are coupled. The Lewis number (Le) relationship between them is:

A Le = Sh/Nu always equals 1

B Le = Sc/Pr, determines coupling extent

C Le is independent of fluid properties

D Le increases with temperature

Correct Answer: B. Le = Sc/Pr, determines coupling extent

EXPLANATION

Lewis number Le = Sc/Pr = (ν/D_AB)/(α/ν) = ν/(D_AB·α). For air, Le ≈ 1, meaning heat and mass transfer are equally significant in coupled processes.

Test

Q.5 Hard Mass Transfer

The effective diffusivity in porous solids is given by the Knudsen diffusion model when:

A Pore diameter is very large (> 1 μm)

B Molecular mean free path exceeds pore diameter

C Pressure is very high (> 10 atm)

D Temperature is below 273 K

Correct Answer: B. Molecular mean free path exceeds pore diameter

EXPLANATION

Knudsen diffusion dominates when molecular mean free path λ >> pore diameter d_p, causing molecules to collide with walls more than with other molecules, typical in microporous materials.

Test

Q.6 Hard Mass Transfer

In a packed column used for gas absorption, the NTU (Number of Transfer Units) approach is preferred over HTU when:

A The liquid flow rate is constant

B The equilibrium relationship is linear

C The gas and liquid flow rates are both variable

D The column diameter is very large

Correct Answer: C. The gas and liquid flow rates are both variable

EXPLANATION

NTU accounts for changing concentrations along the column height and is more versatile for varying flow rates, while HTU is better for constant flow conditions with linear equilibrium.

Test

Q.7 Hard Mass Transfer

The flux N_A for component A diffusing through stagnant B can be expressed using:

A Fick's first law directly without any correction

B LMCD (Log Mean Concentration Difference) factor

C Molar average velocity concept

D Binary diffusion coefficient only

Correct Answer: B. LMCD (Log Mean Concentration Difference) factor

EXPLANATION

For diffusion through stagnant film, N_A = (D_AB·C_T/y_B,lm)·ln[(1-y_A2)/(1-y_A1)], requiring LMCD correction due to bulk flow of B.

Test

Q.8 Hard Mass Transfer

For a multicomponent gas mixture undergoing absorption with selective removal, the flux of component i is affected by:

A Concentration gradient of i alone

B Gradients of all species and their mutual interactions

C Only the partial pressure of component i

D Exclusively the solubility of component i

Correct Answer: B. Gradients of all species and their mutual interactions

EXPLANATION

In multicomponent systems, the Stefan-Maxwell equations govern diffusion, showing that fluxes depend on concentration gradients of all species and their binary diffusivity interactions.

Test

Q.9 Hard Mass Transfer

The correlation for mass transfer coefficient in agitated vessels (Harriott equation) is k_L·a ∝:

A (Power per unit volume)^0.4 × (Diffusivity)^0.5

B (Interfacial area)^0.5 × (Shear rate)^0.33

C (Reynolds number)^0.67 × (Schmidt number)^0.33

D (Viscosity)^(-0.5) × (Density)

Correct Answer: A. (Power per unit volume)^0.4 × (Diffusivity)^0.5

EXPLANATION

The Harriott correlation shows k_L·a ∝ (P/V)^0.4·D^0.5, indicating dependence on power input intensity and diffusivity, fundamental to oxygen transfer calculations in bioprocess engineering.

Test

Q.10 Hard Mass Transfer

In hollow fiber membrane contactors for gas absorption, the advantage over conventional columns is:

A Higher interfacial area per unit volume and no flooding

B Lower capital and operating costs exclusively

C Better liquid distribution only

D Elimination of mass transfer resistance

Correct Answer: A. Higher interfacial area per unit volume and no flooding

EXPLANATION

Hollow fiber contactors provide 500-1500 m²/m³ interfacial area (vs 100-400 m²/m³ in packed columns), operate without flooding, and enable independent control of gas and liquid flows.

Test

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