In membrane separation processes, the selectivity is expressed as:

The correct answer is A: Ratio of permeance of component A to component B. Selectivity α = (J_A/ΔP_A)/(J_B/ΔP_B). Higher selectivity indicates better separation capability of the membrane.

For gas absorption in a plate column, the gas-phase mass transfer coefficient increases with:

The correct answer is A: Higher gas velocity and lower pressure. Higher velocity increases turbulence (higher k); lower pressure increases diffusivity D. Both increase Sh = kL/D.

For boiling heat transfer, the critical heat flux (CHF) depends on which property most strongly?

The correct answer is C: Surface tension and density difference. CHF correlations (Zuber equation) strongly depend on surface tension (σ) and (ρ_l - ρ_v) density difference.

Which of the following equations is used to calculate the mass transfer coefficient in laminar flow over a flat plate?

The correct answer is A: Sh = 0.664 Re^(1/2) Sc^(1/3). The Ranz-Marshall correlation for laminar flow over a flat plate is Sh = 0.664 Re^(1/2) Sc^(1/3), derived from boundary layer theory.

What does the selectivity parameter in a competing reaction system measure?

The correct answer is B: The ratio of desired product to undesired product formation rates. Selectivity S = rate of desired product formation / rate of undesired product formation, used to optimize reactor conditions for maximum desired product yield

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Chemical Engineering

Process design, thermodynamics, reactions

117 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 91–100 of 117

Topics in Chemical Engineering

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

Q.91 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.92 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.93 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.94 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.95 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

Q.96 Hard Mass Transfer

For simultaneous diffusion and reaction (fast reaction regime) in a porous catalyst, the effectiveness factor η approaches:

A 1 (no limitation)

B 1/Φ where Φ is the Thiele modulus

C 0.3 × 1/Φ² (approximately)

D Φ² (for diffusion-limited regime)

Correct Answer: B. 1/Φ where Φ is the Thiele modulus

EXPLANATION

In the fast reaction regime where diffusion limits the overall rate, η ≈ 1/Φ for spherical pellets, indicating severe internal diffusion limitations.

Test

Q.97 Hard Mass Transfer

In gas-solid adsorption, the Langmuir isotherm assumes:

A Multilayer adsorption with uniform enthalpy

B Monolayer adsorption with constant enthalpy of adsorption

C Linear variation of adsorption with pressure

D Exponential increase in adsorption capacity with pressure

Correct Answer: B. Monolayer adsorption with constant enthalpy of adsorption

EXPLANATION

The Langmuir model assumes monolayer adsorption on homogeneous surfaces with constant heat of adsorption, leading to saturation at high pressures.

Test

Q.98 Hard Mass Transfer

The asymptotic solutions for mass transfer in turbulent flow predict that the Sherwood number varies with Reynolds number as:

A Sh ∝ Re^0.5

B Sh ∝ Re^0.8·Sc^0.33

C Sh ∝ Re^1.0

D Sh ∝ Re^0.25·Sc^0.25

Correct Answer: B. Sh ∝ Re^0.8·Sc^0.33

EXPLANATION

For turbulent flow, empirical correlations show Sh ∝ Re^0.8·Sc^0.33 (or 0.5 depending on flow regime), derived from boundary layer theory and mass transfer analogies.

Test

Q.99 Hard Mass Transfer

For a non-isothermal diffusion process with coupled heat and mass transfer, the Lewis number Le is defined as:

A Ratio of thermal diffusivity to mass diffusivity

B Product of Schmidt and Prandtl numbers

C Ratio of heat capacity to molar volume

D Product of Grashof and Rayleigh numbers

Correct Answer: A. Ratio of thermal diffusivity to mass diffusivity

EXPLANATION

Lewis number Le = α/D_AB, where α is thermal diffusivity. It indicates the relative importance of heat and mass transfer in coupled processes. Le ≈ Sc/Pr for most gases.

Test

Q.100 Hard Mass Transfer

In foam fractionation for protein separation, the enrichment factor depends on:

A Interfacial tension and protein hydrophobicity

B Foam drainage rate only

C pH and temperature exclusively

D Column diameter only

Correct Answer: A. Interfacial tension and protein hydrophobicity

EXPLANATION

Foam fractionation enrichment factor depends on surface activity (interfacial tension), protein hydrophobicity, and preferential adsorption of proteins at gas-liquid interfaces.

Test

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