In gas-solid adsorption, the Langmuir isotherm assumes:
Answer: B
The Langmuir model assumes monolayer adsorption on homogeneous surfaces with constant heat of adsorption, leading to saturation at high pressures.
Q.22Hard
For simultaneous diffusion and reaction (fast reaction regime) in a porous catalyst, the effectiveness factor η approaches:
Answer: B
In the fast reaction regime where diffusion limits the overall rate, η ≈ 1/Φ for spherical pellets, indicating severe internal diffusion limitations.
Q.23Hard
In hollow fiber membrane contactors for gas absorption, the advantage over conventional columns is:
Answer: A
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.
Q.24Hard
The correlation for mass transfer coefficient in agitated vessels (Harriott equation) is k_L·a ∝:
Answer: A
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.
Q.25Hard
For a multicomponent gas mixture undergoing absorption with selective removal, the flux of component i is affected by:
Answer: B
In multicomponent systems, the Stefan-Maxwell equations govern diffusion, showing that fluxes depend on concentration gradients of all species and their binary diffusivity interactions.
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Q.26Hard
The flux N_A for component A diffusing through stagnant B can be expressed using:
Answer: B
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.
Q.27Hard
In a packed column used for gas absorption, the NTU (Number of Transfer Units) approach is preferred over HTU when:
Answer: C
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.
Q.28Hard
The effective diffusivity in porous solids is given by the Knudsen diffusion model when:
Answer: B
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.
Q.29Hard
In evaporative cooling, the heat and mass transfer are coupled. The Lewis number (Le) relationship between them is:
Answer: B
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.
Q.30Hard
For a gas diffusing into a liquid droplet, the internal circulation (if present) causes:
Answer: B
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.
Q.31Hard
In industrial scale-up of absorption columns, the L/G ratio (liquid to gas ratio) is adjusted to:
Answer: B
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.
Q.32Hard
For membrane distillation with a hydrophobic membrane, the mass transfer resistance in the gas phase across the membrane pores is overcome by:
Answer: C
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.
Q.33Hard
For a long thin fin with adiabatic tip condition, the fin efficiency is given by:
Answer: A
For adiabatic tip condition: η_fin = tanh(mL)/(mL), where m = √(hP/kA_c). This represents the efficiency of heat transfer through the fin.
Q.34Hard
In a parallel plate channel with constant heat flux q'' on both walls, what is the thermal entrance length relationship with hydrodynamic entrance length?
Answer: A
Thermal entrance length L_th ≈ L_h/Pr for laminar flow, where Pr < 1 for metals means thermal entrance is shorter than hydrodynamic entrance.
Q.35Hard
The Graetz number in laminar heat transfer represents:
Answer: A
Graetz number Gz = (D_h/L) × Re × Pr indicates relative importance of entrance effects. Gz > 100 means entrance region dominates.
Q.36Hard
Which statement best describes radiation heat transfer in participating media?
Answer: B
In participating media (like CO₂, H₂O vapor), radiation is absorbed and re-emitted by gas molecules, affecting net radiative heat transfer significantly.
Q.37Hard
For a microfin tube (enhanced surface), the heat transfer improvement comes primarily from:
Answer: C
Microfins provide dual benefits: (1) increased surface area A, and (2) enhanced convection coefficient h by disrupting boundary layer development, thus improving overall heat transfer coefficient.
Q.38Hard
In a counter-flow double pipe heat exchanger, the outlet temperature of hot fluid becomes lower than the outlet temperature of cold fluid. This is:
Answer: C
In counter-flow, the temperature profiles allow hot outlet to be lower than cold outlet with sufficient heat transfer area, limited by second law of thermodynamics.
Q.39Hard
For radiation heat exchange between two parallel plates at temperatures T₁ and T₂ with emissivities ε₁ and ε₂, the radiation heat transfer is reduced by factor:
Answer: B
For two parallel plates, Q = σA·F(T₁⁴ - T₂⁴) where F = ε₁ε₂/(ε₁ + ε₂ - ε₁ε₂), derived from network resistance analogy.
Q.40Hard
For boiling heat transfer, the critical heat flux (CHF) depends on which property most strongly?
Answer: C
CHF correlations (Zuber equation) strongly depend on surface tension (σ) and (ρ_l - ρ_v) density difference.
