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.
Q.82Easy
In equimolar counter-diffusion between two gases A and B, what is the relationship between their molar fluxes?
Answer: A
In equimolar counter-diffusion, the molar fluxes of the two components are equal in magnitude but opposite in direction, hence N_A = -N_B.
Q.83Easy
The Schmidt number (Sc) in mass transfer is defined as the ratio of:
Answer: A
Schmidt number Sc = ν/D_AB = (μ/ρ)/D_AB, representing the ratio of kinematic viscosity to mass diffusivity, analogous to Prandtl number in heat transfer.
Q.84Medium
In a binary gas mixture, if component A is highly soluble in the liquid phase compared to component B, which statement is true regarding absorption?
Answer: C
High solubility of A means low liquid-phase resistance for A, making the gas-phase resistance dominant. Thus, gas-phase mass transfer resistance controls the overall rate for A.
Q.85Easy
The Sherwood number (Sh) in mass transfer correlations represents the dimensionless:
Answer: B
Sherwood number Sh = k_c·L/D_AB represents the ratio of convective mass transfer to diffusive mass transfer, analogous to Nusselt number in heat transfer.
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Q.86Medium
For absorption of sparingly soluble gas in a liquid film, the operating line and equilibrium curve relationship determines:
Answer: B
The vertical distance between operating and equilibrium lines represents the concentration driving force, determining whether absorption occurs and in which direction.
Q.87Medium
In distillation, the Murphree plate efficiency is always:
Answer: B
Murphree plate efficiency accounts for non-ideal mixing on a plate, therefore it is always less than or equal to the overall efficiency which includes effects of all plates.
Q.88Medium
A liquid film of thickness δ has a component A diffusing through it with first-order reaction. The Thiele modulus is defined as:
Answer: A
For first-order reaction in a film, the Thiele modulus φ = δ√(k₁/D_AB), which compares reaction rate to diffusion rate. It determines whether the process is reaction-controlled or diffusion-controlled.
Q.89Medium
In the penetration theory of mass transfer, the mass transfer coefficient varies with contact time (t) as:
Answer: C
According to penetration theory, k_c = 2√(D_AB/(πt)), showing k_c is inversely proportional to √t. Shorter contact times give higher coefficients.
Q.90Hard
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.91Hard
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.92Medium
For drying of a wet solid in the constant rate period, the controlling mechanism for mass transfer is:
Answer: B
During constant rate drying period, surface moisture is continuously replenished from inside, so the external convective mass transfer from wet surface to air is the limiting step.
Q.93Hard
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.94Medium
In a wetted wall column for gas absorption, if the gas film resistance is negligible (R_g ≈ 0), the overall mass transfer coefficient K_g is primarily controlled by:
Answer: B
When gas-film resistance is negligible, K_g ≈ k_l/H where H is Henry's constant. The liquid-phase resistance becomes dominant in overall mass transfer.
Q.95Medium
For the absorption of multiple components from a gas stream into an organic solvent, the selectivity depends on:
Answer: B
Selectivity = (k_l,A/k_l,B)·(H_B/H_A). While diffusivity ratio affects k_l, the Henry's constant ratio H_B/H_A determines preferential absorption, making partition coefficients critical.
Q.96Hard
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.97Medium
In reverse osmosis (RO), the membrane flux equation (Darcy's law for membranes) predicts that flux is:
Answer: D
RO flux J = (k_m/μ)·ΔP/Δx, where flux is directly proportional to pressure difference and inversely to membrane thickness, following modified Darcy's equation.
Q.98Hard
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.99Hard
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.100Hard
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.
