What is the primary difference between diffusion and convection in mass transfer operations?
Answer: A
Diffusion is the movement of species due to concentration gradient (molecular level), while convection is the bulk movement of fluid carrying dissolved or suspended species.
Q.42Easy
In Fick's second law of diffusion, which parameter represents the rate of change of concentration with time?
Answer: A
Fick's second law: ∂C/∂t = D(∂²C/∂x²), where ∂C/∂t represents the temporal change in concentration.
Q.43Easy
The mass transfer coefficient is typically expressed in units of:
Answer: A
Mass transfer coefficient (k) has dimensions of velocity: m/s. It represents the driving force (concentration difference) per unit area per unit time.
Q.44Medium
In a packed column absorption operation, the overall mass transfer coefficient KG is related to individual phase coefficients by:
Answer: A
For gas-phase controlling resistance: 1/KG = 1/kG + (m/kL), where m is Henry's Law constant and accounts for equilibrium relationship.
Q.45Medium
The Sherwood number (Sh) in mass transfer is analogous to which number in heat transfer?
Answer: A
Sh = kL·L/D is analogous to Nu = h·L/k. Both represent dimensionless transfer coefficients for mass and heat respectively.
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Q.46Hard
For a binary gas mixture, the Stefan flow correction factor accounts for:
Answer: A
When one component condenses/reacts, equal molar counter-diffusion assumption breaks down. Stefan factor (B_m) corrects for net molar flow.
Q.47Medium
In a continuous contacting device, the operating line is obtained from which fundamental principle?
Answer: A
Operating line represents all possible compositions entering and leaving the device based on material balance: Y₁ - Y₂ = (L/V)(X₁ - X₂)
Q.48Medium
The penetration theory for mass transfer assumes that:
Answer: A
Penetration theory (Higbie) assumes eddies contact interface, remain for time θ, then submerge. More realistic than film theory for turbulent systems.
Q.49Medium
For simultaneous heat and mass transfer in a cooling tower, the Lewis number (Le = α/D) typically has a value of:
Answer: A
For air-water system: Le = α/D ≈ 0.8-1.0. This means heat and mass transfer coefficients are comparable, important for cooling tower design.
Q.50Hard
In ion exchange resins, the mass transfer resistance primarily occurs in:
Answer: A
For ion exchange, intraparticle diffusion through pores is the rate-limiting step. External film resistance is negligible in comparison.
Q.51Hard
The Colburn analogy relates dimensionless groups for mass, heat, and momentum transfer as:
Answer: A
Colburn analogy: j_D = (Sh)/(Re·Sc^(31)) ≈ j_H ≈ f/2 for smooth surfaces. Useful for correlating data when only one phenomenon is studied.
Q.52Medium
In distillation, the Murphree plate efficiency is defined as the ratio of:
Answer: A
E_mv = (y_n - y_(n-1))/(y*_n - y_(n-1)), where y* is equilibrium vapor composition. Accounts for non-equilibrium stage behavior.
Q.53Medium
For gas absorption in a plate column, the gas-phase mass transfer coefficient increases with:
Answer: A
Higher velocity increases turbulence (higher k); lower pressure increases diffusivity D. Both increase Sh = kL/D.
Q.54Medium
The concept of 'equivalent theoretical stages' in absorption is based on:
Answer: A
Used to compare different types of contactors. Actual stages divided by Murphree efficiency gives equivalent theoretical stages needed.
Q.55Medium
In evaporative cooling, the effectiveness (ε) is defined as:
Answer: A
ε = (t_in - t_out)/(t_in - t_wb). Higher effectiveness indicates better utilization of air capacity, crucial for cooling tower design.
Q.56Hard
For a pseudosteady-state diffusion with heterogeneous reaction, the effectiveness factor η accounts for:
Answer: A
η = (actual reaction rate)/(rate if all surface at bulk concentration). Important in catalytic reactors where diffusion and reaction compete.
Q.57Hard
The Thiele modulus (Φ) for a catalyst pellet determines whether the reaction is:
Answer: A
Φ = L√(k/D). When Φ >> 1, diffusion controls; when Φ << 1, kinetics controls. Critical for reactor design optimization.
Q.58Medium
In membrane separation processes, the selectivity is expressed as:
Answer: A
Selectivity α = (J_A/ΔP_A)/(J_B/ΔP_B). Higher selectivity indicates better separation capability of the membrane.
Q.59Hard
For a reverse osmosis process, the concentration polarization effect results in:
Answer: A
Due to higher solute rejection, solutes accumulate near membrane surface, increasing local osmotic pressure and reducing driving force.
Q.60Hard
In coal gasification combustion analysis (2024-25 focus), the mass transfer of oxygen to solid particles is characterized by:
Answer: A
Shrinking core model applies: external diffusion, ash diffusion, and chemical reaction all contribute. Modern focus on clean energy requires optimized design.
