Chemical Engineering questions for GATE and PSU exams are built on a handful of core subjects applied in many ways. Practice spans fluid mechanics, heat transfer, mass transfer, chemical reaction engineering, thermodynamics, process control and instrumentation, and plant design economics. Numerical solutions carry the assumptions written out, because the assumption is usually what separates a correct answer from a plausible one.
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.22Medium
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.23Medium
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.24Medium
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.25Medium
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.26Medium
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.27Medium
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.28Medium
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.29Medium
The Fick's first law of diffusion in binary mixtures states that the molar flux J_A is proportional to:
Answer: B
Fick's law: J_A = -D_AB(dC_A/dz) + C_A(J_A + J_B). The total flux includes both diffusive and convective contributions due to bulk flow.
Q.30Medium
For equimolar counter-diffusion in a binary system, the molar fluxes satisfy:
Answer: D
In equimolar counter-diffusion, moles of A diffusing in one direction equal moles of B diffusing in the opposite direction, making J_A = -J_B and their sum zero.
Q.31Medium
The Stefan problem in mass transfer involves:
Answer: A
The Stefan problem deals with unsteady diffusion with a moving interface, commonly encountered in evaporation from droplets and sublimation processes where the interface position changes with time.
Q.32Medium
In a countercurrent absorption column, if the gas-phase mass transfer coefficient is k_G = 0.05 kmol/(m²·s·atm), the interfacial area 'a' = 200 m²/m³, and column cross-section = 5 m², the volumetric overall mass transfer coefficient is:
Answer: B
Overall volumetric mass transfer = k_G × a × Volume = 0.05 × 200 × (5 × height). For unit height, K_G·a·V = 0.05 × 200 × 5 = 50 kmol/(s·atm).
Q.33Medium
The penetration theory for mass transfer assumes that the contact time between fluid elements is:
Answer: B
Penetration theory assumes fluid elements contact the interface for a short time, then move away. Mass transfer is modeled using unsteady diffusion into a semi-infinite medium.
Q.34Medium
For absorption of a sparingly soluble gas in a liquid with fast reaction kinetics, the enhancement factor 'E' is determined by:
Answer: B
The enhancement factor E = k_L,with reaction/k_L,without reaction depends on the Hatta number (Ha = √(k·C_A0·D_A/k_L²)), which incorporates reaction kinetics and mass transfer parameters.
Q.35Medium
In membrane separation, the permeate flux in reverse osmosis is given by:
Answer: B
The reverse osmosis permeate flux follows: J = A(ΔP - Δπ), where A is membrane permeability, ΔP is applied pressure, and Δπ is osmotic pressure difference.
Q.36Medium
A gas A diffuses through a stagnant film of gas B. If the diffusivity D_AB = 0.1 cm²/s, film thickness δ = 0.01 cm, and concentration difference = 0.05 mol/cm³, the diffusive flux is:
Answer: B
Using Fick's law for stagnant diffusion: J = D_AB·ΔC/δ = 0.1 × 0.005.01 = 0.5 × 10⁻² = 5 × 10⁻³ mol/(cm²·s).
Q.37Medium
The height of a transfer unit (HTU) in a packed absorption column represents:
Answer: B
HTU = V·G/(k_G·a·A), where a smaller HTU indicates more efficient mass transfer. It represents the column height needed to achieve one logarithmic unit of concentration driving force reduction.
Q.38Medium
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.39Medium
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.40Medium
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.
