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 membrane reactor for an equilibrium-limited reaction, what is the primary advantage?
Answer: B
By selectively removing product through the membrane, the equilibrium constant expression is displaced, allowing conversion beyond the thermodynamic limit.
Q.182Medium
The mean residence time in a reactor can be calculated from RTD data using:
Answer: D
Both equations are valid: the integral formulation and the volumetric flow definition give the same mean residence time.
Q.183Medium
In a tubular reactor with axial dispersion, increasing the Peclet number (Pe) results in:
Answer: A
Peclet number Pe = uL/D. High Pe means low diffusion relative to convection, approaching ideal PFR behavior with narrow RTD.
Q.184Medium
In a heterogeneous catalytic reaction, the overall rate is limited by which step if the external mass transfer coefficient is very small?
Answer: C
A very small external mass transfer coefficient creates a large resistance to diffusion from bulk to particle surface, making external mass transfer the rate-limiting step.
Q.185Medium
In a plug flow reactor (PFR), what is the relationship between conversion and reactor volume for a first-order reaction?
Answer: B
For a first-order reaction in PFR, X = 1 - exp(-kτ), showing exponential relationship with residence time and thus volume.
Q.186Medium
Which reactor configuration provides the highest conversion for an endothermic reaction at equilibrium?
Answer: D
Membrane reactors shift equilibrium by removing products, overcoming equilibrium limitations in endothermic reactions.
Q.187Medium
In a CSTR operating at steady state, if volumetric flow rate increases while keeping concentration constant, what happens to conversion?
Answer: B
Increased flow rate reduces residence time τ = V/F. For CSTR, X = kτ/(1+kτ), so increased τ means decreased conversion.
Q.188Medium
What does the Damköhler number (Da) represent in reactor design?
Answer: B
Da = reaction rate/flow rate, determining whether reaction or flow dominates; Da >> 1 means reaction-limited.
Q.189Medium
In a reactor with catalyst deactivation following first-order decay, what is the effect on reactant conversion over time?
Answer: B
Catalyst activity decays as a = exp(-k_d·t), causing effective rate constant to decrease, reducing conversion over time.
Q.190Medium
For competitive-consecutive reactions: A → B (k₁), A → C (k₂), B → D (k₃), selectivity of B over C is defined as S_B/C = ?
Answer: A
For parallel reactions, instantaneous selectivity S_B/C = k₁/k₂, independent of time at low conversions.
Q.191Medium
In microbial fermentation kinetics, the Monod equation models specific growth rate. What happens when substrate concentration >> K_s?
Answer: A
When [S] >> K_s, μ ≈ μ_max, making growth zero-order in substrate (Monod equation simplification).
Q.192Medium
For isothermal batch reactor with r = -dC_A/dt = kC_A^n, what is the integrated rate law for n=2?
Answer: A
For second-order: ∫dC_A/C_A² = -k∫dt gives 1/C_A - 1/C_A0 = kt.
Q.193Medium
For a reaction with activation energy E_a = 50 kJ/mol, by what factor does rate constant increase if temperature increases from 300K to 310K? (R = 8.314 J/mol·K)
Answer: B
Using Arrhenius: ln(k₂/k₁) = (E_a/R)(T₂-T₁)/(T₁T₂) ≈ 1.96, so k₂/k₁ ≈ 2.0
Q.194Medium
In a CSTR operating at steady state with a first-order irreversible reaction A → B, if the volumetric flow rate is doubled while keeping reactor volume constant, how does the conversion of reactant A change?
Answer: B
In a CSTR, conversion depends on residence time (τ = V/Q). When volumetric flow rate Q doubles while V remains constant, residence time τ decreases by half. Since conversion X_A = kτ/(1+kτ) for first-order reaction, decreased τ leads to decreased conversion. This is a fundamental principle in reactor design for 2024-25 competitive exams.
Q.195Medium
Which thermodynamic potential is most useful for constant temperature and pressure processes?
Answer: C
Gibbs Free Energy (G = H - TS) is the appropriate thermodynamic potential for processes at constant T and P. ΔG = 0 at equilibrium and ΔG < 0 for spontaneous processes.
Q.196Medium
For an ideal gas undergoing isothermal expansion, the work done is given by:
Answer: A
For isothermal process of ideal gas, W = nRTln(V₂/V₁) = nRTln(P₁/P₂). This is derived from the first law with ΔU = 0 for isothermal ideal gas process.
Q.197Medium
For a process where ΔG < 0 at all temperatures, the process must be:
Answer: B
From ΔG = ΔH - TΔS, for ΔG < 0 at all T: ΔH < 0 (exothermic) and ΔS > 0 (entropy increases). This is a spontaneous process at all temperatures.
Q.198Medium
The heat of vaporization of water is 40.66 kJ/mol at 373 K. The entropy of vaporization is approximately:
Answer: C
ΔS_vap = ΔH_vap/T = 40660 J/mol / 373 K ≈ 109 J/mol·K. This follows Trouton's rule (~85-105 J/mol·K for most liquids).
Q.199Medium
For a reversible process, the Clausius inequality states:
Answer: B
For reversible processes, ΔS = Q_rev/T. For irreversible processes, ΔS > Q_irrev/T. This is the Clausius inequality: dS ≥ dQ/T.
Q.200Medium
A cyclic heat engine operates between hot reservoir at 500 K and cold reservoir at 300 K. Maximum theoretical efficiency is:
Answer: B
Maximum efficiency is Carnot efficiency: η_max = 1 - T_cold/T_hot = 1 - 500300 = 0.40 = 40%. No heat engine can exceed this efficiency.
