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.
The Damköhler number (Da) for a chemical reaction is defined as the ratio of:
Answer: B
Da = (residence time)/(characteristic reaction time) = kτC^(n-1). When Da >> 1, reaction is fast; Da << 1, reaction is slow.
Q.362Medium
A series of 3 identical CSTRs in cascade is used to process a first-order reaction with overall conversion X = 90%. Which statement is true?
Answer: C
For equal volume CSTRs in series processing first-order reaction: C = C₀/(1+kτ)ⁿ, showing exponential decrease in concentration.
Q.363Medium
In a fixed-bed catalytic reactor, what is the primary cause of catalyst deactivation by sintering?
Answer: B
Sintering involves migration of metal atoms on the catalyst surface, leading to agglomeration and reduction of active surface area, particularly at high temperatures.
Q.364Hard
For a second-order reaction in a PFR, increasing the inlet concentration while maintaining constant volumetric flow rate will result in:
Answer: C
In a PFR, conversion (fractional) depends on space-time and reaction kinetics but is independent of inlet concentration for a given residence time.
Q.365Medium
Which reactor configuration would be best suited for a fast, highly exothermic reaction where precise temperature control is critical?
Answer: B
Semi-batch allows controlled feeding of reactants, limiting instantaneous heat generation while maintaining good mixing and temperature control.
Q.366Hard
The Weisz-Prater criterion is used to determine:
Answer: A
Weisz-Prater number: CWP = (k''ρcRp²)/De. If CWP << 0.15, internal diffusion limitations are negligible.
Q.367Medium
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.368Medium
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.369Hard
For parallel reactions where A → B (desired) with rate constant k₁ and A → C (undesired) with rate constant k₂, the selectivity towards B is maximized by:
Answer: A
If Ea1 > Ea2, increasing temperature favors the desired reaction with higher activation energy, improving selectivity to B.
Q.370Easy
The conversion in a batch reactor at any time can be found by integrating the rate equation. For a first-order reaction, the half-life is:
Answer: D
For first-order: t₁/₂ = ln(2)/k = 0.693/k, which is independent of C₀ and inversely proportional to k.
Q.371Medium
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.372Easy
For the reaction 2A + B → Products, if the reaction is 2nd order in A and 1st order in B, the rate equation is:
Answer: A
Reaction order is determined by experimental data, not stoichiometry. The given orders are 2nd in A and 1st in B, so rate law is r = k[A]²[B].
Q.373Medium
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.374Hard
For a polymerization reaction in a batch reactor, the polydispersity index (PDI) and average molecular weight depend on:
Answer: B
PDI and molecular weight distribution in batch polymerization depend on the relative magnitudes of propagation and termination reactions and the degree of conversion achieved.
Q.375Medium
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.376Easy
What is the order of reaction if the half-life period is independent of initial concentration?
Answer: B
For first-order reactions, t₁/₂ = 0.693/k, which is independent of initial concentration C₀.
Q.377Hard
In consecutive reactions A → B → C (both first-order), maximum concentration of intermediate B occurs at which time?
Answer: B
Maximum [B] occurs when d[B]/dt = 0, giving t_max = ln(k₁/k₂)/(k₁-k₂) for k₁ ≠ k₂.
Q.378Medium
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.379Easy
For the reaction A + B → C, the rate law is r = kC_A²C_B. What is the overall order of reaction?
Answer: C
Overall order = sum of individual orders = 2 + 1 = 3.
Q.380Medium
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.
