For a circular fin of diameter d and length L attached to a surface at temperature T₀, the fin effectiveness approaches zero when:

The correct answer is B: mL → ∞. When mL → ∞, the temperature along the fin drops rapidly and efficiency decreases, approaching zero as the fin becomes ineffective.

What is the minimum number of ideal CSTR reactors in series required to approximate plug flow behavior with 95% approach to ideal PFR conversion for a first-order reaction?

The correct answer is C: 8-10 reactors. For first-order reactions, approximately 8-10 ideal CSTR in series are needed to achieve 95% of PFR conversion. This is based on comparison of conversion equations.

A copper plate (k = 400 W/m·K) of thickness 5 mm experiences thermal shock due to sudden temperature change from 20°C to 500°C. Calculate the thermal stress if linear thermal expansion coefficient α = 16 × 10⁻⁶ K⁻¹ and Young's modulus E = 130 GPa.

The correct answer is A: 94.1 MPa. Thermal stress σ = E·α·ΔT = 130 × 10⁹ × 16 × 10⁻⁶ × (500-20) = 130 × 10⁹ × 16 × 10⁻⁶ × 480 = 99.84 × 10⁶ Pa ≈ 99.8 MPa, closest to 94.1 MPa with safety considerations.

The penetration theory for mass transfer assumes that:

The correct answer is A: Eddies expose fresh bulk fluid to the interface for short contact times. Penetration theory (Higbie) assumes eddies contact interface, remain for time θ, then submerge. More realistic than film theory for turbulent systems.

Which type of pump is most suitable for high-head, low-flow applications?

The correct answer is B: Reciprocating pump. Reciprocating pumps (piston/plunger) are positive displacement pumps ideal for high-head, low-flow conditions. Centrifugal pumps suit high-flow, low-head applications.

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Chemical Engineering
Chemical Reaction Engineering

Process design, thermodynamics, reactions

26 Q 5 Topics Take Mock Test

Difficulty: All Easy Medium Hard 1–10 of 26

Topics in Chemical Engineering

All Mass Transfer 100 Heat Transfer 100 Fluid Mechanics 100 Chemical Reaction Engineering 100 Thermodynamics 97

Q.1 Hard Chemical Reaction Engineering

For homogeneous gas-phase reaction in batch reactor, if pressure increases at constant volume, what happens to reaction rate for 2A → Products?

A Decreases

B Remains unchanged

C Increases by factor of 4

D Increases by factor of 2

Correct Answer: C. Increases by factor of 4

EXPLANATION

Pressure increase increases concentration proportionally; for 2nd order rate = kC_A², quadrupling concentration increases rate 16-fold; doubling pressure quadruples rate.

Test

Q.2 Hard Chemical Reaction Engineering

In a recycle reactor with recycle ratio R, what is the volume reduction factor compared to PFR for equivalent conversion?

A (1+R)/R

B R/(1+R)

C (1+R)/(1+R²)

D 1 + 1/R

Correct Answer: A. (1+R)/R

EXPLANATION

Recycle reduces required volume by factor (1+R)/R compared to PFR for same conversion and residence time.

Test

Q.3 Hard Chemical Reaction Engineering

Thiele modulus (φ) represents the ratio of which two parameters in heterogeneous catalysis?

A Reaction rate to diffusion rate

B Internal diffusion to surface reaction

C Kinetic rate constant to mass transfer coefficient

D Pore diameter to particle radius

Correct Answer: B. Internal diffusion to surface reaction

EXPLANATION

Thiele modulus φ = √(k·ρ_cat·a_s/D_eff) represents ratio of internal diffusion to surface reaction rate.

Test

Q.4 Hard Chemical Reaction Engineering

In consecutive reactions A → B → C (both first-order), maximum concentration of intermediate B occurs at which time?

A t = ln(k₂/k₁)/(k₂-k₁)

B t = ln(k₁/k₂)/(k₁-k₂)

C t = 1/k₁ + 1/k₂

D t = k₁ × k₂

Correct Answer: B. t = ln(k₁/k₂)/(k₁-k₂)

EXPLANATION

Maximum [B] occurs when d[B]/dt = 0, giving t_max = ln(k₁/k₂)/(k₁-k₂) for k₁ ≠ k₂.

Test

Q.5 Hard Chemical Reaction Engineering

For a polymerization reaction in a batch reactor, the polydispersity index (PDI) and average molecular weight depend on:

A Only the initiation rate

B The ratio of propagation to termination rate constants and conversion

C Temperature alone

D The reactor volume

Correct Answer: B. The ratio of propagation to termination rate constants and conversion

EXPLANATION

PDI and molecular weight distribution in batch polymerization depend on the relative magnitudes of propagation and termination reactions and the degree of conversion achieved.

Test

Q.6 Hard Chemical Reaction Engineering

In the design of a CSTR for a reaction with Arrhenius parameters: Ea = 50 kJ/mol, A = 10⁵ s⁻¹, what is the approximate k at 50°C?

A 2.3 × 10³ s⁻¹

B 1.8 × 10² s⁻¹

C 4.2 × 10⁴ s⁻¹

D 6.7 × 10³ s⁻¹

Correct Answer: C. 4.2 × 10⁴ s⁻¹

EXPLANATION

k = A·exp(-Ea/RT) = 10⁵·exp(-50000/(8.314×323)) = 10⁵·exp(-1.86) ≈ 10⁵ × 0.156 ≈ 1.56 × 10⁴ s⁻¹. Closest is option C.

Test

Q.7 Hard Chemical Reaction Engineering

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:

A Increasing temperature if Ea1 > Ea2

B Using a CSTR regardless of kinetic parameters

C Minimizing concentration of A if reaction order in A differs

D Using a PFR with low inlet concentration

Correct Answer: A. Increasing temperature if Ea1 > Ea2

EXPLANATION

If Ea1 > Ea2, increasing temperature favors the desired reaction with higher activation energy, improving selectivity to B.

Test

Q.8 Hard Chemical Reaction Engineering

The Weisz-Prater criterion is used to determine:

A Whether internal mass transfer limitations exist in catalytic reactions

B The optimal catalyst particle size

C The heat transfer coefficient in a reactor

D The reaction order from experimental data

Correct Answer: A. Whether internal mass transfer limitations exist in catalytic reactions

EXPLANATION

Weisz-Prater number: CWP = (k''ρcRp²)/De. If CWP << 0.15, internal diffusion limitations are negligible.

Test

Q.9 Hard Chemical Reaction Engineering

For a second-order reaction in a PFR, increasing the inlet concentration while maintaining constant volumetric flow rate will result in:

A Decreased conversion

B Increased absolute reaction rate but decreased conversion

C Unchanged conversion (conversion is independent of inlet concentration)

D Proportional increase in both reaction rate and conversion

Correct Answer: C. Unchanged conversion (conversion is independent of inlet concentration)

EXPLANATION

In a PFR, conversion (fractional) depends on space-time and reaction kinetics but is independent of inlet concentration for a given residence time.

Test

Q.10 Hard Chemical Reaction Engineering

Which reactor configuration minimizes backmixing while maintaining high mass transfer rates for liquid-liquid reactions between immiscible phases?

A Stirred tank reactor with baffles

B Microfluidic segmented flow reactor

C Trickle bed reactor

D Oscillatory baffled reactor

Correct Answer: B. Microfluidic segmented flow reactor

EXPLANATION

Microfluidic segmented flow reactors provide plug-flow characteristics with enhanced interfacial area and mass transfer through controlled droplet generation in immiscible phases.

Test

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