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HKIE Professional Assessment - Chemical Discipline study materials
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HKIE Professional Assessment - Chemical Discipline practice questions

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Question 1Chemical Engineering Fundamentals and Applied Thermodynamics

In a steady-flow process with negligible kinetic and potential energy changes, the first law for an open system with one inlet and one outlet is best written as which expression?

  1. Q + W = ΔU
  2. Q − W = ΔU + Δ(PV)
  3. Q − W = ΔH + ΔKE + ΔPE Correct answer.
  4. ΔU = Q + W − ΔH

Explanation

For a single-stream steady-flow device the energy balance reduces to heat in minus shaft work out equals the change in enthalpy plus kinetic and potential energy between inlet and outlet. Internal energy alone (ΔU) applies to closed systems, so the flow-work term PV must appear.

Question 2Chemical Engineering Fundamentals and Applied Thermodynamics

For an ideal gas undergoing an isothermal reversible expansion, which statement is correct?

  1. The enthalpy change equals the work done by the gas.
  2. The change in internal energy is zero and all heat absorbed is converted to work. Correct answer.
  3. The entropy change of the gas is zero because temperature is constant.
  4. The heat absorbed is zero because no temperature change occurs.

Explanation

For an ideal gas internal energy depends only on temperature, so an isothermal process has ΔU = 0 and the first law gives Q = W. Entropy still increases because the volume increases even though temperature is constant.

Question 3Chemical Engineering Fundamentals and Applied Thermodynamics

Which quantity is an intensive thermodynamic property?

  1. Total enthalpy of the stream
  2. Specific volume Correct answer.
  3. Mass of the system
  4. Total internal energy

Explanation

Intensive properties are independent of the amount of material, and specific volume (volume per unit mass) satisfies this. Total enthalpy, total internal energy and mass all scale with the size of the system and are therefore extensive.

Question 4Chemical Engineering Fundamentals and Applied Thermodynamics

A Carnot engine operates between a hot reservoir at 600 K and a cold reservoir at 300 K. What is its thermal efficiency?

  1. 0.50 Correct answer.
  2. 0.80
  3. 0.67
  4. 0.25

Explanation

Carnot efficiency equals 1 − Tc/Th using absolute temperatures, giving 1 − 300/600 = 0.50. No real engine operating between these reservoirs can exceed this value.

Question 5Chemical Engineering Fundamentals and Applied Thermodynamics

Which statement correctly describes the critical point of a pure substance?

  1. It is the point where the vapour pressure equals atmospheric pressure.
  2. It is the point where the saturated liquid and saturated vapour densities become identical and the latent heat of vaporisation vanishes. Correct answer.
  3. It is the temperature above which a liquid can still be liquefied by pressure alone.
  4. It is the triple point where solid, liquid and vapour coexist.

Explanation

At the critical point the saturated liquid and saturated vapour states become indistinguishable, so densities coincide and the enthalpy of vaporisation falls to zero. Above the critical temperature no applied pressure can produce a distinct liquid phase.

Question 6Chemical Engineering Fundamentals and Applied Thermodynamics

Which expression is the correct statement of the Lewis–Randall (fugacity) rule for vapour–liquid equilibrium of a component i?

  1. y_i P = x_i P_i^sat / φ_i
  2. y_i P = x_i P_i^sat
  3. y_i φ_i P = x_i γ_i f_i^0 Correct answer.
  4. x_i φ_i P = y_i γ_i f_i^0

Explanation

The general VLE relation equates the vapour-phase fugacity y_i φ_i P to the liquid-phase fugacity x_i γ_i f_i^0, where f_i^0 is the standard-state fugacity. Simplified forms such as Raoult's law follow only under ideal assumptions.

Question 7Chemical Engineering Fundamentals and Applied Thermodynamics

In a binary mixture at constant temperature and pressure, the partial molar Gibbs energy (chemical potential) of component i is defined as which derivative?

  1. (∂H/∂n_i) at constant T and V
  2. (∂G/∂T) at constant P and composition
  3. (∂G/∂P) at constant T and composition
  4. (∂G/∂n_i) at constant T, P and other mole numbers Correct answer.

Explanation

The chemical potential is the partial molar Gibbs energy, defined as the derivative of total Gibbs energy with respect to the moles of component i while temperature, pressure and other amounts are held constant. The other derivatives give molar volume or negative molar entropy, not the chemical potential.

Question 8Chemical Engineering Fundamentals and Applied Thermodynamics

The excess Gibbs energy of a binary mixture is given by G^E/RT = A x1 x2 with A constant. What is the activity coefficient of component 1?

  1. ln γ1 = A x1 x2
  2. ln γ1 = A x2
  3. ln γ1 = A x2^2 Correct answer.
  4. ln γ1 = A x1

Explanation

Applying the relation ln γi = [∂(nG^E/RT)/∂n_i] to the symmetric one-parameter Margules form yields ln γ1 = A x2^2 and ln γ2 = A x1^2. The infinite-dilution limits are then ln γ1∞ = A.

Question 9Chemical Engineering Fundamentals and Applied Thermodynamics

For a reaction with standard Gibbs energy change ΔG° = −20 kJ/mol at 298 K, the equilibrium constant K is closest to which value?

  1. about 3
  2. about 3 × 10^3 Correct answer.
  3. about 3 × 10^9
  4. about 30

Explanation

Using ΔG° = −RT ln K, ln K = 20000/(8.314 × 298) ≈ 8.07, so K ≈ 3.2 × 10^3. The positive ΔG° magnitude drives a strongly favourable equilibrium.

Question 10Chemical Engineering Fundamentals and Applied Thermodynamics

A gas mixture contains 2 mol of A and 3 mol of B at total pressure 5 bar. Treating the mixture as ideal, what is the partial pressure of A?

  1. 3.0 bar
  2. 2.0 bar Correct answer.
  3. 5.0 bar
  4. 1.5 bar

Explanation

The mole fraction of A is 2/(2+3) = 0.4, and for an ideal mixture the partial pressure equals y_A times total pressure, giving 0.4 × 5 = 2.0 bar. Partial pressures of all components sum to the total pressure.

Question 11Chemical Engineering Fundamentals and Applied Thermodynamics

Which condition defines a mixture at its azeotropic point?

  1. The total pressure equals the sum of the pure-component vapour pressures.
  2. The activity coefficients of all components equal unity.
  3. The vapour and liquid compositions are identical. Correct answer.
  4. The relative volatility of the components is very large.

Explanation

At an azeotrope the equilibrium vapour and liquid phases have the same composition, so ordinary distillation cannot separate the mixture further. The activity coefficients are generally not unity, which is precisely why the azeotrope forms.

Question 12Chemical Engineering Fundamentals and Applied Thermodynamics

In a throttling (Joule–Thomson) process through an insulated valve, which quantity is conserved?

  1. Internal energy
  2. Enthalpy Correct answer.
  3. Entropy
  4. Gibbs energy

Explanation

A throttling process is adiabatic with negligible work and kinetic energy change, so the enthalpy of the fluid is essentially constant across the valve. Entropy increases irreversibly while internal energy changes as pressure falls.

Question 13Chemical Engineering Fundamentals and Applied Thermodynamics

A real gas has a Joule–Thomson coefficient that is positive at a given temperature and pressure. What happens to the gas as it is throttled?

  1. Its temperature remains constant.
  2. Its temperature rises.
  3. Its temperature falls. Correct answer.
  4. It condenses completely.

Explanation

The Joule–Thomson coefficient equals (∂T/∂P) at constant enthalpy, so a positive value means temperature decreases as pressure falls during throttling. Cooling by throttling therefore occurs below the inversion temperature.

Question 14Chemical Engineering Fundamentals and Applied Thermodynamics

An ideal-gas mixture is compressed isothermally and reversibly from P1 to P2. Which expression gives the change in Gibbs energy per mole?

  1. RT ln(P2/P1) Correct answer.
  2. 0
  3. −RT ln(P2/P1)
  4. RT (P2 − P1)

Explanation

For an isothermal change of an ideal gas, dG = V dP, and integrating V = RT/P gives ΔG = RT ln(P2/P1). Because P2 exceeds P1 the change is positive, reflecting the work of compression.

Question 15Chemical Engineering Fundamentals and Applied Thermodynamics

Which statement about the fugacity coefficient φ of a real gas is correct?

  1. It equals the compressibility factor at all pressures.
  2. It is always greater than one.
  3. It equals unity for all gases at all conditions.
  4. It approaches unity as pressure approaches zero. Correct answer.

Explanation

Fugacity coefficient measures departure from ideal-gas behaviour and approaches unity as pressure tends to zero, where the gas becomes ideal. At finite pressures it may be greater or less than one depending on the sign of the residual property.

Question 16Chemical Engineering Fundamentals and Applied Thermodynamics

For an adiabatic steady-flow compressor with negligible heat loss, the shaft work per unit mass is given approximately by which expression?

  1. ΔG
  2. ΔS
  3. ΔH Correct answer.
  4. ΔU

Explanation

With negligible heat transfer and kinetic/potential energy, the steady-flow balance gives shaft work equal to the enthalpy rise across the compressor. For an ideal gas this becomes the integral of Cp dT along the compression path.

Question 17Chemical Engineering Fundamentals and Applied Thermodynamics

A binary mixture shows a minimum-boiling azeotrope at 1 atm. Which statement about separation by ordinary distillation is correct?

  1. The azeotrope can be separated because the components have different boiling points.
  2. The azeotrope composition is a barrier that ordinary distillation cannot cross. Correct answer.
  3. The azeotrope disappears at higher reflux ratio.
  4. The azeotrope can be broken by simply adding more trays.

Explanation

At a minimum-boiling azeotrope the vapour and liquid compositions coincide, so the equilibrium curve touches the diagonal and no finite number of stages can cross that composition. Breaking it requires a change of pressure, an entrainer, or another separation method.

Question 18Chemical Engineering Fundamentals and Applied Thermodynamics

The compressibility factor Z of a gas is 0.85 at a given state. Which conclusion follows?

  1. The gas cannot be liquefied.
  2. The gas has a negative absolute temperature.
  3. The gas occupies less volume than an ideal gas would at the same T and P. Correct answer.
  4. The gas is at its critical point.

Explanation

Z = PV/RT, so Z below unity means the actual molar volume is smaller than the ideal-gas value at the same temperature and pressure, reflecting net attractive forces. A value of 0.85 is entirely consistent with a real gas below its Boyle temperature.

Question 19Chemical Engineering Fundamentals and Applied Thermodynamics

Which equation of state is a cubic equation that explicitly incorporates the critical temperature and critical pressure of the substance?

  1. The Peng–Robinson equation Correct answer.
  2. The virial equation truncated at the second coefficient
  3. Dalton's law of partial pressures
  4. The ideal-gas law

Explanation

The Peng–Robinson equation is a cubic equation of state whose parameters a and b are evaluated from the critical temperature and pressure and the acentric factor. This allows it to predict both vapour and liquid behaviour near the critical region.

Question 20Chemical Engineering Fundamentals and Applied Thermodynamics

A closed rigid vessel contains saturated liquid water and saturated vapour in equilibrium at 150 °C. Heat is added until only saturated vapour remains. Which quantity is directly obtained from the steam tables to find the heat added per unit mass?

  1. The specific heat of liquid water at 25 °C
  2. The entropy of the saturated liquid only
  3. The gas constant of water vapour
  4. The enthalpy of vaporisation at 150 °C Correct answer.

Explanation

In a rigid closed vessel the volume is fixed, so the heat added equals the change in internal energy; however the enthalpy of vaporisation, together with the specific volumes, allows the internal energy change to be evaluated from tabulated saturation data. The latent heat at 150 °C is the key tabulated quantity for the phase change.

Question 21Chemical Engineering Fundamentals and Applied Thermodynamics

Which relationship correctly expresses the Clausius–Clapeyron equation for vapour pressure?

  1. d(ln P^sat)/dT = RT/ΔH_vap
  2. d(ln P^sat)/dT = ΔH_vap/(RT^2) Correct answer.
  3. dP^sat/dT = ΔH_vap/(RT)
  4. dP^sat/dT = −ΔH_vap/(RT^2)

Explanation

The Clausius–Clapeyron equation relates the temperature derivative of the logarithm of vapour pressure to the enthalpy of vaporisation divided by RT squared. Integrating it with constant ΔH_vap gives the familiar linear plot of ln P against 1/T.

Question 22Chemical Engineering Fundamentals and Applied Thermodynamics

Two streams are mixed adiabatically: 1 kg/s of water at 80 °C and 2 kg/s of water at 20 °C, with equal specific heats. What is the outlet temperature?

  1. 60 °C
  2. 40 °C Correct answer.
  3. 35 °C
  4. 50 °C

Explanation

An adiabatic mixing energy balance with equal specific heats gives a mass-weighted average: (1×80 + 2×20)/(1+2) = 120/3 = 40 °C. Entropy is generated in the mixing even though enthalpy is conserved.

Question 23Chemical Engineering Fundamentals and Applied Thermodynamics

A heat pump delivers 12 kW of heating to a building while drawing 4 kW of electrical power. What is its coefficient of performance for heating?

  1. 0.33
  2. 3 Correct answer.
  3. 2
  4. 4

Explanation

The heating COP is the heat delivered divided by the work input, giving 12/4 = 3. This is consistent with the second law since it is finite and positive.

Question 24Chemical Engineering Fundamentals and Applied Thermodynamics

For a binary system at equilibrium, the Gibbs phase rule gives the number of degrees of freedom F as which expression, where C is the number of components and P the number of phases?

  1. F = C + P + 2
  2. F = C − P + 2 Correct answer.
  3. F = C + P − 2
  4. F = C − P

Explanation

The Gibbs phase rule states F = C − P + 2 for a system where temperature and pressure are both variable intensive properties. For a single-component, two-phase system this gives one degree of freedom, consistent with saturation pressure fixing temperature.

Question 25Reaction Engineering and Catalysis

In a continuous stirred-tank reactor, a first-order reaction is carried out under steady isothermal conditions. Which statement correctly describes the relationship between space time and conversion?

  1. Conversion increases linearly with space time at all values of space time.
  2. Conversion reaches unity at a finite space time equal to the rate constant.
  3. Conversion increases with space time and approaches unity only as space time becomes very large. Correct answer.
  4. Conversion is independent of space time for a first-order reaction.

Explanation

For a CSTR with first-order kinetics, the design equation gives conversion X = kτ/(1+kτ), where τ is space time. This is a saturating function that approaches but never reaches unity at finite τ. Linearity holds only at very small τ where kτ is much less than one.

Question 26Reaction Engineering and Catalysis

A gas-phase reaction A → 2B is run in a plug-flow reactor at constant temperature and pressure. The volumetric flow rate changes with conversion. Which expression correctly gives the concentration of A as a function of conversion?

  1. C_A = C_A0 (1 − X)
  2. C_A = C_A0 (1 − X)/(1 + X) Correct answer.
  3. C_A = C_A0 (1 − X)/(1 + 2X)
  4. C_A = C_A0 (1 + X)/(1 − X)

Explanation

For a gas-phase system with changing moles, the volumetric flow follows v = v0(1 + εX), where ε is the fractional volume change at full conversion. Here ε = (2 − 1)/1 = 1, so C_A = C_A0(1 − X)/(1 + X). The denominator accounts for expansion of the gas mixture as reaction proceeds.

Question 27Reaction Engineering and Catalysis

Which quantity is correctly described by the term turnover frequency (TOF) in heterogeneous catalysis?

  1. Fraction of active sites occupied by adsorbed species at steady state
  2. Moles of reactant converted per active site per unit time Correct answer.
  3. Ratio of product concentration to reactant concentration at equilibrium
  4. Total moles of product formed per unit mass of catalyst

Explanation

Turnover frequency is defined as the number of catalytic cycles (or molecules converted) per active site per unit time, typically reported in s⁻¹. It normalises activity to the number of active sites rather than to catalyst mass. This makes TOF a site-specific measure of intrinsic catalytic activity.

Question 28Reaction Engineering and Catalysis

A reaction follows Langmuir-Hinshelwood kinetics on a solid catalyst with a single adsorbed reactant. As the partial pressure of the reactant increases without limit at constant temperature, what happens to the reaction rate?

  1. It falls to zero because of product inhibition.
  2. It approaches a constant maximum value. Correct answer.
  3. It increases without bound proportional to pressure.
  4. It passes through a maximum and then declines to zero.

Explanation

In Langmuir-Hinshelwood kinetics the rate is proportional to fractional coverage, r = kKp/(1+Kp). At high pressure the surface becomes saturated, Kp dominates the denominator, and the rate approaches the constant k. The order shifts from first order at low pressure to zero order at high pressure.

Question 29Reaction Engineering and Catalysis

The Arrhenius activation energy of an elementary step is 80 kJ/mol. By what factor does the rate constant increase when the temperature is raised from 300 K to 310 K, assuming the pre-exponential factor is constant?

  1. About 10
  2. About 2.8 Correct answer.
  3. About 0.5
  4. About 1.1

Explanation

Using k2/k1 = exp[(Ea/R)(1/T1 − 1/T2)], with Ea = 80,000 J/mol and R = 8.314 J/mol·K, the exponent is (9625)(1/300 − 1/310) ≈ 9625 × 1.075e-4 ≈ 1.035. The exponential of 1.035 is about 2.8, so the rate constant roughly triples.

Question 30Reaction Engineering and Catalysis

Which statement best explains why a catalyst increases the rate of a thermodynamically favourable reaction?

  1. It increases the equilibrium constant of the reaction.
  2. It raises the temperature of the reacting mixture.
  3. It provides an alternative pathway with a lower activation energy. Correct answer.
  4. It increases the concentration of reactants at the surface above the bulk value at equilibrium.

Explanation

A catalyst accelerates a reaction by offering a reaction pathway with a lower activation energy, so a larger fraction of molecules can react at a given temperature. It does not change the overall thermodynamics, so the equilibrium constant is unaffected. The catalyst is regenerated and does not shift the final equilibrium position.

Question 31Reaction Engineering and Catalysis

A second-order irreversible reaction 2A → products with rate −r_A = kC_A² is carried out in a CSTR. Which design equation correctly relates space time to conversion?

  1. τ = X/(kC_A0²(1 − X))
  2. τ = X/(k(1 − X)²)
  3. τ = X/(kC_A0(1 − X)²) Correct answer.
  4. τ = X/(2kC_A0(1 − X)²)

Explanation

For a CSTR the design equation is τ = (C_A0 − C_A)/(−r_A) evaluated at outlet conditions. With C_A = C_A0(1 − X) and −r_A = kC_A² = kC_A0²(1 − X)², the ratio gives τ = X/(kC_A0(1 − X)²). The C_A0 appears once in the denominator after cancellation of one power.

Question 32Reaction Engineering and Catalysis

In a packed-bed catalytic reactor, which condition is most consistent with the absence of significant external mass-transfer limitation?

  1. The observed rate is proportional to the pellet surface area per unit bed volume.
  2. The observed rate doubles when the pellet diameter is doubled.
  3. The observed rate is independent of the linear velocity past the pellet at constant temperature and composition. Correct answer.
  4. The observed activation energy is approximately half the intrinsic value.

Explanation

External mass-transfer resistance produces a dependence of observed rate on the gas velocity past the pellet, because higher velocity reduces the boundary-layer thickness. If the rate is unchanged over a range of velocities, external diffusion is not controlling. A strong dependence on pellet size or a halved apparent activation energy would signal transport limitation.

Question 33Reaction Engineering and Catalysis

The Thiele modulus for a first-order reaction in a porous spherical pellet is doubled while all other parameters are held constant. For strong diffusion limitation (large modulus), how does the effectiveness factor change?

  1. It remains unchanged.
  2. It increases by a factor of four.
  3. It is approximately halved. Correct answer.
  4. It is approximately doubled.

Explanation

In the strong-diffusion regime the effectiveness factor for a first-order reaction scales inversely with the Thiele modulus, η ≈ 3/φ for a sphere. Doubling φ therefore roughly halves η. This reflects shallower penetration of reactant into the pellet as the reaction outpaces diffusion.

Question 34Reaction Engineering and Catalysis

Which statement correctly describes the role of the equilibrium constant in the design of a reversible exothermic reaction carried out in an adiabatic reactor?

  1. The equilibrium conversion falls as temperature rises, so the adiabatic temperature rise limits attainable conversion. Correct answer.
  2. The equilibrium constant is independent of temperature for exothermic reactions.
  3. The equilibrium constant depends only on total pressure.
  4. The equilibrium conversion rises with temperature, favouring adiabatic operation.

Explanation

For an exothermic reaction, the equilibrium constant decreases as temperature rises according to the van't Hoff relation. In an adiabatic reactor the temperature rises with conversion, driving the equilibrium conversion down and limiting the maximum attainable value. This is why interstage cooling or a cooled reactor is often used.

Question 35Reaction Engineering and Catalysis

Which type of reactor model is most appropriate for describing severe non-ideal flow with significant bypassing and dead zones in a continuous vessel?

  1. A single ideal CSTR operating at the mean residence time
  2. An ideal plug-flow model with no axial dispersion
  3. A tanks-in-series model combined with a dead-volume and bypass compartment Correct answer.
  4. A batch reactor model with uniform composition

Explanation

Severe non-ideality such as bypassing and dead zones cannot be captured by a single ideal model. A compartment model combining a tanks-in-series or CSTR region with explicit bypass and dead-volume elements reflects the actual residence-time distribution. This allows the measured tracer response to be matched more faithfully.

Question 36Reaction Engineering and Catalysis

In a catalytic converter operating at high temperature, the apparent reaction rate begins to decrease with further temperature increase even though intrinsic kinetics predict an increase. Which phenomenon most likely explains this?

  1. Thermodynamic reversibility combined with declining equilibrium conversion at high temperature Correct answer.
  2. A change from first-order to zero-order kinetics
  3. An increase in the pre-exponential factor
  4. A decrease in the activation energy at high temperature

Explanation

For exothermic reversible reactions, the equilibrium conversion decreases as temperature rises. At high temperature the reactor approaches equilibrium and the net forward rate falls even though the intrinsic forward rate constant continues to rise. This produces an apparent maximum in conversion or rate with temperature.

Question 37Reaction Engineering and Catalysis

Which condition is required for a catalyst to exhibit high selectivity toward a desired product in a parallel reaction network?

  1. The catalyst should maximise the total conversion regardless of product distribution.
  2. The catalyst should increase the equilibrium constant of all reactions equally.
  3. The catalyst should accelerate the desired pathway relative to competing pathways. Correct answer.
  4. The catalyst should operate at the highest possible temperature.

Explanation

Selectivity is governed by the relative rates of competing pathways, so a selective catalyst lowers the activation energy of the desired route more than that of the undesired routes. Increasing total conversion alone does not improve selectivity and may increase by-product formation. Temperature affects selectivity only through the differing activation energies of the pathways.

Question 38Reaction Engineering and Catalysis

A first-order reaction with rate constant 0.2 min⁻¹ is carried out in a plug-flow reactor. What space time is required to achieve 90% conversion?

  1. About 50 min
  2. About 11.5 min Correct answer.
  3. About 0.2 min
  4. About 5.0 min

Explanation

For a first-order PFR, τ = −ln(1 − X)/k. With X = 0.90 and k = 0.2 min⁻¹, τ = −ln(0.10)/0.2 = 2.303/0.2 ≈ 11.5 min. This is the mean residence time needed for 90% conversion in plug flow.

Question 39Reaction Engineering and Catalysis

Which statement correctly distinguishes homogeneous from heterogeneous catalysis in terms of phase behaviour?

  1. In heterogeneous catalysis the catalyst dissolves in the reaction mixture.
  2. In homogeneous catalysis the catalyst is always a solid; in heterogeneous catalysis it is always a gas.
  3. In homogeneous catalysis the catalyst is consumed stoichiometrically.
  4. In homogeneous catalysis the catalyst and reactants are in the same phase; in heterogeneous catalysis they are in different phases. Correct answer.

Explanation

Homogeneous catalysis involves catalyst and reactants in a single phase, typically a liquid-phase solution, while heterogeneous catalysis involves a catalyst in a different phase, usually a solid contacting fluid reactants. This phase distinction drives differences in separation, heat transfer, and active-site character. A catalyst is not consumed overall in either case.

Question 40Reaction Engineering and Catalysis

The rate of an enzyme-catalysed reaction follows Michaelis-Menten kinetics. At a substrate concentration equal to twice the Michaelis constant, what fraction of the maximum rate is achieved?

  1. One-half
  2. One-third
  3. Three-quarters
  4. Two-thirds Correct answer.

Explanation

The Michaelis-Menten rate is r = r_max S/(K_m + S). Substituting S = 2K_m gives r = r_max(2K_m)/(3K_m) = (2/3)r_max. Thus two-thirds of the maximum rate is reached at twice the Michaelis constant.

Question 41Reaction Engineering and Catalysis

Which measurement would most directly provide the number of active sites on a supported metal catalyst for turnover frequency calculations?

  1. Chemisorption of a probe gas such as carbon monoxide or hydrogen Correct answer.
  2. Thermogravimetric analysis of coke deposition
  3. BET surface area from nitrogen physisorption
  4. Mercury porosimetry pore-size distribution

Explanation

Selective chemisorption of probe molecules such as CO or H₂ titrates exposed metal atoms, giving the number of accessible active sites needed to normalise rates into turnover frequencies. Physisorption gives total surface area but not site count. Porosimetry and coke analysis provide structural or deactivation information rather than site density.

Question 42Reaction Engineering and Catalysis

In a gas-solid catalytic reactor, which observation would most strongly indicate that pore diffusion is limiting the observed rate?

  1. The observed rate is unchanged when the catalyst is crushed to a fine powder.
  2. The observed rate is independent of pellet size and gas velocity.
  3. The observed rate is proportional to the external surface area of the pellet.
  4. The apparent activation energy is about half the intrinsic activation energy and the rate depends on pellet size. Correct answer.

Explanation

Strong pore-diffusion limitation produces an effectiveness factor that scales inversely with the Thiele modulus, halving the apparent activation energy relative to the intrinsic value. The rate then depends on pellet size because the diffusion path length changes. Crushing the catalyst to a fine powder removes this limitation and increases the rate.

Question 43Reaction Engineering and Catalysis

Which statement correctly describes the effect of increasing the residence time on the product distribution in a series reaction A → B → C, where B is the desired product?

  1. The yield of B passes through a maximum as residence time increases. Correct answer.
  2. The yield of B is independent of residence time.
  3. The yield of B decreases monotonically with residence time.
  4. The yield of B increases monotonically with residence time.

Explanation

In a series network, B is formed from A and consumed to form C, so its concentration rises initially as A converts and then falls as B is further converted. The intermediate yield therefore passes through a maximum at an optimum residence time. Operating beyond that optimum sacrifices B to the consecutive reaction.

Question 44Reaction Engineering and Catalysis

Which statement correctly describes the role of axial dispersion in a packed-bed reactor described by the axial dispersion model?

  1. The Peclet number is independent of bed length and particle size.
  2. Axial dispersion increases the conversion of a positive-order reaction compared with plug flow.
  3. A small Peclet number indicates ideal plug flow behaviour.
  4. A small Peclet number indicates significant axial dispersion and deviation from plug flow. Correct answer.

Explanation

In the axial dispersion model the Peclet number expresses the ratio of convective to dispersive transport. A low Peclet number means dispersion is large relative to convection, so the reactor deviates from plug flow. For positive-order reactions, dispersion generally lowers conversion relative to ideal plug flow at the same space time.

Question 45Transport Phenomena, Separation Processes and Unit Operations

In a packed absorption column, the operating line relates the bulk mole fractions of solute in the gas and liquid phases. Which statement correctly describes the slope of the operating line when expressed as y versus x for dilute conditions?

  1. It equals Gs/Ls, the inverse of the solute-free molar flow ratio.
  2. It equals the ratio of the column cross-sectional area to the packing surface area.
  3. It equals Ls/Gs, the ratio of solute-free liquid to solute-free gas molar flow rates. Correct answer.
  4. It equals the equilibrium slope m from Henry's law.

Explanation

A solute mass balance over the column gives y = (Ls/Gs)x + constant, so the operating line slope is Ls/Gs, the ratio of solute-free liquid to solute-free gas flows. The equilibrium line, not the operating line, has slope m for a linear isotherm. Column geometry does not set the operating line slope.

Question 46Transport Phenomena, Separation Processes and Unit Operations

For a binary distillation with constant relative volatility, which quantity does the McCabe-Thiele operating line for the rectifying section intersect the y-axis at?

  1. xD/(R+1) Correct answer.
  2. xD·R/(R+1)
  3. xD·(R+1)/R
  4. xF/(R+1)

Explanation

The rectifying operating line is y = [R/(R+1)]x + xD/(R+1). Setting x = 0 gives the intercept xD/(R+1), which is the standard construction point for McCabe-Thiele stepping. The feed composition xF does not appear in this intercept.

Question 47Transport Phenomena, Separation Processes and Unit Operations

A centrifugal pump delivers water through a system whose friction losses scale with the square of flow. If the impeller speed is doubled while the pump remains in the same efficiency regime, the developed head at the corresponding operating point changes by what factor?

  1. 0.5
  2. 8
  3. 4 Correct answer.
  4. 2

Explanation

Pump affinity laws state that head varies with the square of impeller speed at geometrically similar points, so doubling speed multiplies head by four. Flow scales linearly and power with the cube of speed. The factor of eight applies to power, not head.

Question 48Transport Phenomena, Separation Processes and Unit Operations

Which dimensionless group is the correct criterion for characterizing whether flow in a pipe is laminar or turbulent?

  1. Nusselt number
  2. Prandtl number
  3. Reynolds number Correct answer.
  4. Sherwood number

Explanation

The Reynolds number compares inertial to viscous forces and sets the laminar-turbulent transition, roughly Re = 2100 for pipe flow. Prandtl, Nusselt, and Sherwood numbers describe momentum-thermal diffusivity ratios and heat or mass transfer rates respectively.

Question 49Transport Phenomena, Separation Processes and Unit Operations

In a shell-and-tube heat exchanger, baffles are installed on the shell side primarily to serve which function?

  1. To reduce the tube-side pressure drop by straightening the tube flow.
  2. To increase shell-side velocity and turbulence, raising the shell-side heat transfer coefficient. Correct answer.
  3. To prevent leakage between the shell and tube passes.
  4. To allow thermal expansion of the tubes within the shell.

Explanation

Baffles force the shell-side fluid across the tube bundle, raising velocity and turbulence and thus the shell-side film coefficient, at the cost of pressure drop. Tube-side flow is unaffected by shell baffles. Expansion and leakage are handled by other components such as expansion joints and gaskets.

Question 50Transport Phenomena, Separation Processes and Unit Operations

In drying, which moisture is removed at a constant rate during the constant-rate period?

  1. Free (unbound) surface moisture Correct answer.
  2. Water of crystallization chemically bound in the solid
  3. Equilibrium moisture remaining at the drying air condition
  4. Bound moisture held within the solid matrix

Explanation

During the constant-rate period, unbound surface moisture evaporates at a rate limited by external heat and mass transfer. Bound and chemically held moisture require internal diffusion and are removed in the falling-rate period. Equilibrium moisture is never removed at the given air condition.