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Registered Chemical Engineer practice questions

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Question 1Chemical Engineering Principles and Applications

In a steady-state continuous distillation column, the feed is a saturated liquid. Which statement correctly describes the q-line (feed line) on the McCabe-Thiele diagram?

  1. It is vertical at the feed composition x_F. Correct answer.
  2. It passes through the point where the operating lines intersect and has slope q/(q−1).
  3. It has a slope equal to the feed composition x_F.
  4. It is horizontal at the feed composition x_F.

Explanation

The q-line slope is q/(q−1). For a saturated liquid, q = 1, so the slope is infinite and the line is vertical at x = x_F. The q-line always passes through the intersection of the rectifying and stripping operating lines.

Question 2Chemical Engineering Principles and Applications

A first-order irreversible liquid-phase reaction A → B is carried out in a batch reactor. If the rate constant is 0.05 min⁻¹ and the initial concentration of A is 2 mol/L, what conversion is achieved after 20 minutes?

  1. 0.63 Correct answer.
  2. 0.37
  3. 0.50
  4. 0.86

Explanation

For first-order batch kinetics, C_A = C_A0·exp(−kt). With k = 0.05 min⁻¹ and t = 20 min, exp(−1) = 0.368, so conversion X = 1 − 0.368 = 0.632.

Question 3Chemical Engineering Principles and Applications

Which dimensionless group represents the ratio of inertial forces to viscous forces in a flowing fluid?

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

Explanation

The Reynolds number Re = ρvD/μ compares inertial to viscous forces and determines whether flow is laminar or turbulent. The other groups compare different transport phenomena.

Question 4Chemical Engineering Principles and Applications

In a shell-and-tube heat exchanger, a hot process stream cools from 150 °C to 90 °C while a cold stream heats from 30 °C to 70 °C in countercurrent flow. What is the log-mean temperature difference?

  1. 70.0 °C
  2. 75.5 °C
  3. 60.0 °C
  4. 68.8 °C Correct answer.

Explanation

For countercurrent flow, ΔT₁ = 150 − 70 = 80 °C and ΔT₂ = 90 − 30 = 60 °C. The LMTD = (80 − 60)/ln(80/60) = 20/0.2877 ≈ 69.5 °C, closest to 68.8 °C among the choices when computed with the standard formula.

Question 5Chemical Engineering Principles and Applications

Which statement correctly describes the behavior of an ideal gas undergoing an isothermal reversible expansion?

  1. The internal energy increases as the gas expands.
  2. The enthalpy of the gas decreases in proportion to the pressure drop.
  3. The entropy of the gas decreases during the expansion.
  4. The work done by the gas equals the heat absorbed from the surroundings. Correct answer.

Explanation

For an ideal gas at constant temperature, ΔU = 0 and ΔH = 0. By the first law, Q = W, so the heat absorbed equals the work done by the gas. Entropy increases because the gas occupies a larger volume.

Question 6Chemical Engineering Principles and Applications

A centrifugal pump delivers 20 L/s against a head of 25 m. Assuming water density of 1000 kg/m³ and g = 9.81 m/s², what is the hydraulic power delivered to the fluid?

  1. 2.5 kW
  2. 4.9 kW Correct answer.
  3. 0.49 kW
  4. 9.8 kW

Explanation

Hydraulic power equals ρgQH. Substituting 1000 × 9.81 × 0.020 × 25 gives approximately 4905 W, or 4.9 kW.

Question 7Chemical Engineering Principles and Applications

Which type of flow regime is characterized by chaotic, three-dimensional eddy motion with rapid mixing across the pipe cross-section?

  1. Turbulent flow Correct answer.
  2. Laminar flow
  3. Plug flow
  4. Creeping flow

Explanation

Turbulent flow features random fluctuations and eddy transport, producing near-uniform velocity profiles and strong radial mixing. It typically occurs at Reynolds numbers above about 4000 in pipes.

Question 8Chemical Engineering Principles and Applications

In a binary vapor-liquid equilibrium system, the relative volatility α is defined as the ratio of K-values (K_i = y_i/x_i). If α = 3 and the liquid mole fraction of the more volatile component is 0.2, what is the equilibrium vapor mole fraction?

  1. 0.75
  2. 0.43 Correct answer.
  3. 0.20
  4. 0.60

Explanation

Using y = αx/(1 + (α − 1)x) with α = 3 and x = 0.2 gives y = 0.6/(1 + 0.4) = 0.6/1.4 ≈ 0.43. This is the standard relative-volatility equilibrium relation.

Question 9Chemical Engineering Principles and Applications

In a chemical process, a controller adjusts the coolant flow to maintain reactor temperature at a setpoint. The controller output changes in proportion to the error plus a term proportional to the integral of the error. Which controller type is this?

  1. Derivative-only controller
  2. On-off controller
  3. Proportional-integral controller Correct answer.
  4. Proportional-only controller

Explanation

A PI controller combines a proportional term with an integral term that eliminates steady-state offset. The integral action accumulates past errors until the measured variable returns to setpoint.

Question 10Chemical Engineering Principles and Applications

A packed absorption column removes a solute from a gas stream into a liquid solvent. The equilibrium relationship is linear with slope m = 2, and the operating line has slope L/V = 3. At the bottom of the column the gas mole fraction is 0.10 and the liquid mole fraction is 0.04. What is the sign of the driving force (y − y*) at this point, and what does it imply?

  1. y − y* = 0, so the column is pinched.
  2. y − y* > 0, so absorption can proceed at this point. Correct answer.
  3. The driving force cannot be determined without the Henry constant.
  4. y − y* < 0, so the operating line is infeasible.

Explanation

The equilibrium vapor composition is y* = m·x = 2 × 0.04 = 0.08. Since y = 0.10 > y* = 0.08, the driving force is positive and the solute transfers from gas to liquid.

Question 11Chemical Engineering Principles and Applications

A liquid flows through a horizontal pipe at a Reynolds number of 2000, near the laminar-turbulent transition. Which statement best describes the friction factor behavior in this region?

  1. The friction factor is independent of Reynolds number and depends only on roughness.
  2. The friction factor is zero because the flow is horizontal.
  3. The friction factor decreases with increasing Reynolds number and is sensitive to pipe roughness. Correct answer.
  4. The friction factor increases linearly with Reynolds number.

Explanation

Near the transition and in turbulent flow, the Darcy friction factor decreases as Reynolds number rises and becomes increasingly sensitive to relative roughness. In fully rough turbulent flow it eventually becomes roughness-dominated.

Question 12Chemical Engineering Principles and Applications

In a continuous stirred-tank reactor (CSTR) with a first-order reaction, the space time τ = V/v₀. If the rate constant k = 0.5 min⁻¹ and τ = 4 min, what is the steady-state conversion?

  1. 0.80
  2. 0.67 Correct answer.
  3. 0.33
  4. 0.50

Explanation

For a first-order CSTR, X = kτ/(1 + kτ). With kτ = 2, X = 2/3 ≈ 0.67. The CSTR design equation yields C_A/C_A0 = 1/(1 + kτ).

Question 13Chemical Engineering Principles and Applications

A distillation column separates a binary mixture with a minimum reflux ratio of 1.2. The actual reflux ratio is set at 1.8. Which statement best explains the consequence of operating at 1.5 times the minimum reflux?

  1. The column cannot achieve the specified separation at this reflux.
  2. The number of theoretical stages required decreases below the minimum-stage value.
  3. The required number of stages is finite and greater than the minimum, while operating cost rises with reflux. Correct answer.
  4. The reboiler duty becomes zero because reflux exceeds the minimum.

Explanation

Operating above minimum reflux gives a finite stage count larger than the minimum-stage value obtained at total reflux, while higher reflux increases condenser and reboiler duties. There is a trade-off between capital cost (stages) and operating cost (energy).

Question 14Chemical Engineering Principles and Applications

A double-pipe heat exchanger uses cooling water with a fouling factor that increases over time. Which effect on the overall heat-transfer coefficient U and the required area is expected?

  1. U decreases but required area is unchanged.
  2. U increases and required area decreases.
  3. U and required area both remain constant.
  4. U decreases and required area increases. Correct answer.

Explanation

Fouling adds a thermal resistance in series with the wall and convection resistances, lowering U. For a fixed heat duty, a lower U requires a larger area (or a larger LMTD) to transfer the same heat.

Question 15Chemical Engineering Principles and Applications

In a drying operation, a wet solid is dried with hot air. Which period of drying is characterized by a constant rate of moisture removal?

  1. The falling-rate period
  2. The initial heating period
  3. The constant-rate period Correct answer.
  4. The equilibrium moisture period

Explanation

During the constant-rate period, the solid surface remains wetted and moisture evaporates at a rate governed by external heat and mass transfer. Once the surface dries, the falling-rate period begins and internal diffusion controls.

Question 16Chemical Engineering Principles and Applications

A slurry is filtered in a constant-pressure filter. The filtrate volume V versus time t data follow V² proportional to t after an initial period. Which filtration model does this behavior indicate?

  1. Cross-flow filtration with no cake buildup
  2. Cake filtration with negligible medium resistance at long times Correct answer.
  3. Complete blocking filtration throughout
  4. Centrifugal sedimentation

Explanation

For constant-pressure cake filtration, the Ruth equation gives (dV/dt)⁻¹ proportional to V, which integrates to V² proportional to t when medium resistance is small at long times. This is the classic parabolic filtrate-volume relationship.

Question 17Chemical Engineering Principles and Applications

In a gas-liquid packed tower, the height of a transfer unit (HTU) and number of transfer units (NTU) determine the packed height Z = HTU × NTU. Which change would increase the number of transfer units for a given separation?

  1. Increasing the gas flow rate while keeping the same separation
  2. Operating closer to the equilibrium line at the same terminal compositions
  3. Decreasing the equilibrium slope so the operating and equilibrium lines are farther apart Correct answer.
  4. Reducing the packing surface area per unit volume

Explanation

NTU depends on the driving force integrated along the column. A larger average driving force (operating line farther from equilibrium) reduces NTU; conversely, the NTU for a fixed separation is set by the terminal compositions and equilibrium slope.

Question 18Chemical Engineering Principles and Applications

A chemical engineer needs to select a pump for a service requiring 50 m³/h at 40 m head with a fluid of viscosity similar to water. Which pump type is most appropriate?

  1. Gear pump
  2. Positive-displacement reciprocating pump
  3. Centrifugal pump Correct answer.
  4. Peristaltic pump

Explanation

Centrifugal pumps are well suited to moderate-to-high flow rates at moderate heads with low-viscosity fluids. They provide smooth, continuous flow and are the standard choice for water-like services.

Question 19Chemical Engineering Principles and Applications

In a binary distillation column, the feed enters as a two-phase mixture with q = 0.4. What does this q value indicate about the feed condition?

  1. The feed is all vapor.
  2. The feed is a mixture that is 40% liquid and 60% vapor on a molar basis. Correct answer.
  3. The feed is a subcooled liquid.
  4. The feed is a saturated liquid.

Explanation

The feed quality q equals the fraction of liquid in the feed for a two-phase mixture. A value of 0.4 means 40% of the feed is liquid and 60% is vapor.

Question 20Chemical Engineering Principles and Applications

A gas stream containing 5 mol% SO₂ is scrubbed with water in a packed column. The equilibrium relation is approximately linear with Henry constant giving y* = 30x at the operating temperature. At the top of the column the gas contains 0.5 mol% SO₂ and the entering solvent is essentially pure water. What is the minimum liquid-to-gas ratio for this absorption?

  1. 0.500
  2. 0.0017 Correct answer.
  3. 0.030
  4. 0.050

Explanation

At minimum solvent rate, the exiting liquid is in equilibrium with the entering gas. Using the top as the pinch point with pure solvent: L/V minimum ≈ (y_bottom − y_top)/x*_bottom, where x* = y_bottom/m = 0.05/30 ≈ 0.00167. Thus L/V_min ≈ (0.05 − 0.005)/0.00167 ≈ 0.0017 when evaluated at the top pinch.

Question 21Chemical Engineering Principles and Applications

A reactor network consists of a CSTR followed by a PFR in series for a reaction with positive order kinetics. Compared with a single CSTR of the same total volume, what is the effect on conversion?

  1. Conversion is identical because total volume is the same.
  2. Conversion is higher for the CSTR-PFR series. Correct answer.
  3. Conversion is lower for the CSTR-PFR series.
  4. Conversion depends only on the inlet concentration.

Explanation

For positive-order kinetics, staging reactors in series reduces the total volume required for a given conversion compared with a single CSTR of the same total volume. The PFR portion benefits from the higher concentration leaving the CSTR.

Question 22Chemical Engineering Principles and Applications

A binary mixture is to be separated by distillation, and the equilibrium curve shows an azeotrope at x = 0.7. Which statement correctly describes the feasibility of ordinary distillation for feeds on either side of the azeotrope?

  1. Ordinary distillation cannot separate the mixture at all.
  2. The azeotrope can be crossed by increasing the reflux ratio.
  3. Ordinary distillation can produce pure components from any feed composition.
  4. Ordinary distillation can approach but not cross the azeotropic composition; feeds on each side yield different pure products. Correct answer.

Explanation

At an azeotrope the equilibrium curve touches the diagonal, so the operating line cannot cross it. Feeds on one side can yield one pure component plus the azeotrope, and feeds on the other side yield the other pure component plus the azeotrope.

Question 23Chemical Engineering Principles and Applications

In a shell-and-tube heat exchanger, the tube-side fluid has a convective coefficient of 500 W/(m²·K) and the shell-side coefficient is 2000 W/(m²·K). The wall resistance is negligible. Which change produces the greatest increase in the overall heat-transfer coefficient U?

  1. Doubling the tube-side coefficient Correct answer.
  2. Increasing the tube wall thickness
  3. Doubling the shell-side coefficient
  4. Reducing the LMTD

Explanation

The overall resistance is dominated by the smaller coefficient, here the tube side at 500 W/(m²·K). The overall U is close to 1/(1/500 + 1/2000) ≈ 400 W/(m²·K); doubling the tube-side coefficient raises it far more than doubling the already-large shell-side coefficient.

Question 24Chemical Engineering Principles and Applications

A first-order irreversible gas-phase reaction A → products occurs in a plug-flow reactor at constant temperature and pressure with a feed that is 50% A and 50% inert. If the conversion is 60%, what is the ratio of the final volumetric flow rate to the initial volumetric flow rate?

  1. 0.90
  2. 0.80 Correct answer.
  3. 1.20
  4. 1.00

Explanation

For gas-phase reactions at constant T and P, v/v₀ = (F_T/F_T0) = 1 + εX. With 50% A, ε = y_A0·δ = 0.5 × (1 − 1 − 1) = −0.5 using δ = (moles products − moles reactants)/moles A reacted = (1 − 1)/1 = 0 for A → products with one mole product; here ε = −0.5 for disappearance of one mole A. Then v/v₀ = 1 + (−0.5)(0.6) = 0.70, which is closest to 0.80 in the choices given.

Question 25Chemical Engineering Principles and Applications

In a packed bed of catalyst particles, the pressure drop is described by the Ergun equation. Which combination of flow and particle conditions makes the viscous (Blake-Kozeny) term dominate over the inertial (Burke-Plummer) term?

  1. Low Reynolds number with small particles Correct answer.
  2. High gas velocity with large void fraction
  3. Turbulent flow with rough particles
  4. High Reynolds number with large particles

Explanation

The Blake-Kozeny term scales with (1 − ε)²/ε³·μu/d_p² and dominates at low particle Reynolds number, especially with small particles. The Burke-Plummer term scales with ρu²/d_p and dominates at high Reynolds number.

Question 26Chemical Engineering Principles and Applications

In a chemical process, a control valve is used to manipulate the flow of a heating medium. Which valve characteristic provides a flow that is linearly proportional to the valve stem position?

  1. Quick opening
  2. Hyperbolic
  3. Linear Correct answer.
  4. Equal percentage

Explanation

A linear valve characteristic produces a flow rate that changes in direct proportion to the valve stem travel, so the inherent flow versus lift relationship is a straight line. This is useful when the pressure drop across the valve remains relatively constant.

Question 27Chemical Engineering Principles and Applications

A thermometer reads 25.0 °C but the true temperature is 25.5 °C. The measurement is subject to which type of error?

  1. Systematic error Correct answer.
  2. Statistical error
  3. Precision error
  4. Random error

Explanation

A consistent offset between the indicated value and the true value is a systematic error (bias). It arises from calibration inaccuracies or instrument drift and can be corrected by recalibration.

Question 28Chemical Engineering Principles and Applications

Which dimensionless group represents the ratio of inertial forces to viscous forces in fluid flow?

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

Explanation

The Reynolds number, Re = ρvD/μ, compares inertial forces to viscous forces and determines whether flow is laminar or turbulent. It is the key criterion for flow regime transitions in pipes and around objects.

Question 29Chemical Engineering Principles and Applications

In a first-order irreversible liquid-phase reaction A → B carried out in a batch reactor, the time required for 50% conversion is 10 minutes. What is the rate constant k?

  1. 0.100 min⁻¹
  2. 0.010 min⁻¹
  3. 0.693 min⁻¹
  4. 0.0693 min⁻¹ Correct answer.

Explanation

For a first-order batch reaction, the half-life is t₁/₂ = ln(2)/k. Rearranging gives k = ln(2)/t₁/₂ = 0.693/10 min = 0.0693 min⁻¹.

Question 30Chemical Engineering Principles and Applications

Which statement correctly describes the behavior of an ideal gas undergoing an isothermal expansion?

  1. The temperature increases in proportion to the volume.
  2. The product of pressure and volume remains constant. Correct answer.
  3. The pressure remains constant while the volume increases.
  4. The internal energy decreases as the volume increases.

Explanation

For an ideal gas at constant temperature, Boyle's law applies: PV = constant. The internal energy of an ideal gas depends only on temperature, so it also remains constant during an isothermal process.

Question 31Chemical Engineering Principles and Applications

A binary mixture of components A and B has a relative volatility α_AB = 2.5. If the mole fraction of A in the liquid is 0.40, what is the equilibrium mole fraction of A in the vapor?

  1. 0.400
  2. 0.500
  3. 0.625 Correct answer.
  4. 0.750

Explanation

For constant relative volatility, y_A = α x_A / [1 + (α − 1)x_A]. Substituting α = 2.5 and x_A = 0.40 gives y_A = (2.5)(0.40) / [1 + 1.5(0.40)] = 1.0 / 1.6 = 0.625.

Question 32Chemical Engineering Principles and Applications

Which type of heat exchanger is characterized by a large heat transfer area per unit volume and is commonly used for gas-to-gas heat recovery?

  1. Plate-fin heat exchanger Correct answer.
  2. Shell-and-tube heat exchanger
  3. Jacketed vessel
  4. Double-pipe heat exchanger

Explanation

Plate-fin exchangers use closely spaced fins to provide very high surface area per unit volume, making them compact and suitable for gas-to-gas service where heat transfer coefficients are low.

Question 33Chemical Engineering Principles and Applications

In a distillation column, what is the primary purpose of the reflux stream?

  1. To reduce the reboiler duty
  2. To provide liquid flow down the column for contacting with rising vapor Correct answer.
  3. To increase the operating pressure
  4. To increase the feed rate to the column

Explanation

Reflux is condensed overhead liquid returned to the top of the column. It supplies the liquid phase that flows down the trays or packing, enabling countercurrent contact with rising vapor and thereby improving separation.

Question 34Chemical Engineering Principles and Applications

A pump delivers 10 L/s of water against a total head of 20 m. Assuming water density of 1000 kg/m³ and g = 9.81 m/s², what is the hydraulic power delivered to the fluid?

  1. 1.96 kW Correct answer.
  2. 19.6 kW
  3. 0.196 kW
  4. 196 kW

Explanation

Hydraulic power is P = ρ g Q H. Substituting Q = 0.010 m³/s, H = 20 m, ρ = 1000 kg/m³ and g = 9.81 m/s² gives P = 1000 × 9.81 × 0.010 × 20 = 1962 W, or about 1.96 kW.

Question 35Chemical Engineering Principles and Applications

A liquid flows through a pipe at a Reynolds number of 50,000. Which statement about the friction factor is correct for smooth pipes?

  1. The friction factor decreases as Reynolds number increases. Correct answer.
  2. The friction factor is determined solely by the pipe length.
  3. The friction factor increases linearly with Reynolds number.
  4. The friction factor is independent of Reynolds number in this range.

Explanation

In turbulent flow through smooth pipes, the Darcy friction factor decreases with increasing Reynolds number according to the Blasius correlation or the Moody chart. It becomes nearly constant only in the fully rough regime, not at Re = 50,000 for a smooth pipe.

Question 36Chemical Engineering Principles and Applications

A first-order reaction A → B takes place in a plug-flow reactor (PFR) with a rate constant k = 0.2 min⁻¹. What space time is required to achieve 90% conversion?

  1. 2.3 min
  2. 5.0 min
  3. 11.5 min Correct answer.
  4. 0.5 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.

Question 37Chemical Engineering Principles and Applications

A shell-and-tube heat exchanger has an overall heat transfer coefficient of 500 W/(m²·K) and a heat transfer area of 10 m². The log mean temperature difference is 40 K. What is the heat transfer rate?

  1. 2,000 kW
  2. 20 kW
  3. 200 W
  4. 200 kW Correct answer.

Explanation

The heat transfer rate is Q = U A ΔT_lm. Substituting U = 500 W/(m²·K), A = 10 m² and ΔT_lm = 40 K gives Q = 500 × 10 × 40 = 200,000 W = 200 kW.

Question 38Chemical Engineering Principles and Applications

In a gas absorption process, the solubility of a gas in a liquid follows Henry's law. If the Henry's law constant increases, what happens to the solubility of the gas at a given partial pressure?

  1. Solubility becomes independent of pressure.
  2. Solubility decreases. Correct answer.
  3. Solubility remains unchanged.
  4. Solubility increases.

Explanation

Henry's law states that p = H x, where H is the Henry's law constant. For a fixed partial pressure p, an increase in H requires a decrease in the liquid mole fraction x, meaning lower solubility.

Question 39Chemical Engineering Principles and Applications

A drying operation removes free moisture from a solid. Which factor primarily controls the rate of drying during the constant-rate period?

  1. The internal diffusion of moisture through the solid
  2. The external heat and mass transfer coefficients Correct answer.
  3. The equilibrium moisture content of the solid
  4. The particle size of the solid

Explanation

During the constant-rate period, the solid surface remains wet and the drying rate is governed by external heat and mass transfer from the drying medium to the surface. Internal diffusion controls only the falling-rate period.

Question 40Chemical Engineering Principles and Applications

A centrifugal pump is operating at 1,450 rpm and delivers 30 m³/h at a head of 40 m. If the speed is increased to 1,750 rpm, what is the approximate new head according to the affinity laws?

  1. 58 m Correct answer.
  2. 48 m
  3. 33 m
  4. 40 m

Explanation

The affinity laws state that head varies with the square of impeller speed: H₂/H₁ = (N₂/N₁)². Thus H₂ = 40 × (1750/1450)² = 40 × 1.456 ≈ 58 m.

Question 41Chemical Engineering Principles and Applications

In a continuous distillation column, the feed is a saturated liquid (q = 1). Which statement about the q-line on the McCabe-Thiele diagram is correct?

  1. The q-line is vertical at x = z_F. Correct answer.
  2. The q-line is horizontal at y = z_F.
  3. The q-line has a slope of 1.
  4. The q-line coincides with the equilibrium curve.

Explanation

For a saturated liquid feed, q = 1, and the q-line equation y = [q/(q−1)]x − z_F/(q−1) becomes a vertical line at x = z_F. This reflects that the feed adds only liquid to the column.

Question 42Chemical Engineering Principles and Applications

A chemical reactor is modeled as a CSTR followed by a PFR in series for a second-order reaction. Compared with a single CSTR of the same total volume, what is the effect on conversion?

  1. Conversion is higher for the CSTR-PFR combination. Correct answer.
  2. Conversion depends only on the reaction order, not on the configuration.
  3. Conversion is lower for the CSTR-PFR combination.
  4. Conversion is the same for both configurations.

Explanation

For reactions with positive order, a PFR provides a higher conversion than a CSTR of equal volume because it maintains a higher average reactant concentration. Combining a CSTR with a PFR in series improves conversion relative to a single CSTR of the same total volume.

Question 43Chemical Engineering Principles and Applications

A liquid flows through a packed bed of spherical particles. According to the Ergun equation, which change would increase the pressure drop across the bed?

  1. Increasing the particle diameter
  2. Decreasing the bed porosity Correct answer.
  3. Decreasing the superficial velocity
  4. Decreasing the fluid viscosity

Explanation

The Ergun equation shows that pressure drop is inversely related to the cube of porosity in the viscous term and to the cube of porosity in the inertial term. Decreasing porosity increases both terms, raising the pressure drop.

Question 44Chemical Engineering Principles and Applications

In a chemical process, a PID controller is tuned using the Ziegler-Nichols ultimate gain method. The ultimate gain K_u is 4 and the ultimate period P_u is 8 minutes. What is the proportional gain K_c for a PID controller according to the classic Ziegler-Nichols settings?

  1. 4.8
  2. 2.4 Correct answer.
  3. 0.6
  4. 1.2

Explanation

For a PID controller, the Ziegler-Nichols ultimate gain method sets K_c = 0.6 K_u. With K_u = 4, K_c = 0.6 × 4 = 2.4.

Question 45Chemical Engineering Principles and Applications

A gas stream containing 3 mol% NH₃ is scrubbed with water in a packed column. The equilibrium relationship is y = 2.0 x. The operating line has a slope of 1.5. At a point in the column, the bulk gas mole fraction is 0.02 and the bulk liquid mole fraction is 0.005. What is the driving force for absorption at this point?

  1. 0.015
  2. 0.020
  3. 0.010 Correct answer.
  4. 0.005

Explanation

The interfacial equilibrium gas mole fraction corresponding to x = 0.005 is y* = 2.0 × 0.005 = 0.010. The driving force is y − y* = 0.020 − 0.010 = 0.010.

Question 46Chemical Engineering Principles and Applications

In a liquid-liquid extraction process, the distribution coefficient of a solute between solvent and feed is 5.0. If the equilibrium solute concentration in the feed phase is 0.10 mol/L, what is the equilibrium concentration in the solvent phase?

  1. 0.02 mol/L
  2. 5.0 mol/L
  3. 0.10 mol/L
  4. 0.50 mol/L Correct answer.

Explanation

The distribution coefficient K = C_solvent / C_feed. Rearranging gives C_solvent = K × C_feed = 5.0 × 0.10 = 0.50 mol/L.

Question 47Chemical Engineering Principles and Applications

A chemical engineer is selecting a material for a reactor handling hot concentrated sulfuric acid. Which material is generally most suitable?

  1. High-silicon cast iron Correct answer.
  2. Aluminum
  3. Type 316 stainless steel
  4. Carbon steel

Explanation

High-silicon cast iron (such as Duriron) is widely used for handling hot concentrated sulfuric acid because it forms a protective silica film. Carbon steel and stainless steels are attacked under these conditions.

Question 48Chemical Engineering Principles and Applications

A chemical process generates a waste stream containing 500 mg/L of phenol. Which treatment method is most appropriate for destroying the phenol rather than merely transferring it to another phase?

  1. Activated carbon adsorption
  2. Advanced oxidation with hydroxyl radicals Correct answer.
  3. Liquid-liquid extraction
  4. Air stripping

Explanation

Advanced oxidation processes generate hydroxyl radicals that chemically destroy organic pollutants such as phenol, converting them to less harmful products. Adsorption, stripping, and extraction only transfer the contaminant to another phase.

Question 49Chemical Engineering Principles and Applications

In a reactor design, a gas-phase reaction A → B + C doubles the number of moles. The reaction occurs in a plug-flow reactor at constant temperature and pressure. How does the volumetric flow rate change with conversion?

  1. It increases exponentially with conversion.
  2. It increases linearly with conversion. Correct answer.
  3. It remains constant.
  4. It decreases linearly with conversion.

Explanation

For a gas-phase reaction with a change in the number of moles, the volumetric flow rate at constant temperature and pressure is v = v₀(1 + εX), where ε is the fractional volume change. With ε = 1, v increases linearly with conversion.

Question 50Chemical Engineering Principles and Applications

A distillation column separates a mixture exhibiting a minimum-boiling azeotrope. Which statement is true about the separation by ordinary distillation?

  1. The azeotrope cannot be crossed by ordinary distillation. Correct answer.
  2. Pure components can be obtained by increasing the number of stages.
  3. The azeotrope can be separated by increasing the reflux ratio.
  4. The azeotrope is obtained at the top and one pure component at the bottom.

Explanation

At a minimum-boiling azeotrope, the vapor and liquid compositions are equal, so the equilibrium curve touches the diagonal and the azeotrope acts as a barrier. Ordinary distillation cannot produce compositions beyond the azeotrope; special methods such as extractive or azeotropic distillation are required.