Free Mass Transfer Operations MCQs with Answers

• A. Concentrated solution and non-isothermal operation
• B. Dilute solution and non-isothermal operation
• C. Dilute solution and isothermal operation
• D. Concentrated solution and isothermal operation

Explanation: For dilute mixtures under isothermal conditions, the equilibrium relation is approximately linear and the operating-line material balance…

• A. Drying cost per unit weight of dried product is less
• B. Product with more uniform moisture content is produced
• C. Equipment size required is smaller
• D. All A , B. & C.

Explanation: Continuous dryers generally reduce drying cost through steadier operation, provide more uniform product moisture, and require less…

• A. Nature of the vapor
• B. None of these
• C. Composition of the mixture
• D. Temperature of the mixture

Explanation: Relative saturation is the ratio of actual vapour partial pressure to saturation vapour pressure at the same temperature.

• A. T2
• B. √T
• C. T
• D. 1/T

Explanation: For liquids, the simplified temperature dependence of diffusivity is D proportional to T, although the complete relation also includes the…

• A. Fixed bed absorption
• B. Moving bed absorption
• C. Fixed bed adsorption
• D. Moving bed adsorption

Explanation: Hypersorption is an adsorption process carried out with a moving bed of adsorbent, allowing near-continuous solids circulation and…

• A. Unbound
• B. Bound
• C. Critical
• D. Free

Explanation: Equilibrium moisture is the amount retained when the solid is in equilibrium with the surrounding gas; moisture above this value can be…

• A. Extractive
• B. Azeotropic
• C. Molecular
• D. Flash

Explanation: Extractive distillation adds a relatively nonvolatile solvent that changes component relative volatilities without necessarily forming a…

• A. 0.31
• B. 0.107
• C. 0.017
• D. 0.215

Explanation: Humid heat is the heat capacity of dry gas plus the vapour contribution: c_H = c_g + Hc_v = 0.2 + (0.01)(1.5) = 0.215 kJ/(kg dry gas·K).

• A. 100
• B. None of these
• C. < 100
• D. > 100

Explanation: With a long liquid flow path, the liquid composition changes substantially across the tray, so the reference equilibrium vapour…

• A. Uses molal units for material and energy balance
• B. Uses weight fractions to express liquid and vapour composition
• C. Can use any type of units
• D. Is more accurate than Ponchon-Savarit method

Explanation: The McCabe-Thiele construction is based on mole balances and the constant-molal-overflow assumption, so liquid and vapour compositions are…

• A. 1
• B. 0.24
• C. 3.97
• D. 600

Explanation: For air-water vapour, thermal and mass diffusivities are of similar magnitude, so Sc and Pr are both close to unity.

• A. And humidified
• B. And dehumidified with increase in wet bulb temperature
• C. And dehumidified with decrease in wet bulb temperature
• D. At the same relative humidity

Explanation: Because the entering dew point is 18°C and the spray water is at 15°C, the air is cooled below its dew point and water condenses from it.

• A. Azeotropic mixture is
• B. More
• C. Same
• D. Less
• E. Either more or less; depends on the system

Explanation: At an azeotrope, the equilibrium liquid and vapour compositions are identical, so the mixture distils without composition change.

• A. None of these
• B. Moles transferred/ [(time) (area) (mass A/mass B)]
• C. Moles transferred/ [(time) (area) (pressure)]
• D. Moles transferred/ [(time) (area) (mole fraction)]

Explanation: A mass-transfer coefficient has units of molar flux divided by a concentration, mole-fraction, mass-ratio, or pressure driving force.

• A. 1
• B. ∞
• C. 0.5
• D. 2

Explanation: In equimolal counterdiffusion, the molar fluxes satisfy N_A = -N_B, so the total molar flux N_A + N_B is zero.

• A. High pressure
• B. Low pressure
• C. Extractive
• D. Steam

Explanation: Rose oil is volatile and heat-sensitive, so it is recovered by steam distillation at a temperature lower than its normal boiling point.

• A. Decreases tray efficiency
• B. Increases tray efficiency
• C. Reduces pressure drop
• D. Is desirable, as it provides improved vapour-liquid contact

Explanation: Back-trapping causes liquid or vapour to move undesirably back to a preceding tray, increasing mixing and reducing the effective driving…

• A. Cracks develop on the surface of the solid
• B. Surface evaporation of unbound moisture occurs
• C. Rate of drying decreases abruptly
• D. None of these

Explanation: During the constant-rate period, the exposed surface remains saturated with unbound moisture, so evaporation occurs at an essentially…

• A. 0 to 0.5
• B. 0 to 3
• C. 1.25 to 2
• D. 5 to 15

Explanation: An absorption factor of about 1.25 to 2 gives a practical balance between solvent circulation and absorber size.

• A. Isothermal operation
• B. Dilute solutions
• C. None of these
• D. Dilute solutions and isothermal operation

Explanation: For dilute solutions at constant temperature, the equilibrium relation is approximately linear, such as y* = mx, and constant physical…