Free Mass Transfer Operations MCQs with Answers

• A. 2000
• B. 4000
• C. 5000
• D. 6000

Explanation: The material contains 100 kg of dry solid, so its exposed area is 100 × 0.04 = 4 m².

• A. Nusselt number
• B. Sherwood number
• C. Stanton number
• D. Schmidt number

Explanation: The Schmidt number, Sc = ν/D, compares momentum diffusivity with mass diffusivity.

• A. Super-saturation
• B. Viscosity
• C. Concentration
• D. Density

Explanation: Crystal growth requires a supersaturated solution, which provides the chemical-potential driving force for solute molecules to deposit on…

• A. High pressure
• B. Vacuum
• C. High residence time
• D. None of these

Explanation: Vacuum evaporation lowers the boiling temperature, allowing concentration at a lower thermal load and reducing damage to heat-sensitive…

• A. Facilitate draining out liquid from a tray when the unit is not in operation
• B. Are normally located near the overflow weir so that any delivery of liquid during operation
• C. follows approximately the same path as the overflow fluid
• D. Must be large enough (usually 1/4″ to 5/8″ dia) to prevent plugging but should not deliver
• E. All A., B. and C.
• F. excessive amount of fluid during operation

Explanation: Weep holes drain liquid from trays when the column is shut down and are placed and sized to avoid undesirable liquid bypass during…

• A. 0
• B. ∞
• C. > 1
• D. <1

Explanation: For the rectifying section, the operating-line slope is R/(R+1), where R is the reflux ratio.

• A. Murphree plate efficiency
• B. Overall plate efficiency
• C. Point efficiency
• D. None of these

Explanation: Murphree plate efficiency can exceed 100% because the exiting vapor or liquid may reach a composition beyond the equilibrium value used in…

• A. Providing a higher skirt clearance or a higher weir
• B. All A., B. and C.
• C. velocity of liquid flow past the caps or by reducing the distance along the tray through which
• D. the liquid must flow
• E. Using split flow, radial flow or cascade flow for column diameter large than 4 ft
• F. Decreasing the number of rows of caps through which the liquid flows or by decreasing the

Explanation: Liquid gradient is reduced by lowering liquid resistance and travel distance, for example by reducing cap rows, liquid velocity, or flow…

• A. 0 to 1.5
• B. 0 to 0.8
• C. 0 to 0.08
• D. 0 to 2.2

Explanation: Counter-current rotary dryers are given only a slight slope, typically about 0 to 0.08 m per metre of dryer length, so solids move…

• A. Cross
• B. Split
• C. Cascade
• D. Reverse

Explanation: Cross-flow trays give effective contact because liquid travels across the tray while vapour passes upward through the liquid, with a…

• A. Lower limit of vapour velocity
• B. Upper limit of the vapour velocity
• C. Hole diameter of the sieve tray
• D. Height of the plate spacing

Explanation: Sieve-tray diameter is selected from the allowable vapour velocity, especially its upper limit, which is governed by entrainment and…

• A. 5.5
• B. 12.5
• C. 30.5
• D. 45.5

Explanation: The total liquid-level difference caused by gradient across a tray should generally be limited to about 12.5 mm.

• A. 19
• B. 38
• C. 67
• D. 95

Explanation: A clearance of about 38 mm between the column wall and the nearest bubble cap is normally maintained to allow proper liquid and vapour…

• A. 0.25
• B. 0.50
• C. 0.75
• D. 1.25

Explanation: In a plate-and-frame filter, washing is commonly carried out at about one-fourth of the final filtration rate because the wash liquid must…

• A. 250 to 500
• B. 100 to 150
• C. 50 to 100
• D. 600 to 700

Explanation: Rotary vacuum filters normally operate with a vacuum corresponding to roughly 250 to 500 mm Hg differential pressure.

• A. Sensible heat differences are small, because the temperature changes from tray to tray is small
• B. All A., B. and C.
• C. Liquid/vapor loading across the column remains constant
• D. Trouton's rule is applicable

Explanation: The McCabe-Thiele method uses constant molal overflow: sensible-heat effects are neglected, vapour and liquid molar flow rates remain…

• A. 2.5″
• B. 50% of the plate spacing
• C. 5″
• D. 0.5″

Explanation: A bubble-cap tray commonly maintains about 0.5 inch of static liquid submergence over the cap slots at atmospheric pressure.

• A. 3 to 12.5
• B. 4 to 8
• C. 1 to3
• D. 12.5 to 18.5

Explanation: Sieve-tray perforations commonly fall in the range of about 3 to 12.5 mm.

• A. is more accurate in finding the number of equilibrium stages
• B. All A, B. & C.
• C. Facilitates direct calculation of heat load on reboiler & condenser from, the diagram used in
• D. this method
• E. accounts for the enthalpy changes in the process

Explanation: Ponchon-Savarit uses enthalpy-composition data, so it includes heat effects, gives a more rigorous stage calculation, and permits direct…

• A. 0
• B. ∞
• C. > 1
• D. < 1

Explanation: The stripping-section operating-line slope is normally written as the liquid-to-vapour flow ratio, L′/V′, and is conventionally greater…