Free Mass Transfer Operations MCQs with Answers

• A. Thermal diffusivity to the mass diffusivity
• B. Momentum diffusivity to the mass diffusivity
• C. Momentum diffusivity to the thermal diffusivity
• D. None of these

Explanation: The Schmidt number is Sc = ν/DAB = (μ/ρ)/DAB, so it is the ratio of momentum diffusivity to mass diffusivity.

• A. Liquid is stripped of high boiler
• B. Vapour is stripped of low boiler
• C. Liquid is enriched with high boiler
• D. None of these

Explanation: In the stripping section, the liquid flows downward while the more volatile component is removed into the rising vapour.

• A. The pressure drop and hence the pumping cost will be very high
• B. It cannot be recovered by distillation
• C. It will decompose while recovering by distillation
• D. Its recovery cost by distillation may be prohibitively high

Explanation: A solvent with high latent heat requires substantial energy for vaporisation during distillation recovery.

• A. Increased capacity of the absorber
• B. Decrease in equilibrium solubility
• C. Increase in equilibrium solubility
• D. None of these

Explanation: Heat evolution raises the liquid temperature, and gases generally become less soluble as temperature increases.

• A. Mass transfer is accompanied by reaction
• B. Diffusion of A is in stagnant medium B
• C. There is equimolar counter diffusion
• D. Molecular mean free path is high

Explanation: The stationary-point flux equals the molecular diffusion flux when there is no net bulk molar movement.

• A. Increases Linearly
• B. Decreases
• C. Remain unaffected
• D. Increases

Explanation: Increasing temperature usually improves leaching by increasing solute solubility and molecular diffusivity while reducing liquid…

• A. Increase in pressure drop
• B. Decrease in pressure drop
• C. Decrease in liquid hold up in the column
• D. Increase in Murphree plate efficiency

Explanation: Flooding causes excessive liquid accumulation and resistance to vapour flow in the column.

• A. Dissolving gold from ores
• B. Removing nicotine from its water solution by kerosene
• C. Dissolving sugar from the cells of the beet
• D. Dissolving pharmaceutical products from bark or roots

Explanation: Leaching removes a soluble constituent from a solid using a liquid solvent, as in ores, roots, and beet cells.

• A. 1
• B. 0.5
• C. 2
• D. 8

Explanation: A standard Raschig ring is approximately a short cylindrical ring whose height equals its diameter.

• A. May increase or decrease; depends on individual design
• B. Decreases
• C. Remain same
• D. Increases

Explanation: Bubble-cap trays retain their liquid seal at reduced vapour rates, while lower throughput reduces entrainment and can improve tray…

• A. Calculated
• B. Known
• C. Unknown
• D. Not required

Explanation: In a wetted-wall tower, the liquid film covers a geometrically measurable wall surface.

• A. D0.5
• B. D2
• C. D
• D. 1/D

Explanation: In film theory, k is approximately D divided by the film thickness, k = D/δ, so it is directly proportional to molecular diffusivity.

• A. Concentration of liquid and vapour leaving a plate will be same
• B. Reflux ratio will be maximum
• C. None of these
• D. Number of plates required will be maximum

Explanation: At minimum reflux, the operating and equilibrium lines pinch, so an infinite number of ideal stages would be required for the specified…

• A. Force
• B. Pressure
• C. Concentration
• D. Thermal

Explanation: Thermal diffusion separates isotopes because the components respond differently to a temperature gradient due to their mass difference.

• A. Increases
• B. Decreases
• C. Remain unaffected
• D. Passes through a maximum

Explanation: Increasing reflux increases the internal liquid and vapour flow and moves the operating lines closer to the desired high-purity limit.

• A. Reducing the total pressure
• B. Both B. and C.
• C. Reducing the temperature
• D. Increasing the total pressure

Explanation: Relative saturation is the vapour partial pressure divided by the saturation pressure at the same temperature.

• A. Substances from non-volatile impurities
• B. Volatile impurity from still higher boiling substances
• C. Both A. and B.
• D. Neither A. nor B.

Explanation: Steam distillation allows a high-boiling, steam-volatile substance to distil below its normal boiling point, separating it from…

• A. Molar heats of vaporisation of components are nearly equal
• B. Linear crystal growth rate depends on the degree of super-saturation
• C. Linear crystal growth rate depends on the crystal size
• D. Linear crystal growth rate does not depend on the crystal size

Explanation: McCabe’s ΔL law states that, at a given supersaturation, the linear crystal growth rate is independent of crystal size.

• A. Avoid channelling
• B. Reduce the overall pressure drop
• C. Reduce liquid hold-up
• D. Avoid flooding

Explanation: Very tall packed towers are split into beds with liquid redistributors so that liquid is spread uniformly again.

• A. Reverse osmosis
• B. Thermal diffusion
• C. Leaching
• D. Solvent extraction

Explanation: Helium isotopes can be separated by thermal diffusion because the lighter and heavier isotopes diffuse differently under a temperature…