Free Mass Transfer Operations MCQs with Answers

• A. 90
• B. 91.5
• C. 95
• D. 99

Explanation: The atmospheric-pressure ethanol-water azeotrope is commonly approximated as 95% alcohol by volume, which prevents ordinary distillation…

• A. 15 min
• B. 51 min
• C. 71 min
• D. 94 min

Explanation: During the falling-rate period, drying time is proportional to (Xc − Xe) ln[(Xc − Xe)/(X − Xe)].

• A. Material to be distilled is thermally unstable or has the tendency to react with other
• B. Liquids decompose, if distilled directly at atmospheric pressure
• C. Material cannot be distilled by indirect heating even under low pressure, because of the high
• D. boiling temperature
• E. Azeotropic mixture is to be separated and the final product is miscible with water
• F. components associated with it, at the boiling temperature

Explanation: Steam distillation is useful for thermally sensitive or water-immiscible materials because the mixture boils below the normal boiling…

• A. Decrease in air temperature
• B. Increase in air humidity
• C. Neither A. nor B.
• D. Both A. and B.

Explanation: The constant-rate drying rate depends on the external heat and mass-transfer driving forces rather than the solid's moisture content.

• A. Height < 6
• B. Both A. & B.
• C. Diameter < 0.6
• D. Neither A. nor B.

Explanation: Packed distillation columns are generally used for relatively small columns, commonly limited to about 0.6 m diameter and 6 m height in…

• A. Solids are drained to the same extent
• B. Both A. & B.
• C. Ratio of the insoluble to solution is constant
• D. Neither A. nor B.

Explanation: Constant underflow means that the amount of solution retained with the insoluble solids remains constant per unit mass of solids.

• A. High evaporation losses of water
• B. Difficulty in its fabrication
• C. High pressure drop of the gas
• D. None of these

Explanation: Bubble-cap trays provide effective gas-liquid contact but impose a relatively high pressure drop because gas must pass through the liquid…

• A. (1 - Ri) = Ro/Ri
• B. (1 - Ro) = Ro/Ri
• C. (1 + Ri) = Ro/Ri
• D. (1 + Ro) = Ro/Ri

Explanation: For a total condenser, the internal reflux ratio is Ri = Ro/(1 + Ro), where Ro is the external reflux ratio.

• A. Both A. and B.
• B. Humidity at constant temperature
• C. Temperature at constant humidity
• D. Neither A. nor B.

Explanation: Humid volume increases with temperature because the gas expands, and it also increases with humidity because water vapour adds volume per…

• A. Using higher reflux ratio
• B. Use of increased tray spacing
• C. Increasing the liquid flow rate
• D. Increasing the vapour flow rate

Explanation: Increasing tray spacing provides more disengagement space and permits a higher allowable vapour velocity before entrainment becomes…

• A. Hydrochloric acid
• B. Nitric acid
• C. Alkalis
• D. Sulphuric acid

Explanation: Porcelain is resistant to many acids, including hydrochloric and nitric acids, but concentrated alkalis attack silica-containing ceramic…

• A. Decreases with temperature
• B. Increases with temperature
• C. May increase or decrease with temperature
• D. Is independent of temperature

Explanation: Liquid molecular diffusivity generally increases with temperature because molecular motion and molecular mobility increase.

• A. 100
• B. 50
• C. 0
• D. Between 0 and 100

Explanation: Humidity is related to the amount of water vapour present, often through its partial pressure.

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

Explanation: A non-hygroscopic solid does not retain moisture strongly in its structure, so its moisture is classified as unbound moisture.

• A. 1
• B. ∞
• C. 0.5
• D. 2

Explanation: For diffusion of A through stagnant, non-diffusing B, NB = 0. Thus NA/(NA + NB) = NA/NA = 1.

• A. High temperature distillation
• B. Both B. and C.
• C. Very low pressure distillation
• D. For heat-sensitive materials

Explanation: Molecular distillation operates at extremely low pressure, giving a very short mean free path and allowing heat-sensitive materials to be…

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

Explanation: Dividing a tall packed tower allows liquid to be collected and redistributed between sections, preventing maldistribution and channel…

• A. Absorption
• B. Continuous distillation
• C. Evaporation
• D. Solvent extraction

Explanation: Acetic acid is present in dilute aqueous solution, and distilling it from water is relatively energy-intensive because of their volatility…

• A. Truck drier
• B. Fluidised bed drier
• C. Agitated pan drier
• D. Vacuum shelf drier

Explanation: Truck, agitated-pan, and vacuum-shelf driers are commonly operated batchwise.

• A. 2
• B. 0.5
• C. 50
• D. 150

Explanation: The reflux ratio is defined as reflux returned to the column divided by distillate withdrawn: R = L/D = 100/50 = 2.