Free Mass Transfer Operations MCQs with Answers

• A. Adsorption
• B. Humidification
• C. Gas absorption
• D. Binary distillation

Explanation: The Lewis number compares thermal diffusivity with mass diffusivity and is important in simultaneous heat and mass transfer.

• A. No reflux
• B. Total reflux
• C. Zero reflux ratio
• D. Minimum reflux ratio

Explanation: At total reflux, all overhead liquid is returned and no product is withdrawn, giving the maximum separation per theoretical stage.

• A. √T
• B. T1.5
• C. T
• D. T3

Explanation: During the constant-rate period, the drying rate per exposed area is essentially constant, so drying time is proportional to the amount of…

• A. Tray
• B. Rotary
• C. Fluidised bed
• D. None of these

Explanation: Tray dryers can handle sticky or pasty materials because the material can remain on stationary trays without needing continuous mixing or…

• A. Dry those materials which have very high unbound moisture content
• B. Reduce drying temperature
• C. Increase drying temperature
• D. Dry materials having high bound moisture content

Explanation: Vacuum lowers the boiling point of moisture, allowing evaporation at a lower temperature.

• A. 0.0
• B. 0.2
• C. 0.4
• D. 1.0

Explanation: At equilibrium, the remaining air-phase water is 100C, while the gel holds 10(C/0.0667) kg.

• A. Increases
• B. Depends on the pressure of the system
• C. Remains unchanged
• D. Decreases

Explanation: Axial mixing causes back-mixing, reducing the concentration-driving force toward the ideal counter-current profile.

• A. Vapour is in equilibrium with the liquid at the gas temperature
• B. Vapour is in equilibrium with the liquid at the room temperature
• C. Partial pressure of vapour equals the vapour pressure of the liquid at room temperature
• D. None of these

Explanation: A saturated gas contains vapour at equilibrium with the liquid phase at the gas temperature, so its vapour partial pressure equals the…

• A. Spray the liquid on gas stream
• B. Bubble the gas through liquid
• C. Either A. or B. would suffice
• D. None of these

Explanation: A highly soluble gas is efficiently contacted by dispersing it as bubbles through the liquid, giving repeated gas-liquid contact and rapid…

• A. Parallel
• B. Straight
• C. Both parallel and straight
• D. Neither parallel nor straight

Explanation: The Ellis distillation approximation treats the vapour and liquid enthalpy-concentration relationships as straight, parallel lines.

• A. Viscosity with concentration
• B. Both A. and B.
• C. Degree of ideality of the solution
• D. Neither A. nor B.

Explanation: In concentrated solutions, diffusivity is affected by both concentration-dependent viscosity and departures from ideal solution behaviour.

• A. Pressure
• B. Temperature
• C. None of these
• D. Nature of the components

Explanation: Gas-phase binary diffusivity varies with pressure, temperature and the identities or molecular properties of both components.

• A. Molecular velocity only
• B. Pore radius of the catalyst only
• C. Molecular mean free path only
• D. Molecular velocity and pore radius of the catalyst

Explanation: Knudsen diffusivity is controlled by molecule-wall collisions and is proportional to the pore radius multiplied by the molecular thermal…

• A. The ratio of number of ideal plates to actual plates
• B. The ratio of number of actual plates to ideal plates
• C. Same as the Murphree efficiency
• D. Always more than the point efficiency

Explanation: Overall efficiency compares the number of ideal stages required with the actual stages installed, so it is ideal plates divided by actual…

• A. Low reflux ratio
• B. High reflux ratio
• C. Small cross-section column
• D. Less number of trays

Explanation: Low relative volatility makes separation difficult because the vapour composition differs only slightly from the liquid composition.

• A. Above which it will always become damp
• B. Below which it will always stay dry
• C. Both A. and B.
• D. Above Which it will always become dry and below which it will always stay damp

Explanation: Critical humidity is the boundary humidity: above it, the salt tends to absorb moisture and become damp, while below it, the salt remains…

• A. Spray chamber
• B. Induced draft cooling tower
• C. Natural draft cooling tower
• D. None of these

Explanation: A spray chamber brings recirculated liquid into direct contact with air, allowing simultaneous evaporation and sensible cooling under…

• A. Changes the relative volatility of the original components
• B. Should be of high volatility
• C. Should form azeotropes with the original components
• D. All A., B. and C.

Explanation: An extractive-distillation solvent is added to alter the relative volatility of the original components and make their separation easier.

• A. Relative humidity
• B. Wet bulb temperature
• C. Partial pressure of vapour
• D. None of these

Explanation: Ideal evaporative cooling follows an approximately adiabatic saturation path, along which the wet-bulb temperature remains constant.

• A. Increases with increase in
• B. Is one at unity value of
• C. Both A. and B.
• D. Neither A. nor B.

Explanation: For straight equilibrium and operating lines, HETP/HTUOG is proportional to A ln A/(A−1), where A is the absorption factor.