# Mass Transfer Questions and Answers

## Multiple Choice Questions (MCQ) and Answers on Mass Transfer

Question 1 : In distillation columns, the number of bubble caps per tray primarily depends upon the

1. allowable liquid velocity.
2. allowable gas velocity.
3. allowable gas and liquid velocities.
4. feed composition.

Question 2 : In distillation, overhead product contains

1. only one component
2. two components
3. any number of components
4. only saturated liquid

Question 3 : With increase in pressure, the relative volatility for a binary system

1. increases.
2. decreases.
3. remains same.
4. either (1) or (2), depends on the system.

Question 4 : Low viscosity absorbent is preferred for reasons of

1. rapid absorption rates and good heat transfer characteristics.
2. improved flooding characteristics.
3. low pressure drop on pumping.
4. all (a), (b) and (c).

Question 5 : Mass transfer co-efficient (K) and diffusivity (D) are related according to film theory as

1. K ∝ D
2. K ∝
$\sqrt{D}$
3. K ∝ D1.5
4. K ∝ D2

Question 6 : Which of the following liquid-vapor contacting devices provides maximum contact surface area for a particular duty ?

1. Sieve plate column
2. Bubble cap column
3. Randomly packed column
4. Wetted wall column

Question 7 : The equipment frequently used for adiabatic humidification-cooling operation with recirculating liquid is

1. natural draft cooling tower
2. induced draft cooling tower
3. spray chamber
4. none of these

Question 8 : Chemisorption (chemical adsorption) is

1. same as “Van der Waals” adsorption.
2. characterised by adsorption of heat.
3. an irreversible phenomenon.
4. a reversible phenomenon.

Question 9 : The rate of solid-liquid extraction is limited by the

1. phase change of the solute as it dissolves in the solvent.
2. diffusion of the solute through the solvent in the pores to the outside of the particle.
3. diffusion of the solute from the solution in contact with the particle to the main bulk of the solution.
4. all ‘1’, ‘2’ & ‘3’.

Question 10 : Raoult’s law applies to the

1. all liquid solutions
2. non-ideal solution only.
3. non-volatile solute.
4. solvents.

Question 11 : Make up water is required in a cooling tower to replace the water lost by

1. evaporation
2. drift
3. blowdown and leakage
4. all (a), (b) and (c)

Question 12 : Which of the following quantities need not be fixed before starting the design of a co-current absorber ?

1. Heat gain or loss
2. Flow rate of entering liquid
3. Flow rate of gas
4. None of these

Question 13 : Mass transfer co-efficient of liquid is

1. affected more by temperature than that for gases.
2. affected much less by temperature than that for gases.
3. not affected by the temperature.
4. none of these.

Question 14 : Absorption accompanied with chemical reaction is exemplified by the absorption of

1. ammonia in water.
2. benzol present in coke oven gas by wash oil.
3. SO2 in alkaline solution.
4. all (a), (b) and (c).

Question 15 : Which of the following unit operations makes use of Thiele-Geddes equation ?

1. Liquid-liquid extraction
2. Solid-liquid extraction
3. Multicomponent absorption with chemical reaction
4. Multicomponent distillation.

Question 16 : As the reflux ratio in a distillation coloumn is increased from the minimum, the

1. slope of the operating line in stripping section decreases.
2. number of plates decreases very slowly first and then more and more rapidly.
3. total cost first decreases and then increases.
4. liquid flow increases while the vapor flow decreases for a system.

Question 17 : Absorption factor is defined as (where, S1 = slope of the operating line S2 = slope of the equilibrium curve)

1. S2/S1
2. S1/S2
3. S1 – S2
4. S1 x S2

Question 18 : A plait point is the point on the solubility curve, where the tie line reduces to a point. What is the number of plait point for a ternery system containing twopairs of partially miscible liquids ?

1. 0
2. 1
3. 2
4. 3

Question 19 : Which of the following remains constant during sensible cooling process ?

1. Specific humidity
2. Partial pressure of vapour
3. Both (1) and (2)
4. Neither (1) nor (2)

Question 20 : Physical absorption is

1. an irreversible phenomenon.
2. a reversible phenomenon.
3. accompanied by evolution of heat.
4. both (2) and (3)

Question 21 : The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are same for, NPr = NSc = __________

1. 1
2. 0.5
3. 10
4. 0

Question 22 :Dew point of a gas-vapour mixture

1. increases with temperature rise.
2. decreases with temperature rise.
3. decreases with decrease in pressure.
4. increases with increase in pressure.

Question 23 : In a packed absorption tower, if the equilibrium and operating lines are both straight lines, then the ratio, HETP/HTUOG __________ the absorption factor.

1. increases with increase in
2. is one at unity value of
3. both (1) and (2)
4. neither (1) nor (2)

Question 24 :  Which of the following plays an important role in problems of simultaneous heat and mass transfer ?

1. Lewis number
2. Schmidt number
3. Prandtl number
4. Sherwood number

Question 25 : In case of an absorber, the operating

1. line always lies above the equilibrium curve.
2. line always lies below the equilibrium curve.
3. line can be either above or below the equilibrium curve.

Question 26 : Relative humidity is the ratio of the

1. partial pressure of the vapour to the vapour pressure of the liquid at room temperature.
2. partial pressure of the vapour to the vapour pressure of the liquid at gas temperature.
3. actual humidity to saturation humidity.
4. none of these

Question 27 : In batch distillation with constant reflux, overhead product composition __________ with time.

1. increases
2. decreases
3. does not vary
4. may increase on decrease, depends on the system.

Question 28 : Psychrometry deals with the properties of gas-vapor mixture. Humidity can be determined by the measurement of the __________ of a fibre.

1. electrical resistance
2. thermal conductivity
3. strength
4. none of these.

Question 29 : Heat in BTU necessary to increase the temperature of 1 lb of gas and its accompanying vapour by 1°F is called the

1. latent heat
2. humid heat
3. specific heat
4. sensible heat

Question 30 : To increase the absorption factor, (where, G = gas flow rate, S = solvent flow rate)

1. increase both ‘G‘ and ‘S‘.
2. decrease both ‘G‘ and ‘S‘.
3. increase ‘S‘ and decrease ‘G‘.
4. increase ‘G‘ and decrease ‘S

Question 31 : Drying operation under vacuum is carried out to

1. dry those materials which have very high unbound mositure content.
2. reduce drying temperature.
3. increase drying temperature.
4. dry materials having high bound moisture content.

Question 32 : Air initially at 101. 3 kPa and 40°C and with a relative humidity of 50%, is cooled at constant pressure to 30°C. The cooled air has a

1. higher dew point
2. higher absolute (specific) humidity.
3. higher relative humidity
4. higher wet bulb temperature.

Question 33 : Separation of two or more components of a liquid solution can not be achieved by

1. fractional crystallisation
2. liquid extraction
3. absorption
4. evaporation

Question 34 : Stefan’s law describes the mass transfer by

1. diffusion
2. bulk flow
3. both ‘a’ & ‘b’
4. neither ‘a’ nor ‘b’

Question 35 : At minimum reflux ratio for a given separation

1. number of plates is zero.
2. number of plates is infinity.
3. minimum number of the theoretical plates is required.
4. separation is most efficient.

Question 36 : Pick out the correct statement.

1. In case of liquid-liquid extraction, no separation is possible, if the selectivity of the solvent used is unity.
2. With increase in temperature, the selectivity of the solvent used in solvent extraction decreases.
3. The selectivity of solvent used in solvent extraction is unity at the plait point.
4. all (1), (2) and (3).

Question 37 : __________ columns are used for liquid dispersion in a continuous gas phase.

1. Packed
2. Pulse
3. Bubble cap
4. Sieve plate

Question 38 :  Overall efficiency of the distillation column is

1. the ratio of number of ideal plates to actual plates.
2. the ratio of number of actual plates to ideal plates.
3. same as the Murphree efficiency.
4. always more than the point efficiency.

Question 39 : On addition of solute in the solvent, the __________ of the solution decreases.

1. boiling point
2. freezing point
3. vapour pressure
4. both (2) and (3)

Question 40 : Which is the controlling factor for a drum drier?

1. Diffusion
2. Heat transfer
3. Both (1) and (2)
4. Neither (1) nor (2)

Question 41 : Total reflux in a distillation operation requires minimum

2. number of plates
4. all (1), (2) and (3)

Question 42 : The amount of steam required per unit quantity of distillate in case of steam distillation will be reduced by

1. raising the temperature.
2. lowering the total pressure.
3. both (1) and (2)
4. neither (1) nor (2).

Question 43 : The difference of wet bulb temperature and adiabatic saturation temperature of unsaturated mixture of any system is

1. +ve
2. -ve
3. zero
4. none of these

Question 44 : Fenske’s equation for determining the minimum number of theoretical stages in distillation column holds good, when the

1. relative volatility is reasonably constant.
2. mixture (to be separated) shows negative deviation from ideality.
3. mixture (to be separated) shows positive deviation from ideality.
4. multicomponent distillation is involved.

Question 45 : When the temperature and humidity of air is low, we usually use __________ draft cooling tower.

1. natural
2. forced
3. induced
4. none of these

Question 46 : Mass transfer rate between two fluid phases does not necessarily depend on the __________ of the two phases.

1. chemical properties
2. physical properties
3. degree of turbulence
4. interfacial area

Question 47 : In a solution containing 0.30 Kg mole of solute and 600 kg of solvent, the molality is

1. 0.50
2. 0.60
3. 2
4. 1

Question 48 :  Fenske equation determines the

1. maximum number of ideal plates.
2. height of the distillation column.
3. minimum number of theoretical plates.
4. optimum reflux ratio.

Question 49 : The term “cooling range” in a cooling tower refers to the difference in the temperature of

1. cold water leaving the tower and the wet bulb temperature of the surrounding air.
2. hot water entering the tower and the wet bulb temperature of the surrounding air.
3. hot water entering the tower and the cooled water leaving the tower.
4. none of these