IndiaBIX Chemical Engineering: Mass Transfer Section 7

Question 1

Perforated plate towers are unique for solvent extraction because, they provide
higher contact area.
better contact of the phase.
repeated coalescence & redispersion of the drop phase.
none of these.

Answer 1

repeated coalescence & redispersion of the drop phase.

Question 2

In case of absorption & stripping, the interface of the liquid & gas phases are present in equilibrium, when the diffusional resistance of __________ is zero.
interface
gas phase
liquid phase
all ‘a’, ‘b’ & ‘c’

Answer 2

interface

Question 3

Pick out the wrong statement pertaining to the analogy between equations of heat and mass transfer operations.
Sherwood number in mass transfer is analogous to Nusselt number in heat transfer.
Prandtl number in heat transfer is analogous to Schmidt number in mass transfer.
Reynolds number in mass transfer is analogous to Grashoff number in heat transfer.
Reynolds number remains the same in both heat and mass transfer.

Answer 3

Reynolds number in mass transfer is analogous to Grashoff number in heat transfer.

Question 4

The slope of the operating line for a single component co-current absorber when plotted in terms of mole ratio units is
0
∞
-ve
+ve

Answer 4

-ve

Question 5

The individual mass transfer co-efficients (moles/m2 . s) for absorption of a solute from a gas mixture into a liquid solvent are, KL = 4.5 and KG = 1.5. The slope of the equilibrium line is 3. Which of the following resistance (s) is (are) controlling ?
liquid side
gas side
interfacial
both liquid and gas side

Answer 5

gas side

Question 6

Pore size of the molecular seive 5A is __________ mm.
5
0.5
0.05
50

Answer 6

0.5

Question 7

Swenson-Walker crystalliser is a __________ unit
continuous
batch
semi-batch
cooling (adiabatic)-cum-evaporation

Answer 7

continuous

Question 8

According to the Fenske equation, what will be the minimum number of plates required in a distillation column to separate an equimolar binary mixture of components A and B into an overhead fraction containing 99 mol% A and a bottom fraction containing 98 mol% B ? Assume that relative volatility (αAB = 2) does not change appreciably in the coloumn.
5
9
12
28

Answer 8

12

Question 9

Absorption (liquid-gas system) with evolution of heat as compared to isothermal absorption results in
decreased solute solubility.
large minimum liquid to gas (L/G) ratio.
large number of trays.
all (a), (b) and (c).

Answer 9

all (a), (b) and (c).

Question 10

With increase in temperature, the mutual solubility of two liquids
increases
decreases
remains unchanged
decreases exponentially

Answer 10

increases

Question 11

Pick out the wrong statement.
In solvent extraction, the distribution co-efficient should be as large as possible for solvent economy.
With increase in pressure, the solubility of gases in solvent increases at a fixed temperature.
Decrease in Murphree plate efficiency is an indication of entrainment in distillation coloumn.
The capacity of a distillation coloumn is maximum at total reflux.

Answer 11

The capacity of a distillation coloumn is maximum at total reflux.

Question 12

The relative volatility of a binary mixture at the azeotropic composition is
1
> 1
0
∞

Answer 12

1

Question 13

Relative volatility varies with the concentration of component for an ideal solution. The relative volatity of a binary mixture may be defined as the ratio of vapor pressure of component ‘A’ to that of component ‘B’, when
only vapor phase follows Raoult’s law.
only liquid phase obeys Dalton’s law.
liquid phase obeys Dalton’s law and vapor phase obeys Raoult’s law.
Vapor phase obeys Dalton’s law and liquid phase obeys Raoult’s law.

Answer 13

Vapor phase obeys Dalton’s law and liquid phase obeys Raoult’s law.

Question 14

Bound moisture is that liquid which exerts an equilibrium vapor pressure __________ that of the pure liquid at the given temperature.
less than
more than
equal to
either (o) or (b); depends on the solid

Answer 14

less than

Question 15

Lewis number (Le) is
Sc / Pr
Pr x St
Sh x Pr
St x Sh

Answer 15

Sc / Pr

Question 16

Which of the following is not a continuous leaching equipment ?
Pachuka tank
Kennedy extractor
Bollman extractor
Hydrocyclones

Answer 16

Pachuka tank

Question 17

__________ extractor is used for the concentration of radioactive nuclear waste.
Pulsed column
Seive plate
Mixer-settler
Ballman

Answer 17

Pulsed column

Question 18

Moisture content of a substance when at equilibrium with a given partial pressure of the vapour is called the __________ moisture.
free
unbound
equilibrium
bound

Answer 18

equilibrium

Question 19

All moisture in a non-hygroscopic material is the __________ moisture.
free
equilibrium
unbound
bound

Answer 19

unbound

Question 20

Pick out the correct relationship. (where, Ri = internal reflux ratio Ro = external reflux ratio. )
(1 + Ro) = Ro/Ri
(1 - Ro) = Ro/Ri
(1 + Ri) = Ro/Ri
(1 - Ri) = Ro/Ri

Answer 20

(1 + Ro) = Ro/Ri

Question 21

Separation of __________ employs gaseous diffusion process.
N2 and O2 from air
isotopes of uranium
isotopes of helium
SO2 and air

Answer 21

isotopes of uranium

Question 22

For water-ethanol system, the minimum reflux ratio
is computed from the slope of the upper operating line that is tangent to the equilibrium curve.
is computed from the intercept of the operating line.
cannot be computed.
is the optimum reflux ratio.

Answer 22

is computed from the slope of the upper operating line that is tangent to the equilibrium curve.

Question 23

The interfacial area per unit volume of dispersion, in a gas-liquid contactor, for fractional hold up of gas = 0.1 and gas bubble diameter = 0.5 mm is given by (in m2/m3)
500
1200
900
800

Answer 23

500

Question 24

Though, theoretically the adsorption & desorption isotherms is expected to coincide but sometimes at least a part of the isotherms does not coincide and this phenomenon is known as adsorption hysterisis. In such cases, the desorption equilibrium pressure is __________ that obtained for adsorption.
lower than
higher than
same as
either same or higher than

Answer 24

lower than

Question 25

Experimental determination of __________ is done by wetted wall column method.
diffusion co-efficient
mass transfer co-efficient
NTU
none of these

Answer 25

mass transfer co-efficient

Question 26

Pick out the wrong statement:
Absorption factor is constant, when the equilibrium and operating lines are straight.
In case of a stripper, the equilibrium curve is always below the operating line.
In case of an absorber, the operating line is always above the equilibrium curve.
In the absorption of low solubility gases, the liquid film is the controlling resistance.

Answer 26

In case of a stripper, the equilibrium curve is always below the operating line.

Question 27

In liquid extraction, if the selectivity is unity, then
separation of the constituents is the most effective.
no separation will occur.
amount of solvent required will be minimum.
solvent flow rate should be very low.

Answer 27

no separation will occur.

Question 28

Paper industry employs __________ driers.
cylinder
rotary
spray
fluidised bed

Answer 28

cylinder

Question 29

Which of the following has the same dimension as mass diffusivity ?
Momentum flux
Kinematic viscosity
Thermal diffusivity
Both (b) and (c)

Answer 29

Both (b) and (c)

Question 30

Pick out the wrong statement pertaining to the cooling towers.
In case of induced draft cooling tower, the fan is placed at the top of the tower.
Effectiveness of forced draft cooling tower is reduced by the recirculation of the humid and hot discharged air.
A natural draft cooling tower is recommended to be used, when the air humidity is low and air temperature is also low.
Effectiveness of a mechanical draft cooling tower is reduced with increase in air wet-bulb temperature.

Answer 30

Effectiveness of a mechanical draft cooling tower is reduced with increase in air wet-bulb temperature.

Question 31

Experiments were conducted to determine the flux of a species A in a stagnant medium across a gas-liquid interface. The overall mass transfer co-efficient based on the liquid side for dilute systems for the above was estimated to be 4 x 10-3 kg mole/m2.s. The equilibrium data for the system is given as y = 2x. The flux across the interface (in kg mole/m2 .s) for bulk concentrations of A in gas phase and liquid phase as y = 0.4 and x = 0.01 respectively is
5.6 x 10-4
8.5 x 10-4
5.6 x 10-3
8.5 x 10-3

Answer 31

8.5 x 10-4

Question 32

Leaching of coarse solid lumps is also termed as
decoction
dissolution
percolation
agitation and settling

Answer 32

percolation

Question 33

The main reason for dividing a tall packed tower into series of small towers is to
minimise the overall pressure drop.
avoid flooding.
reduce liquid hold up.
avoid channeling.

Answer 33

avoid channeling.

Question 34

Co-current absorbers are usually used when the gas to be dissolved in the liquid is
sparingly soluble
highly soluble
a pure substance
a mixture

Answer 34

a pure substance

Question 35

At what percentage (by volume) of alcohol composition, it forms an azeotrope with water ?
90
91.5
95
99

Answer 35

95

Question 36

Measurement of the interfacial area of mass transfer is achieved easily & accurately in case of a __________ coloumn.
spray
packed
bubble cap plate
wetted wall

Answer 36

wetted wall

Question 37

With the lowering of equiliDrium pressure, at a given temperature, the amount of ad-sorbate on the adsorbent
increases.
decreases.
remains same.
either (a) or (b), depends on the system.

Answer 37

decreases.

Question 38

Mass transfer co-efficient is defined as
Flux = Co-efficient/concentration difference.
Co-efficient = Flux/concentration difference.
Flux=concentration difference/co-effi-cient.
none of these.

Answer 38

Co-efficient = Flux/concentration difference.

Question 39

When an unsaturated air-water mixture is heated at constant pressure, then
the partial pressure of water vapour increases.
the specific humidity decreases.
The relative humidity increases.
the relative humidity decreases.

Answer 39

the relative humidity decreases.

Question 40

During sensible cooling process
relative humidity decreases
relative humidity increases
wet bulb temperature decreases
both (b) and (c)

Answer 40

both (b) and (c)

Question 41

At a fixed pressure, the humidity depends upon the partial pressure of vapor in the mixture. Humidity of a vapour free gas is __________ percent.
100
0
50
between 0 and 100

Answer 41

0

Question 42

In case of physical adsorption, the difference between heat of adsorption and heat of normal condensation is
equal to the heat of formation of surface compound.
equal to the heat of wetting.
zero.
called integral heat of adsorption.

Answer 42

equal to the heat of wetting.

Question 43

Pick out the wrong statement:
Change in barometric pressure does not affect the relative humidity.
In case of a packed tower, the operating velocity is about half of the flooding velocity, generally.
‘Elution’ means the desorption of the adsorbed solute by a solvent.
The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity.

Answer 43

The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity.

Question 44

The value of L/mG ratio for economical absorption operation ranges from
0.5 to 0.8
0.8 to 1.2
1.25 to 2.0
2.5 to 3.5

Answer 44

1.25 to 2.0

Question 45

Large scale usage of flash distillation is practised in
soap manufacturing
ammonia synthesis plant
polymerisation industry
petroleum refinning

Answer 45

petroleum refinning

Question 46

High pressure at the bottom of a distillation tower handling heat sensitive materials results in
thermal decomposition of bottoms.
increased relative volatility.
erosion of the tower.
very efficient operation.

Answer 46

thermal decomposition of bottoms.

Question 47

Alcohol is dehydrated using __________ distillation.
extractive
azeotropic
steam
molecular

Answer 47

azeotropic

Question 48

Separation of two volatile liquids by distillation makes use of their
selectivity
relative volatility
solubility
density difference

Answer 48

relative volatility

Question 49

Which of the following increases during evaporative cooling process with recirculated water supply ?
Wet bulb temperature
Relative humidity
Partial pressure of vapour
Both (b) and (c)

Answer 49

Both (b) and (c)

Question 50

Dried finished product sampled from various parts of a __________ dryer exhibits non-uniformity in the moisture content.
rotary
tray
freeze
none of these

Answer 50

tray