IndiaBIX Chemical Engineering: Mass Transfer Section 11

Question 1

__________ developed the film theory.
Higbie
Fick
Ergun
Levenspeil

Answer 1

Higbie

Question 2

(NSh/NRe.NSc) is termed in mass transfer operation as the
Stanton number
Peclet number
thermal diffusivity
momentum diffusivity

Answer 2

Stanton number

Question 3

1 kg of a saturated aqueous solution of a highly soluble component A at 60°C is cooled to 25°C. The solubility limits of A are (0.6 kgA/kg water) at 60°C and (0.2 kgA/kg water) at 25°C. The amount in kg of the crystals formed is
0.4
0.25
0.2
0.175

Answer 3

0.2

Question 4

A mixture of 10% C6H6 vapour in air at 25°C and 750 mm Hg has a dew point of 20°C. Its dew point at 30°C and 700 mm Hg will be around __________ °C.
21.7
20
27.3
18.7

Answer 4

18.7

Question 5

An ideal tower packing should not
have low weight per unit volume.
have large surface area per unit volume.
have large free cross-section.
hold up large amount of liquid in the packing.

Answer 5

hold up large amount of liquid in the packing.

Question 6

As per Gilliland’s equation, the diffusivity is
directly proportional to ‘T1.5’.
inversely proportional to ‘P’.
dependent on the molecular volume & molecular weight of components.
all ‘a’, ‘b’ & ‘c’.

Answer 6

all ‘a’, ‘b’ & ‘c’.

Question 7

At a fixed total pressure, humidity depends only on the
partial pressure of vapour in the mixture.
heat capacity of the vapour.
density of the vapour.
none of these.

Answer 7

partial pressure of vapour in the mixture.

Question 8

Berl saddle made of carbon cannot be used for
alkalis
SO2
H2SO4
oxidising atmosphere

Answer 8

oxidising atmosphere

Question 9

CO2 can be absorbed by
hot cupric oxide
heated charcoal
cold Ca(OH)2
alumina

Answer 9

heated charcoal

Question 10

Coffee is prepared from coffee beans by leaching with
cold water
hot water
dilute hot caustic solution
naphtha

Answer 10

hot water

Question 11

Corresponding to Prandtl number in heat transfer, the dimensionless group in mass transfer is the __________ number.
Schmidt
Sherwood
Peclet
Stanton

Answer 11

Schmidt

Question 12

Desirable value of absorption factor in an absorber is
1
< 1
> 1
0.5

Answer 12

> 1

Question 13

During drying of a solid, the critical moisture content increases with
decreased drying rate
increased thickness of the solid
increased drying rate
both (b) and (c).

Answer 13

both (b) and (c).

Question 14

During the constant rate period of drying of a solid,
increased air humidity decreases the rate of drying.
increasing the air temperature decreases the drying rate.
surface evaporation of unbound moisture occurs.
none of these.

Answer 14

surface evaporation of unbound moisture occurs.

Question 15

Extraction of pencillin employs a __________ extraction coloumn.
centrifugal
packed
plate
None of these.

Answer 15

centrifugal

Question 16

Fenske-Underwood equation used for calculating the minimum number of theoretical plates in distillation column is valid for
constant molal overflow
total reflux
constant relative volatility
all (a), (b) and (c)

Answer 16

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

Question 17

Flash distillation is
same as differential distillation.
used for multicomponent systems like crude refining.
same as simple distillation.
most useful for handling binary systems.

Answer 17

used for multicomponent systems like crude refining.

Question 18

For __________ drying, the effects of velocity, temperature and humidity of the gas and the thickness of the solid are the same.
unsaturated surface
constant rate
both (a) and (b)
neither (a) nor (b).

Answer 18

both (a) and (b)

Question 19

For absorbers, high pressure drop results in
increased efficiency.
decreased efficiency.
high operating cost.
better gas liquid contact.

Answer 19

high operating cost.

Question 20

For the same process conditions, the reflux ratio of an unlagged distillation column
decreases in winter
increases in winter
increases in summer
none of these

Answer 20

increases in winter

Question 21

Gaseous diffusion co-efficient increases with increase in the
pressure
temperature
both (a) & (b)
neither (a) nor (b)

Answer 21

temperature

Question 22

If a two phase system is in physical equilibrium
then it means that, the
escaping tendency of each component from the liquid phase to the vapor phase is exactly equal to that from vapor phase to liquid phase.
temperature of the liquid phase is equal to that of the vapor phase.
total pressure throughout the liquid phase is equal to that throughout the vapor phase.
all (a), (b) and (c).

Answer 22

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

Question 23

If moisture content of solid on dry basis is
X/(X+1)
X/(1-X)
(1+X)/X
(1-X)/X

Answer 23

X/(X+1)

Question 24

If the amount of the steam used in steam distillation is increased, the temperature of distillation
increases
decreases
remains unchanged
depends on relative volatility

Answer 24

decreases

Question 25

If xD = overhead product molal composition and RD = reflux ratio, then slope and intercept of the operating line for rectifying section are respectively
xD/(RD+1), RD/(RD+1)
RD/(RD+1), xD/(xD+1)
xD/(RD+1), (RD+1)/RD

Answer 25

RD/(RD+1), xD/(xD+1)

Question 26

In a binary system, separation is very efficient, when the relative volatility is
1
> 1
< 1
0.5

Answer 26

> 1

Question 27

In case of gases, the binary diffusivity is proportional to (where, P = pressure)
P
1/P
1/sqrt(P)
sqrt(P)

Answer 27

1/P

Question 28

In saturated gas, the
vapour is in equilibrium with the liquid at the gas temperature.
vapour is in equilibrium with the liquid at the room temperature.
partial pressure of vapour equals the vapour pressure of the liquid at room temperature.
none of these.

Answer 28

vapour is in equilibrium with the liquid at the gas temperature.

Question 29

Inside the distillation columns, the
highest temperatures is near the feed plate.
driving force for the liquid flow is its weight.
vapors are not always at their dew points.
all (a), (b) and (c)

Answer 29

driving force for the liquid flow is its weight.

Question 30

L/mGis the expression for
absorption factor
Brinkman number
slope of operating line in absorber
slope of operating line in stripper.

Answer 30

absorption factor

Question 31

Langmuir equation is associated with
leaching
adsorption
steam distillation
multicomponent absorption.

Answer 31

adsorption

Question 32

McCabe-Thiele method of binary distillation does not assume that the
sensible heat changes are negligible compared with latent heat changes.
molar latent heats of all components are equal.
heat of mixing is negligible.
none of these

Answer 32

none of these

Question 33

Moisture in a solid exerting an equilibrium vapour pressure equal to that of the pure liquid at the same temperature is called the __________ moisture.
unbound
critical
free
bound

Answer 33

unbound

Question 34

Pick out the wrong statement.
When the concentration difference for the mass transfer becomes zero at the bottom of the gas absorption tower, then the upper end of the operating line touches the equilibrium curve.
The average distance between the operating line and equilibrium line is large, when the liquid-gas ratio is also large in case of a counter current gas absorption tower.
The operating line lies above the equilibrium curve in case of a gas desorber.
With decrease in the liquid flow rate for a fixed gas flow rate, the slope of the operating line decreases, in case of gas absorption process.

Answer 34

The operating line lies above the equilibrium curve in case of a gas desorber.

Question 35

Pick out the wrong statement. Raoult’s law does not apply, when the
size of component molecules are not equal.
attractive forces between like and unlike molecules in the solution are approximately equal.
component molecules are nonpolar.
chemical combination or molecular association between unlike molecules takes place in the formation of solution.

Answer 35

component molecules are nonpolar.

Question 36

Priming in a distillation column
results from very low gas velocity.
is desirable from point efficiency consideration.
is characterised by the presence of foam throughout the space between trays.
reduces the overall pressure drop.

Answer 36

is characterised by the presence of foam throughout the space between trays.

Question 37

Refractory bricks are usually dried in a __________ dryer.
tray
tunnel
conveyor
festoon

Answer 37

tunnel

Question 38

Sticky material can be dried in a __________ dryer.
tray
rotary
fluidised bed
none of these

Answer 38

tray

Question 39

The binary diffusivity in gases does not depend upon the
pressure
temperature
nature of the components
none of these

Answer 39

none of these

Question 40

The commonly used solvent in supercritical extraction is
methyl ethyl ketone.
water.
carbon tetrachloride.
carbon dioxide.

Answer 40

carbon dioxide.

Question 41

The Hatta number is important in
multicomponent distillation.
binary distillation.
gas absorption without chemical reaction.
gas absorption with chemical reaction.

Answer 41

gas absorption with chemical reaction.

Question 42

Water-ethyl alcohal solution can be concentrated (in respect of alcohal concentration) by __________ distillation.
atmospheric
vacuum
high pressure
none of these

Answer 42

vacuum

Question 43

What is the degree of freedom of an absorption process in which only one component is transferred between phases ?
3
2
4
1

Answer 43

3

Question 44

When a multistage tray tower uses a minimum reflux ratio, it implies
infinite trays and maximum reboiler heat load.
infinite trays and minimum reboiler heat load.
minimum trays and minimum reboiler heat load.
minimum trays and maximum reboiler heat load

Answer 44

infinite trays and minimum reboiler heat load.

Question 45

When the gas to be dissolved in liquid is a/an __________ then normally co-current adsorber are used.
mixture of two gases
pure gas
ideal gas
sparingly soluble gas

Answer 45

pure gas

Question 46

When the liquid over a plate is of uniform concentration, then
Murphree efficiency > point efficiency.
Murphree efficiency < point efficiency.
Murphree efficiency = point efficiency.
Murphree efficiency ≠point efficiency.

Answer 46

Murphree efficiency = point efficiency.

Question 47

When the psychometric ratio is __________ then the adiabatic saturation temperature and wet bulb temperature becomes equal.
1
< 1
> 1
∞

Answer 47

1

Question 48

Which of the following is not fixed by the process requirements, in the design of absorbers ?
Flow rate of the entering gas.
Composition of the entering liquid.
Terminal concentrations of gas stream.
None of these.

Answer 48

None of these.

Question 49

Which of the following parameters remains almost constant during adiabatic saturation of unsaturated air ?
Dry bulb temperature
Dew point
Wet bulb temperature
None of these

Answer 49

Wet bulb temperature

Question 50

With increase in absolute humidity, the dew point of an unsaturated mixture of air and water vapor (at constant pressure and temperature)
increases
remains unchanged
decreases
decreases linearly

Answer 50

increases