IndiaBIX Chemical Engineering: Mass Transfer Section 13

Question 1

In a gas-liquid absorption coloumn, for obtaining the maximum absorption efficiency
liquid stream should be distributed uniformly.
gas stream should be distributed uniformly.
both gas as well as liquid streams should be distributed uniformly.
by passing should be completely avoided.

Answer 1

both gas as well as liquid streams should be distributed uniformly.

Question 2

Reverse osmosis is also called
dialysis
electrodilysis
diffusion
ultrafiltration.

Answer 2

ultrafiltration.

Question 3

Which of the following is not a batch drier?
Truck drier
Agitated pan drier
Fluidised bed drier
Vacuum shelf drier

Answer 3

Fluidised bed drier

Question 4

Steam distillation is not recommended to be used, if the
azeotropic mixture is to be separated and the final product is miscible with water.
liquids decompose, if distilled directly at atmospheric pressure.
material can not be distilled by indirect heating even under low pressure, because of the high boiling temperature.
material to be distilled is thermally unstable or has the tendency to react with other components associated with it, at the boiling temperature.

Answer 4

azeotropic mixture is to be separated and the final product is miscible with water.

Question 5

Rayleigh equation applies to __________ distillation.
differential
flash
equilibrium
molecular

Answer 5

differential

Question 6

Mass transfer co-efficient ‘K’ according to penetration theory varies with mass dif-fusivity as
D^0.5
D
1/D
D^1.5

Answer 6

D^0.5

Question 7

Most important factor to be considered in the selection of packings for absorbers is the __________ of packing.
size
durability
porosity
cost

Answer 7

porosity

Question 8

__________ is the separation technique used for desalination of sea water.
Thermal diffusion
Reverse osmosis
Adsorption
absorption

Answer 8

Reverse osmosis

Question 9

The relative saturation of a partially saturated mixture of vapour and gas can be increased by __________ of the mixture.
reducing the total pressure
increasing the total pressure
reducing the temperature
both (b) and (c)

Answer 9

both (b) and (c)

Question 10

Danckwerts developed the __________ theory.
penetration
surface renewal
film
none of these

Answer 10

surface renewal

Question 11

Molecular distillation is
high temperature distillation
for heat-sensitive materials
very low pressure distillation
both (b) and (c)

Answer 11

both (b) and (c)

Question 12

Drying of a wet solid under constant drying conditions means the exposure of the wet solid to the air of constant
humidity
velocity
temperature
all (a), (b) & (c)

Answer 12

all (a), (b) & (c)

Question 13

Which of the following provides maximum contact surface for a liquid-vapour system?
Packed tower
Bubble-cap plate column
Seive-plate column
Wetted wall column

Answer 13

Packed tower

Question 14

For experimental determination of mass transfer co-efficient by wetted wall tower, the mass transfer area is
calculated
unknown
known
not required

Answer 14

known

Question 15

In stripping section of continuous distillation column, the
liquid is stripped of high boiler.
liquid is enriched with high boiler.
vapour is stripped of low boiler.
none of these.

Answer 15

liquid is enriched with high boiler.

Question 16

Molality is defined as the number of gm moles of solute per __________ of solvent.
litre
gm
Kg
m3

Answer 16

Kg

Question 17

__________ is concerned with the adsorption equilibria.
Fick’s law
Gibb’s equation
Freundlich equation
none of these

Answer 17

Freundlich equation

Question 18

Which of the following is the correct ternery diagram for liquid-liquid extraction system comprising of one pair partly soluble (e.g. water-benzene-acetic acid in which A-diluent, B - solvent and C - solute)?

Answer 18

0

Question 19

Fractional solvent extraction
employs only one solvent
employs two solvents
results in low interfacial tension
none of these

Answer 19

employs two solvents

Question 20

In the layout plan for a vacuum distillation unit, operating at 60 mm Hg, supported by a barometric condenser, the appropriate place for the location of vacuum drum for collecting the distillate will be
at ground level.
2 metres above the ground.
5 metres above ground.
10 metres above ground.

Answer 20

at ground level.

Question 21

Raschig ring made of procelain cannot be used for treating concentrated
hydrochloric acid
nitric acid
alkalis
sulphuric acid

Answer 21

alkalis

Question 22

H2S present in naphtha reformed gas is removed by absorbing with
ethanolamine
K2CO3
HCl
vacuum gas oil

Answer 22

ethanolamine

Question 23

During constant rate drying period, vaporisation rate per unit drying surface area
decreases with time.
increases with time.
does not change with time,
does not affect the moisture content of the wet solid.

Answer 23

does not change with time,

Question 24

Pick out the wrong statement.
Mass transfer co-efficient does not account for gase-liquid diffusion.
Diffusion co-efficient and mass transfer co-efficient are not same in any mass transfer operation.
Contacting cold vapor-gas mixture with hot liquid exemplifies humidification process.
Contacting cold liquid with warm vapor-gas mixture exemplifies dehumidification process.

Answer 24

Mass transfer co-efficient does not account for gase-liquid diffusion.

Question 25

In rectifying section of a continuous distillation column, the
vapour is enriched with low boilers.
vapour is enriched with high boilers.
liquid is stripped of high boilers.
none of these.

Answer 25

vapour is enriched with high boilers.

Question 26

For a fixed number of ideal stages in a distillation column, as the reflux ratio is increased, the difference in composition between the top and bottom product streams
increases
decreases
remains unaffected
passes through a maximum

Answer 26

increases

Question 27

According to Chilton-Colburn analogy for mass transfer, NSt.NSc^2/3 is equal to
f
f/2
2f
1/f

Answer 27

f/2

Question 28

A pure drug is administered as a sphere and as a cube. The amount of drug is the same in the two tablets. Assuming that the shape and size do not influence the mass transfer, the ratio of rate of dissolution in water at t = 0 for the cubic to spherical tablet is
0.54
1.04
1.24
1.94

Answer 28

1.24

Question 29

Minimum reflux ratio in a distillation column results in
optimum number of trays.
minimum reboiler size.
maximum condenser size.
minimum number of trays.

Answer 29

maximum condenser size.

Question 30

Flooding in a distillation column is detected by a sharp
increase in Murphree plate efficiency.
decrease in pressure drop.
decrease in liquid hold up in the coloumn.
increase in pressure drop.

Answer 30

increase in pressure drop.

Question 31

In extraction, as the temperature increases, the area of hetrogeneity (area covered by binodal curve)
decreases
increases
remains unchanged
none of these

Answer 31

decreases

Question 32

Pick out the wrong statement.
The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are the same for NSc = NPr = 1.
1 Nm3 of dry air is lighter than 1Nm3 of humid air.
The Lewis number of a mixture is unity, when the thermal diffusivity is equal to the mass diffusivity.
An azeotropic mixture of two liquids has boiling point higher than either of them, when it shows negative deviation from Raoult’s Law.

Answer 32

1 Nm3 of dry air is lighter than 1Nm3 of humid air.

Question 33

In case of an unsaturated mixture of gas and vapor, the percentage saturation is __________ its relative saturation.
lower than
higher than
equal to
either (a) or (b), depends on the mixture composition

Answer 33

lower than

Question 34

At constant pressure, with increase of temperature, the dew point will
increase
decrease
remain unchanged
increase/decrease
depends on the temperature

Answer 34

increase

Question 35

For a single component absorber, the operating line is straight only when plotted in term of __________ units.
partial pressure
mole ratio
mole fraction
none of these

Answer 35

mole ratio

Question 36

In case of unsaturated air
dew point < wet bulb temperature
wet bulb temperature < dry bulb temperature
both (a) and (b)
neither (a) not (b)

Answer 36

both (a) and (b)

Question 37

For a binary mixture with low relative volatility,
use steam distillation.
use molecular distillation.
use high pressure distillation.
an azeotrope may be formed during distillation.

Answer 37

an azeotrope may be formed during distillation.

Question 38

Plate efficiency
is a function of the mass transfer between liquid and vapour.
increases due to foaming.
increases due to liquid entrainment.
increases due to weeping and dumping of liquid.

Answer 38

is a function of the mass transfer between liquid and vapour.

Question 39

Absorption accompanied by heat evolution results in
increased capacity of the absorber.
increase in equilibrium solubility.
decrease in equilibrium solubility.
none of these.

Answer 39

decrease in equilibrium solubility.

Question 40

Even though bubble cap towers are very effective for humidification operation, they are not used commonly in industries, because of the
high evaporation losses of water.
high pressure drop of the gas.
difficulty in its fabrication.
none of these.

Answer 40

high pressure drop of the gas.

Question 41

For a distillation column operating at minimum reflux, the
concentration of liquid and vapour leaving a plate will be same.
reflux ratio will be maximum.
number of plates required will be maximum.
none of these.

Answer 41

number of plates required will be maximum.

Question 42

A solid is being dried in the linear drying rate regime from moisture content Xo to XF. The drying rate is zero at X = 0 and the critical moisture content is the same as the initial moisture Xo. The drying time for M = (Ls/ARc) is (where, L = total mass of dry solid, A = total surface area for drying Rc = Constant maximum drying rate per unit area X = moisture content (in mass of water/mass of dry solids))
M(Xo - XF)
M(Xo/XF)
M ln(Xo/XF)
MXo ln(Xo/XF)

Answer 42

MXo ln(Xo/XF)

Question 43

Flash distillation is suitable for the separation of components
having very close boiling points.
which form maximum boiling azeotrope.
having very wide boiling points.
which form minimum boiling azeotrope.

Answer 43

having very wide boiling points.

Question 44

In Mcabe-Thiele method, at infinite reflux ratio
number of plates is maximum.
overhead product is maximum.
both the operating lines coincide with the diagram.
none of these.

Answer 44

both the operating lines coincide with the diagram.

Question 45

Leaching rate is independent of the
agitation
temperature
particle size
none of these

Answer 45

none of these

Question 46

Use of packed towers for distillation is generally limited to the
small sizes.
multicomponent distillation.
high pressure operation.
vacuum distillation.

Answer 46

small sizes.

Question 47

Pick out the wrong statement.
The binary diffusivity in liquids is of the order of 10-5 cm2 /sec.
Molecular diffusion in solid is much faster than that in liquids.
Particles movement from higher concentration to lower concentration takes place in diffusion process.
According to Poiseuille’s law, the permeability decreases with increase in temperature for flow of a gas through a given capillary.

Answer 47

Molecular diffusion in solid is much faster than that in liquids.

Question 48

Which of the following decreases during evaporative cooling process with recirculated water supply ?
Wet bulb temperature
Relative humidity
Partial pressure of vapour
None of these

Answer 48

None of these

Question 49

Laboratory determination of the dif-fusivity of vapor is done by __________ method.
wetted wall column
Gilliland’s
Winkel mann’s
Wike’s

Answer 49

Winkel mann’s

Question 50

The minimum liquid rate to be used in an absorber corresponds to an operating line
of slope = 1
ofslope = 0.1
tangential to the equilibrium curve
none of these

Answer 50

tangential to the equilibrium curve