IndiaBIX Chemical Engineering: Mass Transfer Section 4

Question 1

Boiling point diagram is
not affected by pressure.
affected by pressure.
a plot of temperature vs. liquid composition.
a plot of temperature vs. vapour composition.

Answer 1

affected by pressure.

Question 2

When acetone is added in a two layer mixture of methyl isobutyl ketone and water at 30°C, the acetone distributes between the two layers and the composition of the layer follows two solubility curves. For this system,
as the acetone concentration increases, the solubility curves approach each other.
both the phases become identical at a particular common point on both the solubility curves called the plait point.
the distribution curve which is a plot between x (acetone concentration in one phase) and y (acetone concentration in other phase) is analogous to x-y curve used in distillation and absorption terminates on the x = y diagonal at the concentration of the plait point.
all (a), (b) and (c).

Answer 2

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

Question 3

Steam distillation is used to
reduce the number of plates.
avoid thermal decomposition of a component.
increase the efficiency of separation.
increase the total pressure of distillation.

Answer 3

avoid thermal decomposition of a component.

Question 4

Freundlich equation applies to the adsorption of solute from
dilute solutions, over a small concentration range.
gaseous solutions at high pressure.
concentrated solutions.
none of these.

Answer 4

dilute solutions, over a small concentration range.

Question 5

For ethanol-water system, the lowering of distillate quality from 95% to 92% will cause __________ plate requirement.
no change in theoretical
marginal decrease in the number of
major decrease in the number of
none of these

Answer 5

marginal decrease in the number of

Question 6

The apex of an equilateral-triangular coordinate (in ternary liquid system) represents a/an
pure component
binary mixture
ternary mixture
insoluble binary system

Answer 6

pure component

Question 7

The diffusivity (D) in a binary gas mixture is related to the pressure (P) as
D ∝ P^0.5
D ∝ 1/^P0.5
D ∝ 1/P
D ∝ 1/^P1.5

Answer 7

D ∝ 1/P

Question 8

A slurry is to be dried to produce flaky solid. Which dryer would you recommend ?
Spray dryer
Drum dryer
Rotary dryer
Dielectric dryer

Answer 8

Spray dryer

Question 9

Which of the following psychrometric processes is followed by water vapour laden unsaturated air, when it is passed through solid or liquid adsorbent ?
Cooling and dehumidification.
Heating and dehumidification at almost constant wet bulb temperature.
Dehumidification with dry bulb temperature remaining constant.
None of these.

Answer 9

Heating and dehumidification at almost constant wet bulb temperature.

Question 10

In gas-liquid contact operation, the number of ideal stages, N = (xa-xb)/(xb-x*b). This is true when the stripping factor ‘S’ is
S > 1
S < 1
S = 1
S = ∞

Answer 10

S > 1

Question 11

The cooling effect in a cooling tower can not be increased by
increasing the air velocity over the wet surfaces.
reducing the humidity of entering air.
lowering the barometric pressure.
none of these.

Answer 11

none of these.

Question 12

The value of NA/(NA + NB) for steady state molecular diffusion of gas ‘A’ through non-diffusing gas ‘B’ is
1
∞
0.5
2

Answer 12

1

Question 13

The humid volume may be increased by increasing the
temperature at constant humidity.
humidity at constant temperature.
both (a) and (b).
neither (a) nor (b).

Answer 13

both (a) and (b).

Question 14

Pick out the wrong statement
Co-current absorbers are usually used, when the gas to be dissolved in the liquid is a pure substance.
In case of gas absorption, HETP is equal to HTU, when both the operating line & the equilibrium lines are parallel.
Hatta number is important in case of gas absorption with chemical reaction.
In actual practice, absorption is an en-dothermic process, while stripping is an exothermic process.

Answer 14

In actual practice, absorption is an en-dothermic process, while stripping is an exothermic process.

Question 15

When the mixture to be distilled has a very high boiling point and the product material is heat sensitive, the separation technique to be used is __________ distillation.
continuous
steam
azerotropic
none of these

Answer 15

steam

Question 16

Large scale drying of wheat is done in a __________ dryer.
fluidised bed
spouted bed
tray
vacuum rotary

Answer 16

spouted bed

Question 17

Azeotropic distillation is employed to separate
constant boiling mixture.
high boiling mixture.
mixture with very high relative volatility.
heat sensitive materials.

Answer 17

constant boiling mixture.

Question 18

The drying time between fixed moisture content within diffusion controlled ‘falling rate period’ is proportional to (assuming that drying occurs from all surfaces of the solid) (where, T = thickness of the solid )
sqrt(T)
T
T^2
T^3

Answer 18

T2

Question 19

At the boiling point of the liquid at the prevailing pressure, the saturated absolute humidity becomes
1
0
∞
none of these

Answer 19

∞

Question 20

Use of natural draft cooling tower is practised, when the air has low
humidity
temperature
both ‘a’ & ‘b’
neither ‘a’ nor ‘b’

Answer 20

both ‘a’ & ‘b’

Question 21

Pressure drop through plate tower as compared to that through packed tower, for the same duty will be
less
more
equal
either (a) or (b); depends on the packing height

Answer 21

more

Question 22

Dimension of mass diffusivity is the same as that of
kinematic viscosity
dynamic viscosity
surface tension
pressure

Answer 22

kinematic viscosity

Question 23

Dorr thickener is an equipment used for
classification
sedimentation
clarification
leaching

Answer 23

sedimentation

Question 24

Steam distillation is used to separate
azeotropes.
high boiling substances from nonvolatile impurities.
heat sensitive materials.
mixtures of low relative volatility.

Answer 24

high boiling substances from nonvolatile impurities.

Question 25

In distillation where q is defined as the moles of liquid flow in the stripping section per mole of feed introduced, for saturated liquid feed
q > 1
q < 1
q = 1
q = 0

Answer 25

q = 1

Question 26

Which of the following is an undesirable property in a tower packing ?
Large surface per unit volume.
Large free cross-section.
Low weight per unit volume.
Large weight of liquid retained.

Answer 26

Large weight of liquid retained.

Question 27

Water entrained by circulating air in cooling towers is termed as
drift
blow down
vapor load
none of these.

Answer 27

drift

Question 28

A 25 cm x 25 cm x 1 cm flat sheet weighing 1.2 kg initially was dried from both sides under constant drying rate conditions. It took 1500 seconds for the weight of the sheet to reduce to 1.05 kg. Another 1m x 1m x 1cm flat sheet of the same material is to be dried from one side only.Under the same constant drying rate conditions, the time required for drying (in seconds) from its initial weight of 19.2 kg to 17.6 kg is
1000
1500
2000
2500

Answer 28

1000

Question 29

With increase in temperature, the rate of leaching increases, because the
diffusivity of solute increases.
viscosity of solvent decreases.
thermal conductivity of solute increases.
none of these.

Answer 29

diffusivity of solute increases.

Question 30

Humidity chart is useful for the solution of problems concerning condensation, vaporisation and air conditioning. At a given dry bulb temperature, value of humidity obtained from the humidity chart directly, is in terms of __________ humidity.
molal
relative
percentage
none of these

Answer 30

molal

Question 31

The Kundsen diffusivity is proportional to (where, T = absolute temperature)
sqrt(T)
T^2
T
T^4

Answer 31

sqrt(T)

Question 32

In case of constant underflow extraction operation, the __________ at all solute concentration.
solids are drained to the same extent
ratio of the insoluble to solution is constant
both(a) & (b)
neither (a) nor (b)

Answer 32

both(a) & (b)

Question 33

In an absorption coloumn, the flooding velocity for random packing is __________ that for stacked/regular packing.
greater than
smaller than
equal to
either (a), or (b) ; depends on the type of packing

Answer 33

smaller than

Question 34

The relation among various mass transfer co-efficients (M.T.C) for ideal gases is given by (where, Kc & Km are M.T.C. for equimolar counter diffusion with concentration & mole fraction respectively as the driving force. and, Kp = M.T.C. for diffusion of a gas through a stagnant inert gas with pressure as driving force.)
Kc = Kp = Km
Kc = Kp/RT = Km . RT/P
Kc = Kp . RT = Km . RT/p
none of these

Answer 34

Kc = Kp . RT = Km . RT/p

Question 35

With increase in temperature, the leaching rate increases because of the
increased diffusivity.
decreased viscosity of liquid.
both (a) and (b).
neither (a) nor (b).

Answer 35

both (a) and (b).

Question 36

Leaching of uranium ore and gold ore is done in
centrifugal extractor
Pachuka tanks
Bollman extractor
none of these

Answer 36

Pachuka tanks

Question 37

Blowdown in a cooling tower
means discarding a small fraction of circulating water to prevent and limit the concentration of salt and impurities.
increases the scale forming tendencies of water.
is undesirable.
all (a), (b) & (c).

Answer 37

means discarding a small fraction of circulating water to prevent and limit the concentration of salt and impurities.

Question 38

During constant rate period, the rate of drying decreases with the
decrease in air temperature
increase in air humidity
both (a) and (b)
neither (a) nor (b)

Answer 38

both (a) and (b)

Question 39

Antibiotics are best handled in a
Podbielniak extractor.
sieve tray contractor.
rotating disc contactor.
pulsed column extractor.

Answer 39

Podbielniak extractor.

Question 40

The fixed cost of a distillation column at minimum reflux ratio is
minimum
infinite
optimum
none of these

Answer 40

infinite

Question 41

__________ extractor uses centrifugal force for separating the two phases.
Treybal
Schiebel
Podbielniak
None of these

Answer 41

Podbielniak

Question 42

In a binary distillation column, if the feed contains 40 mole % vapour, the q line will have a slope of
1.5
-0.6
-1.5
0.6

Answer 42

-1.5

Question 43

Pick out the wrong statement.
Mechanical agitation is required, if the system has low interfacial tension.
Seive tray towers are generally used for systems having low interfacial tension.
When Henry’s law constant is very small, then the mass transfer rate is controlled by the gas film resistance.
Taylor-Prandtl analogy for heat and mass transfer considers the transfer through both laminar and turbulent layers.

Answer 43

Mechanical agitation is required, if the system has low interfacial tension.

Question 44

Acetic acid will be most economically separated from a dilute solution of acetic acid in water by
solvent extraction
continuous distillation
evaporation
absorption

Answer 44

solvent extraction

Question 45

Design calculation for multiple component distillation is done by
Ponchon-Savarit method
Mccabe-Thiele method
enthalpy concentration method
tray to tray calculations.

Answer 45

tray to tray calculations.

Question 46

To get high tray efficiency
interfacial surface between liquid and gas phase should be large.
time of contact between the two phases should be less.
gas velocity should be very low.
liquid entrainment should be severe.

Answer 46

interfacial surface between liquid and gas phase should be large.

Question 47

The mass transfer co-efficient for a solid sphere of radius ‘a’ dissolving in a large volume of quiescent liquid, in which D is the diffusivity of solute, is
D/a
D/2a
proportional to sqrt(D)
dependent on the Reynolds number

Answer 47

dependent on the Reynolds number

Question 48

Pick out the wrong statement. Generally for physical adsorption, a gas of
higher molecular weight is adsorbed in preference to a gas of low molecular weight.
high critical temperature is adsorbed in preference to a gas of low critical temperature.
low volatility is adsorbed in preference to a gas of high volatility.
none of these.

Answer 48

none of these.

Question 49

In most of the mechanically agitated liquid-liquid extractors, baffles or horizontal compartmental plates are provided, which helps in
reducing the axial mixing.
increasing the rate of extraction.
maintaining the concentration difference between the two phases.
all ‘a’, ‘b’ & ‘c’.

Answer 49

all ‘a’, ‘b’ & ‘c’.

Question 50

In a vapor-liquid contacting equipment, the overall gas phase mass transfer co-efficient (M.T.C), KG is related to individual co-efficients (KG and KL) as
KG = 1/KG + m/KL
1/KG = 1/KG + m/KL
1/KG = 1/KL + m/KG
KG = 1/KL + m/KG

Answer 50

1/KG = 1/KG + m/KL