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Mass Transfer
Quiz 7
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Q.1
Acetic acid will be most economically separated from a dilute solution of acetic acid in water by
Solvent extraction
Continuous distillation
Evaporation
Absorption
Q.2
Minimum possible diameter of a packed coloumn is decided mainly by the
Flooding
Gas viscosity
Liquid density
Liquid hold up
Q.3
In triangular co-ordinates, the ternery composition point falls __________ of the triangle.
In the corners
Inside
On the sides
None of these
Q.4
The dew point of a saturated gas phase equals the __________ temperature.
Gas
Room
Wet bulb
None of these
Q.5
The critical moisture content in case of drying indicates the __________ rate period.
Beginning of falling
Beginning of constant
End of falling
None of these
Q.6
Rayleigh's equation applies to __________ distillation.
Continuous
Steam
Differential
Flash
Q.7
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
Q.8
The wet and 'dry bulb temperature for a vapour-gas mixture are 25°C and 30°C respectively. If the mixture is heated to 45°C at constant pressure, the wet bulb temperature will be __________ °C.
25
> 25
< 25
- 25
Q.9
The Kundsen diffusivity is proportional to (where, T = absolute temperature)
√T
T²
T
T⁴
Q.10
Langmuir equation is associated with
Leaching
Adsorption
Steam distillation
Multicomponent absorption
Q.11
Large scale drying of wheat is done in a __________ dryer.
Fluidised bed
Spouted bed
Tray
Vacuum rotary
Q.12
In case of steam distillation, the steam leaving the liquid is not completely saturated with distillate vapour, because
Temperature is less
Total pressure is less
Mixing of steam with the material being vaporised is not so intimate as to result in equilibrium condition
Saturated steam is used for steam distillation
Q.13
__________ is concerned with the adsorption equilibria.
Fick's law
Gibb's equation
Freundlich equation
None of these
Q.14
The change in enthalpy per unit weight of adsorbed gas when adsorbed on gas free or "outgassed" adsorbent to from a definite concentration of adsorbate is called its
Integral heat of adsorption
Heat of wetting
Differential heat of adsorption
Heat of normal condensation
Q.15
Total reflux in a distillation operation requires minimum
Reboiler load
Number of plates
Condenser load
All of the above
Q.16
When the solvent dissolves very little of solute, then
Solvent of low latent heat of vaporisation should be used
Solvent of low freezing point should be used
Large quantity of solvent is required to extract the solute
Very small quantity of solvent is required
Q.17
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 & B
Neither A nor B
Q.18
Ion exchange process is similar to
Absorption
Adsorption
Extraction
Leaching
Q.19
Pick out the wrong statement pertaining to the rotary dryer.
Flights (located in the inside shell of rotary dryer) lift the material being dried and shower it down through the current of hot air/gases. It extends from the wall to a distance which is about 8-12%
Hold up of a rotary drier is defined as the fraction of the dryer volume occupied by the solid at any instant. The best performance for rotary drier is obtained, when the hold up is in the range of 0.
Rotary dryer is suitable for drying sticky material
Recommended peripherial speed of a rotary drier is in the range of 10 to 30 metres/minute
Q.20
To remove all the moisture from a wet solid it requires exposure to __________ air.
Perfectly dry
Highly humid
High temperature
None of these
Q.21
Large scale drying of sugar is done in a __________ dryer.
Spouted bed
Tray
Rotary
Truck
Q.22
Weeping in a distillation column
Increases tray efficiency
Provides large interfacial surface for mass transfer
Results due to very high gas velocity
Results due to very low gas velocity
Q.23
Steady state equimolal counter diffusion is encountered in
Separation of a binary mixture by distillation
Absorption of NH₃ from air by water
All liquid-liquid diffusion systems
All of the above
Q.24
For turbulent mass transfer in pipes, the Sherwood number depends on the Reynolds number as
Re
0.33
Re
0.53
Re
0.83
Re
Q.25
Make up water is supplied during cooling tower operation to compensate for water losses due to
Evaporation
Blowdown
Entrainment
All of the above
Q.26
The Reynolds number of the liquid was increased 100 fold for a laminar falling film used for gas-liquid contacting. Assuming penetrating theory is applicable, the fold increase in the mass transfer co-efficient (Kc) for the same sytem is
100
10
5
1
Q.27
In distillation, overhead product contains
Only one component
Two components
Any number of components
Only saturated liquid
Q.28
When the __________ the relative humidity of air decreases, despite an increase in the absolute humidity.
Temperature rises
Pressure rises
Temperature falls
Pressure falls
Q.29
Molecular diffusivity of a liquid
Increases with temperature
Decreases with temperature
May increase or decrease with temperature
Is independent of temperature
Q.30
Mass transfer co-efficient (K) and diffusivity (D) are related according to film theory as
K ∝ D
K ∝ √D
K ∝ D
1.5
K ∝ D
2
Q.31
During drying operation, it is easier to remove the __________ moisture.
Equilibrium
Critical
Unbound
Bound
Q.32
At constant pressure, with increase of temperature, the dew point will
Increase
Decrease
Remain unchanged
Increase/decrease ; depends on the temperature
Q.33
Steady state equimolal counter diffusion occurs in case of
Leaching
Absorption
Binary phase distillation
Liquid-liquid extraction
Q.34
In liquid-liquid extraction, the number of phases at plait point is
1
2
3
4
Q.35
An alkaline solution is used to reduce the concentration of carbon dioxide in a stream from 10% to 0.1% by absorption with irreversible chemical reaction. The overall number of transfer units based on gas phase is
9.21
4.605
100
0.001
Q.36
Flash distillation is suitable for separating the constituents of a binary system, which
Form minimum boiling azeotrope
Have very wide boiling points
Have very close boiling points
Form constant boiling azeotrope
Q.37
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
Q.38
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
Q.39
The temperature to which a vapour gas mixture must be cooled (at varying humidity) to become saturated is called the __________ temperature.
Dew point
Wet bulb
Dry bulb
None of these
Q.40
Steady state temperature reached by a small amount of liquid evaporating into a large amount of unsaturated vapour-gas mixture is called the __________ temperature.
Dry-bulb
Wet-bulb
Dew point
Adiabatic saturation
Q.41
Sand is dried in foundaries in a___drier.
Rotary
Fluidisedbed
Vacuum
Spray
Q.42
Which of the following adsorbant is used in the refinning of sugar?
Bone charcoal
Wood charcoal
Silica gel
Activated clay
Q.43
Absorption accompanied with chemical reaction is exemplified by the absorption of
Ammonia in water
Benzol present in coke oven gas by wash oil
SO₂ in alkaline solution
All of the above
Q.44
In gas-liquid contact operation, the number of ideal stages, N = (x
a
- x
b
)/(x
b
- x
*b
) . This is true when the stripping factor 'S' is
S > 1
S < 1
S = 1
S = ∞
Q.45
The term "approach" in a cooling tower refers to the difference in the temperature of the
Cold water leaving the tower and the wet bulb temperature of the surrounding air
Hot water entering the tower and the wet bulb temperature of the surrounding air
Hot water entering and the cooled water leaving the cooling tower
None of these
Q.46
Hypersorption refers to a __________ process.
Fixed bed absorption
Moving bed absorption
Fixed bed adsorption
Moving bed adsorption
Q.47
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
Q.48
At 750°K and 1 atm, the approximate value of Schmidt number for air is
0.01
0.1
1
10
Q.49
The reason for preferring packed towers over plate towers in distillation practice is that the packed tower operation gives
Low pressure drop and high hold up
High pressure drop and low hold up
Low pressure drop and low hold up
High pressure drop and high hold up
Q.50
Which of the following gas-liquid contacting devices incurs the least pressure drop for a particular duty?
Grid tray tower
Perforated tray tower
Wetted wall tower
Bubble cap tower
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