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Chemical Engineering
Mass Transfer
Quiz 8
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Q.1
Heat sensitive materials with very high latent heat of vaporisation may be economically separated using
Liquid extraction
Distillation
Evaporation
Absorption
Q.2
Pick out the wrong statement.
Critical moisture content is not a property of the material itself
A sharp increase in pressure drop gives an indication of the flooding in a distillation column
Separation of components is not possible in liquid extraction, if selectivity is unity
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum
Q.3
Plate efficiency in a distillation column is reduced due to the __________ of liquid.
Entrainment
Foaming
Weeping & dumping
All of the above
Q.4
Relative humidity is the ratio of the
Partial pressure of the vapour to the vapour pressure of the liquid at room temperature
Partial pressure of the vapour to the vapour pressure of the liquid at gas temperature
Actual humidity to saturation humidity
None of these
Q.5
Which of the following is considered equivalent to one theoretical stage in Mc-Cabe Thiele's method?
Partial condenser
Total condenser
Reboiler
Both B and C
Q.6
Absorption factor method is used to calculate the number of ideal stages, when
Operating line lies above the equilibrium line
Operating line lies below the equilibrium line
Both operating and equilibrium lines are parallel
Pressure drop in the column is very high
Q.7
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
Q.8
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
Q.9
Inside the distillation column, the
Driving force for the vapour flow is the pressure drop
Liquids are not always at their bubble points
Pressure increases gradually from bottom to the top of the column
None of these
Q.10
The partial pressure distribution of an ideal gas diffusing through another stagnant ideal gas at steady state follows a/an __________ law.
Exponential
Parabolic
Linear
Cubic
Q.11
Leaching of coffee from coffee beans is done by
Hot water
Hexane
Lime water
Dilute H₂SO₄ (hot)
Q.12
The Hatta number is important in
Multicomponent distillation
Binary distillation
Gas absorption without chemical reaction
Gas absorption with chemical reaction
Q.13
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
Q.14
Stage efficiency in a leaching process depends on the
Time of contact between the solid and the solution
Rate of diffusion of the solute through the solid and into the liquid
Both A & B
Vapour pressure of the solution
Q.15
Berl saddle made of carbon cannot be used for
Alkalis
SO₂
H₂SO₄
Oxidising atmosphere
Q.16
During constant rate period, the rate of drying decreases with the
Decrease in air temperature
Increase in air humidity
Both A & B
Neither A nor B
Q.17
Drift in a cooling tower is
The water entrained by the circulating air
Dependent on the water lost by evaporation
Desirable
All of the above
Q.18
Pick out the correct statement.
Ponchan-Savarit method is more accurate than McCabe-Thiele method to determine the number of theoretical stages required for the separation of non-ideal binary system by distillation
A sharp decrease in pressure drop is an indication of flooding in a distillation coloumn
Solvent used in extractive distillation should be of high volatility
Flash distillation is suitable for separating components which have very close boiling temperature
Q.19
Flooding in a column results due to
High pressure drop
Low pressure drop
Low velocity of the liquid
High temperature
Q.20
The operating line for an absorber is curved, when plotted in terms of
Mole fractions
Mole ratios
Partial pressure
Mass fractions
Q.21
During dehumidification of unsaturated air wet bulb temperature & partial pressure of vapor are not constant. However, during evaporative cooling process with recirculated water supply, the __________ remains constant.
Wet bulb temperature
Partial pressure of water vapor
Relative humidity
None of these
Q.22
Which of the following liquid-vapor contacting devices provides maximum contact surface area for a particular duty?
Sieve plate column
Bubble cap column
Randomly packed column
Wetted wall column
Q.23
In a distillation operation, what is the effect of the temperature of the reflux stream (given below) on the condenser and reboiler loads? Reflux conditions:
(i) Reflux stream is completely liquid and is at its bubble point.
(ii) Reflux stream is below its bubble point.
Condenser and reboiler loads are the same in both the cases
Reboiler load is the same in both the cases but condenser load is higher in case (ii)
Condenser load is the same in both the cases but reboiler load is higher in case (ii)
Both condensor and reboiler loads are higher in case (ii) as compared to case (i)
Q.24
In the constant rate period of the rate of drying curve for batch drying,
Cracks develop on the surface of the solid
Rate of drying decreases abruptly
Surface evaporation of unbound moisture occurs
None of these
Q.25
The suitable evaporator for concentration of foaming liquids is a __________ evaporator.
Long tube
Short tube
Vacuum
Falling film
Q.26
At a given equilibrium pressure, with increase in temperature, the concentration of adsorbed gas on solid adsorbant
Increases
Decreases
Remains unchanged
Increases exponentially
Q.27
Leaching rate is independent of the
Agitation
Temperature
Particle size
None of these
Q.28
Which of the following remains constant during sensible cooling process ?
Specific humidity
Partial pressure of vapour
Both A & B
Neither A nor B
Q.29
The cooling effect in a cooling tower can be speeded up by
Increasing the air velocity and area of exposed wet surface
Reducing the barometric pressure
Reducing the humidity of the air
All of the above
Q.30
It takes 6 hours to dry a wet solid from 50% moisture content to the critical moisture content of 15%. How much longer it will take to dry the solid to 10% moisture content, under the same drying conditions? (The equilibrium moisture content of the solid is 5%).
15 min
51 min
71 min
94 min
Q.31
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
Q.32
Sticky material can be dried in a __________ dryer.
Tray
Rotary
Fluidised bed
None of these
Q.33
In steam distillation, the
Temperature is 100°C
Temperature is more than 100° C
Product must be immiscible with water
Temperature is higher than the boiling point of either component
Q.34
The commonly used solvent in supercritical extraction is
Methyl ethyl ketone
Water
Carbon tetrachloride
Carbon dioxide
Q.35
Gaseous diffusion co-efficient increases with increase in the
Pressure
Temperature
Both A & B
Neither A nor B
Q.36
With increase in solvent rate, the number of transfer units, NtoG, for a fixed degree of absorption from a fixed amount of gas
Increases
Decreases
Decreases linearly
Remains unaffected
Q.37
Which of the following has the same dimension as mass diffusivity?
Momentum flux
Kinematic viscosity
Thermal diffusivity
Both A & B
Q.38
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
Q.39
A packed tower compared to a plate tower for a particular mass transfer operation
Incurs smaller pressure drop
Allows installation of cooling coils
Is less costly when built in large sizes/diameters
Is more suitable, if suspended solids are present in fluid streams
Q.40
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
Q.41
Drying of refractory is normally carried out in a __________ dryer.
Open pan
Tray
Rotary vacuum
Truck/tunnel
Q.42
Which of the following quantities need not be fixed before starting the design of a co-current absorber?
Heat gain or loss
Flow rate of entering liquid
Flow rate of gas
None of these
Q.43
Pick out the wrong statement.
The Reynolds analogy for mass transfer is given by Lewis relation and is applicable, when Schmidt number is one
Sherwood number for flow in pipes can be expressed as the ratio of the concentration gradient at wall to the overall concentration difference
According to film theory for equimolar counter diffusion, the mass transfer coefficient is given by √D
AB
(b)P - 3, Q - 2
The z-component of the total mass flux of a component A in binary mixture of A and B is given by - Dab'.dC
A
/dz
Q.44
Stacked packing compared to dumped packing
Provides poorer contact between the fluids
Gives lower pressure drop
Both A & B
Gives higher pressure drop
Q.45
At __________ reflux, theoretical number of plates in a distillation coloumn is calculated using Fenske-Underwood equation.
Operating
Total
Minimum
Maximum permissible
Q.46
In case of unsaturated air
Dew point < wet bulb temperature
Wet bulb temperature < dry bulb temperature
Both A & B
Neither A nor B
Q.47
Water entrained by circulating air in cooling towers is termed as
Drift
Blow down
Vapor load
None of these
Q.48
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
Q.49
The Knudsen diffusivity is dependent on the
Molecular velocity only
Pore radius of the catalyst only
Molecular mean free path only
Molecular velocity and por radius of the catalyst
Q.50
Separation of the isotopes of helium for scientific investigations is done by
Reverse osmosis
Thermal diffusion
Leaching
Solvent extraction
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