IndiaBIX Chemical Engineering: Mass Transfer Section 8 Flashcards
__________ is used as an ‘entrainer’ for the separation of acetic acid-water mixture by distillation.
Furfural
Benzol
Butyl acetate
Hexane
Butyl acetate
Rotary dryers cannot handle __________ materials.
free flowing
dry
sticky
grannular
sticky
Pick out the correct statement.
Diffusivity decreases with increase in temperature.
Diffusivity increases with increase in molecular weight.
Diffusivity increases with the size of the individual molecule.
none of these.
none of these.
An example of elutioh is
separation of uranium oxide from its ore by H2SO4 in a Pachuca tank.
separation of sugar from sugar beet by hot water.
dissolution of tannin out of tree barks by water.
recovery of vegetable oils from seeds.
separation of uranium oxide from its ore by H
The operating line for an absorber is curved, when plotted in terms of
mole fractions
mole ratios
partial pressure
mass fractions
partial pressure
Relative volatility does not change appreciably with the change in
temperature
vapour pressure of either component
total pressure
none of these
temperature
Boiling point diagram is not affected by the ambient
pressure
humidity
temperature
both (b) and (c)
both (b) and (c)
The dew point of a saturated gas phase equals the __________ temperature.
gas
room
wet bulb
none of these
gas
A solid material shows case hardening properties while drying. Which of the following should be controlled to control the drying process ?
Flow rate of inlet air
Relative humidity of outlet air
Humidity of inlet air
Temperature of the solid
Temperature of the solid
In case of liquid-liquid binary diffusion, diffusivity of one constituent into another is not dependent on the
temperature and pressure
concentration
nature of the constituents
none of these
none of these
For continuous drying of grannular or crystalline material, the dryer used is the __________ dryer.
tunnel
tray
rotary
none of these
rotary
When adsorption hysteresis is observed, the desorption equilibrium pressure is __________ that obtained by adsorption.
always lower than
always higher than
same as
dependent on the system; can be either lower or higher than
always lower than
Moisture in a substance exerting an equilibrium vapour pressure less than that of pure liquid at the same temperature is called the __________ moisture.
bound
unbound
critical
none of these
bound
For a binary mixture with low relative volatility, continuous rectification to get pure products will require
low reflux ratio
less number of trays
small cross-section column
high reflux ratio
high reflux ratio
The efficiency of a plate column will be maximum, when the flow is ideal __________ the plate.
across
above
both (a) & (b)
neither (a) nor (b)
both (a) & (b)
Heat load in a cooling tower
means the amount of heat thrown away (KCal/hr.) by the cooling tower.
is equal to the number of kg, of water circulated times the cooling range.
both (a) & (b).
neither (a) nor (b).
both (a) & (b).
In case of absorption with exothermic reaction, for fluids having
Pr = Sc; percentage change in heat and mass transfer flux will be the same for a given change in the degree of turbulence.
Pr = Sc = 1; total mass, momentum and thermal diffusivity will be the same.
both (a) and (b)
Pr = Sc ; there won’t be any change in heat and mass transfer flux with changes in degree of turbulence.
both (a) and (b)
Detergent solution is dried to a powder in a
spray dryer
spouted bed dryer
tunnel dryer
pan open to atmosphere
spray dryer
Which of the following mixtures does not form an azeotrope at atmosheric pressure?
Water-alcohol
Methyl alcohol-acetone
Butyl acetate-water
None of these
None of these
During bottling of cold drinks (e.g. Pepsi), the main resistance to mass transfer for the absorption of carbon dioxide in water lies in the
gas film
liquid film
liquid-gas interface
none of these
liquid film
The overall mass transfer co-efficient for the absorption of SO2 in air with dilute NaOH solution can be increased substantially by
increasing the gas film co-efficient.
increasing the liquid film co-efficient.
increasing the total pressure.
decreasing the total pressure.
increasing the gas film co-efficient.
Flooding results in
high tray efficiency
low tray efficiency
high gas velocity
good contact between the fluids
low tray efficiency
For the air water system under ambient conditions, the adiabatic saturation temperature and the wet bulb temperature are nearly equal, because
water has a high latent heat of evaporation.
Lewis number is close to unity.
they are always equal under all circumstances.
solubility of the components of air in water is very small.
water has a high latent heat of evaporation.
In an operating distillation column, the
vapors and liquids are at their dew point and bubble point respectively
driving force for the liquid flow is its specific weight.
driving force for the vapor flow is the pressure drop, as the pressure decreases gradually from the bottom to the top of the column.
highest temperature is encountered at the top of the column.
driving force for the vapor flow is the pressure drop, as the pressure decreases gradually from the bottom to the top of the column.
In a forced draft cooling tower, water is cooled from 95 to 80°F by exposure to air with a wet bulb temperature of 70°F. In this case, the
range is 15°F.
approach is 10°F.
both (a) & (b).
neither (a) nor (b).
both (a) & (b).
The diffusivity (D) in a binary gas mixture is related to the temperature (T) as
D ∠T
D ∠T^0.5
D ∠T^1.5
D ∠T^2
D ∠T^1.5
For which of the following unit operations, Lewis number is of significance ?
Adsorption
Binary distillation
Gas absorption
Humidification
Humidification
Tray spacing in a distillation column of dia 10-12 ft. used in petroleum refinery may be around __________ inches.
5
30
60
100
30
If a1 and a2 are the relative volatilities when the pressure in the distillation column is 1 and 2 atm respectively. Pick out the correct statement.
a1 = a2
a1 = 2a2
a1 = 0.5 a2
none of these
none of these
The dimension of diffusivity is same as that of the
density
molal concentration
kinematic viscosity
velocity head
kinematic viscosity
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
none of these
Entrainer used in azeotropic distillation should
form a low boiling azeotrope with one of the constituents of the mixture.
form a new azeotrope of low relative volatility with one of the constituents of the mixture.
have high latent heat of vaporisation.
have high viscosity to provide high tray efficiency.
form a low boiling azeotrope with one of the constituents of the mixture.
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.
mixing of steam with the material being vaporised is not so intimate as to result in equilibrium condition.
Pick out the wrong statement.
The operating line always lies above the equilibrium solubility curve in case of a single component absorber.
The operating line always lies below the equilibrium solubility curve in case of a single component stripper.
The operating pressure in solvent extraction system should be below vapor pressures of all components.
The solubility of a particular gas in ideal solution in any solvent is always the same.
The operating pressure in solvent extraction system should be below vapor pressures of all components.
Leaching of coffee from coffee beans is done by
hot water
hexane
lime water
dilute H2SO4 (hot)
hot water
For contacting a highly soluble gas with a liquid
bubble the gas through liquid.
spray the liquid on gas stream.
either (a) or (b) would suffice.
none of these.
bubble the gas through liquid.
During drying operation, it is easier to remove the __________ moisture.
equilibrium
critical
unbound
bound
unbound
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
decreases
Ion exchange process is similar to
absorption
adsorption
extraction
leaching
adsorption
A distillation column with N plates is being operated under normal conditions. At some point of time, the operation is shifted to total reflux condition (i.e., no product and residue are being withdrawn and feed to the column is stopped). At the new steady state,
composition of vapors and that of liquid do not vary throughout the column.
reboiler load and condenser load are minimum.
the top and bottom compositions are unchanged with and without total reflux.
the top and bottom compositions correspond to the maximum enrichment available.
the top and bottom compositions correspond to the maximum enrichment available.
Dryer widely used in a textile industry is __________ dryer.
festoon
cylinder
conveyor
tunnel
conveyor
Solvent used in extractive distillation
changes the relative volatility of the original components.
should be of high volatility.
should form azeotropes with the original components.
all (a), (b) and (c)
changes the relative volatility of the original components.
Air at a particular humidity is heated in a furnace. The new dew point
decreases
increases
depends on the extent of heating
remain unchanged
increases
In a packed tower, the value of HETP equals HTUOG, when the equilibrium and the operating lines are (where, HETP = height equivalant to a theoretical plate HTUOG = overall gas phase height of a transfer unit)
straight
parallel
both (a) & (b)
neither (a) nor (b)
both (a) & (b)
The vapour pressure exerted by the moisture contained in a wet solid depends upon the
nature of the moisture.
temperature.
nature of the solid.
all (a), (b) and (c).
all (a), (b) and (c).
The absorption factor is defined as (where, L = liquid flow rate, G = gas flow rate and, m = slope of the equilibrium line)
L/mG
G/mL
mL/G
LG/m
L/mG
Smoke is an example of
solid dispersed in liquid
solid dispersed in gas
liquid dispersed in gas
gas dispersed in liquid
solid dispersed in gas
Which of the following adsorbant is used in the refinning of sugar ?
Bone charcoal
Wood charcoal
Silica gel
Activated clay
Bone charcoal
Stacked packing compared to dumped packing
provides poorer contact between the fluids.
gives lower pressure drop.
both (a) and (b).
gives higher pressure drop.
both (a) and (b).
In McCabe-Thiele method of theoretical plate calculation for a distillation column, the operating lines of stripping and rectifying sections coincide with the diagonal at __________ reflux.
total
minimum
operating
maximum permissible
total
