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SECTION 3
On moving the feed line (q-line) from saturated liquid feed (vertical
position) to saturated vapor feed (horizontal feed), if the slope of
both the operating lines are to be increased, then it will result in
o greater degree of separation with a fixed number of trays
o increased reboiler load
o increased reflux ratio
o None of these.
__________ column is the most suitable for achieving the best
performance for mass transfer operations involving liquid with
dispersed solids.
o Watted wall
o Packed
o Plate
o Spray
Deairation (removal of O 2
) of water is done by
o rectification
o absorption
o ion-exchange
o adsorption
Make up water is supplied during cooling tower operation to compensate
for water losses due to
o evaporation
o blowdown
o entrainment
o all (a), (b)&(c)
Pick out the wrong statement.
o The mole fraction of solute is directly proportional to the
molality of a 'dilute' solution.
o For a non-reacting binary mixture of ideal gases, the partial
pressure distribution of both components is nonlinear in the case
of steady state unimolal unidirectional diffusion.
o Azeotropes obey Raoult's law at all temperature.
o The relative volatility of a binary mixture at the azeotropic
composition is 1
A minimum boiling azeotrope is exemplified by
o toluene-benzene
o acetone-carbon disulphide
o water-isobutyl alcohol
o none of these
Which of the following solutions will follow Raoult's law most closely
o A solution of benzene, toluene and oxylene.
o 35% solution of camphor in water.
o 35% solution of NH 3
in water.
o A solution of polar organic compounds (not of homologs of a
series).
Use of raschig rings in place of crushed stones as packing in packed
beds (other things being same)
o increases pressure drop, increases surface area.
o increases pressure drop, decreases surface area.
o decreases pressure drop, increases surface area.
o decreases pressure drop, decreases surface area.
Critical humidity of a solid salt means the humidity
o above which it will always become damp.
o below which it will always stay dry.
o both (a) and (b).
o above Which it will always become dry and below which it will
always stay damp.
The assumption made in Elis method in distillation is that enthalpy
concentration lines of vapor and liquid are
o parallel
o straight
o both parallel and straight
o neither parallel nor straight
As complete saturation of an adsorbent is approached, the differential
heat of adsrop-tion approaches
o that of normal condensation.
o integral heat of adsorption.
o zero.
o none of these.
'Absorption with chemical reaction' is involved in the removal of
o carbon dioxide from gaseous stream using alkaline solution.
o benzol from coke oven gas using solar oil/wash oil.
o ammonia from coke oven gas using refrigerated water.
o single spiral
When the feed to a distillation column is a saturated liquid, slope of
the feed line is
o zero
o unity
o infinity
o none of these
Leaching of sugar from sugar beets is done by
o hot water
o hexane
o dilute H 2
SO
4
o lime water
Absorption factor method is used to calculate the number of ideal
stages, when
o operating line lies above the equilibrium line.
o operating line lies below the equilibrium line.
o both operating and equilibrium lines are parallel.
o pressure drop in the column is very high.
Flights in a rotary dryer are provided to
o lift and shower the solids thus exposing it thoroughly to the
drying action of the gas.
o reduce the residence time of solid.
o increase the residence time of the solid.
o none of these.
If f = moles of vapor present per mole of feed, then the slope of feed
line is (Mcabe-Thiele method)
o
o
o
o
If the solubilities of different components (in a liquid-liquid
extraction system) increase with rise in temperature, then the
temperature above which they dissolve completely is known as the
critical solution temperature (CST or consolute temperature). If
solubilities increase with decrease in temperature, then CST is the
temperature below which they dissolve completely. If a binary system
has no critical solution temperature, it implies that
o the system comprises of partially mis-cible liquids.
o the system comprises of miscible liquids.
o the system comprises of an azeotrope.
o on heating, a vapor phase will appear ; while on cooling, a solid
phase will appear.
The rate of mass transfer is not dependent upon the
o degree of dispersion of one fluid in the other.
o interfacial surface exposed between the phases.
o both (a) & (b).
o none of these.
At equilibrium the concentration of water in vapor phase (C
)in
kg/m
3
of air space and the amount of water (m) adsorbed per kg of dry
silica gel are related by, C
= 0.0667m. To maintain dry conditions in
a room of air space 100m
3
containing 2.2 kg of water vapor initially,
10 kg of dry silica gel is kept in the room. The fraction of initial
water remaining in the air space after a long time (during which the
temperature is maintained constant) is
o 0.
o 0.
o 0.
o 1.
Dry bulb temperature of unsaturated air is more than its __________
temperature.
o dew point
o wet bulb
o both 'a' & 'b'
o neither 'a' nor 'b'
For the case of flow of air past a wet bulb thermometer (air water
vapor system), the approximate value of h 2
/ k y
. C
z
is around
o 0.
o 0.
o 1
o 0.
The reciprocal of stripping factor is termed as
o selectivity index
o relative volatility
o absorption factor
o Murphree efficiency
For absorbing a sparingly soluble gas in a liquid
o gas side mass transfer co-efficient should be increased.
o liquid side mass transfer co-efficient should be increased.
o liquid side mass transfer co-efficient should be decreased.
o mass transfer co-efficient must be kept constant.
Channelling in a packed tower results from the
o high pressure drop
o maldistribution of liquid
o non-uniformity of packing
o both (b) and (c)
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
o integral heat of adsorption.
o heat of wetting.
o differential heat of adsorption.
o heat of normal condensation.
The minimum number of theoretical plates is required for achieving a
given separation in distillation column with
o no reflux
o total reflux
o zero reflux ratio
o minimum reflux ratio
A good solvent used for absorption should not have very high
o viscosity
o vapor pressure
o freezing point
o all (a), (b) & (c)
In a counter-current liquid extractor
o both liquids flow at fixed rate.
o both liquids can have any desired flow rate.
o only one of the liquids may be pumped at any desired rate.
o liquid's flow rate depends upon the temperature and pressure.
Which of the following is an undesirable property for an absorbing
solvent?
o Low vapor pressure
o Low velocity
o Low freezing point
o None of these
The wetted wall tower is used to determine
o individual mass transfer co-efficient (M.T.C.) in gaseous system.
o M.T.C. of individual components in a liquid-liquid system.
o M.T.C. of liquid in liquid-gas system.
o the overall M.T.C. of the system.
Weight of 1 m
3
of humid air as compared to 1 m
3
of dry air, under the
same conditions, is
o less
o more
o same
o unpredictable
Positive deviation from Raoult's law means a mixture whose total
pressure is
o greater than that computed for ideality.
o less than that computed for ideality.
o less than the sum of the vapor pressure of the components.
o none of these.
With increase in gas rate, the number of transfer units, N toG
, for a
fixed degree of absorption by a fixed amount of solvent
o increases
o decreases
o decreases linearly
o remains unaffected
Schmidt number is given by
o
o Re.Pe
o Sh x Pe
o Re/Pe
The caking of crystals can be prevented by
o maintaining high critical humidity.
o maintaining low critical humidity.
o coating the product with inert material.
o both (a) and (c)
Prandtl number for water at 20°C is about
o 7
o 70
The apex of an equilateral-triangular coordinate (in ternary liquid
system) represents a/an
o pure component
o binary mixture
o ternary mixture
o insoluble binary system
The diffusivity ( D ) in a binary gas mixture is related to the pressure
P
) as
o D ∝ P
o
o
o
A slurry is to be dried to produce flaky solid. Which dryer would you
recommend?
o Spray dryer
o Drum dryer
o Rotary dryer
o Dielectric dryer
Which of the following psychrometric processes is followed by water
vapor laden unsaturated air, when it is passed through solid or liquid
adsorbent?
o Cooling and dehumidification.
o Heating and dehumidification at almost constant wet bulb
temperature.
o Dehumidification with dry bulb temperature remaining constant.
o None of these.
- In gas-liquid contact operation, the number of ideal
stages,. This is true when the stripping factor ' S ' is
o S > 1
o S < 1
o S = 1
o S = ∞
The cooling effect in a cooling tower can not be increased by
o increasing the air velocity over the wet surfaces.
o reducing the humidity of entering air.
o lowering the barometric pressure.
o none of these.
The value of N A
/( N
A
+ N
B
) for steady state molecular diffusion of gas
' A ' through non-diffusing gas ' B ' is
o 1
o ∞
o 0.
o 2
The humid volume may be increased by increasing the
o temperature at constant humidity.
o humidity at constant temperature.
o both (a) and (b).
o neither (a) nor (b).
Pick out the wrong statement
o Co-current absorbers are usually used, when the gas to be
dissolved in the liquid is a pure substance.
o In case of gas absorption, HETP is equal to HTU, when both the
operating line & the equilibrium lines are parallel.
o Hatta number is important in case of gas absorption with chemical
reaction.
o In actual practice, absorption is an endothermic process, while
stripping is an exothermic process.
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.
o continuous
o steam
o azeotropic
o none of these
o surface tension
o pressure
Dorr thickener is an equipment used for
o classification
o sedimentation
o clarification
o leaching
Steam distillation is used to separate
o azeotropes.
o high boiling substances from nonvolatile impurities.
o heat sensitive materials.
o mixtures of low relative volatility.
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
o q > 1
o q < 1
o q = 1
o q = 0
Which of the following is an undesirable property in a tower packing?
o Large surface per unit volume.
o Large free cross-section.
o Low weight per unit volume.
o Large weight of liquid retained.
Water entrained by circulating air in cooling towers is termed as
o drift
o blow down
o vapor load
o none of these.
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
o 1000
o 1500
o 2000
o 2500
With increase in temperature, the rate of leaching increases, because
the
o diffusivity of solute increases.
o viscosity of solvent decreases.
o thermal conductivity of solute increases.
o none of these.
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.
o molal
o relative
o percentage
o none of these
The Kundsen diffusivity is proportional to (where, T = absolute
temperature)
o T
o T
2
o T
o T
4
In case of constant underflow extraction operation, the __________ at
all solute concentration.
o solids are drained to the same extent
o ratio of the insoluble to solution is constant
o both(a) & (b)
o neither (a) nor (b)
In an absorption coloumn, the flooding velocity for random packing is
__________ that for stacked/regular packing.
o greater than
o smaller than
o equal to
o either (a), or (b) ; depends on the type of packing
The relation among various mass transfer co-efficients (M.T.C) for
ideal gases is given by (where, K c
& K
m
are M.T.C. for equimolar
counter diffusion with concentration & mole fraction respectively as
the driving force. and, K p
= M.T.C. for diffusion of a gas through a
stagnant inert gas with pressure as driving force.)
o K c
= K
p
= K
m
o K c
= K
p
/ RT = Km. RT / P
o K c
= K
p
. RT = Km. RT / p
o None of these
In a binary distillation column, if the feed contains 40 mole % vapor,
the q line will have a slope of
o 1.
o - 0.
o - 1.
o 0.
Pick out the wrong statement.
o Mechanical agitation is required, if the system has low
interfacial tension.
o Seive tray towers are generally used for systems having low
interfacial tension.
o When Henry's law constant is very small, then the mass transfer
rate is controlled by the gas film resistance.
o Taylor-Prandtl analogy for heat and mass transfer considers the
transfer through both laminar and turbulent layers.
Acetic acid will be most economically separated from a dilute solution
of acetic acid in water by
o solvent extraction
o continuous distillation
o evaporation
o absorption
Design calculation for multiple component distillation is done by
o Ponchon-Savarit method
o Mccabe-Thiele method
o enthalpy concentration method
o tray to tray calculations.
To get high tray efficiency
o interfacial surface between liquid and gas phase should be large.
o time of contact between the two phases should be less.
o gas velocity should be very low.
o liquid entrainment should be severe.
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
o D/a
o D/2a
o proportional to D
o dependent on the Reynolds number
Pick out the wrong statement. Generally for physical adsorption, a gas
of
o higher molecular weight is adsorbed in preference to a gas of low
molecular weight.
o high critical temperature is adsorbed in preference to a gas of
low critical temperature.
o low volatility is adsorbed in preference to a gas of high
volatility.
o none of these.
In most of the mechanically agitated liquid-liquid extractors, baffles
or horizontal compartmental plates are provided, which helps in
reducing the axial mixing.
o increasing the rate of extraction.
o maintaining the concentration difference between the two phases.
o all 'a', 'b' & 'c'.
In a vapor-liquid contacting equipment, the overall gas phase mass
transfer co-efficient (M.T.C), K G
is related to individual co-
efficients ( K G
and K L
) as
o
o
o
o
SECTION 5
Solvent extraction is the terminology applied to the liquid-liquid
extraction, which is preferred for the separation of the components of
liquids, when
o extracting solvent is cheaply & abundantly available.
o one of the liquid components is heat sensitive.
o viscosity of liquid components is very high.
o one of the liquid components has very high affinity towards the
solvent.
Inside the distillation column, the
o driving force for the vapor flow is the pressure drop.
o liquids are not always at their bubble points.
An azeotropic mixture is a __________ mixture.
o binary
o ternary
o constant boiling point
o none of these
Pick out the wrong statement.
o In the McCabe-Thiele diagram for binary distillation, vertical
feed line represents saturated liquid feed and horizontal feed
line represents saturated vapor feed.
o In small columns, local efficiency is larger than Murphree
efficiency; in large columns local efficiency is smaller than
Murphree efficiency.
o For Laminar flow over a plate of length L, the local mass
transfer co-efficient at a distance L from the leading edge is
1.5 x 10
m/s. Then the average mass transfer co-efficient for the
plate is 2 x 10
m/s.
o The concentration and hydrodynamic boundary layers over a flat
plate are of equal thickness, if Schmidt number is equal to
unity.
Molecular seives are regenerated by heating to __________ °C.
o < 150
o 200 - 330
o > 500
o > 1000
In continuous distillation, the internal reflux ratio ( R i
) and the
external reflux ratio ( R e
) are related as
o
o
o R i
= R
e
o
In a counter-current extractor, as the axial mixing increases, the
extraction efficiency
o increases
o decreases
o remains unchanged
o depends on the pressure of the system
Heat sensitive materials like certain pharmaceuticals and food stuffs
can be dried in a/an __________ dryer.
o indirect tray
o spray
o freeze
o none of these
In extractive distillation, solvent is
o added to alter the relative volatility of the mixture.
o of high volatility.
o present in overhead stream.
o of high viscosity to give high tray efficiency.
Radioactive nuclear waste is treated in
o mixer-settler extractor
o rotating-disc contactor
o pulsed column extractor
o Bollman extractor
Large scale drying of sugar is done in a __________ dryer.
o spouted bed
o tray
o rotary
o truck
__________ temperature is the steady state temperature attained by a
small amount of liquid evaporating into a large quantity of
unsaturated gas-vapor mixture.
o Dry bulb
o Wet bulb
o Dew point
o none of these
The drying time between fixed moisture contents within the 'constant
rate period' is proportional to (assuming that drying occurs from all
surfaces of the solid) (where, T = thickness of the solid )
o T
o T
o T
o T
3
__________ equation applies to diffusivities in liquids.
o Gilliland
o Hirschfelder
o Wilke and Chang
o none of these
