Mass Transfer Part 4 Flashcards
Which of the following is a suitable absorbent for removal of H2S from natural gas?
Vetrocoke solution
Monoethanol amine (MEA)
Dilute
Hot water
Monoethanol amine (MEA)
Acetone is to be removed from air in an isothermal dilute absorber using pure water as solvent. The incoming air contains 5 mole% of acetone (yin = 0.05). The design equation to be used for obtaining the number of trays (N) of the absorber is, N+2 = 6 log (yin/yout).For 98% recovery of acetone, the number of trays required is/are
1
8
9
10
9
Y-out = 0.05 - (0.05 * 0.98) = 0.001
N = 6 * log(0.05/0.001)-2
N = 9
Schimdt number and Lewis number for pure air at atmospheric conditions are respectively
< 1 and = 1
1 and 0
>1 and = 1
0 and = 1
< 1 and = 1
Below is given an equilibrium moisture curve of a substance. State which regions represent free moisture and unbound moisture respectively.
image of curve
II, III
II, IV
IV, III
IV, II
II, III
Molecular diffusion is caused by the
transfer of molecules from low concentration to high concentration region.
thermal energy of the molecules.
activation energy of the molecules.
potential energy of the molecules.
thermal energy of the molecules.
molecular difusion thermal energy
At __________ reflux, theoretical number of plates in a distillation coloumn is calculated using Fenske-Underwood equation.
operating
total
minimum
maximum permissible
total
Separation of a mixture of two gases by absorption in the liquid solvent depends upon the difference in their
viscosity
density
solubility
relative volatility
solubility
Components, having widely different boiling point in a binary mixture, can be separated using __________ distillation.
molecular
extractive
steam
simple
simple
Which of the following equipments is not used in liquid-liquid extraction?
Pachuka tank
Agitated vessels
Centrifugal extractors
Packed towers
Pachuka tank
Pick out the wrong statement.
Packed towers are preferred for vacuum operation, because the pressure drop through the packing is less and they (packings) also lessen the possibility of tower wall collapse.
Packed towers are preferred over plate towers for handling corrosive and foamy liquids.
Diameter of randomly packed tower is normally more than 1.2 metres.
Due to uneven supply and improper distribution of liquid, problem of channeling, loading & flooding is generally encountered in packed towers.
Diameter of randomly packed tower is normally more than 1.2 metres.
Equilibrium-moisture curves of different solids
are different
are same
depend on the humidity of the gas
none of these
are different
Azeotropism for ethyl alcohol-water system disappears by
increasing temperature
decreasing pressure
increasing pressure
no means
decreasing pressure
Degree of freedom for leaching and solvent extraction process is
1
3
0
2
3
Diffusion co-efficient generally depends upon the temperature, pressure & the nature of the components of the system. Its dimension is not the same as that of the
mass transfer co-efficient.
thermal diffusivity.
kinematic viscosity.
volumetric diffusivity.
mass transfer co-efficient.
Equilibrium moisture of a solid is equal to the total moisture minus free moisture. Which of the following substances will have maxi-mum equilibrium moisture?
Anthracite coal
Rag paper
Bamboo
Inorganic solids
Bamboo
BMEM bamboo maximum equilibrium moisture
In the absorption of ammonia in water, the main resistance to absorption is by the __________ phase.
liquid
gas
both (a) & (b)
neither (a) nor (b)
gas
Drying of refractory is normally carried out in a __________ dryer.
open pan
tray
rotary vacuum
truck/tunnel.
truck/tunnel.
RTT refractory truck/tunnel
The expression, ln(F/W) = https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/314-454-1.png is
differential distillation equation
for NtoG in absorption for dilute mixture
slope of q-line in McCabe-Thiele method
for relative volatility
differential distillation equation
Heat sensitive materials with very high latent heat of vaporisation may be economically separated using
liquid extraction
distillation
evaporation
absorption
liquid extraction
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.
large quantity of solvent is required to extract the solute.
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
4.605
HETP is numerically equal to HTU, only when the operating line
lies below the equilibrium line.
lies above the equilibrium line.
and equilibrium lines are parallel.
is far from the equilibrium line.
and equilibrium lines are parallel.
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).
Both condensor and reboiler loads are higher in case (ii) as compared to case (i).
Steady state equimolal counter diffusion occurs in case of
leaching
absorption
binary phase distillation
liquid-liquid extraction
binary phase distillation
JD factor for mass transfer is a function of Reynolds number. Mass transfer by molecular diffusion from a single drop to surrounding still air is given by
Nsh = 2
Nst = 2
Nsc = 2
none of these
Nsh = 2
JD Nsh = 2
The relative saturation of an unsaturated mixture of gas and vapor is independent of the
nature of the vapor.
temperature of the mixture.
composition of the mixture.
none of these.
none of these.
The adiabatic saturation curve for a vapour-gas mixture is
straight line.
slightly concave upward.
slightly concave downward.
none of these.
slightly concave upward.
Pick out the wrong statement.
Cellulose acetate membranes are used in dialysis process.
Elutriation process refers to when the soluble material is largely concentrated on the surface of an insoluble solid and is simply washed off by the solvent.
Cascade’ means a group of stages interconnected so that various streams flow from one to another.
Zirconia probe is used for the humidity measurement.
Zirconia probe is used for the humidity measurement.
Zirconia wrong
The value of NA/(NA + NB), for steady state equimolal counter diffusion of two gases ‘A’ and ‘B’ is
1
∞
0.5
2
∞
Drift in a cooling tower is
the water entrained by the circulating air.
dependent on the water lost by evaporation.
desirable.
all (a), (b) & (c).
the water entrained by the circulating air.
With increase in the concentration of the adsorbate, the integral heat of adsorption
increases.
decreases.
remains unchanged.
may increase or decrease ; depends on the type of adsorbate.
decreases.
Small dia distillation column can be a
packed column
sieve tray
bubble cap
any of these
packed column
If mass diffusivity in a mixture is equal to the thermal diffusivity, then the Lewis number is
0
1
< 1
> 1
1
An air-water vapour mixture has a dry bulb temperature of 60 C and a dew point temperature of 40 C. The total pressure is 101.3 kPa and the vapour pressure of water at 40 C and 60 C are 7.30 kPa and 19.91 kPa respectively.The humidity of air sample expressed as kg of water vapour/kg of dry air is
0.048
0.079
0.122
0.152
0.152
__________ column is used in gas absorption process.
Wetted wall
Seive/perforated tray
Bubble cap
Packed
Packed
Nitrobenzene (boiling point = 210.6 C) is steam distilled at 1 atm pressure.Nitrobenzene will distill off __________ C.
at < 100
at > 210.6
between 100 and 210
none of these
at < 100
Pick out the wrong statement.
With decrease in feed temperature, the number of plates required for binary distillation decreases for a given L/V ratio.
The extraction per unit of total height for a short packed column is more effective than for a tall column, in case of liquid-liquid extraction.
Natural draft cooling towers are made of ferro-concrete and its cross-section is wider at the bottom.
Vanderwall adsorption is also called chemisorption.
Vanderwall adsorption is also called chemisorption.
wrong vanderwall chemisorption
At what value of reflux ratio, number of theoretical plates in a distillation column is minimum?
0
1
infinite
< 1
infinite
Batch tray dryers suffers from the disadvantage of
high maintenance cost.
non-uniform & low production rate.
high labour cost & low heat economy.
all ‘a’, ‘b’ & ‘c’.
high labour cost & low heat economy.
In a moderately sized packed absorption tower, channeling (which is most severe with stacked packings) can be minimised by taking the ratio of tower diameter to packing diameter
> 4 :1
< 8 : 1
> 8:1
< 6:1
> 8:1
__________ column is preferred to be used, when a high liquid hold up is required in a reactor for gas-liquid reaction.
Packed
Bubble
Spray
Tray
Bubble
The ‘shanks system’ of leaching (i.e., counter-current multiple contact leaching) is used for
recovery of tannis from the tree barks and woods.
leaching sodium nitrate from Chilean nitrate bearing rock.
both (a) and (b).
neither (a) nor (b).
both (a) and (b).
Penetration model (theory) for mass transfer was enunciated by
Danckwerts
Toor and Marchello
Higbie
Kissinevskii
Higbie
HP Higbie Penetration
Surface renewal model of mass transfer was enunciated by
Toor and Marchello
Fick
Danckwerts
Stefan
Danckwerts
When the __________ the relative humidity of air decreases, despite an increase in the absolute humidity.
temperature rises
pressure rises
temperature falls
pressure falls
temperature rises
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
Re^0.83
The relative humidity of air decreases in-spite of an increase in the absolute humidity, when the
temperature rises.
pressure rises.
temperature falls.
pressure falls.
temperature rises.
Unit of molal diffusivity is
cm2/sec gm . mole
gm moles/cm2 . sec
gm moles/cm. sec
gm moles/cm2. sec
gm moles/cm. sec
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
cold water leaving the tower and the wet bulb temperature of the surrounding air.
Binary distillation involves the mass transfer by __________ at the gas-liquid interface.
unidirectional diffusion from liquid to gas phase
unidirectional diffusion from gas to liquid phase
either ‘a’ or ‘b’
a counter diffusion at an almost equal molar rate
a counter diffusion at an almost equal molar rate
In a gas-liquid absorption coloumn, for obtaining the maximum absorption efficiency
liquid stream should be distributed uniformly.
gas stream should be distributed uniformly.
both gas as well as liquid streams should be distributed uniformly.
by passing should be completely avoided.
both gas as well as liquid streams should be distributed uniformly.
Reverse osmosis is also called
dialysis
electrodilysis
diffusion
ultrafiltration.
ultrafiltration.
Which of the following is not a batch drier?
Truck drier
Agitated pan drier
Fluidised bed drier
Vacuum shelf drier
Fluidised bed drier
Steam distillation is not recommended to be used, if the
azeotropic mixture is to be separated and the final product is miscible with water.
liquids decompose, if distilled directly at atmospheric pressure.
material can not be distilled by indirect heating even under low pressure, because of the high boiling temperature.
material to be distilled is thermally unstable or has the tendency to react with other components associated with it, at the boiling temperature.
azeotropic mixture is to be separated and the final product is miscible with water.
Rayleigh equation applies to __________ distillation.
differential
flash
equilibrium
molecular
differential
Mass transfer co-efficient ‘K’ according to penetration theory varies with mass dif-fusivity as
D^0.5
D
1/D
D^1.5
D^0.5
Most important factor to be considered in the selection of packings for absorbers is the __________ of packing.
size
durability
porosity
cost
porosity
__________ is the separation technique used for desalination of sea water.
Thermal diffusion
Reverse osmosis
Adsorption
absorption
Reverse osmosis
The relative saturation of a partially saturated mixture of vapour and gas can be increased by __________ of the mixture.
reducing the total pressure
increasing the total pressure
reducing the temperature
both (b) and (c)
both (b) and (c)
Danckwerts developed the __________ theory.
penetration
surface renewal
film
none of these
surface renewal
DSR Danckwerts Surface Renewal
DLSR camera
Molecular distillation is
high temperature distillation
for heat-sensitive materials
very low pressure distillation
both (b) and (c)
both (b) and (c)
Drying of a wet solid under constant drying conditions means the exposure of the wet solid to the air of constant
humidity
velocity
temperature
all (a), (b) & (c)
all (a), (b) & (c)
Which of the following provides maximum contact surface for a liquid-vapour system?
Packed tower
Bubble-cap plate column
Seive-plate column
Wetted wall column
Packed tower
For experimental determination of mass transfer co-efficient by wetted wall tower, the mass transfer area is
calculated
unknown
known
not required
known
In stripping section of continuous distillation column, the
liquid is stripped of high boiler.
liquid is enriched with high boiler.
vapour is stripped of low boiler.
none of these.
liquid is enriched with high boiler.
Molality is defined as the number of gm moles of solute per __________ of solvent.
litre
gm
Kg
m3
Kg
__________ is concerned with the adsorption equilibria.
Fick’s law
Gibb’s equation
Freundlich equation
none of these
Freundlich equation
FA Freundlich Adsorption
Fractional solvent extraction
employs only one solvent
employs two solvents
results in low interfacial tension
none of these
employs two solvents
In the layout plan for a vacuum distillation unit, operating at 60 mm Hg, supported by a barometric condenser, the appropriate place for the location of vacuum drum for collecting the distillate will be
at ground level.
2 metres above the ground.
5 metres above ground.
10 metres above ground.
at ground level.
Raschig ring made of procelain cannot be used for treating concentrated
hydrochloric acid
nitric acid
alkalis
sulphuric acid
alkalis
H2S present in naphtha reformed gas is removed by absorbing with
ethanolamine
K2CO3
HCl
vacuum gas oil
ethanolamine
ethanolamine H2S ethanolamine
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.
does not change with time,
Pick out the wrong statement.
Mass transfer co-efficient does not account for gase-liquid diffusion.
Diffusion co-efficient and mass transfer co-efficient are not same in any mass transfer operation.
Contacting cold vapor-gas mixture with hot liquid exemplifies humidification process.
Contacting cold liquid with warm vapor-gas mixture exemplifies dehumidification process.
Mass transfer co-efficient does not account for gase-liquid diffusion.
wrong does not account for gase-liquid diffusion
In rectifying section of a continuous distillation column, the
vapour is enriched with low boilers.
vapour is enriched with high boilers.
liquid is stripped of high boilers.
none of these.
vapour is enriched with high boilers.
For a fixed number of ideal stages in a distillation column, as the reflux ratio is increased, the difference in composition between the top and bottom product streams
increases
decreases
remains unaffected
passes through a maximum
increases
According to Chilton-Colburn analogy for mass transfer, NSt.NSc2/3 is equal to
f
f/2
2f
1/f
f/2
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
1.24
Minimum reflux ratio in a distillation column results in
optimum number of trays.
minimum reboiler size.
maximum condenser size.
minimum number of trays.
maximum condenser size.
Flooding in a distillation column is detected by a sharp
increase in Murphree plate efficiency.
decrease in pressure drop.
decrease in liquid hold up in the coloumn.
increase in pressure drop.
increase in pressure drop.
flooding increase pdrop
In extraction, as the temperature increases, the area of hetrogeneity (area covered by binodal curve)
decreases
increases
remains unchanged
none of these
decreases
Pick out the wrong statement.
The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are the same for NSc = NPr = 1.
1 Nm3 of dry air is lighter than 1Nm3 of humid air.
The Lewis number of a mixture is unity, when the thermal diffusivity is equal to the mass diffusivity.
An azeotropic mixture of two liquids has boiling point higher than either of them, when it shows negative deviation from Raoult’s Law.
1 Nm3 of dry air is lighter than 1Nm3 of humid air.
In case of an unsaturated mixture of gas and vapor, the percentage saturation is __________ its relative saturation.
lower than
higher than
equal to
either (a) or (b), depends on the mixture composition
lower than
At constant pressure, with increase of temperature, the dew point will
increase
decrease
remain unchanged
increase/decrease ; depends on the temperature
increase
For a single component absorber, the operating line is straight only when plotted in term of __________ units.
partial pressure
mole ratio
mole fraction
none of these
mole ratio
In case of unsaturated air
dew point < wet bulb temperature
wet bulb temperature < dry bulb temperature
both (a) and (b)
neither (a) not (b)
both (a) and (b)
For a binary mixture with low relative volatility,
use steam distillation.
use molecular distillation.
use high pressure distillation.
an azeotrope may be formed during distillation.
an azeotrope may be formed during distillation.
Plate efficiency
is a function of the mass transfer between liquid and vapour.
increases due to foaming.
increases due to liquid entrainment.
increases due to weeping and dumping of liquid.
is a function of the mass transfer between liquid and vapour.
Absorption accompanied by heat evolution results in
increased capacity of the absorber.
increase in equilibrium solubility.
decrease in equilibrium solubility.
none of these.
decrease in equilibrium solubility.
Even though bubble cap towers are very effective for humidification operation, they are not used commonly in industries, because of the
high evaporation losses of water.
high pressure drop of the gas.
difficulty in its fabrication.
none of these.
high pressure drop of the gas.
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.
number of plates required will be maximum.
A solid is being dried in the linear drying rate regime from moisture content Xo to XF. The drying rate is zero at X = 0 and the critical moisture content is the same as the initial moisture Xo. The drying time for M = (Ls/ARc) is (where, L = total mass of dry solid, A = total surface area for drying Rc = Constant maximum drying rate per unit area X = moisture content (in mass of water/mass of dry solids))
M(Xo - XF)
M(Xo/XF)
M ln(Xo/XF)
MXo ln(Xo/XF)
MXo ln(Xo/XF)
Flash distillation is suitable for the separation of components
having very close boiling points.
which form maximum boiling azeotrope.
having very wide boiling points.
which form minimum boiling azeotrope.
having very wide boiling points.
In Mcabe-Thiele method, at infinite reflux ratio
number of plates is maximum.
overhead product is maximum.
both the operating lines coincide with the diagram.
none of these.
both the operating lines coincide with the diagram.
Leaching rate is independent of the
agitation
temperature
particle size
none of these
none of these
Use of packed towers for distillation is generally limited to the
small sizes.
multicomponent distillation.
high pressure operation.
vacuum distillation.
small sizes.
Pick out the wrong statement.
The binary diffusivity in liquids is of the order of 10-5 cm2 /sec.
Molecular diffusion in solid is much faster than that in liquids.
Particles movement from higher concentration to lower concentration takes place in diffusion process.
According to Poiseuille’s law, the permeability decreases with increase in temperature for flow of a gas through a given capillary.
Molecular diffusion in solid is much faster than that in liquids.
Which of the following decreases during evaporative cooling process with recirculated water supply?
Wet bulb temperature
Relative humidity
Partial pressure of vapour
None of these
None of these
Laboratory determination of the dif-fusivity of vapor is done by __________ method.
wetted wall column
Gilliland’s
Winkel mann’s
Wike’s
Winkel mann’s
The minimum liquid rate to be used in an absorber corresponds to an operating line
of slope = 1
ofslope = 0.1
tangential to the equilibrium curve
none of these
tangential to the equilibrium curve
Unsaturated air (with dry and wet bulb temperatures being 35 and 25 C respectively) is passed through water spray chamber maintained at 35 C. The air will be
cooled
humidified
both (a) & (b)
dehumidified
humidified
For absorbing a sparingly soluble gas in a liquid, the
gas side co-efficient should be increased.
liquid side coefficient should be increased.
gas side co-efficient should be decreased.
liquid side co-efficient should be decreased.
liquid side coefficient should be increased.
A natural draft cooling tower is filled with wooden grids, which covers about __________ percent of the tower height.
10-15
30-40
70-80
90-100
10-15
The boiling points of pure components of a binary system __________ with increase in total pressure of the system.
decreases
increases
remains unchanged
may increase or decrease, depends on the system.
increases
The change in enthalpy, when a unit quantity of gas is absorbed by relatively large quantity of adsorbent (on which a definite concentration of the adsorbed gas already exists) is termed as the
differential heat of adsorption.
heat of wetting.
integral heat of adsorption.
none of these.
differential heat of adsorption.
Penetration theory relates the average mass transfer co-efficient (K) with diffusivity (D) as
K is proportional to D
K is proportional to sqrt(D)
K is proportional to D^1.5
K is proportional to D^2
K is proportional to sqrt(D)
penetration sqrt(D)
Desulphurisation of petrol is done in a
Bollman extractor
rotating disc extractor
packed extraction column
plate type extraction column
rotating disc extractor
PDRD Petrol Desulphurisation Rotating Disc
Chemisorption compared to physical absorption has
lower adsorption rate.
lower capacity of the solvent for the solute gas.
increased utilisation of stagnant zones of the liquid phase.
none of these.
increased utilisation of stagnant zones of the liquid phase.
Rate of adsorption increases as the
temperature increases.
temperature decreases.
pressure decreases.
size of adsorbent increases.
temperature decreases.
Pick out the system with maximum boiling azeotrope at 1 atm.
acetone chloroform
ethyl alcohol-water
benzene-ethyl alcohol
none of these
acetone chloroform
maximum boiling acetone chloroform
At 750 K and 1 atm, the approximate value of Schmidt number for air is
0.01
0.1
1
10
0.1
Calculation of mass transfer co-efficient is mostly/normally done using __________ theory.
surface renewal
film
penetration
none of these.
film
The velocity, concentration and temperature boundary, for the boundary layer development on a flat plate during connective mass transfer will be same, if
NSc = 1
NSc = NPr = NLe
NPr = NLe
NSc = NLe
NSc = NPr = NLe
For the gas absorption, the height of a transfer unit, based on the gas phase is given by (G: superficial molar gas velocity, L: superficial molar liquid velocity, FG:mass transfer co-efficient, mols/m2 , a: in-terfacial area per unit volume of tower)
G/FG.a
FG/G.a
G.a/FG
L/FG.G
G/FG.a
Which of the following is used for the concentration of rubber latex?
Agitated film evaporator
Long tube vertical evaporator
Short tube evaporator
Calendria
Agitated film evaporator
As the reflux ratio, in a continuous counter-current extraction is increased, the number of stages
increase
decrease
remain unchanged
can either increase or decrease, depends on the system
decrease
In triangular co-ordinates, the ternery composition point falls __________ of the triangle.
in the corners
inside
on the sides
none of these.
inside
In a single stage extraction process, 10 kg of pure solvent S (containing no solute A) is mixed with 30 kg of feed F containing A at a mass fraction xf = 0.2. The mixture splits into an extract phase E and a raf-finate phase R containing A at xB = 0.5 and xR = 0.05 respectively. The total mass of the extract phase is (in Kg)
6.89
8.89
10
8.25
8.89
Penetration theory states that the mass transfer co-coefficient is equal to (where, De is diffusivity and ‘t’ is time)
(De.t)^1/2
(De/t)^1/2
(4De/πt)^1/2
(4De/t)
(4De/πt)^1/2
Reboiler is considered as one theoretical plate, because
of the assumption that vapour and liquid leaving the reboiler are in equilibrium.
vapour is recycled to the column.
reboiler itself contains one plate.
none of these.
of the assumption that vapour and liquid leaving the reboiler are in equilibrium.
