Mass Transfer 2 Flashcards
Hypersorption refers to a __________ process.
moving bed adsorption
Extraction of coffee from its seed is done by
leaching.
In azeotropic mixture, the equilibrium vapour composition is
same as liquid composition.
With lapse of time, the overhead composition of light component in case of batch distillation with constant reflux
decreases
Lewis number, which is important in problems involving simultaneous heat and mass transfer, is the ratio of
thermal diffusivity to mass diffusivity.
According to the film theory of mass transfer, the mass transfer co-efficient is proportional to (where, D = molecular diffusivity)
D
Dry bulb temperature of the gas is __________ the wet bulb temperature.
more than
Decoction refers to the use of solvent
at its boiling point
Flash distillation is suitable for separating the constituents of a binary system, which
have very wide boiling points.
The ratio of Murphree plate efficiency to point efficiency is 1 in a __________ flow model.
perfectly mixed
The rough value of diffusion co-efficient of water vapor into air at 25°C may be about __________ cm2/sec.
0.25
Flooding in a column results due to
high pressure drop
Which of the following gas-liquid contacting devices incurs the least pressure drop for a particular duty?
Wetted wall tower
A liquid mixture contains 30% o-xylene, 60% p-xylene and 10%. m-xylene (all percentages in w/w). Which of the following statements would be true in respect of this mixture?
The mixture exhibits an azeotrope at 101.3 kPa.
Unsaturated air (with dry bulb temperature and dew point being 35°C and 18°C respectively) is passed through a water spray chamber maintained at 15°C. The air will be cooled
and dehumidified with decrease in wet bulb temperature.
The process employing desorption of the absorbed solute by a solvent is called
elution
If the overall efficiency and Murphree plate efficiency are equal, then both the equilibrium and operating lines are
both (a) & (b)
Which of the following operations does not involve leaching?
Removing nicotine from its water solution by kerosene.
The recovery of pencillin from the acidified fermentation broth is done by
liquid extraction
Unit operation involved in the prilling of urea is
both (b) and (c)
Molecular seives are porous
synthetic zeolites crystals/metal alumino-silicates.
Calcium ammonium nitrate (a fertiliser) is dried in a __________ dryer.
rotary
A binary mixture of benzene and cyclohexane is separated by
extractive distillation
Component A is diffusing in a medium B. The flux NA relative to a stationary point is equal to the flux due to molecular difusion, if
there is equimolar counter diffusion.
Cox chart is useful in the design of
distillation column
Dew point of an air-water vapor mixture
all ‘a’, ‘b’ & ‘c’.
Can a cooling tower cool water below the wet bulb temperature of inlet air?
no
The solvent used in liquid-liquid extraction should have __________ less than one.
neither (a) nor (b).
The suitable evaporator for concentration of foaming liquids is a __________ evaporator.
long tube
The ratio of Murphree plate efficiency to point efficiency is always > 1 in a __________ flow model.
plug
Tea percolation employs
leaching
Which of the following assumes constant molal vaporisation and overflow?
McCabe-Thiele method
Absorption factor, for a fixed degree of absorption from a fixed amount of gas should be
> 1
Selectivity of the solvent used in solvent extraction should be
> 1
Pick out the wrong statement pertaining to leaching.
Fine solids can not be leached in a Pachuka tank.
Wet bulb and dry bulb temperatures becomes identical at __________ percent saturation curve.
100
NSc/NPr is called the
Lewis number
McCabe-Thiele method
uses molal units for material and energy balance.
The film penetration model of mass transfer was enunciated by
Toor and Marchello
The most economical range of absorption factor is
1.25 to 2
Channelling is most severe
in towers packed with stacked packing.
Out of two equilibrium curves, first one lies above the diagonal and the second lies below the diagonal in the x-y plot. It means that separation by distillation is
more readily done in the second case compared to first one.
ADDUCTS’ and ‘CLATHRATES’ are used in case of
crystallisation.
Adsorption is an unit operation which generally involves __________ systems.
fluid-solid
In case of __________ distillation, a solvent is added to alter the relative volatility of the mixture to be separated.
extractive
Separation of the isotopes of helium for scientific investigations is done by
thermal diffusion
In a binary system, separation is very efficient, when the relative volatility is
> 1
Fenske-Underwood equation used for calculating the minimum number of theoretical plates in distillation column is valid for
all (a), (b) and (c)
The binary diffusivity in gases does not depend upon the
none of these
Berl saddle made of carbon cannot be used for
oxidising atmosphere
1 kg of a saturated aqueous solution of a highly soluble component A at 60°C is cooled to 25°C. The solubility limits of A are (0.6 kgA/kg water) at 60°C and (0.2 kgA/kg water) at 25°C. The amount in kg of the crystals formed is
0.2
During drying of a solid, the critical moisture content increases with
both (b) and (c).
Priming in a distillation column
is characterised by the presence of foam throughout the space between trays.
The commonly used solvent in supercritical extraction is
carbon dioxide.
When the liquid over a plate is of uniform concentration, then
Murphree efficiency = point efficiency.
In saturated gas, the
vapour is in equilibrium with the liquid at the gas temperature.
As per Gilliland’s equation, the diffusivity is
all ‘a’, ‘b’ & ‘c’.
Pick out the wrong statement.
The operating line lies above the equilibrium curve in case of a gas desorber.
A mixture of 10% C6H6 vapour in air at 25°C and 750 mm Hg has a dew point of 20°C. Its dew point at 30°C and 700 mm Hg will be around __________ °C.
18.7
Which of the following is not fixed by the process requirements, in the design of absorbers?
None of these.
If the amount of the steam used in steam distillation is increased, the temperature of distillation
decreases
Moisture in a solid exerting an equilibrium vapour pressure equal to that of the pure liquid at the same temperature is called the __________ moisture.
unbound
Inside the distillation columns, the
driving force for the liquid flow is its weight.
When the psychometric ratio is __________ then the adiabatic saturation temperature and wet bulb temperature becomes equal.
1
Desirable value of absorption factor in an absorber is
> 1
Refractory bricks are usually dried in a __________ dryer.
tunnel
For __________ drying, the effects of velocity, temperature and humidity of the gas and the thickness of the solid are the same.
both (a) and (b)
What is the degree of freedom of an absorption process in which only one component is transferred between phases?
3
Sticky material can be dried in a __________ dryer.
tray
Langmuir equation is associated with
adsorption
During the constant rate period of drying of a solid,
surface evaporation of unbound moisture occurs.
At a fixed total pressure, humidity depends only on the
partial pressure of vapour in the mixture.
If moisture content of solid on dry basis is X, then the same on wet basis is
X/X+1
Flash distillation is
used for multicomponent systems like crude refining.
__________ developed the film theory.
Higbie
For absorbers, high pressure drop results in
high operating cost.
Which of the following parameters remains almost constant during adiabatic saturation of unsaturated air?
Wet bulb temperature
Coffee is prepared from coffee beans by leaching with
hot water
Gaseous diffusion co-efficient increases with increase in the
temperature
With increase in absolute humidity, the dew point of an unsaturated mixture of air and water vapor (at constant pressure and temperature)
increases
Pick out the wrong statement. Raoult’s law does not apply, when the
component molecules are nonpolar.
Water-ethyl alcohal solution can be concentrated (in respect of alcohal concentration) by __________ distillation.
vacuum
L/mG is the expression for
absorption factor
Extraction of pencillin employs a __________ extraction coloumn.
centrifugal
In case of gases, the binary diffusivity is proportional to (where, P = pressure)
1/P
When the gas to be dissolved in liquid is a/an __________ then normally co-current adsorber are used.
pure gas
Corresponding to Prandtl number in heat transfer, the dimensionless group in mass transfer is the __________ number.
Schmidt
If a two phase system is in physical equilibrium; then it means that, the
all (a), (b) and (c).
McCabe-Thiele method of binary distillation does not assume that the
none of these
(NSh/NRe.NSc) is termed in mass transfer operation as the
Stanton number
The Hatta number is important in
gas absorption with chemical reaction.
When a multistage tray tower uses a minimum reflux ratio, it implies
infinite trays and minimum reboiler heat load.
CO2 can be absorbed by
heated charcoal
For the same process conditions, the reflux ratio of an unlagged distillation column
increases in winter
An ideal tower packing should not
hold up large amount of liquid in the packing.
Which of the following is a suitable absorbent for removal of H2S from natural gas?
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
9
Schimdt number and Lewis number for pure air at atmospheric conditions are respectively
< 1 and ≈ 1
Below is given an equilibrium moisture curve of a substance. State which regions represent free moisture and unbound moisture respectively. https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/292-170-1.png
II, III
Molecular diffusion is caused by the
thermal energy of the molecules.
At __________ reflux, theoretical number of plates in a distillation coloumn is calculated using Fenske-Underwood equation.
total
Separation of a mixture of two gases by absorption in the liquid solvent depends upon the difference in their
solubility
Components, having widely different boiling point in a binary mixture, can be separated using __________ distillation.
simple
Which of the following equipments is not used in liquid-liquid extraction?
Pachuka tank
Pick out the wrong statement.
Diameter of randomly packed tower is normally more than 1.2 metres.
Equilibrium-moisture curves of different solids
are different
Azeotropism for ethyl alcohol-water system disappears by
decreasing pressure
Degree of freedom for leaching and solvent extraction process is
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.
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?
Bamboo
In the absorption of ammonia in water, the main resistance to absorption is by the __________ phase.
gas
Drying of refractory is normally carried out in a __________ dryer.
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
Heat sensitive materials with very high latent heat of vaporisation may be economically separated using
liquid extraction
When the solvent dissolves very little of solute, then
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
4.605
HETP is numerically equal to HTU, only when the operating 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.
Both condensor and reboiler loads are higher in case (ii) as compared to case (i).
Steady state equimolal counter diffusion occurs in case of
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
The relative saturation of an unsaturated mixture of gas and vapor is independent of the
none of these.
The adiabatic saturation curve for a vapour-gas mixture is
slightly concave upward.
Pick out the wrong statement.
Zirconia probe is used for the humidity measurement.
The value of NA/(NA + NB), for steady state equimolal counter diffusion of two gases ‘A’ and ‘B’ is
∞
Drift in a cooling tower is
the water entrained by the circulating air.
With increase in the concentration of the adsorbate, the integral heat of adsorption
decreases.
Small dia distillation column can be a
packed column
If mass diffusivity in a mixture is equal to the thermal diffusivity, then the Lewis number is
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
0.152
__________ column is used in gas absorption process.
Packed
Nitrobenzene (boiling point = 210.6°C) is steam distilled at 1 atm pressure.Nitrobenzene will distill off __________ °C.
at < 100
Pick out the wrong statement.
Vanderwall adsorption is also called chemisorption.
At what value of reflux ratio, number of theoretical plates in a distillation column is minimum?
∞
Batch tray dryers suffers from the disadvantage of
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
> 8:1
__________ column is preferred to be used, when a high liquid hold up is required in a reactor for gas-liquid reaction.
Bubble
The ‘shanks system’ of leaching (i.e., counter-current multiple contact leaching) is used for
both (a) and (b).
Penetration model (theory) for mass transfer was enunciated by
Higbie
Surface renewal model of mass transfer was enunciated by
Danckwerts
When the __________ the relative humidity of air decreases, despite an increase in the absolute humidity.
temperature rises
For turbulent mass transfer in pipes, the Sherwood number depends on the Reynolds number as
Re^0.83
The relative humidity of air decreases in-spite of an increase in the absolute humidity, when the
temperature rises.
Unit of molal diffusivity is
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.
Binary distillation involves the mass transfer by __________ at the gas-liquid interface.
a counter diffusion at an almost equal molar rate
In a gas-liquid absorption coloumn, for obtaining the maximum absorption efficiency
both gas as well as liquid streams should be distributed uniformly.
Reverse osmosis is also called
ultrafiltration.
Which of the following is not a batch 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.
Rayleigh equation applies to __________ distillation.
differential
Mass transfer co-efficient ‘K’ according to penetration theory varies with mass dif-fusivity as
D^0.5
Most important factor to be considered in the selection of packings for absorbers is the __________ of packing.
porosity
__________ is the separation technique used for desalination of sea water.
Reverse osmosis
The relative saturation of a partially saturated mixture of vapour and gas can be increased by __________ of the mixture.
both (b) and (c)
Danckwerts developed the __________ theory.
surface renewal
Molecular distillation is
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
all (a), (b) & (c)
Which of the following provides maximum contact surface for a liquid-vapour system?
Packed tower
For experimental determination of mass transfer co-efficient by wetted wall tower, the mass transfer area is
known
In stripping section of continuous distillation column, the
liquid is enriched with high boiler.
Molality is defined as the number of gm moles of solute per __________ of solvent.
Kg
__________ is concerned with the adsorption equilibria.
Freundlich equation
Fractional solvent extraction
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.
Raschig ring made of procelain cannot be used for treating concentrated
alkalis
H2S present in naphtha reformed gas is removed by absorbing with
ethanolamine
During constant rate drying period, vaporisation rate per unit drying surface area
does not change with time,
Pick out the wrong statement.
Mass transfer co-efficient does not account for gase-liquid diffusion.
In rectifying section of a continuous distillation column, the
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
According to Chilton-Colburn analogy for mass transfer, NSt.NSc2/3 is equal to
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
1.24
Minimum reflux ratio in a distillation column results in
maximum condenser size.
Flooding in a distillation column is detected by a sharp
increase in pressure drop.
In extraction, as the temperature increases, the area of hetrogeneity (area covered by binodal curve)
decreases
Pick out the wrong statement.
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
At constant pressure, with increase of temperature, the dew point will
increase
For a single component absorber, the operating line is straight only when plotted in term of __________ units.
mole ratio
In case of unsaturated air
both (a) and (b)
For a binary mixture with low relative volatility,
an azeotrope may be formed during distillation.
Plate efficiency
is a function of the mass transfer between liquid and vapour.
Absorption accompanied by heat evolution results in
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 pressure drop of the gas.
For a distillation column operating at minimum reflux, the
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))
MXo ln(Xo/XF)
Flash distillation is suitable for the separation of components
having very wide boiling points.
In Mcabe-Thiele method, at infinite reflux ratio
both the operating lines coincide with the diagram.
Leaching rate is independent of the
none of these
Use of packed towers for distillation is generally limited to the
small sizes.
Pick out the wrong statement.
Molecular diffusion in solid is much faster than that in liquids.
Which of the following decreases during evaporative cooling process with recirculated water supply?
None of these
Laboratory determination of the dif-fusivity of vapor is done by __________ method.
Winkel mann’s
The minimum liquid rate to be used in an absorber corresponds to an operating line
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
humidified
For absorbing a sparingly soluble gas in a liquid, the
liquid side coefficient should be increased.
A natural draft cooling tower is filled with wooden grids, which covers about __________ percent of the tower height.
15-Oct
The boiling points of pure components of a binary system __________ with increase in total pressure of 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.
Penetration theory relates the average mass transfer co-efficient (K) with diffusivity (D) as
K ∝sqrt(D)
Desulphurisation of petrol is done in a
rotating disc extractor
Chemisorption compared to physical absorption has
increased utilisation of stagnant zones of the liquid phase.
Rate of adsorption increases as the
temperature decreases.
Pick out the system with maximum boiling azeotrope at 1 atm.
acetone chloroform
At 750°K and 1 atm, the approximate value of Schmidt number for air is
0.1
Calculation of mass transfer co-efficient is mostly/normally done using __________ theory.
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 = 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
Which of the following is used for the concentration of rubber latex?
Agitated film evaporator
As the reflux ratio, in a continuous counter-current extraction is increased, the number of stages
decrease
In triangular co-ordinates, the ternery composition point falls __________ of the triangle.
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)
8.89
Penetration theory states that the mass transfer co-coefficient is equal to (where, De is diffusivity and ‘t’ is time)
(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.
