Mass Transfer Flashcards
1.
Mass transfer rate between two fluid phases does not necessarily depend on the __________ of the two phases.
chemical properties
physical properties
degree of turbulence
interfacial area
chemical properties
2.
In a solution containing 0.30 Kg mole of solute and 600 kg of solvent, the molality is
0.50
0.60
2
1
0.50
Fenske equation determines the maximum number of ideal plates.
height of the distillation column.
minimum number of theoretical plates.
optimum reflux ratio.
minimum number of theoretical plates.
The term “cooling range” in a cooling tower refers to the difference in the temperature of cold water leaving the tower and the wet bulb temperature of the surrounding air.
a. hot water entering the tower and the wet bulb temperature of the surrounding air.
b. hot water entering the tower and the c. cooled water leaving the tower.
c. none of these
hot water entering the tower and the cooled water leaving the tower
Which of the following is the most suitable for extraction in a system having very low density difference ?
Centrifugal extractor
Total reflux in a distillation operation requires minimum
number of plates
The amount of steam required per unit quantity of distillate in case of steam distillation will be reduced by
a. raising the temperature.
b. lowering the total pressure.
both (a) and (b).
The difference of wet bulb temperature and adiabatic saturation temperature of unsaturated mixture of any system is
+ve
Fenske’s equation for determining the minimum number of theoretical stages in distillation column holds good, when the:
a. relative volatility is reasonably constant.
b. mixture (to be separated) shows negative deviation from ideality.
c. mixture (to be separated) shows positive deviation from ideality.
d. multicomponent distillation is involved.
relative volatility is reasonably constant.
When the temperature and humidity of air is low, we usually use __________ draft cooling tower.
natural
12.
__________ columns are used for liquid dispersion in a continuous gas phase.
Packed column
Overall efficiency of the distillation column is
a. the ratio of number of ideal plates to actual plates.
b. the ratio of number of actual plates to ideal plates.
c. same as the Murphree efficiency.
d. always more than the point efficiency.
the ratio of number of ideal plates to actual plates.
14.
On addition of solute in the solvent, the __________ of the solution decreases.
boiling point
freezing point
vapour pressure
both (b) and (c)
both (b) and (c)
15.
Which is the controlling factor for a drum drier?
Diffusion
Heat transfer
Both (a) and (b)
Neither (a) nor (b)
Both (a) and (b)
16.
Drying operation under vacuum is carried out to
reduce drying temperature.
17.
Air initially at 101. 3 kPa and 40°C and with a relative humidity of 50%, is cooled at constant pressure to 30°C. The cooled air has a
higher relative humidity
18.
Separation of two or more components of a liquid solution can not be achieved by
fractional crystallisation
liquid extraction
absorption
evaporation
absorption
Stefan’s law describes the mass transfer by
diffusion
bulk flow
both ‘a’ & ‘b’
neither ‘a’ nor ‘b’
both ‘a’ & ‘b’
At minimum reflux ratio for a given separation
number of plates is infinity.
- Relative humidity is the ratio of the
a. partial pressure of the vapour to the vapour pressure of the liquid at room temperature.
b. partial pressure of the vapour to the vapour
c. pressure of the liquid at gas temperature.
actual humidity to saturation humidity.
d. none of these
partial pressure of the vapour to the vapour pressure of the liquid at gas temperature.
22.
In batch distillation with constant reflux, overhead product composition __________ with time.
decreases
23.
Psychrometry deals with the properties of gas-vapor mixture. Humidity can be determined by the measurement of the __________ of a fibre.
electrical resistance
24.
Heat in BTU necessary to increase the temperature of 1 lb of gas and its accompanying vapour by 1°F is called the
specific heat
25.
To increase the absorption factor, (where, G = gas flow rate, S = solvent flow rate)
a. increase both ‘G’ and ‘S’.
b. decrease both ‘G’ and ‘S’.
c. increase ‘S’ and decrease ‘G’.
d. increase ‘G’ and decrease ‘S’
increase ‘S’ and decrease ‘G’.
26.
The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are same for, NPr = NSc = __________
1
27.
Dew point of a gas-vapour mixture
increases with temperature rise.
decreases with temperature rise.
decreases with decrease in pressure.
increases with increase in pressure.
decreases with decrease in pressure.
28.
In a packed absorption tower, if the equilibrium and operating lines are both straight lines, then the ratio, HETP/HTUOG __________ the absorption factor.
increases with increase in
is one at unity value of
both (a) and (b)
neither (a) nor (b)
both (a) and (b)
29.
Which of the following plays an important role in problems of simultaneous heat and mass transfer ?
a. Lewis number
b. Schmidt number
c. Prandtl number
d. Sherwood number
Lewis number
- In case of an absorber, the operating
a. line always lies above the equilibrium curve.
b. line always lies below the equilibrium curve.
c. line can be either above or below the equilibrium curve.
d. velocity is more than the loading velocity.
line always lies above the equilibrium curve.
As the reflux ratio in a distillation coloumn is increased from the minimum, the
total cost first decreases and then increases.
Absorption factor is defined as (where, S1 = slope of the operating line S2 = slope of the equilibrium curve)
S2/S1
S1/S2
S1 - S2
S1 x S2
S1/S2
A plait point is the point on the solubility curve, where the tie line reduces to a point. What is the number of plait point for a ternery system containing two pairs of partially miscible liquids ?
0
Which of the following remains constant during sensible cooling process ?
Specific humidity
Partial pressure of vapour
Both (a) and (b)
Neither (a) nor (b)
Both (a) and (b)
Physical absorption is
an irreversible phenomenon.
a reversible phenomenon.
accompanied by evolution of heat.
both (b) and (c)
both (b) and (c)
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
Flow rate of entering liquid
- Mass transfer co-efficient of liquid is ________.
affected more by temperature than that for gases.
affected much less by temperature than that for gases.
not affected by the temperature.
none of these.
affected more by temperature than that for gases.
Absorption accompanied with chemical reaction is exemplified by the absorption of
SO2 in alkaline solution.
which of the following unit operations makes use of Thiele-Geddes equation ?
Liquid-liquid extraction
Solid-liquid extraction
Multicomponent absorption with chemical reaction
Multicomponent distillation
Multicomponent distillation.
41.
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
Wetted wall column
The equipment frequently used for adiabatic humidification-cooling operation with recirculating liquid is
spray chamber
Chemisorption (chemical adsorption) is ______.
a. same as “Van der Waals” adsorption.
b. characterised by adsorption of heat.
c. an irreversible phenomenon.
d. a reversible phenomenon.
an irreversible phenomenon.
- Raoult’s law applies to the _______.
all liquid solutions
non-ideal solution only.
non-volatile solute.
solvents.
solvents.
In distillation columns, the number of bubble caps per tray primarily depends upon the _____.
allowable liquid velocity.
allowable gas velocity.
allowable gas and liquid velocities.
feed composition.
allowable gas and liquid velocities.
In distillation, overhead product contains
any number of components
With increase in pressure, the relative volatility for a binary system
decreases.
Mass transfer co-efficient (K) and diffusivity (D) are related according to film theory as
K ∝ D
K ∝D
K ∝ D^1.5
K ∝ D^2
K ∝ D
The McCabe Δ L law states that the
molar heats of vaporisation of components are nearly equal.
linear crystal growth rate depends on the degree of supersaturation.
linear crystal growth rate does not depend on the crystal size.
linear crystal growth rate depends on the crystal size.
linear crystal growth rate does not depend on the crystal size.
Bollman extractor
is used for extraction of oil from oilseed.
The relation, Sc = Pr = 1, is valid, when the mechanism of __________ ansfer is same.
heat and mass
For the same feed, feed quality and separation (in a distillation column), with the increase of total pressure, the number of ideal plates will
increase
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.
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.
When the liquid phase and vapour phase of a binary system obeys Raoult’s and Dalton’s law respectively, the relative volatility is the ratio of
vapour pressure of component A to that of component B.
vapour pressure of component A to the total pressure.
vapour pressure of component A to the partial pressure of A.
partial pressure of component A to the total pressure.
vapour pressure of component A to that of component B.
The operation involved when the soluble material is largely on the surface of an insoluble solid and is merely washed off by the solvent is called
elutriation or elution
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.
surface evaporation of unbound moisture occurs.
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%).
71 min
__________ diffusion is used for separating the isotopes of methane.
Thermal
Ordinary diffusion process is also called __________ diffusion.
concentration
Pick out the wrong statement.
a. Different solids have different equilibrium moisture curves.
b. Total condenser or reboiler is considered equivalent to one theoretical plate in McCabe-Thiele method of theoretical plate calculation for distillation column.
c. Heat removal load on cooler remains constant, even with increase of the reflux ratio in a distillation column.
d. Even a forced draft cooling tower can not cool water below the wet bulb temperature.
Heat removal load on cooler remains constant, even with increase of the reflux ratio in a distillation column.
Pick out the wrong statement pertaining to the rotary dryer.
a. Flights (located in the inside shell of rotary dryer) lift the material being dried and shower it down through the current of hot air/gases. It extends from the wall to a distance which is about 8-12% of the inside diameter of shell.
b. Hold up of a rotary drier is defined as the fraction of the dryer volume occupied by the solid at any instant. The best performance for rotary drier is obtained, when the hold up is in the range of 0.05 to 0.15.
c. Rotary dryer is suitable for drying sticky material.
d. Recommended peripherial speed of a rotary drier is in the range of 10 to 30 metres/minute.
Rotary dryer is suitable for drying sticky material.
The reason for preferring packed towers over plate towers in distillation practice is that the packed tower operation gives
low pressure drop and high hold up.
high pressure drop and low hold up.
low pressure drop and low hold up.
high pressure drop and high hold up.
low pressure drop and low hold up.
If the path of liquid across the plate is very long as in case of large diameter tower, Murphree efficiency can be __________ percent.
100
> 100
< 100
none of these
> 100
Compound A is extracted from a solution of A + B into a pure solvent S. A Co-current unit is used for the liquid-liquid extraction. The inlet rate of the solution containing A is 200 moles of B/hr.m2 and the solvent flow, rate is 400 moles of S/m2. hr. The equilibrium data is represented by Y = 3X2 , where Y is in moles of a A/moles of B and X is in moles A/moles of S. The maximum percentage extraction achieved in the unit is
25%
50%
70%
90%
50%
Raoult’s law is applicable to the
ideal solutions
Weeping in a distillation column
results due to very low gas velocity.
In case of binary distillation, increasing the reflux ratio above optimum does not result in the increase of
area between operating line and 45° diagonal x-y diagram.
condenser and reboiler surfaces.
tower cross-section.
none of these.
area between operating line and 45° diagonal x-y diagram.
Wetted wall tower experiment determines the
mass transfer co-efficient.
In McCabe-Thiele method, at infinite reflux ratio
the overhead product is minimum.
With decrease in the throughput (compared with the design capacity) for a bubble cap distillation column, its efficiency
increases
Plate towers are preferred to packed towers, when large temperature changes are involved, because of the reason that the
A. packing may be crushed due to thermal expansion/contraction of the components of the equipments.
B. pressure drop in the equipment will be . very high.
C. viscosity variation of the fluids may cause channeling/flooding.
D. none of these.
packing may be crushed due to thermal expansion/contraction of the components of the equipments.
The equilibrium liquid composition compared to the vapor composition in case of azeotropic mixture is
same
Out of the following gas-liquid contacting devices, for a given set of operating conditions, gas pressure drop is the least in __________ tower.
wetted wall
bubble cap
perforated tray
packed
wetted wall
Pick out the system with minimum boiling azeotrope at 1 atm.
benzene-toluene
ethyl alcohol-water
hydrochloric acid-water
all (a), (b) and (c)
ethyl alcohol-water
Free flowing granular materials can be best dried in a __________ drier.
rotary
cylinder
drum
freeze
rotary
In case of an ideal solution, the total vapor pressure varies __________ with the composition expressed as mole fraction,
linearly
Corresponding to Nusselt number in heat transfer, the dimensionless group in mass transfer is the __________ number.
Sherwood
Schmidt
Peclet
Stanton
Sherwood
If the specific heats of a gas and a vapor are 0.2KJ/Kg.°K and 1.5 KJ/Kg.°K respectively, and the humidity is 0.01; the humid heat in KJ/°Kg. is
0.215
Liquid diffusivity is of the order of __________ cm2 /second.
0.01
0.1
10-5 to 10-6
> 1
10-5 to 10-6
The solvent used in liquid extraction should not have high latent heat of vaporisation, because
A. the pressure drop and hence the pumping cost will be very high.
B it cannot be recovered by distillation.
C its recovery cost by distillation may be prohibitatively high.
D it will decompose while recovering by distillation.
its recovery cost by distillation may be prohibitatively high.
At the same gas flow rate, the pressure drop in a packed tower being irrigated with liquid __________ that in dry packed tower.
is greater than
Molarity is defined as the number of gm moles of solute per __________ of solvent.
litre
The value of Lewis number (Le = Sc/Pr) for air-water vapour system is around
1
0.24
3.97
600
1
Which of the following T-H diagram is valid for dehumidification operation ?
operating line above equilibrium curve
An aqueous solution of methanol is to be distilled in a tray column. High pressure steam is available as a source of heat. For a given reflux ratio and overhead composition, two options are being explored; (i) a reboiler is used and (ii) no reboiler is used but steam is fed directly to the bottom of the column. As compared to option (i), in option (ii)
less number of trays are required.
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
exponential
Which of the following binary systems is an example of a maximum boiling azeotrope ?
Water-hydrochloric acid.
Acetone-carbon disulphide.
Water-ethyl alcohol.
n-heptane-n-octane.
Water-hydrochloric acid.
(NRe . NSc) is termed in mass transfer operation as the __________ number.
Stanton
Sherwood
Peclet
none of these
Peclet
The binary diffusivity in gases and liquids vary respectively as
T^3/2 and T
Pick out the correct statement.
A. The slope of the stripping line is always less than unity.
B. For a given separation in a binary distillation column, with increase in reflux ratio (above the minimum reflux ratio), the fixed cost of the distillation column first increases and then decreases.
C. In Ponchon-Savarit method of no. of theoretical plate determination, the stripping and rectifying operating lines are vertical at total reflux.
D. Kremsor-Brown-Souder’s equation is used to calculate the efficiency of a plate tower.
In Ponchon-Savarit method of no. of theoretical plate determination, the stripping and rectifying operating lines are vertical at total reflux.
Pick out the correct statement.
A Dialysis process is used for the production of fresh water from brine.
B Electrodylsis is used in the manufacture of NaOH.
C Electrodylsis is used for the refin-ning/purification of petrol.
D both (b) and (c).
Electrodylsis is used in the manufacture of NaOH.
Pick out the correct statement.
A Bound moisture is removed during constant rate drying period.
B Free moisture is removed during falling rate drying period.
C The sum of free moisture and equilibrium moisture in a solid is called the critical moisture.
D The driving force for mass transfer in a wet solid is its free moisture content.
The driving force for mass transfer in a wet solid is its free moisture content.
Very tall packed towers are divided into series of beds to
avoid channelling
Moisture contained by a substance in excess of the equilibrium moisture is called the __________ moisture.
unbound
free
critical
bound
free moisture (“excess of”)
With increase in temperature, the solubility of gases in liquids, at fixed pressure
decreases
In most of the vacuum crystalliser, vacuum is generally produced by means of a
a. suction pump
b. compressed air jet
c. steam jet ejector with a barometric condenser
d. none of these
steam jet ejector with a barometric condenser
Rose oil is extracted from rose leaves using _________ distillation
a. low pressure
b. extractive
c. high pressure
d. steam distillation
steam distillation
When the solvent dissolves very little of solute then
a. solvent of low freezing point should be used
b. very small quantity of solute is required
c. large quantity of solvent is required to extract the solute
d. solvent of low latent heat of vaporization should be used
large quantity of solvent is required to extract the solute
The triangle whose one corner coincides with the enriching line and the outer corner is on the stripping line is called the
a. feed plate
b. middle plate
c. partial condenser
d. ideal stage
feed plate
Relative volatility varies with the concentration of component for an ideal solution. The relative volatity of a binary mixture may be defined as the ratio of vapor pressure of component ‘A’ to that of component ‘B’, when
vapor phase obeys Dalton’s law and liquid phase obeys Raoult’s law.
Flash distillation is suitable for the separation of components
a. having very close boiling points.
b. which form minimum boiling azeotrope.
c. which form maximum boiling azeotrope.
d. having very wide boiling points.
having very wide boiling points.
Large scale usage of flash distillation is practiced in
ammonia synthesis plant
petroleum refinning
polymerization industry
soap manufacturing
petroleum refinning
In an operating distillation column, the
Group of answer choices
driving force for the vapor flow is the pressure drop, as the pressure decreases gradually from the bottom to the top of the column.
driving force for the liquid flow is its specific weight.
highest temperature is encountered at the top of the column.
vapors and liquids are at their dew point and bubble point respectively
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 case of steam distillation, the steam leaving the liquid is not completely saturated with distillate vapor, because
Group of answer choices
temperature is less.
mixing of steam with the material being vaporised is not so intimate as to result in equilibrium condition.
total pressure is less.
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.
Which of the following is the most suitable for extraction in a system having very low density difference ?
Group of answer choices
Mixer-settler extractor
Packed extraction tower
Pulsed extractor
Centrifugal extractor
Centrifugal extractor
Total reflux in a distillation operation requires minimum
Group of answer choices
reboiler load
condenser load
number of plates
all of these
number of plates
In batch distillation with constant reflux, overhead product composition with time.
Group of answer choices
does not vary
decreases
increases
may increase on decrease, depends on the system.
decreases
Raoult’s law applies to the
all liquid solutions
non-ideal solution only.
solvents.
non-volatile solute.
solvents.
In distillation, overhead product contains
only saturated liquid
two components
any number of components
only one component
any number of components
For the same feed, feed quality and separation (in a distillation column), with the increase of total pressure, the number of ideal plates will
Group of answer choices
remain same
data insufficient, can’t be predicted
increase
decrease
increase
The operation involved when the soluble material is largely on the surface of an insoluble solid and is merely washed off by the solvent is called
decoction
elutriation or elution
percolation
none of these
elutriation or elution
Pick out the wrong statement.
Different solids have different equilibrium moisture curves.
Even a forced draft cooling tower can not cool water below the wet bulb temperature.
Heat removal load on cooler remains constant, even with increase of the reflux ratio in a distillation column.
Total condenser or reboiler is considered equivalent to one theoretical plate in McCabe-Thiele method of theoretical plate calculation for distillation column.
Heat removal load on cooler remains constant, even with increase of the reflux ratio in a distillation column.
An aqueous solution of methanol is to be distilled in a tray column. High pressure steam is available as a source of heat. For a given reflux ratio and overhead composition, two options are being explored; (i) a reboiler is used and (ii) no reboiler is used but steam is fed directly to the bottom of the column. As compared to option (i), in option (ii)
more number of trays are required but the residue composition remains unchanged.
more number of trays are required and the residue composition is more dilute in methanol.
less number of trays are required.
composition of the residue remains unchanged.
less number of trays are required.
Ponchan-Savarit method analyses the fractional equipment based on
both enthalpy and material balances.
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.
azerotropic
none of these
steam
continuous
steam
The fixed cost of a distillation column at minimum reflux ratio is
none of these
minimum
optimum
infinite
minimum
extractor uses centrifugal force for separating the two phases.
None of these
Treybal
Podbielniak
Schiebel
Podbielniak
In most of the mechanically agitated liquid-liquid extractors, baffles or horizontal compartmental plates are provided, which helps in
maintaining the concentration difference between the two phases.
increasing the rate of extraction.
All of these
reducing the axial mixing.
All of these
Which of the following applies to a feed at dew point?
f = 0
f > 1
0 < f < 1
f = 1
f = 1 (saturated vapor or dew point q=0)
f=1-q
f=1-0
f=1
Pick out the wrong statement.
With increase in pressure, the solubility of gases in solvent increases at a fixed temperature.
Decrease in Murphree plate efficiency is an indication of entrainment in distillation column.
In solvent extraction, the distribution coefficient should be as large as possible for solvent economy.
The capacity of a distillation column is maximum at total reflux.
The capacity of a distillation column is maximum at total reflux.
The relative volatility of a binary mixture at the azeotropic composition is
1
0
∞
> 1
azeotrope: x=y
relative volatility =1
Relative volatility varies with the concentration of component for an ideal solution. The relative volatity of a binary mixture may be defined as the ratio of vapor pressure of component ‘A’ to that of component ‘B’, when
vapor phase obeys Dalton’s law and liquid phase obeys Raoult’s law.
Raoult’s = xi (liquid)
Dalton’s = yi (vapor)
Which of the following is not a continuous leaching equipment?
Kennedy extractor
Bollman extractor
Hydrocyclones
Pachuka tank
Pachuka tank
High pressure at the bottom of a distillation tower handling heat sensitive materials results in
thermal decomposition of bottoms.
very efficient operation.
increased relative volatility.
erosion of the tower.
thermal decomposition of bottoms.
For a binary mixture with low relative volatility, continuous rectification to get pure products will require
less number of trays
small cross-section column
low reflux ratio
high reflux ratio
high reflux ratio
In steam distillation, the
temperature is higher than the boiling point of either component.
product must be immiscible with water.
temperature is more than 100° C.
temperature is 100°C.
product must be immiscible with water.
