Mass Transfer Flashcards
The term “cooling range” in a cooling tower refers to the difference in the temperature of
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
When the temperature and humidity of air is low, we usually use __________ draft cooling tower.
natural
__________ columns are used for liquid dispersion in a continuous gas phase.
Packed
Which is the controlling factor for a drum drier?
Heat transfer
Drying operation under vacuum is carried out to
reduce drying temperature.
Relative humidity is the ratio of the
partial pressure of the vapour to the vapour pressure of the liquid at gas temperature.
In batch distillation with constant reflux, overhead product composition __________ with time.
decreases
Psychrometry deals with the properties of gas-vapor mixture. Humidity can be determined by the measurement of the __________ of a fibre.
electrical resistance
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
Dew point of a gas-vapour mixture
decreases with decrease in pressure.
Which of the following plays an important role in problems of simultaneous heat and mass transfer ?
Lewis number
In case of an absorber, the operating
line always lies above the equilibrium curve.
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 twopairs of partially miscible liquids ?
0
Which of the following quantities need not be fixed before starting the design of a co-current absorber ?
Flow rate of entering liquid
Mass transfer co-efficient of liquid is
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 ?
Multicomponent distillation.
Which of the following liquid-vapor contacting devices provides maximum contact surface area for a particular duty ?
Wetted wall column
The equipment frequently used for adiabatic humidification-cooling operation with recirculating liquid is
spray chamber
Chemisorption (chemical adsorption) is
an irreversible phenomenon.
Raoult’s law applies to the
solvents
In distillation columns, the number of bubble caps per tray primarily depends upon the
allowable gas and liquid velocities.
In distillation, overhead product contains
any number of components
Mass transfer co-efficient (K) and diffusivity (D) are related according to film theory as
K ∝ D
The McCabe Δ L law states that the
linear crystal growth rate does not depend on the crystal size.
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.
Pick out the wrong statement.
Heat removal load on cooler remains constant, even with increase of the reflux ratio in a distillation column.
The reason for preferring packed towers over plate towers in distillation practice is that the packed tower operation gives
low pressure drop and low hold up.
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
50%
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.
Wetted wall tower experiment determines the
mass transfer co-efficient.
In McCabe-Thiele method, at infinite reflux ratio
the overhead product is minimum.
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
Which of the following binary systems is an example of a maximum boiling azeotrope ?
Water-hydrochloric acid.
Pick out the system with minimum boiling azeotrope at 1 atm.
ethyl alcohol-water
In case of an ideal solution, the total vapor pressure varies __________ with the composition expressed as mole fraction,
linearly
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
The solvent used in liquid extraction should not have high latent heat of vaporisation, because
its recovery cost by distillation may be prohibitatively high.
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
(NRe . NSc) is termed in mass transfer operation as the __________ number.
Peclet
Pick out the correct statement.
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.
Electrodylsis is used in the manufacture of NaOH.
Pick out the correct statement.
The driving force for mass transfer in a wet solid is its free moisture content.
With increase in temperature, the solubility of gases in liquids, at fixed pressure
decreases
__________ column is the most suitable for achieving the best performance for mass transfer operations involving liquid with dispersed solids.
Plate
Deairation (removal of O2) of water is done by
ion-exchange
Pick out the wrong statement.
Azeotropes obey Raoult’s law at all temperature.
A minimum boiling azeotrope is exemplified by
acetone-carbon disulphide
The assumption made in Elis method in distillation is that enthalpy concentration lines of vapor and liquid are
both parallel and straight
Pick out the wrong statement:
Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition.
In paper industry, paper is dried in a __________ dryer.
heated cylinder
Pick out the wrong statement.
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum.
Only small amount of evaporation of water produces large cooling effects because of its
large latent heat
When the feed to a distillation column is a saturated liquid, slope of the feed line is
infinity
Absorption factor method is used to calculate the number of ideal stages, when
both operating and equilibrium lines are parallel.
Flights in a rotary dryer are provided to
lift and shower the solids thus exposing it thoroughly to the drying action of the gas.
If f = moles of vapour present per mole of feed, then the slope of feed line is (Mcabe-Thiele method)
-1/f
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
on heating, a vapor phase will appear ; while on cooling, a solid phase will appear.
At equilibrium the concentration of water in vapour phase (C)in kg/m3 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 100m3 containing 2.2 kg of water vapour 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
0.4
The binary diffusivity in gases at atmospheric conditions is about
10-1 cm2/sec
For achieving rapid drying rate in a spray dryer, the diameter of the particles in the feed should be in the range of __________ microns (1 mm = 1000 microns).
10-60
The minimum number of theoretical plates is required for achieving a given separation in distillation column with
total reflux
In a counter-current liquid extractor
only one of the liquids may be pumped at any desired rate.
The wetted wall tower is used to determine
individual mass transfer co-efficient (M.T.C.) in gaseous system.
Weight of 1 m3 of humid air as compared to 1 m3 of dry air, under the same conditions, is
less
Positive deviation from Raoult’s law means a mixture whose total pressure is
greater than that computed for ideality.
Prandtl number for water at 20°C is about
7
Schmidt number is given by
u/pDab
The apex of an equilateral-triangular coordinate (in ternary liquid system) represents a/an
pure component
The diffusivity (D) in a binary gas mixture is related to the pressure (P) as
D ∝ 1/P
A slurry is to be dried to produce flaky solid. Which dryer would you recommend ?
Spray dryer
In gas-liquid contact operation, the number of ideal stages, . This is true when the stripping factor ‘S’ is
S > 1
The cooling effect in a cooling tower can not be increased by
none of these.
Pick out the wrong statement
In actual practice, absorption is an en-dothermic 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.
steam
Large scale drying of wheat is done in a __________ dryer.
spouted bed
Azeotropic distillation is employed to separate
constant boiling mixture.
At the boiling point of the liquid at the prevailing pressure, the saturated absolute humidity becomes
∞
Pressure drop through plate tower as compared to that through packed tower, for the same duty will be
more
Dorr thickener is an equipment used for
sedimentation
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
1000
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.
molal
The Kundsen diffusivity is proportional to (where, T = absolute temperature)
sqrt T
In an absorption coloumn, the flooding velocity for random packing is __________ that for stacked/regular packing.
smaller than
The relation among various mass transfer co-efficients (M.T.C) for ideal gases is given by (where, Kc & Km are M.T.C. for equimolar counter diffusion with concentration & mole fraction respectively as the driving force. and, Kp = M.T.C. for diffusion of a gas through a stagnant inert gas with pressure as driving force.)
Kc = Kp . RT = Km . RT/p
Blowdown in a cooling tower
means discarding a small fraction of circulating water to prevent and limit the concentration of salt and impurities.
Antibiotics are best handled in a
Podbielniak extractor.
__________ extractor uses centrifugal force for separating the two phases.
Podbielniak
In a binary distillation column, if the feed contains 40 mole % vapour, the q line will have a slope of
-1.5
Pick out the wrong statement.
Mechanical agitation is required, if the system has low interfacial tension.
Design calculation for multiple component distillation is done by
tray to tray calculations.
To get high tray efficiency
interfacial surface between liquid and gas phase should be large.
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
dependent on the Reynolds number
In a vapor-liquid contacting equipment, the overall gas phase mass transfer co-efficient (M.T.C), KG is related to individual co-efficients (KG and KL) as
1/kG = 1/kG + m/kL
Solvent extraction is the terminology applied to the liquid-liquid extraction, which is preferred for the separation of the components of liquids, when
one of the liquid components has very high affinity towards the solvent.
Occurrence of ‘case hardening’ during drying of a high moisture solid cake __________ the drying rate.
decreases
__________ is the temperature at which a gas-vapor mixture becomes saturated, when cooled at constant total pressure out of contact with a liquid.
Dew point
Rate of leaching increases with increasing
temperature
Pick out the wrong statement.
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 10-2m/s. Then the average mass transfer co-efficient for the plate is 2 x 10-2m/s.
In a counter-current extractor, as the axial mixing increases, the extraction efficiency
decreases
In extractive distillation, solvent is
added to alter the relative volatility of the mixture.
Radioactive nuclear waste is treated in
pulsed column extractor
Large scale drying of sugar is done in a __________ dryer.
rotary
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 )
T
__________ equation applies to diffusivities in liquids.
Gilliland
Which of the following is the most commonly used leaching solvent in vegetable oil industry ?
Hexane
In an absorber, HETP does not vary with the
none of these
Large quantity of silica gel is normally dried in a __________ dryer.
through circulation
For the same system, if the same liquid used in an absorber is decreased, the tower height will
increase
Hydrogenation of oil is carried out in a/an __________ in Vanaspati manufacturing plant.
agitated vessel
Sides of equilateral-triangular co-ordinates (on which ternary liquid system is plotted) represent
a binary mixture.
In distillation column design, the McCabe-Thiele procedure is inadequate and a Ponchan-Savarit procedure is needed, when
a total condenser is used.
One mole feed of a binary mixture of a given composition is flash vaporised at a fixed P and T. If Raoult’s law is obeyed, then changing the feed composition would effect
the fraction vaporised but not the product composition.
With increase in the liquid flow rate at a fixed gas velocity in a randomly packed counter current gas-liquid absorption column, the gas pressure drop
increases
Which of these columns incurs the lowest pressure drop ?
Packed column (with stacked packing)
In the desorption of highly soluble gas from the liquid, the main resistance will be in the __________ phase.
liquid
For stripping of a gas in a counter current stripper, the operating line
can lie above or below the equilibrium curve.
The Knudsen diffusivity is dependent on the
molecular velocity and por radius of the catalyst.
Minimum possible diameter of a packed coloumn is decided mainly by the
flooding
Solvent used in extractive distillation
forms a low boiling azeotrope.
When both the fluids flow concurrently in an absorber, the slope of the operating line is
- ve
In a distillation column, with increase in the reflux ratio, the heat removed in the cooler
increases
The falling rate period in the drying of a solid is characterised by
increasing temperatures both on the surface and within the solid.
In which of the following unit operations, the selectivity is an important parameter ?
Solvent extraction
Which of the following equipments is not used for liquid dispersion ?
Agitated vessels
Diameter of raschig rings used in packed tower in industry is normally around __________ inches.
2
In case of non-ideal gases and liquids, the molal diffusivity
is independent of pressure.
Condensation of a vapour-gas mixture just begins, when (where, p = partial pressure of the vapour P = vapour pressure of the liquid )
p = P
__________ number indicates the ratio of the rates of the heat and mass transfer incase of a cooling tower.
Lewis
In physical terms, Schmidt number means
momentum diffusivity/mass diffusivity.
In case of liquids, the binary diffusivity is proportional to (where, T = temperature)
T
The diffusivity, DAB (for component A diffusing in B) is equal to the diffusivity DBA (for component B diffusing in A) for a binary mixture of
ideal gases
Mass transfer co-efficient is directly proportional to DAB0.5, according to __________ theory.
penetration
Which of the following employs an accessory known as ‘drift eliminator’ ?
Mechanical draft cooling tower.
Pick out the correct statement.
For a non-reacting binary mixture of ideal gases, the partial pressure distribution of both components is linear in the case of steady state equimolal counter-diffusion.
Diameter to height ratio for a rashig ring is
1
Gaseous diffusivity at atmospheric pressure is of the order of __________ cm2/second.
1
In a liquid-liquid extraction, 10 kg of a solution containing 2 kg of solute C and 8 kg of solvent A is brought into contact with 10 kg of solvent B. Solvent A and B are completely immiscible in each other whereas solute C is soluble in both the solvents. The extraction process attains equilibrium. The equilibrium relationship between the two phases is Y* = 0.9X, where Y* is the kg of C/kg of B and X is kg of C/kg of A. Choose the correct answer.
The entire amount of C is transferred to solvent B.
The surface renewal frequency in Danckwerts’ model of mass transfer is given by (KL: mass transfer co-efficient)
aqrt KL^2.DA
If Rm is the minimum reflux ratio, the optimum reflux ratio may be around __________ Rm.
1.2 to 1.5
During dehumidification of unsaturated air wet bulb temperature & partial pressure of vapor are not constant. However, during evaporative cooling process with recirculated water supply, the __________ remains constant.
wet bulb temperature
Mass transfer co-efficient varies as DAB0.5, according to the __________ theory.
surface renewal
A distillation column separates 10000 kg/hr of a benzene-toluene mixture as shown in the figure below :
In the figure xF, xD and xW represent the weight fraction of benzene in the feed, distillate and residue respectively. The reflux ratio is
0.6
Unsaturated air (with dry bulb and wet bulb temperatures being 45°C and 25°C respectively) is passed through a water spray chamber maintained at 23°C. The air will be cooled and
humidified with wet bulb temperature decreasing.
Humid volume, at a given temperature is a/an __________ function of humidity.
linear
If f is defined as above, then which of the following applies to a feed at dew point ?
f = 1
Perforated plate towers are unique for solvent extraction because, they provide
repeated coalescence & redispersion of the drop phase.
In case of absorption & stripping, the interface of the liquid & gas phases are present in equilibrium, when the diffusional resistance of __________ is zero.
interface
The individual mass transfer co-efficients (moles/m2 . s) for absorption of a solute from a gas mixture into a liquid solvent are, KL = 4.5 and KG = 1.5. The slope of the equilibrium line is 3. Which of the following resistance (s) is (are) controlling ?
gas side
Pore size of the molecular seive 5A is __________ mm.
0.5
Swenson-Walker crystalliser is a __________ unit
continuous
According to the Fenske equation, what will be the minimum number of plates required in a distillation column to separate an equimolar binary mixture of components A and B into an overhead fraction containing 99 mol% A and a bottom fraction containing 98 mol% B ? Assume that relative volatility (αAB = 2) does not change appreciably in the coloumn.
12
Pick out the wrong statement.
The capacity of a distillation coloumn is maximum at total reflux.
The relative volatility of a binary mixture at the azeotropic composition is
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.
Lewis number (Le) is
Sc / Pr
All moisture in a non-hygroscopic material is the __________ moisture.
unbound
Pick out the correct relationship. (where, Ri = internal reflux ratio Ro = external reflux ratio. )
(1 + Ro) = Ro/Ri
Separation of __________ employs gaseous diffusion process.
isotopes of uranium
Pick out the wrong statement:
In case of a stripper, the equilibrium curve is always below the operating line.
In liquid extraction, if the selectivity is unity, then
no separation will occur.
Pick out the wrong statement pertaining to the cooling towers.
Effectiveness of a mechanical draft cooling tower is reduced with increase in air wet-bulb temperature.
Experiments were conducted to determine the flux of a species A in a stagnant medium across a gas-liquid interface. The overall mass transfer co-efficient based on the liquid side for dilute systems for the above was estimated to be 4 x 10-3 kg mole/m2.s. The equilibrium data for the system is given as y = 2x. The flux across the interface (in kg mole/m2 .s) for bulk concentrations of A in gas phase and liquid phase as y = 0.4 and x = 0.01 respectively is
8.5 x 10»-4
Leaching of coarse solid lumps is also termed as
percolation
Co-current absorbers are usually used when the gas to be dissolved in the liquid is
a pure substance
At what percentage (by volume) of alcohol composition, it forms an azeotrope with water ?
95
Measurement of the interfacial area of mass transfer is achieved easily & accurately in case of a __________ coloumn.
wetted wall
With the lowering of equiliDrium pressure, at a given temperature, the amount of ad-sorbate on the adsorbent
decreases
At a fixed pressure, the humidity depends upon the partial pressure of vapor in the mixture. Humidity of a vapour free gas is __________ percent.
0
In case of physical adsorption, the difference between heat of adsorption and heat of normal condensation is
equal to the heat of wetting.
Pick out the wrong statement:
The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity.
The value of L/mG ratio for economical absorption operation ranges from
1.25 to 2.0
Alcohol is dehydrated using __________ distillation.
azeotropic
Dried finished product sampled from various parts of a __________ dryer exhibits non-uniformity in the moisture content.
tray
__________ is used as an ‘entrainer’ for the separation of acetic acid-water mixture by distillation.
Butyl acetate
Rotary dryers cannot handle __________ materials.
sticky
Pick out the correct statement.
Diffusivity decreases with increase in temperature.
Diffusivity increases with increase in molecular weight.
Diffusivity increases with the size of the individual molecule.
none of these.
none of these
An example of elutioh is
separation of uranium oxide from its ore by H2SO4 in a Pachuca tank.
The operating line for an absorber is curved, when plotted in terms of
partial pressure
Relative volatility does not change appreciably with the change in
temperature
The dew point of a saturated gas phase equals the __________ temperature.
gas
A solid material shows case hardening properties while drying. Which of the following should be controlled to control the drying process ?
Temperature of the solid
In case of liquid-liquid binary diffusion, diffusivity of one constituent into another is not dependent on the
none of these
Detergent solution is dried to a powder in a
spray dryer
Which of the following mixtures does not form an azeotrope at atmosheric pressure?
None of these
During bottling of cold drinks (e.g. Pepsi), the main resistance to mass transfer for the absorption of carbon dioxide in water lies in the
liquid film
For the air water system under ambient conditions, the adiabatic saturation temperature and the wet bulb temperature are nearly equal, because
water has a high latent heat of evaporation.
For which of the following unit operations, Lewis number is of significance ?
Humidification
If a1 and a2 are the relative volatilities when the pressure in the distillation column is 1 and 2 atm respectively. Pick out the correct statement.
none of these
The temperature to which a vapour gas mixture must be cooled (at varying humidity) to become saturated is called the __________ temperature.
none of these
Pick out the wrong statement.
The operating pressure in solvent extraction system should be below vapor pressures of all components.
With increase in solvent rate, the number of transfer units, NtoG, for a fixed degree of absorption from a fixed amount of gas
decreases
A distillation column with N plates is being operated under normal conditions. At some point of time, the operation is shifted to total reflux condition (i.e., no product and residue are being withdrawn and feed to the column is stopped). At the new steady state,
the top and bottom compositions correspond to the maximum enrichment available.
Dryer widely used in a textile industry is __________ dryer.
conveyor
Solvent used in extractive distillation
changes the relative volatility of the original components.
Air at a particular humidity is heated in a furnace. The new dew point
increases
Which of the following parameters remains constant during chemical dehumidifica-tion?
Dry bulb temperature
The dew point of an unsaturated gas-vapor mixture does not depend upon the __________ of the mixture.
temperature
Removal of __________ exemplifies an adsorption unit operation.
water from petrol
Can the efficiency of a plate in the distillation tower be greater than 100% ?
Never
The operating cost of a distillation column at minimum reflux ratio is
minimum
The reflux to a distillation column is 100 moles/hr, when the overhead product rate is 50 moles/hr. The reflux ratio is
2
Which of the following processes is not an example of adsorption applied to gaseous separations ?
Decolouration of yellow glycerene.
For a binary mixture distillation process, the degree of freedom is 2. However, if the pressure is fixed in this process, the number of independent variables in this process will be
1
H2S is being absorbed in a gas absorber unit. The height of the transfer unit based on the overall mass transfer coefficient on the gas side is 0.4 m. The equilibrium data is given by, y = 1.5 x. The bulk concentration of H2S has to be reduced from 0.05 to 0.001 mole fraction in the gas side. The height of the tower (in metres) corresponding to an operating line given by, y = 5x + 0.001 is
1
2
The most efficient cooling tower out of the following is
induced draft
Sodium bicarbonate is produced during soda ash manufacture using a __________ tower.
baffle
Powdery materials can be guarded against caking tendency on storage by
none of these.
In case of cooling towers, the ratio of the rates of heat and mass transfer is indicated by the __________ number.
Lewis
Which one of the following devices is not used for both the absorption as well as the liquid-liquid extraction processes ?
Wetted wall columns
The temperature of water cooled in cooling tower is always __________ temperature of entering air.
more than the wet bulb
Tower diameter may be decreased by
use of increased tray spacing.
Rayleigh’s equation applies to __________ distillation.
differential
Steady state equimolal counter diffusion is encountered in
separation of a binary mixture by distillation.
Polar organic compounds are normally used as separating agents for the azeotropic and extractive distillation. Which of the following is the most important factor to be considered for the choice of the separating agent for extractive distillation ?
Selectivity
As the reflux ratio increases, the slope of the operating line for rectifying section
increases
Cox chart, which is useful in the design of distillation column particularly for petroleum hydrocarbons, is a plot of (where, P = vapor pressure, T = temperature)
log P vs . T
For a ternery mixture, in which equilateral triangular co-ordinate is used in leaching and extraction, a __________ of the equilateral triangular co-ordinates.
pure component is represented by the apex.
The Reynolds number of the liquid was increased 100 fold for a laminar falling film used for gas-liquid contacting. Assuming penetrating theory is applicable, the fold increase in the mass transfer co-efficient (Kc) for the same sytem is
10
The slope of 9-line is determined by the
thermal condition of the feed
In steam distillation, the
product must be immiscible with water.
Which of the following can be calculated theoretically using steam table and Dalton’s law of partial pressure; if the relative humidity and dry bulb temperature of moist air is known ?
Wet bulb temperature
Hypersorption refers to a __________ process.
moving bed adsorption
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
Decoction refers to the use of solvent
at its boiling point
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
Which of the following gas-liquid contacting devices incurs the least pressure drop for a particular duty ?
Wetted wall tower
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
Which of the following operations does not involve leaching ?
Removing nicotine from its water solution by kerosene.
Calcium ammonium nitrate (a fertiliser) is dried in a __________ dryer.
rotary
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.
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 ratio of Murphree plate efficiency to point efficiency is always > 1 in a __________ flow model.
plug
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
The film penetration model of mass transfer was enunciated by
Toor and Marchello
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
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
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.
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
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.
What is the degree of freedom of an absorption process in which only one component is transferred between phases ?
3
Langmuir equation is associated with
adsorption
At a fixed total pressure, humidity depends only on the
partial pressure of vapour in the mixture.
__________ 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
Gaseous diffusion co-efficient increases with increase in the
temperature
Pick out the wrong statement. Raoult’s law does not apply, when the
component molecules are nonpolar.
L/mGis the expression for
absorption factor
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
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.
CO2 can be absorbed by
heated charcoal
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
Molecular diffusion is caused by the
thermal energy of the molecules.
Components, having widely different boiling point in a binary mixture, can be separated using __________ distillation.
simple
Pick out the wrong statement.
Diameter of randomly packed tower is normally more than 1.2 metres.
Azeotropism for ethyl alcohol-water system disappears by
decreasing pressure
Degree of freedom for leaching and solvent extraction process is
3
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.
Heat sensitive materials with very high latent heat of vaporisation may be economically separated using
liquid extraction
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
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).
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
Pick out the wrong statement.
Zirconia probe is used for the humidity measurement.
Drift in a cooling tower is
the water entrained by the circulating air.
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
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.
Batch tray dryers suffers from the disadvantage of
high labour cost & low heat economy.
__________ column is preferred to be used, when a high liquid hold up is required in a reactor for gas-liquid reaction.
Bubble
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
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
Reverse osmosis is also called
ultrafiltration
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
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 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.
Fractional solvent extraction
employs two solvents
Raschig ring made of procelain cannot be used for treating concentrated
alkalis
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.
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
Flooding in a distillation column is detected by a sharp
increase in pressure drop.
Pick out the wrong statement.
1 Nm3 of dry air is lighter than 1Nm3 of humid air.
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
For a binary mixture with low relative volatility,
an azeotrope may be formed during distillation.
Absorption accompanied by heat evolution results in
decrease in equilibrium solubility.
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)
Use of packed towers for distillation is generally limited to the
small sizes.
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.
10-15
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 ∝ «_space;D
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
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
Reboiler is considered as one theoretical plate, because
of the assumption that vapour and liquid leaving the reboiler are in equilibrium.
