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
