mass transfer Flashcards
Mass transfer rate between two fluid phases does not necessarily depend on the __________ of the two phases.
chemical properties
Fenske equation determines the
minimum number of theoretical plates.
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
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
relative volatility is reasonably constant.
When the temperature and humidity of air is low, we usually use __________ draft cooling tower.
natural
Pick out the correct statement.
-In case of liquid-liquid extraction, no separation is possible, if the selectivity of the solvent used is unity.
-With increase in temperature, the selectivity of the solvent used in solvent extraction decreases.
-The selectivity of solvent used in solvent extraction is unity at the plait point.
-all (a), (b) and (c).
all (a), (b) and (c).
__________ columns are used for liquid dispersion in a continuous gas phase.
Packed
Overall efficiency of the distillation column is
the ratio of number of ideal plates to actual plates.
Which is the controlling factor for a drum drier?
Heat transfer
Drying operation under vacuum is carried out to
reduce drying temperature.
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
Separation of two or more components of a liquid solution can not be achieved by
absorption
At minimum reflux ratio for a given separation
number of plates is infinity.
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
To increase the absorption factor, (where, G = gas flow rate, S = solvent flow rate)
increase ‘S’ and decrease ‘G’.
The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are same for, NPr = NSc = __________
1
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.
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)
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 twopairs of partially miscible liquids ?
0
Make up water is required in a cooling tower to replace the water lost by
evaporation
drift
blowdown and leakage
all (a), (b) and (c)
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
Chemisorption (chemical adsorption) is
an irreversible phenomenon.
The rate of solid-liquid extraction is limited by the
phase change of the solute as it dissolves in the solvent.
diffusion of the solute through the solvent in the pores to the outside of the particle.
diffusion of the solute from the solution in contact with the particle to the main bulk of the solution.
all ‘a’, ‘b’ & ‘c’.
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
With increase in pressure, the relative volatility for a binary system
decreases.
Low viscosity absorbent is preferred for reasons of
rapid absorption rates and good heat transfer characteristics.
improved flooding characteristics.
low pressure drop on pumping.
all (a), (b) and (c).
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.
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
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.
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,
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.
Different solids have different equilibrium moisture curves.
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.
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.
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.
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.
Rotary dryer is suitable for drying sticky material.
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 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
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%
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.
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
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
Pick out the system with minimum boiling azeotrope at 1 atm.
ethyl alcohol-water
Free flowing granular materials can be best dried in a __________ drier.
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
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.
10-5 to 10-6
The solvent used in liquid extraction should not have high latent heat of vaporisation, because
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
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
Which of the following binary systems is an example of a maximum boiling azeotrope ?
Water-hydrochloric acid.
(NRe . NSc) is termed in mass transfer operation as the __________ number.
Peclet
The binary diffusivity in gases and liquids vary respectively as
T3/2 and T
Pick out the correct statement.
Dialysis process is used for the production of fresh water from brine.
Electrodylsis is used in the manufacture of NaOH.
Electrodylsis is used for the refin-ning/purification of petrol.
both (b) and (c).
Electrodylsis is used in the manufacture of NaOH.
Pick out the correct statement.
Bound moisture is removed during constant rate drying period.
Free moisture is removed during falling rate drying period.
The sum of free moisture and equilibrium moisture in a solid is called the critical moisture.
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.
free
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
steam jet ejector with a barometric condenser
