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
Total reflux in a distillation operation requires minimum
number of 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.
The most suitable for extraction in a system having very low density difference ?
Centrifugal extractor
The amount of steam required per unit quantity of distillate in case of steam distillation will be reduced by
both (a) and (b).
Fenske’s equation for determining the minimum number of theoretical stages in distillation column holds good, when the
relative volatility is reasonably constant.
The difference of wet bulb temperature and adiabatic saturation temperature of unsaturated mixture of any system is
+ve
When the temperature and humidity of air is low, we usually use __________ draft cooling tower.
natural
Pick out the correct statement.
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.
the controlling factor for a drum drier
Heat transfer
On addition of solute in the solvent, the __________ of the solution decreases.
both (b) and (c)
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
Stefan’s law describes the mass transfer by
both ‘a’ & ‘b’
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’.
In a packed absorption tower, if the equilibrium and operating lines are both straight lines, then the ratio, HETP/HTUOG __________ the absorption factor.
both (a) and (b)
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
case of an absorber, the operating
line always lies above the equilibrium curve.
the reflux ratio in a distillation column 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
the following remains constant during sensible cooling process ?
Both (a) and (b)
Physical absorption is
both (b) and (c)
Make up water is required in a cooling tower to replace the water lost by
all (a), (b) and (c)
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.
he following unit operations makes use of Thiele-Geddes equation
Multicomponent distillation.
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.
The rate of solid-liquid extraction is limited by the
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.
distillation, overhead product contains
any number of components
Low viscosity absorbent is preferred for reasons of
all (a), (b) and (c).
The McCabe Δ L law states that the
linear crystal growth rate does not depend on the crystal size.
increase in pressure, the relative volatility for a binary system
decreases.
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
A coalescer in a mixer-settler liquid-liquid extraction coloumn
all ‘a’, ‘b’, & ‘c’.
For the same feed, feed quality and separation (in a distillation column), with the increase of total pressure, the number of ideal plates will
Incease
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.
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
Agitator is provided in a crystalliser for
all (a), (b) and (c).
__________ diffusion is used for separating the isotopes of methane.
Thermal
Ordinary diffusion process is also called __________ diffusion.
concentration
Pick out the wrong statement.
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.
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.
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
The solvent used in liquid extraction should not have high latent heat of vaporization, because
its recovery cost by distillation may be prohibitatively high.
Molarity is defined as the number of gm moles of solute per __________ of solvent.
litre
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
The partial pressure distribution of an ideal gas diffusing through another stagnant ideal gas at steady state follows a/an __________ law.
exponential
the following binary systems is an example of a maximum boiling azeotrope
Water-hydrochloric acid.
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.
(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.
The driving force for mass transfer in a wet solid is its free moisture content.
Pick out the correct statement.
Electrodylsis is used in the manufacture of NaOH.
Very tall packed towers are divided into series of beds to
avoid channeling
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
On moving the feed line (q-line) from saturated liquid feed (vertical position) to saturated vapor feed (horizontal feed), if the slope of both the operating lines are to be increased, then it will result in
increased reflux ratio
__________ column is the most suitable for achieving the best performance for mass transfer operations involving liquid with dispersed solids.
Plate
Deliration (removal of O2) of water is done by
ion-exchange
Make up water is supplied during cooling tower operation to compensate for water losses due to
all (a), (b)&(c)
Pick out the wrong statement.
Azeotropes obey Raoult’s law at all temperature
A minimum boiling azeotrope is exemplified by
acetone-carbon disulphide
the following solutions will follow Raoult’s law most closely
A solution of benzene, toluene and oxylene.
Use of raschig rings in place of crushed stones as packing in packed beds (other things being same)
decreases pressure drop, increases surface area.
Critical humidity of a solid salt means the humidity
both (a) and (b).
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.
Only small amount of evaporation of water produces large cooling effects because of its
large latent heat
The-relative volatility for separation of a non-ideal binary mixture by distillation should be
> 1
In paper industry, paper is dried in a __________ dryer.
heated cylinder
When the feed to a distillation column is a saturated liquid, slope of the feed line is
infinity
As complete saturation of an adsorbent is approached, the differential heat of adsrop-tion approaches
that of normal condensation.
Pick out the wrong statement.
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum.
The most common packing used in industrial operations is __________ rings.
raschig
Leaching of sugar from sugar beets is done by
hot water
Absorption with chemical reaction’ is involved in the removal of
carbon dioxide from gaseous stream using alkaline solution.
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.
The rate of mass transfer is not dependent upon the
none of these.
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.
Dry bulb temperature of unsaturated air is more than its __________ temperature.
both ‘a’ & ‘b’
For the case of flow of air past a wet bulb thermometer (air water vapour system), the approximate value of h2/ky.Cz is around
1
