Mass Transfer (Set 6) Flashcards
As one of the transport phenomena, the concepts in mass transfer are crucial for design, operation, and maintenance of mass or heat & mass transfer equipment such as mixing and separation processes.
In a binary system, separation is very efficient, when the relative volatility is
> 1
Fenske-Underwood equation used for calculating the minimum number of theoretical plates in distillation column is valid for
all (a), (b) and (c)
The binary diffusivity in gases does not depend upon the
none of these
Not pressure, temperature, and nature of components
Berl saddle made of carbon cannot be used for
oxidising atmosphere
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
During drying of a solid, the critical moisture content increases with
a. decreased drying rate
b. increased thickness of the solid
c. increased drying rate
d. both (b) and (c).
both (b) and (c).
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.
In saturated gas, the
vapour is in equilibrium with the liquid at the gas temperature.
As per Gilliland’s equation, the diffusivity is
all ‘a’, ‘b’ & ‘c’.
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
Which of the following is not fixed by the process requirements, in the design of absorbers?
None of these.
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.
When the psychometric ratio is ____ then the adiabatic saturation temperature and wet bulb temperature becomes equal.
1
Desirable value of absorption factor in an absorber is
> 1
Refractory bricks are usually dried in a ____ dryer.
tunnel
For ____ drying, the effects of velocity, temperature and humidity of the gas and the thickness of the solid are the same.
both (a) and (b)
What is the degree of freedom of an absorption process in which only one component is transferred between phases?
3
Sticky material can be dried in a ____ dryer.
tray
Langmuir equation is associated with
adsorption
During the constant rate period of drying of a solid,
surface evaporation of unbound moisture occurs.
At a fixed total pressure, humidity depends only on the
partial pressure of vapour in the mixture.
If moisture content of solid on dry basis is X, then the same on wet basis is
X/X+1
Flash distillation is
used for multicomponent systems like crude refining.
____ 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
Coffee is prepared from coffee beans by leaching with
hot water
Gaseous diffusion co-efficient increases with increase in the
temperature
With increase in absolute humidity, the dew point of an unsaturated mixture of air and water vapor (at constant pressure and temperature)
increases
If xD = overhead product molal composition and RD = reflux ratio, then slope and intercept of the operating line for rectifying section are respectively
https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/287-102-2.png
Pick out the wrong statement. Raoult’s law does not apply, when the
component molecules are nonpolar.
Water-ethyl alcohal solution can be concentrated (in respect of alcohal concentration) by ____ distillation.
vacuum
L/mG is the expression for
absorption factor
Extraction of pencillin employs a ____ extraction coloumn.
centrifugal
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
Corresponding to Prandtl number in heat transfer, the dimensionless group in mass transfer is the ____ number.
Schmidt
If a two phase system is in physical equilibrium; then it means that, the
all (a), (b) and (c).
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.
When a multistage tray tower uses a minimum reflux ratio, it implies
infinite trays and minimum reboiler heat load.
CO2 can be absorbed by
heated charcoal
For the same process conditions, the reflux ratio of an unlagged distillation column
increases in winter
An ideal tower packing should not
hold up large amount of liquid in the packing.
Which of the following is a suitable absorbent for removal of H2S from natural gas?
Monoethanol amine (MEA)
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
Below is given an equilibrium moisture curve of a substance. State which regions represent free moisture and unbound moisture respectively. https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/292-170-1.png
II, III
Molecular diffusion is caused by the
thermal energy of the molecules.
At ____ reflux, theoretical number of plates in a distillation coloumn is calculated using Fenske-Underwood equation.
total
Separation of a mixture of two gases by absorption in the liquid solvent depends upon the difference in their
solubility
Components, having widely different boiling point in a binary mixture, can be separated using ____ distillation.
simple
Which of the following equipments is not used in liquid-liquid extraction?
Pachuka tank
Pick out the wrong statement.
Diameter of randomly packed tower is normally more than 1.2 metres.
Equilibrium-moisture curves of different solids
are different
Azeotropism for ethyl alcohol-water system disappears by
decreasing pressure
Degree of freedom for leaching and solvent extraction process is
3
Diffusion co-efficient generally depends upon the temperature, pressure & the nature of the components of the system. Its dimension is not the same as that of the
mass transfer co-efficient.
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.
The expression, ln(F/W) = https://www.indiabix.com/_files/images/chemical-engineering/mass-transfer/314-454-1.png is
differential distillation equation
Heat sensitive materials with very high latent heat of vaporisation may be economically separated using
liquid extraction
When the solvent dissolves very little of solute, then
large quantity of solvent is required to extract the solute.
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
HETP is numerically equal to HTU, only when the operating line
and equilibrium lines are parallel.
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).
Steady state equimolal counter diffusion occurs in case of
binary phase distillation
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
The relative saturation of an unsaturated mixture of gas and vapor is independent of the
none of these.
The adiabatic saturation curve for a vapour-gas mixture is
slightly concave upward.
Pick out the wrong statement.
Zirconia probe is used for the humidity measurement.
The value of NA/(NA + NB), for steady state equimolal counter diffusion of two gases ‘A’ and ‘B’ is
∞
Drift in a cooling tower is
the water entrained by the circulating air.
With increase in the concentration of the adsorbate, the integral heat of adsorption
decreases.
Small dia distillation column can be a
packed column
If mass diffusivity in a mixture is equal to the thermal diffusivity, then the Lewis number is
1
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
____ column is used in gas absorption process.
Packed
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.
At what value of reflux ratio, number of theoretical plates in a distillation column is minimum?
∞
Batch tray dryers suffers from the disadvantage of
high labour cost & low heat economy.
In a moderately sized packed absorption tower, channeling (which is most severe with stacked packings) can be minimised by taking the ratio of tower diameter to packing diameter
> 8:1
____ column is preferred to be used, when a high liquid hold up is required in a reactor for gas-liquid reaction.
Bubble
The ‘shanks system’ of leaching (i.e., counter-current multiple contact leaching) is used for
both (a) and (b).
Penetration model (theory) for mass transfer was enunciated by
Higbie
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
For turbulent mass transfer in pipes, the Sherwood number depends on the Reynolds number as
Re^0.83
The relative humidity of air decreases in-spite of an increase in the absolute humidity, when the
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
