Mass Transfer Operations Question bank
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Register Number: _________________
Answer all the Questions
PART A (10 x 1 = 10 Marks)
Objective Types & Fill in the Blanks
A naphthalene hemisphere is placed on a glass plate and a naphthalene ball of identical diameter is
kept suspended in the stagnant air in two large rooms of same volume. The respective shapes of the
balls are maintained while they sublime and reduce in size. Which of the following statements are
true? (K3)
a) They take the same time for complete b)
The sphere disappears first
sublimation
c) The hemisphere disappears first
d)
No sublimation occurs
The unit of diffusivity in SI system is __________ (K1)
Which of the following diffusion phenomenon can be explained by a hopping mechanism? (K3)
a) Knudsen diffusion
b)
Surface diffusion
c) Multicomponent diffusion
d)
No diffusion
The ratio between mass flux and molar flux is __________ (K2)
Which of the following mechanism is observed in breathing process? (K2)
a) Knudsen diffusion
b)
Surface diffusion
c) Multicomponent diffusion
d)
No diffusion
__________ (mass transfer operation) is an example for equimolar counter diffusion. (K2)
If the mass transfer is expressed in terms of mass transfer coefficient, the driving force should be
(K3)
a) Concentration gradient at the interface
b)
Concentration gradient at the interface
and the bulk
c) Interfacial concentration
d)
Bulk concentration
The driving force of momentum transfer is __________ (K1)
For gas absorption from a bubble in steady rise through a stagnant liquid, which of the following
theories is more appropriate? (K2)
a) Surface renewal theory
b)
Film theory
c) Penetration theory
d)
Film penetration theory
__________ = Driving force/Resistance (K1)
PART B - [Answers not more than 40 words]
(5 x 2 = 10 Marks)
Short Answer Questions
The species A diffuses through gas mixture A-B from point x=0, y=y o to x=l, y=y at temperature T
and pressure P. The total pressure is now doubled, but the mole fractions, T and P remains same as
before. What would be the fractional increase in the flux of A? (K2)
Equimolar counter diffusion occurs in binary mixture A-B through a spherical film of ineer
diameter r and thickness t. The flux of A is NA at r. The flux through a flat film of same thickness is
NB. What is the fractional increase in thickness if NA is greater than NB by 5%? (K3)
Classify mass transfer operations with suitable examples. (K1)
Define interphase mass transfer. (K1)
Write short notes on individual mass transfer coefficient. (K2)
PART C [Answers not more than 300 words]
(3 x 10 = 30 Marks)
[Question No. 16 is compulsory]
Mark
Q.
no.
16. Through the accidental opening of a valve, water has been spilled on the floor of an 10
industrial plant in a remote, difficult to reach area. Estimate the time required to
evaporate the water into surrounding quiescent air. The water layer is 1 mm thick and
may be assumed to remain at constant temperature of 24C. The air is also at 24C and 1
1
atm pressure with an absolute humidity of 0.002 kg water vapour / kg dry air. The
evaporation is assumed to take place by molecular diffusion through a gas film of 5 mm
thickness. Diffusivity of water vapour in air is 0.259 sq. cm/s. (K3)
17. a) Ethanol is diffusing through a water layer of 3 mm thickness at 20C. Diffusivity of 10
alcohol in water is 520 m2/s. The concentrations on opposite sides of water film are
4% and 10% (w/w) alcohol respectively are 0.99 and 0.98 g/cc. Assume that water
film is stagnant, estimate: (K3)
(i)
The flux of alcohol
(ii)
Concentration of alcohol in the middle of water film.
(OR)
b) In O2-N2 mixture at 1 atm and 20C, the concentrations of O2 at two planes 2 mm 10
apart are 20% and 10% (v/v) respectively. Calculate the rate of diffusion of O 2 in
kg/m2.s for the case N2 is non-diffusing. Diffusivity is 0.181 cm 2/s. Calculate the rate
of diffusion of O2 assuming equimolal counter diffusion. (K3)
18. a)
In an experimental study of absorption of ammonia by water in a wetted wall 10
column, the overall gas phase mass transfer coefficient was 0.000272 kmol/m2.s.atm.
At one point in the column, the gas contained 10% (v/v) ammonia and the liquid
phase concentration was 0.0642 kmol NH 3/m3 of solution. Process occurs at 20C
and 1 atm. 85% of resistance to mass transfer lies in gas phase. If Henry’s law
constant is 0.00953 atm.m3/kmol, calculate the individual film coefficients and the
interfacial composition. (K3)
(OR)
b) At 20C, the solubility of ammonia in water is given by Henry’s law p = 0.3672 C 10
where p is in atm and C is in kmol/m3. A mixture of 15% ammonia-85% air by
volume at 1 atm is in contact with an aqueous solution containing 0.147 M. Air
velocity such that kg/kL = 0.9. Find the concentration of ammonia and partial pressure
at interface. (K3)
******************** All the Best *********************
2
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Register Number: _________________
Answer all the Questions
PART A (10 x 1 = 10 Marks)
Objective Types & Fill in the Blanks
A naphthalene hemisphere is placed on a glass plate and a naphthalene ball of identical diameter is
kept suspended in the stagnant air in two large rooms of same volume. The respective shapes of the
balls are maintained while they sublime and reduce in size. Which of the following statements are
true? (K3)
a) They take the same time for complete b)
The sphere disappears first
sublimation
c) The hemisphere disappears first
d)
No sublimation occurs
The unit of diffusivity in SI system is __________ (K1)
Which of the following diffusion phenomenon can be explained by a hopping mechanism? (K3)
a) Knudsen diffusion
b)
Surface diffusion
c) Multicomponent diffusion
d)
No diffusion
The ratio between mass flux and molar flux is __________ (K2)
Which of the following mechanism is observed in breathing process? (K2)
a) Knudsen diffusion
b)
Surface diffusion
c) Multicomponent diffusion
d)
No diffusion
__________ (mass transfer operation) is an example for equimolar counter diffusion. (K2)
If the mass transfer is expressed in terms of mass transfer coefficient, the driving force should be
(K3)
a) Concentration gradient at the interface
b)
Concentration gradient at the interface
and the bulk
c) Interfacial concentration
d)
Bulk concentration
The driving force of momentum transfer is __________ (K1)
For gas absorption from a bubble in steady rise through a stagnant liquid, which of the following
theories is more appropriate? (K2)
a) Surface renewal theory
b)
Film theory
c) Penetration theory
d)
Film penetration theory
__________ = Driving force/Resistance (K1)
PART B - [Answers not more than 40 words]
(5 x 2 = 10 Marks)
Short Answer Questions
The species A diffuses through gas mixture A-B from point x=0, y=y o to x=l, y=y at temperature T
and pressure P. The total pressure is now doubled, but the mole fractions, T and P remains same as
before. What would be the fractional increase in the flux of A? (K2)
Equimolar counter diffusion occurs in binary mixture A-B through a spherical film of ineer
diameter r and thickness t. The flux of A is NA at r. The flux through a flat film of same thickness is
NB. What is the fractional increase in thickness if NA is greater than NB by 5%? (K3)
Classify mass transfer operations with suitable examples. (K1)
Define interphase mass transfer. (K1)
Write short notes on individual mass transfer coefficient. (K2)
PART C [Answers not more than 300 words]
(3 x 10 = 30 Marks)
[Question No. 16 is compulsory]
Mark
Q.
no.
16. Through the accidental opening of a valve, water has been spilled on the floor of an 10
industrial plant in a remote, difficult to reach area. Estimate the time required to
evaporate the water into surrounding quiescent air. The water layer is 1 mm thick and
may be assumed to remain at constant temperature of 24C. The air is also at 24C and 1
1
atm pressure with an absolute humidity of 0.002 kg water vapour / kg dry air. The
evaporation is assumed to take place by molecular diffusion through a gas film of 5 mm
thickness. Diffusivity of water vapour in air is 0.259 sq. cm/s. (K3)
17. a) Ethanol is diffusing through a water layer of 3 mm thickness at 20C. Diffusivity of 10
alcohol in water is 520 m2/s. The concentrations on opposite sides of water film are
4% and 10% (w/w) alcohol respectively are 0.99 and 0.98 g/cc. Assume that water
film is stagnant, estimate: (K3)
(i)
The flux of alcohol
(ii)
Concentration of alcohol in the middle of water film.
(OR)
b) In O2-N2 mixture at 1 atm and 20C, the concentrations of O2 at two planes 2 mm 10
apart are 20% and 10% (v/v) respectively. Calculate the rate of diffusion of O 2 in
kg/m2.s for the case N2 is non-diffusing. Diffusivity is 0.181 cm 2/s. Calculate the rate
of diffusion of O2 assuming equimolal counter diffusion. (K3)
18. a)
In an experimental study of absorption of ammonia by water in a wetted wall 10
column, the overall gas phase mass transfer coefficient was 0.000272 kmol/m2.s.atm.
At one point in the column, the gas contained 10% (v/v) ammonia and the liquid
phase concentration was 0.0642 kmol NH 3/m3 of solution. Process occurs at 20C
and 1 atm. 85% of resistance to mass transfer lies in gas phase. If Henry’s law
constant is 0.00953 atm.m3/kmol, calculate the individual film coefficients and the
interfacial composition. (K3)
(OR)
b) At 20C, the solubility of ammonia in water is given by Henry’s law p = 0.3672 C 10
where p is in atm and C is in kmol/m3. A mixture of 15% ammonia-85% air by
volume at 1 atm is in contact with an aqueous solution containing 0.147 M. Air
velocity such that kg/kL = 0.9. Find the concentration of ammonia and partial pressure
at interface. (K3)
******************** All the Best *********************
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