Department of Chemical Engineering, NEC, Erode.
MASS TRANSFER I
UNIT 1 DIFFUSION
TWO MARKS:
1. State Fick’s law of diffusion. (June 12/ Dec 14)
It states that molar flux is directly proportional to the concentration gradient.
JA = - DAB dCA/dZ
Where
JA = molar flux (moles/ m2 sec)
DAB= diffusivity or diffusion coefficient (m2/ s)
dCA/dZ = concentration gradient
The –ve sign indicates drop in concentration from high to low concentration.
2. Define molecular diffusion and eddy diffusion. (Dec’11,13)
Molecular diffusion: When diffusion results from the random movement/motion of the
molecules it is called molecular diffusion. The movement of individual molecules is highly
zigzag manner/ random walk process through another fluid.
Eddy diffusion: To enhance the rate of mixing, mechanical agitation is provided and this will
cause a turbulent motion. This method of mass transfer is known as eddy diffusion.
3. What is the effect of temperature and pressure on diffusivity of gases? (Dec’13)
DAB decreases with increase in pressure.
DAB α 1/P
DAB increases with increase in temperature.
DAB α T1.5
4. Define Diffusivity.
It is defined as the ratio of the mass flux to its concentration gradient.
Unit – m2 / s
5. Define Knudsen diffusion. (Nov’11)
At low pressure, the mean free path of molecules may be larger than the diameter of passage
when the diffusion occurs inside the fine pore of the solids. The collision within the wall
becomes important compared to collision among molecules. This kind of diffusion is known
as Knudsen diffusion.
6. What is meant by pseudo steady state diffusion?
If the length of the diffusion path changes over a period of time, pseudo steady state
develops.
7. Explain briefly about film theory.
Mass transfer co efficient is directly proportional to diffusivity.
k α DAB
The concentration gradient is assumed to be linear in these films.
The theory assumes that the turbulence in the bulk fluids dies at the interface.
8. What are the assumptions of penetration theory? (Dec 2016)
The mass transfer coefficient is proportional to the square root of diffusivity.
Contact between gas and liquid occurs in series of intermittent steps and the length of time
that each element remains in contact with gas is constant.
The transfer is largely due to fresh material brought to the interface by the eddies.
Each fluid element resides for the same time interval at the surface.
Question Bank – Mass Transfer I
Page 1
Department of Chemical Engineering, NEC, Erode.
9. Define flux in mass transfer.
The mass transfer flux of a given component is defined as the amount of that component, in
either mass or molar units, that crosses a unit area per unit time.
Flux = concentration × velocity
10. Name different theories of mass transfer / Compare and contrast the three theories
of mass transfer.
Film theory – Mass transfer coefficient proportional to diffusivity
Penetration theory - Mass transfer coefficient proportional to square root of diffusivity
Surface renewal theory - Mass transfer coefficient is proportional to square root of diffusivity
multiplied by surface renewal rate.
11. Write down the relation between NA and JA.
Rate of diffusion is expressed in terms of molecular flux in moles per area and time. N is
defined with respect to fixed locations in space. J is defined with respect to the relative
velocity of all components.
NA= JA+ NxA
where N= NA+ NB
N- Total mass flux, NA= Mass flux of component A, NB= Mass flux of component B
xA = No of moles of A
12. How does diffusion occur in solids?
Diffusion through polymers
Diffusion through crystalline solid
Diffusion in porous solid
13. What are the methods for measurement of gas phase diffusion coefficient?
Twin bulb method
Stefan tube method
14. Define Mass Transfer.
The transfer of molecules from one phase to another phase / or within a phase.
Driving force is concentration gradient.
The removal of a component from mixture of components is done (Gas, liquid and solid) by
the principle of vapor pressure or solubility.
15. Define concentration gradient.
The change in concentration of molecules transferred per unit time per unit area.
13 MARKS:
1. Derive an expression for finding the mass flux of a component in a binary gas mixture.
(i) Diffusion of gas A through non-diffusing B.
(ii) Equimolar counter diffusion under steady state. (May’16)(Nov’16)
2. Calculate the rate of diffusion of acetic acid (A) across a film of non-diffusing water (B)
solution 2 mm thick at 17⁰C, when the concentrations (by weight) on opposite sides of the
film are 10% and 4% acid. The diffusivity of acetic acid in the solution is 0.000095 m 2/s.
Density of 10% and 4% acid (by weight) are 1013 kg/m3 and 1004 kg/m3 respectively.
Molecular weight of acetic acid is 60, molecular weight of water is 18.
3. Explain the types of diffusion in solids. (Write a note on diffusion in solids)
4. A vertical glass tube of diameter 0.3 cm is filled with benzene at 30⁰C to a depth of 2 cm
from top end. After 24 hours, the liquid level in the tube had fallen to 2.5 cm from the top
Question Bank – Mass Transfer I
Page 2
Department of Chemical Engineering, NEC, Erode.
end. Estimate the diffusivity of benzene into air if the air above the liquid surface in the tube
is stagnant. The vapour pressure and density of benzene at 30⁰C are 60 mm Hg and 800
kg/m3 respectively.
5. Alcohol is diffusing from gas to liquid and water from liquid to gas under conditions of
equimolar counter diffusion at 35⁰C and 1 atm pressure. The molar concentration of alcohol
on the two sides of a gas film 0.2 mm thick is 90% and 10% respectively. Assuming the
diffusivity of alcohol-water vapour to be 0.18 cm2/s, calculate the rate of diffusion of alcohol
and water in kilograms per hour through an area of 100 cm2. Molecular weight of alcohol =
7.41.
6. Water in the bottom of a narrow metal tube is held at a constant temperature of 298K. The
dry ambient air outside the tube is at 1atm and 298 K. Water evaporates and diffuses through
the air in the tube and the diffusion path z2-z1 is 50 cm long. Calculate the rate of evaporation
at steady state in moles/ cm2 s. The diffusivity of water vapour (A) in air (B) at 1 atm and
298 K is 0.250 cm2/s. Assume that air is insoluble in water.
7. Discuss the various mass transfer theories have been used to predict mass transfer
coefficients.
8. In an oxygen-nitrogen gas mixture at 1 atm, 25⁰C, the concentrations of oxygen at two
planes 0.2 cm apart are 10% and 20% volume respectively. Calculate the rate of diffusion of
oxygen expressed as gmol/cm2 for equimolar counter diffusion of the two gases. Diffusivity
of oxygen in nitrogen at 25⁰C and 1 atm is 0.206 cm2/sec.
9. Explain the method of measurement and calculation of diffusivities.
Question Bank – Mass Transfer I
Page 3
Department of Chemical Engineering, NEC, Erode.
UNIT 2 INTERPHASE MASS TRANSFER
Two Marks:
1. Define JD factor and give its significance.
The experimental data on mass transfer coefficient and friction factor is correlated
empirically by Chilton and Colburn. JD = f/2
2. Define mass transfer coefficient.
Mass transfer coefficient is defined as the rate of mass transfer per unit area per unit
concentration difference.
3. What is meant by HTU and NTU?
HTU - Height of transfer unit is a measure of the separation effectiveness of the particular
packings for a particular separation process.
NTU – Number of transfer unit is a measure of the difficulty of separation.
4. Differentiate between murphree efficiency and point efficiency in gas-liquid contact
operations.
Overall efficiency = (theoretical/ideal plates required)/actual plates required × 100
The murphree stage efficiency is defined as the ratio of the actual concentration change
effected in one phase to the concentration change attainable if that phase leaves in
equilibrium with the actual concentration of the other leaving stream.
Point efficiency pertains to a particular place on the plate surface.
5. State Henry’s law. Where can it be applied?
pA = H CA. Henry’s constant H is very small means ( a case with high soluble gas) total
resistance equal to the gas film resistance. If H is very large (a case with a insoluble gas) the
total resistance equal to the liquid film resistance.
6. What do you mean by equilibrium line and operating line?
An ideal stage is a device in which the phases are allowed to remain in contact for a period
long enough to establish thermodynamic equilibrium then the phases leave with equilibrium
compositions. An ideal stage is also known as equilibrium stage or a theoretical stage. In an
actual operation, we will not be able to provide such a long time of contact and therefore, the
concentration change in an actual stage (operating line) will be less than that achieved with
an ideal stage.
7. Give four examples of common tower packings.
Raschig rings, Lessing ring, Berl saddle, Intalox saddle, Metal pall ring.
8. Explain the term Foaming in packed column.
Foaming refers to the expansion of liquid due to passage of vapour or gas. Although it
provides high interfacial liquid-vapour contact, excessive foaming often leads to liquid build
up on trays. In some cases, foaming may be so bad that the foam mixes with liquid on the
tray above. Whether foaming will occur depends primarily on physical properties of the
liquid mixtures, but is sometimes due to tray designs and condition. Whatever the cause,
separation efficiency is always reduced.
9. Explain the term Entrainment
Entrainment refers to the liquid carried by vapour up to the tray above and is again caused by
high vapour flow rates. It is detrimental because tray efficiency is reduced: lower volatile
material is carried to a plate holding liquid of higher volatility. It could also contaminate high
purity distillate. Excessive entrainment can lead to flooding.
Question Bank – Mass Transfer I
Page 4
Department of Chemical Engineering, NEC, Erode.
10. Explain the term Weeping/Dumping.
This phenomenon is caused by low vapour flow. The pressure exerted by the vapour is
insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through
perforations. Excessive weeping will lead to dumping. Weeping is indicated by a sharp
pressure drop in the column and reduced separation efficiency.
11. Explain the term Flooding. (Dec’13/June’16)
Flooding is brought about by excessive vapour flow, causing liquid to be entrained in the
vapour up the column. The increased pressure from excessive vapour also backs up the liquid
in the down comer, causing an increase in liquid holdup on the plate above. Depending on
the degree of flooding, the maximum capacity of the column may be severely reduced.
Flooding is detected by sharp increases in column differential pressure and significant
decrease in separation efficiency.
12. Distinguish between co current and counter current process. (June’16)
The extent of mass transfer can be improved over and above that which is possible with a
single stage by multiple contact in which several stages are interconnected. The co-current
operation, in which both streams are flowing in same direction and in counter current
operation they flow in opposite direction. For a given ratio of flow rates the percent recovery
of solute, is higher in counter current process than any other arrangements.
13. Define channeling.
At low liquid rates much of the packing surface may be dry or atmost covered by stagnant
film of liquid (unequal wetting of packed tower). This effect is known as channeling.
14. Define loading. ( Dec’13)
As gas flow rate is increased, the pressure drop per unit length of packing increases, pressure
drop is low when the packing is dry. With increase in liquid flowrate, pressure drop increases
as it reduces the space available for gas flow. The point at which liquid hold (liquid held in
tray) start increases is known as loading point.
15. Define priming.
In case of gas liquid contact, high gas velocities may lead to a condition of foaming is known
as priming. In such cases the foam is present in the space between trays and there is a great
deal of liquid getting entrained with gas. The liquid carried is recirculated between trays and
the added liquid give rise to pressure drop.
13 Marks:
1. Derive the relation between local and overall transfer coefficient. (Nov/Dec 2015, Nov/
Dec 2014)
2. Discuss the working principle of a Tray towers with neat sketch. (Nov/Dec 2014)
3. Obtain an expression for overall mass transfer coefficient in terms of film coefficients.
4. Compare Tray towers with packed towers based on its operation and applications.
(May/June 2016, Nov/Dec 2014)
5. Explain with neat sketch about cross current and counter current arrangements. Also
obtain expressions for operating lines for each.
6. At 293 K the solubility of ammonia in water is given by p = 0.00127C, where, p is in
atmosphere and C is in kmole/m3. A mixture of 15% ammonia and 85% air by volume at 1
atm is in contact with an aqueous solution containing 0.147 g mole/lit. The air velocity is
such that hg/hl = 0.9, Find the concentration of ammonia and partial pressure at interface.
7. Compare plate and packed columns for merits and demerits.
Question Bank – Mass Transfer I
Page 5
Department of Chemical Engineering, NEC, Erode.
8. Discuss in detail about the differential and stage wise contact operations.
9. What is meant by murphree stage efficiency and plate efficiency?
10. Explain the essential properties of good tower packing used in gas-liquid contact.
Question Bank – Mass Transfer I
Page 6
Department of Chemical Engineering, NEC, Erode.
UNIT III HUMIDIFICATION
1. Define wet bulb temperature. (Nov’17) (May’18)
It is the temperature of air measured by a thermometer when its bulb is covered with wet
cloth and is exposed to a current rapidly moving air.
2. Define humidity and dew point. (Nov’16)
Humidity is defined as the mass of water vapour present in one kg of dry air.
Dew point - It is the temperature at which the water vapour present in air begins to
condense when the air is cooled. For saturated air, the dry bulb, wet bulb and dew point
temperature are all same.
3. Distinguish between spray pond and spray tower. (May’18)
A spray pond is a reservoir in which warmed water (e.g. from a power plant) is cooled
before reuse. This is done by spraying the warm water with nozzles into the cooler air, where
cooling takes place by exchange of heat with the ambient air, involving both conductive heat
transfer between the water droplets and the surrounding air.
A spray tower (or spray column or spray chamber) is gas-liquid contactor used to
achieve mass and heat transfer between a continuous gas phase (that can contain dispersed
solid particles) and a dispersed liquid phase.
4. Absolute humidity & Molal humidity.
It is defined as the weight of water vapour, present in a unit weight of dry (noncondensable) gas.
It is defined as the ratio of moles of vapour (condensable) and to the moles of dry
(non-condensable) gas.
5. Relative humidity/percentage relative humidity (relative saturation).
It is defined as the ratio of the partial pressure of condensable vapour in the gas phase
to the vapour pressure of liquid at DB.
13 Marks:
1. Explain in detail on the construction, advantages and limitations of both Natural draft and
forced draft cooling towers with a neat sketch of it. (May’18)
2. A mixture of nitrogen-acetone vapour at 800 mmHg and 25⁰C has percentage saturation of
80%. Calculate
i.
absolute humidity,
ii.
partial pressure of acetone,
iii.
absolute molal humidity and
iv.
volume percent of acetone. Assuming vapour pressure of acetone at 25⁰C is 190
mmHg.
8. The air supply for a drier has dry bulb temperature of 26⁰C and a wet bulb temperature of
17⁰C. It is heated to 85⁰C by heating coils and introduced into the drier. In the dryer, it cools
along the adiabatic cooling line and leaves the dryer fully saturated.
i.
What is its humidity initially and after heating?
ii.
What is the dew point of the initial air?
iii.
How much water will be evaporated per 100 cubic meter of entering air?
iv.
How much heat is needed to heat 100 cubic meter air to 85⁰C?
v.
At what temperature does the air leave the drier?
3. Explain the principle of cooling tower.
4. Explain about spray chamber and spray ponds.
Question Bank – Mass Transfer I
Page 7
Department of Chemical Engineering, NEC, Erode.
5. An air (B) – water vapour (A) sample has dry bulb temperature 55⁰C and an absolute mass
humidity of 0.030 kg of water/kg of dry air at 1 standard atmospheric pressure. Find the
percentage humidity, absolute molal humidity, humid volume, humid heat, enthalpy and
partial pressure of water vapour.
Vapour pressure of water at 55⁰C = 118 mmHg
Heat capacity of air = 1005 J/kg⁰C
Heat capacity of water vapour = 1884 J/kg⁰C
Latent heat of water vapour = 2502.3 kJ/kg
(Nov’13)
6. Estimate the wet bulb and adiabatic saturation temperatures for a toluene-air mixture of
60⁰C dry-bulb temperature, Y´= 0.050 kg vapour/kg air, 1 std atm. (April 18)
UNIT IV DRYING
Question Bank – Mass Transfer I
Page 8
Department of Chemical Engineering, NEC, Erode.
Two Marks:
1. Define drying.
Drying refers to the removal of moisture of a substance by thermal means (hot air). It
involves the transfer of liquid from wet solid into an unsaturated gas medium.
2. List down the reasons to carry out drying operation.
i. For reducing the transport cost
ii. For purifying a crystalline product so that the solvent adhering to the crystals is
removed.
iii. For making a material more suitable for handling and storage.
iv. Prevention of corrosion
3. Define moisture content on wet basis and dry basis.
Moisture content, wet basis
It is expressed as the ratio of the weight of the moisture substance to the weight of the
wet feed material.
Moisture content, dry basis
It is expressed as the ratio of the weight of the moisture to the weight of the dry solids present
in the wet feed material.
4. Define bound and unbound moisture. (Nov’14)
Bound moisture
It is the moisture content of a substance which exerts a vapour pressure less than that of the
pure liquid at the same temperature.
Unbound moisture
It is the moisture content in a substance that exerts an equilibrium vapour pressure equal to
that of the pure liquid at the given temperature.
5. Define equilibrium moisture content.
It is the moisture content of a substance that is in equilibrium with its vapour in the gas phase
under specified humidity and temperature of hot gas or air.
6. Define critical moisture content. (Nov’15)
It is the moisture content of a material at which the constant rate period ends and the falling
rate period starts.
7. Define free moisture content. (Nov’15) (Nov’16)
It is the moisture in the substance which is in excess of the equilibrium moisture content.
8. Indicate the various regimes of drying. (May’16)
Constant drying rate
It is the part of drying at which the rate of drying is
constant.
Falling rate period
It is the part of drying during which rate of drying
varies with time.
Question Bank – Mass Transfer I
Page 9
Department of Chemical Engineering, NEC, Erode.
9. What is the effect of gas velocity/ flow rate and temperature on drying? (May’16)
(Nov’16)
When the velocity of gas or air is high, the rate of drying will also be high. When the
temperature of gas increases, its relative humidity decreases and thus it increase the driving
force. So rate of drying also increases.
10. What is the principle involved in fluidised bed drier? (Nov’13)
In this dryer, hot air is passed through a wet material at a velocity sufficiently high to fluidise
the wet material but not too high enough to cause pneumatic conveying. It is used for very
fine size free flowing materials.
11. What are the parameters affecting drying rate during constant rate drying period?
a. Gas velocity: When the velocity of air is high, the rate of drying will also be high.
b. Humidity of gas: Lesser the relative humidity, the more will be the rate of drying.
c. Area of drying surface: If the area of wet surface exposed to air is more, the rate of drying
will also be more.
d. Temperature: If the temperature of the air is increased, its relative humidity decreases and
thus increases driving force and so the rate of drying increases.
12. What is meant by free moisture and equilibrium moisture? (Nov’16)
13. What is the effect of temperature and air flow rate on drying rate during constant
rate period? (Nov’16)
a. Gas velocity: When the velocity of air is high, the rate of drying will also be high.
b. Temperature: If the temperature of the air is increased, its relative humidity decreases and
thus increases driving force and so the rate of drying increases.
14. List out some industrial drying equipment.
Tray dryer, rotary dryer, drum dryer, fluidised bed dryer and tunnel dryer.
15. Sketch the typical drying characteristics curves.
13 Marks:
1. A porous solid is dried in a batch dryer under constant drying condition. 6 h is required to
reduce the moisture content from 30% to 10%. Critical moisture content was found to be
16%. Equilibrium moisture was 2%. All moisture contents are in the dry basis assuming that
the rate of drying during falling rate period is proportional to the free moisture period. How
long will it take to dry a sample from 35 to 6% under the same drying condition.
2. It is desired to dry a certain type of fibre board in sheets 0.131 m × 0.162 m × 0.071 m
from 59% to 4% moisture (wet basis) content. The rate of drying at constant rate period was
found to be 8.0 kg/m3h. The critical moisture content was 24.8% and the equilibrium
moisture content was 1.0%. The fibre board is to be dried from one side only and has a bone
dry density of 299 kg/m3. Determine the time required for drying. Assume the falling rate is
linear.
3. Classify the dryers. With the help of neat sketch explain the construction and working of
different types of dryers. (Tray, rotary, spray, drum, vacuum & special drying methods)
5. Explain drying rate curve and calculation of drying rates under constant rate and falling
rate. (Nov’17)
6. A batch of the solid, for which the following table o data applies, is to be dried from 25%
to 6% moisture under conditions identical to those for which the data were tabulated. The
initial weight of the wet solid is 350 kg, and the drying surface is 1 sq. m/8 kg dry weight.
Determine the time for drying.
Question Bank – Mass Transfer I
Page 10
Department of Chemical Engineering, NEC, Erode.
𝑘𝑔 𝑚𝑜𝑖𝑠𝑡𝑢𝑟𝑒
X ×100, 𝑘𝑔 𝑑𝑟𝑦 𝑠𝑜𝑙𝑖𝑑
35 25 20 18
16
14
12 10 9
8 7
6.4
N × 100, kg moisture
evaporated / hr.m2
30 30 30 26.6 23.9 20.8 18 15 9.7 7 4.3 2.5
7. A rotary dryer using counter current flow is to be used to dry 12000 kg/hr of wet salt
containing 5% water (wet basis) to 10% water (wet basis). Heated air at 147⁰C with a WBT
of 50⁰C is available. The specific heat of the salt is 0.21 kcal/kg ⁰C. The outlet temperature
of air and salt are 72⁰C and 93⁰C respectively. Calculate the diameter of the dryer required.
(Nov’17).
8. A tunnel dryer is being designed for drying apple halves from initial moisture content of
70% (wet basis) to final moisture content of 5% (wet basis). An experimental drying curve
or the product indicates that the critical moisture content is 25% (wet basis) and the time for
constant drying is 5 min. Based on the information provided, estimate the total drying time
for product. (May’18)
9. A stock containing 1.526 kg moisture per kg of dry solid is dried to 0.099 kg moisture per
kg dry solid by counter current air flow. Fresh air entering contains 0.0152 kg water per kg
dry air and the exit air has 0.0526 kg water per kg dry air. What fraction of air is recycled if
52.5 kg of dry air flows per 1 kg of dry solid inside the drier? (May’18)
10. What is the purpose of drying operations? Explain with suitable examples.
11. Write short notes on liquid diffusion and capillary movement theory to explain the
moisture movement within the solids.
12. Write a note on drying rate curve.
Question Bank – Mass Transfer I
Page 11
Department of Chemical Engineering, NEC, Erode.
UNIT V CRYSTALLIZATION
Two Marks:
1. What are the methods of achieving supersaturation? (Nov’16)
a) Supersaturation by cooling a concentrated, hot solution through indirect heat
exchange
b) Supersaturation by evaporation of a part of solvent/ by evaporating a solution
c) Supersaturation by adiabatic evaporation and cooling
d) Supersaturation by adding a new substance
e) Supersaturation by chemical reaction with third substances.
2. Define crystallisation.
It is a unit operation used to separate a solute from its solution in the form of crystals.
3. Define solubility.
The solubility of a solute in a given solvent is the concentration of the solute in a saturated
solution at a given temperature.
4. What is the purpose of agitator in a crystallizer? (Nov’17)
The agitator helps in increasing the rate of heat transfer and in maintaining uniformity of the
temperature throughout the solution; it also keeps the growing crystals in suspension so that
these can grow uniformly.
5. Yield
Yield may be defined as the weight of the substance crystallised per unit weight of the
substance in the feed solution.
6. Processes involved in the formation of crystals.
Supersaturation, Nucleation and crystal growth: Birth of very small bodies of molecules
gather together in clusters in a defined manner. Nuclei that have successfully achieved the
“critical cluster size” begin to increase in size.
7. The delta-L law of crystal growth:
According to the law of crystal growth, all geometrically similar crystals of the same material
suspended in the same solution grow at the same rate and the rate is independent of crystal
size, provided all crystals in the suspension are treated alike.
13 Marks:
1. Write in detail about different types of cooling towers with neat diagram. (Nov’17)
2. Explain about the different types of crystallizer. (Nov’14)
3. Explain why homogeneous nucleation is difficult to occur. (Nov’17)
4. Explain the principle of wet bulb thermometry. (Nov’17)
5. 2500 kg of KCl are present in a saturated solution at 80⁰C. The solution is cooled to 20⁰C
in an open tank. The solubility of KCl at 80⁰C and 20⁰C are 55 and 35 parts per 100 parts of
water. Assuming water equal to 5% by weight of solution is lost by evaporation; calculate
the weight of crystals obtained. (Nov’17)
6. A crystallizer is charged with 7500 kg of an aqueous solution containing 29.6% of
anhydrous sodium sulphate. The solution is cooled and 10% of the initial water is lost by
evaporation. Na2SO4.10H2O crystallizes out. If the mother liquor (after crystallization) is
found to contain 18.3% anhydrous Na2SO4, calculate the yield of crystals and weight of the
mother liquor. (Dec’17)
7. Differentiate between batch and continuous crystallization.
(Nov’13)
Question Bank – Mass Transfer I
Page 12