Mass Transfer Flashcards
1
Q
With increase in pressure, the relative volatility for a binary system
A
decreases
2
Q
The McCabe Δ L law states that the
A
linear crystal growth rate DOES NOT depend on the crystal size.
3
Q
For the same feed, feed quality and separation (in a distillation column), with the increase of total pressure, the number of ideal plates will
A
increase
4
Q
In the constant rate period of the rate of drying curve for batch drying
A
surface evaporation of unbound moisture occurs.
5
Q
The reason for preferring packed towers over plate towers in distillation practice is that the packed tower operation gives
A
low pressure drop and low hold up
6
Q
Weeping in a distillation column
A
results due to very low gas velocity.
7
Q
With decrease in the throughput (compared with the design capacity) for a bubble cap distillation column, its efficiency
A
increases
8
Q
Pick out the system with minimum boiling azeotrope at 1 atm
A
ethyl alcohol-water
9
Q
Free flowing granular materials can be best dried in a __________ drier.
A
rotary
10
Q
Corresponding to Nusselt number in heat transfer, the dimensionless group in mass transfer is the __________ number.
A
Sherwood
11
Q
Liquid diffusivity is of the order of __________ cm2 /second.
A
10-5 to 10-6
12
Q
The partial pressure distribution of an ideal gas diffusing through another stagnant ideal gas at steady state follows a/an __________ law
A
exponential
13
Q
The binary diffusivity in gases and liquids vary respectively as
A
T3/2 and T
14
Q
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).
A
Electrodylsis is used in the manufacture of NaOH.
15
Q
Moisture contained by a substance in excess of the equilibrium moisture is called the __________ moisture.
A
free
16
Q
With increase in temperature, the solubility of gases in liquids, at fixed pressure
A
decreases
17
Q
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
A
increased reflux ratio
18
Q
__________ column is the most suitable for achieving the best performance for mass transfer operations involving liquid with dispersed solids.
A
Plate
19
Q
Use of raschig rings in place of crushed stones as packing in packed beds (other things being same)
A
decreases pressure drop, increases surface area.
20
Q
Pick out the wrong statement:
Reboiler is not used in open steam distillation.
The separation of solutes of different molecular sizes in a liquid solution by use of a suitable membrane is called dialysis.
Freeze drying is used for the drying of fish.
Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition.
A
Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition.
21
Q
Pick out the wrong statement.
Critical moisture content is not a property of the material itself.
A sharp increase in pressure drop gives an indication of the flooding in a distillation column.
Separation of components is not possible in liquid extraction, if selectivity is unity.
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum.
A
Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum.
22
Q
Absorption factor method is used to calculate the number of ideal stages, when
operating line lies above the equilibrium line.
operating line lies below the equilibrium line.
both operating and equilibrium lines are parallel.
pressure drop in the column is very high.
A
both operating and equilibrium lines are parallel.
23
Q
Flights in a rotary dryer are provided to
A
lift and shower the solids thus exposing it thoroughly to the drying action of the gas.
24
Q
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
A
on heating, a vapor phase will appear ; while on cooling, a solid phase will appear.
25
Under conditions of flooding in packed tower, the gas pressure drop
is maximum
26
For absorbing a sparingly soluble gas in a liquid
liquid side mass transfer co-efficient should be increased.
27
Channelling in a packed tower results from the
high pressure drop
maldistribution of liquid
non-uniformity of packing
both (b) and (c)
both (b) and (c)
28
The change in enthalpy per unit weight of adsorbed gas when adsorbed on gas free or "outgassed" adsorbent to from a definite concentration of adsorbate is called its
integral heat of adsorption.
29
The wetted wall tower is used to determine
individual mass transfer co-efficient (M.T.C.) in gaseous system.
30
The caking of crystals can be prevented by
maintaining high critical humidity.
maintaining low critical humidity.
coating the product with inert material.
both (a) and (c)
both (a) and (c)
31
The diffusivity (D) in a binary gas mixture is related to the pressure (P) as
D ∝ P0.5
D∝ 1/P0.5
D ∝ 1/P
D∝ 1/P1.5
D ∝ 1/P
32
Which of the following psychrometric processes is followed by water vapour laden unsaturated air, when it is passed through solid or liquid adsorbent ?
a. Cooling and dehumidification.
b. Heating and dehumidification at almost constant wet bulb temperature.
c. Dehumidification with dry bulb temperature remaining constant.
d. None of these.
b. Heating and dehumidification at almost constant wet bulb temperature.
33
The value of NA/(NA + NB) for steady state molecular diffusion of gas 'A' through non-diffusing gas 'B' is
1
∞
0.5
2
1
34
Large scale drying of wheat is done in a __________ dryer.
spouted bed
35
The drying time between fixed moisture content within diffusion controlled 'falling rate period' is proportional to (assuming that drying occurs from all surfaces of the solid) (where, T = thickness of the solid )
T2
36
At the boiling point of the liquid at the prevailing pressure, the saturated absolute humidity becomes
∞
37
Pressure drop through plate tower as compared to that through packed tower, for the same duty will be
more
38
Dorr thickener is an equipment used for
sedimentation
39
Steam distillation is used to separate
high boiling substances from nonvolatile impurities.
40
Water entrained by circulating air in cooling towers is termed as
drift
41
Humidity chart is useful for the solution of problems concerning condensation, vaporisation and air conditioning. At a given dry bulb temperature, value of humidity obtained from the humidity chart directly, is in terms of __________ humidity.
molal
42
The Kundsen diffusivity is proportional to (where, T = absolute temperature)
sqrtT
43
In an absorption coloumn, the flooding velocity for random packing is __________ that for stacked/regular packing.
smaller than
44
The relation among various mass transfer co-efficients (M.T.C) for ideal gases is given by (where, Kc & Km are M.T.C. for equimolar counter diffusion with concentration & mole fraction respectively as the driving force. and, Kp = M.T.C. for diffusion of a gas through a stagnant inert gas with pressure as driving force.)
Kc = Kp . RT = Km . RT/p
45
The fixed cost of a distillation column at minimum reflux ratio is
infinite
46
In a binary distillation column, if the feed contains 40 mole % vapour, the q line will have a slope of
-1.5
47
Pick out the wrong statement.
A. Mechanical agitation is required, if the system has low interfacial tension.
B. Seive tray towers are generally used for systems having low interfacial tension.
C. When Henry's law constant is very small, then the mass transfer rate is controlled by the gas film resistance.
D. Taylor-Prandtl analogy for heat and mass transfer considers the transfer through both laminar and turbulent layers.
A. Mechanical agitation is required, if the system has low interfacial tension.
48
Acetic acid will be most economically separated from a dilute solution of acetic acid in water by
A. solvent extraction
B. continuous distillation
C. evaporation
D. absorption
A. solvent extraction
49
Design calculation for multiple component distillation is done by
tray to tray calculations.
50
To get high tray efficiency
Interfacial surface between liquid and gas phase should be large.
51
The mass transfer co-efficient for a solid sphere of radius 'a' dissolving in a large volume of quiescent liquid, in which D is the diffusivity of solute, is
dependent on the Reynolds number
52
Pick out the wrong statement. Generally for physical adsorption, a gas of
A. higher molecular weight is adsorbed in preference to a gas of low molecular weight.
B. high critical temperature is adsorbed in preference to a gas of low critical temperature.
C. low volatility is adsorbed in preference to a gas of high volatility.
D. none of these.
D. none of these.
53
Solvent extraction is the terminology applied to the liquid-liquid extraction, which is preferred for the separation of the components of liquids, when
one of the liquid components has very high affinity towards the solvent.
54
Inside the distillation column, the
driving force for the vapour flow is the pressure drop.
55
Occurrence of 'case hardening' during drying of a high moisture solid cake __________ the drying rate.
decreases
56
Which of the following is not a continuous drier ?
Tray drier
57
Humidification involves mass transfer between a pure liquid phase and a fixed gas, which is
insoluble in the liquid
58
Pick out the wrong statement.
A. In the McCabe-Thiele diagram for binary distillation, vertical feed line represents saturated liquid feed and horizontal feed line represents saturated vapour feed.
B. In small columns, local efficiency is larger than Murphree efficiency; in large columns local efficiency is smaller than Murphree efficiency.
C. For Laminar flow over a plate of length L, the local mass transfer co-efficient at a distance L from the leading edge is 1.5 x 10-2m/s. Then the average mass transfer co-efficient for the plate is 2 x 10-2m/s.
D. The concentration and hydrodynamic boundary layers over a flat plate are of equal thickness, if Schmidt number is equal to unity.
For Laminar flow over a plate of length L, the local mass transfer co-efficient at a distance L from the leading edge is 1.5 x 10-2m/s. Then the average mass transfer co-efficient for the plate is 2 x 10-2m/s.
59
In a counter-current extractor, as the axial mixing increases, the extraction efficiency
decreases
60
The drying time between fixed moisture contents within the 'constant rate period' is proportional to (assuming that drying occurs from all surfaces of the solid) (where, T = thickness of the solid )
T
61
In an absorber, HETP does not vary with the
flow rate of liquid
flow rate of gas
type and size of packing
none of these
none of these
62
In an interphase mass transfer process, the lesser the solubility of a given solute in a liquid, the higher are the chances that the transfer process will be
gas phase resistance controlled.
63
The equation, is for
steady state equimolar counter diffusion
Fick's first law of diffusion
steady state diffusion for stagnant case
liquid M.T.C. by penetration theory.
steady state equimolar counter diffusion
64
Which of the following remains constant during evaporative cooling process with recirculated water supply ?
Wet bulb temperature
65
Minimum possible diameter of a packed coloumn is decided mainly by the
flooding
66
Solvent used in extractive distillation
Forms a low boiling azeotrope.
67
When both the fluids flow concurrently in an absorber, the slope of the operating line is
- ve
68
Back-trapping in a distillation column
decreases tray efficiency.
69
Flooding in a vapor-liquid contacting equipment occurs in a tray, when the pressure drop through a tray is __________ the liquid head available in the downcomer.
more than
70
At a given equilibrium pressure, with increase in temperature, the concentration of adsorbed gas on solid adsorbant
decreases
71
Pick out the wrong statement.
Relative volatility of a binary mixture changes appreciably with the minor change in temperature.
72
Diameter of raschig rings used in packed tower in industry is normally around __________ inches.
2
73
__________ number indicates the ratio of the rates of the heat and mass transfer incase of a cooling tower.
Lewis
73
In case of non-ideal gases and liquids, the molal diffusivity
is independent of pressure.
74
In physical terms, Schmidt number means
momentum diffusivity/mass diffusivity.
75
In case of liquids, the binary diffusivity is proportional to (where, T = temperature)
T
76
Mass transfer co-efficient is directly proportional to DAB0.5, according to __________ theory.
penetration
77
Pick out the correct statement.
For a non-reacting binary mixture of ideal gases, the partial pressure distribution of both components is linear in the case of steady state equimolal counter-diffusion.
78
Humid volume is the total volume in m3 of 1 kg of
gas plus its accompanying vapour at 1 atm. and gas temperature.
79
Diameter to height ratio for a rashig ring is
1
80
In a liquid-liquid extraction, 10 kg of a solution containing 2 kg of solute C and 8 kg of solvent A is brought into contact with 10 kg of solvent B. Solvent A and B are completely immiscible in each other whereas solute C is soluble in both the solvents. The extraction process attains equilibrium. The equilibrium relationship between the two phases is Y* = 0.9X, where Y* is the kg of C/kg of B and X is kg of C/kg of A. Choose the correct answer.
The entire amount of C is transferred to solvent B.
81
Mass transfer co-efficient varies as DAB0.5, according to the __________ theory.
surface renewal
82
Perforated plate towers are unique for solvent extraction because, they provide
repeated coalescence & redispersion of the drop phase.
82
In case of absorption & stripping, the interface of the liquid & gas phases are present in equilibrium, when the diffusional resistance of __________ is zero.
interface
83
The slope of the operating line for a single component co-current absorber when plotted in terms of mole ratio units is
-ve
84
The individual mass transfer co-efficients (moles/m2 . s) for absorption of a solute from a gas mixture into a liquid solvent are, KL = 4.5 and KG = 1.5. The slope of the equilibrium line is 3. Which of the following resistance (s) is (are) controlling ?
gas side
85
Swenson-Walker crystalliser is a __________ unit
continuous
86
Pick out the wrong statement.
The capacity of a distillation coloumn is maximum at total reflux.
87
Moisture content of a substance when at equilibrium with a given partial pressure of the vapour is called the __________ moisture.
equilibrium
88
Pick out the correct relationship. (where, Ri = internal reflux ratio Ro = external reflux ratio. )
(1 + Ro) = Ro/Ri
89
Though, theoretically the adsorption & desorption isotherms is expected to coincide but sometimes at least a part of the isotherms does not coincide and this phenomenon is known as adsorption hysterisis. In such cases, the desorption equilibrium pressure is __________ that obtained for adsorption.
lower than
90
Paper industry employs __________ driers.
cylinder
91
Co-current absorbers are usually used when the gas to be dissolved in the liquid is
a pure substance
92
Pick out the wrong statement pertaining to the cooling towers.
Effectiveness of a mechanical draft cooling tower is reduced with increase in air wet-bulb temperature.
93
Experiments were conducted to determine the flux of a species A in a stagnant medium across a gas-liquid interface. The overall mass transfer co-efficient based on the liquid side for dilute systems for the above was estimated to be 4 x 10-3 kg mole/m2.s. The equilibrium data for the system is given as y = 2x. The flux across the interface (in kg mole/m2 .s) for bulk concentrations of A in gas phase and liquid phase as y = 0.4 and x = 0.01 respectively is
8.5 x 10-4
94
At a fixed pressure, the humidity depends upon the partial pressure of vapor in the mixture. Humidity of a vapour free gas is __________ percent.
0
95
In case of physical adsorption, the difference between heat of adsorption and heat of normal condensation is
equal to the heat of wetting.
96
Pick out the wrong statement:
The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity.
97
The value of L/mG ratio for economical absorption operation ranges from
1.25 to 2.0
98
Alcohol is dehydrated using __________ distillation.
azeotropic
99
__________ is used as an 'entrainer' for the separation of acetic acid-water mixture by distillation.
Butyl acetate
100
An example of elutioh is
separation of uranium oxide from its ore by H2SO4 in a Pachuca tank.
101
The operating line for an absorber is curved, when plotted in terms of
partial pressure
102
Relative volatility does not change appreciably with the change in
temperature
103
The dew point of a saturated gas phase equals the __________ temperature.
gas
104
A solid material shows case hardening properties while drying. Which of the following should be controlled to control the drying process ?
Temperature of the solid
105
During bottling of cold drinks (e.g. Pepsi), the main resistance to mass transfer for the absorption of carbon dioxide in water lies in the
liquid film
106
The overall mass transfer co-efficient for the absorption of SO2 in air with dilute NaOH solution can be increased substantially by
increasing the gas film co-efficient.
107
Entrainer used in azeotropic distillation should
form a low boiling azeotrope with one of the constituents of the mixture
108
In case of steam distillation, the steam leaving the liquid is not completely saturated with distillate vapour, because
mixing of steam with the material being vaporised is not so intimate as to result in equilibrium condition.
109
Pick out the wrong statement.
The operating pressure in solvent extraction system should be below vapor pressures of all components.
110
For contacting a highly soluble gas with a liquid
bubble the gas through liquid.
111
A distillation column with N plates is being operated under normal conditions. At some point of time, the operation is shifted to total reflux condition (i.e., no product and residue are being withdrawn and feed to the column is stopped). At the new steady state,
the top and bottom compositions correspond to the maximum enrichment available.
112
Solvent used in extractive distillation
changes the relative volatility of the original components.
113
The absorption factor is defined as (where, L = liquid flow rate, G = gas flow rate and, m = slope of the equilibrium line)
L/mG
114
Which of the following adsorbant is used in the refinning of sugar ?
Bone charcoal
115
In McCabe-Thiele method of theoretical plate calculation for a distillation column, the operating lines of stripping and rectifying sections coincide with the diagonal at __________ reflux.
total
116
Which of the following parameters remains constant during chemical dehumidifica-tion?
Dry bulb temperature
117
Mechanism of moisture removal in case of freeze drying of food stuff is by
sublimation
118
The dew point of an unsaturated gas-vapor mixture does not depend upon the __________ of the mixture.
A. composition
B. temperature
C. total pressure
D. all (a), (b) and (c)
temperature
119
Which is not concerned directly with mass transfer ?
Froude number
120
Removal of __________ exemplifies an adsorption unit operation.
water from petrol
121
Can the efficiency of a plate in the distillation tower be greater than 100% ?
Never
122
With increase in the mass velocity of the gas, the rate of drying during the constant rate period __________ , if the conduction and radiation through the solid are negligible.
increases
123
The operating cost of a distillation column at minimum reflux ratio is
minimum
124
According to Poiseuille's law, the permeability for gas flow through a capillary is proportional to (where, μ = gas viscosity)
μ
1/μ
125
For a binary mixture distillation process, the degree of freedom is 2. However, if the pressure is fixed in this process, the number of independent variables in this process will be
1
126
H2S is being absorbed in a gas absorber unit. The height of the transfer unit based on the overall mass transfer coefficient on the gas side is 0.4 m. The equilibrium data is given by, y = 1.5 x. The bulk concentration of H2S has to be reduced from 0.05 to 0.001 mole fraction in the gas side. The height of the tower (in metres) corresponding to an operating line given by, y = 5x + 0.001 is
2.0
127
Sodium bicarbonate is produced during soda ash manufacture using a __________ tower.
baffle
128
Powdery materials can be guarded against caking tendency on storage by
A. providing irregular grain size.
B. providing minimum percentage of voids.
C. having maximum possible points of contact.
D. none of these.
none of these.
128
In case of cooling towers, the ratio of the rates of heat and mass transfer is indicated by the __________ number.
Lewis
129
Which one of the following devices is not used for both the absorption as well as the liquid-liquid extraction processes ?
Wetted wall columns
129
Mass transfer co-efficient is directly proportional to DAB according to the __________ theory.
film
130
The relative volatility of a binary mixture at constant temperature __________ the total pressure.
decreases with increase in
131
132
133
The unit of volumetric diffusivity is
cm2/sec
133
As the reflux ratio increases, the slope of the operating line for rectifying section
increases
134
Tower diameter may be decreased by
use of increased tray spacing.
135
The slope of 9-line is determined by the
thermal condition of the feed
136
In steam distillation, the
product must be immiscible with water.
137
Which of the following can be calculated theoretically using steam table and Dalton's law of partial pressure; if the relative humidity and dry bulb temperature of moist air is known ?
Wet bulb temperature
138
