SO2
from air is absorbed by pure water in a counter current packed column operating
at constant pressure. The compositions and the flow rates of the streams are
shown in the figure.
In
addition, use the following data and assumptions
·
Column operates under isothermal
conditions
·
At the operating temperature of the
column, y* = 40x, where y* is the mole fraction of SO2
in the gas that is in equilibrium with water containing SO2 at a
mole fraction of x
·
Solution is dilute and the operating
line is linear
·
Negligible amount of water evaporates
The number of
transfer units (NTU) for this column is
(A)
0.5
(B)
1.0
(C)
1.5
(D)
2.0
GATE
2020
Answer: (D)
Ratio
of momentum diffusivity to thermal diffusivity is
(A)
Reynolds number
(B)
Prandtl number
(C)
Nusselt number
(D)
Peclet number
GATE
2020
Answer: (D)
Two
film theory applies for absorption of a solute from a gas mixture into a liquid
solvent. The interfacial mass transfer coefficient (in mol m-2 s-1) for the gas
side is 0.1 and for the liquid side is 3. The equilibrium relationship is y* =
2x, where x and y* are mole fractions of the solute in the liquid and gas
phases, respectively. The ratio of the mass transfer resistance in the liquid
film to the overall resistance is
(A)
0.0161
(B)
0.0322
(C)
0.0625
(D)
0.0645
GATE
2020
Answer: (C)
Consider
the equilibrium data for methanol-water system at 1 bar given in the figure
below.
A distillation column operating at 1.0
bar is required to produce 92 mol% methanol. The feed is a saturated liquid. It
is an equimolar mixture of methanol and water. The minimum reflux ratio is
(A)
0.33
(B)
0.50
(C)
0.54
(D)
1.17
GATE
2020
Answer: (B)
Two
ideal cross-current stages operate to extract P from a feed containing P and Q,
as shown below.
The
mass flow rates of P and Q fed to Stage 1 are 1000 kg h-1 and 10,000
kg h-1, respectively. Pure solvent (S) is injected at mass flow rate
of 5000 kg h-1 and 15000 kg h-1 to Stages 1 and 2,
respectively. The components Q and S are immiscible. The equilibrium relation
is given by Y* = 1.5X, where X is the mass of P per unit mass of Q
in the raffinate, and Y* is mass of P per unit mass of S in the
extract, which is in equilibrium with the raffinate. The mass flow rate of P
(in kg h-1) in the raffinate from Stage 2 is ________ (round off to
nearest integer).
GATE
2020
Answer: 176
Consider
a vertically falling film of water over an impermeable wall. The film is in
contact with a static pool of non-reactive pure gas. The gas diffuses into the
water film over the entire height of the falling film. The height of the film
is m, and its thickness is m. The velocity of water, averaged over the film
thickness, is 0.01 m s-1. The gas concentration (in kg m-3),
averaged over the film thickness is
Where
y is the vertical position in meters measured from the top of the wall.
In
addition, use the following data and assumptions
o
The flow is fully developed
o
The width of the film is much larger
than the thickness of the film, and the dissolved gas concentration is
invariant over this width
o
The solubility of the gas in water, CAi,
is constant
o
Pure water enters at y = 0
o
The evaporation of water in negligible
The mass
transfer coefficient on the liquid side (in mm s-1), averaged over
the entire height of the falling film is _______ (round off to 3 decimal
places).
GATE
2020
Answer: 0.03
The correct
expression for the Colburn j-factor for mass transfer that relates Sherwood
number (Sh), Reynolds number (Re) and Schmidt number (Sc) is
GATE
2019
Answer: (A)
Three
distillation schemes for separating an equimolar, constant relative volatility
ABC mixture into nearly pure components are shown. The usual simplifying
assumptions such as constant molal overflow, negligible heat loss, ideal trays
are valid. All the schemes are designed for minimum total reboiler duty. Given
that the relative volatilities are in the ratio αA:αB:αC
≡ 8:2:1, the correct option that arranges the optimally-designed schemes in
ascending order of total reboiler duty is
(A)
I, II, III
(B)
III, I, II
(C)
II, I, III
(D)
III, II, I
GATE
2019
Answer: (B)
In the drying of non-dissolving
solids at constant drying conditions, the internal movement of moisture in the
solid has a dominant effect on the drying rate during
(A)
The
initial adjustment period only
(B)
The
constant rate period only
(C)
The
falling rate period only
(D)
Both
the initial adjustment and constant rate periods
GATE 2019
Answer: (C)
A binary mixture with components A
and B is to be separated in a distillation column to obtain 95 mol% A as the
top product. The binary mixture has a constant relative volatility αAB = 2. The column feed is a saturated liquid containing 50 mol%
A. Under the usual simplifying assumptions such as constant molal overflow,
negligible heat loss, ideal trays, the minimum reflux ratio for this separation
is____________________ (rounded off to one decimal place). GATE 2019
Answer: 1.7
A countercurrent absorption tower is designed to remove 95%
of component A from an incoming binary gas mixture using pure solvent B. The
mole ratio of A in the inlet gas is 0.02. The carrier gas flow rate is 50
kmol/h. The equilibrium relation is given by Y = 2X, where Y and X are the mole
ratios of A in the gas and liquid phases, respectively. If the tower is
operated at twice the minimum solvent flow rate, the mole ratio of A in the
exit liquid stream is________________(rounded off to three decimal places).
GATE
2019
Answer: 0.005
100 kg of a feed containing 50 wt. %
of a solute C is contacted with 80 kg of a solvent containing 0.5 wt.%
of C in a mixer-settler unit. From this operation, the resultant extract and
raffinate phases contain 40 wt.% and 20 wt.% of C, respectively. If E and R
denote the mass of the extract and raffinate phases, respectively, the ratio
E/R is
(A)
¼
(B)
½
(C)
2/3
(D)
1
GATE
2019
Answer: (C)
Two spherical camphor particles of
radii 20 cm and 5 cm, far away from each other, are undergoing sublimation in a
stream of air. The mass transfer coefficient is proportional to
Where, r(t) is the radius of the
sphere at time t. Assume that the partial pressure of camphor far away from the
surface of the particle is zero. Also, assume quasi-steady state, identical
ambient conditions, and negligible heat effects. If t1 and t2
are the times required for complete sublimation of the 20 cm and 5 cm camphor
particles, respectively, the ratio t1/t2 is _________
(rounded off to one decimal place).
GATE
2019
Answer: 8
Two packed
towers are designed for the same mass velocity of the gas. The first has liquid
and gas flow rates of 30 kg/s and 1.2 kg/s, respectively, while the
corresponding flow rates in the second tower are 67.5 kg/s and 1.8 kg/s. The
ratio of the design diameter of the wider tower to that of the narrower tower
is
(A) 2
(B) 1.8
(C) 1.5
(D) 1.225
GATE
2018
Answer: (D)
According to the
surface renewal theory, the unit of fractional rate of surface renewal is
(A) m2
s-2
(B) m2
s-1
(C) m
s-1
(D) s-1
GATE
2018
Answer: (D)
For absorption
of H2S from a mixture with hydrocarbon vapour into an aqueous
alkanolamine solution, the liquid phase mass transfer resistance is
(A) Significantly
higher than that of the gas phase
(B) Negligible
compared to that of the gas phase
(C) Equal
to that of the gas phase
(D) Dependent
on the gas phase mass transfer resistance
GATE
2018
Answer: (B)
Pitot tube is used to measure
(A) Liquid
level in a tank
(B) Flow
velocity at a point
(C) Angular
deformation
(D) Vorticity
GATE
2018
Answer: (B)
The humidity of
air at a dry-bulb temperature of 65°C is 0.025 kg water/kg dry air. The latent
heat of vaporization of water at 0°C is 2500 KJ/kg. The psychometric ratio of
air is 0.95 KJ (kg dry air)-1 K-1. Considering 0°C as reference temperature,
the enthalpy of air (in KJ/kg) at its adiabatic saturation temperature of 35°C
is _________ (rounded off to two decimal places).
GATE
2018
Answer: 124
It is decided to extract A from a feed containing 20
mol% A and 80 mol% B in two ideal cross-current stages as shown below, using
equal amount of pure solvent C in each stage.
Components B and C are immiscible. 60% of A in the
feed is extracted in stage 1. The equilibrium relation is given by Y*
= 1.5 X wher,
X = moles of A per mole of B in raffinate
Y* = moles of A per mole of C in extract
in equilibrium with raffinate
The mol % of A in raffinate from stage 2 is
____________ (rounded off to second decimal place).
GATE
2018
Answer: 3.85
A binary distillation column is designated by
McCabe-Thiele method to get a distillate mole fraction of 0.9. The enriching
section operating line has an intercept with y-axis at 0.3 mole fraction. The
ratio of liquid to vapour molar flow rate in the enriching section is _______
(rounded off to third decimal place).
GATE
2018
Answer: 0.667
