A distillation
column with n plates is being operated under normal conditions. At some point
in time, the operation is shifted to total reflux conditions (i.e. no product
and residue are being withdrawn and feed to the column is stopped). At the new
steady state,
(A) Composition
of vapours and that of liquid do not vary throughout the column
(B) Reboiler
load and condenser load are minimum
(C) The
top and bottom compositions are unchanged with and without total reflux
(D) The
reflux and bottom compositions correspond to the maximum enrichment achievable.
GATE
2004
Answer: (D)
An aqueous
solution of methanol is to be distilled in a tray column. High pressure stem is
available as a source of heat. For a given reflux ratio and overhead
composition, two options are being explored: (i) a reboiler is used and (ii) no
reboiler is used but steam is fed directly to the bottom of the column. As
compared to option (i), in option (ii)
(A)
Less number of trays required
(B)
Composition of the residue remains
unchanged
(C)
More number of trays are required but
the residue composition remains unchanged
(D)
More number of trays are required and
the residue composition is more dilute in methanol
GATE 2004
Answer: (A)
A 25 cm×25 cm×1
cm flat sheet weighing 1.2 kg initially was dried from both sides under
constant drying rate conditions. It took 1500 seconds for the weight of the
sheet to reduce to 1.05 kg. Another 1 m× 1 m×1 cm flat sheet of the same
material is to be dried from one side only. Under the same constant drying rate
conditions, the time required for drying (in seconds) from its initial weight
of 19.2 kg to 19.6 kg is
(A)
1000
(B)
1500
(C)
2000
(D)
2500
GATE
2004
Answer: (C)
Acetone is to be
removed from air in an isothermal absorber using pure water as solvent. The
incoming air contains 5 mol% 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
(A)
1
(B)
8
(C)
9
(D)
10
GATE
2004
Answer: (C)
Common data for the next two
questions
An air-water vapour 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 pressures 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
(A)
0.048
(B)
0.079
(C)
0.122
(D)
0.152
GATE
2004
Answer: (A)
The
wet bulb temperature Tw for the above mixture would be
(A)
Less than 40°C
(B)
40°C
(C)
40°C < Tw <60°C
(D)
60°C
GATE
2004
Answer: (C)
Common data for the next two
questions
Pure aniline is
evaporating through a stagnant air film of 1 mm thickness at 300 K and a total
pressure of 100 Kpa. The vapor pressure of aniline at 300 K is 0.1 KPa. The
total molar concentration under these conditions is 40.1 mol/m. The diffusivity
of aniline in air is 0.74×10-5 m2/s.
The numerical
value of the mass transfer coefficient is 7.4×10-3. Its units are
(A)
m/s
(B)
cm/s
(C)
mol/(m2 s Pa)
(D)
kmol/(m2 s Pa)
GATE
2004
Answer: (A)
The rate of
evaporation of aniline is 2.97×10-4. Its units are
(A)
mol/s
(B)
mol/(cm2 s)
(C)
mol/(m2 s)
(D)
kmol/(m2 s)
GATE
2004
Answer: (D)
A distillation
column operates 10,000 kg/hr of a benzene-toluene mixture as shown in the
figure. In the figure xF, xD and xW represent
the weight fractions of benzene in the feed, distillate and residue
respectively.
The reflux ratio
is
(A)
0.5
(B)
0.6
(C)
1.0
(D)
2.0
GATE
2004
Answer: (B)
Minimum reflux ratio in a distillation column
results in
(A)
Optimum number of trays
(B)
Minimum reboiler size
(C)
Minimum condenser size
(D)
Minimum number of trays
GATE
2003
Answer: (B)
Component A is diffusing in a medium B. The flux NA
relative to a stationary point is equal to the flux due to molecular diffusion
if
(A)
Diffusion of A is in stagnant medium B
(B)
Mass transfer is accompanied by reaction
(C)
Molecular mean free path is high
(D)
There is equimolar counter diffusion
GATE
2003
Answer: (D)
The
diffusion coefficient, in m2/s, of acetic acid in benzene (liquid in liquid) is
(A)
2.09×10-4
(B)
2.09×10-5
(C)
2.09×10-9
(D)
2.09×10-12
GATE
2003
Answer: (C)
In
distillation column sizing calculations by short cut methods, match the
following
|
P |
Underwood’s equation |
1 |
Number of real trays |
|
Q |
Fenske equation |
2 |
Column diameter |
|
R |
Gilliland’s equation |
3 |
Minimum number of ideal trays |
|
S |
Vapour velocity at flooding |
4 |
Actual number of ideal trays |
|
|
|
5 |
Minimum reflux ratio |
|
|
|
6 |
Tray efficiency |
(A)
P-1, Q-3, R-4, S-6
(B)
P-2, Q-5, R-1, S-3
(C)
P-5, Q-3, R-6, S-2
(D)
P-5, Q-3, R-4, S-2
GATE
2003
Answer: (C)
Compound
A is extracted from a solution of A + B into a pure solvent S. A co-current
unit is used for the liquid-liquid extraction. The inlet rate of the solution
containing A is 200 mol of B/h-m2 and the solvent flow rate is 400
mol of S/h-m2. The equilibrium data is represented by Y = 2X, where
Y is in mol of A/mol of B and X is in mol of A/mol of S. The maximum percentage
extraction achieved in the unit is
(A)
25%
(B)
50%
(C)
70%
(D)
90%
GATE
2003
Answer: (B)
The
following plot gives the saturated-humidity (H) versus temperature (T)
Line joining (H1, T1) and (H2, T2) is the constant enthalpy line. Choose the correct one from the following options.
(A)
T1 = Dew point temp, T2
= Dry bulb temperature, T3 = Wet bulb temp
(B)
T1 = Dew point temp, T2
= Wet bulb temperature, T3 = Dry bulb temp
(C)
T1 = Wet bulb temp, T2
= Dry bulb temperature, T3 = Dew point temp
(D)
T1 = Dry bulb temp, T2
= Wet bulb temperature, T3 = Dew point temp
GATE
2003
Answer: (D)
A
solid is being dried in the linear drying rate regime from moisture content X0
to XF. The drying rate is zero at X = 0 and the critical moisture
content is the same as the initial moisture, X0. The drying time for
m = (LS/ARC) is
(A)
M(X0 - XF)
(B)
M(X0/XF)
(C)
M ln(X0/XF)
(D)
mX0 ln(X0/XF)
Where,
LS = Total mass of dry solids, A = total surface area for drying, RC
= constant maximum drying rate per unit area and X = moisture content (in mass
of water/mass of dry solids).
GATE
2003
Answer: (D)
A
pure drug is administered as a sphere and as a cube. The amount of drug is the
same in the two tablets. Assuming that the shape and size do not influence of
mass transfer, the ratio of rate of dissolution in water at t = 0 for the cubic
to spherical tablet is
(A)
0.54
(B)
1.04
(C)
1.24
(D)
1.94
GATE
2003
Answer: (C)
The Reynolds number of the liquid was
increased 100 fold for a laminar falling film used for gas liquid contacting.
Assuming the penetration theory is applicable, the fold-increase in the mass
transfer coefficient (Kc) for tha same system is
(A)
100
(B)
10
(C)
5
(D)
1
GATE
2003
Answer: (B)
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 given by y
= 1.5x. The bulk concentration of H2S has to be reduced from 0.05
and 0.001 mole fraction in the gas side. The height of the tower (in meter)
corresponding to an operating line given by y = 5x + 0.001 is
(A)
2.0
(B)
1.56
(C)
1.0
(D)
0.56
GATE
2003
Answer: (A)
Experiments were conducted to determine the flux of
a species A in a stagnant medium across gas-liquid interface. The overall mass
transfer coefficient based on the liquid side for dilute systems for the above
was estimated to be 4×10-3 kg-mol/m2-s. The equilibrium
data for the system is given as Y = 2X. The flux across the interface X = 0.01,
respectively is concentration of A in gas phase and liquid phase as Y = 0.4 and
X = 0.01, respectively is
(A)
5.6×10-4
(B)
8.5×10-4
(C)
5.6×10-3
(D)
8.5×10-3
GATE
2003
Answer: (B)
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