Sunday, August 22, 2021

GATE Chemical Engineering Questions and Answers - Mass Transfer


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.

GATE 2020

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.

GATE 2020

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.

GATE 2020

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

GATE 2020

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

                                                                                                                  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

GATE 2019

(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

GATE 2019

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.

GATE 2018

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


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