IndiaBIX Chemical Engineering: Mass Transfer Section 9

Question 1

Which of the following parameters remains constant during chemical dehumidifica-tion?

Dry bulb temperature

Partial pressure of vapour

Wet bulb temperature

None of these

Answer 1

Dry bulb temperature

Question 2

Drying of a solid involves __________ transfer.

only heat

only mass

both heat and mass

none of these

Answer 2

both heat and mass

Question 3

Mechanism of moisture removal in case of freeze drying of food stuff is by

evaporation

sublimation

dehydration

adsorption

Answer 3

sublimation

Question 4

The dew point of an unsaturated gas-vapor mixture does not depend upon the __________ of the mixture.

composition

temperature

total pressure

all (a),

(b) and (c)

Answer 4

temperature

Question 5

Which is not concerned directly with mass transfer ?

Schmidt number

Sherwood number

Lewis relationship

Froude number

Answer 5

Froude number

Question 6

Removal of __________ exemplifies an adsorption unit operation.

uranium from its ore

water from petrol

ammonia from coke oven gas

mustard oil from mustard seed

Answer 6

water from petrol

Question 7

Can the efficiency of a plate in the distillation tower be greater than 100% ?

Yes

Normally not; but is possible if infinite number of plates are put.

Never

Yes ; if the reflux ratio is maximum.

Answer 7

Never

Question 8

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

decreases

remains same

increases linearly

Answer 8

increases

Question 9

For gas absorption,

low viscosity solvents are preferred,

because of their

better flow characteristics.

low pumping pressure drop.

rapid absorption rates.

all (a),

(b) and (c).

Answer 9

all (a),

(b) and (c).

Question 10

To remove all the moisture from a wet solid it requires exposure to __________ air.

perfectly dry

highly humid

high temperature

none of these

Answer 10

perfectly dry

Question 11

The operating cost of a distillation column at minimum reflux ratio is

minimum

maximum

infinite

zero

Answer 11

minimum

Question 12

The reflux to a distillation column is 100 moles/hr,

when the overhead product rate is 50 moles/hr. The reflux ratio is

2

0.5

50

150

Answer 12

2

Question 13

Molecular diffusivity of a liquid

increases with temperature.

decreases with temperature.

may increase or decrease with temperature.

is independent of temperature.

Answer 13

increases with temperature.

Question 14

The wet and ‘dry bulb temperature for a vapour-gas mixture are 25°C and 30°C respectively. If the mixture is heated to 45°C at constant pressure,

the wet bulb temperature will be __________ °C.

25

> 25

< 25

• 25

Answer 14

> 25

Question 15

Henry’s law states that the

partial pressure of a component over a solution is proportional to its mole fraction in the liquid.

partial pressure of a component over a solution is proportional to the mole fraction in the vapour.

vapour pressure is equal to the product of the mole fraction and total pressure.

partial pressure is equal to the product of the mole fraction and total pressure.

Answer 15

partial pressure of a component over a solution is proportional to its mole fraction in the liquid.

Question 16

Ammonia present in the coke oven gas is removed by washing with

caustic solution

dilute ammoniacal liquor

dilute HCl

ethanolamine

Answer 16

dilute ammoniacal liquor

Question 17

With increase in temperature,

the leaching rate increases due to

decreased liquid viscosity

increased diffusivity

both (a) and (b)

neither (a) nor (b)

Answer 17

both (a) and (b)

Question 18

Which of the following processes is not an example of adsorption applied to gaseous separations ?

Recovery of valuable solvent vapors from dilute mixture with air and other gases.

Dehumidification of air and gas.

Removal of objectionable odour and impurities from industrial gases.

Decolouration of yellow glycerene.

Answer 18

Decolouration of yellow glycerene.

Question 19

The critical moisture content in case of drying indicates the __________ rate period.

beginning of falling

beginning of constant

end of falling

none of these

Answer 19

beginning of falling

Question 20

Pick out the wrong statement

A horizontal line on the humidity chart indicates the temperature changes at constant molal humidity.

When water is cooled from 80 to 70°C by exposure to the air with a wet bulb temperature of 60°C,

then both the approach & the range would be 10°C.

For unsaturated air-water vapor mixture at atmospheric conditions,

the wet bulb temperature & adiabatic cooling lines are the same.

Relative saturation of unsaturated mixture of water vapor & air can not be increased by either reducing the mixture temperature or by increasing the total pressure.

Answer 20

Relative saturation of unsaturated mixture of water vapor & air can not be increased by either reducing the mixture temperature or by increasing the total pressure.

Question 21

According to Poiseuille’s law,

the permeability for gas flow through a capillary is proportional to (where,

μ = gas viscosity)

μ

1/μ

sqrt(μ)

μ^2

Answer 21

1/μ

Question 22

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

0

2

3

Answer 22

1

Question 23

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

1.56

1.0

0.56

Answer 23

2

Question 24

Packed column distillation is limited to the column __________ metres.

height < 6

diameter < 0.6

both (a) & (b)

neither (a) nor (b).

Answer 24

both (a) & (b)

Question 25

For the flow of gases through a capillary, with increase in temperature, the permeability __________ as per Knudsen law. increases decreases remains same either (a) or (b); depends on the gas

Answer 25

increases

Question 26

Which of the following is directly concerned with psychrometry ? Lewis reltionship Galileo number Weber number Dean number

Answer 26

Lewis reltionship

Question 27

Plate efficiency in a distillation column is reduced due to the __________ of liquid. entrainment foaming weeping & dumping all (a), (b) & (c)

Answer 27

all (a), (b) & (c)

Question 28

The most efficient cooling tower out of the following is induced draft forced draft natural draft atmospheric

Answer 28

induced draft

Question 29

Sodium bicarbonate is produced during soda ash manufacture using a __________ tower. plate baffle packed none of these

Answer 29

baffle

Question 30

Powdery materials can be guarded against caking tendency on storage by providing irregular grain size. providing minimum percentage of voids. having maximum possible points of contact. none of these.

Answer 30

none of these.

Question 31

In case of cooling towers, the ratio of the rates of heat and mass transfer is indicated by the __________ number. Lewis Grashoff Sherwood none of these

Answer 31

Lewis

Question 32

Which one of the following devices is not used for both the absorption as well as the liquid-liquid extraction processes ? Packed towers Plate towers Spray towers Wetted wall columns

Answer 32

Wetted wall columns

Question 33

The temperature of water cooled in cooling tower is always __________ temperature of entering air. more than the wet bulb less than the wet bulb equal to the wet bulb equal to the dry bulb

Answer 33

more than the wet bulb

Question 34

Which of the following processes is followed by unsaturated air (with dry bulb temperature 12°C and relative humidity 47%) passing through water spray washer (temperature of water being constant at 40°C? Humidification only Heating only Both heating and humidification Evaporative cooling

Answer 34

Both heating and humidification

Question 35

Mass transfer co-efficient is directly proportional to DAB according to the __________ theory. film penetration surface-renewal none of these

Answer 35

film

Question 36

The relative volatility of a binary mixture at constant temperature __________ the total pressure. decreases with increase in increases with increase in is independent of none of these

Answer 36

decreases with increase in

Question 37

Tower diameter may be decreased by using higher reflux ratio. use of increased tray spacing. increasing the liquid flow rate. increasing the vapour flow rate.

Answer 37

use of increased tray spacing.

Question 38

Rayleigh's equation applies to __________ distillation. continuous steam differential flash

Answer 38

differential

Question 39

Steady state equimolal counter diffusion is encountered in separation of a binary mixture by distillation. absorption of NH3 from air by water. all liquid-liquid diffusion systems. all liquid-solid diffusion systems.

Answer 39

separation of a binary mixture by distillation.

Question 40

Which of the following is considered equivalent to one theoretical stage in Mc-Cabe Thiele's method ? Partial condenser Total condenser Reboiler Both (b) and (c)

Answer 40

Both (b) and (c)

Question 41

Polar organic compounds are normally used as separating agents for the azeotropic and extractive distillation. Which of the following is the most important factor to be considered for the choice of the separating agent for extractive distillation ? Cost Availability Toxicity Selectivity

Answer 41

Selectivity

Question 42

As the reflux ratio increases, the slope of the operating line for rectifying section increases decreases remains constant data insufficient, cannot be predicted

Answer 42

increases

Question 43

Cox chart, which is useful in the design of distillation column particularly for petroleum hydrocarbons, is a plot of (where, P = vapor pressure, T = temperature) log P vs . T log P vs. log T T vs. P P vs . log T

Answer 43

0

Question 44

The unit of volumetric diffusivity is cm2/sec cm/sec cm3/sec cm2/sec2

Answer 44

cm

Question 45

For a ternery mixture, in which equilateral triangular co-ordinate is used in leaching and extraction, a __________ of the equilateral triangular co-ordinates. binary mixture is represented by the apex binary mixture is represented by any point inside ternery mixture is represented by the sides pure component is represented by the apex.

Answer 45

pure component is represented by the apex.

Question 46

Steady state temperature reached by a small amount of liquid evaporating into a large amount of unsaturated vapour-gas mixture is called the __________ temperature. dry-bulb wet-bulb dew point adiabatic saturation

Answer 46

wet-bulb

Question 47

The Reynolds number of the liquid was increased 100 fold for a laminar falling film used for gas-liquid contacting. Assuming penetrating theory is applicable, the fold increase in the mass transfer co-efficient (Kc) for the same sytem is 100 10 5 1

Answer 47

10

Question 48

The slope of 9-line is determined by the reflux ratio plate efficiency to be achieved thermal condition of the feed relative volatility.

Answer 48

thermal condition of the feed

Question 49

In steam distillation, the temperature is 100°C. temperature is more than 100° C. product must be immiscible with water. temperature is higher than the boiling point of either component.

Answer 49

product must be immiscible with water.

Question 50

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 ? Dew point Humidity ratio Wet bulb temperature None of these

Answer 50

Wet bulb temperature