Mass Transfer Quiz 8

1 / 50

Pick out the wrong statement?

Separation of components is not possible in liquid extraction, if selectivity is unity

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

Dehydration of ethyl alcohol is most economically done by molecular distillation at very high vacuum

2 / 50

Pick out the wrong statement ?

Reboiler is not used in open steam distillation

Fenske-Underwood equation is used for calculating theoretical number of plates in a distillation column at normal operating reflux condition

Freeze drying is used for the drying of fish

The separation of solutes of different molecular sizes in a liquid solution by use of a suitable membrane is called dialysis

3 / 50

Pick out the correct statement ?

Dialysis process is used for the production of fresh water from brine

Electrodialysis is used for the refining/purification of petrol

Electrodialysis is used in the manufacture of NaOH

Both B. and C.

4 / 50

Pick out the correct statement ?

The selectivity of solvent used in solvent extraction is unity at the plait point

All A., B. and C.

In case of liquid-liquid extraction, no separation is possible, if the selectivity of the solvent used is unity

With increase in temperature, the selectivity of the solvent used in solvent extraction decreases

5 / 50

Pick out the correct statement ?

For the same duty, plate columns weigh less than packed columns

Plate column can handle greater liquid loads without flooding than packed column, but the liquid hold up is more in the case of plate column

All A., B. and C.

For identical gas flow rates, less pressure drop occurs through a plate tower than through a packed tower

6 / 50

Pick out the wrong statement ?

Heat removal load on cooler remains constant, even with increase of the reflux ratio in a distillation column

Total condenser or reboiler is considered equivalent to one theoretical plate in McCabe-Thiele method of theoretical plate calculation for distillation column

Even a forced draft cooling tower cannot cool water below the wet bulb temperature

Different solids have different equilibrium moisture curves

7 / 50

Penetration theory states that the mass transfer co-coefficient is equal to (where, De is diffusivity and ?t? is time) ?

(De.t)1/2

(De/t)1/2

(4De/?t)1/2

(4De/t)

8 / 50

Pick out the wrong statement ?

Taylor-Prandtl analogy for heat and mass transfer considers the transfer through both laminar and turbulent layers

When Henry?s law constant is very small, then the mass transfer rate is controlled by the gas film resistance

Mechanical agitation is required, if the system has low interfacial tension

Sieve tray towers are generally used for systems having low interfacial tension

9 / 50

Paper industry employs ________ driers?

Rotary

Cylinder

Spray

Fluidised bed

10 / 50

Pick out the wrong statement ?

With increase in pressure, the solubility of gases in solvent increases at a fixed temperature

In solvent extraction, the distribution co-efficient should be as large as possible for solvent economy

Decrease in Murphree plate efficiency is an indication of entrainment in distillation column

The capacity of a distillation column is maximum at total reflux

11 / 50

Pick out the wrong statement. Raoult?s law does not apply, when the________?

Attractive forces between like and unlike molecules in the solution are approximately equal

Chemical combination or molecular association between unlike molecules takes place in the formation of solution

Size of component molecules are not equal

Component molecules are non-polar

12 / 50

Pick out the wrong statement ?

?Cascade? means a group of stages interconnected so that various streams flow from one to another

Zirconia probe is used for the humidity measurement

Cellulose acetate membranes are used in dialysis process

Elutriation process refers to when the soluble material is largely concentrated on the surface of an insoluble solid and is simply washed off by the solvent

13 / 50

Pick out the wrong statement?

In small columns, local efficiency is larger than Murphree efficiency; in large columns local efficiency is smaller than Murphree efficiency

In the McCabe-Thiele diagram for binary distillation, vertical feed line represents saturated liquid feed and horizontal feed line represents saturated vapour feed

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 ? 10-2m/s. Then the average mass transfer co-efficient for the plate is 2 ? 10-2m/s

14 / 50

Pick out the wrong statement?

According to Poiseuille?s law, the permeability decreases with increase in temperature for flow of a gas through a given capillary

Molecular diffusion in solid is much faster than that in liquids

Particles movement from higher concentration to lower concentration takes place in diffusion process

The binary diffusivity in liquids is of the order of 10-5 cm2/sec

15 / 50

Overall efficiency of the distillation column is _______?

The ratio of number of ideal plates to actual plates

The ratio of number of actual plates to ideal plates

Same as the Murphree efficiency

Always more than the point efficiency

16 / 50

Pick out the wrong statement ?

In actual practice, absorption is an endothermic process, while stripping is an exothermic process

Co-current absorbers are usually used, when the gas to be dissolved in the liquid is a pure substance

Hatta number is important in case of gas absorption with chemical reaction

In case of gas absorption, HETP is equal to HTU, when both the operating line & the equilibrium lines are parallel

17 / 50

Pick out the correct statement?

Diffusivity increases with increase in molecular weight

Diffusivity increases with the size of the individual molecule

None of these

Diffusivity decreases with increase in temperature

18 / 50

Pick out the wrong statement ?

With decrease in feed temperature, the number of plates required for binary distillation decreases for a given ?L/V? ratio

The extraction per unit of total height for a short packed column is more effective than for a tall column, in case of liquid-liquid extraction

Vander wall adsorption is also called chemisorption

Natural draft cooling towers are made of ferroconcrete and its cross-section is wider at the bottom

19 / 50

Pick out the wrong statement ?

Constant pressure distillation cannot separate an Azeotropic mixture

Flash distillation is practised on wide spread scale in petroleum refinery

Relative volatility of a binary mixture changes appreciably with the minor change in temperature

The relative volatility of a binary mixture at Azeotropic composition is unity

20 / 50

Physical absorption is___________?

A reversible phenomenon

Accompanied by evolution of heat

Both B. and C.

An irreversible phenomenon

21 / 50

Pick out the correct statement?

Kremser-Brown-Souder?s equation is used to calculate the efficiency of a plate tower

The slope of the stripping line is always less than unity

In Ponchon-Savarit method of no. of theoretical plate determination, the stripping and rectifying operating lines are vertical at total reflux

For a given separation in a binary distillation column, with increase in reflux ratio (above the minimum reflux ratio), the fixed cost of the distillation column first increases and then decreases

22 / 50

Pick out the wrong statement ?

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 cannot be increased by either reducing the mixture temperature or by increasing the total pressure

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

23 / 50

Penetration model (theory) for mass transfer was enunciated by _______?

Kissinevskii

Higbie

Toor and Marcello

Danckwerts

24 / 50

Pick out the wrong statement ?

The mole fraction of solute is directly proportional to the molality of a ?dilute? solution

Azeotropes obey Raoult?s law at all temperature

The relative volatility of a binary mixture at the Azeotropic composition is 1

For a non-reacting binary mixture of ideal gases, the partial pressure distribution of both components is nonlinear in the case of steady state unimolal unidirectional diffusion

25 / 50

pertaining to the analogy between equations of heat and mass transfer operations ? Pick out the wrong statement

Sherwood number in mass transfer is analogous to Nusselt number in heat transfer

Reynolds number in mass transfer is analogous to Grashoff number in heat transfer

Reynolds number remains the same in both heat and mass transfer

Prandtl number in heat transfer is analogous to Schmidt number in mass transfer

26 / 50

Pick out the wrong statement. Generally for physical adsorption, a gas of_________?

None of these

Higher molecular weight is adsorbed in preference to a gas of low molecular weight

Low volatility is adsorbed in preference to a gas of high volatility

High critical temperature is adsorbed in preference to a gas of low critical temperature

27 / 50

Pick out the wrong statement?

Packed towers are preferred over plate towers for handling corrosive and foamy liquids

Packed towers are preferred for vacuum operation, because the pressure drop through the packing is less and they (packings) also lessen the possibility of tower wall collapse

Due to uneven supply and improper distribution of liquid, problem of channelling, loading & flooding is generally encountered in packed towers

Diameter of randomly packed tower is normally more than 1.2 metres

28 / 50

Pick out the wrong statement ?

In case of an absorber, the operating line is always above the equilibrium curve

Absorption factor is constant, when the equilibrium and operating lines are straight

In case of a stripper, the equilibrium curve is always below the operating line

In the absorption of low solubility gases, the liquid film is the controlling resistance

29 / 50

Pick out the wrong statement ?

?Elution? means the desorption of the adsorbed solute by a solvent

Change in barometric pressure does not affect the relative humidity

In case of a packed tower, the operating velocity is about half of the flooding velocity, generally

The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity

30 / 50

Pick out the system with minimum boiling azeotrope at 1 atm ?

Hydrochloric acid-water

Benzene-toluene

Ethyl alcohol-water

All A., B. and C.

31 / 50

Pick out the wrong statement?

Molecular diffusion in solid is much faster than that in liquids

The binary diffusivity in liquids is of the order of 10-5 cm2/sec

According to Poiseuille?s law, the permeability decreases with increase in temperature for flow of a gas through a given capillary

Particles movement from higher concentration to lower concentration takes place in diffusion process

32 / 50

Out of two equilibrium curves, first one lies above the diagonal and the second lies below the diagonal in the x-y plot. It means that separation by distillation is__________?

Not possible in the second case unless vacuum is applied

None of these

Not possible in the second case unless high pressure is applied

More readily done in the second case compared to first one

33 / 50

Perforated plate towers are unique for solvent extraction because, they provide_______?

Repeated coalescence & redispersion of the drop phase

Better contact of the phase

None of these

Higher contact area

34 / 50

Percentage saturation is ________ the relative saturation?

Always smaller than

None of these

Always greater than

Not related to

35 / 50

Packed column distillation is limited to the column ______ metres?

Height < 6

Both A. & B.

Diameter < 0.6

Neither A. nor B.

36 / 50

Pick out the wrong statement pertaining to the rotary dryer ?

Recommended peripheral speed of a rotary drier is in the range of 10 to 30 metres/minute

Rotary dryer is suitable for drying sticky material

Hold up of a rotary drier is defined as the fraction of the dryer volume occupied by the solid at any instant. The best performance for rotary drier is obtained, when the hold up is in the range of 0.05 to 0.15

Flights (located in the inside shell of rotary dryer) lift the material being dried and shower it down through the current of hot air/gases. It extends from the wall to a distance which is about 8-12% of the inside diameter of shell

37 / 50

Pick out the wrong statement pertaining to the analogy between equations of heat and mass__________? transfer operations.

Reynolds number remains the same in both heat and mass transfer

Reynolds number in mass transfer is analogous to Grashoff number in heat transfer

Sherwood number in mass transfer is analogous to Nusselt number in heat transfer

Prandtl number in heat transfer is analogous to Schmidt number in mass transfer

38 / 50

Paper industry employs ______ driers ?

Rotary

Fluidised bed

Spray

Cylinder

39 / 50

Pick out the wrong statement pertaining to leaching ?

Door classifier can be used for leaching of coarse solids

Vegetable seeds can be leached in either of Bollman extractor, Rotocel or Kennedy extractor

Very high temperature is not needed for the leaching of sugar beet

Fine solids cannot be leached in a Pachuca tank

40 / 50

Pick out the correct statement?

Free moisture is removed during falling rate drying period

Bound moisture is removed during constant rate drying period

The driving force for mass transfer in a wet solid is its free moisture content

The sum of free moisture and equilibrium moisture in a solid is called the critical moisture

41 / 50

Penetration theory relates the average mass transfer co-efficient (K) with diffusivity D. as______?

K ?D2

K ??D

K ?D

K ?D1.5

42 / 50

Pick out the wrong statement?

Change in barometric pressure does not affect the relative humidity

?Elution? means the desorption of the adsorbed solute by a solvent

The equilibrium moisture content of the solid can be reduced by increasing the absolute humidity

In case of a packed tower, the operating velocity is about half of the flooding velocity, generally

43 / 50

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

Total reflux in case of distillation operation requires infinite number of plates for a binary system separation

The ?Hatta number? is important in problems involving gas absorption without chemical reaction

Higher hold up of the solid in the rotary dryer results in better exposure of the solids to the gas

44 / 50

Pick out the wrong statement?

The Lewis number of a mixture is unity, when the thermal diffusivity is equal to the mass diffusivity

An Azeotropic mixture of two liquids has boiling point higher than either of them, when it shows negative deviation from Raoult?s Law

1 Nm3 of dry air is lighter than 1Nm3 of humid air

The mass diffusivity, the thermal diffusivity and the eddy momentum diffusivity are the same for NSc = NPr= 1

45 / 50

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

A natural draft cooling tower is recommended to be used, when the air humidity is low and air temperature is also low

Effectiveness of forced draft cooling tower is reduced by the recirculation of the humid and hot discharged air

In case of induced draft cooling tower, the fan is placed at the top of the tower

46 / 50

Out of the following gas-liquid contacting devices, for a given set of operating conditions, gas pressure drop is the least in _______ tower?

Perforated tray

Wetted wall

Bubble cap

Packed

47 / 50

Pick out the wrong statement?

Diffusion co-efficient and mass transfer co-efficient are not same in any mass transfer operation

Contacting cold vapor-gas mixture with hot liquid exemplifies humidification process

Mass transfer co-efficient does not account for gas-liquid diffusion

Contacting cold liquid with warm vapor-gas mixture exemplifies dehumidification process

48 / 50

Pick out the correct statement ?

Flash distillation is suitable for separating components which have very close boiling temperature

A sharp decrease in pressure drop is an indication of flooding in a distillation coloumn

Ponchan-Savarit method is more accurate than McCabe-Thiele method to determine the number of theoretical stages required for the separation of non-ideal binary system by distillation

Solvent used in extractive distillation should be of high volatility

49 / 50

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

A natural draft cooling tower is recommended to be used, when the air humidity is low and air temperature is also low

In case of induced draft cooling tower, the fan is placed at the top of the tower

Effectiveness of forced draft cooling tower is reduced by the recirculation of the humid and hot discharged air

50 / 50

Pick out the correct relationship. (Where, Ri = internal reflux ratio Ro = external reflux ratio.) ?

(1 ? Ri) = Ro/Ri

(1 ? Ro) = Ro/Ri

(1 + Ro) = Ro/Ri

(1 + Ri) = Ro/Ri

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