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CHE > Separation Processes > Flashcards

Separation Processes Flashcards

1
Q

The removal of relatively small amounts of solvent, at temperatures below its boiling
point, by circulating air or some other gas over the material in order to carry away the
solvent vapor

A

Drying

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2
Q

The vapor pressure exerted by the moisture contained in a wet solid depends upon the…

A

-nature of the moisture
-temperature
-nature of the solid

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3
Q

To remove all the moisture from a wet solid requires exposure to…

A

Perfectly dry air

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4
Q

Milk is dried usually in a…

A

Spray dryer

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5
Q

Equilibrium-moisture curves of different solids are the same or different?

A

Different

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6
Q

Detergent solution is dried to a powder in a…

A

Spray dryer

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7
Q

If moisture content of a solid on dry basis is X then the same on wet basis is…

A

X/X+1

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8
Q

In paper industry, paper is dried in…

A

Heated cylinder dryer

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9
Q

Moisture content of a substance when at equilibrium with a given partial pressure of the vapor is called…

A

Equilibrium moisture

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10
Q

Calcium ammonium nitrate (a fertilizer) is
dried in a…

A

Rotary dryer

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11
Q

Moisture in a substance exerting an
equilibrium vapor pressure less than that of pure liquid at the same temperature is…

A

Bound moisture

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12
Q

Rotary dryer cannot handle…

A

Sticky materials

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13
Q

Moisture in a solid exerting an equilibrium vapor pressure equal to that of the pure liquid at the same temperature is…

A

Unbound moisture

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14
Q

Refractory bricks are usually dried in a…

A

Tunnel dryer

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15
Q

Hest sensitive materials like certain
pharmaceuticals and food stuff can be dried by…

A

Freeze drying

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16
Q

Moisture contained by a substance in excess of the equilibrium moisture is called…

A

Free moisture

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17
Q

Flights in a rotary dryer are provided to…

A

Lift and shower the solids thus exposing it thoroughly to the drying action of the gas

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18
Q

What occurs in the constant rate period of the rate of drying curve for batch drying?

A

Surface evaporation of unbound moisture

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19
Q

The falling rate period in the drying of a solid is characterized by…

A

Increasing temperatures both on the surface and within the solid

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20
Q

Dryer widely used in a textile industry is…

A

Festoon dryer

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21
Q

Sticky material can be dried in…

A

Tray dryer

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22
Q

Drying of a solid involves…

A

Both heat and mass transfer

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23
Q

A solid material shows case hardening properties while drying. What should be controlled to control the drying process?

A

Temperature of the solid

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24
Q

Capacity of a rotary dryer depends on…

A

-r.p.m
-inclination with ground surface

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25
Q

Air at a particular humidity is heated in a furnace. The new dew point…

A

Increases

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26
Q

The critical moisture content in case of drying indicates…

A

At the beginning of falling rate period

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27
Q

A starry is to be dried to produce flaky solid. What type of dryer would you recommend?

A

Spray dryer

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28
Q

For continuous drying of granular or crystalline material, the dryer used is…

A

Rotary

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29
Q

A moisture in a non-hygroscopic material is…

A

Unbound moisture

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30
Q

During the constant rate period, the rate of drying decreases with…

A

-decrease in air temperature
-increase in air humidity

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31
Q

What is the controlling factor for a drum dryer?

A

Heat transfer

32
Q

Paper industry employs…

A

Cylinder dryers

33
Q

Sand is dried in foundaries in a…

A

Vacuum

34
Q

Free flowing granular materials can be best dried in…

A

Rotary dryer

35
Q

In the usual drying or dehumidification process, water is (1) and air is (2)

A

(1) Solvent
(2) Drying

36
Q

The drying process cools the air adiabatically at…

A

Constant wet bulb

37
Q

It is usually the final step in a series of operations.

A

Drying

38
Q

What are the mechanisms of drying?

A
  • Mass transfer from interior of the solid to the surface – diffusion or capillarity.
  • Heat transfer by vaporization of the liquid moisture at the surface.
  • Mass transfer of the vapor into the bulk gas stream.
39
Q

What are the two approaches of drying?

A

-Equilibrium relationships
-Rate relationships

40
Q

In an adiabatic drying process in which heat required for the
vaporization of solvent comes solely from the…

A

Sensible

41
Q

Typically, the solids is put in contact with (1) to remove (2)

A

(1) Carrier gas (usually air)
(2) Vapor (usually water)

42
Q

What are the factors affecting drying?

A

EXTERNAL FACTORS:
- Temperature of the Gas Stream
- Humidity of the Gas Stream
- Velocity of the Gas Stream
INTERNAL FACTORS:
- Nature of Solids
- Movement of moisture with the solid

43
Q

METHODS OF DRYING OPERATIONS:

A
  • BATCH DRYING
  • CONTINUOUS DRYING
44
Q

Actually a semi-batch process wherein a quantity of the substance to be dried is exposed
to a continuously flowing stream of air into which the moisture evaporates

A

Batch Drying

45
Q

The substance to be dried as well as the gas passes continually through the equipment; no
typically stagewise methods are ordinarily used, and all operations involve continuous
contact of the gas and the drying substance

A

Continuous Drying

46
Q

MAJOR DIVISION OF DRYERS:

A
  • Dryers in which the solid is directly exposed to a hot gas.
  • Dryers in which heat is transferred to the solid from an external medium.
  • Dryers that are heated by a dielectric, radiant, or microwave energy.
47
Q
  • also known as direct dryers; where solids are exposed to a hot gas.
  • heat is supplied entirely by direct contact of the substance with the hot gas into which
    the evaporation takes place
A

Adiabatic Dryers

48
Q
  • also known as indirect dryers; heat is transferred from an external medium.
  • heat is supplied quite independently of the gas used to carry away the vaporized
    moisture
A

Non-adiabatic Dryers

49
Q

Drying Equipment is used for…

A

-DRYERS FOR SOLIDS AND PASTES
-DRYERS FOR SOLUTIONS AND SLURRIES

50
Q

DRYERS FOR SOLIDS AND PASTES:

A

-Tray Dryers
-Screen Conveyor Dryers
-Tower Dryers
-Rotary Dryers
-Screw Conveyor Dryers
-Fluid Bed Dryers
-Flash Dryers

51
Q

DRYERS FOR SOLIDS AND PASTES:

A

-Tray Dryers
-Screen Conveyor Dryers
-Tower Dryers
-Rotary Dryers
-Screw Conveyor Dryers
-Fluid Bed Dryers
-Flash Dryers

52
Q

DRYERS FOR SOLUTIONS AND SLURRIES

A

-Spray Dryers
-Thin Film Dryers
-Drum Dryers

53
Q
  • Consists of a rectangular chamber of sheet metal containing two trucks that supports
    racks; each rack carries a number of shallow trays that are loaded with the material to be
    dried
  • Heated is circulated at 2 – 5 m/s between trays by fan and motor and passes over heaters;
    air is distributed uniformly over the stack of trays through baffles.
  • Useful on small production rate; they find most frequent application for valuable products
    like dyes and pharmaceuticals
A

Tray Dryers

54
Q
  • A layer (25 mm to 150 mm) thick of material to be dried is slowly carried on a travelling
    metal screen through a long drying chamber or tunnel.
  • The chamber consists of series of separate sections, each with its own fan and air heater.
    At the inlet end of the dryer, the air usually passes upward through the screen and the
    solids; near the discharge end, where the material is dry and may be dusty, air is passed
    downward through the screen. The air temperature and humidity may differ in the various
    sections to give optimum conditions for drying at each point.
  • Typically 2 m wide and 4 – 50 m long, giving drying times of 5 – 120 minutes; the minimum
    screen size is about 30 mesh.
  • Handles variety of solids continuously and with a very gentle action; particularly applicable
    when the drying conditions must be appreciably changed as the moisture content of the
    solid is reduce
A

Screen Conveyor Dryers

55
Q
  • Contains a series of circular trays mounted one above the other on a central rotating
    shaft.
  • Solid feed is dropped on the topmost tray is exposed to a stream of hot air or gas that
    passes across the tray. The solid is then scrapped off and dropped to the tray below. The
    flow of solids and gas may be either parallel or counter-current
A

Tower Dryers

56
Q
  • Consists of a revolving cylindrical shell, horizontal or slightly inclined toward the outlet.
    -Wet feed enters one end of the cylinder; dry material discharges from the other.
  • Rotary dryers are heated by direct contact of gas with the solids, by hot gas passing
    through an external jacket, or by steam condensing in a set of longitudinal tubes mounted
    on the inner surface of the shell.
A

Rotary Dryers

57
Q
  • A continuous indirect-heat dryer, consisting essentially of a horizontal screw conveyor (or
    paddle conveyor) enclosed in a cylindrical jacketed shell.
  • Solid fed in one end is conveyed slowly through the heated zone and discharges from the
    other end.The vapour evolved is withdrawn through pipes set in the roof of the shell.
  • Handles solids that are too fine and too sticky for rotary dryers; they are completely
    enclosed and permit recovery of solvent vapors with little or no dilution by air.
A

Screw Conveyor Dryers

58
Q
  • Solid particles are fluidized by air or gas in a boiling-bed unit; mixing and heat transfer are
    very rapid; wet feed is admitted to the top of the bed; dry product is taken out from the
    side, near the bottom.
A

Fluid Bed Dryers

59
Q
  • Wet pulverized solid is transported for a few seconds in a hot gas stream.
  • The rate of heat transfer from the gas to the suspended solid particles is high and drying is
    rapid so that no more than 3 or 4 s is required to evaporate substantially all the moisture
    from the solid.
  • Flash drying may be applied to sensitive materials that in other dryers would have to be
    dried indirectly by a much cooler heating medium.
A

Flash Dryers

60
Q
  • A slurry or liquid solution is dispersed into a stream of hot gas in the form of a mist of
    fine droplets. Moisture is rapidly vaporized from the droplets, leaving residual particles of
    dry solid, which are then separated from the gas stream. The flow of liquid and gas may be
    co-current, counter current or a combination of both in the same unit.
  • Droplets are formed inside a cylindrical drying chamber by pressure nozzles, two-fluid
    nozzles, or, in large dryers, high speed spray disks
A

Spray Dryers

61
Q

Competitive with spray dryers but relatively expensive.

A

Thin Film Dryers

62
Q

Competitive with spray dryers but relatively expensive.

A

Thin Film Dryers

63
Q

Consist of one or more heated metal rolls on the outside of which a thin layer of liquid is
evaporated to dryness. Dried solid is scraped off the rolls as they slowly revolve.

A

Drum Dryers

64
Q

A material with no moisture content

A

. Bone-Dry Material (𝒎𝑺)

65
Q

the mass fraction of the moisture in the wet solid;
expressed as percentage. It is also expressed as kg moisture per kg wet solid or kg moisture per combined kg of dry solid and moisture.

A

Moisture Content (wet basis)

66
Q

the ratio of the mass of the moisture to the mass of
the bone-dry solid

A

Moisture Content (dry basis)

67
Q

Is the moisture content of a substance which exerts an equilibrium vapor pressure less than that of the pure liquid at the same temperature; it is the moisture difficult to remove, but which can be removed only under special conditions. It can exist in various forms such as liquid water in very fine capillaries, moisture in cell or fiber walls, and water in natural organic structures – in physical and chemical combination

A

Bound Moisture (𝑿𝒃)

68
Q

Refers to the moisture content of a substance which exerts an equilibrium vapor pressure equal to that of the pure liquid at the same temperature. It is
also the moisture in the solid in excess of the equilibrium moisture content corresponding
to saturation humidity

A

Unbound Moisture (𝑿𝒖)

69
Q

A material that contains bound moisture.

A

Hygroscopic Material

70
Q

Is the limiting moisture to which a given
material can be dried under specific conditions of air temperature and humidity; corresponds to bound moisture. It depends on the direction where equilibrium was reached (i.e. desorption or absorption)

A

Equilibrium Moisture Content (𝑿𝒆)

71
Q

Moisture content of a substance in excess of the
equilibrium moisture; only free moisture can be evaporated, and the free moisture content
of a solid depends upon the vapor concentration in the gas

A

Free Moisture Content (𝑿 − 𝑿𝒆)

72
Q

The average moisture content at the end of
constant rate drying period or at the start of the falling rate period

A

Critical Moisture Content (𝑿c)

73
Q

The drying period during which the rate of water
removal per unit of drying surface is constant.

A

Constant Rate Drying Period

74
Q

The drying period during which the instantaneous
drying rate continually decreases

A

Falling Rate Drying Period

75
Q

May kulang pa sa Drying pero prio Humid q&a and slides

A

CHE (5 decks)

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