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
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
3
Q
To remove all the moisture from a wet solid requires exposure to…
A
Perfectly dry air
4
Q
Milk is dried usually in a…
A
Spray dryer
5
Q
Equilibrium-moisture curves of different solids are the same or different?
A
Different
6
Q
Detergent solution is dried to a powder in a…
A
Spray dryer
7
Q
If moisture content of a solid on dry basis is X then the same on wet basis is…
A
X/X+1
8
Q
In paper industry, paper is dried in…
A
Heated cylinder dryer
9
Q
Moisture content of a substance when at equilibrium with a given partial pressure of the vapor is called…
A
Equilibrium moisture
10
Q
Calcium ammonium nitrate (a fertilizer) is
dried in a…
A
Rotary dryer
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
12
Q
Rotary dryer cannot handle…
A
Sticky materials
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
14
Q
Refractory bricks are usually dried in a…
A
Tunnel dryer
15
Q
Hest sensitive materials like certain
pharmaceuticals and food stuff can be dried by…
A
Freeze drying
16
Q
Moisture contained by a substance in excess of the equilibrium moisture is called…
A
Free moisture
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
18
Q
What occurs in the constant rate period of the rate of drying curve for batch drying?
A
Surface evaporation of unbound moisture
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
20
Q
Dryer widely used in a textile industry is…
A
Festoon dryer
21
Q
Sticky material can be dried in…
A
Tray dryer
22
Q
Drying of a solid involves…
A
Both heat and mass transfer
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
24
Q
Capacity of a rotary dryer depends on…
A
-r.p.m
-inclination with ground surface
25
Air at a particular humidity is heated in a furnace. The new dew point...
Increases
26
The critical moisture content in case of drying indicates...
At the beginning of falling rate period
27
A starry is to be dried to produce flaky solid. What type of dryer would you recommend?
Spray dryer
28
For continuous drying of granular or crystalline material, the dryer used is...
Rotary
29
A moisture in a non-hygroscopic material is...
Unbound moisture
30
During the constant rate period, the rate of drying decreases with...
-decrease in air temperature
-increase in air humidity
31
What is the controlling factor for a drum dryer?
Heat transfer
32
Paper industry employs...
Cylinder dryers
33
Sand is dried in foundaries in a...
Vacuum
34
Free flowing granular materials can be best dried in...
Rotary dryer
35
In the usual drying or dehumidification process, water is (1) and air is (2)
(1) Solvent
(2) Drying
36
The drying process cools the air adiabatically at...
Constant wet bulb
37
It is usually the final step in a series of operations.
Drying
38
What are the mechanisms of drying?
- 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
What are the two approaches of drying?
-Equilibrium relationships
-Rate relationships
40
In an adiabatic drying process in which heat required for the
vaporization of solvent comes solely from the...
Sensible
41
Typically, the solids is put in contact with (1) to remove (2)
(1) Carrier gas (usually air)
(2) Vapor (usually water)
42
What are the factors affecting drying?
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
METHODS OF DRYING OPERATIONS:
- BATCH DRYING
- CONTINUOUS DRYING
44
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
Batch Drying
45
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
Continuous Drying
46
MAJOR DIVISION OF DRYERS:
- 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
- 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
Adiabatic Dryers
48
- 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
Non-adiabatic Dryers
49
Drying Equipment is used for...
-DRYERS FOR SOLIDS AND PASTES
-DRYERS FOR SOLUTIONS AND SLURRIES
50
DRYERS FOR SOLIDS AND PASTES:
-Tray Dryers
-Screen Conveyor Dryers
-Tower Dryers
-Rotary Dryers
-Screw Conveyor Dryers
-Fluid Bed Dryers
-Flash Dryers
51
DRYERS FOR SOLIDS AND PASTES:
-Tray Dryers
-Screen Conveyor Dryers
-Tower Dryers
-Rotary Dryers
-Screw Conveyor Dryers
-Fluid Bed Dryers
-Flash Dryers
52
DRYERS FOR SOLUTIONS AND SLURRIES
-Spray Dryers
-Thin Film Dryers
-Drum Dryers
53
- 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
Tray Dryers
54
- 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
Screen Conveyor Dryers
55
- 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
Tower Dryers
56
- 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.
Rotary Dryers
57
- 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.
Screw Conveyor Dryers
58
- 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.
Fluid Bed Dryers
59
- 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.
Flash Dryers
60
- 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
Spray Dryers
61
Competitive with spray dryers but relatively expensive.
Thin Film Dryers
62
Competitive with spray dryers but relatively expensive.
Thin Film Dryers
63
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.
Drum Dryers
64
A material with no moisture content
. Bone-Dry Material (𝒎𝑺)
65
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.
Moisture Content (wet basis)
66
the ratio of the mass of the moisture to the mass of
the bone-dry solid
Moisture Content (dry basis)
67
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
Bound Moisture (𝑿𝒃)
68
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
Unbound Moisture (𝑿𝒖)
69
A material that contains bound moisture.
Hygroscopic Material
70
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)
Equilibrium Moisture Content (𝑿𝒆)
71
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
Free Moisture Content (𝑿 − 𝑿𝒆)
72
The average moisture content at the end of
constant rate drying period or at the start of the falling rate period
Critical Moisture Content (𝑿c)
73
The drying period during which the rate of water
removal per unit of drying surface is constant.
Constant Rate Drying Period
74
The drying period during which the instantaneous
drying rate continually decreases
Falling Rate Drying Period
75
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