Terms
related to drying materials
• Feed: Wet input material to the dryer is termed as feed.
• Hygroscopic /non hygroscopic materials: material that has ability to
absorb and bind moisture by hygroscopic forces (depending on nature of the
product and temperature/ humidity of the surroundings is termed as hygroscopic.
Material, which does not contain any bound moisture, is called non-hygroscopic.
Terms
related to level/nature of moisture in drying materials
• Bone Dry Material: Any material, which has been dried at
sufficiently high temperature for a prolonged time by well-established methods
till it is deviled of all traces of moisture, is called ‘Bone Dry Material’.
• Moisture Content: The loss of moisture under standard prescribed
drying condition till bonedry stale is reached is termed as the ‘moisture
content’ of the material and is usually expressed as a fraction of moisture per
kg of wet material (wet basis) or expressed as fraction of moisture per kg of
bone-dry material (bone dry basis). Moisture refers to water, although other
liquids may follow the same testing techniques.
• Equilibrium moisture content: It is the level of bound moisture in a
given material which is attained on stabilization under specified conditions of
temperature and humidity either by loosing excess moisture by drying or by
absorbing moisture from surroundings.
• Bound Moisture: Liquid bound in the solid in its capillaries, by
solution in its cells/walls, by solution and by chemical/physical adsorption.
• Free moisture: In a hygroscopic material, it is the moisture in
excess of the equilibrium moisture content at existing humidity and temperature
and includes unbound as well as bound moisture which can be removed.
Terms
related to drying process
• Periods of Drying: As drying proceeds, moisture content and rate of
drying change with respect to time as follows.
Initially
the moisture evaporates from the saturated surface of a solid. In this phase,
the rate of drying per unit drying area is CONSTANT. At the end of this, there
is a decrease in the area of saturated surface and a transition level called
CRITICAL MOISTURE CONTENT is reached. Finally, the water diffuses from the
interior and then evaporates. In this phase called FALLING RATE PERIOD of
drying, the instantaneous rate of drying continuously
decreases.
The
drying curve is a graphical representation of moisture content of the product
vs. time during the process of drying and it identifies the constant, critical
and falling rate regimes of drying.
Terms
Related to Heat and Mass Transfer/Psychrometric Processes.
•
Absolute Humidity: It is the amount of liquid (e.g. water) vapour in a given
gas stream expressed as weight of liquid per weight of dry gas, expressed as kg
of liquid /kg of dry air
•
Relative Humidity: It is the ratio of the partial pressure of the condensable
vapour in the gas to the vapour pressure of the pure vapour at the same
temperature expressed as a percentage.
•
Wet Bulb Temperature: It is the dynamic equilibrium temperature attained by a
liquid surface when the rate of heat transfer to the surface by convection
equals the rate of mass transfer away from the surface.
•
Sensible heat: It is the energy involved in changing the temperature of a given
substance.
•
Latent heat: It is the energy involved in a phase change (e.g. liquid to
vapour), which does not result in a temperature change, expressed as kJ/kg.
•
Humid Heat: Is the heat necessary to cause a unit temperature increase in a
unit mass of humid air (dry air + moisture)
•
Material Balance: It is an account of material entering a system, which must
equal the material leaving a system if no hold up occurs. Care must be taken to
account for the various means through which material can leave a system. For
example, in a spray dryer, dried powder can come out through the main dryer as
well as though the dust collector.
•
Heat Balance: It is an account of the heat supplied to the system and the heat
used. The heat required in the dryer is generally made up of the following:
• Sensible heat to for raising the material to the drying
temperature.
• Heat required for raising the temperature and then the evaporation
of the liquid
• Heat losses through the equipment losing by radiation and
convection.
• Heat lost in exhaust or due to air leakage
and in the rejected heating medium like condensate if it is not
ecovered/recycled.
•
Thermal Efficiency: Is the percentage of total energy supply that is used to
evaporate water (or solvent).
Equipments:
The
letter symbols in the code may be used with appropriate subscript, which may
designate a place in space or time a system of units or a constant or reference
value. The terminology refers principally to the unit operation of drying to
remove water, though often drying of other solvents is also involved.
The
definitions conform generally to common usage but as there are many types of
dryers and many modes of dryer operation there are exceptions to some
definitions.
• Dryer: It is an assembly of equipments used for removal of moisture
from solids by evaporation.
• Continuous Dryers: These are those in which the feed, moisture
evaporations are continuous and uniform
• Batch Dryers: These are those in which either the feed operation or
discharge operation or both are intermittent.
• Direct Dryers: Heat is transferred from hot gases by direct contact
with wet solids. Hot gases carry the vaporized liquid away. These are
hot-air/convection dryers.
• Indirect Dryers: Heat is transferred to the wet solid through a
retaining wall. The rate of drying depends on good contact of wet materials
with hot surfaces. These are conduction/contact dryers.
REFERENCES
1.
Energy Conservation in Pulp & Paper Industry- CADDET Analysis Series,
Netherlands
2.
APV Dryer Handbook-Invensys APV Technical Centre, USA
3.
Handbook of Drying Technologies- Arun Mujumdar-Marcel Decker Publications
4.
Chemical Engineering handbook- Perry
5.
Industrial Drying- A. Williams Gardner- George Godwin Ltd.
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