Drying seems to be an important technology! This is just a very very very short notes of mine on drying. Colorful for better retention of memory!

1. Drying Methods: Principles, Advantages & Disadvantages
Drying Method & Tools
Solar Drying
Solar Tray Cabinet
Tent solar dryer
Brace Solar Dryer
Tunnel Solar Dryer
Chimney Solar Dryer
Drum Drying
Principle
Open Air:
Crops generally spread on the ground where they receive short
wavelength solar energy during day.
A part of energy is reflected back, and the remaining is absorbed by
the surface of the crops depending on the color of the crops.
The absorbed radiation is converted into thermal energy and the
temperature of the material starts to increase.
Loses like the long wavelength radiation loss from the surface of crop
due to ambient air through moist air and also convective heat loss due
to the blowing wind through moist air over crop surface.
Direct Solar Drying:
Solar radiation impinging on the glass cover, a part is reflected and
remaining transmitted inside cabinet.
A part of transmitted radiation is reflected back from the crop surface
and the rest is absorbed by the surface of the crop.
This causes the temperature to increase and thereby emitting long
wavelength radiation which are not allowed to escape the glass cover.
This cause the temperature above crops higher.
The glass cover serves in reducing direct convective losses to the
ambient.
Indirect Solar drying
Crops are located in trays inside a drying cabinet.
Separate unit, solar collector is used for heating of the air entering the
cabinet.
Heated air  floe through provides heat for moisture evaporation.
Consist of a drum, heated internally, usually by steam and rotated on
its longitudinal axis.
Drum dips into fed pan  heated drum rotates  Liquid applied to
the surface and spread to a film  product scrapped from surface of
the drum by means of a doctor knife.
Single drums  greater flexibility, a larger proportion of the drum
area, easier to access for maintenance, no risk of damage caused by
metal objects falling between the drums.
Advantage
No cost & inexpensive
technologies.
Ideal for products where
little or no value added.
Food usually dried close to
the home.
Simple and cheap
equipments.
Protection from rains,
dews, debris & etc.
Better control over drying.
Not exposed to sun directly.
Good for photo sensitive
crops.
Can be operated at high T.
Rapid drying and higher
energy efficiencies.
Compact equipment, less
space consumption.
Heating time is short.
Product: flake form,
convenient for many
purposes.
Disadvantage
Contamination.
Dependent of weather.
Slow drying rate.
Large labor force.
May not possible to dry to
sufficient low level moisture
to prevent MO growth.
Over-heating, causing crop
damage.
Poor vapor removal rates.
Small scale application.
Discoloration.
Moisture condensation
reduces its transmittivity.
More capital, investment
due to expensive
equipments.
Operating conditions are
critical.
Careful control on feed rate,
film thickness, speeds of
drum rotation and drum
temperature.
Heat damage to sensitive
foods

2. . Drying Method
Microwave Drying
Principles
Exposed to high frequency electromagnetic waves.
High frequency water, water molecules are polarized and thend to
change orientation.
In the process of orientation, sufficient heat to expel moisture from
the product is generated.
Advantages
Rapid drying at Low T.
High thermal efficiency.
Uniform heating
Highly efficient equipment
Increased rate of killing
bacteria due to speedy T
rise
Good quality attributes of
material.
Disadvantages
Batch size is small.
Microwave radiation is
deadly, proper safety control
should be taken.
Fluidized bed drying
Drying air introduced at high velocities through the particulate
material.
Velocity is raised to the point that the particulate fully suspended in
the hot air stream.
All particles are completely exposed to drying air, resulting in high
rates of heat transfer.
The high rate of heat transfer results in instantaneous evaporation of
moisture at the entry point.
Dryer consist of a chamber with perforated bottom.
High drying rates and short
drying time.
Constant drying rate
Uniform temperature
The free movement of
individual particles
eliminates the risk of
soluble materials migrating,
as may occur in static beds.
Fine particles may become
entrained in the fluidizing air
and must be collected by bag
filters, leading to segregation
and loss of fines.
Spray Drying
The spray dryer provides a large surface area for heat and mass
transfer by atomizing the liquid to small droplets.
Type of atomizer: Centrifugal atomizer
: Pressure Nozzle atomizer
: Two-fluid nozzle atomizer
: Ultrasonic nozzle atomizer
These are then sprayed into a stream of hot air, so that each droplet
dries to a solid particle.
.The dry powder is collected at the base of drier & removed by screw
conveyor with a cyclone separator.
May be fitted with fluidized bed facilities to finish powders taken from
the drying chamber.
High capital costs
Requirement for a relatively
high-feed moisture content
to ensure that food can be
pumped into the atomizer
(higher energy costs & higher
volatile losses)
Limited to food that can be
atomized, such as liquids and
low viscosity pastes or
purees.
Vacuum Drying
Example of conduction drier
Consist of jacketed vessel to withstand vacuum within the oven.
There are supports for the shelves giving a larger surface area for
Rapid drying
Large scale continuous
production
Low labor cost
Relatively simple operation
and maintenance.
The characteristics particle
form gives the product high
bulk density, in turn ready
solubility.
Powder, will have uniform
and controllable particle
size.
Drying takes place at low
temperature.
Little air present, minimal
Expensive drying method.
Yields hygroscopic products,
proper packaging needed

3. conduction heat transfer.
Oven closed by a door.
Oven is connected through a condenser & liquid receiver to a vacuum
pump,
Low pressure, at with water boils at 25-35˚C.
Drying is usually rapid as moisture movement is by total pressure flow
rather than diffusion.
The water content is first converted to ice and then changed into
vapor without passing back through the water phase.
Water vapor pressure of food held below 610.5 Pa & the water is
frozen, when food is heated the solid ice sublimes directly to vapor
without melting  vapor removal with a vacuum pump &
condensation on refrigeration coils.
Frozen food remains rigid during sublimation, resulting in a porous
sponge like dried structure.
Food  Vacuum Chamber  Evaporating the ice (low T) freeze
dried food sealed in moisture & oxygen proof packaging water
replaced  food regains its original quality.
Freeze Drying
(Lyophilization)
risk of oxidation.
Suitable for heat-sensitive
products such as ascorbic
acid containing product.
Less damaging drying
process.
Conserves energy.
Drying at low
temperature chemical
decomposition is
minimized. (oxidation,
hydrolysis)
Product is light and porous
(gives it ready solubility).
Suited for dry solid foods 
delicate flavors, color
textural & appearance
attributes.
Reference:
Handbook of industrial drying by Arun S Mujumdar
.
Yields a very hygroscopic
product (proper packing is
vital)
Very slow process
Very complicated plant
Very expensive
Limited to certain type of
valuable products.