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Drying Equipment Selection
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DRYERS
CONTENTS: DRYING PURPOSE PRINCIPLE TYPES ON THE BASIS OF HEATING MODE TYPES ON THE BASIS OF FEED SELECTION OF DRYING EQUIPMENT TROUBLESHOOTING APPLICATION
DRYING
PURPOSE
In pharmaceutical technology, drying is carried out for one or more of the following reasons:
To avoid or eliminate moisture which may lead to corrosion and decrease the product or drug stability
To improve or keep the good properties of a material, e.g. flow ability, compressibility
To reduce the cost of transportation of large volume materials ( liquids)
To make the material easy or more suitable for handling
Preservative
The final step in: Evaporation- Filtration- Crystallization
PRINCIPLE
Basic Mechanisim:
Migration Of Moisture From The Interior Of An Individual Grain To The Surface
Evaporation Of Moisture From The Surface To The Surrounding Air
TYPES OF DRYER ON THE BASIS OF HEATING MODE
Convective/Direct dryers/ADIABATIC
Conductive /Indirect dryers/NON-ADIABATIC
Dryers by radiant energy
TYPES OF DRYER ON THE BASIS OF FEED
•TRAY DRYERS
•SCREEN-CONVEYOR DRYER
•ROTARY DRYER
•TOWER DRYER
•SREW-CONVEYOR DRYER
•FLUID BED DRYER
•FLASH DRYER
•SPRAY DRYER
•THIN FILM DRYER
•DRUM DRYER
DRYERS FOR SOLIDS
o TRAY DRYERS
oScreen-conveyor dryer• A layer 25 to 150 mm(1 to 6 in) thick of material to be dried is
slowly carried on a traveling metal screen.
• The chamber consists of a series of separate sections, each with
its own fan and air heater
• At the inlet end of the dryer, the air usually passes upward 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
o Advantage:It is quick method because of its large surface area, material is dried in few seconds.
oRotary Dryer
• A rotary dryer consists of a revolving cylindrical shell, horizontal or slightly
inclined towards the outlet.
• Wet feed enters one end of the cylinder; dry material discharges from the
other end.
• As the shell rotates, internal flights lift the solids and shower them
down through the interior of the shell.
• 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.
oAdvantage:
◦ It is a batch process rather than continuous. Each batch can be
dried separately and same drying equipment can be used for
drying the variety of materials.
oTower Dryers
• Tower dryer contains a series of circular trays mounted one above the
other on central rotating shaft.
• Solid feed drop on the top most tray is exposed to a stream of hot air gas
that passes across the tray.
• The feed travel in this way through the dryer, discharging as product from
the bottom of the dryer.
oAdvantage
These type of dryers are used for continuous drying of powders and granules
on large scale and several tones of drugs can be dried per hour(production
capacity is increased).
oScrew-conveyor dryer
◦ Shell diameter is 3 to 24 inches
◦ Shell length is up to 6 meters
◦ Coefficient of heat transfer depends upon
◦ Loading of shell (10-60%)
◦ Speed of screw(2-30 rpm)
◦ Range of heat transfer coefficient 15-60 W/m²-ºC
oAdvantages:
◦ Screw-conveyor dryers are used
◦ For too fine and too sticky materials
◦ For recovery of solvent ,that is why these conveyors are also
called “Desolventizers”
oFluid-Bed Dryer
oFlash Dryer•Residence time 3-4s
•Max. temperature of gas=650ºC
•Used for heat sensitive materials
•A=feed entrance
•B=furnace
•C=hammer mill
•D=cyclone
•E=vent fan
•F=star feeder
•G=solid divider
DRYERS FOR SOLUTIONS &
SLURRIES
o SPRAY DRYERS
◦ A spray dryer mixes a heated gas with an atomized (sprayed)
liquid stream within a vessel (drying chamber) to accomplish
evaporation and produce a free flowing dry powder with a
controlled average particle size.
o ADVANTAGES:
• Short drying time,
• Single step drying process
o THIN-FILM DRYERS
• High thermal efficiency
• Relatively expensive
• Limited in heat transfer
◦ area
o DRUM DRYERS
◦ The rotating drums drag the slurry around their slowly-revolving
peripheries while knives peel off the dried product, which falls into
a conveyor.
Selection of Drying Equipment◦ It depends on;
• Physical /Chemical properties of the product.
• Production Capacity (Kg/hr)
• Initial Moisture Content and final moisture required.
• Particle Size Distribution.
• Temperature & Drying Characteristics.
• Explosion & Toxicological Characteristics.
The following are some general considerations for the selection of dryer.
◦ The dryer must also operate reliably, safely & economically
◦ Operating & maintenance costs must not be excessive
◦ Pollution must be controlled
◦ Energy consumption must be minimized
It is always wise in case of uncertainty, to run preliminary tests to
ascertain both design & operating data & also suitability of dryer for
particular application.
The final choice is then made on the basis of operability ,capital &
operating costs.
Industrial Dryers
TroubleshootingProblem Cause Solution
Material Degrades Very High Temperature low down the temperature or distribute the particles
High Drying Time Material contains high moisture content
Prefer using twice drying
Reduced Particle Size High residence time Reduce residence time
High Moisture content in Dried solids
Due to internal diffusion of particles
Reduce residence time
Material not fluidizing Powders develop electrostatic charge
Use efficient electrical earthingof drying chamber
Surface shrinkage Due to high temperature Make temperature optimized
Applications◦ Milk Powder
◦ Ceramic Slurry
◦ Magnesium Chloride
◦ Molasses
◦ Poly Aluminum Chloride
◦ Enzymes
◦ Catalysts
◦ Calcium Chloride
◦ Instant Tea/Coffee
◦ Instant Coffee
◦ Pastes
◦ Zeolites
◦ UF Resin
◦ Fruit Juice Powder
◦ Protein Hydrolysate
◦ Polyvinyl Acetate
◦ Dyes
◦ Detergent Powder
◦ Herbal Products
◦ Polymers (PVC,PP ,PS,PMMA,PE)
Extruded PET pelletsUrea prills
Roasting cereal