selection of drying equipment

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