Spray drying is transformation of feed from a fluid state into dried particulate form by spraying the feed into a hot drying medium. It is a continuous particle-processing drying operation. The feed can be a solution, suspension, dispersion, emulsion or slip. The dried product can be in the form of powders, granules, or agglomerates depending upon the physical and chemical properties of the feed, the dryer design and final powder properties desired.
Feed is finely atomized and is introduced in the drying chamber along with hot air. The mixture of atomized feed and hot air moves towards the air exhaust of drying chamber. The time taken by this mixture to travel up to air exhaust is called as residence time of drying. During this residence time, the feed droplets loose its moisture to the hot air and is converted into dry powder particles. Hot air absorbs this moisture, so its absolute humidity increases while its temperature is reduced. The feed droplets while loosing its moisture to hot air, remain at temperatures much below the hot air temperature and it is exposed to hot air for a very short time. Hence spray drying is essentially known as "Low Temperature Drying". The temperature of feed droplet is the wet bulb temperature at the prevailing relative humidity.
The dry powder falling on to the conical portion of drying chamber slides down to bottom of drying chamber with the assistance of vibrators/knockers and is collected in collection bag/bottle through the rotary valve located at the bottom of drying chamber.
The air leaving the drying chamber entrains some of the dry powder which is recovered in the cyclone separator and collected through rotary airlock. The air leaving the cyclone separator still has traces of dry powder, which is further recovered by scrubbing in wet scrubber or passing it through bag filter.
In wet scrubber the dry powder particle are wetted by water and the resultant mixture is separated in straight cyclone. The air coming out of the straight cyclone is dust free and absolutely clean, which is exhausted to atmosphere through induced draft fan. In the bag house the exhaust air is filtered in fabric filter bags. Dust laden bags are cleaned during normal running cycle by compressed air pulse jet or vibrators or shakers.
Air from scrubber cyclone or bag house is sucked by induced draft fan. The forced draft fan at entry point of air circuit and induced draft fan at exhaust point of air circuit operate the air handling system on push-pull principle which avoids extreme pressures (either positive or vacuum) in the drying chamber, cyclone, scrubber/filter.
Ideally Suited for:
- If the product is heat sensitive and non sticky powder is desired.
- Free flowing powder is required which is a unique feature of spray drying.
- Free flowing powder is desired from the extremely sticky slurry.
- For bulk production of powder.
Granulated powder is to be produced from slurry/slip.