SPRAY DRYING

 

OPEARATION

In spray dryers the liquid feedstock is first pumped into the atomization unit, which is located on top of the drying chamber. The atomization unit, wheel or nozzle, produces a spray of desired droplet size distribution. As the droplets come into contact with hot drying air, which is introduced into top part of the drying chamber through drying air distributor, evaporation takes place. The moisture removal continues as the droplets (particles) are conveyed by the drying air towards the bottom part of the drying chamber. Due to short product contact times, the powder (particle) temperatures are kept at low level, making spray drying a suitable drying technique for heat sensitive products.

 

ATOMIZATION

In spray dryers two main type of atomizers are used, which are wheel atomization (centrifugal) and pressure nozzle atomization.

 

Wheel atomization

In wheel atomization the feed is pumped into the centre of a wheel rotating at high speed. The wheel contains vanes that direct the feed into the peripheral part of the wheel. As the feed leaves the wheel edge, atomization occurs immediately. Droplet particle size, for a given feed, is affected both by the wheel geometry and also by the wheel rotation speed.

 

Pressure nozzle atomization

The pressure nozzle atomizers have two functions 1) to create liquid rotation at the nozzle head and 2) to provide an orifice through which the liquid is discharged as a conical spray. The size of the above mentioned components and the physical liquid properties define the droplet size distribution. Thus both the nozzle geometry and the pumping pressure affect the distribution.

 

DRYING AIR

The drying air can be direct of indirect, depending on the process needs.

 

AIR BROOM AND SONIC HORN

Depending of the physical properties of the product to be dried, the powder might have a tendency to start accumulating on the drying chamber walls. In order to avoid the accumulation of the powder an air broom and/or sonic horn can be utilized.

 

POWDER HANDLING AND MULTIPLE STAGES

The majority of the powder (particles) formed during the drying process fall to the outlet of the drying chamber. A small amount of the material remains in the drying air and is recovered by externally mounted cyclones. Subsequent powder handling depends whether single, double or multi-stage processing is used. If no subsequent drying of the product is needed, in a single stage system the product is normally passed to a pneumatic conveying system.

If the desired powder specifications are not achieved in the single stage (spray drying), fluid bed can act as a second and/or third stage.

 

 

RALLI OY (Ltd.) designs and manufactures single or multi-stage spray dryers with centrifugal and nozzle atomization. With a high degree of perfection Ralli Oy (Ltd.) has delivered these drying systems for several industrial applications. Full control over powder properties and an aggravated efficiency in energy consumption are the main features among others, like easy operation etc.