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.
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.
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.
If the desired powder specifications are not achieved in a single stage spray drying, multi stage operations are necessary. Fluid bed assemblies act as the second and/or third stages to conduct product agglomeration and/or further conditioning of the product.
In the conventional two-stage processing, vibrating fluid beds are installed outside of the drying chamber and act as the second-stage.
In the modern three-stage processing, the drying chamber has an integrated fluid bed as a second-stage. The third stage is an externally mounted vibrating fluid bed. Multi-stage drying gives the possibilities of product agglomeration, better control over residual moisture of the powder and improved thermal efficiency.
SPRAY DRYING AGGLOMERATION
Agglomerate is composed of two or more particles adhering to each other. Agglomeration is often desired in the spray-drying operation due to the physical characteristics of agglomerates, such as improved dispersibility and free flowing properties.
Agglomerates can be created, when specifically desired, by
- Contacting the evaporation spray with dry product fines (mixed-flow)
- Installing special agglomeration equipment directly to the drying chamber (e.g fluid bed)
Fluidized bed dryers are one of the most common types of dryers used for producing dried particles (powders). Their advantages are low initial capital investment, simple structure and low maintenance costs.
Consider a bed of particles initially at rest, supported by a perforated plate. Gas is introduced through the perforated plate and as the gas flow increases, at some point the particles become freely suspended in the air stream and the bed begins to expand.
VIBRO-FLUIDIZED BED DRYERS
If order to overcome problems of the fluidization, especially with products that have significant amount of oversized particles, in vibro-fluidized bed dryers the distributor is vibrated. This allows keeping the gas flow velocities low, since the large particles are kept moving by the vibrations of the distributor. For this same reason, this design is often used for beds consisting entirely of large particles.
VIBRATING FLUID BED DRYERS
With some difficult to handle products, one option is to install a mechanical agitator to the base of the dryer. The agitator causes to break up most of lumps and mix them into the dry powder.
Fluid bed drying in both stationary and vibrating bed types is another RALLI OY (Ltd.) specialty. Plug flow, mixed flow and multiple-stage designs are available. Selection is based on through test work directed to find the best design for the desired product. Many times fluidized bed dryers are used in combination with spray dryers or flash dryers to form multi-staged drying systems.