In spray drying there are two ways of agglomeration: the spontaneous and the forced, both in a primary and secondary form. (Fig. 86)
During the spray drying process the aim is to produce particles with a big surface/mass ratio, i.e. small particles. The reconstitution in water of a powder consisting of small particles is however difficult and requires intensive mixing in order to disperse the powder, before it is totally dissolved. Bigger particles exhibit a better dispersion, but the solubility is negatively affected during the drying operation, as discussed on page 123 forward.
By agglomeration both a good dispersion and a complete solution are obtained.
TYPE DEFINITION EXAMPLES
Spontaneous primary Random, unprovoked collision of primary spray particles All atomization devices
Forced
primary Intended collision between primary spray particles from different atomization devices Collision of sprays from different nozzles
Spontaneous secondary Random, unprovoked collision of primary spray particles and fines Multi-Stage or Integrated Filter Dryers
Forced secondary Intended collision between primary spray particles and fines returned to the atomization zone Normal type when Fines Return is applied
THE SPONTENEOUS PRIMARY AGGLOMERATION
This typer of agglomeration is a result of a random unprovoked collision of particles in a single atomizer cloud due to particles of different diameter having different deceleration paths. It takes place in both nozzle and rotary atomizers. See Fig. 86a.
Fig. 86a Spontaneous primary agglomeration
THE FORCED PRIMARY AGGLOMERATION
This type of agglomeration is a controllable means for production of an agglomerated product with certain properties, by for example collision of particles from two or more atomization clouds, typically in a multi-nozzle unit, where the sprays from the individual nozzles are forced into each other. See Fig. 86b.
Fig. 86b Forced primary agglomeration
THE SPONTANEOUS SECONDARY AGGLOMERATION
The type of agglomeration is a result of a venturi effect at the drying air inlet to the chamber, whereby dry single particles are sucked into the wet atomizer cloud. Moist particles colliding with air-borne dry particles contained in the exhaust air on its counter-current way out of the MSD/IFD dryer. See Fig. 86c.
Fig. 86c Spontaneous secondary agglomeration
THE FORCED SECONDARY AGGLOMERATION
This type of agglomeration is a controllable means for agglomeration by returning fines to the atomizer cloud, via the fines return. The spontaneous agglomeration, which will always exist, is enforced by the agglomeration applied by returning the fines to the atomizer cloud. By definition fines are the cyclone or bag filter fractions and consist of the smallest particles, which are returned to the process. The small dry particles are introduced into the dryer near the atomizing device, where they will meet and collide with atomized wet droplets thus forming agglomerates consisting of many particles stuck together having a size of 100-500 microns, depending on the parameters selected. See Fig. 86d.
Fig. 86d Forced secondary agglomeration
Due to the special air flow pattern in an MSD/IDF plant a considerable spontaneous, secondary agglomeration takes place. For production of high quality instant whole or skim milk powder this spontaneous agglomeration suffices, and the fines are just returned to the integrated fluid bed, from where they will get airborne again and reach the atomizing zone again. However, the agglomeration may be further enhanced by forced, primary agglomeration (collision of sprays overlapping each other from different nozzles in a multi-nozzle atomization unit) and/or by returning the fines to the atomization zone (forced, secondary agglomeration). Further flexibility can be gained by designing the atomization unit in a way that allows the distance between the single nozzles or between the nozzles and the fines return tube to be altered.
Depending on the atomization device the fines return is designed in different ways:
For Rotary Atomization
The aim is to bring the fines as close as possible to the atomizer wheel. This can be done from below, see Fig. 87, via a pressure conveying system using a 3-4" dosing pipe with a fines distributor at the end inside the drying chamber. However, deposits are easily formed on this pipe, if the air disperser is not adjusted to avoid it. This adjustment is, however, not necessarily optimal from the drying point of view.
Fig. 87 Fines return rotary atomizer "old type"
In modern dryers fines are therefore introduced from above in through the air disperser (FRAD System) via 4 fines pipes situated just above the atomizer cloud. Deflector plates at the end of each fines pipe ensure a correct introduction and distribution of the fines. See Fig. 88.
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Fig. 88 Forced secondary fines return for rotary atomizer FRAD
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For Nozzle Atomization
The fines return is an integral part of the nozzle unit with the fines duct in the centre surrounded by nozzles at the periphery. The fines are introduced tangentially into the fines distribution duct or through a center pipe, see Fig. 89. The nozzles can be welded to the nozzle rod at a certain angle, so that by turning the nozzle rod around its axis the collision point can be altered.
Fig. 89 Forced secondary fines return for nozzle atomizer
SEPARATION
is the process separating the part of fines which is entrained in the main drying air leaving the drying chamber. The efficiency of separation is determined by the air flow pattern and air velocities in the drying chamber and is therefore closely related to the chamber design and can only be marginally affected at normal running conditions, f.inst. by air disperser adjustments and variations in drying air rates.
The agglomerated powder leaves the chamber at the base or from the integrated fluid bed and enters the Vibro-Fluidizer. During the passage down the wall of the chamber cone some stabilization of the already induced agglomeration takes place. In the static fluid bed and/or the Vibro-Fluidizer the powder is met by a warm airstream evaporating the excess moisture content, as was the case in the two-stage drying process.
ATTRITION
is defined as the partial break down of agglomerates in fluid beds or powder conveying systems resulting in creation of either fines and smaller agglomerates (abrasion) or of a number of smaller sized particles (fragmentation). This often overlooked phenomenon is the result of mechanical motion between the agglomerate and another body which may be the walls of the fluid bed or another particle. The most likely cause of attrition in fluid beds is particle/particle interaction, as interparticle impact velocities can be very high, caused by high air jet velocities out of the holes in the perforated plate that forms the bottom of the fluid bed. Factors affecting the extent of attrition is the jet velocity, determined by the pressure difference across the perforated plate, the fluidization velocity and the actual design of the perforated plate.
CLASSIFICATION
is defined as the separation of fines in fluid beds. The efficiency of classification is mainly determined by the fluidization air velocity, but also fluid bed design features are of importance in securing that separated fines are kept airborne and entrained in the exhaust fluid bed air.
After the final drying the powder enters the cooling section where the powder is cooled by means of air at ambient temperature followed by cooled, dehumidified air. The powder is finally passed over a sifter where any oversize particles are removed. It is also possible to install a sifter with two nets thus removing any remaining particles/agglomerates of small diameter. Together with the fines, this fraction may be returned to the atomizing device thus producing a powder with a well defined agglomerate size distribution. The fines removal in the fluid beds is, however, regarded as sufficient from a product point of view, and plants with above mentioned sifter are only used when particular product specifications have to be met. Fig. 90 shows a plant set-up featuring evaporator and spray dryer.´
Fig. 90 Complete plant for production of agglomerated products (MSDTM)
AGGLOMERATE STRUCTURE AND POWDER PROPERTIES
Depending on the design and adjustment of the fines return system - particularly the location of the introduction of the fines in relation to the atomization device - different agglomerate structures result, which influences certain powder properties, such as bulk density, mechanical stability, dispersibility and slowly dispersible particles.
The relation between agglomerate structure and certain powder properties is illustrated in Fig. 90a.
AGGLOMERATE STRUCTURE:
Onion ---> Raspberry ---> Compact grape ---> Loose grape
PARTICLE MOISTURE CONTENT AT COLLISION:
High -----------------------------------------> Low
MECHANICAL STABILITY:
High-----------------------------------------> ;Low
BULK DENSITY (no attrition):
High-----------------------------------------> &nbs p;Low
BULK DENSITY (after attrition):
High----------------> Low------------------> &nbs p; High
SLOWLY DISPERSIBLE PARTICLES:
Many-----------------------------------------> FewDISPERSIBILITY (after attrition):
Poor ----------------> Good----------------> Poor
Fig. 90a Agglomerate structure/powder properties relationship
If the fines are introduced close to the atomizing devise the moisture content of the primary spray particles is high and thereby their plasticity and stickiness, and the fines particles may penetrate primary particles or be completely covered by concentrate (Fig. 90b). Such agglomerates have been termed 'onion'-structured. When collision takes place at a progressively longer distance, away from the atomizing device, less compact agglomerate structures are obtained. Such structures have been termed 'raspberry'- and 'grape'-structures in decreasing order of compactness.
Fig. 90b Onion-structured agglomerate
'Onion'-structured agglomerates are characterized by a high mechanical stability and a high bulk density, but they will often appear as slowly dispersible particles after reconstitution. They may also be collected during the different dispersibility tests in use and jeopardize the general quality evaluation of the product.
With progressively looser agglomerate structures the bulk density and mechanical stability decrease gradually, and the overall instant properties improve. However, if a 'loose grape'-structure is eventually obtained, the mechanical stability may be so low that the powder becomes very susceptible to attrition resulting in deteriorated instant properties. A 'compact grape'-structure (Fig. 90c) is regarded as the ideal compromise where the powder has simultaneously good instant properties and sufficient mechanical strength to enable necessary transport and packaging.
Fig. 90c Compact grape structured agglomerate
The agglomeration is improved by:
- High solids content in the concentrate
- Bigger quantity of fines returned to the atomizing device
- Fines introduction closer to the atomizing device
- Shorter distance from nozzle to fluidized layer in a static fluid bed
- Higher moisture content from the primary drying stage
- Bigger primary particles
- Lower pasteurization temperature of the milk prior to the evaporation
When leaving the sifter the powder should not be exposed to strong mechanical conveying, neither by means of air nor by fast moving mechanical screws. However, today's lenient vacuum-low speed air systems are used without too much damage to the agglomerates. The best thing, however, is to install the plant so high that filling into bags or tote-bins is possible by gravity.