Fig. 91. Peebles' Rewet Instantizer
means getting smaller particles to adhere to each other to form a powder consisting of bigger conglomerates/agglomerates, which are essential for easy reconstitution in water.
Since many powders may become instant by mere agglomeration, many processes have been developed over the years to agglomerate ordinary powders consisting of single spray particles usually produced in plants with a pneumatic conveying system.
The historic development started with the pioneering research of Mr. David D. Peebles in the beginning of the fifties and instantized non-fat dry milk was marketed from 1954. Soon it replaced regular spray dried products on the retail market. Peebles' instantizer is shown in fig. 91.
Another example of a rewet agglomeration plant is shown in fig. 92, which illustrates the principle of a method developed by NestlÚ for agglomerating milk powders, chocolate-flavored beverages and soups. When agglomerating skim milk powders, the tank (1) would contain a 10% solution of skim milk solids in water cooled to about 6 ║C. The liquid is sprayed at a pressure of 20 bar by means of the pump (2) and nozzle (3), which is designed to give a flat jet with a high velocity of up to 8 m/sec. Skim milk powder, which is fed by means of a vibrator/screw conveyor (4), is dispersed over the jet where the single powder particles become rewetted to a moisture sufficiently high (10-15 %) to make them sticky on the surface, so that any collision between particles will result in an agglomeration in the chamber (5). The powder is finally dried at 90 ║C to 3 % moisture (6) and bagged off.
Fig. 92. REWET AGGLOMERATOR™ plant (NestlÚ)
The main features of the REWET AGGLOMERATOR™ process are:
- Wetting of the surface of the particles
Wetting of the surface of the particles is done with humid air, steam, atomized water either pure or containing milk solids, sugar or other soluble components. The water may further contain additives such as vitamins (water soluble), minerals, colour and surface-active agents. The atomization of the moistening agent can be carried out by means of nozzles or a rotary atomizer. If a rotary atomizer with two feed pipes is used, it is possible to use a combination of steam and water or use two moistening agents, which cannot be mixed for various reasons.
If the product is insoluble, an adhesive can be dissolved in the moistening agent. When doing so products otherwise impossible to agglomerate can be agglomerated with a good result.
Agglomeration, whereby the moist sticky particles collide due to the turbulence and adhere to each other forming agglomerates, is essential for the rewet process.
As powders with different compositions do not behave in the same way during the rewetting and agglomeration process, different equipment is needed to obtain an optimal agglomeration.
In principle there are two ways of performing the agglomeration:
- Droplet agglomeration
- Surface agglomeration
In the droplet agglomeration process the powder particles are wetted with droplets of liquid atomized by means of a nozzle or a rotary atomizer while suspended in air, as described in Fig. 93. The powder may either be introduced around the rotary atomizer or the nozzle by means of gravity or pressure air conveying, or from below by means of pressure conveying.
Fig. 93. Feed system for a REWET plant (wetting by means of sarm water atomized by an atomizer)
The actual agglomeration takes place by collision of the single particles wetted and sticky on the surface, and when they reach the agglomeration chamber wall they will roll down whereby the compactness of the agglomerates is obtained.
Droplet agglomeration can also be performed by spraying the wetting agent through a number of nozzles positioned right above the fluidized powder layer in a VIBRO-FLUIDIZER™. To obtain stable agglomerates the powder should contain sufficient binding material, like carbohydrates.
Some powders (containing a high content of fat and sugar) become so sticky when wetted, that heavy deposits in the conical part of the agglomeration chamber develop. A mechanically revolving scraper or similar is therefore necessary in order to get the powder out of the chamber. Another and technically better solution is to let the conical part of the agglomeration chamber rotate slowly and have a stationary scraping device such as a knife scraping off the powder. The rotating cone is mainly used when the powder contains cereals and starch, such as baby foods.
The droplet agglomeration process is especially used for powders containing fat such as whole milk powder and powders with a high content of sugar such as cocoa-milk-sugar mixtures. If cold-water instant whole milk powder is produced, a lecithin dosing equipment is installed between two Vibro-Fluidizers (see page 235). However, it should be mentioned that the final product quality will never be as good as that produced on a drying plant equipped with fines return, fluid beds and lecithin dosing equipment.
In surface agglomeration, either steam or warm moist air with a high relative humidity is used as the moistening agent. The surface of the individual dry particles is wetted due to condensation of the water vapour on the colder particles, whereby the stickiness required for the agglomeration is created. The subsequent agglomeration will take place, if the particles are exposed to sufficient mechanical impact. The impact can for example be accomplished in a vortex tube into which the moistening medium is introduced tangentially and the powder to be agglomerated along the centre axis. It is very important that the humid air/steam used for the rewetting does not contain any droplets of water, as that, in combi-nation with the intensive mechanical impact, will result in overwetted agglomerates being too compact for a quick rate of rehydration.
The warm humid air is usually made by spraying steam into warm air at a given temperature to obtain a relative humidity of 100%. Any water droplets are re-moved in a demister, after which the air is heated further to give the desired relative humidity. By changing the air temperature prior to the steam injection and the subsequent air temperature, it is possible to obtain air with a given temperature and relative humidity. See fig. 94.
Fig. 94. Feed system for a REWET plant. Wetting by means of warm moist air
Surface agglomeration is mainly used for skim milk powder when large agglomerates are aimed at. The final product properties depend to a great extent on the raw material used for the rewetting, and below is shown a specification for a recommendable basis powder:
|Bulk density, tapped 1250x:
||0.80 g/cm3 |
||2-3 mg/g powder *)|
||< 0.1 ml|
|Mean particle size:
||approx. 50 μm|
|Amount above 100 μm:
*) corresponding to a pasteurization temperature of
≈90 ║C prior to evaporation.
The surface agglomeration can also be used for whole milk powder, but the agglomerates get too compact for obtaining a powder with good rehydrating properties.
As the basis powder used for agglomeration is remoistened to obtain the desired surface characteristic for an optimal stickiness, this additional moisture has to be evaporated again in order to reach the specified moisture content.
The agglomerates may break down again, if they are exposed to extensive mechanical handling, such as in a pneumatic conveying system. It is therefore necessary to perform the drying in a VIBRO-FLUIDIZER™ as mentioned on page 119. However, as the rewetted powder is to be compared to a layer of wet snow when entering the VIBRO-FLUIDIZER™, a relatively high air velocity is required to fluidize the powder. Otherwise, lumps will be formed, and the drying efficiency of the VIBRO-FLUIDIZER™ will decrease.
Like the re-drying the cooling is performed best in a VIBRO-FLUIDIZER™.
Usually, there is a well defined requirement to the agglomerate size distribution of the final powder. It is therefore necessary to sift the powder. This is done in a sieve with two different net sizes placed above each other. Thus it is possible to remove any agglomerates/lumps considered to be too big in the final product. This oversize fraction may be milled and returned to the process. Powder passing through the upper net may be further fractionated on the lower net into a main fraction and a fines fraction consisting of single particles and agglomerates being too small. This fines fraction is together with the cyclone fraction from the Vibro-Fluidizer recycled back to the process.
The actual powder feeding system consists of a silo and a screw conveyor with variable speed. The powder is by means of a rotary blow-through valve fed into a high-pressure air line conveying the powder to the agglomerating module.
It is usually a requirement to most rewet installations that they are product flexible. Therefore, both droplet and surface agglomeration methods are often installed in the same plant. The rotating cone may also easily be incorporated in the plant thus giving the possibility of agglomerating any milk based product. Lecithination may naturally also be applied to this process, if cold water fat-containing products are aimed at. In fig. 95 a flow sheet of a rewet plant NIRO design is shown, with the possibility of all above mentioned agglomeration systems.
Fig. 95. GEA Niro multi-purpose REWET agglomeration plant