How O'Brien Agglomerators Work
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All agglomerators are used to increase the particle size of powders. There are two basic types of true agglomerators: compaction and noncompaction. Other unit operations such as crystalizers or high shear dispersers, such as Shugi or Spray Towers, are not true agglomerators.
The compaction types use mechanical pressure (and often very high pressures) to "press" the powders together. Binders are usually not needed for these. Compaction agglomerators are highly specialized, and not addressed here. Noncompaction agglomerators always need a binder.
Noncompaction agglomerators keep the powder base in motion during the binder spray. Powders of different size or density have a natural tendency to "demix" or segregate as they move. O'Brien agglomerators induce a gentle mixing in three directions. These directions are (1) as particles normally move the length of the agglomerator, (2) as the particles "fall" off the lifting bars, and (3) a recycle motion due to a internal spiral. Taken together, these motions keep the powder base well mixed using a gentle. low shear action. The binder is sprayed onto this well mixed powder bed to permanently glue the powders as single beads This is unique among all other agglomerators on the market.
Binders are specially selected to hold the base powders together as they create a larger agglomerate. As a general rule, more binder makes a larger particle. Binders can be any material that forms a solid from a sprayable liquid. Binders can be amorphous liquids (such as liquid silicates), that harden when a small amount of water evaporates, gels that "set" as they cool or dry, poly-organics that form a solid when drying, sugars that bind as they crystallize, lignon that hardens with evaporation, and many others. Depending upon the process, binders are usually no more then a few formula percent.
O'Brien Agglomerator Equipment Details
Below are two drawings of an O'Brien Agglomerator cross section:
The left drawing is from the original agglomerator patent. The right drawing is a simplified version for ease of explanation.
These cross sections show the lifting bars , "Bars" in the right hand drawing, #60 in the patent drawing. These bars slowly rotate with the drum. They lift the powder a short distance. As the powder falls off the bars, it forms the Falling Powder Curtain. This stops demixing.
Not shown is the internal spiral ribbon. It is a continuous metal spiral ribbon positioned between the bars and the shell of the drum. When the drum rotates counterclockwise, the ribbon pushes powder towards the back of the drum. When the drum rotates clockwise, it pushes the powder bed towards the front of the drum to empty the product.
The O'Brien ribbon keeps the powder bed well mixed at all times. Other agglomerators and mixes exhibits "plug-flow" powder flow characteristics, (powders first in are the powders first out - no recycle mixing). This eliminates any recycle of fine, unagglomerated particles in that other equipment.
The agglomerator has a single motor, usually one to eight hp, depending upon drum size. A slight negative air flow is used to reduce the nuisance dust. In some cases, larger airflows are used to help the powder to dry. The agglomerator can be completely sealed if required.
The O'Brien design provides process and economic advantages for both large scale, commodity type materials, (such as detergents, fertilizers and other large volume organic and inorganic chemicals) as well as single batches as small as one cubic foot of material. O'Brien is the only agglomerator that provides the user with maximum control over powder characteristics. Depending upon the chemicals used, this control includes density and porosity while providing a homogenous mix.
Control of Particle Final Characteristics
Physical properties of the final product are controlled by the amount, position and spray nozzle size, the intensity of the spray (spray pressure), rotation speed of the agglomerator drum, and other chemical and physical conditions. Fifty years of experience has shown that the fastest and surest way to determine final properties is through test runs in a Benchtop Agglomerator.
The O'Brien design minimizes routine maintenance. Since only the powder bed of particles are sprayed, the amount of "over-agglomeration" and build-up within the agglomerator is reduced, thus reducing the frequency of clean-ups.
Why and when an O'Brien type agglomerator is better.
This unique powder movement within the O'Brien produces a more consistent particle size, product density, porosity and other desired characteristics. This also allows a control over final product properties not available in any other equipment.
The internal recycle provides a residence time within the agglomerator of 20 minutes and more. In comparison, other agglomerator types, such as a pan agglomerator or Shugi-type mixers, provide only a few seconds of residence time. Hardly enough time to let any reactions come to equilibrium, much less provide a good mix. The O'Brien Agglomerator works best in situations where low investment, low operating and low maintenance costs are significant. Personnel training time is low due the simple, intuitive nature of the agglomerator.
The same O'Brien agglomerator can be used for batch processing or on a continuous production basis. Flexibility and potential reuse of equipment is high.
Scale up from a Benchtop Agglomerator to a larger unit is relatively easy. The internals and liquid/powder contact characteristics are so-called "1:1 dynamically similar" among all units.
The O'Brien design was first patented in 1965. Every O'Brien agglomerator ever made is either still in operation, or capable of operation. These things are rugged and a good investment.
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