Rewet agglomerator chamber
Rewet agglomerator chamber

Agglomeration is getting smaller particles to adhere to each other to form bigger conglomerates/granules or agglomerates. Agglomeration is performed as a one step operation during the spray drying “straight through” process with fines return or in the Fluidized Spray Dryer FSD™ with or without fines return.

 

 

Agglomeration can also be performed in a two step operation as in the rewet agglomeration process. Or as a combination of the two in the MSD-GRANULATOR™.

Generally the process of any type of agglomeration involves:

  • Bringing independent particles into contact with each other
  • Enhancing or controlling particle adhesion
  • Stabilizing the created agglomerates – in some cases (REWET agglomeration) under the action of external forces

Assuming that agglomeration during the one step spray drying process occurs as a series of binary interactions, the rate of agglomeration – Gij – of I and j type particles can be expressed by Hogg´s formula from 1992:

Gij = Kij Eij ni nj

  • Kij is the collision frequency rate constant, determined by fines return system designs
  • Eij is a collision efficiency factor i.e. probability of particle adhesion and their ability to “glue” together depending on their chemical composition, moisture content, temperature and thermoplasticity
  • ni and nj are the respective number concentration of the two types of particles e.g. the concentration of the returned fines particles ni and primary spray particles nj

There are many requirements for powdered products. Some of the most important ones and those that are controlled by agglomeration are that powders must be:

  • Instantly soluble in hot and/or cold water 
  • Free flowing 
  • Non dusty

These properties can be met if the powder is present in agglomerated form.

During the spray drying process, the type of agglomeration is defined as

  • Primary or secondary 
  • Spontaneous or forced

...and their combinations are described below:

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 atomizer zone Normal type when Fines Return is applied

After the agglomeration, the agglomerated powder usually needs to be re-dried and cooled. This is typically done in a VIBRO-FLUIDIZER™ and to avoid attrition and/or braking down the agglomerates, the perforated plate is selected carefully. It is recommended to convey the final agglomerated powder carefully in the subsequent powder handling system.

Many powdered materials, for example coffee powder, consist of single particles when they are discharged from the coffee spray dryer. Agglomeration by the REWET method produces an agglomerated instant coffee powder similar to that from a freeze dryer.

The REWET agglomeration process is divided into the following processing steps:

  • Wetting of the surface of the particle (droplet agglomeration using a liquid, e.g. water or surface agglomeration using steam or humid air that condenses on the surface of the particle)
  • Agglomeration with or without mechanical force depending on the composition of the product 
  • Re-drying and cooling typically in a VIBRO-FLUIDIZER™ 
  • Sizing

Learn more about REWET agglomeration here.

The two different agglomeration methods can be combined as in the MSD-GRANULATOR™, where a liquid can be sprayed onto the fluidized powder layer to enhance the agglomeration and non agglomerated powders can be added to the process via the fines return system.