Spray Cooling / Spray Congealing

Spray Cooling

The transition of a melt from a soft or fluid state to a rigid or solid state by cooling is called congealing. The various droplet formation techniques and the efficient droplet / air contact make the spray drying concept ideal for making spherical particle powder by congealing of melts.

Reference Products:

• Encapsulated materials
• Fats
• Glycerides
• Hydrates
• Inorganic / organic melts
• Quaternary ammonium compounds
• Stearic acid/stearates
• Waxes

Flash animation of a spray cooler with prilling atomizer

The selection of a spray cooling plant design depends upon the particle size specified as well as the need for post-cooling / crystalization for the cooled product:

Theoretical Solidification Curve

• Fine particle products with mean particle size 50-150 micron;co-current conical based cooling chamber with high-speed rotary atomization.
• Coarse particle products with mean particle size 150-300 micron;conical based cooling chamber with fountain nozzle.
• Extra coarse particle products with mean particle size 300-1000 micron; Atomization by low-speed rotary atomizer in cooling chambers of large diameters with flat-base or a 2-stage fluidized bed base design. In the flat bottom execution, the product particles fall onto the floor of the chamber and leave with the exhaust air through the slowly-rotating broom-like exhaust air duct. In the 2-stage fluidized bed design a rotary cup- or Bucket-type atomizing.
• Slowly solidifying products: In cases where melts are slow to solidify, the cooling time available in the spray chamber may be insufficient and particles can leave the chamber without being completely solid, stable and cool, which leads to product caking and lumping during storage. A second stage is then required to complete the operation, either by fluid bed cooling or pneumatic conveying.

Recent developments in spray cooling include the use of congealing chambers with integrated bag filters for congealing and separation of product in one unit. This design is very compact and thus space-saving.

Spray Cooling

Nozzle Atomization

Rotary Atomization

Spray cooling (alternatively spray congealing) is defined as a process by which a melt is formed into particles of spherical shape by spraying the melt into a cooling chamber through which ambient or cooled, low temperature air is passing. Rotary (wheel) atomizers and nozzles are used to spray the melt. Spray cooling finds applications in the chemical, food, and pharmaceutical industries. It is a most convenient method of transforming melted feedstocks into free-flowing particulates of controlled particle size.

Importance of melt properties

Figure 3 Solidification Curve

Products that form melts have a solidification curve. When a spray of melt droplets contacts a cool air environment, the melt cools to a solidification temperature. Congealing then takes place at a constant temperature during release of the product's heat of solidification. When no longer in a fluid state, the droplets further cool to give stable solid particulate forms.

Some products do not have a clearly defined solidification point. The phase change may take place over a range of temperatures or the product may go from a melt phase to solid amorphous form without the release of heat of solidification, since a non-crystalline formation occurs. The droplet may sub-cool below the solidification temperature before hardening suddenly. Data on the physical properties of the melt and behaviour during solidification is important to size the cooling chamber, select the atomizer, and determine whether the cooling should be conducted in one or two stages.

Design selection based upon the solidification properties of the melt

The most suitable second stage cooler is a fluid bed supplied with cooled ambient air. Alternatively, a pneumatic transport system with cooled conveying air might be suitable.

Utilizing closed cycle spray cooling enables optimization of cooling energy requirements and product quality

Typical Products from Nozzle Atomization and Rotary Atomization

Plant operation in warm and humid climates will normally require the ambient air supply to the spray cooler to be dehumidified and cooled. To reduce the extra energy consumption involved, it is common practice to recycle the process air, thereby minimizing the load on the dehumidification/cooling unit.

The recycle system is designed to permit a bleed of air from the system, enabling an intake of ambient air in cases, where there is a degree of evaporation associated with the spray cooling. The multi-way damper incorporated in the recycle system also allows the plant to run in open cycle without the dehumidification / cooling unit in operation during the colder and dryer seasons of the year. This reduces operational costs.

Particles

Design selection is based upon particle size requirement, ranging from fine over coarse to extra coarse products.

Plants

Over the years Niro have supplied a number of plants specially designed for spray cooling and / or congealing of various products. The majority of the plants has been specially designed for individual needs of the clients.

It is the intention of the Niro organization to suit the special demands or needs in this area of processing.

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