Feature

A Novel Way to Convert Molten Material to Granules

Posted on 08 February, 2010 | Tags: Particle Processing

Spray Cooling is a one-step continuous particle-processing operation to transforms feed from a molten state to solid form.  The process is used by many industries to meet the desired product specifications.

-Amol Atre & Sourabh Gupte

Spray Cooling or Spray Congealing is the transformation of feed from a molten state into a solidified form by spraying the feed into a cold drying medium. It is a one-step, continuous particle-processing operation involving solidification. The feed has to be in a molten state above its Melting Point. The resulting cooled product conforms to powders, granules or agglomerates, the form of which depends upon the physical and chemical properties of the feed and the cooler design and operation. Spray Cooling is a procedure, which in many industries is meeting product specifications most desirable for subsequent processing or direct consumer usage.

What is Involved in Spray Cooling?
Spray Cooling involves atomization of the feed into a spray and contact between spray and cooling medium resulting in solidification. The cooling of the spray proceeds until the desired temperature in the cooled particles is obtained and the product is then recovered from the cooling medium. These four stages are illustrated by reference to the open-cycle, co-current spray-cooler layout, the most common type of Spray Cooler in industry (see Figure 1).

Atomization
The formation of spray (atomization) and the contacting of the spray with air are the characteristic features of Spray Cooling. The selection and operation of atomizer is of supreme importance in achieving economic production of top quality products. The atomization stage must create a spray for optimum solidification conditions leading to a dried product of required characteristics.
Rotary Atomizers and Nozzles are used to form sprays. With rotary atomizers centrifugal energy is used and with nozzle atomization pressure energy is utilized.

• Rotary Atomizers
  Feed is introduced centrally on to a wheel or disc rotating at speed. The feed flows outwards over the surface, accelerating to the periphery. Feed, on leaving the periphery readily disintegrates into a spray of droplets. Rotary atomizers are reliable, easy to operate and can handle fluctuating feed rates. Atomizer wheels have negligible clogging tendencies due to large flow ports. One of the most important features is the ease of particle-size control merely through wheel-speed control.
• Pressure Nozzles
  The feed is fed to the nozzle at high pressure. Pressure energy is converted to kinetic energy and the feed issues from the orifice as a high-speed film that readily disintegrates into a spray as the film is unstable. The feed is made to rotate in the nozzle, resulting in cone-shaped spray patterns. Variation in pressure gives control over feed rate and spray characteristics.

Spray-Air Contact
A Spray Cooler provides a means of heat removal from the feed. The cooling air can be normal atmospheric air or dehumidified and cooled air depending on the feed properties.
The manner in which spray contacts the cooling air is an important factor in spray cooler design, as this has great bearing on product properties by influencing droplet behavior during solidification. The basic types are co-current, counter-current and mixed flow patterns. The chamber design and method of air introduction via the air disperser is selected according to (a) the required particle size and (b) the required particle form.
While most of the spray cooling applications utilize air and exhaust the air to atmosphere, there are cases when an inert gas (e.g.: Nitrogen) is used as the drying medium instead of air. The inert gas prevents fire, explosion or chemical reaction of the feed or product.

Solidification
As soon as the droplets of the spray come into contact with the cooling air, heat is transferred from the droplets to the air. The air has to remove heat in the form of (a) specific heat of liquid for cooling the feed, (b) latent heat of solidification of the product and (c) specific heat of solid for cooling the product. This is shown in a Theoretical Solidification Curve (see Figure 2).
The heat removal depends on the rate of heat transfer in the droplet and the rate of heat transfer between the solid and the air.
During solidification, the atomised spray distribution undergoes change. Different products exhibit different evaporation characteristics. Some tend to expand; others collapse, fracture or disintegrate, leading to porous, irregularly shaped particles. Others maintain a constant spherical shape or even contract, so that particles become denser. The extent of any change in particle shape and hence the powder characteristics, are closely related to the Solidification Curve.

Separation of Product from Cooling Medium
Product separation from the cooling air follows completion of the Solidification stage, when the product particles remain suspended in the air. In the system shown in Figure 1, majority of the product falls to the base of the chamber, while a small fraction passes out entrained in the air and is recovered in the separation equipment. Such equipment is usually cyclones followed by bag filter or wet scrubber. The choice of equipment depends upon the powder loading of the air leaving the chamber and acceptable recovery efficiencies.

Applications and Advantages
Typical applications of spray cooling include waxes, fats, glycerides, hydrates, inorganic and organic melts, steric acid or stearates, quaternary ammonium compounds and encapsulated materials etc.
Though there spray cooling has a wide scope of applications, but high installation costs are a constraint. Moreover industrial units are physically larger per unit output than other methods of obtaining powder that makes fabrication of spray coolers expensive. Furthermore these require expensive buildings and / or supporting structures.
However the spray cooling offers many advantages over grinding or crushing for producing powders. This is a continuous and easy process and the operation is adaptable to fully automatic control with fast response time. The specification or powder quality remains constant throughout the entire cooling operation irrespective of the time when cooling conditions are held constant. Product specifications can be met through selection of appropriate design and operation for the availability of wide range of designs and layouts. Feedstock in the melt form can be handled and pumped irrespective of the fact whether it is abrasive or corrosive. These systems can be designed as per individual capacity requirement.

Amol Atre is CEO Elite Thermal Engineers Pvt. Ltd.

 

 

Sourabh Gupte is Business Development Executive Elite Thermal Engineers Pvt. Ltd.