Spray Drying and Atomization Overview

Spray drying

Spray drying has become the most important method for the dehydration of fluid foods such as milk, coffee and egg powder, and is also used extensively in the pharmaceutical and chemical industries. It is a method whereby solutions or slurries are rapidly dried to particulate form by atomizing the liquid in a heated chamber. Typically performed using aqueous systems, spray drying can also be undertaken with solvent-based systems under controlled conditions.

Spray drying consists of the following unit operations:

• Pre-concentration of liquid (for more economic operation, since evaporation is expensive)
• Atomization (creation of droplets)
• Drying in stream of hot, dry gas (usually air)
• Separation of powder from moist gas
• Cooling
• Packaging of product

Relatively high temperatures are needed for spray drying operations. Heat damage to products is generally only slight, however, because of an evaporative cooling effect during the critical drying period and because the subsequent time of exposure of the dry materials to high temperatures may be only very short. The typical surface temperature of a particle during the constant drying period is 45-50 °C. For this reason, it is possible to spray dry some bacterial suspensions without destroying the organisms. The physical properties of the products are intimately associated with the powder structure generated during spray drying. It is possible to control many of the factors that influence powder structure in order to obtain the desired properties.

The cyclone spray dryer is the most commonly used. Typically, a liquid product concentrate is pumped to the atomizing device where it is broken into small droplets. These droplets meet a stream of hot gas and lose moisture very rapidly while suspended in the drying air. The dry powder is separated from the moist air in cyclones by centrifugal action. The atomizer comprises either a spinning disc (rotating at 2000-20,000 rpm) or static high velocity jet nozzles.

Average operating conditions for spray drying vary depending on the application and on the dryer system used. These conditions must be adjusted to produce the desired uniform moisture content. The final moisture content and final particle size can be controlled by changing the operating conditions. Final moisture content is determined by the relative humidity of the outlet air and if that value is too high, powder particles will absorb rather than lose moisture.

The primary conditions which may be controlled directly by the operator are:

• Inlet temperature
• Flow rate of liquid feed (pump speed and pressure)
• Air flow rate
• Particle size (adjustment of atomizer)

Outlet temperature and relative humidity of the outlet air are also important, but can only be controlled indirectly through the adjustment of primary conditions.

Atomization

Atomization is of particular interest to the metal industry for generating fine powders. Generally it is carried out using an inert carrier gas. This gas forces molten powder (notably aluminium or solder) through a nozzle and then cools the droplets into highly spherical powders. The physics of gas atomization of metal powders involves multiphase gas flow, heat transfer, droplet formation, and solidification. Interaction between these individual mechanisms complicates understanding and modelling of the process. In addition, the chemical activity between some metals introduces further complexity.