SPRAY Dryers:

A spray dryer, as the name implies, is a device for drying, utilizing a spray.
A spray dryer mixes a heated gas with an atomized (sprayed) liquid stream within a vessel (drying chamber) to accomplish evaporation and produce a free flowing dry powder with a controlled average particle size.
The unit operation of spray drying includes the following key components:

A method for atomizing a solution or slurry
An air/gas heater, or a source of hot air such as a waste flue gas
A gas/spray mixing chamber with adequate residence time and droplet trajectory distance for achieving the heat and mass transfer
A means for recovering the solids from the gas stream
A fan to induce the required air/gas through the spray drying system
The unique feature of a spray dryer is the surface area per unit weight generated by atomization of the liquid feed. It is this fact that enables a spray dryer to work. For example, feed atomized to 100 micron average droplet size generates approximately 15,400 ft2/lb of surface area. The same feed atomized to an average of 20 micron generates approximately 77,021 ft2/lb! This is like spreading one gallon of feed over 14 football fields!

DESIGN CONSIDERATIONS
The most important variables in the design of a spray dryer is the evaporation rate and the required particle size distribution of the product. The evaporation rate and the dryer ΔT (inlet air temperature minus the outlet air temperature) dictates the amount of drying air needed, which dictates the sizing and cost of almost all of the system components. The particle size requirement affects the choice of atomization and can also affect the size of the spray dryer.

For a given evaporation rate, the higher the design ΔT, the less process air is required, and the smaller and less costly the spray drying system will be. Many products, though, cannot use high inlet temperatures due to the potential of thermal degradation of the product.

ALBUM